Investigation of Layer-by-Layer Assembly and M13 Bacteriophage
Nanowires for Dye-Sensitized Solar Cells
by
ARCHIVES
Rebecca L. Ladewski
B.S. Chemical Engineering, B.A. Philosophy
University of Notre Dame, 2007
SUBMITTED TO THE DEPARTMENT OF CHEMICAL ENGINEERING IN PARTIAL
FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY IN CHEMICAL ENGINEERING
AT THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
JUNE 2012
C 2012 Massachusetts Institute of Technology. All rights reserved
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Signature of Author:
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Department of Chemical Engineering
May 4, 2012
Certified by:
Certified by:
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Paula T. Hammond
Professor of Chemical Engineering
Thesis Supervisor
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Angela M. Belcher
Professor of Materials Science and Biological Engineering
Thesis Supervisor
Accepted by: .................................................................
PT----..........
Parc S.....
oyle.
Patrick S. Doyle
Professor of Chemical Engineering
Chairman, Committee for Graduate Students
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Investigation of Layer-by-Layer Assembly and M 13 Bacteriophage
Nanowires for Dye-Sensitized Solar Cells
by
Rebecca L. Ladewski
Submitted to the Department of Chemical Engineering
on May 7, 2012 in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy in Chemical Engineering
at the Massachusetts Institute of Technology
ABSTRACT
A number of challenges related to the development of new organic-inorganic photovoltaic
systems exist, including the ability to enhance the materials interface and improve the control
required in development of nanoscale materials. Layer-by-layer (LbL) assembly allows for the
incorporation of a wide range of functional materials into structured thin films based on the
alternate adsorption of cationic and anionic species. Biomolecules, and in particular viruses,
show great potential as components of functional materials due to their capacity for molecular
recognition and self-assembly. Here we report that by substituting a negatively charged variant
of M13 bacteriophage for the negatively charged polymer during the dip LbL assembly process,
phage can be incorporated into a hybrid material with characteristics of both its biological and
polymeric components. The resulting mesoporous polymer films can be used as a template for
the construction of the titania photoanode of dye sensitized solar cells (DSSCs) with a novel
nanowire architecture to enhance electron transport. The biotemplated nanowires are shown to
significantly increase device electron diffusion length and increase device efficiency as
compared to LbL-templated titania photoanodes made without bacteriophage.
Spray LbL is also investigated as an assembly method for the construction porous templates for
titania photoanodes. The necessary porous transition is shown to occur on flat substrates, like
those normally utilized for DSSCs, and on porous metal meshes, substrates that have been
proposed as lower-cost DSSC current collectors. Spray LbL is demonstrated to coat metal to
different degrees of conformality as a function of mesh pore size. The conformality of the
coating, in turn, determines which functions it could assume within a LbL-based DSSC.
Thesis Supervisor:
Thesis Supervisor:
Paula Hammond
Professor of Chemical Engineering
Angela Belcher
Professor of Materials Science and Biological Engineering
4
5
For my wonderful parents,
Bruce and Barb Ladewski
who taught me love, respect and curiosity
and my equally wonderful husband,
David Couling,
who has continued the lessons
6
7
Acknowledgements
I would like to thank my advisors, Paula Hammond and Angela Belcher for their academic and
research guidance. They have both been incredibly supportive of my explorations throughout
graduate school, allowing me to pursue unrelated interests in technology policy and teaching.
They have also, and each in her own way, been wonderful models of successful women in
science. Angie gave a speech during graduate orientation about her research and her own
experiences in graduate school that inspired me and led me to reconsider my declaration, leftover
from my undergraduate prejudices, to never pursue biological research. She has always
impressed me with her vision, not limiting herself to the current state of the art, but imagining
and helping her students see how things could be if... This vision, combined with her down-toearth personality and humility are things that I will try to take with me to whatever endeavors I
undertake next. Paula has shown me what an incredibly focused person can accomplish, even in
the face of extensive, multifaceted demands. I have continually been impressed with her
kindness, memory, and intelligence. She has been a wonderful example for me of what
successful leadership and management can entail and produce. Also, it didn't hurt that these two
women made it possible for me to shake hands with the President of the United States.
I would also like to thank my other committee members: Tonio Buonassisi and Karen Gleason.
Their intellectual input has been invaluable. Tonio has always helped me think about solar cell
device physics in new and deeper ways. Karen has always prompted me to understand my own
results on a deeper level. I must also thank William Tisdale for agreeing late in the hour to
preside at my thesis defense. I deeply appreciate the facilitation from all of these MIT faculty
members.
I would also like to thank the professors at Notre Dame who helped me early in my research
career, particularly, Professors Joan Brennecke and David Leighton. Dr. Brennecke sent me an
email inviting me to participate in research in her lab group during my sophomore year at Notre
Dame. That email and the subsequent meeting started me down a path that I had did not know
was open to people with my background. Dr. Leighton advised me on my senior thesis and
shared his significant knowledge about fluids and index notation. He left me with these wordsto-live-by for experimentalists (though I have not always followed his advice as well as I wish I
had), "I have never regretted writing something down. I have often regretted not writing
something down." I have done both and couldn't agree more.
I would like to thank everyone who has worked on different aspects of these projects with me,
Dr. Friederike Fleischhaker for her help getting the solar project going from scratch and my
UROPS Alex, Sunshine and Matt for their contributions to different aspects of this work. I
would like to single out Dr. Rebekah Miller for being my comrade in arms during a very
productive period of activity on the bacteriophage-DSSC project. Our discussions and research
interactions will be some of my fondest memories of graduate school and resulted in more than a
8
few of the figures in Chapters 2 and 3. I must also single out Po-Yen Chen, whose tenacious
spirit and talent for making DSSCs has pushed the bacteriophage-DSSC work to a new and
deeper level of understanding. The ENI solar subgroup has given very good research feedback
and shared interesting papers and research techniques, including Jifa Qi, Forrest Liau, Xiangnan
Dang, Nick Orf, and Noemie Dorval Courchesne.
I would like to thank the entire Hammond and Belcher lab groups 2008-2012 for help and advice
during group meetings and in the lab. I must acknowledge Nasim Hyder for his invaluable help
with ANOVA and nanoindentation experiments (resulting in more than a few of the figures in
Chapter 3), David Liu for taking over lab managing responsibilities, Kevin Krogman for building
me a sprayer, Nicole Davis, Kevin Huang, and Megan O'Grady for fun in the office, and Nathan
Ashcraft, Avni Argun, and the Dans (Bonner and Schmidt) for helping me get started as a lowly
first year graduate student in the lab. The people in the Belcher lab have also been very helpful
and available, particularly Mark Allen, who always goes above and beyond whenever you seek
him out for help and John Casey, for helping to keep us all safer.
Alan Schwartzman in the Nanomechanical Technology Laboratory has been incredibly helpful to
me in the last part of my thesis with making and understanding nanoindentation experiments.
Bill Dinatale in the ISN has always been friendly and incredibly knowledgeable about electron
microscopy methods. The staff of the VWR stockroom was always helpful and wonderful (and
almost always had a candy dish out!). Christine Preston, Linda Mousseau, Liz Galoyan, and
Jared Embelton have been wonderful administrative assistants to Paula and Angie, helping me
with orders, payments, and all of the other stuff that needs to happen in order to get to do
research. Suzanne Maguire has also been tremendously helpful with the facilitation of the
necessary academic/administrative hurdles involved in pursuing a PhD. I have always been
impressed by and grateful for the competence that Gwen Wilcox exuded whenever I interacted
with her to schedule meetings. I must also thank Professors Bill Deen and Yang Shao-Horn,
who helped me with an academic petition form that changed my status from would-like-tograduate to is-academically-eligible-to-graduate. Barry Hughes was always a wonderful help
whenever we needed to install something, power something, or fix a mechanical problem in the
76 lab. Steve Wetzel was a good resource for many years regarding how to get things fixed or
updated in the 66 lab.
I would like to gratefully acknowledge my mentors while I was at MIT, which included my
advisors and my thesis committee, but also people from other organizations I had the good
fortune to be involved in. Tony Lee taught me about effective leadership, team management,
and perseverance through volleyball and his example as a talented and kind coach. Reen Gibb
taught me responsibility and various strategies for effectively presenting material to a diverse
group of people. Along these lines I must also thank Lynsey Kraemer, who supervised my
student teaching at Watertown high school and helped me become a better teacher.
9
I would be remiss not to single out my friends near and far in helping me preserve my sanity
throughout this process. The Women's Volleyball Club of MIT has afforded me many fun
afternoons and evenings of stress-relieving volleyball, particularly with the Darcys, Jen, and
Courtney. My non-sports related friends at MIT have played equally important roles in the
maintenance of my sanity. Bradley and Blair (my killer B's) have given me sympathy, empathy,
fun, and good study buddies (particularly during first year and before quals). Caley Burke was
always a source of fun and interesting conversations (about women in science and engineering
research mixed in with a melange of other topics). My double, triple, and sometimes (if we were
very lucky) quadruple date group: CJ, Stef, Meredith, Jerry, Kevin, and Sarah for the life,
laughter, and food. I am very grateful for these wonderful people and sustaining experiences.
I find it difficult to convey how important my family has been to getting me to this place and
getting me through the ups and downs here. My parents, Bruce and Barb Ladewski, have been
amazingly supportive of my quirky interests my whole life. They fostered in me a love of and
respect for the natural world and all its wonders. Sarah and Brett Crawford (my sister and
brother-in-law) have been incredibly supportive of me, particularly during the thesis drafting and
revising stage. In fact, my whole family, Hutchins, Ladewski and more recently, Couling (+3
Crawfords) has always made me feel accepted and comfortable making the choices that were
right for me and that led me to science, to chemical engineering and philosophy at Notre Dame,
and ultimately to MIT for further exploration of these interests. My husband, David Couling, has
been a boon companion throughout college and graduate school. He does an excellent job
propping me up when I am in a slump and grounding me when I impatiently start building
castles in the sky. I am inspired by his kindness, humbled by his intelligence, and grateful for his
partnership (and facility with MATLAB). There is no one else quite like him.
I feel incredibly lucky to have had these experiences and people in my life. Thank you again to
everyone for all the help and support.
10
Financial support for this work came from ENI and MITEI. I also gratefully acknowledge the
NSF Graduate Research Fellowship, NSF Grant #0645960, which allowed Paula and Angie to
hire me before other funding was certain. This work also utilized the imaging facilities at the
MIT Institute for Soldier Nanotechnologies and the shared experimental facilities at the CMSE
for X-ray diffraction and X-ray photoelectron spectroscopy. The mechanical testing studies were
performed at the Nanomechanical Technology Laboratory at MIT.
1
Table of Contents
List of Figures
16
List of Tables
23
Introduction and Background ............................................................................
24
1.1
Current Technology for Harnessing the Power of the Sun........................................
24
1.2
Dye-Sensitized Solar Cells ........................................................................................
24
1.3
Layer-by-Layer Assembly ........................................................................................
28
Chapter 1.
1.3.1
LbL Incorporation of a Nanowire Template: M13 Bacteriophage ......................
30
1.3.2
Cost Reduction through Faster Processing and Metal Current Collectors ......
33
1.4
Thesis O verview .......................................................................................................
1.5
R eferen ces.....................................................................................................................
Chapter 2.
. 34
35
Layer-by-Layer Deposition of Engineered M13 Bacteriophage for the Construction
of Dye-Sensitized Solar Cells with Novel Titania Architectures .............................................
38
2 .1
In tro d uctio n ...................................................................................................................
38
2.2
DSSC Materials and Physics ...................................................................................
40
Active Components of DSSCs.............................................................................
2.2.1
41
2.2.1.1
Titania - Photoanode....................................................................................
41
2.2.1.2
Dye - Photoactive Absorber.........................................................................
41
2.2.1.3
Electrolyte - Dye Regeneration....................................................................
43
2.2.2
DSSC Device Performance Parameters...............................................................
44
2.2.2.1
Origin of DSSC Photovoltage and Photocurrent.........................................
44
2.2.2.2
Electron Diffusion Length as Motivation for this Work .............................
45
Bacteriophage Incorporation......................................................................................
2.3
2.3.1
2.3.1.1
Literature Precedent .............................................................................................
LbL Involving Viruses as Functional Components......................................
47
47
47
12
2.3.1.2
Previous Applications of M13 Bacteriophage to DSSCs .............................
48
2.3.1.3
Our Approach ...............................................................................................
49
2.3.2
M aterials and M ethods........................................................................................
49
M13 Bacteriophage Amplification, Modification with Oregon Green, and
2.3.2.1
Quantification of Labeling.............................................................................................
49
2.3.2.2
Preparation of Polym er Solutions.................................................................
50
2.3.2.3
Dip Layering Process....................................................................................
51
2.3.2.4
AFM Imaging ..................................................................................................
52
2.3.2.5
Bacteriophage Depletion Study....................................................................
52
2.3.3
2.4
2.4.1
Results and D iscussion ........................................................................................
54
DSSC Photoanode Generation from LbL Thin Film s...................................................
57
Literature Precedent .............................................................................................
57
2.4.1.1
Direct LbL of Titania for D SSC Applications.............................................
57
2.4.1.2
Polyelectrolyte-only Layering Followed by Titania Conversion .................
58
Porous Transition......................................................................................................
58
Previous Work Using a Porous LbL Film as a Titania Template ..............................
60
2.4.1.3
2.4.2
Our Approach ...............................................................................................
60
M aterials and M ethods........................................................................................
60
2.4.2.1
Porous Transition.........................................................................................
61
2.4.2.2
Top Skin Rem oval........................................................................................
61
2.4.2.3
Titania Conversion ......................................................................................
63
2.4.2.4
High Temperature Annealing ......................................................................
64
Film characterization ....................................................................................................
66
Surface Area Characterization ....................................................................................
66
2.4.2.5
Redipping ......................................................................................................
67
13
Results and D iscussion ........................................................................................
2.4.3
69
A ssem bly and Testing of LbL-based D SSC D evices ...............................................
70
2.5.1
Literature precedent .............................................................................................
70
2.5.2
M aterials and M ethods.........................................................................................
70
Solar Cell Construction ...............................................................................
70
Titania N anoparticle Control ....................................................................................
71
M aterials .......................................................................................................................
71
2.5
2.5.2.1
2.5.2.2
D evice Testing Procedures ...........................................................................
72
JV Curves......................................................................................................................
72
EIS testing and Ldiff Extraction ..................................................................................
72
Dye Loading..................................................................................................................
72
Results and D iscussion ........................................................................................
72
2.5.3
2.6
Conclusions...................................................................................................................
76
2.7
References.....................................................................................................................
77
Chapter 3.
Coating Planar and Non-planar Structures via Spray Layer-by-Layer Assembly.. 82
82
3.1
Introduction...................................................................................................................
3.2
Investigating the Porous Transition in Spray-deposited Weak Polyelectrolyte Films . 83
3.2.1
Introduction.............................................................................................................
83
3.2.2
M aterials and M ethods.........................................................................................
84
3.2.2.1
Preparation of Polym er Solutions................................................................
84
3.2.2.2
Substrate Cleaning.........................................................................................
84
3.2.2.3
Spray Layer-by-Layer A ssem bly..................................................................
84
3.2.2.4
D ip Layer-by-Layer A ssembly....................................................................
85
3.2.2.5
Film Post-treatm ent Conditions and A nalysis .................................................
85
3.2.2.6
AN OV A and Design of Experim ents ...........................................................
85
14
3.2.3
Results and D iscussion ........................................................................................
3.3
Spray LbL Pore Bridging of M etal M eshes...............................................................
86
92
3.3.1
Introduction.............................................................................................................
92
3.3.2
M aterials and M ethods........................................................................................
93
3.3.2.1
M esh Substrates.............................................................................................
93
3.3.2.2
Solution Preparation ......................................................................................
94
3.3.2.3
Spray Layer-by-Layer Assembly..................................................................
95
3.3.2.4
Drying Conditions ........................................................................................
95
3.3.2.5
% Bridging Coverage Analysis .......................................................................
95
3.3.2.6
Imaging............................................................................................................
96
3.3.2.7
Film Thickness Analysis .................................................................................
96
3.3.2.8
N anoindentation...........................................................................................
97
Results and Discussion ........................................................................................
98
3.3.3
3.3.3.1
Comparison of Weak and Strong Polyelectrolyte Systems ..........................
3.3.3.2
Comparison of "Normal" LbL Assembly and Bridged Film Assembly ....... 101
98
Proposed Mechanism for Spray Bridged Film Formation...................
102
Film Growth Behavior ................................................................................................
102
3.3.3.3
Controlling the Am ount of Bridging on M eshes ...........................................
104
Effects of Surfactant and Pore Size on M esh W etting................................................
105
3.4
Conclusions.................................................................................................................
110
3.5
References...................................................................................................................
111
Chapter 4.
Recom mendations for Future W ork......................................................................
114
4.1
Brief Sum m ary of Results...........................................................................................
114
4.2
LbL Incorporation of Other Components for DSSC Applications .............................
114
4.3
Spray LbL Deposition for DSSC Photoanodes...........................................................
115
15
4.4
Device Design Improvement ......................................................................................
116
4.5
Non-conformal Spray LbL Deposition .......................................................................
116
4 .6
R eferen ces...................................................................................................................
117
DSSC Device Assembly and Testing................................................................
Appendix A.
JV Testin g ...................................................................................................................
A .1
119
119
A. 1.1
Reading and Understanding JV Curves ................................................................
119
A.1.2
JV Performance of Various Iterations of Device Design......................................
121
A.1.3
Effect of Surlyn or Parafilm Separator Size .........................................................
126
M easurement Transience ...............................................................................
128
A. 1.4
M asking and Framing Effects...............................................................................
130
A.1.5
MATLAB Code for JV Data Analysis..................................................................
131
A.1.3.1
A.2
IPCE M easurements....................................................................................................
135
A.2.1
Explanation of IPCE M easurements .....................................................................
136
A.2.2
M aking Accurate IPCE Measurements on DSSCs ...............................................
136
Electrochemical Impedance Spectroscopy (EIS) of DSSCs.......................................
137
Explanation of EIS Measurements for DSSCs .....................................................
137
A.3
A.3.1
A.3.1.1
General EIS Review ......................................................................................
137
A.3.1.2
Transmission Line M odel..............................................................................
138
A.3.2
M aking Accurate EIS M easurements for Ldif Determination ..............................
139
A.3.3
MATLAB Code for Fitting EIS Measurements with the Equivalent Circuit ....... 141
A .4
R eferen ces...................................................................................................................
14 3
16
List of Figures
Figure 1.1 Images of DSSC devices that are a) flexible, b) transparent and multicolored, and c)
manufactured in a roll-to-roll printing process. (a) and (c) are reproduced from [5]. (b) is
reproduced from [6]......................................................................................................................
25
Figure 1.2 Schematic of DSSC device operation. Light is absorbed by a sensitizer that is
adsorbed to the surface of a nanostructured TiO2 electrode. The excited sensitizer injects the
electron into the TiO 2 phase where it diffuses to a transparent conducting oxide (TCO) current
collector, travels through an external load, and re-enters the device at the platinum
counterelectrode. An electrolyte redox mediator (Ij/I-) shuttles the charge from the
counterelectrode to the dye to regenerate its neutrality. Reproduced with permission from [9].
C opyright 2010 Wiley-V C H .....................................................................................................
27
Figure 1.3 Schematic of Dip LbL Assembly Process. Reprinted with Permission from [12].
C opyright 2004 Wiley-V C H .....................................................................................................
28
Figure 1.4 Cross-sectional SEM image of LbL deposited film that has been made porous. Scale
bar is 1 pm . ...................................................................................................................................
29
Figure 1.5 Schematic of DSSC device containing nanowires as ID "highways" for electron
diffusion to the current collector...............................................................................................
30
Figure 1.6 Schematic of M13 bacteriophage with the DNA shown in red and the various coat
proteins (PVIII, PIII, PVI, PVII, PIX) shown surrounding it...................................................
31
Figure 1.7 TEM images of titania templated bacteriophage.........................................................
32
Figure 1.8 Schematic of BCE solar cells that do not use a transparent conducting oxide layer as a
current collector. CE is the platinum counterelectrode. Adapted with permission from [39].
Copyright 2008 Am erican Chem ical Society. ...........................................................................
34
Figure 2.1 Schematic of the nanostructured titania photoanode containing phage-templated
nan ow ires. .....................................................................................................................................
39
Figure 2.2 Scheme of DSSC operation in a bilayer device. TCO is a common abbreviation for
transparent conducting oxide. Electrons are shown in red circles. Charged and uncharged anions
are show n as pink circles. .............................................................................................................
40
Figure 2.3 Chemical structures of the common Ru-based DSSC dye molecules a) red dye and b)
black dy e .......................................................................................................................................
42
17
Figure 2.4 Energy level diagram for a DSSC. For clarity, the color scheme of the phases
matches that shown in Figure 2.2. Load refers to the resistance of the external circuit. Redox
refers to th e electrolyte..................................................................................................................
44
Figure 2.5 Illustrations of electron diffusion through different device types showing (a) an ideal
bilayer-type DSSC, (b) an ideal sintered nanoparticle DSSC, and (c) a more realistic picture of
the sintered nanoparticle DSSC. rt refers to the rate of electron transport out of the device. rr
refers to the rate of electron loss due to recombination. Arrows show potential paths for
electron s to trav erse. .....................................................................................................................
45
Figure 2.6 UV-Vis spectroscopy of Oregon Green modified bacteriophage............................
50
Figure 2.7 Schematic of the dip layering process with (tetralayers) and without (bilayers)
b acteriop h ag e ................................................................................................................................
52
Figure 2.8 Bacteriophage dipping bath concentration as a function of tetralayers deposited.
Black diamonds represent the concentration of the phage bath before doping. Gray triangles are
the concentrations of the bath after phage doping. All concentrations are normalized to the same
bath volum e of 34 m L ...................................................................................................................
53
Figure 2.9 Tracking film fluorescence increase and phage loading per film area (from the bath
depletion study) as a function of tetralayers applied. ................................................................
54
Figure 2.10 AFM images (amplitude) of the tetralayer films after M13 bacteriophage deposition
(left) and after LPEI deposition (right). ....................................................................................
55
Figure 2.11 AFM Images of Tetralayer Films from 5 mM NaOAc buffer at (a) pH 4.60, (b) pH
4.75, (c) pH 4.90, and (d) pH 5.05. All images are a 3 pm x 3pm square. ..............................
56
Figure 2.12 Charge density analogy between polyelectrolytes and acids or bases. PDAC is
poly(diallyldimethylammonium) chloride. PSS is poly(styrene sulfonate)...............................
58
Figure 2.13 Scheme depicting the pH dependence of the charge density of LPEI and PAA.
Green bars represent where the polymer is more neutral. Blue and yellow bars indicate where
the polymer has more charged units. pKa values for LPEI and PAA came from
[60]
and
[61]
resp ectiv ely . ..................................................................................................................................
59
Figure 2.14 Schematic of film post-treatment procedure steps. ................................................
60
Figure 2.15 LbL growth curves for films without (b) and with (a) bacteriophage before (black
diamonds) and after (gray squares) the porous transition. Note that the polymer only data are
18
shown as a function of bilayers while the bacteriophage-containing films are shown as a function
o f tetralay ers..................................................................................................................................
61
Figure 2.16 SEM micrograph of a scratch in an LPEI/PAA film after the porous transition. The
silicon substrate is visible to the left of the scratch, the polymer skin to the right.................... 62
Figure 2.17 SEM image of and LPEI/PAA film after top skin removal..................
63
Figure 2.18 SEM image of 8 ptm polymer and titania film after annealing. The lighter colored
islands are what remains of the polymer film. The darker spaces between them are the
underlying silicon substrate. .........................................................................................................
63
Figure 2.19 XPS data for titania films templated using the method outlined above and for a
commercially available nanoparticulate titania paste. .............................................................
64
Figure 2.20 Top down optical microscopy of LPEI/PAA films annealed at 150'C under dry
conditions (a) and in a w ater bath (b). ......................................................................................
65
Figure 2.21 Top-down SEM images of annealed titania films in (a) dry conditions or (b) humid
conditions described below ...........................................................................................................
65
Figure 2.22 XRD data for template titania before and after high temperature annealing ......
66
Figure 2.23 Dye loading of titania films as a function of film thickness..................................
67
Figure 2.24 Cross-sectional SEM images of LPEI/PAA films at (a) 20 bL, (b) 30 bL, (c) 40 bL,
(d) 60 bL , and (e) 80 bL ................................................................................................................
68
Figure 2.25 SEM image of a redipped LPEI/PAA film. The top crust of the lower film was not
removed in order to highlight the existence of the two separate films. ....................................
68
Figure 2.26 SEM images of porous, annealed titania films templated from a) PAH/PAA (10 bL)
by Shiratori et al.56 , b) PAH/PAA (15 bL) by Shiratori et al.", and c) LPEI/PAA by Hammond et
al.5 0 and d) LPEI/PAA in this work (not redipped). (a), (b) and (c) are reproduced from [56]
Copyright 2003 with permission from Elsevier, [57] Copyright 2006 with permission from
Elsevier, and [50] Copyright 2005 with permission from Wiley-VCH, respectively. .............. 69
Figure 2.27 Generation IV of device architecture. The picture is an image of real device, and the
adjacent scheme depicts the way the active components are aligned. In the scheme, the dark gray
color represents the platinum counterelectrode. The light gray color represents the FTO current
collector. The dark green represents the dyed titania film. The yellow is a spacer layer. The
dashed lines outline the active/working portion of the device.................................................
71
19
Figure 2.28 JV curve for doctor bladed nanoparticle devices (NP) and for LbL devices made
73
without bacteriophage (L) and with bacteriophage (P).............................................................
Figure 2.29 Electron diffusion length data normalized to film thickness (d) as a function of
75
ap plied bia s. ..................................................................................................................................
Figure 3.1 Growth curves for spray (gray diamonds) and dip LbL (black triangles) films before
(filled points) and after (empty points) the porous transition. Data were fit with linear regression
equations, the equations of which are shown in the boxes. The inset is a cross-sectional SEM of
87
100 bL spray-deposited porous film . .........................................................................................
Figure 3.2 Film thickness as a function of LPEI assembly pH. The line is the ANOVA
89
prediction. The diamonds are the raw thickness data for 100 bL films....................................
Figure 3.3 Contour plot of film porosity as a function of LPEI assembly pH and film transition
pH. The gray region represents the range of assembly pH where the LPEI/PAA film grew
slowly due to the high charge density of LPEI. The numbers represent the calculated porosity of
91
the film at that elevation. ..............................................................................................................
Figure 3.4 Schematic of the spray LbL setup and mesh substrate holder (a) and picture of a mesh
95
m ounted on the spray holder (b). ..............................................................................................
Figure 3.5 Visual sequence for ImageJ analysis. The first step involves taking a digital picture of
the flat mesh (a), cropping the picture to the active area (b), and then applying the thresholding
96
(c) to make the image consist of only black or white pixels....................................................
Figure 3.6 Cross-sectional SEM images of a bridged polymer film. The red rectangle in (a) is
shown at a higher magnification in (b). The three pseudo-circular shapes in (a) and the two in
(b) are the cross-sections of the metal w ire. .............................................................................
97
Figure 3.7 Optical microscopy of bridged films on the 240 mesh. Scale bars are not shown in all
images because the spacing (240 pim) and diameter (22 pim) of the wires acts as a reference..... 99
Figure 3.8 Cross-sectional SEM images of spray LbL deposited polymer films of a) 150 bL
PDAC/PSS pH 2.00, b) 100 bL PDAC/PSS pH 2.00, 0.20 M NaCl, c) 100 bL LPEI/PAA pH
4.40, d) 150 bL PDAC/PSS pH 2.00, e) 100 bL PDAC/PSS pH 2.00, 0.2 M NaCl, and f) 150 bL
LPEI/PAA pH 4.40. The top row of images is shown at low magnification to give an indication
of the overall character of the film and mesh. The bottom row of images is at higher
magnification and shows the film conformation in more detail. ................................................
100
20
Figure 3.9 Images of a 150 bL LPEI/PAA film, pH 4.75 after a porous transition at pH 2.25
taken via a) bottom illumination optical microscopy, b) top illumination optical microscopy, and
c) scanning electron m icroscopy.................................................................................................
100
Figure 3.10 Scheme depicting the LbL steps that are proposed to explain the formation of a
bridged film on a mesh (shown as the orange #s). a) Mesh is wetted (blue droplet). A meniscus is
formed. b) Positive polyelectrolyte (red droplet) is introduced. c) Some polyelectrolyte adheres
to the grid. Some is entrained in the meniscus. d) Negative polyelectrolyte (yellow droplet) is
introduced. e) Some polyelectrolyte adheres to the grid. Some is entrained in the meniscus.
Electrostatic crosslinks begin to form in the meniscus. f) The LbL process is iterated. After
iteration, the LbL film is spans across the mesh grid because of the electrostatic crosslinks that
span the meniscus. g) A balance of film tensile strength (due to electrostatic crosslinks) and
strain induced from film deswelling (due to loss of water) determines whether the bridged film in
each # survives the film drying...................................................................................................
102
Figure 3.11 Growth curves for sprayed films assembled on glass substrates (a) and bridged films
assembled on mesh substrates (b). In (a), the film thickness increases with the addition of more
bilayers, and the thickness increase per bilayer is dependent upon the polymer system studied.
.....................................................................................................................................................
10 3
Figure 3.12 Digital camera images of pore bridging for (a) 200 bL PDAC/PSS, 0 M NaCl, (b)
150 bL PDAC/PSS, 0.2 M NaCl, and (c) 150 bL LPEI/PAA, pH 4.75. The images were taken of
freeze dried meshes to maximize the contrast between the bridged (light colored or white) and
unbridged (gray or brown) portions of the m esh. .......................................................................
104
Figure 3.13 Percent bridging coverage as a function of polyelectrolyte system and bilayers
applied. All films were dried at 75% relative humidity and ambient temperature.................... 105
Figure 3.14 Digital camera images of (a) 240 mesh wetted with pure water, (b) 118 mesh wetted
with pure water, (c) 240 mesh wetted with 1 mM Triton X-100 solution, (d) 240 mesh wetted
with 10 mM Triton X- 100 solution. Only images bounding the wetting/non-wetting transition
are included. Droplets indicate a tendency toward the instability. ............................................
106
Figure 3.15 Digital camera images of (a) 240 mesh, (b) 118 mesh, and (c) 47 mesh coated with
150 bL of PDAC/PSS, 0 M NaCl. Films were freeze dried in order to maximize contrast for
imag in g . ......................................................................................................................................
10 7
21
Figure 3.16 Quantification of percent bridging study on the effects of mesh pore size and rinse
solution surfactant concentration. All films were freeze-dried to eliminate the potentially
deleterious effects of film drying on bridging coverage. Results for 240 mesh are shown as
diamonds, 118 mesh as triangles, and 47 mesh as squares. All data for meshes that were rinsed
with any surfactant are shown with filled points (black).
Empty points are from meshes that
w ere rinsed w ith pure w ater........................................................................................................
108
Figure 3.17 Mesh bridging coverage as a function of pore size over the entire range of pore sizes
studied. Bridging occurs above the threshold 30-40% level at metal mesh pore sizes of 10 pm
and 240 pm. Image insets correspond to the data points indicated by the arrows..................... 109
Figure 3.18 Top-down SEM of a) an uncoated membrane, b-d) 150 bL PDAC/PSS, 0 M NaCL
film bridging the 10 pm pores of a track-etched polycarbonate membrane with a razorblade
scratch. Image (b) is of the spray-facing side of the film, image (c) is of the opposite side, and
im age (d) is a close up of the outlined are in (c).........................................................................
110
Figure 4.1 Schematic of mid-contact solar cells. Back contact solar cells are shown for
comparison. MCE and BCE indicate the mid-contact and back-contact electrodes respectively.
Adapted with permission from [10]. Copyright 2008 American Chemical Society...................
117
Figure A. 1 JV curve (black line) generated from the ideal diode equation with m= 1.6, Jc= 15
mA/cm 2, and Jd = 10-7 mA/cm 2. The short circuit current density (Jsc), open circuit voltage (Voc),
current density at maximum point (Jmp) and voltage at maximum power (Vmp) are defined as
sh ow n in the figure......................................................................................................................
120
Figure A.2 Pictures and schematics of the 4 generations of device architecture utilized
throughout this work. Shapes outlined in dashed lines in the schematics represent the active
dom ain in the device...................................................................................................................
122
Figure A.3 JV curves for best devices made with Generation I architecture. Both devices were
750 nm th ick . ..............................................................................................................................
12 3
Figure A .4 Generation II best perform ing devices. ....................................................................
124
Figure A.5 Mask application on Generation II device architectures. .........................................
125
Figure A.6 JV data for best performers of the Generation III device architecture. LbL devices
without phage are shown in blue. M13-LbL devices (with phage) are shown in green.
N anoparticle devices are show n in red. ......................................................................................
125
22
Figure A.7 Cross-sectional schemes of DSSC devices showing different options for separator
coverage. In all of these schemes, the blue rectangles represent the FTO-coated glass slides (the
one on top being the platinum counterelectrode and the one on the bottom being the photoanode
current collector). The green rectangles represent the dyed titania. The orange rectangles
represent the electrolyte, and the gray areas represent the polymer separator............................
126
Figure A. 8 JV performance results for nanoparticle devices made with the same active area but
different separator schem es.........................................................................................................
128
Figure A.9 JV measurement transience as a function of separator scheme, shown for schemes a
and c. The transience of type-b is the similar to type-c, so it is not shown here. The red circles
represent the m axim um pow er point...........................................................................................
129
Figure A.10 Effects of masks of different sizes on a type-c nanoparticle DSSC device. In the
scheme in the upper right hand corner, the orange circle corresponds to where the electrolyte and
the dyed titania were, the green circle underneath the orange circle corresponds to where the
dyed titania was not covered with electrolyte, the gray square represents the underlying current
collector. Dashed lines indicate which areas were shaded by a particular mask. For example, the
small mask shaded the entirety of the green and gray areas, which the medium mask shaded only
the gray area. Small, medium, and large correspond to circles of the following diameters: 5/32",
6/32", and 7/32". .........................................................................................................................
13 0
Figure A. 11 External quantum efficiency of a nanoparticle DSSC as a function of chopping
frequency. These data were taken with a white bias light. ........................................................
137
Figure A. 12 DSSC equivalent circuit . Zd represents the Warburg component that is
characteristic of ion diffusion. Rs is the series resistance of the device. RcO and Cco, RTCO and
CTCO,
and Rpt and Cpt are the resistance and capacitance of the TCO-TiO 2 interface, TCO-
electrolyte interface and electrolyte-Pt interface respectively. The extended TiO 2 interface is
characterized with a transmission line element containing the three repeated elements rt, ret, and
c,. This figure has been reproduced with permission from [14] Copyright 2006 American
C hem ical S ociety . .......................................................................................................................
139
Figure A. 13 Suggested simplified equivalent circuit for DSSCs. CPEpt is the constant phase
element of the platinum (characterized by a capacitance and ideality factor). ZTL represents the
transmission line element (identical to the one depicted in Figure A. 11, which in the ZView
softw are w as the extended elem ent type 6..................................................................................
140
23
List of Tables
Table 2.1 Absorbance values for selected wavelengths of Oregon Green modified M13
bacteriop h age ................................................................................................................................
50
Table 2.2 Bacteriophage loss per 5 TL between 5 and 25 TL. ................................................
53
Table 2.3 Device performance parameters extracted from Figure 2.28. ..................................
74
Table 3.1 Summary of input parameters studied in ANOVA....................................................
86
Table 3.2 Values of coefficients given by the JMP software for the prediction of film thickness
and height ratio. Starred p-values indicate significant parameters within the 95% confidence
in terv al. .........................................................................................................................................
88
Table 3.3 Summary of mesh pore and wire sizes sorted by steel type as determined by optical
microscopy. The lengths and diameters presented are pm. .....................................................
94
Table 3.4 Elastic moduli of PDAC/PSS and LPEI/PAA films assembled on glass via spray LbL
(Normal Sprayed Film) or via dropeasting (Dropcast Polyplex Film) or on a metal mesh (Spray
Bridged Film). The values for the spray bridged film are starred because they represent a lower
threshold of elastic modulus, instead of the estimated average value. .......................................
101
Table A.1 List of DSSC Device Parameters and Related Loss Mechanisms .............................
121
Table A .2 JV data extracted from Figure A .3.............................................................................
123
Table A .3 JV data from Figure A .4. ...........................................................................................
124
Table A .4 JV data extracted from Figure A .6.............................................................................
125
Table A.5 Model parameters, suggested ranges and descriptions. The ranges given for rt, ret, and
C are valid only for n= 100. ........................................................................................................
140
24
Chapter 1. Introduction and Background
Every hour, more energy strikes the surface of the Earth than what humans currently use in the
course of a year. This statistic translates to roughly 3 x 1024 J/year (95 PW) of incident solar
radiation'. Given the unequal geographic distribution of fossil fuel resources and growing public
concern about the greenhouse effect, the scientific community has had an increasing interest in
developing economically viable and technologically robust methods for harnessing solar energy.
1.1
Current Technology for Harnessing the Power of the Sun
Semiconductors are the basis for modern solar cells, and there has been much focus on pnjunction devices. A pn-junction is created by intimately contacting p-doped and n-doped
semiconductors. Such devices have motivated much of the research and development of
photovoltaic technology so far, with the highest reported efficiency of 28.3% being achieved for
a single-junction device in August 20112. Unfortunately, such high efficiencies come at an
economic and environmental cost. For example, the input materials into most semiconductor
solar cells must be extremely pure (>99.99%) so that the dopant concentrations can be strictly
controlled and the resulting material highly crystalline 3. The high purity requires manufacturing
techniques that are costly in addition to being chemically and thermally intensive. Therefore, a
significant amount of current research is directed at finding different material morphologies or
semiconductor combinations that would allow cheaper manufacture while maintaining high
conversion efficiency. Such devices could consist of lower grade, polycrystalline
semiconductors or new materials that replace the inorganic semiconductor phases altogether
(with conjugated organic molecules, conducting polymers, electrolytes or etc.). Regardless of
the materials, the ideal solar energy converter would exhibit both a low cost of manufacturing
and a high conversion efficiency.
1.2
Dye-Sensitized Solar Cells
The dye-sensitized solar cell (DSSC) 4 , has aroused considerable interest as a promising low-cost
photovoltaic technology because its unique architecture and material components allow cheaper
manufacturing techniques to be used for fabrication and exciting new solar module designs for
photovoltaic applications.
25
DSSCs present unique opportunities in solar module fabrication and design. Some of these
advantages are pictured in Figure 1.1 below.
Figure 1.1 Images of DSSC devices that are a) flexible, b) transparent and
multicolored, and c) manufactured in a roll-to-roll printing process. (a) and (c)
are reproduced from [5]. (b) is reproduced from [6].
DSSC devices can be made that are flexible, lightweight, transparent, multi-colored, and/or
manufactured via roll-to-roll printing 7 . Each of these attributes has great import on the design
and eventual cost of DSSC modules. Roll-to-roll printing, for example, is well known to be a
cheaper way to manufacture layered devices. Flexibility and module weight can also have a
large impact module costs by lowering installation costs7. Current solar modules are quite heavy
and bulky, and thus more difficult for workers to lift and install. Lightweight, flexible devices
could be rolled up for transport, easily lifted, and the installed simply by unrolling. Also, multicolored and transparent module designs are currently being investigated for commercialization
by two different companies for a new building integrated photovoltaic technology 5' 8 . Such
modules would be installed as power-generating windows (similar to stained glass) or artwork
for buildings. These designs are exciting because they unite beauty and function for the purposes
of green engineering.
These exciting and desirable properties of DSSCs partially stem from their new and different
operational principles. DSSCs are functionally different from pn-junction devices in that these
devices separate the three main mechanisms of photovoltaic activity-light absorption, electron
conduction, and hole conduction-into different materials within the device. In pn-junction
devices, all three mechanisms are performed within the same medium-the doped
semiconductor. Separating these mechanisms into distinct phases has some major design
26
advantages, including requiring lower material purity than other solar technologies do, limiting
recombination of charge carriers by putting electrons and holes in different phases, and creating
the possibility of independently tuning each phase for its specific function. To elaborate, each
phase can be tuned separately from the others in order to maximize its individual performance
and in a way that is not possible for pn-junction devices. For example, the light is absorbed by a
molecule, called the dye (hence dye-sensitized solar cells), dedicated solely to this task. Since
the dye has such a specific purpose, its chemical makeup can be altered without regard to how
easily it conducts electrons or holes, and its energy levels can be tuned to maximize energetic
driving forces for electron transfer, minimize electron transfer losses due to poor energy level
matching between donor and acceptor phases, and absorb a specific spectrum of light as
determined by the application. This tunability is undoubtedly an advantage, but it has limitations
as well. One limitation is that a degree of matching must be established among the phases in
order to maintain stability and match energy levels to create a successful device. Another is that
DSSCs have three different materials that must be in intimate contact at the nanoscale. Because
DSSCs rely on a series of interfacial events to collect a current from incoming photons; it is
crucial to have good interfaces and fine engineering control over them. A schematic of the
DSSC is shown in Figure 1.2 below.
27
Figure 1.2 Schematic of DSSC device operation. Light is absorbed by a
sensitizer that is adsorbed to the surface of a nanostructured TiO2 electrode. The
excited sensitizer injects the electron into the TiO 2 phase where it diffuses to a
transparent conducting oxide (TCO) current collector, travels through an external
load, and re-enters the device at the platinum counterelectrode. An electrolyte
redox mediator (I3-/I-) shuttles the charge from the counterelectrode to the dye to
regenerate its neutrality. Reproduced with permission from [9]. Copyright 2010
Wiley-VCH.
Generally, the fabrication of the DSSC photoanode is achieved through coating a thick paste of
titania nanoparticles onto a TCO-coated glass slide using doctor blading or screen printing. A
high temperature annealing step sinters the particles together and burns off the paste binder. The
resulting structure is that of tightly packed spheres. However, the titania architecture is crucial to
the performance of the device, determining the surface area available for light absorption and the
efficiency of electron transport, among other functions; this packed-sphere structure is not
necessarily optimized for these functions'0 . Thus, engineering the architecture of the titania
phase in order to optimize this set of functions is important and requires very fine, nanoscale
control over the titania film morphology that is not necessarily available through the previously
mentioned coating methods. One coating method that is well known to yield such nanoscale
control over the morphology and chemical composition of films is Layer-by-Layer (LbL)
assembly". The object of this thesis was, therefore, to apply this morphological control to the
nanoscale architecture of a titania photoanode.
28
1.3
Layer-by-Layer Assembly
LbL is a powerful assembly technique that presents several advantages, including the ability to
control the composition of the materials at the nanoscale, to harness distinct properties of
separate materials into one film, and to blend polymers and nano-objects that might otherwise be
immiscible1 2 . Also, the assembly conditions are inherently benign. Thus, this technique is very
desirable from an industrial scale-up perspective because it is a technically straight-forward
water-based assembly method that yields intimate materials mixing with delicate controls even
while the processing is performed at ambient conditions. Also, the method is easily scalable
from very small to very large volumes. It has been used to create films for various applications,
including drug delivery systems, battery separators, fuel cell membranes, ultrahard materials,
etc.13 , .
LbL works by intimately combining dissimilar materials with complementary functionality into
one thin film". Complementary functionality can include electrostatic interactions, hydrogen
bonding, or covalent associations 5 . This technique is illustrated in Figure 1.3 below for the case
of electrostatic interactions between positively and negatively charged polymers
(polyelectrolytes), known as polycations and polyanions respectively.
substrate
polycation
solution
polyanion
solution
Polyelectrol e
Mutilayer Film
Figure 1.3 Schematic of Dip LbL Assembly Process. Reprinted with Permission
from [12]. Copyright 2004 Wiley-VCH.
Figure 1.3 depicts a charged substrate being alternatingly dipped into a solution of polycation
and polyanion. Some polyelectrolytes adhere to the surface, and the excess are washed off in a
rinse bath. The surface charge is reversed16, and the process is iterated to build up conformal
films of varying thicknesses (depending in large part on the number of bilayers, the charge
density of the polymers, and the ionic strength of the solution)' 7. The process is not limited to
29
polyanions and polycations, but has also been proven to work for other materials, including
charged nanoparticles or macromolecules 5 . Also, although dip LbL is pictured in the figure
above, other application methods exist, including spray LbL' 8 '19, spin LbL, and others20 that
affect the processing time and film assembly characteristics.
Thus, LbL is an extremely versatile coating technique that provides many different handles for
controlling the properties of the deposited film. Total film thickness is easily controlled by the
number of layers deposited. The thickness of each layer and the degree to which in interacts
with the other layers is highly tunable depending on the pH, ionic strength, component molecular
weight, and polymer choice 7 . Film thickness and morphology (porosity, roughness) can also be
controlled by adjusting the charge density of the materials, the concentration of polymer, and the
conformation of the charged nano-object or polymer being incorporated. These are precisely the
controls required for creating a DSSC photoanode with nanostructure architecture. However,
this assembly method generally creates condensed films with controlled composition and
morphology but that are not porous. DSSC photoanodes also require some porosity. This can be
attained in LbL films by judicious polymer selection and solution pH control, creating pores up
to several microns in diameter, like those shown in Figure 1.4 below.
Figure 1.4 Cross-sectional SEM image of LbL deposited film that has been made
porous. Scale bar is 1 pm.
A liquid phase deposition procedure 2 ' can be used to coat the now-porous films with titania,
turning them into a viable titania photoanode precursor. Thus, the LbL technique can be used to
create titania films with finely controlled porous morphologies. These porous films can be
created on a variety of substrates and can include charged nano-objects in the layering process.
In this thesis, these porous films were augmented with additional photoanode functionality via
LbL in two ways. First, LbL was used to combine the porous polymeric template with a
nanowire template, M13 bacteriophage, creating a thin film that had the tunable porosity of the
30
polymeric LbL component and a biologically tunable nanowire template. The resulting hybrid
film, as will be explained in Section 1.3.1, has the potential to increase device performance
through enhanced electron collection efficiency. Second, LbL was used to combine the porous
polymeric template with non-traditional current collectors, metal meshes. As will be explained
in Section 1.3.2, this substitution could reduce the cost of manufacturing for DSSCs.
1.3.1
LbL Incorporation of a Nanowire Template: M13 Bacteriophage
The highest performing DSSCs have efficiencies between 11 and 12.3%2
22
. These
efficiencies must be improved in order to make these devices a viable alternative to existing solar
technologies. One way to increase the conversion efficiency of the DSSC device is to increase
the efficiency of electron collection from the titania phase. The packed-sphere morphology that
is the current state-of-the-art DSSC photoanode architecture is optimized for dye adsorption (by
maximizing available surface area), but not for electron conduction. One strategy for increasing
the speed with which electrons travel through the titania film, and thus their probability of
reaching the current collector, is to incorporate of 1-dimensional structures, such as nanowires,
into the photoanode to create electron "highways". Such a scheme is depicted in Figure 1.5
below.
eredox
El
Iye
couple
liquid electrolyte
Figure 1.5 Schematic of DSSC device containing nanowires as ID "highways"
for electron diffusion to the current collector.
By reducing the time electrons spend in the titania photoanode, these pathways should reduce
recombination of charge carriers and lead to higher short circuit current densities and thus,
higher efficiencies. Many others have looked into making films out of nanowires or nanowire-
31
like components only with the objective of increasing the electron collection efficiency and
electron diffusion length (Ldiff, the average distance an electron can travel before suffering a
recombination event) 23 . Grimes et al. synthesized 17.6 ptm long single crystal titania nanotubes
via anodization on titanium metal foil to achieve device efficiencies up to 6.9%24. Structures like
sputter-deposited "nanotrees" and AC anodized "nanobamboo" have given device efficiencies of
4.9%25 and 2.96%26 , respectively. Cellulose fibers have been used as a sacrificial template and
coated with titania via a liquid phase deposition. The resulting hollow nanofibers were mixed
with titania nanoparticle paste and doctor bladed, giving a 7.2% efficient device2 7 . Even though
each of these architectures was shown to enhance electron collection efficiency, none were able
to overcome the loss of surface area for dye loading in order to achieve device efficiencies on par
with the best packed-sphere nanoparticle devices. In other words, none of these devices
achieved an appropriate balance between controlling the nanostructured titania photoanode
interface (and thus the area for dye adsorption) and the conduction of electrons out of the film
through the nanowire component. This failure indicates that the ability to independently control
both the relative content of nanowire component in the film and the surface area of the film
would be required to truly optimize the titania photoanode architecture to maximize device
electron collection efficiency. This object of this work was to create a system wherein this
independent optimization was possible.
The nanowire-template chosen for incorporation was M13 bacteriophage (phage). Phage is a
virus, virulent to E. coli. Phage consists of a single stranded DNA core surrounded by coat
proteins2 8. A schematic of the phage is shown in Figure 1.6 below.
pill, pV
pV
I, lPX
pVIll
6-7 nm
880 nm
Figure 1.6 Schematic of M13 bacteriophage with the DNA shown in red and the
various coat proteins (PVIII, PIII, PVI, PVII, PIX) shown surrounding it.
32
As the dimensions of this figure illustrate, the phage exhibit an unusually high aspect ratio. This
high aspect ratio was one reason they were chosen for study-they have the dimensions of
nanowires29. However, phage were chosen for two other important reasons. First, they are
readily subject to genetic engineering. Taking advantage of the molecular recognition and selfassembly behavior demonstrated by viruses, M13 bacteriophage can be engineered to nucleate
and assemble a wide variety of inorganic materials 30 31 . Harnessing the power of nature to finely
control nanostructures based upon the aggregation of weak interactions with peptide sequences
has been increasingly investigated for the templating of nanomaterials 32 . The M13
bacteriophage is a tunable nanowire template that has been engineered to scaffold a variety of
materials, including iridium oxide for water splitting 33 , gold and cobalt oxide for lithium battery
electrodes 34 , and porphyrins for photochemical devices3 5. The biotemplating method allows
milder fabrication conditions to be employed than would be required to achieve similar nanowire
shapes (via anodization or other methods23 ).
Creating a library of bacteriophage that displays a great diversity of peptide sequences on the
outside protein coat allows the researcher to conduct directed evolution, literally performing
billions of experiments at a time to develop a nanowire template by mimicking nature. This
library can be applied to the desired material template and then subjected to more and more
vigorous rinsing, allowing the selection of bacteriophage that adhere strongly to the material
template 28 . This adhesion has been shown to correspond to directed assembly of that material on
the bacteriophage in solution. Using these methods, a bacteriophage peptide sequence was
identified that templated titania in solution. Images of the templated bacteriophage are shown in
Figure 1.7 below.
a
b
Figure 1.7 TEM images of titania templated bacteriophage.
The other important reason bacteriophage were chosen for this study is that they are inherently
negatively charged, which implies that LbL methods can be used to arrange them in a nanoscale
blend. The phage templated titania nanowires for incorporation into DSSC photoanodes; the
33
LbL polymeric component added a surrounding porous structure and nanoscale assembly
control. Thus, we employ layer-by-layer (LbL) assembly to fabricate a highly porous titania
photoanode in which the phage morphology imparts enhanced electron diffusion characteristics
to a dye-sensitized solar cell (DSSC). Such a titania photoanode would be subject to
independent engineering of its porous morphology and nanowire content. It would also take
advantage of the water-based ambient processing conditions that are inherent to LbL assembly
and bacteriophage templating. These titania photoanodes were put into DSSC devices and tested
in order to document the effects of this addition.
1.3.2
Cost Reduction through Faster Processing and Metal Current Collectors
The incorporation of nanowires would improve device efficiency, but both efficiency and
cost must be considered in any discussion of viable alternative solar energy technologies. The
next portion of the thesis focused on expanding the applicability of the LbL titania template
described in Section 1.3.1 via spray LbL to metal current collectors. These two adjustments,
spray LbL instead of dip LbL and metal current collectors instead of TCO current collectors,
were both investigated as methods to reduce the final cost of any commercial systems based
upon the LbL titania template.
Spray LbL assembly works the same way as the dip LbL assembly described in Section
1.3, but the materials are deposited onto a stationary substrate as a fine mist instead of moving a
substrate between stationary dipping baths3 6 . Spray LbL is also easily scalable and has the
additional advantage that it is up to 25 times faster than dip LbL36 . Processing speed is an
important parameter in determining the ease of manufacture, and is thus deeply related to cost.
Metal mesh current collectors were investigated because the TCO component of DSSCs
is expected to contribute up to 24% of the final module cost 37. Metals have been proposed as a
flexible, conductive, and cheaper alternative to transparent conducting oxide current collectors3 8 .
Although metals are not transparent, the DSSC devices can be made with porous metals that are
applied as a back contact electrode (BCE) to DSSCs as is shown in Figure 1.8.
34
dpB
C
INr
Figure 1.8 Schematic of BCE solar cells that do not use a transparent conducting
oxide layer as a current collector. CE is the platinum counterelectrode. Adapted
with permission from [39]. Copyright 2008 American Chemical Society.
In light of the potential for metal electrodes, the latter portion of this thesis explores LbL thin
film assembly on stainless steel meshes, with the objective of determining whether the work
undertaken on the first topic could be extended to these substrates. Metal meshes were chosen
because of the other interesting DSSC device morphologies that have been achieved on mesh
substrates such as wire DSSCs or cylindrical DSSCs with a coaxial platinum wire cathode.
1.4
Thesis Overview
In this thesis, LbL deposition of thin films is explored as a DSSC assembly technique to create
devices with the potential to have higher conversion efficiencies or lower manufacturing costs.
Chapter 2 details how M13 bacteriophage were incorporated as a regular component of a dip
LbL film of weak polyelectrolytes. The hybrid system underwent a porous transition and was
then templated with titania. It functioned as a photoanode for a DSSC, and Ldiff and JV data for
the resulting devices was determined. The associated appendix, Appendix A, details the various
device making and testing procedures associated with the DSSC-specific methods employed in
Chapter 2.
Chapter 3 explores the extension of the dip LbL work presented in Chapter 2 to spray LbL.
This chapter includes an ANOVA to determine the viability of a porous transition in sprayed
systems and then explores the possibility of nonconformally coating metal meshes as a
replacement for the transparent conducting oxide layer in DSSCs. Chapter 4 discusses new
directions for study that these results suggest may be interesting or profitable.
35
1.5
References
1.
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Izquierdo, A.; Ono, S. S.; Voegel, J. C.; Schaaf, P.; Decher, G., Dipping versus spraying:
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Schaaf, P.; Voegel, J.-C.; Jierry, L.; Boulmedais, F., Spray-Assisted Polyelectrolyte
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Deki, S.; lizuka, S.; Horie, A.; Mizuhata, M.; Kajinami, A., Liquid-phase infiltration
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Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Gratzel, M., Porphyrin-Sensitized Solar Cells
with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency. Science 2011, 334
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Varghese, 0. K.; Paulose, M.; Grimes, C. A., Long vertically aligned titania nanotubes on
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Sauvage, F.; Di Fonzo, F.; Li Bassi, A.; Casari, C. S.; Russo, V.; Divitini, G.; Ducati, C.;
Bottani, C. E.; Comte, P.; Graetzel, M., Hierarchical TiO2 Photoanode for Dye-Sensitized Solar
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Ghadiri, E.; Taghavinia, N.; Zakeeruddin, S. M.; Gratzel, M.; Moser, J. E., Enhanced
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Barbas, C.; Burton, D.; Scott, J.; Silverman, G., Phage Display: A Laboratory Manual.
Cold Spring Harbor Laboratory Press: Cold Spring Harbon, NY, 2001.
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Spontaneous assembly of viruses on multilayered polymer surfaces. Nat. Mater. 2006, 5 (3),
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Flynn, C. E.; Lee, S. W.; Peelle, B. R.; Belcher, A. M., Viruses as vehicles for growth,
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Mao, C. B.; Solis, D. J.; Reiss, B. D.; Kottmann, S. T.; Sweeney, R. Y.; Hayhurst, A.;
Georgiou, G.; Iverson, B.; Belcher, A. M., Virus-based toolkit for the directed synthesis of
magnetic and semiconducting nanowires. Science 2004, 303 (5655), 213-217.
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Briggs, B. D.; Knecht, M. R., Nanotechnology Meets Biology: Peptide-based Methods
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Weitz, D. A.; Belcher, A. M., Biologically templated photocatalytic nanostructures for sustained
light-driven water oxidation. Nat. Nanotechnol. 2010, 5 (5), 340-344.
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Nam, K. T.; Kim, D. W.; Yoo, P. J.; Chiang, C. Y.; Meethong, N.; Hammond, P. T.;
Chiang, Y. M.; Belcher, A. M., Virus-enabled synthesis and assembly of nanowires for lithium
ion battery electrodes. Science 2006, 312 (5775), 885-888.
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Nam, Y. S.; Shin, T.; Park, H.; Magyar, A. P.; Choi, K.; Fantner, G.; Nelson, K. A.;
Belcher, A. M., Virus-Templated Assembly of Porphyrins into Light-Harvesting Nanoantennae.
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Palomares, E.; Pettersson, H.; Gruszecki, T.; Walter, J.; Skupien, K.; Tulloch, G. E.,
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Nanocrystalline dye-sensitized solar cells having maximum performance. Progressin
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Toivola, M.; Peltola, T.; Miettunen, K.; Halme, J.; Aitola, K.; Lund, P. D., Large Area
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the View Point of Light Harvesting and Charge Collection. J. Sol. Energy Eng. Trans.-ASME
2010, 132 (2), 7.
38
Chapter 2.
Layer-by-Layer Deposition of Engineered
M13 Bacteriophage for the Construction of DyeSensitized Solar Cells with Novel Titania
Architectures
Portions of this chapter are reproduced from Chen, P.*, Ladewski, R.L.*, Miller, R.A., Liau,
F.W., Belcher, A.M., Hammond, P.T., Novel Titania Architectures for Dye-Sensitized Solar
Cells Achieved through Layer-by-Layer Deposition and Incorporation of Engineered M13
Bacteriophage, In Preparationfor Advanced Materials.
*These two authors contributed equally to the work.
2.1
Introduction
The overall goal of this aspect of the thesis was to develop an alternative architecture for the
titania photoanode of a dye-sensitized solar cell (DSSC) 1 through the addition of biologicallytemplated nanowires2, 3 . A schematic of the desired device architecture is shown in Figure 2.1
below.
39
Conducting
Titania Photoanode
Glass
-----+
--
Conducting
Electrolyte Glass
-e--
e
+
e- -
Black
Dye
e-
Redox
Couple
e-
Electrical
Work
Figure 2.1 Schematic of the nanostructured titania photoanode containing phagetemplated nanowires.
M13 bacteriophage (explained in greater detail in Section 1.3.1) was used as a titania nanowire
template in the photoanode of a dye-sensitized solar cell to increase device performance as
measured by electron diffusion length, Ldiff and thus to increase Jsc and device efficiency.
Because of its fine nanoscale assembly control 4 and the negative charge of the bacteriophage5 ,
Layer-by-Layer (LbL) assembly 6 was employed to incorporate the bacteriophage into thin films
on FTO-coated glass. The resulting templated photoanode would theoretically increase electron
collection efficiency over the traditional photoanode through the addition of these M 13
bacteriophage-templated nanowires. It would also allow for independent control of the titania
surface area available for dye adsorption because the polymeric LbL components and pH
treatment conditions would control the final morphology of the porous titania template, which
would also be embedded throughout with the phage nanowire template. Achieving the desired
photoanode structure required that several different challenges be overcome. First, a process for
the regular incorporation of bacteriophage into a porous LbL film needed to be developed and
adsorption and conformation parameters investigated. Second, the resulting hybrid film needed
to be faithfully templated with an anatase titania coating at device relevant thicknesses (1030pm). Third, these titania electrodes needed to be placed into functional DSSCs and tested to
determine efficiency and electron diffusion length. Because each of subsequent step was
40
dependent upon success in the preceding steps, the research results are divided into three parts.
Each section has its own literature review, methodology, and results and discussion subsections.
Section 2.3 covers the incorporation of bacteriophage into a LbL film. Section 2.4 covers the
generation of a thick titania photoanode from the phage-containing LbL films. Section 2.5
covers the application of these photoanodes in DSSC devices. Furthermore, a more thorough
treatment of DSSC materials selection and device physics, Section 2.2, is presented before these
sections as a detailed overview of the current device capabilities and functions. This section
explains the link between electron diffusion length Ldiff and device performance while also
providing a more thorough overview of DSSC operation. Also, an appendix to section 2.5.3
covers different strategies for testing and designing effective device architecture and may be
useful to future researchers trying to develop DSSC device capabilities.
2.2
DSSC Materials and Physics
Figure 2.2 below shows the different phases of a DSSC and how each is connected to the others.
e-only
Ejonsonly
Figure 2.2 Scheme of DSSC operation in a bilayer device. TCO is a common
abbreviation for transparent conducting oxide. Electrons are shown in red circles.
Charged and uncharged anions are shown as pink circles.
As illustrated in Figure 2.2, the active components of the DSSC are generally encapsulated by
glass (though other materials have been used 7 ~9). Adjacent to the glass on the left-hand side of
the schematic is the TCO (transparent conducting oxide), which, as its name indicates, is a
transparent conductor. This is typically fluorine-doped tin oxide (FTO) because of its heat
stability'0 and lower cost".
41
2.2.1
Active Components of DSSCs
DSSCs have three main active components: the titania (or any wide band-gap semiconductor1214), the dye, and the electrolyte. As was stated in Chapter 1,
each phase has a distinct purpose.
The titania performs the electron conduction, the dye performs the light absorption, and the
electrolyte acts as the hole conductor (though it only shuttles ions). Because each phase has a
different function and chemical makeup, they will be covered individually in the following
subsections.
2.2.1.1 Titania - Photoanode
The titania is sintered to the TCO in order to achieve an intimate electrical contact. Although
anatase titania is typically used for this phase, devices have been made with different phases of
titania". The record efficiencies for these types of devices, however, has so far been lower than
that of anatase titania, (12.3%16) with ZnO record devices giving a maximum of 6.5% conversion
efficiency14 and rutile titania devices giving 5.0
%15).
Anatase titania has a band gap of 3.2 eV
and absorbs very little light without a sensitizing molecule 7 . It is this property that makes it
possible to create transparent devices. Also, electrons move through the conduction band of the
titania strictly through a diffusive mechanism because the effects of migration (motion due to the
presence of an electric field) are minimized by ionic shielding caused by the small crystallite size
and immersion in a high ionic strength electrolyte''18
2.2.1.2 Dye - PhotoactiveAbsorber
Figure 2.3 shows the two most common DSSC dye molecules, which, by design, strongly
chemisorb to the titania phase 19,20. Thus, the dye creates a physical barrier between the titania
and the electrolyte. The most successful DSSC dyes usually have the added benefit that the
majority of their lowest unoccupied molecular orbital (LUMO) electron density is located on the
portion of the molecule that is chemisorbed to the surface of the titania. When light is absorbed
by a molecule, an electron is promoted from its highest occupied molecular orbital (HOMO) to
its LUMO. The electron clouds associated with the HOMO and the LUMO will very likely be
different. In the case of DSSCs, this difference works to our advantage because upon adsorption
of light, electron density is shifted to the LUMO and thus toward the semiconducting titania
phase.
42
N
N*
?d
L
HO
j
a
b
Figure 2.3 Chemical structures of the common Ru-based DSSC dye molecules a)
red dye and b) black dye.
Each molecule shown in Figure 2.3 has carboxylic acid groups which adhere to exposed titanium
on the surface of the titania phase and contain most of the electron density of the LUMO.
Similarly, the HOMO electron density sticks out away from the titania phase as the electron-rich
NCS ligands that point away from the titania when the dye is adsorbed' 9 . When the dye absorbs
a photon, it undergoes a metal-to-ligand charge transfer reaction. Once the electron has been
pushed into the pyridine ligands, the transfer to the titania is almost instantaneous (happening on
the femtosecond timescale)21 22 . The positive charge remains localized on the ruthenium metal
center until an electron is transferred from the adjacent electrolyte phase. The energy difference
between the LUMO and HOMO of black dye is approximately 1.4 eV.23 All photons with
energies less than 1.4 eV (wavelengths greater than ~885 nm) cannot be absorbed by this dye.
This is termed a non-absorption loss. Also, though photons with energy greater than 1.4 eV will
be absorbed, the extra potential energy is likely to be lost quickly to heat, either in the
conduction band of the titania or in the LUMO of the dye. This is termed a thermalization loss.
The HOMO-LUMO gaps of DSSC dyes are tailored to limit the effects of non-absorption and
thermalization losses. All single-wavelength sensitive solar cells suffer from non-absorption and
thermalization losses, and the maximum theoretical efficiency (limited by these types of loss)
occurs at approximately 1.3 eV.
They dye also acts to blanket the TiO 2 phase, serving as a physical barrier between the titania
and the electrolyte and helping to limit one major loss mechanism in DSSCs: recombination.
43
Recombination refers to electrons in the titania conduction band recombining with positive
charges before traveling to the current collector. This could be the result of any of the following
mechanisms: 1. the excited state of the dye donating electrons directly to the electrolyte, 2. the
conduction band of the titania donating electrons directly to electrolyte, or 3. the excited state of
the dye emitting energy and losing its ability to donate electrons to the titania. Of the three cases
listed, the second loss mechanism strongly overwhelms any contributions from the other two in
well-designed DSSCs24. Spatial arguments based upon the localization of the HOMO and
LUMO show that electron injection from the dye into the titania much more likely than injection
into the electrolyte (the same arguments show that electron donation from the electrolyte to the
HOMO of the dye is more likely). Case two is made much less likely because the dye acts as an
insulator coating the surface of the titania. Therefore, losses due to case two are more likely to
occur on surfaces that are not coated with dye. Case three losses are simply unlikely due to the
long-lived excited states of these molecules. For example, the average lifetime of the dye's
excited state is 20 ms2 , as compared to the femtosecond electron injection process21,22
2.2.1.3 Electrolyte - Dye Regeneration
The electrolyte phase of the DSSC, shown in pink in Figure 2.2, can either be liquid or solid2 6
and can function through either hole transport or ion diffusion 27,28 . The most efficient devices to
date have been made with liquid electrolytes. Although iodide-tri-iodide (-/ I3 ) has been the
most commonly employed electrolyte in high performing devices, the current DSSC device
record of 12.3% is held by a cobalt complex 16. Ferrocene/ferrocinium has also been used as a
redox couple to achieve efficiencies up to 7.5%29. The solid-state device record of 5.0% is held
by spiro-OMeTAD, a hole transport material30 . Others have looked into Br-/Br3~ based
electrolytes3
1
and fully organic electrolytes based on disulfide thiolate redox couples 32 , 33 , but
the efficiencies for devices made with these alternative ions have not exceeded 6.5%. Because
this work does not focus on electrolyte makeup and all devices made in this titania photoanode
investigation were constructed with a liquid I/ I3 electrolyte, the rest of the device physics
explanation in this section will focus on this redox couple.
The I/ I3-electrolyte is generally a liquid solution that fills all the space between the dye and the
platinum counterelectrode. The I ions shuttle electrons from the counterelectrode to the
positively charged dye molecules that have already injected electrons into the titania phase.
44
After donating this charge, these ions diffuse back to the counterelectrode as
13
ions. At the
counterelectrode, the b- ions receive electrons and split back into three I ions. These reactions
are shown in the Equations 2.1 and 2.2 below.
32 1 +dye*
I3 + 2e-,
"'0 >dye+
2.1
I-
2.2
P' >3I-
The mechanism of the reaction shown in equation 2.1 is not well understood 8 , but the overall
transfer equations are accurate representations of the charge transfer phenomena.
2.2.2
DSSC Device Performance Parameters
The interaction of these three components, titania, dye and electrolyte, determines the
performance of the DSSC. The effect of these interactions on photovoltage, photocurrent, and
electron diffusion length (Ldiff) are explained below.
2.2.2.1 Origin of DSSC PhotovoltageandPhotocurrent
The flow of electrons through these devices determine their overall efficiency, current density
and voltage output. Therefore, it is useful to lay out the steps of electron transport in an ideal
device. Figure 2.4 below is an energy level diagram for the DSSC that illustrates these steps.
TiO)
E
redox
Figure 2.4 Energy level diagram for a DSSC. For clarity, the color scheme of the
phases matches that shown in Figure 2.2. Load refers to the resistance of the
external circuit. Redox refers to the electrolyte.
The flow of electrons starts from the excited state of the dye, depicted in Figure 2.4 as dye*. The
driving force for electron injection into the titania is the difference between the LUMO of the
dye and the conduction band of the titania phase, labeled as TiO 2'
. Upon
photoexcitation of
45
the dye, electrons are injected into the titania, leaving a positively charged dye molecule
adsorbed to the titania surface. The electrons then flow through the titania, into the FTO and
then into the external circuit. They are reintroduced into the device as a redox reaction at the
platinum counterelectrode (Equation 2.2 above), changing an electron-poor triiodide molecule
into three electron-rich iodide ions. These iodide ions diffuse to the positively charged dye
molecules, donate electrons, and return to the platinum counterelectrode as electron-poor triiodide, completing the circuit. As is shown in Figure 2.4, the energy levels of the conduction
band of the titania and the redox potential of the electrolyte determine what voltage is applied to
the external circuit. Thus, the photovoltage is more sensitive to materials selection than to
device-specific features.
The current output of the device, however, is more dependent upon architecture. For example,
the dye loading of the device should be directly related to the amount of light absorbed and
therefore to the amount of electrons generated. Thus, we can assume that the bilayer device
depicted in Figure 2.2 would likely generate less current than a device that had more surface area
for dye adsorption, like a bilayer device with a rough surface and much less current than a
nanostructured photoanode.
2.2.2.2 Electron Diffusion Length as Motivationfor this Work
Because of the surface area dependence explained in the previous section, the best performing
DSSC devices are not bilayer devices at all, but thin films of sintered nanoparticles that are
coated uniformly with dye. An idealized cross section of such sintered nanoparticles is shown in
Figure 2.5b below.
ron
only
only
r,
r
a
b
c
r,
Figure 2.5 Illustrations of electron diffusion through different device types
showing (a) an ideal bilayer-type DSSC, (b) an ideal sintered nanoparticle DSSC,
and (c) a more realistic picture of the sintered nanoparticle DSSC. r refers to the
rate of electron transport out of the device. r, refers to the rate of electron loss due
to recombination. Arrows show potential paths for electrons to traverse.
46
Figure 2.5 illustrates the trade-offs involved in switching from a bilayer architecture to a sintered
nanoparticle architecture. The bilayer device suffers from low dye loading because of low
surface area. In fact, the best performing DSSCs generally use a 10-15 prn thick layer of a
combination of 20-300 nm anatase particles3 5 . However, the path for electrons in the bilayer
device from the titania to the current collector is very short and direct. Once the titania
architecture is switched to sintered nanoparticles, as shown in Figure 2.5b and c, the path
becomes longer and more tortuous. In this architecture, the ratio between the rate of electron
transport out of the titania phase rt and the rate of electron loss to recombination rr becomes a
much more important parameter. The relationship between these two rates is more precisely
expressed as the electron diffusion length, Ldiff, which is most simply expressed as equation 2.3
below36
Ldlff = V-C-f2.3
In this equation, Deff is the effective diffusivity of electrons in the titania phase and - is the
characteristic timescale of recombination 37 . Ldiff is related to the architecture of the titania phase,
the conformality of the dye coating, the crystallinity of the titania, and the chemical makeup of
the electrolyte. For example, if recombination were a first order process in electron density or
tri-iodide concentration, - would be the inverse of the rate constant for this reaction. If it were
possible to increase the effective diffusivity of the titania phase or to increase the electron
lifetime, then one would expect a commensurate increase in device efficiency, Ldiff, and current
output. In fact, the electron collection efficiency ijcoi (the efficiency for injected electrons to
make it to the current collector) of the DSSC depends upon Ldiff according to equation shown
below', where d is the thickness of the titania thin film within the DSSC36, 38
COL
cosh(j)+sinh(A)+0.368y]
0.632[1 -
cosh(d)
Assuming 90% incident light absorption and linear recombination kinetics.
y
47
The electron collection efficiency is important because it is directly related to the short circuit
current density of the device, Js, which is the maximum current that is produced by a device
normalized to the area of the device36
J. (A)=q9(A)qLH
in)j
2.5
co,(A)
Equation 2.5 expresses Jsc as a function of wavelength X,the elementary charge of an electron q,
the intensity of the incident light p, the light harvesting efficiency
Tinj , and
TLH,
the injection efficiency
the collection efficiency. The injection efficiency is most dependent upon the dye
chosen. The light harvesting efficiency depends on the optical characteristics of the titania
photoanode. Collection efficiency also depends most strongly on titania architecture (and
crystallinity) 36 . Because diffusion in a sintered nanoparticle phase requires electron transport
between adjacent nanoparticles and likely across disordered crystal arrays, it is conceivable that
the introduction of ordered crystallinity (e.g. titania nanowires) would increase the effective
diffusivity of electrons in the titania phase. It is this thought that motivated the present work.
2.3
Bacteriophage Incorporation
In order to make a DSSC according to the scheme in Figure 2.1, a method to incorporate the
nanowire template, M13 bacteriophage, into a LbL film needed to be developed. This section
covers this portion of the research only.
2.3.1
Literature Precedent
Other biological components have been added to LbL films before and some others have also
used M13 bacteriophage in the photoanode of a DSSC. These two relevant literature precedents
are discussed here: virus assembly within LbL films and the incorporation of M13 bacteriophage
in DSSC photoanodes via other assembly methods. The differences and advantages of our
approach are highlighted in Section 2.3.1.3, in light of the previous work explained in Sections
2.3.1.1 and 2.3.1.2.
2.3.1.1 LbL Involving Viruses as FunctionalComponents
In 1994, the spherical Carnation Mottle virus was incorporated with poly(styrene sulfonate) into
39
a multilayer film to demonstrate that the LbL technique could be expanded to charged particles
48
Since then, other viruses, such as cowpea chlorotic mottle virus (CCMV) 40 and M13
bacteriophage 5 have been investigated in conjunction with the LbL technique. CCMV was
incorporated as a regular component of LbL film growth, like the Carnation Mottle virus. As a
LbL component, it was proposed but not demonstrated to be a scaffold for materials generation
or an encapsulant for sequential drug delivery. The incorporation of M13 bacteriophage,
however, differs from the two other virus-LbL combinations mentioned above in that
bacteriophage have been shown to assemble in an ordered fashion on top of a LbL film, not as a
regular LbL component. This process has been demonstrated only for one combination of
polymers: linear poly(ethylenimene) (LPEI) and poly(acrylic) acid (PAA). This combination is
important because the LPEI in these LbL systems has been shown to interdiffuse through the
polymer film and cause exponential growth and porous transition behavior 41'
42.
The
bacteriophage adsorb to the top layers of the film at certain pH conditions. However, when more
polyelectrolyte is deposited on top of the bacteriophage, the bacteriophage will "float" to the top
of the film instead of remaining within the film'
43.
This floating is only observed after the
complete application of the next three layers (LPEI, PAA, LPEI) or five layers (LPEI, PAA,
LPEI, PAA, LPEI) in sequence. No floating was observed after any PAA deposition steps. This
dependence upon polymer type indicates that the mobile LPEI are diffusing down to the
bacteriophage, preferentially binding to the charges present, and thus freeing the bacteriophage
to diffuse to the top layer of the film 5 . When the mobility of the LPEI species was reduced
through deposition pH modifications, the bacteriophage floating and ordering were reduced4 3 .
The ordered bacteriophage on this LbL film were shown to be functional when they were
successfully used as a template for Co 3 0 4 in a lithium-ion battery electrode 44
2.3.1.2 Previous Applications of M13 Bacteriophageto DSSCs
In the literature, M13 bacteriophage have been employed twice in the photoanode of a DSSC.
The most straightforward modification was achieved by simply adding phage to an existing
titania paste as a sacrificial template that would burn away during the annealing process and
leave behind interconnected channels for ion diffusion 4 5 . The phage increased device
performance from 4.67% to 6.32% which the authors attributed to increased scattering and ion
transport from the channel voids created by the bacteriophage. Around the same time and in a
more sophisticated approach, Belcher and coworkers genetically engineered the M13
bacteriophage to complex with carbon nanotubes. These researchers then mixed complexes of
49
phage and semiconducting single-walled carbon nanotubes with titania nanoparticles in a paste
and deposited onto the TCO via doctor blading. This photoanode modification improved the
efficiency of DSSCs from 8.3% without virus-CNT complex to 10.6% with it38 .
2.3.1.3 Our Approach
The LbL templating method chosen for this work brings all of the advantages of the LbL process
to bear on the titania photoanode assembly process. As was explained in detail in Section 1.3,
these advantages include the use of water-based assembly methods at ambient conditions,
scalability, and fine nanoscale control over film component concentration and conformation. For
this work, it was desired to achieve bacteriophage loading within the LbL thin film in order to
template titania nanowires that would traverse the depth of the film. Therefore, a scheme to limit
the mobility of the bacteriophage within the film (preventing the floating that had been
previously observed) had to be developed and tested. The pH control method for limiting this
mobility (mentioned in Section 2.3.1.1) could not be used here due to concerns about the pH
stability of the phage.
2.3.2
Materials and Methods
2.3.2.1 M13 BacteriophageAmplification, Modification with Oregon Green, and Quantification
of Labeling
M13 bacteriophage were amplified using a well-documented procedure4 6 . For one study, the
M13 bacteriophage were covalently modified with the fluorophore Oregon Green (Oregon Green
488 carboxylic acid, succinimidyl ester 5-isomer, Invitrogen) using EDC chemistry47 and a
commonly available protocol 48. Instead of using the gel columns recommended in the protocol,
PEG precipitation and dialysis, as described by Barbas46 , were used to separate the bacteriophage
from the unattached dye molecules. UV-Vis spectroscopy of the modified bacteriophage in
solution, shown below in Figure 2.6, confirms the presence of modified bacteriophage with up to
15 fluorophores per bacteriophage.
50
1 0.8 0.6 0.40.2-
250
300
350
550
450
500
400
Wavelength (nm)
600
Figure 2.6 UV-Vis spectroscopy of Oregon Green modified bacteriophage.
Peaks at 269 and 320 nm are used to calculate the concentration of bacteriophage in solution4 6
The peak at 494 nm is from the Oregon Green4 9 . The table below summarizes specific important
values taken from the data.
Table 2.1 Absorbance values for selected wavelengths of Oregon Green
modified M13 bacteriophage
Wavelength (nm)
269
320
494
600
Absorbance
0.770
0.055
0.154
0.023
~ [nipfC
phge
= (ti
(49
nn -A A
320 n
)*6440077
2.6
Equation 2.6 above, based on Beer's law, shows the formula used to calculate the concentration
of the bacteriophage in solution 46. The data in Table 2.1, when used in conjunction with
Equation 2.6 and Beer's law for Oregon green with an extinction coefficient of 75000 cm-1M-1 at
its 494 nm 4 9 indicate that there is an average of 15 fluorophores per bacteriophage.
2.3.2.2 Preparationof Polymer Solutions
LPEI (250,000 Mw, Polysciences) and PAA (250000+ Mw, 25 % aqueous solution,
Polysciences) were used as received and dissolved in deionized water (18.2 MQ-cm, Milli-Q
Ultrapure Water System, Millipore) at 20 mM concentration with respect to the repeating unit.
51
5
42
This PAA is the same that was used in previous studies by Lowman 0 and Lutkenhaus , but was
previously mislabeled by the supplier, Polysciences, as 90,000 Mw. The pH was adjusted to
4.75 using 1 M and 0.1 M HCl or NaOH. The M13 bacteriophage, variant E4, was purified from
solution through PEG precipitation and redissolved in pH 4.9 10 mM sodium acetate (NaOAc)
buffer to generate a stock solution. Aliquots of this stock solution were further diluted with
water and 100 mM NaOAc buffer, pH 4.90, to achieve a final dipping bath concentration of
2x 10
phage particles per mL (pfu) in 5 mM NaOAc.
2.3.2.3 Dip Layering Process
The layer-by-layer films were constructed on silicon and glass substrates for analytical purposes
and on fluorine-doped tin oxide (FTO) substrates (2.3 mm, TEC8, Pilkington) for incorporation
into photovoltaic devices. The substrates were cleaned by sequential ultrasonication in 1%
alconox solution, water, acetone, and propanol for 15 minutes each. Before use, the substrates
were plasma etched in a Harrick PCD 32G plasma cleaner with oxygen bleed for ten minutes. A
modified Carl Zeiss DS50 programmable slide stainer was used to accumulate the film
components in a tetralayer repeat architecture denoted as (LPEI/PAA/LPEI/M 13)n or a bilayer
architecture (LPEI/PAA)n, where n represents the number of deposited tetralayer or bilayer
repeats. Each polymer deposition step involved 4 separate dipping baths at the following
conditions: 3 minute submersion in the polymer solution at pH 4.75, followed by three Milli-Q
water rinse steps (30 seconds in the first one, 1 minute in the second, then 1
minutes with
agitation in the third). For the bacteriophage deposition, substrates were submerged for 15 min
in the phage bath and rinsed in two separate rinse baths (5 mM NaOAc buffer at pH 4.9) for 1/2
minutes each. After every 5 tetralayers, 10% of the original phage amount in solution was added
to the phage dipping bath to maintain the original concentration. The substrates were dried at
ambient conditions after the deposition process finished. This process is depicted in Figure 2.7
below.
52
R nse
PAA
e
14..
Figure 2.7 Schematic of the dip layering process with (tetralayers) and without
(bilayers) bacteriophage.
2.3.2.4 AFM Imaging
The diffusion of phage particles through the film and the adhesion characteristics of the
bacteriophage were monitored using a Dimension 3100 Nanoman AFM (Veeco Metrology,
Santa Barbara, CA) in tapping mode.
2.3.2.5 BacteriophageDepletion Study
To determine the bacteriophage loading per tetralayer (TL) by subtraction, an experiment was
performed wherein a small aliquot (100 ptL) of the bacteriophage dipping bath was removed after
every 5 TL of film deposition in order to measure the concentration of the bacteriophage in the
dipping solution. Previous experiments had indicated that the phage concentration was depleted
by about 10% every 5 TL. Therefore, after the aforementioned 100 pL aliquot of solution was
removed for analysis, 10% of the initial bacteriophage concentration was added into the
bacteriophage solution in order to restore the concentration of bacteriophage in the dipping bath
to its original value. The dipping solution was mixed and then another aliquot of 100 pL was
removed to determine the present concentration of bacteriophage before next 5 TL of film were
deposited. In effect, two data points were taken after every 5 TL of film were deposited, one
before additional bacteriophage were added (a number that should reflect how much
bacteriophage were incorporated into the film) and one after. The results of this experiment are
shown in Figure 2.8 below.
53
2-
A
---------
A
-A--------------
--------
1-
0.5
-
0
5
0
20
15
10
Number of Tetralayers Deposited
25
Figure 2.8 Bacteriophage dipping bath concentration as a function of tetralayers
deposited. Black diamonds represent the concentration of the phage bath before
doping. Gray triangles are the concentrations of the bath after phage doping. All
concentrations are normalized to the same bath volume of 34 mL.
The concentrations of bacteriophage in the rinse baths were also monitored and determined not
to be significant compared to the losses experienced by the bacteriophage dipping baths (two
orders of magnitude smaller). Table 2.2 below summarizes the change in bacteriophage
concentration every 5 TL.
Table 2.2 Bacteriophage loss per 5 TL between 5 and 25 TL.
Tetralayer Range
Concentration Change (*1010 pfu/iL)
0-5
6.4
5-10
4.6
10-15
13.4
15-20
9.5
20-25
15.2
Table 2.2 shows that more bacteriophage were incorporated into the LbL film after 10 TL than
before, which corresponds to the switch in film growth from linear to superlinear growth (see
54
Figure 2.15 for the growth curve). These data indicate that the bacteriophage are being depleted
from the dipping bath at rates that would be expected if regular LbL incorporation were
occurring.
2.3.3
Results and Discussion
Although the depletion study described in section 2.3.2.5 was a good negative control to
demonstrate bacteriophage incorporation into the LbL films, a positive control was also desired.
Thus, a batch of bacteriophage was generated with a fluorescent tag. When these bacteriophage
were layered in the tetralayer architecture, the fluorescence of the films was shown to increase
linearly with the number of tetralayers applied. The results of this experiment are shown in
Figure 2.9 below.
70 -
-
y = 2.1796x + 18.666
R2= 0.983 1
60
10000
-8000
LI
-
8 40
S30
J20
-4000
-
10
6000
2000
.
y = 275.98x - 648.35
R2= 0.9399
-
0 -
0
0
5
10
15
20
Number of Tetralayers
25
30
Figure 2.9 Tracking film fluorescence increase and phage loading per film area
(from the bath depletion study) as a function of tetralayers applied.
This figure illustrates that as the phage are being depleted from the dipping bath (gray squares),
the fluorescence of the films is increasing (black circles), demonstrating the regular
incorporation of bacteriophage within the film. An AFM imaging study of these films after the
55
LPEI deposition (the layering step where previous work indicated that bacteriophage floating had
occurred 5 ) and after the bacteriophage deposition is shown in Figure 2.10 below.
Figure 2.10 AFM images (amplitude) of the tetralayer films after M13
bacteriophage deposition (left) and after LPEI deposition (right).
The AFM image on the right was taken three layers (LPEI, PAA, and LPEI) after the previous
M13 bacteriophage deposition. The worm-like shapes in the left image are the bacteriophage; no
such shapes are present in the image on the right, indicating that the increased molecular weight
of the LPEI decreases its interdiffusing mobility and thus suppresses the mobility of the
bacteriophage that had been previously observed.
Previous studies have shown that the density and conformation of bacteriophage deposition was
dependent upon solution pH due to induced changes in surfaces charge of the bacteriophage and
LPEI5'4 . For this work, fine control of bacteriophage surface loading and conformation was
desired. Therefore, further AFM imaging studies were performed to investigate the effects of
solution pH and buffering conditions on the conformation of the deposited bacteriophage.
56
Figure 2.11 AFM Images of Tetralayer Films from 5 mM NaOAc buffer at (a)
pH 4.60, (b) pH 4.75, (c) pH 4.90, and (d) pH 5.05. All images are a 3ptm x 3pm
square.
As expected, the results of the imaging study, shown in Figure 2.11, agree with those obtained in
the previous work5'
. These images show that the bacteriophage are prone to denser deposition
at pH 4.60 and 4.75. However, the bacteriophage in Figure 2.11 (a) and (b) appear to have a
larger diameter than those pictured in (c). Because the diameter of the bacteriophage is invariant
and the scale bars of these images are the same, this result indicates that the bacteriophage are
forming rope-like bundles as they deposit at lower pH. Bacteriophage bundling reduces their
aspect ratio and thus their nanowire-like character. Even though the amount deposited was
increased at pH 4.6 and 4.75, the bundling made these pHs unattractive for this project. At pH
5.05, shown in Figure 2.11 (d), the deposition of bacteriophage becomes much sparser. Thus,
57
the intermediate pH of 4.90, Figure 2.11 (d), was chosen because a uniform layer of
bacteriophage would be deposited without bundling. These results are in good agreement with
the previous study of bacteriophage deposition on the top layer of and LPEI/PAA film 5 . The
zeta potential experiments showed that the bacteriophage are very weakly charged below pH
4.75 and thus bundle together, but deposit in very thick layers to achieve appropriate charge
compensation. While at higher pH the bacteriophage become strongly charged and adsorb only
sparsely. The key to managing the bacteriophage deposition was to deposit them at an
intermediate pH where neither effect dominates.
2.4
DSSC Photoanode Generation from LbL Thin Films
2.4.1
Literature Precedent
The goal of this aspect of the project was to make the LbL thin film developed in Section 2.3 into
a suitable template for a DSSC titania photoanode. The previous research relevant to this aspect
of the work involves the use of LbL assembly for DSSC applications. LbL has been used for
DSSCs in two ways. First, it has been employed to directly layer titania nanoparticles onto a
substrate. Second, it has been employed, as it is in this work, to create a porous template for
titania conversion. Each of these strategies is reviewed, and then this work is explained in the
context of the previous work.
2.4.1.1 Direct LbL of TitaniaforDSSC Applications
LbL assembly has been applied in several different ways to DSSCs. It has been applied as a
means to coat the semiconductor with absorber molecules.
More common applications relate
to direct modification of the titania photoanode. Direct layering of titania nanoparticles with
polyelectrolytes has been used extensively and takes advantage of the amphoteric character of
titania. For example, Iha and coworkers have used it to deposit a compact multilayer film of
titania nanoparticles and poly(styrene sulfonate) at the TCO as a blocking layer to prevent
recombination. When a doctor bladed titania film was applied on top of this blocking layer, the
efficiency was increased from 5.7% without the blocking layer to 7.3%52. Li and coworkers
have also used direct LbL application of titania nanoparticles with a polyelectrolyte (polyacrylate
sodium) to achieve an efficiency of 1.29%53. Kumar et al. achieved an efficiency of 7.2% for
titania nanoparticle/PDAC films 54 . In a similar approach, Taghavinia and coworkers used LbL
58
to coat cellulose fibers with titania nanoparticulate films 5 5 . The resulting titania fibers were
incorporated via the doctor blading method into a titania film in order to increase film scattering
and ion diffusion properties. An efficiency increase from 2.46% to 3.75% was observed in
similar films with and without the LbL-based structures.
2.4.1.2 Polyelectrolyte-only Layering Followed by Titania Conversion
Still others have investigated the use of LbL for DSSCs through a polyelectrolyte-only
deposition followed by a titania conversion step that uses the existing film morphology as a
template 50' 56'57. These schemes exploit the porous transition behavior that is observed in
specific assemblies of weak polyelectrolytes. In order to explore the work that utilizes this
porous transition, it must be explained in more detail.
Porous Transition
Previous work has shown that when LbL is performed with weak polyelectrolytes, it is possible
to reproducibly modulate the porosity of the resulting films after they have been assembled by
taking advantage of the dependence of polymer charge density on solution pH4 2 ,58 ,5 9
Weak Polyelectrolytes: Weak Acids/Bases:: Strong Polyelectrolytes : Strong Acids/Bases
PDAC
Hydrochloric Acid
LPE1
Ammonia
1A
C'-
C
NH,+/NH 3
(U
HCI
H3C
PAA
C
.
Acetic Acid
PSS
n
coo
H3 COO-/ H3COOH
CH3
/)
I+
Sodium Hydroxide
Na
NaOH
Figure 2.12 Charge density analogy between polyelectrolytes and acids or bases.
PDAC is poly(diallyldimethylammonium) chloride. PSS is poly(styrene
sulfonate).
Strong acids and bases completely dissociate when dissolved in water. Weak acids and bases,
however, display a characteristic ratio of charged to uncharged molecules depending on their
pKa and the pH of the solution in which they are dissolved. The strong polyelectrolytes PDAC
and PSS are like strong acids and bases. Once they are put into solution, they immediately and
59
completely dissociate; every repeat unit along their backbone is charged. However, LPEI and
PAA are like weak acids and bases. They display a characteristic ratio of charged to uncharged
monomers along the polymer backbone that depends upon the pH of the solution and their pKa.
The dependence of the charge density of the polymer backbone on solution pH helps to cause the
porous transition behavior, as is depicted in Figure 2.13 below.
LPEI, pKa = 4.8 PAA, pKa = 6.5
Low
[H+]
11
ZO. 9
C
.0
7
"Z 5
3
0
OH
High
[H+]
1+
Figure 2.13 Scheme depicting the pH dependence of the charge density of LPEI
and PAA. Green bars represent where the polymer is more neutral. Blue and
yellow bars indicate where the polymer has more charged units. pKa values for
LPEI and PAA came from [60] and [61] respectively.
If polymer films are assembled at a pH between 4 and 5 (near the center of Figure 2.13, where
both polyelectrolytes are weakly charged) and after assembly are exposed to a much lower pH
(where the PAA becomes mostly uncharged, shown at the bottom of Figure 2.13 where the
yellow and green regions overlap), then some of the electrostatic interactions between the LPEI
and PAA are erased. The film undergoes entropically-driven swelling when counterions and
water enter the film to compensate for the available positive charge. Also, there is charge
repulsion from the excess positive charges on the protonated amine groups. At certain pH
conditions, the film undergoes syneresis as well as swelling, leading to the creation of water
filled pores throughout the film, surrounded by crosslinked polyelectrolyte domains62 . During
drying, the water is replaced by air, yielding structures like those shown in Figure 2.16, Figure
2.17, Figure 2.24, and Figure 2.25 in later sections. The next section explains how such porous
polymer structures have been used as templates for titania photoanodes in DSSCs.
60
Previous Work Using a PorousLbL Film as a Titania Template
Shiratori et al. have used porous films made from poly(allylamine) (PAH) and PAA for
templating titania for DSSCs. Thin polymer films were deposited (10 bilayers (bL) 56 and 15
bL 57 ) and then made porous in a low pH silver acetate solution. An aqueous liquid phase
deposition of titanium tetrafluoride and ammonium hydroxide was used to template titania on the
films. After annealing, the 10 bL films were 300 nm thick and yielded a device efficiency of
0.7% while the 15 bL films were 4 ptm thick and gave a device efficiency of 2.66%. Both
devices utilized a liquid -/I3~ based electrolyte. Hammond et al. used LPEI and PAA as the LbL
template to create a 5 ptm thick film of porous polymer, yielding an solid-state device efficiency
of 1.1%50. Titania was templated with a similar aqueous liquid phase deposition procedure, but
this work utilized a solid-state LbL deposited I/
3
electrolyte.
2.4.1.3 Our Approach
As was stated in section 2.2.2, nanoparticulate titania films must be 10-15 gm thick to achieve
the highest efficiencies. Therefore, our aim was to create porous templated titania films in this
thickness range with and without bacteriophage. As was stated in Section 2.4.1.2, previous
efforts to utilize the templating method had only achieved device thickness of up to 5 pm.
2.4.2
Materials and Methods
Figure 2.14 depicts the 4-step post treatment process for generating an open mesoporous titania
film from the thin conformal LbL coating. Each step is described in its own section below. One
step not pictured in Figure 2.14 is redipping. This step occurs after high-temperature annealing
and is explained in Section 2.4.2.5.
9j
Figure 2.14 Schematic of film post-treatment procedure steps.
61
2.4.2.1 Porous Transition
Dry substrates that had been coated with the (LPEI/PAA/LPEI/M 13), or (LPEI/PAA)n films
were exposed to 5 mM (NH4 )2TiF 6 (Alfa Aesar) aqueous solution, pH 2.25, to achieve a porous
microstructure 42. The effects of the porous transition on film thickness are shown below in
Figure 2.15
100
80M
16 -
(a)
*
1
12
80.-
y =0.2083x - 1.8707
R 2 = 0.9893
60
(b
14
y = 0.2858x - 3.0238
10
V 8
- 6
40
20
R 2=0.9677
2
0
50
Bilayers
100
5
10
20
15
Tetralayers
25
Figure 2.15 LbL growth curves for films without (b) and with (a) bacteriophage
before (black diamonds) and after (gray squares) the porous transition. Note that
the polymer only data are shown as a function of bilayers while the
bacteriophage-containing films are shown as a function of tetralayers.
Figure 2.15 demonstrates that the bacteriophage films tend to be thinner than polymer only films
after the porous transition (note the scale bars). The 40 bilayer polymer only film should be
comparable to the 20 tetralayer phage-containing film. However, their thicknesses after the
porous transition are 59 ± 6 pim and 8.0 ± 0.4 pim respectively. This expected behavior indicates
that the reduced content of the pH-reactive PAA (which has been partially replaced with
bacteriophage) decreases the effect that low pH has on the porous transition of the LbL film.
2.4.2.2 Top Skin Removal
As the films are made porous, a previously reported "skin" forms on top of the structure4 2 . An
SEM micrograph of a typical "polymer skin" is shown in Figure 2.16 below.
62
Figure 2.16 SEM micrograph of a scratch in an LPEI/PAA film after the porous
transition. The silicon substrate is visible to the left of the scratch, the polymer
skin to the right.
Because this polymer skin would impede diffusion into and out of the pores of the polymeric
matrix, it would have a negative impact on the titania conversion process (see section 2.4.2.3)
and on the diffusion of electrolyte ions into and out of the titania phase. Therefore, a method for
its removal was devised based upon previous reports in the literature that pH extremes 63'64, high
ionic strength65, and oxygen plasma treatment 66 had been shown to delaminate or destroy
polymer films. The challenge for adapting these procedures was to selectively and uniformly
remove only the top layer of the polymeric thin film while leaving the underlying structure
intact. Oxygen plasma treatment on the high setting etched the film down to the surface in spots,
and on the low setting did not etch the film much at all. Short oxygen plasma treatment on the
medium setting, however, appeared to affect only the skin of the film, causing it to begin to peel
back from the underlying layers. Based upon some preliminary experiments, we began to follow
the short plasma treatments with short salt soaks to further weaken the interaction between the
skin and the underlying film. To ensure complete removal by burning away any remnants of the
skin we added a final, longer oxygen plasma treatment step. It is very likely that a separate set of
conditions involving oxygen plasma treatment and salt soaking could remove this top layer. The
set of conditions described below are shown to work for this system. A 2 minute oxygen plasma
treatment on the medium setting, followed by a 10 minute soak in pH 2.25 2 M NaCl solution,
distilled water rinse, air dry, and a further 10 minute oxygen plasma treatment were used to
achieve the desired morphology, which is shown in Figure 2.17 below.
63
Figure 2.17 SEM image of and LPEI/PAA film after top skin removal.
2.4.2.3 Titania Conversion
The previous study employing LPEI/PAA films for DSSC photoanodes used a liquid phase
deposition of titania 5. Attempts to use the identical conditions for thick porous films (>5 pm) of
LPEI/PAA resulted in catastrophic collapse of the titania film during annealing, as is shown in
Figure 2.18. This collapse was likely not apparent in the previous study because all reported
films were 1-5 im thick50' 56'57.
Figure 2.18 SEM image of 8 gm polymer and titania film after annealing. The
lighter colored islands are what remains of the polymer film. The darker spaces
between them are the underlying silicon substrate. The scale bar is 10 pm.
However, for reasons explained in more detail in Section 2.2.1, devices must be 10-15 pm thick
in order to yield acceptable efficiencies and currents. The original literature source for the liquid
phase deposition used much higher concentrations of the titania precursor solutions than previous
64
LbL-DSSC studies 67. Switching to these original deposition conditions yielded robust titania
coatings that did not collapse upon annealing. XPS data, shown in Figure 2.19 and taken on a
PHI Versaprobe II XPS at the CMSE Materials Analysis SEF, show that the resulting templatedtitania-on-LbL films are atomically indistinguishable from commercially available anatase titania
nanoparticles. Specific procedures and conditions for the conversion process are listed below.
60
"" TiO 2 nanoparticle film
40
"' Templated TiO 2 film
E)201
1000
800
600
400
200
0
Binding Energy (eV)
Figure 2.19 XPS data for titania films templated using the method outlined above
and for a commercially available nanoparticulate titania paste.
Titania was templated on the films after the porous transition by immersing the films into an
aqueous bath of 100 mM (NH 4)2TiF 6 for 30 minutes, after which an equal volume of 200 mM
B(OH) 3 (puratronic, Alfa Aesar) was added and the mixture allowed to sit for 6 hours at 350 C
then ramped up to 500 C for at least 10 more hours. Extreme caution was used to minimize the
chances of skin exposure when handling the boric acid mixtures as this material is a known toxic
teratogen. The films were then removed from solution, rinsed with water to remove
homogeneously nucleated titania that had settled onto the top surface of the films and then dried
in air.
2.4.2.4 High TemperatureAnnealing
As has been noted in the literature, titania is prone to cracking during the annealing step68, and
therefore templated titania films above 1 pm are infrequently observed69 . Suggested methods for
remedying this cracking range from hot oil or paraffin treatment during annealing 69' 70 to a justbelow-melting-point temperature soak followed by a high temperature anneal 71 . The annealing
method of a long 300"C followed by a high temperature phase at the end had the strongest
impact in reducing the cracking in these systems. Also, the LbL-templated titania films
exhibited significant cracking when annealed at the same conditions as were suggested for a
nanoparticulate paste. However, it was noted that much of this cracking occurred due to the
65
drying out of the polymer film before temperatures above 150'C were reached, as is shown in
Figure 2.20.
Figure 2.20 Top down optical microscopy of LPEI/PAA films annealed at 150'C
under dry conditions (a) and in a water bath (b).
Therefore, films were placed in water baths during annealing, which eliminated films cracks that
went down to the underlying FTO layer and significantly reduced the size of surface cracks.
This result is illustrated in Figure 2.21 below.
Figure 2.21 Top-down SEM images of annealed titania films in (a) dry conditions
or (b) humid conditions described below.
To burn off the polymer and anneal the titania under initially humid conditions, the films were
sintered in a furnace using the following procedure. The films were placed in a Pyrex glass
container and covered with approximately 1 inch Milli-Q water which was then placed into the
oven. Another glass jar was placed next to the films filled with approximately 300 mL Milli-Q
water. The oven was then programmed to ramp to 300 0 C in 1 hour, hold 300'C for 6 hours,
execute a 30 minute ramp to 450"C, hold 450*C for 1 hour, and then allowed to cool to a
66
temperature between 800 and 100C in the same furnace or in a different oven set in that same
temperature range. X-ray diffraction data, shown in Figure 2.22 below, confirm that the
resulting templated titania is anatase phase.
Binding Energy (eV)
SAnnealed at 450*C
SAmorphous Tio2
25
20
30
35
40
20
45
50
55
60
Figure 2.22 XRD data for template titania before and after high temperature
annealing
The titania conversion and high-temperature annealing steps are depicted schematically in Figure
2.14.
Film characterization
Film thickness was monitored using a surface profilometer (Veeco Dektak). Films were
scratched with a razor blade, and the surface profilometer was then run over the film and the
scratch. The deposition of bacteriophage within the film was monitored using AFM (Dimension
3100 Nanoman, Veeco Metrology). Films were taken from the dipper after various polymer or
bacteriophage depositions steps directly to the AFM for imaging. SEM micrographs were
obtained of films made on silicon or FTO (JEOL instruments: JSM-6060 and 6700F) and
sputtered with approximately 4 nm of Pd and Au before imaging. Porosity was determined using
the method outlined in a previous publication using the film thickness before and after the porous
transition
58
Surface Area Characterization
The surface area of films was determined through a dye desorption measurement. Titania films
were immersed into black dye solution for 24 hours. Black dye solution was 0.2 mM black dye
(N749, Ruthenium 620-IH3TBA, Solaronix) and 20 mM chenodeoxycholic acid (>97%, Sigma)
in dry ethanol 72 . The amount of adsorbed dye is directly proportional to the available titania
surface area because the dye adsorbs as a monolayer. The total amount of dye in the device was
67
determined by eluting the black dye from the devices into a known amount of 0.1 M NaOH and
using a UV-Vis calibration curve to determine the concentration of dye in solution. Dye loading
was then calculated by dividing that total amount by the thickness and area of the titania film.
2.4.2.5 Redipping
Dye loading experiments, described in the previous section, were performed to determine the
available surface area of the annealed titania photoanodes. The results of these experiments are
shown in Figure 2.23 below.
70 ~ Nanoparticle
65 -g
LbL
60
E
P 55
E 50
45
M 40
~35
~30
025 20
0
j
5
1
-
10
15
20
25
Film Thickness
Figure 2.23 Dye loading of titania films as a function of film thickness.
Figure 2.23 illustrates two things. First, the dye loading and thus surface area of the LbL films is
about 50% of nanoparticle films. Second, once film thickness increases above about 10 tm,
there is a significant, sustained decline in dye loading. This cause of this decrease was
investigated with cross-sectional SEM. These images are shown in Figure 2.24.
68
pun 20
25 pm
Figure 2.24 Cross-sectional SEM images of LPEI/PAA films at (a) 20 bL, (b) 30
bL, (c) 40 bL, (d) 60 bL, and (e) 80 bL.
These images illustrate that as film thickness is increased, the average pore size also increases.
This asymmetric porosity has been observed by previous researchers, and the cause is not well
understood4 2 . Regardless of the cause, the increase in pore size translates to less surface area
available for dye loading. Lower dye loading, in turn, translates to decreases in Jsc and device
efficiency. Therefore, to achieve thicknesses of 12-14 microns with good porosity, the dip LbL
and post-treatment procedure were repeated twice. Initial attempts to deposit more tetralayers on
top of a film before the titania conversion process all resulted in complete pore collapse.
However, no collapse was observed when films were deposited, made porous, templated with
titania, annealed as mentioned above and then subjected to further LbL deposition. One such
"redipped" film is shown below in Figure 2.25.
Figure 2.25 SEM image of a redipped LPEI/PAA film. The top crust of the
lower film was not removed in order to highlight the existence of the two separate
films.
69
All LbL DSSC devices discussed in section 2.5 were made with redipped films 12-14 ptm in total
thickness and made with or without bacteriophage.
2.4.3
Results and Discussion
Based upon the literature review of previous work in templated LbL films, one major challenge
in the creation of effective LbL-based titania photoanodes for DSSCs is film thickness after
annealing. Previous researchers in this area had not been able to demonstrate films at the
thicknesses required for high performance DSSCs in the range of 10-15 pm. The results of the
adjusted annealing and titania templating conditions (covered in Sections 2.4.2.3 and 2.4.2.4) on
the annealed film morphology are shown in Figure 2.26 below. For comparison, these images
are shown side-by-side with images from the previous LbL templating work of Shiratori et al.se,
5
and Hammond et al.50 .
Figure 2.26 SEM images of porous, annealed titania films templated from a)
PAH/PAA (10 bL) by Shiratori et al.se, b) PAH/PAA (15 bL) by Shiratori et al.5 ,
and c) LPEI/PAA by Hammond et al. 50 and d) LPEI/PAA in this work (not
redipped). (a), (b) and (c) are reproduced from [56] Copyright 2003 with
permission from Elsevier, [57] Copyright 2006 with permission from Elsevier,
and [50] Copyright 2005 with permission from Wiley-VCH, respectively.
These images clearly show that the current templated titania films can achieve much higher
titania film thicknesses after templating and annealing than previous work in this area. This is
70
likely due to two factors. The higher concentrations of titania precursor in solution lead to more
robust coatings of titania on the LbL template. Also, the humid annealing conditions prevent
cracks from forming in the LbL film at low temperatures. Although the film shown in Figure
2.26 is thicker than the desired range for DSSC applications, it is meant to highlight to
capabilities of the new templating and annealing conditions.
2.5
Assembly and Testing of LbL-based DSSC Devices
2.5.1
Literature precedent
Because the literature presents a myriad of different approaches to DSSC device assembly 73 and
testing74 76 , the challenge was to winnow down the suggested procedures to create assembly and
testing methodology the produced reproducible, reliable data for these systems. Gratzel et al.
created the most basic resources available for learning device construction techniques by
publishing a DSSC lab sequence for high school77 or college students78 . Several common
strategies for creating higher performing devices will be discussed in Section 2.5.2. More details
on device construction and testing, including discussions of the following topics: various device
architectures, tips for making efficient devices, JV testing, IPCE testing, and EIS testing for
electron diffusion length determination can be found in Appendix A.
2.5.2
Materials and Methods
2.5.2.1 Solar Cell Construction
Several generations of device architecture were investigated in order to create the best
performing DSSCs possible with these devices. Because this section includes results from only
Generation IV devices, a picture and schematic of this device type are shown in Figure 2.27
below. All device generation pictures and schematics are included in Figure A.2 in Appendix A)
and details of the performance of each generation are also presented.
71
Figure 2.27 Generation IV of device architecture. The picture is an image of real
device, and the adjacent scheme depicts the way the active components are
In the scheme, the dark gray color represents the platinum
aligned.
counterelectrode. The light gray color represents the FTO current collector. The
dark green represents the dyed titania film. The yellow is a spacer layer. The
dashed lines outline the active/working portion of the device.
TitaniaNanoparticleControl
Nanoparticulate titania films were made by doctor blading Solaronix titania paste (13/400 nm,
Ti-Nanoxide D/SP) using a glass rod and one layer of scotch tape. Films were dried on a hot
plate at 100 "C for 5 minutes, then sintered in a 450 "C furnace for 30 minutes and cooled in an
80"C oven. If thicker films were desired, doctor blading was repeated after the annealing step.
Materials
A dye solution of 0.2 mM black dye (N749, Ruthenium 620-1H3TBA, Solaronix) and 20 mM
chenodeoxycholic acid (>97%, Sigma) was made in dry ethanol 72 . Titania films were moved
directly from the 80"C oven and submerged in the dye solution for at least 9 hours. Upon
removal, the films were rinsed with dry ethanol. The films were shaved into rectangles and the
FTO was then covered with 25 micron thick surlyn spacer (Solaronix), cut to leave the active
area and the external circuit contact exposed. A flat piece of Teflon was then placed over the
surlyn and the substrates were clipped together using binder clips (Staples) and place into a
100"C oven for 5-10 minutes or until the surlyn had good contact with the FTO. The clips and
Teflon were removed and the photoanode was allowed to cool. After cooling, 10-20 pL of
electrolyte (1.0 M butyl-methylimidazolium iodide (Iolitec), 0.05 M lithium iodide, 0.1 M
guanidinium thiocyanate, 0.03 M iodine, and 0.5 M tert-butyl pyridine (Sigma Aldrich) in a
72
solvent of 85% acetonitrile and 15% valeronitrile v/v 7 3 were dropped onto the active area, and a
platinum counterelectrode (constructed by sputtering 100 nm of Pt onto a clean FTO substrate)
was sandwiched on top. Binder clips were used to prevent leakage, and devices were tested
immediately after assembly to minimize degradation effects. Contacts were made to the
platinum counterelectrode and FTO electrode using copper tape (Mcmaster-carr).
2.5.2.2 Device Testing Procedures
JV Curves
Device performance was characterized with a Keithly 2400 source meter and a Class B solar
simulator (Photo Emission Tech., Inc., Camarillo, CA), calibrated to 1 sun with a silicon
photodiode. JV curves were measured from forward to reverse bias at a 100 ms voltage settling
time. Care was taken to minimize framing effects by using rubber pieces to shade the FTO glass
and inactive areas of the device. Many more details of JV curve generation are covered in
Section A.l.
EIS testing and Ldiff Extraction
Electron diffusion length measurements were taken with a Solartron 1460 impedance
spectrometer in the dark under forward bias in a faraday cage. The applied AC voltage was 10
mV. The applied DC voltage was varied in 25 mV increments within 100 mV of the voltage at
the maximum power point for the solar cell (data taken from the JV curve). Impedance data
were fitted using the ZView software and the commonly available transmission line model 36.
EIS testing and methodology are discussed in more detail in Section A.3.
Dye Loading
The total amount of dye in the device was determined by eluting the black dye from the devices
into a known amount of 0.1 M NaOH and using a UV-Vis calibration curve to determine the
concentration of dye in solution. Dye loading was then calculated by dividing that total amount
by the thickness and area of the device.
2.5.3
Results and Discussion
To precisely determine the effects of the bacteriophage on titania photoanode function as distinct
from the effects of the porous LbL film morphology, control devices were made. The
73
photoanodes of the control devices were fabricated simply by replacing the phage with PAA
during LbL deposition, yielding photoanodes with similar pore structure and surface area. The
discussion below compares the device performance (JV and Ldiff characteristics) and of a phagetemplated titania photoanode (termed P device) to the LPEI/PAA only control photoanodes
(termed L device). Devices were fabricated with identical construction techniques and films
were made to be between 12 and 14 pm thick. A device made with a commercial titania
nanoparticle paste ("NP Device") was also employed as an efficiency benchmark for
comparison. A highly scattering paste was chosen to more closely match the scattering properties
of the LbL-based titania architecture. The performance of these devices under simulated sunlight
is shown in Figure 2.28 (and extracted data are in Table 2.3).
15
E 10
51
E 5
0
C
0.8
U
-5
Applied Bias (V)
Figure 2.28 JV curve for doctor bladed nanoparticle devices (NP) and for LbL
devices made without bacteriophage (L) and with bacteriophage (P).
74
Table 2.3 Device performance parameters extracted from Figure 2.28.
Thickness Area
ri
FF V0c
(pm)
(cm2)
(%) (% (mV)
14.8
0. 186 5. 10 59.9 695
L
1 0.1 t 0.003i± 0.05 0.
2
Dye
Jsc
Loading
2
(mA/cm ) (Vmol/cm3)
12.25
34.3
0.07
0.7
In general, the short circuit current density (Jsc) of a DSSC is determined by the light harvesting,
charge injection, and charge collection efficiencies (see Equation 2.5). Higher short circuit
current density is indicative of higher electron collection efficiency, which is the expected effect
of an embedded titania nanowire component. In general, the charge injection efficiency of
DSSCs approaches unity, and it is necessary to examine the effect of light harvesting and
electron collection efficiencies. The NP device yields a higher 11than the L device, likely
resulting from more efficient light harvesting due to higher dye loading than the L device.
However, the P and NP devices have the same fl, indicating that the presence of the phage can
further increase the electron collection efficiency and compensate for the loss of light harvesting
(through the lower dye loading). The randomly packed sphere morphology of the nanoparticle
films is not ideal for efficient electron collection. Thus, even with 52% and 54% of the adsorbed
dye of NP device, the L and P devices yields 95% and 110% of its Jsc. Electron collection
efficiency is directly related to the ratio Ldiff (see Equation 2.4) to film thickness d. This ratio
was measured for each of these devices and is shown in Figure 2.29 below.
75
12
*+ Nanoparticle Device (NP)
1:
n LbL-only Device (L)
A Bacteriophage Device (P)
8
21
500
600
700
800
Applied Bias (mV)
Figure 2.29 Electron diffusion length data normalized to film thickness (d) as a
function of applied bias.
The nanoparticle control device (NP) exhibited the same efficiency as the P device. Although
the original goal of this work was to surpass the efficiency of NP devices, the P devices still
show promise in that the dye loading for the NP device is twice that of the P and L devices, but
the P devices alone achieve the same efficiency as the NP device. This result is not from the
morphology effect, because the P and L devices have identical morphologies. It is expected
when the templated-bacteriophage component is acting to increase electron collection efficiency.
These data show that the P devices have a higher electron collection efficiency than L and NP
devices, as expected, and are in good agreement with the conclusion that the bacteriophagenanowire template is acting as expected within the templated film. Surprisingly, the L devices
also have a larger Ldiff than the NP devices, although it is still smaller than the P devices. Ldiff is
a function of the effective diffusivity (see Equation 2.3), and effective diffusivity, in turn, is a
function of film porosity (0), tortuosity ((o), and pure component diffusivity (Do), as is shown in
Equation 2.7 below).
Deff =
o
)Do
2.7
76
From porosity measurements (not shown here, determined from thickness measurements before
and after the porous transition), we know that the pore volume fraction of the L and P devices is
identical within error and equal to a value of 76 ± 7%. Also, because the XPS data for L, P and
NP devices were identical, the chemical makeup of the resulting titania films is identical. This
result indicates that the pure component diffusivity, Do should be identical for the P, L and NP
devices. This leaves the tortuosity o as the component that could differ between NP devices and
P and L devices. Tortuosity is introduced as a factor to account for morphology differences that
affect diffusion through obstructed spaces. Therefore, the tortuosity of the L and P devices is
likely smaller than that for the NP devices, leading to a higher Deff and thus, Ldiff.
2.6
Conclusions
With this work we have successfully developed a method to incorporate the M13 bacteriophage
in a highly controlled manner as a functional component of a LbL film. AFM imaging,
fluorescent labeling, and dipping bath concentration depletion all confirm that bacteriophage is
assembling within the tetralayer film architecture. AFM imaging was also used to show that the
bacteriophage can be deposited with various surface conformations, from bundled thick layers to
sparse layers depending on solution pH and buffer conditions. The demonstrated LbL
incorporation technique could easily be extended to other genetic variants of the M 13
bacteriophage that would nucleate different materials for different applications.
Porous film architecture was faithfully templated with titania using an in situ liquid phase
deposition step. A method was developed to remove the top skin that forms during the LbL film
porous transition. Titania films 12-14 microns thick were made using a redipping procedure that
also increased the film surface area. The effects of the bacteriophage template were tested in the
templated titania photoanode of a DSSC. The phage devices yielded higher efficiencies and
electron diffusion lengths than non-phage LbL devices, indicating that the incorporated
bacteriophage template was active. Comparable gains in device efficiency as compared to
nanoparticle systems were not realized due to smaller photoanode surface area, as demonstrated
by dye loading measurements. LbL devices without phage also yielded an increase in electron
diffusion length, which is likely due to changes in tortuosity from sintered nanoparticles to the
open porous morphology.
77
2.7
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Langmuir 2003, 19 (6), 2169-2174.
55.
Rahman, M.; Tajabadi, F.; Shooshtari, L.; Taghavinia, N., Nanoparticulate Hollow TiO2
Fibers as Light Scatterers in Dye-Sensitized Solar Cells: Layer-by-Layer Self-Assembly
Parameters and Mechanism. ChemPhysChem 2011, 12 (5), 966-973.
56.
Takenaka, S.; Maehara, Y.; Imai, H.; Yoshikawa, M.; Shiratori, S., Layer-by-layer selfassembly replication technique: application to photoelectrode of dye-sensitized solar cell. Thin
Solid Films 2003, 438, 346-351.
57.
Tsuge, Y.; Inokuchi, K.; Onozuka, K.; Shingo, 0.; Sugi, S.; Yoshikawa, M.; Shiratori, S.,
Fabrication of porous TiO2 films using a spongy replica prepared by layer-by-layer selfassembly method: Application to dye-sensitized solar cells. Thin Solid Films 2006, 499 (1-2),
396-401.
58.
Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F.,
Fabrication of Microporous Thin Films from Polyelectrolyte Multilayers. Langmuir 2000, 16
(11), 5017-5023.
59.
Shiratori, S. S.; Rubner, M. F., pH-dependent thickness behavior of sequentially adsorbed
layers of weak polyelectrolytes. Macromolecules 2000, 33 (11), 4213-4219.
60.
Clark, S. L. Engineering the Microfabrication of Layer-by-Layer Polyelectrolyte
Assembly. MIT, Cambridge, MA, 1999.
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Choi, J.; Rubner, M. F., Influence of the degree of ionization on weak polyelectrolyte
multilayer assembly. Macromolecules 2005, 38 (1), 116-124.
62.
Chia, K. K.; Rubner, M. F.; Cohen, R. E., pH-Responsive Reversibly Swellable Nanotube
Arrays. Langmuir 2009, 25 (24), 14044-14052.
63.
Cho, J.; Caruso, F., Polymeric Multilayer Films Comprising Deconstructible HydrogenBonded Stacks Confined between Electrostatically Assembled Layers. Macromolecules 2003, 36
(8), 2845-2851.
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Sukhishvili, S. A.; Granick, S., Layered, Erasable, Ultrathin Polymer Films. J. Am. Chem.
Soc. 2000, 122 (39), 9550-9551.
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Dubas, S. T.; Schlenoff, J. B., Polyelectrolyte Multilayers Containing a Weak Polyacid:
Construction and Deconstruction. Macromolecules 2001, 34 (11), 3736-3740.
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Chernik, V. N.; Paskhalov, A. A.; Gaidar, A. I., Polymer surface erosion under an oxygen
plasma stream. J. Surf Ingestig.-X-Ray Synchro. 2009, 3 (2), 215-217.
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Deki, S.; lizuka, S.; Horie, A.; Mizuhata, M.; Kajinami, A., Liquid-phase infiltration
(LPI) process for the fabrication of highly nano-ordered materials. Chem. Mat. 2004, 16 (9),
1747-1750.
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Hou, K.; Tian, B.; Li, F.; Bian, Z.; Zhao, D.; Huang, C., Highly crystallized mesoporous
TiO2 films and their applications in dye sensitized solar cells. J. Mater. Chem. 2005, 15 (24).
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Chen, W.; Geng, Y.; Sun, X.-D.; Cai, Q.; Li, H.-D.; Weng, D., Achievement of thick
mesoporous TiO2 crystalline films by one-step dip-coating approach. Microporousand
Mesoporous Materials2008, 111 (1-3), 219-227.
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Chen, W.; Sun, X.; Cai, Q.; Weng, D.; Li, H., Facile synthesis of thick ordered
mesoporous TiO2 film for dye-sensitized solar cell use. Electrochem. Commun. 2007, 9 (3), 382385.
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A.; Brunet-Bruneau, A.; Amenitsch, H.; Albouy, P. A.; Sanchez, C., Highly Porous TiO2
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A.; Gratzel, M., Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO2Based Solar Cells. J. Am. Chem. Soc. 2001, 123 (8), 1613-1624.
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M., Fabrication of thin film dye sensitized solar cells with solar to electric power conversion
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82
Chapter 3.
Coating Planar and Non-planar
Structures via Spray Layer-by-Layer Assembly
3.1
Introduction
Layer-by-Layer (LbL) is a powerful thin film assembly technique that has the ability to
intimately blend polymers or nano-objects in ways that can combine the desirable characteristics
of the different materials into one thin film
3.
Traditionally, dip LbL has been the main
deposition method for LbL assembly 4 . The previous chapter explored one particular application
of dip LbL to the blending of a nanowire template with the mesoporosity of a weak
polyelectrolyte system. In this chapter, we will extend the analysis to spray LbL systems on
planar and non-planar substrates, both of which could be used as current collectors for DSSCs.
Spray LbL also involves the sequential adsorption of components with complementary
functionality, but it differs from dip LbL in two major ways. First, the application involves a
fine mist of the active components directed at a stationary substrate, not static dipping baths that
a mobile substrate is immersed into sequentially 5 . Second, this process is up to 25 times faster
than dip assembly but can still coat large substrate areas6 . The dip LbL device assembly method
as described in Chapter 2 requires eight days from start to finish for the fabrication of dip LbL
devices. This timeline is impractical if devices such as these are ever to be commercialized.
Because spray LbL could considerably shorten this timeline, it is an attractive technique for
improving manufacturability of LbL photoanodes.
Spray LbL is investigated in two different ways in this analysis. First, the potential of these
systems to undergo a porous transition, and thus also be a suitable means for DSSC photoanode
templates as discussed in Chapter 2, is investigated as a function of assembly and treatment pH.
Second, spray LbL is applied to metal meshes of various pore sizes to determine if this
deposition method could be used to coat solid state electrolytes or titania photoanodes onto
porous or non-planar surfaces. These two topics are addressed in sections 3.2 and 3.3
respectively, each with its own introduction, materials and methods, and results and discussion
sections.
83
3.2
Investigating the Porous Transition in Spray-deposited Weak
Polyelectrolyte Films
3.2.1
Introduction
Dip LbL films of certain weak polyelectrolytes have been shown to undergo a porous transition
upon exposure to extreme pH conditions that induce changes in the charge density of the
polyelectrolyte 7 '0 . The porous transition has been shown to depend upon the assembly pH of the
polyelectrolyte solutions and on the transition pH of a (generally) acidic solution that the
assembled film is exposed to after assembly. Both of these parameters affect the ratio of free
(ionized or ionizable groups not currently involved in electrostatic bonding with polyelectrolyte
groups of opposite charge) to bound (ionized or ionizable groups currently involved in
electrostatic bonding with polyelectrolyte groups of opposite charge) groups in the
polyelectrolyte film. This ratio determines the conformation of polyelectrolyte chains within the
LbL film, as well as the manner in which excess charge is compensated for in the film. Polymer
adsorption conformation and charge compensation, in turn, are known to differ between films
deposited via spray and dip LbL 5 , . Spray LbL films are thought to exhibit effects from kinetic
trapping during deposition", while polyclectrolytes in dip LbL films deposit in an equilibrium
conformation 4 . In this work, we demonstrate a porous transition in spray LbL deposited systems
of weak polyelectrolytes, indicating that the conformation and charge compensation differences
between spray and dip LbL do not eliminate the porous transition behavior in LPEI/PAA films.
Specifically, we investigate the range of deposition and treatment pHs that will induce a porous
transition in spray-deposited LbL systems. Due to the large range of parameter space,
experimental design techniques were utilized to most efficiently capture the effects of these
parameters on film outcomes.
Spray LbL assembly of polyelectrolytes can be affected by various materials factors (polymer
concentration, solution ionic concentration, etc.) and processing conditions (nozzle pressure,
flow rate, spray time, spray distance, droplet size etc.) 2 . Many of these factors are at least
weakly dependent on the others. By controlling one factor at a time and keeping the others fixed,
it is possible to probe the effects of each systematically using statistical design of experiments 3 .
The spray porous transition was a good candidate system for analysis of variance (ANOVA)
84
because the inputs and outputs were quantifiable and independent. Thus, an ANOVA design of
experiments was performed to quantify the dependence of film thickness and porosity on
assembly pH and transition pH (including first order interactions between them). This approach
allowed a focused set of experiments to be performed and their results used directly to create
equations relating the input parameters to the measured outcomes. The design equations
resulting from this analysis could be used to guide future research in spray LbL for the creation
of porous LbL-based DSSC photoanode templates.
3.2.2
Materials and Methods
3.2.2.1 Preparationof Polymer Solutions
LPEI (25,000 MW, Polysciences) and PAA (250,000+ Mw, 25 % aqueous solution, Polysciences)
were used as received and dissolved in Milli-Q water at 20 mM concentration with respect to the
repeating unit. The pH of the polymer solutions and water (for the porous transition step) was
adjusted using 1 M and 0.1 M HCl or NaOH. Unless specified otherwise, the pH of the polymer
solutions was 4.40 and the rinse solution was untreated Milli-Q water.
3.2.2.2 Substrate Cleaning
The layer-by-layer films were constructed on 2" x 2" glass substrates (VWR) and cleaned by
sequential ultrasonication in 1%alconox solution, water, acetone, and ethanol for 15 minutes
each. For dip layering, substrates were cracked into two 1"x 2" pieces before cleaning. Before
use, the substrates were plasma etched in a Harrick PCD 32G plasma cleaner with oxygen bleed
for five minutes.
3.2.2.3 Spray Layer-by-Layer Assembly
An automated spray LbL deposition system was used to construct films with the bilayer
architecture (LPEIx/PAAy)n, where n represents the number of deposited bilayer repeats, x is the
pH of the LPEI solution and y is the pH of the PAA solution. For the ANOVA analysis, 100
bilayers were always deposited with the following deposition time structure. LPEI was sprayed
first for 5 seconds, followed by a 4 second drain time. A Milli-Q water rinse (18.2 Me-cm,
Milli-Q Ultrapure Water System, Millipore) was then sprayed for 6 seconds, followed by a 4
second drain time. PAA was then sprayed for 5 seconds, followed by a 4 second drain time, 6
second Milli-Q water rinse, and further 4 second drain time. To increase the uniformity of the
85
deposited film, the substrate, which was oriented parallel to gravity, was rotated at a rate of 35
rpm. After deposition, the films were dried in ambient conditions and then stored in a vacuum
desiccator until further use. For all spray studies, the source pressure was maintained at 20 psi
and the flow rate of each nozzle was calibrated to be 0.15 mL/s.
3.2.2.4 Dip Layer-by-Layer Assembly
A modified Carl Zeiss DS50 programmable slide stainer was used to accumulate the film
components. Similar nomenclature, (LPEIX/PAAy)n, for the dip films was used. Each polymer
deposition step involved 4 separate dipping baths at the following conditions: 3 minute
submersion in the polymer solution at pH 4.40, followed by three Milli-Q water rinse steps (30
seconds in the first one, 1 minute in the second, then 11/2 minutes with agitation in the third).
After deposition, the films were dried in ambient conditions and then stored in a vacuum
desiccator until further use.
3.2.2.5 Film Post-treatmentConditions and Analysis
After assembly and drying, the thickness and roughness of the sprayed films were measured
using a surface profilometer (Veeco Dektak) at 1, 2, 5, and 9 mm from the center of the slide. A
linear regression was performed with each sprayed data set as a function of distance from the
center of the film. All data shown in the results section are the fitting results from the films at 5
mm. The thickness and roughness of the dipped films were also measured with profilometry
close to the middle of the dipped film area. Once initial film thickness measurements had been
taken, the films were immersed into a solution at a low transition pH for 25 minutes. After this
time had elapsed, the films were blown dry with nitrogen and then cross-linked in an oven at
120 0C for 1 hour. Film thickness and roughness were determined after the crosslinking step with
the same surface profilometer and techniques mentioned above. Height ratios were calculated by
dividing the film height after assembly by its height after the porous transition.
3.2.2.6 ANOVA and Design of Experiments
A central composite rotatable design of experiments was performed with axial points added to
estimate curvature using the JMP software package. The input parameters of interest were the
LPEI rinse time, varied from 1.0 to 11.0 seconds, the assembly pH of the LPEI solution, varied
from pH 3.00 to 5.80, and the transition pH, varied from pH 1.80 to 3.00. All ANOVA films
consisted of 100 deposited bilayers with a PAA assembly pH of 4.40. Data at the center point
86
were repeated six times to give an estimate for appropriate fitting error bars. The measured
outcomes were film thickness and roughness before and after the porous transition. A height
ratio was then calculated from the film thickness results before and after the porous transition.
ANOVA was performed for each measured parameter, but only outcomes that showed evidence
of effect by the input parameters are shown in Section 3.2.3 below. Table 3.1 below summarizes
the range of parameters investigated in this work.
Table 3.1 Summary of input parameters studied in ANOVA.
Variation levels, a
Variation intervals
Spray parameters
Xi,-G
X1,-
X1,0
Xj,+
Xj,+a
AX
X, - LPEI Assembly pH
3.00
3.57
4.40
5.23
5.80
0.83
X2 - Transition pH
1.80
2.04
2.40
2.76
3.00
0.36
X3 - LPEI Rinse Time (s)
1.0
3.0
6.0
9.0
11.0
3.0
a is known to be 1.682 for a 3 parameter experiment' . The maximum and minimum values of
the dimensionless parameters X, were set equal to + a and - a and then variation intervals, Axi,
were calculated from the given maximum and minimum dimensional parameter values, xi, using
Equation 3.1 below.
X. =
x.
-
x
Ax.
iO
3.1
Outcome data, Yj, were fitted using Equation 3.2, which estimated the first and second order
effects of each input parameter and the first order interaction effects of the parameters.
3
Y =bo +
i=1
3.2.3
3
bX,+
2
3
bA
X+
i=1
b,,XX,
i=1
3.2
j>i
Results and Discussion
The main difference between dip LbL and spray LbL films is the deposition timescale. Dip LbL
films are generated with adsorption over a much longer time scale, where polyelectrolytes or
phage diffuse from the bulk solution and deposit on the surface in an equilibrium conformation;
87
spray LbL deposition is much faster because a microns-thick liquid phase reduces the distance
charged groups must travel to adsorb to the surface. Assembly time is reduced, yielding little
time for polyelectrolyte rearrangement and a kinetically-trapped adsorbed morphology5 . A
reduction in the interdiffusion of LPEI or an increase in its charge density (by lowering
deposition pH) is expected to correspond to smaller thickness increases per bilayer14. This effect
is noted in Figure 3.1 below, which shows the growth curves for the (LPEI 4 .4 /PAA 4 .4 )n films
before and after the porous transition for dip and spray deposition at different values of n. These
porous transitions were performed at the ANOVA center point value of pH 2.40.
y
6
- 0.4471:
R2 = 0.989
=0.0968x
51
E~
0~R
10
2=0.9949
y =0.0149x - 0.0894
30
ADip LbL after Assembly
+ Spray LbL after Assembly
40
50
60
70
Bilayers
ADip LbL after Porous Transition
Spray LbL after Porous Transition
Figure 3.1 Growth curves for spray (gray diamonds) and dip LbL (black
triangles) films before (filled points) and after (empty points) the porous
transition. Data were fit with linear regression equations, the equations of which
are shown in the boxes. The inset is a cross-sectional SEM of 100 bL spraydeposited porous film.
As expected, the spray LbL system (before the porous transition) grows much more slowly than
the dip LbL system at a rate of 15 nm/bL as compared to the 97 nm/bL exhibited by dipped
systems. Others, who have also noted the slower growth of spray LbL films as compared to dip
LbL films, have attributed this trend to the decreased contact time of spray LbL 5 .
88
To further probe the effect of switching from dip to spray deposition, LPEI assembly pH and
LPEI rinse time were varied from 3.00 to 6.00 and 1.0 to 11.0 seconds respectively. Assembly
pH affects the charge density along the polymer backbone, which, in turn, affects its deposition
conformation. Shorter polymer rinses limit the time available for the polymers to interdiffuse
and could limit both the interdiffusion of LPEI and increase kinetic trapping, which would
further slow film growth per bilayer. The effect of assembly pH, LPEI rinse time and the acidic
post-treatment transition pH (varied from 1.80 to 3.00) on the weak polyelectrolyte porous
transition (explained in Section 2.4.1.2) was also measured by calculating a height ratio (HR)
according to Equation 3.3 below where d is the film thickness before or after the porous
transition, depending on the subscript.
HR =
3.3
dbefore
dafter
The results of the film thickness ANOVA based upon equations 3.1 and 3.2 are shown in Table
3.2 below.
Table 3.2 Values of coefficients given by the JMP software for the prediction of
film thickness and height ratio. Starred p-values indicate significant parameters
within the 95% confidence interval.
Thickness
Height Ratio
Coefficient
p- value
Coefficient
p- value
1.773
<0.0001*
0.421
<0.0001*
1.29
<0.0001*
-0.277
0.0001*
b2
-0.046
0.535
0.184
0.0001*
b3
-0.005
0.95
0.018
0.322
bl,
0.02
0.781
0.266
0.0001*
b22
-0.006
0.926
0.098
0.0002*
b33
0.017
0.809
0.032
0.089
b 12
-0.065
0.505
0.054
0.041
b13
0.038
0.691
0.002
0.92
b23
0.004
0.965
-0.029
0.238
bo
S
89
Table 3.2 indicates that the assembly pH is the only parameter, of those investigated, that has a
statistically significant effect on 100 bL film thickness. As expected, film thickness right after
assembly,
dbefore,
showed no dependence on transition pH because the films had not yet been
exposed to that pH to undergo a porous transition. LPEI rinse time also showed no effect,
indicating that within the measured film area, a 1.0 s rinse and a 11.0 s rinse will yield the same
film thickness. Equation 3.4 summarizes these effects where A is assembly pH and d is film
thickness in pm.
3.4
d =1.773+1.29X = -5.766+1.675A
Figure 3.2 shows Equation 3.4 graphically across the range of parameters that were investigated.
4.5
4
3.5
3
2.5
~1.5
1
0.5
3
4
5
6
LPEI Assembly pH
Figure 3.2 Film thickness as a function of LPEI assembly pH. The line is the
ANOVA prediction. The diamonds are the raw thickness data for 100 bL films.
Films deposited at lower LPEI pH were the slowest growing, indicating that LPEI was deposited
in a very flat conformation as would be expected at high LPEI charge densities. When the LPEI
is weakly charged at higher pHs, the growth conditions are dominated by loopy and excess
adsorption, leading to large thickness increases of approximately 30 nm/bilayer above pH 5.23 as
compared to the 3 nm/bL thickness increases seen below pH 3.57. Between pH 5.23 and 5.80,
the LPEI pH effect saturates, and film thickness deviates from its linear dependence on LPEI
assembly pH; film thicknesses at 5.23 and 5.80 are nearly the same. Because the ANOVA as
designed can only predict first and second order effects, it misses the third order effect that these
90
low and high pH values induce and predicts a film thickness of -0.4 ± 0.3, which is clearly not a
possible outcome.
The results of the ANOVA for HR are summarized in Equation 3.5 below.
HR = 0.421-0.277X +0.184X 2 +0.266X1 + 0.098X
X2
3.5
=14.254 - 4.397A -3.1185T + 0.4487A 2 + 0.756T 2
This equation shows that the film swelling during the porous transition depends both on
assembly and transition pH and not on LPEI rinse time. This fitted parameter is directly related
to the porosity P that can be calculated from the film thickness (d) before and after the porous
transition8 according to equation 3.6 below.
P =1- HR =
dft-d
d'efore
b''
d after
3.6
Equation 3.5 indicates that assembly pH and transition pH have first and second order effects on
film porosity. The data and ANOVA predication are graphed together in the contour plot below,
Figure 3.3.
91
3
2.8
q2
Q1
~2.6
2.42.2
21.81
3
3.5
5
4
4.5
LPEI Assembly pH
5.5
6
Figure 3.3 Contour plot of film porosity as a function of LPEI assembly pH and
film transition pH. The gray region represents the range of assembly pH where
the LPEI/PAA film grew slowly due to the high charge density of LPEI. The
numbers represent the calculated porosity of the film at that elevation.
Within the range of parameters investigated, the second order effect of transition pH on film
porosity has been noted in previous work on dipped films of LPEI and PAA 7 . The measured
height ratios clearly show that the films underwent a porous transition, as would be expected for
similar dip LbL systems. It is interesting to note that the gray region (assembly pH <3.5),
corresponding to the slower growth rates associated with higher LPEI charge density, does not
exhibit a porous transition in this treatment pH range. The area encompassed by the green circle
indicates the region where the porous swelling behavior is expected (P > 0). The largest possible
porosity of 73.9% is predicted to occur at an assembly pH of 4.90 and a transition pH of 2.06.
These results can be used to design a spray-deposited porous template for a DSSC photoanode
with a specific pore volume.
92
3.3
Spray LbL Pore Bridging of Metal Meshes
3.3.1
Introduction
LbL assembly produces a thin conformal coating of polymer. This characteristic of LbL has
been touted as an advantage over other coating techniques and used to conformally coat porous
bandages with drug for wound healing applications 15 , to conformally coat microneedles for
vaccine applications16 , and to create a pinhole-free titania blocking layer on an FTO substrate for
DSSC applications 17. Recently, however, techniques have been developed that expand the LbL
process beyond conformal coating. Farhat et al. coated over 100 nm pores using dip LbL, noting
that this phenomenon occurred only when the molecular weight and solution conditions caused
the effective size of the polymer chains to be as big as or larger than the pores18. Krogman et al.
was able to bridge the 10 and 20 im spaces between fibers of a porous electrospun mat using
spray LbL, postulating that bridging occurred when the spray droplet size was on the same order
as the spacing between the fibers19 . The ability to choose when porous substrates are
conformally coated or bridged by polyelectrolytes would add another tunable function to the LbL
arsenal.
It would be useful to know how to maintain conformal coating if, for example, a solid state
electrolyte were to be applied to a titania photoanode for a DSSC. Solid state LbL electrolytes
have been developed for battery and DSSC applications20 . Recently, vacuum-assisted spray LbL
has been introduced to yield conformal coatings on thick porous structures.
However, the
vacuum-assisted technique only works on porous substrates, and so is limited in its applications.
If conformal coatings could be achieved without the use of a vacuum assembly, the range of
substrates this technique could be applied to would increase.
Alternately, if non-conformal coatings could be controllably achieved, then they could be used to
create a porous LbL-templated titania photoanode (like those discussed in Chapter 2) that
bridged the pores of a metal mesh current collector. Many others have looked into various
metals as cheaper current collectors for DSSCs. Kroon et al. have shown that the TCO would
account for 15-25% of the final cost of a commercial DSSC 2 1 . In addition to cost reduction,
metals are attractive as current collectors because of their potential to be impermeable and
flexible22 , their inherent decreased sheet resistance (8 ohms per sq for TCOs compared to 1 ohms
93
per sq for metal films) 2 3, their potential for innovative device architectures
25,
and their
potential for increased transmittance 2 in the IR and near IR26 . Han et al. have deposited a porous
titanium layer as a current collector on the back side of a titania photoanode, achieving a DSSC
conversion efficiency of 8.4%27. Also, two companies, Tatasteel and Dyesol, have a partnership
to commercialize DSSCs on steel substrates 28. Because of the desirable attributes of metal
current collectors, LbL coating methods on porous metal substrates were investigated for DSSC
applications. However, LbL assembly is known for its ability to conformally coat substrates of
all shapes and sizes. In order to use the titania templating method outlined in Chapter 2, a
nonconformal coating on a metal mesh would be required.
Thus, the object of this portion of the study was twofold: to take advantage of the faster
processing afforded by spray LbL and to investigate the ability to conformally and
nonconformally coat porous substrates. It was desired to determine what controlled the extent of
pore bridging to see if such deposition could be utilized to apply a titania photoanode using
methods outlined in chapter 2 or as an electrolyte application method for DSSC or battery
applications. Because this work was directly related to electrolyte or titania photoanode
deposition, both strong and weak polyelectrolytes (see Section 2.4.1.2 for a discussion of the
differences) were used. The weak polyelectrolyte chosen was the LPEI/PAA pair discussed in
Chapter 2 and the first part of Chapter 3 for the titania photoanode application. The strong
polyelectrolytes chosen were Poly(diallyldimethylammonium chloride) (PDAC) and Poly(4styrenesulfonic acid) (PSS), which have been extensively studied and used either in electrolyte
applications or as cheaper analogs of electrolyte polymers.
3.3.2
Materials and Methods
3.3.2.1 Mesh Substrates
Stainless steel meshes (type 304 wire cloth assortment from McMaster-Carr or type 316 from
TWP Inc.) were cleaned by sequential ultrasonication in 1%alconox solution, water, acetone,
Because metals are opaque, they would be used as back contacts on the titania phase or deposited as a grid.
Neither conformation requires light to first go through a TCO, which is known to absorb in the IR and near IR.
2
94
and ethanol for 15 minutes each. Optical microscopy image analysis was performed and the
mesh names and pore and wire sizes are shown in the table below.
Table 3.3 Summary of mesh pore and wire sizes sorted by steel type as
determined by optical microscopy. The lengths and diameters presented are prm.
Mesh
Name
Steel
Type
240 mesh
316
240
2
22 ±2
118 mesh
316
118
3
53 ± 2
47 mesh
316
47
3
35± 3
mesh
304
111 8
68 ± 3
88 mesh
304
88
7
50.± 2
65 mesh
304
65 ±3
40.±2
42 mesh
304
42 ± 3
39± 4
32 mesh
304
32 ± 3
32±2
26 mesh
304
26 ±2
25 ± 1
1l
Square Pore
Side Length
Wire
Diameter
Immediately before deposition, each substrate was subjected to a five minute plasma treatment
with oxygen bleed in a Harrick PCD 32G plasma cleaner. Track-etched polycarbonate meshes
(Millipore, Isopore Membrane,17.8 pm thickness) with 10 pm pores were obtained to probe pore
sizes that were inaccessible with metal meshes. These membranes were used as received and
plasma treated for only 90 seconds before spray deposition.
3.3.2.2 Solution Preparation
Triton X-100 (VWR) was diluted to a stock solution of 100 mM with pH 2.00 water and stirred
vigorously until all gel had been broken up. LPEI (25,000 M., Polysciences), PAA (250,000+
Mw, 25 % aqueous solution, Polysciences), PDAC (200,000-350,000 Mw, 20% aqueous solution,
Aldrich), and PSS (~75,000 M, 18% aqueous solution, Aldrich) were used as received. LPEI
and PAA were diluted with Milli-Q water to a 20 mM concentration with respect to the repeating
unit while PDAC and PSS were diluted to a concentration of 10 mM. The pH of the polymer
solution and water rinse was adjusted using 1 M and 0.1 M HCl or NaOH. Unless specified
otherwise, the pH of the LPEI and PAA polymer solutions was 4.40 and their rinse solution was
95
untreated Milli-Q water. The pH values of the PDAC, PSS, and corresponding rinse solutions
were all adjusted to 2.00.
3.3.2.3 Spray Layer-by-Layer Assembly
All polymer solutions were deposited for 4, 5, or 6 seconds. All substrates were rinsed for 2 or 3
seconds. Polycation solutions were always sprayed for the same amount of time as polyanion
solutions. Polymer and rinse solutions were elevated to utilize hydrostatic pressure to eliminate
the burst of air that occurs when the nozzles leak air into the solution space during inactive
periods between spray bursts. The substrates were rotated at a rate of 35 rpm. A mesh substrate
holder was designed with a 1"wide trench so that meshes could be sprayed without a backing.
This holder and spray setup is depicted in Figure 3.4. All meshes were cut to 1" strips and
affixed to the holder with the 1"trench. Thus, the active area for analysis was always a l"x 1"
square defined by the mesh edge and the trench of the holder.
ma
Figure 3.4 Schematic of the spray LbL setup and mesh substrate holder (a) and
picture of a mesh mounted on the spray holder (b).
3.3.2.4 Drying Conditions
In order to minimize the effects of ambient conditions on the rate of film drying, films were
either dried in a humidity chamber at 75% relative humidity or were immersed into liquid
nitrogen and dried via sublimation on a Labconco FreeZone 2.5 Plus lyophilizer. For the
imaging studies performed in Figure 3.5, Figure 3.12, Figure 3.15, and Figure 3.16, all films
shown were freeze-dried in order to enhance contrast for imaging and to eliminate film drying
and deswelling effects on film bridging coverage.
3.3.2.5 % Bridging CoverageAnalysis
The fraction of mesh area that was covered with a bridging LbL film was calculated using an
ImageJ analysis of a digital camera image of the bridged film. Thresholding was applied and the
bridged portions of the film were made black. ImageJ was used to calculate the total number
96
pixels in the mesh area and the total number of black pixels. The pixel counts were used to
calculate the % bridging coverage. The sequence of images shown in Figure 3.5 below illustrate
the steps of this sequence.
Figure 3.5 Visual sequence for ImageJ analysis. The first step involves taking a
digital picture of the flat mesh (a), cropping the picture to the active area (b), and
then applying the thresholding (c) to make the image consist of only black or
white pixels.
The final thresholded images were visually compared to the original film. If the thresholding
was deemed inaccurate, the bridged areas of the film were manually outlined and used to
generate a black and white image that accurately reflect the bridged portions of the film.
3.3.2.6 Imaging
Optical microscopy was performed using a Leica Microscope, Leitz DMRX. SEM micrographs
were taken on a JEOL JSM-6060 microscope after being sputtered with 12-16 nm of gold
coating.
3.3.2.7 Film Thickness Analysis
The thickness of bridged films was determined via cross-sectional SEM images taken a low
magnification and high working distance (to achieve a good depth of field). Example images are
shown in Figure 3.6.
97
Figure 3.6 Cross-sectional SEM images of a bridged polymer film. The red
rectangle in (a) is shown at a higher magnification in (b). The three pseudocircular shapes in (a) and the two in (b) are the cross-sections of the metal wire.
Normally, cross-sectional images would be generated via freeze-fracturing. However, because
of the difference in mechanical properties between the metal mesh and the film (and the fragility
of the bridged films), cross-sectional mesh samples were cut with metal shears along bridged
portions of the film. ImageJ was then applied to these images to determine the number of
microns per pixel and then number of pixels thick the film was at points that were close to the
center of the pore in the mesh.
3.3.2.8 Nanoindentation
Nanoindentation experiments were performed using a Hysitron TriboIndenter to determine the
bridged film maintained the mechanical stiffness of films deposited in a normal LbL fashion. All
films were thick enough that a 200 nm indent could be applied. A Berkovich model was used to
fit the load data. Young's modulus was calculated from the reduced modulus using Equation
(3.7) below, where Er is the reduced modulus of the sample, Ei and Es are the Young's moduli or
the indenter and the sample, respectively, and vi and vs are Poisson's ratios of the indenter and
the sample, respectively29
S=(1- v,2)E, +(1-
vs) E,
(3.7)
For the indenter, vi and Ei are 0.07 and 1.41 respectively. The value of Poisson's ratio used to
analyze the polymer samples was 0.35, a generally accepted value for polymers. Three different
sample types were tested with nanoindentation: normal sprayed films, spray bridged films, and
dropcast polyplex films. The normal sprayed films and dropcast polyplex films were constructed
on glass substrates. Dropcast polyplex films were constructed by mixing the same volumes of a
polyanion and polycation and then pouring the solutions into a petri dish that contained a clean
98
glass slide. The solution was allowed to evaporate, leaving behind a condensed polyelectrolyte
film. Spray bridged films were constructed on the 240 mesh. Care was taken to ensure that
measurements of the mechanical stiffness of the mesh films were measured only in a 50x50 pm
square in the middle of the pores. Despite these precautions, nanoindentation experiments on the
spray bridged films were performed on polyelectrolyte surfaces that did not have a solid
substrate backing behind them. Such substrate backings were present in the other two sample
types (glass). Even though the backing does not affect the measured properties of the material if
the indent is less than 10% of the film thickness 30 , the measurement on spray bridged films
includes a trampoline effect, wherein the whole film can deform upon indentation like a
trampoline does under the weight of a single item. Therefore, nanoindentation measurements on
the metal mesh can show minimum values for elastic modulus, but not average film property
values.
3.3.3
Results and Discussion
3.3.3.1 Comparison of Weak and Strong Polyelectrolyte Systems
Pore bridging was observed to occur on the 240 mesh in LbL films made with weak and strong
polyelectrolytes. Optical microscopy confirms the existence of the bridging, as shown in Figure
3.7.
99
PDAC/PSS, pH 2, OM NaCI
200bL
PDAC/PSS, pH 2, 0.2M NaCI
200bL
LPEI/PAA, pH 4.75
150bL
0
E
0
0
CL
Figure 3.7 Optical microscopy of bridged films on the 240 mesh. Scale bars are
not shown in all images because the spacing (240 pim) and diameter (22 pm) of
the wires acts as a reference.
These images demonstrate that a polymer film can be induced to span pores of the 240 mesh for
both weak and strong polyelectrolyte systems. Also, the two strong polyelectrolyte polymer
films appear smooth compared to the rough or rope-like structure of the weak polyelectrolyte
system. This structural difference was investigated with cross-sectional SEM, shown in Figure
3.8 below.
100
Figure 3.8 Cross-sectional SEM images of spray LbL deposited polymer films of
a) 150 bL PDAC/PSS pH 2.00, b) 100 bL PDAC/PSS pH 2.00, 0.20 M NaCl, c)
100 bL LPEI/PAA pH 4.40, d) 150 bL PDAC/PSS pH 2.00, e) 100 bL
PDAC/PSS pH 2.00, 0.2 M NaCl, and f) 150 bL LPEI/PAA pH 4.40. The top
row of images is shown at low magnification to give an indication of the overall
character of the film and mesh. The bottom row of images is at higher
magnification and shows the film conformation in more detail.
These SEM images confirm the differences in structure between the weak and strong
polyelectrolyte systems. The reason for this difference is not well understood. It may be related
to differences in polymer conformation during adsorption.
The weak polyelectrolyte systems were further investigated to determine if these films would
undergo a porous transition like LPEI/PAA films do when built on a flat surface at identical
conditions. As is shown below in Figure 3.9, these films will undergo a porous transition.
Figure 3.9 Images of a 150 bL LPEI/PAA film, pH 4.75 after a porous transition
at pH 2.25 taken via a) bottom illumination optical microscopy, b) top
illumination optical microscopy, and c) scanning electron microscopy. Scale bars
are not shown in (a) and (b) because the wire spacing (240 tm) acts as a
reference.
101
This behavior indicates that the electrostatic cross-linking structure of the bridged film is similar
enough to "normal" LbL on a flat substrate that the reorganization behavior during exposure to
low pH conditions induces a porous transition in both systems.
3.3.3.2 Comparison of "Normal" LbL Assembly andBridgedFilm Assembly
Mechanical stiffness testing via nanoindentation
was also performed on PDAC/PSS films at 0
M NaCl and LPEI/PAA films assembled at pH 4.40. If the mechanical stiffness of the bridged
films was much lower than that of the normal LbL film, it would suggest that the electrostatic
cross-linking of the bridged film was altered and lessened by the deposition method. The elastic
modulus of films deposited via dropcasting a solution of mixed polycation and polyanion was
used to determine the modulus difference between a film deposited in a sequential way (sprayed)
and one deposited with disordered electrostatic interactions (dropcast). The stiffness of the
dropcast film was expected to more closely resemble the bridged films because both are
assembled via polyplex aggregation. The elastic moduli, as determined from the nanoindentation
experiments, are shown in the table below.
Table 3.4 Elastic moduli of PDAC/PSS and LPEI/PAA films assembled on glass
via spray LbL (Normal Sprayed Film) or via dropcasting (Dropcast Polyplex
Film) or on a metal mesh (Spray Bridged Film). The values for the spray bridged
film are starred because they represent a lower threshold of elastic modulus,
instead of the estimated average value.
Young's Modulus (Gpa)
PDAC/PSS, 0 M NaCl
LPEI/PAA, pH 4.40
Normal Sprayed Film
4.5 ± 0.5
11.0 ± 1.3
Dropcast Polyplex Film
3.4 ± 0.4
10.0 ± 2.3
Spray Bridged Film
1.6* ± 0.9
1.6* ± 1.1
The measured values for the spray bridged films are much lower than that of the other film types,
indicating that the trampoline effect (explained in Section 3.3.2.8) is likely affecting the
measurement. These values do indicate that the films do have a measureable mechanical
stiffness, however, indicating the presence of electrostatic crosslinks and interwoven polymer
chains.
102
ProposedMechanismfor Spray BridgedFilm Formation
These data and observations led us to propose the scheme for bridged film formation depicted in
Figure 3.10 below.
a
b
c
Figure 3.10 Scheme depicting the LbL steps that are proposed to explain the
formation of a bridged film on a mesh (shown as the orange #s). a) Mesh is
wetted (blue droplet). A meniscus is formed. b) Positive polyelectrolyte (red
droplet) is introduced. c) Some polyelectrolyte adheres to the grid. Some is
entrained in the meniscus. d) Negative polyelectrolyte (yellow droplet) is
introduced. e) Some polyelectrolyte adheres to the grid. Some is entrained in the
meniscus. Electrostatic crosslinks begin to form in the meniscus. f) The LbL
process is iterated. After iteration, the LbL film is spans across the mesh grid
because of the electrostatic crosslinks that span the meniscus. g) A balance of
film tensile strength (due to electrostatic crosslinks) and strain induced from film
deswelling (due to loss of water) determines whether the bridged film in each #
survives the film drying.
This scheme could easily accommodate a mixed mechanism of polyplex formation in
combination with LbL film formation.
Film Growth Behavior
One hallmark of "normal" LbL film assembly is a growth curve, where the film thickness
increases monotonically with the number of bilayers deposited. Such a growth curve is shown
for sprayed films on glass substrates in Figure 3.11 a below. Growth curves, with thickness data
taken from cross-sectional SEM images, for bridged films on 240 mesh is shown in Figure 3.1 lb.
103
+ PDAC/PSS, 0.2 M NaC
1.4
1.2
A
1
PDAC/PSS, 0 M NaCI
A LPEI/PAA, pH 4.40+
1
0.8
0.6-A
E0.4
0.2
0
EA
0
A
100
50
150
200
Bilayers
b
30
AA
+ PDAC/PSS, 0.2 M NaCl
25
mPDAC/PSS, 0 M NaCl
A LPEI/PAA, pH 4.40
,20
15
0 10
5
0
100
125
150
Bilayers
175
200
Figure 3.11 Growth curves for sprayed films assembled on glass substrates (a)
and bridged films assembled on mesh substrates (b). In (a), the film thickness
increases with the addition of more bilayers, and the thickness increase per bilayer
is dependent upon the polymer system studied.
The sprayed films on glass show the expected smooth growth curve behavior. However, the
spray bridged films depicted in Figure 3.1 lb do not exhibit a neat growth curve. The thickness
difference between bridged films and sprayed films on glass should also be noted. The glass
films achieved a maximum thickness below 1.4 tm, while the mesh films achieved thicknesses
of up to approximately 25 ptm for the same range of bilayers applied. This result strongly
suggests the involvement of polyplex aggregates in the formation of the bridged film structures
because the growth per bilayer is much too fast to be explained by simple sequential adsorption.
It should also be noted that all films, regardless of the number of bilayers applied, exhibit a large
range of film thickness (including very thin spots, usually in the middle of the mesh pore).
These thin spots are likely the result of the drying and deswelling behavior depicted in Figure
3. 10g. The film is pinned to the mesh, so as it deswells due to the loss of water, it likely induces
104
a strain that can most easily pull material from the middle of the film toward the edges of the
film to compensate. This creates a thickness gradient across the pore. Also, this thinning
behavior could happen to different degrees in different pores, causing scatter in the film
thickness measurements. It should also be noted that the LPEI/PAA films show an increasing
trend for the maximum film thickness that is not noted in the PDAC/PSS films. This difference
is likely related to the conformational, and hence assembly and adsorption, differences noted in
Section 3.3.3.1.
3.3.3.3 Controllingthe Amount of Bridgingon Meshes
The imaging and comparisons shown in Sections 3.3.3.1 and 3.3.3.2 were all performed on
portions of the mesh that exhibited film bridging. However, the bridging did not occur over the
whole sprayed mesh area. Instead, it occurred in fingerlike protrusions across the film, as is
shown in Figure 3.12.
Figure 3.12 Digital camera images of pore bridging for (a) 200 bL PDAC/PSS, 0
M NaCl, (b) 150 bL PDAC/PSS, 0.2 M NaCl, and (c) 150 bL LPEI/PAA, pH
4.75. The images were taken of freeze dried meshes to maximize the contrast
between the bridged (light colored or white) and unbridged (gray or brown)
portions of the mesh.
The percent bridging coverage (ratio of the area where bridging occurred to the total mesh area)
of the film was measured using such images and never exceeded 60% for any polymer system on
240 mesh. In order to use this bridging technique in solar cells, batteries, or other devices, 100%
coverage would be required. Therefore, techniques for increasing bridging coverage were
investigated.
Bridging coverage was measured as a function of bilayers applied, and showed a generally
increasing trend, as is shown in the figure below.
105
70 -* PDAC/PSS, 0.2 M
NaCI
m PDAC/PSS, OM NaCI
A LPEI/PAA, pH 4.40
60
50
640
.A30
A
A
20
10
0
__--
50
1
-Ti__
75
100
125
Bilayers
150
175
200
Figure 3.13 Percent bridging coverage as a function of polyelectrolyte system and
bilayers applied. All films were dried at 75% relative humidity and ambient
temperature.
Regardless of the number of layers applied, however, the coverage always appeared in patterns
like those shown in Figure 3.12. Based upon the shape of the coverage, it was hypothesized that
a Rayleigh-Taylor instability32 was occurring and preventing portions of the film from remaining
wetted, maintaining a meniscus, and thus from being effectively coated. This instability is
partially responsible for the phenomenon called "tears of wine" and occurs when a fluid of
higher density is suspended above a fluid of lower density 33. The more dense fluid, instead of
flowing uniformly down will create fingers as it displaces the less dense fluid under the action of
gravity. Two schemes were devised to reduce the effect of the instability on the bridging
coverage of the mesh: smaller mesh pore sizes and the addition of a surfactant. Both of these
parameters have been suggested to reduce the dominance of the instability in other systems3.
These changes were studied only with the PDAC/PSS polyelectrolyte pair without salt because,
as Figure 3.13 shows, this system achieved the highest bridging coverage of the three systems
studied.
Effects of Surfactant andPore Size on Mesh Wetting
240, 118, and 47 meshes were studied visually during spraying with Milli-q water solution to
determine at what point full wetting was achievable through the elimination of the instability.
Similar experiments were also performed on 240 mesh with surfactant solutions of
concentrations ranging from 0.1 to 100 mM. The results of this study are shown in Figure 3.14
below.
106
Figure 3.14 Digital camera images of (a) 240 mesh wetted with pure water, (b)
118 mesh wetted with pure water, (c) 240 mesh wetted with 1 mM Triton X- 100
solution, (d) 240 mesh wetted with 10 mM Triton X-100 solution. Only images
bounding the wetting/non-wetting transition are included. Droplets indicate a
tendency toward the instability.
Comparing Figure 3.14a and b, it becomes obvious that the threshold where the instability begins
to occur happens when pore sizes are between 118 and 240 im. Similarly, the images from
Figure 3.14c and d indicate that this threshold occurs for 240 mesh between 10 mM and 1 mM
Triton X- 100, a range that is near the critical micelle concentration of this surfactant, 189 ppm 35
Based upon the results shown in Figure 3.14a and b, bridging coverage experiments were
performed on 240, 118, and 47 mesh without surfactant. It was expected that the resulting
increase in film wetting would in turn lead to complete bridging coverage of the film; the
meniscus would be formed everywhere, allowing polymers to span the pores everywhere.
However, the results of this experiment, shown in Figure 3.15 below, do not support this
hypothesis.
107
Figure 3.15 Digital camera images of (a) 240 mesh, (b) 118 mesh, and (c) 47
mesh coated with 150 bL of PDAC/PSS, 0 M NaCl. Films were freeze dried in
order to maximize contrast for imaging.
Figure 3.15 shows that when the Rayleigh-Taylor instability is eliminated, as it is in b and c,
bridging only occurs at the edges of the mesh. These results suggest that the bridging coverage
phenomenon is due to polyplex accumulation associated with solution droplet formation and not
to the presence of a meniscus that LbL films build on. The edges of the mesh are prone to
droplet accumulation as the mesh drains. Apart from wetting, one effect of the Rayleigh-Taylor
instability on the meshes is to create droplets in the middle portion of the mesh (as is
demonstrated in Figure 3.14). The residence time of polyelectrolytes within a droplet is much
higher than the case when draining is good and only a thin film of liquid, wholly connected to
the edges, is present in the center of the mesh. The higher residence time allows for the
formation of polyplexes in the droplets, and the data indicate that it is these droplet-induced
polyplexes that ultimately bridge the pores of the mesh. This droplet theory also explains the
thickness of the bridged films as compared to films built on a flat substrate. Excess polymers on
the flat substrate are rinsed off, leaving behind only adsorbed polymers. In a droplet, however,
both excess and adsorbed polymer contribute to the thickness of the film. This polyplex droplet
accumulation theory suggests that longer rinse times could reduce pore bridging.
The effect of surfactant on bridging coverage was also investigated by replacing the pH 2.00
water rinse step with a pH 2.00 surfactant solution rinse. The surfactant decreases the liquid
surface tension, increase mesh wetting , and thus decreases the influence of the Rayleigh Taylor
instability. The results of this study were quantified and are shown below in Figure 3.16,
combined with those from the previous pore size study.
108
65
60
+ Surfactant, 240 mesh
o No Surfactant, 240 mesh
55
50 -0
A No Surfactant, 118 mesh
No Surfactant, 47 mesh
45
O40
35
30
0
0.1
1
10
Triton-X 100 Concentration (mM)
100
Figure 3.16 Quantification of percent bridging study on the effects of mesh pore
size and rinse solution surfactant concentration. All films were freeze-dried to
eliminate the potentially deleterious effects of film drying on bridging coverage.
Results for 240 mesh are shown as diamonds, 118 mesh as triangles, and 47 mesh
as squares. All data for meshes that were rinsed with any surfactant are shown
with filled points (black).
Empty points are from meshes that were rinsed with
pure water.
These results confirm that when the Rayleigh-Taylor instability is reduced (via changing mesh
pore size or liquid surface tension), the film bridging coverage is also reduced. This result was
expected in light of the residence-time droplet formation theory mentioned above.
At some small pore diameter, it is expected that surface interaction forces from a hydrophilic
mesh would eventually induce wetting on a scale where pores could be easily bridged by the
polymer complexes. Thus, if pore sizes were shrunk enough, bridging coverage would
eventually increase again, not through the effects of droplet-induced polyplex formation but
through polyelectrolyte pore bridging like that observed previously by Krogman et al.19 .
Therefore, a final study was performed to investigate whether or not bridging coverage would
increase again when pore sizes became small enough. The results of this study are shown in
Figure 3.17 below.
109
100 -
I75~
.
50
25
---
0
50
100
150
200
250
Mesh Pore Size (pm)
Figure 3.17 Mesh bridging coverage as a function of pore size over the entire
range of pore sizes studied. Bridging occurs above the threshold 30-40% level at
metal mesh pore sizes of 10 ptm and 240 pm. Image insets correspond to the data
points indicated by the arrows and an area of approximately 1 inch2 .
As this figure demonstrates, the pore size required for PDAC/PSS systems to achieve complete
bridging is somewhere between 10 and 25 pm, and the coverage at 10 pm is higher than
anything seen for the 240 meshes because it is not limited by the droplet size of the RayleighTaylor instability. Previous studies have postulated that when the pore size of the mesh becomes
smaller than or approximately equal to the 5 pim droplet size of the spray, pore bridging occurs19.
The bridging of 10 pm pores suggests that slightly larger pores may be bridged when favorable
interfacial interactions occur. SEM images of a bridged coating over the 10 pm pores of a tracketched membrane are shown below in Figure 3.18.
110
Figure 3.18 Top-down SEM of a) an uncoated membrane, b-d) 150 bL
PDAC/PSS, 0 M NaCl film bridging the 10 pm pores of a track-etched
polycarbonate membrane with a razorblade scratch. Image (b) is of the sprayfacing side of the film, image (c) is of the opposite side, and image (d) is a close
up of the outlined are in (c).
This image illustrates that pores are covered by the LbL film when the pore size is 10 prm, as
expected.
3.4
Conclusions
We have demonstrated that the weak polyelectrolyte system, LPEI and PAA, will undergo a
porous transition when deposited via spray LbL and treated at certain pH conditions. ANOVA
was performed to determine the effects of transition pH, LPEI assembly pH, and LPEI rinse
time. The rinse time did not affect the parameters studied, but the assembly pH had a large first
order effect on film thickness and also appears to have a higher order effect that could not be
probed using this ANOVA. The porosity of the resulting film depended on both transition and
LPEI assembly pH in a quadratic manner.
111
Metal meshes of varying pore sizes were coated with weak and strong polyelectrolytes.
PDAC/PSS films were shown to bridge small pores (<10 micron), likely through the action of
surface forces (surface tension). Both weak and strong polyelectrolyte systems were shown to
bridge large pores (>200 microns) through polyelectrolyte complex accumulation in droplets
created by a Rayleigh-Taylor instability. The morphology of these systems was observed to be
quite different depending on the polymer system studied, with LPEI/PAA systems yielding a
rope-like rough bridged film and PDAC/PSS films yielding smooth bridged films. Finally, these
results show that a mesoporous LPEI/PAA template is also achievable via the faster spray-LbL
assembly method and that it is accessible on porous geometries.
Recalling that nonconformal coatings would be desired for titania photoanode templates and
conformal coatings for solid state electrolyte applications. The results outlined in Chapter 3
suggest a set of conditions for the spray LbL of LPEI/PAA films that could be useful for
generating DSSC photoanodes on flat substrates. Because these systems were also shown to
undergo a porous transition on mesh substrates, this technique could likely be applied to mesh
films as well. Finally, 100% nonconformal bridging coverage was induced for 10 pm pore sizes.
The droplet theory of bridging formation on the larger pore sizes suggests that film draining
controls the conformality of the coating.
3.5
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13.
Montgomery, D. C., Design and Analysis ofExperiments. Wiley: New York, 2004.
14.
Zacharia, N. S.; DeLongchamp, D. M.; Modestino, M.; Hammond, P. T., Controlling
diffusion and exchange in layer-by-layer assemblies. Macromolecules 2007, 40 (5), 1598-1603.
15.
Shukla, A.; Fang, J. C.; Puranam, S.; Hammond, P. T., Release of vancomycin from
multilayer coated absorbent gelatin sponges. J. Control.Release 2012, 157 (1), 64-71.
16.
DeMuth, P. C.; Su, X. F.; Samuel, R. E.; Hammond, P. T.; Irvine, D. J., Nano-Layered
Microneedles for Transcutaneous Delivery of Polymer Nanoparticles and Plasmid DNA. Adv.
Mater. 2010, 22 (43), 485 1-+.
17.
Patrocinio, A. 0. T.; Paterno, L. G.; Iha, N. Y. M., Layer-by-layer TiO2 films as efficient
blocking layers in dye-sensitized solar cells. J. Photochem. Photobiol.A-Chem. 2009, 205 (1),
23-27.
18.
Farhat, T. R.; Hammond, P. T., Designing a new generation of proton-exchange
membranes using layer-by-layer deposition of polyelectrolytes. Advanced FunctionalMaterials
2005, 15 (6), 945-954.
19.
Krogman, K. C.; Lowery, J. L.; Zacharia, N. S.; Rutledge, G. C.; Hammond, P. T.,
Spraying asymmetry into functional membranes layer-by-layer. Nat Mater 2009, 8 (6), 512-518.
20.
Hammond, P. T., Engineering materials layer-by-layer: Challenges and opportunities in
multilayer assembly. AIChE Journal2011, 57 (11), 2928-2940.
21.
Kroon, J. M.; Bakker, N. J.; Smit, H. J. P.; Liska, P.; Thampi, K. R.; Wang, P.;
Zakeeruddin, S. M.; Gratzel, M.; Hinsch, A.; Hore, S.; Wtirfel, U.; Sastrawan, R.; Durrant, J. R.;
Palomares, E.; Pettersson, H.; Gruszecki, T.; Walter, J.; Skupien, K.; Tulloch, G. E.,
Nanocrystalline dye-sensitized solar cells having maximum performance. Progressin
Photovoltaics:Research and Applications 2007, 15 (1), 1-18.
22.
Kang, M. G.; Park, N. G.; Ryu, K. S.; Chang, S. H.; Kim, K. J., A 4.2% efficient flexible
dye-sensitized TiO2 solar cells using stainless steel substrate. Sol. Energy Mater. Sol. Cells 2006,
90 (5), 574-581.
23.
Chua, J.; Mathews, N.; Jennings, J. R.; Yang, G. W.; Wang, Q.; Mhaisalkar, S. G.,
Patterned 3-dimensional metal grid electrodes as alternative electron collectors in dye-sensitized
solar cells. Phys. Chem. Chem. Phys. 2011, 13 (43), 19314-19317.
24.
Uzaki, K.; Nishimura, T.; Usagawa, J.; Hayase, S.; Kono, M.; Yamaguchi, Y., DyeSensitized Solar Cells Consisting of 3D-Electrodes-A Review: Aiming at High Efficiency From
113
the View Point of Light Harvesting and Charge Collection. J. Sol. Energy Eng. Trans.-ASME
2010, 132 (2), 7.
Fan, X.; Chu, Z. Z.; Wang, F. Z.; Zhang, C.; Chen, L.; Tang, Y. W.; Zou, D. C., Wire25.
shaped flexible dye-sensitized solar cells. Adv. Mater. 2008, 20 (3), 592-+.
26.
Usagawa, J.; Pandey, S. S.; Ogomi, Y.; Noguchi, S.; Yamaguchi, Y.; Hayase, S.,
Transparent conductive oxide-less three-dimensional cylindrical dye-sensitized solar cell
fabricated with flexible metal mesh electrode. Progressin Photovoltaics:Research and
Applications 2011, n/a-n/a.
Fuke, N.; Fukui, A.; Komiya, R.; Islam, A.; Chiba, Y.; Yanagida, M.; Yamanaka, R.;
27.
Han, L. Y., New approach to low-cost dye-sensitized solar cells with back contact electrodes.
Chem. Mat. 2008, 20 (15), 4974-4979.
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Fischer-Cripps, A. C., The IBIS Handbook ofNanoindentation.Fischer-Cripps
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Laboratories Pty Ltd: Forestville NSW, 2005.
Fischer-Cripps, A. C., Nanoindentation. 3rd ed.; Springer: New York, 2011.
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Perpendicular to their Planes .1. Proceedingsof the Royal Society of London Series aMathematicaland PhysicalSciences 1950, 201 (1065), 192-196.
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epath=surfactants/pdfs/noreg/ 119-01882.pdf&fromPage=GetDoc.
114
Chapter 4. Recommendations for Future Work
4.1
Brief Summary of Results
The object of this work was to apply a LbL coating technique to a DSSC photoanode template,
adding functionality through the incorporation of M13 bacteriophage and porous polymeric
template. The investigation presented in Chapter 2 utilized dip LbL assembly to achieve this
objective. DSSC devices were made and tested, confirming that the addition of bacteriophage to
the porous dip LbL template increased device electron diffusion length. As was presented in
Chapter 3, however, spray LbL has some processing advantages over dip LbL, including up to 25
times faster processing speeds'. Therefore, experiments were performed to determine whether a
spray-deposited LbL film would undergo a porous transition; porous transitions were observed
on both flat glass and porous mesh substrates. Additionally, the conformality of spray LbL mesh
coatings was investigated and shown to be dependent upon mesh pore size and the effects of a
Rayleigh-Taylor instability.
4.2
LbL Incorporation of Other Components for DSSC
Applications
One major strength of LbL assembly is its ability to incorporate materials with different
characteristics and functions into the same thin film with some control over the spacing and
placement of the materials2 . The results outlined in Chapter 2 explored LbL incorporation of
bacteriophage; however, this assembly method could incorporate other promising materials into
DSSC photoanodes. One such material is plasmon enhanced core-shell nanoparticles that have
been shown to increase device performance even while the device thickness was decreased 3 by
increasing the activity of the nearby dye molecules. Although the titania itself is not expensive,
the adsorbed dye is, potentially accounting for up to 22% of the DSSC module cost4 . Because
these nanoparticles have the surface characteristics of titania, they could simply be incorporated
along with other titania nanoparticles into the LbL assembly process. However, these core-shell
nanoparticles could further enhance the device response if they were precisely and purposely
spaced. Such spatial arrangement could likely be accessed through controlling the deposited
115
layer thickness during LbL assembly via ionic strength and the concentration of plasmonenhanced nanoparticles in the solution phase. It could also be achieved through interaction with
a charged macromolecule that would wrap up the core-shell nanoparticle and then self-assemble
into regularly spaced packets similar to the histone DNA wrapping that results in the well-known
beads-on-a-string morphology5 . Histones have already been shown to interact with positively
charged dendrimers yielding tunable morphologies in one to three dimensions depending on
solution ionic strength 6 . A titania photoanode that incorporated both bacteriophage and these
core-shell nanoparticles could combine the desirable properties of decreased film thickness and
increased electron diffusion length. These advantages, in turn, could lead to more efficient and
less expensive DSSCs.
4.3
Spray LbL Deposition for DSSC Photoanodes
In Chapter 2, we demonstrated that the LPEI/PAA/bacteriophage system could template titania
architectures with longer electron diffusion lengths than traditional nanoparticle systems. In
Chapter 3 we demonstrated that spray-deposited LPEI/PAA films would still undergo a porous
transition and determined an equation to predict the porosity of the film based upon assembly pH
and transition pH. Spray LbL can be performed on TCOs to yield porous spray-LbL titania
templates for DSSC photoanodes. Film thickness and porosity can be optimized using Equations
3.4 and 3.5. Because spray assembly is faster, it is recommended that this deposition technique
be adopted for all future work involving LbL templates for DSSCs. Similar fluorescent labeling
experiments (Section 2.3.2.1) could be performed to determine if the bacteriophage would
assemble within films via spray deposition. If so, all of the work in Chapter 2 could be extended
to spray LbL on FTO to take advantage of the decreased processing time1 . It is recommended
that the transition pH range be explored again with spray LbL because the decreased processing
time makes it possible to add the additional layers of polymer required to achieve very thick,
nanoporous films instead of fewer dip-LbL layers yielding a microporous architecture (due to
time constraints). Lower transition pHs are known to create nanopores instead of micropores in
dip LbL systems, 7 and an alternative LbL method for creating nanopores8 has also been
116
developedt. Using this spray LbL method and 2 separate transition pHs (or one nanoporous film
generation method), the transparent + scattering photoanode architecture that is commonly used
in the most efficient light harvesting DSSCs9 could be produced as a porous polymer template.
Hundreds of bilayers or tetralayers could be deposited on FTO and made nanoporous to create a
transparent underlayer for titania nucleation. This structure could be locked-in with UV or heat
crosslinking, and then more layers could be deposited and made microporous to create a
scattering overlayer. The resulting device would truly take advantage of the independent
nanowire-content and pore-morphology tuning offered by the LbL-bacteriophage system and
could yield extremely efficient devices that have very high light harvesting and charge collection
efficiencies.
4.4
Device Design Improvement
It is recommended that a strategy for controlling the series resistance of the LbL devices be
developed. A comparison of the dark and light JV curves for these devices (Figure 2.28) shows
a significant series resistance effect. This effect could be caused by exposure to water, titania
nucleation on the FTO external electrode contacting surface during the liquid phase deposition
procedure, poor control over final FTO dimensions, or FTO selection. Regardless of its cause,
the higher series resistance greatly decreases device fill factor and efficiency. Discovering the
cause and eliminating it would easily increase the measured performance of the DSSCs
investigated here.
4.5
Non-conformal Spray LbL Deposition
One unique aspect of the spray deposited titania template on metal meshes is that the resulting
titania film would fully encapsulate the metal mesh, resulting in a mid-contact titania
photoanode, as opposed to the typical front contact architectures or the developing back-contact
architectures.
A schematic of this device architecture is presented in Figure 4.1 below.
: Instead of charge density induced swelling, these systems are assembled with both electrostatic and hydrogen
bonded polymers. After deposition, the hydrogen bonded polymers can be selectively released from the films,
leaving behind a nanoporous film.
117
ir
Figure 4.1 Schematic of mid-contact solar cells.
i~r
Back contact solar cells are
shown for comparison. MCE and BCE indicate the mid-contact and back-contact
electrodes respectively. Adapted with permission from [10]. Copyright 2008
American Chemical Society.
A mid-contact mesh electrode, with wires smaller than the resolution of the human eye
(approximately 50 pim) 1, could be used in conjunction with a transparent platinum
counterelectrode
2
to make a device that could generate power when illuminated from either side,
perhaps as a frosted-type glass building-integrated photovoltaic.
Many others have looked into
transparent DSSC devices that could be illuminated bi-directionally,14-18 though none of their
electrodes were embedded within the titania photoanode. The advantage of the mid-contact
architecture over back-contact or bi-directionally illuminated DSSCs is that the distance
electrons would need to travel to the current collector could be reduced by half. Figure 3.13
suggests that pore bridging could be induced at very high bL numbers of LPEI/PAA films (410
bL). These bridged films could then be templated with titania, using the methods of Chapter 2,
and used to make DSSC photoanodes. A good metal protection strategy, such as the one
developed by Yanagida et al., would need to be employed to prevent metal oxidation and thermal
separation of the titania from the metal' 9 .
4.6
References
1.
Krogman, K. C.; Zacharia, N. S.; Schroeder, S.; Hammond, P. T., Automated process for
improved uniformity and versatility of layer-by-layer deposition. Langmuir 2007, 23 (6), 3137-
3141.
2.
Hammond, P. T., Form and function in multilayer assembly: New applications at the
nanoscale. Adv. Mater. 2004, 16 (15), 1271-1293.
Qi, J. F.; Dang, X. N.; Hammond, P. T.; Belcher, A. M., Highly Efficient Plasmon3.
Enhanced Dye-Sensitized Solar Cells through Metal@Oxide Core-Shell Nanostructure. ACS
Nano 2011, 5 (9), 7108-7116.
118
4.
Kroon, J. M.; Bakker, N. J.; Smit, H. J. P.; Liska, P.; Thampi, K. R.; Wang, P.;
Zakeeruddin, S. M.; Gratzel, M.; Hinsch, A.; Hore, S.; Wtirfel, U.; Sastrawan, R.; Durrant, J. R.;
Palomares, E.; Pettersson, H.; Gruszecki, T.; Walter, J.; Skupien, K.; Tulloch, G. E.,
Nanocrystalline dye-sensitized solar cells having maximum performance. Progressin
Photovoltaics:Research andApplications 2007, 15 (1), 1-18.
5.
Zlatanova, J.; Leuba, S. H., Chromatin fibers, one-at-a-time. J. Mol. Biol. 2003, 331 (1),
1-19.
6.
Dootz, R.; Toma, A. C.; Pfohl, T., PAMAM6 dendrimers and DNA: pH dependent
"beads-on-a-string" behavior revealed by small angle X-ray scattering. Soft Matter 2011, 7 (18),
8343-8351.
7.
Lutkenhaus, J. L.; McEnnis, K.; Hammond, P. T., Nano- and microporous layer-by-layer
assemblies containing linear poly(ethylenimine) and poly(acrylic acid). Macromolecules 2008,
41 (16), 6047-6054.
8.
Wang, Y.; Angelatos, A. S.; Caruso, F., Template Synthesis of Nanostructured Materials
via Layer-by-Layer Assembly. Chem. Mat. 2008, 20 (3), 848-858.
9.
Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Gratzel, C.; Nazeeruddin, M. K.; Gratzel,
M., Fabrication of thin film dye sensitized solar cells with solar to electric power conversion
efficiency over 10%. Thin Solid Films 2008, 516 (14), 4613-4619.
10.
Fuke, N.; Fukui, A.; Komiya, R.; Islam, A.; Chiba, Y.; Yanagida, M.; Yamanaka, R.;
Han, L. Y., New approach to low-cost dye-sensitized solar cells with back contact electrodes.
Chem. Mat. 2008, 20 (15), 4974-4979.
11.
Yang, Y.; Jeong, S.; Hu, L. B.; Wu, H.; Lee, S. W.; Cui, Y., Transparent lithium-ion
batteries. Proc.Natl. Acad Sci. U. S. A. 2011, 108 (32), 13013-13018.
12.
Papageorgiou, N.; Maier, W. F.; Gratzel, M., An iodine/triiodide reduction electrocatalyst
for aqueous and organic media. Journalof The ElectrochemicalSociety 1997, 144 (3), 876-884.
13.
Benemann, J.; Chehab, 0.; Schaar-Gabriel, E., Building-integrated PV modules. Sol.
Energy Mater. Sol. Cells 2001, 67 (1-4), 345-354.
14.
Fan, X.; Chu, Z. Z.; Wang, F. Z.; Zhang, C.; Chen, L.; Tang, Y. W.; Zou, D. C., Wireshaped flexible dye-sensitized solar cells. Adv. Mater. 2008, 20 (3), 592-+.
15.
Chua, J.; Mathews, N.; Jennings, J. R.; Yang, G. W.; Wang, Q.; Mhaisalkar, S. G.,
Patterned 3-dimensional metal grid electrodes as alternative electron collectors in dye-sensitized
solar cells. Phys. Chem. Chem. Phys. 2011, 13 (43), 19314-19317.
16.
Fan, X.; Wang, F. Z.; Chu, Z. Z.; Chen, L.; Zhang, C.; Zou, D. C., Conductive mesh
based flexible dye-sensitized solar cells. Applied Physics Letters 2007, 90 (7), 3.
17.
Usagawa, J.; Pandey, S. S.; Ogomi, Y.; Noguchi, S.; Yamaguchi, Y.; Hayase, S.,
Transparent conductive oxide-less three-dimensional cylindrical dye-sensitized solar cell
fabricated with flexible metal mesh electrode. Progressin Photovoltaics:Research and
Applications 2011, n/a-n/a.
18.
Wang, Y. H.; Yang, H. X.; Lu, L., Three-dimensional double deck meshlike dyesensitized solar cells. J. Appl. Phys. 108 (6), 6.
19.
Yanagida, S.; Nakajima, A.; Kameshima, Y.; Yoshida, N.; Watanabe, T.; Okada, K.,
Preparation of a crack-free rough titania coating on stainless steel mesh by electrophoretic
deposition. Mater. Res. Bull. 2005, 40 (8), 1335-1344.
119
Appendix A. DSSC Device Assembly and Testing
This appendix is meant to illuminate the details of current device construction and testing
procedures. Many details of and explanations for the final device architecture can be found in
section A. 1, in addition to a general primer on JV testing. Section A.2 gives a brief overview of
IPCE testing, including some references for Ldiff extraction. Section A.3 explains the impedance
spectroscopy technique and its application to DSSCs and Ldiff extraction. MATLAB codes for
JV data analysis and EIS fitting are also given in sections A.1.5 and A.3.3 respectively.
A.1
JV Testing
A. 1.1 Reading and Understanding JV Curves
The output of solar cells is usually measured by constructing a JV curve, and several
performance parameters can be calculated from such a measurement. The measurement is
performed at a particular illumination intensity and light spectrum that have been adjusted to
closely match the sun's illumination on the surface of the earth (AM 1.5 light at <D =100
mW/cm 2 ) . The light is generally collimated to ensure identical illumination over the whole area
and depth of the beam. Once the device is placed in this beam of light, it is connected to a
source meter that applies a voltage and measures a current. This voltage application is iterated
over a range of biases (for DSSCs a good range is -10 to 900 mV). Care should be taken to
avoid reverse biases over -10 mV because of potential damage to the DSSC devices. DSSCs
respond very slowly to applied light as compared to other types of solar cells. Therefore, one
parameter to pay particular attention to when making a JV measurement on a DSSC is how long
the instrument waits for the device response to equilibrate after it applies the new voltage but
before it takes the current measurement. 100 ms has been recommended as an appropriate
voltage settling time for DSSCs2
The form of the response of a solar cell to illumination is well known, and, in its simplest form,
follows the equation shown below, which Bisquert et al. have shown can be derived for DSSCs
120
simply by assuming selective contacts 3 . This equation is commonly referred to as the ideal diode
equation.
(qV
J = Jsc - Jd
>
A.1
emkT1
In Equation A. 1, Jsc represents the short circuit current density of the device, Jd represents the
dark current density, q is the elementary charge, J and V are response current density and applied
voltage, respectively, m is the device ideality factor, k is Boltzmann's constant, and T is the
temperature. The graphical results of this equation are shown in Figure A.1 below.
16 ,cVee
-f14
12
10
*8
P
0
Q
6
isc
4
2
0
0
0.2
0.4
Applied Bias (V)
0.6
0.8
Figure A.1 JV curve (black line) generated from the ideal diode equation with
m=1.6, Jsc=15 mA/cm 2 , andJd = 10-7 mA/cm 2 . The short circuit current density
(Jsc), open circuit voltage (Voc), current density at maximum point (Jmp) and
voltage at maximum power (Vmp) are defined as shown in the figure.
As indicated in Figure A.1, Jsc is the current density value when the applied bias is 0 V, and VOc
is the voltage when the current density is 0 mA/cm 2 .
Jmp
and Vmp are the (J,V) pair that
correspond to the highest power output by the cell (Pmp =Jmp*Vmp or the area of the purple
rectangle). Because the incident power flux, is determined by the illumination intensity, fixed at
(D, the efficiency of the device is simply Pmp/ (D. The fill factor of the device is Pmp/(Jsc*Voc),
which is also the ratio of the area of the purple rectangle to that of the red rectangle in Figure
121
A.1. Table A.1 below lists some of the common DSSC device parameters and the loss
mechanisms that contribute to them.
Table A.1 List of DSSC Device Parameters and Related Loss Mechanisms
Name
Definition
Dominant Loss Mechanisms
Illumination
Incident light energy per time per area
Reflection, Non-absorption
Intensity
(mW/cm 2)
Voc
Open Circuit
Voltage
Measured potential drop across the cell
under illumination when current no
longer flows (mV)
Thermalization, Energy
(band) level misalignment
ISC/
Short Circuit
Current/ Short
Circuit Current
Current output by the cell under
illumination when there is no voltage
drop across the device terminals (mA,
Recombination (e.g.
shunting), Non-absorption
of photons, Dye Loading
Density
mA/cm 2)
Fill Factor
Measure of actual solar cell
performance as compared to maximum
possible performance
ID(X)
Jsc
FF
FF -
(%)
-
"'
IS
~
SC
O~C
Power output of cell divided by power
input to cell (%)
q
Efficiency
Pmax
Maximum Power The largest achieved power output of
Ldiff
Shunt Resistance, Series
Resistance
Output
the cell (mW or mW/cm 2)
Electron
Diffusion Length
Distance electrons can travel before
their concentration is attenuated to 1/e
of the original value (pm)
All of the above
All of the above
Carrier lifetime, carrier
diffusivity (mobility)
The incorporation of bacteriophage was shown to increase device Ldiff, yielding devices that
generated a larger current density with less adsorbed dye.
A.1.2
JV Performance of Various Iterations of Device Design
The generations of device architecture are depicted in Figure A.2 below.
122
Figure A.2 Pictures and schematics of the 4 generations of device architecture
utilized throughout this work. Shapes outlined in dashed lines in the schematics
represent the active domain in the device.
Generation I devices required a solid state electrolyte to stick the platinum counterelectrode to
the photoanode. Spacing between the counterelectrode and the photoanode was difficult to
control because no spacer was used. The gel electrolyte used poly(epichlorohydrin-co-ethylene
oxide) and the iodide/tri-iodide redox couple, which, using 8 ptm thick nanoparticulate titania
photoanodes, had yielded efficiencies up to 2.6%4. The best performance of this generation of
devices is shown below in Figure A.3 with extracted JV data in Table A.2.
123
2.5
-Bacteriophage
TiO2-Nanowire DSSC
Tio2-Nanoparticle DSSC
2.
E
1.00.5
0.00
100
200
300
400
500
600
700
mV
Figure A.3 JV curves for best devices made with Generation I architecture. Both
devices were 750 nm thick.
Table A.2 JV data extracted from Figure A.3.
V0 e
r
FF
(mA/cm )
(mV)
(%)
(%)
2.10
612
2.35
55
1.87
611
1.96
51
Jac
Bacteriophage
2
LbL Device
Nanoparticle
Device
Both of these devices were measured to be 750 nm thick and were made on ITO, not FTO. The
deposition method for this generation of devices made with nanoparticles was not doctor blading,
but spin-coating. Commercial titania paste (Ti-Nanoxide D/SC, Solaronix) was spin coated onto
ITO (which was sold from Solaronix at the time, but is no longer available). Once dry, the thin
coating was sintered for 10 minutes at 450'C, and then another layer was applied. This process
was iterated until films of 750 nm thickness were achieved. As was stated in section 2.4.2.4, the
LbL film was likely only 750 nm thick due to collapse during annealing.
In order to achieve higher efficiencies, all subsequent generations of devices were made using a
liquid iodide/tri-iodide redox couple with iodide ionic liquids and acetonitrile as the solvents5
and so should yield higher efficiencies than Generation I by this fact alone. The best performing
devices from Generation II devices are shown in Figure A.4 below, with the extracted JV data in
Table A.3.
124
14
E 12
E
10 -
4-
--
2 0
0
M13 LbL Film
-N P Film
200
400
Voltage (mV)
600
Figure A.4 Generation II best performing devices.
Table A.3 JV data from Figure A.4.
VO
JC
(mVmA/cm 2
M13 LbL 566
NP Film 568
12.5
14.1
FF
ri
%
64
67
%L
d
4.55 10 ± 1
5.35 10± 1
The marked improvement in device performance from Generation I to II is due to the switch
from the liquid state electrolyte and the increased device thickness (from 750 nm to 10 pm). The
next generation of devices, Generation III, was developed to eliminate the contribution of all of
the excess titania surface area (that was not illuminated once a mask was applied, like the one
shown in Figure A.5 below) to the device dark current, Jd.
125
Mask
Figure A.5 Mask application on Generation II device architectures.
Thus, Generation III devices also had better performance than Generation II devices. This
performance is shown in Figure A.6 and Table A.4.
16
12
E
8
U
'Z
4
0
-4
200
400
600
Potential (mV)
800
Figure A.6 JV data for best performers of the Generation III device architecture.
LbL devices without phage are shown in blue. M13-LbL devices (with phage) are
shown in green. Nanoparticle devices are shown in red.
Table A.4 JV data extracted from Figure A.6.
d
V0 C
Jsc
Jpm mV mA/cm 2
Lb 20 708
11.6
M13-Lb 12 684
15.1
np 13 702
14.5
FF r1
% %
63 5.15
56 5.77
60 6.07
126
The results for Generation IV devices are shown in Chapter 2 and are all higher than those
obtained for Generation I-III. Also, for Generations II-III, the electrolyte was deposited through
holes that were drilled (with a water jet) into the platinum counterelectrode. For Generation IV,
the surlyn separator was melted in a 100"C oven for 5-10 minutes (less time is better for the dye
stability) between the photoanode current collector and a block of Teflon. After removal from
the oven and cooling, a drop of electrolyte was placed onto the dyed titania photoanode.
A.1.3
Effect of Surlyn or Parafilm Separator Size
The separator used in Generation II and III devices was parafilm. Parafilm can be stretched
down to 10-25 microns thick (from its original thickness of hundreds of microns). In its thinner
form, it can be applied as the separator that keeps the platinum counterelectrode from touching
the FTO and shorting completed devices. Surlyn is the commercially available (Solaronix)
product that is sold in sheets of specific thickness (25 microns, 60 microns, etc). It is more
expensive than parafilm, but also is more uniform and exhibits better long term adhesion
properties than parafilm. Therefore, it was employed in all Generation IV devices. It is not the
sole cause of the improved performance from Generation III to Generation IV, but it did improve
device stability during testing.
When applying a separator, several different strategies for its coverage on the FTO photoanode
can be used. Figure A.7 below shows 3 options for separator coverage.
a.
b.
c.
Figure A.7 Cross-sectional schemes of DSSC devices showing different options
for separator coverage. In all of these schemes, the blue rectangles represent the
FTO-coated glass slides (the one on top being the platinum counterelectrode and
the one on the bottom being the photoanode current collector). The green
rectangles represent the dyed titania. The orange rectangles represent the
electrolyte, and the gray areas represent the polymer separator.
As Figure A.7 shows, there are at least three different ways to apply the separator to a titania
photoanode and the separator controls the spacing between the FTO electrodes. In a-type
127
devices, the hole cut in the separator is larger than the dyed titania, so the electrolyte contacts the
photoanode current collector on the sides of the dyed titania. In b-type devices, the separator
hole is made exactly the same size as the dyed titania, and there is no direct path between the
electrolyte and the photoanode current collector. In c-type devices, the separator hole is made
smaller than the dyed titania. In this instance, there is also no direct path between the electrolyte
and the photoanode current collector, but the space between the counterelectrode and the
photoanode current collector is now equal to the thickness of the separator plus the thickness of
the dyed titania layer. B-type devices are expected to be the most desirable architecture, but due
to practical difficulties with matching separator holes to device active areas, all Generation IV
devices likely have contributions from a-type and c-type devices. Therefore, it is worthwhile to
investigate the contributions of those morphologies.
The performance of each of these device types was tested (in a Generation III setup but with 25
micron surlyn instead of parafilm as the separator). It was expected that the b-type devices
would perform better than a-type because b-type devices eliminate the direct path between the
two electrodes through the electrolyte and thus the potential for shunting while minimizing the
distance for ions to diffuse, thus decreasing series resistance.
The JV results are shown in Figure A.8 below.
128
---------------*..
-------1 4 .....-----... e.. .....
12
..... .
-
**
...
. - .......
....-------
A -ty p
E
-
(4
-
A-type
--
C-type
n(
711
-type
2
.-...........
-...
FF
Jsc
Voc
2
~(mV) (mA/cm ) (%) (%)
Active Area
11.79
-~-~
702
4.2
-- -
*
--
50
----
12.52
5.8
66
729.13.82
729
13.82
5.3
5.3
53
53--
. . . . . . . . . . . . . . .. .
-
-
- - -----------------------------------------
00
-2
0
0.1
0.2
...........
------ ...
0.3
0.4
Potential, V
0.5
0.6
~
00
0.7
0.8
Figure A.8 JV performance results for nanoparticle devices made with the same
active area but different separator schemes.
As expected, the best performing device was the b-type device. The a-type device exhibited a
significant loss of shunt resistance (noted by the negative slope of the JV curve at the yintercept). Also, the c-type device exhibited a higher series resistance than the other devices
(noted by the slope of the blue data at the x-intercept). These resistance effects lowered the fill
factors for both the a-type and c-type devices, as compared to b-type.
A.].3.J Measurement Transience
The differences between these separator schemes also extended to measurement transience, or
the amount of time it took the device to come to steady state performance after initial exposure to
light during JV testing. This effect is illustrated in Figure A.9 below.
129
12
Time=5 minutesA-type
10.
40
E
4-
U
..---
-
10,2
0
1.2*
10
.....
Time =0 minutes
E
6
C4
4-0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Potential (V)
Figure A.9 JV measurement transience as a function of separator scheme, shown
for schemes a and c. The transience of type-b is the similar to type-c, so it is not
shown here. The red circles represent the maximum power point.
The cause of this transience is unknown, but commonly occurs in most assembled DSSC devices
to different degrees. All reported measurements were taken once device performance no longer
appreciably depended upon time. Also, for the reasons explained throughout this section, the
separator scheme chosen for device making in Generation IV was b-type.
130
A.1.4 Masking and Framing Effects
When making JV measurements, it is important to mask the non-active areas of the solar cell to
ensure accurate testing results6 . This effect is shown in Figure A.10 below.
14.........
0
.
........
........................................
..
.............
*~**e~se@
**
.......
.
I =::
8:.....
**.................
.............
.................
........
.......
Voc
Jsc
ri
(mV)
(mA/cm 2 )
(%)
(%)
No Mask
729
13.82
5.3
52.9
Large Mask
725
12.98
5.2
55.6
2
Medium Mask
709
12.68
5.8
64.6
n
Small Mask
- -
06
0
0.1
-
-
716 -
0.2
-
-
0.3
10.33
-
-
4.7-
-
0.4
Potential, V
...;40.........
FF
-
64.2
0.5
0.6
0.7
0.8
Figure A.10 Effects of masks of different sizes on a type-c nanoparticle DSSC
device. In the scheme in the upper right hand corner, the orange circle
corresponds to where the electrolyte and the dyed titania were, the green circle
underneath the orange circle corresponds to where the dyed titania was not
covered with electrolyte, the gray square represents the underlying current
collector. Dashed lines indicate which areas were shaded by a particular mask.
For example, the small mask shaded the entirety of the green and gray areas,
which the medium mask shaded only the gray area. Small, medium, and large
correspond to circles of the following diameters: 5/32", 6/32", and 7/32".
As is shown in Figure A.10, when only the active area is masked (data shown in red), the device
performance is markedly smaller than when any other mask size is applied. However, since this
data represents the most accurate measurement of device performance, this masking condition
was used for all Generation IV devices (where the mask covered all device area that was not
131
active). Also, the efficiency data do not increase monotonically with exposed area; there is a
peak in performance when the medium mask is applied. This counterintuitive effect has also
been noticed by others who attempted to explain in it terms of light reflection and scattering7.
A.1.5
MA TLAB Code for JV Data Analysis
The solar simulator outputs data in text files like the one shown below.
V
IcorrI
0.8000
0.7910
0.7831
0.7741
0.7661
0.7571
0.7481
0.7401
0.7311
0.7231
0.7141
0.7061
0.6971
0.6882
0.6802
0.6712
0.6632
0.6542
0.6452
0.6371
0.6281
0.6201
0.6111
0.6031
0.5941
0.5851
0.5771
0.5681
0.5601
0.5511
0.5421
0.5341
0.5251
0.5171
0.5081
0.4991
0.4911
0.4821
0.4741
0.4651
IV 110511-131105.txt
-1. 1859E-3 -1. 1859E-3
-8.9694E-4 -8.9694E-4
-7.6129E-4 -7.6129E-4
-6. 5941E-4 -6. 5941E-4
-5.8470E-4 -5. 8470E-4
-5. 0764E-4 -5. 0764E-4
-4.3614E-4 -4.3614E-4
-3. 7693E-4 -3. 7693E-4
-3.1205E-4 -3.1205E-4
-2. 5837E-4 -2. 5837E-4
-1. 9933E-4 -1. 9933E-4
-1. 5050E-4 -1.5050E-4
-9. 7068E-5 -9. 7068E-5
-4.8355E-5 -4. 8355E-5
-5. 9148E-6 -5. 9148E-6
3.8600E-5
3.8600E-5
7.4295E-5
7.4295E-5
1. 1583E-4
1. 1583E-4
1. 5273E-4
1. 5273E-4
1. 8413E-4
1. 8413E-4
2.1774E-4
2. 1774E-4
2.4480E-4
2. 4480E-4
2. 7549E-4
2.7549E-4
2 .9952E-4
2. 9952E-4
3.2775E-4
3. 2775E-4
3.5147E-4
3.5147E-4
3.7361E-4
3.7361E-4
3.9402E-4
3.9402E-4
4.1228E-4
4.1228E-4
4 .3014E-4
4.3014E-4
4 .4827E-4
4 .4827E-4
4.6160E-4
4.6160E-4
4.7484E-4
4.7484E-4
4.8643E-4
4.8643E-4
4.9669E-4
4.9669E-4
5 .0522E-4
5. 0522E-4
5.1529E-4
5. 1529E-4
5. 2288E-4
5. 2288E-4
5.2970E-4
5. 2970E-4
5. 3368E-4
5. 3368E-4
Wed,
May 11,
2011 1:11 PM
132
0.4571
0.4481
0.4391
0.4312
0.4222
0.4142
0.4052
0.3962
0.3882
0.3792
0.3712
0.3622
0.3542
0.3452
0.3361
0.3281
0.3191
0.3111
0.3021
0.2931
0.2851
0.2761
0.2681
0.2590
0.2511
0.2421
0.2331
0.2251
0.2161
0.2081
0.1991
0.1901
0.1821
0.1732
0.1652
0.1562
0.1472
0.1392
0.1302
0.1222
0.1132
0.1052
0.0962
0.0872
0.0791
0.0701
0.0621
0.0531
0.0441
0.0361
0.0271
0.0191
0.0100
0.0020
-0.0070
-0.0161
5.3929E-4
5.4416E-4
5.4535E-4
5.4926E-4
5. 5283E-4
5. 5570E-4
5. 5625E-4
5. 5722E-4
5. 6143E-4
5.6087E-4
5. 6049E-4
5. 6199E-4
5. 6384E-4
5. 6303E-4
5. 6635E-4
5. 6495E-4
5. 6617E-4
5. 6781E-4
5.6792E-4
5. 6812E-4
5.6731E-4
5. 6943E-4
5. 6971E-4
5. 6954E-4
5. 7180E-4
5.7069E-4
5. 7288E-4
5. 7228E-4
5. 7388E-4
5. 7489E-4
5. 7433E-4
5.743 0E-4
5. 7331E-4
5. 7583E-4
5.7640E-4
5. 7573E-4
5. 7536E-4
5. 7699E-4
5. 7611E-4
5. 7695E-4
5. 7653E-4
5. 7759E-4
5. 7859E-4
5. 7768E-4
5. 7885E-4
5. 7760E-4
5.8050E-4
5. 7959E-4
5. 7859E-4
5.8055E-4
5. 7928E-4
5.8051E-4
5. 8223E-4
5. 8028E-4
5. 8236E-4
5. 8257E-4
5.3929E-4
5.4416E-4
5.4535E-4
5.4926E-4
5. 5283E-4
5. 5570E-4
5. 5625E-4
5. 5722E-4
5.6143E-4
5.6087E-4
5.6049E-4
5. 6199E-4
5.6384E-4
5. 6303E-4
5.6635E-4
5.6495E-4
5. 6617E-4
5.6781E-4
5.6792E-4
5. 6812E-4
5.6731E-4
5.6943E-4
5.6971E-4
5.6954E-4
5.7180E-4
5.7069E-4
5. 7288E-4
5. 7228E-4
5. 7388E-4
5.7489E-4
5. 7433E-4
5.7430E-4
5.7331E-4
5. 7583E-4
5.7640E-4
5. 7573E-4
5. 7536E-4
5. 7699E-4
5. 7611E-4
5. 7695E-4
5. 7653E-4
5.7759E-4
5. 7859E-4
5. 7768E-4
5. 7885E-4
5.7760E-4
5. 8050E-4
5. 7959E-4
5. 7859E-4
5. 8055E-4
5.7928E-4
5. 8051E-4
5. 8223E-4
5.802 8E-4
5.8236E-4
5.8257E-4
133
-0.0241
5.8161E-4
5.8161E-4
-0.0331
5.8278E-4
5.8278E-4
-0.0411
5.8417E-4
5.8417E-4
-0.0501
5.8308E-4
5.8308E-4
end
0.6790
Voc (V)
678.968
Voc/seg (mV)
Isc (A)
5.8075E-4
7.743 Jsc (mA/cm2)
(mA/cm2)
7.743 Jsc/seg
64.18 Fill
Factor (%)
2.531E-1
Pmax (mW)
Vmax (V)
4.911E-1
Imax (mA)
5.153E-1
1.46E+4
Rshunt est (ohms)
Voc slope (ohms)
2.02E+2
(%)
3.37 Efficiency
0.00 Monitor Cell Reading (mV)
0.00 Monitor Cell Calibration Value
0.0649
Test device area (cm2)
1
(mV)
Number of series-connected stacks
Meter Integration (NPLC)
1.000000
(mW/cm2)
100.0 Source Irradiance
100
Voltage Settling Time (ms)
F -> R
Sweep Direction
None Irradiance variation correction
Wed, May 11, 2011 1:11 PM
Ll mask
Device ID
These data files can be processed by the MATLAB code shown below. To read the preceding
datafile (saved as IV 110511-131105.txt) shown above, the MATLAB input would be the
following:
[titlename
area xall
IVdata]
= solarsimread('IV 110511-131105.txt');
The output variables are titlename (the name given to the file by the user), area (the device area),
xall (column 1 is the applied bias in V, column 2 is the measured current density in A/cm 2 ), and
IVdata is described in the code below.
function [titlename area xall IVdata] = solarsim read(input)
takes comma delimited text data from the potentiostat, and turns it into
a n x 2 matrix with voltage and current being the two column headers and
input must be a string, end in .txt, and
the units being mV and mA.
either be fully designated
(i.e. c://users/desktop.. .etc OR else is the
The program only works when all 100 points
same folder as the program) .
IVdata = [Pmax Voc Jsc Isc efficiency Rshunt Voc slope]
have been taken.
134
fidtxt = fopen(input);
Ctxt = textscan(fidtxt,'%ss*\n]');
fclose('all');
legendtxt = fopen(input);
Ltxt = textscan(legendtxt,'%s','Headerlines',
124,
'Delimiter',',');
fclose ('all');
% convert cell arrays to 2 different strings
sltxt = char(Ctxt{1});
s2txt =
char(Ctxt{2});
% find the title of the curve to go into the legend of the final graphs
fullname = char(Ltxt{1});
name = fullname(1,:);
position = findstr('Device ID', name);
titlename = name(1:position-2); % gets rid of D and space before it
long = 100;
col 1 = zeros(long,1); col_2 = zeros(long,1);
% Select the numbers from the string
for i = 2:long+1
col_1(i-1) = str2double(sltxt(i,:));
col_2(i-1) = str2double(s2txt(i,:));
end
% converting to J from I
area = str2double(sltxt(117,:));
col_2 = col_2/area;
% ensuring that data are in Amps and Volts
if max(col_1) >=2000
col_1 = col_1/1000;
end
if max(col_2) >= 30
col_2 = col_2/1000;
end
% determining the voltage sweep direction
if col_2(1) <= col_2(long)
xVall = [col_1(1:long), col_2(1:long)];
else
xVall = [col_1(long:-1:1), col_2(long:-1:1)];
end
% finding where the current becomes positive
sign 2 = sign(xVall(:,2));
for j = 1:long
if sign_2(j) == 0;
index = j-1;
break
elseif sign_2(j) == -1
index = j-1;
135
break
end
end
getting rid of reverse biases
xVsign = sign(xVall(:,1));
for k = 1:length(xVsign)
if xVsign(k) == 0;
indexV = k;
break
elseif xVsign(k) == -1
indexV = k;
break
elseif k == length(xVsign)
indexV =
k;
break
end
end
x = xVall(1:indexV-1,:);
xall = [col_1 col_2];
% Determine IVdata.
IVdata = [Pmax Voc Jsc Isc efficiency Rshunt
% Voc slope]
These should be located in rows 109, 104, 106, 105, 114,
% 113 respectively and have units of mW, mV, mA/cm2, A, %, ohms, ohms.
% Pmax = str2double(sltxt(109,:));
Jsc
P = col 1.*col 2;
112,
% taken out of use for same reason as
Pmax = max(P)/area*10^3;
Voc = str2double(sltxt(104,:));
Jsc = str2double (sltxt(106,:));
taken out of use b/c areas can be
changed within the text file if they were initially entered incorrectly
Isc = str2double(sltxt(105,:));
Jsc = Isc/area*10^3;
efficiency = Pmax/100; %str2double(sltxt(114,:));
Rshunt = str2double(sltxt(112,:));
Vocslope = str2double(sltxt(113,:));
IVdata =
return
A.2
[Pmax Voc Jsc Isc efficiency Rshunt Vocslope];
IPCE Measurements
Incident photon-to-electron current efficiency (IPCE) measurements on DSSCs were originally
investigated because several references indicated them to be an accurate method for deriving the
electron diffusion length of devices' 9. IPCE-based methods have subsequently been shown to
be less accuratel0 than small-perturbation techniques like IMVS, IMPS, or EIS measurements"
136
because the IPCE methods rely on the assumption that recombination is first order in electron
concentration for the analytical solution used in data fitting. These measurements, however, are
still useful for determining the spectral sensitivity of DSSC devices.
A.2.1
Explanation of IPCE Measurements
IPCE, also known as quantum efficiency (QE), is essentially a measure of what percentage of
incident photons are harvested by the device and turned into electrons that go through the
external circuit, where J is the current, q is elementary charge, and P is the illumination intensity
at a particular wavelength.
electrons
IPCE =YrP(A)U==
htnn
2
cin
/s'-CM~
A.2
The IPCE is dependent upon the wavelength of the incident light because the photosensitizer will
be more or less sensitive to excitations at different wavelengths. For most types of solar cells,
including all those discussed in this thesis, the maximum possible value of IPCE is 100%. At
100% quantum efficiency, each incident photon is converted into 1 electron that is transported to
the current collectors with no losses. It should be noted that IPCE measures only current effects
of the device under shorted conditions. No voltage effects can be gleaned from these numbers
unless the measurement can be taken at different applied biases.
There are two types of quantum efficiency-internal (lqI) and external (1q,E). The former
accounts for only photons that actually make it to the active area of the device (optical losses due
to reflectance (R) are removed from the calculation). The latter includes all photons that are
directed at the active area of the device. The two quantities are related via the equation below.
77q,I
A.2.2
=
(1- R )7q,E
A.3
Making Accurate IPCE Measurements on DSSCs
IPCE measurements are taken by applying a small beam of light at a particular wavelength to the
active area of the solar device at a known frequency. The device will produce a measurable
137
amount of current at the same frequency (with an offset). For most solar devices, the device
response is independent of frequency, so measurements can be taken quickly at high frequencies
(approximately 100 Hz). DSSCs, however, have a very slow response compared to other solar
devices (and no response without a white bias light in the background to fill the traps in the
titania photoanode)12 . Therefore, the response of these devices is dependent upon the frequency
used to make the measurement. An example of this dependence is shown in Figure A.11 below.
50
45
40
--
100 Hz
C
-80
Hz
E 30 4
-60
Hz
35
'U
-40Hz
25
+
-30
Hz
-20
20
15
Hz
-15Hz
-10Hz
6 Hz
10
4Hz
5,
300
400
500
600
700
900
800
Wavelength (nm)
Figure A.11 External quantum efficiency of a nanoparticle DSSC as a function of
chopping frequency. These data were taken with a white bias light.
It is possible to calculate the short circuit current density of the device from the external quantum
efficiency (EQE) by integrating the EQE over the solar spectrum. Such calculations show that
lower frequencies give more accurate estimates of the short circuit current density13 .
A.3
Electrochemical Impedance Spectroscopy (EIS) of DSSCs
A.3.1
Explanation of EIS Measurements for DSSCs
A.3. 1. General EIS Review
138
EIS for DSSCs is similar to JV curve testing in that the equipment applies a bias to the device
and measures the resulting current (no solar simulator is used here). The trick of this technique
is that it applies a constant bias (DC voltage) with a sinusoidal perturbation on top of it (AC
voltage). The frequency of the AC voltage can be varied across a wide range, depending on the
instrument. When the frequency corresponds to the timescale of some process in the DSSC,
there is a current response associated with it that the EIS system records. Thus, this technique is
very helpful for probing different time domains in the device. In principle, intricately
intertwined processes can be observed separately using this technique, provided that the
processes occur on different timescales.
EIS analysis generates datasets that are characterized by three parameters (f, Z', Z") where f is
the frequency, and Z' and Z" are impedances that relate to the real and imaginary parts of the
device response. An equivalent circuit is required that reflects the internal processes of the
device. This equivalent circuit can then be theoretically modeled to generate an equation relating
f, Z' and Z" to a smaller set of independent variables (usually resistances, capacitances, and
ideality factors). The equation is fit to the data, and the values of the independent variables can
be extracted.
A.3.1.2 TransmissionLine Model
In order to utilize EIS for any technical analysis, an equivalent circuit for the electrochemical
system must be developed. Without this equivalent circuit, no quantitative information can be
extracted from the data. The generally accepted equivalent circuit for DSSCs is shown below in
Figure A.12.
r,
AA
r,
AAA
-1L
Try- ir
r,
vor
AI
139
Figure A.12 DSSC equivalent circuit 14. Zd represents the Warburg component
that is characteristic of ion diffusion. Rs is the series resistance of the device. Rco
and Cco, RTCO and CTCO, and Rpt and Cpt are the resistance and capacitance of the
TCO-TiO 2 interface, TCO-electrolyte interface and electrolyte-Pt interface
respectively. The extended TiO 2 interface is characterized with a transmission line
element containing the three repeated elements rt, ret, and c,. This figure has been
reproduced with permission from [14] Copyright 2006 American Chemical
Society.
As is typical of electrochemical analysis, every interface is characterized by a parallel RC circuit.
The three elements, rt, ret, and c., are used to determine the three commonly referenced quantities
transport resistance (Rt= rtL), charge transfer resistance (Rct = retL) and chemical capacitance
(C,=cpL), where L is the number of repeats within the transmission line (typically 50 or larger).
Ret and R1 can be used to determine the device electron diffusion length according to the
Equation A.4 below, where d is the thickness of the titania photoanode.
Ld 1=
R
A.4
d
Good theoretical discussions of EIS analysis for
Ldiff
determination is solar cells can be found in
references by Halme et al.", Bisquert et al.16 and Gratzel et al.".
A.3.2
Making Accurate EIS Measurements for Ldiff Determination
It is recommended that data be taken at forward bias in the dark. Others have taken data under a
solar simulator 8 , but this condition is undesirable for two reasons. First, it heats the cell when
no external cooling is applied, and the temperature change affects the performance
characteristics. Second, the light output by any solar simulator is sensitive to underlying
variations in the grid power supply. These small variations can add to the noise in the data. The
forward biases recommended for testing are ± 100 mV from the voltage at the maximum power
point at 25 mV increments. The AC signal amplitude recommended is 10 mV. At these
conditions, the different contributions of the transmission line should be visible. During fitting,
the RC contributions of the TCO are typically neglected in the model because they are rarely
visible in the device response over the frequency ranges of interest (60,000 Hz to 0.01 Hz).
Also, to increase the speed with which measurements can be taken, whenever the electrolyte is
140
not under investigation, the smaller frequency range can be shrunk to 60,000 Hz to 0.1 Hz.
Speed can be an important experimental parameter when testing DSSC devices that are sensitive
to environmental conditions. When using the smaller frequency range, as we did for these
studies, it is recommended that the model be shrunk to the one depicted in Figure A.13 below.
Rs
ZTL
Rpt
CPEPt
Figure A.13 Suggested simplified equivalent circuit for DSSCs. CPEp is the
constant phase element of the platinum (characterized by a capacitance and
ideality factor). ZTL represents the transmission line element (identical to the one
depicted in Figure A. 11, which in the ZView software was the extended element
type 6.
Symmetric characterization of the platinum counterelectrode is recommended to determine
appropriate values for Cpt and Rpt 19 . Also, it is recommended that constant phase elements
replace all capacitors in the model to reflect the plurality of surface conditions in the system.
Table A.5 below lists all of the parameters that are fitted from the equivalent circuit shown in
Figure A.13 and shows suggested values and ranges for them.
Table A.5 Model parameters, suggested ranges and descriptions.
given for rt, ret, and c, are valid only for n=100.
Parameter
Suggested Range
Rs
5 - 100 Q
The ranges
Description
Series resistance (contributions from the FTO and external
contacts)
rt
cP
0.1 -5 Q
10-1 - 10-5 F
Transport resistance to electron diffusion within the titania phase
Chemical capacitance of the double layer at the TiO 2 -electrolyte
interface
0.8 - 1.2
ret
104 - 106 K2
n
100
Ideality of the capacitive behavior at the TiO 2 -electrolyte
interface
Charge transport resistance to electron transfer from the TiO 2 to
the electrolyte phase
Number of repeats the software uses to model the "infinite"
transmission line
Rpt
0.05
Cpt
10-5
prt
-
0.8
-
20 2
Resistance to charge transfer at the platinum counterelectrode
10-4 F
Double-layer capacitance at the platinum counterelectrode
1.2
Ideality of the capacitive behavior at the Pt-electrolyte interface
141
Finally, it is important to remember that the equation for fitting the DSSC, Equation A.5 below,
corresponding to the equivalent circuit shown in Figure A.13, is complex and nonlinear.
is the complex impedance value for the DSSC, and co is the frequency
ZDSSC = R
Z DSS
+'
1+
( ic)fl R,, C,,
+
coth L r
I+ ( ic) '' re ,
ZDSSC
20
re,
+ i)8 C
A.5
Therefore, all values of Ldiff/d obtained should be compared with expected values to ensure that
the minimum of the objective function achieved through fitting is not one that yields aphysical
results.
A.3.3
MATLAB Code for Fitting EIS Measurements with the Equivalent Circuit
The ZView software package was used for all of the fitting to get values for the Ldiff calculation.
However, this package is limited in that upper and lower bounds cannot be specified for the
parameters. This limitation can lead to situations where the calculated minimum does not yield
physical solutions for the parameters. Therefore, a MATLAB code was developed to fit
Equation A.5 while applying upper and lower bounds for the parameters. This code is included
in the text below. Before fitting, it is recommended that the initial guesses (ig) and upper and
lower bounds (ub and lb) be modified to appropriate values for the system under consideration.
Also, the variable 'name' must be changed to the name of the ZView file that contains the raw
data and must be saved in the same directory as the code.
function params = zfittxt_v_single
%David Couling, 5/26/11
modified by Becky Ladewski 5/30/1
name =
'n3
525mv.z';
Rs, Rpt,
ig
= loglO([27
Cpt,
4.2 2.8e-5
Ppt,
Rct,
Cmu,
.97
43313
9e-7
Pmu,
Rt
0.96
.34]');
.8 1E+05 1E-6
0.8 0.11');
lb = loglO([5 1 1E-6
ub = loglO([100 20 1E-5 1.1 1E+06 1E-05 1.1 10]');
Function using fmincon to find optimal parameters Rs, Rpt, Cpt, Ppt, Rct.,
Cmu, Pmu, Rt
L is always 100
142
ig,
lb, ub
should
come from
the
calling function
multiple fitting
and are
% the initial guess, lower and upper bounds
L=100;
%InputMatrix
%Z" are the
be 3 columns,
will
column I = omega, 2 = Z',
x and y coordinates
(real and imaginary parts,
3 = Z";
respectively)
Z'
readoptions = ['MultipleDelimsAsOne',l,'Headerlines',125];
fid = fopen(name);
InputMatrix
= textscan
(fid,'s
s ss sis s*[^\n] ,readoptions);
fclose ('all');
omegastr
char(InputMatrix{1});
Z_realstr = char(InputMatrix{5});
Z imagstr = char(InputMatrix{6});
skip first 125 lines because they are just text
[row col] = size(omegastr);
omeganum = str2num(omegastr(126:row,:));
Z_real num = str2num(Zrealstr(126:row,:));
Z imagnum = str2num(Z imag str(126:row,:));
w = 2*pi*omeganum;
Z = Zreal num + li*Zimagnum;
opt = optimset('Disolay','Iter','MaxFunEvals',5e4,'TolFun',le8,'Maxlter',5000,'TolX',1e-6);
%opt = optimset('MaxFunEvals',2e4,'TolFun',1e-9,'MaxIter',5000,'TolX',1e-9);
logparams = fmincon(@(y) minfun(y,w,Z,L),ig,[],[],[],[],lb,ub,[],opt);
Jlogparams
fminsearch(@ (y) min-fun(y,w,Z,L),ig,opt);
params = 10.^(logparams);
Rs =
Rpt
Cpt
Ppt
Rct
Cmu
Pmu
Rt =
x1
x3
params(l);
params(2);
params(3);
params(4);
params(5);
= params(6);
= params(7);
params(8);
Rt;
= Rct./(l+Rct*Cmu*(li*w).^Pmu);
-
Z_calc = Rs + Rpt./(l+Rpt*Cpt*(li*w).^Ppt) +
sqrt(xl*x3).*coth(L*sqrt(xl./x3));
Z_calcreal = real(Zcalc);
Z_calcimag = imag(Zcalc);
figure (1)
plot(real(Z),-imag(Z),Zcalc
title(name);
real,-Z
calc
imag)
Need magnitude and angle for
making bode elots
mag data = sqrt(real(Z).^2+imag(Z).^2);
mag fit = sqrt(Z calcreal.^2+Zcalcimag.^2);
angledata = -rad2deg(atan(imag(Z)./real(Z)));
angle fit
=
-rad2deg(atan(Z calcimag./Z calcreal));
f = w./(2*pi);
figure (2)
and
143
subplot (2,1, 1)
loglog(f,mag data,f,mag fit)
title (name);
subplot
(2, 1, 2)
semilogx(f,angle data,f,anglefit)
function output = min _fun(y,w,Z,L)
This is the function that is optimized, we are going to assume that we are
minimizing the sum of squares of the function
Define the parameters
y
10.^(y);
Rs
y(1);
Rpt =y(2);
=
=
=
=
Cpt
Ppt
Rct
Cmu
y(3);
y(4);
y(5);
y(6);
Pmu = y(
Rt
7
);
= y(8);
x1 = Rt;
x3 = Rct./(l+Rct*Cmu*(li*w).^Pmu);
Z_calc = Rs + Rpt./(l+Rpt*Cpt*(li*w).^Ppt) +
sqrt(xl*x3).*coth(L*sqrt(xl./x3));
Z calc real = real(Z calc);
Z calcimag = imag(Zcalc);
Z real = real(Z);
Z imag = imag(Z);
diff sq real = w.*(Zreal-Zcalc_real).^2;
diff sqimag = w.*(Z_imag-Z_calcimag).^2;
%diffsq
high _real
0;
diff sq high imag
0;
diffsqhigh real
600000*w(1:29).^-1.*(Zreal(1:29) Z calcreal(1:29)).^2;
diff sq high imag = 600000*w(1:29).^-1.*(Z imag(1:29) Z calc imag(1:29)).^2;
%dif f sq high
output =
imag =(
imrag (1: 25)
-
Z_calcimag (1: 25))
.^2;
sum(diffsq real)+sum(diffsqimag)+sum(diffsq highreal)+sum(diffsqhigh_i
mag);
return
A.4
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