Photovoltaic Devices using Photosynthetic Protein Complexes
By
Rupa Das
Submitted to the Department of Electrical Engineering and Computer Science
in partial fulfillment of the requirements for the
Degree of Master of Science in Electrical Engineering and Computer Science
Massachusetts Institute of Technology
February, 2004
Copyright Massachusetts Institute of Technology 2004. All rights reserved.
Author
Rupa Das
Department of Electrical Engineering and Computer Science
8 th Jan, 2004
Certified by
Marc Baldo
Assistant Professor of Electric-al+-ngineering and-Computer Science
bT7esis Avisor
Accepted by
Arthur C. Smith
Professor of Electrical Engineering and Computer Science
Chairman, Department Committee on Graduate Thesis
MASSACHUSETTS
S
OF TECHNOLOGY
BARKER
APR 15 2004
LIBRARIES
E
Photovoltaic Devices using Photosynthetic Protein Complexes
By
Rupa Das
Submitted to the Department of Electrical Engineering and Computer Science
February, 2004
in partial fulfillment for the requirements for the
Degree of Master of Science in Electrical Engineering and Computer Science
Abstract
Photosynthetic proteins have been used as an active material in design of organic solar
cells. Traditional organic solar cells have the limitation of not being able to absorb light
in the visible-NIR region of the solar spectrum. This region corresponds to over 70%
power of the total solar radiation. Using molecular proteins obtained from nature these
limitations can be overcome. Biological photosynthetic complexes contain reaction
centers with a quantum yield of >95% and a bandgap of less than 1.1eV allowing
absorption in the 600-11 00nm visible-NIR range.
Two types of photosynthetic complexes are employed to demonstrate the generality of
the solid state integration technique to make solar cells. The simplest photosynthetic
complex used is a bacterial reaction center (RC), isolated from the purple bacterium R.
sphaeroides. The other protein being used is Photosystem I (PSI), a much larger complex,
which is isolated from spinach chloroplasts.
Electronic integration of devices is achieved by depositing organic semiconducting
protective layer over a self-assembled monolayer of photosynthetic reaction centers
oriented via an engineered metal-affinity polyhistidine tag. Various analytical and
spectroscopic techniques have been used to examine solution spectrum and solid state
device characteristics. Reasonable efficiencies have been obtained which demonstrates
applicability of such techniques. The efficiency obtained is higher than a wet cell made
using same proteins. The next immediate goal is to optimize processing conditions and
therefore improve efficiency to reach levels comparable traditional organic solar cells.
i
Acknowledgements
I am extremely grateful to Prof. Marc Baldo. I found Marc not only a research
advisor but also a mentor. Marc's guidance and discussion helped me to feel at ease with
the project in a relatively short period of time enabling me to complete the project with
success.
I would like to thank Prof. Terry Orlando, Prof. Hank Smith and Ms. Marilyn
Pierce for providing me administrative as well as moral support during my stay here at
MIT. I would especially like to thank Marilyn Pierce for always making time in order to
listen to me and provide helpful advice.
My thanks are due to Libby Shaw and Kurt Broderick who helped me during the
first part of my thesis project. Libby Shaw for training me on the equipment in the CMSE
lab and helping me understand the technical details pertaining to them. Kurt Broderick
for training me in MTL and helping me understand the technical aspects of materials
processing, which was of extreme value in my scientific endeavors.
A number of graduate students, colleagues and friends have helped make my stay
at MIT a memorable, eventful and an inspiring one. With Michael Segal, I spent endless
hours testing devices in the optics lab. It was a delight to work with him. I would like to
thank him for all his support and advice. Patrick Kiley, Elizabeth Broadwater, Victoria
Chou and Benjie Limketkai for helping me in the project. I would like to thank John
Kymissis, Seth Coe, Yaakov Tischler, Debbie Mascaro and Alexi Ara for helping me
learn my way through the LOOE lab. I would especially like to thank John Kymissis, for
his remarkable willingness to teach me new processing tools as and when I need his help.
I would like to mention a special thanks to my friends Charu, Sapna, Sajan,
Mahesh, Ashish, Vipin, and Lalit for being extraordinary friends all through the tough
times as well as the good times.
Finally, I would like to take a few lines to acknowledge the ones who have been
the nearest and dearest to me. My parents for their support and for being the wonderful
parents they are. Manish for his constant support, advice and inspiration which made me
believe in myself. Their unconditional support has helped me to emerge victorious in my
endeavors and I will forever remain grateful.
ii
Table of Contents
Abstract .......................................................................................................................................................... i
Acknowledgem ents ....................................................................................................................................... ii
Table of Contents ......................................................................................................................................... iii
List of Figures .............................................................................................................................................. iv
1
Introduction ......................................................................................................................................... 1
2
Molecular electronicsIMotivation ............................................................................................... 1
So lar Cells ................................................................................................................................... 1
PhotosyntheticProtein Molecular Complexes ............................................................................. 3
1.3.1
Photosynthesis ........................................................................................................................ 4
Classification of Photosynthetic Organism ............................................................................. 5
1.3.2
Organizationof Thesis................................................................................................................. 9
1.4
Photovoltaic Characterization ......................................................................................................... 11
3
2.1
Characteristicsof Solar Cells....................................................................................................
2.2
Power Conversion Efficiency ....................................................................................................
2.3
Equivalent CircuitDiagram (ECD) ...........................................................................................
2.3.1
Open Circuit Voltage ............................................................................................................
2.3.2
Short Circuit Current Isc .......................................................................................................
Protein Complex ................................................................................................................................
1.1
1.2
1.3
3.1
3.2
4
11
13
16
19
22
25
Photosystem I ............................................................................................................................ 25
Reaction Centers ....................................................................................................................... 29
Device Design & Fabrication ............................................................................................................ 35
D esign ........................................................................................................................................
4 .1
4.2
Self-assembly .............................................................................................................................
4.3
Evaporation...............................................................................................................................
5
Results for RC ...................................................................................................................................
35
36
39
40
6
Absorption Spectrum using UV-Vis ...........................................................................................
Laser Measurements..................................................................................................................
Spectrum Measurements using Tunable Laser..........................................................................
Results for PSI ..................................................................................................................................
40
41
47
49
7
Absorption Spectrum using UV-Vis ...........................................................................................
6.1
62
FluorescenceMeasurements .....................................................................................................
63
Laser Measurements..................................................................................................................
Conclusion .........................................................................................................................................
49
50
54
57
5.1
5.2
5.3
References ................................................................................................................................................... 59
iii
List of Figures
Figure 1.1: Structure of the chloroplast. (a) Electron micrograph of a plant
chloroplast. Chloroplast is about 6 A long. Inside the chloroplast is the
photosynthetic membrane, which is organized into stacked and
unstacked regions. It is not known why the photosynthetic membrane
forms such a complicated architecture. The stacked regions are linked
by unstacked membranes. (b) Model of the chloroplast showing the
................................................................................
photosynthetic membrane.
6
Figure 1.2: Illustration of the four major protein complexes on the thylakoid
membrane. Model of the photosynthetic membrane of plants showing
the electron transport components and the ATP Synthase enzyme (cross
sectional view). The complete membrane forms a vesicle. The pathways
of electrons are shown by solid arrows. The membrane bound electron
transport protein complexes involved in transferring electrons are the
photosystem II and I reaction centers (PSII and PSI) and the
cytochrome bf complex (Cyt b6 f)..........................................................................
8
Figure 1.3: Bacterium Rhodobactersphaeroides,shown here undergoing cell division,
belongs to the a-subdivision of the Proteobacteria. 15............................................. 9
Figure 2.1 The interface between two different organic semiconductors (a) facilitate
charge transfer (D/A interface), (b) energy transfer (no exciton splitting).
What kind of interface forms depends on the position of the HOMO and
LUMO levels and the direction in which band bending occurs. The latter
is determined by the relative position of the Fermi levels...............................13
Figure 2.2: Current versus applied voltage of a solar cell. The extracted current is
negative. The fourth quadrant represents the voltage and current that is
generated by the cell. An externally applied voltage is necessary to
obtain data points in the first and third quadrant. .............................................
16
Figure 2.3 : ECD of a solar cell. Circuit consists of the following ideal components:
Current source IL that considers the light-generated current, a diode D
that accounts for the nonlinear voltage dependence and a shunt as well
as a series resistor Rsh and R, respectively. Load resistor RL and its
voltage drop V and current I across it. I is negative if V > Vo, and it
flows "into" the device - otherwise it is positive . .................................................
17
Figure 3.1 : View of PSI from the top. Fe 4 S 4 cluster can be clearly seen to be in the
middle of the protein. Also, the radial arrangement of the chlorophylls
is readily seen. (NCBI M M DB)...........................................................................
26
Figure 3.2: Structural model of the PS I trimer derived from the electron density
map at 4 A resolution. (a) View perpendicular to the membrane plane
from the stromal side. (b) Rotated 900 about the horizontal. Helices
represented by cylinders and Chla molecules by wire models of their
.. .. .. .. . .. .. .. .. .. .. .. .. . .. .. .. ..
dihydroporphyrin head groups.
Figure 3.3: Photosystem I electron transport pathways and rates. The vertical axis
shows the midpoint potential of the electron carriers. Abbreviations are
iv
27
given in the legend of (FA and FB are equivalent names for FeSA and
FeSB).3 .....................................................................................................................
29
Figure 3.4: An illustration depicting the membranes of Rb. sphaeroides. (a)
Representation of the intracytoplasmic membrane system formed in Rb.
sphaeroides as a result of a reduction in the oxygen tension33 (b) Random
distribution of RC/LH1/LH2; indication of PufX inserted into LH1.
Abbreviations: OM = outer membrane; PG = peptidoglycan layer; PPS
= periplasmic space; CM = cytoplasmic membrane; p = protein; pp =
porin protein; ps = phospholipid bilayer; lp = lipoprotein; lps =
lipopolysaccharide; A = fully formed invagination; B = site of initiation
of m embrane growth............................................................................................
30
Figure 3.5 : View of bacterial reaction center (RC). (NCBI MMDB)3 4 ...............
31
. . . . .. . . . .. . . . . .
Figure 3.6: Structure of photosynthetic reaction center of Rb. Sphaeroides. (a)
Location of the bound cyt c2 (lavender), the heme prosthetic group
(turquoise), the RC L subunit (yellow), the RC M subunit (blue), the RC
H subunit (green), the RC primary donor (red), and non-heme Fe atom
(red). (b) Pigment molecules in RC (details in text)........................................32
Figure 3.7 : Near-IR absorption spectra for the photosynthetic reaction center of
Rhodobactersphaeroides. The three major bands are labeled as resulting
from absorptions due to the bacteriopheophytins at 760 nm (H),
accessory bacteriochlorophylls at 800 nm (B), and the special pair (Py+
and Py-) at 825 and 870, respectively. The dotted line represents a
sample laser spectrum. The center wavelength of the spectra is tunable
from 750 to 850 nm . 3 ..........................................................................................
34
Figure 4.1: Schematic Energy Level Diagram for device design. (a) RC (b) PSI...............36
Figure 4.2 : Self Assembly of photosynthetic complexes on ITO substrates. (a)
Functionalization of ITO/Au surface. (b) RC assembly. (c) PSI assembly..........37
Figure 5.1 : RC absorption spectrum in pure buffer and with stabilizers dissolved in
the buffer...................................................................................................................
41
Figure 5.2 : Photocurrent response of pure RC device........................................................
43
Figure 5.3 : RC with DEAE-dextran stabilizer photocurrent. (a) I-V curve of device
with RC and DEAE-dextran. Inset zooms in on the origin. (b) Control
device response (device without RC)..................................................................
44
Figure 5.4 : Photocurrent of RC device with Polyethyleneimine (PEI) stabilizer. (a)
I-V curve of device with RC and PEI. Inset zooms in on the origin. (b)
Control device response (device without RC)...................................................
45
Figure 5.5: Photocurrent of RC device with PEG and sucrose stabilizer..........................
47
Figure 5.6 : Comparison between the photocurrent spectrum of solid-state (i) and
wet electrochemical cell devices (m), and the solution absorption
spectrum of the bacterial reaction centers (-o-), demonstrates that the
observed photocurrent originates in the RCs. Wet cell device data is
from a prior research article 56...............................................................................
48
Figure 6.1 : PSI solution spectrum. (Data courtesy Patrick Kiley) .....................................
49
V
Figure 6.2: Fluorescence spectrum of PSI in buffer solution. (Data Courtesy Patrick
K iley) ..........................................................................................................................
51
Figure 6.3: Fluorescence spectrum for PSI film. (Data courtesy Patrick Kiley).........52
Figure 6.4: Peptide nanotube formation. 60 ............................................................................
53
Figure 6.5 : Fluorescence spectrum of PSI stabilized using A6K and V6D peptide.
(Data Courtesy Patrick Kiley)............................................................................
54
Figure 6.6 : Photocurrent characteristic of PSI. (a) Exchanged PSI with peptide. (b)
Exchanged PSI without peptide..........................................................................
56
vi
1
Introduction
1.1
Molecular electronics/Motivation
Solar photo-voltaic cells commonly known as solar cells are used to harness solar
radiation, the most abundant source of energy on Earth. As the search for alternative
energy sources continues, design of efficient and inexpensive solar cells is of special
interest. Solar cells fabricated using organic materials have several advantages over
traditional inorganic solar cells due to much lower production costs, flexibility and large
area applications even if they do not reach the energy conversion efficiencies of inorganic
materials. Using molecules from nature in organic solar cells is expected to increase the
total amount of solar radiation absorption achieved in such devices.
Bulk-heterojunction organic solar cells in which a maximum power efficiency of upto 3%
has been achieved absorb in the 300-700nrn range. These solar cells can not absorb all the
photon flux, as they do not absorb infrared radiation. One way to achieve better
harvesting of sunlight is to use low bandgap protein complexes with a bandgap of less
than 1.1eV which allow absorption in the 300-900nm range. The absorption of such
molecular materials will match the solar spectrum better and can enhance the efficiency
of organic solar cells. In this thesis we demonstrate integration of such biomolecular
complexes into solid state devices.
1.2 Solar Cells
The conversion of solar light into electric power requires the generation of both negative
and positive charges as well as a driving force that can push these charges through an
1
external electric circuit. At present, solar cells comprising of inorganic semiconductor
such as mono- and multi-crystalline silicon are used commercially for small scale devices
such as solar panels on roofs, pocket calculators and water pumps. These conventional
solar cells can harvest up to as much as 24% 1 of the incoming solar energy which is
already close to the theoretically predicted upper limit of 33%
2.
The relatively low
absorption of inorganic solar cells suggests that new technologies which allow for low
fabrication costs need to be explored instead of merely focusing on increasing conversion
efficiencies. With traditional silicon materials, production of solar cells requires many
energy intensive processes at high temperatures (400-1400'C) and high vacuum
conditions with numerous lithographic steps leading to relatively high manufacturing
costs.
Solar cells comprising of organic semiconductors require considerably less production
energy because of simpler processing at much lower temperatures (20-200 0 C). For
example, electro-chemical solar cells using titanium dioxide in conjunction with an
organic dye and a liquid electrolyte 3 have already exceeded 6% power conversion
efficiencies and are expected to be commercially available in near future since they have
relatively low production costs.
When organic materials are excited using photons, the strong electron-phonon interaction,
does not lead to the generation offree charge carriers, but to bound electron-hole pairs
(excitons) with a binding energy of about 0.4eV 4. These excitons need to dissociate
before the charges can be transported through the film and collected at the electrodes. For
example, exciton dissociation can occur at a rectifying interface (Schottky contact) in
2
single layer devices or at the interface between an electron-donor and an electronacceptor semiconductor material. The larger this interface area the more excitons can
reach it and then dissociate. In addition, the small diffusion range of the excitons
(typically about 10nm) 5-7 compared to the film thickness necessary to absorb the major
portion of light (typically >100nm) makes it difficult to reach high power conversion
efficiencies in organic solar cells.
Photosynthetic complexes such as Photosystem I and Reaction Centers (detailed
description in Chapter 3) offer a possible solution. Photosynthetic protein complexes
contain precisely engineered excitonic circuits that guide the energy from an absorbed
photon directly to the dissociation center (the special pair) at the base of the complex.
The overall quantum yield is seen to be greater than 95%
8-10
and the potential power
conversion efficiency of a solid-state photosynthetic device may approach or exceed 20%
providing an economical alternative to crystalline photovoltaic cells.
1.3
Photosynthetic Protein Molecular Complexes
Biological structures such as protein-molecular complexes have evolved and adapted to
optimize the molecular circuitry over the years. Photosynthetic reaction centers harvest
photons with an overall quantum yield of greater than 95%
8-10.
They self assemble with
remarkable precision and repeatability.
Protein-molecular complexes incorporate molecular circuits with very high resolution
features. The simplest model of a solid-state photosynthetic device consists of uniformly
oriented photosynthetic reaction centers deposited between two metallic contacts. After
3
absorption of a photon and rapid charge separation within a complex, a potential of up to
1. 1V can be developed across the metal contacts.'
1.3.1
11
Photosynthesis
Photosynthesis is the physico-chemical process by which plants, algae and photosynthetic
bacteria use light energy to drive the synthesis of organic compounds. In plants, algae and
certain types of bacteria, the photosynthetic process results in the release of molecular
oxygen and the removal of carbon dioxide from the atmosphere that is used to synthesize
carbohydrates (oxygenic photosynthesis). Other types of bacteria use light energy to
create organic compounds but do not produce oxygen (anoxygenic photosynthesis).
The Sun is the source of almost all energy that runs life on Earth. Each minute our Sun
converts 120 million tons of its mass into electromagnetic radiation and dumps it out into
space. Only one billionth of that reaches Earth. The visible portion of this
electromagnetic radiation is captured by plants, algae and cyanobacteria. Plants are green
because chlorophyll, the key pigment that captures light, absorbs blue and red efficiently,
but reflects most of the green light. Photosynthetic organisms then use the absorbed light
energy to make food and oxygen.
The photosynthetic process in plants and algae occurs in small organelles known as
chloroplasts that are located inside cells. The more primitive photosynthetic organisms,
for example oxygenic cyanobacteria, prochlorophytes and anoxygenic photosynthetic
bacteria, lack organelles. The photosynthetic reactions are traditionally divided into two
stages - the "light reactions," which consist of electron and proton transfer reactions and
4
the "dark reactions," which consist of the biosynthesis of carbohydrates from carbon
dioxide. The light reactions occur in a complex membrane system (the photosynthetic
membrane) that is made up of protein complexes, electron carriers, and lipid molecules.
The photosynthetic membrane is surrounded by water and can be thought of as a twodimensional surface that defines a closed space, with an inner and outer water phase. A
molecule or ion must pass through the photosynthetic membrane to go from the inner
space to the outer space. The protein complexes embedded in the photosynthetic
membrane have a unique orientation with respect to the inner and outer phase. The
asymmetrical arrangement of the protein complexes allows some of the energy released
during electron transport to create an electrochemical gradient of protons across the
photosynthetic membrane.
1.3.2
Classification of Photosynthetic Organism
Oxygenic Photosynthetic Organisms
In plants the photosynthetic process occurs inside organelles known as chloroplasts.
Chloroplasts provide the energy and fix carbon needed for plant growth and development,
while the plant provides the chloroplast with carbon dioxide, water, nitrogen, organic
molecules and minerals necessary for the chloroplast biogenesis. Most chloroplasts are
located in specialized leaf cells, which often contain 50 or more chloroplasts per cell.
Each chloroplast is defined by an inner and an outer envelope membrane and is often
shaped like a meniscus convex lens that is approximately 5-10 microns in diameter
(Figure 1.1), although many different shapes and sizes can be found in plants. Details of
chloroplast structure, are presented in previous extensive reviews 12. The inner envelope
5
membrane acts as a barrier, controlling the flux of organic and charged molecules in and
out of the chloroplast. Water passes freely through the envelope membranes, as do other
small neutral molecules like carbon dioxide and oxygen. There is evidence that
chloroplasts were once free living bacteria that invaded a non-photosynthetic cell long
ago. They have retained some of the DNA necessary for their assembly, but much of the
DNA necessary for their biosynthesis is located in the cell nucleus. This enables a cell to
control the biosynthesis of chloroplasts within its domain.
(a)
Thylakold
gran. stacks
gnnr
Thylakoid
stroma Iamellae
amban
membrane
nner
Figure 1.1: Structure of the chloroplast. (a) Electron micrograph of a plant chloroplast. Chloroplast
is about 6 A long. Inside the chloroplast is the photosynthetic membrane, which is organized into
stacked and unstacked regions. It is not known why the photosynthetic membrane forms such a
complicated architecture. The stacked regions are linked by unstacked membranes. (b) Model of the
chloroplast showing the photosynthetic membrane. 12,13
Inside the chloroplast is a complicated membrane system, known as the photosynthetic
membrane (or thylakoid membrane), that contains most of the proteins required for the
light reactions. The proteins, required for the fixation and reduction of carbon dioxide,
6
are located outside the photosynthetic membrane in the surrounding aqueous phase. The
photosynthetic membrane is composed mainly of glycerol lipids and protein. The
glycerol lipids are a family of molecules characterized by a polar head group that is
hydrophilic and two fatty acid side chains that are hydrophobic. In membranes, the lipid
molecules arrange themselves in a bilayer, with the polar head toward the water phase
and the fatty acid chains aligned inside the membrane forming a hydrophobic core
(Figure 1.2). The photosynthetic membrane is vesicular, defining a closed space with an
outer water space (stromal phase) and an inner water space (lumen). The organization of
the photosynthetic membrane can be described as groups of stacked membranes called
grana, interconnected by non-stacked membranes that protrude from the edges of the
stacks (Figure 1.1).
Anoxygenic Photosynthetic Or2anisms
Some photosynthetic bacteria can use light energy to extract electrons from molecules
other than water. These organisms are of ancient origin, presumed to have evolved before
oxygenic photosynthetic organisms. Anoxygenic photosynthetic organisms occur in the
domain bacteria and have representatives in four phyla - Purple Bacteria, Green Sulfur
Bacteria, Green Gliding Bacteria, and Gram Positive Bacteria.
Anoxygenic photosynthetic bacteria differ from oxygenic organisms in that each species
has only one type of reaction center. In some photosynthetic bacteria the reaction center
is similar to Photosystem II and in others it is similar to Photosystem I. However, neither
of these two types of bacterial reaction center is capable of extracting electrons from
water, so they do not evolve oxygen. Many species can only survive in environments that
7
have a low concentration of oxygen. To provide electrons for the reduction of carbon
dioxide, anoxygenic photosynthetic bacteria must oxidize inorganic or organic molecules
available in their environment.
CYTOSOL
10
Ribphute
'ADP
Asynh
STROMA
te
syAbre aP700
ADP +sP,
sI
NADPH
F*R
2H*
2+
NADP*
+H+
2H
Fe+
th ctochom bfcopex(Ctb" )
respctiely
PhphOptn;
bFiffrent fost
boudpatqioes
QA, andQ$eue
of -tye
oro j
r
F, ionlfur
Q
nTte
patqioe
2
ytchmdP
y
C,
e). oxidoedcpteNADPH, niorinormid esin e diacthaseo
erdoxseinal
ferrex enzy
protein complexesmembrane. In ti
bdus otetlaot
b
oD the lihn
H poton
phosae
I reaction centers (PSII and PSI) and
and
II
involved in transferring electrons are the photosystem
Fd,
the
cytochrome bf complex (Cyt bef).
Al,physlsoqingone; eFXroFAtandsporcon slu enters;th
tptoyn;eAtichlorphll
I,
photosystem
and
11
Abbreviations: P680 and P700, the reaction center chlorophyll of photosystem
respectively; Ph, pheophytin; QA, and QB bound plastoquinones; PQH2, reduced plastoquinone; Cyt
be, different forms
of b-type cytochromes; FeS, iron-sulfur centers; Cyt f, cytochome f PC,
plastocyanin; AO, chlorophyll; Al, phylloquinone; FX, FA and FB, iron sulfur centers; Fd,
ferredoxin; FNR, ferredoxin/NADP+ oxidoreductase; NADPH, nicotinomide adenine dinucleotide
phosphate (reduced form); ADP, adenosine diphosphate; ATP, adenosine triphosphate; Pi, inorganic
phosphate; H+, protons; DY, the light-induced electrical potential across the membrane. In this
diagram, plastoquinone (PQ,PQH2) and plastocyanin (PC) are shown with feet to indicate that they
are mobile. 14
8
Despite these differences, the general principles of energy transduction are the same in
anoxygenic and oxygenic photosynthesis. Anoxygenic photosynthetic bacteria depend on
bacteriochlorophyll, which absorbs strongly in the infrared between 700 and 1000 nm.
The antenna system consists of bacteriochlorophyll and carotenoids that serve as a
reaction center where primary charge separation occurs. The electron carriers include
quinone (e.g. ubiquinone, menaquinone) and the cytochrome bc complex, which is
similar to the cytochrome bf complex of oxygenic photosynthetic apparatus.
Figure 1.3: Bacterium Rhodobacter sphaeroides, shown here undergoing cell division, belongs to the
zx-subdivision of the Proteobacteria. 15
1.4 Organization of Thesis
This thesis is presented in five chapters which is then followed by the conclusion.
Chapter 2 is on photovoltaic characterization. First, a summary is presented of the key
steps involved in generating photocurrent from photon absorption to eventual charge
collection at the electrodes. In order to facilitate understanding of current generation
mechanisms used for data analysis a brief theoretical background on characteristic solar
9
cell parameters and relations is presented. Chapter 3 describes the structure and
photocurrent generation mechanism of the two protein complexes being studied: RC and
Photosystem I.
In Chapter 4 device design is examined in detail. This is followed by the experimental
protocol used to fabricate the devices which is divided two parts: self assembly of the
proteins and deposition of organic protective layers. Chapter 5 contains results of
experiments conducted using RC as the active material in the device. Solution absorption
spectra results obtained using UV-VIS. I-V characteristics obtained by using an 808nm
diode laser on the device are presented. Finally, results for device absorption obtained
using Ti-Sapphire tunable laser are presented. Chapter 6 contains results of experiments
conducted using Photosystem I as the device active material. Solution absorption
spectrum results using UV-VIS. Fluorescence measurements at 20 K using a cryogenic
system and device I-V absorption data obtained using a 670 nm diode laser are presented.
10
2 Photovoltaic Characterization
2.1 Characteristics of Solar Cells
In organic semiconductors, absorption of photon leads to the creation of bound electron
hole pair (excitons) rather than free charges. These excitons, which carry energy but no
net charge, may diffuse to dissociation sites where their charges can be separated. The
separated charges then need to travel to the respective device electrodes, holes to the
anode and electrons to the cathode to provide voltage and be available for injection into
an external circuit. The key conversion steps in solar cells are discussed in further detail:
Absorption of photons: In most organic devices only a small portion of the incident light
is absorbed for the following reasons:
1) The semiconductor bandgap is too high. A bandgap of 1.1eV (1100nm) is
required to absorb 77% of the solar radiation on earth whereas the majority of
organic semiconductors have bandgaps higher than 2.0eV (600nm) limiting the
possible absorption to about 33%
2
2) The organic layer is too thin. Typically exciton diffusion lengths are on the order
of 10nm. Fortunately the absorption coefficient of organic materials is generally
much higher than that of silicon, so only a layer of about 1 00nm is required to
absorb between 60 to 90% if a reflective back contact is used.
Exciton diffusion: Ideally, all photoexcited excitons should reach a dissociation site.
Thus their diffusion length should be at least equal to the required layer thickness (for
11
sufficient absorption) - otherwise they recombine and photons are wasted. Exciton
diffusion ranges in polymers and pigments are typically around 1Onm 5-7. However, some
crystalline materials such as perylenes are believed to have exciton diffusion lengths of
several 100nm
16
Char2e separation: Charge separation is known to occur at organic semiconductor/metal
interfaces, impurities (e.g. oxygen) or between materials with sufficiently different
electron affinities (EA) and ionization potentials (IA). In the latter case, one material can
then act as electron acceptor (A) while the other keeps the positive charge and is referred
to as electron donor (D). If the difference in IA and EA is not sufficient, the exciton may
just hop onto the material with the lower bandgap without splitting up its charges.
Eventually it will recombine without contributing charges to the photocurrent (Figure
2.1).
Charge transport: The transport of charges to the electrodes is affected by
recombination, particularly if the same material serves as transport medium for both
electrons and holes. Also, disorder in the organic semiconductor and interaction with
other charges may slow down the travel speed and thereby limit the current.
Charge collection: In order to enter an electrode material with a relatively low work
function (e.g. Al, Ca) the charges may have to overcome the potential barrier of a thin
oxide layer. In addition, the metal may have formed a blocking contact with the
semiconductor so that they can not immediately reach the metal.
12
Both exciton and charge transport in organic materials usually require hopping from
molecule to molecule. Thus, close packing of the molecules is assumed to increase the
intermolecular overlap and a
7t-7t
stacking should generally lead to better transport
properties than bulky 3 dimensional molecules. PL is often quenched considerably if
close packing occurs indicating the presence of rapid exciton transport (loss) channels.
Note that dense packing also favors a higher absorption coefficient.
a) charge transfer
D
Toj
b) energy transfer
A
-5
Al
-4
ITO
Al
Figure 2.1 The interface between two different organic semiconductors (a) facilitate charge transfer
(D/A interface), (b) energy transfer (no exciton splitting). What kind of interface forms depends on
the position of the HOMO and LUMO levels and the direction in which band bending occurs. The
latter is determined by the relative position of the Fermi levels.' 7
2.2 Power Conversion Efficiency
In practice the photoexcited electron can decrease its potential energy by losing energy to
phonons until it reaches the lowest unoccupied molecular orbital (LUMO). This process
of energy dissipation of a phonon into heat is known as thermalization. As a consequence
of thermalization the semiconductor energy gap is often regarded as a measure for the
achievable voltage. The higher the energy gap the higher the voltage can be.
13
On the other hand, a low energy gap material can absorb more photons and thus increase
the number of photogenerated charge carriers i.e. the photocurrent. Therefore the lower
the energy gap, higher the photocurrent. Hence, there must be an optimal energy gap for a
given illumination spectrum. Shockley and Queisser were the first who calculated the
maximum power conversion efficiency for a semiconductor with a given bandgap
assuming only radiative recombination. For silicon devices with a bandgap of 1.1eV the
maximum power conversion efficiency 30%. 18
Semi empirical limits take into account realistic loss mechanisms e.g. by assuming
realistic values for the fill factor. For a semiconductor bandgap between 1.3 and 1.5eV,
the power conversion efficiency is around 30%. Since a high load resistor reduces the
current flux the charges need more time to get out of the semiconductor. Therefore the
recombination rate increases and the extracted external current decreases. This behavior
can be seen in the fourth Quadrant of the IV characteristic in Figure 2.2.
Thus considering the voltage dependence of the IV curve - the maximum power is the
maximum product of I and V that can be found amongst the data points in the fourth
quadrant. The larger the maximum area, the more the IV curve resembles a rectangle with
the area [Voc x Isc]. The ratio between these two areas represents a measure of the
quality of the shape of the IV characteristics:
FF= (
"V
ISC
(1)
OC
thus:
Pmax = (IV)max = Voc 'SC
14
-FF
(2)
Higher the FF (fill factor), the more the IV characteristics resembles to that of a constant
current source with a maximum voltage and higher electric power that can be extracted.
The voltage/current (Vp /Ip) combination that gives the largest power rectangle is called
the maximum power point. Thus, any appliance connected to a solar cell can utilize the
maximum output power only if the supply voltage is around Vp. Hence the load resistor
R1 = Vp/Ip. Otherwise power would be wasted in heating the series resistor (V < Vp) or
via increased current losses through the ideal diode and shunt resistor (V > Vp).
In order to describe the power conversion efficiency / of a solar cell the maximum output
power Pmax has to be related to the power of the incident light Plight. Using (2) to express
Pmax considering the wavelength dependence of the parameters involved we can write:
TW=Isc (k) -Voc (k) -FF(k)(3
Plight
Due to the wavelength and intensity dependence 19 power conversion efficiencies are
only meaningful for a given spectral distribution and intensity.
15
2.Quadrant
I.Quadrant
SC
3.Quadrant
4.Quadrant
0
voltage
Figure 2.2: Current versus applied voltage of a solar cell. The extracted current is negative. The
fourth quadrant represents the voltage and current that is generated by the cell. An externally
applied voltage is necessary to obtain data points in the first and third quadrant.
2.3 Equivalent Circuit Diagram (ECD)
ECDs are frequently used to describe the electric behavior of more complex
semiconductor devices with a network of ideal electrical components such as diodes,
current or voltage sources and resistors.
Figure 2.3 shows the ECD that is typically used for inorganic solar cells. Although the
specific physical processes in organic semiconductors may be different and therefore lead
to other parameters, the principal loss mechanisms are the same and we may therefore
apply the same circuit.
16
R,
U,-
U
I
/00
0.
ho
f
T
I
F
Vd~ 7D
U
R'h
LWV
U
Figure 2.3 : ECD of a solar cell. Circuit consists of the following ideal components: Current source IL
that considers the light-generated current, a diode D that accounts for the nonlinear voltage
dependence and a shunt as well as a series resistor Rsh and R, respectively. Load resistor RL and its
voltage drop V and current I across it. I is negative if V > VOc and it flows "into" the device otherwise it is positive.' 7
The current source generates current IL upon illumination. IL is the number of dissociated
excitons per second, which equals the number of free electron-hole pairs immediately
after generation and just before recombination can take place.
The shunt resistor Rsh is due to recombination of charge carriers near the dissociation site.
Rsh can be derived by taking the inverse slope around OV:
R
~
(4)
This is because at very small voltages the diode D is not conducting and the current
driven by the external voltage is only determined by Rsh + R, with Rsh (typically) being
much larger.
17
The series resistor R, considers conductivity i.e. mobility of the specific charge carrier in
the respective transport medium. The mobility can be affected by space charges and
traps.17 R, is also increased with a longer traveling distance of the charges in e.g. thicker
transport layers. R, can be estimated from the (inverse) slope at a positive voltage > Voc
if the IV curves becomes linear:
R ~(5)
This is because at high positive external voltages V, the diode D becomes much more
conducting than Rsh so that R, can dominate the shape of the IV curve.
The ideal diode D is a voltage dependent resistor that takes into account the asymmetry
of conductivity due to the built in field in the cell (difference between the acceptor
LUMO and the donor HOMO) or the nature of the semiconductor electrode interface in
single layer cells. This diode is responsible for the nonlinear shape of the IV curves. The
diode characteristic is not necessarily Shockley type. Note that the IV characteristic of the
ideal diode D is only equal to the IV characteristic of the entire cell if R, = 00 and Rsh
oc9.
The solar cell voltage V is the voltage that the cell can generate between 0 and Voc
depending on the size of the load resistor. In order to obtain IV curve data in other
voltage ranges (V < 0 and Voc < V) in the IV curve an external voltage source is required.
We note that also the voltage drop across a load resistor - the range between 0 and Voc can be simulated by the same voltage source so that the entire range can be scanned by
applying an external voltage.
18
Note that the current for V > Voc and the extra current for V < 0 V is delivered from the
external voltage source. With an external voltage source, the device may act as a
photodetector increasing the photon to current conversion efficiency.
Using the ECD model in Figure 2.3 and Kirchoff's laws, the following relation can be
formulated:
%sh
(IL
(6)
I)Rsh =V +IRS
d
R
I(Rs +1)=I L -Id
V
V(7
Rsh
Rsh
The Shockley diode equation describes the voltage dependence of the current
Id
through
the ideal diode D:
V-IR,
Id
= Io -(e nkI/
(8)
-1)
Replace Idin (7):
IL-
V
Rsh
1+
2.3.1
10
s
Rsh
1+
V-IRs
.(e nkT/q
1)
(9)
s
Rsh1
Open Circuit Voltage
In the preceding Section we have deduced a formula that allows calculating the output
current of a solar cell. In this Section we derive an expression for the open circuit voltage
Voc and consider the effects of the single components of the ECD. We will also discuss
how PL efficiency can positively affect the solar cell performance.
19
Ideal Solar Cell
First we consider the case of an ideal solar cell that comprises only the ideal diode D in
the dark so that R, = 0 and Rh
=1.
Hence (9) becomes:
(10)
Id = I0(eqV/nkT -1)
The current through such a cells or photodiode is determined only by the current through
the ideal diode D - here represented by the Shockley equation. For positive voltages, I
increases exponentially. Upon illumination the light generates a photo current IL that is
simply superimposed (added) upon the normal rectifying IV characteristics of the diode
D:
I = IJe qV/ nk1)
__
1
(11
The addition of IL results in a region of the fourth quadrant where electrical current and
voltage can be extracted from the solar cell. The highest voltage in this quadrant develops
at the electrodes when IL just manages to cancel the dark current. Thus, given a constant
IL the Voc increases with decreasing dark current. Here, Io determines the height of the
characteristics. Canceling of Id by means of IL can be considered in Eq.(1 1) by setting 1=0.
Voc can than be derived quantitatively using:
= nkT
q
I
(12)
I
Note that the effect of the IL=10 ratio is relatively small. Reduction of 1o by a factor 1 Ox
increases Voc only by 25mV-ln(1O) (58mV). Typical Voc of silicon cells under solar
conditions are around 550mV."
20
Effect of Reh
A formula for Voc that considers the influence of Rsh can be deduced from (9) by setting
the output current to zero (I 0) to give:
=
nkT I(L -VOc /Rsh +
q
(13)
I0
This equation relates the maximum output voltage not only to the light generated current
IL but also to the reverse saturation current and the shunt resistor. Now, also the formula
for Voc becomes transcendent and requires numerical modeling to find solution. The
value for Voc still depends on the ratio IL=Io but now IL is decreased by the presence of a
finite Rsh whereas R, is unimportant since there is no current flowing through it that can
create a voltage drop (loss).
Both the ideal diode D and Rsh are now the components that determine Voc: Suppose Rsh
is not very high and the device is in dark. If we apply a positive voltage across the cell
electrodes we create a voltage drop across Rsh that is equal to the voltage Vd across the
ideal diode D. The current that can pass through the diode D at Vd is determined by its IV
characteristic. The sum of the currents through D and Rsh yields the current through the
electrodes of the solar cell for a given applied voltage.
Upon illumination, the current source generates the current IL, some of which passes
through the diode where a voltage drop is generated that is big enough to allow the rest of
IL to go through Rsh - if the electrodes are open. The same voltage can be measured with a
voltmeter with high internal resistor across the device electrodes and is then termed open
circuit voltage Voc.
21
Note that n which determines the shape of the IV curve stands outside the logarithm in
Eq.(13) and has therefore a stronger influence than a variation of Rsh or 1o. The latter
controls the "height" of the IV curve of D. However, since Rsh can vary considerably it
can seriously decrease Voc if Voc= Rsh is not much smaller than IL. We note that Io can be
related to PL efficiency as discussed later in this thesis.
If there is no external circuit (open circuit mode) the charges accumulate at the electrodes
to build up Voc before they decay showing their maximum PL efficiency. Indeed, it has
been shown that the PL of e.g. chlorophyll is about 3% in a living plant, and 30% if the
chlorophyll molecules are separated from the rest of the electron transport chain, which
corresponds to operation under load and open circuit mode, respectively
2.3.2
1
Short Circuit Current Isc
From the ECD in Figure 2.3 it can be seen that the short circuit current is simply the light
current IL reduced by the current through the shunt Ish and the diode D, Id:
Isc = IL
'sh
'd
(14)
Isc is the highest current that can be extracted from a solar cell. The light current
increases linearly with the illumination intensity. In order to extract a quantity that
describes how efficiently photons are converted into charges in the external circuit, Isc
has to be related to light intensity. This leads to the definition of the spectral response:
SR()
22
= isc ()
(D(k)
(15)
with (D being the light intensity per illuminated area (W/m 2 ) and Jsc representing the
current density (A/m 2 ) in short circuit mode.
The spectral response (SR) thus gives the current in the external circuit per watt incident
light (A/W). This quantity is frequently used to characterize the light sensitivity of
photodiodes since the product SR -Jsc gives immediately the light intensity.
For the characterization of solar cells a quantity that considers the actual fraction of the
incident photons that can be converted into electrons in an external circuit is of higher
interest:
EQE =- number of electrons in external circuit
number of incident photons
(16)
EQE stands for external quantum efficiency and represents, together with the power
conversion efficiency ri, an important parameter of a solar cell. The EQE can be derived
from the spectral response considering that the energy of a photon Ep = hc/k with h being
Planck's constant, c the speed of light and q the elementary charge using the following
formula:
EQE(X) = SR(k)---
(17)
Note that the EQE gives higher values for shorter wavelengths when compared with the
SR spectrum. This is because the spectral response relates to the energy of photons
whereas the EQE refers only to the number of particles. Since the output current is
determined by the number of electrons that can be "pumped" into the CB regardless of
23
their energy, the EQE represents a true measure for the photon to current conversion
efficiency in contrast to SR.
The EQE can be converted into the internal quantum efficiency IQE if only the fraction
of the actually absorbed photons is considered:
IQE(X) =-
EQE()
1- R(k)- T()
where R(k) denotes the fraction of reflected light and T(X) the fraction of the transmitted
light.
The IQE is very helpful and frequently used to investigate the physical processes in the
semiconductor material. In calculations of IQE interference effects may have to be
considered because of the very thin films employed in organic solar cells.
24
3 Protein Complex
3.1 Photosystem I
Photosynthesis is a fundamental biological process that supplies the Earth's biosphere
with oxygen and highly energetic reducing equivalents for CO 2 assimilation.2 2
Photosystems I and II (PSI and PSII respectively) in chloroplasts of cyanobacteria, algae
and higher plants are the main parts of the molecular photosynthetic machinery. In
thylakoid membranes of higher plants and green algae each photosystem (PS) has two
structurally distinct parts, the core complex and the peripheral (or distal) light-harvesting
complex (LHC). 2 3 In cyanobacterial PSI and PSII core complexes, a lack of membranebound peripheral lightharvesting antennas is compensated by an extrinsic phycobilisome
complex. 24 PSI is a large protein complex embedded (Figure 1.2) within the
photosynthetic thylakoid membrane. The function of the chlorophyll (Chl) a-containing
core complexes in thylakoid membranes is the capture of solar energy and the efficient
delivery of the excitation to the reaction center (RC), where this energy is converted into
the electrochemical energy of separated charges in a series of transmembrane electron
transfer reactions. The functions of the LHC antenna are to engage in accessory lightharvesting in a broader spectral region than the core antenna and supply energy to the
core structures, to regulate excitation flows between the two photosystems and to provide
a physiological response to the quality of illumination.2
PSI is a light-driven
plastocyanin:ferreedoxin oxidoreductase sensitized by excited chlorophyll molecules.
Decades of extensive studies of energy transfer processes in the PSI
25
26
has led to
significant progress in our understanding of the mechanisms of fast and efficient energy
conversion in PSI.
Figure 3.1 : View of PSI from the top. Fe 4 S4 cluster can be clearly seen to be in the middle of the
protein. Also, the radial arrangement of the chlorophylls is readily seen. (NCBI MMDB)
Structure of the Photosystem I core
The Photosystem I core of cyanobacteria, algae and higher plants belongs to a group of
iron-sulfur containing RCs. It is widely believed that the principles of the PSI core
organization apply to all Fe-S-type reaction centers
27,
although the structural model of
the complex is available with 4 A resolution only for the cyanobacterium Synechococcus
elongates.9,28
In photosynthetic membranes of cyanobacteria, the PSI complex lacks a peripheral
chlorophyll a/b antenna and exists largely as trimers. 9 Each monomer of the trimer
consists of 11 protein subunits with total molecular mass of about 250 kDa and binds
about 100 Chl a molecules, 15-25 carotenoids and cofactors of electron transfer in the
26
RC. 9 The majority of Chis (~ 00) are coordinated by the two largest protein subunits of
the PSI multisubunit complex, PsaA and PsaB (molecular mass of 83 kDa each),
although minor subunits, PsaF and PsaL, were also shown to bind chlorophylls. The PsaL
subunit is a trimer-forming domain in cyanobacterial PSI. 29 It is the chlorophyll a antenna
network of the monomeric PSI that captures energy from light and delivers it to the PSI
reaction center, thus performing an intrinsic light-harvesting function. An available
structural model of the PSI with 4 A resolution 9 identified in each monomer 89
molecules of Chl a, of which 83 molecules form a relatively disorganized irregular sphere
of the core antenna. In contrast, six ChIs of the RC located in the center of the core
antenna are symmetrically aligned relative to a pseudo C2 symmetry axis running in a
lumenal-stromal direction.
a)
b)
C (AB)
L
Stroma
Pshydopor hsri
Thylakoid
PsaK PsPs
-A/B
-- _Ps aF/J
Lumen
Figure 3.2: Structural model of the PS I trimer derived from the electron density map at 4 A
resolution. (a) View perpendicular to the membrane plane from the stromal side. (b) Rotated 900
about the horizontal. Helices represented by cylinders and Chla molecules by wire models of their
dihydroporphyrin head groups.9
27
Electron Transfer System of PSI
Historically the primary electron donor and terminal electron acceptor of PSI were
discovered first, as these exist for the longest time periods and are therefore most easily
detected. Structural features of the PSI core that are important for understanding energy
transfer processes in the PSI antenna are the distance of P700 from the antenna network,
the connection of P700 to the antenna via monomeric chlorophylls in the reaction center
and the presence of clustered pigments in the structurally disordered antenna. The
available structural model of the cyanobacterial PSI core with medium 4 A resolution is
inadequate to identify all polypeptides and ligands of the chlorophylls, nor does it show
precise interpigment distances and details of the molecular organization of the pigments.
The primary donor P700, primary acceptor AO, phylloquinone molecule Al and ironsulfur clusters Fx and FAB represent spectroscopically identified redox cofactors of the
electron transfer in the RC .30'31
28
P7QQ*
3 ps
AO
-1.2
40-200 ps
A1
-0.8
15-200 ns
F I
FA/ FB -2 gs
-0.4
-PSI
0
E
0.0
hv
200
0.4
P
Ps
-': P710
0.8-
Figure 3.3: Photosystem I electron transport pathways and rates. The vertical axis shows the
midpoint potential of the electron carriers. Abbreviations are given in the legend of (FA and FB are
equivalent names for FeSA and FeSB). 32
3.2 Reaction Centers
Rhodobacter (Rb.) sphaeroides (shown in Figure 1.3) belongs to the alpha-subdivision of
the Proteobacteria. This group of bacteria is among the most metabolically diverse
organisms known, being able of grow in a wide variety of environmental conditions.
These bacteria possess an extensive range of energy acquiring mechanisms including
photosynthesis, lithotrophy, aerobic and anaerobic respiration. It can also fix molecular
nitrogen, synthesize important tetrapyrroles, chlorophylls, heme, and vitamin B12. This
bacterium has extremely facile methodologies for genetic manipulations, gene transfer,
genetic analyses, chromosomal mobilization, etc.
29
ca
CM
OM
---
-)
Cell interior
r-
A
b)
Reaction Center (RC)
-
Light-Harvesting I Complex (LI')
Light-Harvesting
II Complex (LH2)
ATPase
PufX protein
Cynochrome bacteria complex
Cytoplasmic membrane
Figure 3.4: An illustration depicting the membranes of Rb. sphaeroides. (a) Representation of the
intracytoplasmic membrane system formed in Rb. sphaeroides as a result of a reduction in the
oxygen tension33 (b) Random distribution of RC/LH1/LH2; indication of PufX inserted into LH1.
Abbreviations: OM = outer membrane; PG = peptidoglycan layer; PPS = periplasmic space; CM =
cytoplasmic membrane; p = protein; pp = porin protein; ps = phospholipid bilayer; lp = lipoprotein;
lps = lipopolysaccharide; A = fully formed invagination; B = site of initiation of membrane growth.
In 1994, Ermler et al. (1994)34 reported the 2.65 A resolution X-ray crystal structure of
bacteria photosynthetic reaction center (RC) for Rb. sphaeroides. To date, 2.2 ~ 2.3
resolution X-ray crystal structure of bacteria RC has also been detected.35 ' 36
30
A
Figure 3.5 : View of bacterial reaction center (RC). (NCBI MMDB)34
The bacterial reaction center of Rhodobacter sphaeroides is a pigment-protein complex
designed to convert optical excitation into the initial electron-transfer event in
photosynthesis. The reaction center contains three polypeptides (subunits L, M and H),
six
chlorophyll-like
pigments
(four
bacteriochlorophyll-a
(BChl-a),
two
bacteriopheophytin-a (BPheo-a) ) arranged with approximate C2 symmetry. The relative
arrangement of these pigments within the protein matrix is well known,
34,37,38
consisting
of two strongly interacting bacteriochlorophyll molecules known as the special pair (P),
two accessory bacteriochlorophylls (B), and two bacteriopheophytins (H). 39 RC also has
two ubiquinone (QA and QB) molecules, one carotenoid species and a non-heme iron
center. Some RCs contain a fourth subunit known as C (cytochrome). The C subunit is a
four-heme containing c-type cytochrome.
31
a)
Special pair (2BChl)
b)
BCh1 aL
ABChl
M-Subunit,
BChl aM
aM
ABChl aL
BPheo aM
BPheo aL
QB
QA
H-Subunit
Fe
Figure 3.6: Structure of photosynthetic reaction center of Rb. Sphaeroides. (a) Location of the bound
cyt c2 (lavender), the heme prosthetic group (turquoise), the RC L subunit (yellow), the RC M
subunit (blue), the RC H subunit (green), the RC primary donor (red), and non-heme Fe atom
(red). (b) Pigment molecules in RC (details in text).
The pseudo-C2 symmetry structure gives rise to cofactor binding in two branches
referred to as A and B sides of RC 34,'41 . Despite this apparently symmetrical structural
arrangement, the light induced vectorial electron transfer from P* to QA in RCs from
purple bacteria comprises only the chromophores on the A side including the monomeric
BChl-a (BA) and BPheo-a (HA) as intermediate redox carriers
4
, 41,42
Following optical excitation of P, either directly or through energy transfer, an electron is
transferred to H with a 3ps time constant;
42-47
this process is facilitated by the spatially
intermediate chromophore, B. Recent electronic
structure calculations treat the
photosynthetic reaction center as a supermolecule, with all of the chromophores
excitonically coupled to some extent. It is commonly accepted that the two BChl
molecules making up the special pair interact strongly. The excitonic coupling of the PL
and PM bacteriochlorophyll monomers leads to two distinct bands corresponding to
32
symmetric and antisymmetric combinations of the two monomer Qy transitions. For Rb.
sphaeroides, the lower excitonic state (antisymmetric combination termed Py-) produces
the strong, broad absorption near 870 nm. It is optical excitation of this state that results
in the initial charge separation.42-47
The near-IR
absorption spectrum
of the
photosynthetic reaction center is shown in Figure 3.7. The absorption bands due to the Qy
transitions of the bacteriopheophytins (H) and accessory bacteriochlorophylls (B) are
labeled, as are the two excitonic states of the special pair, Py- at 865 nm and Py* in the
region of 810 nm to 825 nm. The upper excitonic state, Py*, has proven to be more
problematic to characterize than Py-, with the orientation, extinction coefficient, and
energy of this state being well established only at low temperatures; the most reliable
information about Py comes from linear dichroism and hole-burning results. 48 ' 49 Linear
dichroism measurements for Rb. sphaeroides gave similar results, with P -+ Py- at 890
and P -* Py* at 810 nm. The results obtained by Breton 48 were supported by holeburning studies from Small and co-workers. 49 The hole-burning and linear dichroism
results provide fairly convincing proof that Py+ absorbs near 810 nm for Rb. sphaeroides
at 15 K. The principal reason for the lack of room-temperature information is the spectral
congestion in the (anticipated) region of P -- Py+ absorption due to overlap with the
strong B and Py~ absorption bands, making it very difficult to deconvolute individual
spectra.
33
1.L
B
Linear Absorption Spectrum
----
Laser Spectrum
*
N9
9
S
P
y
0
70*
600
800
700
900
Wavelength (nm)
Figure 3.7
Near-IR absorption spectra for the photosynthetic reaction center of Rhodobacter
sphaeroides. The three major bands are labeled as resulting from absorptions due to the
bacteriopheophytins at 760 nm (H), accessory bacteriochlorophylls at 800 nm (B), and the special
pair (Py+ and Py-) at 825 and 870, respectively. The dotted line represents a sample laser spectrum.
The center wavelength of the spectra is tunable from 750 to 850 nm.
34
4 Device Design & Fabrication
4.1 Design
The simplest model for a solid-state photosynthetic device consists of uniformly
oriented photosynthetic reaction centers deposited between two metallic contacts. After
absorption of a photon and rapid charge separation within a complex, a potential of up to
1. 1V can be developed across the metal contacts.'50
51 However,
this model of a solid-state
photosynthetic device must overcome several practical obstacles. First, the optical cross
section of a single layer of photosynthetic reaction centers may be fairly low because of
the density of proteins on the surface. Secondly, deposition of the top metallic contact
might damage the complexes as the metal molecules could denature them due to their
high velocity during evaporation. Thirdly, defects in the layer of photosynthetic reaction
centers may permit electrical shorts between the metallic contacts.
In the present design, these issues are addressed by depositing a thin (<1 OOOA) layer of
an amorphous organic semiconductor between the photosynthetic reaction centers and the
top metal contact (detailed description shown in Figure 4.1). Using a thin film of an
organic semiconductor is an established technique in photovoltaic applications 3 and as
such they can be employed as solid-state antennae for photosynthetic complexes,
resulting in enhancement of the optical absorption of the device.
35
--I
5.OeV ..
-- ev
ITO/AU
IT/A
3.1 eV
3.0eV
ROb)
a)
L4.7eV
4.3eV
/7
ETL Ag
psI
~Fe 4 S1~
II
ETL
.......
BCP
1.1eV
Ceo
5.OeV~~
ITO/Au.
ETL
4
4.3eV
Ag
Alq3
P7Q..
5.8eV
6.8eV 6.7eV
Figure 4.1: Schematic Energy Level Diagram for device design. (a) RC (b) PSI.
Device fabrication is carried out with minimum exposure to ambient light. Half inch
indium-tin oxide (ITO) coated glass slides are used as the substrate with the ITO serving
as the bottom electrode. The transparent ITO allows for a conducting substrate which lets
a chosen laser light to excite the protein layer. The substrates are cleaned using a standard
glass cleaning procedure. Reaction centers in PSI and RC have been genetically modified
such that they can be self assembled on Au. To facilitate self assembly a thin layer of Au
is deposited on ITO. After assembling reaction centers on Au, the organic semiconductor
layer and top electrode are evaporated onto the device.
4.2 Self-assembly
The self assembly process is engineered so that the protein can attach to the Au
layer by using a selected set of reagents. Oriented films of RC and PSI photosynthetic
complexes are self assembled on the ITO/Au surface as shown in Figure 4.2. The Ni252
NTA binding to the 6xHis tag on the photosynthetic complexes is used for the assembly .
36
Functionalization of ITO/Au (Figure 4.2a) is achieved by incubating the slides in a
solution of 6.1 mg/ml DTSSP (Pierce) for 10 minutes.53 Subsequently the slides are
washed with DI H20 followed by incubation of the surface in 0.33 mg/ml NTA Ligand
(Qiagen) for another 10 minutes.5 The NTA-functionalized surface is then charged with
200 mM nickel sulfate solution.
a)
Na03S
CH
##CH
0o
C
CH2
CH 2
2
-
Gold
DTSSP
3
CH 2
Ni2+
X 0
0
-Co
NAI
c)
b)
6xHis
NTA
PSI
psaD~
ps r---...- ---,
6xHis cA
--
PSI
6xHis
NTA
O--------
NTA
GOLD
GOLD
GOLD
Figure 4.2 : Self Assembly of photosynthetic complexes on ITO substrates. (a) Functionalization of
ITO/Au surface. (b) RC assembly. (c) PSI assembly.
Poly-histidine tagged RCs are then expressed and isolated from R. spheroids
strain SMpHis with the tag constructed at the C-terminal end of the RC M-subunit.55 The
expression and purification procedure has been performed as described earlier. 56 RCs are
then immobilized by incubating the functionalized ITO surface with approximately 100
37
mL of RC solution (0.2 mg/mL in 10 mM phosphate buffer pH 7.4, 0.05% LDAO) for 1
hour at 4'C in the dark (Figure 4.2b).
Native PSI complexes are isolated from spinach leaves as described earlier.57 A 6xHis tag
is then introduced to native PSI complexes in a three step process; see Figure 4.2c
1.
Gene psaD is cloned into pET-21b (Novagen) and recombinantly expressed in E.
coli to produce a 6xHistidine 58 tagged PsaD subunit of PS1.
2. Genetically modified protein (psaD*) is immobilized on the functionalized
ITO/Au surface.
3. After washing to remove excess psaD*, the surface is incubated with native PSI in
50mM Tris, 25mM NaCl, 2 M sucrose, .02% Triton X-100 for 1 hour at 4 'C in
the dark.
psaD is exchanged from the native complexes and replaced by the immobilized psaD*,
thereby immobilizing the PSI. This assembly technique is highly specific since psaD*
can only bind specifically with PSI. In principle, the capability of PSI to exchange
subunits allows this type of electronic device to be regenerated if the PSI degrades. It is
postulated that plants use similar techniques to replace photodamaged PSI. 1
After self-assembly of RCs or PSI, the substrates are washed with DI water and
dried in an N2 stream. The washing/drying is done while making sure that there are no
visible residues on the surface of the slide.
38
4.3 Evaporation
In order to self-assemble the photosynthetic reaction centers on the ITO, thermal
evaporation is used to deposit a 1 OA thick layer of Cr followed by a 40A layer of Au.
Both materials are evaporated at a rate of 0.5A/s at 3E-6 torr pressure. The Cr layer
promotes adhesion of Au to ITO and prevents it from peeling off.
Since the protein assembles on gold islands which are not continuous, an organic
layer is deposited on top to obtain a flat surface above which the top electrode layer can
be deposited. The organic layer helps in preventing a short between the two electrodes as
well as protects the protein from any damage during deposition of the top electrode. For
the device consisting of RC as the active material, the organic layer consists of 600A of
C6 0
deposited at a rate of 3A/s followed by 120A of BCP deposited at a rate of 2A/s. For
the device consisting of PS1 as the active protein, 800A of Alq 3 is deposited as the
organic protective layer. Subsequent to the organic layer deposition, 800A of silver,
which acts as the top electrode, is deposited at a rate of 3A/s. All depositions have been
carried out at < 3x 10-6 torr pressure.
39
5
Results for RC
5.1 Absorption Spectrum using UV-Vis
The methods used to process RC proteins may make it susceptible to molecular
degradation. In order to verify that the RC complex does not suffer any such degradation,
a relatively straightforward procedure of examining the absorption spectrum of the
protein complex in its buffer solution is carried out. The absorption peaks correspond to
the subunits of the complex. If the complex is not intact then the peaks should be shifted.
All the solution spectra for the RC protein were obtained using the UV-Vis spectrometer.
A comparison of Figure 5.1 to Figure 3.7 indicates that the RC in buffer is intact. The
buffer solution used is 10 mM phosphate buffer, pH 7.5 with .05 % LDAO (N,Ndimethyldodecylamine N-oxide, a surfactant).
In order to stabilize RC protein, stabilizers such as polyethylene glycol (PEG) with
sucrose, polyethyleneimine and DEAE-dextran have been added to the RC-buffer
solution. Absorption spectrum obtained using the UV-Vis (shown in Figure 5.1) showed
that none of the three buffers caused any degradation to the protein structure.
40
0.007-
0.006
0.005-
RCBuffer
RCPEGsucrose
RCpolyethyleneimine
RCDEAEdextran
0.0040
.
0.003-
0
<
0.0020.001 0.000-0.001
400
500
700
600
800
900
1000
Wavelength (nm)
Figure 5.1 RC absorption spectrum in pure buffer and with stabilizers dissolved in the buffer.
5.2 Laser Measurements
Photocurrent is measured using a laser at the peak absorption wavelength of the protein
complex. For the RC complex, a diode laser at 808nm was used since the peak absorption
is at about 800nm as seen in Figure 3.7. I-V characteristic for the RC device, in response
to the laser, is achieved by sweeping the voltage across the contacts from -lV to lV. The
experiment also can show if the laser causes any permanent changes to the device
characteristics. A control device was fabricated in the same way as the RC device as
described in Chapter 4 with no RC complexes being added.
41
On photoexcitation using the 808nm laser, the photocurrent generated in the device is
shown in Figure 5.2. Results show a pronounced difference between the dark and laser
(laser is shown on the device) I-V measurements with room lights switched off at all
times. At O.4V the difference is larger than 360nA, IDark
=
1. 5nA and Iaser= 365nfA.
INSET shows that there is no photovoltaic effect for the Dark response but there is a
Voc=-0.2V and Isc=20nA for the Laser response.
A few stabilizers were tested to examine which produced the maximum difference in
photocurrent for the Dark and Laser response. Figure 5.3(a) shows the Dark and Laser
response of the RC with DEAE-dextran. There is a difference of more than 0.4ptA
between the Dark and Laser photocurrent response at a bias of 1V. The Inset shows that
in the Dark the device does not exhibit photovoltaic effects, but for the Laser response
there is a Voc=0.04V and Isc=lOnA. Figure 5.3(b) shows the response of the control
device. The currents are about 0.6pA bigger than the RC device. However, there seems to
be no Voc or Isc for both the Dark and Laser response for this device. This experiment
demonstrates that the device shows photovoltaic effect only in the presence of RC
complexes.
42
400n
-
Zoom In
3010
300n
2XI0+ --
-
o01x10*-
200n
-
1x10 -2x10* --
a)
-
100n -
0.0
-0.2
0.2
O
Voltage (V)
0
C.
0
0Dark1
--_
Laser1
-1On
-0
-__Dark2
Laser2
-200n
-C.6
-0.4
-0.2
0.2
0.0
0.4
0.6
Voltage (V)
Figure 5.2 : Photocurrent response of pure RC device.
. , i , i ,
8x10-7
6x10
C
-
2x10-
-
,
i
Dark1
Dark2
Laser1
Laser2
-
4x10-
i
Zoom In-
0-
/-
0
-2x10
wo*-
-4x10 7 -
-IM0-
-6x104.20
-8x10
41
-3xiD010 .. 0
.6
-
-1.0
06
.10
0;
VO00egM
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Voltage (V)
43
0.4
0.6
0.8
1.0
1.2x10'
*
1.0x10 8.Oxl 0
6.0x10
-
q
4.0x10 7 -
0
2.0x10-
0
0
S
0.0--
-2.Ox 10-7
-___Dark1
Dark2
Laser1
Laser2
-4.Ox1- -6.Ox 10-7
-8.Ox1
0-1.0
-0.5
0.0
0.5
1.0
Voltage (V)
Figure 5.3 : RC with DEAE-dextran stabilizer photocurrent. (a) I-V curve of device with RC and
DEAE-dextran. Inset zooms in on the origin. (b) Control device response (device without RC).
Next we tested the stabilizer polyethyleneimine (PEI). Figure 5.4(a) shows the Dark and
Laser response of the RC with PEI device. There is a difference of more than 25pA
between the dark and laser at a bias of V. Inset shows no presence of Voc or Isc for both
Dark and Laser responses. Figure 5.4(b) shows the response of the control device. The
difference in dark and laser currents is about 10pA. This experiment demonstrates that
the difference in dark and laser currents seems to be increased by the presence of the RC
complexes.
44
.
35p
Zoom In
-
4.1-
30p -
/O
Dark
- Dark2
Laser1
Laser2
25p 20p -
-
10.
15p 0
Votage N)
.2 lop 0L
5p
-
0-
-1.0
tI
-0.5
1
1
0.0
0.5
10
0.5
1.0
Voltage (V)
35p
Dark1
Dark2
Laser1
Laser2
30p 25p -
C
20p -
:3
U)
15p -
.2
-c
0
lop 5p -
0-5p
:
-1.0
-0.5
0.0
Voltage (V)
Figure 5.4 : Photocurrent of RC device with Polyethyleneimine (PEI) stabilizer. (a) I-V curve of
device with RC and PEI. Inset zooms in on the origin. (b) Control device response (device without
RC).
45
Polyethylene glycol (PEG) and sucrose is used as a stabilizer for the RC device. Figure
5.5 shows the photoexcitation response for this device. It can be seen that the difference
between the Dark and Laser current is about 28OnA at a bias of lV, IDark
=
25nA and ILaser
= 294nA. Inset shows that the Dark current exhibits no photovoltaic effect, where as the
Laser photocurrent response does, Voc = 0.085V and Isc = 3.4nA under an excitation
intensity of 0.6 W/cm 2 at k = 800 nm. Assuming a perfectly formed RC monolayer of
density 3x10- 2 mol/cm 2 , and given an extinction coefficient of 2.9x10
5
M-cm-1,
59
we
calculate the optimum photocurrent as > 0.7 mA/cm2 . Here we have ignored possible
microcavity effects due to reflections from the ITO/Au electrode, and assumed 100%
reflection of the optical pump by the Ag cathode. Therefore, at a -IV bias, an estimated
internal quantum efficiency of 5% is achieved. A solid-state charge extraction efficiency
of 60% is obtained by comparing the solid-state external quantum efficiency of 0.01% at
an excitation intensity of 0.6 W/cm 2 at k = 800 nm to a similar wet cell with an external
quantum efficiency of 0.016% under an excitation intensity of 6mW/cm 2 at k = 800 nm.5 6
46
Zoom In
3.0x10 7 9X10
2.5x10 7
6x10
2.0x10 -
r
0
0
1.5x10
-3x10
7
0
.-
C.)
-
.
.)
.10
1.0x10 7
-0.3
-0.2
0.1
0.0
Voltage (V)
0.2
0.1
0.~3
5.0x1 0 -
0.0-
Dark
Laser
-5.0x104-8
-1.0
-0.5
0.0
0.5
-
1.0
Voltage (V)
Figure 5.5: Photocurrent of RC device with PEG and sucrose stabilizer.
5.3
Spectrum Measurements using Tunable Laser
In Figure 5.6, the activity of the RC complexes is confirmed by spectrally resolving the
short circuit current using a Ti-Sapphire CW laser tunable between X=790nm and
X=890nm. The photocurrent spectrum is compared with both the solution absorption
spectrum of the RC complexes, and a photocurrent spectrum of identical RC complexes
in an electrochemical cell reproduced from a prior research article 6 . With the exception
of a region near )= 860nm the spectra overlap closely. It is possible that the optical
absorption of the special pair at X = 860nm is influenced by its proximity (< 2 nm) to
nanoclusters of Au on the ITO electrode.
47
£
1.0
Cd
0.
a3 0.6
CL
0
0oc
o00
M-
C
-- '
(DO.
E .
locbc
0.0'-
700
I
750
800
850
I
900
950
Wavelength [nm]
Figure 5.6 : Comparison between the photocurrent spectrum of solid-state (M) and wet
electrochemical cell devices (m), and the solution absorption spectrum of the bacterial reaction
centers (-o-), demonstrates that the observed photocurrent originates in the RCs. Wet cell device
data is from a prior research article 56.
48
6 Results for PS1
6.1 Absorption Spectrum using UV-Vis
Similar procedures as described in Section 5.1 have been used to ascertain any molecular
degradation of the protein PSI. The solution spectrum of the PSI in buffer solution (50
mM tris, pH 7.8, 25 mM NaCl, 0.02% triton x-100) is obtained using UV-Vis
spectrometer. Figure 6.1 shows the solution absorption.
I
*
I
I
I
I
600
650
700
750
1.0
0.8
LM
0.6
.0
0.4
0.2
nn
500
550
Wavelength (nm)
Figure 6.1 : PSI solution spectrum. (Data courtesy Patrick Kiley)
49
800
6.2
Fluorescence Measurements
The stability of PSI through the drying process after the self assembly is examined using
low temperature fluorescence. PSI fluorescence spectrum was measured at 20K (±0.2K).
The sample is transferred into a cryogenic system and cooled to 20K. A 405 nm pump
laser was shined on the sample through one of the viewing windows. The laser was
oriented such that it would hit the glass first and then the PSI. A fiber from the
spectrometer was placed at another viewing window just in front of the PSI. A lens was
used to focus the fluorescence from the sample onto the fiber. A 530nm filter was
attached to the front of fiber detector to filter out the laser and the room light.
Spectrasense spectrometer software was used for data acquisition.
The fluorescence spectrum for PSI in the buffer solution was obtained by using a cuvette
and transferring it into the cryogenic system (shown in Figure 6.2).
50
1.00.8-
0.6S0.4U)
SO-
0
0.2-
0.0 -.
400
500
700
600
800
90
Wavelength (nm)
Figure 6.2: Fluorescence spectrum of PSI in buffer solution. (Data Courtesy Patrick Kiley)
Next the fluorescence spectrum of the PSI film was obtained. PSI in the buffer solution
was dry cast on a slide in a desiccator. This sample was then transferred into the
cryogenic system. PSI film fluorescence spectrum shown in Figure 6.3 was taken at 20K
with an integration time of 40ms. This spectrum when compared to the spectrum in
Figure 6.2, shows a lot of degradation due to the drying process.
51
I
-
I
1.0-
T=2 K12U-
0.8
U
0
0.6
0.4-
U)
I
*
-
0
0.20.0 -'-400
500
700
600
89
800
900
Wavelength (nm)
Figure 6.3: Fluorescence spectrum for PSI film. (Data courtesy Patrick Kiley)
Surfactant like peptides are employed for the stabilization of PSI. They are composed of
six hydrophobic residues joined via peptide (amide) bonds to either one or two charged
residues. The resulting hexa- or hepta- peptide therefore has a charged domain and a
hydrophobic domain, and so bears a resemblance to a typical surfactant molecule shown
in Figure 6.4.60 Like other surfactants these peptides can form discrete structures, such as
vesicles and nanotubes, in solution. They display dynamic behavior dependent upon pH,
temperature, and ion strength.
52
..-
SOnm
--
o
Figure 6.4: Peptide nanotube formation.
Figure 6.5 shows the fluorescence spectrum of the PSI film stabilized using a
combination of two peptides, A6K and V6D. This spectrum shows very little degredation
when compared to the fluorescence solution spectrum of PSI in Figure 6.2. This confirms
that peptides are able to stabilize the PSI complex during the drying process.
53
-
I
-
i
I
1.00.8S 0.6-
S0.40
0.0
400
500
600
700
800
900
Wavelength (nm)
Figure 6.5 : Fluorescence spectrum of PSI stabilized using A6K and V6D peptide. (Data Courtesy
Patrick Kiley)
6.3 Laser Measurements
For the photosynthetic complexes, the locations of their respective poly-histidine tags,
cause PSI and RC complexes to bind in opposite orientations. In the case of PSI
complexes, the special pair, P700, is oriented away from the substrate. When a
preferentially electron transporting material Alq 3 (see Figure 4.1 B) is used as a protective
overlayer, holes are trapped on P700 and the device acts as a photodetector. Hence no
Voc or Isc can be seen in the I-V characteristics.
54
The current-voltage characteristics of the PSI photodetector are shown in Figure 6.6. The
excitation source here is a k= 680 nm laser with an intensity of 0.5 W/cm 2 . At a bias of
-4V, the maximum external quantum efficiency is 3x10-6 %. Note that Alq 3 is transparent
at this wavelength. To estimate the limiting performance of a PSI-based device, we
assume that including external light harvesting complexes, each P700 dimer may be
coupled to up to 100 chlorophylls with an approximate extinction coefficient of < lx107
cm-1 per molar concentration of P700. Assuming a perfectly formed PSI monolayer of
density 4x10-1 3 mol/cm 2 , we calculate the optimum external quantum efficiency for aflat
PSI interfacial layer as 0.8%. Here we have ignored possible microcavity effects due to
reflections from the ITO/Au electrode, and assumed 100% reflection of the optical pump
by the Ag cathode. Thus, at a bias of +1V the device in Figure 6.6 generates
approximately 3x10-4% of the photocurrent of an ideal flat monolayer of PSI coupled to
antenna complexes. As in the RC-based device, performance is probably limited by the
density of PSI particles on the Au/ITO surface.
55
0.0
-
-5.0x1 0
C
-1.0x10 .,
0
-1.5x10 -
Dark
---
-2.0x10-
-
Laser
I*
I'
-2.5x 0*-
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
Voltage (V)
1'1'
1'1*1.
0-
9
-1x10 -
C.,
0
0
-C
9
-2x10-
9
-3x10 -
-4x10
--
9
'I
I
-3.0
-2.5
-2.0
-1.5
-0.5
-1.0
Dark1
Laser1
0.0
0.5
1.0
Voltage (V)
Figure 6.6 : Photocurrent characteristic of PSI. (a) Exchanged PSI with peptide. (b) Exchanged PSI
without peptide.
56
7 Conclusion
We have demonstrated here that photosynthetic complexes may be integrated in solidstate devices. These proteins are known to sustain large open circuit voltages of 1.LV
without significant electron-hole recombination and they may be self-assembled into an
insulating membrane. Using these properties to our advantage, we have designed
photosynthetic organic solar cells. One of the challenges working with these materials is
that they are very susceptible to degradation on changing environmental conditions.
Using specific buffers and peptides such as PEG and sucrose and A6K and V6D
respectively, we have demonstrated that these proteins can be used in their non-native
state.
For the RC device, we attained an efficiency of 0.01% with an excitation energy of
0.6W/cm 2 at X=800nm. For the PSI device, we attained an efficiency of 3x10-6 % with an
excitation energy of 0.5W/cm 2 at k=680nm.
The advantages for these photosynthetic complexes originate in their molecular scale
design and comparable molecular circuitry is currently beyond the capability of
alternative technologies. Since we have taken the first step of demonstrating device
operation, albeit with low quantum efficiency, our next task would be to improve device
design and achieve quantum efficiencies comparable to traditional solar cells. Several
future directions possible are described below; many of which are currently being
examined.
57
For the PSI devices, we would like to explore the photovoltaic effect in a manner similar
to the effect observed in RC devices. In order to achieve this goal, CuPc can be used as
the protective organic layer instead of Alq 3. One of the challenges with exploring the
photovoltaic effect in PSI is that the absorption spectrum of the CuPc overlaps with the
PSI absorption spectrum making it difficult to establish whether the photocurrent
detected is due to PSI or CuPc.
Currently the PSI is assembled such that the electron is ejected towards the bottom
contact. Another method to explore photovoltaic effects would be to engineer the
assembly process such the orientation of the PSI is reversed and the electron is ejected
towards the top contact. This procedure has the advantage that Alq 3 can still be used as
the organic protective layer.
Extensive surface roughness studies need to be carried out. The surface roughness at each
of the interface is expected to have significant effect on the open circuit voltage. One of
the future goals would be to optimize the surface roughness. For example when
assembling the proteins on top of the gold surface, a smooth surface is not desirable.
Increase in the surface area of the exciton dissociation interface can be achieved by either
roughening the surface 3or by stacking planar interfaces.61 On the other hand the interface
between the organic layer and the metal, a smooth surface would be desirable as it would
decrease the number of trap states. Another attempt would be to use stabilizing peptides
to increase protein lifetime. Such peptides have been shown to increase the lifetime in
DNA stabilization.
58
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