Towards High Efficiency Dye Sensitized Solar Cells with a Photoanode Fabricated Via
Simple Water Based Formulation
Robert Lupitskyy1,a, Venkat Kalyan Vendra2,a, Jacek Jasinski,1 Delaina A. Amos, 1,2 Mahendra K.
Sunkara,1,2 Thad Druffel1,*
1
Conn Center for Renewable Energy Research, University of Louisville, Louisville KY
Department of Chemical Engineering, University of Louisville, Louisville KY
a
Authors contributed equally towards this work
*Corresponding author email: thad.druffel@louisville.edu
2
Supplementary Information
Figure S1 shows the chemical structure of the titanium bis(ammonium lactato) dihydroxide
(TALH) in its ionized form. Bidentate lactato ligands, with strong affinity to Ti atom, make this
molecule resistant to hydrolysis, compared to monodentate ligands such as alkoxides.
Figure S1. Structural formula of TALH
S1
The preparation of a typical paste is shown schematically in figure S2a, compared to the TALH
formulation discussed in this paper in figure S2b. The preparation of the formulation discussed
here takes approximately 15-20 minutes with significantly fewer processing steps.
Figure S2. Comparison between: (A) conventional ethylcellulose-based paste preparation
(adapted from Ito 2007)1 and (B) our aqueous formulation.
S2
Figure S3 are the lower magnification SEM micrographs that complement those in figure 4 and
represent the change in porosity as the concentration of TALH is varied.
Figure S3. SEM images of sintered mesoporous TiO2 films prepared from formulations
containing no TALH (A), 0.14 M TALH (B), 0.43 M TALH (C), and 1.30 M TALH (D).
S3
X-ray photoelectron spectroscopy (XPS) was used to study chemical composition in a series of
P25-TAHL films with varying TAHL concentration. The films were deposited on FTO glass
slides and sintered at 500oC. XPS spectra were measured using a VG Scientific MultiLab 3000
ultra-high vacuum surface analysis system equipped with a dual-anode (Mg/Al) X-ray source
and a CLAM4 hemispherical electron energy analyzer. The measurements were conducted at the
base chamber pressure in the 10-9 Torr range and non-monochromatized Mg K X-ray beam
(h 1253.6 eV) was used for the excitation of photoelectrons. The XPS spectra reported here
were collected at an electron emission angle of 54.7o relative to the surface normal. For the
binding energy calibration, the position of the Sn3d5/2 peak originating from FTO was set to
486.2 eV.
Survey XPS spectra of the sintered P25-TALH films with varying TALH concentration are
shown in Figure S4A. Inset in the figure shows nitrogen region, indicating that no nitrogen from
TALH is present in the films after thermal treatment.
XPS was also used to study changes in Ti oxidation states as a function of TALH concentration.
High-resolution Ti2p XPS spectra acquired for all four P25-TAHL films used in this study are
shown in Figure S4B. For the ease of comparison, background was subtracted from each of the
spectrum and they were normalized at the Ti2p3/2 peak. No significant difference was observed
between different films. All Ti2p spectra exhibited a single component and its binding energy of
458.4 eV agreed with the value reported for Ti4+.2 This indicates that the trap state population
does not change with TALH concentration.
S4
6
6x10
A
6
5x10
Sn3d
No TALH
P1
P3
0.14 MP6
6
4x10
6
Intensity (cps)
5x10
6
6
3x10
6
2x10
1.30 M
Ti2p
no N1s
6
1x10
4x10
Intensity (cps)
P7
0.43 M
0
500
C1s
450
400
350
300
250
BE (eV)
6
3x10
6
2x10
No
P1TALH
0.14
P3 M
0.43
P6 M
6
1x10
P7 M
1.30
0
1000
800
600
400
200
0
BE (eV)
Theoretical evaluation of the effect of TALH on the porosity of the TiO2 films
We can theoretically estimate the void fraction (φ) that will be created in the mesoporous P25
film upon thermal decomposition of TALH during sintering as a function of TALH
concentration in the formulation using Eq.1:
𝜑=
𝑉𝑇 − 𝑉𝐴
,
𝑉𝑃 + 𝑉𝑇
(𝐸𝑞. 1)
where 𝑉𝑇 , 𝑉𝐴 , and 𝑉𝑃 is volume of TALH in the formulation, volume of anatase formed upon
thermal decomposition of TALH, and volume of P25, respectively. Taking into account that P25
consists of 80% anatase form and 20% rutile form, 𝑉𝑃 can be calculated using Eq.2:
𝑉𝑃 = 𝜐𝑃 (
𝑀𝑤𝑃 0.8 𝑀𝑤𝑃 0.2
+
),
𝜌𝐴
𝜌𝑅
(𝐸𝑞. 2)
where 𝜐𝑃 is the amount of P25 (based on TiO2 molecules, not particles) in the formulation, 𝑀𝑤𝑃
is molecular weight of titanium oxide (79.866 g/mol), 𝜌𝐴 and 𝜌𝑅 are densities of anatase (3.895
g/cm3) and rutile (4.274 g/cm3) respectively. Volume of TALH is calculated using Eq.3:
𝑉𝑇 =
𝜐𝑇 𝑀𝑤𝑇
,
𝜌𝑇
S5
(𝐸𝑞. 3)
where 𝜐𝑇 is the amount of TALH in the formulation, 𝑀𝑤𝑇 is molecular weight of TALH (294.08
g/mol), and 𝜌𝑇 is density of TALH (1.22 g/cm3). The volume of anatase formed upon thermal
decomposition of TALH, 𝑉𝐴 , is calculated as follows:
𝑉𝐴 =
𝜐𝐴 𝑀𝑤𝑃
,
𝜌𝐴
(𝐸𝑞. 4)
where 𝜐𝐴 is the amount of anatase formed upon thermal decomposition of TALH. Since 1 mole
of TALH theoretically yields 1 mole of TiO2, the term 𝜐𝐴 in the above equation can be
substituted with 𝜐𝑇 :
𝑉𝐴 =
𝜐𝑇 𝑀𝑤𝑃
,
𝜌𝐴
(𝐸𝑞. 4𝑎)
Using the above equations, void fractions of the P25 films made from formulations containing
different amounts of TALH were calculated and summarized in the Table S1. From the data we
can see that the void fraction formed using formulation with the lowest TALH concentration is
less than the theoretical void fraction of the P25 film with no TALH (for simplicity, we assume
that our P25 nanoparticle film is a random close packing (RCP) of uniform spheres; the void
fraction of such arrangement (φRCP) was experimentally found to be 0.36).3
Table S1. Void fraction φ of the empty space formed upon thermal decomposition of TALH
during sintering of P25 films made from formulations at different TALH concentration
φ
0.16
0.37
0.58
0.73
TALH, mol/l
0.04
0.14
0.43
1.30
Thus, this formulation has little effect on porosity of the P25 film. Formulation containing 0.43
M TALH gives void fraction which is within the range of reported optimal porosity (50-60%).4
This correlates with the best DSC performance obtained using this formulation. Void fraction
formed from the formulation with the highest TALH concentration is higher than optimal, which
may explain the decrease in DSC performance due to fewer interconnections between the
particles and decrease of charge collection efficiency.
References
S6
1.
2.
3.
4.
S. Ito, P. Chen, P. Comte, M. K. Nazeeruddin, P. Liska, P. Péchy and M. Grätzel, Progress in
photovoltaics: research and applications, 2007, 15, 603-612.
P. M. Kumar, S. Badrinarayanan and M. Sastry, Thin Solid Films, 2000, 358, 122-130.
G. D. Scott and D. M. Kilgour, J. Phys. D: Appl. Phys., 1969, 2, 863-866.
K. D. Benkstein, N. Kopidakis, J. Van de Lagemaat and A. Frank, The Journal of Physical Chemistry
B, 2003, 107, 7759-7767.
S7