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 Kļ” X-ray beam (hļ®ļ ļ» 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: š= šš − šš“ , šš + šš (šøš. 1) where šš , šš“ , and šš 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, šš can be calculated using Eq.2: šš = šš ( šš¤š 0.8 šš¤š 0.2 + ), šš“ šš (šøš. 2) where šš is the amount of P25 (based on TiO2 molecules, not particles) in the formulation, šš¤š is molecular weight of titanium oxide (79.866 g/mol), šš“ and šš are densities of anatase (3.895 g/cm3) and rutile (4.274 g/cm3) respectively. Volume of TALH is calculated using Eq.3: šš = šš šš¤š , šš S5 (šøš. 3) where šš is the amount of TALH in the formulation, šš¤š is molecular weight of TALH (294.08 g/mol), and šš is density of TALH (1.22 g/cm3). The volume of anatase formed upon thermal decomposition of TALH, šš“ , is calculated as follows: šš“ = šš“ šš¤š , šš“ (šøš. 4) where šš“ is the amount of anatase formed upon thermal decomposition of TALH. Since 1 mole of TALH theoretically yields 1 mole of TiO2, the term šš“ in the above equation can be substituted with šš : šš“ = šš šš¤š , šš“ (šøš. 4š) 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