Electrophoretic deposition of TiO 2 /Nb 2 O 5 composite thin films for photovoltaic applications

1. Electrophoretic deposition of TiO2/Nb2O5 composite thin films for photovoltaic applications J. N. Nguu, B. O. Aduda, F. W. Nyongesa, & R. J. Musembi Department of Physics, School of Physical Sciences, University of Nairobi, P. O. Box 30197-00100, Nairobi Corresponding author; J. N. Nguu, jnguu@students.uonbi.ac.ke or jnguu@daystar.ac.ke Poster presented at the International Conference on Solar Energy for World Peace, held in Istanbul, Turkey (17TH -19TH AUG 2013) 1. Introduction Numerous studies have dealt with development and application of renewable energy technologies particularly Dye sensitized solar cells (DSSC) [1,2] to mitigate against climatic change and possible depletion of fossil fuels. The Challenge with DSSC has been to reduce recombination effects in order to improve efficiency which currently stands at 11.1% [2]. Research efforts are geared towards modification of photoelectrode through use of large band gap semiconductors and through use of composites [3] In this research, we explore the use of composite material in deposition of TiO2/Nb2O5 composite electrode fabricated using Electrophoretic deposition (EPD) technique which is inexpensive with short deposition times [4,5]. Visual inspection and SEM images of the composite electrode thin films showed that porous films of high quality with well controlled morphology were deposited using the EPD technique 3. Results and discussion The % transmittance highest for a) time= 90s, b) concentration = 0.01g/40ml, and c) applied DC voltage=35V (fig 3). These values produced best quality porous films Transmittance useful in testing the transparency of the films to allow harvesting of photons for charge generation in DSSC. Transmittance decreased with increased voltage, concentration and deposition time as shown in fig. 3 b a c Figure 5 XRD spectra of TiO2/Nb2O5composite thin films annealed at 4000C for 30 minutes under atmospheric conditions Figure 2 Transmittance spectra for varied (a) deposition time (b) concentration and (c) applied voltages. • The XRD micrographs confirmed that TiO2 and Nb2O5 particles were present in the composite film. • Dominant peak of TiO2 was at 2-theta of 25.5 deg while that of Nb2O5 was at 26.5 deg. However the peak (counts=1600) of TiO2 was longer than that of Nb2O5 (900 counts) which translates to ratio 1.78 to 1. • The length of peaks corresponds to degree of crystallinity [8]. It follows therefore that the crystallinity of TiO2 is higher than that of Nb2O5 in the composite films. 2. Materials and methods b a c Figure 1 A schematic drawing of the EPD setup showing TiO2/Nb2O5/propan-2-ol suspension after [6]. Figure 3 Variation of Transmittance (at 1508nm) with (a) applied voltage, (b) concentration, and (c) deposition time. 4. Conclusions This work shows that TiO2/Nb2O5 composite thin films can be fabricated by EPD technique EPD process parameters which include concentration, applied voltage and deposition time, were successfully optimized in this study. Further work is needed to evaluate the potential of TiO2/Nb2O5 composite films deposited by EPD for solar cell applications • MATERIALS USED: • TiO2 nanopowder (Ca No. 13463-67-7 Aldrich), • Nb2O5 nanopowder (Cas No. 1313-96-8 Acros Organics), • Mg(NO3)2.6H2O, 99.9%, Aldrich), and • propan-2-ol or Isopropyl alcohol: IPA, (Scharlau chemie) . • Fluorine doped tin oxide (FTO) coated glass substrates (16mmx25mmx1mm) (Pilkington, Hartford Glass Co. Inc., USA) having sheet resistances of 8Ω/square were used as electrodes in EPD setup. • Key EPD deposition (process) parameters namely applied DC electric field (V/l), deposition time, solid concentration in suspension, were optimized by visual inspection of deposited films, by UV-Vis-NIR spectrophotometer spectra (fig 2), by SEM images (fig 4) and by XRD (fig 5). • Method: 40mL of Propan-2-ol in a Pyrex beaker; solid concentration varied : 0.01 to 0.3g, deposition time: 60 to 180 s, DC Voltage: 25 to 60V and (FTO) electrode separation=12mm in accordance with Hamaker’s equation [7]. • (1) • Where the deposit yield (m) is a function of process-related parameters: concentration (C), electric field (E=V/L), and deposition time (t). • (2) Table 1. Optimized electrophoretic deposition parameters Concentration of Applied DC voltage Distance between Deposition TiO2 & Nb2O5 across electrodes electrode time 0.01g/40mL (=0.25g/L) 35V12 (mm) 90 sec • References • B. O’Regan, and M. Gratzel, Nature, vol. 353, pp. 737–740, 1991. • Chiba, et al., Japanese Journal of Applied Physics, vol. 45 (25), pp. L638-640, 2006 • A. Taotao, Journal of Wuhan University of Technology, vol. 24 (5), pp. 732-735, 2009. • J-H. Yum, S-S. Kim, D-Y. Kim, and Y-E. Sung, J. Photochemistry and Photobiology A: Chemistry, vol. 173, pp. 1-6, 2005. • W. Jarernboon, et al., Thin Solid Films (2009). DOI: 10.1016/j.tsf.2009.02.129. • P. Sarkar, and P. S. Nicholson, J. Am. Ceram. Soc. vol. 79, pp. 1987–2002, 1996. DOI:10.1111/j.1151- 2916.1996.tb08929.x • L. Besra, and M. Liu, Progress in materials Science, vol. 52, pp 1-61, 2007. • Chang, et al., Materials Transactions, vol. 50, (12), pp. 2879-2884, 2009. Figure 4 SEM image of thin films of a) Nb2O5, b) TiO2, and c) composite of TiO2/Nb2O5 electrophoretically deposited with 0.01g/40mL, 35V, and 90s at magnification of 200K Acknowledgement: The financial assistance to purchase chemicals used in research was provided by Daystar University, Kenya.