Conduction Band Edge of TiO 2 -SnO 2 Solid Mixtures Tuning for Photoelctrochemical Applications
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1171-S05-04
Conduction Band Edge of (Ti,Sn)O2 Solid Mixtures Tuning for Photoelectrochemical Applications J. Simiyu*, B. O. Aduda and J. M. Mwabora, 1
University of Nairobi, Dept. of Physics, P.O. Box 30197 - 00100, Nairobi, Kenya
ABSTRACT We report investigation of effect of conduction band edge on the dye injection and transport by preparation of (Ti,Sn)O2 solid mixtures in ratios of 80:20 and 90:10 as possible applications in dye sensitized solar cells. SEM micrographs showed highly porous with nanometer sized particles of around 6 - 10µm diameter. X-ray diffraction patterns showed strong TiO2 anatase peaks with crystal orientation directions (101) being the strongest in both the solid mixtures and in pure TiO2. XPS studies have shown an apparent chemical shift for Ti 2p and O1s core level spectra with an energy difference between the unmodified and the solid mixture being 0.65eV. Initial I-V studies have shown high open circuit potential (Voc) but low short circuit photocurrent, showing a possible unfavorable band edge shift between the semiconductor and the dye LUMO level. INTRODUCTION Photoelectrochemical solar cells with wide-band-gap oxide semiconductors have received much attention since the development of dye-sensitized solar cell with porous TiO2 thin film by Gratzel’s group [1, 2]. The chemical and physical processes involved in the operation of these cells take place in two-phase system consisting of porous TiO2 film and I3-/I- redox electrolyte. Charge injection from the photoexcited dye and regeneration of the dye by electron transfer from I- lead to transport of electrons in the TiO2 as well as transport of I3- and I- ions in the electrolyte with electron transfer from I- to the oxidized dye and regeneration of I- from I3- at the counter electrode linking the two transport processes. To achieve a favorable electron injection efficiency requires proper matching of the dye molecule’s LUMO (Lowest Unoccupied Molecular Orbital) level (in excited state) with the conduction band of the semiconductor. Various approaches have been applied, among them using dye molecules that have high LUMO levels in excited state, modifying the conduction band edge of the semiconductor to either lower or raise it [3 - 6] and using redox couples with high molecular orbital levels while maintaining the same band edge for the semiconductor. These studies have been done extensively on single semiconductors like TiO2, and to lesser extent ZnO and SnO2 [7, 8]. Recently dye sensitized nanoporous semiconductor materials comprising more than one material have been studied as a possible way to improve on performance [9 – 12]. Tennakone et al. [13] reported the suppression of charge recombination for the mixture (referred to as composite) of SnO2 with small crystalline size of 15nm and ZnO with large crystalline size of 2 mm. Tai et al [14 - 16 ] studied widely SnO2-TiO2 coupled and composite solar cells using various sensitization dyes. They have reported higher values of Incident Photon to Current
conversion Efficiency (IPCE) in
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