PREPARATION AND CHARACTERISTIC CONTROL OF CONDUCTING POLYMER/METAL OXIDE NANO-HYBRID FILMS FOR SOLAR ENERGY CONVERSION

We review our recent investigation of photoinduced polymerization of thiophene inside TiO2 nanopores to form nanostructured polythiophene/TiO2 heterojunction films and the related kinetic studies. The nanohybrid film possesses dense heterojunction and electronic connection within the TiO2 nanoporous domain. Photo-polymerization proceeded in 3 stages, (i) photoexcitation of bithiophene covalently attached to the TiO2 surface, (ii) an electron injection reaction from the surface attached thiophene to the TiO2 and (iii) an electron transfer from a thiophene reactant in an electrolyte to the surface attached oxidized bithiophene. The nanohybrid film was applied to a sensitized-type photoelectrochemical solar cell, substantiating direct application of the nanohybrid film to electronic devices. Despite increase in light harvesting efficiency, wavelength dependent incident photon-to-current conversion efficiency decreased with the light irradiated polymerization time. In order to identify a factor controlling photocurrent efficiency, kinetic studies at TiO2/bithiophene/electrolyte interfaces were conducted, and their parameters to the solar cell functions were related. Comparison of emission studies between the bithiophene adsorbed TiO2 and Al2O3 revealed the electron injection from the excited bithiophene into the TiO2 with the efficiency of nearly 100 %. The charge recombination between the bithiophene cation and the electron in the TiO2 appeared to be fast with a half decay time of 70 mu s in comparison to the ruthenium dye sensitized TiO2 film (similar to 1 ms). The bithiophene regeneration kinetics was slightly faster, clarifying the inferior photocurrent performance.