Characterization of nanocrystalline and thin film TiO2 solar cells with poly(3-undecyl-2,2′-bithiophene) as a sensitizer and hold conductor

We report on the use of poly(3-undecyl-2,2'-bithiophene) and three different types of TiO2 film layers to determine the dependence of photovoltaic performance on the morphology of the TiO2 films. It was observed that the TiO2 film morphology plays an important role in the performance of photovoltaic solar cells with polythiophene used as both sensitizer and hole conductor. The polymer was tested on a flat TiO2 layer made from a sol-gel process, a larger surface area TiO2 layer derived from an aqueous TiF4 solution, and a nanocrystalline thin TiO2 film. We observe over twice the improvement in short circuit current density in the nanocrystalline cells (j(sc) = 233.6 muA cm(-2)) over that of the flat titania cells (89.8 muA cm(-2)). The best photocurrent density (448 muA cm(-2)) performance was from the TiF4 derived titania based cells when using the polymer, It was noted that soaking the nanocrystalline and TiF4 cells in the polymer for at least 24 h improves their overall performance, while no noticeable improvement is evident for the flat sol-gel derived cells. We explain our results by proposing that pore filling is likely to be easier in the TiF4 derived TiO2 due to larger pore sizes. Multiple reflections of light may also play an important role in the TiF4 titania films by enhancing the amount of light absorbed by the polymer. These observations suggest that the relative size of the sensitizer molecule and the pores of nanocrystalline films may be a critical factor to consider in designing photovoltaic devices such as solar cells based on nanoporous materials. (C) 2002 Elsevier Science B.V. All rights reserved.