Hybrid nanocrystalline TiO2 solar cells with a fluorene-thiophene copolymer as a sensitizer and hole conductor

We report the effects of layer thickness, interface morphology, top contact, and polymer-metal combination on the performance of photovoltaic devices consisting of a fluorene-bithiophene copolymer and nanocrystalline TiO2. Efficient photoinduced charge transfer is observed in this system, while charge recombination is relatively slow (similar to100 mus-10 ms). External quantum efficiencies of 13% and monochromatic power conversion efficiencies of 1.4% at a wavelength of 440 nm are achieved in the best device reported here. The device produced an open-circuit voltage of 0.92 V, short-circuit current density of about 400 muA cm(-2), and a fill factor of 0.44 under simulated air mass 1.5 illumination. We find that the short-circuit current density and the fill factor increase with decreasing polymer thickness. We propose that the performance of the indium tin oxide/TiO2/polymer/metal devices is limited by the energy step at the polymer/metal interface and we investigate this situation using an alternative fluorene-based polymer and different top contact metals. (C) 2004 American Institute of Physics.