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 (∼100 μs–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 μA 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.