Optical and electrochemical characterization of poly(3-undecyl-2,2′-bithiophene) in thin film solid state TiO2 photovoltaic solar cells

A poly(3-undecyl-2,2′-bithiophene) in thin film TiO2 photovoltaic has been characterized electrochemically and optically. An oxidation potential and onset of oxidation of the polythiophene were determined by cyclic voltammetry to be 1.0 and 0.8 V (versus Ag|AgCl|sat KCl), respectively. From the onset oxidation, the HOMO has been estimated to be at 1.0 V versus NHE. A bandgap of 2.04 eV (610 nm) was determined from UV-Vis spectroscopy and yielded a LUMO of −1.04 V versus NHE. The energetics relative to TiO2 make electron injection possible under the absorption of light. Polymer/TiO2 solar cells were constructed yielding Voc reaching 0.92 V and Jsc approaching 100 μA cm−2 under 100 mW cm−2 light intensity. The solar cells showed a logarithmic increase in Voc under increasing light intensity while the Jsc showed a sublinear increase indicating the presence of trap states. Photocurrent enhancement is observed over time under illumination possibly due to both an annealing process occurring between the metal oxide and polymer and a possible increase of polymer photoconductivity. The diode quality factor was as good as 2.9 with a J0 of 2 nA cm−2. A p–n junction is thought to be formed due to the energetics, type of materials (n-type TiO2 and p-type polythiophene) present, and the formation of a depletion region, which is most likely between the TiO2/P3UBT interface. It was found that the sol–gel derived TiO2 used in this study was electrically blocking from cyclic voltammetric measurements and hence dense and non-porous. This suggests that such films can be used as a dense blocking layer in solid state nanocrystalline solar cells.