Interfacial properties of photovoltaic TiO2/dye/PEDOT-PSS heterojunctions

Systems comprising a dense TiO2 film electrode, a ruthenium polypyridine dye and a PEDOT-PSS (poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate)) film were prepd. The heterojunctions have photovoltaic properties, with the dye absorbing the light, the TiO2 acting as an electron conducting material and PEDOT-PSS acting as a hole transport material. Dyes were used to study their influence on the photocurrent and the photovoltage characteristics of the heterojunction. These results were compared to a photoelectrochem. system in which the PEDOT-PSS was replaced by a liq. electrolyte contg. triiodide/iodide redox-couple. Photoelectron spectroscopy (PES) was used to monitor the interfacial properties of the heterojunction and the study points out effects of importance when assembling the materials together to a functional unit. Specifically, the interaction with the dye clearly affects the structure of PEDOT-PSS, both with respect to the surface compn. of PSS relative to PEDOT and with respect to the chem. state of the sulfur in the polymers. Also, a comparison of the Ru3d and the valence band spectra of the two different interfaces (dye/TiO2 and dye/PEDOT-PSS) indicates that the energy level structure of the dyes compared to the substrate is different for the two surfaces. Thus, in the combined energy level picture under dark conditions, the energy levels in TiO2 relative to the energy levels in PEDOT-PSS depend on the dye.