Photoinduced Formation of Polythiophene/TiO<sub>2</sub> Nanohybrid Heterojunction Films for Solar Cell Applications

Formation of nanostructured polythiophene/TiO<sub>2</sub> heterojunction films, using photoinduced polymerization of thiophene inside TiO<sub>2</sub> nanopores, was investigated. The resultant film possesses nanohybridization and electronic connection within the TiO<sub>2</sub> nanoporous domain. Photopolymerization proceeded in three stages:  (i) photoexcitation of bithiophene covalently attached to the TiO<sub>2</sub> surface, (ii) an electron injection reaction from the surface attached thiophene to the TiO<sub>2</sub>, and (iii) an electron transfer from a thiophene reactant in an electrolyte to the surface-attached bithiophene. Initial rapid photopolymerization and subsequent slow polymer growth were explained by analysis of a series of experiments, e.g., with respect to light irradiation time, applied bias, electrolyte types, thiophene reactant type, and their morphology. Electrochemical measurements for the bithiophene adsorbed on TiO<sub>2</sub> revealed a wide distribution of redox potentials. This was explained by influence of the local electric field on the TiO<sub>2</sub> surface in addition to strong interaction between the surface-bound bithiophene and the TiO<sub>2</sub>. The nanohybrid film was applied to a sensitized-type photoelectrochemical solar cell, substantiating direct application of the nanohybrid film to electronic devices. The solar cell performance was closely associated with the interfacial structure in the nanohybrid film and the photopolymerization degree.