Effect of photo- and thermo-oxidative degradation on the performance of hybrid photovoltaic cells with a fluorene-based copolymer and nanocrystalline TiO2

Abstract

This work proposes a new fluorene-based copolymer containing a spiroanthracenefluorene unit, poly[10,10-bis(2-ethylhexyl)-10H-spiro(anthracene-9,9′-fluorene)-2′,7′-diyl-co-bithiophene] (SFT2), for hole transporting materials in photovoltaic (PV) cells consisting of conjugated polymers and nanocrystalline TiO₂. This polymer was synthesized to avoid photo- and thermo-oxidative degradation of poly[9,9′-dioctyl-fluorene-co-bithiophene] (F8T2) used in hybrid PV cells. The occurrence of oxidative degradation after irradiation with simulated sunlight or severe heating over the glass transition temperature in air on F8T2 and SFT2 films was confirmed by an increase of low energy emission at 600 nm in the photoluminescence emission spectra—originating from the generation of a keto defect site in the C-9 position of the fluorene units. In conclusion, we found that the defects play a vital role in degradation of the device performance of PV cells, especially in the decline of short-circuit current density (J_sc), consisting of fluorene-based polymers.