Direct observation of UV-induced charge accumulation in inverted-type polymer solar cells with a TiO_x layer: Microscopic elucidation of the light-soaking phenomenon

The mechanism of light-soaking phenomenon in inverted-type organic solar cells (IOSCs) with a structure of indium-tin-oxide/TiO_x/P3HT:PCBM/Au was studied by electron spin resonance (ESR) spectroscopy. Charge accumulation in the cell during UV-light irradiation was observed using ESR, which was clearly correlated with the light-soaking phenomenon. The origin of the charge accumulation is clarified as holes that are deeply trapped at p-type P3HT polymer-chain ends with bromine after hole transfer from the band excitation in the TiO_x layer. The holes are considered to be electrostatically attracted to trapped electrons in the TiO_x layer after the band excitation. These accumulated charges are the origin of the light-soaking phenomenon. Our results strongly suggest that passivation of the residual OH groups in the TiO_x layer is needed to avoid the light-soaking phenomenon by preventing electron trappings, a step that is indispensable in the operation of highly stable IOSCs without UV-light irradiation based on a low-cost and low-temperature device fabrication process using flexible plastic substrates.