Enhanced charge extraction in organic solar cells through electron accumulation effects induced by metal nanoparticles

Metal nanoparticles (NPs) have been used to enhance the performance of thin-film devices such as organic photovoltaics. In this paper, we propose and demonstrate electron extraction enhancement induced by charge accumulation effects of metal NPs. The metal NPs (Au and Ag NPs) are embedded in a titanium oxide (TiO2) layer, functioning as a highly efficient transport layer for improving the performances of inverted organic solar cells (OSCs), which leads to significantly increased photocurrent and power conversion efficiency reaching 8.20%. Importantly, our results show that the optical plasmonic effect of metal NPs (Au NPs and Ag NPs) in the electron transport layer is a minor factor in improving the OSCs' efficiency. Instead, the charge extraction enhancement under solar illumination can be explained by the transfer of UV-excited electrons from the TiO2 electron transport layer to metal NPs and the enhanced accumulation of the electrons in metal NPs-TiO2 composites. The electron accumulation reduces the work function of the electron transport composite layer after UV illumination. The redistribution of charges in the UV-irradiated metal NPs-TiO2 system can assist the charge extraction in OSCs. Multiphysics study is also conducted to explain the effects of the charge accumulation on device performances (i.e. improving short-circuit current without degrading the open-circuit voltage). Consequently, by incorporation of metal NPs, our experimental and theoretical results show that the NPs-TiO2 transport layer, in which case the effect of doping is different from the conventional doping effects in semiconductors, exhibits very good charge extraction and collection at the electrode for efficient organic optoelectronic devices.