Theoretical analysis on effect of band offsets in perovskite solar cells

The effect of band offsets in CH3NH3PbI3-xClx perovskite-based solar cells with planar junction configuration was analyzed using one-dimensional device simulator. As widely known in thin-film compound solar cells, the band offset between buffer/absorber layers is a decisive factor for carrier recombination at the interface, determining open-circuit voltage (Voc). In this study, the impact of two kinds of band offsets, i.e., the conduction band offset of buffer (or blocking layer)/absorber layers and the valence band offset of absorber/hole transport material (HTM) were examined. When the conduction band of the buffer was lower than that of the absorber, the interface recombination became prominent and Voc decreased. In contrast, when the conduction band of the buffer was higher than that of the absorber by more than 0.3 eV, the collection of photo-generated carriers, i.e. electron in this case, was impeded by the spike formed by the conduction band offset. Thus, the optimum position of the conduction band of the buffer was 0.0~0.3 eV higher than that of the absorber. Also, the optimum position of the valence band of the HTM was derived to be 0.0~0.2 eV lower than that of the absorber. These findings will be useful for new material choice and optimization of buffers and HTMs.