Polarization suppresses local photocurrent in triple-cation mixed-halide perovskite

Triple-cation mixed-halide perovskite Cs-0.05(FA(0.85)MA(0.15))(0.95)Pb(I0.85Br0.15)(3) (CsFAMA) has emerged as one of the most promising candidates for future high-efficiency solar cells. Ferroelectricity has been recognized as a critical issue in perovskite photovoltaics, although its existence and influence on photovoltaic performance remain controversial. We verify the ferroelectric polarization and reveal that it can suppress local photocurrent in CsFAMA through photoconductive atomic force microscopy (pc-AFM) and piezoresponse force microscopy (PFM). The time evolution of pc-AFM and PFM mappings first demonstrates that the photocurrents decreased with increasing electromechanical responses. Systematic characterization based on the first and second harmonic point wise measurement as well as mappings shows that piezoelectricity primarily contributes to the measured electromechanical responses, while the ionic activity takes a back seat. Clear hysteresis loops and relaxation behaviors further confirm that the piezoresponse originates from ferroelectric polarization. Finally, based on temperature-dependent studies, we conclude that it is the ferroelectric polarization that weakens local photocurrent. This work provides insights into optimizing the photovoltaic performance of triple-cation mixed-halide perovskite solar cells.