Carrier-Activated Polarization In Organometal Halide Perovskites

Organometal halide perovskite solar cells exhibit a strong polarization effect under light illumination. This unique property, although widely observed, has not been well understood. In this work, we carried out a systematic investigation on this phenomenon by varying the perovskite composition and device configurations. We find that the light-enhanced strong polarization is a general phenomenon that occurs in all tested perovskite materials. The organic molecular dipoles affect the polarization at high frequency range, while the photoexcited free carriers and the interface between perovskite and its neighboring layers dominate the low-frequency response. In particular, our study suggests that the giant low-frequency capacitance enhancement originates from native defects and their accompanying defect dipoles, which need to be activated by photogenerated charge carriers. The high flexibility of the perovskite lattice facilitates the formation of these defects, and the dipole effect is enhanced at the interface regions where an electric double-layer capacitor is formed. This work sheds light on the understanding of the light-induced polarization mechanisms of perovskite materials.