Room temperature ferroelectricity of hybrid organic-inorganic perovskites with mixed iodine and bromine

Ferroelectricity has been reported in organic-inorganic perovskites, and it can enhance the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) because ferroelectricity can facilitate charge carrier separation and charge transport through the perovskite layer. However, the existence of ferroelectricity in perovskites has been in hot debate, particularly at room temperature. Here, we report the ferroelectric polarization switching of MAPb(I_1-xBr_x)₃ which showed a high dependence on its composition. The ferroelectric behavior of MAPbI₃-50% PbBr₂ is confirmed with domain switching by Piezoelectric Force Microscopy (PFM) imaging and bias-off "butterfly-like" amplitude loops and piezoresponse hysteresis loops at room temperature. The possible factors, such as film electrical properties attributed to the enhanced room-temperature ferroelectricity in MAPb(I_1-xBr_x)₃ films are clarified by using conductive atomic force microscopy (C-AFM). In addition, the charge separation and charge transport in the perovskite MAPb(I_1-xBr_x)₃ films are further investigated by Kelvin probe force microscopy (KPFM). Finally, the possible influences of polarization orientations, trapping effects and ion migrations within the MAPb(I_1-xBr_x)₃ films on the J-V characteristics of PSC devices are discussed in detail. It discovers that the polarization switching under the positive tip biased condition in the PSCs with 50% PbBr₂, which could hinder the photovoltaic performance. These findings help better understand the electronic structure of hybrid organic-inorganic perovskites and provide guidance for the improvement of the PSC performance and other electronic applications of perovskites.