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Evidence for polarization-induced phase transformations and degradation in CHNHPbI
In solar cells, hybrid halide perovskites operate under constant bias, thus their stability towards electric field-induced degradation is of key importance. Here we report on evidence of previously unidentified electric field-induced transitions and degradation path of CHNHPbI (MAPbI) using elemental and phase mapping. Thin films of MAPbI were deposited onto 1β2 Β΅m-pitch interdigitated electrodes and subjected to direct current (DC)-polarization. The MAPbI layer polarized with < 0.8 V/Β΅m DC electric field undergoes pronounced ion redistribution to methylammonium-rich MAPbI (y < 0.6) and iodine-rich MAPbI (x < 0.3) regions. Polarization-induced loss of both methylammonium and iodine provokes degradation of MAPbI. Using nanofocus grazing-incidence wide-angle X-ray scattering (GIWAXS), we unambiguously showed that the bias voltage induces the transformation of Ξ²-MAPbI to metastable Ξ΄-MAPbI polymorph via alignment of polar organic cation with the electric field. This transformation is partially reversible upon field removal. However, once formed, Ξ΄-MAPbI disrupts the morphology of pristine film and undergoes decomposition to Ξ²-MAPbI (Ξ²-MAPI) and PbI. With the aforementioned compositional and phase changes, only MA-rich part serves as the charge separation layer, while the I-rich excitation is blocked with the PbI barrier serving as holes trapping layer. These observations reveal the intermediate steps in electric-field-driven degradation of halide perovskites and show the role of polar cations in the process, which is instructive for further material design with higher stability metrics