Evidence for polarization-induced phase transformations and degradation in CH₃NH₃PbI₃

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 CH₃NH₃PbI₃ (MAPbI₃) using elemental and phase mapping. Thin films of MAPbI₃ were deposited onto 1–2 µm-pitch interdigitated electrodes and subjected to direct current (DC)-polarization. The MAPbI₃ layer polarized with < 0.8 V/µm DC electric field undergoes pronounced ion redistribution to methylammonium-rich MAPbI_3−y (y < 0.6) and iodine-rich MA_1−xPbI₃ (x < 0.3) regions. Polarization-induced loss of both methylammonium and iodine provokes degradation of MAPbI₃. Using nanofocus grazing-incidence wide-angle X-ray scattering (GIWAXS), we unambiguously showed that the bias voltage induces the transformation of β-MAPbI₃ to metastable δ-MAPbI₃ polymorph via alignment of polar organic cation with the electric field. This transformation is partially reversible upon field removal. However, once formed, δ-MAPbI₃ disrupts the morphology of pristine film and undergoes decomposition to β-MAPbI₃ (β-MAPI) and PbI₂. 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 PbI₂ 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.