Evidence for polarization-induced phase transformations and degradation in CH3NH3PbI3

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Abstract
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 MAPbI3−y (y < 0.6) and iodine-rich MA1−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.

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