Quantification of Ionic Diffusion in Lead Halide Perovskite Single Crystals

Lead halide perovskites are mixed electronic–ionic semiconductors that have recently revolutionized the photovoltaics field. The physical characterization of the ionic conductivity has been rather elusive because of the high intermixing of ionic and electronic current. In this work, the synthesis of low defect density monocrystalline MAPbBr3 (MA = methylammonium) solar cells free of a hole transport layer (HTL) suppresses the effect of electronic current. Impedance spectroscopy reveals the characteristic signature of ionic diffusion (the Warburg element and transmission line equivalent circuit) and ion accumulation at the interface of MAPbBr3 with the external contacts. Diffusion coefficients are calculated based on a good correlation between thickness of MAPbBr3 and characteristic diffusion transition frequency. In addition, reactive external interfaces are studied by comparison of polycrystalline MAPbBr3 devices prepared either with or without a HTL. The low-frequency response in impedance spectroscopy measurements is correlated with the chemical reactivity of moving ions with the external interfaces and diffusion into the HTL.