Doping Profile in Planar Hybrid Perovskite Solar Cells Identifying Mobile Ions

The role of ions in organo-metal halide perovskites and its impact on optoelectronic device performance remains one of the most intriguing issues in the field. The current–voltage hysteresis often observed in hybrid perovskite solar cells has been assigned to the presence of mobile ions inside the perovskite layer. The difficulty in studying electronic properties of solar cells results from the screening effects as well as slow dynamics of mobile ions, particularly if they are located at the interfaces. In this work, we addressed the distribution of charged species in planar-type methylammonium lead iodine (MAPI) as well as formamidinium lead iodine (FAPI) solar cells by using a modified capacitance–voltage (CV) method without illumination in combination with a Mott–Schottky (MS) analysis of experimental data. The characteristic Mott–Schottky behavior with a linear dependence of C–2(V), distinctive for a pn-junction, is not visible in pristine devices. Surprisingly, biasing the device in the forward direction results in MS behavior, which is due to the field-driven redistribution of mobile ions from the interface toward the absorber bulk. From the MS analysis, we deduced space charge concentrations of 2.5 × 1016 and 2.8 × 1016 cm–3 for the FAPI and the MAPI devices, respectively. However, the junction formation effect is not sustainable, since mobile ions relax to their initial location at ambient conditions. However, if the prebiasing is done at temperatures slightly below room temperature, the pn-junction can be stabilized for the FAPI device. In contrast, the MAPI device shows a rapid redistribution of mobile ions back to the transport layers during the measurement even at lower temperatures. This can be observed in the quite different doping profiles for the two perovskite devices.