Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells

The partial replacement of methylammonium by formamidinium and cesium in organolead trihalide materials is of great importance to improve the performance and stability of photovoltaic solar cells. However, the effect of multiple cations on the cell functioning and their electrical characteristics remains to be clarified. By using the impedance spectroscopy technique, we have investigated the electrical response to a small ac perturbation applied to solar cells implementing hybrid perovskites with various compositions, polarized over a large potential range. The solar cell preparation protocols have been optimized to reach power conversion efficiencies higher than 17%. The impedance response has been investigated both under light and in the dark to discriminate the light sensitive parameters. The spectra have been carefully analyzed using an ad hoc equivalent circuit, and the data have been discussed in the light of the existing literature. The spectra showed no intermediate frequency inductive loop due to the absence of multistep charge transfer involving surface states. A large inductive loop is found to be the signature of poorly functioning solar cells. Except for the high frequency capacitance, which is the bulk response of perovskite, the other parameters are influenced by interface and contact phenomena, ionic conductivity and charge accumulations. The scaling of the low frequency capacitance with the hysteresis amplitude is clearly stated by our comprehensive study. Moreover, no diffusion impedance due to the diffusion of ionic species is observed. However, ion mobility results in a strong effect on recombinations and has a strong influence on the low frequency impedance response of the system.