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 <i>ad hoc</i> 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

Impact of Organic Hole Transporting Material and Doping on the Electrical Response of Perovskite Solar Cells

The hole transport material (HTM) layer is a key component of the perovskite solar cells (PSCs) that must be optimized to reach high efficiency. The development of new HTMs alternative to Spiro-OMeTAD and the understanding of the role of doping agents on these layers are important research axes in the field. It requires the use of appropriate characterization tools enabling us to discriminate the bulk and interface effects. In the present paper, we fully analyze the effect of HTM doping and of the material on the impedance response of PSCs. The approach has been implemented on two different molecular HTMs, Spiro-OMeTAD and a new molecular carbazole HTM, called B186, and with various doping levels. We show that limitations by poor doping are characterized by an extra high frequency impedance loop for which capacitance and resistance analysis gives the dielectric constant and conductivity of the material, respectively. However, the low-frequency part of the spectra provides important information on the charge accumulation/outflow and on the recombination levels. More generally, the presented approach is of high practical interest for the development of new organic HTMs and for the optimization of the layer doping