Using Monovalent- to Trivalent-Cation Hybrid Perovskites for Producing High-Efficiency Solar Cells: Electrical Response, Impedance, and Stability
Daming Zheng, Tao Zhu, Thierry Pauporté
Abstract
Production of high-efficiency solar cells based on organometal halide perovskites has emerged recently as the most promising technology. During the past years, innovative film processings have been developed for increasing the device’s performance and stability. The two most efficient strategies are (i) to complexify the perovskite composition by mixing monovalent cations and (ii) to add additives in the precursor solution, which are eliminated upon the film annealing and postdeposition processing steps. In the present work, perovskite films varying in their monovalent cation content, from one to three (MAPbI3, Cs0.08FA0.80MA0.12Pb(I0.88Br0.12)3, and FA1–xMAxPbI3), and being optimized to reach a high efficiency up to 22.2% have been thoroughly compared for their steady-state and impedance electrical responses. Much information has been extracted from the electrical impedance spectroscopy data after fitting using equivalent electrical circuits. With the help of complementary measurements (photoluminescence, scanning electron microscopy, Voc measurements, etc.), the spectral features have been assigned to physical phenomena. The relative permittivities of MAPbI3, Cs0.08FA0.80MA0.12Pb(I0.88Br0.12)3, and FA1−xMAxPbI have been extracted from the high-frequency capacitance C2. The high-frequency resistance R2 has been related to the slow lifetime of the time-resolved photoluminescence and assigned to the bulk recombination resistance of the perovskite. The low-frequency resistance R4 has been assigned to the recombinations occurring at and near the interfaces. The low-frequency capacitance C4 has been related to the J–V curve hysteresis and assigned to the recombinations. The recombination in perovskite solar cells is phase delayed due to the dynamics of ions and related defect formation. It induces an imaginary component in the impedance spectra that is analyzed as C4. Overall, the strategy of mediating the growth of FA1–xMAxPbI3 perovskite double cations by additives is found to be the best one to suppress recombinations, suppress ion mobility, reach high efficiency with low hysteresis, and stabilize the perovskite compound.