Improved Understanding of the Electronic and Energetic Landscapes of Perovskite Solar Cells: High Local Charge Carrier Mobility, Reduced Recombination, and Extremely Shallow Traps

The intriguing photoactive features of organic–inorganic hybrid perovskites have enabled the preparation of a new class of highly efficient solar cells. However, the fundamental properties, upon which the performance of these devices is based, are currently under-explored, making their elucidation a vital issue. Herein, we have investigated the local mobility, recombination, and energetic landscape of charge carriers in a prototype CH₃NH₃PbI₃ perovskite (PVK) using a laser-flash time-resolved microwave conductivity (TRMC) technique. PVK was prepared on mesoporous TiO₂ and Al₂O₃ by one or two-step sequential deposition. PVK on mesoporous TiO₂ exhibited a charge carrier mobility of 20 cm² V^–1 s^–1, which was predominantly attributed to holes. PVK on mesoporous Al₂O₃, on the other hand, exhibited a 50% lower mobility, which was resolved into balanced contributions from both holes and electrons. A general correlation between crystal size and mobility was revealed irrespective of the fabrication process and underlying layer. Modulating the microwave frequency from 9 toward 23 GHz allowed us to determine the intrinsic mobilities of each PVK sample (60–75 cm² V^–1 s^–1), which were mostly independent of the mesoporous scaffold. Kinetic and frequency analysis of the transient complex conductivity strongly support the superiority of the perovskite, based on a significant suppression of charge recombination, an extremely shallow trap depth (10 meV), and a low concentration of these trapped states (less than 10%). The transport mechanism was further investigated by examining the temperature dependence of the TRMC maxima. Our study provides a basis for understanding perovskite solar cell operation, while highlighting the importance of the mesoporous layer and the perovskite fabrication process

Anomalous Dielectric Behavior of a Pb/Sn Perovskite: Effect of Trapped Charges on Complex Photoconductivity

Organic–inorganic metal halide perovskites (MHPs) exhibit prominent electronic and optical properties benefiting the performance of solar cells and light-emitting diodes. However, the dielectric properties of these materials have remained poorly understood, despite probably influencing delayed charge recombination and device capacitance. Herein, we characterize the unprecedented dielectric behavior of MHPs comprising methylammonium cations, Pb/Sn as metals, and Br/I as halides using time-resolved microwave conductivity (TRMC) measurements. At specific compositions, the above MHPs exhibit negative real and positive imaginary photoconductivities, the polarities of which are opposite those observed for conventional photogenerated charge carriers. Comparing the observed TRMC kinetics with that of inorganic perovskites (SrTiO₃ and BaTiO₃) and characterizing its dependence on temperature, frequency, and near-infrared second push pulse, we conclude that the above behavior is due to the trapping of polaronic holes/electrons by oriented dipoles of organic cations, which opens a hitherto unexplored route to the dynamical control of dielectric permittivity by photoirradiation