Slow Diffusion and Long Lifetime in Metal Halide Perovskites for Photovoltaics

Metal halide perovskites feature excellent absorption, emission, and charge carrier transport properties. These materials are therefore very well suited for photovoltaic applications where there is a growing interest. Still, questions arise when looking at the unusual long carrier lifetime that, regarding the micrometric diffusion length, would imply a very low diffusion coefficient as compared to the commonly used photovoltaic absorbers. In this paper, we point out the slow in-depth diffusion mechanism in the time resolved photoluminescence decay interpretation by using an experimental setup with a wide-field excitation condition and an appropriate model of the carrier dynamics. The model includes charge carrier transport, photon recycling, and trap dynamics. It is verified on various excitation intensities, different interface properties, and various perovskite absorbers such as mixed alloys with the cesium content. For most of the perovskite-based materials we analyzed, the band-to-band recombination rate remains close to the radiative limit, leading to the expected sub-microsecond lifetime. The slow diffusion of charge carriers is observed with values of the diffusion coefficient D around 5 x 10(-3) cm(2)/s. Nonetheless, the power conversion efficiency remains high as the diffusion length is still found in the micrometric range.