Free Carrier Radiative Recombination and Photon Recycling in Lead Halide Perovskite Solar Cell Materials

Organic-inorganic hybrid lead halide perovskites are currently a most attractive class of materials since they have emerged as a solar cell material that realizes both high efficiency and simple low-cost fabrication. The power conversion efficiencies of perovskite solar cells now exceed 22%, which is comparable to that of commercially available CIGS and CdTe thin film solar cells. The key to further improvement is understanding the physical origin of the high efficiency of the perovskite solar cells, and a tremendous effort to come closer to this target has been made through numerous experiments. In this review article, we discuss the optoelectronic properties of perovskite CH3NH3PbX3 (X = I and Br) solar cell materials. Special attention is given to the free carrier recombination and photon recycling (the re-absorption of photons emitted by radiative recombination of photocarriers) processes in CH3NH3PbX3 single crystals, because a deep understanding of these processes is crucial for improving the solar cell performance. Lead halide perovskites show unique optical properties, e.g., extremely high quantum efficiency of luminescence, small Urbach tail in the absorption spectra, and long lifetime of photocarriers, which all suggest a low density of defects in the crystals. Because of these features, photon recycling efficiently occurs and dominates the optical processes of thick crystals.