Photoluminescence from Radiative Surface States and Excitons in Methylammonium Lead Bromide Perovskites.

In view of its band gap of 2.2 eV and its stability, methylammonium lead bromide (MAPbBr) is a possible candidate to serve as a light absorber in a subcell of a multijunction solar cell. Using complementary temperature-dependent time-resolved microwave conductance (TRMC) and photoluminescence (TRPL) measurements, we demonstrate that the exciton yield increases with lower temperature at the expense of the charge carrier generation yield. The low-energy emission at around 580 nm in the cubic phase and the second broad emission peak at 622 nm in the orthorhombic phase originate from radiative recombination of charges trapped in defects with mobile countercharges. We present a kinetic model describing both the decay in conductance as well as the slow ingrowth of the TRPL. Knowledge of defect states at the surface of various crystal phases is of interest to reach higher open-circuit voltages in MAPbBr-based cells.