Metal halide perovskites such as methylammonium lead iodide (CH₃NH₃PbI₃) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron–hole recombination in these materials. Here, we propose that the bandgap in CH₃NH₃PbI₃ has a direct–indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron–hole recombination of photo-excited charges is retarded at lower temperature. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temperature orthorhombic phase, fast quenching of mobile charges occurs independent of the temperature and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyse spectroscopic data.

Nature Materials (2016) doi:10.1038/nmat4765
Received 27 April 2016 Accepted 05 September 2016 Published online 03 October 2016

http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4765.html