The material exhibits strong absorptivity characteristic of a direct-gap semiconductor, while showing extraordinarily slow radiative recombination times, reminiscent of an indirect-gap semiconductor. This is widely thought to be the key to the very high efficiencies in the device.
In APL Materials 4, 091501 (2016), we provide a simple explanation for the origin of this effect, as a consequence of large spin-orbit coupling combined with large internal electric fields. Together, they create a Rashba effect which slightly spin-splits the bands at the direct-gap point. We develop a theory of photoluminescence based on the Quasiparticle Self-Consistent GW approximation, and obtain good agreement for the recombination rate with experiment.