Direct Bandgap Behavior in Rashba-Type Metal Halide Perovskites.

The generation and recombination of charge carriers in semiconductors through photons controls photovoltaic and light-emitting diode operation. Understanding of these processes in hybrid perovskites has advanced, but remains incomplete. Using femtosecond transient absorption and photoluminescence, it is observed that the luminescence signal shows a rise over 2 ps, while initially hot photogenerated carriers cool to the band edge. This indicates that the luminescence from hot carriers is weaker than that of cold carriers, as expected from strongly radiative transitions in direct gap semiconductors. It is concluded that the electrons and holes show a strong overlap in momentum space, despite recent proposals that Rashba splitting leads to a band offset suppressing such an overlap. A number of possible resolutions to this, including lattice dynamics that remove the Rashba splitting at room temperature, and localization of luminescence events to length scales below 10 nm are considered.The authors acknowledge financial support from the Engineering and Physical Sciences Research Council of the UK (EPSRC). J.M.R. thanks the Winton Programme for the Physics of Sustainability (University of Cambridge). J.M.R. thanks the Cambridge Home European Scheme for financial support. B.M. thanks Robinson College and the Cambridge Philosophical Society for a Henslow Research Fellowship. F.D. acknowledges funding from a Herchel Smith Research Fellowship and a Winton Advanced Research Fellowship