Methylammonium lead iodide perovskites are considered direct bandgap semiconductors. Here we show that in fact they present a weakly indirect bandgap 60 meV below the direct bandgap transition. This is a consequence of spin-orbit coupling resulting in Rashba-splitting of the conduction band. The indirect nature of the bandgap explains the apparent contradiction of strong absorption and long charge carrier lifetime. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure induced reduction in local electric field around the Pb atom. The nature of the bandgap becomes increasingly more direct, resulting in five times faster charge carrier recombination, and a doubling of the radiative efficiency. At hydrostatic pressures above 325 MPa, MAPI undergoes a reversible phase transition resulting in a purely direct bandgap semiconductor. The pressure-induced changes suggest epitaxial and synthetic routes to higher efficiency optoelectronic devices.
Bibliographical noteFunding Information:
This work is part of the research program of the Foundation for Fundamental Research on Matter (FOM), which is part of The Netherlands Organization for Scientific Research (NWO). The work at ICL was supported by the EPSRC (grant no. EP/K016288/1 and EP/M009580/1) and the ERC (grant no. 277757). The authors thank Sarah Brittman for comments on the manuscript. T. W. thanks Dr. Michiel Petrus, Henk-Jan Boluijt, Ricardo Struik, Marc Duursma and mechanical workshop at AMOLF for help with the experiments. B. D. thanks Gede Adhyaksa for help with sample preparation.
© The Royal Society of Chemistry 2017.