Solar cells based on organic-inorganic hybrid perovskites have reached an efficiency of 22%. For a long time, the prototype material in this family, methylammonium lead iodide perovskite (MAPI) has been considered a direct bandgap semiconductor. However, the lifetime of free carriers in these materials is unusually long for a direct bandgap semiconductor, and theoretical calculations have predicted a slight indirect bandgap for MAPI as a consequence of spin-orbit coupling resulting in Rashba-splitting of the conduction band. Currently there is limited experimental evidence to support this theoretical prediction.
Using pressure- and time-dependent absorption and emission measurements, we show that a weakly indirect bandgap around 60 meV below the direct bandgap transition is present. The indirect transition arises from Rashba splitting of the conduction band. The splitting occurs due to the local electric field generated by the absence of inversion symmetry around the Pb site, which acts on the 6p orbitals of the lead-atom where most of the conduction band minimum is located. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure-induced reduction in electric field around the Pb atom. The indirect nature of the bandgap is suppressed, leading to five times faster charge carrier recombination, and a two-fold increase of the radiative efficiency. At hydrostatic pressures above 325 MPa, a reversible phase transition of MAPI occurs, resulting in a purely direct bandgap semiconductor.
The finding of an indirect bandgap in MAPI sheds light on the apparent contradiction of strong absorption and long charge carrier lifetime. Novel epitaxial and synthetic routes to solar cells with higher open-circuit voltage might be developed based on the pressure-induced changes we observe.