Indirect to direct bandgap transition in methylammonium lead halide perovskite
Bruno Ehrler a, Erik Garnett a, Sander Mann a, Benjamin Daiber a, Tianyi Wang a, Jarvist Frost b, Aron Walsh b
a Center for Nanophotonics, AMOLF, The Netherlands, Science Park, 104, Amsterdam, Netherlands
Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
València, Spain, 2017 March 1st - 2nd
Organizers: Henk Bolink and David Cahen
Oral, Tianyi Wang, presentation 063
Publication date: 18th December 2016

Methylammonium lead iodide perovskites (MAPI) are generally considered direct bandgap semiconductors. However, 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-dependent absorption and emission measurements, we show that a weakly indirect bandgap around 60 meV below the direct bandgap transition is present. 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 higher efficiency optoelectronic devices might be developed based on the pressure-induced changes we observe.

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