Indirect to direct bandgap transition in methylammonium lead halide perovskite

TL;DR

This study reveals that methylammonium lead iodide perovskites have a weakly indirect bandgap caused by spin-orbit coupling, which can be tuned to a direct bandgap under pressure, enhancing optoelectronic performance.

Contribution

The paper demonstrates the indirect nature of the bandgap in methylammonium lead iodide perovskites and how hydrostatic pressure can induce a transition to a direct bandgap, improving device efficiency.

Findings

Weakly indirect bandgap due to Rashba splitting

Pressure reduces Rashba splitting and makes the bandgap more direct

Reversible phase transition to a purely direct bandgap at high pressure

Abstract

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 ordering of the crystal structure. 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.