Indirect to direct bandgap transition in methylammonium lead halide perovskite

Methylammonium lead iodide perovskite has potential applications in solar cells, lasers, light-emitting diodes, and photodetectors. Methylammonium lead iodide perovskite is considered a direct bandgap semiconductor, however, the long minority carrier lifetime with similar values as those of the indirect bandgap semiconductors, as well as the associated long charge carrier diffusion length is a mystery in this field.

Various theoretical studies have indicated that methylammonium lead iodide perovskite has an indirect bandgap 60meV below the direct band gap which should be visible in emission and absorption spectra. The indirect gap could be responsible for the long carrier lifetime owing to the fact that thermalized carriers are protected against recombination through the fast direct transition. This indirect transition is reference to Rashba splitting of the conduction band.

Researchers led by Dr. Bruno Ehrler from the AMOLF Institute in Amsterdam, The Netherlands studied the optoelectronic changes of thin films of polycrystalline methylammonium lead iodide perovskite exposed to mild hydrostatic pressure of up to 400MPa which was below and above the phase transition at 325MPa. They demonstrated that the bandgap changed with pressure and the direct transition was enhanced. Above the phase transition, methylammonium lead iodide perovskite behaved like a direct bandgap semiconductor. Their work is published in Energy & Environmental Science.

In their studies the hybrid methylammonium lead iodide had an indirect bandgap which the authors explained because of the distortion of the lead iodide framework and this led to an electric field across the lead atom therefore splitting the conduction band. They observed that the position of the indirect transition 60meV below the direct transition was in keeping with an absorption spectrum that resembled an indirect-direct bandgap semiconductor. Charges thermalized in the conduction band were protected from recombination considering that recombination required a change in crystal momentum. For this reason, the indirect bandgap explained the contradiction between the long charge carrier lifetime and strong absorption. The electric field across the lead atom was observed to reduce when under pressure. This led to increased strength of the direct transition, which led reduced the carrier charge lifetime and improved the radiative efficiency. Tianyi Wang, lead author of the study, says “We first could not explain the apparent contradiction between an increased radiative efficiency, and a shorter lifetime under pressure, but with the direct-indirect bandgap it makes perfect sense.”

The outcome of the study opens a new path for development of new perovskite semiconductors with engineered band structure. The new material will be suitable for solar cells with high photovoltage, low lasing lasers, and efficient light emitting diodes.