Quasiparticle band gap of organic-inorganic hybrid perovskites: Crystal structure, spin-orbit coupling, and self-energy effects

Abstract

The quasiparticle band gap is one of the most important materials properties for photovoltaic applications. Often the band gap of a photovoltaic material is determined (and can be controlled) by various factors, complicating predictive materials optimization. An in-depth understanding of how these factors affect the size of the gap will provide valuable guidance for new materials discovery. Here we report a comprehensive investigation on the band gap formation mechanism in organicinorganic hybrid perovskites by decoupling various contributing factors which ultimately determine their electronic structure and quasiparticle band gap. Major factors, namely, quasiparticle selfenergy, spin-orbit coupling, and structural distortions due to the presence of organic molecules, and their influences on the quasiparticle band structure of organic-inorganic hybrid perovskites are illustrated. We find that although methylammonium cations do not contribute directly to the electronic states near band edges, they play an important role in defining the band gap by introducing structural distortions and controlling the overall lattice constants. The spin-orbit coupling effects drastically reduce the electron and hole effective masses in these systems, which is beneficial for high carrier mobilities and small exciton binding energies.

Molecular orientational melting within a lead-halide octahedron framework: The order-disorder transition in CH3NH3PbBr3

“…Photoluminescence spectra results show that there is a change in the energy of the maximum photoluminescence intensity at the 150 K transition 61 , in agreement with the observation made here that the key change in whole body molecular dynamics is at this temperature. This suggests that it is the whole body rotations of the MA cation and their interactions with the surrounding inorganic framework 12 that is key to understanding the photovoltaic properties. However, it is important to note that the change in pho-toluminescence observed at 150 K is not large.…”

“…The hydrogen bonding has been identified as strongly linked with the octahedral tilts of the system, and the phase transitions within the system, with the hydrogen bonding stronger in the low-T phase where the molecular motion is lower 17,18,22,62 . Due to the fact that the structure is so heavily dependent on the cation dynamics, this then influences the band gap of the system, with the states near the top of the valence band stabilized by the octahedral tilting in the low temperature phase with fewer cation dynamics 12,17,18 .…”

“…The behavior of the organic cation in these hybrid organic-inorganic compounds is recognized to be key to understanding their overall properties, including the photovoltaic properties and how this material can be implemented in solar panel devices 5,12,13 . Importantly, the molecule is able to rotate in the perovskite cage, and the exact nature of this rotation and how it depends on the structural phase of the system is still yet to be understood.…”

Accurate and efficient band gap predictions of metal halide perovskites using the DFT-1/2 method: GW accuracy with DFT expense

“…This is especially true for realistic structures in working devices, which often are compounds with mixtures of more than one organic cation, metal cation or halide anion with sophisticated surfaces and interfaces [1][2][3][4][5][6][7] . The challenges to fundamental understanding of the electronic structure of AMX 3 perovskites stem from their rich chemical and physical properties and the interplay of these properties 6,7 Several research groups [8][9][10][11][12][13][14][15][16] have successfully applied the GW method to predicting electronic structures of AMX 3 perovskites. Even et al 8 have identified the importance of a giant spin-orbit coupling (SOC) effect (about 1.0 eV) in the lead iodide perovskites, acting mainly on the conduction band (mainly consisting of Pb states).…”

“…The same group of researchers also looked at the substitution of halide ions in CH 3 NH 3 PbX 3 11 and nicely reproduced experimental findings in optical behavior, such as an increase of the band gap when moving from I to Cl. A few other theoretical studies at GW level include the investigation of polar phonons 12 , crystal structure effects and phase transitions 13,14 , exciton binding energies 15 , and band gap trends 16 in AMX 3 . The above mentioned GW studies have been very important in understanding the chemistry and physics of these materials and providing materials design inspirations.…”