Revealing the role of organic cations in hybrid halide perovskites CH3NH3PbI3

TL;DR

This study uses advanced calculations to show that the orientation of organic molecules in hybrid halide perovskites significantly influences their electronic properties and efficiency in solar cells.

Contribution

It reveals the fundamental role of organic cation orientation in determining the electronic structure and performance of CH3NH3PbI3 perovskites.

Findings

Organic molecule orientation affects band gap nature.

Molecular rotations may cause dynamical band structure changes.

Orientation-dependent distortions influence electronic properties.

Abstract

The hybrid halide perovskite CH$_{3}$NH$_{3}$PbI$_{3}$ has enabled solar cells to reach an efficiency of about 18\%, demonstrating a pace for improvements with no precedents in the solar energy arena. Despite such explosive progress, the microscopic origin behind the success of such material is still debated, with the role played by the organic cations in the light-harvesting process remaining unclear. Here van-der-Waals-corrected density functional theory calculations reveal that the orientation of the organic molecules plays a fundamental role in determining the material electronic properties. For instance, if CH$_{3}$NH$_{3}$ orients along a (011)-like direction, the PbI$_{6}$ octahedral cage will distort and the band gap will become indirect. Our results suggest that molecular rotations, with the consequent dynamical change of the band structure, might be at the origin of the slow carrier recombination and the superior conversion efficiency of CH$_{3}$NH$_{3}$PbI$_{3}$.