Effect of Orientation of Cation CH3NH3PbI3 on Ambipolarity, Open-Circuit Voltage, and Excitons Lifetime

TL;DR

This study uses density functional theory to show how the orientation of methylammonium cations in MAPbI3 influences its electronic properties, explaining its ambipolarity, open-circuit voltage, and exciton lifetime.

Contribution

It reveals the role of MA cation orientation in Fermi level variation, linking it to key photovoltaic properties of MAPbI3, a novel insight into perovskite behavior.

Findings

Fermi level varies significantly with MA orientation

Fermi level variation explains ambipolarity and open-circuit voltage

Effective diode behavior accounts for long exciton lifetime

Abstract

The effect of the orientation of the cation MA (i.e., CH3NH3) on the physical properties of MAPbI3 are investigated using the density functional theory. We report that the Fermi energy level exhibits a large variation with MA orientation, and it is this Fermi level variation that makes this material unique and so different from the non-organic perovskites. The Fermi level variation with orientation of MA is proposed to be responsible for the experimentally observed intrinsic open-circuit voltage V_OC, ambipolarity, and long lifetime of the excitons. Based on our results, ferroelectric domains in the low-temperature orthorhombic phase or clusters of MA molecules not rotating in unison (as a consequence of e.g. impurities and/or defects) in the higher-temperature tetragonal and cubic phases are proposed to be responsible for ambipolarity. This is because any given two adjacent domains or clusters with different MA s average orientations may have very different Fermi levels, causing them to behave as an effective diode. Also, the measured significant lifetime of excitons in MAPbI3 finds a natural explanation using this effective diode. Thus, our work provides a unified manner for explaining at least these three important properties in terms of the rotation-induced Fermi level variations. We used the density functional theory to get our results.