Universal rules for visible-light absorption in hybrid perovskite materials

TL;DR

This study establishes universal rules for understanding visible-light absorption in hybrid perovskites, highlighting the significant role of A-site cations and X-anions in optical properties, supported by DFT calculations and systematic analysis.

Contribution

The paper introduces universal rules that interpret optical absorption in hybrid perovskites, emphasizing the critical influence of A-site cations and X-anions, supported by theoretical and systematic analysis.

Findings

A-site cation significantly affects light absorption.

Replacing CH3NH3+ with HC(NH2)2+ halves the absorption coefficient.

The variation in absorption spectra can be described by the sum rule.

Abstract

A variety of organic-inorganic hybrid perovskites (APbX3) consisting of mixed center cations [A = CH3NH3+, HC(NH2)2+, Cs+] with different PbX3- cages (X = I, Br, Cl) have been developed to realize high-efficiency solar cells. Nevertheless, clear understanding for the effects of A and X on the optical transition has been lacking. Here, we present universal rules that allow the unified interpretation of the optical absorption in various hybrid perovskites. In particular, we find that the influence of the A-site cation on the light absorption is rather significant and the absorption coefficient (alpha) reduces to half when CH3NH3+ is replaced with HC(NH2)2+ in the APbI3 system. Our density functional theory (DFT) calculations reproduce all of the fine absorption features observed in HC(NH2)2PbI3 and CH3NH3PbBr3, allowing the unique assignment of the interband transitions in the Brillouin zone. In contrast to general understanding that the A-site cation involves weakly in the optical process, our theoretical calculations reveal that the center cation plays a critical role in the interband transition and the absorption strength in the visible region is modified by the strong A-X interaction. Furthermore, our systematic analyses show that the variation of the absorption spectrum with X can be described simply by the well-known sum rule. The universal rules established in this study explain the large reduction of alpha in HC(NH2)2PbI3 and predict CsPbI3 as the highest alpha material.