Hybrid Organic-Inorganic Perovskites on the Move.

David A Egger,Leeor Kronik,Andrew M Rappe

doi:10.1021/acs.accounts.5b00540

Abstract

ConspectusHybrid organic–inorganic perovskites (HOIPs) are crystals with the structural formula ABX3, where A, B, and X are organic and inorganic ions, respectively. While known for several decades, HOIPs have only in recent years emerged as extremely promising semiconducting materials for solar energy applications. In particular, power-conversion efficiencies of HOIP-based solar cells have improved at a record speed and, after only little more than 6 years of photovoltaics research, surpassed the 20% threshold, which is an outstanding result for a solution-processable material. It is thus of fundamental importance to reveal physical and chemical phenomena that contribute to, or limit, these impressive photovoltaic efficiencies.To understand charge-transport and light-absorption properties of semiconducting materials, one often invokes a lattice of ions displaced from their static positions only by harmonic vibrations. However, a preponderance of recent studies suggests that this picture is not sufficient for HOIPs, where a variety of structurally dynamic effects, beyond small harmonic vibrations, arises already at room temperature.In this Account, we focus on these effects. First, we review structure and bonding in HOIPs and relate them to the promising charge-transport and absorption properties of these materials, in terms of favorable electronic properties. We point out that HOIPs are much “softer” mechanically, compared to other efficient solar-cell materials, and that this can result in large ionic displacements at room temperature. We therefore focus next on dynamic structural effects in HOIPs, going beyond a static band-structure picture. Specifically, we discuss pertinent experimental and theoretical findings as to phase-transition behavior and molecular/octahedral rearrangements. We then discuss atomic diffusion phenomena in HOIPs, with an emphasis on the migration of intrinsic and extrinsic ionic species. From this combined perspective, HOIPs appear as highly dynamic materials, in which structural fluctuations and long-range ionic motion have an unusually strong impact on charge-transport and optical properties. We highlight the potential implications of these effects for several intriguing phenomenological observations, ranging from scattering mechanisms and lifetimes of charge carriers to light-induced structural effects and ionic conduction.

Highlights

Hybrid organic−inorganic perovskites (HOIPs), the structure of which is defined below, are materials which are solution processable, comprise earth-abundant elements, and yet exhibit outstanding semiconducting and light-absorption properties
We show that the static band-structure picture, possibly with harmonically vibrating ions, still leaves important phenomena, for example, lightinduced effects or hysteresis in the current−voltage curves of HOIP solar cells, unexplained
We focus on dynamic structural ef fects in HOIPs, highlighting important molecular and octahedral rearrangements, as well as ion migration phenomena, discuss how those could explain some of the remaining mysteries of HOIPs, and suggest further routes for progress

Summary

INTRODUCTION

Hybrid organic−inorganic perovskites (HOIPs), the structure of which is defined below, are materials which are solution processable, comprise earth-abundant elements, and yet exhibit outstanding semiconducting and light-absorption properties (see refs 1−3 for overviews) These features render HOIPs promising for (opto-) electronic applications in general and photovoltaic (PV) devices in particular,[4−7] as power-conversion efficiencies surpassed 20% after some 6 years of PV research.[8] The microscopic origin of this success lies partly in the electronic structure of HOIPs, which is well understood:[9,10] it resembles that of a good inorganic semiconductor, with optical gaps that can be close to optimal for solar absorption, small exciton binding energies, and low effective masses. The local presence of secondary phases is especially relevant to HOIPs in which phase transitions can occur around room temperature, notably MAPbI3 (Figure 1a)

MOLECULAR ROTATIONS AND OCTAHEDRAL DISTORTIONS

THE ROLE OF ION MIGRATION

SUMMARY AND CONCLUSIONS

■ REFERENCES