Position-locking of volatile reaction products by atmosphere and capping layers slows down photodecomposition of methylammonium lead triiodide perovskite.

Fengshuo Zu,Patrick Amsalem,Thorsten Schultz,Dieter Neher,Norbert Koch,Dongguen Shin,Lennart Frohloff,Christian M Wolff

doi:10.1039/d0ra03572f

Abstract

The remarkable progress of metal halide perovskites in photovoltaics has led to the power conversion efficiency approaching 26%. However, practical applications of perovskite-based solar cells are challenged by the stability issues, of which the most critical one is photo-induced degradation. Bare CH3NH3PbI3 perovskite films are known to decompose rapidly, with methylammonium and iodine as volatile species and residual solid PbI2 and metallic Pb, under vacuum under white light illumination, on the timescale of minutes. We find, in agreement with previous work, that the degradation is non-uniform and proceeds predominantly from the surface, and that illumination under N2 and ambient air (relative humidity 20%) does not induce substantial degradation even after several hours. Yet, in all cases the release of iodine from the perovskite surface is directly identified by X-ray photoelectron spectroscopy. This goes in hand with a loss of organic cations and the formation of metallic Pb. When CH3NH3PbI3 films are covered with a few nm thick organic capping layer, either charge selective or non-selective, the rapid photodecomposition process under ultrahigh vacuum is reduced by more than one order of magnitude, and becomes similar in timescale to that under N2 or air. We conclude that the light-induced decomposition reaction of CH3NH3PbI3, leading to volatile methylammonium and iodine, is largely reversible as long as these products are restrained from leaving the surface. This is readily achieved by ambient atmospheric pressure, as well as a thin organic capping layer even under ultrahigh vacuum. In addition to explaining the impact of gas pressure on the stability of this perovskite, our results indicate that covalently “locking” the position of perovskite components at the surface or an interface should enhance the overall photostability.

Highlights

Metal halide perovskites, owing to their remarkable optoelectronic characteristics[1,2,3,4] have shown great potential in photovoltaic applications[5,6,7,8] and in light-emitting devices[9,10] and photodetectors.[11,12,13] In particular, perovskite solar cells have achieved tremendous accomplishments over the past decade, reaching a power conversion efficiency of over 25% for single junction cells.[14]
We demonstrate that upon white light illumination the rapid decomposition process of MAPbI3 into PbI2 and metallic Pb in ultrahigh vacuum (UHV) starts predominantly at the surface, resulting in signi cant changes in the structure and optoelectronic properties
By employing charge selective and non-selective organic capping layers, the fast photodecomposition process in UHV can be tremendously reduced by restraining the volatile methylammonium and iodine from leaving the surface, indicating the effect of surrounding gas pressure being one of the key factors contributing to stability

Summary

Introduction

In a previous study,[23] we demonstrated that upon white light illumination the methylammonium lead mixed halide perovskites (MAPbIxCl3Àx, MA: CH3NH3+) degrade rapidly under ultrahigh vacuum (UHV) conditions. We demonstrate that upon white light illumination the rapid decomposition process of MAPbI3 into PbI2 and metallic Pb in UHV starts predominantly at the surface, resulting in signi cant changes in the structure and optoelectronic properties. By employing charge selective and non-selective organic capping layers, the fast photodecomposition process in UHV can be tremendously reduced by restraining the volatile methylammonium and iodine from leaving the surface, indicating the effect of surrounding gas pressure being one of the key factors contributing to stability. While certainly great care must be taken when characterizing bare perovskite materials under (ultrahigh) vacuum conditions, our results suggest that the back-reaction of volatile degradation products, when kept in proximity, to the parent perovskite is very efficient

Impact of environmental conditions on the photodecomposition process

Impact of organic capping layers on the photodecomposition process

Conclusions

Sample preparation

Photoemission spectroscopy