Degradation mechanism of CH3NH3PbI3 perovskite materials upon exposure to humid air

TL;DR

This study investigates the degradation mechanisms of CH3NH3PbI3 perovskite layers in humid air using spectroscopic ellipsometry, revealing rapid surface phase changes and the formation of PbI2/hydrate crystals, which impact stability.

Contribution

It provides real-time spectroscopic ellipsometry analysis of perovskite degradation, identifying two competing reactions and surface phase changes not detected by traditional methods.

Findings

Rapid initial layer thickness reduction within 1 hour at 40% humidity

Formation of hexagonal PbI2/hydrate crystals with 0.65 nm multilayer structure

Conventional X-ray diffraction less sensitive to surface phase changes

Abstract

Low stability of organic-inorganic perovskite (CH3NH3PbI3) solar cells in humid air environments is a serious drawback which could limit practical application of this material severely. In this study, from real-time spectroscopic ellipsometry characterization, the degradation mechanism of ultra-smooth CH3NH3PbI3 layers prepared by a laser evaporation technique is studied. We present evidence that the CH3NH3PbI3 degradation in humid air proceeds by two competing reactions of (i) the PbI2 formation by the desorption of CH3NH3I species and (ii) the generation of a CH3NH3PbI3 hydrate phase by H2O incorporation. In particular, rapid phase change occurs in the near-surface region and the CH3NH3PbI3 layer thickness reduces rapidly in the initial 1-h air exposure even at a low relative humidity of 40%. After the prolonged air exposure, the CH3NH3PbI3 layer is converted completely to hexagonal platelet PbI2/hydrate crystals that have a distinct atomic-scale multilayer structure with a period of 0.65 nm. We find that conventional x-ray diffraction and optical characterization in the visible region, used commonly in earlier works, are quite insensitive to the surface phase change. Based on results obtained in this work, we discuss the degradation mechanism of CH3NH3PbI3 in humid air.