Critical role of water in defect aggregation and chemical degradation of perovskite solar cells

TL;DR

This study reveals how water influences defect formation and accelerates chemical degradation in methylammonium lead iodide perovskite solar cells, highlighting the importance of moisture control for stability.

Contribution

It provides a first-principles thermodynamic analysis of defect processes in water-intercalated perovskites, uncovering water-mediated vacancy-pair formation mechanisms.

Findings

Water facilitates  vacancy complex formation.

Hydrous phases create deep charge transition levels.

Moisture exposure leads to halide perovskite degradation.

Abstract

The chemical stability of methylammonium lead iodide (\ce{MAPbI3}) under humid conditions remains the primary challenge facing halide perovskite solar cells. We investigate defect processes in the water-intercalated iodide perovskite (\ce{MAPbI3}\_\ce{H2O}) and monohydrated phase (\ce{MAPbI3}$\cdot$\ce{H2O}) within a first-principles thermodynamic framework. We consider the formation energies of isolated and aggregated vacancy defects with different charge states under I-rich and I-poor conditions. It is found that a \ce{PbI2} (partial Schottky) vacancy complex can be formed readily, while the \ce{MAI} vacancy complex is difficult to form in the hydrous compounds. Vacancies in the hydrous phases create deep charge transition levels, indicating the degradation of halide perovskite upon exposure to moisture. Electronic structure analysis supports a novel mechanism of water-mediated vacancy-pair formation.