Impact of Grain Boundaries on Efficiency and Stability of Organic-Inorganic Trihalide Perovskites

TL;DR

This study uses nanoscale imaging to analyze how grain boundaries affect the efficiency and stability of perovskite solar cells, revealing that bulk properties and water diffusion impact device performance and degradation.

Contribution

First quantitative nanoscale photoconductivity imaging of MAPbI3 films showing uniform response across grains and boundaries, and elucidating degradation mechanisms.

Findings

Photo-response is uniform across grains and boundaries.

Carrier mobility and lifetime depend on crystallinity.

Degradation starts with large grain disintegration due to water diffusion.

Abstract

Organic-inorganic perovskite solar cells have attracted tremendous attention because of their remarkably high power conversion efficiencies (PCEs). To further improve the device performance, however, it is imperative to obtain fundamental understandings on the photo-response and long-term stability down to the microscopic level. Here, we report the first quantitative nanoscale photoconductivity imaging on two methylammonium lead triiodide (MAPbI3) thin films with different PCEs by light-stimulated microwave impedance microscopy. The intrinsic photo-response is largely uniform across grains and grain boundaries, which is direct evidence on the inherently benign nature of microstructures in the perovskite thin films. In contrast, the carrier mobility and lifetime are strongly affected by bulk properties such as the sample crystallinity. As visualized by the spatial evolution of local photoconductivity, the degradation due to water diffusion through the capping layer begins with the disintegration of large grains rather than the nucleation and propagation from grain boundaries. Our findings provide new insights to improve the electro-optical properties of MAPbI3 thin films towards large-scale commercialization.