Discovery of Twin Defects in Hybrid Perovskite Nanocrystals: Atomic Structure and Impact on Photovoltaic Performance and Stability

Colloidal organic-inorganic hybrid perovskite nanocrystals (PNCs) have attracted considerable interest for their potential in fundamental research and optoelectronic applications. However, understanding the role of structural defects in the properties of optoelectronic devices remains particularly challenging, due to their delicate nature under light, electric bias and in air. In this study, we discover face-sharing twin defects in photoactive Cs_1-<i>x</i>FA_<i>x</i>PbI₃ PNCs, previously thought to be single-crystal. By reducing nanocrystal size, we reduce the prevalence of these twin defects and show this significantly enhances power conversion efficiency, carrier lifetimes and stability (while maintaining other device parameters constant). Using ultralow dose electron microscopy, we determine the atomic structure of the twin boundary and reveal at the atomic-level why it is harmful to both performance and structural stability. These findings establish at the atomic-level the detrimental role of face-sharing twin defects in perovskite photovoltaics and demonstrate how they can be eliminated by nanocrystal size control for improved photophysical properties and structural stability.