Light-Induced Defect Dynamics in Lead-Halide Perovskites Revealed by X-Ray Photon Correlation Spectroscopy

The efficiency of solution-processed lead-halide perovskite (LHP) thin films in solar cells has surged in recent years, with power conversion efficiencies now exceeding 25%. However, their long-term stability remains a major challenge for commercial adoption. A key factor limiting stability is the high concentration of point defects, such as Frenkel defects, in these materials. These electrically charged and mobile defects significantly impact solar cell performance, contributing to transient phenomena like hysteresis, burn-in, and degradation at interfaces and at electrical contacts.
To address the underlying mechanisms of defect dynamics, we have employed X-ray Photon Correlation Spectroscopy (XPCS), a synchrotron-based technique capable of probing the motion of defects in real time. Our experiments reveal a novel light-induced effect in perovskite thin films, where illumination triggers diffusion of charged defects, likely ions or vacancies, due to locally induced potential differences. XPCS directly measures this collective defect motion through changes in the diffuse scattering “speckle” pattern, offering new insights into the timescales and reversibility of these processes.
Unlike traditional techniques such as photoluminescence (PL) or capacitance spectroscopy, which rely on indirect measurements or require full devices, XPCS provides a direct probe of defect dynamics in thin films, enabling the measurement of phenomena on timescales as short as milliseconds. The observed effect is highly reversible, with defect motion rapidly returning to equilibrium after changes in light or voltage conditions, suggesting that defect dynamics in perovskites may be more tunable than previously understood.
This discovery raises important questions about the role of light-induced defect motion in perovskite stability and performance and opens new avenues for engineering defect tolerance in these materials. Furthermore, the application of XPCS to probe defect dynamics could be extended to other materials, such as ionic conductors, where similar light-induced effects may be prevalent. Our work lays the foundation for using coherent X-ray scattering methods to study materials with complex defect behavior, advancing both fundamental understanding and practical material design.