Imaging Functional Microstructures to Understand the Working Mechanism of Perovskite Solar Cells in Operation

The performance and stability of solar cells are typically assessed through macroscopic photophysical and electrical measurements. These observed bulk properties result from the convolution of microscopic structural, chemical, and functional properties, which are influenced by defects, carrier transport, and chemical reactions under external stimuli such as light, bias, and ambient conditions. This is especially true for soft and dynamic light-harvesting materials like halide perovskites. Therefore, understanding the functionality of the microstructures in a device, particularly under operational conditions, is crucial for accurately interpreting and enhancing device performance. Conventional techniques such as scanning electron microscopy, and electron/ x-ray-based analytical methods can provide high spatial resolution but are mostly limited to structural characterization. Moreover, these surface-sensitive or invasive techniques often alter the material properties.
To address these challenges, we have implemented a microscale functional imaging method (CLIM) that utilizes photoluminescence fluctuations to reveal contrasts associated with defect dynamics in semiconductor materials. CLIM images correlated with SEM reveal crucial information about the structure-function relationship in the bare thin films. Particularly noteworthy is the large amplitude fluctuation of photoluminescence of these films when incorporated in a solar cell. The local functional regions in a solar cell are much larger as compared to the bare film. Moreover, the fluctuation amplitude and functional regions strongly depend on the device's operational regime. From the statistical analysis of intensity fluctuations, we provide insights into the type of metastable defects responsible for fluctuating non-radiative recombination processes in thin film and operational solar cells.
The insights gained from microscale functional imaging contribute to a deeper understanding of device efficiency, structure, and durability, which are crucial for the rational engineering of the next generation of devices.