Strain Evolution in Hybrid Perovskite Thin Films Revealed by Simultaneous In Situ Multi-Modal Substrate Curvature, Spectroscopic and Structural Characterizations

Hybrid metal halide perovskites are promising to displace other thin film photovoltaic technologies thanks to their high power conversion efficiency (PCE) up to 25.7%, low materials and fabrication costs, and overall ease of manufacturing. Hybrid perovskite thin films deposited on inorganic substrates are believed to exhibit a tensile strain due to mismatch of coefficient of thermal expansion (CTE), which is more than one order of magnitude larger for the hybrid perovskite than typical inorganic substrates like glass and silicon. For photovoltaic devices, the film strain is inevitably present, and several studies have linked device performance and stability to the presence of residual strain. Much effort has therefore been made to engineer the film strain, relying primarily on ex situ characterization methods. Therefore, significant questions remain about the origins of strain, its low magnitude in some instances, and its relationship to solution processing and halide diffusion in hybrid perovskites.<br/><br/>In this work we report, for the first time, integration of multi-beam optical sensor (MOS) experiments together with the entire solution processing and annealing workflows of metal halide perovskite thin films to measure in real time the changes in substrate curvature during solution processing and thin film formation. By combining <i>in situ</i> curvature measurements with <i>in situ</i> photoluminescence, absorbance, and GIWAXS measurements we reveal, for the first time how the anti-solvent drip triggers the phase transformation of hybrid perovskite near the surface of a supersaturated sol. Phase transformation leads to considerable volume reduction which is felt by the substrate within seconds. This insight reveals that even conversion of a small fraction of the sol into perovskite can cause strain evolution in the substrate, indicating substantial mechanical coupling to the substrate. We go on to demonstrate that thermal annealing causes compressive strain to form due to CTE mismatch, with tensile strain developing during subsequent cooling of the sample. The magnitude of the strain is considerably lower than predicted by the CTE mismatch and points to stress relaxation behavior at elevated temperature, which we will discuss briefly.