Manipulating phase distribution and transformation kinetics by substrate clamping for DMAxCs1-xPbI3 perovskite solar cells

The black-phase CsPbI₃, a Br-free wide-bandgap perovskite with promising light and thermal stability, tends to transition to a non-perovskite yellow phase at room temperature, posing a significant challenge for its application in solar cell devices. To tackle this issue, dimethylammonium iodide (DMAI) is frequently included to form black phase DMA_xCs_1-xPbI₃ to accommodate the tolerance factor, however, its phase behavior and effect on device stability are still not well understood. In this work, we employed transient reflection spectroscopy with front and back laser excitation to investigate the phase transformation kinetics and the vertical phase distribution of DMA_xCs_1-xPbI₃ films during thermal annealing. Our results indicate that these properties are influenced by the choice of substrate. During prolonged annealing, DMAI partially evaporates, causing lattice reorganization and leading to surface degradation. For substrates with a strong clamping effect, the surface DMA_xCs_1-xPbI₃ transitions to a black phase CsPbI₃, whereas for those with a weak clamping effect, it degrades directly to the yellow phase. Importantly, films of DMA_xCs_1-xPbI₃ exhibit improved stability when annealed close to the boundary where the black CsPbI₃ phase emerges at the surface, particularly when supported by an organic substrate that has a suitable thermal expansion coefficient and strong coordination with perovskite.