Bypassing the non-perovskite yellow phase: Revealing and regulating the crystallization pathways for efficient all-inorganic perovskite solar cells

All-inorganic CsPbX3 perovskites have shown great potential in the photovoltaic community due to their excellent thermal stability and suitable bandgap for tandem applications. However, the formation of high-quality all-inorganic perovskite films remains a grand challenge, which requires delicate control over the high-temperature annealing process as the photo-inactive orthorhombic δ-phase is more stable than the photoactive perovskite phase at room temperature. This study revealed the formation of a black γ-phase at low temperature (LT-γ-phase), which is temporarily stabilized by the dimethyl sulfoxide (DMSO) solvent. It collapses to δ-phase when DMSO evaporates and restores to γ-phase at high temperature (HT-γ-phase). Remarkably, we found that an additional methylammonium chloride (MACl) treatment further stabilizes the LT-γ-phase, allowing a direct transition from LT-γ-phase to HT-γ-phase while bypassing the δ-phase, resulting in a uniform and pinhole-free film with reduced defects and released strain. An efficiency as high as 20.1 % was obtained and the device stability under illumination and humidity were both enhanced. This work demonstrates that understanding and control over the crystallization pathway are essential for developing high-performance and high-stability perovskite solar cells for real-life applications.