All-inorganic halide perovskites hold promise for improving the thermal stability of perovskite solar cells (PSCs). However, so far, the moisture sensitivity of all-inorganic perovskites significantly limits scalable fabrication of high-quality perovskite thin films over large areas.  

Upscaling of uniform and pinhole-free coatings is further complicated by the fluid dynamics of the ink and its solidification mechanisms. Therefore, it’s necessary to understand and control the phase transformation behavior, fluid dynamics, and moisture sensitivity for inorganic perovskites to be upscaled and amenable to ambient manufacturing.  

Recently, Prof. LIU Shengzhong (Frank) from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences, in collaboration with Prof. ZHAO Kui from Shaanxi Normal University and Prof. Aram Amassian from King Abdullah University of Science and Technology, demonstrated that good-controlled film formation dynamics during ambient scalable blading not only helped in constructing highly-crystalline uniform pinhole-free all-inorganic perovskite (CsPbI₂Br) films, but also had strong capability to enhance the ambient phase stability in contrast to conventional spin-coated films.  

They investigated the fluid dynamics and structural evolution during film fabrication and a high-performance large (1.0 cm²) solar cell was achieved. This study was published in Joule.  

"For the first time, we investigated the coupling between the fluid dynamics and the structural evolution during controlled film formation for ambient-air scalable fabrication of CsPbI₂Br perovskite films using blade-coating. We simultaneously overcame the negative influences of moisture attack and the Bénard-Marangoni instability in the drying ink, and achieved an ideal sequential crystallization with changing halide composition during film formation." Prof. ZHAO said. 

The qualitative change in the solid-state microstructural outcome pointed to the critical role of the film formation dynamics. The fluid could be affected by the Bénard-Marangoni convection, which was driven by temperature-gradient induced surface tension variations. This suggested the Bénard-Marangoni convection induced by the thermal gradient was very strong and could not be neglected in CsPbI₂Br fluid.  

These results highlighted the importance of the film formation dynamics for scalable ambient fabrication of high-quality CsPbI₂Br film. Large-area, highly-crystalline and uniform pin-hole free CsPbI₂Br film could be achieved when overcoming the negative influences of moisture attack and the Bénard-Marangoni instability in the drying ink. 

"We overcame the challenges of film formation and crystallization during ambient scalable fabrication of all-inorganic CsPbI₂Br perovskite films, demonstrating impressive PCEs of 14.7% for 0.03 cm² cell and a 12.5% for large-area (1.0 cm²) cell in planar perovskite solar cells," Prof. ZHAO said. 

This study represents a significant step toward industrial implementation of perovskite solar cells by providing important lessons on controlling the solidification of all-inorganic perovskite inks to achieve superior thin film quality and PCE than lab-scale processes all the while using scalable meniscus-guided coating processes in ambient air.