Suppressing “Coffee ring effect” to deposit high-quality CsPbI₃ perovskite films by drop casting

Inorganic CsPbI₃ perovskite has shown much promise as a light absorber in perovskite solar cells (PSCs) due to its appropriate optical bandgap and high chemical stability. However, it is still a great challenge to deposit large-area CsPbI₃ perovskite films by existing methods (e.g. spin coating). Herein, a facile and scalable strategy based on a drop-casting process is developed. To mitigate the coffee ring effect (CRE) that commonly occurs during drop casting, a vacuum-assisted (VA) process is exploited to boost solvent extraction, which significantly reduces the difference in solvent evaporation rates at the edge and middle regions of droplets. As a result, uniform CsPbI₃ perovskite films with higher crystallinity and lower defect density are deposited by drop casting. By using the CsPbI₃ perovskite film as a light absorber, the carbon-based CsPbI₃ PSCs without hole transport layer achieves an efficiency of 16.33 %, a new record for inorganic C-PSCs. After finely controlling deposition processes, a large-area CsPbI₃ perovskite film (5 × 5 cm²) with excellent uniformity can be easily prepared by the drop casting method.