Growing high-quality CsPbBr ₃ by using porous CsPb ₂ Br ₅ as an intermediate: A promising light absorber in carbon-based perovskite solar cells

CsPbBr ₃ with a large band gap (∼2.3 eV) is a promising material for fabricating perovskite solar cells (PSCs) with a high open-circuit voltage (V _oc ) and high stability. However, a suitable method is still lacking for depositing high-quality CsPbBr ₃ films. Herein, we develop a novel strategy to deposit high-quality CsPbBr ₃ films by employing a porous CsPb ₂ Br ₅ film as an intermediate layer. Highly porous CsPb ₂ Br ₅ films composed of lamellar crystals were obtained by immersing Pb-Br precursor layers in a low-concentration CsBr IPA solution. The low CsBr concentration helped to widen the processing window of CsPb ₂ Br ₅ , while the low-polarity IPA solvent served to keep the film structure stable during the reaction. High-quality CsPbBr ₃ films with full coverage, high purity and low defect density were obtained by further converting the CsPb ₂ Br ₅ films in a high-concentration CsBr solution. After introducing the optimized CsPbBr ₃ film into a carbon-based PSC without a hole transporting material (C-PSC), a power conversion efficiency (PCE) of 6.1% and a V _oc of 1.38 V were achieved. Moreover, the devices without encapsulation show almost no PCE decay after 200 days of storage in ambient air (25-85% relative humidity, 20-30 °C) and at 80 °C (10-20% relative humidity) for over 1080 h.