Growing high-quality CsPbBr3 by using porous CsPb2Br5 as an intermediate: a promising light absorber in carbon-based perovskite solar cells

CsPbBr3 with a large band gap (approximate to 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 CsPbBr3 films. Herein, we develop a novel strategy to deposit high-quality CsPbBr3 films by employing a porous CsPb2Br5 film as an intermediate layer. Highly porous CsPb2Br5 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 CsPb2Br5, while the low-polarity IPA solvent served to keep the film structure stable during the reaction. High-quality CsPbBr3 films with full coverage, high purity and low defect density were obtained by further converting the CsPb2Br5 films in a high-concentration CsBr solution. After introducing the optimized CsPbBr3 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 degrees C) and at 80 degrees C (10-20% relative humidity) for over 1080 h.