Over 24% efficient MA-free CsxFA1−xPbX3 perovskite solar cells

Abstract

<h2>Summary</h2> Methylammonium (MA)-free perovskites such as Cs_xFA_1−xPbX₃ (CsFA) show excellent thermal stability, favoring their use in perovskite solar cells (PSCs). However, the power conversion efficiency (PCE) of CsFA lags behind that of MA-containing formulations due to the difficulty in controlling the film morphology. Herein, we report innovative morphology engineering using solution deposition of a mixture of CsBr, PbI₂, and PbCl₂ followed by vacuum evaporation of formamidinium iodide (FAI) to fabricate CsFA films. The introduction of a chloride precursor leads to larger grain sizes, forming a high-quality CsFA film under annealing. This enables the realization of robust CsFA-based PSCs with the highest reported PCE up to 24.1% (certified 23.9%) for 0.1 cm² and 22.8% for 1 cm² devices. Moreover, the unencapsulated evaporation device shows excellent stability with negligible efficiency decline after 20,000 h of storage in dry air or 1,000 h of exposure to 50% humidity, which is much better than the device prepared using the solution procedure.