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

Context & scaleBased on their excellent optoelectronic properties and low cost, metal halide perovskites have become a rising star for next-generation photovoltaics. Methylammonium (MA)-containing perovskites are popular compositions for perovskite solar cells (PSCs) because of their high efficiency. Nevertheless, MA tends to evaporate, especially under heating, reducing the thermal stability of the perovskite material. Here, we combined the traditional solution deposition method with a vacuum deposition procedure to prepare an MA-free CsxFA1−xPbX3 (CsFA) perovskite layer. Our new process guarantees a homogeneous film, and 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 high efficiency and excellent stability.Highlights•Vacuum evaporation of FAI enables uniform MA-free CsxFA1−xPbX3 perovskite film•Introduction of chlorine to the precursor improves perovskite film quality•Over 24% efficient CsxFA1−xPbX3 PSCs•95% of the initial PCE maintained after storage in dry air for over 20,000 hSummaryMethylammonium (MA)-free perovskites such as CsxFA1−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.Graphical abstract