Thermally evaporated perovskite/silicon tandems via formamidinium eutectic

Solution processing remains the dominant route to high-performance perovskite/silicon tandems, but it remains challenging to simultaneously achieve industrial scalability and long-term reliability.1-3 Thermal evaporation is more industrially viable, yet it has not been successfully demonstrated for large-area perovskite/Si tandems, largely due to the thermal degradation of formamidinium iodide (FAI) during high-temperature evaporation. Here, we synthesize a formamidinium-based eutectic (Eu) that lowers the effective evaporation temperature of FAI by an average of 36 °C - below its degradation threshold - thereby enabling stable FAI evaporation without thermal degradation. As a result, the evaporated perovskite films exhibit enhanced crystallinity and atomic-scale compositional homogeneity. Sequentially evaporated perovskite/Si tandems achieve a steady-state efficiency of 31.5% (1 cm2). Benefiting from the uniformity of evaporation, we further demonstrate the first thermally evaporated large-area perovskite/Si tandem on a commercial half-cut G12 wafer, delivering a steady-state efficiency of 30.0% (200 cm2). Scaling the device area from 1 to 200 cm2 incurs only a 3.99% relative efficiency loss, representing the lowest reported efficiency penalty for area scaling in perovskite-based tandems. The Eu-based tandem retains 95% of its initial efficiency after 2000 hours of damp-heat aging (85 °C/85% RH) and exhibits negligible power loss after two months of real-world outdoor operation.