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/silicon tandems, largely due to the thermal degradation of formamidinium iodide (FAI) during high-temperature evaporation. Here we synthesize a formamidinium-based eutectic 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 have enhanced crystallinity and atomic-scale compositional homogeneity. Sequentially evaporated perovskite/silicon tandems achieve a steady-state efficiency of 31.5% (1 cm²). Benefiting from the uniformity of evaporation, we further demonstrate a thermally evaporated large-area perovskite/silicon tandem on a commercial half-cut G12 wafer, delivering a steady-state efficiency of 30% (200 cm²). Scaling the device area from 1 cm² to 200 cm² incurs only a 3.99% relative efficiency loss, representing, to our knowledge, the lowest reported efficiency penalty for area scaling in perovskite-based tandems. The eutectic-based tandem retains 95% of its initial efficiency after 2,000 h of damp-heat ageing (85 °C and 85% relative humidity) and has negligible power loss after 2 months of real-world outdoor operation.