Thermal evaporation enables 31.5% efficient perovskite/silicon tandem solar cells

Researchers have developed a formamidinium-based eutectic that reduces the effective evaporation temperature of formamidinium iodide by an average of 36°C, bringing it below the material's degradation threshold. This advance enables thermal evaporation of perovskite films without the thermal decomposition that previously prevented this manufacturing method from being used for high-performance tandem solar cells.

The evaporated perovskite films show enhanced crystallinity and atomic-scale compositional homogeneity compared to solution-processed alternatives. Using sequential thermal evaporation, the team fabricated perovskite/silicon tandem cells achieving a steady-state efficiency of 31.5% on a 1 cm² active area.

The uniformity of the evaporation process allowed demonstration of the first thermally evaporated large-area perovskite/silicon tandem on a commercial half-cut G12 wafer. The 200 cm² device delivered a steady-state efficiency of 30.0%, with scaling from 1 cm² to 200 cm² incurring only a 3.99% relative efficiency loss.

This scaling penalty represents the lowest reported efficiency loss for area scaling in perovskite-based tandem devices. The eutectic-based tandem retained 95% of its initial efficiency after 2,000 hours of damp-heat aging at 85°C and 85% relative humidity.

The devices also showed negligible power loss after two months of real-world outdoor operation. Thermal evaporation is considered more industrially viable than solution processing for large-scale manufacturing, but had not previously been successfully demonstrated for large-area perovskite/silicon tandems.

The formamidinium eutectic approach addresses the key barrier of formamidinium iodide thermal degradation during high-temperature evaporation. By enabling stable evaporation of this critical perovskite component, the method combines the manufacturing advantages of thermal evaporation with the high performance of formamidinium-based perovskites.