The most efficient perovskite-on-silicon tandem solar cells are made by solution processing, a method that works in the lab but struggles on industrial production lines, where uniformity and reliability matter more than a record number. Thermal evaporation, the technique the semiconductor industry already knows, should be the scalable route, but it has always hit a wall: the formamidinium iodide that gives perovskites their best properties degrades at the temperatures needed to evaporate it.
A team led by the Solar Energy Research Institute of Singapore (SERIS) at the National University of Singapore, with Trina Solar as industry partner, has removed that wall. In Nature, they report synthesizing a formamidinium-based eutectic that lowers the effective evaporation temperature of formamidinium iodide by an average of 36 degrees Celsius, dropping it below its degradation threshold. The result is stable evaporation without thermal damage, films with enhanced crystallinity and atomic-scale compositional homogeneity, and the first-ever thermally evaporated large-area perovskite/silicon tandem on a commercial wafer.
The numbers that matter
The small-area device reached 31.5 percent steady-state efficiency on 1 square centimeter. On a commercial 200-square-centimeter half-cut G12 wafer, the tandem hit 30.0 percent. The loss in scaling from 1 to 200 square centimeters was a 3.99 percent relative efficiency penalty, the lowest reported for perovskite-based tandems and a direct payoff of evaporation’s inherent film uniformity.
Reliability, historically the weak point of perovskites, held up under stress: the devices retained 95 percent of initial efficiency after 2,000 hours of damp-heat aging at 85 degrees Celsius and 85 percent relative humidity, and showed negligible power loss after two months of outdoor operation. The milestone, the authors note, is not just the efficiency but the proof that an industry-compatible deposition method can carry it.
Why the eutectic is the trick
Formamidinium iodide, the A-site cation that gives formamidinium-based perovskites their superior thermal stability and bandgap, has a high evaporation temperature and decomposes before it evaporates cleanly. A eutectic, a mixture whose melting point is lower than either component’s, changes the thermodynamics. By forming a formamidinium-based eutectic, the team lowered the effective evaporation temperature by 36 degrees, keeping the material below its degradation threshold during deposition.
The choice of material was not arbitrary. The work draws on the broader push toward formamidinium-caesium metal halide perovskites and on the observation that volatile loss and cation segregation under high-temperature operation are the main threats to stability. By fixing the evaporation problem at the source, the team made the rest of the process (film growth, interface engineering, cell integration) behave like a mature manufacturing technology.
What it means
The significance is industrial. Solution processing of perovskite/silicon tandems has produced the headline efficiencies of recent years, but manufacturing lines favor evaporation, which is already used in established photovoltaic and display industries and offers better thickness control over large areas. This paper demonstrates that the performance gap between the two routes can be closed, and that the evaporated route carries an area-scaling advantage.
The tandem design stacks a perovskite cell on a silicon cell to capture more of the solar spectrum. With 30 percent efficiency demonstrated on a commercial-sized wafer and damp-heat stability at 2,000 hours, the technology clears two of the three hurdles (performance and durability) that have kept perovskite tandems out of volume production. The third, cost at scale, is where evaporation’s industrial familiarity helps most.
The work involved collaborators across Singapore, China, and the United States, including IMRE at A*STAR, Peking University, the University of Electronic Science and Technology of China, the Institute of High Energy Physics of the Chinese Academy of Sciences, Soochow University, and Caltech-adjacent computation by Wan-Jian Yin’s group. The paper is published as an early-access manuscript, meaning it will undergo final editing before definitive publication.
Sources
- Luo, C., He, R., Ran, L., Hu, J., Wang, Y. et al. “Thermally evaporated perovskite/silicon tandems via formamidinium eutectic.” Nature (2026). DOI: 10.1038/s41586-026-10970-1