Bigger Solar

Stacking sunlight without the solvent mess

Solar panels already hum on rooftops and in fields, but researchers keep chasing a smarter stack: a thin perovskite layer riding atop everyday silicon. Together, the pair can catch a wider slice of sunlight than silicon alone. The catch has been how to build that upper layer at factory scale without sacrificing quality.

Solution coating—the workhorse that has delivered many lab records—still struggles to marry industrial throughput with long-term reliability. Thermal evaporation looks far more factory-friendly. Materials are gently heated in vacuum until they rise as vapor and settle into ultrathin, even films. Yet one stubborn ingredient kept getting in the way.

Formamidinium iodide, or FAI—a salt that helps the perovskite crystal form—tends to break down when heated hard enough to evaporate cleanly. That thermal bruise left films less crystalline and less uniform, and it blocked truly large tandem devices made entirely by vapor.

A mixture that lowers the temperature

A team led by researchers including Chao Luo, Rui He, Ran Luo, and Yi Hou has now reported a formamidinium-based eutectic that changes the math. A eutectic is a carefully chosen blend that softens or vaporizes at a lower temperature than its pure ingredients—think of salt lowering the melting point of ice, only here the goal is cooler, cleaner evaporation.

Their eutectic trims the effective evaporation temperature of FAI by an average of 36 °C, pushing the process below the salt’s degradation threshold. FAI can now leave the crucible as intact vapor instead of a partially cooked mess.

With that barrier cleared, the group grew perovskite films by sequential thermal evaporation. The films showed sharper crystallinity and atomic-scale compositional homogeneity—meaning the atoms sat where they belonged, film after film, without the patchy chemistry that can sap performance.

From lab square to half-cut wafer

Those films were married to silicon in tandem solar cells. On a 1 cm² device the stack reached a steady-state efficiency of 31.5 percent. Efficiency here simply means how much of the incoming sunlight becomes usable electricity under continuous operation.

Because evaporation coats large surfaces evenly, the same process scaled onto a commercial half-cut G12 silicon wafer. The resulting 200 cm² tandem delivered a steady-state 30 percent efficiency. Stretching the active area two hundredfold cost only a 3.99 percent relative efficiency drop—the smallest area-scaling penalty the authors know of for perovskite-based tandems.

That uniformity is the quiet hero. In manufacturing, a process that stays honest from a postage-stamp cell to a full wafer half is exactly what factories need.

Holding up when the weather turns

High numbers on day one matter less if the cells wilt. The eutectic-based tandems kept 95 percent of their starting efficiency after 2,000 hours of damp-heat ageing—85 °C and 85 percent relative humidity, a harsh standard stress test. After two months of real outdoor operation they showed negligible power loss.

Those results do not claim a finished product on every rooftop tomorrow. They do show that a vapor route once limited by FAI’s heat sensitivity can now produce large, efficient, and resilient perovskite/silicon tandems on industry-standard wafers.

What the cooler chemistry opens

The advance is less about a single magic number and more about removing a materials bottleneck. When a critical salt can evaporate without cooking itself, the entire vacuum line becomes more trustworthy. Crystallinity improves. Composition stays even across centimeters and, apparently, across hundreds of square centimeters.

Next steps will keep testing how these stacks behave over still longer outdoor campaigns and how the eutectic recipe integrates with full production toolsets. For now, the work offers a hopeful proof: industrial-style thermal evaporation and high-performance perovskite tandems no longer have to be strangers.

Sunlight is free. The craft is learning to catch more of it, more evenly, on the silicon already rolling out of factories—and to do it with a little less heat and a little more patience for the chemistry that makes the catch possible.