Tandem Solar Cells Keep 90% After 1,000 Hours

Laboratory perovskite/silicon tandem solar cells led by researchers at King Abdullah University of Science and Technology (KAUST) kept 90% of their starting efficiency after more than 1,000 hours under full sunlight-strength light at 65C, according to a paper published in Nature Energy on 29 October.

The team, with senior author Stefaan De Wolf, says the result comes from choosing perovskite recipes with a “softer” crystal lattice that copes better with heat. The paper calls this property lattice compliance: “the ability of the crystal lattice to elastically accommodate stress”.

The same findings appear in the public abstract of a KAUST PhD dissertation by Xuechun Zhang, one of the paper’s three joint first authors, which De Wolf supervised. Fuzong Xu and Jiang Liu are the other joint first authors. Co-authors include researchers at Soochow University, Southeast University and Huazhong University of Science and Technology in China, and Marmara University in Turkey.

These are small lab cells, not solar panels. The paper and thesis report device results, and none of the sources gives a timeline for products.

What the team built and tested

A tandem cell stacks a perovskite layer on top of a silicon cell so that each captures a different part of the sunlight. A bifacial tandem also collects light that reaches its back face, such as sunlight reflected from the ground.

The researchers used perovskite absorbers with a band gap of about 1.55 electronvolts. That choice needs less bromide in the recipe, which the paper says “largely” avoids a known problem in which light separates the mixture of halide ions inside the material. With that problem reduced, the main remaining threat becomes how well the perovskite survives light and heat together.

Across the recipes they studied, the team found that more compliant lattices were more stable. A lattice that can flex under stress built up less strain when heated, its defects sat at shallower, less harmful energy levels, and the edges of its energy bands were less disordered. The thesis abstract adds that compliant lattices “suppress Urbach-tail broadening and nonradiative recombination”, two signs of energy being lost as heat rather than electricity.

Using the best recipe, the bifacial tandems produced 36.2 milliwatts per square centimetre with rear light equal to 20% of the front (a 0.20 albedo), or about 362 watts per square metre by simple conversion. They kept 90% of their initial efficiency after more than 1,000 hours of ISOS-L-2 testing, a standard lab stability protocol run here at 1-sun light and 65C. That is about six weeks of continuous stress. The thesis also reports a single-junction perovskite cell at 26.3% efficiency.

The work was funded by KAUST, the National Natural Science Foundation of China and others, and the authors declare no competing interests.

Why heat, not halide segregation, is the next test for tandems

This is TSN’s analysis. Perovskite/silicon tandems already beat silicon alone on efficiency in the lab. What has held them back is durability, because perovskites degrade under light, heat and moisture far faster than silicon. This paper does not solve that, but it narrows the problem in a useful way.

By moving to a lower-bromide perovskite, which bifacial tandems can use because the rear side adds extra current, the team sidesteps one failure mode and is left with another: heat. Instead of testing recipes by trial and error, they offer a property that can be measured in advance. If lattice compliance holds up as a predictor in other labs, it gives researchers a quicker way to rule out recipes that will not last.

The limits are clear. A 1,000-hour indoor test is weeks; panels on a roof are expected to last decades, outdoors, through cycles of heat, cold and humidity. The thesis abstract also notes that temperature and rear light can shift which layer limits the cell’s current, which matters in real installations. Scale is another step. Japan’s Ricoh is preparing to mass-produce inkjet-printed perovskite cells, and flexible perovskite modules have reached 20.2% at commercial size, but each move from cell to module has cost efficiency. Like Oxford’s recent work on bismuth sulfide charge losses, this is a materials advance, and the field test comes later.