A problem with the current generation of silicon solar cells is their relatively low efficiency at converting solar energy into electricity. The best silicon solar cells in the lab are about 26% efficient while commercial cells are about 15%. That means bigger systems are necessary to produce a given amount of electricity and bigger systems mean higher costs.

That has researchers looking for new ways to raise efficiency and decrease costs. One idea that could boost efficiency by as much as 50% is a tandem structure that stacks two kinds of cells on top of each other, each using different, complementary parts of the solar spectrum to produce power.

Researchers have recently started looking at hybrid organic-inorganic perovskite materials as a good tandem partner for silicon cells. Perovskite cells have efficiency rates nearing 25%, have a complementary bandgap, can be very thin (just a millionth of meter), and can easily be deposited on silicon. But the hybrid perovskite solar cells break down when exposed to high temperatures.

That's a problem for solar arrays placed where the sunshine is — such as hot, dry deserts. Ambient temperatures in such places can hit 120 to 130 °F and solar cell temperatures can hit 200 °F.

Engineers have found a way to take advantage of perovskite's useful properties while stabilizing the cells at high temperatures. First, the engineers tweaked the makeup of the perovskite material, doing away with organic components in the material — particularly cations, materials with extra protons and a positive charge — and substituted inorganic materials such as cesium. That made the material stable at higher temperatures.

They also developed a fabrication technique that builds the perovskite material one thin layer — just a few billionths of a meter — at a time. This vapor deposition technique is consistent, leaves no contaminants, and is already used in other industries, so it can be scaled up for commercial production.

The resulting perovskite solar cells showed no thermal degradation, even at 390 °F for more than three days — temperatures far more than what the solar cell would have to endure in real-world environments. The new inorganic perovskite solar cells have a photoconversion efficiency of 11.8%.

The engineers replaced the iodine common in perovskite materials with bromine. That made the cells much less sensitive to moisture, solving another problem with standard hybrid perovskites. But that substitution changed the cells’ properties, reducing efficiency and how well they work in tandem with silicon cells.

For more information, contact Vikram Dalai, Microelectronics Research Center, at This email address is being protected from spambots. You need JavaScript enabled to view it.; 515-294-2664.