Metal halide perovskites (ABX3, where A is typically Cs, methylammonium (MA), or formamidinium (FA), B is Pb or Sn, and X is I, Br, or Cl) are one of the most promising thin-film solar-cell materials thanks to the fact that they can absorb light over a broad range of solar spectrum wavelengths. What is more, the power conversion efficiency (PCE) of solar cells made from these materials has gone up from just 3% to more than 20% in the last five years, which now makes their PCE comparable to that of silicon.

Previous research has shown that a layer of perovskite added to silicon solar cells can actually improve their efficiency, but a tandem device that consists of two all-perovskite cells would be cheaper and easier to make. However, this has proved to be no easy task because it requires materials with band gaps that have not yet been realized in the lab. Indeed, the highest efficiency tandem devices would require a rear cell with a band gap of 0.9 to 1.2 eV and a front cell with a band gap of 1.7 to 1.9 eV.

Two ideally matched cells

A typical perovskite cell harvests photons from solar energy, which cause electrons in the material to jump across the band gap and produce an electric current. A solar cell with a smaller energy gap would absorb most of the photons but produce a very low voltage. A cell with a larger band gap, on the contrary, would generate a higher voltage but let lower energy photons pass right through it.

A good tandem device would consist of two ideally matched cells, explains team member Giles Eperon from Oxford. “The cell with the larger energy gap would absorb higher energy photons and generate an additional voltage, and the cell with the smaller energy gap would harvest photons that are not collected by the first cell.”

Combined efficiency of 20.3%

Making a smaller-gap perovskite has proved to be the most difficult for researchers so far, but Eperon and colleagues have now made a good material from tin, lead, caesium, iodine and organic compounds. Their novel 1.2 eV band gap perovskite, which has the chemical formula FA0.5Cs0.25Pb0.5Sn0.5I3, absorbs lower energy infrared light and has a PCE of 14.8%. When combined with a perovskite made of similar material, FA0.83Cs0.17Pb(I0.5Br0.5)3, but with a larger energy gap of 1.8 eV, the tandem device has a combined efficiency of 20.3%.

While the PCE is obviously important, a photovoltaic material also needs to be stable for long periods when exposed to sunlight, and this has unfortunately not been the case for many perovskites – especially for ones containing tin. However, Eperon’s team tested its cells at elevated temperatures of 100°C for four days and found that they had “excellent thermal and atmospheric stability, unprecedented for tin-based perovskites”.

The researchers, led by Michael McGehee of Stanford and Henry Snaith at Oxford, say they are now busy optimizing the composition of the materials they made so that they absorb more light and generate an even higher current.

The new perovskite material is detailed in Science DOI: 10.1126/science.aaf9717.