Researchers at the Adolphe Merkle Institute in Fribourg and the Ecole Polytechnique Fédérale de Lausanne have developed a new technique to replace one of the least stable components in perovskite solar cells, which could be a major step towards commercialization.

Perovskites are seen as promising thin-film solar-cell materials because they can absorb light over a broad range of solar spectrum wavelengths thanks to their tuneable bandgaps. Charge carriers (electrons and holes) can also diffuse through them quickly and over long lengths. The most efficient perovskite solar cells usually contain bromide and MA, which is thermally unstable. To overcome this problem, researchers tried replace MA with FA since it is not only more thermally stable but also has an optimal redshifted bandgap. Unfortunately, because of its large size, FA does distort the perovskite lattice and tends to produce a photoinactive “yellow” phase at room temperature. The other photoactive “black phase” can only be seen at high temperatures. However, the researchers in this new work have now found a way to stabilize the black phase of FA at room temperature.

They did this by replacing the bromine with iodine and the MA with a combination of rubidium and caesium. The resulting cells have a PCE of 20.35%, which is one of the best-reported efficiencies yet for a non-MA perovskite solar cell. “As well as being thermally stable, the new material is also compatible with a planar architecture, which means that flexible solar cells are an exciting possibility” the team says.

Replacing the organic MA with the inorganic FA was only one step, the team adds. “Previous attempts to stabilize the photoactive black phase of FA came at the cost of a blue-shifted bandgap, which absorbs less light and thus reduces device performance. We succeeded in removing Br and still achieved a phase-stable perovskite with a smaller bandgap that can collect more light and therefore produce more electricity.”

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