Inorganic Perovskite Absorbers for Use in Thin-Film Solar Cells

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A team at the Helmholtz Center Berlin has succeeded in producing co-evaporation of inorganic perovskite thin films at moderate temperatures - no post-heat treatment at high temperatures is necessary. 

Image Caption: By coevaporation of cesium iodide and lead iodide, thin layers of CsPbI ₃ can be prepared even at moderate temperatures. A cesium excess leads to stable perovskite phases. Copyright: J. Marquez-Prieto / HZB

As a result, it is much easier to produce thin-film solar cells made of this material. Inorganic perovskites are more thermally stable than the hybrid organometallic perovskites. 

Teams around the world are working hard to develop perovskite solar cells. The focus is on so-called metalorganic hybrid perovskites, whose crystal structure is composed of both inorganic elements such as lead and iodine and an organic molecule.

Fully inorganic perovskite semiconductors such as CsPbI₃ have the same crystalline architecture as hybrid perovskites, but contain an alkali metal such as cesium instead of an organic molecule. This makes them much more stable than hybrid perovskites but usually requires a very high temperature manufacturing step of several hundreds of degrees Celsius. For this reason, it has been difficult to integrate inorganic perovskite semiconductors into thin-film solar cells that can not tolerate high temperatures. Now it's a team around Dr. Thomas Unold succeeded in producing inorganic perovskite semiconductors at moderate temperatures so that they could also be used in various thin-film cells in the future.

For this, the physicists designed an innovative experiment that allowed them to synthesize and analyze many combinations of materials within a single sample. Co-evaporation of cesium iodide and  lead iodide  produced thin layers of CsPbI₃, systematically varying the excess of elements in the atmosphere. The substrate temperature was below 60 degrees Celsius.

"Such a combinatorial approach will allow finding optimal manufacturing parameters for new material systems much faster than the traditional approach, which typically requires 100 samples to be made for 100 compositions," explains Unold. Through careful analysis during the synthesis and subsequent measurements of the optoelectronic properties, they were able to determine how the composition of the thin film affects the material properties.

Their measurements show that both the structural and important optoelectronic material properties are sensitive to the ratio between cesium and lead. Thus, a cesium excess allows a stable perovskite phase with good mobility and lifetime of the charge carriers.

In collaboration with the junior research group of Prof. Steve Albrecht at HZB, these optimized CsPbI₃ layers demonstrated perovskite solar cells with an efficiency of more than 12% and a stability of more than 1200 hours. "We have shown that even inorganic perovskite absorbers could be suitable for use in thin-film solar cells, if they can be produced accordingly. We currently assume that such components can still be optimized very strongly," says Unold.