Stability

A collaborative research team from Los Alamos National Laboratory, Rice University, Purdue University, Northwestern University, Institut FOTON CNRS UMR 6082 (France) and Argonne National Laboratory has created a number of hybrid perovskite solar cells with a FA_0.7MA_0.25Cs_0.05PBI₃ composition and measured them using a variety of techniques including grazing-incidence wide-angle x-ray scattering (GIWAXS) maps at the X-ray Science Division 8-ID-E x-ray beamline of the APS (an Office of Science user facility at Argonne).

The experimental setup (top left) and the corresponding light-induced lattice expansion effect, which leads to curing defects and relieving of lattice strain (bottom left) and as a result an increase in the open circuit voltage of a solar cell

In most of the cells, the researchers noted a substantial improvement in PCE from 18.5% to 20.5% under continuous light soaking with a 1-sun (100 mW/cm²) source as the lattice structure of the hybrid cells uniformly expanded. This expansion relieved local strains in the bulk material and better aligned the crystal planes, as evidenced by narrowing and uniform shifting of the Bragg peaks toward lower scattering values as seen by GIWAXS. The researchers explain that constant illumination generates electron-hole pairs in the perovskite material, decreasing the distortions of some bonds while elongating others, resulting in a generalized lattice expansion and relaxation. A similar phenomenon was seen with pure MAPbI₃ thin films, suggesting that such lattice expansion under light is common for hybrid perovskite materials.

Researchers at the Eindhoven University of Technology in the Netherlands have found a way to address the issue of stability in perovskite solar cells by adding a small amount of fluoride during the production process, which was found to increase the stability of such cells.

Fluoride stablizes perovskite solar cells by encouraging the formation of strong hydrogen bonds and ionic bonds on the surface of the perovskite material.

The scientists found the fluoride ions form a protective layer around perovskite crystals, preventing the ill effects of light, heat and moisture. "Our work has improved the stability of perovskite solar cells considerably," said Shuxia Tao, assistant professor at Eindhoven University of Technology's Center for Computational Energy Research. "Our cells maintain 90% of their efficiency after 1,000 hours under extreme light and heat conditions. This is many times as long as traditional perovskite compounds. We achieve an efficiency of 21.3%, which is a very good starting point for further efficiency gains."