Hybrid perovskites have already shown very high photovoltaic efficiencies of greater than 25%. Achieving this impressive performance is essential because they are believed to suppress defect-assisted carrier recombination.
Although this assumption is incorrect, suggests a new study by UC-Santa Barabara. It also suggests that all-inorganic materials have the potential for outperforming hybrid perovskites.
Xie Zhang, a lead researcher on the study, said, “To compare the materials, we performed comprehensive simulations of the recombination mechanisms. When light shines on a solar-cell material, the photo-generated carriers generate a current; recombination at defects destroys some of those carriers and lowers the efficiency. Defects thus act as efficiency killers.”
Scientists studied two prototype materials to compare inorganic and hybrid perovskites. Both materials had lead and iodine atoms. However, one of the materials had a crystal structure completed by the inorganic material cesium, whereas, in the other, the organic methylammonium molecule is present.
Experimentally sorting the processes is quite difficult. Hence, they went with advanced quantum-mechanical calculations for the prediction of recombination rates.
Mark Turansky, a senior graduate student in the group, said, “Our methods are potent for determining which defects cause carrier loss. It is exciting to see the approach applied to one of the critical issues of our time, namely the efficient generation of renewable energy.”
When they ran simulations, they found that common defects in both materials give rise to comparable (and relatively benign) levels of recombination. However, the organic molecule in the hybrid perovskite can break up; when loss of hydrogen atoms occurs, the resulting “vacancies” strongly decrease efficiency. The molecule’s presence is thus a detriment rather than an asset to the overall efficiency of the material.
Van de Walle said, “Why, then, has this not been noticed experimentally? Mainly because it is more difficult to grow high-quality layers of all-inorganic materials. They tend to adopt other crystal structures, and promoting the formation of the desired structure requires greater experimental effort. Recent research has shown, however, that achieving the preferred structure is feasible. Still, the difficulty explains why the all-inorganic perovskites have not received as much attention to date.”
“We hope that our findings of the expected efficiency will stimulate more activities directed at producing inorganic perovskites.”
- Xie Zhang et al. All-inorganic halide perovskites as candidates for efficient solar cells. DOI: 10.1016/j.xcrp.2021.100604