Team shows great promise with inorganic perovskite solar cells to improve solar cell efficiency

Inorganic perovskite compares well with its hybrid counterparts in terms of efficiency. credit: Xie Zhang

The organic and inorganic hybrid perovskite has already demonstrated a high efficiency in photovoltaic cells of more than 25%. The prevailing wisdom in the field is that organic molecules (containing carbon and hydrogen) in the material are essential to achieve this amazing performance because they are believed to prevent defect-assisted carrier recombination.

New research in the Department of Materials at the University of California, Santa Barbara, shows not only that this assumption is incorrect, but also that all inorganic materials have the potential to outperform hybrid perovskite. The results were published in an article titled “All-inorganic halide perovskites as candidates for efficient solar cells,” which appeared on the cover of the October 20 issue of the journal. Cell Reports Physical Sciences.

“To compare the materials, we performed comprehensive simulations of the recombination mechanisms,” explained Xie Zhang, the study’s principal investigator. “When light is shone on the material of the solar cells, the photo-generated carriers generate current; recombination at defects destroys some of those carriers and thus reduces efficiency. Thus, defects act as an efficiency killer.”

To compare the inorganic and hybrid perovskite, the researchers studied two primary model materials. Both substances contain atoms of lead and iodine, but in one substance the crystal structure is completed by the inorganic element cesium, while in the other there is the organic molecule methylammonium.

Experimentally sorting out these processes is very challenging, but the latest quantum mechanical calculations can accurately predict recombination rates, thanks to the new methodology developed at UCSB Materials Group Professor Chris Van de Waal, who credits Mark. Turiansky, a graduate student in the group, helps write the code to calculate recombination rates.

“Our methods are very powerful for identifying defects that cause carrier loss,” Turiansky said. “It is exciting to see the approach applied to one of the critical issues of our time, the efficient generation of renewable energy.”

Running the simulations showed that the common defects in both materials result in comparable (and relatively benign) levels of recombination. However, the organic molecule can dissociate in the hybrid perovskite; When the loss of hydrogen atoms occurs, the resulting “vacancies” severely reduce efficiency. Thus, the presence of the molecule is detrimental to the overall efficiency of the material, not an asset.

Why, then, is this not observed experimentally? This is mainly due to the difficulty of growing high-quality layers of completely inorganic materials. They have a tendency to adopt other crystal structures, and to enhance the formation of the desired structure requires greater experimental effort. Recent research has shown, however, that achieving the preferred structure is certainly possible. However, the difficulty explains why inorganic perovskite has not received as much attention so far.

“We hope that our findings on the expected efficiencies will stimulate further activities directed at the production of inorganic perovskite,” Van de Waale concluded.

The hydrogen in a hybrid perovskite is less innocent than it looks

more information:
Xie Zhang et al, Inorganic halide perovskite as candidates for efficient solar cells, Cell Reports Physical Sciences (2021). DOI: 10.1016 / j.xcrp.2021.100604

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