Comparing the photovoltaic efficiencies of all-inorganic perovskites with their hybrid counterparts. Image: Xie Zhang.
Comparing the photovoltaic efficiencies of all-inorganic perovskites with their hybrid counterparts. Image: Xie Zhang.

Hybrid organic-inorganic perovskites have already demonstrated very high photovoltaic efficiencies of greater than 25%. The prevailing wisdom in the field is that the organic (carbon- and hydrogen-containing) molecules in the material are crucial to achieving this impressive performance by suppressing defect-assisted carrier recombination.

New work by the materials department at the University of California (UC) Santa Barbara has not only shown that this assumption is incorrect, but also that all-inorganic materials have the potential for outperforming hybrid perovskites. The researchers report their findings in a paper in Cell Reports Physical Science.

“To compare the materials, we performed comprehensive simulations of the recombination mechanisms,” explained Xie Zhang, lead researcher on the study. “When light shines on a solar-cell material, the photo-generated carriers generate a current; recombination at defects destroys some of those carriers and hence lowers the efficiency. Defects thus act as efficiency killers.”

To compare all-inorganic and hybrid organic-inorganic perovskites, the researchers studied two prototype materials. Both materials contain lead and iodine atoms, but in one material the crystal structure is completed by the inorganic element cesium while in the other it is completed by the organic molecule methylammonium.

Sorting out these processes experimentally is exceedingly difficult. But state-of-the-art quantum-mechanical calculations can accurately predict the recombination rates, thanks to a new methodology developed in the group of Chris Van de Walle, a materials professor at UC Santa Barbara. Van de Walle credited Mark Turiansky, a senior graduate student in his group, with helping to write the code to calculate the recombination rates.

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

Running the simulations showed that defects common to both materials give rise to comparable (and relatively benign) levels of recombination. However, the organic molecule in the hybrid perovskite can break up; this results in the loss of hydrogen atoms, creating 'vacancies' that strongly decrease efficiency. The presence of the organic molecule is thus a detriment, rather than an asset, to the overall efficiency of the material.

Why, then, has this not been noticed experimentally? Mainly because of the difficulty of growing high-quality layers of all-inorganic materials. They have a tendency to adopt alternative 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 definitely feasible. Still, this difficulty explains why all-inorganic perovskites have not received as much attention to date.

“We hope that our findings about the expected efficiency will stimulate more activities directed at producing inorganic perovskites,” concluded Van de Walle.

This story is adapted from material from the University of California, Santa Barbara, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.