A team of researchers, led by South Korea's UNIST and KIER, and Switzerland's EPFL, has reached 25.6% power conversion efficiency of perovskite solar cells by introducing an anion engineering concept that uses pseudo-halide anion formate to suppress anion-vacancy defects and augment crystallinity.

Perovskite derivatives have been investigated to overcome instability issues with lead-based organic perovskite materials in ambient air and reduce the use of lead. Researchers have introduced various methods to improve conversion efficiency. An engineering concept using formate, which is the anion derived from formic acid, was introduced by researchers.

"It is significant in that it is the world's first to discover that a formate can interact with surrounding elements in anionic position within a perovskite crystal," said Jeong Jae-ki, a UNIST professor of energy engineering. "This study suggests a new direction for perovskite material research."

"The cubic α-phase of formamidinium lead triiodide has emerged as the most promising semiconductor for highly efficient and stable perovskite solar cells, and maximizing the performance of this material in such devices is of vital importance for the perovskite research community," the team said.

The anion engineering concept uses the pseudo-halide anion formate to suppress anion-vacancy defects that are present at grain boundaries and at the surface of the perovskite films and to augment the crystallinity of the films. As a result, solar cell devices attained an efficiency of 25.6 percent and showed long-term operational stability, the team said.

"Our findings provide a direct route to eliminate the most abundant and deleterious lattice defects present in metal halide perovskites, providing a facile access to solution-processable films with improved optoelectronic performance," the team said.