Origin of Efficiency Enhancement by Lattice Expansion in Hybrid-Perovskite Solar Cells.

It has been experimentally observed that light-induced lattice expansion could enhance the solar conversion efficiency in hybrid perovskites, but the origin remains elusive. By performing rigorous first-principles calculations for a prototypical hybrid-perovskite FAPbI_{3} (FA: formamidinium), we show that 1% lattice expansion could already reduce the nonradiative capture coefficient by one order of magnitude. Unexpectedly, the suppressed nonradiative capture is not caused by changes in the band gap or defect transition level due to lattice expansion, but originates from enhanced defect relaxations associated with charge-state transitions in the expanded lattice. These insights not only provide a rationale for the efficiency enhancement by lattice expansion in hybrid perovskites, but also offer a general approach to the manipulation of nonradiative capture via strain engineering in a wide spectrum of optoelectronic materials.