Defect physics in perovskite solar cells

Organometal halide perovskites have rapidly emerged as promising materials for thin-film solar cells. Last year, just four years after the materials’ 2009 debut , two groups independently developed perovskite-based solar cells with an impressive 15% power-conversion efficiency. But many mysteries remain, including the perovskites’ unusually long charge-carrier lifetimes and the extreme sensitivity of device performance to preparation conditions. Now Yanfa Yan and colleagues at the University of Toledo in Ohio have shed some light on the physical origins of both those effects through their density-functional calculations on the perovskite methylammonium triiodide plumbate. As shown in the crystal structure in the figure, that material is composed of three ion types: positively charged methylammonium (MA, a polyatomic organic ion) and lead and negatively charged iodide. Yan and colleagues systematically calculated the energies of each possible point defect, including ion vacancies, interstitials, and substitutions. They found that the defects with the lowest energies of formation are a Pb vacancy and an interstitial MA. And because both those defects create charge-carrier states near the edges of the semiconductor bandgap, they don’t facilitate electron–hole recombination the way defects with energy levels near the middle of the bandgap do. Furthermore, the Toledo researchers’ calculations showed that by changing the partial pressures of the constituent materials during perovskite film growth, one can change which of the two defects is dominant. Because the Pb vacancy is an electron acceptor and the interstitial MA is an electron donor, the preparation conditions determine whether the perovskite film has p-type or n-type conductivity. (W.-J. Yin, T. Shi, Y. Yan, Appl. Phys. Lett. 104, 063903, 2014 .)