Why are hybrid metal-halide perovskites so defect tolerant?
Hybrid lead halide perovskites are causing a revolution in photovoltaics, reaching light conversion efficiencies in excess of 25% after less than a decade of intense research. In nanocrystal form, these materials exhibit light-emission quantum yields close to 100%, which make them excellent candidates for light emitting devices too. However, a feature making hybrid perovskites so attractive for commercialization is the fact that they are produced by low-temperature, hence low-cost, scalable solution-based methods. In part as a consequence, hybrid perovskites are mechanically soft ionic solids with low energetic barriers for point-defect formation and yet they exhibit excellent optoelectronic properties. This can be explained if most native point defects (vacancies, interstitials and antisite defects) are shallow. Unlike deep traps, which are highly localized centers exhibiting high charge-carrier trapping rates, shallow defects are benign as far as the charge-carrier recombination is concerned. Shallow defects can be studied by means of low-temperature photoluminescence (PL). In PL experiments, correlated electron-hole pairs, called excitons, are excited with a laser. Excitons move freely through the crystal, like a binary-stars system, but they can become bound to different shallow defects. Free or bound excitons recombine radiatively emitting photons with precise energies, constituting an optical fingerprint (see Fig. 1).
In a recent work , we performed the first systematic study of the evolution of shallow-defect signatures observed in low-temperature PL spectra of mixed organic-cation lead iodide perovskite single crystals. Based on state-of-the-art ab initio calculations, we were able to provide a first assignment for all PL features to different shallow-defects (vacancies & interstitials) of hybrid perovskites.
In this way, our results provide a deeper insight into fundamental aspects of the photo-physics of native shallow defects in metal halide perovskites. This is instrumental for the optimization and further development of photovoltaic as well as light-emitting devices, based on this class of extraordinary semiconductor materials.