Apparent Defect Densities in Halide Perovskite Thin Films and Single Crystals

Non-radiative recombination via defects is a major loss mechanism for nearly all photovoltaic technologies. (1) Despite their frequently quoted “defect tolerance”, (2,3) halide perovskites are no exception to this rule, given that it remains difficult to exceed luminescence quantum efficiencies of a few percent at photovoltaic working conditions in devices. (4−6) Given the importance of non-radiative recombination, the experimental detection of the culprits, i.e., the most recombination-active defects, is of substantial importance for controlled optimization of devices but also for long-term strategic decisions. One such strategic decision is the assessment of possible performance benefits associated with going from polycrystalline thin films to single crystals (7,8) as active elements in perovskite solar cells. A substantial amount of experimental data (9−17) indicates that polycrystalline thin films of lead halide perovskites typically have defect densities on the order of 1015–1016 cm–3, while single crystals are typically reported (9,14,18−20) to have bulk defect densities of 1012 cm–3 or lower. These findings support an intuitive rationale, namely that single crystals have orders of magnitude lower defect densities than thin films that should contain a certain density of defects at their grain boundaries. This narrative has even inspired paper titles such as the one from Brenes et al., (21) who write about “Metal Halide Perovskite Polycrystalline Films Exhibiting Properties of Single Crystals” while reporting exceptionally long charge-carrier lifetimes in perovskite thin films