Organic-inorganic perovskite solar cells (PSCs) have shown enormous success in the past decade, increasing in power conversion efficiency from ~4% in 2009 to >22%. One of the critical properties that has been attributed to this success is “defect tolerance”: in organic-inorganic perovskites, the majority of point defects with low formation energy lie within or near the conduction or valance band. Defects with deep states, which act as electronic traps, are expected to be much less common due to their high formation energies. We demonstrate that, despite the preference for shallow defects, point defects play an integral and role in materials properties and PSC device performance.
We first study the role of point defects on nanoscale luminescence properties of inorganic-organic perovskites by using cathodoluminescence in scanning transmission electron microscope (STEM). By correlating local luminescence properties with compositional variations using STEM, we demonstrate that iodide segregation induced by electron beam is correlated with a spatially-localized high-energy emission. Similar high-energy emission has been observed in photoluminescence (PL) measurements for films made in the presence of excess methyl ammonium iodide, demonstrating that the observed defect segregation is relevant to practical device design.
Next, we study the effects of directional point defect segregation under an applied electric field on current extraction from PSCs. Specifically, we use electron beam induced current measurements in a scanning electron microscope to measure the inhomogeneity in current extraction before and after forward biasing the device. These measurements point to preferential defect migration at extended defects and allow us identify low frequency capacitive elements related to compensation of charge defect segregation under applied biasing.
Finally, we directly track the migration of deep defects in PSCs through intensity dependent PL mapping of laterally biased perovskite films. Using Monte Carlo simulations of defect drift and diffusion to model these time dependent luminescence maps, we provide evidence for four deep level defects in these films, extract their mobilities, and demonstrate their significant impact on PL intensity. Particularly, removal of defect states by mild voltage biasing results in over an order of magnitude increase in luminescence. Overall, our work demonstrates the ways in which deep and shallow defects play a critical role in PSCs and suggest that, despite their “defect tolerance,” the ultimate stability and performance of PSCs will be dependent on either minimizing the presence of point defects in these materials or inhibiting defect migration.