The uncommon nature of point defects in organic-inorganic Perovskite solar cells

Abstract

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 contributed to this success is "defect tolerance": in organic-inorganic perovskites, the majority of point defects with low formation energy are shallow, with energies within or near the conduction or valence band. Defects with deep states, which act as electronic traps, are expected to be much less common due to their high formation energies. In this thesis, we demonstrate that, despite the preference for shallow defects, point defects play an integral 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 microscopy (STEM). By correlating local luminescence properties with compositional variations using STEM, we demonstrate that iodide segregation induced by the 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 charged defect segregation under applied biasing. Finally, we directly track the migration of deep defects in PSCs through photoluminescence mapping of laterally biased perovskite films. Removal of defect states by mild voltage biasing results in over an order of magnitude increase in luminescence. Using Monte Carlo simulations of defect drift and diffusion to model these time dependent luminescence maps, we extract the mobility of these point defects and provide evidence of demonstrates the ways in which deep and shallow defects play a critical role in PSCs and suggests 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.