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

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 133-148).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.National Science Foundation (U.S.) under award number DMR-141-9807by Olivia Dolores Hentz.Ph. D

Impacts of Ion Segregation on Local Optical Properties in Mixed Halide Perovskite Films

Despite the recent astronomical success of organic–inorganic perovskite solar cells (PSCs), the impact of microscale film inhomogeneities on device performance remains poorly understood. In this work, we study CH₃NH₃PbI₃ perovskite films using cathodoluminescence in scanning transmission electron microscopy and show that localized regions with increased cathodoluminescence intensity correspond to iodide-enriched regions. These observations constitute direct evidence that nanoscale stoichiometric variations produce corresponding inhomogeneities in film cathodoluminescence intensity. Moreover, we observe the emergence of high-energy transitions attributed to beam induced iodide segregation, which may mirror the effects of ion migration during PSC operation. Our results demonstrate that such ion segregation can fundamentally change the local optical and microstructural properties of organic–inorganic perovskite films in the course of normal device operation and therefore address the observed complex and unpredictable behavior in PSC devices

Dimension- and Surface-Tailored ZnO Nanowires Enhance Charge Collection in Quantum Dot Photovoltaic Devices

The use of zinc oxide (ZnO) nanowires improves charge collection, and consequently power conversion efficiency, in quantum dot (QD) based photovoltaic devices. However, the role of the nanowire geometry (e.g., density, length, and morphology, etc.) relative to the QD properties remains unexplored, in part due to challenges with controlled nanowire synthesis. Here, we independently tailor nanowire length and the active device layer thickness to study charge collection in lead sulfide (PbS) QD photovoltaic devices. We then demonstrate consistently high internal quantum efficiency in these devices by applying quantum efficiency and total reflectance measurements. Our results show that significant losses originate from ZnO nanowire–QD interfacial recombination, which we then successfully overcome by using nanowire surface passivation. This geometry-tailored approach is generally applicable to other nanowire–QD systems, and the surface passivation schemes will play a significant role in future development of nanostructured photovoltaics

Transition Metal-Oxide Free Perovskite Solar Cells Enabled by a New Organic Charge Transport Layer

Various electron and hole transport layers have been used to develop high-efficiency perovskite solar cells. To achieve low-temperature solution processing of perovskite solar cells, organic n-type materials are employed to replace the metal oxide electron transport layer (ETL). Although PCBM (phenyl-C₆₁-butyric acid methyl ester) has been widely used for this application, its morphological instability in films (i.e., aggregation) is detrimental. Herein, we demonstrate the synthesis of a new fullerene derivative (isobenzofulvene–C₆₀–epoxide, IBF–Ep) that serves as an electron transporting material for methylammonium mixed lead halide-based perovskite (CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>) solar cells, both in the normal and inverted device configurations. We demonstrate that IBF–Ep has superior morphological stability compared to the conventional acceptor, PCBM. IBF–Ep provides higher photovoltaic device performance as compared to PCBM (6.9% vs 2.5% in the normal and 9.0% vs 5.3% in the inverted device configuration). Moreover, IBF–Ep devices show superior tolerance to high humidity (90%) in air. By reaching power conversion efficiencies up to 9.0% for the inverted devices with IBF–Ep as the ETL, we demonstrate the potential of this new material as an alternative to metal oxides for perovskite solar cells processed in air