Development of colloidal quantum dot and lead halide perovskite light emitting devices

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, February, 2021Cataloged from the official PDF of thesis. Page 139 blank.Includes bibliographical references (pages 118-138).In recent years, optically active semiconductors, such as organic molecules, colloidal quantum dots (QDs) and lead halide perovskites, have emerged as top candidates for light emitting materials. One key feature of these materials is their bandgap tunability, e.g. via size or chemical composition, allowing for their emission color to be turned throughout the entire visible spectrum. Thin-film light emitting devices (LEDs) based on these luminophores are promised to deliver the next-generation display technologies that are ultrathin and light, high-color-quality, and energy efficient with new form factors (e.g. foldable and flexible). In this thesis, we present the work performed to improve the understanding and performance of colloidal nanocrystal QDs and lead halide perovskites as visible luminophores in optically- and electrically-driven thin-film LEDs. First, we create an efficient voltage-controlled optical down-converter by operating a quantum dot light emitting diode (QD-LED) under reverse bias. Using field-induced luminescence quenching to our advantage, we show that a large electric field can strongly modify QD carrier dynamics, resulting in stable and reversible QD photoluminescence (PL) modulation. Next, we address the QD's toxicity issue by developing a synthesis of heavy-metal-free ZnSe/ZnS core-shell QDs with narrow spectral linewidth and high PL quantum yield. By employing these QDs as emitters, we demonstrate QD-LEDs with efficient and saturated blue electroluminescence (EL). Finally, we present a new way of depositing compact CsPbBr₃ perovskite thin films by thermal co-evaporation and demonstrate all vacuum-processed perovskite LEDs with efficient green EL emission. Our results show that evaporative deposition can be a viable alternative to solution-based deposition for fabricating high-quality perovskite thin films for LEDs.by Sihan Xie.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Materials Science and Engineerin