Development and Application of Novel Optoelectronic Materials for Photodetectors and Solar Cells

Thesis (Ph.D.)--University of Washington, 2020Modern society has shown increasing demands of optoelectronic devices with lightweight, flexible, and scalable features. Novel materials such as conducting polymer and perovskite are developed to replace the traditional group III-V inorganic materials and counter the new challenges. Ultraviolet (UV) photodetectors play a critical role in scientific, commercial, civil, and military areas. Perovskite materials with tunable bandgap have been successfully applied to fabricate high-performance solar cells. However, their potential for UV detection has not been demonstrated yet. In this work, pinhole-free methylammonium lead trichloride (CH3NH3PbCl3) perovskite thin films were fabricated via a two-step spin coating and solvent-vapor-assisted thermal annealing method under low temperature for the first time. Prototypical UV photodetectors based on CH3NH3PbCl3 perovskite thin films were fabricated and showed strong photoresponse in the 300-400 nm region. All the results demonstrated that low-temperature solution-processed CH3NH3PbCl3 perovskite thin films offer great potential for making flexible, lightweight visible-blind UV-A photodetectors. Narrowband UV photodetectors are highly desired in multiple areas. Photodetectors based on organic-inorganic nanocomposite offer high sensitivity, widely adjustable response range, lightweight, and low-temperature solution processibility. However, the broad absorption range of organic and inorganic semiconductor materials make it difficult to achieve a narrowband detection feature. Nanocomposite thin films containing the conjugated polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(bithiophene)] (F8T2) blended with ZnO nanoparticles (NPs) were applied as the active layers of the photodetectors. Narrowband UV photodetectors with high gain and low driving voltage were demonstrated by adopting a symmetric device structure, controlling the active layer composition and microstructure, and manipulating the light penetration depth in the active layer. Our method offers a pathway to design and fabricate narrowband UV photodetectors. Hybrid organic-inorganic perovskite has shown its potential as high-performance solar cell active layer. However, its instability, including intrinsic and operational instability, strongly hinders its real-life applications. Two types of methylammonium (MA)-free, formamidinium (FA)-based perovskite, Rb0.05Cs0.1FA0.85PbI3 and Cs0.15FA0.85PbI3, was fabricated and the effect of Rb+ on the device performance and long-term stability were investigated. By simply mixing the precursor solution before spin coating, the defect states in both types of perovskite were significantly reduced and device stabilities against the electric field were improved. The modified precursor solution provided devices with Rb0.05Cs0.1FA0.85PbI3 and Cs0.15FA0.85PbI3 active layer that retained 68% and 92% of their initial PCE, respectively, over 30 days under N2 protection. The work sheds a light on the solution of the operational instability of perovskite solar cells and paves the pathway for their industrialization. To further improve the optoelectronic device performance based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) conducting polymer, a hydroxymethyl (-MeOH) and chloromethyl (-MeCl) function group was introduced to the oxyethylene ring of EDOT monomer. Oxidative chemical polymerization was applied to synthesize PEDOT:PSS and functionalized PEDOT:PSS. The polymerization mechanism of PEDOT:PSS and effects of functional group on the polymerization were revealed. The study sheds light on the polymerization of PEDOT with functional groups and provides a guideline for the synthesis of functionalized PEDOT conducting polymers with polyelectrolyte counterions using oxidative chemical polymerization