Solution routes to metal chalcognide and zinc oxide for device applications
Semiconductor thin films and their fabrication methods have been intensively investigated in order to fulfill a wide spectrum of applications in electronic devices, optical devices, and photoelectronic devices. Solution deposition of semiconductor thin films draws increasing attention since it offers low cost, high-throughput production, and better compatibility with flexible or polymeric substrates compared to the vacuum deposition. This dissertation mainly focuses on the solution routes for depositing two groups of seminconductor thin films: kesterite copper zinc tin sulfide/selenide/sulfoselenide (Cu 2ZnSn(S1-xSex)4(0≤x≤1) (CZTSSe) and zinc oxides (ZnO). The corresponding thin film devices based on the solution routes are also presented. Compared to fossil fuels, the higher cost of photovoltaic electricity is the main barrier for its extensive application. To advance photovoltaic electricity as a promising energy alternative, the candidate materials and processing routes should be cost effective for beyond terawatt scale. Kesterite CZTSSe is an attractive absorber material since it contains earth-abundant elements (i.e. copper, zinc, tin, and sulfur) and has a high light absorption coefficients (i.e. 104~105 cm-1). The molecular solution routes, which offers a facile control of the film composition and the scalability for mass production, has the demonstrated advantages in fabricating CZTSSe thin films. In this study, metal salt precursor solutions and metal-metal chalcogenide precursor solutions based on versatile amine-thiol solvent mixtures are developed to deposit high-quality CZTSSe thin films, and solar cells with power conversion efficiencies (PCEs) more than 8% and 7% are fabricated based on the corresponding solution routes, respectively. The effects of the metal salts, the solvents, and the precursor compositions (e.g. Cu/Sn ratios) on the PCEs are studied systematically in order to achieve better solar cell performances. The microstructural evolution of solution-processed CZTSSe precursor films during the selenization is examined, and a mechanism is proposed to explain the rapid microstructural and compositional changes as well as the formation of fined-grained layers during the selenization. In addition, the versatility of using the amine-thiol solvent system in depositing semiconductor thin films is illustrated in terms of the rapid dissolution of bulk metals, metal salts, organometallic compounds, and metal chalcogenides at room temperature in this solvent system, and the deposition of binary, ternary, and quaternary metal chalcogenide thin films from the as-dissolved or combined precursor solutions. The molecular precursor route based on this versatile solvent system opens a door for low-cost fabrication of semiconductor thin films. Solution-processed ZnO thin films are promising candidates for cost-effective thin film electronics. In this study, an aqueous solution route is utilized to grow ZnO thin films with different morphologies at low temperature (90°C). Effects of the seed layers and solution conditions on morphologies of individual ZnO nanorods and thin films are investigated. Particularly, this study presents a solution method to modify the anisotropic growth behavior of hexagonal ZnO, which successfully results in a two-dimensional continuous polycrystalline film within a short-time growth. This method is then applied to the fabrication of ZnO thin film transistors. Continuous active layers with thicknesses of ~ 100 nm are acquired with solution growth for 15 min. The applicability of this method for low-cost transparent thin film transistors is demonstrated in terms of film microstructure, film transmittance, and device I-V measurements.
Agrawal, Purdue University.
Chemical engineering|Materials science
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