Rapid exploration of multinary compound semiconductors through a novel spray deposition technique

Steven J Gaik, Purdue University

Abstract

Solar energy represents the single largest source of renewable energy available on Earth. Photovoltaic (PV) technology is one of the most common methods in which to harness solar energy for electricity generation, but the market penetration of PV technology is impeded by high capital costs and low conversion efficiencies. One approach to address high manufacturing costs, which has shown some success industrially, is to employ a liquid based processing scheme, in which thin film photovoltaic devices are manufactured in a roll to roll manner. Semiconductors appropriate for this processing are subject to stringent and specific material requirements however, which are only found in a few known material systems, such as CdTe and CuInxGa 1-xSe2 (CIGS). Additionally, concerns regarding the supply and demand for tellurium, indium, and gallium are fueling interest in new semiconductor alloys that are comprised of earth abundant elements but retain similar properties to CdTe or CIGS. Despite this interest, the solution space of potential semiconductor alloys is vastly unexplored. Furthermore, the role of intrinsic and extrinsic elements (i.e. dopants) in these multinary semiconductors is of paramount importance for controlling bulk optoelectronic properties, and the composition range of interest for studying dopants and alloying agents spans several orders of magnitude (10 - 100,000 ppm). Conventional combinatorial or high throughput techniques are not suitable for investigating such wide composition ranges with high fidelity, and insufficient fundamental data is available for first principles based predictions. To address these challenges, a novel spray deposition technique capable of depositing continuous gradient libraries spanning multiple orders of magnitude in concentration is presented. The instrument design and construction is discussed, and an instrument model is developed to identify the limits of operation in various operating modes. The instrument model suggests that the onset of pump pulsation cannot be neglected, and that a maximum of one order of magnitude change in concentration is achievable with any given pump stage regardless of solution concentrations or pump operating mode. A Taylor dispersion model is utilized to address gradient distortion in the fluid delivery system, and results are confirmed quantitatively through experiments. A proof of concept is demonstrated by depositing films of Cu2ZnSnS4 (CZTS) containing intrinsic element composition gradients. The instrument is then utilized in the first systematic investigation of the role of Group IA elements in the Cu2ZnSn(SxSe 1-x)4 (CZTSSe) material system. Films are characterized using photoluminescence (PL) mapping, and the data is analyzed using a custom Mathematica code to extract statistical trends in the PL data. The Wurfel-Planck emissivity model is applied to PL data to extract additional information about the maximum theoretical open circuit voltage (Voc) achievable from devices incorporating Group IA dopants, and an in depth discussion of the model sensitivities and nuances associated with confocal PL measurements is presented. Lithium, which has the smallest ionic radius in the dopant series, is found to exhibit the most dramatic trends of all the dopants studied, particularly above 10,000 ppm, which is likely related to its larger diffusion coefficient. Lithium is found to enhance the optical bandgap and blue shift the PL spectra but not yield a statistically significant increase in Voc. Sodium doping is found to not exhibit any statistically significant tends up to 1,000 ppm, while potassium is shown to systematically reduce both Voc and optical bandgap, suggesting enhanced crystallographic disorder and nonradiative recombination pathways with potassium loading.

Degree

Ph.D.

Advisors

Litster, Purdue University.

Subject Area

Chemical engineering

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