Study of new materials, device structures, and characterization techniques for photovoltaics [electronic resource]
Keywords: Electrical Engineering.

In Chapter 1, three reasons are provided for embracing solar photovoltaic energy conversion: (1) its feasibility given the Sun's incident power magnitude as well as the amount of land needed on Earth, (2) its versatility to be centralized or decentralized, and (3) its non-toxic and waste-free nature. Our study of new solar cell materials, device architectures and characterization tools are all motivated by the need to develop solar cells that outperforms and outcompetes other energy generation technologies on performance and cost. The photovoltaic physics background necessary to understand this dissertation is briefly covered. Three basic requirements of solar cells are explained, i.e. light enters the absorbing layer to transfer energy to electrons, the photo-excited electrons and holes maintain the energy gained, and charge carrier collection at designated contacts. Chapter 2 describes an aqueous bath process developed for Cu2ZnSnS4 (CZTS). Chemical bath deposition and ion exchange were used to incorporate copper, zinc, tin and sulfur into a thin film precursor stack. The stack was then sulfurized to form the photovoltaic absorber material CZTS. The morphology and elemental composition of the films at each process stage were analyzed by Auger electron spectroscopy and scanning electron microscopy, and the structural and optical properties of the sulfurized film were determined by a combination of X-ray diffraction, Raman scattering, and diffuse reflectance UV-Vis spectroscopy. Compositionally uniform microcrystalline CZTS with kesterite structure and a bandgap of 1.45 eV were observed. A preliminary solar cell device was produced exhibiting photovoltaic and rectifying behavior. Chapter 3 covers four different SnS deposition techniques: chemical bath deposition, molecular precursor pyrolysis deposition, sputter deposition and vapor transport deposition (VTD). VTD was found to be most promising for making SnS solar cells and was thus characterized in most detail. Polycrystalline SnS, Sn2S3, and SnS2 were deposited onto glass substrates by VTD, with the stoichiometry controlled by deposition temperature. In addition, epitaxial growth of orthorhombic SnS(010) films on NaCl(100) with thicknesses up to 600 nm was demonstrated. The in-plane [100] directions of SnS and NaCl are oriented approximately 45° apart, and the translational relationship between SnS and NaCl was predicted by density functional theory. The epitaxial SnS is p-type with carrier concentration on the order of 1017 cm−3 and Hall hole mobility of 385 cm2 V−1 s−1 in-plane. It has indirect and direct bandgaps of 1.0 and 2.3 eV, respectively. Based on the SnS VTD method characterized in Chapter 3 and the standard CIGS solar cell architecture, Chapter 4 summarizes the numerous material and process combinations tested to make functional SnS solar cells. The device stack consisting of Si/Mo/SnS/ZnO/ITO was the first generation device, with its performance limited by high surface texture. To reduce the SnS texture, an FTO substrate was used. The first bifacial SnS solar cell was thus demonstrated using the device stack consisting of glass/FTO/SnS/CdS/ZnO/ITO, resulting in front- and back-side efficiencies of 1.2% and 0.2%, respectively. To improve the performance of solar cells, Chapter 5 describes a model is developed for three-dimensionally nanostructured photovoltaic devices, distinguishing between isolated radial p-n junctions and interdigitated p-n junctions. We examine two specific interdigitated architectures, the point-contact nanojunction and the extended nanojunction, which are most relevant to experimental devices reported to date but have yet to be distinguished in the field. The model is also applied to polycrystalline CdTe devices with inverted grain boundaries. We demonstrate that for CdTe/CdS solar cells using low-quality materials, the efficiency of the extended nanojunction geometry is superior to other designs considered. In Chapter 6, the Generalized Impedance Spectra Analysis of Non-ideal Diodes (GISAND) is described for the purposes of process optimization and understanding key diode properties of devices of new materials. From a non-destructive 30-minute, room temperature impedance measurement of a diode in the dark the following can be extracted: series resistance, shunt resistance, depletion width, depletion capacitance, Debye length, doping concentration, built-in voltage, diffusion capacitance, diffusion conductance, minority carrier lifetime, minority carrier mobility, and diode ideality factor. The presence of extra series-connected diodes, such as a Schottky back contact, may also be detected and considered. Light-emitting diodes, photodiodes, and solar cells of CZTSSe and PbS colloidal quantum dots were tested to demonstrate the utility of GISAND. Chapter 7 summarizes the efforts above and suggests future directions for improving upon the work. One strategy to economically fabricate the 3D nanojunction solar cells described in Chapter 5 is by phase-segregation of chalcogenides to form heterojunctions such as CZTS/ZnS and SnS/ZnS. Two routes for producing solar cells with epitaxial SnS films are suggested. In one method, the NaCl is maintained as the transparent superstrate and selective contacts are both deposited on the exposed SnS surface. In the other method, the SnS is isolated from the NaCl by dissolving the latter in water once the SnS and a back contact is bonded to a glass substrate with epoxy. For both methods, the thermal mismatch between NaCl and SnS must be solved to mitigate the detrimental cracking that causes the device to be shunted.