Fabrication and characterization methods for growth of CZTS as a promising material for thin film solar cells

Current research trends are moving towards earth-abundant and low toxicity materials. Cu2ZnSnS4 (CZTS), which consists not only earth-abundant and non-toxic elemental constituents, but also possesses a nearly optimum band gap of 1.5 eV and a high absorption coefficient, has the potential to be a leading material for large scale generation of solar energy.;Although theoretical calculations estimated the feasibility of achieving an efficiency between 30 to 40 %, for CZTS solar cells, unfortunately, there is no standard approach to fabricate CZTS cells to reach an efficiency even close to these numbers. In this dissertation, several potential methods from vacuum based techniques such as sputtering to a new solution-based process to deposit CZTS films have been explored, studied and developed. To further improve the CZTS solar cell performance, other layers essential for CZTS solar cells have also been analyzed and optimized. Lastly, the performance and efficiencies of the final integrated cells are presented.;First, we report a two-step process with sputtering of elemental precursors followed by sulfurization in dilute H2S. Structural and optical properties of CZTS thin films at various temperatures are studied. The CZTS films formed at 550°C exhibited a compact void-free structure yields the highest efficiency of 5.75%.;Since long duration annealing processes are not practical for industry and result in the formation of voids due to the sublimation of secondary phases, fast annealing under sulfur vapor atmosphere has further been investigated. Since the H2S annealing is cleaner and more controllable than dealing with sulfur vapor pressure. We report a two-step process with sputtering of elemental precursors followed by fast sulfurization in dilute H2S. The electrical characteristics and the efficiencies of the respective solar cells were analyzed and compared. The films annealed at 580°C for 30 minutes exhibited the highest efficiency of 3.8%.;Another approach to form a uniform precursor is the use of a target consisting of Zn, Sn and Cu. Here we studied the practical issues of using an alloy targets (particularly Cu-Sn alloy target) to deposit CZT layers.;Although high-vacuum techniques have already shown promising results, the complex equipment required for such the processes results in costly implementation to scale up. CZTS growth by solution-based methods could potentially provide a low-cost alternative method to fabricate CZTS solar cells in large scale. Here, we introduced and developed a new non-toxic solution-based method to deposit CZTS. Efficiencies of 1.3% have been achieved up to now.