Copper zinc tin sulfide (Cu2ZnSnS4 or CZTS) is a promising candidate as a sunlight absorbing layer in thin film solar cells. CZTS is comprised of earth abundant and non-toxic elements, and power conversion efficiencies in CZTS based solar cells have risen quickly. However, there is still a need to develop low-cost and scalable synthesis methods. Furthermore, the rapid rise in CZTS solar cell efficiencies is largely due to a trial-and-error approach to assembling devices, which has led to a knowledge gap in the fundamental material properties of CZTS, which this thesis aimed to help close. We first developed a low cost synthesis method by ex situ sulfidation of Cu-Zn-Sn thin metal films. The metal films are exposed to sulfur vapor at elevated temperatures to form CZTS. This takes place within an isothermal, sealed quartz ampoule. We found that phase pure CZTS films may be achieved at sulfidation temperatures of 600o C. With this method, we also found that the Sn content within the CZTS films was largely self regulating such that the Cu-to-Sn ratio always approached two. The mechanism behind this ultimately gave us strategies to carefully control stoichiometry. We also developed methods to control the film microstructure. Large grains are achieved by the introduction of Na and K. This occurs through diffusion from certain substrates, or via the vapor phase through NaOH and KOH coatings on the quartz ampoule. Finally, we examined how stoichiometry, grain size and Na content affect electronic properties of CZTS. The hole concentration decreases rapidly with small decreases in the Cu-to-Zn ratio. Increases in grain sizes led to gains in mobility, and the introduction of Na generated increases in hole concentrations. All films exhibited variable range hopping transport, however, Cu rich films had high levels of compensation making them unsuitable for solar cell devices.
University of Minnesota Ph.D. dissertation. November 2014. Major: Chemical Engineering. Advisor: Prof. Eray S. Aydil. 1 computer file (PDF); x, 141 pages.
Johnson, Melissa C..
The synthesis and characterization of thin film copper zinc tin sulfide for solar cell applications.
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.