Synthesis and characterization of kesterite Cu2ZnSnS4 (CZTS) thin films for solar cell application.

摘要

The quaternary compound Cu2ZnSnS4 (CZTS) gained considerable attention in the last decade due to its potential as an active-layer semiconductor for low-cost thin-film solar cells. The material is composed of nontoxic and Earth-abundant constituents, has optical properties suitable for photovoltaic application, and can be synthesized using a wide variety of methods.;Polycrystalline CZTS was grown in this work using vacuum-based deposition to first deposit metal films (precursors) of Cu, Zn, and Sn. In a subsequent step, the precursors underwent an annealing treatment in sulfur vapor environment (sulfurization) to form CZTS. Using sputtering, a physical vapor deposition (PVD) technique, two different kinds of metal precursors were deposited: a) stacked precursors, in which a stack of metal layers are sequentially deposited on a glass substrate, and b) co-sputtered precursors, in which the three metals are deposited simultaneously.;The effect of sulfurization time and temperature was investigated to optimize the process and to study the impact of both factors on the morphological and structural properties of the synthesized compound. CZTS films based on a stacked precursor were prepared and characterized by X-ray fluorescence, energy-dispersive X-ray spectroscopy, electron probe microanalysis, X-ray diffraction, scanning electron microscopy , ultraviolet-visible-near-infrared spectrophotometry, and Auger electron spectroscopy. Films with dense morphology, good crystallinity, and optical energy bandgap close to 1.5 eV were obtained using annealing temperatures of 500°C and 550°C. Furthermore, the study addressed the issue with Cu2-xS phases segregated on the film surface and suggested a route to avoid their evolution.;The secondary-phase dependence on the chemical composition was studied, focusing on suppressing Cu2-xS phases. In addition, we addressed the importance of Sn/Cu ratio as a controlling factor to avoid developing these detrimental phases. An electron backscatter diffraction study confirmed the role of increased Cu content in achieving larger grains.;Studying the influence of the stack sequence in the precursor revealed that depositing Zn as the first or second layer leads to better adhesion, and depositing Cu as top layer minimizes the loss of Sn and allows better control of film composition. Solar cells based on CZTS films prepared with the sulfurization of precursors with different stacking orders were produced, and the best device showed power conversion efficiency (PCE) of 4.43%.;CZTS films based on co-sputtered precursors showed very promising results in terms of large grains, dense morphology, and smooth surface; however, they suffered from poor adhesion to the Mo layer due to compressive intrinsic stresses. Further investigation is needed to identify the stress origin and improve the contact to the back electrode. The best device based on CZTS films from co-sputtered precursors yielded a PCE of 1.72%.