Investigations Of The Properties Of Cu₂ZnSnS₄ Related Thin Films And The Performance Of Their Photovoltaic Devices

Cu2Zn Sn S4, Cu2 Zn Sn Se4 and Cu2 Zn Sn(S,Se)4 are suitable absorber materials for the emerging thin film solar cell technology. They are all direct bandgap semiconductors with high absorption coefficient in visible light region. Their bandgap energies can be linearly tuned in the range of 1.0~1.5e V by control of Se content in the material, which cover the optimum single-junction bandgap range according to Shockley-Queisser limit. Thus, these materials are potential candidates for high-performance photovoltaic devices. In addition, chemical elements of the compounds are non-toxic, friendly and Earth-abundant. Therefore, the material cost can be drastically reduced in mass production. Recently, Cu2 Zn Sn S4 and its related compounds have been intensively investigated. Historically, substantial progress has been made in improving the performance of Cu2 Zn Sn S4-based device. However, the conversion efficiency has remained the same level in the past two years. It seems that the Cu2 Zn Sn S4 technology has encountered a bottleneck. Therefore, it is very important to study the underlying physical properties of the material, to develop a manufacturing process with extreme low costs, and to propose feasible suggestions for resolving the problems in the present Cu2 Zn Sn S4 technology. In this thesis, I will report our theoretical and experimental investigations about the abovementioned problems.First of all, we studied a low cost fabrication process of Cu2 Zn Sn S4. The cheap metal sulfides were used as raw materials. Cu2 Zn Sn S4 was synthesized by ball milling, which is suitable for mass production. The phase transition as well as the reaction mechanism in the synthesis process were investigated. According to our analysis, ball collision promotes inter-diffusion of raw materials by repeatedly welding and fracturing the powders. At the same time, local temperature induced by steel ball collision drives the thermodynamic formationof Cu2 Zn Sn S4. The reaction steps in the ball milling process follows the reaction equations Cu2 S + Sn S2 ? Cu2 Sn S3 and Zn S + Cu2 Sn S3 ? Cu2 Zn Sn S4.Secondly, we proposed a low cost and friendly hybrid ink process to prepare Cu2 Zn Sn S4 thin film. By dispersing the milled Cu2 Zn Sn S4 powder in a Cu-, Zn- and Sn-chalcogenide precursor solution, a hybrid ink was fabricated. With the hybrid ink, a precursor Cu2 Zn Sn S4 film was deposited on Mo coated soda-lime glass by spin-coating. In order to obtain Cu2 Zn Sn(S,Se)4 absorber film with kesterite structure, the Cu2 Zn Sn S4 film was annealed in Se ambient. Solar cell device was fabricated by using the annealed Cu2 Zn Sn(S,Se)4 film as absorber layer, and its conversion efficiency reached 4.2%.Thirdly, we proposed a solution to relieve the interface recombination and insufficient collection of minor carriers simultaneously. CZTSSe thin films with non-uniform selenium distribution through the depth of the films was fabricated by sequential heat treatment of precursor films. The top region of the film becomes selenium rich, while the bottom region is selenium poor. This structure has higher absorption coefficients than the uniform ones. In addition, the uneven composition distribution may lead to a bandgap gradient in the film. Device simulation demonstrated that graded absorber layer device relieved the electrical loss by improved collection efficiency of minority carriers and suitable band alignment at the absorber/buffer interface.Finally, we carried out first-principles calculations on the electronic structures and optical properties of Cu2 Zn Sn S4. As a heterojunction device, the absorber layer and the buffer layer(or the substrate) of a solar cell are different in lattice constant and thermal expansion coefficient. Thus, it is inevitable that stress(or strain) appears in Cu2 Zn Sn S4 film. Therefore, it is significant to study the influence of strains on the electronic structure, crystal quality and optical properties of Cu2 Zn Sn S4. It is found that the fundamental bandgap at the ? point decreases linearly with increasing tensile biaxial strain perpendicular to c-axis. However, a bandgap maximum occurs as the compressive biaxial strain is-1.5%. At the same time, the split energy of crystal field changes from negative to positive.