Fabrication Of Cu₂ZnSnS₄Thin Film Solar Cells Via Sol-gel Route

Abstract:Chalcogenide semicondouctor material, Cu2ZnSnS4(CZTS) semiconductor has been confirmed as a promising alternative for thin film solar cells because it not only exhibits suitable optical and electronical properties similar to CIGS, but also consists of earth-abundant and non-toxic elements. As a result, CZTS materials have attracted increasing attention and remarkable progresses have been made over the past few years by lots of groups using different fabrication approaches. To This dissertation proposed a low-cost, non-roxic and high-throughput method based on the thermal decomposition and reaction of metal-thiourea-oxygen sol-gel complexes to synthesize CZTS thin film and solar cells and demonstrated a highest power conversion efficiency of6.32%(active cell area) in this way. Detailed innovative research contents are shown as follows:(1) Based on the analysis of conventional non-aqueous sol-gel reaction process, the alcoholysis and polycondensation reaction mechanism between metal thiourea complexes and organic solvent (ethylene glycol monomethyl ether) was revealed; and the molecular model of metal-thiourea-oxygen complexes was proposed and its thermal decomposition reaction process and mechanism of CZTS nanocrystal was revealed. On this basis, the preheating optimization parameters and phase composition of CZTS precursor film were obtained and it provided theoretical and experimental basis for subsequent sulfurazing thin film.(2) Research revealed the influence of preparation conditions on the properties of CZTS thin films; and the distributions of secondary phase and the impurity were indicated. The synthesized condition for CZTS cell with best conversion efficiency was obtained thorugh the photovoltaic tests on CZTS thin films with different properties. The optimized preparation process of CdS was achieved by studying the influence of buffer layer CdS on the performance of CZTS cells.(3) The CZTS thin film solar cell was modified by doping Na, etching secondary phase and improving Mo back contact. Appropriate Na doping not only can improve the crystalline quality of the CZTS film but also can reduce the content of impurity C in CZTS film therefore the power conversion efficiency was enhanced significantly. In addition the short-circuit current of CZTS thin film solar cell was increased after etching while the open circuit voltage was reduced, so the cell conversion efficiency was improved slightly, which indicated that the trace of ZnS in the surface of CZTS film favored the improvement of problems of interface recombination between absorber layer and buffer layer. Moreover the conventional Mo back contact was treated by sulfurizing substrate before depositing CZTS thin films and the results showed that the secondary phase of ZnS at the bottom of CZTS was suppressed and interface problem between CZTS and Mo was improved, thereby the open-circuit voltage and fill factor were increased and cell conversion efficiency was correspondingly improved.