Preparation And Properties Of The Absorber Layer Of Cu₂ZnSnS₄Thin Film Solar Cell

Cu2ZnSnS4(CZTS) thin film solar cell is a hot topic in the research field of solar cells, whose constituents are abundant, cheap and non-toxic. The theoretical limit conversion efficiency of CZTS thin film solar cells can reach32.2%. However, the highest conversion efficiency is only about12%at present. The key problem is that the quality and property of CZTS thin films are bad. Therefore, finding the preparation techniques and methods, and improving the quality and property are the key scientific problems. CZTS precursor thin films were prepared by sol-gel and magnetron sputtering in this paper. Combining with heat treatment or sulfurization technology, the study on CZTS thin film was developed. The structure, composition and properties were investigated. The factors and mechanism are studied, which effects on the crystal structure and physical properties of CZTS. The contents and results are as follows:1.CZTS thin films were prepared by sol-gel sulfurizing method with copper chloride, zinc chloride, tin chloride and thiourea as the raw material. The effect of the heat treatment conditions and the Zn content on the structure, composition and properties were studied. A part of chlorine was observed in the low temperature. The Cl may be exist at the S site and replaced by the S during the sulfurization process. When the precursor thin film(the content of Zn in the precursor solution was11.5mmol) was pre-annealed at250℃and then annealed at480℃for2h in sulfur vapor, the thin film formed a single Cu2ZnSnS4phase with kesterite structure and showed a continuous and dense surface. The band gap was estimated to be1.52eV and the carrier concentration was2.46×1015cm-3.2. The Cu-Zn-Sn-S amorphous precursor thin film was prepared by sputtering CZTS quaternary target and then annealed under different conditions. The effect of the heat treatment conditions on the structure, composition and properties was studied. When the temperature of precursor was530℃and the mass of sulfur was3g, the thin film exhibited good electrical and optical properties. The band gap was estimated to be1.52 eV and the carner concentration was1.59×1016cm-3.3. The stacked metallic films with the Cu/Sn/ZnS sequence were deposited by magnetron sputtering and then annealed under different sulfurization conditions. The reaction routes of CZTS and the optimum sulfurization conditions were studied. The binary and the ternary phases were formed in the beginning. The sulfides reacted each other and transformed into CZTS phase at last. If the sulfur source mass was little or the sulfur source temperature was low, it would lead to inadequate react between elements during the sulfurization process. If the sulfur source mass was much, it would lead to more porous in the surface. If the Sulfur source temperature was high, it would lead to the precipitation of impurity phase. When both the precursor and sulfur source were at500℃, and the sulfur source quality was2g, the thin film after sulfurization formed a single Cu2ZnSnS4phase with kesterite structure, exhibited compact and uniform grain characteristics. The band gap of the film was estimated to be1.54eV.4. Six different sequence of the precursor thin film deposited by magnetron sputtering. The CZTS thin films were prepared when the precursors were annealed in sulfur vapor at500℃. The effect of the sequence on the structure, composition and properties was studied. The surface of the Cu/Sn/ZnS thin film after sulfurizing was compacter and the component was closer to the stoichiometric composition. On the basis of the above, stacking orders of Cu/Sn/ZnS/Cu/Sn/ZnS and Cu/Sn/ZnS/Cu/Sn/ZnS/Cu/Sn/ZnS were prepared. The effect of the cycle number on the structure, composition and properties of the films was studied. After sulfurizing at500℃, only the CZTS phase with kesterite structures was observed in Cu/Sn/ZnS and Cu/Sn/ZnS/Cu/Sn/ZnS thin films. The ZnS was observed in Cu/Sn/ZnS/Cu/Sn/ZnS/Cu/Sn/ZnS thin film. The increase of cycle number promoted the formation of CZTS phase, increased the particle size and the band gap of the thin films.Through the research of the above several methods, the cycle sputtering deposition is more beneficial to prepare the CZTS thin film with kesterite structure, big crystalline grain and high properties.