| Inorganic nanocrystals have been widely applied in thin film solar cells for their ideal bandgap and high absorption coefficient. Thin film solar cells of CdTe and Cu(In, Ga)Se2 systems have already achieved the efficiency over 20% in industry scale. But Cd, Te and Se are toxic and In and Ga are scarce elements, which confined their application in the future.Cu2ZnSnS4(CZTS) is a promising quaternary kesterite compound,which has an optimal direct band gap of 1.5 eV and high absorption coefficient. The advantage of CZTS is that it comprised of non-toxic and earth abundant elements. CZTS is considered as substitution for Cu(In,Ga)Se2 because it has similar band gap and crystal structure with Cu(In,Ga)Se2. Compared with binary and ternary compounds, the quaternary compounds have their unique advantages because the category and concentration of benign lattice defects could be tailored to optimize the optical and electrical properties. Vacuum based approaches with high costs and complicated apparatus confined large scale application for synthesizing thin films. Electrodeposition is regarded as a feasible method for synthesizing various functional nanostructured materials in industry scale.In this thesis, targeted off-stoichiometry CZTS thin films were synthesized by one-step electrodeposition. Electrodeposition mechanism was investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV)and Electrochemical Impedance Spectrum (EIS). The nuclei-growth process of CZTS thin films was revealed. Based on such mechanism, electrolyte design principle was proposed and a novel electrolyte was developed. The CZTS thin films was successfully electrosynthesized from this novel electrolyte. According to the optimization of heat treatment, CZTS thin films consisted of rice-like particles was obtained, which have both high carrier concentrations and high carrier mobility. The CZTS thin film solar cells,which reached power conversion efficiency of 0.39%, were fabricated with Mo back electrode, CdS buffer layer, ZnS window layer and Pt grid. The main achievements were as follows:(1)The targeted off-stoichiometry CZTS thin films were synthesized by one-step electrodeposition. Based on the different kinetics of the metallic ion reduction, the chemical composition was controlled by varying deposition time. The Cu/(Zn+Sn) and Zn/Sn ratios of the precursor films were precisely controlled in the range of 0.71 -1.05 and 0.99-1.17, respectively. After annealing, pure kesterite structure, which carrier type was hole, was obtained. The grain size was in the range of 20-30 nm and the band gap was in the range of 1.43-1.52 eV. The carrier mobility reached a value of 28.20 cm2/V · s with carrier concentration of 2.09×1018 cm-3 when Cu/(Zn+Sn) and Zn/Sn ratios were 0.97 and 1.13, respectively. The high carrier mobility was attributed to the electronic interactions between Cu and Sn.(2) The electrodeposition mechanism of pure phase CZTS thin film was investigated. Electrochemical studies indicated that trisodium citrate and tartaric acid could narrow the co-deposition potential range of the four elements to -0.8 V?-1.2 V (vs. SCE). The cause was the synergetic effect that trisodium citrate inhibited the reduction of Cu2+and Sn2+ and tartaric acid promoted the reduction of Zn2+. The reduction of S2O32- was mainly attributed to the induction effect of the metallic ions. The mechanism studies provided a path of electrolyte design for multicomponent compounds. To achieve co-deposition,complexing agent and organic acid should be added in the electrolyte to narrow the potential gaps among the main salts.(3) Potassium pyrophosphate and sulfosalicylic acid were chosen as additives in the novel bath for synthesizing CZTS thin films with high S content of 43.15 at%. The obtained CZTS thin films had pure kesterite structure and suitable band gap. Potassium pyrophosphate and sulfosalicylic acid were verified to have synergetic effect narrowing the potential gaps among the main salts. They also promoted the reduction of S2O32- to ensure the stoichiometry.(4) The heat treatment is inevitable because the as-deposited thin film was in amorphous nature. In this thesis, the heat treatment was optimized by controlling of Ar flow rate and multi-step annealing. Then the CZTS thin films consist of rice-like particles were obtained. The grain size changed a little compared with one step annealing. However,those with optimized annealing have both high carrier concentrations and high carrier mobility because the holes mainly generated in the grain and transported in the grain boundary. The actual optimization of annealing is the optimization of grain boundary. Therefore, the carrier mobility increased in the circumstance of high carrier concentration.(5) In the fabrication of CZTS thin film solar cells, the CZTS absorber layer was synthesized by electrodeposition-annealing route on Mo substrate. Then CdS buffer layer and ZnS window layer were prepared via chemical bath deposition methods. The area of the solar cell was 0.2 cm2. The photovoltaic properties of the solar cell were as follows:VOC = 0.53 V, ISC=1.76 mA/cm2, FF=0.42, and PCE=0.39%. |
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