Preparation And Photovoltaic Properties Of Cu₂ZnSnS₄ Nanoparticles And Thin Films

With the continuous growth of word population and economy, the demand for energy is more and more. In the current energy consumption structure, fossil fuel is main part which pollute environment and is nonrenewable. In recent years, the energy crisis also has forced people to find clean and renewable energy sources. Solar energy is one of the most ideal renewable energy sources. Solar cells can convert sunlight to electricity. However, the traditional crystalline silicon solar cells are high cost. So, the low-cost and new solar cell materials such as Cu（In,Ga）（S,Se）2（CIGS）, Cu2 ZnSn S4（CZTS）, CdTe and so on, are attracting widespread attention.CZTS retains the similar structure to CIGS, and contains earth-abundant, low-cost, and nontoxic elements. Moreover, CZTS has an optical band gap（¹.5 eV） and a large absorption coefficient（>104 cm-1）. The theoretical limit for CZTS thin film solar cells is nearly 32.2%. Thus, CZTS solar cell is one of the most promising solar cells.In this paper, flower-like CuS, lamellar SnS and sphere-like ZnS binary sulfide nanoparticles were synthesized by microwave irradiation assisted solvothermal method first. Based on the prepareation of binary sulfide nanoparticles, irregular sphere-like Cu₂SnS₃ nanoparticles were successfully synthesized with thiourea as sulfur source, ethylene glycol as solvent, polyvinylpyrrolidone（PVP） as surfactant. With increasing amount of PVP, agglomeration of as-prepared Cu2 Sn S3 nanoparticles was less and the size gradually became smaller. When L-cysteine was used as sulfur source, hollow Cu₂SnS₃ nanoparticles were obtained. Its band gap was about 1.25 eV.According to the experience of preparation of ternary Cu₂SnS₃ nanoparticles, single phase and satisfactory stoichiometry CZTS nanoparticles were prepared by thiourea as sulfur source and PVP as surfactant. The average size of CZTS nanoparticles was ⁴00 nm. Using sodium dodecyl benzene sulfonate（SDBS） and hexadecyl trimethyl ammonium bromide（CTAB） to replace PVP as surfactant, peanut-like and sphere-like CZTS nanoparticles were synthesized, respectively. Moreover, with increasing the amount of PVP, the size of CZTS nanoparticles became gradually smaller and the sheet size on the surface was gradually larger. To understand the mechanism of formation of CZTS nanoparticles, the effect of reaction time and temperature on properties of CZTS nanoparticles was studied. The results showed that Cu2-xS nuclei firstly formed, and served as the starting point for the nucleation and growth of CZTS. The effect of sulfur source was also studied. Using L-cysteine as sulfur source, ⁵0 nm hollow CZTS nanoparticles were obtained. CZTS quantum dots were obtained with thioacetamide as sulfur source. The average size was about 3 nm. The band gap had obvious blue shift and was about 1.8 eV. In order to tailor the band gap of as-prepared CZTS nanoparticles, Cu2（FexZn1-x）SnS4 nanoparticles were prepared by Fe partial substitution of Zn. The band gap of as-prepared nanoparticles decreased from 1.54 eV to 1.23 eV.The as-prepared CZTS nanoparticles with different size were dispersed in n-propanol to form CZTS nano-ink, respectively. CZTS nano-ink prepared with large size nanoparticles had bad disperision. Thin films prepared by spin-coating method had obvious large holes. However, the thin films prepared with small size nanoparticles ink were uniform and compact. To remove oxygen element and improve crystalline, as-prepared thin films were sulfurized. The sulfurization process was optimized by changing sulfurization temperature, time and pressure. When sulfurization temperature was low, CZTS thin films had some holes. With increasing sulfurization temperature, the compactness and crystalline of as-prepared thin films were improved. When sulfurization temperature was 600 ℃, photoelectric performance was the best and the efficiency of corresponding solar cell reached 1.3%. The sulfurization time had great impact on properties of as-prepared thin films. The CZTS thin films prepared for short sulfurization time had worse crystalline. If sulfurization time was too long, secondary phase existed in the as-prepared thin films. When sulfurization time was 40 min, the as-prepared CZTS thin films were best. The sulfurization pressure also had influence on the performance of thin films. The CZTS thin films prepared at 300 mbar had a best crystalline and compactness. Based on optimized sulfurization process, the spin-coating times were also optimized. The results showed thickness of CZTS thin films spin-coated 20 times was ⁸10 nm. The efficiency of CZTS solar cell reached 2.1%. In order to further enlarge grain size, selenization process was also optimized. The final results indicated CZTSSe thin films prepared at a ramp rate of 50 ℃/min to 520 ℃ for 20 min were compact and the as-prepared solar cell had 2.5% efficiency.To solve the problem of cracks in the nano-ink derived thin films, CZTS thin films were fabricated by combination of CZTS nano-ink and molecular solution method. Firstly, thin films with certain thickness were prepared with CZTS nano-ink. And then molecular solution was spin-coated on the thin films for different times. When the molecular solution was spin-coated for 3 times, the as-prepared CZTS thin films were cracks-free but had some holes. However the CZTS thin films prepared with 5 layers molecular solution had compactness and best crystallinity. The as-prepared CZTS thin film with 5 layers molecular solution was selenized. After selenization, the cystallinity of thin films were improved and the CZTSSe solar cell had 4.18% efficiency. Secondly, hybrid ink was prepared by combination CZTS nano-ink and molecular solution. After sulfurization, the as-prepared CZTS thin films had single-phase and best crystallity. But the efficiency of as-prepared CZTS solar cell was only 1.27%. After selenization, grain size of CZTSSe thin film reached micron scale. A 6.39% efficienct CZTSSe thin film solar cell was obtained. The band gap of CZTSSe thin film was about 1.14 eV.