Preparation Of Copper, Zinc, Tin And Sulfur Powders And Films By Solution Metho

Cu2ZnSnS4(CZTS),a p-type photovoltaic absorber material has received significant attention due to its attractive features such as utilization of abundant metals,a suitable band gap(1.4～1.6 eV)which matches well with the best band gap(1.5 eV)that required by the single solar cells and its high absorbance coefficient(>104 cm-1).It has become a very potential thin film solar cell absorption layer material.In this paper,phase and morphology-controlled CZTS powders have been successfully synthesized by a facile and effective two-step-heating route under the open-air condition by adjusting the ratio composition of solvent,reaction time,reaction temperature and other reaction conditions.In our experiments,low-cost metal salts and thiourea are chosen as raw materials,organic solvent ethylene glycol(EG)and Triethylenetetramine(TETA)are priority selected as the reaction solvent during the synthesis process.Two-step-heating method could effectively distinguish a nucleation process from a crystal growth process and obtain relatively homogeneous CZTS nanoparticles.TETA is well known as a common chelating ligand for the metal ions[Mn+,Cu2+,Zn2+and Sn4+],it could cap on the surface of generated nanocrystals to reduce its energy,adjust the chemical environment of the reaction system,and may prohibit the phase transition from metastable wurtzite CZTS to stable kesterite CZTS,its use is the key factor for the formation of metastable wurtzite CZTS rather than kesterite CZTS.In order to furthe understand the phase-selective growth mechanism for CZTS nanocrystals synthesized with different ratio of VEG:VTETA,time-dependant experiments and analyses of XRD,EDS,Raman and SEM focus on the phase composition evolution under the condition of VEG:VTETA=1:39 and VEG:VTETA=1:1,respectively.In the EG and TETA mixtures with a volume ratio of 39:1,firstly,during the heating at 130℃ as the nucleation process,CuS NCs were formed by the decomposition of(Tu-M-TETA)n+complex.Then,the formed CuS would aggregate and form a ball cactus-like shape through the assembly process because of the characteristic growth pattern of CuS in solution phase routes and the selective adsorption of TETA.In case of VEG:VTETA=1:1,Cu7S4 nanoparticles were formed as first seeds.As the reaction progressed,Sn and Zn entered into the Cu7S4 lattice to form W-CZTS,small particles grow to larger ones via an Ostwald ripening process.The optical properties of CZTS nanoparticles were performed,they present strong optical absorption in the range of visible light,and their band gap values agree well with the ideal value for photovoltaic applications,which showing their potential applications in high-efficiency solar cells as well as other photovoltaic devices.Then,CZTS thin films were prepared through coating nano-ink followed by sulfurization.After annealing,the crystallinity of CZTS is obviously improved,the particle size increases,and the film is denser with the increase of sulfurization temperature.The metastable wurtzite CZTS would transform into a stable kesterite phase.A low sulfurization temperature is harmful to the formation of dense film,a high will lead to decomposition of CZTS,and the appropriate annealing temperature is 500～550℃.The films prepared with the W-CZTS nanoparticles were more homogeneous and dense than the films made with K-CZTS nanoparticles.These results reveal the suitability of these films as an absorber layer for thin film solar cells.In order to further study the nucleation of metastable W-CZTS,Cu7S4 was prepared by one-step-heating route,and the influence of the ratio of raw materials and reaction time on the product was studied.The sulfur-rich raw materials is favorable for the synthesis of Cu7S4 with high purity and good crystallinity.