The Preparation And Characterization Of Cu₂ZnSn(S_1-xSe_x)₄ Films

Thin films solar cells have developed rapidly in recent years. In particular, the current powder conversion efficiencies of Copper Indium Gallium Selenide (CIGS) thin films solar cells have been pushed beyond 20%. However, indium and gallium used for preparation of CIGS are rare and expensive, which would inhibit a cost-effective large scale production. Cu2ZnSn(S1-x,Sex)4(0≤x≤1) has a structure similar to CIGS. It is also a p-type semiconductor with a bandgap about 1.0-1.6 eV, which is very close to the optimum value for being used as an absorbed layer in solar cells. In addition, the high absorption coefficient in the visible region (over 104 cm-1). Moreover, it is nontoxic, and the elements it contained are abundant in the crust in comparison with those of CIGS. Owing to these special properties, Cu2ZnSn(S1-x,Sex)4 is considered to be a promising material for application in low cost and environmentally friendly thin films solar cells. In this paper, we prepared Cu2ZnSn(S1-x,Sex)4 films and nanocrystal vis the facile solution chemical method, and the preparation conditions are discussed and analyzed in detail. In addition, the photoelectric properties of the films have also been investigated. This thesis includes the following main aspects:(1) The CZTS films are directly prepared on FTO glass substrates by SIALR, the temperature (50 ℃) and pH (3.0 and 6.0) are optimized to improve repeatability. Further, the investigation concerning the number of deposition cycles and annealing conditions and the manner in which they determine CZTS properties. The characterization results reveal that the optimal preparation condition should be executed with approximately 50 deposition cycles and then be subjected to annealing in sulfur vapor, the film has high purity, the thickness is about 500 nm, the band-gap is 1.4 eV, as well as sulfurization annealing improves the crystallinity and stability of photocurrent.(2) For the first time, the CZTS films are directly prepared on FTO glass substrates by hydrothermal method. The influence of reaction time on the properties is investigated. The optimized CZTS films with 12 h reaction time and sulfurization annealing appear high purity and best performance. The peak of the current density is about 2.5 μA·cm-2 and stable at 2.0 μA·cm-2. After selenization annealing, the film present CZTSSe crystal and the band-gap range 1.4 eV to 1.3 eV and photocurrent increses four times.(3) For the first time, the CZTS films are directly prepared on FTO glass substrates by ethanol-thermal method. After sulfurization annealing, the films defectes are reduced, the band-gap is 1.42 eV and the photocurrent is 1.0 μA·cm-2 ; after selenization annealing, the film present CZTSe crystal, the band-gap is 1.15 eV and the photocurrent increase 2 times. The results reveal the importance of the sulur or selenium vapor in the annealing process.(4) CZTS nanocrystals with kesteries structure are synthesized by a simplified hydrothermal method and the films are prepared by using the brush-coating technique. The influence of reaction time on the properties is investigated. The results reveal reaction time has significant impact. The optimized CZTS nanocrystal with 9h reaction time appear high purity, the band-gap is 1.58 eV and photocurrent is 15 μA·cm-2 .