Synthesis And Photoelectric Properties Study Of Cu₂ZnSn（S,Se）₄ Thin Films

Cu2ZnSnS4(CZTS) is a new semiconductor material. It shares similar structure with the chalcopyrite material CuInS2 except that half of the In is replaced with Zn and another half with Sn. Its crystal structure and optical properties(band-gap energy of ～1.5 eV; absorption coefficient of ～104 cm-1) are similar to Cu(In,Ga)Se2(CIGS), one of the most successful thin-film new photovoltaic materials. Moreover, it is composed of abundant elements. Owing to these special properties, CZTS has attracted increasing attention in low cost and environmentally friendly thin film solar cells.Nowdays, CZTS thin film can be fabricated by vacuum methods and non-vacuum methods High-quality thin films are often fabricated by vacuum methods, but the fabrication procedure requires relatively expensive equipment, which limits the use in large-scale preparation of CZTS thin films. Electro-deposition methods, sol-gel methods, hot-injection methods and solution based methods are typical non-vacuum methods. The molecular precursor-based solution approach without complex nanocrystal synthesis is particularly suitable for low-cost and high-speed deposition of metal sulfide thin film. Recently, Mitzi and co-workers developed a general hydrazine-based solution approach to fabricate CZTSSe films and received the highest efficient of 12.6%. However, hydrazine is a highly toxic and explosive solvent. In view of the existing shortcomings of hydrazine-based solution method, we developed a general solution method for the direct fabrication of CZTSSe thin films using diamine-dithiol mixture as solvent. The fabricated CZTSSe films were used in thin-film solar cells and DSSCs according to their features.This dissertation consists of three chapters follow as:In chapter one, CZTSSe precursor solution was obtained by using ethylenediamine as main solvent. Then, the CZTS thin ?lm with thickness of ～1.5 μm was obtained by repeating three spin-casting/sintering cycles. The X-ray diffraction patterns revealed that the main diffraction peaks intensity become strong after post-treatments. By selenization, large grains appeared and XRD diffraction changed. This is attributed to the expansion of the unit cell volume by the partial replacement of S with Se. Through surface contrast of precursor film and selenization film, and we can find that selenization is beneficial for the grow up of small grains. Ethylenediamine is highly volatile and easy to react with acids in air, which lead to the porous CZTSSe precursor films and non-continue CZTSSe films.In chapter two, porous CZTSSe films were obtained as above and used as counter electrode(CE) in DSSCs. The influence of the different post-treatments on performance of CZTSSe counter electrodes based DSSCs was evaluated. The CZTSSe CE selenizated in a tube furnace at 500 °C for 10 min showed the highest energy conversion efficiency of 6.13%, which is comparable to that of Pt CE(6.20 %).In chapter three, diethylenetriamine, instead of ethylenediamine, was used as the main solvent to form CZTSSe precursor solution. By changing the spin speed, dense CZTSSe precursor films were obtained. After selenylation, dense CZTSSe films with good crystallinity were obtained.As the absorber layer, The CZTSSe films were used to fabricate CZTSSe based solar-cells. The active area of the as-fabricated devices is 0.19 cm2, and the best energy conversion efficiency of 3.28 % was achieved.