| Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) have attracted more and more attention as ideal candidate absorber materials for thin film solar cells with the optimal band-gap for the single junction photovoltaic device of 1.4-1.5 eV, high absorption coefficient (>104 cm-1), abundant elemental components, which are not constrained by the drawbacks of CdTe and CuIn1-xGaxSe2 (CIGS). Several groups have studied the fabrication of CZTS solar cells using a variety of methods such as thermal evaporation, sputtering deposition, and soft chemistry method.The efficiency of CZTS based solar cells has increased from 0.66% in 1997 to 8.4% in 2011. Recently, IBM have fabricated Cu2ZnSnSxSe4-x (CZTSSe)thin film solar cells with 9.6% efficiency using a hydrazine-based solution via spin coating, and Guo et al. fabricated CZTSSe solar cells by selenization of CZTS nanocrystallines, with 7.2% efficiency using a simple film coating method. Those encouraging results demonstrate that the Se composition plays an important role in increasing the power efficiency of CZTS based solar cells. However, hydrazine poses safety challenges from a manufacturing perspective, and the loss of Sn at temperatures higher than 400℃during selenization of CZTS films makes it difficult to control film composition and impede film homogeneity. Thus, it is very important to synthesize pure phase Cu2ZnSnSxSe4-x(0<x<4) solid solution nanocrystallines for fabrication of thin film solar cells.Nowadays, CZTS and CZTSe nanoparticles have been synthesized via wet chemical method and those solution-based nanoparticles prepared by a rigmarole process and use some expensive reagent. In this Theses, a simple and facile solvothermal approach has been developed to prepare Cu2ZnSnS4, Cu2ZnSnSe4 and Cu2ZnSnSxSe4-x(0<x<4) solid solution nanocrystallines. Furthermore, the as-synthesized Cu2ZnSnSxSe4-x(0<x<4)) solid solution nanocrystallines are used as raw materials to prepare the Cu2ZnSnSxSe4-x absorber layer. The details are introduced as following:1. High quality Cu2ZnSnSe4 and Cu2ZnSnS4 nanocrystallines have been synthesized via solvothermal route in ethylenediamine using CuCl, ZnCl2, SnCl4·5H2O, Se or S powers as reaction reagents. The as-synthesized products were characterized by X-ray powder diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. The results confirm that the Cu2ZnSnSe4 and Cu2ZnSnS4 nanocrystallines are synthesized with single phase and uniform composition and display plate-like morphology with average diameters of 6-8 nm and 8-12 nm, respectively. Study the influence of the products by reaction temperature, time and explore the possible growth mechanism.2. Based on the experience for Cu2ZnSnSe4 nanocrystallines, Cu2ZnSnSxSe4-x(0<x<4) solid solution nanocrystallines have been synthesized via solvothermal route using mixture (Se power and S powers) instead of Se power. The nanoparticles Cu2ZnSnSxSe4-x(0<x<4) are apt to aggregate and single crystalline nanoplates with larger average diameters of 20-50 nm. From the XRD patterns, it is observed that the peaks gradually move to the higher deg as the increasing quantity of Se powders, which demonstrates the S with a low diameter has been mixed into the Cu2ZnSnSe4 and pure phase Cu2ZnSnSxSe4-x (0<x<4) solid solution nanocrystallines are synthesized. From the high resolution transmission electron microscope (HRTEM) images of the products, it is observed that the d-spacing of (112) plane decrease as the increasing quantity of the S element in the solid solution nanocrystallines, which may further proves that the S element has been successfully mixed into the Cu2ZnSnSe4 nanocrystallines and formed the Cu2ZnSnSxSe4-x (0<x<4) solid solution nanocrystallines. Similar result is also found in the Raman spectra of the Cu2ZnSnSxSe4-x nanocrystallines, in which the peaks get a blue-shift as the increasing quantity of S. It demonstrates that the light element has successfully substituted for the heavy. The optical property of the solid solution nanocrystallines are also investigated by photoluminescence spectra (PL) and ultraviolet-visible (UV) absorption spectra. It is deduced that the band-gap value is modulated by the quantity of S.3. The CZTSSe absorber thin films have been fabricated under non-vacuum conditions by screen printing method using the prepared Cu2ZnSnSxSe4-x (0<x<4) solid solution nanocrystallines as the raw materials. Mo-coated SLG (Mo/SLG) were used as substrates for these thin films. The screen printing has advantages of low-cost, simple and reliable process, larger size, simple equipment and non-vacuum condition. The as-synthesized thin films were characterized by X-ray powder diffraction(XRD) and scanning electron microscopy(SEM). The factors which influence the quality of thin films are also studied. CdS is deposited onto the selected CZTSSe absorber thin films with 2～3μm thickness by chemical bath method. Conductive silver adhesives are used as the electrode. From theⅠ-Ⅴcurve of the CZTSSe/CdS thin film, it is found that the CZTSSe thin film has p-type conductivity. |
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