Chemically Synthesized Cu2ZnSnS4 (CZTS) Thin Films for 2nd Generation Photovoltaics

Abstract

The fabrication of solar cell with low cost, higher efficiency and with environmental friendly synthesis process is a current challenging task. The second-generation thin film solar cells are an alternative approach for reducing manufacturing cost with facile fabrication process as compared to first generation solar cells. The second-generation thin film solar cells are based on CIGS, CdTe, CdS and CZTS compound, known as polycrystalline thin film solar cells. CZTS acquired considerable attentions of the researchers for past several years over CIGS, CdS and CdTe due to its cost-effectiveness, natural abundance and non-toxicity of composing elements. This thesis reports the study of the effect of different copper concentration on the electrical and optical properties of the CZTS thin film as an absorber layer. CZTS thin films were deposited onto the SLG substrate by spin coating method and characterized by X-ray diffraction, scanning electron spectroscopy, energy-dispersive spectroscopy, UV-VIS NIR spectroscopy, and hall effect measurements. X-ray diffraction measurements for structural analysis confirms that the crystal structure and crystallite size of CZTS thin films improved by increasing the copper concentration from 0.15M Cu to 0.25M Cu and then slightly decrease for 0.30M Cu. It is observed from SEM micrographs that increasing the copper concentration leads to more dense and compact surface morphology, while EDX measurements revealed that by increasing copper concentration the CZTS thin film becomes Cu-poor and Zn-rich from 0.15M Cu to 0.25M Cu and vice versa for 0.30M Cu. UV-VIS NIR spectroscopy for optical energy band gap analysis reveals the improvement in the energy band gap value of CZTS thin films by increasing copper concentration from 0.15M to 0.25M Cu. For electrical measurements hall effect analysis shows the charge carrier concentration, mobility and resistivity enhanced by increasing the copper concentration. The results confirm that the optimum values for high quality CZTS thin film is achieved by using 0.25M Cu concentration. Because at this concentration pure kesterite structure with compact and dense film structure with Cu-poor and Zn-rich composition is formed. Furthermore, a favorable optical energy band gap of 1.5 eV with absorption coefficient of 106 cm-1 is obtained, which is most promising and suitable for an absorber material for CZTS thin film solar cells