The effect of dopant concentration on properties of transparent conducting Al-doped ZnO thin films for efficient Cu2ZnSnS4 thin-film solar cells prepared by electrodeposition method

Al-doped ZnO (AZO) thin films were potentiostatically deposited on indium tin oxide substrates. The influence of the doping level of the ZnO:Al films was investigated. The results of the X-ray diffraction and scanning electron microscopy analysis revealed that the structural properties of the AZO films were found polycrystalline with a hexagonal wurtzite-type structure along the (002) plane. The grain size of the AZO films was observed as approximately 3 mu m in the film doping with 4 mol% ZnO:Al concentration. The thin films also exhibited an optical transmittance as high as 90 % in the wavelength range of 100-1,000 nm. The optical band gap increased from 3.33 to 3.45 eV. Based on the Hall studies, the lowest resistivity (4.78 x 10(-3) Omega cm) was observed in the film doping with 3 mol% ZnO:Al concentration. The sheet resistant, carrier concentration and Hall mobility values were found as 10.78 Omega/square, 9.03 x 10(18) cm(-3) and 22.01 cm(2) /v s, respectively, which showed improvements in the properties of AZO thin films. The ZnO:Al thin films were used as a buffer layer in thin-film solar cells with the structure of soda-lime glass/Mo/Cu2ZnSnS4/ZnS/ZnO/Al grid. The best solar cell efficiency was 2.3 % with V-OC of 0.430 V, J(SC) of 8.24 mA cm(-2) and FF of 68.1 %.