Thermal evaporated undoped and na-doped CuInS2 with copper contact for photovoltaic applications

Undoped and Na-doped CuInS2 powder has been synthetized by direct fusion of precursor elements. We have deposited those two materials in thin films on glass substrate and on copper layer by vaccum thermal evaporation method followed by an annealing in air atmosphere at 250°C and 450°C. We have investigated their structural, optical and electrical properties that show good results for photovoltaic applications. The undoped CuInS2 layer exhibit an n-type conductivity whereas Na-doped CuInS2 is p-type both on glass and on copper layer. X-ray analyses of the as deposited samples show amorphous structures. The annealed samples in air showed a crystalized CuInS2 layer with a major peak of the (112) chalcopyrite phase with the presence of CuO and In2S3 impurity phases for the 450° annealed. Optical gap of both undoped and Na-doped CuInS2 where between 1.4 and 1.6 eV, that is close to the optimal gap value 1.5 eV of the CuInS2 suitable for a good absorption of solar spectrum. Electrical characterizations showed good ohmic contacts with low potential barrier height, and good ideality factor of respectively (0.75 eV, 1.37) for undoped and (0.66 eV, 1.27) for Na-doped CuInS2/Cu contacts. Also, we observed a low value of series resistance 3.51 and 1.75 Ohms respectively that suppose a good current flow in the contact.