Cu1−xInxSe2 thin films: Deposition by spray pyrolysis and characteristics

Abstract

Deposition of copper indium selenide (Cu1−xInxSe2 (0⩽x⩽1)) thin films onto the spectroscopic grade amorphous glass substrates was carried out by a chemical spray deposition technique. The deposition parameters, viz. temperature (400°C), time (8min), solution spray rate (5ml/min), nozzle to substrate distance (30cm), air pressure (1.2kg/m2), etc. were optimized so as to obtain good quality samples. The chemical composition and structure of these as-deposited films were determined by the EDS and XRD analyses techniques. The films are nonstoichiometric. Copper and indium contents in the films vary in proportion with the x and 1−x values. Selenium content in the films (for low In-concentration) was less than that of the expected (50%) whereas it exceeds 50% for higher In-concentration in the films. The crystal structure of these films as-revealed by an X-ray diffraction technique showed that the deposits are composites of the CuSe2, CuInSe2, and In2Se3 phases. The observed data have fair match with the JCPD data. The optical studies conducted in the range of wavelengths between 300nm and 1300nm showed absorption coefficient of the order of 104–105cm−1 and the transitions to be of the direct type. The energy band gap decreased continuously from 2.10eV to 1.15eV as x was varied from 0 to 1. The surface morphology revealed a web of needle like long and tapered at one end, randomly oriented crystallites. Some globule like overgrowth has also been observed. The electrical conductivity studies showed that the films are semiconducting in nature and conductivity is found to be increased with increasing In-content up to x=0.4 and decreased thereafter. A highest electrical conductivity of 1.1×10−4 (Ωcm) has been observed for the films with x=0.4. The activation energies of an electrical conduction were then determined for these samples. Thermo power measurements showed same trend of variations analogous to the electrical conductivity and the samples exhibit n as well as p-type conduction.