Deposition and characterization of graded Cu(In1-xGax)Se2 thin films by spray pyrolysis

Abstract

Cu(In1-xGax)Se2 (CIGS) thin films and their graded (x = 1 to 0) layer were grown on soda lime glass substrates using chemical spray pyrolysis (CSP) at different substrate temperatures (Ts). After optimization of Ts, depositions were carried out at different gallium composition (x) at optimized temperature of 350 °C. All the films deposited at Ts ≥ 350 °C were polycrystalline chalcopyrite structure, with a preferential orientation of (112), including the graded layer. With increase in x, lattice parameters a and c were observed to decrease. Line scan of the CIGS layer showed intersection of gallium and indium concentrations, revealing the graded nature of the film. Composition dependence of Raman peak for CuInSe2 (CIS) deposited by CSP was analyzed. Optical transmittance at a wavelength of 800 nm of the film with x = 0 (CIS) (30%) was found lower than that of the film grown with x = 0.82 (CIGS) (50%). Cusp-shape of the resistivity was observed with an increase of x leading to steep rise in resistivity of the films (1.61–71.68 Ω-cm) till x = 0.42 and then decreased to 4.78 Ω-cm at x = 0.82. Carrier concentrations of the films were evaluated in the order of 1016–1019 cm−3 with p-type conductivity. These results indicate that graded CIGS thin films with modulated gallium composition can be prepared by CSP.