Ultrasonic spray deposition of CuInS2 absorber thin films: effect of nozzle frequency

Copper indium disulphide (CuInS2) thin films have a huge potential as absorber layer for thin film photovoltaic applications. In this work, a very simple, and therefore, cost-effective technique, ultrasonic spray pyrolysis, have been used to deposit CuInS2 films on soda lime glass substrates. Two different nozzles, with 48 and 120 kHz frequencies, were used to control the droplet size of the precursor solution. It has been observed that nozzle frequency had a dramatic effect on both solution consumption and the surface morphology of the films. Scanning electron microscopy (SEM) analysis of the films revealed that 48 kHz nozzle resulted very rough surfaces and low adhesion. On the other hand, 120 kHz nozzle resulted lower deposition yield than 48 kHz nozzle due to the smaller droplet size. Energy Dispersive X-Ray Analysis (EDAX) has been used to determine the chemical structure of sprayed CuInS2 films. Films deposited using 20 ml precursor solution and 120 kHz nozzle had the closest Cu:In:S atomic ratio to the targeted 1:1:2 value. Crystal structure of the spray-deposited films was confirmed using X-ray Diffraction (XRD) analysis. Regardless of the solution amount and the nozzle frequency, polycrystalline CuInS2 thin films were observed. All sprayed films showed nanocrystalline nature. As a general trend, optical transmission and band gap energy values of the films decreased with increasing the solution amount for both 48 and 120 kHz nozzles. The absorption coefficient of sprayed CuInS2 films was found to be in the order of 10(5)-10(3) cm(-1) in the wavelength range 200-2000 nm. As a result, in terms of solution consumption 48 kHz nozzle was superior to the 120 kHz nozzle. However, 120 kHz nozzle allows us to obtain more homogenous and well crystallized films.