Conventional and Rapid Thermal Annealing of Spray Pyrolyzed Copper Indium Gallium Sulfide Thin Films

Abstract

With this study for the first time effects of post annealing on morphological, structural, optical and electrical properties of spray pyrolyzed copper-indium-gallium-sulfide (CuInGaS2) thin films have been investigated. Pros-and-cons of conventional (CA) and rapid thermal annealing (RTA) have been discussed to obtain the high quality thin film absorbers for solar cell applications. X-ray diffraction analysis revealed that all of the spray pyrolyzed CuInGaS2 thin films have chalcopyrite structures with a highly (112) preferential orientation. Raman spectra also confirmed this structure. However, metal oxide secondary phases such as copper oxide and gallium oxide were detected when the temperature ramp rate was increased during RTA process. Energy dispersive X-ray measurements revealed that both copper and gallium diffused through the surface after annealing processes. Moreover, copper diffusion became pronounced especially at high annealing temperatures. Optical transmission measurements in the wavelength range between 600 and 1100 nm showed that band gap energy of CuInGaS2 thin films was ranging between 1.36 and 1.51 eV depending on the annealing conditions. Very high mobility values have been observed for both processes. The maximum electrical mobility, 30.9 cm(2)/V s, was observed for the films annealed at 600 degrees C via CA. This is the highest reported value among the CuInGaS2 thin film absorbers deposited by both solution and vacuum based techniques. As a result, post-annealing of spray pyrolyzed CuInGaS2 thin films without usage of highly toxic gases, reported in this study, is very promising and environmentally friendly method to produce solar cell quality absorber layers. (C) 2014 Elsevier B.V. All rights reserved.