Study Of The Annealing Treatments Of Electrodeposited CuInSe₂ And Cu(In,Ga)Se₂ Thin Films

CuInSe2-basedⅠ-Ⅲ-Ⅵchacolpyrite semiconductor compounds are very promising materials for photovoltaic applications due to their excellent optical-electrical properties. Mature techniques to synthesize these compounds are expensive vacuum-based methods, such as evaporation and sputtering. Electrodeposition of CuInSe2 thin films has advantages of low cost, non-vacuum, and large area deposition which provides the possibility of large-scale industrial applications. However, as-deposited CuInSe2 and Cu(In,Ga)Se2 by electrodepositon method are amorphous and nanocrystalline that post-heat-treatment is needed to improve the crystallinity for fabricating devices.The aim of this paper is to show how composition, morphology, structure and optical properties of the one-step electrodeposited films are modified by different post-treatments and characterized by SEM, EDS, XRD, AES and Raman analysis. The main research results are as follows:(1) Effects of rapid thermal annealing (RTA) in Ar ambient on properties of ED-Cu(In,Ga)Se2 thin films have been studied. Results show that RTA treatments significantly improve crystallinity while preserving the film composition, but the surface morphology turns bad. Higher annealing temperatures improve the crystallinity of films. However, above 500℃it results in the loss of In and Se species. Both larger heating rate and less holding time enhance Ga incorporating into the crystal lattice of chalcopyrite during the RTA process.(2) Wide band-gap quaternary CuIn(S,Se)2 and pentanary Cu(In,Ga)(Se,S)2 thin films have been fabricated by sulfurizing the CuInSe2 and Cu(In,Ga)Se2 in H2S/Ar ambient at elevated temperature for 30 min. Results show that an increase of the annealing temperature facilitates the incorporation of S into the film and widens the band-gaps of as-deposited films. At 500℃, sulfurization of CuInSe2 precursor produces a homogeneous single-phase CuIn(Se,S)2 thin film with relatively homogeneous S depth profile. Above or below 500℃, it corresponds to inhomogeneous diffusion of S along the depth and results in a phase separation. In addition, the compositions of the films can strongly affect the incorporation of S since Cu-rich films favor more S incorporation. This is probably related to the Cu(Se,S) secondary phases aggregated on the surface of the sulfurized films.(3) Wide band-gap CuIn(S,Se)2 and Cu(In,Ga)(Se,S)2 thin films have been prepared by evaporating element S on the surface of as-deposited films and then RTA treatment. Phase identifications indicate that the evaporated sulfur crystallizes as S8 molecules on the surface of the precursors, and reacts with the precursors to form wide band-gap thin films during the RTA process. The band-gaps determined by transmittance spectra further confirm that these films have wider band-gaps than those films without addition of S.