Diffusion of gallium in CuInSe2/CuGaSe2 bilayer thin films

Abstract

At present, CuIn1-xGaxSe2 (CIGS) thin films are one of key materials for an absorber layer for photovoltaic devices. The energy gap of CIGS is in the range between 1.02 to 1.66 eV which can be adjusted by the Ga composition. The conversion efficiency of 20% for the CIGS thin film solar cells has been achieved which is due to the appropriate Ga content and the profile of Ga distribution in the film. An understanding of the diffusion of Ga is necessary to develop and increase the performance of the CIGS thin film solar cells. The investigation of Ga diffusion is proposed through the studies of bilayer structures, CIS/CGS and CGS/CIS, fabricated on molybdenum-coated soda-lime glass (Mo/SLG) by Molecular Beam Deposition (MBD) technique. The compositions of the bilayers are varied for both Cu-rich and Cu-poor in each layer with the substrate temperature at 600C. The XRD results show that higher Ga diffusion is observed in the Cu-rich CIS/CGS bilayer rather than other bilayers. The diffusion of Ga seems to be enhanced by the excess Cu-Se phase that is dependent upon the substrate temperature. The Na from the SLG is one of the important factors affecting the Ga diffusion. The aluminum oxide (Al2O3) thin film is used to block Na from the SLG. The results show the alloying CIGS patterns rather than separated CIS and CGS patterns as for the bilayers with Na. Ga can diffuse through either Cu or group-III sites where Na may seize these sites and obstruct the diffusion of Ga. Apart from the SLG substrate, GaAs(001) is used for the growth of bilayers to observe the diffusion of Ga under the constrains of crystal orientation. The diffusion of Ga is more difficult in Cu-rich bilayers. The defects for enhancing Ga diffusion hardly occur in the high crystallinity of Cu-rich epitaxial bilayers.