The Advantages of Ga-Graded Obtained by Growth Profile Modification and Na Incorporation on Cu(In,Ga)Se<sub>2</sub> Solar Cells

Abstract

Cu (In,Ga)Se2 (CIGS) compound is a p-type semiconductor that has been used as light absorber layer in high efficiency thin film solar cell. The CIGS compound can be adjusted the band gap energy by varying the ratio of [Ga]/([In]+[Ga]) ratio (x). From theoretical and simulation, it was found that band gap grading in CIGS thin films showed the advantages to increase the efficiency of solar cells. Generally, the band gap grading can be done by the growth of non homogeneous x-ratio in depth of CIGS thin films. In this work, we develop two approaches to create band gap grading in CIGS thin films; (1) modifying the growth profile and (2) using Na incorporation in the growth process. The effects of Ga-graded would be revealed and compared with homogeneous CIGS thin films. CIGS thin films were grown on soda-lime glass and Al2O3 coated soda-lime glass substrates by molecular beam deposition method. The growth process was based on 2-stage and 3-stage growth profiles. The as grown films were characterized for their structural property, chemical composition and optical transmission as well as solar cell performance. The Auger electron spectroscopy in depth profiles revealed the variation of x-ratio increasing from the surface toward the back contact in CIGS films with our modified growth profile and Na incorporation. This result indicated Ga-graded in CIGS thin films. The structural property of Ga-graded CIGS films showed the (112) preferred orientation of the chalcopyrite structure with a broad asymmetric spectrum related to the inhomogeneous structure. The optical transmission measurements of the Ga-graded CIGS film showed the broad transition near the absorption edge indicating the effect of the band gap grading as a result of the variation in depth of the Ga-content. From I-V measurements, the solar cell efficiencies significantly increase due to the advantages of Ga-graded constitute.