AVS 53rd International Symposium
    Thin Film Wednesday Sessions
       Session TF-WeM

Paper TF-WeM11
Morphological Changes in CIGS2 Upon Thickness Reduction of Absorber Layer and its Correlation with Device Performance

Wednesday, November 15, 2006, 11:20 am, Room 2022

Session: Thin Films for Photovoltaics and Energy Applications
Presenter: P. Vasekar, Florida Solar Energy Center
Authors: P. Vasekar, Florida Solar Energy Center
N.G. Dhere, Florida Solar Energy Center
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Chalcopyrites are important contenders among thin film solar cells due to direct band gap and higher absorption coefficient. Copper-Indium-Gallium Sulfide (CIGS2) is a chalcopyrite material with a near-optimum band gap of 1.5 eV. At FSEC PV Materials Laboratory, record efficiency of 11.99 % has been achieved on a 2.7 µm CIGS2 film prepared by sulfurization. Copper indium sulfide modules are being commercialized by Sulfurcell in Germany. The availability and cost of Indium can be a limiting factor. The required amounts of metals can be lowered by using thinner films. Efforts are being made to reduce the thickness while maintaining the comparable performance. At NREL, 17.16 % efficiency has been obtained with 1 µm copper-indium-gallium selenide absorber layer. Thickness reduction up to 0.75 µm seems plausible. It has been estimated that thickness can be reduced even to 0.5 microns without light trapping. We have already obtained about 6.4 % efficiency for CIGS2 with thickness of 1 µm. Initially small size grains are formed during the film growth. With continuing growth to large thicknesses, more favorably oriented grains grow faster and coalesce to form compactly packed large-grain morphology. Solar cell performance in smaller grain chalcopyrite absorber deteriorates due to larger fraction of grain boundaries. It is essential to hasten the grain growth through coalescence to retain quality even in thinner films. This work presents a study of morphology of CIGS2 absorber layers of decreasing thicknesses and the assessment of the efficacy of various techniques in improving morphology and thus the device performance and yield even at thicknesses below 1 µm.