Nanopowder Based Printed CIGS Chalcogenide Semiconductor Absorber Layers for Thin Film Solar Cell Applications

The objective of this PhD was the development of a suspension-based fabrication technique for CuIn(S,Se)2 (CIS) and Cu(In,Ga)(S,Se)2 (CIGS) chalcogenide semiconductor light absorber layers for solar cell applications via printing of a nanopowder precursor suspension followed by annealing/selenization. To reach the main objectives of this research, a systematic study was conducted to provide an in-depth understanding of the material science aspects of chalcogenide semiconductor processing. A new synthesis route was developed for high purity chalcogenide alloy nanopowder precursors for printing CuIn(S,Se)2 (CIS) and Cu(In,Ga)(S,Se)2 (CIGS) absorber layers with adjustable composition based on a sequence of dry mechanical alloying and wet ball milling in a sulfur doped-amine solution. The particle size distribution of the mechanically synthesized chalcogenide alloy nanopowders could be optimized by controlling the ball milling process parameters, such as milling speed and milling time. Suspension inks were prepared from mechanically synthesized chalcogenide alloy nanopowders, via solvent exchange and re-dispersing of chalcogenide alloy nanopowders in environmentally friendly solvents suitable for coating. Nanopowder suspension inks were coated on Mo-sputtered glass substrates via doctor blade coating followed by a drying process for the formation of 1-2 μm thin crack-free and organic phase free nanopowder precursor coatings. In order to transform the nanopowder precursor coatings into a 1-2 μm thin large crystalline CuInSe2 and CIGSe semiconductor layer, annealing and heat treatments in controlled selenium vapor (selenization) were performed. A prototype infrared heating furnace for rapid thermal processing (RTP) in controlled selenium vapor (selenization) was designed and built. The effect of the chalcogenide nanopowder precursor composition and selenization conditions on the grain growth and densification of CISe and CIGSe semiconductor phases was investigated. Chalcogenide alloy nanopowder precursors with sub-stoichiometric selenium and sulfur contents (CuInSe0.5 and CuIn0.7Ga0.3S0.5 ) were found to be suitable precursors for the formation of large grain CuInSe2 and CIGSe semiconductor phases during selenization. CISe and CIGSe semiconductor absorber layers were fabricated by inert gas annealing and selenization of printed CuInSe0.5 and CuIn0.7Ga0.3S0.5 nanopowder precursor coatings. The physical and optoelectronic properties of the CuInSe2 and CIGSe semiconductor absorber layers were investigated by several characterization techniques. Thin film solar cell devices with standard stack structure of Glass/Mo/CIGSe/CdS/ZnO/AZO based on the printed CuInSe2 and CIGSe semiconductor absorber layers showed efficiencies of 5.4 % and 6 %. Further optimization of the nanopowder precursor composition and selenization condition are needed to enhance the quality and solar cell performance of the printed CuInSe2 and CIGSe semiconductor absorber layers.status: publishe