Study On Fabrication Of Cu(InGa)Se₂(CIGS) Thin-Film Solar Cells

The Cu(InGa)Se₂(CIGS)-based polycrystalline solar cell is regarded as a promising future generation of solar energy devices due to its low cost,high conversion efficiency and prominent stability.The highest conversion efficiency about 19.9%of CIGS based solar cells have been produced using the multi-stage co-evaporation technique at National Renewable Energy Laboratory(NREL).Despite the demonstration of high efficiency by the co-evaporation technique for small size cells,there still exists some big challenges about its commercial deployment due to film uniformity problems over large areas,the extremely high costs of high vacuum facilities the technique requires and low utilization rates of materials.Industrial applications need cost effective deposition techniques.The related fabrication technologies need to be investigated and optimized.In this dissertation,one simple,reliable and reproductive technology was developed for CIGS solar cells by selenization of sputtering alloy precursors.A home-made magnetron sputtering machine containing three 3-inch diameter targets and a rotatable substrate holder was used.Firstly,～l.0μm thick Mo layers were deposited by DC-magnetron sputter on Corning soda lime substrates.The residual stresses were minimized by using two-layer structure and changing the working pressure.Next,700nm-thick CuInGa precursors were deposited onto Mo-coated glass substrates by Co-sputtering with two CuIn and CuGa alloy targets.A reproducible and easily scalable two-step selenization process of CuIn layers was developed to form high quality CIGS absorbing layers. High-quality CIGS absorbers have been formed by selenization of CuInGa precursors in a partially closed graphite container using selenium pellets at 500℃for 30 mins. The deposited CIGS absorbers exhibited a single phase chalcopyrite structure with a preferential orientation in the(112)direction.These layers were uniform,and the crystals were densely packed with a grain size of about 3～5μm.Then about 70nm thick CdS buffer layers were prepared by chemical bath deposition using cadmium acetate as the cadmium ion source and thiourea as the sulphur source.The synthesis of ZnS buffer layer were also studied for CdS-free CIGS solar cells.Finally,Indium tin oxide(ITO)and Al-doped ZnO films were deposited at room temperature with no oxygen flow on glass substrates by RF magnetron sputtering.The effects of sputtering power and argon ambient pressure were investigated.The deposited TCO films show a high transparency between 80 and 90%in the visible spectrum and 15Ω/â–¡sheet resistance.Those TCO films are suitable for application in CuInGaSe2 thin film solar cell as transparent conductive electrode layers.Mo back contact,CIGS absorber,CdS/ZnS and i-ZnO/n-ZnO(ITO)layers have been investigated systemically and optimized.The properties of the deposited films were analyzed using scanning electron microscopy(SEM),X-ray energy dispersive (EDX)and X-ray diffraction(XRD).CIGS solar cells with the structure of Glass/Mo/CIGS/CdS/i-ZnO/n-ZnO/Al were tested and analysed.Efficiencies of～7.8%have been achieved without antireflective films.The whole technology of CIGS cells is simple,economical,and without using of toxic gases.It could easily be scaled up or integrated into a commercial process.