Preparation and Characterization of Cu(In,Ga)Se2 and Cu(In,Ga)(Se,S) Thin Film Solar Cells at PV Materials Lab, FSEC

Abstract

Summary form only given. Choice of solar cell technology depends on conversion efficiency, reliability, available material resources and environmental effects. At present, CdTe, CuInSe2 (CIS) alloys, and amorphous silicon (a-Si: H), are major contenders for large scale production of thin-film solar cells. CdTe and CIS alloy-based solar cells with the best efficiencies of 16.5% and 19.5% respectively are preferred candidates compared to amorphous silicon (a-Si:H) due to their higher efficiencies and freedom from intrinsic degradation mechanisms. In the PV Materials Lab at FSEC, emphasis is given on the research of solar cell preparation processes that are amenable to sealing-up for economic, large-scale manufacture. Facilities are being developed with the objective of easy scalability to a pilot plant. Research is being carried out on thin-film CuIn1-xGax Se2 (CIGS) solar cells, a high absorption co-efficient material. The CIGS absorber layer is deposited on molybdenum-coated soda lime glass in two steps. The first step is deposition of CuGa-In metallic precursors using DC magnetron sputtering at room temperature. This is followed by selenization of these precursors in a conventional furnace in a controlled ambience. Diethyleselenide is preferred over H 2Se or elemental selenium as the selenium source because of the less stringent safety requirements and also due to its comparatively lower toxicity. To use this technology for space applications, the bandgap of the material needs to be engineered to match the solar spectrum in space. A material with higher bandgap is required in space. This can be achieved by increasing gallium content or by replacing selenium with sulfur. CuIn1-xGaxS2 (CIGS2), a sulfur alloy has an optimum bandgap for space applications of ~1.55 eV. Here the second step consists of sulfurization of the metallic precursors with diluted H2S as the sulfur source. This process is again carried out in a conventional furnace in a controlled ambient. CIGSS solar cells are also prepared using the above techniques to get the advantage of higher bandgap at the interface to increase open circuit voltage (Voc) and low bandgap bulk absorber to increase short circuit current (Isc). In order to further reduce the manufacturing cost, rapid thermal processing (RTP) was initiated at the PV Materials Lab. RTP reduces the time taken for depositing the absorber layer and thus leads to better usage of energy. Another cost-saving approach is to reduce the amount of material used. As CIGS is a direct band gap material, the thickness of the absorber layer can be reduced significantly. This facilitates reduction in the usage of indium, a scarce and costly resource. Ultra-thin CIGSS solar cells are also being developed at the PV Materials Lab. The absorber layer is characterized for better understanding of the material properties and the process is optimized using material characterization techniques such as scanning electron microscopy (SEM), auger electron spectroscopy (AES), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD) at the AMPAC's Materials Characterization Facility, UCF. Electron probe microanalysis (EPMA) is carried out on the absorber layer at the National Renewable Energy Laboratory (NREL). Thin-film solar cells are completed by deposition of CdS buffer layer using chemical bath deposition (CBD). Intrinsic ZnO and Al- doped ZnO window layer are deposited by RF sputtering and the front contact fingers of Cr-Ag are deposited by thermal evaporation. Photovoltaic properties of these solar cells are analyzed using current-voltage measurements and quantum efficiency measurements. Efficiencies of 9% have been achieved on CIGS/CIGSS by RTP and conventional annealing processes while CIGS2 efficiencies have reached 12%. A world record Voc of 830.5 mV has been achieved on CIGS2 absorber based thin-film solar cells prepared by sulfurization of metallic precursors