Effect of Sodium And Absorber Thickness on Cigs2 Thin Film Solar Cells

Chalcopyrites are important contenders among solar cell technologies due to direct band gap and higher absorption coefficient. CuIn1-xGaxS2 (CIGS2) thin-film solar cells are of interest for space power applications because of near optimum bandgap of 1.5 eV for AM0 solar radiation outside the earth\u27s atmosphere. The record efficiency of 11.99% has been achieved on a 2.7 µm CIGS2 thin film prepared by sulfurization at FSEC PV Materials Laboratory. Since CIGS2 films are typically grown in copper-rich regime, excess cuprous sulfide which helps in the formation of CIGS2 is etched away. This makes CIGS2 nearly stoichiometric. However, it is difficult to adjust Cu/(In+Ga) ratio in the desired range 0.7 to 0.9. A solution to this is to grow CIGS2 in copper-deficient regime. However, it is difficult to produce device quality films without the support of cuprous sulfide. This work is one of the very few attempts in which device quality films were formed even in copper-deficient regimes with the addition of sodium. Also, recent research endeavors in the CIGS2 thin film photovoltaic community are directed towards thinner films because the availability and cost of indium as well as gallium are limiting factors. The required amounts of rare and expensive metals can be lowered by using thinner films. The solar cell performance in the thinner absorbers deteriorates due to the detrimental effects of the larger fraction of grain boundaries. It is essential to hasten the grain growth through coalescence to retain high efficiency in devices prepared using thinner films. Large grain size that is desirable for obtaining high efficiency cells can be achieved by creating conditions of fewer nucleation sites and large mobilities of the deposited species. Sodium has been found to play a vital role by enhancing the atomic mobility and improving the coalescence even in thinner films. This work presents a study of morphology and device properties of CIGS2 thin films with Copper-deficient absorbers after minute amounts of sodium are introduced on the Mo-coated substrate in the form of sodium fluoride layer prior to sputter deposition of copper-gallium alloy and indium. Photovoltaic conversion efficiency of 9.15% was obtained for copper-deficient absorbers. In a parallel set of experiments, copper-rich precursors were used to produce absorbers of lower thickness range values and the parameters were optimized. Photovoltaic conversion efficiency of 10.12% was obtained for an absorber of thickness 1.5 µm and an efficiency of 9.62% was obtained for an absorber of thickness 1.2 µm

Technical Paper Session I-B - CIGSeS and CIGS2 Thin Film Solar Cells on Flexible Foils for Space Power

The objective of the research is to develop flexible, lightweight, radiationresistant, high-specific-power, highly efficient CuIn1-xGaxSe2-ySy (CIGSeS) and CuIn1- xGaxS2 (CIGS2) thin-film solar cells for space electric power. The near optimum bandgap, potential for higher specific power, and superior radiation resistance make this technology an ideal candidate for space electric power. The superior radiation resistance of CIGSeS thin-film solar cells relative to the conventional silicon and gallium arsenide single-crystal cells in the space radiation environment would extend mission lifetimes substantially. The conventional rigid Si and GaAs cells must be folded in an accordion style for deployment space. This can cause problems of opening up and folding of the solar array as has happened recently with the International Space Station. On the other hand, the flexible solar cells and modules can be packaged and rolled out more easily. The stainless steel and titanium foil substrate materials are capable of withstanding high temperatures required for preparing good quality CIGSeS absorber layer. They also do not sag easily and hence do not require rigidizing as is the case with plastic sheet substrates. The CIGSeS absorber film is prepared by selenization/sulfurization of DC magnetron sputter-deposited CuGa, In metallic precursors on 10 cm x 10 cm metallic foil substrate coated with molybdenum back contact layer. CdS heterojunction partner is deposited by chemical bath deposition. Transparent and conducting bilayer of intrinsic ZnO and aluminum doped ZnO:Al is deposited by RF magnetron sputtering. Cells are completed by depositing Ni/Al front contact fingers by thermal evaporation. The sputtering technique utilized in the preparation of solar cells provides an added advantage of facilitating easy scale-up of the laboratory size cells for economic large-area manufacture by the roll-to-roll process. Chemical composition, crystallographic structure and morphology of CIGSeS thin films are analyzed by energy dispersive spectroscopy, Auger electron spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The photovoltaic properties of completed cells are studied by measurement of current-voltage characteristics and quantum efficiency. Best efficiencies of 10.4% under AM 1.5 conditions and 8.84% under AM 0 conditions were achieved on small-area CIGS2 thin-film solar cells

Perovskite solar cells: An integrated hybrid lifecycle assessment and review in comparison with other photovoltaic technologies

Solar cells are considered as one of the prominent sources of renewable energy suitable for large-scale adoption in a carbon-constrained world and can contribute to reduced reliance on energy imports, whilst improving the security of energy supply. A new arrival in the family of solar cells technologies is the organic-inorganic halide perovskite. The major thrust for endorsing these new solar cells pertains to their potential as an economically and environmentally viable option to traditional silicon-based technology. To verify this assertion, this paper presents a critical review of some existing photovoltaic (PV) technologies in comparison with perovskite-structured solar cells (PSCs), including material and performance parameters, production processes and manufacturing complexity, economics, key technological challenges for further developments and current research efforts. At present, there is limited environmental assessment of PSCs and consequently, a methodologically robust and environmentally expansive lifecycle supply chain assessment of two types of PSC modules A and B is also undertaken within the context of other PV technologies, to assess their potential for environmentally friendly innovation in the energy sector. Module A is based on MAPbX3 perovskite structure while module B is based on CsFAPbX3 with improved stability, reproducibility and high performance efficiency. The main outcomes, presented along with sensitivity analysis, show that PSCs offer more environmentally friendly and sustainable option, with the least energy payback period, as compared to other PV technologies. The review and analysis presented provide valuable insight and guidance in identifying pathways and windows of opportunity for future PV designs towards cleaner and sustainable energy production

Effect Of Sodium Addition On Cu-Deficient Cuin1-XGaXS2 Thin Film Solar Cells

Chalcopyrites are important contenders among solar-cell materials due to direct band gap and very high-absorption coefficients. Copper-indium-gallium disulfide (CIGS2) is a chalcopyrite material with a near-optimum band gap of 1.5 eV for terrestrial as well as space applications. At FSEC PV Materials Laboratory, record efficiency of 11.99% has been achieved on a 2.7 μm CIGS2 thin film prepared by sulfurization. There are reports of influence of sodium on copper-indium-gallium selenide (CIGS) as well as copper-indium disulfide (CIS2) solar cells. However, this is the first of its kind approach to study the effect of sodium on CIGS2 solar cells and resulting in encouraging efficiencies. Copper-deficient CIGS2 thin films were prepared with and without the addition of sodium fluoride (NaF). Effects of addition of NaF on the microstructure and device electrical properties are presented in this work. © 2008 Elsevier B.V

Beneficial Effects Of Sodium On Cuin1-XGaXS 2 Thin Film Solar Cells

Chalcopyrites are important contenders among solar cell technologies due to their direct band gaps and high absorption coefficients. Copper indium gallium sulfide, CuIn1-xGaxS2 (CIGS2) thin films have been grown by sulfurization with copper rich compositions. However, the desired free carrier concentration range of 1016-1017/cm 3 can be achieved fairly easily with copper deficient compositions avoiding the toxic etching process. Due to the unavailability of cuprous sulfide which acts as a fluxing agent in copper rich films and is responsible for better film morphology, it is difficult to produce device quality films in copper deficient compositions. The effects of sodium addition on the film microstructure and the device performance are being studied. ©2009 IEEE

Effect Of Post-Sulfurization Annealing And Gallium Grading On Thinner Cuin1-XGaXS2 Absorbers

Thinner CuIn1-xGaxS2 (CIGS2) solar cells are being prepared with an aim to reduce the consumption of indium and gallium. Post-sulfurization annealing is being used to enhance the grain size in order to overcome the problem of very small grains that tend to form in thinner films that are not desirable for device quality solar cells. Based on the fact that gallium gradient that is typically found in CIGS and CIGS2 solar cells has beneficial effect on preventing back contact recombination of minority carriers, an attempt to determine the optimum regime for post-sulfurization annealing is made to derive the benefits from larger grains and gallium gradient. An initial set of experiments carried out at PV materials laboratory at FSEC has shown encouraging results with cell efficiencies of 9-10% for thinner (1.2-1.6 μm) films. © 2009 SPIE

Development Of Cigs2 Solar Cells With Lower Absorber Thickness

The availability and cost of materials, especially of indium can be a limiting factor as chalcopyrite based thin-film solar cells advance in their commercialization. The required amounts of metals can be lowered by using thinner films. When the thickness of the film decreases, there is possibility of remaining only in the small grain region because the coalescence of grains does not have an opportunity to enhance the grain size to the maximum. Solar cell performance in smaller grain chalcopyrite absorber deteriorates due to larger fraction of grain boundaries. Efforts are being made to reduce the thickness while maintaining the comparable performance. This work presents a study of preparation, morphology and other material properties of CIGS2 absorber layers with decreasing thicknesses up to 1.2 μm and its correlation with the device performance. Encouraging results were obtained demonstrating that reasonable solar cell efficiencies (\u3e10%) can be achieved even for thinner CIGS2 thin-film solar cells. © 2009 Elsevier Ltd. All rights reserved