Study of molybdenum back contact layer to achieve adherent and efficient CIGS2 absorber thin-film solar cells

Molybdenum is used as back contact layer in I-III-VI2 compound thin-film solar cells. Mo film was sputter deposited on 125-mm-diameter Si wafer having 100 orientation using dc magnetron sputtering. Films with similar parameters were also deposited on 2.5 cm x 10 cm soda-lime glass for studying the adhesion to the substrate and chemical reactivity of molybdenum with H2S gas at 475 degrees C for 20 min. Mo being refractory material develops stresses. It is essential to deposit stress-free and relatively inert Mo films in order to achieve well adherent and highly efficient CuIn1-xGaxS2 absorber thin film solar cells on flexible metallic foil and glass substrates. Earlier have shown that films deposited at sputtering power of 300 W and 0.3 x 10(-3) Torr working argon pressure develop compressive stress, while the films deposited at 200 W and 5 x 10(-3) Torr pressure develop tensile stress. Four sets of experiments were carried out to achieve optimum deposition cycle to deposit stress-free Mo. In the first experiment, Mo thickness of 138 nm was deposited at 300 W power and 0.3 x 10(-3) Torr pressure. In the second experiment Mo thickness of 127 nm was deposited at power of 200 W and pressure of 5 x 10(-3) Torr. Two more experiments were carried out by using alternate layers to reduce the overall stress. In a third experiment, two high power cycles were sandwiched between three low power cycles with total film thickness of 330 nm. In a fourth experiment two low power cycles were sandwiched between three high power cycles resulting in effective thickness of 315 nm. This article describes the wafer bending analysis for stress measurement, x-ray diffraction for crystal quality, scanning electron microscopy for surface morphology and Auger electron spectroscopy for the extent of sulfur diffusion in Mo layer. (c) 2005 American Vacuum Society

Development of CIGS2 Thin Films on Ultralightweight Flexible Large Area Foil Sunstrates

The development of thin film solar cells is aimed at reducing the costs for photovoltaic systems. Use of thin film technology and thin foil substrate such as 5-mil thick stainless steel foil or 1-mil thick Ti would result in considerable costs savings. Another important aspect is manufacturing cost. Current single crystal technology for space power can cost more than $300 per watt at the array level and weigh more than 1 kg/sq m equivalent to specific power of approx. 65 W/kg. Thin film material such as CuIn1-xGaxS2 (CIGS2), CuIn(1-x)Ga(x)Se(2-y)S(y) (CIGSS) or amorphous hydrogenated silicon (a-Si:H) may be able to reduce both the cost and mass per unit area by an order of magnitude. Manufacturing costs for solar arrays are an important consideration for total spacecraft budget. For a medium sized 5kW satellite for example, the array manufacturing cost alone may exceed$ 2 million. Moving to thin film technology could reduce this expense to less than $ 500K. Earlier publications have demonstrated the potential of achieving higher efficiencies from CIGSS thin film solar cells on 5-mil thick stainless steel foil as well as initial stages of facility augmentation for depositing thin film solar cells on larger (6 in x 4 in) substrates. This paper presents the developmental study of achieving stress free Mo coating; uniform coatings of Mo back contact and metallic precursors. The paper also presents the development of sol gel process, refurbishment of selenization/sulfurization furnace, chemical bath deposition (CBD) for n-type CdS and scrubber for detoxification of H2S and H2Se gases

Highly Efficient Cuin1-XGaXSe2-YSY/Cds Thin-Film Solar Cells By Using Diethylselenide As Selenium Precursor

Conventional furnace selenization process was optimized to achieve effective selenization using diethylselenide (C2H5)2Se (DESe). An optimized quantity of Na was added to improve Voc, FF and morphology. Sputter-deposited CuGa and In metallic precursors were homogenized in an inert atmosphere prior to the introduction of DESe followed by rapidly heating to the maximum process temperature to avoid formation of detrimental binary phases. Selenization was carried out in the temperature range 475-515 °C followed by sulfurization in dilute H2S. Solar cells were completed by depositing CdS heterojunction partner layer, i:ZnO/ZnO:Al window-bilayer and metallic contact fingers. PV conversion efficiency of 13.7% with a Voc of 540 mV, Jsc of 38.3 mA/cm2 and FF of 66.3% were obtained on 0.442 cm2 cell areas. The process can be easily scaled-up for economic large-scale manufacture. © 2010

Structural Comparison Of Thin Film Absorber Layer Fabricated On Ss And Ti Substrates

Copper indium gallium selenide sulfide, CuIn1-xGa xSe2-ySy (CIGSS) absorber layer was prepared on stainless steel (SS) and SiO2-coated titanium substrates. Comparative study was carried out to analyze the variation in the crystal quality depending on the substrate. Identical parameters were used for deposition and selenization/sulfurization of metallic precursors. This paper presents the observations made on the basis of surface morphology using scanning electron microscopy, crystal quality by x-ray diffraction and chemical composition and concentration-depth profiles by x-ray energy dispersive spectroscopy and Auger electron spectroscopy respectively. Film fabricated on both the substrate had chalcopyrite structure with variation of stoichiometry as well as morphology. Selenization/sulfurization cycle must be modified depending upon the substrate to obtain high quality CIGSS film. Copyright © 2005 by ASME

Preparation And Characterization Of Thin Solar Cells On Mo Coated Glass

The aim of this study is to review issues related to the requirement of thin CIGSS absorber layers, prepare and characterize thin CIGSS films on molybdenum coated glass, improve understanding of material properties, and further enhance solar cell performance. This paper presents the preparation and properties of thin (∼1 urn thick) CuIn1-xGaxSe 2-ySy (CIGSS) solar cells on molybdenum coated glass substrate. CIGSS films of thickness ∼1 πm were prepared in two steps. Step one involved the deposition of Cu-In-Ga metallic precursors on molybdenum coated glass substrate and step two involves the selenization/ sulfurization of these metallic precursors using diluted diethylselenide (DESe) as selenium source and diluted H2S as sulfur source respectively. Thin film solar cells were completed by the deposition of n-type CdS layer by chemical bath deposition, ZnO/ZnO:Al transparent conducting window bilayer by RF magnetron sputtering and Ni-Al front contact fingers by e-beam evaporation technique through a metal mask. This paper presents the preliminary results obtained on very thin (∼ 1 μm) absorber layer. Copyright © 2005 by ASME

Effect Of Stresses In Molybdenum Back Contact Film On Properties Of Cigss Absorber Layer

Analysis of CuIn1-xGaxSe2-ySy (CIGSS) absorber and molybdenum back contact layer was carried out to understand the changes in the microstructure of CIGSS layer as a function of the deposition conditions and the nature of stress in the underlying Mo film. All the depositions were carried out on 10 cm × 10 cm glass substrates. Compressive and tensile stressed molybdenum films were prepared with combinations of deposition parameters; power and pressure. CIGSS absorber layer was prepared by depositing metallic precursors using DC magnetron sputtering followed by selenization and sulfurization. Molybdenum layer deposited at 300 W and 3 × 10-4 Torr pressure produced compressive stress with compact, well adherent and lower sheet resistance as compared to the tensile stressed film deposited at 200 W and 5 × 10-3 Torr. The crystallinity of the CIGSS film was found not to depend on the stress in the underlying molybdenum film. However, the adhesion at the Mo/CIGSS as well as gallium profile at the Mo/CIGSS interface were affected by the stress. © 2005 Materials Research Society

Role Of I-Zno In Optimizing Open Circuit Voltage Of Cigs2 And Cigs Thin Film Solar Cells

It is a customary in the preparation of CuIn1-xGa xSe2 (CIGS) or CuIn1-xGaxS 2 (CIGS2) solar cells, to use an un-doped layer of ZnO (i-ZnO) on the CdS layer prior to the deposition of a doped layer (ZnO:Al). This paper presents reasons behind the need for i-ZnO layer and also the effect of its thickness on the open circuit voltage of the CIGS2 based thin film solar cells. It was found that thickness of i-ZnO layer must be optimized depending on the surface roughness of CIGS2 absorber layer. CIGS2/CdS solar cells having optimum i-ZnO thickness were prepared and a photovoltaic conversion efficiency of 11.99% with open circuit voltage, VOC of 830.5 mV under AM 1.5 conditions were obtained. The AMO efficiency measured at NASA GRC for the same CIGS2 solar cell was 10.25%. © 2006 IEEE

Performance Enhancement Of Cigss Absorber Layer On Glass Substrate

The article presents the development of radiation resistant, highly efficient, high specific power, copper indium gallium selenide sulfide, CuIn1-xGaxSe2-ySy (CIGSS) absorber thin film solar cells for terrestrial and space application. The effect of post sulfurization treatment and gallium incorporation for performance enhancement are discussed. CIGSS thin films were prepared at the FSEC photovoltaic materials lab in two steps. The first step consisted of deposition of Cu-In-Ga precursors using DC magnetron sputtering on molybdenum back contact. The second step involved selenization and sulfurization of these precursors. Selenization was carried out using diethylselenide (DESe) as a selenium source and H2S as sulfur source. Characterization techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and Auger electron spectroscopy (AES) has confirmed the formation of chalcopyrite CuIn0.96Ga0.04Se0.7S1.3 phase with random grain size

Cigss Thin Films For Photoelectrochemical Water Splitting Using Multiple Bandgap Combination Of Thin Film Photovoltaic Cell And Photocatalyst: Clean And Renewable Hydrogen Source

This paper presents research carried out at Florida Solar Energy Center (FSEC) to develop cheaper and more efficient photoelectrochemical (PEC) cells for production of highly pure hydrogen and oxygen in the requisite proportion by water splitting using multiple bandgap combination of thin film photovoltaic (PV) cell and a photocatalyst. A transparent and conducting layer of ZnO:Al (or ITO)/MoSe2 (or MoS2) is being developed at the back of PV cell to transmit unabsorbed infrared photons onto the photoanode for efficient oxygen evolution. Preparation and characterization of thin ZnO:Al (or ITO)/MoSe2 (or MoS2) transparent conducting layer, CuIn1-xGaxSe2 (CIGS) and CuIn 1-xGaxS2 (CIGS2) cells with a transparent conducting layer, and CuIn1-xGaxSe2-yS y (CIGSS) thin film PV cells for efficient hydrogen and oxygen generation by water splitting are presented