Ankur A. Kadam

Band alignment at the CdS∕Cu(In,Ga)S2 interface in thin-film solar cells

The band alignment at the CdS∕Cu(In,Ga)S2 interface in thin-film solar cells on a stainless steel substrate was investigated using photoelectron spectroscopy and inverse photoemission. By combining both techniques, the conduction and valence band offsets were independently determined. We find an unfavorable conduction band offset of −0.45 (±0.15) eV, accounting for the generally observed low open-circuit voltage and indicating the great importance of the buffer∕absorber conduction band offset for such devices. The surface band gap of the Cu(In,Ga)S2 absorber is 1.76 (±0.15) eV, being increased with respect to the expected bulk value by a copper depletion near the surface.

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 2.5cm×10cm soda-lime glass for studying the adhesion to the substrate and chemical reactivity of molybdenum with H2S gas at 475°C for 20min. 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 studies have shown that films deposited at sputtering power of 300W and 0.3×10−3Torr working argon pressure develop compressive stress, while the films deposited at 200W and 5×10−3Torr 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 thick...

Surface modifications of Cu(In,Ga)S2 thin film solar cell absorbers by KCN and H2O2∕H2SO4 treatments

KCN etching of the CuxS surface layer formed during the production process of Cu(In,Ga)S2 thin film solar cell absorbers as well as subsequent H2O2∕H2SO4 etching of the Cu(In,Ga)S2 surface have been investigated using x-ray photoelectron spectroscopy, x-ray excited Auger electron spectroscopy, and x-ray emission spectroscopy. We find that the KCN etching removes the CuxS layer—being identified as Cu2S—and that there is K deposited during this step, which is removed by the subsequent H2O2∕H2SO4 oxidation treatment. When a CdS buffer layer is deposited on the absorber directly after KCN etching, a K compound (KCO3) is observed at the CdS surface.

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-xGax Se2 (CIGS) or CuIn1-xGaxS2 (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:AI). 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 AM0 efficiency measured at NASA GRC for the same CIGS2 solar cell was 10.25%

Development of sputtering systems for large-area deposition of CuIn1−xGaxSe1−ySy thin-film solar cells

CuIn1−xGaxSe1−ySy (CIGS) thin-film modules are expected to become cheaper than crystalline silicon modules within 5 yr. At present, commissioning and reaching full production of thin film modules is delayed because of nonavailability of turnkey manufacturing plants. Very few universities are conducting research on development of PV plants. CIGS thin-film solar cells are being prepared routinely at Florida Solar Energy Center (FSEC) on glass and metallic foil substrates for terrestrial and space applications. Earlier, the substrate size was limited to 3×3 cm2. This article presents results of development of large-area sputtering systems for preparation of large (15.2×15.2 cm2) CIGS thin-film solar cells. The facilities have the potential of serving as a nucleus of a pilot plant for fabrication of CIGS minimodules. Initial problems of bowing of the brass diaphragm, restriction of effective water flow and consequent heating of the target material were resolved by increasing the thickness of the backing plate and redesigning the structural members. Thickness uniformity was improved by modifying the magnetic field distribution in the middle 15 cm portion of the 10.2×30.5 cm2 magnetron sputtering sources by selectively removing nickel-coated soft-iron pieces at the rear. This resulted in Mo layer thickness uniformity of ±3% over 10.2×10.2 cm2. The magnetic field was boosted at extremities to avoid precipitous ∼15% drop beyond 10.2 cm. With this, thickness uniformities of ±2.5% for Mo and ±4.5% for ZnO over 12.7×10.2 cm2 have been achieved however with a continuing drop beyond 12.7 cm width. Modifying the magnetic field to achieve better distribution by preferentially removing soft irons pieces and also boosting of the magnetic field at the ends are two new concepts introduced and successfully utilized in this study. Scaling up of the large-area uniform deposition of metallic precursor layers was a challenging task. The efforts were directed towards obtaining similar thickness and uniformity that have provided very good photovoltaic efficiencies of 10.4% (Air mass AM 1.5) in small area CIGS thin film solar cells on stainless steel foils in earlier research. Preliminary results on large area CIGS solar cells are encouraging.CuIn1−xGaxSe1−ySy (CIGS) thin-film modules are expected to become cheaper than crystalline silicon modules within 5 yr. At present, commissioning and reaching full production of thin film modules is delayed because of nonavailability of turnkey manufacturing plants. Very few universities are conducting research on development of PV plants. CIGS thin-film solar cells are being prepared routinely at Florida Solar Energy Center (FSEC) on glass and metallic foil substrates for terrestrial and space applications. Earlier, the substrate size was limited to 3×3 cm2. This article presents results of development of large-area sputtering systems for preparation of large (15.2×15.2 cm2) CIGS thin-film solar cells. The facilities have the potential of serving as a nucleus of a pilot plant for fabrication of CIGS minimodules. Initial problems of bowing of the brass diaphragm, restriction of effective water flow and consequent heating of the target material were resolved by increasing the thickness of the backing plate...

Effect of Stresses in Molybdenum Back Contact Film on Properties of CIGSS Absorber Layer

Analysis of CuIn 1-x Ga x Se 2-y S y (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 x 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 x 10 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 x 10 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.

Comparison of Band Alignments at Various CdS/Cu(In,Ga)(S,Se)2 Inter-Faces in Thin Film Solar Cells

The band alignment at the CdS/Cu(In,Ga)(S,Se)2 interface, as derived in our earlier publications, are compared for different absorber compositions. The discussed band alignments were directly determined using a combination of UV- and X-ray photoemission and inverse photoemission. While a flat conduction band alignment can be found for low-gap material, the cell structure with a high-gap absorber shows a cliff-like alignment. The different alignments can be correlated with the respective cell parameters, explaining why the expected linear gain in open circuit voltage for the high-gap absorbers has not yet been achieved

Photoelectrochemical Water Splitting for Hydrogen Production Using Combination of CIGS2 Solar Cell and RuO2 Photocatalyst

This paper presents the development of photoelectrochemical (PEC) setup using multiple bandgap combination of CuIn1−x Gax S2 (CIGS2 ) thin-film photovoltaic (PV) cell and ruthenium oxide (RuO2 ) photocatalyst. FSEC PV Materials Lab has developed a PEC setup consisting of two illuminated CIGS2 cells, a ruthenium oxide (RuO2 ) anode deposited on titanium sheet for oxygen evolution and a platinum foil cathode for hydrogen evolution. With this combination, a PEC efficiency of 4.29% has been achieved.Copyright

Preparation and Characterization of Thin CIGSS Solar Cells on Mo Coated Glass Substrates

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 μm thick) CuIn1−x Gax Se2−y Sy (CIGSS) solar cells on molybdenum coated glass substrate. CIGSS films of thickness ∼1 μm 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 H2 S 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