Sachin S. Kulkarni

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.

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.

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...

Determination of back contact barrier height in Cu(In,Ga)(Se,S) 2 and CdTe solar cells

A relatively straightforward technique has been developed to quantify the energy barrier for holes between a Cu(In,Ga)(Se,S)2 (CIGSeS) or CdTe absorber and the back-contact metallization. The input data is the current-voltage (J-V) curves for the solar cell measured over a range of temperatures. The key parameter is the “turning current” Jt, which is the current at the transition from the positive J-V curvature of a diode to the negative curvature associated with current limitation at a contact barrier. The analytical strategy is to calculate a series of Jt vs. T curves for different values of barrier height and then overlay the experimental values of Jt. Generally the experimental data follow a single barrier-height curve over a wide temperature range. The presentation will describe the turning point technique and apply it to specific solar-cell examples. The range of Jt that can be practically identified extends from approximately 0.1 to 80 mA/cm2. Assuming that temperatures between 220 and 340 K are available, the range of barriers that can be determined is between 0.30 and 0.55 eV. This is also the practical range, since lower barriers do not have a measurable effect on the power quadrant and higher ones effectively kill the performance of the cell. Many CIGSeS and CdTe cells, however, do have a back-contact barrier in the 0.30 to 0.55 eV range, and the ability to determine it can assist both cell analysis and process optimization.

CIGSeS thin film solar cell research and development at the Florida Solar Energy Center

Research and development of CuIn1-xGaxSe2-ySy (CIGSeS) thin film solar cells on various types of substrates and techniques is being carried out at the Florida Solar Energy Center (FSEC) Photovoltaics Materials Laboratory (PV Mat Lab) since 1990. Excellent facilities have been developed over the years for the preparation of the copper chalcogenide thin film solar cells. Highly efficient, CIGSeS thin film solar cells are being prepared and analyzed. This paper presents the facilities and research activities that have led to the preparation of highly efficient CIGSeS thin film solar cells.

Preparation of CIGSS Thin-Film Solar Cells by Rapid Thermal Processing

Rapid thermal processing (RTP) provides a way to rapidly heat substrates to an elevated temperature to perform relatively short duration processes, typically less than 2-3 min long. RTP can be utilized to minimize the process cycle time without compromising process uniformity, thus eliminating a bottleneck in CuIn 1-x Ga x Se 2-y S y (CIGSS) module fabrication. Some approaches have been able to realize solar cells with conversion efficiencies close or equal to those for conventionally processed solar cells with similar device structures. A RTP reactor for preparation of CIGSS thin films on 10 cm× 10 cm substrates has been designed, assembled, and tested at the Florida Solar Energy Centers PV Materials Lab. This paper describes the synthesis and characterization of CIGSS thin-film solar cells by the RTP technique. Materials characterization of these films was done by scanning electron microscopy, x-ray energy dispersive spectroscopy, x-ray diffraction, Auger electron spectroscopy, electron probe microanalysis, and electrical characterization was done by current-voltage measurements on soda lime glass substrates by the RTP technique. Encouraging results were obtained during the first few experimental sets, demonstrating that reasonable solar cell efficiencies (up to 9%) can be achieved with relatively shorter cycle times, lower thermal budgets, and without using toxic gases.

Preparation and properties of CIGS and CIGSS thin films using DESe as a selenium source and H/sub 2/S as sulfur source

Cu(In/sub 1-x/Ga/sub x/)(Se/sub 1-y/S/sub y/)/sub 2/ (CIGSS) thin films were prepared at the FSEC photovoltaic materials lab in two steps. The first step consisted of deposition of CuGa-In precursors using DC magnetron sputtering on molybdenum back contact. The second step involved selenization/sulfurization of these precursors. Selenization was carried out using diethylselenide (DESe) as a selenium source and H/sub 2/S as sulfur source. CIGSS thin-film solar cells were completed by depositing n-type CdS layer by chemical bath deposition (CBD), ZnO/ZnO:Al window bilayer by RF magnetron sputtering and Ni-AI front contacts by e-beam evaporation. This paper describes the new DESe set-up developed at FSEC PV Materials Lab and preparation and properties of CuIn/sub 1-x/Ga/sub x/Se/sub 2/ (CIGS) and CIGSS thin films.