D. Tarrant

The role of gallium in CuInSe/sub 2/ solar cells fabricated by a two-stage method

Incorporation of Ga in CuInSe/sub 2/ thin film solar cells formed by a two-stage method results in improved adhesion between the CuInSe/sub 2/ absorber layer and the Mo back electrode. The original intent of incorporating Ga was to increase the bandgap by forming a homogeneous CuIn/sub 1-x/Ga/sub x/Se/sub 2/ quaternary absorber. Rather than the expected increase in bandgap by uniform substitution of Ga for In, adhesion was improved by the formation of a Ga-rich interfacial layer between the absorber and the molybdenum. In addition to improved adhesion, an increase in efficiency due to improved open circuit voltage was observed. Structural models for the absorber layer are proposed based on Auger analysis and solar cell performance data.<<ETX>>

I-III-VI/sub 2/ multinary solar cells based on CuInSe/sub 2/

Homogeneous bandgap (0.95 to 1.43 eV) and graded bandgap I-III-VI/sub 2/ multinary absorber layers have been fabricated using a two-stage method. Sulfur has been added to increase the bandgap either throughout the absorber or within the depletion region. Open circuit voltages of 728 mV have been achieved with Cu(In,Ga)(Se,S)/sub 2/ high bandgap absorbers. Active area efficiencies of over 15% have been achieved using graded I-III-VI/sub 2/ absorber layers. Absorber composition and structure of these devices are presented. Advantages of graded bandgap absorbers for achieving higher efficiency are discussed. Differences in performance are related to microscopic and macroscopic characteristics that are dependent on sulfur content and absorber structure.<<ETX>>

CuInSe2 module environmental reliability

Abstract Environmental testing data are presented and discussed in relation to the suitability of CuInSe 2 (CIS) as a durable photovoltaic material and to the Interim Qualification Tests and Procedures for Terrestrial Photovoltaic Thin-Film Flat-Plate Modules (IQTP). Groups of modules having no significant change after ten humidity-freeze cycles are reported. Heating during module packaging or during environmental testing to temperatures above those normally encountered by modules in outdoor service may introduce a temporary power loss; the power recovers with time to near the initial power. Data indicate that temperature alone, rather than temperature combined with humidity, causes the temporary power loss and that CIS is not inherently sensitive to humidity. Hermetic seals are not in general necessary for CIS materials. The IQTP may improperly indicate poor performance if the temporary power loss is not considered in electrical performance testing between different sections of the environmental test procedures and at the end of all environmental tests. Data are not available to validate accelerated testing as a means of predicting long-term in-service performance; however, correlations between outdoor and accelerated testing are seen.

Progress in CIS-based module development

Alloys of copper indium diselenide are the most promising candidates for reducing the cost of photovoltaics below the cost of crystalline silicon. Small area, fully integrated modules exceed 13% in efficiency and long-term outdoor stability has been demonstrated. The availability of natural resources and environmental impacts appear acceptable. Challenges remain to scale the process to larger area and to pass accelerated environmental testing. Process scale-up is systematically proceeding from the foundation of a reproducible small area module process with low variation; however, large-area circuit performance has not yet achieved the same level as the baseline process. Differences in performance between the baseline and large area processes have been isolated to differences in the equipment used to form absorbers and is the subject of current development. The impact of larger part size has been tested for each process step using the demonstrated baseline process. Results indicate that larger-area parts w...

Interface properties of CIGS(s)/buffer layers formed by the Cd-partial electrolyte process

A chemical-bath treatment that does not form a CdS layer has been used on CIGS absorbers made at the National Renewable Energy Laboratory (NREL). The resultant cells have moderate to high efficiency, with improved current collection at shorter wavelengths. Room temperature quantum efficiency (QE) and capacitance-voltage (CV) results indicate that the different surface treatments yield electro-optical differences in the bulk of the absorber. Room temperature current density-voltage (JV) and AMPS modeling results are used to compare and contrast the results of the surface treatments, primarily from the NREL devices.

Effects of buffer layers on SSI CIGSS-absorber transient I-V and C-V behavior

Many thin-film CIS photovoltaic devices exhibit a modest level of reversible transient behavior in electrical properties. This paper evaluates this behavior by comparing three cell configurations processed differently. The configurations include CIGSS cells made by Siemens Solar Industries (SSI), SSI absorbers plus buffers/windows done elsewhere, and standard NREL CIGS cells. The buffers used include combinations of the following: KCN solution etching treatment, annealing of the samples after CdS deposition, and/or chemical vapor deposition of ZnO without CdS deposition. All cells made with SSI absorbers showed transient behavior in fill factor, while the NREL cells did not. Post-annealing of devices was found to improve baseline performance and reduce transient behavior.

Manufacturing of low-cost high-efficiency thin-film CuInSe/sub 2/ photovoltaic modules

Successful development of the key cost-limiting factors of thin film CuInSe/sub 2/ (CIS) technology is anticipated to result in manufacturing costs near

Results of recent CuInSe/sub 2/ module investigations

1/W/sub p/ for 10-12% efficient photovoltaic modules at 10-30 MW/year. This cost level is only attainable by adopting manufacturing practices proven in similar industries and by maintaining high device performance over large areas. The key technology development areas for achieving these competitive manufacturing costs are discussed. Improvements in process control, materials utilization, process speed, and manufacturability and their impacts on projected manufacturing costs are examined. Recent progress in the reduction of hazardous materials in the fabrication of CuInSe/sub 2/ modules is reviewed. Results are presented for 14.3% efficient CIS-based solar cells made without the use of cadmium, 14.0% efficient cells made without hydrogen selenide, and 13.7% efficient cells made without cadmium or hydrogen selenide.<<ETX>>

Advances in large area CuInSe/sub 2/ thin film modules

Efforts in large-area CuInSe/sub 2/ module development have focused on simplified, low-cost processing options with improved yield and performance. A large area (3890 cm/sup 2/), 40.8 W, 10.5% aperture area efficiency module has been demonstrated. Durable module performance, as indicated by over 40 months of continuous outdoor exposure at the National Renewable Energy Research Laboratory PV Outdoor Test Facility, has been shown. Two alternate interconnect structures, designed to simplify processing, have been used to fabricate small-area modules of 10.3% and 11.7% efficiency. Transparent conducting films of ZnO:Al, 1.5 /spl mu/m thick, have been sputter deposited at rates greater than 3 nm/sec with bulk resistivities of 1.5/spl times/10/sup -3/ /spl Omega/-cm, and cell efficiencies greater than 10% on 100 cm/sup 2/ substrates have been achieved using this ZnO. Alternate absorber layer formation and substrate selection have led to a 2.5 percentage point increase in average cell performance and a narrower power distribution of these cells. These improvements have resulted in a 3.1 cm/sup 2/ active area device with 15.1% efficiency.<<ETX>>

Investigation of the Trapping Mechanism for Transient Current-Voltage Behavior In CIGSS-Based Solar Cells

Recent CuInSe/sub 2/ (CIS) thin-film module development emphasized process control and module durability. CIS submodules, 0.4 m/sup 2/ in size with aperture-area efficiencies of 9.7% and power outputs of 37.8 W were fabricated using production-compatible processing techniques. Module stability was measured on 0.1 m/sup 2/ prototypes at several outdoor test sites. Modules changed less than 6% after 17/sup 1///sub 2/ months of continuous outdoor exposure. Module durability was also evaluated in accelerated life tests based on the Jet Propulsion Laboratory Block-V environmental tests. Median module power changed less than 8% after 10 humidity-freeze cycles, and good mechanical adhesion existed between the CIS layer and the substrate.<<ETX>>