In this contribution, we report on the current conversion efficiency status for Cu(In,Ga)Se 2 (CIGS) solar cells made on soda lime glass (SLG) and flexible metallic (FM) substrates at DayStar Technologies, Inc. (DSTI). Using our research and development (R&D) line, we report the following cell results: (1) a 16.9% small-area (~1.1 cm 2 ) CIGS cell on SLG using DSTI Gen I process; (2) a 15.7% small area (~1.1 cm 2 ) CIGS cell on FM substrate using Gen I process; (3) a 15.2% small area (~1.1 cm 2 ) CIGS cell on FM substrate using a rapid activation process; and (4) a 13.5% large area (~14 cm 2 ) CIGS cell on FM substrate using our rapid activation process. Using our Gen II production process, we have made a 13.0% small-area (1.1 cm 2 ) CIGS cell on FM substrate. All the cells have anti-reflection coating. In this contribution, we present material and device characterization for these cells

Discrete CIGS solar cells on flexible metal substrates offer an alternative to wafer-Si based cells. The manufacture of this cell technology additionally offers capital cost, throughput and yield advantages over the manufacture of either wafer-Si or traditional monolithically integrated thin-film modules. Economies of scale and volume can be achieved with the implementation of gigawatt-scale manufacturing. Likewise, vertical integration within the production line to include In refinement, steel substrate finishing, source material synthesis and formation, as well as module packaging materials (either glass and Al or plastic) provide for optimization of cost and supply- chain issues.

Photovoltaic arrays have played a key role in power generation in space. The current technology will continue to evolve but is limited in the important mass specific power metric (MSP or power/weight ratio) because it is based on bulk crystal technology. The objective of this research is to continue development of an innovative photovoltaic technology for satellite power sources that could provide up to an order of magnitude saving in both weight and cost, and is inherently radiation-tolerant through use of thin film technology and thin foil substrates such as 5-mil thick stainless steel foil or 1-mil thick Ti. 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 CuIn(1-x),Ga(x)S2, (CIGS2), CuIn(1-x), G(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 $500 K. Previous work at FSEC 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 (6x 4) substrates. This paper presents further progress in processing on metal foil substrates. Also, previous work at DayStar demonstrated the feasibility of flexible-thin-film copper-indium-gallium-diselenide (CIGS) solar cells with a power-to-weight ratio in excess of 1000 W/kg. We will comment on progress on the critical issue of scale-up of the solar cell absorber deposition process. Several important technical issues need to be resolved to realize the benefits of lightweight technologies for solar arrays, such as: monolithic interconnects, lightweight array structures, and new ultra-light support and deployment mechanisms. Once the technology has gained spaceflight certification it should find rapid acceptance in specific satellite markets.

Our effort towards the attainment of high performance devices has yielded several devices with total-area conversion efficiencies above 16%, the highest measuring 16.8% under standard reporting conditions (ASTM E892-87, Global 1000 W/m‘). The first attempts to translate this development to larger areas resulted in an efficiency of 12.5% for a 1 6.8-cm2 monolithically interconnected submodule test structure, and 15.3% for a 4.85-cm’ single cell. Achievement of a 17.2% device efficiency fabricated for operation under concentration (22-sun) is also reported. All high efficiency devices reported here are made from graded bandgap absorbers. Bandgap grading is achieved by compositional Ga/(ln+Ga) profiling as a function of depth. The fabrication schemes to achieve the graded absorbers, the window materials and contacting will be described.

Crystallographic, optical, and electrical properties of defect chalcopyrite Cu(ln,,Ga,),Se, (O<x<l) materials in polycrystalline thin-film form are reported. Also, an energy band alignment between such materials and CdS has been calculated from X-ray photoelectron spectroscopy data. A comparison of some properties against published data on similarly prepared chalcopyrite Culn,,Ga$e, absorber materials is presented. Considering the chalcopyrite/defect chalcopyrite junction model, we postulate that the traditionally poor device performance of uniform high-Ga-content absorbers (x>0.3) is due to a relatively inferior character-both structural and electrical-at the very chalcopyrite/defect chalcopyrite interface. We demonstrate that this situation can be circumvented (for absorbers with x>0.3) by properly engineering such an interface by reducing Ga content in the region near the surface of the absorber.

The thin-film solar cell program at NASA GRC is developing solar cell technologies for space applications which address two critical metrics: specific power (power per unit mass) and launch stowed volume. To be competitive for many space applications, an array using thin film solar cells must significantly increase specific power while reducing stowed volume when compared to the present baseline technology utilizing crystalline solar cells. The NASA GRC program is developing two approaches. Since the vast majority of the mass of a thin film solar cell is in the substrate, a thin film solar cell on a very lightweight flexible substrate (polymer or metal films) is being developed as the first approach. The second approach is the development of multijunction thin film solar cells. Total cell efficiency can be increased by stacking multiple cells having bandgaps tuned to convert the spectrum passing through the upper cells to the lower cells. Once developed, the two approaches will be merged to yield a multijunction, thin film solar cell on a very lightweight, flexible substrate. The ultimate utility of such solar cells in space require the development of monolithic interconnections, lightweight array structures, and ultra-lightweight support and deployment techniques.

The design features of a patented 4.5-sun, "flat-plate" concentrating module are presented. Various cell interconnect methodologies are considered. Optical characterization indicates Ag is the reflecting material of choice. Thin-film Cu(In,Ga)Se-2 (GIGS) solar cells on metallic substrates out-perform non-optimized wafer-Si in reflectivity and performance under concentration. Wafer-Si cells are easily incorporated into the module with minor variability in performance.

Thin-film CIGS solar cells have been investigated for use in the space environment. The study includes deposition on lightweight metal substrates, measurement under AMO light, and irradiation by electrons and protons. A 15.2% AMO cell performance has been achieved, resulting in a specific power of 1433 W/kg and 1235 W/kg for bare and SiO/sub x/ "coverglass" cells, respectively. No degradation has been observed after e/sup -/ irradiation. A 24-cm/sup 2/ cell design has achieved 1100 W/kg specific power.

DayStar Technologies is developing a PV module technology using low-level concentration (2-8 suns) that can package existing industry cell materials into a lower cost/ higher value product suitable for both low-power (IOW) solar lantern and I-KW and greater power generation applications. Cell materials incorporated to date include Cu(In,Ga)Se-2 (CIGS), a-Si, and c-Si. The use of thin-film cell materials in a concentrator application is the first of its kind. The performance and reliability of CIGS and a-Si under concentration has been demonstrated. The efficacy of the proprietary optics developed by DayStar has been demonstrated. Cell integration and subsequent mating to optics has proven to be nearly lossless. A 7.2% active-area CIGS-based mini-module has been measured.

DayStar Technologies is developing a PV module technology using low-level concentration (2[endash]8 suns) that can package existing industry cell materials into a lower cost/higher value product suitable for both low-power (10W) solar lantern and 1-KW and greater power generation applications. Cell materials incorporated to date include Cu(In, hthinsp;Ga)Se[sub 2](CIGS), a-Si, and c-Si. The use of thin-film cell materials in a concentrator application is the first of its kind. The performance and reliability of CIGS and a-Si under concentration has been demonstrated. The efficacy of the proprietary optics developed by DayStar has been demonstrated. Cell integration and subsequent mating to optics has proven to be nearly lossless. A 7.2[percent] active-area CIGS-based mini-module has been measured. [copyright] [ital 1999 American Institute of Physics.]