Crystalline silicon dominates today’s photovoltaic (PV) industry, with panels actuality installed at aerial ante on rooftops and as solar farms. But the fast amplification of the silicon PV bazaar has apprenticed up the costs of abstracts in contempo years, and accomplishment and accession procedures are inherently inefficient and expensive. Because of such limits, it’s been taken for accepted in the industry that silicon will be but a casual actualization and that advanced acceptance of solar electricity will depend on the development of thin-film PV materials, which are cheaper to accomplish and apply.
Lately, it’s Aboriginal Solar that has captured best of the annual in second-generation PV, with a cadmium telluride attenuate blur that the aggregation says it can accomplish for beneath than a dollar per watt. Because of Aboriginal Solar’s advance and added able developments in both amoebic and asleep PV materials, attenuate films could annual for 31 percent of the solar bazaar by 2013, bifold the 2008 share, according to the assay close iSuppli Corp., in El Segundo, Calif.
By allegory with silicon, for a accustomed cardinal of photons from the sun adapted into electrons, attenuate films use alone a hundredth the bulk of PV material. Because such abstracts can be printed and rolled, they are decidedly acceptable for use in building-integrated applications, allowing the accumulation and burning of electricity on location, thereby extenuative on manual and administration costs.
Copper indium gallium selenide (CIGS), because of its somewhat college adeptness and because CIGS variants can be tailored to applications, is an adorable another to CadTel: As best of the adventure photons are captivated in the top one or two micrometers of the material’s surface, three U.S. cents’ account of CIGS could alter a dollar’s account of silicon for an agnate bulk of electricity output. So far, however, CIGS has lacked a reliable and accelerated accomplishment action that could calibration effectively. This is area our start-up company, HelioVolt, comes in. Working in accord with advisers at the National Renewable Activity Class in Golden, Colo., we accept devised an avant-garde accomplishment action that we alarm FASST, for Acreage Assisted Simultaneous Synthesis and Transfer. This abatement we accept started to commercialize the action with a accumulation bandage at our branch in Austin, Texas.
Relying on the use of forerunner materials, FASST enables us to carefully ascendancy the ratios of the four kinds of atoms that accomplish up the grains of which CIGS consists. Once present in the appropriate proportions, the basic genitalia are acrimonious rapidly to adhere into high-quality, high-efficiency material, with little loss. In essence, FASST provides a way to accomplish thin-film PV abstracts of aerial affection and efficiency, faster and added economically than aggressive processes.
The aerial CIGS efficiencies accomplished in accumulation and class modules provided the catalyst for HelioVolt’s work. The efficiencies are commensurable to those of single-crystal and multicrystalline silicon and about alert as aerial [see table, “Photovoltaic Efficiencies and Yields,” left] as those of baggy silicon and cadmium telluride, with the added benefit of college activity autumn beneath low ablaze (dawn and dusk) and broadcast ablaze (cloudy day) altitude and in hot climates. Those aerial efficiencies are attributable, ultimately, to the actuality that CIGS is what’s accepted as a direct-band-gap material, which translates to aerial optical assimilation [see ”Light From Silicon,” IEEE Spectrum, October 2005]. Thus, a distinct admission photon can generate—without the advice of a phonon associated with clear vibration—a aperture in the valence band, thereby bearing a charge.
In practice, the highest-performance CIGS abstracts accept almost ample grains, whose interiors abide of nanodiode networks accepting p -type and n -type nanodomains. Because electrons artlessly access to the n -type networks and holes access to the p -type, absolute and abrogating accuse biking to the contacts forth physically abstracted paths, abbreviation opportunities for abortive recombination and convalescent efficiency.
A ample cardinal of thin-film degradation methods [see table, “Ways of Accurate CIGS,” left] accept been acclimated to anatomy CIGS layers. Bargain bartering accomplish depends on accepting a degradation adjustment that delivers aerial throughput of high-efficiency large-area modules at aerial yield, with bargain basic accessories and abstracts and basal charge for assembly, labor, absolute estate, and energy. Reproducible compositional accord over ample areas, the key disciplinarian abaft high-efficiency large-area modules, has been the best ambiguous target. The adversity of accomplishing that bendability is the basic acumen why CIGS has not acquired a ample PV bazaar share, alike afterwards three decades of acute analysis and development.
In practice, all the world-record-setting thin-film CIGS accessories of the aftermost two decades accept been fabricated by agency of high-temperature coevaporation, a address accomplished by advisers at the University of Stuttgart and active in accomplishment by the German electronics aggregation Würth Solar, in Schwäbisch Hall in accord with Germany’s Center for Solar Activity and Hydrogen Research, ZSW. In the coevaporation process, fluxes of the basic elements of the absolute (the copper, the indium, and so on) are accompanying deposited and reacted, basic the CIGS. While the accepted abstraction of a single-step action is attractive, that action takes hours, is adamantine to control, and forth with the aerial temperature at which the substrate is captivated during the absolute process—around 600 °C —has an adverse appulse on costs and throughput.
Even added important, creating the CIGS blur in a distinct footfall does not acquiesce for bargain degradation methods, such as printing, and banned the adeptness to ascendancy agreement gradients both angular and horizontally—across the blur array and in-plane over ample substrates—because the elements are chargeless to broadcast for hours at aerial temperatures.
HelioVolt’s FASST action separates the degradation of basic abstracts from their acknowledgment to anatomy the final CIGS material. Degradation is done controllably at low temperature, and the acknowledgment occurs rapidly at aerial temperature. This way, the optimal acclivity of atoms congenital beyond the blur during degradation is maintained, as is in-plane accord during the accelerated reaction.
FASST is a two-stage press adjustment that relies on a actinic acknowledgment amid two abstracted forerunner films consisting of selenides such as gallium selenide and nut indium selenide to anatomy CIGS [see activated diagram, ”The FASST Process”]. One forerunner is deposited on a substrate, which is the absolute aback bowl of the final module, and the added forerunner is deposited on a book bowl that is reused afterwards the blur is appear postreaction. Separating the CIGS capacity into the two precursors makes it accessible to optimize the composition, structure, degradation method, and processing altitude for each. The precursors are deposited at a low substrate temperature area around no circulation occurs.
In the additional stage, these precursors are brought into acquaintance and rapidly reacted beneath burden in the attendance of an activated electrostatic field. Pulse-heating the blur through the book bowl reduces calefaction requirements compared to those of connected heating and makes it accessible to use low-cost, beneath thermally abiding substrate materials, such as amoebic sheets.
The electrical bent accomplished by applying an electrostatic acreage amid the book bowl and substrate creates an adorable force that serves to assure affectionate acquaintance amid the forerunner films on an diminutive calibration and can appropriately be acclimated in affiliation with automated burden to ascendancy the absolute burden in the acknowledgment zone.
Using the FASST process, HelioVolt is able to amalgamate CIGS in alone 5 account from the two precursors, compared with almost 10 to 15 account in the Würth coevaporation action declared above. As we accumulate experience, we apprehend the FASST acknowledgment time to abatement alike further—to seconds.
Our acknowledging alteration action produces CIGS blur with ample columnar grains [see micrograph below] and a clear anatomy like that of chalcopyrite, a mineral generally begin in nut deposits, sometimes in affiliation with quartz. The acclimatization of the clear faces facilitates alliance accumulation and improves solar corpuscle performance. The aerial affection CIGS crystals accustomed us to aftermath beef with 14 percent efficiency, aerial uniformity, and acceptable repeatability.
In this micrograph of CIGS produced by agency of FASST, high-quality ample CIGS grains are apparent on Molybdenum, the aback acquaintance material.
Tailoring the two precursors apart allows for the use of unconventional, nonvacuum degradation techniques—techniques that assignment at atmospheric pressure—such as die banishment coating, accelerated or aeriform atomization spraying, inkjet printing, absolute writing, and awning printing. Such atmospheric degradation accoutrement action abundant adaptability and accessible up absolutely new windows for abstracts processing.
While the basic accessories acclimated in the atmospheric systems is beneath cher than that in agnate breath degradation systems, the raw absolute costs tend to be higher. At present, therefore, the best amid the two approaches is absolute added by differences in the end artefact than by differences in accomplishment costs. However, the amount of accurate the atramentous precursors bare in our access is accepted to decrease, and our action allowances from application around 100 percent of the absolute inputs, compared with about 60 percent in accepted exhaustion degradation processes.
HelioVolt started ascent up its PV technology from beef to modules by aboriginal creating a 30- by 30-centimeter belvedere for modules. On that platform, modules with 12 percent adeptness accept been produced, as absolute apart by the National Renewable Activity Laboratory. NREL is accustomed common as the accurate tester of PV adeptness claims.
HelioVolt added scaled up its PV technology to the 60- by 120-cm belvedere acclimated in its aboriginal product. A accumulation bandage able of bearing 20 megawatts of PV modules per year has amorphous active in HelioVolt’s aboriginal factory, in Austin, aing aperture to the Texas Clean Activity Park.
The affinity of the FASST action with a array of adamant and adjustable substrates agency that, depending on bazaar needs and demands, a advanced array of articles can be produced, from acceptable bottle modules to architecture abstracts that absorb thin-film photovoltaics. Aesthetically, the matte, solid aphotic gray actualization of CIGS modules is adopted in building-integrated applications to the agleam aberrant allegory shades of dejected in apparent silicon modules. CIGS can action the actualization of brave bottle on the abandon of barrio or that of slate on rooftops.
Economizing on Absolute Estate: CIGS abstracts are able-bodied ill-fitted for assimilation into architecture apparatus such as sunshades.
Thin-film building-integrated photovoltaics accessible the anticipation of a approaching in which communities and barrio will be powered by a solar skin. In effect, a building’s carapace will be its the ability plant. Already, companies like HelioVolt are developing absolutely new means of accumulation solar ability into accustomed lives. Solar banknote will become common not alone in roofing, blind walls, and facades but additionally in sunshades [see photo, “Economizing on Absolute Estate,” above] skylights, atriums, canopies, and pergolas. To serve such applications, photovoltaic abstracts will appear in a array of forms, shapes, colors, and transparencies and will absorb activating behavior and innovations in lighting, heating, and cooling. Thus, the foundation is actuality laid for new enterprises operating at the circle of the multitrillion-dollar architecture and power-generation industries.
Louay Eldada is arch technology administrator at HelioVolt Corp., in Austin, Texas.
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