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CIGS Photovoltaics

The benefits of TFPV over crystalline silicon PV, namely the reduced quantity of materials usage, the potential for lower-cost manufacturing techniques, reduced bulk and weight, and flexibility, not to mention reduced sensitivity to the volatile silicon market, promise to make TF modules competitive for the applications now dominated by c-Si, and favorable over c-Si for a wide range of other applications. TFPV is particularly suited to such higher value-added products as building-integrated PV systems and lightweight, portable devices.
Countering these advantages, TFPV technologies suffer from low efficiencies, both in general and compared to the standard c-Si modules. CIGS, however, shines above the rest in this respect, as it boasts efficiencies approaching that of c-Si, and its performance does not degrade over long-term exposure to sunlight. CIGS also has an additional benefit when it comes to options for boosting efficiency: The bandgap (and hence the portion of the solar spectrum absorbed) is determined by composition, mainly the indium:gallium ratio. Among other benefits, this makes CIGS particularly suited to multi-junction cells because each of the multiple absorber layers can be made of CIGS, differing only in composition. This contrasts with the other TFPV technologies, which would require entirely different materials, and the associated manufacturing steps and concerns, in order to build multi-junction cells.
Furthermore, the other thin-film technologies appear to have disadvantages that are less surmountable than those related to CIGS. Amorphous silicon, even though it has been in production for decades, still does not achieve better than about half the efficiency of c-Si, and degrades significantly with long-term exposure to sunlight. CdTe, in addition to also being less efficient than CIGS despite greater maturity, suffers from the environmental and health hazards of the requisite quantity of cadmium. OPV is barely out of the gate and there is still tremendous uncertainty that it can achieve anywhere near the efficiency of c-Si.
On top of the positioning of CIGS relative to other PV technologies, CIGS is also buoyed by the recent resurgence in solar power in general. Supply and pricing concerns with global fossil fuels markets, environmental concerns with the production and burning of fossil fuels, and government subsidies to drive development in light of these concerns have elevated demand for renewable energy sources, including PV. While crystalline silicon has historically been the mainstay of the PV industry, thin-film photovoltaics are rising quickly in terms of adoption and capacity. CIGS is the "rising star" of thin-film PV.
Indeed, the majority of the TFPV community appears to recognize the advantages of CIGS. Capacity for CIGS cells, established, under construction, and projected, is growing at a tremendous clip. The CIGS PV industry is set to take off as technology developments reduce cost and market conditions develop in favor of CIGS or TFPV in general. Many CIGS manufacturers are talking about grid parity, and even prices of less than $1 per watt, in the near future.
CIGS is not quite there, however, in terms of reaching its potential, which opens up opportunities for improvement, namely cost reduction and product improvement. Manufacturing costs still have a considerable way to come down, as the ramp up to high-volume manufacturing goes on. Efficiency, too, still leaves room for improvement. There is still a long way to go to get to 15.6 percent module efficiency, the target set by NREL at 80 percent of champion cell efficiency. Many, if not all, of the opportunities for improvement deal directly or indirectly with the materials used for CIGS. Advances in handling the component materials, such as printing them with inks, have the potential to drop the cost of CIGS dramatically and expand the useful applications of CIGS PV. In addition, the use of new materials in CIGS manufacture could allow breakthroughs in pricing and application development.
There are, of course, also downsides to the outlook for TFPV and CIGS in particular. One is the recent entry into the c-Si PV market of heavy-duty silicon titans such as Intel. While materials costs will still be just as important to the c-Si market and PV does not benefit directly from Intel&#25s expertise in miniaturization, Intel's strength in manufacturing performance and yield are still likely to drive down the cost of c-Si PV. Another downside is CIGS' dependence on indium. CIGS competes for indium mainly with ITO, the dominant transparent conductor used for products such as displays. Such displays currently account for 70 percent of worldwide consumption of indium. Furthermore, indium supply is, in large part, strategically controlled by China.
CIGS Cost Advantages
Several factors are at play to put CIGS PV on a trend toward lower cost per watt. First, CIGS PV can already compete on a cost basis with c-Si PV, which is at about $5 per watt. And, several CIGS manufacturers claim that they will achieve $1 per watt in the near future, which will make CIGS competitive with grid power in most markets.
One major cost-related advantage of CIGS, and of TFPV in general over c-Si PV, is in the quantity of materials used. While the silicon thickness in c-Si cells is measured in hundreds of microns (typically 200-300 microns), TFPV absorber layers are on the order of single microns. Thus, the quantity of absorber layer material per unit area is two orders of magnitude less for TFPV than for c-Si PV. CIGS in particular has an extremely high absorption coefficient, allowing a CIGS absorber layer of only one micron in thickness to absorb 99 percent of available light. While the other TFPV technologies are also attempting to use absorber layers as thin as 1 micron, the higher efficiency of CIGS PV makes it superior to any of the other PV technologies (TF or otherwise) in terms of absorber layer material usage per watt.
Because the cost of PV electricity generation consists almost entirely of the initial equipment and installation outlays, the overall cost per watt is strongly dependent on the life of the modules. Unlike other TFPV technologies, CIGS has no intrinsic degradation mechanisms, so efficiency loss over time can be limited to a very small proportion by effective encapsulation. CIGS PV manufacturers are currently targeting module lives of 30 years.
In addition to the PV modules themselves, the installation cost of the modules is a significant portion of the total cost of the PV power generation system. This is an area where the thinner, lighter, and flexible TFPV modules can really outperform c-Si. Lightweight modules for residential, commercial, and even utility users may be installed with reduced labor costs, less supporting structure, and fewer specialized tools.
There is also tremendous potential to reduce manufacturing costs for TFPV, including CIGS. Aside from the reduced material usage, the wide range of available substrates presents a wide range of manufacturing process possibilities. An especially tantalizing opportunity is roll-to-roll manufacturing. Flexible substrates, already in use by several CIGS manufacturers, permit continuous, rather than batch, processing to be done in several of the manufacturing steps.
Hand in hand with flexible substrates goes low-temperature processing. This is a mixed blessing; while enabling the development of lower-temperature manufacturing processes, these developments are driven precisely because the temperature stability is limited. Nonetheless, lower temperature processes, including low-temperature sputtering, evaporation, non-vacuum deposition techniques, and "flash" heating affecting only a thin surface layer of material, allow significant savings on equipment and energy consumption in manufacturing.
It is true that a-Si and CdTe TFPV manufacturers also claim similar benefits relating to cost. The two major advantages of CIGS versus these other technologies are the efficiency of the cells and the stability of the cells over time and with exposure to sunlight. These factors directly benefit the relative cost of CIGS compared with a-Si and CdTe because of their impact on required surface area and life of the modules.
CIGS Applications Advantages
In general, the advantages of TFPV—light weight and low bulk—are favorable for the traditional open-field and rooftop solar power installations currently dominated by c-Si PV modules. Those same properties, in addition to the flexibility of some TFPV modules, give rise to entirely new applications. In the BIPV market alone, PV devices have the potential to be incorporated into tough, flexible roofing shingles and wall facades. With semitransparent TFPV modules, even window options become available. Integrated modules such as these, with favorable aesthetics, are crucial to the widespread acceptance of BIPV in architecture. Beyond the stationary power generation applications, the properties of TFPV cells and modules open up entirely new portable power applications. While a-Si has dominated the solar calculator market for decades, flexible cells promise additional applications and improved usability. Panels no longer need to be incorporated into a flat, rigid surface, but may be placed as a flexible "skin" on electronics cases or devices themselves, or into a rollable sheet for portability. For these applications, the major advantages of CIGS over the other TFPV technologies are in power per unit surface area and cell life. For portable devices, smaller is better, particularly if performance is not sacrificed. And, as anyone who has experienced the deterioration of a mobile phone battery knows, rapid reduction in performance is quite bothersome.
CIGS Materials Opportunities
In its most basic construction, a CIGS PV cell must contain a CIGS absorber layer, a base electrode, and a top electrode. If a conductive substrate does not serve double duty as the base electrode as well, then a separate substrate is also needed. Currently, a separate junction layer is used between the absorber and the top electrode; there is a possibility that new top electrode materials may also double as the junction layer. Encapsulation is also required to protect cells from the elements, and manufacturers also use a variety of interconnection materials and antireflective coatings.
From a materials volume perspective, growth in CIGS PV technology will first and foremost impact the CIGS absorber layer materials, and the markets for the individual components that make up the CIGS layer. This includes elemental copper, indium, and gallium, as well as selenium and sulfur (elemental or as hydrogen selenide and hydrogen sulfide). Also important are the inks, and their components, used by some manufacturers: Nanoparticles (of CIGS and of its component copper, indium, gallium, selenium, and/or sulfur) as well as suspension solvents and additives such as resins and wetting agents. Particularly for indium, worldwide production is currently limited and the additional demand is likely to significantly impact the market. Markets for the other materials currently used in CIGS PV, including substrates, molybdenum, transparent conductive oxides (TCOs), and encapsulation materials, will also benefit to an extent from growth in CIGS PV production, although CIGS PV is expected to remain a smaller portion of their worldwide usage.
Of course, there is also the potential for new materials to become standard in CIGS PV manufacturing. One of the most significant opportunities for a breakthrough is in non-vacuum deposition of the electrode layers. The common, though not universal, assumption is that non-vacuum deposition processes will reduce the cost of manufacturing significantly. If this is true, we can expect a strong motivation to develop such non-vacuum processes. This may bring the use of nanoparticle inks of TCOs or different conductors into play. Encapsulation is another area where new materials are likely. Excluding moisture for 30 years in an outdoor environment is certainly a challenge. As long-term encapsulation failure mechanisms are discovered and studied, new materials for encapsulation are likely to be tried.

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