Bright Opportunities For Indium

NanoMarkets

thin film | organic | printable | electronics  www.na nomarkets.net

Bright Opportunities for Indium  This article is based in part on research from Indium Markets for Photovoltaics  The indium market is dominated by a single product: indium tin oxide (ITO)—the main transparent conductor used by the display industry and others requiring both conductiv ity and transparency in the same material. The photovoltaics (PV) industry does consume some of this ITO, but it is certainly not the largest consumer. Instead, PV represents the fastest growing application for indium, even when excluding the ITO portion of this consumption. Growth in the PV industry in the last decade has yielded tremendous opportunities not only for the companies that make the actual solar cells and modules, but for materials firms, including indium producers, as well. Continued growth, especially in thin-film PV (TFPV), will provide even more such opportunities. This rapid growth in indium consumption is predicted largely because of the emergence and growth of CIGS photovoltaics. Indium is critical to CIGS PV because it is a major component of the absorber layer, the copper indium gallium selenide (CIGS) material. While CIGS PV has not yet achieved volumes as high as some of the other TFPV technologies such as a-Si and CdTe (despite high hopes and expectations for the last few years), it has the highest demonstrated efficiency of all the TFPV technologies (20.0 percent), approaching that of crystalline silicon PV cells. Combined with the benefits generally associated with all of the TFPV technologies—light weight, potential flexibility, and anticipated lower cost—NanoMarkets expects CIGS PV to gain a large share of the TFPV market over the next eight years. As a result, the CIGS material will become the second largest use of indium in that time, accounting for around 10 percent of worldwide indium consumption. ITO's Role in PV Evolves The oldest, and most widely used PV technology—crystalline silicon (c-Si) PV—does not use ITO as the transparent conductor. While the front electrodes of any PV cell must allow light through, there are two general approaches to do this. The first, which most TFPV technologies use, relies on a transparent conductor, mainly ITO. The second approach, the one taken by c-Si PV cells involves silver patterned into fine line structures (called fingers) that only block a portion of the incident light from entering the underly ing cell.

Page | 1 

NanoMarkets

thin film | organic | printable | electronics  www.na nomarkets.net

With the advent of a-Si PV cells in the 1980s in portable, low-power applications like solar calculators, ITO began to make a significant showing in the PV market. These thin-film cells do not have the carrier mobility of doped crystalline silicon, so their surfaces must be unif ormly coated with a conductor to effectively capture the carriers generated in the cell, and the conductor must be transparent to allow light into the cell. ITO was then, as it is now, the established "standard" transparent conductor, which was used for the front electrodes of a-Si PV cells. ITO worked well in these rigid a-Si PV cells on glass substrates, but it was expensiv e. Although the price of indium had come down from its highs of the nuclear power heyday, it was still over $200 per kilogram on average in the late 1980s and early 1990s. Besides its cost (due to its high indium content), other characteristics of ITO make it less than perfect as a transparent conductor. It is neither very transparent nor very conductiv e as materials go; rather it simply represents a good trade-off between transparency and conductiv ity. It is also fairly brittle, and thus not particularly suited to any application in which flexibility is key. Also, ITO does not easily lend itself to low-temperature manufacturing processes, limiting the processes that can be used with it. Still, in the applications that use ITO the most (mostly displays of various types), ITO is by far the dominant transparent conductor. TFPV is an exception to this clear-cut dominance of ITO. ITO's limitations have caused the newst of the PV technologies to shy away from it for the most part. CdTe PV, brought into high-volume manufacturing by First Solar, uses tin oxide as the transparent conductor for its front electrode. CIGS PV, not yet as high in volume but commercialized to a significant extent, uses aluminumdoped zinc oxide (AZO) in most incarnations. Even the a-Si PV that has been around for decades has now shifted significantly away from ITO and toward other transparent conductive oxides (TCOs); ITO still has only about half of this market. Somewhat ironically, the TFPV technology that uses ITO the most is actually the least commercially developed one, organic PV (OPV). But this can be understood when one considers that OPV has had many obstacles to surmount on its path to commercialization, such as its low conversion efficiency and its extreme sensitivity to oxygen and moisture. An adequately performing, off-the-shelf transparent conductor allows OPV developers to focus on other issues as they race to make OPV commercially viable. In the last few years, though, the transparent electrodes have come back to the forefront and there have been numerous developments in terms of substitutes for ITO in the OPV space. NanoMarkets expects OPV to use proportionally less and less ITO as it moves toward lower costs, one of its chief objectives.

Page | 2 

NanoMarkets

thin film | organic | printable | electronics  www.na nomarkets.net

CIGS PV Overview CIGS PV has found itself third in line

(among TFPV technologies) to high-volume

commercialization. This is not because it lacks potential—as already mentioned, it boasts the highest champion cell conversion efficiencies of all the TFPV technologies, at 20.0 percent, and still rising periodically—but rather because it has been difficult to implement in high-volume manufacturing. Technical and operational issues prevented CIGS PV from taking off in 2007 and 2008, and the economic crisis that became evident in late 2008—reducing PV demand overall and making investors more cautious about funding new ventures—has ensured that there will be a stilllonger wait for high-volume commercialization. Notwithstanding these difficulties, NanoMarkets still expects CIGS PV to achieve over a gigawatt of annual production by 2014. While CIGS PV's potential for higher conversion efficiency is the major factor keeping it in play for the TFPV market, it is not the only one. Its performance does not degrade simply due to light or heat exposure—as CdTe PV and a-Si PV cells do—provided that moisture is effectively excluded from the cells. And, as a quaternary semiconductor (actually an alloy of two ternary ones: copper indium selenide and copper gallium selenide), its composition—and bandgap—is infinitely variable based on the relativ e amounts of indium and gallium. This could allow tuning of the bandgap to optimize tandem cells—using CIGS for both absorber layers, dif fering only in composition—or customization of the bandgap in single-junction cells to match different illumination conditions, such as artificial light. CdTe and a-Si PV cells do not have this flexibility; these materials have a fixed bandgap. Multijunction cells based on CdTe or a-Si would require completely different materials for the additional absorbers; for instance, multijunction a-Si cells typically use Si:Ge alloy. Finally, CIGS PV does not suffer from the stigma of incorporating toxic cadmium into cells to nearly the extent that CdTe PV does. While it is true that most CIGS PV cells currently use cadmium in the junction layer, this layer is diminutively thin and the quantity of cadmium used is much smaller than that used by CdTe PV. The junction layer can even be made of a dif ferent, non-cadmiumcontaining material, and at least two CIGS PV manufacturers have already eliminated cadmium from their cells. CIGS PV's potential for higher efficiency also influences its standing among the other TFPV technologies in terms of the advantages shared by all of them. Flexibility and light weight are two of the key potential performance benefits of TFPV over c-Si PV: both characteristics open the door

Page | 3 

NanoMarkets

thin film | organic | printable | electronics  www.na nomarkets.net

to a wider range of applications and installation locations, but flexibility is also expected to permit lower-cost manufacturing methods, such as roll-to-roll processing. The cost reductions anticipated from such processes are a benefit even if the completed cells and modules are not intended to bend. CIGS PV's efficiency magnifies these benefits. For instance, where weight is a concern, the smaller module area required for a given power output reduces CIGS PV's weight proportionally. For flexible applications, size is also an important concern. Imagine a flexible PV power supply that must be rolled or worn; the size of a device producing a giv en power output (or the power output of a device of a giv en size) will be especially apparent in comparison to other, less-efficient devices. In its most basic construction, a CIGS PV cell must contain a CIGS absorber layer, a base electrode, and a top electrode. If a conductiv e 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 antireflectiv e coatings boost efficiency. Among these materials, the absorber layer and the encapsulation materials have proven the most challenging. The absorber layer, naturally, directly relates to indium consumption by these cells. But encapsulation materials are also critical to the growth of CIGS PV especially for flexible applications. As has already been mentioned, robust encapsulation is required to exclude moisture from CIGS PV cells because their performance can degrade in the presence of moisture. Even trace amounts of moisture can enable this degradation, so the encapsulation of the cells must be very effectiv e indeed. Glass is a suitably impervious encapsulant, but does not address cells used in flexible applications where glass would be too rigid. For cells sandwiched between rigid planes of glass, sealing of the edges is not a trivial task but is still less of a problem than sealing entire surfaces that will bend in use. Some CIGS PV manufacturers have recognized this challenge and even turned down contracts to produce flexible cells and modules because of it. Miasolé's founder, David Pearce, has started a new venture to develop encapsulation solutions for flexible cells. From the perspective of the indium industry, growth in CIGS PV technology will naturally result in growth in demand for indium. NanoMarkets expects CIGS to account for around 10 percent of the worldwide indium market by the end of the forecast period. Other materials will be affected too;

Page | 4 

NanoMarkets

thin film | organic | printable | electronics  www.na nomarkets.net

these include the other components of the CIGS absorber layer—copper, gallium, selenium, and (in some cases), sulfur—as well as the intermediate products including sputtering targets, plating baths, inks, and their components. Also included are the other materials used in CIGS PV, including substrates, molybdenum, and transparent conductiv e oxides (TCOs) including ITO and others. In the case of most of these materials, CIGS PV is expected to remain a small portion of worldwide usage, the exceptions being indium and gallium and the CIGS-specif ic intermediates.

Page | 5