Opportunities for Tin in Photovoltaics
Published: June 01, 2009 Category: Advanced Materials Renewable Energy

Rapid growth in the photovoltaic industry has resulted in equally rapid growth in consumption of the materials used for manufacturing PV cells and modules. This, in turn, has led--and will continue to lead--to tremendous opportunities for the suppliers and others involved with these materials, in addition to the device makers themselves.

While a lot of attention is given to the active materials of PV cells, other materials, such as metals that make up the electrodes, also play an important role. One such metal tin, while small in terms of volume when compared to its other industrial uses, nonetheless represent a rapidly growing use of the metal. Tin is cheap by photovoltaic materials standards. In an industry that uses silver, indium, tellurium, and germanium--all in comparable quantities to tin--the cost of tin is one drop in a rather large bucket. But tin is a critical part of the photovoltaic devices for which it is used. Primarily, tin is used in the transparent electrodes of thin-film PV cells as either ITO or as doped tin oxides such as fluorine-doped tin oxide (FTO).

The photovoltaic applications of tin, while very small in terms of volume when compared to the other industrial uses of tin, nonetheless represent a rapidly growing use of the metal. One obvious use of tin in PV devices is in ITO, the dominant transparent conductor used by the display industry and others requiring both conductivity and transparency in the same material. The PV industry consumes some of that ITO to form the transparent electrodes on the front (and in some cases also the back) of TFPV cells, and growth in TFPV is driving up the overall volume of ITO consumed by it (although the penetration of ITO into that market is actually falling). ITO has the drawback that it depends on costly indium, and this is a major reason for its decline in penetration in the TFPV market.

But tin oxide is also used without the indium, also to form transparent electrodes. Tin oxide is generally doped with fluorine (FTO), but can also be doped with other materials such as antimony. Aside from its lower cost, FTO can easily be applied directly to glass and is thus often preferred for TFPV cells built in superstrate configuration--where the front electrode is applied directly to a glass substrate, through which light travels (when in use) to enter the cell.

Photovoltaics represents only a tiny portion of tin consumption, and the portion is still small when only tin oxide is considered. Tin oxide is widely used as a coating in other industries because of its conductivity, transparency, and low cost; for instance, tin oxide is used as an antistatic coating on architectural glass. Tin itself has more uses, in much higher volume. It is a major component in bronze and in many solders and other low-melting alloys. It is used in so-called "tin cans" and "tin roofs," which are actually steel coated with tin for its excellent corrosion resistance. Tin is also used to make "float glass" in which flat panes of glass are formed by allowing the glass to harden while floating on a pool of molten tin. There are also various synthetic chemicals that contain tin.

In one sense, specialty metals are commodities and as such follow traditional supply-demand economics. To this extent, providers focus on competitive advantages in cost, product lines, or service. However, photovoltaics (and especially thin-film photovoltaics) is a high-growth industry, and will drive tremendous growth in demand for the materials used by it, including certain specialty metals. Companies that recognize new or growing opportunities relating to specialty metals in photovoltaics, and capitalize on those opportunities, will stand to benefit ahead of others.

TCOs in PV: Market Evolution

Any photovoltaic cell must both allow light into the cell for conversion to electricity, and also conduct that electricity into a circuit. This means that the conductive electrodes in front of the cell must allow light through. The first high-volume PV cells--crystalline silicon (c-Si) PV cells--did not (and still, for the most part, do not) use TCOs, or other transparent conductors for that matter. Instead, c-Si PV uses finely patterned line structures (called fingers) that only obstruct a portion of the incident light from entering the underlying cell.

With the advent of thin-film amorphous silicon (a-Si) PV cells in the 1980s in portable, low-power applications like solar calculators, transparent conductive oxides (TCOs), specifically tin-doped indium oxide (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 uniformly 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 and it was used for the front electrodes. ITO worked well in these rigid a-Si PV cells on glass substrates, but it was expensive because of its indium content.

During the 2000s, ITO has lost much of its market share within PV to less-costly TCOs, mainly aluminum-doped zinc oxide (AZO) and fluorine-doped tin oxide (FTO). This has been the result of two phenomena. First is the reduced penetration of ITO within a-Si PV as the cost advantages of alternative TCOs have been realized. Also important has been the emergence of CdTe PV and CIGS PV, neither of which uses much ITO at all. CdTe PV, brought into high-volume manufacturing by First Solar, uses FTO, and CIGS PV, not yet as high in volume but commercialized to a significant extent, uses mostly AZO. NanoMarkets expects these trends to continue as CdTe PV and CIGS PV gain market share, and as a-Si PV continues to focus on cost reductions.

The Other Component of ITO

ITO contains about 10 percent tin oxide by weight (the rest being indium oxide). While all the recent attention on ITO (mainly relating to its cost) has naturally focused on the indium component, its tin component is also important. Some alternative indium-containing materials, indium oxide and indium zinc oxide, have been proposed for some of the applications for which ITO is used, however, they have not caught on largely because they still involve similar quantities of indium and are thus at least as costly as ITO.

Besides its cost, other characteristics of ITO make it less than perfect as a transparent conductor. It is neither very transparent nor very conductive as materials go; rather it simply represents a good trade-off between transparency and conductivity. It is also fairly brittle, and thus not particularly suited to any application in which flexibility is key. And, ITO does not generally lend itself easily 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. This dominance has also caused ITO to, somewhat ironically, dominate the market for the transparent electrodes of organic PV (OPV), which is the least commercially developed of the PV technologies NanoMarkets covers.

OPV's strong reliance on ITO 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.

Tin Oxide

Tin oxide typically requires higher deposition temperatures than ITO, making it generally less suitable than ITO where the high-temperature processing would be a problem. However, tin oxide has found significant use where the deposition temperature is not an issue, as in the case when the TCO is applied directly to a glass substrate. This is generally done for TFPV cells in superstrate configuration, including CdTe PV cells and many a-Si PV cells.

Tin oxide is well-suited to direct application onto glass, and FTO can even be applied to glass while the glass is being made, eliminating a process step. First Solar's CdTe PV cells use FTO, as do many a-Si PV cells built with the superstrate configuration. CIGS PV cells, however, are built in substrate configuration and tin oxide is thus more problematic because it would be applied onto the underlying device layers (which would be impacted by the high-temperature processing). For CIGS PV, AZO has become the TCO of choice, except in some cases where ITO is used.

TFPV on glass is growing rapidly and NanoMarkets expects the use of tin oxide to grow along with the superstrate configurations of a-Si PV and CdTe PV. But on flexible substrates--which are gaining favor in order to enable flexible applications or simply to take advantage of the cost-saving benefits of roll-to-roll processing--the high-temperature deposition conditions required of tin oxide are again an issue, even when applied directly to a flexible (polymer, for transparency) substrate.