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Indium: a critical component for future PV

Until now indium has played a relatively minor role in the photovoltaics (PV) market, but the material is set to take on an expanding role as new technologies come online and gain market share

Skies the limit: Indium is taking solar technology to new heights
Skies the limit: Indium is taking solar technology to new heights

PV “will be the fastest-growing significant application for indium over the next eight years at least,” says industry analysts NanoMarkets, based in the US. “The main reason for this is the expected rise of CIGS photovoltaics.” Indium is a by-product of base metal (such as tin or copper) production. “It is very malleable and ductile and can be easily formed into a wide variety of fabrications,” explains Indium Corporation, which supplies indium to the solar industry along with copper/indium/gallium (CIG) alloy targets and a range of other services and products for the sector.

“It retains its softness to temperatures approaching absolute zero, making it ideal for cryogenic and vacuum applications.”

Today, the main use of indium is for the production of indium tin oxide (ITO), which currently accounts for 80% of all the indium consumed around the world. ITO is the main transparent conductor used by the panel display industry. It is also used in this way for PV too, mostly in organic PV and dye sensitized cell technology systems, which are limited in use still but almost exclusively use ITO as conductors.

“When indium, in the form of indium-tin oxide, is coated onto various materials such as glass or plastic films, it acts as a transparent electrical conductor and an infrared reflector,” says Indium Corporation. “When architectural or photovoltaic glass is coated with ITO it keeps the harmful infrared rays of the sun from passing through.”

It is used for thin film amorphous silicon (a-Si) PV too, but it’s deployment here as a conductor is declining, with other materials becoming more popular due to ITO’s relatively high cost. In the case of crystalline silicon (c-Si) PV, a silver grid is used instead of a transparent conducting oxide for a front electrode.

ITO is not, however, indium’s only application for PV. It is critical to thin film copper indium gallium selenide (CIGS) PV, forming a major component of the absorber layer. As a material, CIGS strongly absorbs sunlight and so a much thinner film is required compared to other semiconductor material (the absorber is placed on a glass backing, along with electrodes to collect current).

“While CIGS PV has yet to achieve the high volumes that were once expected of it, it does seem to have much going for it,” notes NanoMarkets’ recent report, Markets for Indium-Based Materials in Photovoltaics, published in September. Indeed, CIGS has the highest demonstrated efficiency of all the thin film PV (TFPV) technologies, approaching that of crystalline silicon PV cells and it is expected to rapidly penetrate conventional PV panels, BIPV and portable PV segments.

The collapse of CIGS manufacturer Solyndra notwithstanding, NanoMarkets expects CIGS PV to gain a large share of the TFPV market over the next eight years.” While the technology has taken longer to take off commercially than many of its supporters hoped, it can “now be considered commercialised” with annual production capacity expected to reach more than 2 GW by 2014, it says. Indeed, it expects “CIGS will be the only TFPV technology that might be able to compete with conventional c-Si PV technology where C-Si is already established”.

The major factor driving CIGS PV development is the potential for higher conversion efficiency over traditional thin film technologies. The major task ahead is improving deposition methods (such as sputtering and evaporation) for these films to achieve lower costs and increased throughput and producing high efficiency cells on a commercial scale.

“Performance depends on quality, so when developing production methods, the difficulty and cost of creating CIGS films must be balanced with quality and performance,” says Nanomarkets.

German engineering firm, Manz AG, agrees and, in developing its CIGSfab production line, it is just one of the companies working to improve production processes globally and recently hit a world record for thin film CIGS module efficiency.

“CIGS technology is the technology with the largest cost-cutting potential among the competing thin-film technology such as amorphous silicon or cadmium telluride,” it insists.

Additional CIGS benefits

Meantime, CIGS shares the advantages traditionally associated with TFPV too. It’s light-weight, flexible and, as already said, is widely expected to be low cost. In addition, its performance does not degrade simply due to light or heat exposure if moisture is effectively excluded from the cells.

“In addition, CIGS' bandgap is infinitely variable based on the relative amounts of indium and gallium,” stresses Nanomarkets. “This could allow tuning of the bandgap to optimize tandem cells.” Significantly too, CIGS PV “does not suffer from the stigma of incorporating toxic cadmium into cells to nearly the extent that CdTe PV does”.

The potential growth in indium use via the CIGS market will “almost wholly” be realized at the absorber layer, rather than as a transparent conductor too. Most CIGS makers use aluminium-doped-zinc oxide as the conductor.

For the future too, Indium phosphide (InP) PV systems – currently used in a limited sense in the aerospace industry – could also emerge, Nanomarkets notes.

Supply constraints

In planning to take this technology forward, a key issue for the solar industry is whether there is sufficient indium actually available to the market and at what cost? “Indium prices have risen recently and are expected to continue to climb over the next several years,” notes NanoMarkets. China is the main source for the material, while North America and Australia also have stockpiles of concentrates they could sell into the market. “It is uncertain how quickly such activities would impact the market, however,” says the analyst. “Historically the industry has been slow to respond to supply/demand swings.”

Indium Corporation also notes that investments are being made in by zinc producers in South America to extract more by-products like indium too. “These investments will result in gradually increasing indium outputs from 2012 onwards,” suggests the company’s Claire Mikolajczak, Director of Metals & Chemicals, and Bill Jackson, Director of Solar Products in a September paper. “They will eventually increase the world output by 15% or 75 mt year in that continent alone.”

In a technical note issued by the pair in September, entitled Availability of Indium and Gallium, they add: “Price volatility and short-term availability may continue intermittently due to numerous factors, including the time-lag required to install additional capacity, government regulation, and the lack of information suppliers receive about future demand.” Overall, however, they anticipate adequate indium (and gallium) supply and “continued price affordability for current, emerging, and new applications”.