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Efficient Solar Cells from Cheaper Materials

IBM researchers have greatly increased the performance of a novel thin film solar cell.

Researchers at IBM have increased the efficiency of a novel type of solar cell made largely from cheap and abundant materials by over 40 percent. According to an article published this week in the journal Advanced Materials, the new efficiency is 9.6 percent, up from the previous record of 6.7 percent for this type of solar cell, and near the level needed for commercial solar panels. The IBM solar cells also have the advantage of being made with an inexpensive ink-based process.

Copper power: This prototype solar cell uses a copper-based material and has achieved record efficiencies for a cell of its kind.

The new solar cells convert light into electricity using a semiconductor material made of copper, zinc, tin, and sulfur–all abundant elements–as well as the relatively rare element selenium (CZTS). Reaching near-commercial efficiency levels is a “breakthrough for this technology,” says Matthew Beard, a senior scientist at the National Renewable Energy Laboratory, who was not involved with the work.

The IBM solar cells could be an alternative to existing “thin film” solar cells. Thin film solar cells use materials that are particularly good at absorbing light. The leading thin film manufacturer uses a material that includes the rare element tellurium. Daniel Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley, says the presence of tellurium could limit the total electricity such cells could produce because of its rarity. While total worldwide electricity demand will likely reach dozens of terawatts (trillions of watts) in the coming decades, thin film solar cells will likely be limited to producing about 0.3 terawatts, according to a study he published last year. In contrast, the new cells from IBM could produce an order of magnitude more power.

The new cells could also have advantages compared to cells made of copper indium gallium and selenium (CIGS), which are just starting to come to market. That’s because the indium and gallium in these cells is expensive, and while the selenium used in the IBM cell is rarer than indium or gallium, its cost is a tenth of either.

A new ink-based manufacturing process solves some of the key challenges to making efficient CZTS cells. A common approach to making any type of high-quality solar material is to dissolve a precursor substance in a solvent. This isn’t possible with the CZTS cells because the zinc compounds required in the new cells aren’t soluble. To get around this, the researchers used a combination of dissolved materials and suspended particles, creating a slurry-like ink that could then be spread over a surface that’s been heat-treated to produce the final materials. The particles prevent the material from cracking and peeling as the solvent evaporates.

The IBM researchers are also investigating ways to improve the efficiency of the new solar cells, with the goal of reaching about 12 percent in the laboratory–high enough to give manufacturers confidence that they could be mass produced and still have efficiency levels of around 10 percent, says David Mitzi, at IBM Research, who led the work. Beard recommends targeting 15 percent efficiency in the lab, and Mitzi says this should be possible by improving other parts of the solar cell besides the main CZTS material, or by doping the semiconductor with other trace elements (which is easy with the ink-based process).

What’s more, commercial cells will likely use different materials for conducting electrons. The experimental cells used indium tin oxide, which is limited by the availability of indium. But Mitzi says several other conductors could work as well.

One key next step is to completely replace the selenium in the solar cells with sulfur. For the record-efficiency cell, the researchers replaced half of the selenium used in a previous experimental cell. If all of the selenium could be replaced, the cells could, in theory, supply all of the electricity needs of the world. (Provided there are suitable means for storing and redistributing power for use at night or on cloudy days.)

The new type of solar cell will have several competitors, Beard says. For example, non-crystalline silicon is cheaper to make than crystalline silicon, and the efficiency of the resulting cells is improving. Researchers are also finding ways to use less expensive grades of crystalline silicon, and large-scale production has decreased the overall cost of producing such cells, making it difficult for new solar materials to gain a foothold.

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