Spurred on by a DARPA mission several years ago, a number of materials labs have jumped into the solar race. As it stands now, the Univ. of Delaware claims the top mark: 42.8% efficiency for a third-generation multijunction solar cell its team built in 2007. Earlier this year, the Fraunhofer Institute for Solar Energy Research claimed 41.1% and in 2008 before that the National Renewable Laboratory (NREL) claimed 40.8% (using a technology that earned the lab an R&D 100 Award in 2007).
These impressive results seemed to herald the coming solar revolution. Such efficiencies, some of us believed, would justify the cost of making and buying expensive cells from silicon. But the reality is that these cells remain too expensive for profitable production. While first-generation crystalline silicon cells still rule the roost, more and more attention is being paid to thin films. Indeed, even as labs like NREL chase multijunction records, they are working with outside companies to bring lower cost thin-film technologies to the marketplace. In 2008, for example, HelioVolt won an R&D 100 Award for its cadmium indium gallium selenide cells that combined low cost with the possibility of integration into construction materials.
In an MIT Technology Review article today, start-up Solexant shared its market plan for their low-cost, moderate efficiency solar cells. Most existing thin-film cells rely on combinations of fancy semiconducting metals such as indium, gallium, or cadmium. Solexant, using technology from Berkeley Lab, was able to rely on an ink mixture of nano-sized semiconducting rods. The key to making it work, they say, was to prevent electrons from getting trapped between the rods, and that’s what allows their cells to achieve 10% efficiency at very low cost. By printing roll-to-roll on a non-glass substrate, Solexant has lowered its production cost to 50 cents per watt as compared to 85 per watt for vacuum-deposition thin-film processes. It’s not just the technology—the estimated cost for setting up a 100-MW plant to make these cells is a fraction of what is normally required.
The accomplishments of Solexant impressed R&D Magazine’s R&D 100 Awards judges, who gave Berkeley and Solexant an award this year for Nanocrystal Solar Cells. At a real-world 10% conversion of photons-to-electrons, efficiency has apparently bowed to cost, but that doesn’t mean the efficiency race will stop: NREL also claims the record efficiency for cadmium indium gallium selenide (CIGS) cells at nearly 20% and undoubtedly we’ll see someone approach 25% before long.