Chance PV Discovery Looks Promising

An unexpected discovery just might change the face of the solar photovoltaic industry.

Berkeley, California – November 26, 2002 [] Researchers in the Materials Sciences Division (MSD) of Lawrence Berkeley National Laboratory, working with crystal-growing teams at Cornell University and Japan’s Ritsumeikan University, discovered that the band gap of the semiconductor indium nitride is not 2 electron volts (2 eV) as previously thought, but instead is a much lower 0.7 eV. The serendipitous discovery means that a single system of alloys incorporating indium, gallium, and nitrogen can convert virtually the full spectrum of sunlight — from the near infrared to the far ultraviolet — to electrical current. “It’s as if nature designed this material on purpose to match the solar spectrum,” said MSD’s Wladek Walukiewicz, who led the collaborators in making the discovery. What began as a basic research question points to a potential practical application of great value. For if solar cells can be made with this alloy, they promise to be rugged, relatively inexpensive — and the most efficient ever created, said Walukiewicz. The real question now is whether or not solar cells can actually be made from the alloy. “It’s difficult to tell,” said Walukiewicz. “Cost is a very important factor, and the main question is whether improved efficiency would offset the cost. I think if history could be any guide, gallium nitride is used widely for industrial purposes and it seems to be a material that can be made cheap enough for mass applications.” Scientists at the National Renewable Energy Laboratory’s (NREL) National Center for Photovoltaics are skeptical at such an early stage. “It’s a very significant discovery to know that its .07 eV and not 2 eV but there’s a whole lot of research that has to be done,” said Sarah Kurtz from the National Center for Photovoltaics. “This is just in the preliminary stages and so far as I know people haven’t yet grown a solar cell with this alloy.” Scientists have pursued the possibility of converting the entire light spectrum for years. Today’s silicon solar cells can only convert a portion of the visible light spectrum thus losing out on the potential energy in the lower and upper tiers of the light spectrum. To do better, researchers and manufacturers have succeeded in stacking different band gap materials to create multi-junction cells. Each material converts a certain portion of the light spectrum and when stacked together in the right directions they can achieve higher efficiencies – as much as 34 percent. Multi-junction technology however, is complicated and expensive and therefore used primarily for specialized projects such as solar arrays for spacecraft and satellites. Finding a way to harness the entire light spectrum at a reasonable price would be a boon for the solar industry. Walukiewicz and his team made this accidental discovery while studying the properties of indium nitride as a component of Light Emiting Diodes (LEDs). “We didn’t expect this at all,” Walukiewicz said. “There were a lot of surprises” The first clue came when Walukiewicz and his colleagues were studying the opposite problem — not how semiconductors absorb light to create electrical power, but how they use electricity to emit light. At first glance, indium gallium nitride is not an obvious choice for solar cells. Its crystals are riddled with defects, hundreds of millions or even tens of billions per square centimeter. Ordinarily, defects ruin the optical properties of a semiconductor, trapping charge carriers and dissipating their energy as heat, Walukiewicz said. In studying LEDs, however, the Berkeley Lab researchers found that the way indium joins with gallium in the alloy leaves indium-rich concentrations thatemit light efficiently. Such defect-tolerance in LEDs holds out hope for similar performance in solar cells. To exploit the alloy’s near-perfect correspondence to the spectrum of sunlight will require a multi-junction cell with layers of different composition. Walukiewicz said adjusting the right proportions of indium, gallium and nitride should be able to convert the full spectrum of sunlight to electricity to make multi-junction solar cells that could exceed 70 percent theoretical efficiency. And how much might this all cost? “If it works, the cost should be on the same order of magnitude as traffic lights,” Walukiewicz said. “Maybe less. Solar cells so efficient and so relatively cheap could revolutionize the use of solar power not just in space but here on Earth.” Despite their excitement, both Walukiewicz and Kurtz professed there remain many questions to be answered before it becomes clear if this discovery will lead to a dramatic breakthrough for the solar industry. “At this stage, I’m quite hesitant,” Walukiewicz said. “We’ll have to wait and see where this goes.”