Pinpointing solar-cell efficiency.

Pinpointing solar-cell efficiency

Guided by a detailed theoretical model, a team of Stanford University researchers has squeezed a record performance out of a novel solar cell. Their prototype "point-contact" silicon cell recently achieved a 27.5 percent efficiency in converting concentrated sunlight into electricity. This is the highest efficiency yet attained by any photovoltaic device.

"This work is a real benchmark in establishing what silicon technology can do," says Don Schueler, solar programs manager at the Sandia National Laboratories in Albuquerque, N.M. "This efficiency is much higher than what was believed to be the practical, achievable efficiency just a few years ago."

"It has come the closest in performance to what we feel needs to be achieved for photovoltaic cells used in utility systems," says Edgar DeMeo of the Electric Power Research Institute (EPRI) in Palo Alto, Calif., a utilities-sponsored research center that funded much of the Stanford work. "What encourages us is that . . . it really looks like the cell can be made using techniques that are well established within the electronics industry for making integrated circuits."

The idea is to use lenses to concentrate sunlight onto small photovoltaic cells specially designed to operate efficiently in high-intensity sunlight. The point-contact cell has several features that make it particularly efficient.

First, each single-crystal silicon chip, about one-fourth the size of a postage stamp and only 0.1 millimeter thick, has a "texturized" upper surface to spread out incoming light. The mirrorlike lower surface helps trap light within the material so that more can be absorbed.

Furthermore, all of the surfaces have a thin silicon dioxide layer except at the points where the current is conducted out of the cell. This layer reduces the chance of light-ejected electrons recombining with the "holes" left behind. Otherwise, less current is generated. In conventional solar cells, both the top and bottom surfaces must be coated with conducting materials, which tend to increase such losses.

In the point-contact cell, a polka-dot pattern of tiny doped-silicon regions is scattered across the silicon crystal's lower surface just above the silicon dioxide layer. Fine aluminum threads that penetrate the silicon dioxide layer collect the current from each of these points.

"All of this combines to give us a much higher current from this cell than from a conventional cell," says electrical engineer Richard Swanson, leader of the Stanford group.

The only experimental photovoltaic devices that now come close to the Stanford cell's efficiency are ones made from gallium arsenide. However, gallium arsenide is much more costly and difficult to process. Commercially available silicon solar-cell panels without concentrators rarely exceed an efficiency of 12 percent.

The Stanford researchers are now refining their design to improve their cell's performance to the 29 percent level that their calculations show is possible. "In addition," says Swanson, "we're going to be working on ways of mounting the cell."

Meanwhile, EPRI is putting together a program to see if this solar cell can be manufactured at a sufficiently low cost. "It looks good," says DeMeo, "but we've got to be sure that we're not dealing with just a laboratory curiosity. It'll be three or four years before we know what we've got." If the initial investigations work out, then utilities may start testing the use of these solar-cell arrays for large-scale power generation.

"The improvement of efficiency from the 22 to 24 percent region, where we were a number of months ago, up to 27.5 percent is very significant," says Schueler. "It certainly brings down the overall cost per watt of electricity produced. And we're probably not at the end of what can be achieved yet."