Striking up the intermediate band

In the world of solar photovolatics, today's high performance multijunction cell is the undisputed rock star. And where there are rock stars, contenders lurk in the wings.

The leading advocate of a promising new high performance solar cell technology, Dr. Antonio Luque of the Polytechnic University of Madrid, today introduced his "intermediate band" cell design to an audience of largely polo-shirted researchers in a lecture at Xerox's fabled Palo Alto Research Center (PARC) in Palo Alto, California. [ed.: as a matter of fact, we only counted one suit in the room.]

While still largely theoretical, Luque's intermediate band design could promise efficiencies even higher than today's highest performing multijunction cells.

Multijunction cells are made of layers of materials far more expensive than the silicon conventional cells are made of, but they deliver efficiencies about double those of everyday solar panels.

As such, they're only economically viable in military applications, where cost is not an issue, and high concentration applications in which their expense can be justified by converting large volumes of sunlight.

Late last year, Boeing's Spectrolab introduced a 40.7% efficient cell (see Spectrolab solar cell breaks 40% efficiency barrier), but that's close to the theoretical limit of multijunction cells as architected today, according to Luque.

Multijunction cells are complex; the three layers of cells used to harvest electricity from different bands of the spectrum are actually made up of a total of twenty layers of material.

To make the jump from 40 percent efficiency to a potential upper limit of 45 percent projected in 2009, scientists' roadmaps call for potentially adding a fourth cell, complicating things further.

By contrast, Luque's design calls for essentially three layers of materials: a conventional "n+ emitter" layer, a "intermediate band material" and a "p+ emitter layer." When exposed to high amounts of concentrated sunlight (the design is inefficient unless used with multi-sun concentration), the cell harvests photons from across the spectrum with only a minimal compromise in today's tradeoff between current and voltage.

"Most semiconductor devices split into two Fermi levels. We are using three Fermi levels, including one for the intermediate, and this seperation is the secret to how we can get at the same time high voltage and high power."

Luque thinks he can get to 63.2 percent theoretical efficiency in his design, using only three layers.

The chief hurdle, however, is that material for one of those layers, the critical intermediate band middle, hasn't been fully thought out yet. Luque's prototypes to date have used quantum dot material, and he has been looking at gallium arsenide, indium arsenide and other more exotic compounds.

Luque acknowledged he and his researchers had some false starts and got results opposite to what they originally intended. They were initially unsure how to measure whether their design was even working, but are now confident in it.

"We illuminated with lights of different frequencies, and then testing their response to the different frequencies, and then chopped at different frequencies. Then we were able to prove clearly that we were having the process we were looking for."

While promising revolutionary levels of performance, Luque cautioned the crowd that the design will only be applicable in highly concentrated sunlight applications. But that's the direction utility-grade solar power is headed, he affirmed.

"My vision is that in the future, most solar electricity will be produced with systems of very, very high efficiency. We'll be forced to used concentrators, because this will be the only way of making these sophisticated and expensive concepts economical. Concentration is the secret to doing it cheaper."

Luque is so passionate about concentration as the future of solar that he convinced the Spanish government to create and fund ISFOC, the Institute of Concentration Photovoltaic Systems. The institute recently issued a call for tenders to concentrating photovoltaic systems vendors, with a view to buying test systems to help them accelerate their paths to commercialization.

Contracts have been awarded to SolFocus of the U.S., Isofotón of Spain and Concentrix of Germany.

Luque, who himself proudly sported a SolFocus polo shirt today, has been Full Professor of electronic technology at the Polytechnic University of Madrid since 1970, and has led the Instituto de Energía Solar since founding it in 1979. He invented the bifacial solar cell in 1976, brought to market by Isofotón, a company he created in 1982, today present in more than 50 countries.