news digest ♦ Solar


wide-band-gap material responds only to the more energetic segments of the solar spectrum, such as ultraviolet light. By introducing a third band, intermediate between the valence band and the conduction band, the same basic semiconductor can respond to lower and middle-energy wavelengths as well.


This is because, in a multiband semiconductor, there is a narrow band gap that responds to low energies between the valence band and the intermediate band. Between the intermediate band and the conduction band is another relatively narrow band gap, one that responds to intermediate energies. Finally, the original wide band gap is still there to take care of high energies.


“The major issue in creating a full-spectrum solar cell is finding the right material,” says Kin Man Yu. “The challenge is to balance the proper composition with the proper doping.”


In solar cells made of some highly mismatched alloys, a third band of electronic states can be created inside the band gap of the host material by replacing atoms of one component with a small amount of oxygen or nitrogen. In II-VI semiconductors (which combine elements from these two groups of Mendeleev’s original periodic table), replacing some group VI atoms with oxygen produces an intermediate band whose width and location can be controlled by varying the amount of oxygen. Walukiewicz and Yu’s original multiband solar cell was a II-VI compound that replaced group VI tellurium atoms with oxygen atoms. Their current solar cell material is a III-V alloy. The intermediate third band is made by replacing some of the group V component’s atoms, in this case arsenic, with nitrogen atoms.


Finding the right combination of alloys, and determining the right doping levels to put an intermediate band right where it’s needed, is mostly based on theory, using the band anticrossing model developed at Berkeley Lab over the past 10 years.


“We knew that two-percent nitrogen ought to do the job,” says Yu. “We knew where the intermediate band ought to be and what to expect. The challenge was designing the actual device.”


Using their new multiband material as the core of a test cell, the researchers illuminated it with the full


122 www.compoundsemiconductor.net January / February 2011


spectrum of sunlight to measure how much current was produced by different colours of light. The key to making a multiband cell work is to make sure the intermediate band is isolated from the contacts where current is collected.


“The intermediate band must absorb light, but it acts only as a stepping stone and must not be allowed to conduct charge, or else it basically shorts out the device,” Walukiewicz explains.


The test device had negatively doped semiconductor contacts on the substrate to collect electrons from the conduction band, and positively doped semiconductor contacts on the surface to collect holes from the valence band. Current from the intermediate band was blocked by additional layers on top and bottom. For comparison purposes, the researchers built a cell that was almost identical but not blocked at the bottom, allowing current to flow directly from the intermediate band to the substrate (see figure below).


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