Solar ♦ news digest


Semiconductors with wide band gaps respond to shorter wavelengths with higher energies (lower left). A semiconductor with an intermediate band has multiple band gaps and can respond to a range of energies (lower right).


Now Wladek Walukiewicz, who leads the Solar Energy Materials Research Group at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and his colleagues have demonstrated a solar cell which responds to virtually the entire solar spectrum. This cell is different as it can be made using one of the semiconductor industry’s most common manufacturing techniques.


“Since no one material is sensitive to all wavelengths, the underlying principle of a successful full-spectrum solar cell is to combine different semiconductors with different energy gaps,” says Walukiewicz.


One way to combine different band gaps is to stack layers of different semiconductors and wire them in series. This is the principle of current high-efficiency solar cell technology that uses three different semiconductor alloys with different energy gaps.


In 2002, Walukiewicz and Kin Man Yu of Berkeley Lab’s Materials Sciences Division (MSD) found that by adjusting the amounts of indium and gallium in the same alloy, indium gallium nitride (InGaN), each different mixture in effect became a different kind of semiconductor that responded to different wavelengths. By stacking several of the crystalline layers, all closely matched but with different indium content, they made a photovoltaic device that was sensitive to the full solar spectrum.


Kin Man Yu and Wladek Walukiewicz have long been leaders in multiband solar cell technology


However, says Walukiewicz, “Even when the different layers are well matched, these structures are still complex and so is the process of manufacturing them. Another way to make a full- spectrum cell is to make a single alloy with more than one band gap.”


In 2004 Walukiewicz and Yu made an alloy of highly mismatched semiconductors based on a common alloy of zinc zinc (plus manganese) and tellurium. By doping this alloy with oxygen, they added a third distinct energy band between the existing two, thus creating three different band gaps that spanned the solar spectrum. Unfortunately, says Walukiewicz, “to manufacture this alloy is complex and time-consuming, and these solar cells are also expensive to produce in quantity.”


The new solar cell material developed by Berkeley Lab’s MSD, RoseStreet Labs Energy and Sumika Electronics Materials is another multiband semiconductor made from a highly mismatched alloy.


In this case the alloy is gallium arsenide nitride, similar in composition to one of the most familiar semiconductors, gallium arsenide. By replacing some of the arsenic atoms with nitrogen, a third, intermediate energy band is created. The good news is that the alloy can be made by metalorganic chemical vapour deposition (MOCVD), one of the most common methods of fabricating compound semiconductors.


Band gaps arise because semiconductors are insulators at a temperature of absolute zero but inch closer to conductivity as they warm up. To conduct electricity, some of the electrons normally bound to atoms (those in the valence band) must gain enough energy to flow freely – that is, move into the conduction band. The band gap is the energy needed to do this.


When an electron moves into the conduction band it leaves behind a “hole” in the valence band, which also carries charge, just as the electrons in the conduction band; holes are positive instead of negative.


A large band gap means high energy, and thus a January / February 2011 www.compoundsemiconductor.net 121


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