2002 R&D 100 Award Winner
PowerView™ Semi-Transparent Photovoltaic Module
Developers: Harrin Ullal, Ken Zweibel, and Bolko von Roedern, National Renewable Energy Laboratory; Robert S. Oswald and Frank Liu, BP Solar
The PowerView™ module — a BP Solar commercial product — represents the coming of a new era in photovoltaics for buildings. Because it is semi-transparent, it can be used in lieu of architectural glass for many applications, particularly for those that call for sloped glazing, such as awnings, canopies, or slanted roofs. And because it is photovoltaics, the module uses sunlight to generate clean electricity to power a building's electrical needs. As testimony to the PowerView's utility BP Solar has already installed it on canopies of hundreds of its Connect stores — which are combination gasoline stations and multipurpose retail outlets. The canopies generate a significant portion of the electricity required by the stores.
One of the innovations that enables the PowerView™ to act both as architectural glass and a relatively efficient photovoltaic module is BP Solar's unique laser ablation. This is a manufacturing method that employs lasers to selectively remove material from the PV module to allow light through while minimizing the resultant decrease in electrical power. The laser-ablation technique is also programmable, providing the capability to change the density of the ablation or to produce desired patterns without the use of new masks. In this way patterns can be customized to fit customer needs and the amount of light transmitted can be adjusted to as much as 20% or more.
Yet, even at high values of transmitted light the power output remains stable and relatively high. This is because the PowerView™ module employs a two-junction structure with an amorphous silicon device grown on top of an amorphous silicon germanium device (a-Si/a-SiGe). Using germanium in the bottom device lowers the band gap of that device to approximately 1.4 eV. This gives the module a high-band-gap layer (about 1.7 eV) for converting high-energy photons to electricity and a lower-band-gap layer for converting mid- to low-energy photons to electricity. The result, compared to competitors, is higher stable efficiency.