Microsystem-enabled photovoltaic cells
Solar energy is the only renewable-energy source that could supplant fossil fuels. However, despite recent cost reductions, using solar cells to generate electricity is two to four times more expensive than use of fossil fuels. Electricity-generating systems based on solar cells need to be improved to compete with grid power. In particular, cost reductions have to be applied to the overall systems, assembly of photovoltaics (PV) modules, and manufacturing of solar cells.
There are three traditional approaches to solar PV fabrication. The dominant technology uses crystalline-silicon wafers to make solar cells that are 180 to 200μm thick and approximately six inches across. They are then assembled, electrically connected, and encapsulated to make the PV modules that comprise around 80% of the current PV market. An alternative approach employs thin-film PV technology, which uses materials such as cadmium telluride, copper indium gallium diselenide, and amorphous silicon. Solar cells made with this latter technology are less efficient, cheaper, and easier to manufacture than crystalline-silicon devices. Finally, concentrated PV technologies employ lenses or mirrors to focus sunlight onto highly efficient solar cells. They use concentration ratios of up to 1000×, which reduces costs by reducing the amount of semiconductor material used. Overall, however, solar systems that use concentrated PV technologies are more expensive than their thin-film and crystalline-silicon counterparts. The accurate solar trackers, required to keep PV modules pointed toward the sun, significantly increase costs. We are using microsystems technology to explore a new approach to solar-power generation that takes advantage of traditional PV areas, while simultaneously eliminating many of the associated challenges.
Successful microsystems applications enable desirable microscale properties that could not be achieved using macroscale approaches. A classic example are integrated circuits, which offer improved performance, new functionalities, and reduced costs compared to macroscale circuits assembled with discrete components. We looked for scaling effects in solar cells, modules, and systems through development of microsystem-enabled PV cells.1–3 The solar cells we used (see Figure 1) were made of reduced amounts of materials known to provide high-efficiency PV cells—such as crystalline silicon and gallium arsenide (GaAs)—and their small scale allowed us to try development of affordable methods for optical concentration.4
We developed crystalline-silicon solar cells that are 14 to 20μm thick and 250μ across. These cells were created using standard-thickness silicon wafers that were either (111) oriented or silicon-on-insulator wafers. Both approaches allow processing of solar cells on the wafers: it is possible to create junctions and contacts, make metal traces and bond pads, and deposit passivating layers. The cells can then be released from the wafer, leaving its bulk free for further solar-cell processing. These cells are ten times thinner than those currently used in crystalline-silicon modules and have efficiencies of up to 14.9%.
We are also developing crystalline GaAs single-junction solar cells. Since GaAs is a direct-bandgap material, these cells can be even thinner than their silicon counterparts, and we have made cells with lateral dimensions of 250μm and as thin as two micrometers. These structures are unique among III-V-type cells. They have a complete back contact and no metal traces in their optical apertures. Front-side metal traces can lead to 6–10% optical loss in traditional III-V solar cells, but in these GaAs samples we managed to achieve performances of around 11.3%.
We are optimizing the performance of both crystalline-silicon and GaAs solar cells, and anticipate achieving efficiencies in excess of 20% for both technologies. We are also working to create prototype modules that use these microscale cells as building blocks, and tests will hopefully demonstrate some of the scaling benefits that we have identified in solar PV. These benefits will allow improved performance, new functionalities, and reduced costs at the cell, module, and system levels. If successful, we believe that this technology will lead to significantly reduced costs for solar-power generation, with the ultimate goal of making it the cheapest energy source available.
