Anew scalable process for producing ultrahigh-efficiency photovoltaic cells could pave the way for low-cost, utility-scale solar power generation.
"The strategy involves highspeed, printing-based manipulation of thin, microscale solar cells and new interface materials to bond them into multilayer stacks," says
Photovoltaics absorb a specific wavelength of light depending on the type of semiconductors used in the cells. This means that some of the incident light passes through the solar cell without being captured and converted into electricity, limiting the potential efficiency of the device. Conventional silicon-based solar cells, for instance, are unable to absorb the near-infrared part of the solar spectrum, which makes up about 40% of the sunlight that reaches the Earth.
One way to improve the efficiency of photovoltaic systems is to combine multiple cells into one device. Each cell absorbs light of a different wavelength, and together they can capture and convert more of the sunlight than a single-junction cell could. Over the past decade, progress in this area has led to multijunction cells with efficiencies reaching about 44%. Further improvements, however, will require solutions to daunting challenges, the UIUC engineers say.
Mechanical stacking of separately grown single-junction and multijunction materials offers an alternative route to high efficiencies. Key to the success of this technique is a method for bonding the wafers together. Two bonding methods are available: direct, high-temperature wafer fusion techniques, and the use of thick, insulating organic adhesives.
"Despite research and development during the past 25 years, neither of these bonding strategies at present offers a realistic means for manufacturing or for viable multiple stacking operations," Rogers says. "A few simple ideas in materials science and device assembly allow us to bypass many of the limitations of these and other previously explored techniques."
The new process involves epitaxially growing multijunction (and single-junction) solar cells independent of one another, and then using a stamping-and-printing process previously developed by Rogers and his colleagues to layer the cells into stacks. The process allows for the simultaneous fabrication of thousands of stacked solar cells.
The UIUC team used this process to make a quadruple-junction, fourterminal microscale solar cell that consists of a triple-junction cell stacked on top of a single-junction cell with a layer of arsenic triselenide (As2Se3) between them. The triple-junction cell consists of three p-n junctions: indium gallium phosphide (InGaP), gallium arsenide (GaAs), and indium gallium arsenide antimonide nitride (InGaAsNSb). The single-junction cell is made of germanium (Ge).
To eliminate some of the limitations of other multijunction devices, the two cells need to work together but also independently from one another. This requires an interface layer that lets the light not absorbed by the top cell to pass through unimpeded and without loss. Thus, the layer must be optically transparent and have high thermal conductivity, high electrical resistivity, and a refractive index close to that of the two solar materials at the interface.
In the stacked triple-junction/Ge assembly, the top triple-junction cell captures light with wavelengths between 300 nm and 1,300 nm, while light in the range of 1,300-1,700 nm passes through to the bottom cell, where it is absorbed. Light absorbed by both cells is converted into electricity.
"Our specific results already are within a fraction of a percent of world records at the cell level and, in practical modules, our devices outperform anything that has been previously reported," Rogers says. "We are now using these same ideas with even better cells, and more cells, to further improve the efficiency," he says.
"We are also working on advanced anti-reflection coatings and enhanced focusing optics to achieve better efficiencies at the module level." ESI
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