PVMirror Design Enables Higher-Efficiency Tandem Solar Cells and Hybrid Power Generation Options

Achievement date: 
2016
Outcome/accomplishment: 

Researchers at the NSF-funded Quantum Energy and Sustainable Solar Technologies (QESST) Engineering Research Center (ERC) at Arizona State University (ASU), in collaboration with the University of Arizona, introduced a PVMirror tandem solar cell design that promises higher module efficiencies and unprecedented flexibility in coupling photovoltaic (PV) cells of different technologies.

Impact/benefits: 

Module efficiency drives the phovoltaics market, but the silicon cell designs that comprise 90 percent of the available industry are approaching their theoretical single-junction efficiency limit. Tandem solar cells that use silicon as a bottom cell are an attractive route to further increase efficiency; but top-cell materials have not historically integrated well with silicon. QESST’s PVMirror design allows any top cell to be paired with silicon with no modification. Using this design, a 30 percent increase in module efficiency with installed system costs below 0.65 dollars per Watt ($/W) are within reach.

Explanation/Background: 

The heart of the tandem is a PVMirror, which is a curved photovoltaic module that absorbs a portion of the solar spectrum and specularly reflects the remainder of the spectrum. Because of the module’s curvature, the reflected light arrives at a common line or point focus. A photovoltaic receiver (or other solar energy converter) that is complementary to the first may be placed at this point. Modeling of a hypothetical silicon PVMirror/II-VI receiver tandem indicates that a thermal receiver may be used in place of a photovoltaic receiver, allowing photovoltaics and concentrated solar thermal power to be hybridized to generate dispatchable electricity with high efficiency.

As part of an Advanced Research Projects Agency-Energy (ARPA-E) Full-Spectrum Optimized Conversion and Utilization of Sunlight (FOCUS) associated project, QESST researchers fabricated the first PVMirrors using silicon heterojunction solar cells. The prototypes demonstrated that greater than 30 percent of the light transmitted to the photovoltaic cells is converted to electricity and greater than 55 percent of the incident light is reflected to the receiver at the focus.

QESST’s tandem concept was first published in the IEEE Journal of Photovoltaics and was the most downloaded article in the journal in February 2016. QESST researcher Zhengshan (Jason) Yu also won a Best Poster Award at the IEEE Photovoltaic Specialists Conference for his contribution on PVMirrors.