Air-bridge thermophotovoltaic cell with 30% efficiency, 99% reflectance

Researchers from the University of Michigan have demonstrated a thermophotovoltaic (TPV) cell that could be paired with inexpensive thermal storage to provide power on demand.

“The thermal storage can be charged using renewable electricity or high-temperature solar heat,” researcher Andrej Lenert told pv magazine. “As such, the cells are of interest to utilities and grid-scale energy storage developers.”

He said the cells could also be used in a range of other applications, including powering unmanned aerial vehicles and deep-space probes, scavenging electricity from waste heat streams, and supplying decentralized heat and electricity. He and his colleagues presented their findings in “Near-perfect photon utilization in an air-bridge thermophotovoltaic cell,” which was recently published in Nature.

They describe the cell as an air-bridge indium gallium arsenide (InGaAs) TPV cell that can absorb most of the in-band radiation to generate electricity. It can also serve as a nearly perfect mirror, with almost 99% reflectance.

“This high reflectance enables a TPV power conversion efficiency exceeding 30% using a 1,455-Kelvin silicon carbide (SiC) emitter,” the scientists said.

This photovoltaic cell converts thermal radiation into electricity. Typically, such cells have a reflectance of around 95% and efficiencies slightly higher than 20%. The operating temperatures of most thermal emitters used in TPV devices commonly range from 1,000 K to 2,500 K, and that is the main barrier to their development. Many out-of-band (OOB) photons carry energy below the semiconductor bandgap and cannot be used for electricity production.

However, lower energy photons can be “recycled” and brought back to the emitter by using spectral control, which enables the recovery of unconverted energy. An air bridge is created in the cell by replacing the dielectric spacer to the back metal mirror with air from within the thin-film cell. Parasitic absorption in the dielectric material is therefore eliminated and the refractive index mismatch at each interface is maximized, the researchers claimed.

The scientists claimed the air cavity reduces OOB losses by more than four times compared to conventional TPV cell architectures. Many incident photons encounter only the TPV-air interface when they penetrate the device active layers.

“The thickness of the air layer had to be very precise – within a few nanometers – to reflect the lower energy photons,” the scientists said. “The semiconductor film is only 1.5 micrometers (.0015 millimeters) thick, yet it needed to span over 70 micrometers of air between the 8-micrometer-wide gold beams.”

To make the mirror, the research group coated a silicon substrate with gold and cold-welded the gold beams to the gold backing.

“This way, the thickness of the gold beams could accurately control the height of the air-bridge, enabling the near-perfect mirroring,” they said.

The cell has a nearly perfect spectral utilization, which makes it almost insensitive to increases in cell bandgap or decreases in emitter temperature, as the reflectance approaches 100%. This spectral efficiency could make the use of low-cost materials such as silicon possible, the scientists said. They added that a 99.9% reflectance and near-perfect photon utilization are a possible achievement in future research.

“The particular high-power application space occupied by TPVs brings the economics into feasible alignment with practical deployment in systems,” Lenert concluded. “The air bridge technology is compatible with wafer reuse approaches that can drive the cost down further.”