Germanium boosts thermophotovoltaic efficiency

Whereas photovoltaic cells directly convert light to electricity, thermophotovoltaic cells rely on light to drive a heat absorber and then convert the heat to electrical energy. The Belgian-based IMEC (Interuniversity Microelectronics Center) lab has developed a new germanium-based process that offers better efficiency than silicon-based thermophotovoltaic cells. Generally, thermophotovoltaic cells benefit from using a material with a lower bandgap than silicon. Germanium meets the band gap requirement but presents a challenge in surface passivation. IMEC worked with germanium-wafer vendor Umicore (www.umicore.com) to attack the passivation problem because germanium is lower in cost than alternative low-bandgap materials.

The new process forms the shallow emitter of the germanium cell using diffusion from a spin-on oxide. Meanwhile, a thin, plasma-deposited, amorphous silicon layer solves the passivation problem. The result is a cell with an open-circuit voltage higher than 270mV, an airmass-1.5-spectra standard efficiency of nearly 8%, and a broad spectral response of 400 to 1700nm. IMEC claims that the performance significantly exceeds that of cells under the given illumination conditions.

IMEC hopes to move quickly in realizing production of germanium-based cells. The manufacturing process is largely compatible with the organization’s solar-cell process, but the cells require refining for stability for use with higher intensity light.

IMEC, www.imec.be