Semicon Europe --- IMEC’s research breakthrough paves the way for thermophotovoltaic cells based on low-cost Ge bottom cells. Records in open-circuit voltage, AM1.5 efficiency and spectral response were achieved by combining improved surface passivation and novel contacting technologies.

Thermophotovoltaic cells, which are optimized to convert radiation from heat sources at lower temperature as the sun, require materials with a lower bandgap as silicon, which is commonly used for solar cells. Principally, germanium is suited because of its low bandgap but problems related to proper surface passivation were hindering the further development. The significantly lower cost of germanium as compared to the low-bandgap III-V alternatives incited IMEC to tackle the issue again in collaboration with Umicore, the leading manufacturer of germanium wafers.

IMEC combined improved surface passivation and novel contacting technologies, which led recently to Ge cells with an open-circuit voltage over 270mV, an AM1.5 efficiency near 8% and a broad spectral response from 400 to 1700nm. These values exceed significantly the figures reported under the given illumination conditions.

The shallow emitter of the Ge cell was formed by diffusion from a spin-on oxide whereas the improved surface passivation was based on a thin plasma-deposited a-Si:H layer. Before the plasma deposition, the Ge surface was treated with a dedicated ex-situ and in-situ cleaning method. The contacts were formed by diffusion of a metal through the a-Si:H layer ensuring sufficient selectivity to avoid shunting of the device.   

Jef Poortmans, director of the SOLAR+ program at IMEC comments:"The process does not only result in thermophotovoltaic cells with very promising performance, but it is also based on process steps which are to a large extent compatible with silicon solar cell processing.  In fact, the fast progress we made over the recent period resulted largely from the synergies with our silicon solar cell expertise. The next step in IMEC’s development aims at adapting the cell for higher intensities and checking its performance stability and eventually integrating it in a demonstration system."

For more information:

Katrien Marent
Corporate Communication Manager
IMEC, Kapeldreef 75
B- 3001 Leuven, Belgium
Tel +32 16 28 18 80 Fax +32 16 28 16 37
Email: Katrien.Marent@imec.be