IMEC Makes Breakthrough on Ge Cells for Thermophotovoltaic Applications

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.”

About IMEC
IMEC is a world-leading independent research center in nanoelectronics and nanotechnology. Its research focuses on the next generations of chips and systems, and on the enabling technologies for ambient intelligence. Besides its mission as a nanotechnology center, IMEC has always been at the forefront of applying its expertise in semiconductor device physics and processing to the development of photovoltaic devices and power electronics. This dedication to photovoltaics makes IMEC one of the leading European centres in this domain.

IMEC’s research bridges the gap between fundamental research at universities and technology development in industry. Its unique balance of processing and system know-how, intellectual property portfolio, state-of-the-art infrastructure and its strong network of companies, universities and research institutes worldwide position IMEC as a key partner for shaping technologies for future systems.

As an expansion of its wireless autonomous microsystems research, IMEC has created a legal entity in the Netherlands. Stichting IMEC Nederland runs activities at the Holst Centre, an independent R&D institute that develops generic technologies and technology platforms for autonomous wireless transducer solutions and systems-in-foil.

IMEC is headquartered in Leuven, Belgium, and has representatives in the US, China and Japan. Its staff of about 1400 people includes close to 500 industrial residents and guest researchers. In 2005, its revenues are estimated to be close to EUR 200 million. Further information on IMEC can be found at