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A comparison of the fabrication process for standard silicon solar cells (left) with that of black silicon solar cells (right) developed at Fraunhofer HHI. © hhi.fraunhofer.de
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:: New technology doubles efficiency of black silicon solar cells

Researchers at the Fraunhofer Heinrich Hertz Institute have succeeded in using their femtosecond laser pulse technology to double the efficiency of black silicon solar cells. Black silicon solar cells make it possible to harness the infrared spectrum in sunlight for power generation. The infrared part makes up around one third of the spectrum and cannot be harnessed by conventional types of solar cell.

The new technology achieves this high level of efficiency by modification of the raw silicon material and at the same time halves the number of processing steps needed. The research was conducted by the Fraunhofer Heinrich Hertz Institute at the Energie Campus Goslar in the group headed by Prof. Wolfgang Schade. The previous 2.2 percent record of efficiency was held by the team of Prof. E. Mazur at Havard University.

 

With the laser technique developed by Fraunhofer HHI the surfaces of the silicon wafers from which solar cells are manufactured are exposed to ultra-short laser flashes or femtosecond shaped laser pulses. This modifies the surface of the cells, enabling them to absorb the previously unexploited infrared portion of the sun spectrum for power generation. The laser-treated surfaces turn black which is why we speak of 'black silicon' and 'black silicon cells'. Researchers at Fraunhofer HHI have modified the shape of the laser pulses which means they can now produce black silicon solar cells that are twice as effective as before.

 

Developed by Fraunhofer HHI at Goslar, the novel features of this process are that the front side texture and emitter are produced in one single processing step while it also modifies the raw silicon material enabling it to absorb the infrared portion of the sun's spectrum. In addition, the new process also cuts the number of solar cell fabrication stages by approximately half and thus also slashes production costs.

 

Solar cells produced by this process display high current densities in the range of 38mA/cm² < Jsc < 42mA/cm² due to their increased absorption of infrared light. The new record for efficiency was set by the two year research alliance project funded by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU). Researchers see an absolute efficiency increase of 1 percent as a realistic potential for black silicon cells compared to standard silicon cells with a potential of approx. 15 percent.

 

"We think that this technology has a tremendous potential and the first exploratory talks with the German photovoltaic industry have been extremely encouraging," says Professor Dr. Wolfgang Schade, head of the Fraunhofer project group in Goslar. "To achieve even higher solar cell efficiencies, in the next stage we will deepen our understanding of what can be achieved with black silicon."

 

The new developments in Goslar are based on femtosecond laser technology which produces incredibly short femtosecond pulses equal to one millionth of a billionth of a second. This laser technology can be used for pure black silicon solar cells but can equally be applied to obtain single-side solar cell textures or surface area magnification for increasing the mechanical adhesive properties of specific solar cells coatings or contact material.

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