Fraunhofer researchers at the Heinrich Hertz Institute (HHI) have considerably increased the efficiency of silicon solar cells using laser technology. The modification of the surface enables the previously unused infrared portion to be harnessed for generating energy. The surfaces treated with lasers turn black, which is why scientists refer to "black silicon" or "black silicon solar cells".

To generate energy from sunlight, a conventional silicon solar cell uses the visible portion of the spectral range. However, a third of the total sunlight consists of infrared radiation. New types of solar cells known as black silicon solar cells are able to harness the infrared spectrum of sunlight and thus generate more energy. The previously greatest efficiency for pure black silicon solar cells was achieved by researchers at Harvard University with 2.2 per cent. Using a special laser process that modifies the raw silicon material, Fraunhofer HHI has now been able to double the capacity of the black silicon solar cells to 4.5 per cent.

The surfaces of the silicon wafers are treated using ultra-short laser flashes known as femtosecond laser pulses. The modified surface enables the cells to absorb the previously unused infrared portion and generate energy. The surfaces treated with lasers turn black, which is why scientists refer to "black silicon" or "black silicon solar cells". The modified surface enables the cells to absorb the previously unused infrared portion and generate energy.

Potential of black cells

The researchers believe that black silicon solar cells can potentially achieve a one per cent increase in the absolute efficiency when compared with current standard solar cells. Their efficiency is around 15 per cent and they exclusively use the visible portion of the spectral range. “Following the laser treatment of the surface, hardly any light is reflected away but is instead reflected back and forth by the textured, needle-shaped surface and is thus absorbed by this means into the silicon. It is only here in the silicon that the cells can convert the light into charge carriers,” says Stefan Kontermann, head of the "Nanomaterials for Energy Conversion" working group at Fraunhofer HHI, in explaining the process. In addition, sulphur is integrated into the silicon surface. That increases the absorption of light in the infrared portion of the solar spectrum, i.e. above 1100 nanometres. The new black silicon solar cells use the incident light not just more efficiently but are even more cost-effective in the production: only half as many production stages are required compared with standard solar cells.

Use in the dark

Black silicon solar cells cannot only be used like conventional solar cells but can also be used to absorb infrared light in the dark, such as for example from heat. This application is also conceivable in the photovoltaic industry. Fraunhofer HHI is currently working on an industrial-scale project to supplement conventional solar cells with black silicon solar cells on their rear side. “We believe that there is potential to utilise the additional 1 per cent provided by the unused infrared radiation,” explains Stefan Kontermann.

The joint project entitled “Innovative photovoltaics with black silicon and black zinc oxide” has been funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) in the amount of around 850,000 euros.

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