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Panasonic breaks conversion efficiency record for solar cells

11 Apr 2014  | Paul Buckley

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Panasonic Corporation claims it has broken through the 25 per cent conversion efficiency barrier for practical size solar cells at the research level, showing 25.6 per cent conversion efficiency for an area of 143.7cm with the company's HIT solar cells.

The previous record for the conversion efficiency of crystalline silicon-based solar cells of a practical size (100cm and over) was 24.7 per cent, which was announced by Panasonic in February 2013 (cell area: 101.8 cm).

The new record is also an improvement of 0.6 points compared with the previous record for small area crystalline silicon-based solar cells (cell area: 4cm) of 25 per cent.

Panasonic claimed the achievement of the record was made possible by further development of the company's proprietary heterojunction technology to realise the high conversion efficiency and superior high temperature properties of the company's HIT solar cells, as well as adopting a back-contact solar cell structure, with the electrodes on the back of the solar cell, which allows the more efficient utilisation of sunlight.

A key feature of HIT technology is the ability to reduce the recombination loss of charge carriers, particles of electricity generated by light, through laminating layers of high-quality amorphous silicon on the surface of the monocrystalline silicon substrate, where power is generated. By utilising the technology to form a high-quality amorphous silicon film on the monocrystalline substrate while minimising damage to the surface of the substrate, it has been possible to realise a high temperature coefficient*9 of -0.25 per cent per degree Celsius*10 which is able to maintain a high conversion efficiency even with high open circuit voltage (Voc) and at high temperatures.

 Outline of the cell's core technologies

Outline of the cell's core technologies

To increase the current in a solar cell, it is necessary to lead the sunlight, which arrives at the cell's surface to the monocrystalline silicon substrate, which is the layer that generates the power with less loss. By placing the electrodes on the reverse as back contacts allows the light to reach the substrate more efficiently and has led to an improvement in short circuit current density (Jsc) to 41.8mA/cm compared with Panasonic's previous figure of 39.5mA/cm (in the case of a cell with a conversion efficiency of 24.7 per cent).

In solar cells, the generated electrical current is accumulated in the surface grid electrodes and output externally. Previously, the grid electrodes on the light-receiving side were optimised by balancing the thickness of the grid electrodes (thinning the grid electrodes to reduce the amount of light blocked) and the reduction of electrical resistance loss, but by placing the electrodes on the reverse side, it has become possible to reduce the resistive loss when the current is fed to the grid electrodes. In addition, a high fill factor (FF) of 0.827 has been achieved even at a practical cell size by improving resistance loss in the amorphous silicon layer.

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