Panasonic Corp. developed a class of silicon solar cells that convert 25.6 per cent of solar energy into electricity, 0.6 points above the long-standing record of 25 per cent made by the University of South Wales in 1999.

The conversion efficiency record was achieved with the company's HIT photovoltaic module with a cell area of 143.7cm2. This tops the company's previous achievement of 24.7 per cent conversion efficiency with crystalline silicon-based solar cells in February 2013.

The HIT solar cells build on Panasonic's proprietary heterojunction technology, which involves covering the silicon base surface with an amorphous silicon layer. It facilitates superior passivation to compensate for the many flaws around the silicon base surface area.

In addition, these cells adopt a back-contact solar cell structure, with the electrodes on the back of the solar cell, allowing more efficient utilisation of sunlight.

1.Reduction in recombination loss
A key feature of HIT technology is its 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 of -0.25 per cent per degree Celsius, which is able to maintain a high conversion efficiency even with high open circuit voltage and at high temperatures.

2.Reduction in optical loss
In order 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—the layer which generates the power with less loss. Placing the electrodes on the reverse as back contacts allows the light to reach the substrate more efficiently. This has led to a marked improvement in short circuit current density to 41.8mA/cm over Panasonic's previous figure of 39.5mA/cm2 (in the case of a cell with a conversion efficiency of 24.7 per cent).

3.Minimising resistance loss
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 of 0.827, has been achieved, even at a practical cell size by improving resistance loss in the amorphous silicon layer.

Panasonic said it will continue to pursue technology development of its HIT solar cells, aimed at realising higher efficiency, lower costs and the more efficient use of resources, and will work towards mass production.

By: DocMemory
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