sábado, 24 de julio de 2010


The most recent InGaP/GaAs/Ge solar cell design uses a C-doped AlGaAs/ Si-doped InGaP heterostructure tunnel junction with DH AlInP barriers, described previously. A schematic of the device is shown in figure 1 together with light-IV and  quantum efficiency results from a 1cm2 device. Such solar cells have achieved efficiencies of 31.7% (1cm x 1cm) and 31.2% (5cm x 5cm) under 1 sun AM1.5G and were fabricated by Japan Energy Co. [8]. Under AM0, a 29.2% efficient 2x2cm2 device  has been demonstrated; the light-IV.
Other notable cells include:

– InGaP/GaAs/InGaAs mechanical stack that achieved a world record 33.3% efficiency under AM1.5G, following joint work by  Japan Energy Co., Sumitomo Electric Co. and Toyota Tech. Inst.
– AlGaAs/GaAs monolithic tandem cell with a 1 sun AM1.5 efficiency of 27.6% by Hitachi Cable Co.

– GaAs/GaInAsP mechanical stack tandem cell with a 1 sun AM1.5 efficiency of 31.1% by Sumitomo Electric Co.

– InGaP/GaAs/Ge concentrator cell with an efficiency in excess of 36% for concentrations from 100-500 suns; the increase in efficiency under concentration.
Due to the series connection in the multi-junction solar cell, the radiation resistance of the cell as a whole will be dominated by the worst performing layer.
The effect of radiation damage is first a reduction in the minority carrier diffusion length followed by majority carrier removal under heavy irradiation. It is clear that the InGaP is reasonably radiation hard, maintaining a high Isc, while the Ge and GaAs junctions fall quite quickly. However, as the Ge junction is current rich, its effect on the overall multi-junction cell radiation performance is small. It is then the degradation of the GaAs junction that dominates the degradation of the multi-junction cell under irradiation. Various schemes such as thinning the emitter and base layers and field assisted collection can be employed to improve the radiation response of each sub-cell in a multi-junction device. When considering the multi-junction cell as a whole, the standard means for maintaining high efficiency is to design the solar cell such that the GaAs junction is current rich at the beginning of life (BOL) and becomes current matched to the InGaP at the end of life (EOL), as shown in figure 6. Nevertheless, this compromises the BOL efficiency, so it is desirable to replace the GaAs junction with a more radiation resistant material, such as InGaAsP.

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