Christoph Hammerschmidt Courtesy of EE Times Europe (10/12/2007 8:54 AM EST)

MUNICH, Germany — The Belgian research center IMEC has developed a thin-wafer silicon solar cell manufacturing process that yields an efficiency of 17.4 percent. This is not the limit, the researchers say. 20 percent efficiency should be "definitely within reach".

The i-PERC process uses silicon layers with a thickness of less than 180 micron, in which the classical aluminum back surface field which covers the whole rear of the solar cell is passivated by a dielectric stack. In this stack, the contact openings can be realized by laser ablation, the IMEC researchers explain. Subsequently, an Al contact layer is evaporated which is then fired in a belt-line furnace to create local back-surface fields. Further fine-tuning brings the efficiency to 17.4 percent. This was achieved on thin large-area multi-crystalline silicon substrates using screen printing to implement the contacts.

While the iPERC process is described as industrially applicable, the researchers have successfully proved that the efficiency can even be driven up further. In lab trials, an efficiency of 19.4 percent was achieved. The researchers now believe that the process can be used to manufacture solar cells with an efficiency of more than 20 percent.

In related experiments, IMEC researchers also approached the theoretical efficiency limits of Germanium cells for thermophotovoltaic applications or as bottom cell in PV stacks, the researchers succeeded to jump over the hitherto existing barrier of 8 percent and achieved an efficiency of 8.5 percent. For low-bandgap materials such as Ge, this is a high value, they assured. The value of 8.5 percent already comes close to the theoretical limit for this material. The cells proved to be stable under thermal cycling and kept a good performance up to concentration level of about 15 suns, the research institute informs.

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