Si and Ge Solar Cell Efficiency Records Set
Peter Singer, Editor-in-Chief -- Semiconductor International, 10/11/2007 7:31:00 AM
It was announced at SEMICON Europa this week that new records for thin-wafer silicon and germanium solar cells were set by IMEC (Leuven, Belgium). The research consortium realized an impressive efficiency of 17.4% for its thin-wafer silicon solar cells realized with its industrial passivated emitter and rear cells (i-PERC) process. By using the i-PERC process as a method to passivate the rear side and create local contact, efficiencies above 20% are definitely within reach, IMEC reported.
To reduce the cost of solar cells, it is imperative to lower the amount of ultrapure silicon. The most straightforward way to achieve this is the use of very thin silicon wafers. The i-PERC process uses very thin silicon layers (<180 µm) in which the classical aluminum back-surface field, covering the whole rear of the silicon solar cell, is replaced by local aluminum-alloyed contacts. In this process, the rear is passivated by a dielectric stack, in which the contact openings are formed by laser ablation. Subsequently an aluminum contact layer is evaporated and then fired in a belt-line furnace to create local back-surface fields. Further process optimization yielded an efficiency of 17.4% on thin large-area multi-crystalline silicon substrates. Screen printing was used to form the contacts.
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| Large-area i-PERC solar cell on a very thin wafer. |
Progress was also reported in the field of germanium cells for thermophotovoltaic applications or as a bottom cell in photovoltaic stacks. Compared with its previous record of around 8% (which is high for a low-bandgap material like germanium), IMEC jumped over the 8.5% barrier, coming close to the theoretical limits for germanium. The cells proved to be stable under thermal cycling, and kept a good performance up to concentration levels of ~15 suns.
