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 r
educe 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. |
To get more evidence of the efficiency potential of the i-PERC process, IMEC also developed small-area crystalline silicon solar cells with evaporated contacts. Lab-scale silicon solar cells have been realized where, for the first time, the i-PERC process has been used as a method to passivate the rear side and create local contacts. At the front side, random pyramids are used in combination with a standard silicon nitride layer to provide good optical properties and surface passivation. The evaporated Ti/Pd/Ag front-contact grid is patterned by standard lithography. First attempts immediately resulted in an efficiency