October 18, 2007 – IMEC says it has achieved >17% efficiency with thin-film silicon solar cells, and says >20% efficiencies “are definitely within reach.” Meanwhile, the research group says it is nearing the theoretical efficiency limits of Ge solar cells.
IMEC’s work involves an “i-PERC process” that replaces the Al backsurface field on a Si solar cell with thin silicon layers (<180nm) to create Al-alloyed contacts covering the entire rear side of the cell. The rear is passivated with a dielectric stack in which contact openings are realized by laser ablation. An Al contact layer is evaporated and fired in a belt-line furnace to create local back-surface fields.
After further (unidentified) “fine-tuning” of the process, the researchers say they were able to achieve efficiency of 17.4% on thin large-area multi-crystalline Si substrates, using screen-printing to realize the contacts.
Small-area crystalline Si solar cells also have been developed in the lab, also utilizing the i-PERC process to passivate the rear side and to create local contacts. On the front side of the wafer, random pyramids are used in combination with a standard silicon nitride layer “to provide good optical properties and surface passivation,” with the evaporated Ti/Pd/Ag front contact grid patterned by standard lithography. Initial results with the small-area Si solar cells “immediately” showed 19% efficiency and a short-circuit current of >39mA/cm2.
Meanwhile, IMEC says it has made progress with germanium cells for thermophotovoltaic applications or as a bottom cell in photovoltaic stacks, achieving a record 8.5% efficiency (higher for a low-bandgap material like Ge). The cells were shown to be stable under thermal cycling and “kept a good performance up to concentration levels of about 15 suns,” IMEC claims.