Nano-electronics research center imec, reported today an n-type PERT crystalline silicon (Si) solar cell fabricated on a large area wafer (15.6cm x 15.6 cm) reaching a top conversion efficiency of 21.5 percent (calibrated at ISE Callab). This is the highest efficiency achieved for this type of solar cell on an industrial large area wafer size. This result will accelerate the adoption of n-type PERT (Passivated Emitter, Rear Totally diffused) solar cells in the industry as it clearly shows the potential for improved conversion efficiencies for next generation standard two side contacted crystalline silicon solar cells. Additionally imec researchers showed recently that n-type PERT solar cells of imec, having a rear emitter, are not affected by reliability risks originating from a front Ni/Cu plated metallization.
The cell reaching this 21.5 percent conversion efficiency had an open circuit voltage (Voc) of 677mV, a short circuit current (Jsc) of 39.1 mA/cm2, and 81.3% fill factor, and features a rear blanket p+ emitter obtained by boron diffusion. Reliable front metal contacts on an n+ front-surface-field are formed by means of Ni/Cu/Ag plating (3 bus bars grid) using an industrial plating tool from Meco, while the rear local contacts to the p+ emitter were obtained by laser ablation of the rear passivation stack and subsequent physical-vapor-deposition of aluminum. The rear passivation stack includes a thin (<10 nm) Atomic-Layer-Deposited (ALD) Al2O3 layer, deposited with the spatial ALD technique InPassion Lab from SoLayTec. The adoption of ALD Al2O3 based-passivation for the p+ emitter resulted in an average improvement in cell efficiency of about 0.3% absolute with respect to passivation by wet oxidation. This illustrates the excellent capabilities of ALD for passivation layers in next generation cell concepts like PERC and n-type PERT.
"Notwithstanding the early development stage, the result shows very high efficiency potential of n-type PERT solar cells," said Jozef Szlufcik, director of imec's PV department, "Moreover, n-type cells remain unaffected by light induced degradation present in p-type cells due to Boron-Oxygen complex, which results in improved long term energy yield and, therefore, lower total cost/kWh".
These results have been achieved in the framework of the imec's industrial affiliation program on advanced silicon solar cells, dedicated to developing high performance and low cost Si PV-technologies. In this program, imec works closely together with industrial and academic partners along the solar cell value chain. Via participation and contribution to this program, these partners support imec's developments and obtain early access to new technology solutions in this way accelerating their own product development.
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