Advanced surface passivation of epitaxial boron emitters for high-efficiency ultrathin crystalline silicon solar cells

In this work, we demonstrated an enhanced surface passivation of epitaxially grown boron-doped Si emitters by replacing thermal SiO 2 as a passivation layer employed in a 15.9% efficient 21-µm Si solar cell (88 cm 2 ) on stainless steel with a remote-plasma atomic layer deposition (ALD) of an Al 2 O 3 film. A thin Al 2 O 3 film deposited by remote-plasma ALD was very effective at reducing the emitter saturation current density ( J 0e ) of epitaxial p + -emitter to 16.2 fA/cm 2 , compared to the J 0e of 184.9 fA/cm 2 by thermal SiO 2 films. This reduction in J 0e enables an increase in an implied open-circuit voltage (i V oc ) from 630 to 688 mV. Quokka simulation shows that about a 1.1% absolute efficiency increase in the calculated baseline efficiency of a 15.1% of the ultrathin Si solar cell can be achievable by enhancing emitter surface passivation without changing the concentration in either the epi-emitter or epi-base. Finally, our results show that a high efficiency of 17.3% can be reached from the calculated baseline efficiency of 15.1% using the optimized conditions of an epitaxially grown emitter in combination with increasing the base doping concentration and improved base recombination lifetime.