Carrier selective tunnel oxide passivated contact enabling 21.4% efficient large-area N-type silicon solar cells

This paper presents a thermally stable carrier selective back contact for high-efficiency large-area n-type Si solar cells with screen-printed front contact on homogeneous emitter. Our passivated contact structure is based on an ultra-thin (∼15Å) tunnel oxide capped with phosphorus doped n⁺ poly-Si. It is shown that a proper precursor PH₃/SiH₄ ratio and an appropriate crystallization and dopant activation anneal temperature are vital to obtain excellent interface passivation quality with an implied open-circuit voltage (iV_oc) of 728 mV and corresponding back-surface-field saturation current density (J_ob') of < 5 fA/cm². It is found that the tunnel oxide is a critical part of this carrier selective contact, and its absence can result in ∼125 mV drop in iV_oc. Cell efficiency of 21.4% was achieved on 239 cm² commercial grade n-type Cz wafers with screen-printed and fired Ag/Al front contact on ion-implanted homogeneous boron emitter. Detailed analysis of this cell shows that efficiency of this cell is mainly limited by the recombination at the front metal/p⁺ contacts. Our 2-dimentional simulations show that applying fineline metallization on selectively doped boron emitter can raise this cell efficiency to over 22.5%.