Simplification of EWT (emitter wrap-through) solar cell fabrication using Al-P-codiffusion

Back contact solar cells such as EWT (emitter wrap-through) devices with interdigitated pn-junctions offers various advantages like a simplification in module assembly or a reduction of shadowing losses. Our method to form a rectifying p⁺n⁺-junction using an Al-P-codiffusion process-without the necessity of electrical isolation of p- and n-type doping regions-was transferred to EWT cells. We were able to fabricate EWT cells with shunt-values up to 10⁴ Ω cm². The interdigitated pn-junctions of these devices are formed by simply evaporating an Al-finger grid followed by Al-P-tube codiffusion. The effectiveness of the codiffusion for EWT cell fabrication was demonstrated on CZ-Si, mc-Si and EFG (edge-defined film-fed grown)-Si. A main advantage of the developed sequence is a simplification of the EWT cell fabrication process by making single processing steps redundant, particularly the junction isolation, i.e. the separation of the different doping regions at the cell rear. Standard technologies for junction isolation are: the use of a diffusion barrier; plasma etching; locally milling off the rear side emitter; and P-B dopant compensation. For industrial applications it would be interesting to form a rectifying p⁺n⁺-junction without applying one of those additional cost intensive and time consuming processing steps