Development of APCVD BSG and POCl3 Codiffusion Process for Double-Side TOPCon Solar Cell Precursor Fabrication

This article presents a commercially viable process for fabricating a high-quality double-side tunnel oxide passivating contact (DS-TOPCon) cell precursor using atmospheric pressure chemical vapor deposition deposited boron silicate glass and ex situ POCl₃ diffusion in a single high-temperature step, eliminating the need for additional masking and diffusion processes. A two-tier temperature profile was developed, involving a preannealing at above 900 °C in nitrogen ambient followed by POCl₃ diffusion at 840 °C. We investigated the effect of varying preannealing temperatures, ranging from 875 to 950 °C, on the passivation quality and metal-Si contact properties of both n-TOPCon and p-TOPCon layers. The resultant DS-TOPCon cell precursor after silicon nitride passivation exhibited an excellent iV_OC of close to 730 mV. In addition, a rapid asymmetric poly-Si thinning technique, developed in this work, enabled adjustment of the front n⁺ poly-Si thickness while maintaining the rear p⁺ poly-Si thickness. Two types of DS-TOPCon cell architectures can be fabricated: i) full-area thin (≈40 nm) n-TOPCon layer on the front and ii) selective-area thick (≈200 nm) n-TOPCon fingers underneath the metal grid. Device simulations suggest that full-area DS-TOPCon cell with 40 nm n⁺ poly-Si and selective-area DS-TOPCon cell with 200 nm n⁺ poly fingers on the front, fabricated from our current DS-TOPCon cell precursor, can achieve cell efficiencies of 22.1% and 23.5%, respectively. Detailed power loss analysis and device simulation reveal that further improvements in material and device parameters have the potential to push the cell efficiencies of DS-TOPCon cell structure beyond 25%, making it a promising alternative to fabricate high-efficiency next-generation solar cells at low cost.