Stack Diffusion Process for Cost- and Energy-Efficient Boron Emitter Formation

Abstract

The boron emitter formation for tunnel oxide passivated contact (TOPCon) solar cells faces higher costs compared to the POCl3 diffusion for passivated emitter and rear (PERC) solar cells due to the requirement for higher temperatures and longer process times. This work presents an alternative energy-efficient and low cost of ownership boron diffusion approach for TOPCon solar cells, enabling a highly increased throughput compared to the typically used gas phase diffusion. We use an atmospheric pressure chemical vapor deposition borosilicate glass layer as the boron dopant source and combine it with a subsequent thermal anneal in a quartz tube furnace for dopant drive-in. Here, we either use a conventional single-slot quartz boat configuration, or, for highly increased throughput, a vertical wafer stack configuration with the wafer surfaces in direct contact with each other. We show that this approach yields an emitter doping profile comparable to the state-of-the-art gas phase diffusion with sufficient uniformity across the wafer area. We further investigate the emitter dark saturation current densities j0e as well as the energy conversion efficiency of TOPCon solar cells fabricated for each configuration and compare the results to those of a BBr3 reference process. These solar cells achieve energy conversion efficiencies exceeding 23% for the stack diffusion approach. Additionally, we demonstrate a potential reduction in both the cost of ownership and the specific electricity consumption of the presented approach.