Status and perspective of emitter formation by POCl3-diffusion

The advances in contact formation of screen printed Ag pastes and the related progress in emitter profile tailoring has been a key driver for recent efficiency improvements. This paper summarizes the current status of phosphorus-diffused emitters for p-type silicon solar cells and discusses two technological options for advanced tube furnace POCl3-diffusion processes: diluted source and in-situ post oxidation. Lower surface doping concentration levels are identified as key parameter for further reducing carrier recombination in the emitter. Using analytical modelling, we estimate potential efficiency improvements of 0.3%abs for PERC type solar cells from reducing the surface concentration from 2x1020 cm-3 down to 1019 cm-3. We also address the increasing importance of recombination at the metal contacts and investigate the role of the diffusion profile and sheet resistance by analytical modelling. If metal contact recombination is supressed by a selective emitter and/or advanced low recombination metallization pastes, a potential efficiency gain of 0.5%abs is predicted.