N-type multicrystalline silicon solar cells : BBr3-diffusion and passivation of p+-diffused silicon surfaces

The shortage of the p-type silicon (Si) feedstock and the high minority carrier lifetimes in multicrystalline (mc) n-type Si reported by different authors ([1]-[3]) make n-type mc-Si solar cell fabrication more and more interesting. Given the high electronic quality of the material – that is confirmed in our studies again – the task remains to develop an adapted solar cell process. A key feature of the concept presented here is the BBr3diffused emitter on the front side and the surface passivation of this emitter. We show that BBr3 emitter-diffusion is possible without degradation of the high initial carrier lifetimes in the n-type mc-Si material on contrary the diffusion even improves the average lifetime to a large extend. SiO2 provides an excellent surface passivation of the p-Si surface. Application of PECVD SiNx resulted in a decrease of the (implied) Voc measured on lifetime teststructures as well as on solar cell level. As an alternative, a low temperature surface passivation process by PECVD SiCx is investigated. First trials resulted in a very promising value for the emitter saturation current Joe: 180 fA/cm for a 90 Ω/sq emitter. N-type Si solar cells with SiO2-passivated BBr3-emitter were processed in laboratory scale (area of 4 cm) with an efficiency of 15.2% on mc and 16.4% on Cz-Si. With an industrial screen printing process 14.1% and 14.8% were obtained on an area of 12.5 x 12.5 cm on n-type mc-Si and Cz-Si respectively.