Spin-on based process for simultaneous diffusion and passivation for high efficiency LBSF solar cells

Abstract

Low-cost rear surface passivation has been a critical requirement for fabrication of cost effective high efficiency cells with local back surface field (LBSF) structures. This becomes even more important as we move towards next generation thin solar cells. Various dielectrics have been attempted in combination with boron and aluminum BSF to fabricate high efficiency LBSF cells. However there is no commercially viable process compatible with screen-printed contacts. This paper discusses a novel spin-on based process for simultaneous emitter diffusion and front and rear surface passivation in a single high temperature furnace step. A stack of spin-on SiO 2 / PECVD SiN x was optimized to control charge and used for rear passivation and a thin thermal oxide / PECVD SiN x was used for front surface passivation. Limited solid source spin-on phosphorous diffusion was used to obtain emitters. A screen- printed etching paste was used first to open vias in the dielectric followed by Al metal printing to make contact to the rear silicon surface. Local Al-BSF (4 cm2) structures fabricated on FZ silicon using this process resulted in peak cell efficiency of 19.0 % on wafers with a sheet resistance of ∼85 Ω/□. All these cells showed an improved J sc (≫38 mA/cm2) and V oc (≫650 mV) compared to the conventional full area Al BSF cells, indicating improved back surface passivation and reflectance. Optimization of emitter profile and contact firing are expected to result in cell efficiencies in excess of 19.5 %.