Emitter saturation current densities of 22 fA/cm² applied to industrial PERC solar cells approaching 22% conversion efficiency

Passivated Emitter and Rear Cells (PERC) are currently being introduced into mass production. The conversion efficiency of industrial p-type PERC cells is limited by the emitter saturation current density of around 90 fA/cm² of conventional homogeneously POCl₃ diffused emitters. In this paper we investigate two alternative emitter formation technologies. The first approach named in-situ oxidation inserts a short thermal oxidation in-between the phosphorus silicate glass deposition and the drive-in of a conventional homogeneous POCl₃ diffusion thereby reducing the phosphorus surface concentration. The second approach named Gas Phase Etch Back (GEB) selectively removes around 40 nm of the highly doped surface of the POCl₃ diffused emitter by the reactive gas phase of the wet chemical rear polishing bath. Whereas the conventional POCl₃ emitter exhibits a phosphorus surface doping concentration of 3 × 10²⁰ cm⁻³, the in-situ oxidation and the GEB process reduce the doping concentration to 7 × 10¹⁹ cm⁻³ and 4 × 10¹⁹ cm⁻³, respectively. Accordingly, the emitter saturation current density is reduced to excellent values of 22 fA/cm² (in-situ oxidation) and 28 fA/cm² (GEB) compared with 89 fA/cm² for the reference POCl₃ diffusion. Whereas the reference POCl₃ emitter limits the PERC conversion efficiency η to 21.1% and the open circuit voltage V_oc to 655 mV, the in-situ oxidation improves the PERC current–voltage parameters up to 21.3% and 663 mV. The highest efficiency of 21.6% is obtained with the selective GEB emitter. When solving series resistance issues with the most advanced GEB emitter, the measured V_oc and J_sc values would support PERC conversion efficiencies up to 21.9%.