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

Abstract

AbstractPassivated 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 Voc 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 Voc and Jsc values would support PERC conversion efficiencies up to 21.9%. Copyright © 2016 John Wiley & Sons, Ltd.