Nox-Free Solution for Emitter Etch-Back

Abstract

In this paper a novel wet etching solution for emitter etch-back, based on hydrofluoric acid and persulfate, is characterised. The etch rate of this solution is strongly dependent on dopant concentration with an average of 25 nm/min for highly doped emitter surface and below 1 nm/min for lowly doped substrates. The etch rate at a certain dopant concentration is determined by correlation of the removed emitter layer, derived from weight loss, to the emitter profile analysed by secondary ion mass spectroscopy. Through etch-back the highly doped layer is removed, thus recombination losses at the emitter surface are reduced. The improved emitter quality is confirmed by reduced emitter saturation current density with measured values below 30 fA/cm² and corresponding implied open circuit voltages exceeding 690 mV. Standard nitric acid based and new etching processes are compared on cell level with NiCu electroplated front contacts as well as with screen printed contacts using a silver paste for lightly doped emitters, both with passivated rear side. On p-type Cz substrates conversion efficiency of 21,0% has been achieved with a homogenous, etched-back emitter and screen printed silver metallisation. For a cell with electroplated NiCu contacts conversion efficiency of 20,9% has been confirmed