Simultaneous determination of mineral acids, fluoride and silicate in etching baths by ion chromatography with dual detection

Jun 01, 2011
By LCGC Editors

The presented ion chromatographic method is used for the simultaneous determination of HF, HNO 3 , H2SO4, short-chain organic acids and H2SiF6 in acidic texturing baths that are used in the wet chemical etching process of solar cell production. Fluoride, nitrate, sulphate and acetate are determined by conductivity detection after chemical suppression, while the silicon present in the form of hexafluorosilicate is detected spectrophotometrically as molybdosilicic acid after derivatization in the same analysis. The analytical results are validated by titration.

Introduction

Energy production from renewable sources such as biomass, biogas, biofuels, water, wind and solar power is becoming increasingly important in our energy-hungry society. Particular interest is given to solar energy, which by human criteria is inexhaustible. Solar cells used in photovoltaic units convert the radiation energy in sunlight directly into electric energy.


Figure 1: 850 Professional IC Anion – MCS and 858 Professional Sample Processor.
Solar cells are manufactured from ultrapure mono- or polycristalline silicon wafers whose surface is treated in acid etching baths (also known as texturing baths) before being spiked with foreign atoms (P, B). The etching solutions consist of various acids, which act as an oxidizing agent (HNO3), complexing agent (HF), stabilizer and wetting agent (CH3COOH), or buffers (H3PO4, CH3COOH) and determine the surface structure and thus the efficiency of the solar cells. The replenishment of components used up in the etching process extends the bath life and saves costs, though it does require knowledge of the exact composition of the bath, especially the concentration of silicon and hexafluorosilicate. By using titration and ion chromatography (IC), it is possible to determine the key components quickly and precisely.

This article describes an ion chromatographic method that separates all relevant components in the bath on an anion-exchange column and identifies them by dual detection in a single run. After suppressed conductivity detection of the acid anions, the undissociated silicic acid reacts in a post-column reaction (PCR) to form molybdosilicic acid, which is determined spectrophotometrically at 410 nm. The concentrations of fluoride and hexafluorosilicate are determined by way of a simple stoichiometric calculation that is performed by the chromatography software.

Instruments and Reagents

a) Instrument setup

  • 850 Professional IC Anion – MCS with post-column reactor
  • 858 Professional Sample Processor
  • Lambda 1010 UV/VIS Detector
  • 771 IC Compact Interface
  • MagIC Net chromatography software

b) Reagents and eluent
The standard solutions were prepared with CertiPUR standards from Merck (SiO2 in NaOH; solutions of NaF and NaNO3 in ultrapure water) and the TraceCERT standard from Fluka (acetate solution). All the standard and eluent solutions were prepared with ultrapure water with a specific resistance of more than 18 MΩ·cm. Etching bath samples were provided by a solar cell manufacturer from Germany.