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Determination of the Chemical Concentrations American Journal of Analytical Chemistry, 2014, 5, 205-210 Published Online February 2014 (http://www.scirp.org/journal/ajac) http://dx.doi.org/10.4236/ajac.2014.53025 Online Process Control of Alkaline Texturing Baths: Determination of the Chemical Concentrations Martin Zimmer, Katrin Krieg, Jochen Rentsch Department PV Production Technology and Quality Assurance, Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany Email: [email protected] Received December 16, 2013; revised January 20, 2014; accepted January 28, 2014 Copyright © 2014 Martin Zimmer et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In accor- dance of the Creative Commons Attribution License all Copyrights © 2014 are reserved for SCIRP and the owner of the intellectual property Martin Zimmer et al. All Copyright © 2014 are guarded by law and by SCIRP as a guardian. ABSTRACT Almost every monocrystalline silicon solar cell design includes a wet chemical process step for the alkaline tex- turing of the wafer surface in order to reduce the reflection of the front side. The alkaline texturing solution contains hydroxide, an organic additive usually 2-propanol and as a reaction product silicate. The hydroxide is consumed due to the reaction whereas 2-propanol evaporates during the process. Therefore, the correct reple- nishment for both components is required in order to achieve constant processing conditions. This may be sim- plified by using analytical methods for controlling the main components of the alkaline bath. This study gives an overview for a successful analytical method of the main components of an alkaline texturing bath by titration, HPLC, surface tension and NIR spectrometry. KEYWORDS Alkaline Texturisation; Titration; High Performance Liquid Chromatography (HPLC); Surface Tension Measurement; Near-Infrared Spectroscopy (NIR); Silicon Solar Cell Manufacturing 1. Introduction site for a continuous processing at a high quality level. In this paper, we present different approaches for the analy- The wet chemical alkaline texturing is still an important sis of the components of alkaline texturisation baths by process step during the fabrication of monocrystalline titration, high performance liquid chromatography, sur- silicon solar cells [1-3]. The texturing process takes place face tension and near-infrared spectroscopy. In order to in an aqueous solution of potassium or sodium hydroxide and the organic compound 2-propanol (IPA) at a certain evaluate the analytical methods, the component concen- temperature chosen between 70˚C and 80˚C. The hy- trations in the texturisation bath were measured during droxide is consumed due to the reaction with silicon several processes at a semi-industrial batch process plant. whereas 2-propanol is not consumed, but evaporates dur- ing the process due to its low boiling point (82˚C). This 2. Experimental changes the bath’s composition. This texturing process 2.1. Reference Sample Preparation results in a reduction of the reflection of the silicon wafer surface due to a complete coverage of the surface with The validation of the titration was achieved using a set of randomly distributed micro pyramids. The mechanism of reference samples. Samples were prepared from sodium the pyramid formation is not completely understood yet, hydroxide pellets (NaOH p.a., Merck, Darmstadt, Ger- but the influence of the chemical concentrations [4,5] as many), sodium metasilicate powder (Na2SiO3∙5H2O, Carl well as that of the accumulating etching products [6], Roth GmbH, Karlsruhe, Germany), sodium carbonate was subject of many studies and is known as critical. powder (Na2CO3, Carl Roth GmbH, Karlsruhe, Germany) Hence, a complete analysis and control of the chemical and 2-propanol (IPA Puranal, Honeywell, Seelze, Ger- concentrations in alkaline texturing baths is the prerequi- many). The silicon concentration ranged between 0 and OPEN ACCESS AJAC 206 M. ZIMMER ET AL. 50 g/l and sodium carbonate between 0 g/l and 9 g/l, each tassium hydroxide molar ratio exceeded a factor of two, a sample in 25 g/l NaOH and 50 g/l IPA (Table 1). The previous dosing of 1 M sodium hydroxide took place. validation of the high performance liquid chromatogra- phy (HPLC) was achieved using reference samples with 2.3. High Performance Liquid Chromatography 40 g/l IPA in different amounts of KOH and dissolved The determination of 2-propanol (IPA) by high perfor- silicon in KOH in the molar ratio of Si:KOH of 1:1.83 mance liquid chromatography (HPLC) was achieved by a (Table 2). system consisting of the isocratic HPLC pump P680 (Dionex, Sunnyvale, USA) and an Acclaim Organic Acid 2.2. Titration column (4.0 × 150 mm) with a following refraction index All titrations were performed with a Titrando (Deutsche detector at 35˚C (Shodex RI-101, Showa Denko, Tokyo, Metrohm, Filderstadt, Germany) using a 20 ml burette Japan). The mobile phase of the chromatography system and a pH-electrode (Mettler-Toledo, Greifensee, Switzer- was a 30 mM methane sulfonic acid. The injection loop land). Titration curves, equivalence point recognition and contained a volume of about 1 µl for an automated two concentration calculation were evaluated with the titra- position valve (EV700-100-S2, Rheodyne, USA). The tion software tiamo 1.1 assuming K2SiO3. A volume of 5 samples were not diluted for measurements. The chro- to 10 ml of the alkaline solution was titrated against 0.5 matograms were evaluated with the chromatography M hydrochloric acid (Merck, Darmstadt, Germany). software Chromeleon 6.70. Peak area was used for the The titer was determined with certified sodium carbo- quantitative analysis. The external calibration of 2-pro- nate (Merck, Darmstadt, Germany). If the silicon to po- panol was done using 2-propanol in water with the auto- Table 1. Measured NaOH, Si and Na2CO3 concentrations and recovery rates for reference samples, prepared from pure chemicals. The initial target concentration was determined by weighting the used substances. Mean ± standard deviation, n = 3 titrations. Target Titrated NaOH Titrated Si Titrated Na2CO3 Sample NaOH Si Na2CO3 NaOH Recovery Si Recovery Na2CO3 Recovery (g/l) (g/l) (g/l) (g/l) (%) (g/l) (%) (g/l) (%) Ref A1 26.0 0.0 0.0 25.7 ± 0.3 99 No EP Ref A2 25.6 14.9 0.0 24.6 ± 1.5 96 14.3 ± 1.2 96 Ref A3 25.7 30.0 0.0 26.7 ± 1.9 104 27.9 ± 0.8 93 Ref A4 25.4 50.0 0.0 26.4 ± 1.5 104 49.4 ± 0.5 99 Ref A5 26.6 30.0 3.0 24.9 ± 0.1 94 30.4 ± 0.2 101 No EP (0) Ref A6 24.9 30.0 6.0 26.3 ± 0.8 105 29.4 ± 0.9 98 6.1 ± 0.4 102 Ref A7 25.1 30.0 9.1 25.7 ± 1.2 102 29.3 ± 0.6 98 8.6 ± 0.1 94 Table 2. Alkaline samples for HPLC consisting of 2-propanol and as matrix potassium hydroxide and silicate (dissolved sili- con in KOH). Given are target concentrations as well as characteristic peak data. Negative KOH values appear since the sili- con to KOH solution ratio is smaller than 1:2 and silicon is calculated as K2SiO3. Mean ± standard deviation, n = 3 titrations and 4 HPLC analyses. Target concentration Titrated inorganic HPLC inorganic HPLC IPA Sample KOH Si IPA KOH Si peak area peak height peak area peak height IPA (g/l) (g/l) (g/l) (g/l) (g/l) (µRIU) (µRIU min) (µRIU) (µRIU min) (g/l) Ref C1 0 10 40.1 −5.1 ± 0.1 11.0 ± 0.02 27.5 ± 0.6 142 ± 2 33.0 ± 0.3 131 ± 1 39.7 ± 0.4 Ref C2 0 20 40.0 −10.3 ± 0.3 21.1 ± 0.06 55.6 ± 1.0 223 ± 2 32.5 ± 0.5 130 ± 2 39.1 ± 0.6 Ref C3 20 0 40.1 20.9 ± 0.1 No EP 5.1 ± 0.0 48 ± 0 32.6 ± 0.2 125 ± 1 39.3 ± 0.3 Ref C4 15 5 40.1 13.4 ± 0.2 5.1 ± 0.04 27.5 ± 0.6 101 ± 1 33.2 ± 0.7 131 ± 2 40.0 ± 0.9 Ref C5 0 0 41.1 - - - - 33.0 ± 0.6 118 ± 2 39.8 ± 0.7 OPEN ACCESS AJAC M. ZIMMER ET AL. 207 mated injection valve and an auto sampler. 2.4. Surface Tension Measurement The surface tension of the alkaline solution was meas- ured with a SITA Science Line t60 (SITA Messtechnik, Dresden, Germany), using the bubble pressure method. A direct measurement with the capillary in the process bath did not produce the required reproducibility; hence the measurement took place in a customized overflow mea- surement cell (Figure 1, Zitt-Thoma, Freiburg, Germa- ny). 2.5. Near-Infrared Spectroscopy Figure 1. Process bath of the semi-industrial etching plant Near infrared (NIR) spectra were collected continuously and installed analytical devices, surface tension meter and with a FT-NIR process spectrometer FTPA 2000-200 HPLC at the recirculation line and NIR at a bypass of the (ABB, Quebec, Canada) directly through the 12 in recirculation line. PFA tube of the circulation line which is commonly used in industrial etching plants. In this study, 128 scans from An acid base titration of an alkaline texturing solution 8000 to 11,000 cm−1 were averaged for one spectrum at a which contains 25.7 g/l NaOH, 29.3 g/l dissolved Si, 8.6 resolution of 32 cm−1; the measurement cycle was 30 s. g/l Na2CO3 and 50.1 g/l 2-propanol shows three equiva- The calibration set up and partial least square calibration lence points (Figure 2).
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