Publications

  Crystalline Silicon Solar Cell Efficiency Improvement by Advanced Cleaning Technology
David Bohling, Helmuth Treichel, Thierry Lazerand,. Avery Goldstein and Mark GeorgeTechnical Proceedings of the 2010 Clean Technology Conference NANO SCIENCE AND TECHNOLOGY INSTITUTE – Nanotech 2010 Vol. 3
+Abstract
Sunsonix has identified a technology gap in the area of crystalline silicon photovoltaic efficiency and field degradation. The cause of both poor efficiency and rapid degradation may share common roots. Trace contamination at the photovoltaic junctions lead to both mid-level traps and photonic defects. In other words, very small amounts of contamination can result in poor photovoltaic efficiency as well as susceptibility to further efficiency decay when exposed to sunlight after field installation. In recent trials we have conducted best of class cleans that increase the implied open circuit voltage by 3%. In manufacturing split lot trials for both batch and in-line wet cleaning tools a post texturing etch resulted in a cell efficiency increase of 0.3% absolute on multi-crystalline Si wafers.
  Removal of Trace Metals using a Biodegradable Complexing Agent
Helmuth Treichel, Avery Goldstein, Mark George, David Bohling, Jochen Rentsch, Antje Oltersdorf, Martin Zimmer, Sara Ostrowski, Ian Mowat, Larry Wang and Werner KernPhotovoltaics International 12th Edition, May 2011
+Abstract
Processing silicon substrates for PV applications involves texturing, cleaning and/or etching wafer surfaces with chemical solutions. Depending on the cleanliness of the industrial equipment and the purity of the chemical solutions, surface contamination with metals or organic residues is possible [1]. The presence of trace contamination at PV junctions leads to both mid-level traps and photonic defects, which ultimately cause reduced efficiency and rapid cell degradation. Metallic impurities have a greater impact on PV cell lifetime due to their deeper energy levels in the silicon band gap [2]. On the other hand, non-metallic impurities may modify the electrical activity of PV cells because these species involve complex interactions with the host silicon lattice and its structural defects. In other words, very small amounts of contamination can result in poor PV efficiency. This paper presents an overview of the effects of adding a  biodegradable complexing agent in cleaning and rinsing baths to minimize surface contamination and thereby enhance solar cell efficiency.
Surface Contamination Removal From Si PV Substrates Using A Biodegradable Chelating Agent and Detection of Cleaning Endpoints Using UV/VIS Fluorescence Spectroscopy
Mark George, David Bohling, Helmuth Treichel, Avery Goldstein, Herbert Litvak, Sara Ostrowski, Ian Mowat, and Werner KernTechnical Abstracts of the 220th ECS Meeting and Electrochemical Energy Summit, October 2011
+Abstract
Considerable effort has been expended to eliminate contaminants that degrade semiconductor IC performance. These same contaminants have not been adequately addressed in Silicon PV manufacturing processes. Sunsonix has developed a biodegradable and biocompatible cleaning chemistry (SX-E™) that efficiently removes interfacial transition metals and most metal cations in both HF based surface cleans and SC2 HCl based cleans. The chemistry can also be applied directly in a DIW bath, added directly to a KOH texturing bath, or basically to any rinse step in the complete front end process.

REMOVAL OF TRACE ELEMENTS USING A BIODEGRADABLE COMPLEXING AGENT

Helmuth Treichel1, Mark George1, David Bohling1, Avery Goldstein1,Sara Ostrowski2, Ian Mowat2, Werner Kern3; 1 Sunsonix, 859 Pheland Ct, Milpitas, CA 95035, USA; 2 Evans Analytical Group, 810 Kifer Rd., Sunnyvale, CA 94086, USA;3 Werner Kern Associates, 22 Greenways Ln, Lakewood, NJ 08701, USA; 26th EU PVSEC, Hamburg, Germany, September 2011

+Abstract
PV solar cell technologies are in mainstream commercialization and the industry is devoting significant resources toward improving the efficiency of these solar cells.  Processing silicon substrates for PV applications involves texturing, cleaning, and/or etching wafer surfaces with chemical solutions. Depending on the cleanliness of the industrial equipment and the purity of the chemical solutions, surface contamination with metals or organic residues is possible. The presence of trace contamination at PV junctions leads to both mid-level traps and photonic defects, which ultimately cause reduced efficiency and rapid cell degradation. Impurities are the major contributors of electrical activity of defects and are therefore of utmost importance in the control of recombination activity of various defects in solar cells. In this paper, we discuss the application of a biodegradable complexing agent, in both alkaline and acidic cleaning processes. Testing at various customer sites has demonstrated 0.2 to 0.3% absolute cell efficiency improvement for both mc-Si and c-Si production lines. This improvement is not dependent on cell architectures, but addresses a universal complication arising from crystalline silicon photovoltaic wafering manufacture.

Optical Spectroscopy of Chelated Trace Metals for Wet Cleaning Process Control and Optimization

Herbert Litvak1, Helmuth Treichel2, Avery Goldstein2, Mark George2, David Bohling2; 1Lightwind Corporation, 621 Second Street, Suite B, Petaluma CA, 94952, USA;2Sunsonix™, 859 Pheland Ct, Milpitas, CA 95035, USA; 26th EU PVSEC, Hamburg, Germany, September 2011

+Abstract
UV-visible spectroscopy is a technique that readily allows one to determine chemical concentrations of a wide variety of substances over a wide range of conditions. Iron, cobalt, nickel, titanium and copper are important metals that when present on a silicon wafer surface adversely influence the efficiency of resulting PV devices. However, many of these surface ions can be effectively removed during wafer cleaning by the addition of a complexing or chelating agent (such as SX-E™). Since many of the ion-chelate molecular complexes exhibit optical absorption and/or fluorescence in the easily-accessible UV-visible spectroscopy range, they are suitable for chemical analysis by spectroscopic techniques. In this paper, we explore the spectroscopy of several of these metal ion complexes, as well as their potential for chemical analysis and process control, e.g. for endpoint detection, measurement of cleaning bath lifetimes, and as a tool to optimize cleaning sequences.

Pre-Diffusion Clean Optimization for High-Volume Manufacturing of Multi-Crystalline Solar Cells

Helmuth Treichel2, Yen-Ting Lu1, Kirin C. Wang1, Cheng-Chia Wu1, Chao-Min Yang1, Mark George2, David Bohling2, Avery Goldstein2, Werner Kern3; 1Motech, Tainan Science-Base Industrial Park No. 18, Dashun 9th Road, Xinshi Dist., Taiwan; 2Sunsonix™, 859 Pheland Ct, Milpitas, CA 95035, USA; 3Werner Kern Associates, 22 Greenways Ln, Lakewood, NJ 08701, USA; 27th EU PVSEC, Frankfurt, Germany, October 2012

+Abstract
The cost of chemical waste disposal is high. It is therefore important for the PV industry to find ways to reduce chemical consumption and waste generation through chemical usage reduction, recovery, recycle, reuse, and substitution. Iron, cobalt, nickel, titanium and copper are important metals that when present on a silicon wafer surface adversely influence the efficiency of resulting PV devices. However, many of these surface ions can be effectively removed during wafer cleaning by the addition of a complexing or chelating agent (such as SX-E™). In this paper, we explore the process optimization as well as the removal of several of these metal ion complexes.

Investigation of Biodegradable Texturing Chemistries for Crystalline Si

Mark George1, David Bohling1, Helmuth Treichel1, Avery Goldstein1, Werner Kern2, BG. Potter Jr.3; 1Sunsonix™, 859 Pheland Ct, Milpitas, CA 95035, USA; 2Werner Kern Associates, 22 Greenways Ln, Lakewood, NJ 08701, USA; 3 University of Arizona, Arizona Materials Laboratory, 4715 East Fort Lowell Road, Tucson, AZ  85712, USA; 27th EU PVSEC, Frankfurt, Germany, October 2012

+Abstract
The cost of chemical waste disposal is high and the PV industry is discovering ways to eliminate harmful waste and its associated costs. In this paper we report on the results of exploring biodegradable chemistries that are useful for texturing c-Si substrates. We have found the role of additives to the texturing c-Si texturing chemistry to be three-fold.  First the addition of additives stabilizes bath chemistry increasing the number of wafers processed in each bath turn, while at the same time decreasing reflectivity by providing more homogenous generation of texturing features.  Finally these additives are now formulated to sequester efficiency-robbing transition metals like iron, chrome, nickel and copper that when present on a silicon wafer surface adversely influence the efficiency of the resulting PV devices.  The additives come at a relatively high cost and while used in small quantities can be quite toxic to waste water streams.  In the course of this investigation we were able to distinguish the efficiency improvement of the sequestering function and improved texturing function of several texturing additives.  The efficiency gain by improved light trapping from texturing on nominally 18% efficient c-Si solar cells was determined to be about 0.4 % absolute.  The removal of the transition metals offers another 0.15 to 0.25% efficiency improvement, but that gain comes from the use of fully biodegradable and non-toxic systems.

Summer 2011 Newsletter