By Dr. Hans Bell

The composition of pastes for the metallization of solar cells can be highly varied. This does not concern the actual metal content (silver or aluminium powder) but the other additives that are key to the printing properties of the paste and the baking and sintering characteristics. The proportion of these substances can be up to about 25% by weight, of which the largest share is the organic medium in which the solids (metal powders, metal oxides, inorganic binders such as glass frit) are dispersed.

Typically, after being printed onto the solar cell, the paste is dried at temperatures from 200 to 350°C and, in a subsequent firing process, baked into the solar cell at temperatures of 800 to 1,000°C. During drying and firing of the pastes, fumes and smoke are generated. These must be safely extracted from the process chamber to avoid contamination of the system as far as possible. The fumes/smoke arise from the volatile components of the organic medium, which can consist of various organic liquids that may also contain thickeners and stabilisers. Examples of organic liquids are alcohols (Texanol) or alcohol esters (acetic acid and propionate), terpene (pine oil, terpineol), solutions of resins (polymethacrylate), solutions of ethyl cellulose in a solvent (e.g., terpineol) and the monobutylether of ethylene glycol monoacetate. A preferred organic medium is ethyl cellulose in terpineol in combination with a thickening agent mixed with butyl carbitol acetate. In a study, Vogg¹⁾ determined the weight losses using different metallization pastes. Figure 1, for example, shows the weight losses of an Al backside metallization, which here reach >24%.

The operator of a drying plant does not usually know the exact composition of the metallization paste used and its volatile constituents. Therefore, a filter-/collection unit for the vapors/fumes cannot be custom-made, but must be designed for a broad range of deposition of various ingredients. At the same time, it can be expected that after passing through the filter-/collection unit the emissions will fall substantially below the limitations set by legislation, such as the Clean Air Act³⁾.

Very often, so-called condensate separators are used in the manufacturing. On the one hand, the collection efficiency is limited, and on the other hand, the mandatory disposal of the accumulated condensates is expensive.

For these reasons, Rehm has taken the known method of thermal oxidation for solar systems and implemented it in an innovative way. The thermal oxidation is a process that takes the volatile organic components and hydrocarbons from the metallization paste and binds them with oxygen, essentially breaking them down into water vapor and carbon dioxide. The goal is to burn the long-chain molecules in the vapors/fumes and convert them into easily volatile substances that condense only with difficulty. These can then be easily removed from the system, which thus drastically reduces the potential for condensation within the drying system. The thermal oxidation is initiated by the heat of the exhaust gas at a temperature >750°C. At the high temperatures, the molecules break down and bind to the available oxygen in the system. To achieve these high temperatures, Rehm deploys strictly electrical heating systems; the use of open flames is deliberately avoided. The risk of NOx gases forming is thus ruled out as far as possible. Measurements of ILK Dresden²⁾, as shown in Figure 2, document a collection efficiency for the pollutant toluene that reaches 99.5 % in the high temperatures in the oxidizer. With the thermal oxidation, emissions are well below the legal limits set for emissions (e.g., Germany’s Clean Air Act). Good separation behavior was achieved not only for volatile organic hydrocarbons (VOCs), but also for the particulate distribution. A clean gas value of particles below 0.2 microns was gravimetrically determined by ILK (the Institute of Air Handling and Refrigeration in Dresden)²⁾ to be about 2 mg/m³. The oxidizer also copes well with different concentrations, as other measurements by ILK document⁴⁾. At five times the concentration of the model pollutant toluene, the collection efficiency remained > 99.5%.

The oxidizer contributes significantly to minimizing the condensation potential, thus significantly minimizing the cost of maintenance for dryer systems. As no more condensate can be formed on the clean gas side, the main extraction system of the fabrication plant remains visibly cleaner. A highly positive side effect is the markedly lower odor of solar dryers with an integrated oxidizer. Figure 4 shows a functional diagram of the thermal reactor (oxidizer), which is horizontally integrated into the Rehm systems?here, into a dryer. The horizontal integration allows a compact construction of the entire drying system, and through its lengthy gas routing it secures the necessary gas contact time, resulting in an excellent collection efficiency at high temperatures.

With the help of a fan, the hot raw contaminated gas is drawn off from the process chamber of the solar dryer and passed directly through the electrical heating unit of the oxidizer to be warmed to the high temperature of 750°C that is required. The flow is set (~ 170 m³/h actual flow) so that a sufficiently long residence time of the raw gas of >1 s at this high temperature is guaranteed. The oxidizer is optimized for energy use, resulting in an operational power draw of <18 kW.

In general, thermal oxidation systems can be used for almost all organic pollutants and are thus a very good alternative to condensate discharges. The main advantage over the condensate separation is significantly reduced maintenance costs of solar dryers and firing systems.

The main advantages of thermal oxidation are:

-Reduction in maintenance costs, no disposal of condensate,

-Secure compliance with the legal requirements for emissions,

-Universally applicable for various metallization pastes,

-Proven, broadband method,

-Robust systems engineering,

-Low energy consumption, low operating costs,

-Decreased odor of solar dryers. 

Since January 2000, Dr. Hans Bell has been working as Head of the Development and Technology department at the company Rehm Thermal Systems (www.rehm-group.com). After his Abitur (A-Levels equivalent), he studied physics and crystallography at the Berlin Humboldt University. Bell received his doctorate at the Technical University of Munich.

REFERENCES

1) Florian Vogg, Ermittlung und Optimierung von Trocknungsparametern verschiedener Solarpasten, Praxissemesterbericht, Firma Rehm, Februar 2011 [Determination and optimization of drying parameters of different solar pastes, internship report, Rehm, February 2011]

2) Ralf Heidenreich, Oxidizer Rehm, ILK-Bericht, 10.12.2010 [ILK Report, Dec.12, 2010]

3) Bundes-Immissionsschutzgesetz, Technische Anleitung zur Reinhaltung der Luft-TA Luft, 24. Juli 2002 [Federal Pollution Control Act, Technical Instructions on Maintaining Air Quality--Clean Air Act, 24 July 2002]

4) Ralf Heidenreich, Untersuchungen zur Schadstoffabscheidung an einem Trockner fur Solar-Wafer, ILK-Bericht, 31.03.2011 [Ralf Heidenreich, Studies of pollutant deposition in a dryer for solar wafers, ILK Report, March 31, 2011]