Reduction of Organic Emissions from Solar Cell Thermal Processing
Volatile Organic Compounds (VOC) in the Silicon Solar Cell Production
Contacts of silicon solar cells are printed with metallic pastes on the front and backside of the wafer. The organic components in the paste are evaporated after the printing steps in thermal processes and are exhausted from the oven in a controlled flow of air. Depending on the process temperature, some compounds may be partially oxidized. While volatile organic compounds are quickly diluted below their vapor pressure, compounds with low vapor pressure tend to form liquids or solids. These may deposit on the surfaces of the exhaust ducts and over time liquids may oxidize into more viscous or even solid compounds.
It is a typical problem for the exhaust of such thermal processes that liquid and solid residues accumulate in the exhaust ducts. They eventually leak out or flow back into the process tool and solids can block movable parts. Furthermore, the organic load in the exhausted air contributes to the overall organic emission from a production line to the atmosphere.
The development of a technical solution for these problems are described in this publication.
Production Line with Electrostatic Condensate Separator
Three printer-dryer combinations are set-up in a line, followed by a firing process. These lines have multiple exhaust connections where the highest organic loads occur in the connections at the backside drying. This waste gas leaves the production system at a temperature of approximately 200°C. When a conventional condenser was employed to reduce the organcic emissions, condensate accumulated in the exhaust duct.
Development of the Electrostatic Condensate Separator JUNIPER
The newly developed system is an electrostatic condensate separator. It consists of a heat exchanger combined with electrostatic separator columns. The heat exchanger and the separator columns are installed above a tank in which the condensate is collected. Organic carbon compounds with low vapor pressure form aerosols upon cooling. These droplets are then trapped in the particle filter. The entire system is encased by a cabinet. Collected condensate can be automatically drained into a central collection tank. The system has safety interlocks, which in case of an alarm will stop the printer line. It can be connected to a remote monitoring system.
Evaluation Results JUNIPER
After successful evaluation of the prototype, several JUNIPER systems were installed at printing lines in the same solar cell production site. Measurements of the concentrations of total organic carbon before and after the system showed that more than 97% of volatile organic components are removed by the JUNIPER. The prototype system showed stable operation for at least one year without maintenance.
The following examples show measurements of total organic carbon concentration (measured by FID) during production at two different systems.
Summary: Reduction of Organic Emissions
With the electrostatic condensate separator JUNIPER the load of organic carbon in the exhaust after the firing process could be reduced by more than 97%. The installed systems show stable operation. Manual handling of condensate canisters can be avoided by automatically emptying of the condensate with extremely low maintenance requirement.