The presence of a liquid electrolyte requires hermetic sealing of the module in order to prevent evaporation of the solvent as well as intrusion of water and oxygen. The sealing materials have to meet several requirements:
• chemical stability in contact with the liquid electrolyte,
• excellent barrier properties in order to minimise solvent losses (typically nitriles) during the service time of the module, and intrusion of water and oxygen,
• good and stable adhesion of substrates to the TCO and glass,
• processing compatible with other components of the nc-DSC.
Inorganic glass frits can be used to seal glass/glass modules. Hermetic seals have been achieved for module sizes up to 30 cm x 30 cm, by carefully selecting thermal expansion coefficients to match that of the standard float glass [9]. High temperature profiles and equipment similar to those used in industrial glass bending and glass hardening have proved necessary in this case. Consequently, the dye and electrolyte have to be filled through small holes in the glass after the sealing process. These holes must then be closed (see Sect. 7.2), implying additional cost factors. For this reason, low-cost organic adhesives, which can be applied at much lower temperatures, are still very attractive in relation to future industrial production, although organic materials are never completely tight.
Several organic sealants such as epoxy resins, butyl rubber or silicones prove to be more or less permeable and the tiny amount of solvent in the cell is rapidly lost. Suitable organic sealing materials for this technology turn out to be thermoplastic materials, like polyethylene/carboxylate copolymers. So far, Surlyn 1702 ionomer from Dupont has been the main substance used to optimize cell performance and build module prototypes. However, the softening point of Surlyn is rather low (65° C) and at elevated temperatures (> 70° C), serious solvent loss is observed because the bond between Surlyn and TCO-coated glass is substantially weakened [7].
For outdoor applications, solar cells/modules have to survive temperatures up to 80°C, and Surlyn would not be the best material. Therefore, polymer-sealing materials with higher heat resistivity are needed. It has been reported that some linear LDPEs (low-density polyethylenes) and HDPEs (high-density polyethylenes) have shown promising results [10-12]. Bynel (an anhydride modified LLDPE) from Dupont is now regularly used as a sealing material in nc-DSCs.
In the monolithic setup, a non-active flexible foil, such as aluminium, can be used to cover the module. This is impermeable to gases and vapors. The metal foil is laminated with a thermoplastic layer (Surlyn), which is needed for hot sealing, while an additional layer of polyester is used for corrosion protection and electrical insulation.
