Author: Dr. Lena Reinke | lena.reinke@panacol.de
Perovskite-based (PSC) and organic photovoltaic (OPV) cells represent a promising frontier in renewable energy generation technologies due to their potential for lightweight, flexible and potentially cost-effective solar power generation. However one of the critical challenges in the commercialization and long-term reliability of OPV cells is their encapsulation. Effective encapsulation protects the delicate photoactive materials from environmental factors such as moisture, oxygen and mechanical stress. Adhesives play a crucial role in this encapsulation process, offering not only mechanical stability but also environmental protection. This article focuses on the various types of adhesives used in the encapsulation of OPV or PSC cells.
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Encapsulation adhesives for OPV or PSC cells must meet several stringent criteria:
Moisture and Oxygen Barrier: As the active materials are sensitive towards humidity and/or oxygen, the adhesive should therefore provide good barrier properties to prevent a degradation caused by moisture or oxygen ingress. The adhesive barrier properties can be tuned using to different strategies. One can either use quite hydrophobic raw materials to chemically prevent the ingress of water molecules or the cross-linking properties of some multifunctional components can build up a dense polymer network after curing so that the water ingress is blocked mechanically.
UV stability: The adhesive should also withstand prolonged exposure to sunlight without turning yellowish or getting brittle. This is especially important for PV applications that will be used outside.
Adhesion strength: It is important to ensure the mechanical integrity of the devices. Therefore, the adhesive needs to firmly bond the encapsulant material to the PV cell layers. Within initial tests, we check the adhesive performance by conducting T-Peel tests. Often it is very helpful to use a pretreatment technology such as corona or plasma to ensure sufficient surface energy and thus an enhanced T-Peel strength.
Compatibility: While sufficient adhesion strength is crucial, we also need to address the compatibility between the adhesive and the active layers. While an effective binding towards plastic substrates can often be provided by using monomeric raw materials, those can be quite harmful for the sensitive photoactive materials especially if a pronounced polarity is present. To overcome this issue, chemists need to balance out the ratio between oligomeric and monomeric compounds.
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For the encapsulation process, there are several types of adhesives that can be interesting. Epoxy-based formulations are widely used as they typically bring chemical resistance and thermal stability. This is due to the durable bonds that are formed during the polymerization process and accompanied by significantly improved barrier properties.
Acrylic-type adhesives are also commonly used, especially if it comes to more challenging substrates as for example PET. Here, the required flexibility can be achieved more easily as for the usually more rigid epoxies. An additional advantage for acrylic-based adhesives is the typically faster polymerization. As time often is a critical criteria the curing speed should always be taken into consideration. In Table 1 a qualitative comparison between the two adhesive groups is listed.
Table 1 Qualitative comparison of the main features of epoxy- and acrylate-based adhesives.
It is obvious that for both adhesive types there are advantages and drawbacks. This is caused by the adhesive’s properties influencing each other. The best example is that the barrier properties of a flexible acrylate-based adhesive are often worse in comparison to a highly crosslinked material. On the other hand, the flexibility of an adhesive also strongly influences the adhesion properties on flexible substrates positively. Therefore, it is required to balance out the adhesive’s flexibility and barrier properties.
Another interesting binding task for adhesives is the contacting of the PV cells. To address this topic, it is possible to use electrically conductive adhesives. Those adhesives typically have the drawback that a deep frozen storage and delivery is required as the metal particles tend to settle down. As the particles block UV light, a photoinduced curing is also not possible often resulting in a slow curing speed and high temperatures that are required for curing. With Elecolit 3648, Panacol has developed an adhesive that can be cured under relatively mild conditions (see table 2). The comparatively low viscosity of about 10.000 – 15.000 mPas (at shear rate 10, 25 °C) allows it to be dispensed easily.
Table 2 Possible curing parameters for Elecolit 3648.
The previously mentioned adhesive properties are complemented by the adhesives' impressive flexibility which allows Elecolit 3648 to be used within flexible PV devices.
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