ABSTRACT

Figure 9.1 Aging of an orange OLED with insufficient encapsulation. Organic electronic devices show the strongest requirements in comparison to other products that require protection against these gases. These requirements depend on the technology, materials used, and desired product properties and expected product lifetime. In literature, often the water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) are used as quantities to describe on the one hand the performance of an encapsulation system and on the other hand the maximum allowed amount of water to reach an organic device before device failure. These values (WVTR and OTR) describe a mass of water or volume of oxygen permeating through the encapsulation system per time unit (commonly one day) and area (commonly 1 m2). The units of the values are given as [WVTR] = g/(m²day) and [OTR] = cm³/(m²day bar). As we will see below both WVTR and OTR are no material constants but depend on the environmental conditions (mainly temperature and relative humidity) at which they are measured. Therefore also WVTR and OTR requirements given for organic electronic devices depend on their desired application environment. There is a big difference

between indoor use (23°C and about 50% relative humidity) and, for example, outdoor use in tropical conditions (up to 38°C and 90% relative humidity). Browsing the literature about encapsulation of organic electronic devices, the requirement for an encapsulation system with a WVTR of 10-6 g/(m²day) can be found in many papers. This requirement is usually derived for OLEDs, which sometimes include highly reactive low-work function metals as the cathode. The assumption of primary metal oxidation, causing injection/extraction barriers for charge carriers, is well justified and indeed requires very strong encapsulation in the range below 10-5 g/(m²day) for reasonable lifetimes. While OLEDs strongly suffer from local degradation at defects in the encapsulation (see Fig. 9.1), other devices such as organic solar cells are more sensitive to a total amount of water reaching the device over time on a certain surface area. Only 20 mg of water is sufficient to reduce the power efficiency of a small-molecule organic solar cell with a size of 1 m² by 50%. Assuming an expected lifetime of 10,000 hours, that corresponds to a WVTR of 2 × 10-5 g/m²day. This is still less demanding than the 10-6 g/m²day given for OLED. The easiest way to reach such low WVTR is to use glass to encapsulate the device. Glass is impermeable to both water and oxygen and has no local defects that could lead to dead-spot formation. However, it has limited resistance against mechanical impacts and damage and is not flexible. To allow flexible (meaning bendable and formable) devices, coated polymer webs can be used as encapsulation, as shown in the next sections below. 9.2 Types of Encapsulation

Depending on the desired lifetime, the inherent stability of the system and the target market for the organic device, different encapsulation approaches may be chosen, as shown in Fig. 9.2. Among these a glass/glass encapsulation Fig. 9.2a), consisting of a glass substrate with the device and a second glass sheet mounted on top, sealing the device, is still the strongest and most durable encapsulation. However, this type of encapsulation only allows rigid and flat devices. Mostly thin-film barrier-coated polymer webs (e.g., polyethylene terephthalate [PET]) are used to manufacture

flexible devices. These barrier films are laminated on a device that was processed on a rigid glass substrate or metal foil (Fig. 9.2b) or they can be used as a substrate for the device with an additional barrier film laminated on top (Fig. 9.2c). The technologically most challenging option is shown in Fig. 9.2d: thin-film encapsulation (TFE) applied directly on the organic device. This TFE is done by vacuum coating techniques such as plasma-enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD) and is often combined with subsequent barrier film lamination (similar to Fig. 9.2c). In any case, it is important to know which side of the device is the active side. The active side thereby means the side of the device to which the light is emitted by an OLED or from which an organic solar cell or sensor is illuminated. On this side the encapsulation has to be optically transparent. Metal foils or thick metal layers cannot be used on the active side. Glass, many oxide barrier layers, and often used polymer webs fulfill the transparency requirement.