Lamination process encapsulation for longevity of plastic-based organic light-emitting devices
Introduction
Since organic light-emitting device (OLED) was first reported in 1987 [1], much attention has been paid to improvement of long-term stability of OLEDs. Organic materials used to form light-emitting layers of OLEDs are susceptible to water, oxygen and other environmental elements present in ambient conditions [2], [3], [4]. Furthermore, electrode metals formed on the light-emitting layer are also prone to oxidation from exposure to water, oxygen, etc. [2], [5]. As a result, conventional OLEDs exhibit a short lifetime (generally defined as the time necessary to reduce luminance to 50% of initial luminance value at constant current) as a usable device in atmospheric conditions. To increase the lifetime of OLEDs, various methods and techniques have been devised for encapsulating OLEDs with protective layers of varying compositions. At present, a hermetic encapsulation technique using a glass or metal lid attached by a bead of UV cured epoxy resin has been extensively performed to protect OLEDs from oxygen and water vapors [6]. However, these types of seals are very expensive to fabricate and require extensive labor to assemble. In addition, these seals are large and heavy so that they severely limit the applications of OLEDs. For thin and lightweight passivation of OLEDs, encapsulation method based on dry process has been extensively studied [7], [8], [9], [10], [11], [12]. Yamashita et al. [7] and Kho et al. [8] reported a thin film encapsulation technique for OLEDs by plasma-polymerized parylene film. Kwon et al. [9] used multilayer of high-density polyethylene (HDPE) and aluminum (Al)–lithium (Li) alloy. Polymer materials formed by using dry or wet process have a relatively high H2O permeation rate of 100–102 g/m2 [day] and are therefore inadequate for encapsulating OLEDs. In addition, the moisture barrier property below 5×10−3 g/m2 [day] is difficult to achieve using inorganic materials deposited at or near room temperature.
Section snippets
Experimental
In this study, we report a novel encapsulation method by means of an adhesive multilayer formed by a conventional lamination process. The adhesive multilayer consists of polyacrylate-based adhesive (thickness, 15 μm) and Al (thickness, 185 μm), which exhibits water vapor transmission rate below the limit (5×10−3 g/m2 [day]) of a MOCON detection instrument (PERMATRANW® 3/31 MA). The moisture permeation rate of the multilayer was evaluated at 37.8 °C and 100% relative humidity (RH). The device
Results and discussion
We explored the change in electroluminescence (EL) of the device with the passivation layer formed by using the lamination process. The luminance–voltage (L–V) characteristics were examined to compare the electrical behavior of the device before and after forming the passivation layer. As shown in Fig. 2, the L–V characteristic of the encapsulated device was slightly shifted to high voltage, which might be due to the multiply reflected light induced by the passivation layer and/or the modified
Conclusions
We found that the adhesive multilayer could effectively protect the plastic-based devices by using a conventional lamination method. The encapsulation method, which is very simple and convenient to perform, did not influence the EL characteristics of the devices. The encapsulated device showed the lifetime of 229.4 h at the initial luminance of 1840 cd/m2, and its longevity was affected by the structure of the adhesive multilayer. Lamination process encapsulation by means of an adhesive
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