Air-Stable Inverted Organic Light-Emitting Diodes by Katsuyuki Morii & Hirohiko Fukagawa

Author:Katsuyuki Morii & Hirohiko Fukagawa
Language: eng
Format: epub
ISBN: 9783030185145
Publisher: Springer International Publishing

In addition to the initial device characteristics, the long-term storage stability of the conventional and inverted OLEDs in air was examined by using a barrier film with a water vapor transmission rate (WVTR) of about 10−4 g m−2 day−1. Although the short-term air stability of inverted OLEDs was reported by S. Höfle et al. [14], the long-term air stability had not been reported. Both the conventional OLED and the inverted OLED were encapsulated by a glass frame, a barrier film and a UV epoxy resin in a nitrogen atmosphere. The light-emitting areas of the OLEDs, which were kept in air at room temperature and pressure, were observed routinely using an optical microscope. A dc current was applied to the OLEDs during measurements but not under storage conditions. Figure 3.13 shows images of the light-emitting areas of the OLEDs as a function of storage time. The images were obtained without changing the measurement conditions, such as the applied current, diaphragm and exposure time of the optical microscope. Dark-spot formation was clearly observed in the conventional OLED after 15 days of exposure to the atmosphere, and the emitting-area decreased by about half after 103 days. The observed dark-spot formation and emitting-area shrinkage may originate from the degradation/oxidisation of lithium fluoride and/or aluminium, both of which are widely used in conventional OLEDs, and these phenomena are not surprising since it has been proposed that stringent encapsulation (a WVTR of about 10−6 g m−2 day−1) is necessary for conventional OLEDs [18]. On the other hand, the inverted OLED encapsulated by a barrier film exhibited high storage stability in air; no dark-spot formation or emitting-area shrinkage was observed after 250 days. This is because the inverted OLED was fabricated without using alkali metals or aluminium, resulting in its high storage stability in air.

Fig. 3.13Images of light-emitting areas of OLEDs encapsulated by a barrier film as a function of storage time

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