Four color stacked white organic light-emitting diodes utilizing the concept of triplet harvesting
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Four color stacked white organic light-emitting diodes utilizing the concept of triplet harvesting Th. C. Rosenow, S. Olthof, S. Reineke, B. L¨ussem, K. Leoa a Institut
f¨ur Angewandte Photophysik, Technische Universit¨at Dresden, George-B¨ahr-Straße 1, Dresden, Germany ABSTRACT
Organic light-emitting diodes (OLEDs) are developing into a competitive alternative to conventional light sources. Nevertheless, OLEDs need further improvement in terms of efficiency and color rendering for lighting applications. Fluorescent blue emitters allow deep blue emission and high stability, while phosphorescent blue emitter still suffer from insufficient stability. The concept of triplet harvesting is the key for achieving internal quantum efficiencies up to 100 % and simultaneously benefiting from the advantages of fluorescent blue emitters. Here, we present a stacked OLED consisting of two units comprising four different emitters in total. The first unit takes advantage of the concept of triplet harvesting and combines the light emission of a fluorescent blue and a phosphorescent red emitter. The second unit emits light from a single emission layer consisting of a matrix doped with phosphorescent green and yellow emitters. With this approach, we reach white color coordinates close to the standard illuminant A and a color rendering index of above 75. The presented devices are characterized by high luminous efficacies of above 30 lm/W on standard glass substrates without outcoupling enhancement. Keywords: OLED, illumination, white, stacked, triplet harvesting, luminous efficacy
1. INTRODUCTION Stable phosphorescent blue emitters with deep blue color coordinates are not yet available. Hence, the achievable color coordinates and the color rendering index (CRI) of full phosphorescent white OLEDs are limited.1, 2 Fluorescent emitters achieve deep blue emission and improve the color quality of white OLEDs. In order to overcome the low efficiencies implied by quantum statistics, we apply the concept of triplet harvesting. In this approach, the otherwise lost triplets of the fluorescent blue emitter are transferred to a phosphorescent red emitter.3, 4 However, due to the low triplet gap of most fluorescent blue emitters, a triplet transfer to a green phosphor is not possible. OLEDs combining triplet harvesting and the emission of a phosphorescent green emitter are suffering from a loss of singlets to the green emitter.5 This problem can be solved by stacking a triplet harvesting blue/red OLED on top of a yellow phosphorescent OLED. The devices are connected by a charge generation layer, where electrons and holes are generated.6, 7 This approach is equivalent to a monolithic series connection of two OLEDs.
2. EXPERIMENTAL All devices are prepared by thermal deposition at a base pressure of 10−8 mbar in a single chamber tool (Kurt J. Lesker Company). All samples are encapsulated with a second glass and epoxy resin in nitrogen atmosphere subsequently after preparation. We use standard glass substrates coated and structured with 90 nm thic
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