Current Density Dependence of Transient Electroluminescence in Phosphorescent Organic Light-Emitting Diodes
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Current Density Dependence of Transient Electroluminescence in Phosphorescent Organic Light-Emitting Diodes Hirotake Kajii, Noriyoshi Takahota, Yadong Wang and Yutaka Ohmori Center for Advanced Science and Innovation, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
ABSTRACT The transient electroluminescence (EL) of phosphorescent organic light-emitting diodes (OLEDs) was investigated. The behaviors of the transient characteristics are analyzed using the triplet-triplet annihilation model. The device exhibited a gradual decrease in quantum current efficiency owing to the triplet-triplet annihilation at a high current density. At a higher current density, the reduced rise and decay times are due to high-density triplet excitons related to the enhanced triplet-triplet annihilation and the increase of the nonradiative process. The modulation speed of the devices is mainly limited by the phosphorescent recombination lifetime. INTRODUCTION Organic light-emitting diodes (OLEDs) utilizing fluorescent dyes or conducting polymers are capable of emission over a wide visible range, high efficient, and require only a low driving voltage. White light OLEDs [1,2], in particular, polymer-based OLEDs have attracted attention because of their potential for large area flat-panel displays, backlight and other lighting sources by employing solution-processing such as spin-coating and ink-jet printing. Applications for near-infrared OLEDs are particularly interesting for sensors. Recently, the phosphorescent organic light-emitting diodes (OLEDs) using phosphorescent dyes have demonstrated high external quantum efficiencies [1,3]. These materials incorporate heavy metal atoms, such as iridium and platinum, to mix singlet and triplet states by strong spin-orbit coupling. The response time of OLEDs with phosphorescent materials such as Ir complexes is slower than that with fluorescent ones because of their long emission lifetime. The applied-voltage dependence of fundamental transient electroluminescence of OLEDs has been mostly investigated [4-6]. In this study, we focused on the applied-voltage and in particular, current-density dependences of the transient electroluminescence (EL) of the phosphorescent OLEDs. EXPERIMENT We fabricated OLEDs using various phosphorescent dyes, iridium (III) bis(2-(4,6difluorephenyl)pyridinato-N,C2’) [FIrpic], tris(2-phenylpyridine)iridium(III) [Ir(ppy)3], tris(1phenylisoquinoline)iridium(III) [Ir(piq)3] and Pt-tetraphenyltetrabenzoporphyrin [Pt(tpbp)] [7] as blue, green, red and near-infrared emitters, respectively. The substrate was degreased with solvents and cleaned in a UV ozone chamber. To reduce the possibility of electrical shorts within the OLEDs, a 40-nm-thick poly(ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT: PSS) hole-injection layer was spin-coated on an indium tin oxide (ITO)-coated glass substrate and baked at 120oC for 20 min. The layer structure was fabricated by organic molecular beam
deposition to form the EL devices at a background pressure of about 10-5 Pa
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