New Host Materials Containing Carbazole and Naphthyl Moieties for Green Dopant in Phosphorescence Organic Light-Emitting Diodes (OLEDs)

Abstract

Much effort on the development of new host materials has been devoted to increase the emission efficiency of organic light-emitting diodes (OLEDs). In particular, the use of phosphorescent molecules as guest emitters, which harvests both singlet and triplet excitons, leads to internal quantum efficiency (ηint) approaching 100%. 2 Energy transfer from a host to a dopant in OLEDs can result in high external quantum efficiency (ηext). Therefore, the development of suitable hosts for blue, green and red phosphorescent emitters has been one of the major issues in the field of OLEDs. In general, host materials should have a higher triplet energy than that of the dopant molecules due to the energy transfer from the host to the guest while prohibits the energy transfer of triplet excitons from the guest to the host material. From the point of this consideration, carbazole derivatives are often use as host materials, because they have high triplet energies (2.9 eV or less). Polyaromatic compounds bearing naphthyl groups have formed an important class of highly efficient and stable bluelight emitting materials. It has been suggested that nonplanar derivatives of naphthalene due to steric factors may hinder close packing and improve the device performance; hence, the electroluminescence (EL) polyaromatic compounds bearing naphthyl groups have been designed on the basis of this principle. During our ongoing efforts on the development of emitting materials and host for phosphorescence OLEDs, we have observed that naphthyl derivatives linked by 9-ethylcarbazole unit as a core, such as 3,6-di(1naphthyl)-9-ethylcarbazole (1-DNEC) and 3,6-di(2-naphthyl)-9-ethylcarbazole (2-DNEC), have high thermal stability, reversible electrochemical behaviors and larger band gap compared to those of other carbazole derivatives. These facts have prompted us to investigate electroluminescent (EL) characteristics of both compounds. Herein, we discuss electroluminescent characteristics and device data for OLEDs employing both 1-DNEC and 2-DNEC as host. We fabricated multilayer devices with the configuration of ITO/NPB (30 nm)/(device 1) 1-DNEC and 2-DNEC (device 2): Ir(ppy)3 (45 nm: 3%)/Alq3 (25 nm)/LiF (2 nm)/Al. The layers of the device consist of ITO (indium-tin-oxide) as the anode, NPB (N,N'-di-1-naphythyl-N,N'-diphenyl-byphenyl4,4'-diamine) as the hole transporting layer, 1-DNEC or 2DNEC as host, 3%-Ir(ppy)3 (ppy = 2-phenylpyridine) as the emitter, Alq3 (q = 8-hydroxyquinolate) as the electron transporting layer, LiF as the electron injection layer, and Al as the cathode, respectively (Figure 1). In this study, we chose Ir(ppy)3 as the emissive material, which is a prototypical dopant for green OLEDs. The electroluminescence spectra of all devices consist only of green emission (λmax = 458 nm) from Ir(ppy)3 without any emissions from the host and Alq3,