The authors demonstrate that the reduction of quantum efficiency with increasing current density in phosphorescent light emitting diodes ͑PhOLEDs͒ is related to the formation of excitons in hole transporting layer based on the analysis of emission spectra and exciton formation zone. Low roll-off of efficiency in a PhOLED was achieved using dual emitting layers ͑D-EMLs͒ by confining the exciton formation near the interface between the emitting layers. The external quantum efficiency was maintained almost constant up to 22 mA/ cm 2 ͑10 000 cd/ m 2 ͒ by adopting the D-EMLs in Ir͑ppy͒ 3 based PhOLEDs, resulting in high external quantum efficiency ͑ ext = 13.1% ͒ at high luminance. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2745224͔ Phosphorescent organic lighting emitting diodes ͑PhOLEDs͒ have received considerable attention due to their ability of highly efficient emission compared with fluorescent OLEDs.1-4 Through harvesting of both singlet and triplet excitons, the external quantum efficiency ͑ ext ͒ of PhOLEDs has reached above 20% by using the optimized material systems, 5-7 p-i-n structures, 8,9 and microcavity structures.10,11 However, the efficiency roll-off ͑the decrease of efficiency with increasing current density͒ occurs at much lower luminance than required in displays or solid-state lighting. The roll-off of the quantum efficiency is one of the most significant problems facing electrophosphorescent devices and its origin was attributed to the triplet-triplet annihilation coming from long lifetime of triplet excitons, 12,13 electric field induced dissociation of excitons, 14 and tripletpolaron annihilation. 12,13 In this letter, we report that the roll-off of the quantum efficiency with an increasing current density is related to the exciton formation in the hole transporting layer ͑HTL͒. Analysis of the steady state emission spectra indicated that the significant portion of the efficiency reduction is originated from the more and more exciton formation in the HTL with increasing current density. Based on the results, we fabricated devices with double emitting layers ͑D-EMLs͒ in order to confine the exciton formation in the emitting layers. The external quantum efficiency ͑ ext ͒ was maintained constant up to 22 mA/ cm 2 ͑10 000 cd/ m 2 ͒ by adopting the D-EMLs in Ir͑ppy͒ 3 based PhOLEDs, resulting in high external quantum efficiency at high luminescence compared to the devices with single emitting layer ͑S-EML͒. The OLEDs with D-EMLs show significantly lower roll-off of efficiency ͓ ext = 13.1% at 10 000 cd/ m 2 ͑22 mA/ cm 2 ͔͒ than conventional S-EML OLEDs ͓ ext = 7.8% at 5400 cd/ m 2 ͑20 mA/ cm 2 ͒, and 6.9% at 10 000 cd/ m 2 ͑40 mA/ cm 2 ͔͒. The OLEDs were fabricated by thermal evaporation onto a cleaned glass substrate precoated with indium tin oxide ͑ITO͒ without breaking the vacuum. Prior to organic layer deposition, ITO substrates were exposed to UV-ozone flux for 10 min following degreasing in aceton and isoprophyl alcohol. All organic layers were grown by thermal evaporation at the...

The authors report a promising metal oxide-doped hole transporting layer ͑HTL͒ of rhenium oxide ͑ReO 3 ͒-doped N , NЈ-diphenyl-N , NЈ-bis ͑1,1Ј-biphenyl͒-4,4Ј-diamine ͑NPB͒. The tris͑8-hydroxyquinoline͒ aluminum-based organic light-emitting diodes with ReO 3-doped NPB HTL exhibit driving voltage of 5.2-5.4 V and power efficiency of 2.2-2.3 lm/ W at 20 mA/ cm 2 , which is significantly improved compared to those ͑7.1 V and 2.0 lm/ W, respectively͒ obtained from the devices with undoped NPB. Furthermore, the device with ReO 3-doped NPB layer reveals the prolonged lifetime than that with undoped NPB. Details of ReO 3 doping effects are described based on the UV-Vis absorption spectra and characteristics of hole-only devices.

Light extraction efficiency of a conventional organic light emitting diode (OLED) remains limited to approximately 20% as most of the emission is trapped in the waveguide and glass modes. An etchless simple method was developed to fabricate two-dimensional nanostructures on glass substrate directly by using ultraviolet (UV) curable polymer resin and UV nanoimprint lithography in order to improve output coupling efficiency of OLEDs. The enhancement of the light extraction was predicted by the three-dimensional finite difference time domain method. OLEDs integrated on nanoimprinted substrates enhanced electroluminance intensity by up to 50% compared to the conventional device.

Highly efficient deep‐blue fluorescent materials based on phenylquinoline–carbazole derivatives (PhQ‐CVz, MeO‐PhQ‐CVz, and CN‐PhQ‐CVz) are synthesized for organic light‐emitting diodes (OLEDs). The materials form high‐quality amorphous thin films by thermal evaporation and the energy levels can be easily adjusted by the introduction of different electron‐donating and electron‐withdrawing groups on carbazoylphenylquinoline. Non‐doped deep‐blue OLEDs that use PhQ‐CVz as the emitter show bright emission (Commission Internationale de L'Éclairage (CIE) coordinates, x = 0.156, y = 0.093) with an external quantum efficiency of 2.45%. Furthermore, the material works as an excellent host material for 4,4′‐bis(9‐ethyl‐3‐carbazovinylene)‐1,1′‐biphenyl dopant to get high‐performance OLEDs with excellent deep‐blue CIE coordinates (x = 0.155, y = 0.157), high power efficiency (5.98 lm W−1), and high external quantum efficiency (5.22%).

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