Organic light-emitting diodes with 30% external quantum efficiency based on a horizontally oriented emitter
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...
We demonstrate a high-performance flexible organic light-emitting diode ͑OLED͒ employing amorphous indium zinc oxide ͑IZO͒ anode. The amorphous IZO on flexible polycarbonate ͑PC͒ substrate shows similar electrical conductivity and optical transmittance with commercial ͑ITO͒ glass, even though it was prepared at Ͻ50°C. Moreover, it exhibits little resistance change during 5000 bending cycles, demonstrating good mechanical robustness. A green phosphorescent OLED fabricated on amorphous IZO on flexible PC shows maximum external quantum efficiency of ext = 13.7% and power efficiency of p = 32.7 lm/W, which are higher than a device fabricated on a commercial ITO on glass ͑ ext = 12.4% and p = 30.1 lm/W͒ and ITO on flexible PC ͑ ext = 8.5% and p =14.1 lm/W͒. The mechanical robustness and low-temperature deposition of IZO combined with high OLED performance clearly manifest that the amorphous IZO is a promising anode material for flexible displays. There has been increasing activity for flexible organic lightemitting diodes ͑OLEDs͒ over the past few years, and it has focused on developing indium tin oxide ͑ITO͒-coated polymer substrates, such as polyethylene terephthalate ͑PET͒, 1-5 polycarbonate ͑PC͒,polyimide, 8 polyethersulfone ͑PES͒, 9 polyethylene naphthalate ͑PEN͒, 9 and polycyclic olefin ͑PCO͒. 9 However, ITO electrode comes with its own set of problems such as chemical instability in a reduced ambient, poor transparency in the blue region, release of oxygen and indium into the organic layer, imperfect work function alignment with typical hole-transport layers, and easy deterioration of ITO targets. 5 In addition, the optimum properties of ITO film can only be obtained from fully crystallized film deposited at high temperature ͑ϳ300°C͒ or annealed in air or oxygen ambient. 10 With the increasing interest in the development of flexible OLEDs, there is great need for a more mechanically robust and transparent electrode because the resistance of crystallized ITO films on flexible substrate increases with increasing mechanical stain. The increase in resistance is related to the number of cracks generated in the electrode, which depends on applied strain and film thickness. 1,3 For this reason, new transparent conducting materials have been explored to replace ITO for flexible OLED anodes. To achieve better performance of flexible OLEDs, Zn-based transparent conducting oxide ͑TCO͒ films, such as ZnSn 2 O 4 , ZnSnO 3 , Zn 2 In 2 O 5 , and Al͑Ga͒-doped ZnO, have been applied as the anode.11-16 Among various Zn-based transparent conducting oxides, the Zn-doped In 2 O 3 ͑IZO͒ films recently have been recognized as promising TCO materials for OLEDs due to its good conductivity, high transparency, excellent surface smoothness, high etching rate, and low deposition temperature. [13][14][15][16] In particular, it has been confirmed that electrical and optical properties of amorphous IZO ͑a-IZO͒ films can be optimized at Ͻ50°C without a postannealing process. Therefore it is considered that a-IZO anode films can be applied to...
We report that an exciplex is formed at the interface between the N,N′-dicarbazolyl-4-4′-biphenyl (CBP) and the bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM), which are widely used as an emitting layer (EML) host and an electron transporting layer (ETL) for high efficiency, green phosphorescent, organic light-emitting diodes (OLEDs), respectively. The intensity of the exciplex emission is almost proportional to the inverse square of the fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] concentration of the EML. Meanwhile, the efficiency of the OLEDs increases as the concentration of the Ir(ppy)3 increases. This enhancement of the efficiency and the decrease of the exciplex emission originates from the increase in the energy transfer rate from the exciplex to the dopants, due to the decrease in the distance between the exciplex and the dopant. The energy transfer processes were successfully analyzed using the Förster energy transfer mechanism. The high-efficiency OLEDs were obtained through the energy transfer from the exciplex to the dopant at the EML/ETL interface. The external quantum efficiency of the OLED reached 20.1% when the concentration of the Ir(ppy)3 is 6 mol. %. In addition, we investigated the relationship between the efficiency roll-off of the OLEDs and the energy transfer from the exciplex to the dopant by inserting a thin, undoped CBP layer at the EML/ETL interface.
The bulk‐ionized photoconductivity of C60 is reported as an origin of the bias‐dependent linear change of the photocurrent in copper phthalocyanine (CuPc)/C60 planar heterojunction solar cells, based on the observation of the variation of the bias‐dependent photocurrent on excitation wavelengths and the thickness‐dependent photocurrent of the C60 layer. A theoretical model, which is a combination of the Braun‐Onsager model for the dissociation of excitons at the donor/acceptor interface and the Onsager model for the bulk ionization of excitons in the C60 layer, describes the bias‐dependent photocurrent in the devices very well. The bulk‐ionized photoconductivity of C60 must generally contribute to the photocurrent in organic photovoltaics, since fullerene and fullerene derivatives are widely used in these devices.
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