Direct Hole Injection Research Articles

Color tunable white organic light-emitting devices with a hybrid 2-methyl-9,10-di(2-naphthyl)anthracene small molecule/poly(2-methoxy-5-(2-ethyhexoxy)-1,4-phenylenevinylene) polymer emitting layer

White organic light-emitting devices (WOLEDs) were fabricated utilizing a hybrid poly(2-methoxy-5-(2-ethyhexoxy)-1,4-phenylene vinylene) (MEH-PPV)/2-methyl-9,10-di(2-naphthyl) anthracene (MADN) emitting layer. Nano-sized pores were formed on the surface of the MEH-PPV layer by using a selective etching of polystyrene (PS) in the MEH-PPV:PS blending layer due to the enhancement of the direct hole injection from hole injection layer (HIL) into the MADN layer. The contact area between the HIL and the MADN layer was widened with increasing ratio of the PS in the MEH-PPV:PS blending polymer, resulting in the enhancement of the electroluminescence intensity for MADN. One of the WOLEDs utilizing the polymer with the ratio of 1:2 (0.44, 0.31) met the requirements of the Commission Internationale de l'Eclairage for the white color.

Scanning Tunneling Luminescence of Individual CdSe Nanowires

The local luminescence properties of individual CdSe nanowires composed of segments of zinc blende and wurtzite crystal structures are investigated by low-temperature scanning tunneling luminescence spectroscopy. Light emission from the wires is achieved by the direct injection of holes and electrons, without the need for coupling to tip-induced plasmons in the underlying metal substrate. The photon energy is found to increase with decreasing wire diameter due to exciton confinement. The bulk bandgap extrapolated from the energy versus diameter dependence is consistent with photon emission from the zinc blende-type CdSe sections.

Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes

The conventional carrier-blocking design of the exciton formation zone used in nearly all organic light emitting diodes is shown to be problematic, due to exciton quenching from accumulated radical cations. To reduce exciton quenching, a single layer of 4,4′-N,N′-dicarbazole-biphenyl (CBP) is used as hole transport layer, resulting in a dramatically improved device efficiency even at high luminance (e.g., 20.5 cd/A at 100 000 cd/m2 for fluorescent green). Various high work function transition metal oxides (WO3, V2O5, and MoO3) coated on indium tin oxide anodes have been shown to enable direct hole injection into the deep highest occupied molecular orbital of CBP (6.1 eV).

Low voltage red phosphorescent organic light-emitting devices with triphenylphosphine oxide and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl electron transport layers

We have developed red phosphorescent organic light-emitting devices operating at low voltages by using triphenylphosphine oxide (Ph 3PO) and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) electron transport layers. 4,4′-bis( N-carbazolyl)-1,1′-biphenyl (CBP) and tris-(1-phenylisoquinolinolato- C 2, N) iridium(III) [Ir(piq) 3] were used as host and guest materials, respectively. Small voltage drops across the electron transport layers and direct injection of holes from 4,4′,4″-tris[ N-(2-naphthyl)- N-phenyl-amino]-triphenylamine (2-TNATA) hole transport layer into the Ir(piq) 3 guests are responsible for the high current density at low voltage, resulting in a high luminance of 1000 cd/m 2 at low voltages of 2.8–3.0 V in devices with a structure of ITO/2-TNATA/CBP:Ir(piq) 3/DPVBi/Ph 3PO/LiF/Al.

Electroluminescent Cu‐doped CdS Quantum Dots

Incorporating Cu-doped CdS quantum dots into a polymer host produces efficient light-emitting diodes. The Cu dopant creates a trap level that aligns with the valence band of the host, enabling the direct injection of holes into the quantum dots, which act as emitters. At low current densities, the luminance efficiency maximizes at 9 cd A-1, providing an external quantum efficiency of 5%.

Efficient single layer RGB phosphorescent organic light-emitting diodes

We report efficient single layer red, green, and blue (RGB) phosphorescent organic light-emitting diodes (OLEDs) using a “direct hole injection into and transport on triplet dopant” strategy. In particular, red dopant tris(1-phenylisoquinoline)iridium [Ir(piq) 3], green dopant tris(2-phenylpyridine)iridium [Ir(ppy) 3], and blue dopant bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium [FIrpic] were doped into an electron transporting 1,3,5-tris( N-phenylbenzimidazole-2-yl)benzene (TPBi) host, respectively, to fabricate RGB single layer devices with indium tin oxide (ITO) anode and LiF/Al cathode. It is found that the maximum current efficiencies of the devices are 3.7, 34.5, and 6.8 cd/A, respectively. Moreover, by inserting a pure dopant buffer layer between the ITO anode and the emission layer, the efficiencies are improved to 4.9, 43.3, and 9.8 cd/A, respectively. It is worth noting that the current efficiency of the green simplified device was as high as 34.6 cd/A, even when the luminance was increased to 1000 cd/m 2 at an extremely low applied voltage of only 4.3 V. A simple accelerated aging test on the green device also shows the lifetime decay of the simplified device is better than that of a traditional multilayered one.

Efficient bilayer phosphorescent organic light-emitting diodes: Direct hole injection into triplet dopants

In phosphorescent organic light-emitting diodes (OLEDs), a hole transporting layer is traditionally thought to be required to facilitate hole injection into the host molecule. It is found that fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] doped into 4,4′-N,N′-dicarbazole-biphenyl can be used to directly inject and transport holes from an indium tin oxide anode, and thus simplify the device structure and selection of materials. The efficiencies of the simplified bilayer OLEDs exceed 41 lm/W and 57 cd/A at a brightness of 100 cd/m2. We attribute the excellent performance to direct hole injection from the anode to Ir(ppy)3 dopant.

Highly efficient simplified organic light emitting diodes

The authors report on highly efficient organic light emitting diodes (OLEDs) consisting of only two organic layers. The key to the simplification is the direct injection of holes into the wide band gap hole transport material 4,4′,4″-tris(N-carbazolyl)-triphenyl amine (highest occupied molecular orbital is 5.9eV) through an indium tin oxide/tungsten oxide (WO3) anode. Kelvin probe analysis has revealed an extremely high work function of 6.4eV for WO3. The efficiencies of the simplified OLEDs exceed 40lm∕W and 45cd∕A at a brightness of 100cd∕m2, unsurpassed by other comparably simple OLED devices. Therefore, our OLED architecture demonstrates highly efficient, yet easy to fabricate devices.

Photoemission study of direct hole injection into Alq 3 by PEDOT:PSS polymer anode

Combined ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) measurements have been performed on the interface between the electron transport material tris-(8-hydroxy quinoline) aluminum (Alq 3) and the conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS), indium tin oxide (ITO) and polycrystalline gold. The estimated hole injection barrier φ h is smallest at the PEDOT:PSS/Alq 3 contact and largest at the Au/Alq 3 contact. In the absence of chemistry at each interface, as indicated by XPS, φ h are attributed to Fermi level pinning by interface states. The relative differences in φ h are helpful in elucidating recent device measurements that show direct hole injection into Alq 3 from PEDOT:PSS [P.A. Lane, G.P. Kushto, Z.H. Kafafi, Appl. Phys. Lett. 90 (2007) 023511].

Charge-carrier injection characteristics at organic/organic heterojunction interfaces in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) having various guest molecules doped in an organic host matrix layer are fabricated [the OLED structure is anode/hole-transporting layer (HTL)/guest–host emitting layer/hole-blocking layer/electron-transporting layer/cathode], and the dependence of current density–voltage ( J– V) characteristics of the OLEDs on highest occupied molecular orbital (HOMO) levels of guest molecules are investigated. From the J– V characteristics of these OLEDs, we find two important results: (1) J– V characteristics of the OLEDs are controlled by the direct hole injection from the neighboring HTL to guest molecules, and (2) HOMO level alignment between the HTL and guest molecules provides efficient hole injection at this interface.

Efficient, single-layer molecular organic light-emitting diodes

The authors demonstrate efficient molecular organic light-emitting diodes that use direct hole injection from poly(3,4-ethylene-dioxythio-phene):poly(styrene-sulfonate) into a single layer of tris(8-hydroxyquinoline) aluminum (III) for carrier transport and electroluminescence. Single-layer devices have a lower operating bias and higher luminous power efficiency than conventional bilayer devices with a 4,4-bis[N-1-napthyl-N-phenyl-amino]biphenyl hole transport layer. The current density-voltage characteristics of single-layer devices follow Schottky-Richardson behavior and are consistent with an Ohmic contact at the anode.

Efficient Polymer Electrophosphorescent Devices with Interfacial Layers

AbstractIt is shown that several polymers can form insoluble interfacial layers on a poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer after annealing of the double‐layer structure. The thickness of the interlayer is dependent on the characteristics of the underlying PEDOT:PSS and the molecular weight of the polymers. It is further shown that the electronic structures of the interlayer polymers have a significant effect on the properties of red‐light‐emitting polymer‐based electrophosphorescent devices. Upon increasing the highest occupied molecular orbital and lowest unoccupied molecular orbital positions, a significant increase in current density and device efficiency is observed. This is attributed to efficient blocking of electrons in combination with direct injection of holes from the interlayer to the phosphorescent dye. Upon proper choice of the interlayer polymer, efficient red, polymer‐based electrophosphorescent devices with a peak luminance efficiency of 5.5 cd A–1 (external quantum efficiency = 6 %) and a maximum power‐conversion efficiency of 5 lm W–1 can be realized.

Excitation and transport of hot holes in a magnetic tunnel transistor

Spin-dependent transport of nonequilibrium holes has been investigated using a magnetic tunnel transistor in which a magnetic tunnel junction is combined with a p-type semiconductor. The device can be used for direct hole injection and collection, or in reverse mode in which holes are created by inelastic decay of injected hot electrons via electron-hole pair generation. In the latter case, the collected hole current is larger, and a magnetocurrent (MC) of 90% is observed at an emitter bias of −0.8V. This positive and large MC indicates that hot holes generated by hot electrons of majority spin contribute mostly to the collector current.

Polymer electrophosphorescent devices utilizing a ladder-type poly(para-phenylene) host

The properties of light-emitting diodes based on the electrophorescent platinum-porphyrin dye PtOEP blended into a ladder-type poly(para-phenylene) (LPPP) polymer host are presented. Due to the small difference between the highest occupied molecular orbital (HOMO) levels of the guest and the host, the operating voltage of single layer devices is almost independent of the dopant concentration in clear contrast to what has been observed in the case of wide-band gap host polymers. However, the efficiency and the color purity of these single-layer devices is quite poor, which can be attributed to the weak trapping of carriers on the phosphorescent dye. Incorporating an electron-transporting/hole-blocking layer greatly increases the efficiency of the devices but at the same time emission from the host becomes more significant. Adding a hole-transporting/electron-blocking layer further increases the efficiency of the devices. Pure red emission is, however, only obtained if the HOMO of the hole-transporting layer is well above that of the LPPP host. This effect is interpreted in terms of a direct injection of holes from the hole-transporting layer into the HOMO level of the dopant. Further, prompt fluorescence and phosphorescence studies indicate that energy transfer from LPPP singlet excitons contributes to the emission from the dopant, but that the efficiency of singlet exciton transfer is too small to explain the almost complete absence of host emission in the electroluminescence spectra of single- and three-layer devices.

A light-emitting device using a lateral junction grown by molecular beam epitaxy on GaAs (311)A-oriented substrates

A lateral p–n junction allows direct injection of electrons and holes in the active layer of devices such as laser diodes and can reduce carrier relaxation time and increase modulation bandwidth. Light-emitting diodes were made on patterned GaAs (311)A-oriented substrates by using a lateral p–n junction formed in GaAs–silicon-doped epilayers grown by molecular beam epitaxy. Good electroluminescence at room temperature was obtained for both GaAs single layers and GaAs/AlGaAs multiple quantum well structures.

Lateral-junction light emitting devices grown by molecular beam epitaxy on GaAs (311)A-oriented substrates

Light emitting diodes with an unconventional design were made on patterned GaAs (311)A-oriented substrates. A lateral p–n junction was formed in the GaAs–silicon doped epilayers grown by molecular beam epitaxy (MBE). Devices made with this p–n junction show good electroluminescence at room temperature. The lateral p–n junction would allow direct injection of electrons and holes in the active layer of devices such as laser diodes and could improve their characteristics.