Electroluminescent behaviours of polymer/organic heterostructure devices (original) (raw)
Related papers
High performance organic polymer light-emitting heterostructure devices
Applied Physics Letters, 1999
We report a high performance electroluminescence device based on bi-layer conjugated polymer structures consisting of a hole transporting ͑amine-fluorene͒ and an emissive ͑benzothiadiazole-fluorene͒ polymer layers prepared by the spin-coating technique on the glass substrate. Devices showed green emission with an electroluminescence peak located at around 545 nm and a full width at half maximum of about 80 nm. Our devices have also shown a high brightness (ϳ10 000 cd/m 2 at 0.84 mA/mm 2 ), good emission efficiency (ϳ14.5 cd/A͒ and luminous efficiency ͑2.26 lm/W͒, a large external quantum efficiency ͑3.8%͒, and a reasonable forward-to-reverse bias current rectification ratio (Ͼ10 3 at Ϯ25 V͒.
IEEE Journal of Selected Topics in Quantum Electronics, 2004
The characteristics of organic light-emitting diodes depend critically on the arrangement and choice of the constituent organic layers. Diodes constructed using poly(vinylcarbazole) doped with phosphorescent fac tris(2-phenylpyridine) iridium (III) [Ir(ppy) 3 ] as the polymer hole-transport layers and aluminum (III) bis(2-methyl-8-quinolinato) 4-phenylphenolate as hole-blocking and electron-transport layers were investigated. The peak efficiencies of the diodes were sensitive to the concentration of Ir(ppy) 3 . With an optimal 2 wt% concentration, an effective external quantum efficiency of 10% photons/electron, a luminance power efficiency of 7.3 lm/W, and a low turn-on voltage of 6 V were obtained. all in electrical engineering.
A comparison of hole blocking/electron transport polymers in organic LEDs
1999
Three main-chain aromatic polyethers with different electroactive heterocyclic moieties, 1,4-quinoxaline, 1,3,4-oxadiazole and 1,3,5-triazine, have been synthesized. The polymers are amorphous with glass transition temperatures above 200 8C. The polymers with these high electron affinity units were used as hole blocking/electron transport layers (HBETL) in lightemitting diodes (LEDs) having the HBETL casted on top of a hole transport/emitting PPV layer. In order to compare the influence of the different polyethers on the LED characteristics, three multilayer devices (ITO/PPV/HBETL/Al) with different HBETLs were investigated. Relative to the single layer PPV device, quantum efficiencies were improved by two orders of magnitude in all multilayer devices and power efficiency was increased using poly(quinoxaline ether) as HBETL. To investigate the electrochemical behavior of the three HBETLs, cyclic voltammetry measurements were carried out and the HOMO/LUMO energy values determined from redox potentials were used to understand the hole blocking property. Lowering the onset voltage using the poly(quinoxaline ether) as HBETL in two-layer devices is compatible with the high electron affinity of this polymer.
Chinese Journal of Chemistry, 2010
High efficiency organic light-emitting-devices (OLED) have been fabricated by incorporation of a polymeric layer as a controller of the unbalanced charge. In device configuration of ITO/PEDOT:PSS/PVK/Alq 3 /LiF:Al, poly(N-vinylcarbazole) (PVK) was selected as a blocking layer (BL) because it has a hole transporting property and a higher band gap, especially a lower LUMO level than the emitting layer (Alq 3) and a higher HOMO level than the hole injection layer (PEDOT: PSS). As a result, the optimal structure with this bl layer showed a peak efficiency of 6.89 cd/A and 2.30 lm/W compared to the device without the PVK layer of 1.08 cd/A, 0.27 lm/W. This result shows that the PVK layer could effectively block the electrons from metal cathode and confine them in the emitting layer and accomplish the charge balance, which leads to enhanced hole-electron balance for achieving high recombination efficiency.
Electrical and luminescent properties of double-layer oligomeric/ polymeric light-emitting diodes
Synthetic Metals, 1996
We report the use of α-sexithiophene (T6) thin films sublimed onto glass substrates coated with indium-tin oxide in light-emitting diodes. Absolute photoluminescence quantum efficiencies were found to be in the range 10−2–10−3%, and indicate that T6 should be used as a hole injector into an emissive layer, rather than as a luminescent layer. We have fabricated double-layer organic light-emitting diodes where a cyano-substituted derivative of poly(p-phenylenevinylene) (PPV), MEH-CN-PPV, was spun on top of the T6 layer prior to evaporation of Ca-Al cathodes. Turn-on voltages for electroluminescence of about 3 V were found in structures with total thickness between 160 and 230 nm, while internal quantum efficiencies were up to 0.4%, i.e. at least ten times less than those measured in comparative devices where a PPV hole-injecting layer was used in place of T6. The substantial difference is interpreted on the basis of a significant lowering of the barrier to electron ejection from the luminescent layer into the hole-injecting layer when passing from PPV to T6. This allows inefficient recombination to take place in T6, as confirmed by the electroluminescence spectra.
Japanese Journal of Applied Physics, 2019
The performance of organic light-emitting diodes (OLEDs) and their interfacial charge density have been studied in tris-(8-hydroxyquinolate) aluminum (Alq 3)-based OLEDs with four different hole transport layers (HTLs). The four HTLs have almost identical highest occupied molecular orbital (HOMO) energy levels. The performance of the four OLEDs is not greatly changed. The interfacial charge density is determined from the hole injection voltage in the capacitance-voltage characteristics of the OLEDs. Determination of the interfacial charge density is verified by device simulation. The interfacial charge density is dependent on the HTL materials in Alq 3-based OLEDs, and is proportional to the difference in dipole moments between the HTLs and electron transport layers including Alq 3 .
Optical Materials, 2011
ABSTRACT We studied structural and optical properties of 5′ replaced pyrazoline by hindered phenol 2,6-di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H-pyrazol-3-yl)-phenol (HPhP) films for application in organic light-emitting diode (OLED) as a hole transport layer (HTL). Analysis of impedance and current–voltage characteristics of ITO/HPhP/Al structure has shown that the current is limited by a space charged region with exponential distribution of traps near Fermi level. Characteristics of electroluminescence structure ITO/HPhP/Alq3/poly(ethylene glycol) dimethyl ether/Al was studied and analyzed. We performed the comparative analysis of luminescence time decay in two types of electroluminescent devices with HTL from pyrazolines derivative with hindered phenol and without it. We showed that hindered phenol in HTL slows down the degradation processes in OLED.
Organic Light Emitting Materials and Devices XIII, 2009
The properties of phosphorescent fac tris(2-phenylpyridine) iridium [Ir(ppy 3 )]-doped poly(N-vinyl carbazole) (PVK)/4,7-diphenyl-1,10-phenanthroline (Bphen) polymer/small molecular hybrid OLEDs are described. For optimal BPhen thickness, the power efficiency of the devices exceeds 30 lm/W. The low-temperature electroluminescence-detected magnetic resonance (ELDMR) exhibits the well-known negative spin 1/2 resonance attributed to enhanced formation of trions, but the positive spin 1/2 resonance, typically observed at low temperature or at high current density, is not observed. The OLEDs' performance and the ELDMR results are discussed in relation to the nature of the defects and their density in these devices.
Optical and Electrical characteristics of LEDs based on a single organic layer
2005 Spanish Conference on Electron Devices, Proceedings, 2005
Since the discovery of conductivity in polymers and certain organic molecules, remarkable progress has been made in synthesizing organic materials, in understanding their properties and in developing them for use in electronic and optical devices [1]. Currently, polymer and organic light-emitting diodes (LEDs) [2], photovoltaic cells [3] and field effect transistor [4] are being pushed towards commercialization. Among all the conjugated polymers, poly(1,4-phenylenevinylene) or PPV as well as its derivative 2methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene or MEH-PPV have been extensively studied for LED applications. We present here a study of a single layer structure LED with MEH-PPV as the active organic layer. Optical and electrical characterization completes the study of the electroluminescence obtained from such device.
Performance and defects in phosphorescent organic light-emitting diodes
Solid State Sciences, 2010
Phosphorescent heavy metal complexes can utilize both singlet and triplet excitons and thus are interesting for doping polymer to obtain highly efficient organic light-emitting diodes. In this study, we have investigated devices using a new phosphorescent-metal complex containing fluorene and platinum added to a luminescent polymer blend, composed of 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-(1,3,4oxadiazole) (PBD) and poly(9-vinylcarbazole) (PVK). The performance of devices (luminance and yield) is measured in indium tin oxide (ITO)/poly(3-4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/(PVK-PBD-complex)/Al diodes. The devices emit an orange light with a brightness of 607 cd/m 2 and an external quantum efficiency of 0.28 cd/A at 25 V. In order to investigate the structural modifications of the polymer by the incorporation of phosphorescent-metal complex, we have studied the defect states in diodes by charge-based Deep Level Transient Spectroscopy (Q-DLTS). Analysis of Q-DLTS spectra obtained in undoped and doped devices, revealed at least three trap levels distributed in the range 0.2-0.5 eV within the band gap of the hybrid composite with trap density in the range around 10 16 cm À3. Incorporation of Pt complex into the polymer blend modified the trap states by reducing the density of traps in the blend and by creating new trap levels in the band gap.