Polymer Light-Emitting Diodes Efficiency Dependence on Bipolar Charge Traps Concentration (original) (raw)
Related papers
Journal of Polymer Science Part A: Polymer Chemistry, 2010
A series of novel styrene derived monomers with triphenylamine-based units, and their polymers have been synthesized and compared with the well-known structure of polymer of N,N 0 -bis(3-methylphenyl)-N,N 0 -diphenylbenzidine with respect to their hole-transporting behavior in phosphorescent polymer light-emitting diodes (PLEDs). A vinyltriphenylamine structure was selected as a basic unit, functionalized at the para positions with the following side groups: diphenylamine, 3-methylphenyl-aniline, 1-and 2-naphthylamine, carbazole, and phenothiazine. The polymers are used in PLEDs as host polymers for blend systems with the following device configuration: glass/indium-tin-oxide/PEDOT:PSS/polymer-blend/CsF/Ca/ Ag. In addition to the hole-transporting host polymer, the polymer blend includes a phosphorescent dopant [Ir(Me-ppy) 3 ] and an electron-transporting molecule (2-(4-biphenyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole). We demonstrate that two poly-mers are excellent hole-transporting matrix materials for these blend systems because of their good overall electroluminescent performances and their comparatively high glass transition temperatures. For the carbazole-substituted polymer (T g ¼ 246 C), a luminous efficiency of 35 cd A À1 and a brightness of 6700 cd m À2 at 10 V is accessible. The phenothiazine-functionalized polymer (T g ¼ 220 C) shows nearly the same outstanding PLED behavior. Hence, both these polymers outperform the well-known polymer of N,N 0 -bis(3-methylphenyl)-N,N 0diphenylbenzidine, showing only a luminous efficiency of 7.9 cd A À1 and a brightness of 2500 cd m À2 (10 V). V C 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3417-3430, 2010
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.
Applied Physics Letters, 2008
We used a water-soluble bis͑fluorinated phenyl azide͒ to cross-link a poly͑ethylene dioxythiophene͒:poly͑styrene sulphonic acid͒ ͑PEDOT:PSS͒, hole-injection layer, with a view to its future use with water-soluble emitters. To enable direct comparison with conventional PEDOT:PSS, we studied the cross-linked films in diodes incorporating the organic-solvent soluble polymer poly͑9,9Ј-dioctylfluorene-alt-benzothiadiazole͒. Kelvin probe characterization of the PEDOT:PSS and electroabsorption measurements of the devices consistently show a 0.2 eV increase of the PEDOT:PSS work function upon cross-linking. We also observe a 70-fold reduction in resistivity, an increase of the current above threshold and a decrease of the "leakage" current below threshold.
Applied Physics Letters, 2011
Polymer light-emitting diodes based on poly(N-vinylcarbazole) (PVK) with molecular weights M W of 1.1 Â 10 6 and $7.5 Â 10 4 are compared. For devices without an electron transport layer (ETL), the high M W PVK yields higher external quantum efficiency (0.67% vs 0.18%), but for devices with an ETL, the low M W PVK yields higher efficiency (1.13% vs 0.83%). This intriguing difference is believed to result from higher energetic disorder in the high M W polymer and different recombination zone-quenching metal electrode distances, in agreement with Konezny et al. [Appl.
Injection and charge transport processes of polymer light emitting diodes
We investigated thianthrene (TH) and 2-(4-biphenyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD), which is known to be a good scintillator dye and have good electron transport properties, separately by cyclic voltammetry in solution. All electrochemical measurements were performed in solutions of N-methylporrolidone in the reduction region and in acetonitrile (for PBD) and dichloromethane (for TH) in the oxidation direction. We also investigated the current-voltage (I-U) characteristics for testing the diode character and capacitance-voltage (C-V) behaviour of different single and double layer structures to describe the function of such devices. For LED fabrication, TH and PBD were mixed with 20% polycarbonate (PC) dissolved in chloroform, so that a mixed single layer with a thickness of 150 nm as an LED blend structure was built up
Applied Physics Letters, 1999
We report electroabsorption measurements of polymer light-emitting diodes, ͑LEDs͒, fabricated with poly͑4-4Ј-diphenylene diphenylvinylene͒, PDPV, as the emissive layer, Ca-Al cathodes, and indium tin oxide ͑ITO͒ anodes, with and without a doped conducting polymer hole injection/ transport layer, namely poly͑3,4-ethylene dioxythiophene͒, PEDOT, doped with poly͑styrene sulfonate͒, PSS Ϫ . In these structures, the bias at which the electroabsorption signal is null corresponds to the difference between the electrodes' work functions. We find that such a built-in voltage increases by 0.5 V when a PEDOT:PSS film is incorporated between the ITO electrode and the emissive layer. This leads to a marked reduction of the anode barrier height at the hole-injecting interface, and accounts for a variety of improvements brought about by the PEDOT insertion, namely: ͑a͒ the increase of luminescence efficiency, ͑b͒ the reduction of the turn-on voltage, and ͑c͒ the increase of the device lifetime.
Polymeric anodes for improved polymer light-emitting diode performance
Applied Physics Letters, 1997
We have studied polyaniline and polyethylenedioxythiophene transparent electrodes for use as hole-injecting anodes in polymer light emitting diodes. The anodes were doped with a variety of polymer and monomer-based acids and cast from either water or organic solvents to determine the effect of the dopant and solvent on the hole-injection properties. We find that the anodes with polymeric dopants have improved device quantum efficiency and brightness relative to those with small molecule dopants, independent of conductivity, solvent, or type of conducting polymer. For the most conducting polymer anodes ͓Ͼ2(⍀ cm) Ϫ1 ͔, diodes could be made without an indium tin oxide underlayer. These diodes show substantially slower degradation.
Synthetic Metals, 2006
This paper reports on the use of an electron transport layer (ETL) in polymer light-emitting diodes based on poly(2,5-bis(3 ,7 -dimethyloctyloxy)1,4-phenylene-vinylene) (BDMO-PPV). This ETL is inserted between BDMO-PPV and a calcium cathode as a hole blocking layer (HBL). A novel phenyleneethynylene derivative (ImPE) is proposed and compared to well-known materials such as tris(8-hydroxyquinoline) aluminum (Alq 3 ) and bathocuproïne (BCP). Efficient hole blocking is achieved leading to yield improvements at low luminances. With a 8 nmthick ImPE layer, at 1 cd/m 2 , the power efficiency reaches 1.2 lm/W whereas a BDMO-PPV-only PLED exhibits a 0.13 lm/W power efficiency. ImPE enables to reach higher performances than Alq 3 for low luminances (<20 cd/m 2 ). However, for luminances higher than 350 cd/m 2 , it is demonstrated that the hole blocking in no more efficient because of a too strong electric field.