Carbazole Compounds as Host Materials for Triplet Emitters in Organic Light-Emitting Diodes:  Polymer Hosts for High-Efficiency Light-Emitting Diodes

Abstract

A carbazole homopolymer and carbazole copolymers based on 9,9'-dialkyl-[3,3']-bicarbazolyl, 2,5-diphenyl-[1,3,4]-oxadiazole and 9,9-bis(4-[3,7-dimethyloctyloxy]phenyl)fluorene were synthesized and their electrical and photophysical properties were characterized with respect to their application as host in phosphorescent polymer light-emitting diodes. It is shown that the triplet energy of a polymer depends on the specific connections between its building blocks. Without changing the composition of the polymer, its triplet energy can be increased from 2.3 to 2.6 eV by changing the way in which the different building blocks are coupled together. For poly(9-vinylcarbazole) (PVK), a carbazole polymer often used as host for high-energy triplet emitters in polymer light-emitting diodes, a large hole-injection barrier of about 1 eV exists due to the low-lying HOMO level of PVK. For all carbazole polymers presented here, the HOMO levels are much closer to the Fermi level of a commonly used anode such as ITO and/or a commonly used hole-injection layer such as PEDOT:PSS. This makes high current densities and consequently high luminance levels possible at moderate applied voltages in polymer light-emitting diodes. A double-layer polymer light-emitting diode is constructed comprising a PEDOT:PSS layer as hole-injection layer and a carbazole-oxadiazole copolymer doped with a green triplet emitter as emissive layer that shows an efficacy of 23 cd/A independent of current density and light output.

Exciton annihilation in a polyfluorene: Low threshold for singlet-singlet annihilation and the absence of singlet-triplet annihilation

“…If it is assumed that the polymer chain is divided into segments bound by conformational or chemical defects, one normally assumes that the singlet exciton occupies most of the segment, a phenomenon known as the effective conjugation length ͑typically 25-30 repeat units͒. 41,42 Conversely, the triplet exciton is understood to only occupy a segment of a few phenyl rings; [43][44][45] thus, if the reasonable assumption that each chain segment can only hold one excitation ͑either singlet or triplet͒, then the mean distance from the singlet to the neighboring triplet exciton is large and the necessary overlap of the states required for intrachain STA by Dexter transfer would not be possible. Additionally, given this constraint, the singlet would be outside the Förster radius of the triplet and the energy transfer could not proceed by a Forster mechanism either.…”

Novel concept of polymers preparation with high photoluminescent quantum yield

“…Macromolecules end-capped carbazole moieties were also used as luminescent films electrodeposited (ED) with good electropolymerization behaviors and properties [10]. Likewise, polymers bearing carbazole structure has been widely described [14][15][16][17][18][19], e.g., fluorescent carbazolyl-labeled polysiloxanes [14]. A green light-emitting phosphorescent and polymeric light-emitting diode (PLED), containing a layer of a carbazole-oxadiazole copolymer showed a high efficiency, current densities, and luminance levels [15].…”

Side Chain Mediated Electronic Contact between a Tetrahydro‐4H‐thiopyran‐4‐ylidene‐Appended Polythiophene and CdTe Quantum Dots

“…Owing to its electronically insulating character, it prevents electronic contact between quantum dots and other electroactive materials. Nevertheless, hybrid quantum dot/organic systems for optoelectronic applications have been obtained by casting from solutions of mixtures of both constituents [1][2][3]30] or by deposition techniques. [7][8][9][10] Although encouraging results, such as improved electroluminescence efficiencies compared with onecomponent systems and the occurrence of photoinduced charge separation, have been reported, the development of hybrid quantum dot/organic systems with a substantial electronic coupling between the constituents is desired.…”