Hole-transport moiety

Keywords Device performance Electron-transport moiety Hole-transport moiety Polyfluorenes Polymer light-emitting diode... 

Examples of polymers used in photorefractive applications are shown in Fig. 22. PVK and PSX both contain the carbazole moiety that has a donor-like character and provides for hole transport. Carbazole and some other hole transporting moieties have been widely used in the field of photorefractive polymers. The polarity of carbazole makes PVK a good matrix for a lot of the guest electro-active molecules. The choice of a polymer matrix is often dictated by its compatibility with... 

To improve the luminance efficiency of PLEDs, many research groups have attached electron and/or hole transport moieties on the side chain of the polymer backbone or on the main chain of the polymer. Therefore, we reported a new series of multifunctional high-brightness and luminance-efficient EL polymers, poly[2- 4-[5-(4-(3,7-dimethyloctyloxy)phenyl)-l,3,4-oxadiazole-2-yl]phenyloxy -l,4-phenylenevinylene] (Oxa-PPV), DMOP-PPV,... 

Nanostructured semiconducting block copolymers containing triphenylamine as hole transport moiety and perylene bisimide as dye and electron transport, have been investigated in view of applications in photovoltaic devices. The polymers show nanowire like structure which formation is driven by the crystallization of perylene bisimides via n- n stacking and since this self-assembly gives rise to domains size comparable to the exciton diffusion length, these materials offer perspectives for the implementation of organic solar cells [357]. 

The ordered columnar arrangement of the hexapentyloxytriphenylene molecules provides good overlap of the -electrons of the triphenylene moieties along the director axis. This results in efficient hole transport in the mesophase. The hole photocurrent shows nondispersive transport with a high mobihtyup to 1 X 10 cm /Vs (24). 

A new branched carbazole derivative with phenyl ethylene moieties attached, l,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB, 41) (Scheme 3.15), was prepared as a HTM for OLEDs [86], TECEB has a HOMO energy level of —5.2 eV and hole-drift mobility of 1(T 4 cm2/(V s), comparable to NPD. The device performance (maximum luminance of about 10,000 cd/m2 and current efficiency of 3.27 cd/A) in a standard HTL/tris-(8-hydroxyquino-line) aluminum double-layer device is also comparable to NPD, but TECEB has a higher Tg (130°C) and its ease of synthesis is superior to NPD. Distyryl units linked to a TPD derivative, A, A"-bis(4-(2,2-diphenylethenyl)-phenyl)-jY,jV -di(p-tolyl)-bendidine (DPS, 42) (Scheme 3.15), reported by Yamashita and coworkers, showed good hole transport properties and improved thermal stability compared with the parent TPD [87]. 

By introducing the hole transport arylamine as an end cap for an anthracene backbone, Lin et al. designed a series of novel materials (207-212) (Scheme 3.65) [247]. The aim of these dual function materials is to combine the emitting property of the blue anthracene lumino-phore with the hole transport property of the triarylamine to simplify the device fabrication steps. Though the introduction of the arylamino moieties produces moderate QE (f 20%) for these materials, the OLEDs using them as emitters as well as HTMs demonstrate only moderate EL performance with a maximum luminance of 12,922 cd/m2 and 1.8 lm/W with CIE (0.15, 0.15). 

Jen et al. have investigated a copolymer PAE comprising electron-accepting 9,9-bis(2-ethylhexyl)-2,7-diethynyl fluorene and hole-transporting thiophene moieties in an alternating fashion (Table 3, entry 12) [119]. The design was based on the assumption that the combination of fiuorenyl and thiophene moieties would lead to a balanced transport of electrons and holes. The sterically hindered substituents were introduced with the hope that they would prevent intermolecular interactions and, thus, keep the PL quantum efficiency high. 

The hole conductor has a spiro-center (a tetrahedral carbon linking two aromatic moieties) that is introduced in order to improve glass forming properties and prevent crystallization. Crystallization is undesirable since it impairs the formation of a good electrical contact between the 2 surface and the hole transporting... 

Electrical conduction will occur by the hopping of either electrons or holes within these distributions of energy levels. Charge transport can be either of holes by transfer between the LUMO states or of electrons between the HOMO states. These correspond to the formation of either a radical cation by the removal of an electron to an adjacent electrode or an anion by the injection of an electron. The nature of the majority carriers will, therefore, be determined by the ionisation potentials and electron affinities of the conjugated moieties. A low ionisation potential will favour hole transport while a high electron affinity will favour electron transport. Most of the conductive polymers reported in the literature have low ionisation potentials and are hole, conductors. ... 

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