Multiple Functionalities of Pendent Charge ransport Moiety in Electroluminescent Conjugated Polymers
Department of Chemical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan (sachen@che.nthu.edu.tw)
For electroluminescent conjugated polymers, two main factors preventing them from high performance are imbalanced charge fluxes and high barriers for charge injections. To solve the problems, in soluble poly(phenylene vinylene)s (PPVs) for example, the easiest way is to introduce an electron transport molecular material (oxadiazole) by blending with the emitting polymer or dispersing in an inert polymer as an additional layer. However, phase separation or recrystallization of the molecular material from the host polymers often occurs. Incorporation of a proper content of charge transport moiety on side chains of the emitting polymer can overcome the phase separation problem and provide additional merit of easy fabrication.
In addition to charge transport, pendent charge transport moiety grafted on the conjugated polymers including PPVs, polyfluorenes (PFs), poly(p-phenylene)s (PPPs), and poly(3,6-cabazole)s (P(3,6,-Cz) is found to provide multiple functionalities. In triazole (TA)-modified PPVs, TA provides dilution effect giving promoted PL quantum efficiency and prevention from interchain aggregation, tunability for relative charge mobility in complete range, and interfacial dipole for a reduction in barrier for electron injection. In Cz-biphenylene-grafted P(3,6-Cz), Cz is able to tune HOMO and LUMO levels leading to changes in charge injections and a depression of electromer emission. In electron-deficient group (OXD or TA) end-capped PF, OXD or TA can induce a formation of long conjugating length segments near chain ends, leading to a pure blue emission. In Cz-modified PF, Cz-dimer can form of an additional sharp green emitting species by interaction with the main chain under electrical field. In Cz-modified PPP doped with green emitting Ir-complex, Cz serves as a dispersant for the dopant.