Synthesis and Electroluminescence Study of a Novel Copolymer:Poly(Phenylene Vinylene-Co-Quinoline Vinylene)
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INTRODUCTION After the observation of EL in PPV, a number of conjugated homopolymers, copolymers
and polymer blends have been investigated to obtain light emission over the entire visible range
[1-3]. Imbalances in injection and transport properties of charge carriers inside the active polymer are considered to be partly responsible for low efficiencies in polymer LEDs. Strategies to overcome these drawbacks have included the use of multi-layer structures with separate transport layers such as PBD dispersed in PMMA, polyquinoxaline, etc. [4,5]. Other methods involve incorporation of electron withdrawing groups such as cyano- or electron deficient nitrogen atoms in the repeat unit of the emissive polymers to improve the electron accepting property [6,71. Polymers based on the pyridine, quinoline or quinoxaline ring systems fall into this class. The n-type electrical conductivity of this class of polymers is said to originate from the it-deficient nature of the ring system [8]. EL of quinoline based polymers was first investigated by Parker et al [9]. Blue emission was reported from devices in which the active layer consisted of rigid phenylquinoline units spaced by a flexible non-conjugated ether segment containing the hexafluoroisopropylidene group. Internal quantum efficiencies were reported to be 4% indicating the great potential for use of quinoline based polymers in LEDs. Later, electron injection and transport properties of poly(phenylquinoline) (PPQ) were put to use by blending acid protonated PPQ with the emissive polymer in MEH-PPV LEDs [10]. The improvement in emission efficiency was attributed to enhanced electron injection, facilitated by higher electron affinity of PPQ. The complexationmediated solubility of PPQ with protonic acid was reported to yield films that did not show EL
51 Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
when used as active layer in LEDs. Thus the protonated PPQ could only serve the role of electron transporting medium. Recently, we reported the synthesis of PQV by direct polymerization of bis(chloromethyl) quinoline and via the xanthate precursor route. This precursor polymer was converted into the conjugated form by thermal elimination under vacuum [11]. Upon doping with sodium naphthalide, PQV turned from dark red to blue showing the potential for n-dopability. Polymer LEDs fabricated using precursor PQV showed broad emission with the peak wavelength in the yellow region. Literature shows that PPV and its derivatives have yielded the best EL efficiencies so far, despite the associated problems. To utilize the electron transport and EL properties of PQV in conjunction with the hole transport and light emission characteristics of PPV, it was proposed to prepare a copolymer of PPV and PQV via the precursor route. The nonbonding electrons on nitrogen act as shallow traps and prevent positive charge carriers from migrating to the cathode without radiative recombination [12). Such a copolymer, PPVQV, will have great potential for applications in polymer L
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