Nanophase-Separated Blends of Acceptor and Donor Conjugated Polymers. Efficient Electroluminescence from Binary Polyquinoline/Poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) and Polyquinoline/Poly(3-octylthiophene) Blends

Abstract

Binary blends of the acceptor conjugated polymer poly(2,2‘-(3,3‘-dioctyl-2,2‘-bithienylene)-6,6‘-bis(4-phenylenequinoline)) (POBTPQ) with donor conjugated polymer poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (MEH−PPV) or poly(3-octylthiophene) (POT) were nanophase-separated and observed to exhibit efficient electroluminescence in light-emitting-diodes. The 90−120 nm phase-separated morphology of MEH−PPV-containing blends was characteristic of dimixing by spinodal decomposition whereas that of POT-containing blends consisted of nucleation and growth type spherical domains (50−400 nm) dispersed in a matrix. Efficient Förster energy transfer was observed in both blend systems. Voltage-tunable orange-red ↔ yellow ↔ green electroluminescence was observed in the POBTPQ:MEH−PPV blend diodes at a composition of 10−80 wt % MEH−PPV. Only red emission characteristic of POT was observed from POBTPQ:POT blend devices due to efficient energy transfer from POBTPQ. Large enhancements in performance of the bipolar blend light-emitting diodes were observed compared to that of the homopolymer diodes. The compositional dependence of luminance and external quantum efficiency of the blend devices was very different for the MEH−PPV and POT blends, reflecting the difference in morphology. Electric-field-induced photoluminescence quenching confirmed the bipolar charge transport in the blends and associated improved electron−hole recombination and device efficiencies. These results demonstrate that efficient electroluminescence can be achieved in bipolar blends of conjugated polymers and that nanophase-separated morphology is essential to voltage-tunable multicolor light emission in blend devices.