Synthesis and Luminescent Properties of Blue Light Emitting Polymers Containing both Hole and Electron Transporting Units

Abstract

Poly[(oxy-4,4′-octafluorobiphenyloxy)-1,4-phenylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene-1,4-phenylene], POFB-MEH-PPV, poly[(oxy-4,4′-octafluorobiphenyloxy)-1,4-phenylenevinylene-9,9-dihexyl-2,7-fluorenediylvinylene-1,4-phenylene], POFB-PF, and poly[(oxy-4,4′-octafluorobiphenyloxy)-1,4-phenylenevinylene-N-ethylhexyl-3,6-carbazolevinylene-1,4-phenylene], POFB-PK, were synthesized by the well-known Wittig condensation polymerization. We incorporated the high electron affinity (octafluorobiphenyl) and hole-transporting (carbazole, fluorene, and dialkoxyphenyl) units into the conjugated main chain. The conjugation lengths are limited to the blue-emission region by ether linkage. The resulting polymers were completely soluble in common organic solvents such as chloroform, 1,2-dichloroethane, and cyclohexanone, and exhibited good thermal stability up to 300°C. The synthesized polymers showed UV-visible absorbance and photoluminescence (PL) in the ranges of 350–385 nm and 460–490 nm, respectively. The fluorene or carbazole containing POFB-PF and POFB-PK showed blue photoluminescence peaks at 470 and 460 nm, respectively. The single-layer light-emitting diode was fabricated in a configuration of ITO (indium-tin oxide)/polymer/Al. Electroluminescence (EL) emission of POFB-PF and POFB-PK were shown at 475 and 458 nm, respectively, corresponding to the pure blue emissions. And, a dialkoxyphenyl containing POFB-MEH-PPV showed greenish blue light at 494 nm. But, LED devices from synthesized polymers showed poor device performance and high turn on voltage. So, we fabricated light-emitting diodes (LEDs) from blend polymers composed of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and POFB-MEH-PPV (POFB-PF or POFB-PK) as the emitting layers. The EL emission maxima of each blend polymers were in the range of 573–591 nm, which indicates that the emission is mainly due to MEH-PPV and POFB-MEH-PPV (POFB-PF or POFB-PK) contributes to the enhancement of the luminescence. And each blend polymers exhibited higher EL quantum efficiency compared with MEH-PPV at the same current density.