Poly(2-fluoro-1,4-phenylenevinylene) (PFPV), and poly(1,4-phenylenevinylene) (PPV), were synthesized through the water-soluble precursor method. The polymers were characterized by UV-visible spectroscopy, and the light-emitting properties were studied. The absorption maxima of PPVand PFPV are at around 415 nm and 410 nm, respectively. The electroluminescent (EL) devices were fabricated with ITO and aluminum as positive and negative electrodes, respectively. The EL properties of these polymers were studied by EL spectra, current-voltage characteristics, light intensity-current characteristics and Fowler-Nordheim plot. The I-V characteristics of the polymers showed typical diode characteristics and the operating voltages of PPV and PFPV were 6 V and 10 V, respectively. The photoluminescent spectra of these polymers were also obtained, and similar to the EL spectra. The relative EL quantum efficiency of PFPV was 10 times higher than that of PPV. This result may be due to the smaller band offset between the aluminum and the LUMO of the PFPV as compared with that in PPV.
Light-emitting diodes (LEDs) made from blend polymers were investigated. The blend polymers composed of two organic soluble and emittable conducting polymers, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV) and poly [1,3-propanedioxy-1,4- phenylene-1,2-ethenylene-(2,5- bis(trimethylsilyl)-1,4-phenylene)-1,2- ethenylene-1,4-phenylene] (DSiPV) as the emitting layers. The emitting colors of the blend polymers were yellowish- orange, which indicates that the emission is mainly due to MEH-PPV and DSiPV greatly contributes to the enhancement of the luminescence. The efficiencies of the blend polymers increase with decreasing MEH-PPV/DSiPV weight ratio. In blending MEH-PPV with DSiPV, DSiPV is believed to serve as an EL enhancing polymer rather than EL active in our devices. The quantum efficiency is measured as the function of MEH-PPV content in the blend. Especially, the maximum quantum efficiency of MEH-PPV/DSiPV (1/15) blend polymer is about 500 times greater than that of MEH-PPV homopolymer, and the luminance is up to 500 cd/㎡ at 30 V with 23.6 mA/㎠ current density.
Electroluminescence of the polymer blend composed of two organic soluble polymers was also investigated. The blends consist of main chain conjugated poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV) and side chain luminecentable polymer, alkoxy-trifluoromethylstilbene substituted PMA derivative ($CF_3$-PMA). Emitting colors of the polymer blends were varied with the blending ratios of two polymers. Electroluminescent peak of the MEH-PPV-rich polymer blend was ranged over 580-800 nm (orange-red light), and of the $CF_3$- PMA-rich polymer blend was ranged from 380 to 800 nm (white light). Additional new EL peaks of polymer blends at around 380 and 800 nm originated from the $CF_3$-PMA and the exciplex formed by photoinduced electron transfer between two polymers, resulting in the white light emission.
