Synthesis and luminescence properties of poly[2-(9,9-dihexylfluorene-2-yl)-1,4-phenylenevinylene] and its copolymers containing 2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene units

A PPV derivative bearing the dihexylfluorenyl pendants, poly[2-(9,9-dihexylfluorene-2-yl)1,4-phenylenevinylene] (DHF-PPV) and its copolymers containing the dialkoxy-substituted phenylenevinylene units, i.e., poly[(2-(9,9-dihexylfluorene-2-yl)-1,4-phenylenevinylene)-co-(2-(2-ethylhexyloxy )-5-methoxy-1,4-phenylenevinylene)s], were prepared, and luminescence properties of the light-emitting diodes (LEDs) fabricated with them were studied. The copolymers 92.5 DHF/7.5 MEH-PPV and 50.5 DHF/49.5 MEH-PPV contained 7.5 and 49.5 mol % of the dialkoxy comonomer units, respectively. The structure of LED devices was ITO/PEDOT (25 nm)/polymer (80 nm)/Ca (50 nm)/Al (50 nm). The wavelengths of maximum emitted light of the devices were 519 nm (green), 560 nm (yellow), and 585 nm (orange-red) for DHF-PPV and the two copolymers. Turn-on electric fields decreased in the order of DHF-PPV (0.54 MV/cm) > 92.5 DHF/7.5 MEH-PPV (0.43 MV/cm) > 50.5 DHF/49.5 MEH-PPV (0.29 MV/cm). Luminance efficiencies of the three devices were 2.3, 1.0, and 0.9 cd/A for 92.5 DHF/7.5 MEH-PPV, 50.5 DHF/49.5 MEH-PPV, and DHF-PPV, respectively. The maximum luminance for the device of DHF-PPV was 1.6 × 10⁴ cd/m², and the value increased to 2.7 × 10⁴ cd/m² for 97.5 DHF/7.5 MEH-PPV. The maximum luminance of the other copolymer was about 1.9 × 10⁴ cd/m². The study of time-resolved PL of the present polymers strongly suggests that a correlation exists between their PL decay behavior and EL efficiency. The mobilities of the charge carriers, i.e., hole and electron, are better balanced (v_h/v_e ∼ 10) in DHF-PPV than in MEH-PPV (v_h/v_e ∼ 10²).