White polymer light emitting diode using blend of fluorescent polymers

摘要

White polymer light emitting diodes (WPLEDs) are fabricated using poly(9,9-dioctylfluorene-2,7-diyl) (PFO) as host and poly [2-methoxy-5-(2'-ethyl-hexyloxy)]-1,4-phenylene vinylene (MEH-PPV) as guest material having structure ITO(150nm)/PEDOT:PSS(40nm)/PFO:MEH-PPV(75-90nm)/Ca(20nm)/Al(120nm). Photoluminescence spectra of blends with different MEH-PPV concentration reveal that at low doping level of MEH-PPV, blue emission from PFO and yellow emission from MEH-PPV co-exist due to incomplete energy transfer from PFO to MEH-PPV. Surface morphology of the spin coated blend films with different concentrations of MEH-PPV were studied using atomic force microscopy (AFM). It shows segregation/agglomeration of polymers at higher concentration of MEH-PPV (～2.0 wt %). Finally, WPLEDs, having MEH-PPV in the range of 0.8-2.0 wt%, were fabricated. We obtained best device at 0.8 wt% of MEH-PPV, it shows white light with Commission Internationale de I'Enclairage (CIE) coordinate of (0.30, 0.38). Electroluminescence turn-on voltage of the device was 4.0 V and maximum luminance reaches 1234 cd/m~2 at 8.5 V. The luminous current and power efficiency at current density of 22 mA/cm~2 were found to be 2.3 cd/A and 1.1 lm/W, respectively.