White Light-Emitting Diodes from Novel Silicon-Based Copolymers Containing Both Electron-Transport Oxadiazole and Hole-Transport Carbazole Moieties in the Main Chain

Abstract

Silicon-based alternating copolymers containing both electron-transport oxadiazole and hole-transport carbazole moieties in the main chain (SiHMOXD/Cz 10−01) were synthesized by means of the Heck coupling reaction. The resulting polymers exhibit a strong UV−visible absorption band in the range of 345−356 nm in chloroform solution and in film state. Their PL spectra show a maximum band around 435−485 nm in the blue region. The light-emitting diodes of Al (200 nm)/Ca (50 nm)/EL polymer (80 nm)/PEDOT (50 nm)/ITO were successfully fabricated. And, J−V curves show a turn-on voltage of 6−7 V. Their EL properties depend strongly on both the applied voltage and the loading amount of hole-transport carbazole moieties in the present copolymers. With the applied voltage, these emissive EL bands were red-shifted from blue region to red region. Also, the intensity of a blue EL band at the relatively high operating voltages increases with the loading amount of carbazole units. The LED device with the copolymer of SiHMOXD/Cz 19 exhibits the almost same intensity of two bands, like two crests, giving a strong white color. The blue EL color comes from the carbazole units in these silicon-based copolymers. The latter red EL color comes from a specific charge complex with oxadiazole (and carbazole moieties). The new red band is exhibited only in EL but not in PL spectra. The EL device based on SiHMOXD/Cz 19 has a luminescence efficiency of 0.052 lm/W and a power efficiency of 0.13 cd/A at an applied voltage of 9 V. And, the maximum luminance of the white emissive color was 6.04 cd/m2 at an applied voltage of 17 V. In this paper, from the photophysical studies, a specific intramolecular charge complex was proposed.