Blue electrophosphorescence from iridium complex covalently bonded to the poly (9-dodecyl-3-vinylcarbazole): Suppressed phase segregation and enhanced energy transfer

Efficient blue electrophosphorescence was achieved from iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C-2']picolinate (FIrpic) covalently bonded to the carbazole-based wide-band-gap polymer host (poly(9-dodecyl-3-vinylcarbazole) CP0). Number-average molecular weights of CP0-based copolymers containing FIrpic pendants (CPn) ranged over 18 000-33 700 when the molar content of FIrpic-containing monomer units was 0.8-10.6%: CP1 (0-8%), CP2 (2.9%), CP3 (5.3%), and CP4 (10.6%). Phase segregation in the CPn copolymer was significantly suppressed compared to that in the doped analogues (molecular composite of CP0/Flrpic). The photoluminescence spectra of the carbazole-based polymer host (CPO) overlapped with the absorption band of FIrpic, and the T-1 level of CP0 (-2.6 eV) was higher than that of FIrpic (-3.1 eV), ensuring highly efficient exothermic energy transfer. Electroluminescence (EL) devices employing CPn polymers as emitting layer showed exclusive Flrpic emission due to the efficient energy transfer and subsequent exciton confinement in FIpic. The with emission efficiency of 2.23 cd/A. It luminance of preliminary EL devices reached as high as 1450 cd/m(2) was specifically noted that the roll-off of external quantum efficiency in the region of high current density was interestingly suppressed in the CPn polymer devices.