Blue electrophosphorescence from iridium complex covalently bonded to the po̧ly(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,C²′]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 (CP _n) ranged over 18 000-33 700 when the molar content of FIrpic-containing monomer units was 0.8-10.6%: CP₁ (0.8%), CP ₂ (2.9%), CP₃ (5.3%), and CP₄ (10.6%). Phase segregation in the CP_n copolymer was significantly suppressed compared to that in the doped analogues (molecular composite of CPo/FIrpic). The photoluminescence spectra of the carbazole-based polymer host (CP ₀) 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 CP_n polymers as emitting layer showed exclusive FIrpic-emission due to the efficient energy transfer and subsequent exciton confinement in FIpic. The luminance of preliminary EL devices reached as high as 1450 cd/m² with emission efficiency of 2.23 cd/A. It was specifically noted that the roll-off of external quantum efficiency in the region of high current density was interestingly suppressed in the CP_n polymer devices.