Synthesis and characterization of red light-emitting electrophosphorescent polymers with different triplet energy main chain

Red light-emitting electrophosphorescent polymers with poly(fluorene-alt-carbazole) (PFCz) and poly(3,6-carbazole) (PCz) as the main chain, and the quinoline-based iridium (Ir) complex as the side chain have been synthesized by Suzuki and modified Yamamoto polymerization, respectively. The triplet energy of the polymeric backbone is tuned from 2.1 eV of polyfluorene (PF) to 2.3 eV of PFCz and 2.6 eV of PCz. We find that, with the increasing triplet energy, the lifetime of the triplet excitons gradually increases, but the device efficiency becomes worse. The reason is that the alteration of the main chain structure leads to the increase of the highest occupied molecular orbital (HOMO) level of the polymeric host, which can facilitate the efficient hole injection and transportation. As a consequence of this enhancement of the hole current, charge balance in the emitting layer is destroyed, and correspondingly, the poorer device performance is achieved. Our results, we believe, indicate that besides the triplet energy, charge balance is a crucial determinant to develop high efficiency red light-emitting electrophosphorescent polymers.