Highly efficient green-emitting electrophosphorescent hyperbranched polymers using a bipolar carbazole-3,6-diyl-co-2,8-octyldibenzothiophene-S,S-dioxide-3,7-diyl unit as the branch

Abstract

Green-emitting hyperbranched polymers with a bipolar 3,6-carbazole-co-2,8-dioctyl-dibenzothiophene-S,S-dioxide-3,7-diyl [Cz-co-DOSO] as the branch and fac-tris(2-phenylpyridine)iridium[Ir(ppy)₃] as the core were synthesized by Suzuki polycondensation. The Cz-co-DOSO branch with a triplet energy level above 2.5 eV can effectively prevent energy feedback from the phosphorescent core. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the copolymers reduce dramatically by over 0.6 and 0.8 eV with increasing DOSO content up to 45.5 mol% in the polymers, respectively. A maximum luminous efficiency of 50.5 cd A⁻¹ and a maximum external quantum efficiency of 15.3% were obtained for PCzDOSO40Ir3 based on a single layer device of ITO/PEDOT:PSS/polymer/CsF/Al, which were improved by over 13 times compared with PCzIr3. The hole- and electron-only devices show that the hole and electron flux are well balanced, which demonstrates that PCzDOSO40Ir3 is a bipolar polymer with a balanced charge carrier transport.