Novel hyperbranched polymers as host materials for green thermally activated delayed fluorescence OLEDs

A series of novel hyperbranched polymers (HBPs) consisting of a 2,7-subsituted 9-(heptadecan-9-yl)-9H-carbazole unit (A₂+A₂’) and a tetra-substituted green thermally activated delayed fluorescence (TADF) dye of 2,3,5,6-tetra(9H-carbazol-9-yl)-4-pyridinecarbonitrile (4CzCNPy, B4) have been synthesized via Suzuki cross-coupling reaction following an “A₂+A₂’+B₄” method. The polymers are named according to the polymerization ratio of 4CzCNPy monomer (5 mol%, 10 mol% and 15 mol% for HBPs of P2−P4 respectively, and 0 mol% for the control linear polymer P1). Their thermal, optoelectronic and electrochemical properties have been characterized by a combination of techniques. All the polymers exhibit high thermal stability with the decomposition temperatures (T_d) above 400 °C and glass transition temperatures (T_g) up to 98 °C. Unfortunately, the incorporation of TADF moiety into these HBP materials induced non-TADF characteristics. However, when the HBPs functionalized as the host for our previously developed 4CzCNPy TADF dopant in solution processed devices, maximum external quantum efficiency of 5.7% and current efficiency of 17.9 cd/A have been achieved in P3-based device, which is significantly higher than those of 1.5% and 4.2 cd/A for the linear polymer P1.