Novel Hyperbranched Polymers as Host Materials for Green Thermally Activated Delayed Fluorescence OLEDs

A series of novel hyperbranched polymers （HBPs） consisting ofa 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 ＂Az＋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 （Ta） above 400 ℃ and glass transition temperatures （Tg） up to 98℃. 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.