Acridan-Grafted Poly(biphenyl germanium) with High Triplet Energy, Low Polarizability, and an External Heavy-Atom Effect for Highly Efficient Sky-Blue TADF Electroluminescence

We propose the novel σ-π conjugated polymer poly(biphenyl germanium) grafted with two electron-donating acridan moieties on the Ge atom for use as the host material in a polymer light-emitting diode (PLED) with the sky-blue-emitting thermally activated delayed fluorescence (TADF) material DMAC-TRZ as the guest. Its high triplet energy (E_T ) of 2.86 eV is significantly higher than those of conventional π-π conjugated polymers (E_T =2.65 eV as the limit) and this guest emitter (E_T =2.77 eV). The TADF emitter emits bluer emission than in other host materials owing to the low orientation polarizability of the germanium-based polymer host. The Ge atom also provides an external heavy-atom effect, which increases the rate of reverse intersystem crossing in this TADF guest, so that more triplet excitons are harvested for light emission. The sky-blue TADF electroluminescence with this host/guest pair gave a record-high external quantum efficiency of 24.1 % at maximum and 22.8 % at 500 cd m^-2 .