Interfacial Energy Transfer Strategy for Green Circularly Polarized Electroluminescence Based on Chiral Polyfluorene...

Chiral polyfluorene or chiral polyfluorene-based coassembly materials, featuring high carrier mobility and nanofiber structures, are promising for circularly polarized organic light-emitting diodes (CP-OLEDs). However, their relatively low triplet energy level severely limits their utility as chiral hosts due to easily inducing energy back-transfer from the guest emitters to the host. Herein, we present an effective strategy to suppress energy back-transfer from the achiral green emitter to the chiral polyfluorene coassembly host (R/S-PF8) by employing interfacial energy transfer from a cross-linkable, high-triplet-energy hole transport layer (HTL, p-mCBP). The chiral emitting layer consists of R/S-PF8 and achiral green emitters, possessing an ordered arrangement following thermal annealing and solvent vapor exposure. The device without p-mCBP layer exhibited only deep-blue electroluminescence (EL) originating from R/S-PF8. In stark contrast, a multilayer device incorporating p-mCBP as the HTL and anode buffer generated strong green circularly polarized EL signal at 500 nm from the achiral emitter, with a dissymmetry factor of -0.012. This arises from electron accumulation at the p-mCBP/R/S-PF8 interface, shifting the electron-hole recombination zone toward p-mCBP and thereby promoting guest emission. This approach effectively circumvents the intrinsic triplet energy limitation of ordered chiral polyfluorenes, significantly expanding the range of viable chiral polymer hosts for the design of high-performance green CP-OLEDs.