Evaluation of acenes as potential acceptors in thermally activated delayed fluorescence emitters and the promise of a phenoxazine–naphthalene emitter for OLEDs

Abstract

Thermally activated delayed fluorescence (TADF) is one of the most promising technologies for harvesting triplet excitons in all-organic emitters, a property that is essential for achieving high efficiency in devices. Compounds that operate via this mechanism for emission typically rely on a combination of electron donating and accepting moieties separated by an aromatic bridge. Here we demonstrate that although naphthalene is underutilised as an acceptor, it can nonetheless be used in a donor–acceptor TADF emitter when coupled to two phenoxazines in the 1- and 4-positions. The compound 1,4-PXZ-Nap-PXZ emits at 508 nm, has a photoluminescence quantum yield of 48% and a delayed lifetime of 22.7 ms in a 20 wt% doped film in 1,3-bis(N-carbazolyl)benzene (mCP). An organic light-emitting diode (OLED) using this emitter showed a maximum external quantum efficiency (EQE_max) of 11% and green emission at λ_EL of 505 nm, demonstrating for the first time the potential of naphthalene-acceptor based emitters for devices. Finally, we have demonstrated by way of a density functional theory (DFT) study why naphthalene alone amongst linear acenes is suitable for this role.