A 2-phenylfuro[2,3-b]quinoxaline-triphenylamine-based emitter: photophysical properties and application in TADF-sensitized fluorescence OLEDs

Abstract

We have demonstrated that the T₁-state energy of a fluorescence dopant (FD) is close to that of the thermally activated delayed fluorescence (TADF)-type exciplex co-host, and the energy loss caused by the T₁ states of the FD could be suppressed in TADF-sensitized fluorescence (TSF) OLEDs. To further improve device efficiency following this strategy, two compounds, TPA-FQ and dTPA-FQ, were constructed using 2-phenylfuro[2,3-b]quinoxaline (FQ) and triphenylamine (TPA) as the acceptor and donor, respectively. dTPA-FQ, with two ortho-installed TPA units, has a higher photoluminescence quantum yield (PLQY; 100%), a more rapid radiation transition of the S₁ state (K_r = 1.79 × 10⁸ s⁻¹), and a smaller ΔE_S1T1 value (0.24 eV) in comparison to TPA-FQ with only one TPA unit (PLQY: 79%; K_r = 5.03 × 10⁷ s⁻¹; ΔE_S1T1 = 0.47 eV). Transient photoluminescence and absorption spectroscopy experiments revealed that both compounds are green fluorescence emitters. The dTPA-FQ shows a T₁–T₁ difference of only 0.05 eV from the co-host, 4,4′,4′′-tri(N-carbazolyl)triphenylamine and (1,3,5-triazine-2,4,6-triyl)-tris(benzene-3,1-diyl)-tris(diphenylphosphine oxide) (TCTA:PO-T2T), and, thus, T₁ energy loss could be inhibited in the dTPA-FQ doped co-host device. As a result, an external quantum efficiency (EQE) of 10.9% has been achieved, while the device with the TPA-FQ doped co-host presents degraded device efficiency (EQE = 4.8%) due to its relatively large T₁–T₁ difference of 0.1 eV and lower PLQY. In the non-doped devices, the TPA-FQ device shows better device performance than the dTPA-FQ one; its luminance reaches 47 880 cd m⁻² at a low voltage of 8.5 V, its EQE is up to 3.9% at 100 cd m⁻², and it exhibits only 6.2% efficiency roll-off as the luminance is changed from 10 000 cd m⁻² to 30 000 cd m⁻². Our experiments demonstrated that the key considerations for designing a FD for a TSF system should be a high PLQY and small T1 energy difference between the FD and co-host.