Abstract: On the basis of commonly used triphenylamine donor units and dibenzo [a, c] phenazine acceptor units, two red thermally activated delayed fluorescence(TADF) materials TPA-DSP and SPTPA-DSP were designed and synthesized by the steric hindrance effect of spirofluorene group. The spirofluorene group, known for its large steric hindrance, increases the rigidity of the molecule, avoids tight packing between molecules, and effectively reduces non-radiative transition energy loss, which is crucial for improving device performance. By co-doping with 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl(CBP) as organic light emitting layer, TPA-DSP and SPTPA-DSP exhibited excellent performance in organic light-emitting diodes(OLEDs). Both materials emitted red light, with TPA-DSP achieving a maximum external quantum efficiency(EQE) of 17.8% at 580 nm with a 7 wt% doping concentration. SPTPA-DSP, featuring multiple spirofluorene groups, demonstrated superior device performance, achieving a maximum EQE of 19.3% at 580 nm with a 7 wt% doping concentration. The maximum luminance of TPA-DSP and SPTPA-DSP reached 11800 and 12650 cd/m2, the maximum current efficiency(CE) both reached 40.0 cd/A, and the maximum power efficiency(PE) reached 44.3 and 47.2 lm/W, respectively. Notably, SPTPA-DSP, featuring multiple spirofluorene groups, demonstrated superior device performance due to its greater steric hindrance. Our findings underscore the potential of the spirofluorene group to enhance the performance of TADF materials through steric hindrance effects, which not only contributes to the understanding the effect of steric hindrance in TADF but also paves the way for further advancements and applications.

Key words: Thermally activated delayed fluorescence, Red luminescence, Steric hindrance, Spirofluorene