With 9-phenyl-9-phosphafluorene oxide (PhFlOP) moiety as the acceptor (A) and various donors (D), a series of new organic emitters have been synthesized with D-A-D configuration. Their photophysical, electrochemical behaviors and electroluminescent (EL) performances have been characterized in detail. The photophysical results have indicated the PhFlOP-based emitters with acridyl, phenoxazyl and phenothiazyl as donors can show efficient thermally activated delayed fluorescence (TADF) behavior, especially for the TADF emitter with phenoxazyl donor possessing exceptionally high rate constant of reverse inter-system crossing (kRISC) of 4.8×107 s-1. It also has been found that their TADF behavior can be greatly affected by the substitution position of the donors. Different from the reported aryl phosphine oxide (APO) acceptors in TADF emitters, the PhFlOP moiety adopting highly rigid configuration to guarantee photoluminescent quantum yield (PLQY) as high as 0.83, representing the top-ranking emission ability for the TADF emitters with APO-type acceptors. Benefitting from their advanced TADF performances, the doped organic light-emitting diodes/devices (OLEDs) based on these PhFlOP-based TADF emitters can achieve exceptional electroluminescent (EL) performances with the maximum external quantum efficiency (ηext) of 23.3%, current efficiency (ηL) of 83.7 cd A-1 and power efficiency (ηP) of 59.1 lm W-1. These encouraging EL data show the great potential of the PhFlOP moiety in developing highly efficient TADF emitters.