Highly efficient organic light-emitting diodes (OLEDs) with the concurrent achievement of high external electroluminescence quantum efficiency (EQE) and low light amplification thresholds under optical excitation have been considered as a crucial evolution towards the development of high-performance electrically pumped organic semiconductor laser diodes. Herein, a series of 2,6-dicarbonitrile-diphenyl-1 lambda(5)-phosphinine (DCNP) based donor (D)-acceptor (A) type dyes with different electron-withdrawing and donating moieties have been designed and characterized. The well-manipulated D-A strength with tunable optical properties guaranteed the low amplified spontaneous emission (ASE) thresholds of below 10 mu J cm(-2) and furnished a wide-range color-tuning capability in the visible region (485-595 nm). Furthermore, employing a thermally-activated delayed fluorescence (TADF) molecule as a triplet harvester boosted the performance of OLEDs based on mDMCz that exhibits an exceptional EQE value of 18.4% which is an eightfold enhancement as compared with that of standard fluorescence OLEDs. Also, the TADF-assistant fluorescence (TAF) system enables a reduction of the ASE threshold to 3 mu J cm(-2) and excellent ASE stability. These results provide a rational design strategy to construct color-tunable lasing dyes with reduced ASE thresholds and clarify their potentiality as the fluorescent dopant in the TAF system to utilize up-converted triplet excitons via efficient energy transfer.