A highly efficient exciplex with significant thermally activated delayed fluorescence characteristics was realized by combining the electron donor of mTPA-PPI and the electron acceptor of PO-T2T. The exciton dynamics processes, such as the reverse intersystem crossing (RISC), prompt fluorescence, and delayed fluorescence of mTPA-PPI∶PO-T2T were explored through the time-resolved spectra technology. A high-performance phosphorescent OLED based on the mTPA-PPI∶PO-T2T co-host was developed. Due to an efficient reverse intersystem crossing process, the triplet exciton utilization was improved, effectively boosting device efficiency and alleviating efficiency roll-off at high current densities. The maximum current efficiency, power efficiency, and external quantum efficiency of the red phosphorescent device based on the co-host are 20.3 cd/A, 18.6 lm/W, and 11.54%, respectively, which were 1.4, 1.2, and 1.5 times that of single-host device. In addition, the maximum luminescence of the co-host device reaches 25 410 cd/m2, which is 3.9 times that of the maximum luminescence of a single-host device.