The PL spectra of the doped films are shown in Fig. . Interestingly, though their same maximum peak values at 462 nm, a relatively smaller full-width at half-maximum (FWHM) of 27 nm was observed from the deuterated host, in comparison to the value of 29 nm from the non-deuterated one. This result originates from the reduction of those shoulder peaks between 475 and 575 nm. As those shoulder peaks usually come from the large-energy vibration levels of the dopant, those results suggest that deuterated hosts could help to suppress the high frequency vibrations of the dopant . This can be further reflected by a slightly enhanced PLQY of 97% in deuterated host with a relatively shorter radiative decay lifetime of 1.81 μs (Table and Supplementary Fig. ), which were 94% and 1.99 μs in non-deuterated host, respectively. The lowered vibrations could suppress the vibronic coupling between the high vibration levels of the ground states and the low vibration levels of the excited states, thus favouring to suppress non-radiative decay for a high PLQY. To explain those differences, we conducted a molecular dynamic (MD) simulation to ascertain the detailed information of the stacking mode of the host materials. As depicted in Fig. , the average distances between one acceptor and adjacent donors were reduced from 13.751 Å for non-deuterated exciplex to 12.788 Å for deuterated exciplex, accompanied by an increased density from 1134.0 to 1157.4 kg/m 3 , indicating a more compact environment of deuterated exciplex-forming host compared with the non-deuterated one. The enlarged compactness of host molecules could provide a rigid environment for the dopant and inversely suppress the structural vibrations of the phosphorescent dopant. In this way, we can expect a higher colour purity and a higher PLQY with faster radiative decay from the same dopant in deuterated hosts. Diphenyl-1,10-phenanthroline (Bphen, 5 nm)/9,10-Bis(6-phenylpyridin-3-yl) anthracene (DPPyA, 30 nm)/Host (50 nm)/DPPyA (30 nm)/LiF (0.7 nm)/Al (150 nm), respectively. In the HODs and EODs for single donor and acceptor, D-SiCzCz and D-SiTrzCz2 exhibited markedly higher current density than that of the non-deuterated counterparts at the same voltage, as illustrated in Supplementary Fig. 19. In the case of the exciplexes, the increase in current density is still clearly discernible. At a voltage of 11 V, the current density of the HODs based on deuterated exciplexes was approaching saturation, whereas the value of the non-deuterated one remained below 100 mA cm -2 (Fig. ). On the other hand, the current density of EOD was also found to have reached a saturation point at approximately 9 V, which is considerably superior to that of the non-deuterated counterparts (Fig. ). These results demonstrated that the deuteration has a considerable impact on the carrier transport character of the exciplex-forming hosts, with the enhancement of hole transport in donors being particularly notable.