Bearing this in minds, we introduced deuteration strategy to developed the first deuterated exciplex-forming host (D-SiCzCz: D-SiTrzCz2) in literature following the classic protonated SiCzCz: SiTrzCz2 system, of which the underlying physics were thoroughly studied. It was found that deuterated exciplex-forming host could establish i) a better molecular intrinsic stability from the suppressed high-energy molecular vibrations and decreased chemical reactivity, guaranteeing a prolonged device lifetime; ii) an improved charge transport ability due to the reduced molecular reorganization energy, benefiting to lower the device operation voltage; and iii) a denser molecular packing to provide rigid environment for phosphorescent dopant to reduce the shoulder emission intensity of dopant and also accelerate its radiative decay process, blue-shifting the emission colour with even higher device efficiency. The corresponding deep-blue PhOLEDs using D-SiCzCz: D-SiTrzCz2 host simultaneously achieved not only a lower operation voltage and a higher device efficiency, but also a 1.6-fold increase in device lifetime with even bluer electroluminescence colour. To be more specific, a higher maximum external quantum efficiency of 27.4% and a lower operation voltage (decreased by 0.3 V at 1000 cd/m 2 ) were achieved, together leading to a higher power efficiency of 41.2 lm/W. More importantly, a long LT90 of 370 h with a smaller CIEy of 0.165 was obtained at an initial luminance of 1000 cd/m 2 , being a 1.6-time longer than the device with the protonated counterpart with even redshifted CIEy of 0.172, breaking the conventional wisdom that a bluer OLED always companies by a shorter lifetime. To the best of our knowledge, this is the longest lifetime for deep-blue PhOLEDs at this specific colour. Our work demonstrated the great potential of deuterated exciplex-forming host as a better choice for highly efficient and stable deep-blue PhOLEDs, and also provides new scientific findings on the kinetic isotope effect on host materials, deepening our understanding of the deuteration strategy. The molecular structures of D-SiCzCz and D-SiTrzCz2 were provided in Fig. , derived from the previous reported promising SiCzCz: SiTrzCz2 exciplex-system. The hydrogen atoms on the carbazole units were deuterated as those segments were chemically active and functional moieties while the tetraphenylsilane segments maintain the same as they only funct as inert groups without the distribution of molecular orbitals. The synthetic routes of the two compounds were illustrated in Supplementary Scheme 1. For D-SiCzCz, in order to realize Buchwald-Hartwig coupling reaction, the active site of 3-bromo-9H-carbazole-1,2,4,5,6,7,8-d7 was protected by t-butyloxycarbonyl unit. The protective group was removed under acidic condition to ensure subsequent coupling with tetraphenylsilicon moiety. D-SiTrzCz2 were synthesized through nucleophilic substitution and Buchwald-Hartwig coupling reaction according to literature . The chemical structure of the two compounds were confirmed by 1 H NMR, C NMR and MALDI-TOF mass spectrometry (Supplementary Fig. ). Thermal properties of D-SiCzCz and D-SiTrzCz2 were also examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere (Supplementary Fig. ). The decomposition temperatures (corresponding to 5% weight loss) are 380 and 432 o C, respectively, indicating that both materials are sufficiently thermally stable. The glass transition temperatures were observed at 117 and 121 o C for D-SiCzCz and D-SiTrzCz2 on the second heating cycle. The absence of melting point and crystallization temperature may be attributed to the strong amorphous character, which would be beneficial for stable device operation even at high temperatures.