To the best of our knowledge, device A is one of the most stable deep-blue devices with CIEy < 0.2 and EQE over 27% as summarized in Fig. and Supplementary Table . The notable enhancement in device stability naturally originates from the adoption of deuterated exciplexforming host, which possesses not only better intrinsic stability but also improved charge transport ability. Additionally, the electroluminescence decay behaviours of both devices were also measured as shown in Supplementary Fig. . Clearly, similar to the PL results, BD-02 in deuterated host exhibited a faster radiative decay process, thus favouring to suppress the tripletrelated annihilations for extended device lifetime. Of particularly note, device A with deuterated host achieves not only smaller y-colour coordinate, but also significantly prolonged device lifetime. This is contrary to conventional wisdom that a bluer emission would always deteriorate the device stability . As in this case, the bluer colour was achieved not by blue-shift the emission maximum but through reducing the emission shoulder for better colour purity.