The mixed cohosts of electron transport host (E-host): 4,40-bis(carbazol-9-yl)biphenyl (CBP) have been comparatively investigated for an efficient green fluorescent organic light emitting diode (OLED) doped with a thermally activated delayed fluorescence (TADF) emitter (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). The E-host:CBP systems significantly enhance the electroluminescent (EL) properties. After doping E-host, the lifetime of the emissive layer decreases and the surface becomes smoother, together with the impedance decreases for one magnitude and the hole-injection depresses. The charge balance and improved interface both contribute to the EL performance enhancement. Here we develop a universal mixed host system suitable to most of emitters.