The bipolar material 4,4′-N,N′-dicarbazolylbi phenyl (CBP) is often used as a typical host materials in electrophosphorescent devices.In order to compare the relative transport speeds of hole and electron in CBP with CBP used as an active layer,electroluminescent (EL) devices with mul tilayer structures of anode/hole transporting layer/CBP/electron transporting layer/cathode were fabri cated.An ultrathin layer of fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) was inserted into CBP la yer at different positions to obtain EL devices with different colors.Bipolar transport pro perties of CBP are investigated by comparing electroluminescences of differe nt devices instead of mobilities of holes and electrons.The results show that the recombination of electrons and holes tak es place mainly near the anode side,indicating that electrons move much faster than holes in CBP,or in other words,the mobility of an electron in CBP is greater than that of a hole.