An organic electroluminescent device, comprising a substrate and light emitting units formed in sequence on the substrate, wherein each of the light emitting units consists of a first electrode layer, a light emitting layer and a second electrode layer, the light emitting layer consists of a host material and a dye, characterized in that the host material is an exciplex made from an electron transport type thermal activating delayed fluorescence (TADF) material and a hole transport type thermal activating delayed fluorescence (TADF) material; wherein at least one material in the host material has a triplet energy level ( 3 CT) of the excited triplet state with charge transfer character greater than its triplet state energy level of the excited triplet state with ηπ*-transition character (3nπ*) with an energy level difference no more than 0.3eV; or wherein at least one material in the host material has a triplet energy level ( 3 CT) of the excited triplet state with charge transfer character greater than its triplet state energy level of the excited triplet state with ηπ*-transition character ( 3 nπ*) with an energy level difference no less than 1eV, and the difference between its second triplet energy level of the excited second triplet state with ηπ*-transition character and its first singlet energy level of the excited first singlet state with charge transfer character ( 1 CT) is in the range of -0.1eV to 0.1eV; and in that the electron transport type thermal activating delayed fluorescence material and the hole transport type thermal activating delayed fluorescence material have a structure selected from the following structural formulas, to, to, to, to, to,,, and to: the host material is an exciplex made from an electron transport type thermal activating delayed fluorescence (TADF) material and a hole transport type thermal activating delayed fluorescence (TADF) material; wherein at least one material in the host material has a triplet energy level ( 3 CT) of the excited triplet state with charge transfer character greater than its triplet state energy level of the excited triplet state with ηπ*-transition character (3nπ*) with an energy level difference no more than 0.3eV; or wherein at least one material in the host material has a triplet energy level ( 3 CT) of the excited triplet state with charge transfer character greater than its triplet state energy level of the excited triplet state with ηπ*-transition character ( 3 nπ*) with an energy level difference no less than 1eV, and the difference between its second triplet energy level of the excited second triplet state with ηπ*-transition character and its first singlet energy level of the excited first singlet state with charge transfer character ( 1 CT) is in the range of -0.1eV to 0.1eV; wherein at least one material in the host material has a triplet energy level ( 3 CT) of the excited triplet state with charge transfer character greater than its triplet state energy level of the excited triplet state with ηπ*-transition character (3nπ*) with an energy level difference no more than 0.3eV; or wherein at least one material in the host material has a triplet energy level ( 3 CT) of the excited triplet state with charge transfer character greater than its triplet state energy level of the excited triplet state with ηπ*-transition character ( 3 nπ*) with an energy level difference no less than 1eV, and the difference between its second triplet energy level of the excited second triplet state with ηπ*-transition character and its first singlet energy level of the excited first singlet state with charge transfer character ( 1 CT) is in the range of -0.1eV to 0.1eV; and in that the electron transport type thermal activating delayed fluorescence material and the hole transport type thermal activating delayed fluorescence material have a structure selected from the following structural formulas, to, to, to, to, to,,, and to: