Organic electroluminescent device with host materials having same or similar IP, EA and T1 values

A method for fabricating an organic electroluminescent device wherein organic layers containing a light-emitting layer are disposed between an anode and a cathode and the light-emitting layer comprises a phosphorescent dopant material and a host material having a molecular weight of not more than 10,000, which comprises a step where the host material and the phosphorescent dopant material are prepared, the said host material being characterized in that it is composed of a first host material and a second host material that is different from the first host material, the first host material or the second host material is a heterocyclic compound represented by formula (1),

• the ratio by weight of the first host material to the second host material is 90:10 to 10:90, and the first host material differs from the second host material by less than 0.1 eV in the ionization potential (IP), by less than 0.1 eV in the electron affinity (EA), and by less than 0.1 eV in the triplet energy (T1), a step where the host material and the phosphorescent dopant material are dissolved in a solvent to form a coating solution, and a step where the coating solution is applied to an organic layer adjacent to the light-emitting layer and dried;
• wherein in formula (1):
• ring A is an aromatic ring represented by formula (1a) and is fused to the adjacent rings at arbitrary positions and ring B is a heterocycle represented by formula (1b) and is fused to the adjacent rings at arbitrary positions;
• R in formulas (1) and (1a) is independently hydrogen or a monovalent substituent and the adjacent substituents optionally together form a ring;
• L1 in formula (1b) is independently an aromatic hydrocarbon group or an aromatic heterocyclic group;
• L is an n-valent aromatic hydrocarbon group or an n-valent aromatic heterocyclic group;
• n is 1-4;
• and
• when n is 2 or more, the fused heterocycle containing rings A and B are identical with or different from one another.