An OLED is disclosed whose emissive layer has a first host and an emitter, where the emitter is a phosphorescent metal complex or a delayed fluorescent emitter, where EH1T, the T1 triplet energy of the first host, is higher than EET, the T1 triplet energy of the emitter, where EET is at least 2.50 eV, where the LUMO energy of the first host is higher than the HOMO energy of the emitter, where the absolute value of the difference between the HOMO energy of the emitter and the LUMO energy of the first host is ΔE1, where a≤ΔE1−EET≤b; and where a≥0.05 eV, and b≤0.60 eV.