A composition formed of a mixture of two compounds having similar thermal evaporation properties that are pre-mixed into an evaporation source that can be used to co-evaporate the two compounds into an emission layer in OLEDs via vacuum thermal evaporation process is disclosed. The first and second compounds can have an evaporation temperature T1 and T2, respectively, of 150 to 350° C., and the absolute value of T1−T2 can be less than 20° C. The first compound can have a concentration C1 in the mixture and a concentration C2 in a film formed by evaporating the mixture in a vacuum deposition tool at a constant pressure between 1×10−6 Torr to 1×10−9 Torr, at a 2 Å/sec deposition rate on a surface positioned at a predefined distance away from the mixture being evaporated, where the absolute value of (C1−C2)/C1 is less than 5%.