Data from a series of phosphorescent blue organic light-emitting devices with emissive layers consisting of either 4,4'-bis(N-carbazolyl)-2,2'-biphenyl (CBP):6% bis[(4,6-difluorophenyl) pyridinato-N,C-2](picolinato) iridium(III) (Flrpic) or bis(9-carbazolyl) benzene (mCP):6% Flrpic show that the triplet energy of the hole and electron transport layers can have a larger influence on the external quantum efficiency of an operating device than the triplet energy of the host material. A maximum external quantum efficiency of 14% was obtained for CBP: 6% Flrpic devices which is nearly double all other published CBP: 6% Flrpic results. A new host material, 4-(diphenylphosphoryl)-N,N-di-p-tolylaniline (DHM-A2), which has a triplet energy lower than that of Flrpic is also reported. Devices fabricated using DHM-A2 show improved performance (lower drive voltage and higher external quantum efficiency) over devices using 4-(diphenylphosphoryl)-N,N-diphenylaniline (HM-A1), a high performance ambipolar DHM-A2 analogue with a triplet energy greater than Flrpic. Nearly 18% external quantum efficiency was obtained for the DHM-A2:5% Flrpic devices. The results suggest modified design rules for the development of high performance host materials: more focus can be placed on molecular structures that provide good charge transport (ambipolarity for charge balance) and good molecular stability (for long lifetimes) rather than first focusing on the triplet energy of the host material.