On page 4914, J.-J. Kim and co-workers use an exciplex-forming co-host to create phosphorescent organic light-emitting diodes with ultimate efficiencies reported in terms of the external quantum efficiency (EQE), driving voltage, and efficiency rolloff. The image expresses the formation of an exciplex by the recombination of an electron and a hole, followed by energy transfer from the exciplex-forming co-host to the dopant, and the luminescence of the green light from the dopant.