1. Center for Photonics and Optoelectronic Materials, Department of Electrical Engineering and the Princeton Materials Institute, Princeton University, Princeton, New Jersey 08544, USA
2. Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA
3. Permanent address: Department of Chemistry, Goucher College, Baltimore, Maryland 21204-2794, USA.

Correspondence to: S. R. Forrest¹ Correspondence and requests for materials should be addressed to S.R.F. (forrest@ee.princeton.edu).

The efficiency of electroluminescent organic light-emitting devices¹,² can be improved by the introduction³ of a fluorescent dye. Energy transfer from the host to the dye occurs via excitons, but only the singlet spin states induce fluorescent emission; these represent a small fraction (about 25%) of the total excited-state population (the remainder are triplet states). Phosphorescent dyes, however, offer a means of achieving improved light-emission efficiencies, as emission may result from both singlet and triplet states. Here we report high-efficiency (90%) energy transfer from both singlet and triplet states, in a host material doped with the phosphorescent dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP). Our doped electroluminescent devices generate saturated red emission with peak external and internal quantum efficiencies of 4% and 23%, respectively. The luminescent efficiencies attainable with phosphorescent dyes may lead to new applications for organic materials. Moreover, our work establishes the utility of PtOEP as a probe of triplet behaviour and energy transfer in organic solid-state systems.