The authors report the fabrication of organic light-emitting diodes based on N,N′-di-1-naphthalenyl-N, N′-diphenyl-[1, 1′-biphenyl]-4, 4′-diamine (NPB) that emit via singlet excimer states. When the film deposition rate of NPB is reduced from 17 to 1 nm/min, they observe a reduction in intensity of the photoluminescence peak at 437 nm and the evolution of a new, broader peak at 503 nm. From optical absorption and time resolved photoluminescence data the authors attribute this new peak to singlet excimer emission. The authors demonstrate green electroluminescence from organic diodes that utilize this effect. The possible morphological variations resulting in the monomer to excimer transition are discussed. © 2006 American Institute of Physics.