High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer

To obtain the maximum luminous efficiency from an organic material, it is n ecessary to harness both the spin-symmetric and anti-symmetric molecular ex citations (bound electron-hole pairs, or excitons) that result from electri cal pumping. This is possible if the material is phosphorescent, and high e fficiencies have been observed in phosphorescent(1,2) organic light-emittin g devices(3), However, phosphorescence in organic molecules is rare at room temperature. The alternative radiative process of fluorescence is more com mon, but it is approximately 75% less efficient, due to the requirement of spin-symmetry conservation(4) Here, we demonstrate that this deficiency can be overcome by using a phosphorescent sensitizer to excite a fluorescent d ye. The mechanism for energetic coupling between phosphorescent and fluores cent molecular species is a long-range, non-radiative energy transfer: the internal efficiency of fluorescence can be as high as 100%. As an example, we use this approach to nearly quadruple the efficiency of a fluorescent re d organic light-emitting device.