Low Onset Stimulated Emission in Electrically Pumped Organic Light-Emitting Diodes

Long lived stimulated emission is observed from conventional organic light-emitting diodes in extended cavities with onset at current densities of less than 1 mA/cm2. These current densities are more than six orders of magnitude lower than those previously reported for transient stimulated emission in an elaborate structure. The wavelengths at which stimulated emission occurs are determined by a balance between transition strength and population inversion. Because the latter is temperature-sensitive, the electroluminescence spectrum varies as the device progresses toward thermal equilibrium. We explain the spectral dynamics and achieve a good quantitative understanding of the onset current density. At the low operating currents used here, the ability to produce stimulated emission is not limited by absorptive losses, only by the Boltzmann population of vibrational states of the electronic ground state that serve as the lower levels for the emission.