No abstract

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.

Asymptotic efficiency (high output without droop) was recently reported for OLEDS in which a thin emitter layer is located at the anti-node in a resonant microcavity. Here we extend our theoretical analysis to treat multi-mode devices with isotropic emitter orientation. We recover our efficiency equations for the limiting cases with an isotropic emitter layer located at the anti-node where output is linear in current, and for an isotropic emitter located at the node where output can exhibit second order losses with an overall efficiency coefficient that depends on loss terms in competition with a cavity factor. Additional scenarios are described where output is driven by spontaneous emission, or mixed spontaneous and stimulated emission, with stimulated emission present in a loss mode, potentially resulting in cavity driven droop or output clamping, and where the emitter layer is a host-guest system.

We observe stimulated emission from OLEDS at current densities as low as 20 uA/cm2 in a high Q cavity formed between the Ag cathode and exit surface of the substrate. In this regime, inversion is dependent on Boltzman population of vibrational levels serving as the ground state resulting in spectral dynamics on the time scale of thermalization.

Thermally activated delayed fluorescence has been reported in a number of OLED emitter materials engineered to have low singlet-triplet energy gaps. Here we derive closed solutions for steady state and transient behaviors, and apply these results to obtain the singlet and triplet relaxation, forward and reverse crossing rates and the gap energy and reverse crossing prefactor from delayed and prompt relaxation rates measured over a series of temperatures.The primary rates are then used to calculate the fluorescent/phosphorescent ratio and the singlet/triplet population ratio. The method avoids the need for gated yield measurements.Good fits are obtained using previously published data for 4CzIPN and m-MTDATA:t-Bu-PBD and the results appear to be consistent with reported quantities were available, and with reported behaviors of OLEDS that use these materials.