Quantum efficiency roll-off at high brightness in fluorescent and phosphorescent organic light emitting diodes

Abstract

A general technique is demonstrated to quantify the contribution of monomolecular and bimolecular quenching processes to the external quantum efficiency (EQE) roll-off in organic light emitting devices (OLEDs). Based on the photoluminescence transients of electrically driven devices, we identify the relative contributions of quenching and lack of charge balance to the roll-off in four fluorescent and phosphorescent devices containing the dopants 2,3,7,8,12,13,17,18-octaethylporphine platinum (PtOEP), fac tris-2-phenylpyridine iridium $[\mathrm{Ir}{(\mathrm{ppy})}_{3}]$, the laser dye 4-dicyanmethylene-2-methyl-6-($p$-dimethylaminostyryl)-$4H$-pyran (DCM), and neat tris(8-hydroxyquinoline) aluminum. We find that quenching is proportional to the radiative lifetime of the emitting molecule and that it is solely responsible for the roll-off of PtOEP. Roll-off of the EQE for $\mathrm{Ir}{(\mathrm{ppy})}_{3}$ is due primarily to loss of charge balance at low current density, $J$, and only shows significant quenching at $J\ensuremath{\ge}1\phantom{\rule{0.3em}{0ex}}\mathrm{A}∕{\mathrm{cm}}^{2}$. No quenching is observed for the fluorescent doped DCM device, even for $J\ensuremath{\sim}28\phantom{\rule{0.3em}{0ex}}\mathrm{A}∕{\mathrm{cm}}^{2}$. Consequently, doped fluorescent OLEDs that maintain charge balance at high current density enable the elimination of intensity roll-off, which may provide a route to electrically pumped organic lasing.