Quenching effects in organic electrophosphorescence

We examine various electronic processes that underlie the quenching of the emission from highly efficient phosphorescent and electrophosphorescent organic solid-state molecular systems. As an example, we study the luminescent efficiencies from the phosphorescent iridium (III) complex, fac tris (2-phenylpyridine) iridium $\left[\mathrm{Ir}{\mathrm{}\left(\mathrm{ppy}\right)\mathrm{}}_3\right]$ doped into a diamine derivative doped polycarbonate hole-transporting matrix and in the form of vacuum-evaporated films, as a function of electric field. We demonstrate that the observed decrease in electrophosphorescence efficiencies at high electric fields, and electric-field-induced quenching of phosphorescence from neat $\left[\mathrm{Ir}{\mathrm{}\left(\mathrm{ppy}\right)\mathrm{}}_3\right]$ solid films is due to the field-assisted dissociation of Coulombically correlated electron-hole (e-h) pairs. They are formed in a bimolecular recombination process prior to the formation of emissive triplet excitons, or are charge-transfer (CT) states originating from the localized electronic excited states as a result of the initial charge separation upon photoexcitation, respectively. It is found that the high-field dependence of the quenching efficiency in both cases follows the three-dimensional Onsager theory of geminate recombination, the fit yielding the initial intercarrier distance ${(r}_0)$ of the carrier pairs. We find $r_{e-h}>~3.5\mathrm{nm}$ for the triplet exciton precursor pairs in the bimolecular recombination, and $r_{\mathrm{CT}}=1.8\mathrm{}\pm 0.1\mathrm{nm}$ for the initial carrier separation from the photo-excited electronic states. Triplet-triplet and triplet-charge carrier annihilation processes are shown to play major roles in the decrease of the electrophosphorescence efficiency within the lower-field regime at lower current densities. Summarizing the results allows us to point out some emitter features important for identifying phosphors useful for practical electroluminescent devices.