Triplet exciton interactions in solid films of an electrophosphorescent Pt(II) porphyrin

We demonstrate the phosphorescence (PH) from neat Pt (II) porphyrin (PtOEP) films to be quenched by bimolecular reactions of monomeric (M) and dimeric (D) triplet (T) excitons. Their respective rate constants, gamma(TT)((M))congruent to 8x10(-12) cm(3)/s and gamma(TT)((D))congruent to 8x10(-15) cm(3)/s, were found from the kinetics fits to the excitation intensity dependence of the relative PH yield. The exciton decay becomes dominated by mutual annihilation above a critical concentrations of T(M)(crit)congruent to 6.3x10(17) cm(-3) for M, and T(D)(crit)congruent to 1.2x10(19) cm(-3) for D triplet excited states. These data allow us to exclude T-T annihilation as the nonradiative pathway quenching the emission from guest aggregates in PtOEP-doped emitters, and predict the onset of the current roll-off in electrophosphorescence efficiency at a current that exceeds four orders of magnitude the values observed experimentally with light-emitting-diodes based on thin PtOEP-doped emitting layers. (C) 2005 American Institute of Physics.