Abstract
Electric field-modulated photoluminescence (EML) measurements are presented for vacuum-evaporated films of cyclometallated Pt (II) complexes of 1,3-di(2-pyridyl) benzenes used as triplet emitters in organic light-emitting diodes (OLEDs). The excimer phosphorescence is quenched by the external electric field of 2.5MV/cm up to 25% but the same effect on monomer phosphorescence is one order of magnitude smaller. The higher quenching effect for triplet excimers than triplet monomers in solid films of Pt complexes is rationalized assuming excimers to be populated within excimer-active domains of the films through an intermediate stage of geminate (e–h) pairs derived from dissociated monomer excitons. The EML data for excimers are successfully described in the framework of Sano–Tachiya–Noolandi–Hong (STNH) theory of geminate (e–h) pair recombination where the final recombination step (e–h capture) proceeds on a sphere of finite radius (a) with a finite speed. The conventional Onsager theory (a=0) is sufficient to explain the EML quenching effect for monomers. The results are important for explaining the decrease of electroluminescence quantum efficiency observed in OLEDs working under high electric fields.
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