Kinetic Monte Carlo Study of Triplet-Triplet Annihilation in ConjugatedLuminescent Materials

It is well known that in organic solids the collision of two excitons can give rise to delayed fluores-cence (DF). Revived interest in this topic is stimulated by the current endeavor towards the developmentof efficient organic optoelectronic devices such as organic light-emitting diodes (OLEDs) and solar cells,or sensitizers used in photodynamic therapy. In such devices, triplet excitations are ubiquitously presentbut their annihilations can be either detrimental, e.g., giving rise to a roll-off of intensity in an OLED, ormandatory, e.g., if the sensitizer relies on up-conversion of long-lived low-energy triplet excitations. Sincethe employed materials are usually noncrystalline, optical excitations migrate via incoherent hopping.Here, we employ kinetic Monte Carlo simulations (KMC) to study the complex interplay of triplet-tripletannihilation (TTA) and quenching of the triplet excitations by impurities in a single-component systemfeaturing a Gaussian energy landscape and variable system parameters such as the length of the hoppingsites, i.e., a conjugated oligomer, the morphology of the system, the degree of disorder (σ), the concen-tration of triplet excitations, and temperature. We also explore the effect of polaronic contributions to thehopping rates. A key conclusion is that the DF features a maximum at a temperature that scales withσ/kBT. This is related to disorder-induced filamentary currents and thus locally enhanced triplet densities.We predict that a maximum for the TTA process near room temperature or above requires typically adisorder parameter of at least 70 meV