Distinguishing between Triplet-Pair State and Excimer Emission in Singlet Fission Chromophores Using Mixed Thin Films

In the photoluminescence spectra of thinfilms made of singletfission (SF)materials emission features that are red-shifted from the free exciton emission are of particularinterest. They can befingerprints of the correlated triplet-pair state and as such offer insights intothe mechanisms of the multistep SF process. However, excimer formation or trap-state populationcan also cause such features and a clear disentanglement of the various contributions can bechallenging. Here, we use blends of anthradithiophene (ADT) and weakly interacting organicsemiconductors to control the polarizability of the molecular environment and, thus, todistinguish between excimer emission and emission from the correlated triplet-pair state. Usingtime-resolved photoluminescence spectroscopy measurements, we clarify the relation betweenexcimer formation and SF in ADT andfind that excimer formation constitutes a parallel relaxationchannel for the exciton and neither mediates nor hinders SF.