Slowed Singlet Exciton Fission Enhances Triplet Exciton Transport in Select Perylenediimide Crystals

Singlet fission (SF) materials used in light-harvesting devices must not only efficiently produce spin-triplet excitons but also transport them over hundreds of nanometers. N,N’-bis(2-phenylethyl)-3,4,9,10-perylenedicarboximide (EP-PDI) is a promising SF chromophore due to its photostability, large extinction coefficient, and high triplet yield, but the energy transport mechanisms in EP-PDI solids are minimally understood. Herein, we use transient absorption microscopy to directly characterize exciton transport in EP-PDI crystals. We find evidence for singlet-mediated transport in which pairs of triplet excitons undergo triplet fusion (TF), producing spin-singlet excitons that rapidly diffuse. This interchange of singlet and triplet excitons shuttles triplets as far as 205 nm within the first 500 ps after photoexcitation. This enhanced transport comes at a cost, however, as it necessitates favoring triplet recombination and thus requires a fine-tuning of SF dynamics to balance triplet yields with triplet transport lengths. Through numerical modeling, we predict that tuning the ratio of SF and TF rate constants, k_SF/k_TF, to between 1.9 and 3.8 allows for an optimized triplet transport length (425 – 563 nm) with minimal loss (7 – 10%) in triplet yield. Interestingly, by adjusting the size of EP-PDI crystals we find we can subtly tune their crystal structure and thereby alter their SF and TF rates. By slowing SF within small EP-PDI crystals, we are able to boost their triplet transport length by ~20%. Although counter-intuitive, our work suggests that slowing SF by introducing moderate structural distortions can be preferential when optimizing triplet exciton transport, provided singlet exciton transport is not significantly hindered.