Impact of Grain Boundaries on Triplet Exciton Diffusion in Organic Singlet-Fission Materials

Singlet exciton fission is an efficient multiexciton generation process that enables photogenerated singlets to be split into a pair of mobile and long-lived triplets. As interest grows in applications for these materials in photovoltaics, it is essential to develop a clear set of process design rules to maximize the efficiency of triplet diffusion. Here, we probe the dependence of the triplet exciton diffusion length in polycrystalline thin films of the archetypical singlet-fission material pentacene on in-plane grain size. The out-of-plane triplet diffusion length increases from 16.3 +/- 0.5 to 22.1 +/- 1.3 nm for an increase in grain size from 95 +/- 8 to 229 +/- 10 nm. This increase is analyzed in terms of reduced grain boundary quenching, supported by a concomitant increase in the triplet lifetime extracted from transient absorption spectroscopy. Interestingly, the quenching rate for triplets in pentacene is found to be significantly smaller than previously reported values extracted for singlet excitons in fluorescent materials. These results suggest that while grain boundaries impede triplet exciton diffusion in polycrystalline thin films, low-energy triplets are potentially less susceptible to quenching than singlets.