Morphology-Dependent Triplet Exciton Diffusion in Vapor Deposited Pentacene Thin Films

Singlet exciton fission is among the most efficient multiexciton-generation process reported to date. Materials exhibiting singlet fission can potentially generate two triplets for each photogenerated singlet, and are of interest for a variety of applications in photoconversion and light-emission. While the kinetics of fission have been previously examined, fewer works have sought to probe the diffusion length of the resulting non-luminescent triplet state. Here, we probe triplet exciton diffusion in thin films of the archetypical singlet fission material pentacene using a device-based methodology capable of extracting the intrinsic, materials-relevant exciton diffusion length. The dependence of triplet exciton diffusion on morphology and crystalline grain size is probed by varying thin film deposition rate. Increased grain size leads to an increase in exciton lifetime, and a commensurate improvement in the triplet diffusion length. For a change in growth rate from 10 Å/s to 0.05 Å/s, the triplet diffusion length increases from 16.3 nm to 22.0 nm. These results offer an additional handle on how to maximize the triplet exciton diffusion length with optimization of thin film processing conditions.