Singlet-Fission Dynamics Modified through Templated Organic Semiconductor Crystallization

Singlet fission (SF) is a process of multiexciton generation in molecular semiconductor materials that holds promise for improving light-to-charge conversion efficiencies in photovoltaic devices. Molecular packing structure is well-known to impact the electronic coupling that underlies triplet-pair generation, as well as pair separation and triplet transport that occur through singlet fission, ultimately affecting the potential to increase conversion efficiencies in light-harvesting devices. We previously demonstrated that templating the crystallization of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) films on a series of lead-halide perovskites provides a means to control intermolecular packing and alter the kinetics of triplet-pair separation and recombination. Here we contrast triplet transport with the ultrafast interconversion of the singlet exciton to the correlated triplet pair and the dependence of these processes on template-modified crystal packing. We observe that rate constants for the conversion of singlet excitons to correlated triplet pairs likewise are sensitive to the template structure but with a trend that is anticorrelated with those for triplet-pair separation and triplet-triplet annihilation. This observation is consistent with the divergent requisite frontier orbital overlap symmetries of adjacent chromophores that underlie the electronic coupling associated with the generation of triplet pairs and the subsequent triplet transport. We examine how the template-dependent packing structures, as determined by molecular dynamics simulations of a film-template interface, alter figures of merit for electronic interactions (estimated by frontier-orbital overlaps) that underlie each step of the SF mechanism. Our results demonstrate that templating is a general platform for tuning the relative rates of deactivation of correlated triplet pairs in SF-active materials with the potential to favor higher triplet-pair yields or for assisting singlet-mediated triplet-transfer mechanisms without structural modification of the SF-active chromophore.