Exciton Migration in Conjugated Dendrimers: A Joint Experimental and Theoretical Study

Abstract

We report a joint experimental and theoretical investigation of exciton diffusion in phenyl-cored thiophene dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 8 and 17 nm, increasing with the size of the dendrimer. A theoretical methodology is developed to estimate exciton diffusion lengths for conjugated small molecules in a simulated amorphous film. The theoretical approach exploits Fermi's Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Utilization of Poisson's equation in the evaluation of the Coulomb integral leads to very efficient calculation of excitonic couplings between the donor and the acceptor chromophores. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of the material, but do not result in strong couplings between the chromophores which are calculated for more localized transition densities. The molecular structures of dendrimers seem to be playing a significant role in the magnitude of electronic coupling between chromophores. Simulated diffusion lengths correlate well with the experimental data. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors in determining the size of the exciton diffusion length in amorphous films of conjugated materials.

Exciton diffusion in disordered small molecules for organic photovoltaics: insights from first-principles simulations

“…Exciton diffusion in organic materials has been studied theoretically by using a Forster-type energy transfer model; the previous work determined exciton transition rates based on either an empirical Miller-Abrahams form [23,24] or quantum chemical calculations [25][26][27]. Although very powerful in dealing with molecular crystals, these approaches are generally not applicable to amorphous materials such as disordered small molecules as examined in this work.…”

Intrinsic measurements of exciton transport in photovoltaic cells

“…Probing exciton migration often relies on tracking the end-of-life products of excitons, including photons from exciton recombination and charge carriers from exciton dissociation 30,31 . For luminescent materials, L D can be determined from a steady-state or time-resolved photoluminescence (PL)-based measurement 22,32,33 .…”

Thermally activated motion of cold excitons in conjugated polymers

“…The rate constants and are equal due to symmetry of the system. An average of 2 ps −1 is chosen for the rate constant of exciton transfer reaction after following similar studies in literature [23]. The simulation runs until the exciton lifetime (τ) is reached.…”

“…In all the experimental studies summarized above, it is clear that singlet exciton diffusion can be expressed as thermally activated process for certain temperature ranges with activation energy peculiar to the chromophoric system and its environment [17]. Coherent and incoherent exciton migration in conjugated materials has been the subject of many simulation efforts in the literature [18][19][20][21][22][23][24]. Most of these studies focus on a single type of conjugated system and comparative analysis of activation energies for exciton hopping and the relevant dynamics for various types of materials is currently missing in the literature.…”