Dynamic Disorder Drives Exciton Dynamics in Diketopyrrolopyrrole-Thiophene-Containing Molecular Crystals

There is a growing interest in controllable molecular materials for potential nanophotonic and quantum information applications where excitons move beyond the incoherent transport regime. Thus, the ability to identify the key parameters that correlate with the efficiency of the transport of the excitation energy is highly desirable. In this work, we investigate the effects of the dynamic disorder on the transport of the exciton in molecular crystals of several mono- and dialkylated 1,4-diketo-3,6-dithiophenylpyrrolo[3-4-c]pyrrole derivatives. These systems exhibit great potential for photovoltaic applications due to their broad optical absorption and efficient charge transport. The exciton dynamics are studied using a model Hamiltonian, in which the thermal fluctuations of the excitonic coupling (nonlocal electron-phonon coupling), as well as the local exciton-phonon couplings, have been appropriately taken into account. The computed reorganization energies for the most feasible transport pathway (π-πstacking) for the excitons in UBEQUQ and UBEQOK molecular crystals are 0.366 and 0.357 eV, respectively. These values are comparable with the magnitude of the average excitonic coupling ⟨J»(≈ 0.1 eV) for these two molecular crystals. In this instance, the local exciton-phonon coupling is not large enough to form a small exciton-polaron. In addition, substantial coherences are observed on a time scale of less than 100 fs, which indicates that the dynamic disorder is sufficient to overcome quantum dephasing and help drive exciton transport in this class of organic semiconductors. On the other hand, the diffusion of the excitons reduces significantly when the thermal fluctuations of the excitonic coupling are omitted. Thus, dynamic disorder plays a vital role in the transport of the exciton, and the ability to control this inherent property in molecular aggregates will provide valuable tools for the design and development of efficient organic semiconductors.