Ultrafast Excited-State Energy Transfer in DTDCTB Dimers Embedded in a Crystal Environment: Quantum Dynamics with the Multilayer Multiconfigurational Time-Dependent Hartree Method

Photoinduced excited-state energy transfer (EET) processes play a key role in the solar energy conversion of small Molecule organic solar cells. We investigated intermolecular EET dynamics in the 2-[[7-(5-N,N-ditolylaminothiophen-2-yl)-2,1,3-benzothiadiazol-4-yl]methylene]malononitrile (DTDCTB) dimer embedded in a crystal environment using full quantum dynamics, i.e., the multilayer multiconfigurational time-dependent Hartree (ML-MCTDH) method. Two different stacking statuses of the DTDCTB dimers, which occur along the OA axis in the DTDCTB crystal, were considered. We builta vibronic diabatic Hamiltonian using the projection method based on quantum mechanics/molecular mechanics results. Different model Hamiltonian were considered in the full quantum dynamics studies. First, reduced-dimensional models were constructed by simply including, more of the important vibrational modes. Second, we tried to construct a continuous spectral density based on the vibronic coupling strengths of different modes and then created a set of pseudomodes to represent electron-phonon couplings. The dynamics results based on these reduced models were compared with the results obtained with the full dimensional model. Our theoretical descriptions demonstrated that ultrafast intermolecular EET dynamics takes place in the well-stacked DTDCTB dimers. This work deepens our understanding of the photoinduced ultrafast EET dynamics of realistic organic photovoltaic systems at the full quantum mechanical level.