23rd SmoleQ Greeting Seminar (2026.9.18 at 17:00 (JST))

 

Speaker: 

Mauro Cainelli

Title: 

Quantum dynamics of excitons in molecular aggregates: A hierarchical equations of motion approach

Abstract:

Organic semiconductors based on π-conjugated molecules are at the core of photovoltaic cells, light-emitting diodes and photodetectors. Their performance is largely decided within the first hundreds of femtoseconds to a few picoseconds after photoexcitation, when the created exciton has to migrate over several nanometers and reach a donor–acceptor interface before it decays. Predicting this stage is difficult because all the relevant energy scales, namely the intermolecular electronic coupling, the exciton–vibrational reorganization energy and the static and dynamic disorder, are of comparable magnitude, typically tens to a few hundreds of meV. Consequently, neither the weak-coupling limit (Förster or Redfield theory) nor the strong-coupling hopping limit (Marcus-type rates) can be assumed safely, and perturbative rate theories may break down. To treat these effects we employ the hierarchical equations of motion (HEOM), a numerically exact, non-perturbative and non-Markovian approach to open quantum system dynamics. Combined with a Frenkel–Holstein–Peierls Hamiltonian parameterized from electronic-structure calculations, HEOM gives access both to the population dynamics (exciton transfer and dissociation rates, exciton diffusion) and directly to spectroscopic observables such as linear absorption and time- and frequency-gated fluorescence spectra, enabling a direct comparison with time-resolved experiments. Current talk provides an overview of exciton dynamics in organic photovoltaic materials, introduces the HEOM formalism together with its numerical implementation, and discusses how structural effects (vibrational couplings) and charge transfer state mixture affect exciton transport and its optical signatures in molecular aggregates.