Ab Initio Study of Charge Separation Dynamics and Pump–Probe Spectroscopy in the P3HT/PCBM Blend

We develop a bottom-up computational
method for excited-state
dynamics
and time-resolved spectroscopy signals in molecular aggregates, on
the basis of ab initio excited-state calculations. As an application,
we consider the charge separation dynamics and pump–probe spectroscopy
in the amorphous P3HT/PCBM blend. To simulate quantum dynamics and
time-resolved spectroscopy, the model Hamiltonian for single-excitation
and double-excitation manifolds was derived on the basis of fragment-based
excited-state calculations within the GW approximation and the Bethe–Salpeter
equation. After elucidating the energetics of the electron–hole
separation and examining linear absorption spectrum, we investigated
the quantum dynamics of exciton and charge carriers in comparison
with the pump–probe transient absorption spectra. In particular,
we introduced the pump–probe excited-state absorption (ESA)
anisotropy as a spectroscopic signature of charge carrier dynamics
after exciton dissociation. We found that the charge separation dynamics
can be probed by the pump–probe ESA anisotropy dynamics after
charge-transfer excitations. The present study provides the fundamental
information for understanding the experimental spectroscopy signals,
by elucidating the relationship between the excited states, the exciton
and charge carrier dynamics, and time-resolved spectroscopy.