  Cavity quantum electrodynamics has been studied as a potential approach to
modify free charge carrier generation in donor-acceptor heterojunctions because
of the delocalization and controllable energy level properties of hybridized
light-matter states known as polaritons. However, in many experimental systems,
cavity coupling decreases charge separation. Here, we theoretically study the
quantum dynamics of a coherent and dissipative donor-acceptor cavity system, to
investigate the dynamical mechanism and further discover the conditions under
which polaritons may enhance free charge carrier generation. We use open
quantum system methods based on single-pulse pumping to find that polaritons
have the potential to connect excitonic states and charge separated states,
further enhancing free charge generation on an ultrafast timescale of several
hundred femtoseconds. The mechanism involves that polaritons with proper energy
levels allow the exciton to overcome the high Coulomb barrier induced by
electron-hole attraction. Moreover, we propose that a second-hybridization
between a polariton state and dark states with similar energy enables the
formation of the hybrid charge separated states that are optically active.
These two mechanisms lead to a maximum of 50% enhancement of free charge
carrier generation on a short timescale. However, our simulation reveals that
on the longer timescale of picoseconds, internal conversion and cavity loss
dominate and suppress free charge carrier generation, reproducing the
experimental results. Thus, our work shows that polaritons can affect the
charge separation mechanism and promote free charge carrier generation
efficiency, but predominantly on a short timescale after photoexcitation.
