Dynamics of charge separation at an organic donor-acceptor interface

The key process in organic photovoltaic cells is the charge separation at organic donor-acceptor interfaces. However, exactly how the charges separate into free charge carriers still remains a puzzle. We present here simulations of the electron dynamics of this process using a nonadiabatic Ehrenfest method. From these simulations, we give a direct illustration of the charge separation process. The results show that the delocalization of the electronic states involved plays a critical role in order to overcome the Coulomb attraction of the charge transfer (CT) exciton. Charge separation only occurs for sufficiently strong intermolecular interactions. Alternatively, the CT exciton relaxes into a bound polaron pair. The results also show that the “excess energy” of the hot CT exciton facilitates the charge separation process to a certain degree.

Figure 1

Schematic diagram of the D-A blend system.

Figure 2

Time evolution of the mean charge density ${\overline{\rho }}_i\left(t\right)$ with different interchain interactions: (a) $t_{\mathrm{v}}=30$ meV, (b) $t_{\mathrm{v}}=50$ meV, (c) $t_{\mathrm{v}}=70$ meV, and (d) $t_{\mathrm{v}}=100$ meV, $d_{\mathrm{DA}}=6$ Å, $d=5$ Å, $\beta =3.4$, and $\Delta E=0.5$ eV, and $t_{\mathrm{DA}}$ is derived from Eq. (3).

Figure 3

Panels (a) and (b) are the same simulations as Figs. 2 and 2, respectively, but with a shorter interchain distance at the interface, $d_{\mathrm{DA}}=5$ Å.

Figure 4

Panels (a) and (b) are the same simulation as Fig. 2 but with different screening factors: (a) $\beta =4.0$ and (b) $\beta =4.4$.

Figure 5

Panels (a) and (b) are the same simulations as Fig. 2, but with different band offset, (a) $\Delta E=0.6$ eV and (b) $\Delta E=0.7$ eV.