  We demonstrate that efficient and nearly field-independent charge separation
in organic planar heterojunction solar cells can be described by an incoherent
hopping mechanism. We model the separation efficiency of electron-hole pairs
created at donor-acceptor organic interfaces. By using kinetic Monte Carlo
simulations that include the effect of on-chain delocalization we show that
efficient charge extraction to the electrodes requires bipolar transport and
increased dimensionality. This model explains experimental results of almost
field independent charge separation in some molecular systems and provides
important guidelines at the molecular level for maximizing the efficiencies of
organic solar cells.
