Potential Barrier and Excess Energy for Electron-Hole Separation from the Charge-Transfer Exciton at Donor-Acceptor Heterojunctions of Organic Solar Cells

The mechanism of electron-hole separation overcoming Coulomb attraction is one of the important open questions related to the efficiency of organic solar cells. In this work, we have theoretically predicted the potential curves for electron-hole separation from the bound charge-transfer (CT) state at donor-acceptor heterojunctions. The electron-hole potential was calculated considering an oligothiophene-fullerene donor-acceptor complex using the long-range-corrected density functional theory. The screening effect of the medium was taken into account by scaling with the dielectric constants of fullerene and polythiophene. The potential barrier was found to decrease as the pi-conjugation length of the donor increased. We propose a simple analytical formula for the Coulomb potential between an electron localized at a fullerene molecule and a hole distributed along a pi-conjugated chain. We also discuss the possible role of the excess energy due to the preceding exciton dissociation in facilitating the charge separation.