Coulomb barrier for charge separation at an organic semiconductor interface

Charge transfer (CT) excitons across donor-acceptor interfaces are believed to be barriers to charge separation in organic solar cells, but little is known about their physical characteristics. Here, we probe CT excitons on a crystalline pentacene surface using time-resolved two-photon photoemission spectroscopy. CT excitons of 1s, 2s, and 3s characters are bound by Coulomb energies of 0.43, 0.21, 0.12 eV, respectively, in agreement with quantum mechanical modeling. The large binding energy of the 1s CT exciton excludes its participation in photovoltaics. Efficient charge separation in organic heterojunction solar cells must involve a series of hot CT excitons.