Coulomb Barrier for Charge Separation at an Organic Semiconductor Interface

Abstract

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.