Different Dissociation Rates of Singlet and Triplet Excitons of Pentacene at the Interface in Solar Cells

Fission of the lowest-energy singlet exciton (S-1) to two lowest-energy triplet excitons (T-1) in pentacene has been expected to be a promising means for increasing the quantum efficiency of solar cells. Experiments find that S-1 and T-1 dissociate at quite different time scales at the donor/acceptor interface. Using the pentacene/TiO2 heterojunction as the model, we investigate the dissociation of pentacene excitons by a combination of many-body Green's function theory and the time-dependent Schrodinger equation. Singlet and higher-energy triplet (T-n) excitons of pentacene could dissociate at the same timescale of similar to 100 fs, benefiting from their capability to scatter into charge-transfer (CT) states and wealdy bound charge-separated (CS) states across the interface. Resonance of pentacene excitons with CS states could facilitate the creation of free charge carriers. However, dissociation of T-1 is hampered due to its poor density of states projected onto the interfacial states, preventing its scattering into CT and CS states. According to this phenomenon, we suspect that the electron transfer from T-1 to acceptor, as observed in experiments, might undergo two successive processes, promotion of T-1 to T-n by visible light and dissociation of T-n via scattering. Involvement of the additional light absorption process might result in the low dissociation rate of T-1.