Charge transport in organic donor-acceptor mixed-stack crystals: the role of nonlocal electron-phonon couplings.

The charge-transport properties in C8BTBT-FnTCNQ and DMQtT-F4TCNQ mixed-stack crystals have been investigated by means of density functional theory, molecular dynamics and kinetic Monte Carlo simulations. The super-exchange nature of charge transport in these crystals is elucidated by the Larsson partition-based electronic coupling method that was developed recently by us. Compared with hole transport, in addition to the donor HOMO-acceptor LUMO interaction, the interaction between the donor HOMO-1 and the acceptor LUMO will also make an important contribution to electron transport. Moreover, this additional interaction plays an opposite role and results in electron-dominant and hole-dominant transport in the C8BTBT-FnTCNQ and DMQtT-F4TCNQ crystals, respectively. Most importantly, our calculations point out that the nonlocal electron-phonon couplings are very weak and much smaller than the electronic couplings in all the studied crystals. This implies that the nonlocal couplings have little influence on charge transport. In contrast to the experimental measurements, the external reorganization energies are thus expected to play an essential role in determining charge carrier mobilities. These findings pave the way for rational design of high performance organic donor-acceptor mixed-stack semiconductors.