The interface effect between ZIXLIB crystal surface and C₆₀: Strong charge-transfer (CT) vs weak CT state

The charge-transfer (CT) state induced by donors (D) – acceptors (A) interface interactions can dramatically influence the electron transition and charge separation in organic photovoltaic devices. In this work, two D-A interfaces were introduced, which were constructed by {0 1 1} and {1 0 0} crystal surfaces of 2,3,5,6-Bis(ethylenedithio)thieno[3,2-b]thiophene (ZIXLIB) and fullerene (C₆₀) molecules, named {0 1 1}/C₆₀ and {1 0 0}/C₆₀ interface, respectively. The density functional theory (DFT) results show that face-on and edge-on interactions are favorite at {0 1 1}/C₆₀ interface, while only edge-on interactions at {1 0 0}/C₆₀ interface. The geometry, interaction energy, Mulliken charge distribution, visualization of the interactions and charge density difference analysis indicate that the interactions between the D-A interfaces are both non-covalent interactions, but strong CT state was induced by the {0 1 1}/C₆₀ interface interactions, only weak CT state appear at the {1 0 0}/C₆₀ interface. The strong CT state at {0 1 1}/C₆₀ interface results in a remarkable CT absorption peak around 700 nm, while weak CT state at {1 0 0}/C₆₀ surface results in a common absorption peak around 570 nm. Furthermore, the band structure results indicate that a more balanced charge transport presented at {0 1 1}/C₆₀ interface (with strong CT state) than {1 0 0}/C₆₀ interface (with weak CT state). This balanced charge transport are helpful for the charge separation at the D-A interface.