▎ 摘　　要

Due to mechanical flexibility and low cost, heterojunctions consisting of graphene and small organic molecules are regarded as promising candidate materials for vertical organic field-effect transistors (VOFETs), where the charge carrier mobility perpendicular to the graphene plane is crucial to their performance. Herein, through density functional simulations, we find that the vertical charge carrier mobility of the heterojunctions can be greatly adjusted by tuning their pi-pi stacking distances. For the 6,13-dichloropentacene (DCP)/graphene heterojunctions, with the distance between the first DCP layer and graphene decreasing to below 2.4 angstrom, the vertical electron mobility between DCP layers is improved dramatically while the vertical hole mobility is greatly reduced. The strong dependence of vertical charge carrier mobility on the distance between the first molecular layer and substrate for smaller values than the typical pi-pi stacking distance (3.3-3.8 angstrom) was also observed in the perylenetetracarboxylic dianhydride (PTCDA)/graphene and DCP/hexagonal-BN heterojunctions, where the tendency is very different to that of the DCP/graphene heterojunction. Our simulation results enabled us to develop a new strategy to tune the vertical charge transport properties in molecule/graphene heterojunctions, which provides insights into developing efficient VOFETs.