Theoretical insights on tunable optoelectronics and charge mobilities in cyano-perylenediimides: interplays between...

Air-stable perylenediimide (PDI) and its derivatives, in particular the cyano-functionalized ones, have attracted great research attention for their potential use in flexible optoelectronics, organic field-effect-transistors (OFETs) as n-type transport materials and also as non-fullerene acceptors in organic photovoltaics (OPVs). Herein we provide a detailed theoretical study on the optical, electrochemical and charge-transport properties (electron and hole mobilities) in a few CN-substituted PDIs with varied number of -CN at different positions (both symmetric and asymmetric di- and tetra-CN derivatives) using density functional theory (DFT) and time-dependent DFT implementing optimally tuned screened range-separated hybrid (OT-SRSH) combining with kinetic rate theory. All cyano-PDIs studied here are energetically stable and form stable π-stacked structures similar to the pristine one, and also act as better electron acceptors. No significant changes in the PDI optical properties are found with the different ways of CN-functionalization, but, this strongly affects the π-stacked geometry, and thereby the electronic coupling, which greatly modulates the PDI intrinsic carrier mobility. Calculated room-temperature electron mobility for the pristine PDI is in excellent agreement with the reported OFET value (∼0.1 cm2 V-1 s-1). Interestingly, relatively large electronic couplings together with small reorganization energies of the symmetrically substituted tetra-CN PDI result in very large charge mobilities (0.4 cm2 V-1 s-1 for electrons and 5.6 cm2 V-1 s-1 for holes) among the systems studied. Therefore, this may serve as a potential ambipolar transport material and hence, naturally calls for experimental demonstration. This detailed and comprehensive study sheds light on the complex interplays between the -CN numbers and the positions for tailored optoelectronic and charge-transport in several functional PDIs, and also shows routes to molecularly design potential n-type materials.