Theoretical insights on tunable optoelectronics and charge mobilities in cyano-perylenediimides: interplays between –CN numbers and positions

Abstract

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.