Elucidating the role of charge transfer on semiconductor properties in a new donor-acceptor cocrystal 1,5-dihydroxynaphthalene : TCNQ

In this work, we have investigated the semiconducting properties of an unprecedented 1 : 1 pi-stacked donor-acceptor cocrystal of 1,5-dihydroxynaphthalene (DHN) as the pi-donor (D) with 7,7 ',8,8 '-tetracyanoquinodimethane (TCNQ) as the pi-acceptor (A). Molecular semiconductors with electron dominant transport, narrow bandgap, solution processing ability, air-stability are highly sought-after for application in n-channel organic field effect transistors. The DHN : TCNQ cocrystal shows n-type semiconductor nature with a narrow bandgap of around 1 eV, and a low LUMO energy level (-3.8 eV) making it less prone to areal degradation. The electron dominant transport in this cocrystal is described by assuming that electron and hole hop via a super-exchange mechanism along the mixed & ctdot;D-A & ctdot; pi-stack direction. The participation of bridging molecular orbitals other than donor HOMO make a significant contribution to the super-exchange electron transfer, thus resulting in electron hopping from acceptor to acceptor which is four times larger than the value of hole hopping from donor to donor. Detailed analysis of crystal packing and electronic properties demonstrate that the super-exchange charge carrier transport is facilitated by strong pi & ctdot;pi stacking interaction between the donor and acceptor, and prominent charge transfer.