Molecular Doping Induced Charge Transfer Complex Formation and Interfacial Dopant Interdiffusion on Graphite

Doping is a powerful method to optimize the electrical characteristics of organic semiconductors, but a comprehensive picture capturing the phenomena at play is still under development. In this work, combining UV-vis absorbance spectroscopy with ultraviolet and X-ray photoelectron spectroscopy, we investigate the p-type doping of (sub)-monolayer films of two structurally isomeric organic semiconductors on graphite, naphtho-[2,3-<i>b</i>]-thieno-[2‴,3‴:4″,5″]-thieno-[2″,3″:4',5']-thieno-[3',2'-b]-naphtho-[2,3-<i>b</i>]-thiophene (DN4T) and naphtho-[1,2-<i>b</i>]-thieno-[2‴,3‴:4″,5″]-thieno-[2″,3″:4',5']-thieno-[3',2'-b]-naphtho-[1,2b]-thiophene (isoDN4T), with the strong molecular acceptor 2,2'-(perfluoronaphthalene-2,6-diylidene)-dimalononitrile (F6TCNNQ). For DN4T, a hybrid highest occupied molecular level emerges from the hybridization of the DN4T and F6TCNNQ frontier occupied levels, resulting from the formation of DN4T:F6TCNNQ charge-transfer complexes. With increasing F6TCNNQ coverage, the electronic levels of both neutral DN4T and the DN4T:F6TCNNQ complexes shift toward the Fermi level because of an interface dipole that is due to electron transfer from graphite to F6TCNNQ. In comparison, isoDN4T exhibits stronger interaction with F6TCNNQ and increased interfacial disorder, as evidenced by significant spectral broadening. These findings emphasize the profound impact of subtle structural variations on host-dopant interactions and the importance of exploring multicomponent interfaces for advanced organic electronic and optoelectronic applications.