State-of-Matter-Dependent Charge-Transfer Interactions between Planar Molecules for Doping Applications

Controlling the electrical conductivity of organic semiconductors is a key asset for organic electronics, nowadays realized mostly by molecular dopants. Two doping mechanisms have been reported – charge-transfer complex (CTC) and ion pair (IPA) formation. However, their occurrence depending on molecular structure, energy levels, and structure of thin films remains elusive. Here, we study p-type doping of the planar organic semiconductor dibenzotetrathiafulvalene (DBTTF) in combination with the electron acceptors tetracyanonaphthoquinodimethane (TCNNQ) and hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ) as planar dopants. The conductivity of DBTTF films increases by more than two orders of magnitude upon doping with F6TCNNQ and only slightly with TCNNQ. The highest conductivity is reached at about 10 mol % dopant concentration as a result of two counteracting effects: (1) increasing carrier concentration and (2) reduced carrier mobility due to the growing density of structural defects. We identified two different CTCs for DBTTF:TCNNQ blends and both types of charge-transfer interactions (CTC and IPA) in films of DBTTF doped with F6TCNNQ from absorption measurements. No signature of the charge-transfer interaction is found for DBTTF and TCNNQ in solution, whereas IPA formation only is observed for DBTTF and F6TCNNQ. Many-body perturbation theory calculations of the electronic and optical properties of one-dimensional stacks complement the experimental data and help in understanding the behavior of CTCs. The degree of charge transfer turns out to be higher for the DBTTF:F6TCNNQ complexes than for DBTTF:TCNNQ, as derived from the C≡N stretching mode softening in infrared absorption. We discuss the different fundamental semiconductor–dopant interactions in solution as compared to the solid state with the aid of the state-of-matter-dependent energy levels of the materials. The presence of both charge-transfer mechanisms in the material combinations investigated here gives us access to their doping efficiency, which is higher for IPA than for CTC formation. Avoiding the CTC formation by structural imperfections seems to be a way to increase the doping efficiency for crystalline materials. The determination of energy levels both in solution and in thin films is beneficial for understanding charge-transfer behavior.