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 tw...