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

Abstract

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 − chargetransfer 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.

Ab initio modelling of local interfaces in doped organic semiconductors

“…In this way, the type-II alignment between the donor and acceptor species is clearly evident. In the dimers, both the HOMO and the LUMO are hybridized with bonding and antibonding character, respectively, 13,19,20,[82][83][84] (see Fig. 4) and they are energetically comprised between the frontier orbitals of the constituents.…”

Ultrafast charge transfer and vibronic coupling in a laser-excited hybrid inorganic/organic interface

“…The most relevant one concerns the C≡N stretch, which is usually analyzed to estimate the degree of charge transfer in the ground state of donor/acceptor complexes formed by molecules hosting this bond (see Refs. [128,[134][135][136][137]). Indeed, from Fig.…”

Electronic and Optical Properties of Oligothiophene-F4TCNQ Charge-Transfer Complexes: The Role of the Donor Conjugation Length

“…Doping in organic semiconductors is an ubiquitous phenomenon appearing when donor and acceptor molecules are combined together, and crucially determines the electronic, optical, and transport properties of the resulting materials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Two main mechanisms have been identified to be responsible for doping in organic semiconductors: Integer charge transfer (ICT) manifests itself when an electron is transferred from the donor to the acceptor, leading to the formation of ion pairs. Partial charge transfer occurs upon electronic hybridization of the frontier orbitals of the donor and the acceptor, giving rise to a charge-transfer complex (CTC).…”