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

We investigate from the first-principles many-body theory the role of the donor conjugation length in doped organic semiconductors forming charge-transfer complexes (CTCs) exhibiting partial charge transfer. We consider oligothiophenes (nT) with an even number of rings, ranging from four to ten, doped by the strong acceptor, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ). The decrease of the electronic gaps upon increasing the nT size is driven by reduction of the ionization energy with the electron affinity remaining almost constant. The optical gaps exhibit a different trend, being at approximately the same energy regardless of the donor length. The first excitation retains the same oscillator strength and Frenkel-like character in all systems. In 4T-F4TCNQ also, higher-energy excitations preserve this nature, whereas in CTCs with longer nT oligomers, charge-transfer excitations and Frenkel excitons localized on the donor appear above the absorption onset. Our results offer important insights into the structure-property relations of CTCs, thus contributing to a deeper understanding of doped organic semiconductors.