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

Abstract

We investigate from 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 2,3,5,6tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ). The decrease of the electronic gaps upon increasing nT size is driven by the 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. While in 4T-F4TCNQ also higher-energy excitations preserve this nature, 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 insight into the structure-property relations of CTCs, thus contributing to a deeper understanding of doped organic semiconductors.

Microscopic Insight into the Electronic Structure of BCF-Doped Oligothiophenes from Ab Initio Many-Body Theory

“…This situation is representative of the local interactions between donor and acceptor species. 9,10,17 However, it is important to consider the fact that in real samples these systems are embedded in an environment (either in solution or in solid-state) and are therefore subject to non-negligible dielectric screening. In general, going from the gas-to the condensed-phase induces a broadening and a red-shift of the absorption features as an effect of the dielectric screening exerted by the closely-packed molecules.…”

“…Doping represents a crucial process to enable the application of organic semiconductors in optoelectronics. [1][2][3] A consolidated consensus acknowledges the formation of charge-transfer complexes [4][5][6][7][8][9][10] and ion-pairs [11][12][13][14][15][16][17] as the dominant doping mechanisms in organic semiconductors. [17][18][19] The emergence of Lewis acids like tris(pentafluorophenyl)borane, B(C 6 F 5 ) 3 (in short BCF) 20,21 as novel dopant species has opened new routes for efficient p-doping of organic polymers and oligomers.…”

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

“…In contrast, the LUMO of the hybrid corresponds almost identically to the LUMO of the isolated molecule (see, e.g., Refs. [128,131]), with only very small contributions coming from the hybridization with the inorganic part of the interface. A gap of approximately 2 eV separates the LUMO to the higher states in the conduction band, which are localized solely on the H-Si(111) cluster in the energy window shown for the PDOS (see Fig.…”

Ab initio modelling of local interfaces in doped organic semiconductors

“…A common choice in modelling doped OSCs is to consider an isolated dimer formed by a donor and an acceptor molecule interacting with each other. 12,[19][20][21] Representing OSC with an isolated dimer implicitly assumes that the system can be reproduced by a single molecular interface. This model turned out to be successful in describing the level alignment of the donor/acceptor interfaces, 19 the spatial distribution of the resulting frontier states, 12 and also in rationalizing the role of the donor conjugation length.…”

“…This model turned out to be successful in describing the level alignment of the donor/acceptor interfaces, 19 the spatial distribution of the resulting frontier states, 12 and also in rationalizing the role of the donor conjugation length. 20 However, in this way, it is not possible to capture in full the behavior of extended donor/acceptor stacks, nor to reproduce the chemical environment of a molecule surrounded by more than one dopant. 22 This is a non-negligible issue, considering that local interactions at donor/acceptor interfaces are known to critically impact the doping mechanism in organic semiconductors.…”