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

## TL;DR

This study uses first-principles many-body theory to analyze how the length of donor conjugation in oligothiophene-F4TCNQ charge-transfer complexes affects their electronic and optical properties, revealing size-dependent trends and excitations.

## Contribution

It provides new insights into the influence of donor conjugation length on the electronic structure and excitations of doped organic semiconductors from first-principles calculations.

## Key findings

- Electronic gaps decrease with increasing donor length.
- Optical gaps remain approximately constant regardless of donor size.
- Charge-transfer excitations and localized Frenkel excitons appear in longer oligomers.

## 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 2,3,5,6-tetrafluoro-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.