  The first-principles simulation of the electronic structure of organic
semiconductors in solution poses a number of challenges that are not trivial to
address simultaneously. In this work, we investigate the effects and the mutual
interplay of solvation, alkylization, and doping on the structural, electronic,
and optical properties of sexithiophene, a representative organic semiconductor
molecule. To this end, we employ (time-dependent) density functional theory in
conjunction with the polarizable-continuum model. We find that the torsion
between adjacent monomer units plays a key role, as it strongly influences the
electronic structure of the molecule, including energy gap, ionization
potential, and band widths. Alkylization promotes delocalization of the
molecular orbitals up to the first methyl unit, regardless of the chain length,
leading to an overall shift of the energy levels. The alterations in the
electronic structure are reflected in the optical absorption, which is
additionally affected by dynamical solute-solvent interactions. Taking all
these effects into account, solvents decrease the optical gap by an amount that
depends on its polarity, and concomitantly increase the oscillator strength of
the first excitation. The interaction with a dopant molecule promotes
planarization. In such scenario, solvation and alkylization enhance charge
transfer both in the ground state and in the excited state.
