A theoretical study of the absorption spectra of electron-deficient pentacene derivatives using DFT and TDDFT

Non-fullerene acceptor based organic bulk heterojunction solar cells have been a hot topic because their power conversion efficiencies have been up to 16.35%. Functionalized 6,13‑bis (trimethylsilyl alkynyl) pentacenes with strong electron withdrawing groups, which can be easily modified to improve charge transport properties and film morphology, seem to be promising soluble non-fullerene pentacene-based organic acceptors. But how the substitutions of electron withdrawing groups influence their electronic structures, then change the absorption spectra and power conversion efficiencies, is still not clear. In this paper, we utilize density functional theory and time-dependent density functional theory to study the effects of substitutions of different electron withdrawing groups (CN, CF₃, NO₂) and different positions of these groups in 6,13‑bis (trimethylsilyl alkyl) pentacene molecule on their physical and optical properties. We find that the experimental power conversion efficiencies are positively/negatively correlated with calculated dipole moments/exciton binding energies of these functionalized molecules. The computed results indicate that the molecules substituted with CN group have much larger dipole moment than the others. For the same electron withdrawing group, the dipole moment at the R2 position is generally larger than that at the R1 position. Furthermore, we find that the calculated exciton binding energy of these molecules functionalized at the R2 position is lower than that at the R1 position. In addition, the result of absorption spectra confirm that these functionalized 6,13‑bis (trimethylsilyl alkynyl) pentacenes have stronger absorption strength than C60 in the both visible and the ultraviolet regions.