Theoretical investigation on photophysical properties for 2,7-carbazole derivatives as solar cells materials: Effect of substitution

A series of 2,7-carbazole derivatives (ePCDTPT) with heteroatoms, electron-donating, or electron-withdrawing groups was investigated by the PBE0/6-31G(d,p) and the TD-PBE0/6-31+G(d,p) methods to provide an in-depth study for the effects of various substitutions on their optical and electronic properties. The calculated results reveal that, the HOMO–LUMO gaps and the longest wavelengths of maximal absorption of derivatives can be efficiently tuned by heteroatoms (B, P, and Si), electron-donating (–OCH3), or electron-withdrawing (–NO2) groups. Phosphorus, silicon atoms, and –OCH3 substituents in R1, R1′- or R3, R3′-positions can make the absorption spectra have red shifts. Solvent effects on the optical and electronic properties of selected derivatives have also been investigated. The calculated reorganization energies of electron and hole show that phosphorus and silicon atoms can increase the charge transfer rates for derivatives of ePCDTPT, electron-donating or electron-withdrawing groups can decrease the charge transfer rates for them.