Insight into opto-electronic property by modifying optical layers with multi-polar and multi-branched structures

Optical active layers of the multi-polar and multi-branched organic solar cell dye molecules were analyzed by using DFT and TD-DFT methods. Optical active layers contain triphenylamine (TPA) as a donor section and diketopyrrolopyrrole (DPP) as a part of the molecular conjugated bridge. Relative optical and electronic properties have been calculated. The results showed that with the increasing of molecular structure, LUMO energy levels and energy gaps decrease. The decrease of energy gap is beneficial to the red-shift of absorption peak, and TPA(DPP-P)₂ displays the maximum red-shift. Ionization potentials decrease and electron affinities increase with the increase of molecular structure. The hole and electron recombination energies of TPA(DPP-P)₂ is the smallest, so TPA(DPP-P)₂ has a better hole transport capability. For developing their application for DSSC, four molecules are designed on the basis of the original molecules, named after TPA–DPP-1, TPA–DPP-2, TPA(DPP-P)₂-1, TPA(DPP-P)₂-2, respectively. The effect of the number and the site of functional groups on the photoelectric properties of the designed molecules were studied. Absorption spectra, electron injection free energies, light harvesting efficiencies and the dye regeneration free energies were also calculated. The data shows that two –CNs were introduced into the molecules, energy gaps decrease and absorption peak makes red-shift. The introduction of a –CN is favorable to improve the electron injection free energies and dye regeneration free energies. The introduction of a side chain makes LHE increase; however, make absorption peak blue-shift. Through evaluating the absorption ability and charge transfer process for designed molecules, it is found that the photoelectric properties of the molecules can be regulated and improved by introducing appropriate acceptor group, which can provide valuable information for further experiment.