A computational study of anisotropic charge transport in air-stable fluorinated benzobisbenzothiophene (FBBBT) derivatives

A computational study of anisotropical charge transport properties of fluorinated benzobisbenzothiohphene derivatives (FBBBT) is presented. The values of IP_adia of all FBBBTs are found in the range of 6.00-6.20 eV inferring the fact that the investigated compounds have ambient air-stability. In addition, the energy levels of FBBBT s are found to be lower than those of benzobisbenzothiophene (BBBT) compound indicating higher charge carrier stability in the former. Hirshfield surface analyses showed that, in all the studied compounds, the principal identifiable interaction were mostly due to F⋯H and H⋯H intermolecular couplings with no contribution from S⋯S bondings. The calculated maximum μ_hole(μ_elec) value of the compounds FBBBT-a and FBBBT-b was found to be 0.483 (0.794) cm²V^- 1s^- 1 and 0.688 (0.542) cm²V^- 1s^- 1 respectively in the direction of transistor channel (Φ = 93.39 ^∘(273.30^∘) for FBBBT-a and Φ = 92.24 ^∘/272.72 ^∘ for FBBBT-b). For FBBBT-c, the maximum μ_elec(μ_hole) value of 0.933 (0.233) cm²V^- 1s^- 1 appeared for Φ = 0 ^∘/179.90 ^∘. In addition, the compounds FBBBT-a and FBBBT-b possess two additional fluorine atoms attached at the X positions in the backbone, which result in an increment in μ_elec values (1.4 times and 0.78 times higher than μ_hole) in these two compounds at a particular crystal direction.

How to Optimize the Interface between Photosensitizers and TiO2 Nanocrystals with Molecular Engineering to Enhance Performances of Dye-Sensitized Solar Cells?

In this work, the interfacial properties of a series of metal-free organic naphthodithienothiophene (NDTT)-based photosensitizers adsorbed on TiO2 surfaces were investigated by a combination of ab initio calculations and experimental measurements. The calculations and experiments reveal that because of the efficient charge transfer from the adsorbed dyes to TiO2 nanocrystal surface there is an upward shift for the energy levels of dyes and a downward shift for the conduction band of surface TiO2 and that the band gaps for both of them are also reduced. Such electronic level alignments at the interface would lead to increased light absorption range by adsorbed dyes and increased driving force for charge injection but reduced open-circuit potential (V(oc)). More interestingly, we found that molecule engineering of the donor group and introducing additional electron-withdrawing unit have little effect on the electronic level alignments at the interface (because band gaps of the dyes adsorbed on TiO2 surfaces become approximately identical when compared with those of the dyes measured in solution) but that they can affect the steric effect and the charge separation at the interface to tune V(oc) and the short-circuit current density (J(sc)) effectively. All these findings suggest that optimizing the interfacial properties of dyes adsorbed on TiO2 surfaces by synchronously modifying steric effects of dye molecules anchored on TiO2 and charge-transfer and separation properties at the interfaces is important to construct efficient dye-sensitized solar cells.