# Molecular Controlling the Transport Properties for Benzothiadiazole-Based Hole Transport Materials
## 3 Results and Discussion
### Ground-State Geometries and Frontier Molecular Orbitals
The energy diagram of the perovskite, TiO\({}_{2}\) and six investigated HTM molecules are presented in Figure 3. As shown, the HOMO levels of six molecules are quite close, and they are all higher than the HOMO level of perovskite. The LUMO levels of the designed molecules are obviously raised, compared with the experimental molecules, and the LUMO levels of the six molecules are all higher than that of perovskite. It is noted that the LUMO level of perovskite is slightly higher than LUMO level of TiO\({}_{2}\), which can inject the electron from perovskite to TiO\({}_{2}\) successfully. If the HOMO level of HTM is higher than the valence band of perovskite, the hole injection from perovskite to the HTM is advantageous [35]. Meanwhile, if the LUMO level is higher than the conduction band of perovskite, the electron backflow from perovskite to the metal electrodes will be inhibited efficaciously [35; 36]. From the Figure 3, it was found that the HOMO levels of six studied molecules are all beneficial for hole injection from perovskite to them, and the LUMO levels of them can prohibit the electron-backflow effectively. Moreover, the LUMO levels of designed molecules Jy8-Jy10 are higher than those of experimental molecules Jy5-Jy7, which can allow the designed molecules to exhibit better blocking effects on electronic reflux. Six studied molecules have similar hole injection abilities, and for the abilities of preventing electron backflow, we can find the following sequence: Jy8 > Jy9 \(\approx\) Jy10 > Jy5 > Jy6 > Jy7. The results show that the replacement of core structures for Jy5-Jy7 from hydrogen atom to fluorine atom has little influence on hole injection and electron backflow. Moreover, it is obvious that the substitution of the central structures from the electron-withdrawing group to the electron-donating group can significantly increase the ability of preventing electron-backflow of HTM molecules. The FMO plots of molecules were shown in Figure 4. For six molecules, the electronic density of HOMO is mostly concentrated in #A groups of each molecule, and the electronic density of LUMO is mostly concentrated in #B fragments of each molecule. The characteristics of electronic density can explain the results of close energy levels, that is to say, from the electronic density distribution of HOMO (see Figure 4) it was found that the electron densities are located in the two sides of molecular backbone (there are no electron density in core region), meaning no contribution of central cores (BT, FBT and TFBT) region to the HOMO; therefore, there are closed energy levels for the three molecules (JY5-JY7). For LUMO, though electron density resides in the central region, contribution comes from BT, FBT and TFBT cores, and fluorine atom has a little influence on the LUMO. Moreover, for HOMO plots of the investigated molecules, for Jy5, Jy8 and Jy10, their electronic density of HOMO almost belongs to symmetrical characteristics in both sides; while for Jy6, Jy7 and Jy9, the HOMO electronic density distributes asymmetrically on two sides of the molecule. From the distribution of FMO, it was found that the replacement of core structure from the electron-withdrawing group to the electron-donating group will not affect the role of the core structure and the two sides of molecules. The FMOs plots confirm that the #A fragment of each molecule acts as an electron donor and that the B fragment acts as an electron acceptor. The TDOS and PDOS of the six investigated molecules have been presented in Figure 5. The black line represents the TDOS of the molecule, and the red line and blue line represents the PDOS of defined fragments #A and #B, respectively. The DOS diagram of the six molecules shows that almost  
Figure 4: Frontier molecular orbital plots of the HOMO and LUMO for the studied molecules in solvent calculating by density functional theory (DFT).