# Molecular Controlling the Transport Properties for Benzothiadiazole-Based Hole Transport Materials
## 3 Results and Discussion  
### Ground-State Geometries and Frontier Molecular Orbitals  
The chemical structures of three experimental molecules Jy5-Jy7 and three designed molecules Jy8-Jy10 have been presented in Figure 1. As mentioned above, the differences of six studied molecules are their core structures (represented by red groups in Figure 1). In order to clearly explain the functions of each group, we defined the core structure of each molecule as B, and defined the structure of the two sides of each molecule as A. The definition of the fragment is shown in Figure 2. By employing DFT/B3 LYP/6-31G(d) with C-PCM model in dichloromethane solvent, the frontier molecular orbitals (FMOs) energy levels have been calculated, and the results have been listed in Table 1 and Figure 3.  
Figure 2: The fragment’s definitions of six investigated molecules. As shown in Table 1, the HOMO level of three experimental molecules Jy5, Jy6 and Jy7 are \(-\)4.49 eV, \(-\)4.49 eV and \(-\)4.50 eV, respectively; the LUMO level of them are \(-\)2.55 eV, \(-\)2.59 eV and \(-\)2.62 eV, respectively. The energy gap of HTM can be estimated by \(\Delta_{H-L}=E_{LUMO}-E_{HOMO}\); therefore, the \(\Delta_{H-L}\) of Jy5-Jy7 are calculated to be 1.94 eV, 1.90 eV and 1.88 eV, respectively. The computational results show that both HOMO levels and LUMO levels of Jy5\(-\)Jy7 are very close, which leads to small differences in the energy gaps between the three molecules. This result shows that the introduction of fluorine in the core structures of Jy6 and Jy7 have little influence on FMOs levels and energy gaps of them. For the designed molecule Jy8, Jy9 and Jy10, the HOMO levels are \(-\)4.53 eV, \(-\)4.53 eV and \(-\)4.52 eV, respectively; the LUMO levels are \(-\)1.46 eV, \(-\)1.71 eV and \(-\)1.70 eV, respectively; and the \(\Delta_{H-L}\) of them are 3.07 eV, 2.82 eV and 2.82 eV, respectively. It can be seen that the HOMO levels of the three designed molecules are very close. Similarly, there are small differences between the HOMO levels of designed molecules and HOMO levels of experimental parent molecules. The molecular orbital level features can be explained from electron density distribution (Frontier molecular orbital plots) and contribution of molecular orbital (partial density of states (PDOS)) as mentioned below. For the three designed molecules, the LUMO level of Jy8 is the highest, and the LUMO levels of Jy9 and Jy10 are the same. Compared with three experimental molecules, the LUMO levels of three designed molecules have an obviously increase. This means that the substitution of the core structure from the electron-withdrawing group to the electron donating group has little effect on the HOMO level of the studied molecules, but it can significantly increase the LUMO level of the molecule. Compared with experimental molecules, the significant increases of LUMO levels of the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Molecules** & **HOMO (eV)** & **LUMO (eV)** & \(\Delta_{H-L}\) **(eV)** \\ \hline Jy5 & \(-\)4.49 & \(-\)2.55 & 1.94 \\ Jy6 & \(-\)4.49 & \(-\)2.59 & 1.90 \\ Jy7 & \(-\)4.50 & \(-\)2.62 & 1.88 \\ Jy8 & \(-\)4.53 & \(-\)1.46 & 3.07 \\ Jy9 & \(-\)4.53 & \(-\)1.71 & 2.82 \\ Jy10 & \(-\)4.52 & \(-\)1.70 & 2.82 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Calculated energy levels of frontier molecular orbital (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)), and the energy gaps \(\Delta_{H-L}\) of the six studied molecules. Figure 3: Energy level diagram of the perovskite, TiO\({}_{2}\) and the investigated hole transporting materials. three designed molecules lead to notable rises of energy gaps. Taking Jy5 as an example, the \(\Delta_{H-L}\) of Jy8, Jy9 and Jy10 are raised by 1.13 eV, 0.88 eV and 0.88 eV, respectively, when compared with Jy5. Matching energies are very important for the occurrence of charge transfer between the different surfaces in accord with enough driving force, as shown in Figure S1[35].