# Molecular Controlling the Transport Properties for Benzothiadiazole-Based Hole Transport Materials
## 3 Results and Discussion
### Ionization Potentials, Electron Affinities and Absolute Harness  
For organic materials, the barrier of charge transfer can be estimated by ionization potentials (IPs) and electron affinities (EAs) [27; 36]. In general, the lower IP means the easier hole transfer, and the higher EA is more favorable for electron transfer [37]. Based on the optimized cation and anion geometry structures of the six molecules, we calculated their IPs, EAs and absolute hardness (\(\eta\)), as listed in the Table 2.  
Figure 5: Density of states (DOS) of the six investigated molecules. Where the black line represents the total density of states (TDOS), the red line red line represents the partial density of states (PDOS) of defined fragment A, and the blue line represents the PDOS of defined fragment B. The calculated IPs are 4.40 eV for Jy5, 4.39 eV for Jy6, 4.41 eV for Jy7, 4.45 eV for Jy8, 4.37 eV for Jy9 and 4.44 eV for Jy10. The six molecules have similar IPs, and among them the Jy9 has the smallest IP, and Jy8 has the biggest IP. The EAs of three experimental molecules are 2.64 eV for Jy5, 2.68 eV for Jy6 and 2.70 eV for Jy7, respectively; for three designed molecules Jy8-Jy10, the EAs of them are 1.66 eV, 1.96 eV and 1.98 eV, respectively. It can be seen that the EAs of the experimental parent molecules Jy5-Jy7 have little differences. Among the three designed molecules, the Jy8 has the smallest EA, and the EAs of Jy9 and Jy10 are adjacent values. Compared with experimental molecules, the three designed molecules have apparent decreased tendency. As mentioned above, the results show that the six studied molecules should have similar hole injection abilities; and the electron injection abilities of six molecules are in the following order: Jy8 \(<\) Jy9 \(\approx\) Jy10 \(<\) Jy5 \(<\) Jy6 \(<\) Jy7, which means that the abilities to prevent electron-backflow is Jy8 \(>\) Jy9 \(\approx\) Jy10 \(>\) Jy5 \(>\) Jy6 \(>\) Jy7. It was found that for both hole injection abilities and the abilities to prevent electronic backflow, the analysis of FMO and the analysis of IP/EA have the same results. Furthermore, the results also show that the substitution of core structure from electron-withdrawing group to electron-donating group has small influences on hole injection, but it will clearly boost the blocking effect on electron backflow. Although the stability of PVSCs is related to many parameters and aspects, the absolute hardness (\(\eta\)) can be used to evaluate the stability of organic molecule, and the absolute hardness is defined by [38]:  
\[\eta=\frac{IP-EA}{2} \tag{1}\]  
The calculated results of \(\eta\) have been listed in the Table 2. For experimental parent molecules Jy5-Jy7, the absolute hardness are 0.88 eV, 0.86 eV and 0.86 eV, respectively. The results show that the absolute hardness of three experimental molecules is very similar. As a result of the significant drops of EAs for designed molecules, compared with those of experimental molecules, the \(\eta\) of Jy8-Jy10 have evidently increased tendency. As shown in Table 2, the absolute hardness of three designed molecules are 1.40 eV, 1.21 eV and 1.23 eV, respectively. The \(\eta\) is the resistance of the chemical potential to change in the number of electrons. So that the more stable organic molecule should possess a larger absolute hardness [39]. The absolute hardness of six molecules shows that the designed molecules Jy8-Jy10 are apparently more stable than original experimental molecules Jy5-Jy7. Among six molecules, the Jy8 has the highest absolute hardness, which means that the Jy8 has the best stabilization in six studied molecules. And the stabilizations of three designed molecules are clearly better than those of experimental molecules. The stability of six molecules are in the following sequence: Jy8 \(>\) Jy9 \(\approx\) Jy10 \(>\) Jy10 \(>\) Jy5 \(>\) Jy6 \(=\) Jy7. The calculated absolute harness shows that replacing hydrogen by fluorine in the core structure has no obvious effect on the stability of molecules.