# Molecular Controlling the Transport Properties for Benzothiadiazole-Based Hole Transport Materials
## 4 Conclusions  
In this work, firstly we theoretically investigated the structure and transport properties of HTM containing fluorine-substituted benzothiadiazole-based organic molecules (Jy5-Jy7). Subsequently, by replacing the core structures of Jy5-Jy7 from electron-withdrawing group to electron-donating group, we designed three molecules Jy8-Jy10. Some crucial parameters (FMO, IP/EA, absolute hardness \(\eta\), \(\lambda_{e}/\lambda_{h}\), optical absorption and hole mobility) of HTM have been calculated with DFT/TD-DFT associated with CPCM model in dichloromethane solvent.  
The results demonstrated that the replacement of hydrogen atom in the core structures by fluorine atom for Jy5-Jy7 has no obvious influence on the FMO levels, the abilities of hole injection and preventing electron-backflow, molecular stability and optical absorption peak; however, the substitute will significantly increase the hole mobility rate and hole mobility of the molecule because the introduction of the fluorine atom will obviously boost the charge transfer integral \(V\). Furthermore, the substitution of the core structures from the electron-withdrawing group to the electron-donating group has little influence on the HOMO levels, hole injection and electron transport; but it will significantly increase the LUMO level, energy gaps, preventing ability on electron-backflow, molecular stability and oscillator strength of HTM molecule. Finally, the fine structural tuning of the core structures from the electron-withdrawing group to electron-donation group is hoped to be judicious approach for further improving the performance, which provides a possible option in designing the charge transport materials of perovskite solar cells.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Molecules** & **Jy5** & **Jy6** & **Jy7** \\ \hline \(V_{h}\) (eV) & 0.00016 & 0.00141 & 0.01001 \\ \(\lambda_{h}\) (eV) & 0.14 & 0.15 & 0.15 \\ \(r\) (Å) & 4.65 & 7.94 & 7.19 \\ \(k_{h}\) (s\({}^{-1}\)) & 2.96 \(\times\) 10\({}^{8}\) & 2.01 \(\times\) 10\({}^{10}\) & 1.01 \(\times\) 10\({}^{12}\) \\ \(\mu_{h}\) (cm\({}^{2}\)/(V\(\cdot\)s) & 1.24 \(\times\) 10\({}^{-5}\) & 2.45 \(\times\) 10\({}^{-3}\) & 0.1010 \\ \hline \end{tabular}
\end{table}
Table 6: The key parameters related to hole mobility of the three experimental molecules Jy5-Jy7: \(V_{h}\) (eV), \(\lambda_{h}\)(eV), \(r\) (Å), \(k_{h}\) (s\({}^{-1}\)) and \(\mu_{h}\) (cm\({}^{2}\)/(V\(\cdot\)s)).  
**Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/8/9/1461/s1](http://www.mdpi.com/2076-3417/8/9/1461/s1)]([http://www.mdpi.com/2076-3417/8/9/1461/s1](http://www.mdpi.com/2076-3417/8/9/1461/s1)), Figure S1: Working principle of PeSCs, where the perovskite layer sandwiched by an electron transfer materials (ETM) and a hole transfer materials (HTM) as a whole is placed between a transparent electrode (FTO) and a metal electrode (Ag).  
**Author Contributions:** [PERSON], [PERSON] and [PERSON] conceived and designed this work. [PERSON] developed the theoretical model and simulation program and wrote the paper. [PERSON] and [PERSON] analyzed the data.  
**Funding:** This work was supported by the Heilongjiang Postdoctoral Grant (LBH-Z15002), China Postdoctoral Science Foundation (2016 M590270) and the National Natural Science Foundation of China (Grant Nos. 11404055, 61675165 and 11304135). Xinlan Cao thanks the college students' innovation project of NEFU (20170900001) and college student research training Program of NEFU (KY2017001) for this support.  
**Conflicts of Interest:** The authors declare no conflict of interest.