## 4 Conclusions  
In conclusion, based on the comparative studies of the photoexcited charge carrier dynamics in FAPbI3, PTAA/FAPbI3, FAPbI3/TiO2, and PTAA/FAPbI3/TiO2 planar junctions by using ultrafast TAs, we unveiled evidences of quick electron transfer process from FAPbI3 PQDs to TiO2 ETL at the time scale of 20 ps and the transfer efficiency is estimated to be close to 100%. In contrast, no significant hole transfer to the PTAA HTL were observed within \(\sim\)1 ns, which is likely due to the fact that the interface coupling is not strong enough to drive quick hole transport through PQDs and fast hole injection into PTAA. Our results suggest that improving the ultrafast hole transfer at PQDs/HTL interface is essential for the high-performance of FAPbI3 PQDs solar cells.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/16/5553/s1](http://www.mdpi.com/2076-3417/10/16/5553/s1)]([http://www.mdpi.com/2076-3417/10/16/5553/s1](http://www.mdpi.com/2076-3417/10/16/5553/s1)), Figure S1: Temporal evolution of \(\Delta T/T\) for pure FAPbI3 PQDs film with the probe wavelength of 770 nm under different pump fluences. Solid curves represent the fitting results using the formula of \(\Delta T/T=A_{0}\mathrm{e}^{-t/\tau_{0}}+A_{1}\mathrm{e}^{-t/\tau_{1}}+A_{2} \mathrm{e}^{-t/\tau_{2}}\). The fitting parameters are summarized in Table S1; Figure S2: Temporal evolution of \(\Delta T/T\) for pure FAPbI3 PQDs film with the probe wavelength of 570 nm under different pump fluences. Solid curves represent the fitting results using the formula of \(\Delta T/T=A_{0}\mathrm{e}^{-t/\tau_{0}}+A_{1}\mathrm{e}^{-t/\tau_{1}}+A_{2} \mathrm{e}^{-t/\tau_{2}}\). Relative to the case of pump fluence of \(\approx\)14 ul/cm\({}^{2}\), the quick biexciton recombination is absent under the low fluences; Table S1: Parameters used to fit the PB decay processes of the pure FAPbI3 PQDs film probed at 770 nm and excited at 400 nm with different pump fluences, shown in Figure S1.  
Conceptualization, Z.L., J.Y. ([PERSON]), C.S., W.M. and H.Z. ([PERSON]); Formal analysis, Z.L., S.L., J.Y. ([PERSON]), K.J., H.Z. ([PERSON]), H.X. and G.N.; Investigation, Z.L., Methodology, Z.L., J.Y.([PERSON]), C.S., W.M. and H.Z. ([PERSON]); Project administration, G.N., L.C. and H.Z. ([PERSON]); Resources, J.Y. ([PERSON]), K.J., J.Y. ([PERSON]) and W.M.; Supervision, G.N., L.C. and H.Z. ([PERSON]); Validation, Z.L.; Writing--original draft, Z.L. and H.Z.([PERSON]); Writing--review & editing, Z.L., J.Y. ([PERSON]), W.M. and H.Z.([PERSON]) All authors have read and agreed to the published version of the manuscript.  
This work was supported by National Key Research and Development Program of China (2016 YFA0300703, 2016 YFA0202402), National Natural Science Foundation of China (11774064, 51971064, 61674111), the China Postdoctoral Science Foundation (Grant No. 2019M651942) and the \"111\" Project. The authors also acknowledge the financial support from Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).  
The authors declare no conflict of interest.