## 3 Results and Discussion
Moreover, in PTAA/FAPbI3/TiO2, we did not observe the decay component at \(\sim\)1600 ps caused by the bimolecular recombination inside the PQDs. Hence we believe nearly all electrons have been transferred to TiO2, which means the transfer efficiency is close to 100%. On the contrary, in the time window of our experiments (\(\sim\)1 ns), we have not observed substantive hole transfer from the FAPbI3 PQDs to PTAA. To confirm the above discussions that electrons have quick and efficient transfer from FAPbI3 PQDs to TiO2, whereas the hole transfer from FAPbI3 PQDs to PTAA is not significant within 1 ns, we also have measured the time dependences of PA and PB signals for FAPbI3 PQDs covered with only TiO2, allowing only electron transfer, or just PTAA, allowing only hole transfer. The \(\Delta T/T\) spectra of FAPbI3/TiO2 and PTAA/FAPbI3 allow us to separately investigate electron transfer and hole transfer respectively. The measured results are shown in Figure 4a,b. We can see that the PA decaying processes in FAPbI3/TiO2 and PTAA/FAPbI3/TiO2 resemble each other very closely, and the PA decaying processes in PTAA/FAPbI3 and pure FAPbI3 PQDs are quite similar. Consistently, the variations of PB decaying processes in these samples behave in the same manner as those of PA decaying processes. Detailed fitting parameters shown in Table 1 further confirm such behaviors. These comparisons therefore confirm the above discussions of effective electron transfer at the time scale of 20 ps and insignificant hole transfer within 1 ns. The most reasonable explanations for the different electron and hole transfer processes are described below. The strong electronic interactions between the FAPbI3 and the TiO2 is beneficial to the electron transfer [34], and the coupling between PQDs also favors the electron diffusion. Owing to the suitable energy levels and ideal interface between the FAPbI3 PQDs and TiO2 layers, the electron transfer is sufficiently faster than the bimolecular charge recombination. In contrast, the coupling between the PTAA and FAPbI3 is likely not strong enough to yield quick hole injection, though their HOMO energy levels are matched. One possibility is related to the defective physical contact. Less physical contact, due to the roughness of the interfacial planar heterojunction between the FAPbI3 PQDs and PTAA layers, would increase the need of the hole diffusion before finding a suitable site for transfer [35]. Another possibility is that the coupling between PQDs are not strong enough to support the quick transport of holes to reach the interface. Based on the above CT results and the underlying limiting factors, we therefore propose to make proper interface modification between the FAPbI3 PQDs and PTAA to largely enhance the hole injection rate. In addition, the coupling strength affecting the hole transport between neighboring FAPbI3 PQDs also needs to be further examined and improved. If the  
Figure 4: Experiment data of normalized (**a**) PA kinetic traces probed at 570 nm and (**b**) PB kinetic traces probed at 770 nm. All the measurements were carried out under the pump pulses of 400 nm with fluence of \(\approx\)3.5 \(\upmu\)J/cm\({}^{2}\). The solid curves represent the fitting results. The fitting parameters are summarized in Table 1.  
hole transfer rate and efficiency are greatly enhanced, a large increase of PCE in FAPbI3 PQDs based solar cells can be expected.