## 3 Results and Discussion
Transient \(\Delta T/T\) traces of (**c**) pure FAPbI\({}_{3}\) PQDs film (400 nm pump 14 \(\upmu\)J/cm\({}^{2}\)) and (**d**) PTAA HTL/FAPbI\({}_{3}\) PQDs/TiO\({}_{2}\) ETL (400 nm pump 21 \(\upmu\)J/cm\({}^{2}\)) probed at different wavelengths. We then turn to the time dependence of the PA signal at 570 nm for PTAA/FAPbI\({}_{3}\)/TiO\({}_{2}\) shown in Figure 3a. The PA in PTAA/FAPbI\({}_{3}\)/TiO\({}_{2}\) displays a much faster decaying process compared to that of the pure FAPbI\({}_{3}\) PQDs. The fitting using the above formula reveals \(\tau_{1}\approx 20\) ps, and this fast decay component contributes more than 53% of the total PA amplitude. In contrast, the other exponential decay component becomes much slower at the time scale of \(\tau_{2}\approx 2600\) ps, compared to that in the pure FAPbI\({}_{3}\) PQDs. The emergence of fast decay component gives important evidence that free charge carriers in the PQDs are quickly transferred to the charge transport layer. To distinguish which charge carriers, electrons or holes, are effectively transferred within 20 ps, we may turn to the time dependences of PB signals, which are mainly contributed from the holes. The time dependences of the PB signals in Figure 3b clearly show a much slower decay process in PTAA/FAPbI\({}_{3}\)/TiO\({}_{2}\) than in the pure FAPbI\({}_{3}\). The fitting reveals the absence of fast component but only one slow component with \(\tau_{2}\approx 2800\) ps. These results strongly support that electrons are quickly transferred to the TiO\({}_{2}\) layer. Such transfer reduces the conduction electron absorption in the PQDs, and consequently attenuates the PA signals at 570 nm within \(\tau_{1}\approx 20\) ps. The quick electron transfer leaves the holes in the valence band of the PQDs, thus markedly slowing down the PB decaying process. Actually, we also note a small rising signal in the PB of 770 nm at a time scale of 20 ps, as can be better seen in Figure 4b with expanded time scale. This is likely due to the diminishment of conduction electron absorption at this probe wavelength in PQDs as a result of the fast electron transfer to TiO\({}_{2}\). Since the PB signals and the slow decay component of the PA signals have an approximately equal decaying time scale (\(\sim\)2600-2800 ps), we believe that in addition to the origin of conduction electron absorption, the PA signals at 570 nm also have a significant contribution from the BGR caused by the holes. In other words, the slow decay component of the PA at 570 nm in PTAA/FAPbI\({}_{3}\)/TiO\({}_{2}\) is mainly caused by \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Signal** & **Samples** & \(\frac{A_{1}}{A_{1}+A_{2}}\) **(\%)** & \(\tau_{1}\) **(ps)** & \(\frac{A_{2}}{A_{1}+A_{2}}\) **(\%)** & \(\tau_{2}\) **(ps)** \\ \hline \multirow{4}{*}{PA} & FAPbI\({}_{3}\) & 32.89 & 121.65 & 67.11 & 1563.98 \\  & PTAA/FAPbI\({}_{3}\) & 32.31 & 118.53 & 67.69 & 1553.29 \\  & FAPbI\({}_{3}\)/TiO\({}_{2}\) & 53.71 & 18.93 & 46.29 & 2602.56 \\  & PTAA/FAPbI\({}_{3}\)/TiO\({}_{2}\) & 53.68 & 19.97 & 46.32 & 2567.01 \\  & FAPbI\({}_{3}\) & 12.63 & 117.17 & 87.37 & 1656.98 \\  & PTAA/FAPbI\({}_{3}\) & 14.01 & 117.21 & 85.99 & 1667.77 \\  & FAPbI\({}_{3}\)/TiO\({}_{2}\) & — & — & 100 & 2805.96 \\  & PTAA/FAPbI\({}_{3}\)/TiO\({}_{2}\) & — & — & 100 & 2873.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters used to fit the temporal evolution of the PA and PB decay signals of the samples excited at 400 nm with pump fluence of \(\approx\)3.5 \(\mu\)J/cm\({}^{2}\). Figure 3: Normalized kinetic traces of (**a**) PA probed at 570 nm and (**b**) PB probed at 770 nm for the films in vacuum after excitation at 400 nm with pump fluence of \(\approx\)3.5 \(\mu\)J/cm\({}^{2}\). The solid curves represent the fitting results. the holes residing in the PQDs. This implies that the electrons have transfer efficiency much larger than 53% at the time scale of 20 ps.