## 3 Results
### Charge Carrier Dynamic Mechanism of NiO and the PTAA/NiO-Based Device  
The enhanced properties of PSC devices with the bilayer of HTL can be further validated through optical and electrical analyses such as TPV, TPC, trap density (\(N_{\text{trap}}\)), and charge mobility measurements, as shown in Figure 7. As shown in Figure 7a,b, the charge carrier transport, lifetime and extraction ability of the device were obtained by TPV and TPC. As can be seen in Figure 7a, the decay times of the photovoltage (0.6 ms) of the device have a lower decay time than the PTAA/NiO bilayer (1.1 ms) device. The better electronic quality and lower defect concentration in the PTAA/NiO-based device are consistent with the higher Voc of that device [46]. Meanwhile, the TPC decay times  
Figure 6: Contact angle images of water droplets on the surface of (**a**) NiO film, (**b**) PTAA/NiO film. Top-view SEM images of (**c**) perovskite/NiO film, (**d**) perovskite/ PTAA/NiO film.  
were 0.37 \(\upmu\)s and 0.48 \(\upmu\)s in NiO and PTAA/NiO bilayer PSCs, respectively, thus indicating better charge transport in the PTAA/NiO bilayer devices, as shown in Figure 7b. To evaluate the two types of HTL in detail, the \(N_{\text{trap}}\) values of NiO and PTAA/NiO based hole-only devices were measured through space-charge-limited current measurement in the following ways. Dark J-V curves for hole-only devices with Au/PTAA/Perovskite /NiO/ITO and Au/PTAA/Perovskite/PTAA/NiO/ITO are shown in Figure 7c. The \(N_{\text{trap}}\) can be expressed using the following equation [47]:  
\[N_{\text{trap}}=\frac{2\epsilon\epsilon_{0}V_{\text{TFL}}}{eL^{2}} \tag{1}\]  
and can be estimated as the trap-filled limiting voltage (\(V_{\text{TFL}}\)), which is the starting voltage between the ohmic region and the trap-filled region. The \(V_{\text{TFL}}\) values obtained from the J-V curves of the different perovskite devices for NiO and PTAA/NiO were 0.86 V and 0.54 V, respectively. In addition, the \(N_{\text{trap}}\) values of the perovskite film containing NiO and PTAA/NiO obtained through this were 1.34 \(\times\) 10\({}^{16}\) cm\({}^{-3}\) and 8.48 \(\times\) 10\({}^{15}\) cm\({}^{-3}\), respectively. The reduced \(V_{\text{TFL}}\) and \(N_{\text{trap}}\) values in the bilayer perovskite show that the PTAA interlayer has the effect of reducing \(N_{\text{trap}}\) and recombination sites as well as improving overall device performance.  
In addition, a more-detailed investigation of the charge mobility and recombination process of working PSCs was performed using photo-generated charge extraction by linearly increasing voltage (photo-CELIV) method, as shown in Figure 7d. The charge carrier mobility of PSCs with and without a PTAA layer in the HTL were calculated using the equations of current density at maximum peak (\(\Delta\)j) and the time at maximum current density of 3.7 \(\times\) 10\({}^{-3}\) and 1.8 \(\times\) 10\({}^{-3}\) cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\), respectively. In addition, the inset image in Figure 7d shows the extractable charge values, suggesting that the PTAA/NiO bilayer device has a larger extractable charge. This shows that it is possible to inhibit the recombination of charges.