Figure 10. a) impedance of PSCs. The black, green, blue, and red lines represent the pristine PSC, PSC with nanopatterned TiO2 layer, PCBM-PSC, and PCBM-PSC with nanopatterned TiO2 layer. b) hysteresis graphs of PCBM-PSC with nanopatterned TiO2 layer for forward and reverse scanning directions.
Next, to further understand the charge transport process in the pristine PSC, PSC with nanopatterned TiO2 layer, PCBM-PSC, and PCBM-PSC with nanopatterned TiO2 layer, electrochemical impedance spectroscopy (EIS) characterization was carried out and the results are shown in Figure 10 a and tabulated in Table 3 . The series resistance (Rs) can be estimated from the high-frequency intercept on the real axis while the recombination resistance (Rrec) can be assigned to the recombination at the perovskite/TiO2interface.[31] By analyzing the Nyquist plots, it can be observed that the pristine PSC exhibits the largest semicircle, followed by the PSC with nanopatterned TiO2 layer, PCBM-PSC, and PCBM-PSC with nanopatterned TiO2 layer. The largest semicircle indicates low electron-hole density move across the perovskite interface[32], indicating that more charge would take part in the interfacial recombination process.[33] Therefore, as the PCBM-PSC with nanopatterned TiO2 layer has the smallest semicircle, the results further validate that the nanopatterning and PCBM were able to suppress the recombination and increase the electron transport property, owing to the increment in the electron density and the improved surface morphology of PCBM-perovskite layer.
Aside from that, for reverse scanning from positive to negative voltage, the efficiency was 17.336%. When the direction was changed to forward scanning, the efficiency reduced to 16.256%, which is only a 6.23% decrement, indicating that the PCBM-PSC with nanopatterned TiO2 layer has only a small and negligible hysteresis as represented in Figure 10b and Table 4 . These results further showed that PCBM plays a significant role in the suppression of hysteresis. Previous studies have proposed that the origin of hysteresis in PSCs is due to the ion migration and vacancies on perovskite layer. Therefore, through the diffusion of PCBM into the grain boundaries in perovskite, the hysteresis can be suppressed through the formation of physical hindrance for the ion migration or occupying some vacancy space in the perovskite. [34]