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]