Figure 5. FE-SEM images of a) Si master, b) tilted view of PFPE mold, c) top view of nanopatterned TiO2 layer, d) side view of nanopatterned TiO2 layer, e) perovskite layer, and f) PCBM-perovskite layer, g) cross-sectional images of PCBM-PSCs with nanopatterned TiO2 layer, and h) FIB image of PCBM-PSCs with nanopatterned TiO2 layer.
Figure 5 a shows the field emission scanning electron microscopy (FE-SEM) image of the nanopatterned Si master with 250 nm pore sizes and 250 nm interpore distances. For duplication of nanopatterning on the Si master, the PFPE resin was coated on the surface of Si master and polymerized by UV irradiation for hardening, as shown in Figure 5b. A polyethylene terephthalate (PET) film was used as a flexible substrate of PFPE resin. Figure 5c visualized the form of the nanopatterned TiO2 with a 280 nm pore size, which look like the moth-eye nanostructure. The depth of the nanopatterned TiO2 layer is 127 nm, as shown in Figure 5d, which is the most optimized nanopatterning depth.[14]Figure 5e and 5f show the grain boundaries of perovskite and PCBM-perovskite layers. Without PCBM, the perovskite layer has lots of grain boundaries with smaller grain sizes. However, with the incorporation of 0.10 wt% of PCBM, larger grain sizes were formed as PCBM filled the pinholes between the small grain sizes. As a result, the grain of perovskite with PCBM has larger sizes and higher density, that indicates better surface morphology of PCBM-perovskite layer.[19] The cross-sectional image of PCBM-perovskite layer on the PCBM-PSCs with nanopatterned TiO2 layer can be observed from Figure 5g. From the FE-SEM images, the PCBM cannot be confirmed because the resolution of FE-SEM was out of range. Thus, Fourier transform infrared (FTIR) analysis was used to verify the PCBM, which will be discussed in a later paragraph. The thicknesses of nanopatterned TiO2 layer, perovskite, Spiro-OMeTAD, and Au are 131 nm, 400 nm, 250 nm, and 290 nm, respectively, as shown in Figure 5h.