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SEM images of (a) the surface of the naked perovskite material, (b) the surface after coating with Spiro-OMeTAD, (c) cross section of a perovskite solar cell when Spiro-OMeTAD is applied as the hole-transport material, (d) the surface after coating with Y1, (e) the surface after coating with Y2, and (f) the surface after coating with Y3.
