"As widely employed in dye-sensitized, perovskite, and quantum-dot solar cells, the interface between F-doped SnO2 (FTO) and blocking TiO2 (b-TiO2) is essential in understanding the working principles of these types of solar cells. In this work, we have deposited b-TiO2 layers using a simple sol-gel method. While the b-TiO2 layers deposited on Si (100) wafers form pure anatase polymorph, we have found that the rutile structure of the FTO substrates consistently induces the b-TiO2 layers to crystallize into mixed anatase and rutile polymorphs - the same is observed on rutile RuO2 substrates. This indicates that the rutile structural similarity favors the formation of rutile polymorph in b-TiO2 layers; due to the coexistence of both anatase and rutile polymorphs, the interface of FTO/b-TiO2 is essentially inhomogeneous. We also show that the amount of rutile polymorph present in the b-TiO2 layer is a function of layer thickness, with rutile polymorph dominating in thin b-TiO2 layers. As a result, the energetic alignment at the FTO/b-TiO2 interface in general still favors the charge transport. This is confirmed by directly probing an ultra-thin (<10 nm) b-TiO2 layer using X-ray photoelectron spectroscopy (XPS). We emphasize that the rutile structure of FTO substrate plays a significant role in determining the polymorph of successively deposited b-TiO2 layer, which in turn affects the energetic alignment with FTO electrodes and mesoporous nanocrystalline TiO2, and ultimately the performance of solar devices."
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