Polymorphism of the Blocking TiO2 Layer Deposited on F:SnO2 and Its Influence on the Interfacial Energetic Alignment

摘要

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 ultrathin (<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.