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

5 years ago

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

Andreas Klein, Daniel Bellet, Shan-Ting Zhang, Odette Chaix-Pluchery, Hervé Roussel, David Muñoz-Rojas, Michel Langlet

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.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b04893

DOI: 10.1021/acs.jpcc.7b04893