Titanium Dioxide Band-Gap and Interfacial Energy-Level Engineering in Dye-Sensitized Solar Cells – A Narrative Review

Abstract

The wide band gap of TiO₂, its chemical stability, high specific surface area of mesoporous films and favourable electron-accepting states enable efficient dye-sensitization, made TiO₂ preferred photoanode semiconductor in dye-sensitized solar cells (DSSCs). But, recent studies demonstrate that optical band gap alone is not sufficient to explain the role of TiO₂ in photovoltaic (PV) application. All the above methods can influence the position of the conduction band, electron density, trap distribution, dye adsorption, electron transport and recombination at the same time. This review, thus, considers band-gap engineering as one of many aspects of interfacial energy-level engineering. The current literatures are coupled with existing mechanism studies to examine the impact of recent changes on injection, transport, recombination and device output. The special cases of matched control, dye loading, electrolyte effects and difference between optical-gap shifts and actual changes in operating energy alignment are discussed in particular detail. In recent years, the use of In-, Ag-, Mg-, Al- and rare-earth-doped TiO₂ has been reported, as well as TiO₂ that is also doped with Ti³⁺, one-dimensional and hierarchical architectures, and TiO₂/rGO, TiO₂/MoS₂-rGO and TiO₂/g-C₃N₄ composites. Single-layer/bilayer TiO₂ has also been reported. This review suggests that the best design strategy is not to maximize band-gap narrowing or conductivity, but to design an optimized interface between TiO₂, dye and electrolyte that ensures dye loading, an adequate driving force for electron injection, transport and low back-electron transfer, and stability during operation.