pH-Responsive Molecular and Interfacial Engineering of Dye-Sensitized Solar Cells: From Protonation Control to Device Stability

Abstract

Proton activity is often treated as a secondary experimental variable in dye-sensitized solar cells (DSSCs), although it can influence sensitizer protonation, electronic structure, TiO₂ surface chemistry, dye adsorption, interfacial charge transfer, electrolyte behavior, and device stability. This topical review examines the molecular, interfacial, electrochemical, and device-level mechanisms through which proton activity affects DSSC operation, with particular emphasis on the coupling between dye protonation and TiO₂ surface processes. Evidence from primary studies is critically examined to establish how protonation-dependent changes in absorption, redox properties, anchoring geometry, electronic coupling, charge injection, recombination, and photovoltaic parameters contribute to observed device responses. The review further evaluates natural and synthetic pH-responsive sensitizers, anchoring-group chemistry, TiO₂ surface treatment and passivation, electrolyte and solvent effects, and emerging adaptive concepts involving photoacid generators, buffering layers, and dynamic anchors. Rather than proposing a universal optimum pH, the review develops an integrated framework in which sensitizer acidity, semiconductor surface protonation, interfacial coupling, and electrolyte composition are treated as coupled design variables. Particular attention is given to distinguishing reversible proton-responsive switching from irreversible processes such as photobleaching, dye desorption, aggregation, and chemical degradation. An evidence-to-device characterization strategy integrating molecular spectroscopy, surface analysis, electrochemical measurements, photovoltaic testing, and stability assessment is proposed. The review concludes that systematic control of coupled protonation equilibria, supported by independent experimental controls and multiscale modelling, provides a stronger basis for developing DSSCs with reproducible charge-transfer responses and improved operational stability under chemically dynamic conditions.