Sustainable Electrolyte Media in Dye-Sensitized Solar Cells: From Water-Based to Deep Eutectic Solvents and Biopolymeric Approaches

Dye-sensitized solar cells (DSSCs) represent a promising photovoltaic technology for indoor and building-integrated applications due to their colour tunability, semi-transparency, and favourable spectral response. However, the sustainability of conventional devices is hindered by the use of volatile organic solvent-based electrolytes, which raise concerns regarding toxicity, flammability, and long-term stability. This review analyses the evolution of DSSC architecture, with particular focus on electrolyte media, ranging from aqueous systems to deep eutectic solvents and bio-derived quasi-solid architectures. Special attention is focused on the interplay between electrolyte composition, dye design, and interfacial charge-transfer processes. By highlighting recent progress and remaining challenges, this work outlines viable strategies toward safe, durable, and fully sustainable DSSCs tailored for indoor and integrated photovoltaic applications.