Quasi-solid cellulose-based aqueous electrolytes for sustainable DSSCs

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar cells inspired by natural photosynthesis, having low cost and aesthetic features suitable for architectural integration and providing full performance even under low irradiance conditions and/or indoor environments. However, the standard electrolyte system in these devices is based on volatile organic solvents, being also flammable and toxic, which affects long-term stability, eco-sustainability and safety requirements. Recently, green chemistry has contributed significantly to the step forward of DSSC systems into sustainability by developing devices operating in truly aqueous environment. To address the environmental and long-term stability issues, we here demonstrate that DSSCs can be fabricated with bioderived sodium salt of carboxymethylcellulose (CMC) as a green gellifyng agent for a 100% aqueous electrolyte based on iodide/triiodide redox mediator. The aqueous iodide/CMC system displays high ionic conductivity (10-2 S cm-1) even down to −10 °C; hydrogels allow for efficient charge transfer at Pt/electrolyte interface (Rct = 0.14 Ω cm2), as well as a diffusion coefficient of I3 − ions comparable or even higher (5.2×10-6 cm2 s-1) than recent literature reports on ionic liquids and water-based gel electrolytes. In lab-scale solar cells, the best photovoltaic performances (0.72%) were achieved in the presence of 5.5 wt% CMC. The most stable cell demonstrated 93% efficiency retention after one month of dark storage; the CMC-based devices also showed a robust stability when subjected to a further month of thermal aging at 60 °C. This newly proposed electrolyte system, in conjunction with a metal-free sensitizer, easily leads to sustainable, stable, transparent and building integrable solar cells, thus paving the way towards next-generation green solar energy conversion devices based on renewable components