Moving Beyond Choline: Protic Choline Iodide Analogues toward Cosolvent-Free, Self-Contained Deep Eutectic Electrolytes for Dye-Sensitized Solar Cells

Deep eutectic solvents (DESs) remain a hopeful prospect as electrolyte substitutes for traditional organic solvents and ionic liquids in dye-sensitized solar cells. The use of self-contained DES electrolytes that incorporate the redox couple offers simplified systems, aiding in a clearer comprehension of the DES electrolyte behavior. Nevertheless, the majority of DES electrolytes, predominantly choline-based, suffer from excessively high viscosity, resulting in conversion efficiencies that barely reach a fraction of a percent. In this study, we synthesized protic choline iodide analogues, represented as [NH_nMe_3–nEtOH]⁺I^–, and combined them with ethylene glycol (EG) in different molar ratios to develop self-contained DES electrolytes. The performance of the devices was significantly enhanced as the value of n decreased from 3 to 1. Particularly, the dimethyl system (n = 1) exhibited a significantly higher photocurrent (J_SC) and open circuit voltage (V_OC) in comparison to its ethanolammonium homologues, indicating its superior potential for application in solar cell technology. All three protic analogues outperformed their choline iodide counterpart, [NMe₃EtOH]⁺I^–:EG, which required 25 wt % water or more to achieve fluidity at ambient temperature, resulting in a reduced photovoltaic response. The anhydrous 1:2 [NHMe₂EtOH]⁺I^–:EG system exhibited a notable power conversion efficiency of 3.40%. This superior performance was attributed to the excellent fluidity of this eutectic system combined with a favorable interaction with the TiO₂ surface. In contrast, the best-performing choline iodide system achieved only half of this efficiency, measuring at 1.84%. After assembly, a solar cell incorporating the 1:2 [NHMe₂EtOH]⁺I^–:EG electrolyte demonstrated remarkable stability over a period of 1 month, retaining 97% of the conversion efficiency observed in a newly assembled cell. These findings suggest the potential for developing enhanced cosolvent-free DES electrolytes through additional structural adjustments to choline or by departure from that motif entirely.