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 [NHnMe3–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 (JSC) and open circuit voltage (VOC) 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.