Highly Efficient Dye-Sensitized Solar Cells Based on Electrolyte Solutions Containing Choline Chloride/Ethylene Glycol Deep Eutectic Solvent: Electrolyte Optimization

Combined experimental and computational analyses were carried out with the aim of developing a cost-effective and sustainable electrolyte solution based on a deep eutectic solvent (DES) for dye-sensitized solar cells (DSSCs). A mixture of choline chloride (CholCl, 1 mol) and ethylene glycol (EG, 2 mol) as a DES was used in combination with MeCN, lithium iodide, 1-ethyl-3-methylimidazolium iodide (Emim), and iodine as DSSC electrolytes on TiO2 and Pt electrodes. It is noteworthy that in this work the effects of various conditions, namely, the amount of MeCN and the molar concentration of I2, LiI, and EmimI and the 4-tert-butylpyridine (TBP) additive on the photovoltaic performance of DSSCs were evaluated. Optimization of the DSSC performance using photocurrent density–voltage plots allowed reaching an efficiency of 9.26% with an electrolyte solution comprising LiI (1.1 M), EmimI (0.5 M), and I2 (0.04 M), and 60:40 v/v of CholCl/EG DES and MeCN. Addition of TBP to the electrolyte allowed reaching even 9.48%, both are unprecedentedly high values. Density functional theory geometry optimizations and molecular dynamics simulation calculations generated an insightful set of information about the intensity of the interactions between electrolyte components with each other in solution and with the TiO2 or Pt solid surfaces. From mean square displacement curves, diffusion coefficients (×10–11 m2/s) were found to increase along the series Li+ (0.82) < Emim+ (1.01) < Chol+ (1.15) < Cl– (1.29) < I– (1.74) < EG (5.99) < MeCN (102.92) in line with the experimental data. Li+ solvation vs Li+ adsorption on the TiO2 or Pt surface is the main interaction governing the efficiency with a high rate of localization, improving the performance. Our results provide a deep understanding of the molecular interactions at the interfaces of the DES and might pave the way for the fabrication and design of optimized DES-based electrolytes for photovoltaic processes.