First application of bis(oxalate)borate ionic liquids (ILBOBs) in high-performance dye-sensitized solar cells

Abstract

A series of bis(oxalate)borate ionic liquids (ILBOBs) were developed for the first time as the necessary components of the electrolyte system for dye-sensitized solar cells (DSCs). It was found that the photovoltaic performances of DSCs were significantly improved by 26% to 45% with these ILBOBs in the organic solvent electrolytes. Systematic investigations were carried out to reveal the effects of the chain lengths of the quaternary ammonium BOB as well as the cations of the ILBOBs on the performances of the DSCs. Among them, the highest power conversion efficiency (PCE) was achieved by the DSC based on the electrolyte containing 1-ethyl-3-methylimidazolium bis(oxalate)borate (EmimBOB). Specifically, the addition of EmimBOB increased the short circuit current densities (J_sc) from 11.87 mA cm⁻² to 15.99 mA cm⁻², and the open-circuit voltage (V_oc) from 0.721 V to 0.742 V, accompanied by a final 45% improvement of the overall PCE from 6.01% to 8.73% under AM 1.5, 100 mW cm⁻² illumination. As an ionic liquid, EmimBOB was also used to develop a solvent-free electrolyte system. Compared with the one containing pure 1-methyl-3-propylimidazolium iodide (PMII), the hybrid solvent-free electrolyte system based on the EmimBOB/PMII combination notably increased the PCE of the DSCs from 3.48% to 5.38%. Finally, using solvent-free electrolytes, the long-term stability of the DSC devices were remarkably improved.