Quasi-Solid Electrolytes Comprising Cobalt Complexes for Efficient and Durable Dye-Sensitized Solar Cells

Abstract

There has been much attention towards dye-sensitized solar cells (DSSCs) for the past decades due to their attractive features such as reasonable energy conversion efficiency and low energy production cost. Even though a respectable energy conversion efficiency (~12%) has been achieved for photovoltaic cells employing organic liquid electrolyte that consists of I-/I3 - redox couples and volatile solvent (e.g. acetonitrile), the leakage and evaporation of liquid solvent from such cells has been suggested as the one of the critical factors limiting the long-term stability of the DSSC. Moreover, the use of I-/I3 - redox couples limits the energy conversion and even causes some problems such as the metal corrosion and visible light absorption etc. Recently, cobalt complexes (e.g. [Co(bpy)3]2+/3+, bpy=2,2’-bipyridine) have been suggested as one of the most promising candidates for replacing conventional I-/I3 - redox shuttles owing to their high voltage characteristics. Although a high energy efficiency exceeding 12% was reported by employing liquid electrolyte that consists of cobalt complex redox mediator and volatile solvent, the use of volatile liquid solvent cannot ensure the long-term stability of the DSSCs. In this work, therefore, novel quasi-solid electrolytes (QSEs) including cobalt complexes as redox couples have been developed for efficient and long-term stable DSSCs. The physicochemical properties of QSEs were adjusted by selecting proper solvent and functional additives (for improving cobalt complex solubility and cation transference number etc.). Polyethers with polar groups and polymers without polar groups were used as the polymer additives for the gelation of the liquid electrolytes with various blending ratios. The DSSC employing the QSEs showed the respectable energy conversion efficiency comparable with those of the DSSCs with liquid electrolytes. After the thermal aging tests, it was also proven that the long-term stability of the DSSC employing the QSE is much better than that of the cell filled with volatile liquid electrolyte. (This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (2015R1A1A1A05001486).)