Liquid-Crystalline Dye-Sensitized Solar Cells: Design of Two-Dimensional Molecular Assemblies for Efficient Ion Transport and Thermal Stability

Nanostructured liquid-crystalline (LC) electrolytes have been developed for efficient and stable quasi-solid-state dye-sensitized solar cells (DSSCs). Two types of ionic LC assemblies for electrolytes have been designed: (i) noncovalent assemblies of two-component mixtures consisting of I-2-doped imidazolium ionic liquids and carbonate-terminated mesogenic compounds (noncovalent type) and (ii) single-component mesogenic compounds covalently bonding an imidazolium moiety doped with I-2 (covalent type). These mesogenic compounds are designed with flexible oligooxyethylene spacers connecting the mesogenic and the polar moieties. The oligooxyethylene-based material design inhibits crystallization and leads to enhanced ion transport as compared to alkyl-linked analogues due to the higher flexibility of the oligooxyethylene spacer. The noncovalent type mixtures exhibit a more than 10 times higher I-3(-) diffusion coefficient compared to the covalent type assemblies. DSSCs containing the noncovalent type liquid crystals show power conversion efficiencies (PCEs) of up to 5.8 +/- 0.2% at 30 degrees C and 0.9 +/- 0.1% at 120 degrees C. In contrast, solar cells containing the covalent type electrolytes show significant increase in PCE up to 2.4 +/- 0.1% at 120 degrees C and show superior performance to the noncovalent type-based devices at temperature above 90 degrees C. Furthermore, the LC-DSSCs exhibit excellent long-term stability over 1000 h. These novel electrolyte designs open unexplored paths for the development of DSSCs capable of efficient conversion of light to electricity in a wide range of temperatures.