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

Abstract

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 I2-doped imidazolium ionic liquids and carbonate-terminated mesogenic compounds (noncovalent type) and (ii) single-component mesogenic compounds covalently bonding an imidazolium moiety doped with I2 (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 I3 – 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 °C and 0.9 ± 0.1% at 120 °C. In contrast, solar cells containing the covalent type electrolytes show significant increase in PCE up to 2.4 ± 0.1% at 120 °C and show superior performance to the noncovalent type-based devices at temperature above 90 °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.

Functional Liquid Crystals towards the Next Generation of Materials

“…[97][98][99][100][101][102][103][104][105][106][107][108][109] Fore xample,i on-conductive columnar liquid crystals forming 1D ionic nanochannels [102] and bicontinuous cubic liquid crystals forming 3D ionic nanochannels [103][104][105] as well as smectic LC materials, [106][107][108][109] have been developed and studied. By employing specific ion-active groups,L Cm aterials transporting Li + and I À have been designed, which can be used as electrolytes for lithium batteries [110] and dye-sensitized solar cells (DSSCs), [111][112][113] respectively.L Cm aterials con- ducting other ions including H + [114][115][116] have also been examined for the development of fuel cells and the other devices.…”

Liquid crystals in photovoltaics: a new generation of organic photovoltaics

“…FTO, fluoride-doped tin oxide. Reproduced with permission from Högberg et al 169 Copyright 2016, American Chemical Society. A full color version of this figure is available at the Polymer Journal online.…”

“…Very recently, Kato et al 169 developed nanostructures of liquid crystalline electrolytes 34-36 ( Figure 26) to prepare DSSCs. They demonstrated two types of electrolyte assemblies, first as non-covalent assemblies of two-component mixtures consisting of I 2 -doped imidazolium ionic liquids and carbonate-terminated mesogenic compounds and second as single-component mesogenic compounds covalently bonded with an imidazolium moiety doped with I 2 ( Figure 28).…”

Functional liquid-crystalline polymers and supramolecular liquid crystals

“…30,106 Recently, growing attention has been paid to use of these nanosegregated liquid crystals as transport materials because of the formation of well-organized nanochannels structures. 19,[168][169][170][171][172][173][174][175][176][177][178][179][180] These LC low-molecular weight molecules are easy to orient, and show one-dimensional, twodimensional and three-dimensional (3D) transport features. However, they are in general soluble in organic solvents and exhibit poor mechanical properties that compromise their robustness and stability for being applied in devices or as functional coatings and membranes.…”

“…Remarkably, the formation of the LC phases can be, in some extent, controlled by careful molecular engineering to obtain these LC phases. 19,[168][169][170][171][172][173][174][175][176][177][178][179][180] The resulting phase is crucial because it determines the nature of the transport properties including their anisotropic properties.…”