Synthesis Of Cyanobiphenyl-functionalized Imidazolium-type Organic Ionic Crystals For Solid-state DSSC

Ionic liquids (ILs) are organic salts with melting points below or near roomtemperature composed entirely of anion and cation. They have unique properties, such asnegligible vapor pressure, high ionic conductivity, and wide electrochemical window et al.At the same time, the designability makes it easy to obtain specific needs ILs by adjustingthe combination of anion and cation or grafting appropriate functional groups.Dye-sensitized solar cells (DSSC) have recently gained widely interests because oftheir attractive advantages, such as high-efficiency, low-cost, and easy-handling fabrication.Electrolytes play an important role in DSSC. Though the overall photoelectric conversionefficiency up to12%has been reported for organic liquid electrolyte cells, some scientificand technological difficulties, such as the solvent leakage, and long-term instability stillneed to be improved. So ILs has been introduced into DSSC to gain quasi-solid andall-solid state electrolytes before the widespread commercialization of DSSC.Most of ILs applied in DSSC are still liquid materials, which don’t fundamentallysolve the problems brought by liquid electrolytes. At the same time, power conversionefficiency of quasi-solid and solid-state DSSC is far below that of liquid DSSC. Efficientand stable electrolyte materials currently become a hot research. In this work, based on ILs,cyanobiphenyl-functionalized imidazolium-type organic ionic crystals were synthesized,and a new efficient solid DSSC was formed after proper doping.The main contents and conclusions are summarized as follows:(1) Organic ionic crystals [CnBIm][X](n=2,4,6; X=Br, BF4or TFSI) weresynthesized and were characterized by IR, NMR, LCMS, TGA, and DSC. All these ioniccrystals confer a high thermal stability, far beyond the range of interest for practicalapplication in DSSC.1-ethyl-3-methylimidazolium iodide (EMII) matrix was dopeddifferent ionic crystals and I2to prepare solid electrolytes which can be used in DSSC anda series of test were studied, including conductivity, thermal properties, and ion diffusion coefficient. Finally, the fabricated solid-state devices based on ionic crystals electrolytesgive an overall power conversion efficiency (PCE) of about~6.41%at60°C undersimulated radiation (50mW/cm2). The working principle was studied, and it shows that theenhanced photoelectrical parameters of [CnBIm][X] doped electrolytes should due to thelight-scattering properties of cyanobiphenyl-functionalized [CnBIm][X] compounds, whichimproved the light-harvesting efficiency of the dye-sensitized TiO2. To further improve thecell efficiency,1-propyl-3-methylimidazolium iodine (PMII), was added into theelectrolytes as a crystal growth inhibitor. The fabricated devices show enhanced PCE of~6.55%at45°C under simulated radiation (50mW/cm2), and a superior long-timestability during the operating temperature.(2) Organic ionic crystals [C11AIm][X](A=M or B; X=Br, BF4or TFSI) weresynthesized and were characterized by IR, NMR, LCMS, TGA, and DSC. All these ioniccrystals confer a high thermal decomposition temperature, and show different crystalmorphology and curing temperature with the changes in the alkyl chain length and anion.After doped into EMII/PMII, room temperature solid electrolytes were formed. Thefabricated devices can reach a PCE of1.52%at room temperature. The heat resistance ofthe electrolytes under high temperature and photoelectrical parameters of the devices indifferent conditions need further study in order to optimize device performance.