Abstract
A series of polymers with ethylene oxide (EO) functional group were synthesized and analyzed as non-volatile and highly stable electrolyte for dye-sensitized solar cells (DSSCs). The oxide atoms of EO group can coordinate to lithium ions and facilitate ion-pair separation, which favors ionic conduction. In this way, polymer with EO group plays a role as not only ionic transport and reduction helper but also as “solvent,” which can dissolve ions without adding other real solvents. Meanwhile, using polymers as electrolyte take an advantage of high mechanical properties, high stability, extremely low volatility and also being favor of commercial use and so on. Compared with other solvent-free and solid-state electrolytes, this polymer electrolyte is easily synthesized and costs low. Therefore, this EO-containing polymer possesses a great potential candidate as solvent-free and solid-state electrolyte.In our study, we synthesized two novel solid-state co-polymer electrolytes (PSEO) at first, which contain Poly (methyl hydrosiloxane) (PMHS) as main back bone and Poly (ethyl glycol) methyl ether methacrylate (PEGMEMA) as side chain. To solve the high crystallization of Poly (ethylene oxide) (PEO), PMHS, which has a low Tg (Tg= -138oC), was used to modify PEGMEMA so that the ion transport can conduct favorably without solvent. Unfortunately, the viscosity of PSEO is as high as 310.48 Pa-S, about three hundred times higher than water at room temperature. High viscosity makes it difficult for ion transport, so the ionic conductivity of PSEO is only around 10-5 S/cm, which is much lower than conventional organic solvent (~10mS). In order to improve the problem of high viscosity, we used PMHS with less repeating units to obtian a new EO-containing co-polymer (NPSEO) with viscosity as low as 115.94 Pa-S. Mixing NPSEO and redox pairs together and fabricating into DSSCs to observe the cell performance and we found an unexpected results: the conversion efficiency is as low as 0.3%.To get rid of the drawback of high viscosity for solidlike polymers, we used only PEGMEMA instead of using co-polymers and form a binary electrolyte with redox pairs. The results that cell performance of PEGMEMA-based electrolytes in DSSCs presented are η= 3.51%, Jsc= 9.89 mA/cm2, Voc= 0.62 mV, ff= 0.57. Their ambient ionic conductivity is 1.16 mS/cm and the diffusion coefficient of tri-iodide dominates at around 4.98×10-7cm2/S. These two indexed can be compared to EMIBCN/PMII mixing binary ionic liquid reported or other similar PMII-doping system. After long-term stability at room temperature, the cell performance keeps at a stable condition. Except for that, we study the series EO-containing polymers in this study including viscosity, ionic conductivity and diffusion coefficient of tri-iodide and so on. In conclusion, we developed a novel solvent-free polymer-based electrolyte successfully, which has a great potential due to its high ionic conductivity, high stability and cheap properties.