全寡分子聚合物電解質於染料敏化太陽能電池之研究

Study of Solvent-Free Oligomer Electrolytes for Dye-Sensitized Solar Cells

朱家嫺(Chia-Hsien Chu)

指導教授 ： 萬其超 ； 王詠雲 ； 王復民

國立清華大學/工學院/化學工程學系/碩士(2010年)

https://doi.org/10.6843/NTHU.2010.00183

摘要

本論文研究一系列含氧烷基(ethylene oxide)官能基之聚合物作為染料敏化太陽能電池(Dye-sensitized solar cells, DSSC)之非揮發性、高穩定性電解液。此高分子以氧烷基作為電子提供者，藉由和帶正電金屬離子例如鋰離子產生凡得瓦力吸引，幫助碘負離子解離出來。此高分子不僅有助於離子傳遞及氧化還原進行，並且扮以一¨溶劑¨之角色，電解質中毋須再加入其他溶劑來幫助解離，同時保留聚合物之高機械性質、高穩定性、低揮發性且利於加工之諸多優點，別於其他固態(不含溶劑)電解質，其優勢在於原物料便宜且合成容易。因此，此乃極具潛力作為DSSCs中之非揮發性固態電解質。
本研究首先開發出兩種新型固態高分子電解質，主要以聚矽氧烷(poly(methyl hydrosiloxane), PMHS) 為主鏈與含氧烷基之壓克力型寡聚物(Poly(ethyl glycol) methyl ether methacrylate, PEGMEMA)所構成之共聚物(PSEO)，利用聚矽氧烷之低玻璃轉移溫度特性來修飾並改善聚氧烷聚合物高結晶度問題，幫助離子可以在不含溶劑時順利傳遞。合成之固態共聚物其黏度為10.48 Pa-S(為水之三百萬倍)，黏度太高嚴重造成離子傳遞困難，使得導電度在常溫下只有10-5 S/cm，遠低於傳統有機溶劑(~10-2 S/cm)。為了改善此情況，於是藉重複單位(repeating unit)較少之PMHS合成出較小黏度之新共聚物(NPSEO)，其黏度大幅降低至115.94 Pa-S，將其與氧化還原對混合成電解質並實際組成DSSC以測試其光電轉換效能，發現仍然成效不佳，室溫效率僅為0.3%，仍然無法擺脫固態高分子文獻中低轉換效率的困境。
為了擺脫高分子固化造成黏度過高的缺點，改以僅用PEGMEMA混以氧化還原對及PMII(3-propyl-methylimidazolium chloride)成二元電解質。其實驗結果顯示：目前電池效能最高可達Jsc=9.89 mAcm-2, Voc=0.62 mV, ff=0.57,η= 3.51 %，其室溫下導電度約為1.16 mS/cm，I3-之擴散係數高達4.98*10-7cm2/S，其兩項指標可比擬文獻中以EMIBCN/PMII混合之二元離子液體電解質以及其他以純聚合物加上PMII混參之類似系統。在經過室溫下的長效測試後，電池性能也具有一定穩定性。除此之外，論文中並將這一系列聚氧烷聚合物電解質做研究，包含黏度、導電度以及I3-之擴散係數等。綜合以上成果，本研究最後成功開發出一種新型無任何溶劑添加之聚合物電解質，其高導電度以及高穩定性以及物料便宜之特性，實為一極具潛力之電解質。

關鍵字

染料敏化太陽能電池 ； 固態高分子 ； 電解質 ； 聚矽氧烷

並列摘要

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.

並列關鍵字

dye-sensitized solar cells ； solvent-free ； solid-state ； polymer electrolytes ； PMHS

參考文獻

1. P.Wang, B.W., R. Humphry-Baker, J. E. Moser, J. Teuscher, W. kantlehner, j. Mezger, E V. Stoyanov, S. M. Zakeeruddin, and M. Gratzel (2005). Charge separation and efficient light energy conversion ion sensitized mesoscopic silar cells basd on binary ionic liquids. J. Am. Chem. Soc 127, 6850-6856.

3. Grätzel, M. (2004). Conversion of sunlight to electric power by nanocrystalline dye-sensitized solar cells. Journal of Photochemistry and Photobiology A: Chemistry 164, 3-14.

5. Grätzel, M. (2000). Perspectives for dye-sensitized nanocrystalline solar cells,. Prog.Photovolt. Res. Appl. 8, 171-185.

6. Grätzel, M. (2001). Photoelectrochemical cells. Nature 414, 338-344.

7. Q. Wang, J.-E.M., M. Grätzel, (2005). Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells. J. Phys. Chem. B 109, 14945-14953.