天然光敏染料--花青素應用於染料敏化太陽能電池的研究

Studies of dye-sensitized solar cells based on a natural photosensitizer--Anthocyanin.

簡江諭(Chiang-Yu Chien)

指導教授 ： 徐邦達

國立清華大學/生命科學院/生物資訊與結構生物研究所/博士(2015年)

https://doi.org/10.6843/NTHU.2015.00471

摘要

花青素(anthocyanin)是一種廣泛存在於植物細胞中的色素，它能表現多種顏色，並具有抗氧化的能力。在本研究中，萃取了紫高麗菜的花青素作為天然光敏染料，應用在染料敏化太陽能電池(dye-sensitized solar cells, DSSCs)上，我們針對了染料敏化太陽能電池製程上的許多條件做測試並將之最佳化。比起化學性染料，天然染料具有無毒、低成本、低汙染、容易取得的特性。
我們發現當花青素溶液的濃度在3 mM並且將酸鹼值調整至8.0時，電池的光電轉換效率會達到最高。在浸泡電極的時間上，15分鐘是最佳的時間長度。我們發現花青素敏化太陽能電池的表現，在低光度的環境下依然可以保持良好的轉換效率和電壓輸出。在乾燥避光環境下，花青素敏化二氧化鈦電極可以在12週之後保持超過40%的效能，若色素經過純化，效能表現可以保有80%以上。去氧膽酸(deoxycholic acid)可以做為花青素的共同吸附劑(coadsorbent)並可以顯著提升染料敏化太陽能電池的效率，我們發現當花青素和去氧膽酸的濃度比例達到1比40時可以達到最高的光電轉換效率(超過1.4%)，這大約是過去文獻中，用天然花青素做為光敏染料的染料敏化電池效率的將近3倍。
花青素在經過管柱層析純化後，染料敏化太陽能電池的效率會大幅地降低。研究證實，在純化過程中分離出來的無色雜質中，含有某種可以顯著提升電池效率的成份。經過生化測試，在無色雜質中能有效提升電池效率的成份很有可能是屬於醣類(carbohydrates)或是其相關的衍生物。經過測試，我們發現醣類是一種很有效的電池效率促進劑(enhancer)，其促進效率的能力會和染料中的醣類含量成正比。不同種類的醣類具有不同的促進能力，大致上的能力排序是：寡醣>雙醣>單醣>多醣。在寡糖裡，異麥芽寡醣(isomalto-oligosaccharide)和木寡醣(xylooligosaccharide)具有最強的促進能力。染料敏化太陽能電池以醣類做為花青素的共同吸附劑可以將效率提升至超過1.6%，其短路電流密度(JSC)和開路電壓(VOC)各別達到大約4 mA cm-2和630 mV。
同時將醣類和去氧膽酸做為花青素的共同吸附劑，電池的效率被更進一步地提升至將近1.9%，總體的效率成長量大約達到90%。本研究針對了醣類和去氧膽酸在染料敏化太陽能電池上的作用機制做了探討，期望為天然花青素染料敏化太陽能電池的效率促進劑提供一個天然、低成本、好取得的新選擇。

關鍵字

染料敏化太陽能電池 ； 花青素 ； 寡糖

並列摘要

Dye-sensitized solar cells (DSSCs) were fabricated using anthocyanin extracted from red cabbage (Brassica oleracea var. capitata f. rubra), and the conditions that could maximize their performance were explored. The best light-to-electricity conversion efficiency (η) was obtained when the pH and the concentration of the anthocyanin extract were at 8.0 and 3 mM, respectively, and when the immersion time for fabricating sensitized TiO2 film was 15 min. Under low light intensity, the DSSCs sensitized with anthocyanin can keep good VOC and η. More than 40% performance of the TiO2 electrodes sensitized with anthocyanin was maintained after 12 weeks in dark and dry environment, and more than 80% of that was maintained if the anthocyanin was purified. Fabricating the DSSCs in the presence of a coadsorbent, deoxycholic acid, at a molar ratio of dye:coadsorbent of 1:40 was also found to be able to improve the η significantly. The highest η reached was over 1.4%, which is almost three times higher than previously reported conversion efficiencies. In addition, further purification of the anthocyanin extract significantly led to a lower η, and the key elements that greatly affected the performance of the DSSCs based on anthocyanin was found to allocate in the colorless impurities fraction (IPF), which was separated from the purification of anthocyanin extract.
Carbohydrates were found to be excellent performance enhancers that functioned in a dose-dependent manner. Different types of carbohydrates generally had different enhancement capabilities in the order of oligosaccharide > disaccharide > monosaccharide > polysaccharide. The enhancement capability was also highly dependent on the constituents of carbohydrates. Two oligosaccharides, isomalto-oligosaccharide (IMO) and xylooligosaccharide (XOS), showed the highest capability. The DSSCs sensitized with them as additives to anthocyanin exhibited a η over 1.6% (an improvement of 60%) with a short-circuit current (JSC) of about 4 mA cm-2, and open-circuit voltage (VOC) around 630 mV. The mode of interaction of anthocyanin with extrinsic carbohydrates was proposed and discussed. The use of carbohydrates and a common coadsorbent deoxycholic acid (DCA) together could induce an additional enhancement, which boosted the η up to 1.87% (an accumulative improvement of 90%). This study provided a choice of a promising and powerful enhancer for the DSSCs based on natural anthocyanins.

並列關鍵字

dye-sensitized solar cell ； DSSC ； anthocyanin ； oligosaccharide

參考文獻

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