植物色素作為光敏劑應用於染料敏化太陽能電池之研究

本論文利用洛神和黃梔子果實中萃取的天然染料(花青素、梔子藍及梔子黃)作為光敏劑，並將其應用於染料敏化太陽能電池。將工作電極、染料、電解液及對電極以三明治方式組立染料敏化太陽能電池裝置，並以太陽能模擬器量測其光電轉換效率。文中發現當染料由單色之花青素取代為花青素與栀子黃以等比例混色，其短路電流(Isc)值由0.487提高至1.710mA，開路電壓(Voc)由0.300提升至0.340V，而光電轉換效率從0.680提升至1.040%。主要原因為花青素與栀子黃混色後其吸光範圍拓寬且吸收峰位置向短波長方向偏移，形成所謂的藍移現象，這有助於染料能隙的提升，並降低電子躍遷(價電帶電子躍遷至導電帶)所需之能量。此外，藉由色素光譜模擬共混，發現花青素、栀子藍、栀子黃三者以4:4:2比例模擬共混可獲最佳吸光範圍，隨後三種色素以此比例進行實驗，獲得其Isc為1.580 mA，Voc為0.340V，而光電轉換效率為1.007%。此結果並未獲得如預期更高之光電轉換效率，當所有色素共混，其Isc值略低於二種色素共混之值。意即三種色素共混後，栀子黃結構式的C=O與花青素的OH產生氫鍵鍵結，造成花青素的自由基OH減少，進而使色素與電解液之間的氧化還原對數量減少。因此，僅以等比例混合花青素與栀子黃作為光敏劑具有最佳之太陽能電池光電轉換效率。

關鍵字

天然染料；藍移現象；氧化還原；光電轉換

並列摘要

This study employs anthocyanin extract from roselle, blue gardenia and crocin extract from gardenia fruit as the natural dyes for a dye-sensitized solar cell (DSSC). The devices was constructed by the working electrode, dyes, electrolyte and counter electrode, and assembled into a sandwich cell module and illuminated by a light source simulating AM (air-mass) 1.5 with a light strength of 100 mW/cm2 to measure the photoelectric conversion efficiency of the prepared DSSCs. We found that the Isc was increased from 0.487 to 1.710 mA, the Voc was increased from 0.300 to 0.340 V, the photoelectric conversion efficiency was increased from 0.680 to 1.040%, as the dyes were use the mixture of anthocyanin and crocin to replace the anthocyanin as sensitizers for the DSSC. These results were mainly caused by the main peak in the spectra shifts to the short wavelength direction and the absorption range was expanded (this also called the blue shift phenomenon). The blue shift can help to enhance the dye energy gap and reduce the required energy for electron transition (the electronic transitions from valence band to the conduction band). Then three kinds of natural pigments (anthocyanins, blue gardenia and crocin) were blended by the ratio of 4:4:2, the Isc was 1.580 mA, the Voc was 0.340V, while the photoelectric conversion efficiency was only 1.007%. The higher expected photoelectric conversion efficiency was untouchable. When three kinds of pigments were blended, the Isc value was slightly lower than that of two kinds of pigments blending. The lower efficiency of the DSSCs was attributable to the lower value of Isc. The numbers of free bonded OH bonds of the anthocyanin were reduced, since hydrogen bonds were constructed between C=O bonds of the crocin and OH bonds of the anthocyanin for all three kinds of pigments blending. The number of the redox pairs was decreased between the dyes and electrolyte. Therefore, the same ratio of mixture between anthocyanin and crocin was the optimal photosensitizer for the dye-sensitized solar cell.