生物色素太陽電池葉綠素a衍生物之光電性質量子分析

Quantum Analysis of Photo-electronic Characteristics of Chlorophyll a Derivatives in Biological Pigment Solar Cells

王湘靈(Hisang-Ling Wang)

指導教授 ： 洪哲文

國立清華大學/工學院/動力機械工程學系/碩士(2012年)

https://doi.org/10.6843/NTHU.2012.00139

摘要

至今能夠達到高效率的染料敏化太陽電池大多使用釕金屬錯合物作為染料，然而目前為止釕金屬仍相當昂貴。使得許多學者轉而投入同樣具備有吸光特性，並且廣泛存在於自然界中的植物色素研究，期望能以價格低廉且取得容易的植物色素作為替代染料。
日本學者以天然色素葉綠素a （chlorophyll a）為基本結構，做結構上的修飾得到二氫卟吩（chlorin），並更進一步在此結構下添加長碳鏈，得到Chlorins-1－4衍生分子，且產生光電轉換效率上的差異。本研究即利用密度泛函理論（density functional theory, DFT）原理與時間相依泛函密度理論（time-dependent density functional theory, TD-DFT）模擬計算二氫卟吩衍生分子Chlorins-1－4的各項結構與光學特性，如能階分布、能隙、分子軌域與紫外線可見光譜。另外，更使用Marcus Theory計算電子傳遞速率、以熱化學分析與陽極鍵結能力以及分子偶極矩的運算，並將各項特性與其文獻中實驗之光電轉換效率做討論比較，分析影響此類太陽電池光電轉換的關鍵因子。
另外也以二氫卟吩分子為基礎結構，對其結構中心置換金屬離子成衍生物Chlorin-metal，並同樣進行各項結構與光學特性分析，希望藉此結構上的改變達到染料分子各項特性上的改變，進而得到更好的光電轉換效率。本論文經由對Chlorin-metal的分析發現，中心金屬離子與相鄰之氮原子在電負度上的差異大小會造成染料分子中心區域電子雲分布的改變，間接影響分子吸收光譜分別在紫外光與長波長範圍的吸收強度。因此，未來可藉由置換不同電負度的中心金屬離子，達到期望之吸收光譜變化。

關鍵字

太陽電池 ； 生物色素

並列摘要

Though ruthenium dyes provide high power conversion efficiency in photoelectrochemical cells, pollution and costliness are fatal flaws. Therefore, researchers made efforts in biological pigments, which are easy to be collected and environmental friendly. Researchers had modified chlorophyll a structure and obtained a dye with 6.1% power conversion efficiency named chlorin. Furthermore, they added various hydrocarbon chains to chlorin. With diffierent hydrocarbon chains, the chlorin derivatives provide different efficiencies.
This research intends to find out the main factors that affect the power conversion efficiency of chlorin derivaives. By using density functional theory (DFT) with hybrid exchange-correlation functional B3LYP and 6-31G basis set, we calculated the photoelectronic properties of chlorin derivatives, such as energy gap, molecular orbital and UV/VIS absorbance spectrum. We also calculated electron injenction speed, proton affinity, and transition dipole moment. Each property is compared and discussed to analyze the major factors that affect the photoelectronic characteristics.
In addition, we use chlorin as a basic structure and alternate the central metal ion to obtain variations in photoelectrochemical properties. According to our results, the differenece in electronegativity between the central metal ion and the adjacent nitrogen atom causes changes in distribution of electron clouds, which indirectly affect the molecule absorbance magnitude in ultra violet and long wavelength range. Therefore, by alternating central metal ion with different electronegativity, the absorbance spectrum can be controlled.

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