染料在半導體上的吸附量，是決定染料敏化太陽能電池的光捕捉效率的關鍵因素之一。而染料與 TiO2 薄膜之間的界面性質，例如染料的吸附構型及吸附能，會影響到染料的吸附量以及染料敏化太陽能電池的穩定度。所以，從原子的尺度探討染料吸附於 TiO2 薄膜上的吸附性質，以便合理地進行染料分子的結構設計，對於高效率的染料敏化太陽能電池研究是相當重要的。 FT-IR 是廣泛用於分析染料分子吸附在 TiO2 薄膜上之吸附構型的技術。有鑑於其複雜的界面性質，先前對於染料分子吸附構型的研究依然存有爭議，特別是 anchoring group 的 IR 特徵峰鑑定。本研究利用電子密度泛函理論 (Density Functional Theory, DFT) 進行計算，分析不同吸附構型的吸附能、結構以及振動頻率。我們挑選 2-cyano-3-(thiophen-2-yl)acrylic acid 做為模型分子，使用其較穩定的 E-configuration 進行探討。我們將計算得到的 bidentate bridging 及 tridentate 兩種吸附構型的振動特徵峰，同時與實驗得到的 FT-IR 圖譜做比較。先前由孟勝團隊的研究發現， bidentate bridging 和 tridentate 有共存的現象發生，而我們的研究結果亦同；但是，我們鑑定 IR 特徵峰的方式，對於較複雜的染料分子與 TiO2 之間的界面特性，能夠提供更明確的資訊。藉由分析 bidentate bridging 及 tridentate 的能量，探討兩者之間的構型轉換機制，可以推斷 bidentate bridging 是動力學控制的吸附構型，而 tridentate 則是熱力學穩定的吸附構型。最後，我們更進一步地探討 D-π-A 染料分子以 bidentate bridging 及 tridentate 構型吸附於 TiO2 上的光物理性質。 ;The loading of dye on a semiconductor is crucial for determining the light harvesting efficiency of dye-sensitizer solar cells (DSCs). The interfacial properties of dye adsorbed on TiO2 film such as adsorption configuration and adsorption energy can influence the amount of dye that loads and the stability of the DSC device. Therefore, it is important to characterize the adsorption properties of the dye on TiO2 film atomically and electronically to ensure rational structure-based dye design for high-performance DSCs. Fourier Transform Infrared (FT-IR) is a commonly used technique to probe the adsorption configurations of dye on the TiO2 surface. Due to the complex properties of interfacial dyes, previous work on the identification of adsorption configurations of dye on TiO2 has sometimes been controversial, in particular, the essential FT-IR band assignments. In this study, we used density functional theory (DFT) to calculate the adsorption energies, geometries and vibrational frequencies of various adsorption configurations of 2-cyano-3-(thiophen-2-yl)acrylic acid with a stable E-configuration adsorbed on TiO2. We performed a comparative assignment of the calculated vibrational peaks of tridentate and bidentate configurations to the experimental FT-IR spectra at the same time. First, our work supports the coexistence of tridentate and bidentate bridging configurations first proposed by Meng and coworkers. Furthermore, our FT-IR mode assignments provide direction for further studies of the interfacial adsorption properties of dye on TiO2. Study of the transformation mechanisms between tridentate and bidentate modes suggests that the bidentate bridging configuration is a kinetically-trapped adsorption mode and the tridentate configuration is thermodynamically the most stable. Finally, we compared the photophysical properties of a D--A dye in tridentate and bidentate bridging configurations.
