具有D–A’–π–A結構的染料相較於常見的D–π–A結構染料，在染料敏化太陽能電池(Dye-sensitized Solar Cells, DSSC)的應用上有著許多的優點。在D–π–A結構中加入內部的拉電子單元即為內部電子接收者(internal acceptor) 會使其光譜較紅位移且會延伸其吸收範圍，使得吸收光譜範圍變得更加寬廣。更重要的是還能增加染料分子的光穩定度。 在本研究中使用了密度泛函理論 (Density Function Theory, DFT) 、時間密度泛函理論 (TD-DFT)及第一原理方法計算非絕熱分子動態模擬(ab initio nonadiabatic molecular dynamics simulation, MD simulation)計算並分析D–A’–π–A染料在DSSC中所產生的光物理性質，尤其是針對internal acceptor對DSSC造成的影響的分析。本研究結果中呈現出加入強力的拉電子基(A’)會使染料產生新的吸收峰，以及使最大吸收峰產生紅位移的現象，此兩種現象將有助於產生更高比率的光電轉換效率。此外研究結果中也呈現了加入強力的拉電子基(A’)會產生明顯的雙重電子轉移的特徵。雙重電子轉移代表著在激發態時，電子密度會同時轉移至internal acceptor 及末端電子接收者(terminal acceptor)。特別是於光激發時，internal acceptor會捕捉住大量的電子密度。 另外在溫度300K進行的ab initio分子動態模擬的結果中發現了隨著時間經過，在激發態時電荷密度轉移至internal acceptor 及terminal acceptor的變化非常微小。其結果代表著在300K的熱動能還不足以推動電荷密度從internal acceptor轉移至terminal acceptor;或者是沒有產生一個足以推動電荷密度從internal acceptor轉移至terminal acceptor的反應座標。在此研究中還發現了D–A’–pi–A染料具有較低的激發太氧化電位(Excited-state Oxidation Potential, ESOP)，由此也可以證明D–A’–π–A染料具有較佳的光穩定度。由本研究針對D–A’–π–A電荷傳輸的探討，給未來在工程設計方面提供了合理的理論基礎。 ;The dyes with D–A’–π–A configuration which are different to traditional D–π–A framework have several advantages on DSSC applications. Introduction of an internal acceptor into the D–π–A framework red-shifts absorption spectra and also expands its range. More importantly, D–A’–π–A dyes show particular photo-stability. In this study, we employed density function theory (DFT), time-dependent DFT (TD-DFT), as well as ab initio nonadiabatic molecular dynamics (MD) simulations to investigate the photophysical properties of D–A’–π–A dyes on DSSC performance, in particular, the effects of internal acceptors. Our calculations show introduction of a strong electron-withdrawing A’ moiety generates a new band and red-shifts the λmax band, which can potentially, contributes to the photo-to-current conversion. Moreover, the absorption bands exhibit significant characters of dual charge transfer; the excited electron density is transferred to the internal and terminal acceptors at the same time. Particularly, the internal acceptor traps significant amount of electron density upon photo-excitation. The ab initio MD simulations at 300K show small amount of excited electron density is pushing and pulling between the internal and terminal acceptors; the thermal energy is not higher enough or there is no suitable reaction coordinates.to drive the electron density from the internal acceptor to the terminal acceptor. Our calculations shows the D–A’–π–A dyes own a low-lying excited-state oxidation potential, crucial features of photo-stability. Our study investigates the nature of charge-transfer of D–A’–π–A dyes giving theoretical basis for further rational engineering.
