由於Squaraine (SQ) 染料在近紅外光區域具有強烈的吸光特性，且跟太陽光的吸收光 譜高度吻合，因而Squaraine (SQ)染料在染料敏化太陽能電池發展中，市集具有發展潛 力的。在本篇研究中，利用了密度泛函理論(DFT)及time-dependent 密度泛函理論 (TD-DFT)計算了6 個SQ 衍生的染料分子，探討染料分子在溶劑中，以及吸附在鋭鈦 礦相(TiO2)38 cluster (101)面兩種狀態下，其分子結構、光學性質及電賀轉移的性質的 變化。從計算結果中可以得知，分別以carboxylic acid (CA) 及cyanoacrylic acid (CAA)， 作為SQ 染料分子的acceptor/anchor 時，SQ 染料分子會具有不同的吸附及躍遷性質。 以CAA 為anchor 的SQ 染料分子，不僅比以CA 作為anchor 的SQ 染料分子具有較紅 位移的吸收性質，且其轉移至anchor/TiO2 的電荷密度也較多。除此之外，以CAA 分 子取代CA 分子的SQ 染料，還有著另一根有著明顯電荷轉移性質的吸收峰(2)。另一 方面，以dicyanovinyl group a 及 ethyl cyanoacetate group 加入 squarate 部分形成 cis-HSQ2-CA/CAA 和cis-HSQ3-CA/CAA 的官能基化的SQ 染料分子，其轉移至 anchor/TiO2 的電荷密度相較於trans-SQ1-CA/CAA 分子具有著較少比率。除此之外， 在本篇研究中還有探討在D--A 類型的染料中，改變acceptor 對其的影響。我們認為 不同的acceptor 會影響LUMO 的能階，但對HOMO 的能階卻不會造成太大的影響。 在溶劑中，有著拉電子官能基acceptor 的DPA-Th2-AA(NO2)分子，其有著最強的最大 吸收波長(max)及明顯轉移至acceptor 的電荷轉移性質。另一方面AA(H) 及有著推電 子官能基acceptor 的AA(NH2) 分子，其擁有較低的電荷轉移性質。然而DPA– Th2-AA(NO2)/(TiO2)轉移至TiO2 的電荷密度卻十分少。預計的原因可能為，在被光激 發後，較低的LUMO 能階會和較低能量的TiO2 的conduction band 作用。由於較低能 量的TiO2 的conduction band 有著較低的態密度(density of states)，因此導致了較少的 電荷密度轉移至TiO2。本篇研究提供了染料敏化太陽能電池設計新染料分子的依據。;Squaraine (SQ) dye is one of the most promising dyes for dye-sensitized solar cell (DSSC) due to its intense absorption in the red/near-infared (NIR) region closely matches with the solar spectra. In this study, we employed density functional theory (DFT) and time-dependent DFT (TD-DFT) to investigate the structural, optical, and electron transfer properties of six SQ-derived dyes in solution and adsorbed on a (TiO2)38 cluster having an anatase (101) surface, as a model for corresponding DSSCs. We found that SQ dyes using carboxylic acid (CA) as acceptor/anchoring group and SQ dyes using cyanoacrylic acid (CAA) as acceptor/anchoring group have different absorption properties and transition characters. The CAA-based SQ dyes not only own more red-shifted absorption properties but also have enhanced charge transfer (CT) character with electron density transferred to the anchor/TiO2 in relative to those of corresponding CA-based SQ dyes. Moreover, substituent of CA by CAA results in CAA-based SQ dyes having an additional absorption (2), which also have significant CT character. On the other hand, functionalized SQs with dicyanovinyl group and ethyl cyanoacetate group into squarate moiety yielded cis-HSQ2-CA/CAA and cis-HSQ3-CA/CAA are less capable for electron transfer to the anchor/TiO2 in relative to that of corresponding trans-SQ1-CA/CAA. In addition, we also investigate various acceptors of D--A dyes. We identified that different acceptors affect the LUMO energy levels and they have less effect for the energy level of HOMO. In solution, the DPA-Th2-AA(NO2) with electron-withdrawing acceptor has the largest max and its absorption has significant CT character to the acceptor. On the other hand, the AA(H) and electron-donating AA(NH2) groups generate the lowest CT character. However, DPA–Th2-AA(NO2)/(TiO2) have less electron density transferred to TiO2. It is expected that the lower energy LUMOs of dyes (red-shifted absorption) will couple with the lower-energy TiO2 conduction band, which has lower density of states leading to less electron density redistributed to TiO2 upon excitation. This study provides clues for designing new sensitizer for DSSC applications.
