含2,3-Dihydropyrazino[2,3-f][1,10]phenanthroline單元之異配位釕金屬染料敏化太陽能電池之應用

Abstract

本論文合成出一系列(YC)異配位釕金屬染料，它們以2,3-dihydropyrazino[2,3-f][1,10]-phenanthroline (DPPHEN)衍生物作為輔助配位基(ancillary ligand)，以2,2′-bipyridine-4,4′-dicarboxylic acid (H2dcbpy)作為錨配位基(anchoring ligand)為主體，並有兩個NCS配位基。經由Pd(II)進行Stille coupling引入不同共軛片段(如：苯環、thiophene)修飾DPPHEN，對於金屬化合物之吸收光譜以及轉換效率有不同的貢獻。由於DPPHEN屬於較為平面之分子，引入烷基或烷氧基之引入，不僅增加釕金屬染料之溶解性，而且導致輔助配位基之HOMO、LUMO軌域之調變，因其影響整體元件之光收成及光電轉換效率。若修飾烷基或烷氧基在適當位置，可有效阻擋電解液中的I3-接觸到TiO2表面，達到抑制暗電流發生的機率。組裝成染料敏化太陽能電池(Dye-Sensitized Solar Cells, DSSCs)之最高轉換效率(YC-1)為7.95%，已超過標準元件N719的轉換效率(7.69%)。相較於N719 屬於輔助配位基(ancillary ligand)的π-π*躍遷(π-π* transition)，YC系列染料之輔助配位基的π-π*躍遷(π-π* transition)紅位移約15–70 nm。釕金屬t2g軌域至錨配位基π*軌域的電荷轉移(metal-to-ligand charge transfer, MLCT)吸收峰約510 nm，而此吸收峰的莫耳消光係數(ɛ)較N719為大。輔助配位基(ancillary ligand)中引入terthiophene的染料YC-2，雖然在波段800 nm仍有吸收產生，但此為輔助配位基L-2的分子內(intraligand) π-π*躍遷；經電化學數據與理論計算證實，S0→S1躍遷無法將電子注入TiO2導帶，使得效率僅有0.22%。根據理論計算，YC-3–YC-5的LUMO有部份貢獻來自於輔助配位基，可能導致電子注入至TiO2導帶效率降低。

Novel heteroleptic ruthenium (II) dyes (YC-1–YC-5) containing 2,3-dihydropyrazino[2,3-f][1,10]phenanthroline (DPPHEN) derivative as the ancillary ligand, 2,2′-bipyridine-4,4′-dicarboxylic acid (H2dcbpy) as the anchoring ligand, and two NCS ligands, have been synthesized. Conjugated segments such as benzene and thiophene were incorporated into DPPHEN via Stille coupling. The ancillary ligand has significant effect on the absorption spectra and the cell efficiency. Due to the rigidity of the ancillary ligand, introduction of hydrophobic hexyl or hexyloxy chain can greatly increase the solubility of the dye in organic solvents. Moreover, the accompanying change of HOMO and LUMO levels results in variation of light harvesting and consequently cell efficiency. Appropriate disposition of alkyl or alkoxy long chain in DPPHEN can block the electrolyte from approaching the TiO2 surface and suppress the dark current. The best performance of dye-sensitized solar cells (DSSCs) using these dyes as the sensitizers has a conversion efficiency of 7.95%, which surpasses that the standard cell based on N719 (7.69%). Compared to N719, the absorption band of the intraligand π-π* transition of the ancillary ligand in the YC series was red shifted by 15‒70 nm. The metal-to-ligand charge transfer (MLCT) band of these compounds has λmax at ~510 nm, and the molar extinction coefficient is higher than that of N719. YC-2 has an absorption band at an impressively high value, ~800 nm, due to incorporation of terthiophene entity at the ancillary ligand. However, it stems from the intraligand π-π* transition/charge transfer of the ancillary ligand which hampers the injection of electron into the conduction band of TiO2, as evidenced from electrochemical studies and theoretical computations. Consequently, the cell based on YC-2 has a low efficiency at 0.22%. The ancillary ligand has partial contribution to the LUMO in YC-3–YC-5, which could lower the efficiency of electron injection.