一維二氧化鈦奈米結構之製備及染料敏化太陽能電池應用

Preparation of 1D-based TiO2 Nanostructures for the Application of Dye-sensitized Solar Cells

何偲瑜(Shih-Yu Ho)

指導教授 ： 蘇昭瑾

國立臺北科技大學/工程學院/有機高分子研究所/博士(2013年)

摘要

染料敏化太陽能電池（Dye-sensitized solar cells，DSSC）具有低成本、不需精密製程設備及可撓曲等優點，近年來成為學者研究的焦點，其中探討二氧化鈦材料形貌對於DSSC元件表現同樣吸引相當多研究。本研究主要分為三部分，第一、二部分為分別利用陽極氧化法以及水熱法製備出高度有序的一維二氧化鈦奈米結構。在製備二氧化鈦奈米管陣列部分，係利用鈦金屬基板當作陽極，白金當作陰極，以氟化銨當作電解液，搭配外加電壓進行陽極氧化法反應，在鍛燒程序後便可製備出有序的二氧化鈦奈米管陣列。由於二氧化鈦奈米管陣列應用於染料敏化太陽能電池的優勢在於可以快速傳遞電子，但同時也因為奈米結構的比表面積過低造成染料吸附量不高，侷限了效率發展。本研究在二氧化鈦後處理部分以二氧化鈦前驅物：正四丁基氧化鈦以及四氯化鈦對二氧化鈦奈米管陣列做表面處裡，以增加二氧化鈦奈米管陣列比表面積，並且同時提高了光電流轉換效率。而後更利用一步法對未鍛燒前的二氧化鈦奈米管陣列進行吹氮處裡，使二氧化鈦奈米管表面缺陷擴大，在鍛燒後可製備出同時具有奈米顆粒及奈米管結構的二氧化鈦奈米管陣列，省去進行後處理步驟，更有效率的提升了二氧化鈦奈米管染料吸附量以及光電流轉換效率。
第二部分為製備二氧化鈦奈米方形顆粒部分，係以異丙醇鈦當作二氧化鈦前驅物，水解過後加入氨水、氫氧化四甲基銨、氫氧化四乙基銨以及氫氧化四丁基銨，在鹼性環境下進行水熱反應，探討銨鹽結構中碳鏈及氫鍵的有無，以及親水性、親油性等效應對於二氧化鈦結晶過程帶來的影響及改變。研究結果發現，當使用具有碳鏈結構的銨鹽合成二氧化鈦時，會製備出帶有高度緻密排列的二氧化鈦方形顆粒，這是由於銨鹽中的碳鏈－碳鏈具有親油性，在二氧化鈦顆粒形成後因為親油性質關係使得顆粒的堆疊緻密且有序排列，而氨水則不會製備出方形顆粒，並且由於氫鍵的關係使得顆粒堆疊結構相當鬆散。將製備出的二氧化鈦應用於染料敏化太陽能電池後，光電分析結果發現，以氨水製備出的二氧化鈦奈米顆粒因為結構鬆散可以吸附最多染料可以達到7.8 %光電流轉換效率，而具高度排列的二氧化鈦奈米方形顆粒雖然因為結構緻密使得染料吸附量不高，但在阻抗部分卻能有效地降低電子傳遞電阻，提升電子擴散效率。
第三部分是與中央大學化學系生物化學實驗室（李文仁教授指導）合作，將其實驗室合成高效率染料應用於DSSC元件上。2010年其團隊發表藉由含有carbene結構的釕金屬錯合物CBTR作為DSSC光敏化劑，染料CBTR可達9.69 %的高光電轉換效率，而此部分則是沿用CBTR染料結構中的carbene特性，合成出新型染料CB104，增加了高共軛的輔助基團與噻吩結構，以期提升可見光與近紅外光區域吸收，同時提高莫耳消光係數，增加光電轉換效率。不過由於染料結構過大關係，致使染料吸附到二氧化鈦薄膜上以後出現染料團聚堆積現象，因此將染料濃度減半以後，並添加鵝去氧膽酸（CDCA）作為共吸附酸，最終能利用改變染料濃度與添加共吸附酸的方式，將原先0.3 mM的CB104效率6.58 %在濃度減半（0.15 mM）與添加10 mM的CDCA以後將光電轉換效率提升至8.36 %。

關鍵字

水熱法 ； 陽極氧化法 ； 二氧化鈦 ； 銨鹽 ； 奈米管陣列 ； 方形結構 ； 一維結構 ； 電子傳遞 ； 染料敏化太陽能電池

並列摘要

This research has been divided into three parts. In the first part, TiO2 nanotube arrays (TNAs) were prepared and applied in dye-sensitized solar cells (DSSCs). One dimensional TiO2 nanotube structure plays an important role in the application of DSSCs due to its faster electron transport characteristics. The vertically orientated TiO2 nanotube arrays (TNAs) were successfully fabricated by electrochemically anodizing titanium (Ti) foils. The fabrication of photoanode using the TiO2 nanotube structures mixed with the TiO2 nanoparticles was investigated to enhance the photovoltaic efficiency of DSSCs by increasing the surface area of electrode. This can be achieved by Ti-precursor (titanium (IV) n-butoxide and titanium tetrachloride) post-treatment and annealing process or by a simple one-step anodization process. The X-ray diffraction patterns reveal that both TNAs and the decorated TiO2 nanocubes (TNCs) are in anatase phase. The scanning electron microscopy analysis demonstrates that the wall thickness and inner diameter of hexagonal close-packed TiO2 nanotubes from chemically polished Ti foils are 10–15 and 100–120 nm, respectively, and the particle size of TNCs is 60–75 nm. The DSSC fabricated by themixedmorphological TNAs with TNCs shows an enhanced photoconversion efficiency of ~63% than that of TNAs alone, due to the increase of both dye adsorption and electron transportation rate.
In the second part, TiO2 nanocubes were synthesized via hydrolysis of titanium tetra-isopropoxide (TTIP) as a Ti precursor, followed by hydrothermal treatment under the basic condition. Various kinds of TiO2 colloids were prepared by adding ammonium salts with different alkyl group such as ammonium hydroxide (NH4OH), tetramethyl ammonium hydroxide (TMAH), tetraethyl ammonium hydroxide (TEAH) and tetrabutyl ammonium hydroxide (TBAH) before hydrothermal crystallization. The crystal phase, shape, and morphology of TiO2 nanocubes were studied by XRD, TEM, and SEM analysis. It was found that the TiO2 nanocubes were pure anatase and tended to assemble with well-ordered and close-packed domains. Both alkyl length and hydrothermal duration influences the TiO2 nanocube formation efficiency. The ammonium salts with longer alkyl chain could form TiO2 nanocubes in shorter hydrothermal time and vice versa. The above TiO2 nanocubes were applied as photoanode materials in DSSCs. A systematic comparison between alkyl chain length and the photo-conversion efficiency are discussed in this work.
In the third part of this thesis, a new type of carbene-based ruthenium sensitizer, CB104, with a highly conjugated ancillary ligand, diphenylvinylthiophene-substituted benzimidazolepyridine, was designed and developed for dye-sensitized solar cell applications. The influence of the thiophene antenna on the performance of the cell anchored with CB104 was investigated. Compared with the dye CBTR, the conjugated thiophene in the ancillary ligand of CB104 enhanced the molar extinction coefficient of the intraligand ?-?* transition and the intensity of the lower energy metal-toligand charge-transfer band. However, the incident photon-to-current conversion efficiency spectrum of the cell anchored with CB104 (0.15 mm) showed a maximum of 63% at 420 nm. The cell sensitized with the dye CB104 attained a power conversion efficiency of 7.30%, which was lower than that of the cell with nonconjugated sensitizer CBTR (8.92%) under the same fabrication conditions. The variation in the performance of these two dyes demonstrated that elongating the conjugated light-harvesting antenna resulted in the reduction of short-circuit photocurrent density, which might have been due to the aggregation of dye molecules. In the presence of a coabsorbate, chenodeoxycholic acid, the CB104-sensitized cell exhibited an enhanced photocurrent density and achieved a photovoltaic efficiency of 8.36%.

並列關鍵字

Anodization ； Hydrothermal ； TiO2 ； Ammonium hydroxide ； Nanotube array ； Nanocube structure ； One-dimensional ； Electron transport ； Dye-sensitized solar cells

參考文獻

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