染敏太陽能電池之二氧化鈦層結構與電子傳遞機制

蕭博聰

doi:10.6844/NCKU.2011.00660

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染敏太陽能電池之二氧化鈦層結構與電子傳遞機制

Structure and Electron Conveying Patterns of TiO2 Films in Dye-Sensitized Solar Cells

蕭博聰 , Ph.D　　Advisor：鄧熙聖　　

英文 DOI： 10.6844/NCKU.2011.00660

染料敏化太陽能電池；二氧化鈦；電子傳遞；結晶缺陷；銳鈦礦； X光吸收； Rietveld分析；水熱壓力；奈米管陣列；正面照光；電子收集效率； Dye-sensitized solar cells ； Titanium dioxide ； Electron transport ； Crystal defect ； Anatase ； X-ray absorption fine structure spectroscopy ； Rietveld refinement ； Hydrothermal pressure ； Nanotube arrays ； Front illumination ； Electron collecting efficiency

Abstract │ Reference (323) │ Altmetrics

Abstract 〈TOP〉

染敏太陽能電池的光電極是決定電池效能的關鍵元件。電極薄膜的形構必須具備高表面積與孔洞性結構之特性，高表面積提供更多染料分子的吸附，孔洞結構則使電解質易滲透其中進行反應。為了達此要求，電極薄膜通常是由奈米級的顆粒所堆疊而成。然而，顆粒間的晶界容易產生缺陷，不利電子傳遞，使得電子傳遞的速度降低，電子再結合機會上升。因此，如何有效分析薄膜缺陷以及探討缺陷對電子傳遞機制的改變，將提供未來改善電極薄膜的方向。
本論文探討二氧化鈦層的結構缺陷與電子傳遞機制，可分為四個主題：1.二氧化鈦奈米顆粒合成方式與燒結成膜對Ti4+周圍配位數之影響及作為光電極之應用；2.染敏太陽能電池中二氧化鈦奈米粒結晶層的電子傳遞特徵；3.結晶成長的水熱壓力對二氧化鈦顆粒結構與電子傳遞能力之影響；4.二氧化鈦一維奈米管陣列用於前照和背照式染敏太陽能電池的電子傳遞特徵。
第一部分，基於奈米結晶二氧化鈦奈米顆粒結晶層已被廣泛運用在光化學裝置的電子傳導層，此部分利用X光吸收光譜研究二氧化鈦anatase結晶相之奈米顆粒與其成膜後的晶格扭曲。二氧化鈦奈米顆粒經由titanate中間相脫水與溶膠-凝膠法合成，分別命名為AN和AN-br，其中AN為純anatase相，而AN-br除了anatase相外還包含brookite和rutile相。X光吸收光譜顯示這二種樣品在成膜前的Ti-O配位數相差不大，但透過燒結成膜後，Sol-Gel的Ti-O配位數有明顯的下降，表示晶格缺陷增加，不利於電子傳遞。此明顯的下降歸因於額外的結晶相(brookite和rutile)造成晶界處的晶格扭曲增加，也說明形成純結晶相的重要性。
在第二部分中，我們利用Rietveld分析X光繞射圖譜得知結晶結構資訊與結晶成分比例，目標物為AN與AN-br二氧化鈦奈米顆粒與奈米結晶膜。分析結果顯示AN-br的結晶含量為：71.8% anatase、27% brookite和1.2% rutile，以及較多的氧空缺於奈米結晶膜內(相較於AN所構成之薄膜)，導致後續所製備電池的閉環電流與效率隨氧空缺增加而下降。我們亦發現結晶缺陷會形成trap state，使電子的傳遞機制分為trap-free與trap-limited的模式。進一步從交流阻抗分析儀發現trap-free的傳遞方式可延伸電子傳遞距離，增加電子收集效率。
第三部分改變壓力釜內殘餘體積控制結晶成長壓力並固定溫度為250 ºC，研究壓力對二氧化鈦顆粒的結構與電子傳遞特性之影響。電子穿透式顯微鏡和X光繞射圖譜顯示非結晶相二氧化鈦的比例隨合成壓力上升而增加，但Ti空缺率則是下降，這說明適當的合成壓力可獲得適合的結體結構。X光吸收光譜則顯示Ti-O的配位數隨壓力提升而增加，直至壓力達到100 bar才開始下降。因此，在100 bar下合成的二氧化鈦所製備的電池顯現最佳的效率。交流阻抗分析儀亦證實此壓力下合成的顆粒所構成的電極薄膜擁有最高的電子收集效率，說明合成壓力改變薄膜缺陷的多寡，決定最後的電池效率。
最後的部分，我們將一維二氧化鈦奈米管陣列取代奈米顆粒結晶膜作為染敏太陽能電池的光電極材料，奈米管的長度介於17至37 m間，此電池裝置適用於前面照光與背面照光的模式，其中由30 m的奈米管薄膜所製備的電池於前面照光的模式下產生最高的電池效率。儘管奈米管提供直接的電子傳遞路徑，電子的傳遞仍受限於缺陷的影響，這由於管壁內存在晶界結構。此外，不同的照光模式將改變電子傳遞的機制：正面照光下，電子的傳遞包括trap-free與trap-limited的模式；然而，背面照光使電子只能以trap-limited的模式傳遞。對於正面照光的電池而言，trap-free的傳遞模式決定電池的效率。交流阻抗分析儀發現奈米管陣列的薄膜既使長度為30 m仍擁有90%以上的電子收集效率，這歸因於奈米管具有較大尺寸的晶粒造成低密度的缺陷存在，也因此導致較廣泛的trap-free傳遞範圍以利於電子的傳遞。

Parallel Abstract 〈TOP〉

The photoelectrode is a key component determining the efficiency in dye-sensitized solar cells. A film for photoelectrode must have the high surface area and a mesoporous structure. The high surface area can improve dye adsorption and mesoporous structure allows electrolyte penetration to react. Based on these comments, a mesoporous film usually constructed by nanoparticles. However, crystal defects form at the inter-particles, which retard electron transport rate and increase the probability of electron recombination. Thus, how to characterize defects in the film and investigate the influence of defects on electron transport mechanism will provide a knowlegde to develop an advanced electrode.
This dissertation includes four parts: 1. Coordination of Ti4+ sites in nanocrystalline TiO2 films used for photoinduced electron conduction: Influence of nanoparticles synthesis and thermal necking; 2. Electron transport patterns in TiO2 nanocrystalline films of dye-sensitized solar cells; 3. Influence of hydrothermal pressure during crystallization on the structure and electron-conveying ability of TiO2 colloids for dye-sensitized solar cells; 4. Electron transport patterns in TiO2 nanotube arrays based dye-sensitized solar cells under frontside and backside illuminations.
In the first part, we subjected the lattice disorder of TiO2 nanoparticles and the resulting nanocrystalline films to analysis by X-ray absorption fine structure spectroscopy (XAFS). The TiO2 nanoparticles were synthesized from dehydration of a titanate and from a conventional sol-gel method. Although both specimens had similar first shell Ti4+ coordination numbers of ca. 5.7, the AN TiO2 was shown to be phase-pure anatase and the AN-br TiO2 contained a minute amount of brookite impurity. After nanoparticles necking into films, the former TiO2 exhibited a negligible decrease in the coordination number whereas the latter showed a significant decrease to a value of ca. 4.9. As a result, the AN film was more efficient than the AN-br one in transmitting electrons injected from a photoexcited dye. We have demonstrated that synthesis of phase-pure nanoparticles is essentially important in fabricating films with minimal degree of lattice disorder.
The second part reports synthesis and characterization of nanoparticles for fabricating the TiO2 nanocrystalline films used in dye-sensitized solar cells: phase-pure anatase nanoparticles from a titanate-directed route, and brookite (27%) and rutile (1.2%)-containing anatase nanoparticles from a sol-gel route. After nanoparticle-necking into films, XRD pattern simulation shows that the defect density of the anatase (AN) films is less than that of the brookite/rutile-containing anatase (AN-br) films. The defect states in the AN-br films lower the short circuit current and conversion efficiency of the resulting solar cells. Intensity-modulated photocurrent/photovoltage spectroscopic (IMPS/IMVS) analysis demonstrates that electron transport in trap-free and trap-limited diffusion modes, and shows that the defects serve as electron trap state to retard both electron transport and recombination. Electrochemical impedance spectroscopy (EIS) analysis shows that the trap-free mode extends the electron diffusion length in TiO2 films and its contribution magnitude governs the electron collecting efficiency.
The third part synthesizes TiO2 anatase colloids at 250 ºC under varying pressures of 57120 bar by adjusting the residual volume in the autoclaving chamber. Transmission electron microscopy and X-ray diffraction analyses showed that the amorphous phase content of TiO2 powders and films obtained from calcining the colloids increased with the pressure during crystallization, while the Ti vacancy in the crystalline phase decreased. This illustrated a trade-off between lattice distortion and vacancy reduction as a result of an increase in pressure during crystallization. XAFS spectroscopic analysis showed that the coordination number of the Ti4+ sites in the TiO2 increased with the pressure during crystallization to reach a maximum value at 100 bar and then decreased with further increases in pressure. A dye-sensitized solar cell assembled with a TiO2 film from 100bar synthesis exhibited the highest solar energy conversion efficiency. EIS analysis showed that the 100bar film had the highest charge collection efficiency for photogenerated electrons. From these results, we concluded that TiO2 crystallization pressure affects the density of defect in the produced TiO2 films, and therefore the electron-conveying performance in DSSCs.
In the final part, TiO2 nanotube arrays (NTA), of 1737 m in thickness, detached from anodic oxidized Ti foils were used as photo-anodes for dye-sensitized solar cells (DSSCs). Photovoltaic measurements under frontside and backside illumination showed that frontside illumination geometry provided better cell performance than backside illumination did. A cell assembled with 30 m thick NTA film produced the greatest photocurrent and light conversion efficiency. Despite an advantageous architecture for electron transport, electron trapping remained a limiting factor for both illumination geometries, due to the presence of crystal grains in the NTA walls. IMPS analysis showed that electron transport in the front illuminated cells comprises both trap-free and trap-limited diffusion modes, whereas electrons in the back illuminated cells travel only by trap-limited diffusion. The trap-free diffusion mechanism determines front illuminated cell performance. EIS analysis showed the front illuminated NTA based DSSCs have a charge collection efficiency of better than 90%, even at 30 m NTA film thickness. Large crystal size results in low trap state density in the NTA film, and this effect may result in a more extensive trap-free diffusion zone in the films, which facilitates charge collection.

Reference ( 323 ) 〈TOP〉

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