軟性鈦基板染料敏化太陽能電池中緩衝層之研究及老化機制分析

Investigation of Buffer Layer and Degradation Mechanism Analysis of Flexible Ti Substrate Dye-Sensitized Solar Cell

李佳樺(Chia-Hua Lee)

指導教授 ： 吳振名

國立清華大學/工學院/材料科學工程學系/博士(2012年)

摘要

染料敏化太陽能電池(DSSC)因其材料成本低，製程與製程設備簡單，蔚然成為學術界與工業界的研發項目。高效率的染料敏化太陽能電池的光電極(photoelectrode)是利用奈米顆粒金屬氧化物半導體顆粒，塗佈在透明導電玻璃上。金屬箔具有輕、薄且可撓曲的特性，鈦金屬箔具備優異的物理與化學特性，利用鈦金屬箔取代厚重的透明導電玻璃，可擴展染料敏化太陽能電池的應用。
鈦金屬基板與電解液界面產生的再結合反應(recombination)是降低元件效率的因素之一，若要進一步提高光電極中光電子的收集效率，必須降低鈦金屬界面因再結合反應而產生的逆電流。鈦金屬箔做為光電極基板，元件為背照式結構，入射光必須經過對電極與電解液，部分入射光被吸收。對泰金屬光電極元件而言，提高光利用率是提高元件效率的重要關鍵。
在鈦基板上製備緩衝層(buffer layer)，可抑制逆電流產生。本文利用過氧化氫(H2O2)預處理鈦基板，可去除鈦基板表面氧化物鈍化層，並形成海綿狀緩衝層，此緩衝層不但可抑制暗電流，亦增加基板與奈米二氧化鈦顆粒之間接觸面積，增加兩者附著。以電化學交流阻抗分析證實，預處理的鈦基板可大幅降低奈米二氧化鈦與基板之間電荷轉移阻抗，延長電子壽命，增加電子收集效率。與未預處理基板比較，具備海綿狀緩衝層鈦基板元件之填充因子與開路電壓較高，因此有效提高元件轉換效率。
利用過氧化氫添加鹼預處理鈦基板，鈦基板表面形成溝槽狀排列的緩衝層。與未處理基板比較，此溝槽狀排列的緩衝層，可增加與奈米二氧化鈦接觸面積，增加附著，降低電荷轉移阻抗，提高元件填充因子。此緩衝層亦可扮演抑制逆電流腳色，提高元件開路電壓。與未處理鈦基板或海綿結構緩衝層比較，此溝槽狀排列結構，可提高入射單色光子-電子轉換效率與短路電流，元件效率有顯著提升。測量此溝槽狀緩衝層鈦基板之紫外/可見光反射光譜，其反射光譜與N719染料之吸收光譜較一致，證明了溝槽狀緩衝層亦可做為背反射層，增加元件光利用率。最佳化溝槽狀緩衝厚度，鈦基板染料敏化太陽能電池最佳效率達7.28%。
太陽能電池研究除了提高效率，長效穩定性(long term stability)的提升也是研究太陽能電池的一大重點。本文利用電化學交流阻抗分析與掃描式電子顯微鏡分析，鈦基板染料敏化太陽能電池經過熱老化測試失效模式，鈦基板染料敏化太陽能電池經過熱老化，白金對電極劣化造成填充因子下降，是效率下降的主因。將白金對電極浸泡在電解液的個別組成成分，經過老化測試後，後利用循環伏安法(CV)分析期催化能力變化，發現碘(I2)或鋰離子(Li+)和水共存時，是造成白金對電極劣化的因素。

關鍵字

背照式結構 ； 老化分析 ； 染料敏化太陽能電池 ； 可撓式 ； 鈦基板

並列摘要

Dye-sensitized solar cell (DSSC) has received increasing interest, which achieved moderate conversion efficiency using low cost material and simple manufacturing apparatus. The high efficiencies of DSSCs have been achieve using TiO2 nanocrystalline fabricated on heavy, rigid, and expensive fluorine-doped tin-oxide (FTO) glass. Metal foil substrates enable extension of DSSCs to novel applications because they are thin, lightweight and flexible. Ti foil is an excellent alternative due to its superior physical and chemical properties.
Long-term stability and enhancement of conversion efficiency of DSSC are two important subjects for industrializing DSSCs. The recombination occurs at the Ti substrate/electrolyte is one of the factors that limit the conversion efficiency. In order to improve photovoltaic performance, it is essential to suppress the recombination loss at the Ti substrate. With back-illuminated construction, partial incident light is absorbed by counter electrode and electrolyte. The improvement of light harvesting efficiency is particularly important for Ti-based DSSC. This study introduced a feasible and efficient method to prepare blocking layer. The surface of Ti substrate could be transformed into TiO2 thin underlayer by direct oxidation method. The nature of Ti metal was utilized to fabricate underlayer, and no tricky coating process was required.
Introducing underlayer into photoelectrode could reduce the recombination with triiodide ion in the electrolyte. A sponge-like and conformal TiO2 underlayer was successfully fabricated by using hydrogen peroxide oxidation Ti foil. This underlayer serves as a charge recombination barrier layer at the nanocrystalline TiO2/substrate interface, and suppresses recombination reaction. This sponge-like TiO2 underlayer increases the electrical contact area between the Ti substrate and nanocrystalline TiO2 helping nanocrystalline TiO2 attach to the Ti substrate. This study compares the performance of DSSCs that were subjected to different Ti surface treatments. Electrochemical impedance spectroscopy results confirm that the proposed sponge-like TiO2 underlayer increased the open-current voltage (VOC) and fill factor (FF) due to prolonged electron life time (eff), and minimized resistance at TiO2/Ti interface (RCT).
By using hydrogen peroxide (H2O2) with a basic NH4OH agent, a thin TiO2 layer with a grooved structure was formed on Ti substrate, and the Ti substrate was textured. This grooved TiO2 thin layer also increased the electrical contact area at the nanocrystalline TiO2/Ti substrate interface, leading to reduced charge transfer resistance and improved fill factor (FF) of dye-sensitized solar cells. The TiO2 underlayer can also serve as a charge recombination barrier layer at the Ti substrate/electrolyte interface. Compared with DSSCs with non-treated and H2O2-treated Ti substrates, the DSSC with H2O2/NH4OH-treated Ti substrate showed increased conversion efficiency with a significant improvement in short-circuit current density (JSC). Reflection UV-vis spectroscopy and incident photon-to-current efficiency confirmed that the increased JSC was the result of a consistent reflection spectrum with Ru complex dye absorption. Surface modification by H2O2/NH4OH combined with optimized thickness of blocking layer and minimized gap in two electrodes achieved a high efficiency of 7.28 %.
The degradation mechanism of Ti substrate-based DSSCs was studied after a thermal aging test. The deteriorated component of Ti-based DSSCs was clarified by chemical impedance spectroscopy and scanning electron microscope. This indicated that an unfavorable reaction occurred on the Pt counter electrode, leading to a decrease of the fill factor. The device components, that is, counter electrode and electrolyte, were separated from the cell to trace the degradation factor. The factors for catalytic ability degradation of counter electrode were analyzed by cyclic voltammetry. These results indicate that I2 and Li+ coupled with water led to an unfavorable reaction on Pt counter electrode, and that water content in the electrolyte may accelerate Pt degradation.

並列關鍵字

back-illuminated type ； degradation analysis ； dye-sensitized solar cell ； flexible ； Ti substrate

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