二氧化鈦微結構對染料敏化太陽能電池光電效能的影響

Effect of TiO2 microstructure on the photovoltaic performance of dye-sensitized solar cells

蕭光宏(Guang-Hung Shiau)

指導教授 ： 簡淑華

國立臺灣大學/理學院/化學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.03017

摘要

在染料敏化二氧化鈦太陽能電池系統中，二氧化鈦奈米顆粒扮演著吸附染料及傳遞電子的角色。近年來研究學者指出適當地摻入次微米二氧化鈦粒子可增加入射光的散射，進而提升光利用率。近期的報告顯示添加一維奈米結構二氧化鈦除了可改善電子傳遞效率外，亦可增加入射光散射及光利用率。
在本研究中我們將商用二氧化鈦粉體Merck Anatase (MA) 置於強鹼溶液中以水熱法製得氧化鈦奈米管(Tnt)，並應用於染料敏化太陽能電池光陽極。結果顯示，以Tnt奈米管所製得的電極，在AM 1.5模擬太陽光照射下(100 mW/cm2)，其光電轉換效率為4.27%。將Tnt在不同溫度(450、550及650 ℃)鍛燒後發現，550 ℃鍛燒處理之樣品其光電轉換效率為三者間最佳(7.34%)。開環電壓衰退(OCVD)及電化學阻抗分析(EIS)顯示這是因為550℃鍛燒處理樣品擁有最低的表面陷阱密度及不錯的電子傳輸速度所致。
本研究利用三種不同形貌的二氧化鈦材料，分別為溶膠凝膠法所製備的奈米顆粒(SG)、Tnt奈米管及次微米顆粒MA，進行二氧化鈦電極微結構設計，製備出具高效率的染料敏化太陽能電池。實驗結果顯示，將Tnt奈米管和SG奈米粒子以不同比例混合所製得的電極，由於入射光在被染料吸收前已被Tnt散射，電池表現反而降低。改以層狀設計，在SG層上方塗佈上一層Tnt。可以有效將光電轉換效率從7.44%提升至8.65%。於Tnt層上添加具高散射性的次微米顆粒MA層製得的SG/Tnt/MA電池其光電轉換效率可提升至9.04%。
將奈米顆粒SG和Tnt及MA分別以等比例混合作為第二層和第三層電極設計出層狀-混合複合式二氧化鈦電極 SG/SG-Tnt/SG-MA/MA。入射單色光子-電子轉化效率(IPCE)測量實驗顯示，藉著奈米管Tnt以及次微米顆粒MA添加所設計的層狀電極，可使電池光電轉換效率在全波長區域皆有明顯增加。其中短波長區域的增加主因為Tnt的高表面積提供了額外的染料吸附量，而長波長區域的增加則是來自於MA的高散射特性提高了光的使用率。另外，開環電壓衰退和電化學阻抗分析結果皆顯示電子在SG/SG/Tnt/SG-MA/MA中的壽命增長。由於高光使用率及良好的電子傳遞性質，SG/SG/Tnt/SG-MA/MA電池可達9.36%的最佳光電轉換效率。

關鍵字：染料敏化太陽能電池、二氧化鈦微結構設計、氧化鈦奈米管、散射、電子傳遞

關鍵字

染料敏化太陽能電池 ； 二氧化鈦微結構設計 ； 氧化鈦奈米管 ； 散射 ； 電子傳遞

並列摘要

In dye-sensitized solar cell (DSSC) system, TiO2 nanoparticles function as dye adsorption and electron transport. Recently it has been reported that the appropriate addition of submicron TiO2 particles could increase the light-scattering and result in higher light-harvesting efficiency. Recent studies have also revealed that the addition of one-dimensional nanostructured TiO2 could not only improve the electron transport, but also enhance the light-scattering efficiency.
In this study, titania nanotube (Tnt), which was prepared from commercial Merck Anatase TiO2 powders (MA) by alkaline hydrothermal method, was applied to fabricate the DSSC photoanode. It was found that the Tnt-based DSSC exhibited a conversion efficiency of 4.27% under AM 1.5 simulated light irradiation (100 mW/cm2). When Tnt was calcined at different temperatures (450, 550 and 650 oC), the conversion efficiency of the sample calcined at 550 oC was the best (7.34%) among the tested samples. Open-circuit voltage decay (OCVD) and electrochemical impedance analysis (EIS) demonstrated that the better cell performance of 550 oC-calcined sample was attributed to its lower surface traps density and reasonable electron transport property.
TiO2 materials of three different morphologies, i.e., nanoparticle prepared by sol-gel method (SG), nanotube (Tnt) and submicron particle (MA), were utilized for microstructure design of TiO2 electrode to fabricate high-efficiency DSSC. The results showed that when SG was mixed with Tnt with various proportion, it resulted in poor cell performance. It was likely because the incident light was scattered by Tnt prior to the absorption by dye molecules. Layer-design was utilized instead of mix-design to coat a layer of Tnt onto SG layer. The conversion efficiency was elevated from 7.44% (SG-based DSSC) to 8.65% (SG/Tnt-based DSSC). After further coating of high-light-scattering MA layer onto Tnt layer, the conversion efficiency of the resulted SG/Tnt/MA-based DSSC was enhanced to 9.04%.
SG nanoparticles were mixed with Tnt and MA in equal proportion, and then used as the second and third layers, respectively, to fabricate hierarchical TiO2 composites electrode, SG/SG-Tnt/SG-MA/MA. Incident monochromatic photon-to-current efficiency (IPCE) measurements revealed that the utilization of the layered electrode designed by addition of Tnt and MA enabled the conversion efficiency substantially to increase in full wavelength spectrum. The enhancement of the short-wavelength region was mainly due to the supplementary dye adsorption provided by high-surface-area Tnt, and that of long-wavelength region was attributed to the higher light-harvesting efficiency enhanced by MA with high light-scattering property. Moreover, the analysis of OCVD and EIS also indicated that the electron lifetime in SG/SG-Tnt/SG-MA/MA was longer. Due to the enhancement of light-harvesting efficiency and improvement of electron transport property, the SG/SG-Tnt/SG-MA/MA-based DSSC has achieved optimum conversion efficiency of 9.36%.

Keywords: Dye-sensitized solar cell, Microstructure design of TiO2 electrode,
Titania nanotube, Light-scattering, Electron transport

並列關鍵字

Dye-sensitized solar cell ； Microstructure design of TiO2 electrode ； Titania nanotube ； Light-scattering ； Electron transport

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