奈米碳材作為染敏化太陽能電池對電極之研究

Carbon Nanomaterials as Counter Electrodes for Dye-sensitized Solar Cells

楊秉豪(Bing-Hao Yang)

指導教授 ： 謝建德

元智大學/工程學院/化學工程與材料科學學系/碩士(2011年)

https://doi.org/10.6838/YZU.2011.00157

摘要

本論文為奈米碳材作為對電極對染敏化太陽能電池之探討，大致可分為三個部分。第一部分是以一維及二維奈米碳材作為對電極之探討；第二部分是以介瀝青碳微球為主體之複合碳材作為對電極之探討；第三部分是以不同氧化程度之石墨烯作為對電極之探討。碳電極之特性分析則藉由X-ray粉末分析儀、場發射掃描式電子顯微鏡、高解析穿透式電子顯微鏡、拉曼光譜儀、太陽光模擬器及入射光子轉換效率測量儀進行分析。以下為各部分摘要分別敘述如下：

1. 以一維和二維奈米碳材作為對電極
本研究擬以製備一維奈米碳管(1-D carbon nanotubes, CNTs)及二維石墨烯奈米片(2-D graphene nanosheets, GNs)之碳電極應用於染敏化太陽能電池上。由X-射線繞射光譜圖及拉曼分析光譜圖，可發現一些不完整缺陷或表面氧化物附著於奈米碳管側壁與管口兩端及石墨烯奈米片狀結構邊緣。相較於GN對電極，CNT對電極展現更好的入射光子轉換效率及能量轉換效率，此歸因於CNT組成之網絡及MCMB底層結構，有利於催化氧化還原反應、染料分子吸附及提供一維電荷轉移路徑。此CNT/MCMB複合碳材結構作為對電極展現出取代貴重金屬Pt對電極之可能性。

2. 以介瀝青碳微球為主體之複合碳材作為對電極
本研究擬以介瀝青碳微球(mesocarbon microbead, MCMB)為主體之複合電極應用於染敏化太陽能電池上。三種奈米材料鉑、奈米碳管及碳黑(carbon black, CB)的添加皆均勻分散於MCMB表面。在複合電極之設計上，MCMB提供一個優異的電荷轉移平台，使電子從ITO玻璃傳導至染料分子，而Pt、CNT及CB奈米材料的附著，提供大量的觸媒活性位置進行氧化還原反應。此複合電極中，CB複合電極之染敏化太陽能電池，不僅提供大量的觸媒位置，還擁有低電荷轉移之阻抗，促進了氧化還原反應。此類型之複合電極展現出具有取代Pt對電極應用於染敏化太陽能電池之可能性。

3. 以不同氧化程度之石墨烯作為對電極
本研究擬以製備不同氧化程度之二維石墨烯對電極應用於染敏化太陽能電池上。利用熱脫附來調整吸附於石墨烯平面及邊緣上之氧官能基濃度。利用熱脫附來調整吸附於石墨烯平面及邊緣上之氧官能基濃度。經由循環伏安(Cyclic voltammetry, CV)及電化學阻抗譜(electrochemical impedance spectroscopy, EIS)測試可證明隨著O/C原子比下降，GN對電極展現出高觸媒活性於I3￣/I￣之氧化還原反應及低電荷轉移阻抗。GN對電極之染敏化太陽能電池相較於不同氧化程度之GO對電極，明顯提升了入射光子轉換效率及能量轉換效率，其電池效能的增進歸因於GN為二維sp2碳結晶結構，其作為一個半金屬或零能隙之半導體，具有卓越的電子遷移率。

關鍵字

奈米碳材 ； 複合碳材 ； 對電極 ； 染敏化太陽能電池 ； 能量轉換效率

並列摘要

This dissertation can be qualitatively divided into three parts, (i) influence of 1-D carbon nanotubes and 2-D graphene nanosheets carbon, (ii) mesocarbon microbead-based and (iii) graphene nanosheet with different oxidation levels as counter electrodes for dye-sensitized solar cells. The resulting carbon counter electrodes were characteried by XRD, FE-SEM, HR-TEM, Solar simulator and IPCE test.

(i) 1-D carbon nanotubes and 2-D graphene nanosheets carbon as counter electrodes

This study examines the dye-sensitized solar cells (DSSCs) equipped with 1-D carbon nanotubes (CNTs) and 2-D graphene nanosheets (GNs) carbon counter electrodes. Imperfect defects were attached to the sidewall or both the ends of the CNTs, and the edges of the GNs were analyzed by X-ray diffraction and Raman spectroscopy. When compared with the GN-based counter electrode, CNT-based counter electrodes showed a better improvement in the incident photon-to-current efficiency and power conversion efficiency of the cells. This enhancement of cell performance can be attributed to the combination of CNT network and spherical graphite bottom layer, favoring dye adsorption, catalytic redox activity, and 1-D charge-transfer path length. Such carbon configuration as counter electrode provides a potential feasibility for replacing metallic Pt counter electrodes.

(ii) Mesocarbon microbead-based as counter electrodes

The dye-sensitized solar cells (DSCs) equipped with mesocarbon microbead (MCMB)-based counter electrodes were explored to examine their cell performance. Three types of nanosized additives including platinum, carbon nanotubes (CNTs), and carbon black (CB) are well dispersed and coated over microscaled MCMB powders. In the design of the counter electrodes, the MCMB graphite offers an excellent medium that allows charge transfer from the ITO substrate to the dye molecule. The active materials such as Pt, CNT, and nanosize CB act as an active site provider for the redox reaction. Among these counter electrodes, the DSCs fabricated with CB electrode exhibits the highest power conversion efficiency. This improved efficiency can be attributed to the fact that the CB nanoparticles not only offer a large number of catalytic sites but also low charge transfer resistance, facilitating a rapid reaction kinetics. Such design of carbon counter electrode has been confirmed to be a promising candidate for replacing Pt electrodes.

(iii) Graphene nanosheet with different oxidation levels as counter electrodes

This study examines the performance of dye-sensitized solar cells (DSCs) equipped with graphene nanosheet (GN) counter electrodes with different oxidation levels. A thermal deposition is adopted to adjust O/C atomic ratio and surface oxygen functionalities on graphene sheets. With decreasing the O/C ratio, the GN electrode displays high catalytic activity toward the I3￣/I￣ redox reaction and lower charge-transfer resistance, analyzed by cyclic voltammetry and electrochemical impedance spectroscopy. The DSC fabricated with GN counter electrode also offers an improved incident photon-to-current efficiency and power conversion efficiency, in comparison with that equipped with graphene oxide electrodes. This improvement of cell performance could be attributed to the fact that the GN with 2-dimensional crystal of sp2 carbon and π electrons, acts as a semi-metal or a zero-bandgap semiconductor with remarkable high electron mobility.

並列關鍵字

power conversion efficiency ； Carbon nanomaterials ； carbon composite material ； Carbon counter electrode ； Dye-sensitized solar cells

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