以電泳法加壓製備可撓染料敏化太陽能電池

Fabrication of Flexible Dye-Sensitized Solar Cells by the Electrophoretic Deposition and Compression Method

陳信偉(Hsin-Wei Chen)

指導教授 ： 何國川

國立臺灣大學/工學院/化學工程學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.00042

摘要

本論文分為三大部分，主要探討利用以導電塑膠基材，搭配低溫製程製備可撓式染料敏化太陽能電池，分別探討了光電極、對電極以及膠態電解液。
第一部分為利用電泳沉積法製備出介孔洞的氧化鋅以及二氧化鈦商用型顆粒薄膜於導電塑膠基材上作為光電極。首先使用超音波震盪，將商用的氧化鋅粉體(ZnO，20 nm)進行均勻分散於異丙醇(Isopropanol)之中，接著固定電泳沉積之兩極間距，利用電泳沈積法中之定電壓程序控制氧化鋅顆粒沈積到導電塑膠基材上，並且經由不同之沉積時間控制薄膜的厚度，接著使用加壓後處理法進一步強化薄膜顆粒間的連結性。並利用電子顯微鏡觀察加壓後處理法前後，薄膜表面的差異，且最佳化薄膜厚度與加壓之壓力並搭配紫外光臭氧處理，並達到最佳光電轉換效率4%。隨後進一步將其改為商用的二氧化鈦顆粒(P25，25 nm，Degussa)，在普遍的文獻中，使用二氧化鈦取代氧化鋅將有助於提升光電轉換效率，故運用相同的製程方法製備出二氧化鈦薄膜電極，並經由電化學阻抗(EIS)搭配雷射暫態壓降進行元件分析，探討薄膜經過加壓後處理法之顆粒間的連結性與光電轉換效率的關係，最後得到光電轉換效率4.4%。由於使用導電塑膠作為可撓式染料敏化太陽能電池的基材，因此所有的製程都必須低於150 oC，以免造成導電塑膠的毀損或變形。故接下來便引進可耐高溫之金屬鈦板取代原本的塑膠基材，在原有電泳沈積法中加入了高溫鍛燒的製程，並同時最佳化高溫鍛燒之燒結溫度，此步驟將能進一步減少二氧化鈦顆粒中的晶格邊界(grain boundary)，且能使電子在二氧化鈦薄膜中的傳導更加順利，並能降低電子再結合反應的發生，以達到提升光電轉換效率的目的。由於使用不透光的金屬基材，之後量測元件光電轉換效率的入射光必須要從半透明的白金對電極方向照射，而成為背照式元件。另外，因鈦板導電度相比導電塑膠甚佳，故我們也同時最佳化電泳製程並探討顆粒沉積的詳細機制與原理，最後達到了光電轉換效率6.5%。
第二部分為使用自行合成之中孔洞二氧化鈦顆粒(Mesoporous titanium dioxide nanoparticles，MTN)取代商用二氧化鈦顆粒。直徑約為200 nm的中孔洞二氧化鈦顆粒是由粒徑大小約為20 nm的二氧化鈦顆粒所組成的，故其擁有相當大的比表面積，且其又具有大顆粒(>150 nm)的散射能力，因此不僅能吸附更多的染料分子在其二氧化鈦顆粒上，也能提升光在薄膜中被染料吸收的機會。另外，其晶相均為銳鈦礦型(Anatase)且其晶相缺陷(Grain boundaries)相對於商用二氧化鈦小顆粒來較少，這些皆有助於幫助電子在薄膜中的傳輸效率，而能有效提升元件的光電流密度。我們利用電泳法加壓製備中孔洞二氧化鈦顆粒的光電極，同時探討其電泳沉積參數及搭配最佳化元件效能，並使用電化學阻抗分析法與全波段光譜之光電轉換效率進行元件分析，最後達到光電轉換效率5.5%。隨後，為進一步提升轉換效率，我們將奈米金顆粒參雜於中孔洞二氧化鈦顆粒(Au@MTNs)之中，並且合成含有不同奈米金顆粒重量百分率的中孔洞二氧化鈦顆粒，並將其製備成光電極。由於奈米金顆粒擁有表面電漿效應，故我們藉由紫外光可見光(UV-Vis)吸收/反射光譜與全波段光譜之光電轉換效率(IPCE)，探討其對於元件輸出效率的影響，最後達到5.62%的光電轉換效率。
第三部分，主要是使用商用低溫燒結型二氧化鈦漿料，利用刮刀塗佈法搭配低溫燒結進行塑膠基材二氧化鈦薄膜光電極的製備。在此部分將著重於含有中孔洞二氧化矽的膠態電解液，以及一維針狀的硫化鈷塑膠基材對電極之探討。對於膠態電解液部分，我們將中孔洞二氧化矽顆粒與常用含有碘的有機液態電解液進行均勻混合，並且利用EIS、電化學氧化還原掃描法(CV)及UV-Vis反射光譜搭配IPCE分析與探討。在硫化鈷塑膠基材對電極方面，我們使用了化學液相沉積法(Chemical bath deposition，CBD)搭配陰離子置換法進行製備，並且利用X光散射圖譜(XRD)、光電子能譜(XPS)搭配掃描式(SEM)與穿隧式(TEM)電子顯微鏡進行材料的薄膜分析與鑑定，另外也使用CV及EIS進行此種一維針狀硫化鈷對電極的分析與探討。

關鍵字

加壓法 ； 染料敏化太陽能電池 ； 電泳沉積 ； 中孔洞材料 ； 塑膠基板

並列摘要

There are three parts in this dissertation, with particular attention paid to the low-temperature process for the fabrication of the plastic based dye-sensitized solar cells (DSSCs). It is mainly focused on the photoanode, counter electrode, and gel-electrolyte.
For the first part, we focus on the fabrication of photoanode on the conductive plastic substrate using commercially available ZnO and TiO2 nanoparticles by the electrophoretic deposition with compression method. We use the sonication to uniformly disperse the commercial ZnO (20 nm) nanoparticles (NPs) in isopropyl alcohol (IPA), followed by electrophoretic deposition (EPD) at a constant voltage. To control the thickness of the film, we control the deposition period during the EPD process. After the EPD process we then use the compression post-treatment to further strengthen the connection among the particles. Besides, we observe the surface difference of the thin film before and after the compression post-treatment by the scanning electron microscope (SEM) and optimize the film thickness, the compression pressure and the UV ozone treatment, finally reaching a photoelectric conversion efficiency of 4%. In most literatures, the photoelectric conversion efficiency of TiO2 based DSSCs is higher than ZnO based; therefore, we further replace the ZnO NPs by commercial TiO2 NPs (P25, 25 nm, Degussa). By using the same fabrication process to prepare TiO2 thin film electrodes, we further exam the relationship between the compressed particles connection and the photoelectric conversion efficiency by using the pulse laser open-circuit voltage (VOC) decay method and the electrochemical impedance spectroscopy (EIS). Finally we reach the photoelectric conversion efficiency of 4.4%. Due to the conductive plastic substrate, all the process must be carried out below 150 oC to prevent the damage or deformation of the substrates. Therefore, we further introduce the metal titanium (Ti) sheets to replace the conductive plastic substrates, and the Ti sheet can endure the high temperature calcination process after the electrophoretic deposition and the compression post-treatment. We further optimize the calcination temperature, and it then further reduce the grain boundaries in the TiO2 film results in increasing the conduction and making the electrons transport more smoothly. In addition, it reduces the reactions of the electrons recombination resulting in increasing the photoelectric conversion efficiency. However, the illumination of the devices should come from the platinum counter electrode (CE) side due to the opacity of the Ti sheets results in the back-illuminated devices. In addition, the conductivity of the Ti is much higher than the conductive plastic; therefore, we also optimize the EPD and explore the mechanism and principle of the EPD process. Finally, we get the photoelectric conversion efficiency of 6.5%.
For the second part, we use the synthesized mesoporous titanium dioxide nanoparticles (MTNs) to replace the commercial TiO2 NPs. The MTNs is composed of 20 nm TiO2 NPs and approximately 200 nm in diameter, and it has a large specific surface area with the large particles (> 150 nm) scattering property. Therefore, the MTNs based photoanodes can not only absorb more dye molecules on the TiO2 surface, but it can also enhance the light absorption in the film. Besides, the crystal phase of the MTNs are all anatase and the grain boundaries are less than the commercial TiO2 NPs, which is beneficial for the transportation of the electrons in the TiO2 film resulting in effective increasing the photocurrent density of the devices. Therefore, we fabricate the MTNs based photoanodes by electrophoretic deposition and compression post-treatment technique on the conductive plastic substrate. In addition, we also examine the parameters of EPD, analyzing the devices by EIS and the incident photon to current conversion efficiency (IPCE) at different wavelengths. Finally, we reach the photoelectric conversion efficiency of 5.5%. Since, to further increase the conversion efficiency of the MTNs based DSSCs, we combine the Au NPs with the MTNs (Au@MTNs), and we fabricate the MTNs based photoanodes with different wt% of gold NPs. The Au@MTNs have plasmonic effects due to the nanometer size gold NPs in the MTNs, we further analyze the Au@MTNs based photoanode by UV-vis absorption/reflection spectra and the IPCE of the devices. The photoelectric conversion efficiency of 5.62% is obtained.
For the third part, we use the doctor-blade coating technique with the commercial binder-free low temperature sintering TiO2 paste to prepare the plastic based TiO2 photoanodes. In this part, we focus on the silica based gel-electrolyte containing the mesoporous silica nanoparticles (MSNs), as well as the one-dimensional (1D) hollow acicular cobalt sulfide plastic based CEs. For the silica based gel-electrolyte part, we uniformly mix the MSNs with the commonly used organic liquid electrolyte containing iodine redox couples. We use the EIS and the cyclic voltammetry (CV) to analyze the electrolyte, and we also use the UV-Vis reflection spectra and the IPCE to observe the scattering effect of the MSNs in the electrolyte. For the 1D hollow acicular cobalt sulfide plastic based CEs, we use the chemical bath deposition (CBD) with an anion exchanging method to fabricate the CEs. Besides, we use the X-ray deflection spectra (XRD), the photoelectron spectroscopy (XPS), the scanning electron microscopy (SEM) and the tunneling electron microscopy (TEM) to analyze and identify the cobalt sulfide CEs. In addition, we also use the CV and EIS analysis to exam the 1-D hollow acicular cobalt sulfide CEs.

並列關鍵字

Compression post-treatment ； Dye-sensitized solar cells ； Electrophoretic deposition ； Mesoporous materials ； Plastic based

參考文獻

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