奈米材料於染料及鈣鈦礦敏化太陽能電池之應用

Nanomaterials in Dye and Perovskite Sensitized Solar Cells

王映樵(Ying-Chiao Wang)

指導教授 ： 陳俊維

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.01811

摘要

本研究利用材料之特殊性質以獲得低成本與高效率之染料敏化太陽能電池。然而，染敏元件發展近二十年來，其光電轉換效率仍無法有效提升至可商業化的程度。所以，尋找更適合的吸光材料，便成為下一個重要課題。近年來，鈣鈦礦材料因同時具備多種成為光學主動層的優點，使得其元件效能可劇烈的提升至19%。其中，鈣鈦礦的結晶轉化程度更是決定元件效能的重要關鍵，本論文將探討元件中鈣鈦礦晶體的結晶型態。進一步，我們亦將引入奈米晶體添加劑提升鈣鈦礦材料的光電特性，以提供未來研究高效能之鈣鈦礦太陽能電池的依據。
第一部分，首先使用二硫化鐵(FeS2)，別名黃鐵礦。因為黃鐵礦的色澤呈閃閃發亮的金黃色，經常被誤認為是黃金，故俗稱『愚人金』。由於二硫化鐵在地球中含量豐沛、無毒且為低能帶隙的礦材，因此被科學家廣泛討論並視為下一世代有趣的光電材料之一。使用溶液旋轉塗佈法可製備溶膠狀之二硫化鐵奈米晶體，此低廉之奈米顆粒提供很大的潛力以發展二硫化鐵為基底的光伏打元件。近日，我們使用黃鐵礦奈米墨液來製作的光伏打應用設備。這個原料無毒且在大自然更容易取得，不僅能製造出更有經濟效益的對電極，亦能取代在染料敏化太陽能電池中較貴的白金電極。黃鐵礦奈米晶體也展現了非常好的電化學催化活性以及電化學穩定度。另外，以此室溫塗佈法來製備二硫化鐵對電極可應用於軟性基板(PET-ITO)；且因為二硫化鐵對電極呈現半透明，可在背面照光下得到比白金對電極更優越的元件效能。這項研究使得低成本的黃鐵礦二硫化物奈米晶體催化作用於染料敏化太陽能電池及其它電化學電池的相關應用更為清楚明顯。
再者，離子擴散速率差異導致再生染劑的障礙及工作電極/電解液界面的載子再結合現象為染料敏化太陽能電池中能量損耗的一大因素。一般而言，文獻上會以超薄的無機電子阻隔層以達到改善之目的。然而，這些表面覆蓋物會導致染料吸附量下降與阻礙電解液還原染料的機率。因此，我們利用溶液塗佈一具備雙功能之有機共聚物Poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))於含有染料之工作電極上。經修飾過後的工作電極除了有減少電子電洞再結合的功能，更因為P(VDF-TrFE)具備優良的導離性，而促進離子的擴散，進一步增進染料的再生機會。利用此簡易製程之廉價共聚物改善後的元件更因此提升了18.7%的元件效能。
第三部分，在有機無機混成鈣鈦礦組成太陽能電池的領域，因為光電轉換效率快速的躍進，迅速成為下一世代低成本太陽能電池具有潛力的吸光層。然而，高度結晶化的鈣鈦礦才能夠提供更高的吸光效能與形成更有效的載子傳輸媒介。在此，我們利用逐步沉積的技術來控制鈣鈦礦的晶體轉化率。過程中，利用XRD與XPS儀器監測，我們可以獲知PbI2與甲胺碘中解離出來的碘離子形成[PbI6]4-八面體。隨著甲胺碘含量增加，八面體成長為鈣鈦礦之tetragonal結構。晶體完整時，為了降低能量，甲胺陽離子會進入tetragonal的interstitial 位置內，並藉此達到高吸光能力的材料。由於，XRD只能看到tetragonal結構成型的情況，若能搭配XRD 縱深分析與二維GIXRD的綜合探討，可以獲知不同轉化率鈣鈦礦晶體之分布情形，更進一步可推斷初始之PbI2晶體排列將直接影響鈣鈦礦材料的晶體方向，日後亦更可精確控制鈣鈦礦的成長，與有效掌握元件之效能。
第四部分，我們利用化學方法置換二氧化鈦奈米柱之配體，因此在室溫下，鈣鈦礦前驅物即可均勻分散二氧化鈦奈米柱。反應後，可形成塊材混參之結構。此嶄新鈣鈦礦/二氧化鈦奈米柱塊材混參太陽能電池之光電轉換效率可達到12%。此一混參概念，除可提供溫度敏感型基板之應用，亦可引入工業製程以提供未來鈣鈦礦多層複雜結構之製備。
最後，我們將硫化鉛奈米晶體之配體以碘離子置換，用以混入鈣鈦礦前驅物溶液中，並進而改變鈣鈦礦材料的結晶行為。結晶過程中，硫化鉛奈米晶體表面的碘離子配體將與鈣鈦礦前驅物形成螯合反應。由於活化能的降低，鈣鈦礦晶體的反應性將被大幅提升。鈣鈦礦/碘離子配體亦將形成新的集中型晶種，成長時，鈣鈦礦晶體將有較大的成長空間。此一大晶粒之鈣鈦礦平面結構型太陽能電池存在高達76.3%之填充因子與16%之光電轉換效率。此大晶粒之鈣鈦礦使載子傳輸性質提升。為了瞭解晶粒大小與載子傳輸性質之關係，本研究利用時間解析光致螢光與雙極性載子傳輸方程式來分析鈣鈦礦材料的載子擴散長度。結果發現，因為大晶粒的鈣鈦礦單位長度內的晶粒邊界較少，使得載子在其中得以更加順利的擴散至電極，所以具有較長的電子與電洞擴散長度。根據此研究結果，未來可以經由設計適合之奈米晶種，以提升鈣鈦礦之材料性質與提高元件效率。

關鍵字

染料敏化太陽能電池 ； 鈣鈦礦太陽能電池 ； 二硫化鐵 ； P(VDF-TrFE) ； 二氧化鈦奈米柱 ； 硫化鉛

並列摘要

First, we present the colloidal pyrite FeS2 nanocrystals (NCs), which are abundant in nature and nontoxic, have attracted attention for developing low-cost fabrications of photovoltaic (PV) devices using solution processes. This section demonstrates an important PV application using FeS2 nanocrystal pyrite ink to fabricate a cost-effective counter electrode (CE) to replace the expensive Pt counterpart in dye-sensitized solar cells (DSSCs). FeS2 NC ink has exhibited excellent electrochemical catalytic activity and remarkable stability and showed a promising power conversion efficiency (PCE) comparable to that using a Pt CE. Solution-processable and semitransparent FeS2 NC-based CEs also enable the fabrication of flexible and bifacial DSSCs. The results indicate that earth-abundant FeS2 NC ink is an extremely interesting candidate for replacing the precious metal of Pt for employing the iodide/triiodide redox couples, which can substantially lower the cost of DSSCs in future commercial applications.
Next, the impedance of interception of the oxidized dye (S+) by electron donors in the electrolyte, and recombination of the electron in the dye-adsorbed mesoporous electrode with S+ or electrolyte species have been identified as the main cause of energy loss in DSSCs. Generally, an ultrathin inorganic electron blocking material surrounding working metal oxides is required to inhibit their recombination and further promoted electron-transfer reactions. However, the surface passivation interlayers would decrease adsorption of the dye resulting in reduces the interface between the dye molecules and semiconductors or decreases the quantum efficiency for electron injection, which all led to a reduced photocurrent. Here, we demonstrate an important PV application using a dual functional poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer deposited onto the dye penetrant working electrode (WE) by a solution-processed method. The WE after introduced the conformal P(VDF-TrFE) interlayer have both ability of reducing carrier recombination and facilitating ionic mobility, therefore, the further enhancement of 18.7% PCE in DSSCs. These results indicate that the cost-effective P(VDF-TrFE) copolymer is an extremely interesting candidate for promoted dual functions of electron collection efficiency and S+ regeneration rate, which can substantially higher the efficiency of DSSCs in future commercial applications.
In Chapter 5, organometal halide perovskite materials were identified as promising light harvesters to achieve rapidly boosted performance, providing great potential for developing low-cost next-generation photovoltaic devices. The highly crystalline perovskite is required either to absorb most of the sunlight or deliver efficient charge transport pathways for photogenerated carriers. Here, we use a sequential deposition technique for prepared perovskite crystals under various conversion ratios to demonstrate the mechanisms of an extended three-dimensional network of corner-sharing [PbI6]4- octahedral and then filled the methylammonium (MA) to 12-fold iodide coordinated interstitial sites among the octahedral by X-ray diffraction (XRD) spectrum and X-ray photoelectron spectroscopy (XPS), respectively, during crystal growth. Furthermore, the vertical distributions of morphology and crystal structure have important implication for analyzed depth profile of the perovskite structures using the XRD depth profiles and two-dimensional GIXRD spectra measurement. These results indicate that through clearly realized material engineering, and the most significant differences in efficiency are attributed to whether enhances transformation of perovskite by the orderly built the inorganic frameworks and completely inserted the organic molecules.
Furthermore, to replace high-temperature sintered scaffold materials in conventional CH3NH3PbI3-based solar cells, this study demonstrates a new device structure of a bulk intermixing (BI)-typed CH3NH3PbI3/TiO2 nanorods (NRs) hybrid solar cell, where dispersed TiO2 NRs from chemical synthesis are intermixed with the perovskite absorbing layer to form a BI-typed perovskite/TiO2 NRs hybrid for device fabrication. Through interface engineering between TiO2 NR surface and the photoactive perovskite material of CH3NH3PbI3 by ligand exchange treatment, a remarkable power conversion efficiency (PCE) of over 12% was achieved based on the simple BI-typed CH3NH3PbI3/TiO2 NR hybrid device structure. The proposed hybrids not only provide great flexibility for deposition on various substrates through spin coating at low temperatures but also enable layer-by-layer deposition for future development of perovskite-based multi-junction solar cells.
Finally, the utilization of iodide ligand assisted lead sulfide nanocrystal (PbS/I-) as the seeds for heterogeneous-nucleation in perovskite solar cells is demonstrated. Through interface engineering between PbS nanocrystal surface and the perovskite material of CH3NH3PbI3Cl3-x as a result of improvement crystallinity of the perovskite film and further formed large grain sized morphology by ligand exchange treatment, a remarkable power conversion efficiency of 16% was achieved. Both electron and hole diffusion length of large grain perovskite are longer than the pristine sample, indicated that the smaller trap densities in the large grain sized perovskite crystals. Therefore reduced charge transfer resistance across the perovskite material that growth from PbS/I-, so that achieved the higher fill factor and short circuit current density. Our results indicate that PbS nanocrystal could be a simple solution-processable introducing to perovskite precursor solution as the nuclei and multidentate chelation ligands in perovskite solar cells.

並列關鍵字

Dye-sensitized solar cells ； perovskite solar cells ； iron disulfide ； P(VDF-TrFE) ； TiO2 nanorods ； lead sulfide

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