應用製做在透明基材上二氧化鈦奈米管陣列於染料敏化結構太陽能電池之研究

Abstract

本論文研究利用電化學陽極氧化法製備大面積的二氧化鈦奈米管陣列薄膜，並透過轉移奈米管陣列在不同透明基材上製作光電極完成具有正面入射結構的奈米管染料敏化太陽能電池。 首先，分別探討使用二氧化鈦奈米粒子與二氧化鈦溶膠兩種方式轉移奈米管陣列至導電玻璃以製作染敏電池之元件特性。使用奈米粒子連結奈米管形成之光電極結構需要以四氯化鈦後處理增加奈米管表面積同時降低與奈米粒子接觸電阻，並應用CF4/CHF3電漿去除奈米管底部阻障層後才能有效提昇光電轉換效率；相對地，使用二氧化鈦溶膠可直接在奈米管與導電玻璃間形成低阻抗接面，能更有效地完成奈米管的載子收集；應用CF4/CHF3電漿可於短時間內清除奈米管表面收束、破損的結構，可顯著降低二氧化鈦溶膠與奈米管接面阻抗並提高載子收集效率。 接著，本研究進一步觀察在正面入射結構下二氧化鈦奈米管形貌對於染敏電池中載子傳輸特性的影響，使用450C熱處理結晶條件時，N719染敏電池在奈米管長度約為19微米時具有最高的光電轉換效換，在低揮發性3-甲基氧丙腈電解液系統下，奈米管口徑為95奈米時最高可得到約13 mA cm-2光電流密度。透過370C的熱前處理，在稀釋鹽酸溶液中，可在不影響奈米管長度下調整管壁厚度，當奈米管壁減薄時，染敏電池中電解液離子擴散阻抗下降，並且由於底部阻障層的溶解，因此提高奈米管底層二氧化鈦溶膠連結層中離子收集效率，而得到光電流進一步提昇至14 mA cm-2以上。 另一方面，經過400C以上的熱前處理，可提高二氧化鈦奈米管陣列結構強度，因此得以在240C, 高壓的水熱環境下，在保持銳鈦礦晶相時提高奈米管的結晶性質，得到比熱處理結晶條件更高的光電流密度與光電壓，將整體電池效率提高至7%，並且透過x光光電子能譜分析得知可透過改變水熱處理酸鹼條件能夠有效控制奈米管表面氫氧基的比例，提高表面氫氧基在染敏結構下更有利於染料的吸附，因此有效降低載子的再復合速率。 接著研究透過直接在奈米管陣列阻障層上沉積低電阻透明導電薄膜之方法，完成在低溫下轉移奈米管陣列到透明軟性基材。在應用此類型光電極於染敏電池中，經過水熱處理之奈米管陣列，可有效降低染敏電池中透明導電薄膜與電解液間載子的再復合，因此提昇塑膠染敏電池充填因子，最高得以達到5.41%之能源轉換效率。未來如能沉積具高溫及化學穩定性透明導電薄膜，預期可達到接近玻璃基板上相同的能源轉換效率。 以陽極氧化法製備的奈米管陣列具有高度的一維準直特性，在太陽能電池的應用上有利於光激發電子的收集，製做在透明基材上時，必需減低奈米管與透明導電層間的阻抗才能有效地提高奈米管內載子的收集效率。並且透過側向蝕刻的方式改變奈米管陣列間隙，可降低離子擴散阻抗提高電洞的收集，同時在也能較有利於電洞傳輸材料的充填，而在高壓水熱條件下能夠進一步增進奈米管的結晶及表面化學性質，應用上述方法能夠有效地提高奈米管染敏電池的光電特性，未來如能結合高光吸收係數材料及高載子遷移率、高穩定性的電洞傳輸材料，將能完成更高效率、穩定可靠的太陽能發電元件。

This dissertation reports researches on technique of transferring TiO2 nanotube-arrays to transparent substrates and the photovoltaic results of dye-sensitized solar cells (DSSC) utilizing these transferred nanotubes. The first part studies two different bonding mediators, TiO2 nanoparticles and TiO2 gel, to transfer the nanotubes to a conductive glass and compares the resultant performances of the DSSCs. Different post treatments were required to enhance the charge collection of the nanotubes with the different bonding methods. For the nanoparticle-bonded nanotubes, bonding of the nanotubes with inverted orientation resulted in high performance when the barrier layer on top of the nanotubes was fully opened by CF4/CHF3 plasma in which TiCl4 treatment was additionally applied to reduce the contact resistance between the nanoparticles and the nanotubes. Bonding of the nanotubes with the TiO2 gel was achieved with the closed ends of the nanotubes, and oppositely the CF4/CHF3 plasma was applied on the opened tops to clean the surface bundles. This step is of great importance on the charge collection and on the enhancement of the efficiency of DSSCs. The second part examines the geometrical properties of the nanotubes and their influences on the front-side illuminated nanotube DSSCs. With thermally crystallized nanotubes of ca. 19 μm, the DSSC using a low-volatile electrolyte, 3-methoxypropionitrile depicted a higher energy conversion efficiency; the highest photocurrent density of ca. 13 mA cm-2 was observed when the pore size of nanotubes was of ca. 95 nm. Further enhancement of the performance of the nanotube DSSCs was identified as the nanotubes accepted a post sidewall trimming process. The sidewall thickness and the voids among the nanotubes were changed by a 370 C calcinations followed by HCl dissolution. Due to the enlarged voids and the dissolved barrier layer, the ion collection within the bonding layer became more efficient and led to an increase on the photocurrent density more than 1 mA cm-2. A hydrothermal method for crystallization of the nanotubes is introduced in the third part. For the nanotubes with a pre-sintering over 400 C, the dissolution and the collapse of the nanotubes under high pressure hydrothermal conditions were found to be effectively inhibited so that the nanotubes could be hydrothermally crystallized with higher crystallinity of pure anatase. Besides, an influence on the surface hydroxyl coverage of the nanotubes, which related to the anchoring of dye molecules, was identified on the hydrothermal pH level. By optimizing the hydrothermal conditions, the performance of the nanotube DSSCs was enhanced to be over 7%. By directly depositing a low resistance transparent oxide onto the closed tops of the nanotube-array, the transferring of the nanotubes onto a flexible material for an efficient DSSC is demonstrated in the fourth part. Different types of deposition methods and post treatments of the nanotubes were performed. By the hydrothermal treatment, the reduction of possible charge loss from the transparent oxide to the electrolyte in the DSSCs could be efficiently inhibited such that the fill factor of the flexible nanotube DSSCs could be improved and the power conversion efficiency reached over 5%. In summary, an enhanced charge collection can be achieved in the nanotube DSSCs by modifying the sidewall and applying the hydrothermal crystallization method. Lowering the interfacial impedance between the nanotubes and the transparent conductive layer is the most important issue for efficient charge collection of the front-side illuminated nanotube DSSCs. With the optimized nanotube-array, an efficient, stable and full-solid-state photovoltaic system can be expected by integrating high absorbing materials to the one dimensionally ordering nanostructure, which will facilitate the post deposition of solid materials.