聚（3-己基噻吩）/硫化鎘複合物用於異質接面太陽能電池上之合成與特性研究

Abstract

太陽能電池的種類繁多，若以材料種類分類，可以分成：矽太陽能電池、無機化合物太陽能電池、染料敏化太陽能電池、有機太陽能電池等。其中是以單晶矽太陽能電池效率最高，但是其製作成本高，不符合經濟效益。而有機太陽能電池的 在能源危機的世代，研究出適合的再生能源是當務之急。而再生能源中又以太陽能電池受到大眾矚目。在太陽能電池領域中，矽太陽能電池一直是主要的發展對象。雖然它的轉換效率相當的高，但是價格卻不便宜且需要耗費大量能量來製造。因此發展大面積、低成本和低耗費量能的技術就顯得越來越重要。近年來，一種由有機物和無機物所組成的太陽能電池開始被大量的研究，其優點在於製作成本低，只需旋轉塗佈機便可以塗佈上作用層，再用加熱板加熱結晶，屬於低溫製程。造價低加上厚度薄，若是用於塑膠基板上變成可橈式太陽能電池，十分具有實用價值。因此在本論文主要在探討此一類型的太陽電池。 聚（3-己基噻吩）是目前最被廣泛被使用，因為其具有適合的能帶寬，高吸收係數和電洞的傳導係數。在本論文中，聚（3-己基噻吩）/硫化鎘複合物的合成，光學特性與太陽能電池元件的應用是主要的探討的部分。本論文首先利用水溶液法合成氧化鋅奈米柱，接著包覆一層氧化鎂，再利用熱處理得到氧化鎂摻雜的氧化鋅奈米柱。其光學和結構特性以X光繞射儀、穿透式電子顯微鏡、光激發光譜來鑑定。接著合成不同直徑的硫化鎘量子點修飾在氧化鋅奈米柱上再灌入聚（3-己基噻吩）製成太陽電池元件，發現因為量子侷限效應造成能階位移而促使電子躍遷變容易所以元件的效率升提4倍相較於沒有量子點的元件。然而雖然效率獲得提升，但是仍是太低，所以我們發展in-situ方法合成聚（3-己基噻吩）/硫化鎘複合物，在此方法中我們利用聚（3-己基噻吩）的分子鏈作為模板，藉由改變實驗條件合成出不同長寬比的單晶硫化鎘，接著探討其太陽能轉換效率，發現轉換效率隨著長寬比而增加，當長寬比條件為16時可得最佳效率達到2.9%。另外，我們也開始經由熱處理來研究聚（3-己基噻吩）和硫化鎘複之間的作用力和元件效率之間的關係。我們從聚（3-己基噻吩）/硫化鎘複合物的紫外光-可見光吸收光譜中發現藍位移且隨著長寬比增加而增加。此外聚（3-己基噻吩）/硫化鎘複合物的熱性質也會受到長寬比影響。因此可以推論硫化鎘的存在會減少聚（3-己基噻吩）的結晶度，而結晶度是可以經由熱處理得到回復，因此轉換效率有隨著熱處理的溫度和時間增加明顯的提升，然而過久的熱處理使得硫化鎘過度的聚集而造成轉換效率的衰退，而且此現象隨到長寬比減少而更明顯。從核磁共振光譜得知聚（3-己基噻吩）分子鏈和硫化鎘之間的作用力隨著長寬比而增強也因此因造成硫化鎘的聚集速率較慢，而使得元件效率對熱的穩定性較佳。 最後我們嚐試利用三維的電極取代平面電極來提升轉換效率，因此我們利用噴霧裂解法在氧化鋅奈米柱上製備具備低電阻率與高可見光穿透率的含氟氧化錫透明導電薄膜。由結果得知，當奈米柱的長度增長時，轉換效率隨著提升，但當長度長於320 nm，高分子會有填充於奈米柱間隙不完全，造成轉換效率衰退。當使用三維的電極時對具有較長長寬比的硫化鎘而言會有填充於奈米柱間隙的問題，因此最佳的效率為2.6%當長寬比條件等於4。

Conventional solar cells were built from inorganic materials such as silicon. Although the efficiency of such conventional solar cells is high, very expensive materials and energy intensive processing techniques are required. The need to develop and deploy large-scale, cost-effective, renewable energy is becoming increasingly important. In recent years, a hybrid solar cell consisting of a combination of both organic and inorganic materials has achieved good power conversion efficiencies (PCE), which will become the focus in this thesis. A series of P3HT/CdS composites have been synthesized to study the photovoltaic characteristics with ZnO nanorod arrays. First of all, we synthesize highly arrayed ZnO nanorod arrays via chemical process and attempt to fabricate MgO-doped ZnO nanorod arrays. The MgO-doped behavior and PL properties of single-crystal ZnO nanorods were investigated in terms of the annealing temperatures. Second, we evaluate an ordered organic-inorganic solar cell architecture based on CdS QD-decorated ZnO nanorod arrays encased in the hole-conducting polymer P3HT. A photovoltaic device based that has been decorated with CdS QDs yields power conversion efficiency over 4 times greater than that for a similar device without CdS QDs. The best device yields a short circuit current density of 1.38 mAcm-2 under Air Mass (A.M.) 1.5 illumination (100 mW cm-2), resulting in a power conversion efficiency of 0.21%. Third, we developed a novel method used to synthesize CdS single-crystal nanorods directly in the presence of conjugated polymer poly(3-hexylthiophene- 2,5-diyl) (P3HT), where the P3HT is acting as a molecular template for geometrical manipulation of CdS nanocrystals and in the meantime, as an efficient charge conductor in composite form. The mechanism of in-situ growth of high-aspect-ratio CdS nanorod is proposed based on spectroscopic analysis. A considerably improved PL quenching was detected for the nanorods and suggested a result of electronic coupling between the high-aspect-ratio CdS nanorods and the conducting polymer matrix. A photovoltaic device consisting of CdS nanorods with aspect ratio (AR) of ca. 16 and the conjugated polymer poly-3(hexylthiophene) was well assembled and showed a power conversion efficiency of as high as 2.9% under A.M. 1.5 Global solar conditions. We found that the interaction between polymer chains and the CdS nanocrystals increased with the aspect ratio and thus the optical and thermal properties of P3HT/CdS composites change with the aspect ratios. The interaction also affected changes in the morphology of the active layer upon thermal treatment. Therefore, the performances of P3HT/CdS nanocrystals devices dramatically depended on annealing condtions. Finally, hybrid CdS/P3HT photovoltaic devices using F-doped SnO2 (FTO)-coated ZnO nanorod arrays as electrodes were studied. The crystalline FTO made using the low-cost spray pyrolysis deposition (SPD) displayed a nominally complete and uniform coating over the entire outer surface of the ZnO nanorods. The incorporation of CdS into the P3HT much enhanced the Jsc of the devices with a nanorod FTO electrode. The PCE of the device with a ZnO nanorod length of 320 nm was increased from 0.37% for P3HT without CdS to 1.8% with CdS of aspect ratio=1 (spherical shape). With a further increase the AR of CdS nanocrystal to 4, the PCE was further increased up to 2.6%. Longer CdS nanocrystals conversely caused deterioration in PCE as result of the nanorod array morphology. These results indicate that, although increased nanorod length could improve the photocurrent and efficiency, other factors, such as P3HT infiltration, nanorod array morphology and CdS nanocrystal length are required for obtaining optimal performance of these devices.