氧化鋅奈米柱與導電高分子有機無機混成太陽能電池

Abstract

在能源需求大增的時代，太陽能電池成為當前重要的課題；其中高分子太陽能電池不僅生産成本低廉、重量輕，而且能夠橈曲，製成各樣型態的太陽能電池。高分子太陽電池一般最常見高效率的系統是使用塊材異質接面(bulk-heterojunction)結構，以poly(3-hexylthiophene) (P3HT)為電洞傳輸材料，(6,6)-phenyl C61 butyric acid methyl ester (PCBM)為電子傳輸材料的系統。然而在傳統結構的高分子太陽能電池中，由於使用酸性且吸水性的poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)與易氧化的鋁電極，因此存在著元件穩定性的問題；另外，由於高分子材料上的限制，如載子移動率低與激子擴散長度短的問題，當高分子膜增厚時，這些都會使得電子電洞的複合機率大增，因此高分子太陽能電池主動層膜厚約為100-200 nm。在倒置結構的高分子太陽能電池中加入氧化鋅奈米柱則是一種可解決以上的問題的方法，因為倒置結構可以不需要鋁電極與PEDOT:PSS，因此增加元件的穩定性；另外，加入氧化鋅奈米柱於高分子層中，由於氧化鋅擁有電子的高移動率與長擴散長度，因此它可以使高分子層增厚，幫助載子的傳輸與收集。此外由於在本論文中的太陽能電池使用了溶液製程的氧化鋅奈米柱與可網印的銀電極，因此非常適合印刷(roll-to-roll printing)式的製程。 在本篇論文中主要是為了實現低成本與高效率的倒置氧化鋅奈米柱與高分子混成太陽能電池，藉由製程上的最佳化與加入溶液製程的中介層提升元件效率。由於我們研究顯示在熱退火的處理上，會使高分子層與奈米柱間的介面接觸變差，並且使氧化鋅奈米柱斷裂，而無法改善倒置氧化鋅奈米柱與高分子混成太陽能電池的元件表現，因此我們以無需退火的方式改善元件表現，藉由緩慢乾燥法提升高分子層的結晶性；其中，降低塗佈轉速的緩慢乾燥法可大幅改善元件轉換效率從1.85%至3.58%，因為它能同時使高分子層膜厚增加與結晶性提升；隨著高分子層從液態轉變為固態的時間拉長，高分子擁有足夠的時間自組成與滲透進入奈米柱間，亦結合了氧化鋅奈米柱的優勢，因此在慢乾的元件中，高分子層可以增厚至400 nm，卻不會犧牲載子的傳輸，因而突破了高分子太陽能電池其高分子層膜厚約為100- 200 nm的限制；並且隨著膜厚的增加，提升入射光的吸收，增加元件中的光電流，因此元件轉換效率大幅提升至3.58%，這也是目前沒有加入任何蒸鍍中介層的氧化鋅奈米柱/高分子混成太陽電池中最高的效率。另ㄧ方面，為了達到簡單製程與低成本的太陽能電池，我們發展了一系列溶液製程的中介層，其中藉由加入五氧化二釩中介層，並且控制中介層的濃度，使得中介層能夠阻擋電子並不犧牲串聯電阻，有效抑制元件的漏電流進而提升太陽能電池的效率至3 %。 由於本論文中的元件不需要熱退火，因而無需氮氣手套箱，更不需要昂貴真空蒸鍍的中介層，這些都有助於實現低成本、大面積印刷的高分子太陽能電池，對高分子太陽能電池的商業化相當重要。

Solar cells attract great attention owing to the growing need for renewable energy. Polymer solar cells offer the potential for large-scale power generation based on materials that provide flexibility, light weight, low-cost production and low-temperature fabrication. The most common and efficient material system for polymer devices is thus so far the one consisting of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). However, the conventional bulk-heterojunction (BHJ) architecture has limitations in device stability because of the acidic, hygroscopic nature of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and oxidation of the Al electrode. Moreover, there exist problems of inherently poor polymer properties, such as the short exciton diffusion length and the relatively low carrier mobility, which limit the usefulness of thick film. A possible approach to overcome the above difficulties is to use inverted configuration with ZnO nanorod arrays. The inverted structure has the advantage of improved stability by replacing the low work function metal cathode and PEDOT:PSS. The ZnO nanorods provide the possibility of thick active layer for light harvesting without sacrificing the charge transport due to their excellent electron mobility and the long diffusion length. Moreover, ZnO nanorod arrays using solution processing and Ag anode allowing the use of the nonvacuum printing technique provide a route for printed solar cells. The aim of this work is to realize a low-cost and high-efficiency inverted polymer solar cells hybridized with ZnO nanorod arrays by the use of optimized protocols and the introduction of solution-processed interlayer. Our investigation shows that the annealing-free approach, slow drying, improves device performance where thermal annealing is either ineffective or undesirable. As the polymer solidification time is lengthened by lowering the spin-coating rate of the photoactive layer, the photoactive layer becomes thickened, and the polymer chains have enough time to self-organize and effectively infiltrate into ZnO nanorod spacing. While the thickness of the photoactive layer is increased to 400 nm accompanying self-organized polymer, the power conversion efficiency of the device is improved to 3.58% with an enhanced fill factor of 58%. The 400 nm film is composed of the highly ordered polymer and the ZnO nanorod arrays, resulting in increased light harvesting without decreasing the possibility for charge transport. On the other hand, the power conversion efficiency is improved to 3% by the introduction of the solution-processed V2O5 interlayer due to the efficient suppression of the leakage currents at the organic/metal interface. The devices in this study are fabricated not only in an atmosphere environment due to no need of thermal annealing but also without the use of vacuum-deposited interlayer. These are very important for commercial realization of low-cost and large-area printed solar cells.