倒置低能隙高分子太陽能電池及溶液製程氧化鉬的界面修飾研究

Abstract

近年來人們已經意識到能源短缺的危機，新的替代能源成為研究的重點。高 分子太陽能電池因為有達成大面積、低成本生產以及可撓曲的廣泛應用潛力，在 近二十年來已經成為受到關注的一種再生綠能。為了增加太陽光紅外頻譜的吸收， 不同的低能隙高分子(low-bandgap)材料已經被合成來取代 P3HT。而另一種受體材料PC71BM也被用來增加短波長太陽光的吸收並提供更適合低能隙高分子材料的 LUMO能階，取代了PC61BM。 除了效率上的提升，倒置結構太陽能電池也已經被開發來增進元件的生命週 期。在傳統結構中，PEDOT:PSS及Al被分別用來當作電洞傳輸層以及陰電極。 PEDOT:PSS已經被證實擁有腐蝕ITO電極的酸性，而低功函數的Al也很容易氧 化；這些都是造成元件衰退的機制，也降低了元件的穩定度。相對地，倒置結構 高分子太陽能電池在製作上使用高功函數金屬做為陽電極，並且避免了 PEDOT:PSS 與 ITO 的接觸。除此之外，吸光主動層以三明治結構被夾在對空氣穩定的金屬氧化物之間，也產生了類似封裝的效果。

在本篇論文中，為了研究並實現高效率的倒置結構高分子太陽能電池，我們 使用了商業上可以買到的低能隙高分子PBDTTPD來做為主動層予體(donar)材料。經過了一連串最佳化實驗之後，我們發現以蒸鍍的MoOx來做為電洞傳輸層能使 得元件表現有大幅度的提升。同時，研究結果顯示這裡使用的PBDTTPD分子量 過低，很難有好的載子遷移率及表面形態，因而元件表現受到限制。所以接著我 們改用另一個市面上販售的低能隙高分子材料PBDTTT-C與PC71BM搭配作為主 動層，並在主動層已優化的條件下使用溶液製程MoO3作為電洞傳輸層，期許能實 現在低成本、大面積製程下高效率、高穩定的倒置太陽能電池。我們嘗試在 MoO 3 旋鍍完之後使用電漿處理來修飾陽極，不僅還原 MoO 3 以增加電洞傳導效率，也經由界面修飾獲得更好的陽極接觸，使得短路電流、開路電壓等元件表現參數獲得 大幅的提升，光電轉換效率也由從原本溶液製程 MoO 3 的 2.50%上升到 4.01%。接著，我們調變電漿處理時的腔體壓力、電漿處理時的強度以及時間來做最佳化，使得溶液製程MoO3倒置元件的光電轉換效率進一步提升至4.65%。

本論文中使用的溶液製程MoO3不須退火處理也不需高度真空，有助低成本大面積的製作。此研究提供溶液製程金屬氧化物在倒置結構高分子太陽能電池上的 新穎應用方法，不僅能提高效率表現，也有利於高分子太陽能電池的商業化。

In recent years, people have realized the crisis of energy shortage; therefore, replacement energy to fossil fuels becomes an important issue. Polymer photovoltaics (PVs), serving as renewable green energy sources, have attracted much attention in the past two decades due to the potential of achieving low-cost process over a large area and flexibility for versatile applications. To enhance the harvest of infrared solar radiation, different low-bandgap polymers have been synthesized. Further compensating for the short-wavelength light absorption and providing more compatible LUMO level for low-bandgap polymer materials, another acceptor material, [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), is often used to replace [6, 6]-phenyl- C61-butyric acid methyl ester (PC61BM). Except for efficiency improvement, a structure with reverse electron flow, which is named inverted structure, has been investigated to enhance device lifetime. In this work, in order to investigate and attain high efficient inverted polymer PVs, we use a commercial low- bandgap polymer PBDTTPD as electron donar material in photoactive layer. Via a series of experiments for optimization, we find that there is an improvement when thermally deposited molybdenum oxide (MoOx) is used as hole transport layer. At the same time, the research results show that the molecular weight of PBDTTPD we used is too low to have excellent carrier mobility and morphology, thus limiting the device performance. Therefore, we replace the donar material with another commercial low-bandgap polymer PBDTTT-C and blend it with PC71BM as photo-active layer. Solution-processed MoO3 is spin-casted on the optimized photoactive layer to attain highly efficient and stable inverted polymer PVs under low cost and large-area fabrication. We try using plasma treatment to modify the anode after MoO3 is spin-casted. This treatment not only reduces the oxygen ratio of MoO3 for more efficient hole transport, but also modifies the interface for better contact with anode, which leads to the enhancement in short-circuit current-density (Jsc) and open-circuit voltage (Voc), thus raising the power conversion efficiency (PCE) from 2.50% to 4.01%. In the following experiments, we alter the parameter of plasma treatment to optimize, including the pressure of the chamber, the power set of plasma, and the treating time. As a result, the PCE of the device with solution-processed MoO3 enhances again to 4.65%.

As a result, this work provides a novel function for the application of solution-processed metal oxide to inverted polymer PVs. This not only improves the PCE of PVs but also benefits the commercialization of polymer PVs.