藉由處理有機與無機界面以改善有機高分子太陽能電池效率之研究

The Study on Improving Performance of Polymer Solar Cells by Surface Modification at Organic/Inorganic Interface

王膺傑(Ing-Jye Wang)

指導教授 ： 林清富

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.02125

摘要

隨著世界能源需求不斷的擴大，人類開始在尋找替代能源，其中由於地球太陽光充足，所以太陽能產業一直被受期待，且具有相當大的發展潛力。而有機高分子太陽能電池兼具成本低廉、製程容易、重量輕且易撓曲，現已被廣泛研究中。
本篇所研究的倒置結構有機高分子太陽能電池中，使用氧化鋅薄膜層來代替poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)，使得載子傳遞方向不同於傳統結構。同時，這樣的結構又可以改善PEDOT:PSS與ITO電極接觸時，因吸收空氣中的水氣而造成電極腐蝕，大大提升有機太陽能電池的壽命。配合上溶液製程、低成本與相較於傳統有機太陽能電池的長壽命，倒置結構太陽能電池已具有相當的發展潛力與未來性。
目前大多數有機太陽電池都採用體異質接面(bulk heterojunction, BHJ)的構造，也就是將電子施體(donor)與受體(acceptor)兩種材料利用溶劑掺雜，以達到混合相。當有機材料照光後，光子會產生激子(exciton)，激子由混合相分離而產生電子及電洞，並傳導至兩端的電極。這樣可以有效解決載子移動率低與激子擴散長度短等問題，比起其他形式的有機層，更具優勢。當高分子膜增厚時，這些都會使得電子電洞的復合機率大增，因此在我們製程條件下高分子太陽能電池主動層膜厚約為150-200 nm。然而混合相中施體與受體在不同基板及處理方式下，會有不同的垂直分層情況。理想的分布為施體趨近陽極，而受體靠近陰極，以利電子及電洞的傳輸。因此，研究施體與受體在吸光層中的垂直分佈，可以直接了解電子及電洞是否被兩端電極有效地收集，並釐清元件的運作機制。
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薄膜與高分子混成太陽能電池，藉由製程上的最佳化與加入界面處理來提升元件效率。首先，我們所最關注的為有機高分子太陽能電池中，使用氧化鋅薄膜為基板時，電子受體與施體的趨向性，使用X射線光電子能譜原理(XPS)來分析。之後，由於發現其表面有不利於元件運作之缺點，故進一步使用反應式離子蝕刻原理(RIE)做界面改善，以達成目的。經過表面處理之元件，光電轉換效率從3.4%提升至3.9%。又於後退火後，使得效率可以提升到4%以上。我們會如此處理的原因是因為慢乾後的有機層擁有較佳的分層結構，所以我們使用氧氣電漿蝕刻證明可以在不破壞到原來的有機層結構下，將界面處理為較佳的表現。相較於其他方式，如加上一層的電子施體來達到相同效果，我們的方法更具有優勢，且可以融合於工業製程上。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。
另ㄧ方面，為了達到改善界面的目的，我們也針對有機層的下界面，也就是有機/氧化鋅薄膜，進行蒸鍍鎂金屬的處理，期待更進一步提升元件效率。結果顯示，使界面達到費米能階鎖定可以大大提升元件Voc，雖然犧牲了光入射至有機層的強度，但是可以證明我們改善界面的成效遠比損失的多，故能有效抑制元件的漏電流且降低串聯電阻進而提升太陽能電池的效率約20%。

關鍵字

高分子太陽能電池 ； 氧化鋅薄膜 ； 倒置結構 ； 氧氣電漿蝕刻 ； 溶液製程

並列摘要

Various technologies designed to generate power by harnessing solar energy have been gaining interest because of the dwindling supplies of natural resources. In the past decade, organic photovoltaics (OPVs) are attractive because of their compatibility with flexible substrates, roll-to-roll processing, low manufacturing costs, and large area applications.
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.
The state-of-the-art device structure is the polymer BHJ, a conjugated polymer blended intimately with a soluble fullerene derivative. This method generates a maximized interface between the donor and acceptor materials within the whole volume of the photoactive layer. Which in turn, creates maximum exciton dissociation and ensures the creation of continuous, preferably short, pathways for charge transport to both of the electrodes. Recently, many studies have shown that the device performance is related to the morphology of the blend film, including the formation of domains from different compositions and the packing of the molecules. Therefore, morphology optimization of the active layer is an essential way to improve power conversion efficiency (PCE).
The aim of this work is to realize a high-efficiency inverted polymer solar cells with surface modification on two sides of the polymer thin film. Our investigation shows that the plasma-etched polymer surface adjacent to the anode was modified into a more hydrophobic P3HT-rich surface, which leads to better interface contact, fine electrode selectivity, and lower series resistance in the device. Although PCBM still aggregates on the top surface, it can be removed with mild oxygen plasma etching. As a result, the power conversion efficiency increased significantly from 3.4 to 4.3% with an enhanced fill factor of 64%.
Another study is on the surface modification of the ZnO thin film adjacent to the cathode. A thin low work function metal film is sandwiched between the ZnO and organic layer to improve the performance for inverted polymer solar cells. This metal film modifies the organic surface for efficient carrier transport and also eliminates carrier accumulation. In cases where the lowest unoccupied molecular orbital level of the electron acceptor in organic layer is below the Fermi level of the metal substrate, the spontaneous charge transfer from the metal substrate to organic layers leads to the Fermi-level pinning. Therefore, the energy difference at the ZnO/organic interface is reduced so that the electrons can be transported to cathode more efficiently. In addition, the electric field generated by the work function difference between anode and cathode is more significant due to the lower Fermi level of Mg than that of ITO or ZnO; thus, the open circuit voltage is enhanced. Here, we report that inverted polymer solar cells have a power conversion efficiency of 4.63% with Jsc=9.61 mA/cm2, Voc=0.82 V, and FF=58.8% under AM 1.5 G (100mW/cm2) irradiation intensity.

並列關鍵字

Polymer Solar Cells ； ZnO Thin Film ； Inverted Structure ； Oxygen Plasma Etching ； Solution Processing

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