靜電紡絲纖維於光電元件之應用

Electrospun Nanofibers for Optoelectronic Device Applications

陳蓉瑤(Jung-Yao Chen)

指導教授 ： 陳文章

國立臺灣大學/工學院/化學工程學研究所/博士(2016年)

https://doi.org/10.6342/NTU201600331

摘要

光電元件中電荷載子的非等向性傳遞，促使一維奈米結構研究蓬勃地發展。此外，奈米尺度的幾何侷限性更改變了原材料在塊材狀態時的性質。相較其他製備一維奈米結構的方法，靜電紡絲提供了簡便、快速又連續化的製成，對於大量製造上更有其成本上的優勢。除了單一材料奈米纖維，靜電紡絲纖維更是一個良好載體，可導入其他功能性材料來提升光電元件的特性。然而目前將靜電紡絲纖維應用在光電元件的例子仍是少數。在此篇論文中我們製備出各種功能性靜電紡絲纖維，應用於有機場效電晶體和提升有機太陽能電池效能表現。其研究細節分述如下:
第二章中我們在一般靜電紡絲裝置導入二次電場的方式來提升靜電紡絲的拉伸效果來達到順向高分子鍊的效果，由差式掃描量熱法(differential scanningcalorimetry)和X 光繞射光譜(X-ray diffraction)結果顯示聚(3-己烷基噻吩)(P3HT)經由二次電場拉伸後有效拉伸P3HT 高分子鍊並抑制P3HT 進行thiophene 堆疊，且高分子鍊沿纖維長軸方向排列，使載子由原本的π-π 堆疊造成的載子傳遞改成鍊上傳遞，有效提升載子遷移率達1000 倍，此做法可移除原本需加熱後處理提升載子遷移率的問題，使靜電紡絲技術更具有工業化優勢。
第三章中由原本單純的高分子系統延伸至有機/無機複合材料，結合銀還原和靜電紡絲的方法提出一維銀導電奈米纖維的製備，藉由調控電紡參數，可有效提升銀奈米顆粒的結晶度和順向性，且銀線的導電度可高達5.6*105 S m-1。進一步地利用銀奈米纖維的表面電漿共振現象加上靜電紡絲可順向性，製備出埋有順向、相交和不織布型態銀奈米纖維的P3HT:PC61BM 太陽能電池，成功提升太陽能電池效率至4.19 %達到18.7 %的提升效果。
在第四章中，以交聯過的聚甲基丙烯酸(PMAA)為主體製備出含有銀和共軛性茀系衍生物(PFBT)奈米顆粒的靜電紡絲螢光太陽能聚合器(luminescent solar concentrator)，其中PFBT 可當光學轉換材料，銀奈米顆粒則提供表面電漿效應和導電通路。PFBT 可吸收主動層材料吸收效率低的光源並轉換成主動層材料吸收率高的波段，且由於銀的存在可發揮其表面電漿共振效應，除了提升主動層材料激發子的產生速率，更能提升PFBT 的發光效率。此雙功能性複合電紡纖維埋入P3HT:PC61BM 和PTB7:PC71BM 有機太陽能電池中可達到效率上18 %的提升。
以上的研究顯示靜電紡絲纖維可經由調控電紡參數製備出各式功能性奈米纖維，且其不管是巨觀上的纖維順向性抑或是微觀上高分子鍊的順向性皆提供光電元件應用上獨特的特性。

關鍵字

靜電紡絲 ； 二次電場 ； 表面電漿共振 ； 螢光太陽能聚合器 ； 有機場效電晶體 ； 有機太陽能電池

並列摘要

One-dimensional (1D) nanostructures coupled with its possible technological applications have fueled the exponential growth of optoelectronics because of the anisotropic transportation of charge-carrier. The geometry confinement provided by 1D nanostructures also alter the optoelectrical characteristic compared to their bulkcounterpart. Among of the various fabrication of 1D nanostructure, electrospinning is a versatile assembly method for fabricating uniform and ultrafine nanofibers with different patterning via various geometric collectors. Also, electronspun nanofibers can serve as matrix for functional materials such as metallic nanoparticle, conjugated polymer and quantum dot for diverse application in optoelectronic device. However, the applications of such nanofibers in optoelectronic devices have not been fully explored yet. Furthermore, the unique process of electrospinning enable the distinctive characteristic of electrospun nanofiber is also not investigated. In this thesis, we produce diverse functional ES nanofibers and explore their morphology, opto-electrical properties and applications on organic field-effect transistor (OFET) and organic photovoltaic(OPV). The details of each topic are summarized as below:

In chapter 2, an extra electric field below the spinneret in electrospinning setup was introduced for producing poly(3-hexylthiophene) (P3HT) nanofibers. The liquid jet is greatly prolonged by the additional extensional force and thinner fibers can thusbe obtained. The chain conformation and orientation in fibers are probed by differential scanning calorimetry (DSC) and X-ray diffraction techniques. Under the influence of the secondary electric field, P3HT chains are extensively stretched and aligned along the fiber axis. The electrospun P3HT nanofibers are fabricated into field-effect transistors and the charge carrier mobilities of the nanofibers with and without secondary electric field are found to be 1.54×10-4 and 1.62×10-1 cm2 V-1 s-1, respectively. The dramatic enhancement of mobility by more than 1000 times is due to the effective charge transport through the delocalization of electrons along the highly extended and oriented P3HT backbones rather than the ordinary π-π stacking. In addition to P3HT, we find this simple method also works for other poly(3-alkylthiophene).

In chapter 3, the significant enhancement of the P3HT:PC61BM ([6,6]-phenyl C61-butyric acid methyl ester) photovoltaic devices using different patterns of the electrospun Ag/PVP composite nanofibers, including non-woven, aligned-, and crossed-patterns were reported. The composite electrospun nanofibers were prepared through the in-situ reduction of silver (Ag) nanoparticles in Ag/poly(vinyl pyrrolidone) (PVP) through two-fluid coaxial electrospinning technique. The composition, crystalline orientation, and particle size of Ag were successfully manipulated by controlling the core/shell solution concentration (AgF-1, AgF-2 and AgF-3), as evidenced by FE-SEM, TEM and SAED analyses. The smallest diameter of thecomposite nanofibers led to the highest orientation of the Ag nanoparticles and resulted in the largest conductivity comparable to that of ITO, due to the geometrical confinement. Such composite nanofibers exhibited the surface plasmon resonance (SPR) effect, evidenced by the absorption peak around 425 nm, which provide near field enhancement of electromagnetic field around active layer. In addition, the composite nanofibers with the crossed- or nonwoven patterns further enhanced high carrier mobility, compared to those of aligned-pattern. It led to the 18.7% enhancement on the power conversion efficiency of photovoltaic cell (ITO/composite nanofibers/PEDOT:PSS/P3HT:PC61BM/Ca/Al) compared to the parent device. The above results indicated the high conductivity and SPR effect of the Ag/PVP electrospun nanofibers could significantly improve the photocurrent as well as PCE, leading to promising organic solar cell applications.

In chapter 4, plasmonic-enhanced luminescent solar concentrator (LSC) electrospun nanofiber by coaxial electrospinning technique with poly[2,7-(9,9-dihexylfluorene)-alt-4,7-(2,1,3-benzothiadiazole)](PFBT) nanoparticle as the LSC and Ag nanoparticle as the SPR center was explored. The ES nanofiber, crosslinked poly(methacrylic acid) (PMAA),provides a solvent-proof matrix for LSC without changing the conventional OPVs configuration. Besides, the in-situ reduction of Ag nanoparticle simultaneously enhanced the exciton generation of PFBT and active material with the SPR effect. The dualfunctional ES nanofibers allow significant light harvesting through down conversion and enhanced exciton generation, leading to remarkably 18 % enhancement on the PCE for both P3HT:PC61BM and PTB7 (polythieno[3,4-b]-thiophene-cobenzodithiophene): PC71BM([6,6]-phenyl C71-butyric acid methyl ester) [6,6]-phenyl) photovoltaic cells. This configuration provides a novel method to integrate the plasmonicenhanced LSC electrospun nanofiber into OPV device with the combination of two light trapping methods (SPR and LSC) and the maintenance of active area coverage.

The above studies address the characteristics of electrospun nanofibers can be feasibly manipulated the composition and process condition during electrospinning. The orientation property in micro-scope and macro-scope of electrospun nanofibers also indicate the unique application in optoelectronic devices.

並列關鍵字

electrospinning ； secondary electric field ； surface plasmonic resonance ； luminescent solar concentrator ； organic field-effect transistor ； organic photovoltaic device

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