軟性電路板及電極之製備與特性研究及其於有機太陽能電池之應用

Abstract

隨著能源危機的時代漸漸來臨，太陽能電池的發展也逐漸受重視。除了目前廣泛被使用的矽晶圓太陽能電池及無機薄膜太陽能電池技術外，有機太陽能電池因為具有低成本、耐衝擊、高產出及方便攜帶等優點，已成為新世代發展攜帶型電子產品的重點發展技術。本論文主要探討由低成本的濕式製程製備軟性印刷電路板、陰極及陽極，並且利用此兩種濕式製備電極的技術，去製備出全濕式製程的反式有機太陽能電池元件於軟性聚亞醯胺基材上，並探討其中的物性及化性對於元件中光電轉換效率的影響。另一部分，藉由探討不同電極的表面能對於有機小分子的自我組裝型態的不同，去提高有機太陽能電池的光電轉換效率。

本論文研究第一部分主要探討一種全濕式製程的表面鎳金屬化聚亞醯胺基板的製備及特性分析，而此表面的金屬與聚亞醯胺基板間有優異的接著性以期應用在軟性電子的軟性印刷電路板方面。此表面金屬化聚亞醯胺的製作方法為首先利用強鹼對亞醯胺環開環、再依序進行鎳金屬離子的離子交換、強還原劑還原出具有無電解電鍍鎳活性的無鈀系鎳金屬奈米顆粒當作晶種、及最後利用鎳無電解電鍍增厚與平坦化鎳層。由原子力顯微鏡及掃描式電子顯微鏡的觀察，可以了解無鈀系鎳金屬奈米顆粒在鎳無電鍍液下的成長型態；搭配表面電阻的量測，可知鎳奈米顆粒經由無電解電鍍後明顯的導電度提升。綜合穿透式顯微鏡及膠帶接著性測試法的分析，可知此鎳薄膜可以控制在382 nm並且對於聚亞醯胺具有良好的接著性。

本論文研究第二部分是利用一簡單的後續表面改質物的旋轉塗佈處理方法去提升poly(3,4-ethylenedioxythiophene)/poly(styrene-sulfonate) (PEDOT:PSS)薄膜的導電度，並利用此技術應用在無銦錫氧化物(ITO-free)的高分子太陽能電池中。在此研究中，搭配選擇不同改質物(包含一個或多個極性官能基的醇類或醚類)，並利用不同的分析方式去了解此溶劑效應對於PEDOT:PSS薄膜之導電度提升的真實原因。經由不同分析方式的分析，可以推測此溶劑處理之PEDOT:PSS薄膜的三維模型出來，而相信此研究對未來製作ITO-free的元件是有很大的幫助及潛能。

本論文研究第三部分是結合先前兩部分開發的部分技術，由鎳金屬化聚亞醯胺當作陰極及表面溶劑改質的高導電度PEDOT:PSS薄膜當作陽極，並且搭配poly(3-hexylthiophene) (P3HT)及[6,6]-phenyl-C61-buytyric acid methyl ester (PCBM)的混成材料當作光電作用層去製作出全溶劑製程的反式結構ITO-free的高分子太陽能電池。而此顆粒狀導電鎳薄膜對於聚亞醯胺基材有良好的接著性且具有高的導電度(ca. 2778 S cm–1)，經由功函數的量測可知此方法製作出的電極為3.9 eV，此結果有別於一般鎳金屬的功函數(5.4 eV)。製作出此反式結構的高分子太陽能電池製程中，會存在很多介面能不匹配的問題，例如在塗佈光電作用層之前，製程中會導入一層titanium(diisopropoxide)bis(2,4-pentanedionate) (TIPD)當作電洞阻障層並且有利於得到良好成膜性的P3HT:PCBM薄膜於表面鎳金屬聚亞醯胺上。另外，在塗佈PEDOT:PSS薄膜之前，此研究會先使用五秒鐘溫和的氧電漿處理使主動層表面變成較親水的介面，進而有利於後續較親水性的PEDOT:PSS成膜，目前此系統的反式結構高分子太陽能電池製作最好的光電轉換效率約為2.4% (AM 1.5及100 mW cm–2條件下)。此技術的特點為完全濕式的製程。另外藉由UV-vis、Haze effect、AFM及TEM的分析，可以初步推斷此次微米級顆粒狀金屬電極具有幫助光在主動層內的吸收，以期提升反式結構之有機太陽能電池的效率。

本論文研究最後一部分是利用基材效應，藉由不同基材的表面能控制，經由熱蒸鍍方法去製備自我組裝之copper phthalocyanine (CuPc)的奈米柱狀結構當施體，與覆蓋fullerene (C60)當受體，以期達到理想的異質接面型有機太陽能電池。我們可以觀察到柱狀奈米結構的CuPc於indium–tin oxide (ITO)、PEDOT:PSS及Au上的自組裝型態的不同，CuPc可以於Au表面進而形成理想的異質接面型主動層結構。藉由不同分析方法及理論計算去分析表面能對於沉積CuPc型態學的影響。於OPV的光電轉換效率探討方面，可推知由PEDOT:PSS表面造成的平躺及垂直站立混合型態之CuPc奈米柱薄膜，其相對於平面結構之元件的光電轉換效率有50倍以上的提升。

Over the past two decades, satisfying the world’s growing demand for energy is one of the most significant challenges facing society. Therefore, the development of solar energy is viewed as an ideal technology for power generation because it is clean and renewable. Although the photovoltaic (PV) technology platforms of silicon-based PV and thin-film PV are now undergoing a rapid expansion in production, the next generation PV—organic solar cells (OSCs)—could soon be playing a major role with the advantages of ultralow production costs, rugged and lightweight. One part of this dissertation describes the fabrication of cathodes and anodes with low costs and high-throughput-solution processes in attempt to apply in all-solution-processed OSCs on flexible substrates. In another part of this dissertation, the variable self-assembly behavior of small molecular on a diverse range of substrates (surface energies) was used to control the morphology of the interface and the degree of carrier transportation within the active layer in OSCs, and furthermore enhance the power conversion efficiency. All of the phenomena occurring are investigated in this thesis, and those techniques are belived the important roles for developing high efficiency and low-cost OSCs in the future.

In the first part of the dissertation, I discussed the fabrication of surface-nickelized polyimide films using a fully solution-based process, and excellent adhesion between the nickel and polyimide phases was observed. Flexible polyimide substrates were modified by alkaline hydrolysis, ion exchange, reduction and nickel electroless deposition without palladium. Atomic force microscopy and field emission scanning electron microscopy were used to follow the growth of nickel nanoparticles (Ni-NPs) and nickel layers on the polyimide surface. The surface resistances of the Ni-NPs/PI films and Ni/PI films, measured using a four-point probe, were 1.6×107 and 0.83 Ω/cm2, respectively. The thicknesses of Ni-NPs and the Ni layer on the polyimide surface were 82 nm and 382 nm, respectively, as determined by transmission electron microscopy, and the Ni layer adhered well to PI, as determined by the adhesive tape testing method.

In the second part of this dissertation, I devised a simple method to enhance the conductivity of poly(3,4-ethylenedioxythiophene)-poly(styrene-sulfonate) (PEDOT:PSS) films through spin-coating with various surface-modified compounds, and then applied this technique to the preparation of ITO-free polymer solar cells (PSCs). The electrical conductivity of PEDOT:PSS films can be increased by more than two order of magnitudes merely by spin-coating a compound containing one or more polar groups—such as ethanol, methoxyethanol, 1,2-dimethoxyethane, and ethylene glycol—onto the films. The phenomena occurring are discussed through conductivities, morphologies, and chemical properties of the modified PEDOT-PSS films as determined using Raman spectroscopy, a four-point probe, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The schematic 3D morphological model of directly solvent-modified PEDOT:PSS films is presumed for ITO-free devices. The desirable conductivity enhancements of these materials make them attractive candidates for use as anode materials in ITO-free PSCs.

In the third part of this dissertation, I prepared all-solution-processed inverted polymer solar cells (PSCs) incorporating two solution-processed electrodes—surface-nickelized polyimide films (NiPI films) as cathodes and high-conductivity poly(3,4-ethylenedioxythiophene)/poly(styrene-sulfonate) (PEDOT:PSS) films as anodes—and an active layer with a bulk heterojunction morphology of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-buytyric acid methyl ester (PCBM). The granular Ni thin films, which exhibited good adhesion and high conductivity (ca. 2778 S cm–1) on the polyimide (PI) substrates and possessed a work function different from that of pure Ni metal (WF, 5.4 eV). Using ultraviolet photoelectron spectroscopy, we determined that the WF of the NiPI films was ca. 3.9 eV. Prior to the coating of the photoactive layer, the surface of the NiPI films were treated with titanium(diisopropoxide)bis(2,4-pentanedionate) (TIPD) solution to facilitate the deposition of high-quality active layer and further as a hole blocking layer. The solution processed anodes (solvent-modified PEDOT:PSS films) were further coated and subjected to mild oxygen plasma treatment on the active layer. Short exposure (5 s) to the plasma improved the quality of the surface of the active layer for PEDOT:PSS deposition. These inverted PSCs on flexible granular NiPI films provided a power conversion efficiency of 2.4% when illuminated under AM 1.5 conditions (100 mW cm–2). The phenomenon of light absorption enhancement in those inverted PSCs was observed as indicated in reflective UV-vis, haze factor and external quantum efficiency (EQE) responses. The resulting fill factor (FF) of 0.43 is still significantly lower than the FF of 0.64 for standard devices. When compared to the planar structure, the improvement of absorbance of light and good haze factors was obtained for granular structure which suggests NiPI as a better back contact electrode through enhancing the light trapping and scattering in inverted PSCs.

In the last part of this dissertation, I have prepared organic photovoltaic (OPV) cells possessing an ideal bulk heterojunction (BHJ) structure using the self-assembly of copper phthalocyanine (CuPc) as the donor material and fullerene (C60) as the acceptor. The variable self-assembly behavior of CuPc on a diverse range of substrates (surface energies) allowed us to control the morphology of the interface and the degree of carrier transportation within the active layer. We observed rod-like CuPc structures on indium–tin oxide (ITO), poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) (PEDOT:PSS) and Au substrates. Accordingly, the interfaces and continuing transport path between CuPc and fullerene domains could be greatly improved due to the ideal BHJ structure. In this paper, we discuss the mechanisms of producing CuPc rod-like films on ITO, PEDOT:PSS and Au. The OPV cell performance was greatly enhanced when a mixture of horizontal and vertical CuPc rods was present on the PEDOT:PSS surfaces, i.e. the power conversion efficiency was 50 times greater than that of the corresponding device featuring a planar CuPc structure.