Electrochemical Surface Modification for Application in Organic Optoelectronics

本論文主要目的是以電化學方法聚合導電高分子，對現有之有機光電元件進行表面改質，進而用來提升一般有機太陽能電池的效率以及發展混色電致色變薄膜。 第一部分(第三章)，我們使用電化學聚合方法將噻吩(thiophene)單體聚合在高分子Poly(3,4-ethylenedioxythiophene) (PEDOT)電洞傳輸層上，形成與主動層連接的電子予體(electron donor)緩衝層來增進載子收集效率。此緩衝層能夠使主動層中的n-型材料不再接觸到電洞傳輸層，藉此減少電子受體觸碰電洞傳輸層所造成的再結合現像，如此一來可以提升短路電流，進而提升整體元件效率。在塗佈主動層之前，還可以將緩衝層在電解質環境中施加電位使其部份氧化還原來調變緩衝層的功函數(work function)，進而改善太陽能電池元件的開環電壓，將太陽能電池元件更進一步最佳化。 第二部分(第四章)利用聚苯乙烯(polystyrene, PS)小球堆積作成模板，並在模板的縫隙中以電化學聚合的方式成長PEDOT薄膜，再將PS小球的模板用甲苯(toluene)溶劑移除，可以得到有碗狀結構的PEDOT電洞傳輸層，與一般平面電洞傳輸層相比，碗狀電洞傳輸層所擁有的半球狀理論表面積約為兩倍。在此電洞傳輸層上，我們用真空蒸鍍的方式沿著PEDOT的起伏形貌沉積銅酞菁(copper phthalocyanine, CuPc)及奈米碳球(如碳60或碳70)製作成雙層(bilayer)結構的太陽能電池，於是主動層中的電子予體—受體接面也由平面變為半球形，這種有結構的電洞傳輸層使得雙層結構太陽能電池中電子予體—受體的接面大幅增加。原本雙層結構太陽能電池的主動層受限於電荷載子移動性而無法做厚，又因主動層厚度不足而犧牲對可見光的吸收。本章引入有結構之主動層增加了電子予體—受體接面，使激子(exciton)分離機率大增，同時增加入射光在主動層中通過的路徑，使得同樣厚度的主動層能有更強的吸收，避免對入射光能的浪費，改善了雙層太陽能電池最大的缺點。與無結構的雙層太陽能電池相比，有結構的太陽能電池可將元件的短路電流由4.82 mA cm–2提升到9.18 mA cm–2，效率由0.97%提升到2.45%。 第三部分(第五章)則是將第二部分同樣方法所製備出來的PEDOT用作電致色變電極，在碗狀PEDOT層聚合完成後繼續用電化學方法將苯胺(aniline)分子聚合其上。因為PEDOT的碗狀結構有較銳利的邊緣與較平滑的底部，使得電化學聚合的過程中，苯胺單體會因為電荷集中(charge concentration)的效應而優先沉積於較銳利的結構邊緣，結果可以得到PEDOT在下，聚苯胺(polyaniline，PANI)在上的複合膜。對此複合膜施加氧化電位時，因為兩種材料都可接觸到電解質，所以PEDOT與PANI會同時氧化，整體顯示為綠色；施加還原電位時，兩種材料也會同時還原，整體顯示為藍色。結構上的孔洞使得外層的PANI薄膜不會完全覆蓋內層的PEDOT，電解液可以藉由結構上的孔洞順利的接觸外層的PANI薄膜與內層的PEDOT，使得此有結構複合薄膜的著色效率高於無結構的複合薄膜，更接近理論計算值，且變色時間也由於複合薄膜與電解質之間接觸良好而縮短了一半。這個部份的應用也暗示了未來可以用結構來調整兩種電聚合高分子的比例，使得電致色變元件的變色範圍更能預測，也更多樣化。In this dissertation, we use electrochemical process to modify the surface of conventional organic optoelectronics. To improve the performance of unmodified organic photovoltaics and achieving dual-color electrochromic films. In the first part (in Chapter 3), we deposited the thiophene monomer on the surface of poly(3,4-ethylenedioxythiophene) (PEDOT) hole transfer layer through electrochemical polymerization process to form a buffer layer connecting the electron donor domain in the active layer and the hole transfer layer. It increased the charge carrier collection efficiency and reduced the recombination due to the contact of acceptor domain with hole transfer layer. Thus, the cell performance was improved. We also proved that we can tune the work function of the polythiophene buffer layer by applying potential, further improve the open circuit voltage of the solar cell device. In the second part (in Chapter 4), we created a template by spreading polystyrene beads on the ITO substrate. Then grew the PEDOT hole transfer layer bottom-up between the gap of polystyrene beads. After removal of the polystyrene template with solvent, we obtain a PEDOT hole transfer layer featuring bowl-like structure. After depositing the copper phthalocyanine (as the electron donor) and fullerene (C60 or C70 as the electron acceptor) along the feature of the structured PEDOT, we created a bilayer solar cell with structured active layer. Which dramatically increases the donor—acceptor interfaces. Compare with the result of solar cell devices without structure (planar active layer), the structured active layer improves the short circuit current from 4.82 mA cm-2 to 9.18 mA/cm-2; improves the power conversion efficiency from 0.97% to 2.45%. The third part (in Chapter 5), we used the bowl-like structured PEDOT layer as the bottom layer of electrochromic film and electrochemically polymerize the aniline monomer on the rim of the bowl-like structure. As we apply the potential on the structured PEDOT, the aniline monomer will condense on the sharp edge of the rim, but not the smooth bottom of the bowl-like structure, because of the charge concentration effect, forming a PANI-PEDOT composite electrochromic film. By applying an oxidative potential to this structured composite film, PANI and PEDOT change color simultaneously and the color turns green; while applying a reductive potential to this structured composite film, PANI and PEDOT change color simultaneously and turn blue. The hole structure allows both the bottom PEDOT layer and the top PANI layer to have good contact with the electrolyte, thus providing much higher coloration efficiency than the planar composite film. Also the switching time of the structured electrochromic film became shorter due to the easier contact of electrolyte with both electrochromic materials.致 謝 I 摘 要 I Abstract III Table of Contents V List of Tables VIII List of Figures X Chapter 1 Introduction 1 1.1 Preface: Electrochemical Surface Modification 1 1.2 Methods of Polymerization for Conducting Polymers 2 1.2.1 Oxidative polymerizations 2 1.2.2 Electrochemical polymerizations 4 1.2.3 Metal-catalyzed polymerizations 5 1.3 Buffer Layer Used in Solar Cells 7 1.4 Nano Structure Used in Solar Cells 8 1.5 Nano Structure Used in ECDs 10 1.6 Future Directions and Challenges 15 1.7 Objective and Outline of this Dissertation 16 Chapter 2 Experiment 17 2.1 A Strategic Buffer Layer of PT Enhances the Efficiency of BHJ Solar Cells 17 2.1.1 Materials and reagents 17 2.1.2 Electrochemical deposition of the polythiophene buffer layer 17 2.1.3 Fabrication of PV device 18 2.1.4 Characterizations 19 2.2 Organic Solar Cells Featuring Nanobowl Structures 19 2.2.1 Materials and Reagents 19 2.2.2 The preparation of polystyrene beads template 20 2.2.3 Electrochemical polymerization of PEDOT hole transfer layer 20 2.2.4 Fabrication of PV device 22 2.2.5 Morphology and Material Characterizations 23 2.3 Dual-color Electrochromic Films Incorporating a Periodic Polymer Nanostructure 24 2.3.1 Materials and Reagents 24 2.3.2 Fabrication of Electrochromic Films with Nanostructure 24 2.3.3 Characterizations 26 Chapter 3 A Strategic Buffer Layer of PT Enhances the Efficiency of BHJ Solar Cells 28 3.1 A solar cell with polythiophene buffer layer 28 3.2 Geometry and architecture of the device 29 3.3 Solar cell performances of device with or without buffer layer 32 3.4 The morphology of the buffer layer 32 3.5 Switching HOMO level of the buffer layer 36 3.6 Summary 42 Chapter 4 Organic Solar Cells Featuring Nanobowl Structures 43 4.1 The needs of structure in active layer 43 4.2 Preparation of the structured active layer 49 4.3 Morphology and Material Characterizations 50 4.4 The active layer along the structured PEDOT 51 4.5 Performances of the devices 54 4.6 Optical properties enhanced by the structure 58 4.7 Anti-reflection issue 61 4.8 Electrical properties enhanced by tuning the work function of PEDOT 64 4.9 Variations of HOMO level and VOC 69 4.10 Summary 70 Chapter 5 Dual-color Electrochromic Films Incorporating a Periodic Polymer Nanostructure 71 5.1 Concept of Electrochromic Films with Nanostructure 71 5.2 Fabrication of Electrochromic Films with Nanostructure 72 5.3 Characterization 74 5.4 Morphology of the PEDOT Bottom Layer 75 5.5 CV Characteristic of the Composite Film 77 5.6 Optical Properties of the Composite Films 82 5.7 Control Material Ratios with Depositing Charge Densities 84 5.8 Coloration Efficiency Improvement 87 5.9 The Reduction of Ionic Resistance 90 5.10 The Improvement of Switching Time & Stability 95 5.11 Summary 99 Chapter 6 Conclusions 100 6.1 A solar cell with polythiophene buffer layer (Ch 3) 100 6.2 Conclusions of the organic solar cells featuring nanobowl structures (Ch 4) 101 6.3 Conclusions of the dual-color electrochromic films incorporating a periodic polymer nanostructure (Ch 5) 102 Chapter 7 References 104 Appendix A 114 Author’s Profile 11