染料敏化太陽能電池：PEDOT-MeOH薄膜與高分子離子液體之研究

Abstract

本論文旨在發展低成本、高效率並具有長期穩定性的染料敏化太陽能電池(染敏電池)，製備具有特殊結構的poly(hydroxymethyl 3,4-ethylenedioxythiophene) (PEDOT-MeOH)薄膜應用於對電極，與合成無碘高分子型離子液體應用於膠態電解質。本論文主要分為兩大部分：PEDOT-MeOH薄膜應用於對電極(第三章)，和高分子型離子液體應用於膠態電解質(第四章)。 第三章中，我們成功地利用無模板電化學聚合法合成多層次的PEDOT-MeOH，結構類似於管狀珊瑚陣列(TCA)。PEDOT-MeOH TCA具有下列三點特色：(一)因為擁有甲醇推電子基，所以共軛長度比PEDOT還要長。(二)具有管狀的一維電子傳輸路徑。(三)具有珊瑚狀的延伸活性表面積。因為PEDOT-MeOH TCA擁有優異的催化能力來催化三碘離子的還原反應，所以其適用於染敏電池的對電極，並且該電池轉換效率可達9.13±0.06%，且高於使用白金對電極之電池效率(8.94±0.07%)。從旋轉盤電極分析，發現雖然PEDOT-MeOH TCA比白金薄膜的本質電子介面傳輸速率(k0)較低，但活性表面積(Ae)較高出許多。PEDOT-MeOH TCA擁有優異的電催化能力、低成本與簡單的製程方法，我們認為其可以取代昂貴的白金對電極。 第四章中，我們設計並合成一多功能性的高分子離子液體：poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS)，並將其應用於半固態染敏電池的膠態電解質。多功能性的POEI-IS具有下列四點特色：(一)可以當作電解質之膠化劑。(二)可以當作電解質內的氧化還原對：硒氰酸離子(SeCN−)；而硒氰酸氧化還原對之氧化還原電位更高於傳統使用之碘氧化還原對。(三)可以利用主鏈上之氧原子的孤對電子去螯合電解質中的鉀離子。(四)可以利用其鏈段阻擋光陽極上之電子與三硒氰酸離子((SeCN)3−)發生再結合反應。綜觀以上特色，特色(一)可以增加染敏電池的長效穩定性；特色(二)(三)(四)可以提升染敏電池的開路電壓。而我們製備含有30 wt% POEI-IS的膠態電解質於半固態染敏電池，該電池之開路電壓可達825.50±3.51 mV、轉換效率可達8.18±0.02%，且經過1000小時之後，它穩定性測試可以維持95%的起始效率。設計功能性的高分子離子液體是發展高效率與高穩定性的半固態染敏電池之趨勢。

This thesis aimed to develop structured poly(hydroxymethyl 3,4-ethylenedioxythiophene) (PEDOT-MeOH) films as counter electrodes (CEs) and to synthesize an iodide-free polymeric ionic liquid (PIL) as gel electrolyte for the dye-sensitized solar cells (DSSCs) with low-cost, high efficiency, and long-term stability. This thesis is divided into two parts: PEDOT-MeOH films as CEs (Chapter 3) and PIL as gel electrolyte (Chapter 4). In the case of the PEDOT-MeOH films as CEs, a film of the hierarchical PEDOT-MeOH tube-coral array (TCA) was successfully synthesized via a template-free electro-polymerization technique. The PEDOT-MeOH TCA was designed to simultaneously possess (1) the enhanced conjugation on PEDOT main chain due to the electro-donating MeOH group, (2) the tube-like fast one-dimensional charge transfer pathways, and (3) the coral-like extended electro-active sites. For the application in DSSCs, PEDOT-MeOH TCA worked as an outstanding electro-catalytic CE for iodine/triiodine (I–/I3–) reduction. Thus, the DSSCs with the hierarchical PEDOT-MeOH TCA as the CE reached the highest power conversion efficiency (η) of 9.13±0.06%, which was even higher than that of the DSSC with a standard Pt CE (8.94±0.07%). Via rotating disk electrode analysis, the newly synthesized PEDOT-MeOH TCA film was found to have a lower intrinsic heterogeneous charge-transfer rate constant (k0), but extremely larger effective electro-catalytic surface area (Ae) than that of the standard Pt film. The PEDOT-MeOH TCA can be considered as a convincing replacement of the expensive Pt due to its high electro-catalytic ability, low cost, and simple fabrication process. In the case of the PIL as gel electrolyte, a polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was newly synthesized and used for a multifunctional gel electrolyte in a quasi-solid-state dye-sensitized solar cell (QSS-DSSC). POEI-IS has several functions: (1) acts as a gelling agent for the electrolyte of the DSSC, (2) possesses a redox mediator of SeCN−, which is aimed to form a SeCN−/(SeCN)3− redox couple with a more positive redox potential than that of traditional I−/I3−, (3) chelates the potassium cations through the lone pair electrons on the oxygen atoms of its poly(oxyethylene)-imide-imidazolium (POEI-I) segments, and (4) obstructs the recombination of photo-injected electrons with (SeCN)3− ions in the electrolyte through its POEI-I segments. Thus, the POEI-IS renders a high open-circuit voltage (VOC) to the QSS-DSSC due to its functions of (2), (3), and (4), and prolongs the stability of the cell due to its function of (1). The QSS-DSSC with the gel electrolyte containing 30 wt% of the POEI-IS in liquid selenocyanate electrolyte exhibited a high VOC of 825.50±3.51 mV and a high η of 8.18±0.02%. The QSS-DSSC with 30 wt% of POEI-IS retained up to 95% of its initial η after an at-rest stability test with the period of more than 1,000 h. This properly designed PIL paves as promising way for developing highly efficient and durable QSS-DSSCs.