聚苯胺複合材料在染料敏化太陽能電池軟質對電極上之應用

Abstract

本論文使用了三種方法在可撓的石墨基板上合成聚苯胺，其方法分別為---化學沉積法、電化學合成法、化學沉積搭配電化學聚合法。我們使用了掃描式電子顯微鏡、熱重分析法、循環伏安法、四點探針等去檢視此複合材料的性質，也進一步將其作為染料敏化太陽能電池之對電極，製成元件以觀測其表現。 化學沉積方面，我們使用了一個新穎的浸塗方法。搭配不同的浸塗時間，分別為---1.0分鐘、1.5分鐘、2.0分鐘、2.5分鐘、3.0分鐘。化學沉積一層聚苯胺/石墨複合材料以及化學沉積兩層聚苯胺/石墨複合材料兩個群組，將會根據不同時間，合成染料敏化太陽能電池所需要的對電極。以1.5分鐘之化學沉積一層聚苯胺/石墨複合材料作為對電極的染料敏化太陽能電池有最好的表現，其光電轉換率為6.16 ± 0.229 %、短路電流密度為短路電流密度為16.1 ± 0.0777 mA/cm2、開路電壓為0.697 ± 0.0238 V、填充因子為0.571 ± 0.0369。 電化學聚合方面，總共有四個群組，每個群組分別都有五個反應時間，分別為25秒、50秒、˙75秒、100秒、125秒。第一個群組為聚苯胺/石墨複合材料；第二個群組為聚苯胺/奈米碳管/石墨複合材料；第三個群組為聚苯胺/石墨烯/石墨複合材料；第四個群組為聚苯胺/奈米碳管/石墨烯/石墨複合材料。以75秒聚合時間之聚苯胺/奈米碳管/石墨烯/石墨複合材做為對電極之染料敏化太陽能電池有最高的光電轉換率，其值為6.34 ± 0.122 %；其短路電流密度為12.9 ± 0.0570 mA/cm2；開路電壓為0.692 ± 0.0122 V；填充因子為0.674 ± 0.00470。有加入奈米碳管與(或是)石墨烯的聚苯胺/石墨複合材料，在作為對電極時，染料敏化太陽能電池會有較佳之光電轉換率表現。 在化學沉積搭配電化學聚合法方面，總共有四個群組，同上一段所述。聚合方法為先以1.5分鐘化學沉積一層聚苯胺在石墨基材上，接下來的合成方法亦同上一段所述。以50秒之聚苯胺/奈米碳管/石墨複合材料作為對電極者，有最高之光電轉換效率，其值為5.35 ± 0.0981 %；其短路電流密度為13.0 ± 0.0901 mA/cm2；開路電壓為0.694 ± 0.0281 V；填充因子為0.574 ± 0.0185。 從隨聚合時間而下降的染料敏化太陽能電池之Voc值，以及遲滯曲線中電壓下降的觀察，我們可以推測在染料敏化太陽能電池中的聚苯胺可能具有電容效應。電容效應主要源自於聚苯胺的氧化態之改變，來自外電路的電子還原了介於半氧化半還原態的聚苯胺鹽。另外，隨著聚苯胺的聚合時間愈長，其表面的纖維狀結構會愈加複雜並逐漸形成網狀結構，此結構對電解液中的氧化還原對(碘/點三根離子)將造成立體阻隔效應，使得三碘離子的還原速率降低。雖然聚苯胺的網狀結構具有廣大的表面積，但是其孔洞狀的結構反而使得碘/碘三根離子對難以進入到此網狀結構的內部進行氧化還原反應。 電容效應造成一個負向的電壓，隨這聚苯胺的聚合時間愈長，電容效應就越加明顯。在光照之下，以聚苯胺作為對電極之染料敏化太陽能電池的開路電壓和短路電流，會隨著聚苯胺的聚合時間增加而有下降的趨勢。

The main idea of this thesis was to fabricate a flexible and low-cost counter electrode for liquid state dye sensitized solar cells. Three methods were applied---chemical deposition, electrochemical polymerization, and chemical-electrochemical polymerization. We used scanning electron microscopy, thermogravimetry analysis, cyclic voltammetry, four-point probe, etc. to characterize of PANi/graphite composite. We also conducted photovoltaic tests, electrochemical impedance spectroscopy, etc. to observe the device performance. For chemical deposition, we adopted a new dip coating method. Different reaction time was set, which was 1.0, 1.5, 2.0, 2.5, and 3.0 minute. Two groups---1 and 2-layered PANi on Graphite composites---were applied in DSSCs. The DSSC with counter electrode of 1-layered PANi by 1.5 minute has the best performance with power conversion efficiency (PCE) of 6.16 ± 0.229 %, short circuit current density (Jsc) of 16.1 ± 0.0777 mA/cm2, open circuit voltage (Voc) of 0.697 ± 0.0238 V, and fill factor (FF) of 0.571 ± 0.0369. For electrochemical polymerization, there are four groups---PANi, PANi/CNT, PANi/Graphen, and PANi/CNT/Graphene all on graphite substrate---prepared by different reaction time, which was 25, 50, 75, 100, and 125 second. PANi/CNT/Graphene on graphite substrateby 75 second has the highest PCE of 6.34 ± 0.122 %, with Jsc of 12.9 ± 0.0570 mA/cm2, Voc of 0.692 ± 0.0122 V, and FF of 0.674 ± 0.00470. Generally, the DSSCs with CNT or/and Graphene in PANi/Graphite substrate have improved PCE and performance. For chemical-electrochemical polymerization, 1-layered PANi by 1.5 minute was deposited on the counter electrode followed by electrochemical polymerization of PANi with different reaction time as mentioned previously. The DSSCs with PANi followed by PANi/CNT on graphite substrate by 50 second has the highest PCE of 5.35 ± 0.0981 %, with Jsc of 13.0 ± 0.0901 mA/cm2, Voc of 0.694 ± 0.0281 V, and ff of 0.574 ± 0.0185. From the decreasing Voc in I-V curves with reaction time and voltage decrease in hysteresis diagrams, we proposed the possible existence of capacity of PANi in DSSC system, which would impose large impact on the performance of DSSCs. The capacity effect might come from change of oxidation state of PANi. The electrons from external circuit would reduce the protonated emeraldine salt which is at medium oxidation state. Besides, as reaction time for PANi gets longer, the fiber structure interweaves into a network, which imposes steric hindrance for the redox couple (iodide/triiodide) in the electrolyte and thus decrease the reduction rate of triiodide. Although more surface area is created as PANi grows more complexly, the small pores of the network structure prevent redox couples from diffusing into the inside of PANi and undergoing redox reaction. Capacitance effect builds up a reverse potential to the DSSC. Under illumination, Voc and Jsc of DSSCs with PANi as the counter electrode feature a descending tendency with increasing reaction time of PANi.