普魯士藍、導電高分子PEDOT及其電致色變元件： 熱穩定性、長期穩定性與離子進出PEDOT薄膜之傳輸行為研究

Abstract

本論文研究的電致色變材料選定為氧化著色的普魯士藍(Prussian Blue, PB)以及還原著色的poly(3,4-alkylenedioxythiophene)(PEDOT)，在搭配以碳酸丙烯(Propylene carbonate, PC)為溶劑的電解液後，可組成一PEDOT-PB互補式電致色變元件，其顏色變化上可由幾乎透明的淡藍色轉為深藍色。研究內容主要針對PEDOT薄膜、PB薄膜與PEDOT-PB元件在室溫與70℃下的穩定性進行一系列有系統的探討與分析，而離子進出PB薄膜與PEDOT薄膜的行為同樣也是研究的重點。最後，將藉由量測元件在室溫與70℃下的兩極電位分布，對元件長期連續操作穩定性的變化趨勢提出可能的解釋。 對於單極薄膜而言，我們評估了PB薄膜與PEDOT薄膜在70℃下0.1 M LiClO4/PC溶液中的熱穩定性，得知此兩薄膜皆具良好的靜態熱穩定性，研判影響PB薄膜與PEDOT薄膜在70℃下0.1 M LiClO4/PC溶液中之穩定性優劣的關鍵為連續操作穩定性。此外，我們提出當PB薄膜在0.1 M LiClO4/PC溶液中由PB態還原成普魯士白(Prussian White, PW)狀態時，會有部分Li+卡在晶格內的現象產生，使得部份PW態無法氧化回PB態進而導致PB的著色態穿透度呈現隨操作次數增加而衰退的趨勢。在以EQCM分析離子進出PEDOT薄膜方面，得知PEDOT在中性態與摻雜態(p-type)之間進行氧化還原時，薄膜內的電中性是由陰陽離子共同貢獻的。在室溫下0.1 M LiClO4/PC溶液中，Li+是維持薄膜電中性的主要離子，而在室溫下0.1 M TBAClO4/PC溶液中，維持薄膜電中性的主要離子則改變為ClO4-。另外，我們更進一步地提出PEDOT薄膜的氧化還原方程式並計算出陰陽離子的計量係數。 根據PEDOT-PB元件在70℃下的靜態熱穩定性與長期連續操作熱穩定性的評估結果，可判定元件在70℃的存放溫度下相當穩定，需要注意的應該是元件在70℃下的連續操作熱穩定性。此外，藉由比較元件在室溫與70℃下的長期連續操作穩定性可更清楚得知70℃主要是扮演加速元件因連續操作而衰退的角色。在著去色電壓的選擇上，不論在室溫或是70℃下，以-1.2 V為著色電位及0.6 V為去色電位，可得到較佳的穿透度變化值與穩定性。在室溫下，以階梯電位法連續操作10,000圈後，元件的穿透度變化仍維持最大穿透度變化的95.8%；當在70℃下連續操作10,000圈後，元件的穿透度變化仍則保有最大穿透度變化的61.0%。最後，利用量測元件在室溫與70℃下的兩極電位分佈，可知兩極的電位飄移對於元件在室溫與70℃下的長期連續操作穩定性表現不容忽視，並可說明70℃如何扮演加速元件因連續操作而衰退的角色。

In this work, the complementary electrochromic device (ECD) with a color change between nearly transparent light blue and deep blue was assembled by poly(3,4-alkylenedioxythiophene) (PEDOT) and Prussian blue (PB). For the preparation of the electrolyte, the solvent was propylene carbonate (PC) and the salt was LiClO4 or TBAClO4. The objective of this study is focused on the stabilities of PB and PEDOT thin films as well as the PEDOT-PB ECDs at R.T. and 70℃. The ion transport within the PEDOT and PB are also discussed. Furthermore, the potential distributions of PEDOT and PB within the ECDs were measured to explain the change of the transmittance responses in the ECDs with increasing cycle number. After evaluating thermal stabilities of the PEDOT and PB thin films in 0.1 M LiClO4/PC solution at 70℃, the results showed that both PEDOT and PB thin films had good at-rest thermal stabilities but worse cycling thermal stabilities than their at-rest thermal stabilities. Moreover, we proposed that when a PB thin film is switched from the PB state to the Prussian White (PW) state in 0.1 M LiClO4/PC solution, Li+ will be inserted into the lattice and some will be trapped. This phenomenon forbids some of the PW oxidizing to the PB state and causes the decay of the darkened transmittance with increasing cycle number. The EQCM analysis revealed that the electroneutrality of the PEDOT thin film was mantained by the transports of both cation and anion when PEDOT was cycled between its neutral state and p-type doping state. In LiClO4/PC solution, the charge compensation within the PEDOT thin film is dominated by Li+. However, in TBAClO4/PC solution, dominant ion for charge compensation is ClO4-. Furthermore, we proposed an equation to explain the redox process for PEDOT and calculated the corresponding stoichiomtric numbers. From the thermal stability test of PEDOT-PB ECD at 70℃, the results showed that the ECDs had good at-rest thermal stabilities but their cycling thermal stabilities were obviously worse than their at-rest thermal stability. The results revealed that 70℃ would accelerate the cycling instabilities of the ECDs. On the other hand, whether the ECDs were switched at R.T. or 70℃, applying -1.2 V for darkening and 0.6 V for bleaching could achieve better cycling stabilities and larger transmittance differences. When the ECD was cycled potentiostatically between -1.2 and 0.6 V for 10,000 cycles at R.T., the transmittance difference still remained 95.8% of the maximum value (ΔTMax). When cycled at 70℃, its remaining transmittance difference at 10,000th cycle decreased to 61.0% of ΔTMax. Finally, according to the potential distributions of PEDOT and PB thin films within the ECDs, we can explain the changes of the transmittance responses of the ECDs as a function of cycle number and also the reason why the cycling instabilities of the ECDs being accelerated at 70℃.