導電高分子PEDOT:PSS於熱電應用之改質與特性研究

Abstract

近數年來，有機熱電材料已經引起廣泛的注意。由於導電高分子材料具有低導熱率、以及趨近於金屬般高導電率之本質特性，使得導電高分子材料在熱電材料與原件的應用中形成獨立且受矚目的分枝。除了上述優勢之外，無毒性、可撓性、可塗佈性、輕量性、光穿透性等都是有機熱電材料獨具之優勢。本論文主要探討導電高分子材料poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)於常溫下之改質製程，利用各種添加物多面向調控該其熱電性質。除了一般熱電與基礎材料領域之分析外，並輔以各種理論計算相互佐證。 本論文的第一部分主要以甲酸銨(ammonium formate, AF)作為添加劑，製備具有高西貝克系數(Seebeck coefficient)之PEDOT:PSS熱電薄膜。調控此添加物之濃度，PEDOT:PSS之西貝克系數可提升約40倍。霍爾量測証明此效應乃來自於甲酸銨所造成之PEDOT:PSS載子濃度下降。此外，甲酸銨之熱分解性於材料中導入空孔與微結構，進一步使導熱係數下降，提昇整體熱電優值(thermoelectric figure of merit, ZT)。 本論文第二部分主要是以第一部分成果為基礎，除了進一步擴展有效鹽類之種類外，亦同時對PEDOT:PSS進行導電率之改質，以彌補載子濃度降低所導致之熱電性質劣化。本研究以兩種不同方式，包括表面處理法及混摻法，將各種鹽類與導電改質物併用時，PEDOT:PSS之熱電特性呈提升趨勢。在2-methoxyethanol (ME)之表面處理法中，西貝克系數隨鹽類濃度增加而提高，但導電率則呈相反趨勢。而在dimethyl sulfoxide (DMSO)的混摻法中，鹽類亦扮演與表表處理法中相同之化學改質角色，然而因混摻法須加熱以移除溶劑，不同鹽類將產生不同程度之熱分解，故材料中殘留之鹽類將對PEDOT:PSS之導電率造成不同程度之影響。相較於表面處理法，混摻法另一優勢是，盬類熱分解所造成之空孔結構，將可更進一步降低導熱係數，提昇整體之熱電優值。 本論文第三部分主要利用NaOH溶液中和PEDOT:PSS之酸性，產生各種不同酸鹼度之高分子溶液並成膜，同時確認這些薄膜中載子濃度與熱電性質間之關係。在pH值1.5至14的變化過程中，伴隨著載子濃度有限程度之下降，西貝克係數卻可循級數提高約100%，同時導電率則按線性減少。且由理論計算可知若溶液pH值略高於原始溶液，可得最佳之熱電性質。此載子濃度下降乃歸因於PEDOT由雙偏極子(bipolaron)態漸漸轉型成偏極子(polaron)態甚或中性(neutral)態。此理論除可說明鹽類改質之機制外，更可甚至延伸應用至其他有機熱電材料。 本論文最後一部分以串連/並聯之基本電學理論出發，實現以表面處理法製造並聯同質高導電熱電材料的可能性。同時也利用理論與實驗之結合，推估表面處理法之可能處理深度。與ME表面處理可提高導電性質相對，AF之水溶液應用於表面處理法時可提高PEDOT:PSS厚膜之西貝克常數。此外，本研究亦發現將單純甲酸銨水溶液與ME並用於表面處理時，交替二者之處理次序將對材料之熱電性質產生三個級數之差距。

In recent years, organic thermoelectric (TE) materials have drawn a vast amount of attention. Since the ionic conducting polymers (ICPs) are naturally high in electrical conductivity and low in thermal conductivity, they outstand as a promising branch in TE research. Their solubility, flexibility, non-toxic nature and low-density are also their unique properties comparing to inorganic TE materials and make them suitable for portable applications. In this thesis, the TE properties of commercial poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) films are multiply controlled and studied by combining different additives and enhancers under room temperature. In addition to material characterizations and TE measurements, theoretical assumptions and calculations are also proposed throughout this dissertation.

In the first section of this dissertation, the doping effects of thermal decomposable ammonium formate (AF) in different doping amount have been applied to improve the Seebeck coefficient of PEDOT:PSS films. It has been found that the Seebeck coefficient can be effectively enhanced in a very wide range by varying the AF doping concentration, where a maximum value of 436.3 μV/K was obtained, i.e. ~40 times higher in magnitude than the pure PEDOT:PSS films prepared with same process. This effect can be attributed to the decrease of carrier concentration, as characterized by Hall measurement. In addition, AF also plays an important role in the formation of the closed or open pores and channels within the films for phonon scattering, as can be clearly observed in the SEM images.

In the second section of this dissertation, starting from the previous section, a selection of ammonium salts are used in combination with two different conductivity enhancers, namely 2-methoxyethanol (ME) and dimethyl sulfoxide (DMSO) through the surface treatment method and the blending method respectively, to compensate the lowering of carrier concentration. In surface treatment method, ammonium ion and ME played their respective role to enhance Seebeck and electrical conductivity, resulting a good power factor up to 20.02 μW/mK2. Seebeck coefficient and electrical conductivity are increased accordingly with similar mechanism as DMSO and AF are both blended into PEDOT:PSS dispersion. AF plays a more important role by thermal decomposing itself and decreases the thermal conductivity, achieving excellent ZTs of 0.064 at room temperature.

The third section of this dissertation discusses about the carrier concentration effect on TE properties of PEDOT:PSS films at different pH value. With the addition of sodium hydroxide, this ionic conducting polymer (ICP) is dedoped as the pH value increases from 1.5 to 14 and the Seebeck coefficient is exponentially enhanced by ~100% with the decrease of carrier concentration. Furthermore, the decrease of carrier concentration also linearly degraded the electrical conductivity of PEDOT:PSS due to their dependency from Hall effect. A maximum power factor can be obtained at a pH value slightly higher than original, which is evidenced theoretically and experimentally. This dedoping behavior is also proved to be the consequence of the transition of bipolaron PEDOT to a lower charged polaron state or even uncharged neutral state by UV-vis spectra and Raman spectra.

In the final section of the dissertation, the series and parallel model equations are used to describe the respective thermoelectric properties of ME surface treated PEDOT:PSS bulk films. Experimental data points and theoretical lines can be plotted and the treatment depth of the conductivity enhancer ME can thus be evaluated by imaging the actual film thickness. Treatment of AF solution, as contrast, grants the enhancement on Seebeck coefficient of PEDOT:PSS. These two treatment methods can be deployed in sequence and found that in order to obtain higher thermoelectric properties, a correct order of treatment will cause the power factor reaching over 26 μW/mK2.