可熱交聯共軛高分子之合成及其在有機場效電晶體的應用

Synthesis and characterization of thermally crosslinkable conjugated copolymers for use in organic field-effect transistors

蕭妤卉(Hsiao, Yu Hui)

指導教授 ： 堀江正樹

國立清華大學/工學院/化學工程學系/碩士(2015年)

摘要

為了增進高分子材料與有機場效電晶體的熱穩定性，我們製備出可熱交聯的高分子(crosslinkable polymers, Cr-PCPDTBT, Cr-PCDTBT, Cr-PTB7)，並探討其在元件上的表現與老化情形。高分子合成部分，在PCPDTBT, PCDTBT及PTB7合成過程添加5 mol%含有1-hexenyl group的可熱交聯單體，最後加入4-bromoanisole，Heck reaction可使熱交聯單體上的allyl group接上anisole基團，以降低高分子的交聯溫度。我們藉model reaction來了解Heck reaction的條件與活性。為比較高分子特性與元件表現，我們也製備不含可熱交聯單體的高分子(standard polymers, PCPDTBT, PCDTBT, PTB7)當對照組，使用紫外線/可見光光譜儀與循環伏安儀分析standard polymer與crosslinkable polymer的光學性質及電化學性質。
有機場效電晶體的製備在一般大氣環境下完成，且將主動層在不同的溫度下做退火處理(r.t., 140 ºC, 160 ºC, 180 ºC)，結果發現PCPDTBT的載子移動率隨溫度增加而降低，而加熱後的Cr-PCPDTBT可得較高的載子移動率及較長的元件壽命，元件熱穩定性的提升可歸因於Cr-PCPDTBT熱交聯結構; PCDTBT與Cr-PCDTBT的載子移動率皆隨溫度增加而升高，且在相同製備條件下Cr-PCDTBT的載子移動率略低於PCDTBT，因退火處理使高分子呈現較規則結構的影響可能大於Cr-PCDTBT熱交聯結構的影響; PTB7與Cr-PTB7的載子移動率皆隨溫度增加而降低，可能歸因於PTB7加熱後較不規則的分子排列，然而Cr-PTB7的元件壽命卻較PTB7長。本研究使用原子力顯微鏡與X光繞射分析高分子薄膜的表面形態，電子能譜儀則用來分析高分子老化前後的化學結構變化。

關鍵字

熱交聯 ； 共軛高分子 ； 有機場效電晶體 ； 熱穩定性

並列摘要

In this work, thermally crosslinkable conjugated copolymers have been developed to improve lifetime of organic opto-electronic devices. Poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (PCPDTBT), poly[N-9′-heptadecanyl-2,7-carbazole- alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and poly({4,8- bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) are selected as the backbone structures because these polymers show high performances when they are used as active materials in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). In Pd-catalyzed Suzuki or Stille coupling polymerizations, 5 mol% of dibromo-cyclopentadithiophene comprising crosslinkable 1-hexenyl groups was added as a comonomer followed by subsequent addition of 4-bromoanisole to cap the allyl groups on the comonomer by Heck reaction. The purpose of the Heck reaction is to reduce a thermal crosslinking temperature. After purification by Soxhlet extraction, crosslinkable polymers Cr-PCPDTBT, Cr-PCDTBT and Cr-PTB7 were obtained in 56-89% yield. To compare the optical and electrochemical properties and device performances, standard polymers (PCPDTBT, PCDTBT and PTB7) without crosslinkable moiety have also been synthesized. To undergo crosslinking reaction of Cr-PCDTBT and Cr-PTB7 in solid state, polymer films in vial tubes were annealed at 160 ºC for 8-16 hours. After the annealing and addition of chloroform into the vial tubes, insoluble particles were observed in the chloroform solutions. This poor solubility of the polymers results in the thermally crosslinking reaction. UV-vis spectra and cyclic voltammograms (CVs) of the polymers were measured to estimate optical bandgap and HOMO and LUMO levels of the polymers. The polymers show relatively low optical bandgap in a range of 1.40~1.85 eV. The crosslinkable polymers show very similar UV-vis spectra to the standard polymers, indicating that the addition of crosslinkable moiety to the standard polymers does not interrupt their original optical properties. Similarly, CVs of the crosslinkable polymers are similar to the standard polymers.
The OFETs using thermally crosslinkable polymers were fabricated in ambient condition. The best performing device fabricated from Cr-PCPDTBT showed hole mobility of 4.63 × 10-4 cm2 V-1 s-1 after annealing at 160 ºC for 1 hour, which is higher than that from standard PCPDTBT (2.74 × 10-4 cm2 V-1 s-1). In addition, the lifetime of OFET using Cr-PCPDTBT was enhanced by crosslinking. Cr-PCDTBT exhibited the best hole mobility at 1.28 × 10-3 cm2 V-1 s-1 after annealing at 180 ºC for 1 hour. On the other hand, PCDTBT showed slightly higher hole mobility (1.83 × 10-3 cm2 V-1 s-1) than Cr-PCDTBT (1.28 × 10-3 cm2 V-1 s-1) under the same annealing condition (180 ºC for 1 hour). This is probably attributed to more ordered packing structure after annealing. Furthermore, the stability of OFET using Cr-PCDTBT was similar or slightly worse than standard PCDTBT. This is presumably because the effect of thermal annealing dominates over that of structural crosslinking for Cr-PCDTBT. The best performing device fabricated from Cr-PTB7 was measured at 1.21 × 10-4 cm2 V-1 s-1. The decreased hole mobility was observed in Cr-PTB7 after annealing, resulting from the less dense molecular packing. However, the lifetime of OFET using non-annealed Cr-PTB7 was significantly enhanced. The morphological analysis was conducted by Atomic Force Microscopy and X-ray diffraction. The chemical structural changes with ageing were analyzed by X-ray photoelectron spectroscopy.

並列關鍵字

thermal crosslinking ； conjugated polymer ； Heck coupling ； organic field-effect transistor ； thermal stability

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