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  • 學位論文

電子施體/受體高分子系統之合成、形態及元件應用

Syntheses, Morphology, and Device Applications of Donor-Acceptor Semiconducting Polymer Systems

指導教授 : 陳文章教授

摘要


光電高分子由於其在薄膜電晶體、光伏打電池與高分子記憶體上廣泛應用而備受囑目。然而關於電子施體/受體高分子系統之合成、光電特性以及元件應用方面仍缺乏系統性的深入研究,特別是以電子受體為主的共軛高分子、全共軛雙嵌段式以及4,4’-六氟異丙烯二酞酸酐基所組成之高分子結構。此博士論文目標在於合成設計電子施體/受體高分子結構並探討其表面型態對於光電特性、載子傳輸速率、光伏打電池、以及高分子記憶體等元件特性之影響,此論文中所探討之電子施體/受體高分子系統主要包含: (1)一系列電子受體之雙鍵/參鍵共聚高分子,(2)全共軛雙嵌段式polythiophene與奈米碳球之混掺系統,(3)含硫之4,4’-六氟異丙烯二酞酸酐(6FDA)共聚高分子。 本文的第一個部分-低能隙之電子受體-雙鍵/參鍵共軛高分子之合成、光電性質與元件應用(第二章)-進行一系列不同電子受體之雙鍵/參鍵共聚高分子之合成與性質鑑定,包括PAQV,PAQE,PATPV以及PATPE。並研究不同之鍵結形式對於其光電特性及薄膜電晶體性質之影響。實驗發現這四種高分子皆具有非常小的能隙約介於0.90至2.00eV之間,這是由於導入之雙鍵/參鍵降低了分子間的立體障礙並增強電子受體(AQ及ATP)與雙鍵/參鍵間的電荷轉移作用力。此外,在吸收光譜圖的鑑定中我們很明顯地觀察到AQ/ATP雙鍵之共聚高分子比AQ/ATP參鍵共聚高分子具有更強的吸收強度,此現象說明藉由雙鍵鍵結具有比參鍵鍵結更好的pi電子轉移效應因而造成前者具有比後者更好的電子傳輸能力。將PATPV薄膜進行p型態的電晶體測試可以觀察到其電動遷移率約為2.1×10-3至1.1×10-2 cm2V-1s-1之間,電流開關比約為35至7.56×102之間,實驗結果也說明隨著不同的基板表面改良方式,其電動遷移率可以明顯獲得提升。然而PATPE之電動遷移率僅約為1.7×10-6至8.1×10-4 cm2V-1s-1之間,遠小於PATPV之元件特性。由此部份之研究結果可發現,電子受體之雙鍵/參鍵之共聚高分子具有非常小的能隙與良好的電動遷移率,可近一步地應用於遠紅外光光電元件上。 本文的第二個部份-全共軛之噻吩硬桿-硬桿雙嵌段式共聚物之場效電晶體與太陽能電池特性(第三章)-為研究一全共軛之噻吩硬桿-硬桿雙嵌段式共聚物(P3HT-b-P3PT)之光電特性、形態以及其於薄膜電晶體和光伏打電池上的應用。實驗中利用不同比例之溶劑(氯仿以及二氯甲苯)製備所有參數與元件應用。從吸收光譜圖中可以明顯地發現由二氯甲苯所製備得到的光譜具有明顯的長波長吸收,其位於604nm的噻吩結晶特徵吸收峰也較為明顯。此現象亦可從其相對應之表面形態結構得到證明,P3HT-b-P3PT在二氯甲苯中會形成纖維狀的排列進而具有較明顯的結晶特徵吸收峰。將P3HT-b-P3P薄膜進行p型態的電晶體測試可以觀察到其電動遷移率約在6.0×10-3至2.0×10-2 cm2V-1s-1之間,其電動遷移率會隨著製備溶劑中二氯甲苯的含量提高而提高,同樣地,其原子力顯微鏡影像也顯示利用氯仿所製備形成的表面形態結構為無晶型形態,而由二氯甲苯所製備得的表面結構仍呈現纖維狀結晶排列。最後將P3HT-b-P3P薄膜進行光伏打電池測試也發現不同表面結構上的差異會對元件特性產生顯著的影響,元件效率同樣地隨著製備溶劑中二氯甲苯的含量提高而提高。此外,實驗結果也顯示混掺之PCBM比例也會影響表面結構的形態,此乃不同噻吩硬桿所致,由於P3PT硬桿鏈段和PCBM具有良好的互溶性,因此當混掺之PCBM比例降低之0.7,元件效率可提升至2.80%,從原子力顯微鏡影像可清楚的發現,若混掺之PCBM比例提升之2.0會破壞原本由P3HT-b-P3P形成的纖維狀結構。此部份的研究結果清楚說明全共軛之硬桿-硬桿雙嵌段式共聚物之表面結構變化對於其光電特性與元件特性有著決定性的影響,反之可藉由硬桿鏈段的結構設計與調控不同的製備參數達到元件的最適化。 本文的第三個部份-含硫之六氟異丙烯二酞酸酐共聚高分子之合成與記憶體特性(第四章)-為開發兩種新穎性含硫之4,4’-六氟異丙烯二酞酸酐(6FDA)共聚高分子(APTT-6FDA以及3SDA-6FDA)並研究其於高分子記憶上之應用。此兩種含硫單體APTT以及3SDA相較於4,4’-六氟異丙烯二酞酸酐(6FDA)單體而言具有較強的電子提供力,因此設計此兩種分子結構乃為了增強APTT/3SDA與4,4’-六氟異丙烯二酞酸酐(6FDA)間的電子傳遞與轉移能力以利於高分子記憶體應用。其吸收光譜圖顯示APTT-6FDA之能隙為3.51eV而3SDA-6FDA之能隙約為3.46eV,此差異乃由分子間不同的共平面性所造成。將APTT/3SDA-6FDA薄膜利用一簡易之夾層(三明治)結構進行高分子記憶體測試,結果顯示此元件可進行讀、寫、記憶與抹去的功能且其電流開關比在大氣下高達104,此外前者之起始電壓約在1.5伏特左右而後著的起始電壓約在2.5伏特左右。同時,搭配一系列的理論分析計算幫助我們發現其記憶體轉換機制與其兩者的能階大小、能隙以及偶極距有著相當大的關係。相較於現存的三苯胺與4,4’-六氟異丙烯二酞酸酐(6FDA)共聚物之高分子記憶體,含硫之APTT與3SDA由於具有較低的最高電子填滿軌域(HOMO)以及較大的偶極距,因而可形成較穩定的電子/電洞複合物(complex)進而由揮發性記憶體改良成為非揮發性記憶體。此研究結果顯示此類含硫含硫之4,4’-六氟異丙烯二酞酸酐(6FDA)共聚高分子在高分子記憶體領與中具有相當大的應用潛力。 本文的第四個部份-含單一或雙重Phenoxy中介物之三苯胺-六氟異丙烯二酞酸酐共聚高分子的高效能揮發性記憶體特性(第五章)-為開發兩種含不同數目Phenoxy中介物之4,4’-六氟異丙烯二酞酸酐(6FDA)共聚高分子(PI(AAPT-TPA)以及PI(APT-TPA))並研究其於高分子記憶體上之應用。此兩種單體AAPT以及APT相較於4,4’-六氟異丙烯二酞酸酐(6FDA)單體而言同樣扮演電子施體的角色,在分子結構中導入不同數目之Phenoxy中介物乃為釐清AAPT/APT與4,4’-六氟異丙烯二酞酸酐(6FDA)間的電子傳遞與轉移能力受到高分子結構扭轉的影響程度。以AAPT/APT-6FDA製備而成的高分子記憶體測試結果顯示此兩種高分子具有不同的揮發性記憶體特性,並且可在正、負電壓下進行讀寫的動作,其電流開關比在大氣下高達108~109。前者(PI(AAPT-6FDA))於電壓消失後儲存資料亦同時消失,然而,後者(PI(APT-6FDA))於電壓消失後仍可多保存四分鐘的資料儲存週期。這表示前者為動態隨機存取記憶體,而後者為靜態隨機存取記憶體。同時,藉由一系列的理論分析計算幫助我們理解其記憶體轉換機制與中介物造成的高分子扭曲狀態有關,在一外加電場下,對稱(Dual)之Phenoxy中介物相較於不對稱(Mono)的鍵結會造成高分子鏈產生較大扭曲,因此產生一能障延遲了電子/電洞錯合物的回復,因而產生不同程度的資料儲存時間。此研究結果提供了往後應用於高分子記憶體之4-4’六氟異丙烯二酞酸酐(6FDA)高分子一嶄新的分子設計方向。

並列摘要


Optoelectronic polymers are of wide interests for electronic and optoelectronic devices, such as thin film field effect transistors, photovoltaic cells, and polymer memory devices. However, systematic studies on the synthesis, optoelectronic properties, and device characterizations of the donor-acceptor conjugated polymer systems have not been fully explored yet, especially the acceptor based polymers or conjugated diblock copolymers and polyimide based systems. The goals of this thesis is to address the effect of donor-acceptor structure and morphology on the optoelectronic properties, carrier mobility, photovoltaic or memory characteristics, including: (1) acceptor-vinylene/ethynylene copolymers, (2) all-conjugated diblock poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene)/PCBM blends, (3) sulfur-donor containing polyimides, and (4) triphenylamine-based polyimides containing mono- or dual-mediated phenoxy linkages. 1. Syntheses, properties, and field effect transistors of small band gap quinoxaline- and thienopyrazine-vinylene/ethynylene conjugated polymers (Chapter 2): four new conjugated copolymers of quinoxaline (AQ) and thienopyrazine (ATP) with vinylene (V) or ethynylene (E), PAQV, PAQE, PATPV, and PATPE, were synthesized by Stille-coupling reaction. Both the optical and electrochemical band gaps of the PAQV, PAQE, PATPV, and PATPE were quite small around 1.00 to 2.00 eV, which arose from the reduced steric hindrance arising from the incorporation of the V or E linkage or the intramolecular charge transfer between the acceptor and the V or E linkages. Besides, the AQ/ATP-vinylene copolymers exhibited much higher vis/near infrared absorption intensity than the AQ/ATP-ethynylene ones, which suggested the stronger pi-pi transition intensity in the former and led to better charge-transporting characteristics. The saturation field effect hole mobilities of the PATPV were 2.1×10-3, 1.7×10-2, and 1.1×10-2 cm2V-1s-1 on bare, octyltrichlorosilane(OTS)-treated, and octadecyltrichlorosilane(ODTS)-treated SiO2, respectively, with on-off current ratios of 35, 6.02×102, and 7.56×102. On the other hand, the estimated FET hole mobility of the PATPE were in the range of 1.7×10-6 to 8.1×10-4 cm2V-1s-1, which were significantly smaller than that of the PATPV. The small band gaps and high charge carrier mobility of the prepared copolymers suggested their potential applications for near-infrared electronic and optoelectronic devices. 2. All-conjugated diblock copolymer of poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) for field-effect transistor and photovoltaic applications (Chapter 3): the electronic properties, morphology and optoelectronic device characteristics of all-conjugated diblock copolythiophene, poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) (P3HT-b-P3PT), are reported. The polymer properties and structures were explored through different solvent mixtures of chloroform (CHCl3) and dichlorobenzene (DCB). The absorption maximum (λmax) of P3HT-b-P3PT prepared from DCB showed a shoulder peak indicative for the highly crystalline structure around 604 nm while that from CHCl3 did not show it. The field effect hole mobility of P3HT-b-P3PT increased from 6.0×10-3 to 2.0×10-2 cm2V-1s-1 as the DCB content in the solvent mixture increased. The AFM images suggested that CHCl3 processing solvent led to the amorphous surface structures while DCB resulted in the largely crystalline domain. Such difference on the morphology and hole mobility led to the varied power conversion efficiency (PCE) of the photovoltaic cells fabricated from the blend of P3HT-b-P3PT/PCBM (1:1, w/w). The PCE of polymer/PCBM could be improved to 2.80% while the ratio of polymer to PCBM goes to 1:0.7. The present studies suggested that the surface structures and optoelectronic device characteristics of all-conjugated diblock copolymers could be easily manipulated through the processing solvents, the block segment characteristic, and blend compositions. 3. Synthesis and memoy device characteristics of new sulfur-donor containing polyimides (Chapter 4): the synthesis and memory device characteristics of two new poly[2,7-bis(phenylenesulfanyl)thianthrenehexafluoro-isopropylidenediphthalimide] (APTT-6FDA) and poly[4,4’-thiobis(p-phenylene-sulfanyl)-hexafluoroisopropyliden-ediphthalimide] (3SDA-6FDA) are reported. The sulfur-containing APTT and 3SDA as electron-donor were designed to enhance the electron-donating and charge-transporting characteristics for device applications. The optical band gaps of APTT-6FDA and 3SDA-6FDA estimated from the absorption edges were 3.51 and 3.46 eV, which probably resulted from the difference on the structural coplanarity. The memory device with the configuration of ITO/Polymers/Al showed nonvolatile memory characteristics with low turn-on threshold voltages of 1.5 (APTT-6FDA) and 2.5 (3SDA-6FDA) V, probably resulted from the difference on the HOMO energy level. Also, the memory devices could be repeatedly written, read, and erased. The on/off current ratios of the devices were all around 104 in ambient atmosphere. The lower-lying HOMO energy level, larger band gap, and higher dipole moment of the sulfur-containing polyimide compared to the triphenylamine-based polyimide provided a stable CT complex for the flash memory device. The above electronic properties were further confirmed by the density functional theory (DFT) method at the B3LYP level with the 6-31G(d) basic set. The present study suggested that the new sulfur-containing polyimides would have potential applications for memory devices. 4. High performance volatile polymeric memory devices vased on novel triphenylamine-based polyimides containing mono- or dual-mediated phenoxy linkages (Chapter 5): two novel functional polyimides (PIs), PI(AAPT-TPA) and PI(APT-TPA), consisting of electron-donating 4-amino-4’-(p-aminophenoxy)-triphenylamine (AAPT) or 4,4’-bis(p-aminophenoxy)-triphenylamine (APT) and electron-accepting phthalimide moieties, were prepared for the memory device applications. The memory devices with the configuration of ITO/PIs/Al exhibited two conductivity states and could be swept positively or negatively with a high ON/OFF current ratio of 108~109. The PI(AAPT-6FDA) device relaxed from the ON state to the OFF state quickly after the applied voltages was removed, whereas the ON state of the PI(APT-6FDA) device could retain for around 4 min after the power was turned off. It suggested that dynamic random access memory (DRAM) was exhibited for the PI(AAPT-6FDA) device and static random access memory (SRAM) was for the PI(APT-6FDA) device. The volatile memory characteristics were probably attributed to the unstable charge transfer (CT) complex based on the weak theoretical dipole moments of the studied PIs. The dual-mediated phenoxy linkage of PI(APT-6FDA) led to the more twisted conformation compared to the mono-substituted PI(AAPT-6FDA) based on the theoretical analysis by the density functional theory (DFT) method. It thus produced a potential barrier for delaying the back CT process by the electric field and explained the SRAM characteristic. The present study suggested that the importance of the TPA structure on the memory characteristics. The fast switching and high ON/OFF characteristics also indicated the new TPA based polyimides for advanced memory technology.

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