電子施體/受體高分子材料之合成、性質及其記憶體元件應用

Donor-Acceptor Polymeric Materials: Syntheses, Properties, and Their Memory Device Applications

方佾凱(Yi-Kai Fang)

指導教授 ： 陳文章

國立臺灣大學/工學院/高分子科學與工程學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.00210

摘要

近幾年來，高分子材料應用在記憶體是一個新興領域，相較於傳統元件因其本質特型不同而引起了廣泛的研究及討論。含電子施體與受體之高分子為被廣泛研究之項目，透過不同的分子設計結構來調控的光電性質和良好的可加工性，因此在元件應用上擁有極佳之應用潛力。然而，關於不同的高分子結構所造成的影響尚未被深入研究。此論文之研究目標著眼於(1)設計與合成新穎電子施體-受體共軛有機高分子材料 (2) 進行記憶體元件的製備與測試 (3)探討電子施體-受體共軛有機高分子材料結構、形態對於記憶體元件性質。
本論文第一部份(第二章)，為探討雙鏈段之聚噻吩為電子施體-聚噁唑嵌段為電子受體共聚高分子之合成、分子結構、形態、光電性質及其記憶體元件之應用。此新穎共聚物結合經修飾格林納置換反應及原子轉移自由基聚合方式，製備三組不同長度之聚噁唑鏈段(重複單元為5、18與25)，並藉噁唑嵌段長度不同之變化，探討化學結構與微胞形態變化及其相應記憶體元件性質。由廣角及小角X光繞射可看出此雙鏈段高分子隨著噁唑鏈段增加其聚噻吩結晶性隨之下降。從示差掃描熱量分析儀穿、紫外光-可見光光譜儀、循環伏安儀、穿透式顯微鏡及其原子力顯微鏡鑑定下皆得到相似結果。所合成之單聚高分子和雙鏈段共聚高分子製成記憶體元件。聚噁唑鏈段具有較低的最高能階佔有軌域，因此在記憶體元件上扮演著電洞捕捉器，其不同鏈段長度顯示出不同的元件特性。聚噁唑鏈段重複單元為5及25分別顯示是電洞二極體及絕緣體，其中，聚噁唑鏈段重複單元為18顯示是非揮發性與可複寫記憶體性質，這些記憶體性質符合在電子顯微鏡下所觀察到微胞形態之變化。
本論文第二部份(第三章)，以咔唑為電子施體，二種噁唑衍生物為電子受體的單聚高分子與其二類型亂排共聚高分子，並以此二類型亂排共聚高分子以不同電子施體與受體比例分別為 8:2， 5:5和 2:8共聚而得六種亂排共聚高分子。此研究透過分子的電子施/受體設計可以達到記憶體元件的運用。ITO/Polymer/Al所組成的電子元件開關行為可以透過受體電子的比來調整。在實驗及理論上的結果顯示了電子施/受體之間沒有軌域的混成因此電荷轉移相對是很微弱。此外，噁唑衍生物具有較高能接佔有軌域，因此在記憶體元件上扮演著電洞補捉器。從電流電壓圖形的改變可以得知隨著電子受體比例的增加會從揮發性記憶體轉變成絕緣體。P(VPK8OXD2) or P(VPK8BOXD2顯是是靜態隨機讀取記憶體，其其電流開/關比值都104-105，在固定的電壓下其脈衝電壓可達到107、滯留時間達一小時。不穩定的ON state主要是因為當移除外在電場之後，會進行電洞的捕捉並且會自發性地電洞載子的傳輸。而P(VPK8BOXD2)相較於P(VPK8OXD2)擁有較長的ON狀態主要是貢獻於OXD有更低的較高能接佔有軌域。此研究透過分子的電子施/受體設計可以達到記憶體元件的運用。
本論文第三部份(第四章)，合成兩種三苯胺衍生物為電子施體與咔唑為電子授體的單聚高分子及其二類型亂排共聚高分子，並以此二類型亂排共聚高分子以不同電子施體與受體比例分別為 8:2， 5:5和 2:8共聚而得六種亂排共聚高分子，並且探討這些不同比例以及側邊cyano基團對電阻式記憶體性質的影響。隨著電子授體咔唑增加，ITO/P(VTPAxBOXDy)/Al 記憶體元件性質會從揮發性記憶體轉變成絕緣體。ITO/P(VTPAxBOXDy)/Al 及ITO/PVTPA/Al元件分別顯示動態隨機讀取記憶體及靜態隨機讀取記憶體性質，其電流開/關比值都107-108，在固定的電壓下其脈衝電壓可達到108、滯留時間達104s。BOXD較低的HOMO能階為兩個鄰近三苯胺之間電洞阻擋層。電荷捕捉/自發性地電洞載子的傳輸控制著開關行為。另一方面，所有的P(CNVTPAxBOXDy)記憶體元件顯示NDR非發揮性的性質，主要是cyano基團會與金屬鋁產生反應。因此，由這些結果可得知藉由施/受體所組成之側鏈高分子可以進一步應用在記憶體元件。

關鍵字

電子施體/受體 ； 側鏈高分子 ； 記憶體 ； 形態

並列摘要

Donor-acceptor type polymers have attracted a significant interest for memory device applications due to their tunable electronic properties through molecular design. However, the effects of the different polymer structure on the memory characteristics have not yet been explored. In this thesis, we address the above issues by exploring the following subjects: (1) design and synthesis of new donor-acceptor organic polymers, (2) preparation and characterization of the memory devices, and (3) correlation of the donor-acceptor polymer structure or composition with memory device characteristics.
In the first part of this thesis (chapter 2), the synthesis, structures, morphology, optoelectronic and the memory device properties of poly(3-hexylthiophene)-block-poly(vinylphenyl oxadiazole) donor-acceptor rod-coil block copolymers are reported. The novel donor-acceptor rod-coil diblock copolymers of regioregular poly(3-hexylthiophene) (P3HT)-block-poly(2-phenyl-5-(4-vinylphenyl) -1,3,4-oxadiaz-ole) (POXD) were successfully synthesized contain three different the coil lengths of 5, 18 and 25 repeating units by the combination of a modified Grignard metathesis reaction (GRIM) and atom transfer radical polymerization (ATRP). The small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) demonstrate that the decreased crystallinity of the block copolymers were duo to the increasing POXD block ratio was depress the crystallization of P3HT. The DSC, UV-vis, CV, TEM and AFM are similar to the SWAXS results. Homopolymers and block copolymers were synthesized and used to evaluate the memory applications. Mismatch of the metal work function relative to the low-lying HOMO and high-lying LUMO levels of POXD, and served as an insulator and trapping center. These block copolymers depend on different the coil (POXD) lengths exhibited different characteristics of these memory devices. The P3HT44-b-OXD5 and P3HT33-b-OXD25 memory devices are hole-transport diode and insulator, respectively. The P3HT44-b-OXD18 memory devices exhibited non-volatile with NDR behavior.
In the second part of this thesis (chapter 3), a series of non-conjugated random copolymers containing pendent electron-donating 9-(4-vinylhenyl)carbazole (VPK) and electron-withdrawing 2-phenyl-5-(4-vinylphenyl)1,3,4-oxadiazole (OXD) or 2-(4-vinylbiphenyl)-5-(4-phenyl)- 1,3,4-oxadiazole (BOXD) with three different ratio (8/2, 5/5, 2/8) were successfully synthesized by nitroxide-mediated free radical polymerization (NMRP) method. The prepared random copolymers are denoted as P(VPKxOXDy) or P(VPKxBOXDy) with three different electron donor/acceptor (x/y) ratios of 8/2, 5/5, and 2/8. The electrical switching behavior based on the ITO/polymer/Al device configuration could be tuned through the donor/acceptor ratio or acceptor trapping ability. Both experimental and theoretical results indicated that the charge transfer between the pendant donor and acceptor was relatively weak without significant orbital hybridization. In addition, the low-lying HOMO energy level of OXD or BOXD units was employed as charge trapping site. Therefore, distinct electrical current-voltage (I-V) characteristics changed from diode, to volatile memory, and insulator depending on the relative donor/acceptor ratios of 10/0, 8/2, and (5/5, 2/8 and 0/10), respectively. The memory device based on P(VPK8OXD2) or P(VPK8BOXD2) copolymers exhibited volatile static random access memory (SRAM) behavior with an ON/OFF current ratio of approximately 104-105, up to 107 read pulses, and retention time of more than 1 h. The unstable ON state in the device was due to the shallow trapped holes with spontaneously back transferring of charge carriers when the electric field was removed and thus exhibited the volatile nature. The slightly lower HOMO level of OXD moieties than that of BOXD led to P(VPK8OXD2) device storing the charge for a longer period of time. The present study suggested the high performance polymer memory devices could be achieved by changing the donor/acceptor ratio or chemical structure.
In the third part of this thesis (chapter 4), we report the synthesis and resistive-type switching memory characteristics based on new random copolymers of P(VTPAxBOXDy) and P(CNVTPAxBOXDy) containing different donor/acceptor ratios (8/2, 5/5, and 2/8) of pendent electron-donating 4-vinyltriphenylamine (VTPA) or 4,4′-dicyano-4″-vinyl-triphenylamine (CNVTPA) and electron-withdrawing 2-(4-vinylbiphenyl)-5-(4-phenyl)-1,3,4-oxadiazole (BOXD). The effects of donor/acceptor ratios and cyano side group on the memory characteristics were explored and compared with preperties of homopolymers, PVTPA, PCNVTPA, and POXD. The distinct electrical current-voltage (I-V) characteristics of the ITO/P(VTPAxBOXDy) /Al device changed from volatile memory to insulator depending on the relative donor/acceptor ratios. The ITO/P(VTPA8BOXD2) or PVTPA/Al device exhibited DRAM and SRAM behavior, respectively, with an ON/OFF current ratio of 107-108. However, no switching phenomena were observed for a higher BOXD ratio. Moreover, the devices could endure 108 cycles under a voltage pulse and show a long retention time for at least 104 s under a constant voltage stress. The low-lying HOMO energy level of BOXD was employed as hole-blocking units as the charge transport were occurred between the neighboring triphenylamine units. The charge trapping/spontaneously back transferring of trapped carriers controlled the switching behavior. On the other hand, all the P(CNVTPAxBOXDy) memory devices exhibited non-volatile nature with NDR behavior due to the preferred interaction of Al atoms with the cyano group. Here, the results demonstrated that the pendent polymers with specific donor-acceptor chromophores could tune the memory switching characteristics for advanced electronic device applications.

並列關鍵字

Donor-Acceptor ； Pendent ； Memory ； Morphology

參考文獻

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