複合靜電紡絲纖維在非揮發性有機記憶體上之應用與形態分析

Abstract

非揮發性場效電晶體作為儲存資訊的元件具有應用在有機光電元件的潛力。然而直至今日，其在製備方式、型態控制與相關的電性表現上仍有許多探討空間。在本篇論文中，我們採用高分子P4VP混摻半導體小分子PCBM的系統，在靜電紡絲技術下製備複合奈米纖維，並將其作為電介層運用在電晶體式記憶體元件中。複合靜電紡絲纖維對有機半導體分子的影響可主要分為型態和電性二方面，分述如下: 1. 複合靜電紡絲纖維幾何形狀對主動層分子型態與結構之分析: 本研究中的奈米纖維與一般藉由旋轉塗布製成的薄膜最大差異在於改變五聯苯pentacene晶粒型態與結構。由於P4VP與PCBM之間的價電傳輸作用力(charge-transfer interaction)，電紡過程獨特之尺寸抑制效應，故纖維表面非常平滑且均勻。由其形態結果來看，蒸鍍在奈米纖維之上的pentacene其結晶度有明顯被提升的效果，且結晶大小較ODTS改質的SiO_2基板提升約二倍，此顯著差異可歸功於纖維特有的半圓柱表面，使得在蒸鍍過程中的pentacene氣相分子不易吸附在纖維凸面上，造成較低的晶核密度，產生較大的晶粒粒徑。此結果初次提供電介層表面曲率對pentacene結晶成長動力學的研究。 2. 複合靜電紡絲纖維之內電場對場效電晶體記憶儲存之影響:奈米纖維是以高分子/半導體小分子PCBM之混摻系統作為材料，依其功能而言，高分子當作電流阻障層(current blocking layer)；PCBM則扮演捕捉電子的角色，除PCBM混摻比例之外，高分子本身的極性，更重要的是纖維的奈米尺寸產生極大的內電場，注入並儲存載子於纖維內，其製成的元件載子移動率高達2.95 cm^2⁄Vs，電流開關比(ON/OFF current ratio) 超過〖10〗^7，合適的操作條件下，記憶體操作空間(memory window)達到50 V，電性表現遠優於薄膜電晶體。 目前的研究結果顯示電介層幾何形狀對於主動層結晶型態和記憶體元件特性具有極重要的影響。

The nonvolatile transistor memory is a well-known element as an information storage device in a variety of organic electronics. Up to now, the fabrication, morphology control, and electrical properties of devices with the 1-D (one-dimension) configuration remain challenging. In this thesis, new composite nanofibers of polymer and blending with high fraction of small semiconductor molecule are fabricated by electrospinning technique and first applied on organic field-effect transistor memory (OFET memory) as the polymer dielectrics. We studied the effects of electrospun nanofibers as polymer gate dielectrics on pentacene transistor memory as the following: 1. Influences of dielectric geometry on pentacene monolayer growth and finial morphology: On account of the charge-transfer interaction existing in P4VP (Poly (4-Vinyl Pyridine)) and PCBM ([6, 6]-phenyl-C61-butyric acid methyl ester), along with the high evaporation rate and the confinement effect in the electrospinning process, we can obtain uniform and smooth nanofibers. From the results of experiments and analyses, the nucleation behaviors of pentacene molecules appear to be changed on nanofibers. Averaged grain size of pentacene grown on the semicylinder surface of nanofiber becomes about two times larger than that of ODTS (octadecyltrichlorosilane) modified SiO2, suggesting that the grain size is enhanced by eletrospun nanofibers, which leads to a much higher field-effect mobility of 2.95 cm^2⁄Vs compared to thin-film dielectrics whose resulting in field-effect mobility is only 0.04 cm^2⁄Vs. It is because that a smaller amount of nuclei of pentacene formed on the cylindrical nanofibers at the early stage of crystal development, gives rise to larger grain size. The correlation of surface curvature and the growth dynamics of pentacene is provided in this thesis. 2. Effects of the built-in electric field of nanofiber dielectric: In polymer composite nanofibers, polymers are taken as the electron-transporting layer or blocking layers and on the other hand, semiconductor PCBM serves as electron-trapping sites. There are three factors that affect the memory window in nanofiber OFET memory device, i.e. the PCBM corporation, polarity of polymer, and most important of all, the geometry effect of fibers in nanoscale. A higher electrical field generated in the nanofibers allows more electrons to inject into the electrets. With electrospun composite nanofibers as dielectrics, OFET memory exhibits a larger memory window of about 50 V and a higher ON/OFF current ratio of over 〖10〗^7. The present study addresses the importance of dielectric geometry on the pentacene morphology and OFET memory properties.