石墨烯基导电薄膜及其有机半导体二极管器件的制备与性能研究

【作者】 刘举庆；

【导师】 黄维；

【作者基本信息】 南京邮电大学 ， 信号与信息处理， 2011， 博士

【摘要】 石墨烯具有独特的电学性能、优异的力学性能与机械延展性、良好的热稳定性与化学稳定性,是制备高性能导电薄膜的理想替代者,石墨烯基导电薄膜及其有机半导体光电子器件研究引起了人们广泛的关注.当前制备石墨烯基导电薄膜的主要方法包括真空过滤法、旋涂法、化学气相沉积以及外延生长等,基于石墨烯导电薄膜电极已成功应用于有机太阳能电池、有机电致发光二极管以及场效应管等器件中,取得了一些有意义的研究成果.但是,高性能导电薄膜的低成本制备、机理探索及其在新型有机半导体器件方面的研究依然处于初始阶段,相关基础与应用研究还面临着诸多挑战.本论文介绍了石墨烯薄膜的制备技术及其在有机光电二极管领域的应用,总结了有机/聚合物电存储的实现形式与作用机制,重点设计和制备了新型石墨烯基导电功能薄膜及其有机半导体器件,探索了新型导电薄膜及其半导体器件的作用机制.主要研究内容和结果如下:结合当前各种化学还原氧化石墨烯（reduced graphene oxide, rGO）薄膜制备石墨烯导电薄膜技术的优缺点,首次提出一种低温肼润湿还原GO薄膜的方法.此方法采用肼润湿GO薄膜表面来避免还原剂浸泡过程中薄膜剥离问题,同时利用低温（100°C）处理技术提高润湿肼的还原能力,该温度适合绝大多数柔性聚合物衬底.与此同时,我们又首次引入“有效还原深度”概念来评价肼还原GO厚膜的效果,并探索了薄膜的还原机制.与其它常见的化学还原技术相比,肼润湿rGO薄膜具有低的方块电阻与高电导率,其值分别达到160⁵00 ?/□和26 S/cm,这主要因为低温肼润湿还原薄膜具有大的“有效还原深度”（1.46μm）和高的C/O （8.04）.针对肼润湿还原中存在的“有效还原深度”现象,通过多层叠加还原技术实现了不同厚度GO薄膜的彻底有效还原,达到了rGO薄膜方块电阻的可控制备.利用高温热退火GO薄膜的高效性与剥离转移技术,成功制备了基于柔性PET衬底的高导电透明柔性rGO薄膜.为了研究GO尺寸对rGO薄膜剥离可行性的影响,通过差速离心方法获得了不同尺寸（¹μm与⁴μm）的GO.剥离试验表明基于大尺寸GO （⁴μm）制备的rGO薄膜更容易剥离.此外, rGO薄膜剥离的效率受刻蚀剂（NaOH）浓度的影响,快速剥离高质量薄膜的优化摩尔浓度约为2M.这种方法制备的柔性rGO薄膜具有出色的光电性能,方块电阻为8000 /□时的透光率达到80% （550 nm）.在此基础上,通过大尺寸GO与羧基化单壁碳纳米管（SWNT-COOH）直接共混在室内条件下湿法制备了高透明导电全碳类杂化薄膜,方块电阻为4500 /□时的透光率达到90% （550 nm）.所以上述薄膜优良的导电性、高透光性、大面积成膜以及绿色制备使得它们在柔性电子中有着诱人的应用价值.系统分析了rGO薄膜块体的C/O、GO尺寸、薄膜微结构以及有效还原深度等因素与rGO薄膜电导率之间的相互关系,同时研究了GO尺寸、薄膜厚度以及还原方法对rGO薄膜表面形貌的影响.结果显示薄膜的电导率受C/O、GO尺寸、薄膜制备技术及有效还原深度等因素共同影响.当保持GO尺寸、薄膜制备技术以及有效还原深度三个因素相同时,薄膜的电导率伴随着C/O的增大而增高,并最终趋向饱和.类似地,保持其它因素不变时通过改变第四个因素研究其对薄膜电导率的影响,发现基于大尺寸GO （⁴μm）的rGO薄膜电导率（1.1×10⁵ PS/m）约为小尺寸GO （¹μm）时的2倍.同时,制备技术引起的薄膜微结构差异也对电导率有重大影响,石墨烯层层有序叠加结构的薄膜电导率比局部有序叠加结构的薄膜电导率高出一个数量级.此外,由于有效还原深度现象的存在,整体均匀还原rGO薄膜比部分还原薄膜具有更出色的电学性能.形貌研究发现,保持薄膜厚度相同,旋涂制备薄膜时大尺寸GO对应rGO薄膜平整度更小.除了GO尺寸因素外,旋涂制备薄膜表面的平整度还随厚度的增大而粗糙.在还原方法方面,由于热退火技术属于无接触还原,其薄膜表面平整度高.设计和制备了一种结构为rGO/P3HT:PCBM/Al聚合物电存储器件,其中首次将rGO导电薄膜作为电极应用到有机电存储器件中,并首次将体异质结作为功能薄膜用于实现存储功能.该器件的电流电压曲线呈现电学双稳态特性,它具有一次写入多次读取功能.该电存储器件具有高的开关比和低的电转换电压,它们的值取决于rGO导电电极的方块电阻.此外,电流电压的实验与理论拟合数据表明了低导电态时呈现电荷注入限制电流特性,高导电态时呈现欧姆电流特性.这种导电态转变是由于PCBM domain极化引起相邻domains之间局部电场增强所致.除了引用于电存储领域,我们又将湿法制备的GO:SWNT-COOH杂化导电薄膜作为电极应用到有机太阳能电池中,该薄膜的p型半导体性有利于解离空穴的收集.湿法制备的GO:SWNT-COOH/PEDO:PSS/P3HT:PCBM/Al太阳能电池的短路电流与开路电压分别为0.627 mA/cm²P与0.52 V.调制二极管电流电压曲线可以实现对器件功能的调控,指出了场致电荷转移与肖特基势垒是器件实现迟滞整流功能的策略之一.基于此,设计制备了基于后修饰的聚乙烯基咔唑和CdTe纳米晶杂化为电活性材料的二极管,其中咔唑基和纳米晶分别为电子给体和受体.该设计中利用聚合物纳米晶杂化产生场致电荷转移,利用同一有机半导体与不同金属电极接触的能级差来构筑肖特基势垒.电流电压曲线显示负向电压作用时呈现明显的电导转变及磁滞现象,电导转变前后的开关比为¹0³.此外,器件具有整流功能,其整流比为6,最大整流输出电流约为5×10^-5P A.这种非对称性电导转变是场致电荷转移及肖特基势垒共同作用的结果,它不仅可以降低交叉型存储器的误读率,而且CdTe纳米晶的化学搀杂可以实现纳米颗粒在基体中均匀分散,有利于器件长久运行的稳定性.在此基础上,我们分别利用柔性rGO薄膜和GO薄膜充当电极和中间功能层制备了结构为rGO/GO/Al的二极管器件.该器件也具有迟滞与整流效应,其整流比约为3.值得注意是正反扫描时迟滞产生的低阻态在外电场撤除后恢复到初始态,表现出了随机存储特性,拓展了GO薄膜二极管在电存储方面的应用范围.更多还原

【Abstract】 Graphene is a promising conductive material due to its novel electrical properties, extraordinary mechanical property, flexibility, and high chemical and thermal stability, graphene-based conducting electrodes and their applications in organic optoelectronic are emerging as a new field of vital significance. Up to now, various approaches, mainly including vacuum filtration, spin coating, chemical vapor deposition, and expiaxial growth, have been proposed to prepare graphene-based films, much progress has been made toward their use as electrodes in organic solar cells, organic light-emitting diodes, and organic transistors. However, techniques for fabricating graphene films with high performance and low price, mechanisms, and related emerging organic semiconductor devices, are still in their infancy where a lot of challenges remain in fundamental research and practical applications. In this dissertation I introduce the field of fabrication of graphene-based films and related organic optoelectric diodes, progress in organic/polymer memory devices and their mechanisms have been reviewed. While great strides have been made toward the design and fabrication of emerging graphene-based conducting films and their organic electronics. Specially, detailed studies of the conductivity of redued graphene films (rGO) and device mechanisms have been explored. The completed works and the results are mainly summarized as follows:Based on the analysis of advantages and disadvantages of rGO films by chemical reduction, a scheme of hydrazine smearing at low temperature was proposed to reduce GO films. Herein, the hydrazine was smeared on the surface of GO film to avoid fragmentation and delamination of rGO film in hydrazine solution. In order to accelerate the reduction reaction, the hydrazine-smeared GO film was heated to 100°C (low temperature), thus many different polymers can be used as substrates. Moreever, a concept of the“effective reduction depth”for GO film was defined to assess the reduction effect and the reduction mechanism is proposed. The prepared rGO film has a lower sheet resistance (~160-500 ohm/sq) and higher conductivity (26 S/cm) as compared to other rGO films obtained by commonly used chemical reduction methods. The effective reduction can be attributed to the large“effective reduction depth”in the GO films (1.46μm) and the high C/O ratio (8.04). By combining reduction with smeared hydrazine at low temperature and the multilayer stacking technique, rGO film with a tunable thickness and sheet resistance are achievd.A facile approach for fabricating flexible rGO films on PET substrates by high temperature annealing (high reduction ability) and transfer technique was describled. By controlling the differential centrifugation, GO sheets with tunable laterial size have been separated and obtained to study the influence of GO size on the detachment feasibility of rGO film, with the average value of laterial size of ~4μm for large-size GO and ~1μm for small-size GO. GO size dependent the detachment feasibility of rGO films has been observed and rGO films from large-size GO is easily detached. Meanwhile, the concentration (2M) of NaOH etchant, as a critical factor of detachment efficiency, is optimized. The flexible rGO films show outstanding optic-electronic properties, possessing a sheet resistance of 8000 ohm/sq with a transmittance of ~80% at 550 nm. Moreover, by blending the large-size GO and SWNT-COOH directly, a higher conductive and transparent all-carbon hybrid film was solution processed under ambient condition, possessing a sheet resistance of 4500 ohm/sq with a transimittance of ~90% at 550 nm. Thus, combining the remarkable conductivity, high transparency, large-scale, and green production of these conducting films make them of tremendous potential in flexible electronics.The relationship between the conductivity of rGO film and its four contributions, including GO size, C/O ratio, film fabrication process, and effective reduction depth, has been systematically analysed, the effect of GO size, film thickness, and reduction method on the film morphology is investigated as well. By varying the fourth factor with other three factors restricted, measurement results show that the film conductivity is dependent on the four contributions. The conductivity increases with an increasing C/O ratio and tend to saturation finally. Furthermore, rGO films from large-size GO exhibit a higher conductivity with a value of 5P S/m, about two times than that from small-size GO. Moreover, great significance of the difference of conductivity induced by the microstructure of rGO film fabrication is obtained; the conductivity of rGO films with well stacked microstructures is about 10 times higher than that with random stacked microstructures. In addition, the GO films reduced completely and uniformly show a higher electrical property than that reduced partially due to the presence of effective reduction depth. From the point of view of film morphology, the roughness from large-size GO sheets is lower than that from small-size GO when their thickness are similar. Meanwhile, thin film has smooth surface relative to thick film. We also find the film morphology by thermal annealing show the highest quality compared to that by other reduction methods.A unique device structure with a configuration of rGO/P3HT: PCBM/Al has been designed for the polymer memory device, where the rGO film and bulk heterojunction concept are firstly applied as the electrodes and functional layer in organic memory, respectively. The current-voltage curves of the device show the electrical bistable behavior and WORM memory effect, with a high On/Off ratio (10P) and low switching threshold voltage (0.5-1.2 V), which are dependent on the sheet resistance of rGO electrode. Our experimental results confirm that the carrier transport mechanism in the Off and On states are dominated by the thermonic emission current and ohmic current. The polarization of PCBM domains and the localized internal electrical field formed among the adjacent domains are proposed to explain the electrical switching of the memory device. In addition, solution-processable GO: SWNT-COOH hybrid films are also introduced as electrodes in organic solar cells, the hybrid film favors the collection of dissociated hole due to its p-type semiconductor property. The solution-processable polymer solar cell with a configuration of GO: SWNT-COOH/PEDOT: PSS/P3HT: PCBM/Al has a shor-circuit current of 0.627 mA/cmP and open-circuit voltage of 0.52 V.The device function is dependent on its current-voltage characteristics; one of the strategies to tune current-voltage behanviors in organic diodes is to combine field-induced charge transfer with schottky barrier. According to this principle, a diode was fabricated utilizing a hybrid of post-functionalized polystyrene derivative covalently tethered with electron-donar carbazole moieties and electron-acceptor CdTe nanocrystals. Current-voltage curves show an electrical transition with some hysteresis is only observed under a negative bias, with three orders of On/Off ratio. Moreover, the device has rectifying effect with a rectification ratio of 6 and its maximum rectified output current is ~5×10~5P A. The asymmetric switching is interpreted as the result of both field induced charge transfer ad schottky barrier, capable of reducing the misreading of crossbar memory. Meanwhile, chemical doping of CdTe nanocrystals favor their uniform dispersion in matrix and stable operation of device. Based on above analysis, a diode with flexible rGO film as electrode and GO film as functional layer was designed and fabricated, with a configuration of rGO/GO/Al. Both hysteresis effect and rectifying effect are observed in device, with a rectification ratio of 3. Note that the low resistance state by hysteresis under the positive and negative sweeping will back to its initial state when the applied voltage is removed, promising the effect of random acess memory, which extends the scope of applications of GO film- based diodes in electrical memory.更多还原