Design, Fabrication, and Performance Investigation of Organic
Optoelectronic Devices
Chong-an Di
ABSTRACT
Organic optoelectronic materials and devices, which is also called „plastic electronics‟, attrached focus attention in past decade due to their potential application in large area and low cost flexible displays, solid-state lighting, radio frequency identification (RFID) cards and electronic papers etc. As important parts of organic optoelectronic devices, organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic light-emitting transistors (OLEFTs) have made great achievements. The performance of these optoelectronic devices depends not only on the properties of the organic semiconductors involved, but is also dramatically affected by the properties of other functional layers and the nature of the interfaces present. Therefore, interface engineering, a novel approach towards high-performance OFETs, is a vital task for organic optoelectronic devices. Electrode/organic interfaces, dielectric/organic interfaces, organic/organic interfaces and organic/atmosphere interfaces are the three frequently reported interfaces in organic devices. In this dissertation, a systematic research has been carried out centering on the interface engineering of organic optoelectronic devices. With investigation of interface phenomenon and effective interface modification, dramatic decrease of power consumption and cost, obvious ehancement of device performance and improvement of stability are achieved. The main results are obtained as follows:
1: Exploration of novel anode modification approach for OLEDs to reduce the power consumption and enhance the efficiency.
Power consumption and light emitting property are the key parameters for the real application of organic light-emitting diodes. In fact, modification of electrodes is a widely applied approach to improve device performance of OLEDs since it can optimize the devices performance without change of organic functional materials. We demonstrated that the improvement of interface contact between ITO anode and organic semiconductor layer can be realized by the introduction of ultrathin
hexadecafluoro copper phthalocyanine (F16CuPc) layer. Besides, The modification brings on formation of dipole layer on the ITO surface, which in turn leads to workfunction enhancement of ITO anode and dramatic decrease of hole injection barrier. With device design and optimization, we fabricated high performance low-operation voltage single-layer, double-layer and multi-layer OLEDs with tris(8-quinolinolato)aluminum (Alq3) as emissive layer. For the single layer Alq 3 devices, the modification of the anode results in the significant enhancement in the current efficiency by about 30 times. The operation voltage decrease obviously for double layer devices, with minimum turn-on voltage of 2.6 V. As for multilayer OLEDs, the maximum current efficiency up to 7.63 cd/A and low turn-on voltage of 2.89 V are obtained by improving carrier density in the combination zone and optimization of carrier balance. The performance is one of the best one for OLEDs with Alq3 light emitting layer（Patent Number: ZL [1**********]5.X; Di CA, et al. Appl. Phys. Lett. 2007, 90, 133508；Di CA, et al. Appl. Phys. Lett. 2006, 89, 033502）.
2: Development of novel organic light-emitting transistor structure and realization of light emission under ambient atmosphere.
Organic light-emitting transistor is a highly integrated organic optoelectronic devices since both field-effect and light emitting can be realized in the same channel simultaneously. With optimized photolithograph techniques, we fabricated OFETs with Au and Al serves as source and drain electrode, respectively. Then, the laterally arranged heterojunction structures are achieved by successively inclined deposition of the field-effect and light-emitting materials. It has been observed that introduction of Au-Al source-drain electrodes and laterally arranged heterojunction structures result in enhancement of electron injection and improved carrier density of both holes and electrons. Besides, the designed device structure offers an ideal and widely applicable one to realize effective integration of field-effect property and light emission. It is because the two kind of organic semiconductors could take full use of their own advantages. We fabricated both small molecular and polymer based OLEFTs with pentacene, Alq 3 and TPA-PPV , respectively （Patent Number: ZL [1**********]8.0；ZL [1**********]8.8; Di CA, et al. Appl. Phys. Lett. 2006, 88, 121907；Di CA, et al. Adv. Funct. Mater. 2007, 17, 1567. ）. The results constitute first demonstration of organic light-emitting transistor under ambient atmosphere（Cicoira, F. et al. Adv. Funct. Mater. 2007, 17, 3421；Cicoira, F. et al. J. Mater. Chem. 2008, 18, 158）.
3: Exploration of novel approach to fabricate high performance low-cost OFETs.
Low cost plays dominant role in determining the further development of OFETs. Source-drain electrodes are important parts in OFETs. Gold has been the most widely applied source –drain electrode for OFETs to date, due to its high conductivity, good stability, and formation of excellent
contact with many p-type organic semiconductors. However, the high cost of gold is an adverse factor in practical applications. On the other hand, low-cost electrodes such as Cu and Ag, are unsuitable for most p-type OFETs due to their relatively low workfunction. We provide a simple method to modify the bottom contact Cu or Ag electrodes with organic charge transfer compounds (Cu-TCNQ or Ag-TCNQ). The modification enhanced the workfunction of electrodes and improved the electrode/organic semiconductor contact which results in dramatic improvement of carrier injection. Therefore, we fabricated low cost Cu or Ag based OFETs with device performance comparable with the one of Au based OFETs. Besides, we investigated the influence of electrode morphology on the device performance by the formation of nanosized Cu electrodes. It has been discovered that introduction of source-drain electrodes with proper roughness is helpful to reduce the contact resistance. Fabrication of OFETs based on many organic semiconductors proved that it is a universal approach to improve the performance of bottom contact devices （Patent Number: [1**********]1.X ；Di CA, et al. J. Am. Chem. Soc. 2006, 128, 16418; Di CA, et al. Phys. Chem. Phys. Chem. 2008, 10, 2302 (Front Cover) ）. The result possess potential application in the patterning of organic crystals and construction of corresponding devices（Di CA et al. Chem. Mater. 2009, 21, 4873）.
4: Discovery and investigation of high performance top contact OFETs with Cu electrodes. The typical OFET electrode structure, with a bottom gate, can be divided into top-contact and bottom-contact configurations. With varied electrode deposition sequence, the OFETs with different electrode structure required different modification techniques and exhibit varied device performance. Top-contact OFETs usually have a good electrode/organic layer contact and exhibit high device performance. We discovered that many organic semiconductors based OFETs with Cu top-contact electrodes show comparable device performance with the one of Au top-contact devices. The most excellent performance up to 0.8 cm2V -1s -1 can be obtained for pentacene FETs with Cu top-contact. The high performance is result from good electrode/organic layer contact and the formation of Cu x O during the electrode deposition process or device storage in air. The spontaneous formed Cu x O possess matched energy level with many organic semiconductors and bring on improved device performance (Patent Application Number: [1**********]3.6；Di CA, et al. Adv. Mater. 2008, 20, 1286.). The results thus provide an effective way towards high performance low cost top-contact OFETs (High-tech Materials Alert, 2008, 25, 9).
5: Development of novel graphene patterning method and its applications in OFETs.
Graphene, single or few layer of two dimensional graphite, received great interest among condensed physics and material sciences due to its unusual and stable structure. We developed a novel vapor deposition method with ethanol as the carbon source to fabricate patterned graghene
using the patterned copper or silver and demonstrated its application in OFETs. The patterned graphene exhibit good contact with organic semiconductors, with low carrier injection barrier for p-type OFETs and can serve as excellent source-drain electrodes for OFETs. The pentacene based bottom-contact devices with channel length of 5 m can reach high mobility of 0.53 cm 2V -1s -1 which is one of the best result for pentacene bottom contact devices with bare SiO2 dielectric layer (Patent Application Number: [1**********]4.2; Di CA, et al. Adv. Mater. 2008, 20, 3289). The result demonstrates novel approach to fabricate patterned graphene and open a new application of graphene in OFETs (NPG, Asia Materials, http://www.natureasia.com/asia-materials/highlight.php?id=291；Pang, SP et al. Adv. Mater. 2009, 21, 3488；Cao Y, et al. Adv. Funct. Mater. 2009, 19, 2743). The result is the first experimental step towards integrating graphene and conjugated organics （Burghard, M. et al. Adv. Mater. 2009, 21，2586. ）.
6: Discovery of relationship between the device stability and dielectric/organic layer interfaces and fabrication of high performance pentacene FETs.
Device stability, a hot topic in the organic optoelectronic device field, is widely believed to be related to the properties of organic semiconductors. Pentacene is the most widely investigated organic semiconductor for OFETs. However, poor device stability is the key shortcomings that impede its real application. We discovered that the device stability of pentacene OFETs in air is strongly related to the properties of dielectric layers. The device performance of pentacene FETs with bare SiO2 can maintain for 7 months. By the investigation of relationship between the device stability and dielectric layer surface energy, we suggest the pentacene aggregation and phase transfer should be responsible for the device performance degradation for devices with low surface energy dielectric layer (OTS modified SiO2). We obtained high performance pentancene FETs with high mobility up to 1.8 cm2V -1s -1 and excellent stability by the optimization of dielectric layer（Di CA, et al. Phys. Chem. Chem. Phys. 2009, 11, 7268.）.
In summary, centering on investigation of interface phenomenon, we fabricated high performance OLEDs, OFETs and OLEFTs by the device design and optimization. Also, a series of novel interface approaches were explored to improving the device performance and stability, lowering the the fabrication cost and power consumption (Di CA, et al. J. Phys. Chem. B 2007, 111, 14083(Feature Article, Front Cover), Di CA, et al. Acc. Chem. Res. 2009，42，1573). These results might boost further development of organic optoelectronic devices towards real applications.
Key words: organic light-emitting diodes, organic field-effect transistors, organic
light-emitting transistors, interface, electrode modification
中 文 摘 要
被称为“塑料电子学”的有机光电材料与器件因其在大面积和低成本的柔性显示、平板照明、射频标签和电子纸等方面的广阔应用前景在过去二十年中备受关注。近年来，作为有机光电子器件的重要组成部分的有机发光二极管、有机场效应晶体管和有机发光场效应晶体管取得了很大的发展。这些有机光电器件的性能不仅取决于有机半导体材料的固有性能，很大程度上还依赖于器件中的其它功能层的性质以及各功能层之间的界面性质。因此，界面问题是所有有机光电器件面临的重要科学问题。有机光电器件的界面包括机半导体和电极的界面、有机半导体层之间的界面、有机半导体层和绝缘层之间的界面。本论文以有机光电器件的界面问题为研究主线，以提高器件的性能与稳定性、降低成本为目标，以有机发光二极管、有机场效应晶体管和有机发光场效应晶体管为研究对象，通过界面的优化来实现有机器件功耗和成本的降低、器件性能的改善和器件稳定性的提高。论文的主要的研究工作包括以下六个方面：
一：拓展了有机发光二极管的电极修饰方法，大幅降低了器件功耗并提高了发光效率。
器件的功耗和发光性能是有机发光二极管实现应用的关键指标。有机发光二极管的电极修饰是广泛使用的改进器件性能的有效手段，它可以在不改变主要有机半导体材料的同时提高器件的综合性能。我们利用超薄十六氟酞菁铜（F 16CuPc ）层作为有机发光二极管的氧化铟锡（ITO ）阳极修饰层，改进了ITO 阳极和有机半导体的界面接触；此外，F 16CuPc 引入可以和ITO 阳极形成偶极层从而提高ITO 的功函数和降低空穴的注入势垒；通过器件结构的设计与优化，采用三(8-羟基喹啉) 铝(Alq3) 为发光层制备了基于修饰电极的高性能、低操作电压的单层、双层和多层有机发光二极管。单层器件中，器件的发光效率在电极修饰后较未修饰器件提高了30倍；对于双层器件，电极的修饰明显降低了操作电压，最小开启电压为2.6 V ；对于多层器件，我们通过提高复合区载流子密度和改进载流子平衡并结合电极修饰制备了高发光效率、低开启电压的有机电致发光器件，最高效率达到7.63 cd/A，同时开启电压仅为2.89 V ，这是基于Alq 3为发光层的最佳性能之一（专利号：ZL [1**********]5.X; Di CA, et al. Appl. Phys. Lett. 2007, 90, 133508；Di CA, et al. Appl. Phys. Lett. 2006, 89, 033502）。
二：发明了新型的有机发光场效应晶体管结构，实现了有机发光场效应晶体管在空气中的发光。
有机发光场效应晶体管是一种功能高度集成的有机光电器件，它可以在同一器件中实现
场效应调控和光发射两种功能。我们通过光刻技术的改进制备了金-铝不对称源漏电极结构，在此基础上利用倾斜蒸镀方法实现了水平排列异质结的构筑。实验发现，金-铝不对称源漏电极和水平排列异质结增强了电子的注入、提高了异质结附近的两种载流子密度；此外该结构可以利用性能优异的有机场效应材料和发光材料作为半导体层来实现晶体管的场效应调控和发光的集成，使得两种材料在器件中发挥各自的功能优势，降低了有机发光场效应晶体管对有机半导体多功能性质的要求，从而具有广泛的适用性。基于并五苯，三(8-羟基喹啉) 铝和聚苯撑乙烯衍生物（TPA －PPV ）为半导体，我们分别制备了具有水平排列异质结结构的小分子和聚合物发光场效应晶体管（专利号：ZL [1**********]8.0；ZL [1**********]8.8; Di CA, et al. Appl. Phys. Lett. 2006, 88, 121907；Di CA, et al. Adv. Funct. Mater. 2007, 17, 1567.），该结构被认为可以减少激子猝灭并且第一次成功实现了有机发光场效应晶体管在空气中的发光（Cicoira, F. et al. Adv. Funct. Mater. 2007, 17, 3421；Cicoira, F. et al. J. Mater. Chem. 2008, 18, 158）。
三：开发了一种新型高性能低成本有机场效应晶体管制备方法。
降低器件成本是有机场效应晶体管进一步发展和走向应用的关键，电极是有机场效应晶体管的重要组成部分，目前金是高性能有机场效应晶体管广泛应用的源漏电极材料，这使器件成本较高。而低成本的金属材料如铜或银由于功函等原因，不适于作为大部分有机场效应晶体管的源漏电极。我们采用易实用化的下电极结构，从低成本的铜和银源漏电极出发，通过简单的一步溶液法在电极表面上生成具有纳米结构的有机电荷转移复合物（Cu-TCNQ 或Ag-TCNQ ）修饰电极。电极的修饰提高了铜和银电极的表面功函数，改进了电极和有机半导体之间的接触，改善了载流子的注入，实现了和金上电极结构器件性能相当的高性能，同时降低了有机场效应晶体管的制备成本；此外，通过具有纳米结构的电极的构筑研究了电极形貌对于有机场效应晶体管器件性能的影响；发现引入适当的粗糙电极表面有助于器件接触电阻的降低，拓展了降低有机场效应晶体管接触电阻和提高器件性能的途径。同时该类方法具有普适性，可以适用于多种有机半导体材料（专利申请号：[1**********]1.X ；Di CA, et al. J. Am. Chem. Soc. 2006, 128, 16418; Di CA, et al. Phys. Chem. Phys. Chem.2008, 10, 2302 (封面文章) ）。此外，该修饰方法在有机晶体排布及器件制备方面有广泛的应用前景（Di CA et al. Chem. Mater. 2009, 21, 4873）。
四：发现了铜上电极结构有机场效应晶体管的电极自修饰现象并制备了低成本高性能上电极结构有机场效应晶体管。
上电极结构和下电极结构是底栅结构有机场效应晶体管最为常用的两种器件结构。由于器件结构的差异，电极的修饰方法和性能都有明显的差别。其中上电极结构一般具有较好的电极－半导体接触，从而可以展示较高的器件性能，是进行器件研究的理想结构。我们通过多种有机导体的有机场效应晶体管的制备，发现了多种基于铜上电极结构的有机场效应晶体管具有和金上电极器件相当的性能，基于铜上电极结构的并五苯场效应器件的性能可以达到0.8 cm2V -1s -1；通过研究发现，这主要是因为在蒸镀过程中或后续的器件放置过程中少量Cu x O
的形成，Cu x O 的形成会降低空穴的注入势垒，提高器件的性能。此外，铜电极和有机半导体层的良好接触也是导致器件具有高性能的原因(专利申请号：[1**********]3.6；Di CA, et al. Adv. Mater. 2008, 20,1286.) 。值得注意的是，Cu x O 的形成是一自发过程，不需其它的操作，这就为高性能低成本上电极有机场效应晶体管的制备提供了简单有效的途径(High-tech Materials Alert, 2008, 25, 9)。
五：发展了新型石墨烯制备方法并拓展了石墨烯在有机场效应晶体管中的应用。
石墨烯是单层或少数层高结晶性石墨，由于其结构上的独特性使得它在短短几年内就成为了凝聚态物理和材料科学的研究前沿和热点。我们以低成本的铜、银和乙醇为原料，采用气相沉积方法，通过图案化铜和银电极制备了图案化的石墨烯电极，并制备了并五苯场效应晶体管。制备的图案化石墨烯电极和有机半导体层有很好的接触，和p 型有机半导体有很小的注入势垒，从而可以作为高性能有机场效应晶体管的理想的源漏电极。基于图案化石墨烯源漏电极的沟道长度为5 m 的并五苯下电极结构场效应晶体管的场效应迁移率达到了0.53 cm 2.V –1.s –1，该性能是未经绝缘层修饰的下电极结构的并五苯器件中的最好结果之一(专利申请号：[1**********]4.2; Di CA, et al. Adv. Mater. 2008, 20, 3289) 。因此，该结果开发了一种新型的制备图案化石墨烯的方法，拓展并推动了石墨烯在有机场效应晶体管器件方面的应用(NPG, Asia Materials, http://www.natureasia.com/asia-materials/highlight.php?id=291；Pang, SP et al. Adv. Mater. 2009, 21, 3488；Cao Y, et al. Adv. Funct. Mater. 2009, 19, 2743)，被认为是从实验上迈向石墨烯和共轭有机分子集成的第一步（Burghard, M. et al. Adv. Mater. 2009, 21，2586. ）。 六：发现了并五苯场效应晶体管器件稳定性与半导体－绝缘层界面的关系，制备了高性能高稳定性的并五苯场效应晶体管。
稳定性是有机场效应晶体管的研究重点。人们普遍认为有机场效应晶体管的稳定性是由有机半导体的性质决定的。并五苯是有机场效应晶体管实用化研究的明星材料，但是并五苯也存在明显的缺点，那就是它的稳定性较差。我们发现了并五苯薄膜器件在空气中的稳定性和绝缘层的性质密切相关；阐明了通过绝缘层的选择可以制备高稳定的并五苯场效应器件，基于纯SiO 2绝缘层的并五苯器件性能可以在空气中放置7个月没有明显变化；通过并五苯器件稳定性和绝缘层表面能的关系的研究提出了并五苯在低表面能衬底上（OTS 修饰的SiO 2）的聚集和相转变是导致器件性能衰减的关键因素；通过合适绝缘层的选择制备了高性能、高稳定性的有机场效应晶体管，最高性能可以达到1.8 cm2V -1s -1（Di CA, et al. Phys. Chem. Chem. Phys. 2009，11，7268. ）。
总之，我们围绕着有机光电器件中的界面研究，通过有机光电器件的优化和设计，制备了高性能的有机发光二极管、有机场效应晶体管和有机发光场效应晶体管，开发了一系列优化器件性能与稳定性、降低器件成本与功耗的新途径(Di CA, et al. J. Phys. Chem. B 2007, 111, 14083(特写文章、封面文章) ，Di CA, et al. Acc. Chem. Res. 2009，42，1573) ，为有机光电子器件的进一步发展和应用奠定了坚实的基础。
关键词： 有机发光二极管，有机场效应晶体管，有机发光场效应晶体管，界面，电极修饰