【作者】 张丹丹；

【导师】 孙洪波； 冯晶；

【作者基本信息】 吉林大学 ， 微电子学与固体电子学， 2011， 博士

【摘要】 有机电致发光器件(OLED)有着重量轻,响应速度快,视角宽,主动发光无需背光源,易于实现柔性显示等优点,无论是在平板显示还是固态照明领域都展示了广阔的应用和发展前景。然而,虽然近年来OLED取得了突飞猛进的发展,其产业化步伐并不像人们所预测的那样乐观,主要原因是效率和稳定性等关键问题有待于进一步解决。本论文针对这些关键性问题,将磁性材料四氧化三铁应用于OLED,作为阳极缓冲和p型掺杂,提高器件的空穴注入和传输性能；利用其磁性,在磁场作用下,增加单线态激子的形成比例。通过以上工作,从降低驱动电压和提高内量子效率的角度提高器件的性能和稳定性。主要研究内容如下：(1)Fe3O4作为底发射OLED阳极缓冲层,提高空穴注入效率。将过渡型的金属氧化物Fe3O4引入到底发射OLED的阳极ITO表面作修饰层,测试表明修饰层的引入使器件的开启电压由4V降低到2.5V,在12V下的亮度由9040 cd/m2提高到27540 cd/m2。采用X射线光电子能谱(XPS)和紫外光电子能谱(UPS)测试分析缓冲层的引入对空穴注入势垒和界面能级的影响。XPS测试表明缓冲层的引入使得电子从阳极向缓冲层发生转移,形成了界面偶极子层,导致能带向上发生约0.3 eV的弯曲,降低了ITO/Fe3O4/NPB界面的空穴注入势垒。由UPS能谱分析计算得到,引入缓冲层后空穴的注入势垒降低0.22 eV。除此之外,电极的表面平整度也会对载流子的注入效果产生较大的影响,因而对ITO电极生长缓冲层前后进行了原子力(AFM)的测试表征。AFM图片证明Fe3O4的引入改善了ITO电极表面的粗糙度,使其粗糙度从1.04 nm降低到0.72 nm。可见,Fe3O4缓冲层的引入不仅能够明显的降低空穴的注入势垒,而且还能够改善电极表面的平整度,增强空穴的注入能力,从而有效降低器件的驱动电压。(2)Fe3O4作为顶发射OLED阳极缓冲层,提高空穴注入效率。将Fe3O4引入到顶发射器件的银阳极表面,证明了其对银电极有着很好的修饰效果,使器件的开启电压从5V降低到2.5V,最大亮度为108297 cd/m2,提高7倍,效率也得到显著的提高。又进一步将其与常用的缓冲材料三氧化钼制作的器件进行分析对比,证明Fe3O4对银的修饰效果可与MoO3相比拟甚至更佳。采用XPS和UPS测试手段分析了Fe3O4缓冲层对电极/有机界面的影响,结果表明缓冲层的引入降低了空穴的注入势垒和驱动电压,提高了空穴的注入能力,从而大幅度的改善了器件的光电性能。(3)Fe3O4作为p型掺杂剂,提高空穴注入和传输。有机半导体迁移率低,载流子的传输性能差,p(n)型掺杂技术是用来提高载流子传输能力的有效方法之一,同时掺杂剂在电极和有机层界面也能够显著降低载流子注入势垒,提高注入效率。这里采用Fe3O4作为p型掺杂剂,分别掺杂到不同的主体材料中,使器件的性能得到大幅度的改善。将Fe3O4分别掺杂在常用的空穴注入材料m-MTDATA和空穴传输材料NPB中,实验结果证明基于这两种不同掺杂体系(m-MTDATA:Fe3O4和NPB:Fe3O4)的器件性能都得到显著的提高,开启电压分别从3V降低到2.4V；从5V降低到2.5 V。m-MTDATA:Fe3O4的器件在8V下的亮度由未掺杂时的6005 cd/m2提高到29360 cd/m2,NPB:Fe3O4的器件在10V下的亮度由未掺杂时的1680 cd/m2提高到30590 cd/m2,证明Fe3O4是一种很理想的p型掺杂剂。通过XPS和UPS系统分析了上述两种p型掺杂体系的作用机理,基于m-MTDATA:Fe3O4的体系,对空穴传输能力的改善占主导地位,而基于NPB：Fe3O4的体系则是对空穴注入能力的提高占主导。(4)利用磁场效应提高OLED的单线态激子形成几率。Fe3O4本身是一种具有极高自旋极化率的铁磁材料,针对荧光OLED内量子效率极限只能达到25%的问题,从器件的发光机理出发,将这一磁性材料引入到OLED,在外加磁场的作用下,提高单线态激子的形成比例,进而提高器件的内量子效率。我们分别尝试了将Fe3O4以阳极缓冲层和传输层掺杂剂的方式引入到OLED中,研究其磁场效应。作为阳极缓冲层,在外加磁场下器件的内量子效率相对于无外加磁场下器件的内量子效率提高了10.5%,分析得知这应该主要是由于从阳极注入的空穴在经过磁性缓冲层材料时,发生了自旋极化,引起了单线态激子形成比例的增加。将Fe3O4作为传输层掺杂剂掺杂到空穴传输材料NPB中,通过测试对比分析得到在外加磁场下掺杂器件的效率相对于无外加磁场下器件的效率获得了24%的提高。此方法获得的效率增长因子明显高于上述将磁性材料以阳极缓冲层的方式引入到OLED中获得的增长因子。这是由于将磁性材料Fe3O4掺杂在NPB中,能够使磁性材料分布范围更大,增加其与注入空穴的接触机会。上述结果证明将Fe3O4引入到OLED中,在磁场作用下能够有效增强器件的磁场效应,提高器件的内量子效率,为突破荧光OLED内量子效率25%的极限奠定了基础。综上所述,本论文的工作主要致力于将磁性材料Fe3O4应用于OLED,系统研究了Fe3O4作为阳极缓冲和p型掺杂提高OLED的空穴注入和传输,以及作为磁性薄膜和磁性掺杂剂在磁场作用下提高OLED单线态激子形成几率。以上工作对于应用Fe3O4提高OLED性能,解决其效率和稳定性的问题做出了有益的探索。更多还原

【Abstract】 Organic light-emitting devices (OLEDs) has drawn much attention due to its advantages such as light weight, fast response, large viewing angle, active luminescence without background light, easy to realize flexible display and three-dimensional display. In recent years, significant progress has been achieved in OLEDs, however, stability and efficiency is still key issue for its potential applications in flat-panel display and solid-state lighting. In this thesis, Fe3O4 has been applied into OLEDs as electrode modification layer and p type dopant to effectively reduce the driving voltage and power consumption and enhance the luminance of OLEDs. Moreover, we have studied the magnetic field effect of the OLEDs with Fe3O4 to increase the proportion of the singlet excitons, and finally enhance the internal quantum efficiency of the devices.(1) Enhanced hole injection for the bottom-emitting OLEDs (BOLEDs) with the transitional metal oxides Fe3O4 as indium-tin oxide (ITO) anodic buffer. The turn-on voltage of the OLEDs with the anodic buffer is reduced from 4 V to 2.5 V, and the brightness is increased from 9040 cd/m2 to 27540 cd/m2 at 12 V. The x-ray photoemission spectroscopy (XPS) and UV photoemission spectroscopy (UPS) measurements were performed to determine the interfacial energy level. The XPS results showed that the electrons transferred from ITO to Fe3O4 at the interface. The electron transfer across the interface results in a formation of a dipole layer at the interface, leading to an abrupt shift in the potential across the dipole. The core-level shift shows a 0.3 eV up-shift in the vacuum level after depositing 1 nm Fe3O4, which results in a reduced energy barrier at the ITO/Fe3O4/NPB interface and accordingly reduced driving voltage. The UPS spectra showed that the hole-injection barrier at the ITO/NPB interface is reduced by 0.22 eV when the Fe3O4 buffer layer is inserted between them. In addition to the energetics, the morphology of the interface can play a role in determining the injection efficiency. The effect of the Fe3O4 on the interfacial morphology between the ITO anode and the deposited NPB films was investigated by AFM. The AFM images revealed that the Fe3O4 capped ITO surface displays improved smoothness with a root-mean-square (rms) roughness of 0.72 nm compared to the bare ITO with a rms roughness of 1.04 nm. The above results indicates that Fe3O4 is a practical anodic buffer layer to improve the performance of the OLEDs by enhancing the hole injection.(2) Enhanced hole injection for the top-emitting OLEDs (TOLEDs) with the transitional metal oxides Fe3O4 as the silver (Ag) anodic buffer. The turn-on voltage of the Fe3O4 buffered TOLEDs was reduced from 5 V to 2.5 V, and the maximum brightness reached to 108297 cd/m2, which is eight times of that device without buffer layer. In order to study the relative effectiveness of the anodic modification of the Fe3O4, we have fabricated the devices with MoO3 as the buffer layer for comparison, which is one of the most effective anodic buffer materials for the Ag anode. The results indicated that Fe3O4 has comparable and even appreciably superior effect in modifying the Ag anode and improving the properties of the TOLEDs to the MoO3. The XPS and UPS measurements indicated that the introduction of the thin film Fe3O4 can greatly reduce the hole-injection barrier, enhance the hole injection ability and consequently improve the device performances.(3) Fe3O4 as the p-dopant to improve the hole injection and transport of the OLEDs. The mobility of the organic semiconductor is very low, while the p/n doping technology is a powerful solution to improve the charge conductivity, and the carrier injection ability could be improved simultaneously. In our work, the hole injection and transport ability were evidently enhanced by doping the p dopant Fe3O4 into different host materials 4,4’,4"-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) and N,N’-diphenyl-N,N’-bis(1,1’-biphenyl)-4,4’-diamine (NPB), respectively. The experiment results have demonstrated that the luminance, the current density, and the power efficiency of the OLEDs with Fe3O4 doped in two different hosts of m-MTDATA and NPB have been remarkably enhanced compared to those of the undoped devices. The brightness were 29360 cd/m2 and 6005 cd/m2 at 8 V for the devices with doped m-MTDATA and undoped m-MTDATA respectively, and 30590 cd/m2 and 1680 cd/m2 at 10 V for the devices with the doped NPB and undoped NPB, respectively. The turn-on voltage obtained the luminance of 1 cd/m2 was found to be greatly decreased from 3 to 2.4 V and from 5 to 2.5 V, respectively, for the devices with doped m-MTDATA and NPB. The role of the Fe3O4 as the p-dopant in different hosts has been studied. The Fe3O4-doped m-MTDATA layer in the OLEDs is more efficient in improving the hole transportation, while the Fe3O4-doped NPB layer is more efficient in lowering the hole-injection barrier.(4) Magnetic field effect on the OLEDs with the Fe3O4 as the magnetic buffer and dopant. Although 100% internal quantum efficiency can theoretically be achieved by introducing triplet emitter,25% singlet exciton formation ratio has been a bottleneck in improving the efficiency of the conventional fluorescent OLEDs. We chose magnetite Fe3O4 as the magnetic buffer and dopant to improve the singlet exciton formation ratio under an applied magnetic field. The efficiency with the presence of the magnetic field was enhanced by 10.5% compared to that of without the magnetic field for the Fe3O4 buffered OLEDs, this was because that the increase of the singlet exciton fraction due to the hole spin polarization injection. In order to increase the ratio of hole spin polarization, OLEDs with a magnetic dopant of Fe3O4 in a hole-transport layer (HTL) were fabricated and characterized. Magnetic field-dependent electroluminescence (EL) was observed and large enhancement of 24% for the current efficiency was obtained from the magnetic doped devices. Obviously, the efficiency enhancement for the OLEDs with the magnetic dopant was higher than that of the device based on Fe3O4 as the anodic buffer, which is attributed to the increased contact between the holes and magnetic material Fe3O4. We can come to a conclusion that magnetic field effect on the OLEDs employing the Fe3O4 presents an efficient pathway to enhance the EL efficiency of the fluorescent OLEDs.In summary, the application of the Fe3O4 in OLEDs has been systematically investigated. The Fe3O4 has been demonstrated as an effective anodic buffer and p dopant to improve the hole injection and transport. The magnetic field effect on the OLEDs with Fe3O4 as the magnetic buffer and dopant has been explored and enhanced singlet formation ratio has been obtained. Therefore, the use of Fe3O4 in OLEDs presents an efficient pathway to enhance the performance of the OLEDs, and the observational fact we attained may open a new avenue to broad application of Fe3O4, an environmentally benign, inexhaustible, and cheap material in OLEDs.更多还原