有机电致发光二极管阳极的表面改性
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摘要
有机电致发光二极管(OLED)显示技术被认为是最具发展前景的下一代平板显示技术之一。目前,人们研究的热点集中在提高器件的性能和稳定性方面。有机电致发光器件的性能和载流子的注入有着密切地关系。研究表明为了实现空穴的有效注入,阳极的功函数必须与邻近的有机材料的最高占有轨道(HOMO)相匹配。而通常使用的阳极材料ITO与典型的空穴传输材料的HOMO能级并不匹配。对ITO进行表面改性是实现载流子平衡注入的有效手段。
柔性有机电致发光二极管(FOLED)具有可弯曲、重量轻、便于携带等优势,是OLED发展的一个重要方向。但在聚合物衬底上制备的ITO薄膜在弯曲时容易碎裂和剥离,从而造成器件失效。
因此,研究制备工艺相对简单能够替代ITO薄膜的透明导电薄膜就显得非常必要。针对上述问题,我们进行了如下研究工作:
1.利用循环伏安法(CV),以[Fe(CN)6]4-/3-氧化还原对为探针离子,研究了4-氟苯硫醇分子在金电极表面的自组装行为。结果表明:在室温条件下自组装0.5h即可以在电极表面形成稳定的单分子膜。自组装24h可以在电极表面形成致密的单分子层,表面覆盖度约为95%。
2.研究了自组装单分子膜修饰的金属银膜作为有机电致发光器件的空穴注入层。利用四探针(Four Point Probe)、紫外可见光谱(UV-vis)、扫描电镜(SEM)、X-射线光电子能谱(XPS)、原子力显微镜(AFM)等测试手段研究了自组装膜的加入对于ITO性能的影响。制备了器件结构为ITO/Ag/SAM/NPB/ Alq3/LiF/Al的经典双层结构器件。研究了器件的性能并与传统的器件进行了对比。考察了金属薄膜厚度与透光率、表面方口电阻的关系;衬底的表面粗糙度与透光率和表面方口电阻的关系;配合原子力显微镜优化了其厚度。考察了银膜厚度的变化对器件性能的影响。研究结果表明:在ITO表面制备自组装单分子膜修饰的5 nm厚的金属银膜,可以在保持原有阳极透明性的基础上,增强空穴的注入,改善界面的形貌,进而提高器件性能。在此基础上,制备了ITO/Ag/SAM/m-MTDATA/NPB/Alq3/LiF/Al器件。器件的启亮电压4V,最大电流效率6.9 cd/A,最大亮度为34680 cd/m2 (12 V);高于以ITO为阳极的对比器件25300 cd/m2 (12 V)。该内容的探索为后面的柔性器件的制备摸索了条件。
3.研究了自组装单分子膜修饰的金属银膜作为柔性有机电致发光器件的阳极。考察了金属薄膜厚度与透光率、表面方口电阻的关系;采用原子力显微镜观察了衬底的表面粗糙度。制备了柔性底发射和柔性顶发射器件。柔性底发射器件的启亮电压为4V,7V时最大电流效率达到5.6 cd/A,8 V时器件最大亮度达到27000 cd/m2。柔性顶发射器件的启亮电压低于4V,在6.5 V亮度达到了27760 cd/m2,6 V时效率达到最大为7.38 cd/A。
Organic light emitting diodes (OLEDs) have gained great interest due to their potential application in future flat panel display technology. OLED performance, operating voltage and efficiency, is critically affected by carrier injection. For efficient hole injection into OLED devices, the work function of the anode must be aligned with the highest energy occupied molecular orbital (HOMO) of the adjacent organic film. Indium tin oxide (ITO) is generally used as anode materials in OLEDs. However, its work function is not aligned with the typical hole transport layer (HTL) materials. More attentions have been paid to the modification of ITO anode to achieve an effective and balanced injection.
Organic light emitting device fabricated on flexible substrate have the ability to be light weight, extremely rugged, conformable and flexible thus enabling new display product design. However, a number of important issues must first be resolved. However, the use of plastic substrates is restricted by its low processing temperature and high heat-induced shrinkage and high gas permeability. In particular, Indium tin oxide fabricated on plastics substrates tends to be crash when bending or mechanical strain. So, there is a great need for finding a more mechanically robust flexible tans parent conductor to replace the Indium tin oxide as anode of flexible device. Some research work was conduct to resolve these problems.
In this work, a self-assembled monolayer (SAM) of 4-fluorothiophenol is employed to modify the Ag film on the surface of ITO surface. X-ray photoelectron spectroscopy (XPS), Four Point Probe, atomic force microscope (AFM), and UV-vis transmittance spectra were used to characterize the modified anode. Organic light emitting device with the structure of ITO/Ag(x nm)/SAM/α-naphthylphenylbiphenyl diamine (NPB, 60 nm)/ tris-(8-hydroxyquinoline) aluminum (Alq3, 60 nm) /LiF(0.7nm) /Al(100nm) was fabricated. Current density-voltage-luminance characteristics of the devices were investigated using the modified anode and the bare ITO. The effect of Ag layer thickness on the device performance is also investigated. The result revealed that SAM modified ultra-thin Ag film is an effective buffer layer to improve OLED performance. The enhancements are attributed to enhanced hole injection and smooth surface between anode and the organic material, which lead to the more balance of the carriers in emitting zone, then increases the current efficiency. The Ag thickness of 5 nm is chosen as an acceptable compromise between substrate transparency and the device performance. Based on this research, Organic light emitting device with the structure of ITO/Ag(5nm)/SAM/4, 4’, 4’’-tris(3-methylphenylphenylamino) triphenylamine (m-MTDATA)/α-naphthylphenylbiphenyl diamine (NPB)/ tris-(8-hydroxyquinoline) aluminum (Alq3) /LiF) /Al was fabricated. With the presence of the self-assembled monolayer-modified Ag films, the luminance of the device reaches 34680 cd/m2 at 12 V at the current density of 550 mA/cm2, which corresponds to an efficiency of 6.9 cd/A. However, the control device using bare ITO as anode was 25300 cd/m2 with current density of 433 mA/cm2 at the same bias.
Organic light emitting device fabricated on flexible PET substrate with a self-assembled monolayer of 4-fluorothiophenol modified the Ag film as anode is also investigated. The thickness of Ag film on the transmittance and Sheet resistance Rs was investigated by UV-vis transmittance spectra and Four Point Probe. Atomic force microscope was used to measure the surface parameters. Flexible bottom-emitting and top-emitting devices with a self-assembled monolayer of 4-fluorothiophenol modified the Ag film as anode were fabricated. A luminance of 27000 cd/m2 at 8 V, which corresponds to an efficiency of 5.6 cd/A was achieved for Flexible bottom-emitting device. However, the luminance of flexible top-emitting devices was 27760 cd/m2 at the bias of 6.5V, corresponding to an efficiency of 7.38 cd/A.
柔性有机电致发光二极管(FOLED)具有可弯曲、重量轻、便于携带等优势,是OLED发展的一个重要方向。但在聚合物衬底上制备的ITO薄膜在弯曲时容易碎裂和剥离,从而造成器件失效。
因此,研究制备工艺相对简单能够替代ITO薄膜的透明导电薄膜就显得非常必要。针对上述问题,我们进行了如下研究工作:
1.利用循环伏安法(CV),以[Fe(CN)6]4-/3-氧化还原对为探针离子,研究了4-氟苯硫醇分子在金电极表面的自组装行为。结果表明:在室温条件下自组装0.5h即可以在电极表面形成稳定的单分子膜。自组装24h可以在电极表面形成致密的单分子层,表面覆盖度约为95%。
2.研究了自组装单分子膜修饰的金属银膜作为有机电致发光器件的空穴注入层。利用四探针(Four Point Probe)、紫外可见光谱(UV-vis)、扫描电镜(SEM)、X-射线光电子能谱(XPS)、原子力显微镜(AFM)等测试手段研究了自组装膜的加入对于ITO性能的影响。制备了器件结构为ITO/Ag/SAM/NPB/ Alq3/LiF/Al的经典双层结构器件。研究了器件的性能并与传统的器件进行了对比。考察了金属薄膜厚度与透光率、表面方口电阻的关系;衬底的表面粗糙度与透光率和表面方口电阻的关系;配合原子力显微镜优化了其厚度。考察了银膜厚度的变化对器件性能的影响。研究结果表明:在ITO表面制备自组装单分子膜修饰的5 nm厚的金属银膜,可以在保持原有阳极透明性的基础上,增强空穴的注入,改善界面的形貌,进而提高器件性能。在此基础上,制备了ITO/Ag/SAM/m-MTDATA/NPB/Alq3/LiF/Al器件。器件的启亮电压4V,最大电流效率6.9 cd/A,最大亮度为34680 cd/m2 (12 V);高于以ITO为阳极的对比器件25300 cd/m2 (12 V)。该内容的探索为后面的柔性器件的制备摸索了条件。
3.研究了自组装单分子膜修饰的金属银膜作为柔性有机电致发光器件的阳极。考察了金属薄膜厚度与透光率、表面方口电阻的关系;采用原子力显微镜观察了衬底的表面粗糙度。制备了柔性底发射和柔性顶发射器件。柔性底发射器件的启亮电压为4V,7V时最大电流效率达到5.6 cd/A,8 V时器件最大亮度达到27000 cd/m2。柔性顶发射器件的启亮电压低于4V,在6.5 V亮度达到了27760 cd/m2,6 V时效率达到最大为7.38 cd/A。
Organic light emitting diodes (OLEDs) have gained great interest due to their potential application in future flat panel display technology. OLED performance, operating voltage and efficiency, is critically affected by carrier injection. For efficient hole injection into OLED devices, the work function of the anode must be aligned with the highest energy occupied molecular orbital (HOMO) of the adjacent organic film. Indium tin oxide (ITO) is generally used as anode materials in OLEDs. However, its work function is not aligned with the typical hole transport layer (HTL) materials. More attentions have been paid to the modification of ITO anode to achieve an effective and balanced injection.
Organic light emitting device fabricated on flexible substrate have the ability to be light weight, extremely rugged, conformable and flexible thus enabling new display product design. However, a number of important issues must first be resolved. However, the use of plastic substrates is restricted by its low processing temperature and high heat-induced shrinkage and high gas permeability. In particular, Indium tin oxide fabricated on plastics substrates tends to be crash when bending or mechanical strain. So, there is a great need for finding a more mechanically robust flexible tans parent conductor to replace the Indium tin oxide as anode of flexible device. Some research work was conduct to resolve these problems.
In this work, a self-assembled monolayer (SAM) of 4-fluorothiophenol is employed to modify the Ag film on the surface of ITO surface. X-ray photoelectron spectroscopy (XPS), Four Point Probe, atomic force microscope (AFM), and UV-vis transmittance spectra were used to characterize the modified anode. Organic light emitting device with the structure of ITO/Ag(x nm)/SAM/α-naphthylphenylbiphenyl diamine (NPB, 60 nm)/ tris-(8-hydroxyquinoline) aluminum (Alq3, 60 nm) /LiF(0.7nm) /Al(100nm) was fabricated. Current density-voltage-luminance characteristics of the devices were investigated using the modified anode and the bare ITO. The effect of Ag layer thickness on the device performance is also investigated. The result revealed that SAM modified ultra-thin Ag film is an effective buffer layer to improve OLED performance. The enhancements are attributed to enhanced hole injection and smooth surface between anode and the organic material, which lead to the more balance of the carriers in emitting zone, then increases the current efficiency. The Ag thickness of 5 nm is chosen as an acceptable compromise between substrate transparency and the device performance. Based on this research, Organic light emitting device with the structure of ITO/Ag(5nm)/SAM/4, 4’, 4’’-tris(3-methylphenylphenylamino) triphenylamine (m-MTDATA)/α-naphthylphenylbiphenyl diamine (NPB)/ tris-(8-hydroxyquinoline) aluminum (Alq3) /LiF) /Al was fabricated. With the presence of the self-assembled monolayer-modified Ag films, the luminance of the device reaches 34680 cd/m2 at 12 V at the current density of 550 mA/cm2, which corresponds to an efficiency of 6.9 cd/A. However, the control device using bare ITO as anode was 25300 cd/m2 with current density of 433 mA/cm2 at the same bias.
Organic light emitting device fabricated on flexible PET substrate with a self-assembled monolayer of 4-fluorothiophenol modified the Ag film as anode is also investigated. The thickness of Ag film on the transmittance and Sheet resistance Rs was investigated by UV-vis transmittance spectra and Four Point Probe. Atomic force microscope was used to measure the surface parameters. Flexible bottom-emitting and top-emitting devices with a self-assembled monolayer of 4-fluorothiophenol modified the Ag film as anode were fabricated. A luminance of 27000 cd/m2 at 8 V, which corresponds to an efficiency of 5.6 cd/A was achieved for Flexible bottom-emitting device. However, the luminance of flexible top-emitting devices was 27760 cd/m2 at the bias of 6.5V, corresponding to an efficiency of 7.38 cd/A.
引文
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