【作者】 张小文；

【导师】 蒋雪茵；

【作者基本信息】 上海大学 ， 材料学， 2010， 博士

【摘要】 有机电致发光器件（OLED）具有发光效率高、驱动电压低、色彩丰富、超薄便携等优异性能而成为新一代最具发展前景的平板显示技术。与传统的底发射器件相比,顶发射器件可以在Si基或带有复杂电路系统的有源驱动TFT基板上实现高质量显示,迎合了当前高分辨率、大尺寸和全彩显示的需求,是OLED研究的重点。本文侧重于OLED基础理论、性能改善以及在实现全彩显示方面进行的一些基础研究。主要研究内容及结果如下:（1）在底发射器件中,由于OLED的电子注入和传输均比空穴差,因此重点研究了电子注入能力的提高以及电子注入势垒高度的计算。首先开展了Liq/CsO_x作复合电子注入层改善OLED性能的研究,Liq/CsO_x复合电子注入层能使器件的效率提高约30%,电子注入能力的改善还进一步用‘电子-only’器件得到了证实,并用偶极效应和阶梯势垒等理论进行了解释。其次,采用双电子传输层（Bpy-OXD/Alq₃或Bpy-OXD/BPhen）改善了蓝光OLED的色度和电子注入能力,这是由于Bpy-OXD的空穴阻挡作用能有效地将载流子限制在发光层中及其提供的阶梯能级促进了电子注入。最后,用“电流-电压特性”计算了最常用的电子传输材料Alq₃和BPhen与Al形成的“金属/有机”界面的电子注入势垒高度,Alq₃/Al和BPhen/Al的电子注入势垒高度分别为0.66 eV和0.83 eV,而Alq₃/LiF/Al和BPhen/CsO_x/Al的电子注入势垒高度分别为0.1 eV和0.098 eV。（2）在具有微腔结构的顶发射器件中,详细研究了微腔效应及其对器件性能的影响。首先构建了[TBADN:DSA-Ph]作发光层、具有单模共振发射、低驱动电压的高效蓝光顶发射OLED。通过改变空穴传输层的厚度,器件的发光颜色可以从深蓝色[CIE（0.15,0.08）]调节到绿光发射[CIE（0.17,0.57）]。通过在半透明阴极之上引入C₆₀折射率匹配层可以使器件的发光效率提高60%,用传输矩阵理论计算了顶接触多层膜系的透过率和反射率,结果表明器件的最佳性能在顶接触‘最大’和‘最小’透过率之间的某一值获得,这是由于光在微腔内的广角干涉和多光束干涉之间的协调与平衡的结果。用[TBADN:DSA-Ph]/[Alq₃:DSA-Ph]双发光层取代单一发光层[TBADN:DSA-Ph]后,器件的发光效率提高了50%,而CIE色坐标基本保持不变,这主要归功于发光层[Alq₃:DSA-Ph]中Alq₃到DSA-Ph的能量转移以及DSA-Ph直接俘获载流子。其次,用Ag作反射阳极和半透明阴极构建了具有Ag-Ag微腔结构的高效率顶发射OLED,以Alq₃作发光层的器件具有最大发光效率9.21 cd/A,比Al/Ag作半透明阴极的顶发射器件和底发射器件提高了2-3倍,这主要归功于强烈的微腔效应以及从Ag电极的高效载流子注入能力。最后,用新型染料PDT掺杂的发光体系[Alq₃:PDT:rubrene]制备了具有窄光谱发射和没有电流诱导淬灭效应的红光荧光OLED,C₆₀作折射率匹配层的红光顶发射器件的发光效率为3 cd/A、CIE色坐标为（0.64, 0.36）。计算了[Alq₃:PDT:rubrene]中的F?rster能量转移半径,结果表明能量转移的途径主要是从主发光体Alq₃经由辅助掺杂剂rubrene转移到客发光体PDT的。（3）在Si基顶发射器件中重点研究了器件的性能改善和相关机理。首先研究了用MoO_x作阳极缓冲层比SiO2更能有效地提高器件的性能,p-Si/MoO_x器件的效率几乎是p-Si/SiO2器件的两倍。而且,与热氧化的SiO2相比,MoO_x可以采用真空热蒸发方法制备,从而简化了器件制备工艺。然后,用[TPBA:TPA]作发光层构建了高效率荧光Si基顶发射器件,发光效率和功率效率分别为3.3 cd/A和2.3 lm/W,最高亮度达到了1.3×104 cd/m² @12 V,这主要归功于从TPBA到TPA之间非常有效的能量转移以及TPA自身的高荧光发射和器件结构的优化。与Ag作阳极的顶发射器件相比,Si基顶发射器件具有弱微腔效应和非常高的象素对比度,这是由于Si的低反射率引起的,并从理论上进行了详细分析。最后,研究了以廉价Cu为阳极的顶发射OLED,MoO_x修饰能显著提高Cu的功函数,从而改善了器件性能。与Ag作阳极的器件相比,Cu作阳极的顶发射器件的优势主要体现在:较高的象素对比度、较弱的微腔效应以及较低的漏电流。更多还原

【Abstract】 Organic light-emitting devices （OLEDs） are considered to hold the most brilliant promise of next generation of flat-panel displays due to their high luminous efficiency, low driving voltage, a broad range of colors, thinness and portability. In comparison with the conventional bottom emitting OLED （BOLED）, top-emitting OLED （TOLED） paves a feasible way for realizing high-resolution large-size full-color OLED-based displays on Si thin-film-transistor （TFT） substrates or active-matrix backplanes with complicated pixel circuits and thereby drawing great attention of researchers. In this dissertation, we predominantly focused on the fundamental researches such as OLED theory, performance improvement, and some applications to realizing OLED displays. The results are listed as follows:（1） For BOLED, we mainly investigated the enhancement of electron-injection ability and the calculation of electron-injection barrier-height, owing to the fact that the injection and transport ability of electrons is inferior to holes in most OLEDs. Firstly, we studied the improvement of OLED performance by using a composite electron-injection layer （c-EIL） of Liq/CsO_x. The efficiency of device using c-EIL was enhanced by ³0%. The enhancement of device efficiency was further verified by‘Electron-only’devices and explained by dipole effect and step-barrier theory. Then, the chromaticity and electron-injection ability of blue OLEDs were significantly improved by using a dual electron-transport layer （d-ETL, e.g., Bpy-OXD/Alq₃ or Bpy-OXD/BPhen）. This can be attributed to the hole-blocking function of Bpy-OXD which confines the carriers within the emitting layer （EML） and the step-barrier provided by the d-ETL which promotes carrier injection. Lastly, the electron-injection barrier-height of“metal/organic”interface （i.e., between the Al cathode and the most commonly used ETLs of Alq₃ and BPhen） was calculated by using“current-voltage （I-V） characteristics”. The barrier height of 0.66 eV for Alq₃/Al and that of 0.83 eV for BPhen/Al were estimated. While the barrier height of 0.1 eV for Alq₃/LiF/Al and that of 0.098 eV for BPhen/CsO_x/Al were derived.（2） For TOLEDs with microcavity structure, we systematically investigated the microcavity effect and its effect on device performance. We first demonstrated efficient blue TOLEDs with single-mode resonant emission and low voltage by using [TBADN:DSA-Ph] as EML. The chromaticity can be adjusted from deep blue with CIE color coordinates of （0.15, 0.08） to green emission with CIE color coordinates of （0.17, 0.57） by altering the thickness of hole-transport layer. The device efficiency can be enhanced by 60% with the deposition of C₆₀ index-matching layer over the semitransparent cathode. The transmittance and reflectance of top contact through which the light is outcoupled was calculated by using a transfer matrix method, the results indicated that the optimal performance of blue TOLED was obtained in between the maximum and minimum transmittance of top contact as a result of the trade-off between wide-angle interference and multiple-beam interference within the cavity. The device efficiency was enhanced by 50% and the CIE color coordinates were negligibly affected by using a dual EML of [TBADN:DSA-Ph]/[Alq₃:DSA-Ph]. The improved performance of device with dual EML was attributed to the energy transfer from Alq₃ to DSA-Ph and DSA-Ph directly harvesting carriers in the EML of [Alq₃:DSA-Ph]. Then, highly efficient microcavity TOLED based on Ag anode and Ag semitransparent cathode was demonstrated. With Alq₃ as EML, the device showed a maximum luminous efficiency of 9.21 cd/A which is 2-3 times higher than those of the corresponding TOLED with Al/Ag semitransparent cathode and BOLED. This can be attributed to the significant microcavity effect and efficient carrier injection from Ag electrode. Lastly, fluorescent red OLEDs with narrow emission and negligible current-induced quenching by using PDT-doped emitting system of [Alq₃:PDT:rubrene] were demonstrated. With the incorporation of C₆₀ outcoupling layer, the TOLED exhibited excellent red emission with luminous efficiency of ³ cd/A and CIE color coordinates of （0.64, 0.36）. The F?rster’s radius in the EML of [Alq₃:PDT:rubrene] was calculated, and the results indicated that the energy transfer process is predominantly from the host of Alq₃ to the guest of PDT via the intermediation of rubrene.（3） For Si-based TOLEDs, we mainly focused on the performance improvement and related theory. Firstly, MoO_x is proven to be more efficient than SiO₂ in improving device performance, the efficiencies of p-Si/MoO_x device are almost double those of p-Si/SiO₂ device. Moreover, in comparison with the thermally-grown SiO₂ buffer layer, MoO_x can be deposited by conventional evaporation technology under vacuum conditions, which simplifies the fabrication process. Secondly, Si-based TOLED with EML of [TPBA:TPA] was demonstrated possessing superior performance, the luminous and power efficiencies were achieved 3.3 cd/A and 2.3 lm/W, respectively, and the maximum luminance was reached 1.3×104 cd/m2 @12 V. This can be attributed to the efficient energy transfer from the host of TPBA to the guest of TPA, highly fluorescent emission of TPA, and the optimization of device structure. In comparison with the TOLED using conventional Ag anode, the Si-based TOLED shows negligible microcavity effect and giant enhancement of pixel contrast ratio （PCR） as a result of low reflectance of Si anode. The experimetnal results were theoretically analyzed in detail. Lastly, efficient TOLEDs with low-cost Cu anode were investigated. The MoO_x modification can considerably enhance the work function of Cu anode, which accounts for the performance improvement. The TOLED with Cu anode is superior to the counterpart with Ag anode in several respects such as higher PCR, weak microcavity effect and lower leakage current.更多还原