有机电致发光器件的界面效应对器件性能影响的研究
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摘要
尽管有机电致发光器件(OLED)研究与开发工作有了突破性进展，但仍有许多重大基础问题尚不清楚，使得器件寿命短、效率低。要解决这一系列问题，必须深化对薄膜材料发光机理与性能、薄膜材料载流子输运特性、器件失效机制、器件界面特性和界面工程等的认识。本论文重点围绕金属／有机界面层、金属／无机／有机界面层及有机／有机界面层进行了数值模拟研究。
绝缘缓冲层修饰金属／有机界面已成为改善OLED载流子注入的有效手段之一。文中考虑了LiF绝缘缓冲层对电子注入势垒的影响，给出了LiF／金属复合电极注入势垒的表达式，基于载流子的注入和复合过程，建立了双层有机电致发光器件发光效率的理论模型，讨论了器件效率随电压、注入势垒、内界面势垒、有机层厚度的变化关系，成功地模拟了插入不同厚度LiF缓冲层的电流—电压特性曲线，得出了LiF介于不同金属阴极和Alq3之间的最佳厚度值，通过与实验结果的比较，验证了模型的有效范围。这将有助于实验上对缓冲层材料的选择及最佳厚度的确定。
有机／有机界面特性支配着界面处载流子的输运与复合区域，基于该实验事实，文中提出了双层有机电致发光器件中载流子在有机／有机界面复合的无序跳跃理论模型。计算发现了内界面处载流子的有效势垒高度决定OLED中的电子和空穴密度的分布，而电子与空穴密度又决定了电场强度的大小，并研究了电场、跳跃距离与势垒对内界面处复合效率的影响。该模型很好地解释相关的实验现象。
响应延时是衡量器件性能的重要参数之一，根据载流子注入、输运及复合物理机制，文中进一步发展了双层有机电致发光器件的电致发光(EL)延时模型，得出了EL延时的解析表达式，重新评估了注入、输运及复合对EL延时的影响，并揭示了内界面势垒、内建势垒、空穴传输层厚度与复合延时及EL延时的关系，通过比较发现空穴传输层厚度对延时的影响要略弱于内界面势垒与内建势垒。这对快速响应EL器件的功能材料及结构选择有一定的参考意义。
Although the breakthrough of the research and development oforganic light-emitting diodes(OLED) has been gained in last 20 years,some major basic problems remain ambiguous, which lead to shortlifetime and low efficiency of devices. In order to solve a series ofproblems, it should intensify the understanding of luminescencemechanism and the performance of thin film organic materials, transportcharacteristics of carriers of thin film materials, degradation mechanismof devices, interfacial characteristics of devices and interfacesengineering. The numerical simulation was performed mainlysurrounding the metal/organic semiconductor interface, metal/inorganic/organic semiconductor interface and organic/organic semiconductorinterface in this paper.
The insulating buffer layer modification of the metal/organicinterface has been one of the effective ways to improve the carrierinjection of OLED. An analytical model to calculate electroluminescence(EL) efficiency of bilayer organic light-emitting devices, considering theinfluence of introducing LiF insulating buffer layer at metal/organicinterface on the barrier height for electrons injection, was presented. Therelations of EL efficiency versus applied voltage, injection barrier,internal interfacial barrier and the thickness of organic layer werediscussed, the J-V characteristics of OLED inserting LiF buffer layers ofdifferent thickness were simulated successfully, the optimal thickness ofLiF buffer layer was achieved. By comparing with the experimentalresults, the validity of predictions from this model was tested. These willbe conducive to the selection and the optimal thickness design of bufferlayer material.
The organic/organic semiconductor interracial characteristics ofOLED control the transport of carriers between layers and the region ofthe device where recombination takes place. In the light of thisbackground, a disordered hopping model of charge carriers at theorganic/organic interface in bilayer organic light-emitting diodes waspresented. The calculated results indicate that the distribution of electron and hole densities is determined by the effective barrier height at theorganic/organic interface, at the same time, the field strength isdetermined by the distribution of electron and hole densities. Then weanalyzed the influences of the variation of electric field, barrier heightand efficient hopping distance in OLED on recombination efficiency.This model might explain the relative experimental phenomena.
The delay time is an important index of performance evaluation inOLED. Based on the physics mechanism of injection, transport andrecombination of the charge carriers, a model was developed to calculatethe delay time of electroluminescence (EL) from bilayer organic lightemitting diodes. The effect of injection, transport and recombinationprocess on the EL delay time was discussed, possible explanations weregiven. A ground of recombination or EL delay time versus appliedvoltage curves with different internal interface barrier, the built-inpotential and the thickness of hole transport layer are listed, we found thatthe effect of the internal interface barrier and the built-in potential holdthe upper hand over the thickness of hole transport layer. It has a certainreferenced meaning to respond rapidly to the functional materials andstructure options of EL devices.
绝缘缓冲层修饰金属／有机界面已成为改善OLED载流子注入的有效手段之一。文中考虑了LiF绝缘缓冲层对电子注入势垒的影响，给出了LiF／金属复合电极注入势垒的表达式，基于载流子的注入和复合过程，建立了双层有机电致发光器件发光效率的理论模型，讨论了器件效率随电压、注入势垒、内界面势垒、有机层厚度的变化关系，成功地模拟了插入不同厚度LiF缓冲层的电流—电压特性曲线，得出了LiF介于不同金属阴极和Alq3之间的最佳厚度值，通过与实验结果的比较，验证了模型的有效范围。这将有助于实验上对缓冲层材料的选择及最佳厚度的确定。
有机／有机界面特性支配着界面处载流子的输运与复合区域，基于该实验事实，文中提出了双层有机电致发光器件中载流子在有机／有机界面复合的无序跳跃理论模型。计算发现了内界面处载流子的有效势垒高度决定OLED中的电子和空穴密度的分布，而电子与空穴密度又决定了电场强度的大小，并研究了电场、跳跃距离与势垒对内界面处复合效率的影响。该模型很好地解释相关的实验现象。
响应延时是衡量器件性能的重要参数之一，根据载流子注入、输运及复合物理机制，文中进一步发展了双层有机电致发光器件的电致发光(EL)延时模型，得出了EL延时的解析表达式，重新评估了注入、输运及复合对EL延时的影响，并揭示了内界面势垒、内建势垒、空穴传输层厚度与复合延时及EL延时的关系，通过比较发现空穴传输层厚度对延时的影响要略弱于内界面势垒与内建势垒。这对快速响应EL器件的功能材料及结构选择有一定的参考意义。
Although the breakthrough of the research and development oforganic light-emitting diodes(OLED) has been gained in last 20 years,some major basic problems remain ambiguous, which lead to shortlifetime and low efficiency of devices. In order to solve a series ofproblems, it should intensify the understanding of luminescencemechanism and the performance of thin film organic materials, transportcharacteristics of carriers of thin film materials, degradation mechanismof devices, interfacial characteristics of devices and interfacesengineering. The numerical simulation was performed mainlysurrounding the metal/organic semiconductor interface, metal/inorganic/organic semiconductor interface and organic/organic semiconductorinterface in this paper.
The insulating buffer layer modification of the metal/organicinterface has been one of the effective ways to improve the carrierinjection of OLED. An analytical model to calculate electroluminescence(EL) efficiency of bilayer organic light-emitting devices, considering theinfluence of introducing LiF insulating buffer layer at metal/organicinterface on the barrier height for electrons injection, was presented. Therelations of EL efficiency versus applied voltage, injection barrier,internal interfacial barrier and the thickness of organic layer werediscussed, the J-V characteristics of OLED inserting LiF buffer layers ofdifferent thickness were simulated successfully, the optimal thickness ofLiF buffer layer was achieved. By comparing with the experimentalresults, the validity of predictions from this model was tested. These willbe conducive to the selection and the optimal thickness design of bufferlayer material.
The organic/organic semiconductor interracial characteristics ofOLED control the transport of carriers between layers and the region ofthe device where recombination takes place. In the light of thisbackground, a disordered hopping model of charge carriers at theorganic/organic interface in bilayer organic light-emitting diodes waspresented. The calculated results indicate that the distribution of electron and hole densities is determined by the effective barrier height at theorganic/organic interface, at the same time, the field strength isdetermined by the distribution of electron and hole densities. Then weanalyzed the influences of the variation of electric field, barrier heightand efficient hopping distance in OLED on recombination efficiency.This model might explain the relative experimental phenomena.
The delay time is an important index of performance evaluation inOLED. Based on the physics mechanism of injection, transport andrecombination of the charge carriers, a model was developed to calculatethe delay time of electroluminescence (EL) from bilayer organic lightemitting diodes. The effect of injection, transport and recombinationprocess on the EL delay time was discussed, possible explanations weregiven. A ground of recombination or EL delay time versus appliedvoltage curves with different internal interface barrier, the built-inpotential and the thickness of hole transport layer are listed, we found thatthe effect of the internal interface barrier and the built-in potential holdthe upper hand over the thickness of hole transport layer. It has a certainreferenced meaning to respond rapidly to the functional materials andstructure options of EL devices.
引文
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