【作者】 陈平；

【导师】 李峰；

【作者基本信息】 吉林大学 ， 高分子化学与物理， 2014， 博士

【摘要】 有机电致发光器件（OLED）在显示和照明领域显示出极大的应用前景。尽管众多研究机构和公司投入巨资致力于OLED的研发，但是其产业化进程远远低于人们的预料，主要原因是在该领域尚有效率低、稳定性差等许多关键问题没有解决。弄清有机半导体内部的电子结构特点，以及有机光电器件中丰富的激发态过程，对提高OLED的光-电转换效率，以及器件的稳定性具有重要的意义。最近在常规OLED中发现的有机磁场效应（OMFE），成为有机半导体器件物理中新的一个研究热点。在不含任何磁性材料的常规OLED中，外加一个很小的磁场（通常为几十mT），器件的传导电流和发光强度在室温下就可以发生很显著的改变（变化幅度从百分之几到百分之几十）。OMFE的这些特点不仅使它在信息存储、传感技术和手写输入等方面具有非常重要的应用价值，在科学层面它还可以作为探测有机半导体内部的电荷传导、激子行为和自旋输运等微观过程的一种强有力的实验工具。近年来，尽管OMFE已经得到了长足的发展，但是仍有许多科学问题等待科研工作者去理解和挖掘。例如：对于OMFE的物理机制人们还存在很大争议；另一方面，如何利用OMFE来揭示OLED中对光-电转换效率“有用”或“无用”的物理过程，该方面的探索也还尚未引起人们的重视。目前，对OMFE机制的争论，主要集中在“电子-空穴对模型”、“双极化子模型”和“三线态激子-电荷作用模型”这三种解释上。而这三个模型的一个争论焦点就是：磁场到底影响的是激发态过程，还是载流子的迁移率？激基复合物（分子间CT态）的空穴和电子分别局域在不同分子上，其发光对应着分子间电子-空穴对的直接复合过程，因此它是论证“电子-空穴对模型”的理想对象。实验发现，NPB:(dppy)BF激基复合物的MEL要比NPB激子的MEL大2.7倍，而且线型可以用一个“非洛伦兹”线型拟合得很好，这就为“电子-空穴对模型”提供了直接的实验证据。另外，有、无磁场情况下瞬态电致发光曲线的上升沿重合得很好、而缓慢下降沿却彼此分开，说明磁场没有影响载流子的迁移率，而是影响了载流子的复合过程，这就证实了“电子-空穴对模型”在器件开启状态下是主导机制，而“双极化子模型”和“三线态激子-电荷作用模型”等迁移率模型则不占主导。在论证物理模型的基础上，我们就可以用MEL作为研究工具，来探索激基复合物中单、三态的相互转换过程，进而揭示制约激基复合物器件电致发光效率的因素。在激基复合物中，由于电子、空穴之间的空间距离较远，它的自旋交换能很小。这样，单线态就很容易向三线态进行转换（S→T）。通过对比m-MTDATA:Alq3激基复合物器件的MEL和NPB/Alq3激子器件的MEL，我们发现，基于激基复合物的MEL表现为正的效应，并且它的值要比基于激子的MEL大3.2倍，说明激基复合物会比激子存在更显著的S→T转换。该转换会导致单线态激子数目的减少，是激基复合物器件电致发光效率普遍较低的一个重要原因。MEL的实验结果也说明，抑制OLED中S→T的转换，对进一步提高器件的光-电转换效率具有重要意义。除了抑制S→T的转换，实现三线态激子向单线态激子的上转换（T→S），对荧光OLED中三线态激子的有效利用、设计高效率器件同样具有重要的意义。通常有两种途径可以实现T→S的上转换：一种是反向系间窜越过程（RISC），另一种是三线态-三线态相互淬灭过程（TTA）。由于这两种过程都产生延迟荧光，因此传统的时间分辨光谱很难区分这两种过程。但是，RISC和TTA的MEL响应曲线则有明显的不同：在RISC占主导的DCJTB器件中，MEL表现为在低磁场范围（<40mT）内快速下降，而后在高磁场范围（>40mT）饱和的特点。而在TTA占主导的Rubrene器件中，MEL则表现为在低磁场范围（<20mT）内有个很小的增加，而后在高磁场范围（>20mT）呈现出很大的下降的趋势。该实验结果说明，MEL响应能够成为RISC和TTA的“指纹性”特征谱，同时也能为揭示其它自旋相关的相互转换过程提供强有力的实验证据。在追求更高的光-电转换效率的同时，如何降低器件的效率滚降也是OLED研究者们努力思考的问题。先前，人们都是通过优化器件结构或者改善器件工艺，使载流子注入和传输更加平衡，从而达到抑制效率滚降的目的。与此方法不同，我们报道了一种基于dithienylbenzothiadiazole（二噻吩苯并噻二唑）的热致延迟荧光（TADF）材料red-1b，它在材料性质上能够同时满足RISC和TTA的要求。在优化的red-1b器件中，我们实现了31.25%的高激子利用率以及相对较低的效率滚降（在8000cd/m2亮度时仅27%效率滚降）。基于Guassion09理论模拟的前线轨道能级和溶剂化效应，显示red-1b具有分子内电荷转移态特征，从而能进行RISC过程；同时red-1b不同组态的激子的能量满足：S1<2T1<Tn，说明red-1b可以通过TTA作用有效地产生单线态激子。最后，MEL为我们论证了RISC和TTA在总发光中的贡献，以及它们分别在高激子利用率和低效率滚降中扮演的角色。本工作为将来设计高效率、高稳定性的有机荧光材料提供了一种新的思路：即结合RISC和TTA这两种激子过程的特点，在低电流下通过RISC获得高的激子利用率、在大电流下通过TTA抑制器件的效率滚降。更多还原

【Abstract】 Organic light-emitting diodes (OLED) have great potential in the flat-paneldisplays and solid-state lighting. Although many research institutions and companieshave invested heavily on the development of OLED, the industrializations are farbeyond our expectations because of some unresolved issues, e.g., the lowpower-conversion efficiency and the poor device stability. Understandings ofelectronic structure characteristics in organic semiconductors and excitonic processesin organic optoelectronic devices should have important meanings for furtherenhancing the power-conversion efficiency and stability of OLED.Organic magnetic field effects (OMFE) in ordinary OLED without any magneticcomponent have recently become a hot spot in physics of organic semiconductors.When the organic optoelectronic devices are submitted to the external magnetic field(about tens of millitesla), the electroluminescence (EL) intensity and current can belargely changed (from several percents to tens percents). These features of OMFE notonly make it valuable in applications such as Information Storage, Sensor, andPen-based Device, but also to be an effective tool in investigating the charge transfer,excitonic processes, and spin polarization in organic semiconductors. Despite greatprogress has been made in OMFE in recent years, there are still many scientific issues needed to be clarified. First, the underlying mechanisms of OMFE are quite debatable.Second, the exploration of OMFE in revealing the “useful” and “useless” processesfor power-conversion efficiency does not arouse enough attention.Currently, the underlying mechanisms of OMFE are mainly based on three models:“electron-hole pair model”,“bipolaron pair model”, and “exciton-polaron interactionmodel”. One of biggest arguments among these three models is, what does themagnetic field affect: the charge carrier mobility or excitonic processes? The holesand electrons of exciplex (or called intermolecular charge-transfer state) localize ontwo neighboring molecules, and its emission comes from the direct recombination ofintermolecular electron-hole pairs. Thus, the exciplex becomes an ideal object for“electron-hole pair model”. Results show that the magneto-electroluminescence(MEL) of exciplex-based device is larger than that of exciton-based device by a factorof2.2, and its line-shape can be fitted by non-Lorentzian law very well, providingdirect evidence for “electron-hole pair model”. Moreover, the transient EL with andwithout magnetic fields show that the onset of the fast rising edges of EL pulsesoverlap perfectly, while the falling edge of EL pulses separated, confirming themagnetic field has no effect on the charge mobility but on the charge recombinationprocess, implying the charge mobility-related mechanisms may be less dominantabove the turn-on voltage.Based on the clarification of physical mechanisms, we can use the MEL as a toolto investigate the interconversion between singlet and triplet in OLED, and thenreveal the factors limiting the EL efficiency of exciplex. In exciplex, the distancebetween holes and electrons is relative large, leading to very small spin-exchangeenergy between singlet and triplet. Thus, the conversion from singlet to triplet (S→T)should be efficient. By comparing the MEL between m-MTDATA: Alq3exciplex-based device and NPB/Alq3exciton-based device, m-MTDATA: Alq3exciplex exhibits a positive MEL effect, and its amplitude is larger than that ofNPB/Alq3exciton-based device by a factor of3.2, indicating more efficient S→Tconversion in m-MTDATA: Alq3exciplex. This S→T conversion may cause thereduction of singlet exciton, which is an important factor lowering the EL efficiency of exciplex. The MEL results also tell us suppressing S→T conversion is veryimportant for further improving the OLED power conversion efficiency.Besides suppressing the S→T conversion, realizing the up-conversion of T→S isalso critical in utilization of triplet energy in fluorescent OLED. Generally, there aretwo approaches for T→S up-conversion: Reverse Inter-system Crossing (RISC) andTriplet-Triplet Annihilation (TTA). Because both RISC and TTA could generatedelayed fluorescence, it is not an easy task to discern them merely from the traditionaltime-resolved spectra. We note that, RISC and TTA are highly spin-dependentprocesses that can generate distinct MEL responses. In DCJTB device where the CTspecies dominate the light emission, the RISC-mediated MEL shows a rapid decreasein the low-field regime (<40mT) and then tend to be saturated in the high-fieldregime (>40mT). But for the Rubrene device, the TTA-mediated MEL is comprisedof a small increase within the field range of20mT followed by a remarkable decreaseat higher field. Our studies indicate the MEL could serve as a “finger-print” forexploiting the RISC and TTA, and other spin-dependent mutual conversion in OLED.In the pursuit of high power conversion efficiency of OLED, reducing theefficiency roll-off is another key point researchers are thinking about. The balancedcharge carrier injection and transport is previously considered to suppressing theefficiency roll-off. To achieve this goal, optimizing the device structure andimproving the fabrication technique were generally proposed. Different to these twomethods, here we report a new type of thermally activated delay fluorescence-basedof dithienylbenzothiadiazole (red-1b) which combines RISC and TTA up-conversion.In its device performance, high exciton utilization of31.25%and relative lowefficiency roll-off (only~27%efficiency drops when brightness reaches to~8000cd/m2) were realized. The Gaussian09calculated frontier orbital energy levels andthe solvent effects suggest the intra-molecular charge-transfer (ICT) characteristic ofred-1b which is favorable for RISC. In addition, the calculated T1-Tn transition showsthe singlet and triplet energetically satisfy S1<2T1<Tn, indicating efficient singletproduction via TTA. Finally, the MEL measurements demonstrate the co-existence ofRISC and TTA, and the contributions of RISC to high exciton utilization under low current density, and TTA to suppression of efficiency roll-off under high currentdensity, respectively.更多还原