一种苯乙烯基喹啉衍生物的稳态和瞬态光电性质

Steady-state and transient optoelectronic characteristics of styrene-and quinoline-based derivative

苯乙烯和喹啉是有机荧光材料的常用官能基团,已经在有机发光二极管(OLED)中得到了应用.本文用一种苯乙烯基喹啉衍生物2,2'-(2,5-二甲氧基-1,4-苯二乙烯基)双-8-乙酰氧基喹啉(MPV-AQ)同时作为发光材料和电子传输材料,研究了它在OLED器件中的稳态和瞬态光电性质.研究发现,在基于N,N’-二(萘-1-基)-N,N'-二苯基-联苯胺(NPB)/MPV-AQ的双层OLED中,电子以Fowler-Nordheim(FN)隧穿的方式从阴极注入到MPV-AQ层,这与MPV-AQ单电子器件中电子以Richardson-Schottky(RS)热电子发射的注入方式完全不同.这种电子注入方式的差别,主要是由于MPV-AQ的电子迁移率较低,大量空穴在NPB/MPV-AQ界面处形成电荷积累,使得MPV-AQ层的能带发生了弯曲,造成阴极一侧的电子隧穿距离减小,从而导致了FN隧穿的发生.通过拟合稳态电流-电压特性得到了电子注入势垒为0.23 eV,通过瞬态电致发光的延迟时间计算得到MPV-AQ的电子迁移率在10^(-6) cm^(2)/(V·s)数量级,通过瞬态电致发光的衰减获得了复合系数,并发现复合系数随电压增大而减小,与这种发光器件的效率滚降规律一致.本研究为弄清OLED中载流子的注入、传输和复合等基本物理过程提供了基础,能够为提高器件性能提供有益的帮助.

Styrene and quinoline groups are commonly incorporated into the organic fluorescent materials for organic light-emitting diodes(OLEDs). In this work, a type of small molecule derived from styrene and quinoline, with a chemical structure of 2,2’-(2,5-dimethoxy-1,4-phylenedivinylene)bis-8-acetoxyquinoline(MPV-AQ), is employed as the emitter and electron transporting material in the OLEDs, and its optoelectronic characteristics such as charge-carrier injection, transporting and recombination are investigated by the steady-state and transient technologies. It is found that the electron injection from the cathode into the MPV-AQ layer shows the FowlerNordheim(FN) tunneling characteristic in the N,N’-di(naphthalene-1-yl)-N,N’-diphenyl-benzidine(NPB)/MPVAQ bilayer OLED, which is different from the Richardson-Schottky(RS) thermionic emission in the electrononly device based on the MPV-AQ single-layer. The difference in electron injection is attributed to the bend of energy bands of MPV-AQ in the NPB/MPV-AQ device, which can be caused by the charge accumulation at the NPB/MPV-AQ interface. The accumulated charges should mainly be the holes on the side of NPB layer because the electron mobility of MPV-AQ is much lower than the hole mobility of NPB. Owing to the bending of lowest unoccupied molecular orbital(LUMO) of MPV-AQ, the tunneling distance for electrons is significantly reduced, which is favorable for the FN tunneling. The barrier height for electron injection is calculated to be 0.23 eV by fitting the current-voltage curve of the NPB/MPV-AQ bilayer OLED. And the electron mobility of MPV-AQ is determined by the delay time of transient electroluminescence(EL) and shows field-dependence with the value on the order of 10^(-6) cm^(2)/(V·s). In addition, the electron-hole recombination coefficient is obtained from the long time component of the temporal decay of the EL intensity, and the coefficient is found to decrease with the applied voltage increasing, which is consistent with the efficiency roll-off in this bilayer OLED. This study may provide a foundation for understanding the electronic processes of carrier injection,transport and recombination in the OLEDs, which is helpful in improving the device performance.