Enhanced electron injection and efficiency in blended-layer organic light emitting diodes with aluminium cathodes: new 2,5-diaryl-1,3,4-oxadiazole–fluorene hybrids incorporating pyridine units
作者:Stephen Oyston、Changsheng Wang、Igor F. Perepichka、Andrei S. Batsanov、Martin R. Bryce、Jin H. Ahn、Michael C. Petty
DOI:10.1039/b510003h
日期:——
This work focuses on the first 2,5-diaryl-1,3,4-oxadiazoleâfluorene hybrids which incorporate pyridine units within the Ï-electron system, viz. 2,7-bis5-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]-2-pyridyl}-9,9-dihexylfluorene (6) and 2,7-bis5-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]-2-pyridyl}spirobifluorene (7). The thiophene analogue 2,7-bis5-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]-thien-2-yl}-9,9-dihexylfluorene 11 was also synthesised and its X-ray crystal structure was obtained. There is a progressive red shift in the UVâVis absorption and photoluminescence spectra on replacing benzene (8) with pyridine (6) and thiophene (11) consistent with increased planarity of the Ï-system and reduced HOMOâLUMO gap along the series. Calculations at the DFT (density functional theory) level establish that inclusion of the pyridyl rings in 6 and 7 considerably enhances the electron affinity of the system, compared to phenyl analogues. Single-layer organic light-emitting diodes (OLEDs) have been fabricated by spin-coating blends of poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEHâPPV) as the emissive material with added electron transport compounds 6 or 7 to enhance electron injection. The external quantum efficiencies of the devices were greatly enhanced compared to pure MEHâPPV reference devices. ITO/PEDOT â¶ PSS/MEHâPPV : 7 (30 â¶ 70% by weight)/Al devices exhibited an external quantum efficiency (EQE) of 0.5% and a luminous efficiency of 0.93 cd Aâ1 at 9.5 V and a luminance of 100 cd mâ2. The modest increase in efficiency for the same device when Al was replaced by a Ca/Al cathode (EQE 0.6% and 1.2 cd Aâ1 at 10.5 V) suggests that the two methods of enhancing electron injection into the MEHâPPV emitter are mutually exclusive. Utilising blended layers is an attractive alternative to using Ca electrodes, which are highly reactive and are unstable in the atmosphere.
这项工作的重点是在 Ï 电子系统中加入吡啶单元的首批 2,5-二芳基-1,3,4-恶二唑芴杂交化合物,即2,7-bis5-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]-2-pyridyl}-9,9-dihexylfluorene (6) and 2,7-bis5-[2-(4-dodecyloxyphenyl)-1,3,4-oxadiazol-5-yl]-2-pyridyl}spirobifluorene (7).此外,还合成了噻吩类似物 2,7-双5-[5-(4-叔丁基苯基)-1,3,4-恶二唑-2-基]-噻吩-2-基}-9,9-二己基芴 11,并获得了其 X 射线晶体结构。用吡啶(6)和噻吩(11)取代苯(8)后,紫外可见吸收光谱和光致发光光谱逐渐发生红移,这与Ï-系统的平面度增加以及 HOMO- LUMO 间隙沿系列减小是一致的。在 DFT(密度泛函理论)水平上进行的计算表明,与苯基类似物相比,在 6 和 7 中加入吡啶环大大提高了系统的电子亲和力。以聚[2-(2-乙基己氧基)-5-甲氧基-1,4-苯基乙烯](MEHâPPV)为发射材料,并添加电子传输化合物 6 或 7 以增强电子注入,通过旋涂混合物制造出了单层有机发光二极管(OLED)。与纯 MEHâPPV 参考器件相比,这些器件的外部量子效率大大提高。ITO/PEDOT∥PSS/MEHâPPV:7(重量比为30∥70%)/Al器件的外部量子效率(EQE)为0.5%,在9.5 V电压和100 cd mâ2亮度条件下的发光效率为0.93 cd Aâ1。用 Ca/Al 阴极取代 Al 时,同一装置的效率略有提高(EQE 为 0.6%,10.5 V 时为 1.2 cd Aâ1),这表明这两种增强电子注入 MEHâPPV 发射器的方法是相互排斥的。与使用钙电极相比,使用混合层是一种有吸引力的替代方法,因为钙电极具有高活性,在大气中不稳定。