Efficient bright white organic light-emitting diode based on non-doped ultrathin 5,6,11,12-tetraphenylnaphthacene layer

High-performance undoped white organic light-emitting diode (OLED) has been fabricated using an ultrathin yellow-emitting layer of 5,6,11,12-tetraphenylnaphthacene (rubrene) inserted at two sides of interface between two N,N′-bis-(1-naphthyl)-N,N′- biphenyl-1,1′-biphenyl-4,4′- diamine (NPB) layers as a hole transporting and blue emissive layer, respectively. The results showed that a maximum luminance of the device reached to as high as 21,500 cd/m² at 15 V. The power efficiencies of 2.5 and 1.6 lm/W at a luminance of 1000 and 10000 cd/m², respectively, were obtained. The peaks of electroluminescent (EL) spectra locate at 429 and 560 nm corresponding to the Commissions Internationale De L’Eclairage (CIE) coordinates of (0.32, 0.33), which is independent of bias voltage. The performance enhancement of the device may result from direct charge carrier trapping in rubrene. Energy transfer mechanism was also found in the EL process.     

Efficient and stable single-dopant white OLEDs based on 9,10-bis (2-naphthyl) anthracene

Efficient white organic light-emitting diodes (WOLEDs) are fabricated with a thin layer of 9,10-bis (2-naphthyl) anthracene (ADN) doped with Rubrene as the source of white emission. A device with the structure of ITO/NPB (70 nm)/ADN: 0.5% Rubrene (30 nm)/Alq₃ (50 nm)/MgAg shows a maximum current efficiency of 3.7 cd/A, with the CIE coordinates of x=0.33, y=0.43. The EL spectrum of the devices and the CIE coordinates remains almost the same when the voltage is increased from 10 to 15 V and the current efficiency remains quite stable with the current density increased from 20 to 250 mA/cm².     

Spontaneous spin polarization in rubrene studied by density functional theory calculations

We theoretically studied the spontaneous spin polarization properties of organic molecule rubrene by using density functional theory calculations. Our investigations show that normally nonmagnetic molecule rubrene could be spin polarized by spinless-hole injection. Magnetic moment of the molecule increases linearly with the extra hole charge amount only when the injected hole charges reach a certain value. The spin density resides predominantly on the carbon atoms in the tetracene backbone of rubrene molecule and also the bond lengths change differently due to the injected charge. Spontaneous spin polarization can be explained as the preferably filling of the spin-splitted carbon p_z orbitals near the Fermi energy for the injected charge.     

Enhanced blue photoluminescence and new crystallinity of Ag/organic rubrene core-shell nanoparticles through hydrothermal treatment

Light-emitting organic semiconductors have attracted considerable attention for the nanoscale fabrication of organic-based displays and their potential application in optoelectronics, plasmonics, and photonics. In this study, core-shell hybrid nanostructures of organic rubrene coated on Ag nanoparticles (NPs) have been synthesized using a chemical reduction method. The thickness of the rubrene shell was 2.6–6.0 nm and the diameter of the Ag core was 30–70 nm. The optical and structural properties of the Ag/rubrene core-shell NPs were tuned by hydrothermal (HT) treatment at 190 °C. The Ag/rubrene core-shell NPs were characterized by high-resolution transmission electron microscopy and energy-dispersive X-ray (EDX) spectroscopy before and after the HT treatment, and their structural properties were confirmed through X-ray diffraction (XRD) analysis. XRD peaks related to an orthorhombic phase were observed along with the original triclinic crystal structure of the rubrene shell, and the triclinic crystal domain size increased from 28.2 nm to 30.8 nm owing to the HT treatment. Interestingly, the green light emission (λ_em = 550 nm) of the Ag/rubrene core-shell NPs changed to blue light emission (λ_em = 425 nm), increasing in intensity through the HT treatment. This is caused by the crystal change with H-type aggregation and enhanced energy transfer from a surface plasmon resonance.     

白光有机电致发光器件中Rubrene超薄层的发光性能

以黄光荧光染料5,6,11,12-Tetraphenylnaphthacene(Rubrene)作为超薄层, 制备了白光有机电致发光器件, 并采用改变荧光超薄层厚度的方法, 通过表征器件的电致发光光谱, 分析了超薄层中染料浓度对器件性能的影响. 研究结果表明, 在荧光染料Rubrene的厚度为0.3 nm时, 器件可以同时实现黄光和蓝光的等强度发射, 从而得到性能优良的白光器件, 最高亮度达到3700 cd/m², 颜色坐标为(0.32, 0.33). 器件中蓝光来自N,N′-Bis-(1-naphthyl)-N,N′- biphenyl-1,1′-biphenyl-4,4′-diamine(NPB)的电致发光, 而器件优良的光电性能是由于Rubrene分子直接载流子陷阱(DCT)效应和NPB分子向Rubrene分子传递能量的协同作用所致.     

The effect of phenyl groups on the transport properties of tetracene molecule

Electronic transport properties of pure tetracene and rubrene molecules were studied using density functional theory within the non-equilibrium Green's function method. Transmission coefficient and I-V curve were calculated for both molecules. The comparison between transmission coefficients in tetracene and rubrene molecules shows that there are some extra peaks in rubrene that belong to phenyl rings which are attached to tetracene. Besides, we found that up to 2.2 V the current is almost the same in both rubrene and tetracene and above this value, the current in rubrene is increased in comparison to tetracene which is the result of attachment of additional phenyl groups in rubrene molecule. Finally, we detected that these two molecules exhibit negative differential resistance behavior in the range between 1.2 V and 2 V.     

ITO/Rubrene表面及界面的AFM和XPS研究

采用原子力显微镜（AFM）和X射线光电子能谱仪（XPS）研究了氧化铟锡（ITO）/红荧烯（Rubrene）的形貌和表面、界面的电子态。AFM结果显示,ITO上的Rubrene膜有良好的均匀性。XPS结果表明:C1s谱有3个峰,位于282.50,284.70,289.30 eV,对应于C-Si、C O和C-C键。用氩离子束溅射表面,芳香碳对应的峰值逐渐增大,其他两个峰值迅速消失。随着表面O污染的去除,O1s峰先快速减弱然后逐渐增强。界面附近的O原子与C原子结合构成O C、C-O-C和醛基。In3d和Sn3d峰则缓慢增强,在界面附近峰值变得稳定。C1s、In3d、Sn3d谱峰都向低束缚能发生化学位移,表明ITO与Rubrene在界面发生了相互作用,形成一个互扩散层。     

TiO_2阴极缓冲层对Rubrene/C_(70)有机太阳能电池性能的改善

为了提高有机太阳能电池(OSCs)的性能,增强器件在空气中的稳定性,研究了TiO_2薄膜作为阴极缓冲层对OSCs器件性能的影响。制备了结构为ITO/TiO_2/C70/Rubrene/Mo O3/Al的OSCs器件。首先,通过测量器件效率,考察了TiO_2薄膜对Rubrene/C70电池的性能影响。接着,通过控制TiO_2薄膜厚度,研究了TiO_2厚度对器件性能的影响。实验结果显示,当TiO_2修饰层厚度比较薄时,器件的各性能参数较低,随着TiO_2厚度的不断增加,器件的各性能参数呈上升趋势;当TiO_2厚度为81 nm时,器件的各性能参数达到最佳,器件的功率转换效率为1.09%,电流密度为2.55 m A·cm-2,开路电压为0.88 V,填充因子为48.69%;当TiO_2厚度继续增加时,器件的各性能参数开始下降。对比没有TiO_2阴极修饰层的器件,最优时的Jsc、Voc、FF和PCE分别提高了37%、21%、17%和91%,并阐述了性能提高的原因。     

有机电致发光器件中Rubrene掺杂剂的荧光光谱性质

通过改变黄色荧光染料5,6,11,12-tetraphenylnaphthacene(Rubrene)的溶液浓度,研究了溶液的浓度改变对荧光小分子染料的发光光谱的影响.采用测试不同浓度Rubrene溶液光致发光(PL)的光谱强度手段,确定了溶液中染料的最佳浓度,即最大程度避免或减弱浓度效应(浓度猝灭)且光致发光强度最大时的溶液浓度.首先确定极限浓度的范围,然后通过测试范围内的不同浓度的PL光谱强度判断精确的极限浓度位置.结果表明,在Rubrene浓度为2×10-3 mol·L-1时,PL光谱强度最大,并且随着溶液浓度的继续增加,PL光谱强度的降低仅为11%,进而确定Rubrene分子的极限浓度应该在2×10-3 mol.L-1左右.通过对比不同掺杂比例的Rubrene:N,N'-Bis(naphthalen-l-y)-N,N'-bis(phenyl)benzidine(NPB)在溶液中的混合体系的PL光谱发现,NPB与Rubrene分子之间存在小完全的能鞋传递过程,有利于实现NPB分子和Rubrene分子共同的蓝光和黄光发射,从而得到色纯度较高的白色荧光.     

Ir（PPY）3对Rubrene荧光材料的敏化性研究

最近几年,磷光器件是有机电致发光研究领域和产业化的一大热点。在实验中作者发现PVK∶PBD∶Rubrene共掺体系的发光中存在较强的PVK发光,能量传递不充分。由于一些具有重金属离子的有机物,存在强的自旋-轨道耦合作用,引入到共掺体系可以充分利用单线态和三线态的发光,从而获得高于一般有机材料器件所达到的内量子效率。为获得单色性较好的Rubrene发光,作者将磷光敏化剂Ir(ppy)3引入到PVK∶PBD∶Rubrene共掺溶液中,得到了纯正Rubrene发光,Forester能量传递也更加充分。当进一步提高Rubrene掺杂浓度以后,单色性Rubrene发光更加明显,并讨论了Ir(ppy)3所起的作用和器件的发光机理。磷光材料与有机小分子材料共掺的方法,可以有效提高器件的发光亮度及效率。