We fabricate a kind of novel efficient blue fluorescent organic light emitting device(OLED) based on p-n heterojunctions composed of hole transporting layer(HTL) N,N '-bis(naphthalen-1-yl)-N,N '-bis(phenyl)-benzidine(NPB) and electron transporting layer(ETL) 4,7-diphnenyl-1,10-phenanthroline(BPhen),into which a new blue material,DNCA(a derivation of N 6,N 6,N 12,N 12-tetrap-tolylchrysene-6,12-diamine),is partially doped simultaneously,and double emitting layers are configured.With a turn-on voltage of 2.6 V at 1 cd/m 2,this type of OLED presents a maximum luminance efficiency(η max) of 8.83 cd/A at 5.818 mA/cm 2 and a maximum luminance of over 40000 cd/m 2.Meanwhile,the Commission Internationale De L'Eclairage(CIE) coordinates of this device change slightly from(0.13,0.27) to(0.13,0.23) as the driving voltage increases from 3 V to 11 V.This improvement in the electroluminescent characteristics is attributed mainly to the ideal p-n heterojunction which can confine and distribute excitons evenly on two sides of the heterojunction interface so as to improve the carrier combination rate and expand the light-emitting region.
To improve the performance of tandem organic light-emitting diodes(OLEDs), we study the novel NaCl as n-type dopant in Bphen:NaCl layer. By analyzing their relevant energy levels and carrier transporting characteristics, we discuss the mechanisms of the effective charge generation layer(CGL) of Bphen:NaCl(6 wt%)/MoO3. In addition, we use the Bphen:NaCl(20 wt%) layer as the electron injection layer(EIL) combining the CGL to further improve the performance of tandem device. For this tandem device, the maximal current efficiency of 9.32 cd/A and the maximal power efficiency of 1.93 lm/W are obtained, which are enhanced approximately by 2.1 and 1.1 times compared with those of the singleemissive-unit device respectively. We attribute this improvement to the increase of electron injection ability by introducing of Bphen:NaCl layer. Moreover, the CGL is almost completely transparent in the visible light region, which is also important to achieve an efficient tandem OLEDs.
We investigate the electron injection effect of inserting a thin aluminum(Al) layer into cesium carbonate(Cs2CO3)injection layer. Two groups of organic light-emitting devices(OLEDs) are fabricated. For the first group of devices based on Alq3, we insert a thin Al layer of different thickness into Cs2CO3 injection layer, and the device's maximum current efficiency of 6.5 cd/A is obtained when the thickness of the thin Al layer is 0.4 nm. However, when the thickness of Al layer is 0.8 nm, the capacity of electron injection is the strongest. To validate the universality of this approach, then we fabricate another group of devices based on another blue emitting material. The maximum current efficiency of the device without and with a thin Al layer is 4.51 cd/A and 4.84 cd/A, respectively. Inserting a thin Al layer of an appropriate thickness into Cs2CO3 layer can result in the reduction of electron injection barrier, enhancement of the electron injection, and improvement of the performance of OLEDs. This can be attributed to the mechanism that thermally evaporated Cs2CO3 decomposes into cesium oxides, the thin Al layer reacts with cesium oxides to form Al–O–Cs complex, and the amount of the Al–O–Cs complex can be controlled by adjusting the thickness of the thin Al layer.
The driving voltage of an organic light-emitting diode(OLED) is lowered by employing molybdenum trioxide(MoO3)/N,N'-bis(naphthalene-1-yl)-N,N'-bis(phe-nyl)-benzidine(NPB) multiple quantum well(MQW) structure in the hole transport layer.For the device with double quantum well(DQW) structure of ITO/[MoO3(2.5 nm)/NPB(20 nm)]2/Alq3(50 nm)/LiF(0.8 nm)/Al(120 nm)],the turn-on voltage is reduced to 2.8 V,which is lowered by 0.4 V compared with that of the control device(without MQW structures),and the driving voltage is 5.6 V,which is reduced by 1 V compared with that of the control device at the 1000 cd/m2.In this work,the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure,which is attributed not only to the reduced energy barrier between ITO and NPB,but also to the forming charge transfer complex between MoO3 and NPB induced by the interfacial doping effect of MoO3.
We demonstrate high current efficiency of a blue fluorescent organic light-emitting diode (OLED) by using the charge control layers (CCLs) based on Alq3 . The CCLs that are inserted into the emitting layers (EMLs) could impede the hole injection and facilitate the electron transport, which can improve the carrier balance and further expand the exciton generation region. The maximal current efficiency of the optimal device is 5.89 cd/A at 1.81 mA/cm2 , which is about 2.19 times higher than that of the control device (CD) without the CCL, and the maximal luminance is 19.660 cd/m2 at 12V. The device shows a good color stability though the green light emitting material Alq3 is introduced as the CCL in the EML, but it has a poor lifetime due to the formation of cationic Alq3 species.
We chose pentacene as a hole injection layer(HIL) to fabricate the high performance blue fluorescent organic lightemitting devices(OLEDs). We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120?C. Then we performed the tests of scanning electron microscopy, atomic force microscopy, and Kelvin probe to explore the effect of annealing on the pentacene films. The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing. The optimal device with 120?C annealed pentacene film and n-doped electron transport layer(ETL) presents a low turn-on voltage of 2.6 V and a highest luminance of 134800 cd/m^2 at 12 V, which are reduced by 26% and improved by 50% compared with those of the control device.
