Improved property in organic light-emitting diode utilizing two Al/Alq3 layers
Introduction
Since Tang and VanSlyke [1] reported a highly efficient organic light-emitting diodes (OLEDs) with high brightness in 1978. OLEDs have been attracted intensively investigation because of their many potential applications for the next generation flat-panel displays [2], [3], [4], [5], [6], [7], [8], such as unique advantages of high brightness, low power consumption, ability to emit a wide range of colors and application in full color organic displays [9], [10], [11], [12]. A conventional OLED consists of thin flat layer of a transparent anode, organic materials, a metallic cathode, and a glass substrates. Alq3 (tris(8-hydroxyquinoline) aluminum) is the most widely used small organic molecule as the emitting layer due to its good luminescent properties, high electron mobility, and excellent thermal stability. However, since electronic mobility is much lower than hole-mobility, so OLEDs have a fatal problem for applications due to their relatively low efficiencies in comparison with other competitive displays. It is generally considered that the balanced carriers are desirable for high electroluminescence (EL) efficiency.
It has been reported that the hole-injection and transport can be adjusted by inserting a thin Alq3 interlayer into the hole-transport layer (HTL), and to improve the hole-electron balance, and to improve EL efficiency compared to the conventional devices [13].
In this study, in order to enhance the properties of the OLEDS, we recently succeeded in fabricating a novel device with the structure of Al/Alq3/Al/Alq3/TPD/ITO. Al was the cathode, Alq3 represented the emitter layer and electron-transport layer, TPD represented hole-buffer layer, ITO was the anode. We will demonstrate that this OLEDs structure make the number of injected electrons and holes balance, and it exhibit significantly improve the device properties.
Section snippets
Experimental details
In our experiment, The following three structures were made: ITO(60 nm)/TPD(60 nm)/Alq3(65 nm)/Al(110 nm); ITO(60 nm)/TPD(60 nm)/Alq3(50 nm)/Al(110 nm)/Alq3(65 nm)/Al(110 nm); ITO/TPD(45 nm)/TPD:Alq3(5 nm)/Alq3(45 nm)/Al(110 nm). The three devices structures and molecular structure are shown in Fig. 1, Fig. 2, respectively. OLEDs were fabricated on indium–tin-oxide (ITO) coated glass substrates with a nominal surface resistance of 20 Ω/sq. The routine cleaning procedure included sonication in a detergent,
Results and discussion
The J–V characteristics of the OLEDs with various structures are shown in Fig. 3. The current density of Device (a) at the same driving voltage is smaller than that of Device (b), which is attributed to a decrease in the holes mobility resulting from the existence of the 5 nm TPD:Alq3 mixed layer [14]. The current–voltage (J–V) behavior of OLEDs with two Al/Alq3 layers is much superior to that of the Device (a) and Device (b), a good example is that the turn-on voltage is 7 V for the Device (c)
Conclusions
In summary, we have succeeded in fabricating high efficiency of the green OLEDs by utilizing the two Al/Alq3 layers and two electrodes. Such a structure not only improved brightness (the luminance efficiency of the device is approximately improved by 40.7%). but also decreased driving voltage (the device's driving voltage is 7 V). And what's more is that fabrication of this device is also easy.
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