Papers by Keyword: Organic Light Emitting Diodes (OLED)

Abstract: Organic Light Emitting Diodes (OLED) are receiving increased attention due to tremendous application potential these devices hold in the areas of large area displays and lighting applications. However, the problems of efficiency, stability and shelf life are major challenges for making OLEDs an attractive alternative. The simple device structure involving anode, emissive layer and cathode is no longer the norm. Recently, various buffer layers like Hole Injection Layer (HIL), Hole transport Layer (HTL), Electron Injection Layer (EIL), Electron Transport Layer (ETL) etc. are being widely used as integral parts of the OLED architecture to enhance the performance parameters. The nomenclature of these layers is often confusing and sometimes used by different authors to mean different layers and a common and universal nomenclature for layers is still wanting. Applying a buffer layer, often called as the hole injecting layer (HIL) between anode and emissive layer is a general technique for increasing the efficiency and stability of organic light emitting diodes. Poly- (3,4-ethyhylene dioxythiophene): poly- (styrenesulphonate) (PEDOT:PSS) is a very common and popular such HIL used in OLEDs. In this chapter, a basic structure of OLEDs has been discussed in perspective with this HIL material and the effect of annealing this PEDOT: PSS layer on the characteristics of the device at different temperatures ranging from 100°C to 300°C in vacuum. Devices fabricated in clean room conditions are characterized for their electrical and optical properties. Equivalent circuits of the devices are deduced using impedance spectroscopy and discussed. Surface morphology of the HIL layers using atomic force microscopy (AFM) provides reasons for the variation of the device properties with the annealing of HIL.

Abstract: We have employed a simple circuit combining pulse generator and capacitance meter to investigate transient current (TC) and capacitance of organic light-emitting diodes (OLEDs). The dependence of TC on OLEDs structure with the change in thickness of hole transport layer (HTL), and pulse regimes with the change in voltage and frequency have been studied. Results revealed that two different evolution pattern of TC at low and high electric fields, and the change in HTL affected TC much greatly at high electric fields. The capacitance-voltage curve with different frequency was observed and showed trapped- charge state and interfacial charge transport process near the electrode.

Abstract: The aromatic triazole is an interesting electron-injection/transport chromophore because it possesses high electron affinity and thermal stability. In organic lighting-emitting diodes (OLEDs), highly electron-deficient triazole (Tz) derivates have been demonstrated to have more efficient electron-transport and hole-blocking characteristics and have a higher stability to high current density than the oxadiazole derivates. In this study, we have successfully synthesized a star-shaped soluble oligomer,4-(4-butylphenyl)-3,5-bis(4-(5'-hexyl-2,2'-bithiophen-5-yl)phenyl)-4H-1,2,4-triazole(Tz-2T-Hex) via Suzuki coupling reaction. The Tz-2T-Hex shows a red-shifted UV-vis absorption spectrum than Tz-Br with a maximum at 380 nm, due to the introduction of two bithiophene units at both ends. As film, Tz-2T-Hex showed the PL maximum at 530 nm. Tz-2T-Hex emitted light of a white color with the CIE coordinates (0.32 and 0.34).

Abstract: There are several kinds of methods in improving the efficiency of organic light emitting diodes (OLEDs). In this work, we used a co-deposited hole blocking layer to improve the efficiency of OLEDs. The structure of the component is: ITO/ MTDATA(15 nm) /NPB(40 nm) /BCP(10 nm) /BCP: Alq(15 nm) /LiF(0.7 nm)/ Al(180 nm). We changed the mixing rate of the BCP:Alq layer to be capable of hole blocking and electron transporting, and then improved the efficiency of OLEDs. Finally, we prepared white light OLED with doping Rubrene in NPB. When the concentration of the NPB: Rubrene layer was 2.0 wt.%, the device could emit the white light at 100 mA/cm2, and the luminance was above 2300 cd/m2, and the color coordinate was x = 0.36, y = 0.37.

Abstract: This study presents the influences of dye doping and hole blocking layer insertion on the electroluminescent properties of the blue organic light emitting diode. The luminance of the device was significantly improved by BCzVB doping because of the utilization of effective Förster energy transfer and the improvement of carrier injection and trapping; a BCzVB dominant emission was observed. Furthermore, when a BCP layer was inserted between the TBADN:BCzVB and Alq3 layers, a thin BCP layer insertion enhanced the injection of electrons and improved the luminance of the device. In contrast, a thick BCP layer insertion caused a delay in electron transport, resulting in a decrease in current density and a deterioration in luminance.

Abstract: In this investigation, the effect of the thickness in the flexible organic light-emitting diode (FOLED) is studied. The larger luminance and luminance efficiency, 1160 cd/m2 and 2.71 lm/W, can be obtained at NPB thickness of 45 nm with the Alq3 thickness of 35 nm. The luminance dramatically rises when the Alq3 thickness is 45 nm at bias voltage of 19 V. The highest luminance is up to 2190 cd/m2. The extreme characteristic of FOLED may be useful to an organic electrically pumped laser.

Abstract: In this paper, we describe the performance of an organic light emitting devices〔OLEDs〕 with ITO /4,4’,4“-tris{N,- （ 3-methylphenyl ） -N-phenylamino}triphenylamine (m-MTDATA) /N,N-diphenyl-N,N-bis1-naphthyl-1,1-biphenyl-4,4-diamine (NPB) /copper phthalocyanine (CuPc) / NPB / Bathocuproine(BCP) / tris-8-hydroxyquinoline Aluminum (Alq3) / LiF/ AL structure, the CuPc inserted between the two layers of NPB as a hole-consuming layer (HCL), and the BCP as a hole-blocking layer (HBL) . The EL spectrum peak is at 430 nm, indicating that the carrier recombination is confined in the NPB layer, in additional light emission originates from NPB. Compared with the luminous efficiency of the conventional diode without CuPc layer, that of the diode with HCL has been sharply increased up to 2.62 cd /A. It suggested that the CuPc and BCP exactly function as hole-consuming and hole-blocking layers, respectively, which enhance the efficiency of carrier，s recombination and confine the excitation in the EL layer.