Transparent organic light-emitting devices with LiF/Mg:Ag cathode.

Optics Express (Impact Factor: 3.53). 03/2005; 13(3):937-41. DOI: 10.1364/OPEX.13.000937
Source: PubMed

ABSTRACT Transparent organic light-emitting devices (TOLED) based on a stacked transparent cathode of a LiF/Mg:Ag were investigated. The device has a structure of indium-tin-oxide (ITO)/ N, N'-diphenyl-N, N'-(3- methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) (90 nm)/ tris-(8- hydroxyquinoline) aluminum (Alq3) (80 nm)/LiF (0.5 nm)/Mg:Ag (20 nm)/Alq3 (50 nm), where the transparent capping layer of 50 nm Alq3 acts as refractive index matching layer to optimize optical output. The turn-on voltage of the device is as low as 2.8 V. The device also shows high optical transparency and low reflectivity in the visible region, approximately 40% of light can emit from the top cathode side and 60% of the light from bottom ITO glass. At injection current density of 20 mA/cm2, the current efficiency, for bottom emission from ITO anode side and top emission from metal cathode side, is 3.4 cd/A and 2.2 cd/A, respectively. The lower turnon voltage and higher efficiency of device are due to enhancing electron injection by using LiF/Mg:Ag cathode.

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    ABSTRACT: Electrical and optical properties of semitransparent Ag and Al layer were studied, which are used for the electrodes in top-emission organic light-emitting diodes. Sheet resistance and transmittance of visible light through a thin layer were measured and analyzed. Several thin metal layers of Ag and Al were deposited onto a glass substrate up to a thickness of 50 nm using a thermal evaporation. Sheet resistance measurements show that a layer thickness is needed more than 15 nm and 20 nm for Ag and Al, respectively, when a proper sheet resistance is assumed to be less than . From the measurements of transmittance of visible light through a thin-metal layer, metallic behavior was observed when the layer thickness is over 25 nm for both films. Thus, from a study of sheet resistance and transmittance of visible light, a minimum proper thickness of semitransparent metal layer is 20 nm and 25 nm for Ag and Al, respectively.
    Journal of the Korean Institute of Electrical and Electronic Material Engineers. 01/2008; 21(10).
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    ABSTRACT: The inclusion of a capping layer above the cathode can enhance the transmittance and efficiency of a device. Organic LEDs (OLEDs) are light sources that do not produce excessive heat and do not contain toxic materials. 1 The light emitted from OLEDs is UV-free and can therefore be used to illuminate biological substances without causing harmful ef-fects. OLEDs are ultra-thin (about 150nm), have very low weight, and can be tailored to many formats. A specific type of OLED—a so-called transparent OLED (TOLED)—is bidirec-tional, i.e., it emits light from its top and bottom surfaces (see Figure 1). 2–4 It is vital that all components in a TOLED are trans-parent so that the simultaneous top and bottom light emission can be achieved. To ensure transparency of TOLEDs, the thickness of their cath-odes should not exceed 20nm. Transparent cathodes in TOLEDs usually consist of a multilayer metallic thin film. 3, 4 However, limited light transmission through the cathode means that the efficiency of top emission is lower than that of the bottom emis-sion. As such, the asymmetric structure of the light propagation causes different electroluminescence (EL) spectra for the top and bottom emissions from TOLEDs. To address the problem of asymmetry in the EL spectra, we designed a TOLED that uses a capping layer (CL), with a unique optical function. 2 The use of the CL does not disturb the electri-cal characteristics of the device and therefore obviates the need to alter the stack structure. Our CL is an optical dielectric that can enhance the transmittance of the device and the top-emission ef-ficiency. Including the CL on the cathode produces two inter-faces, i.e., air/CL and CL/cathode. Due to the complexity of multilayer interference, it is diffi-cult to establish a straightforward optical principle that achieves the EL spectral matching of bottom and top emissions in our TOLED. We achieve high transmittance and high top-emission efficiency by adjusting the CL thickness so that we created Figure 1. A schematic illustration of a typical transparent organic LED (TOLED). The capping layer and transparent cathode are key compo-nents in the device. destructive interference between the light components that are reflected at the two interfaces (E air/CL and E CL/cathode). In this condition, generated light travels toward the top side of the TOLED without being diminished by the reflected components of E air/CL and E CL/cathode . We can achieve high total efficiency (sum of the top and bottom efficiencies) by adjusting the CL thickness so that constructive interference takes place between E air/CL and E CL/cathode . The constructively interfered light com-ponent therefore adds to the bottom-traveling component to boost the efficiency. However, it is difficult to simultaneously enhance the total and top emissions because the interference conditions for achieving high transmittance and high total ef-ficiency are not the same. We observe that as the CL thickness is changed the peak EL intensity varies, but the peak wavelengths remain constant. The measured intensity ratio can therefore be used to achieve spectral matching. In our experiments, we obtain spectral matching at the CL thickness that corresponds to the Continued on next page
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