Research On Organic Light-emitting Devices Based On Sliver/Germanium/Sliver Transparent Electrode

Recently, products based on the organic light-emitting devices(OLEDs) have come into people’s daily life. The increasing marketshares of the OLEDs products reveal the practical significance of theOLEDs research. Due to the remarkable effect on the optical andelectrical properties, transparent electrode is the significant component ofthe OLEDs. Sliver (Ag) thin films are usually used as the transparentelectrode in the OLEDs because of its fine conductivity, ductility andsimple fabrication process. However, limited its growth mode, Ag thinfilm should be thick enough to form continuous film so that it still hashigh reflectivity. As a result, microcavity effect usually exists in theOLEDs based on the Ag transparent electrode, leading to theangle-dependent characteristic of the electroluminescent spectra. In thispaper, we introduce a germanium (Ge) wetting layer to improve therelevant characteristics of the Ag thin film, demonstrate an efficient Ag/Ge/Ag (AGA) electrode, and study the optical and electrical properties ofOLEDs based on the AGA electrode in detail.Firstly, we use the Ge layer to improve the optical properties of Agthin film. By adding a5nm Ge layer, the AGA electrode with the structureAg (15nm)/Ge (5nm)/Ag (5nm) has an even transmittance in the visiblerange, and it also makes the device’s cavity emission possesses excellentangle stability. Employing the AGA electrode, the red (R), green (G), blue (B) and white (W) top-emitting OLEDs are demonstrated withoutchanging the thickness of the devices. The maximum current efficiency ofthe R, G, B, W device is4.6、15.9、14.6and12.4cd/A, respectively.Besides, the spectra of these devices possess excellent angle stability. Forexample, as the viewing angle changing from0°to60°, the CommissionInternationale de L’Eclairage (CIE) coordinates of B and W device justchange (0.003,0.030) and (0.048,0.046), respectively.Secondly, employing the AGA-based blue top-emitting OLED, thewhite devices based on the down-conversion (DC) layer are demonstrated.The DC layer is composed of red fluorescent dye (diphenylamino) dopedin a host of tris (8-hydroxy-quinolinato) aluminum. When the thicknessof the DC layer reach to200nm, the white device is obtained with theCIE coordinates of (0.240,0.332), a maximum luminance of2992cd/m2,a current efficiency of3.95cd/A and an external quantum efficiency of2.5%. Furthermore, the white device shows excellently stable spectra, asthe bias voltage changes from4V to7V, the CIE coordinates still keep at(0.2360.002,0.3260.004), and they only shift by (0.023,-0.007) asthe vieing angle changes from0°to60°.Finally, we find that the Ge doesn’t only improve the opticalproperties of the Ag film, but also change its growth mode. Byintroducing a1nm Ge layer, the10nm AGA thin films of which thetransmittance is65%in the visible range and the sheet resistance is 26Ω/sq, are demonstrated. Besides, the electrode also presents excellentmechanical resilience as its sheet resistance doesn’t change visibly after1000times bending. Based on the10nm AGA electrode, flexible greenand white top-emitting, bottom-emitting and transparent OLEDs aredemonstrated, with comparable efficiency and color stability to theirITO-based counterparts. This work proves the feasibility of employingthe AGA in flexible OLEDs.