Both Directional Organic Light-Emitting Diodes Using Thin Film Electrodes

Abstract

Recently, transparent organic light-emitting diodes (TOLEDs) have attracted much interest, because they can open new applications for bi-directional displays, transparent displays, wearable displays, smart glass, and window lighting. Typically, the device structure of TOLEDs needs to consist of an ITO bottom anode, emitting layer, and a thin metal cathode. To protect underlying organic layer from sputtering damage, a thin metal-based semi-transparent cathode was necessarily used by thermal evaporation. In this system, there were many problems in application of ITO electrodes to high efficient TOLEDs. First, these TOLEDs usually have a preferential one-sided emission due to differences in reflection between ITO and Ag electrodes. In particular, the ITO-side emission ratio has a strong intensity which is more than 70% of the total emission ratio because of the relatively low reflection of ITO compared with Ag film. Second, the majority of the light generated in the organic layers is confined in the ITO anode and glass substrates due to the large difference in the refractive index between their layers. This results in out-coupling efficiencies of around only 20% due to the total internal reflection at the ITO / glass and glass / air interfaces. These problems can be solved by employing a thin metal (Ag) electrode as an ITO alternative. However, it is very difficult to form smooth and very thin Ag layers for transparent anode. Ag layers exhibit three-dimensional island growth as a result of the poor wettability of Ag on the substrate. A discrete Ag film consisting of an island structure exhibits strong absorption and scattering of incident light, because of localized surface plasmon resonance. Thus, thin Ag film was not considered as a transparent electrode for TOLED. Here, we demonstrate an innovative method to make transparent and high performance OLEDs on substrates using only ultra-thin Ag electrodes as both the top and bottom transparent electrodes. We show that merely a minute-plasma treatment on the glass before the deposition of Ag layer leads to significantly improved growth homogeneity of the Ag layer. The ultra-thin Ag film is formed after oxygen plasma treatment, leading to improved visible range transmittance and sheet resistance. By designing the weak micro-cavity structure with the ultra-thin Ag electrodes, waveguide modes could be suppressed and optical transmittance of TOLEDs could be improved. According to FDTD simulation, waveguide modes in ITO and glass substrate can be remarkably extracted because of weak micro-cavity effect. Consequently, the luminance of TOLED could be improved by 47 %. In addition, the transmittance of TOLED with the ultra-thin Ag electrode showed a high transmittance (73.84 %), which was similar than that of ITO (73.19%). The optimized weak micro-cavity structured shows the high optical transmittance, identical emission rate and reduced wave-guided mode were achieved, enhancing the luminance of devices.