Design Criteria for Transparent Organic Light-emitting Diodes using Ultra-thin Ag Films

Abstract

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. Early studies on TOLEDs used transparent conducting oxides (TCOs) such as indium tin oxide (ITO) and aluminum zinc oxide (AZO) electrodes for both the cathode and anode. Typically, these TCO films are obtained with sputtering techniques, known to significantly damage the underlying organic layers. To protect underlying organic layer from sputtering damage, a thin metal-based semi-transparent cathode was prepared by thermal evaporation. Thus, the device structure of TOLEDs needs to consist of an ITO bottom anode, emitting layer, and a thin Ag cathode for commercialization. There are some drawbacks in transparent ITO and semi-transparent thin Ag electrode for TOLEDs. First, these TOLEDs usually have a preferential one-sided emission due to differences in reflection between ITO and Ag electrodes. In particular, the bottom-side (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 thin Ag-based TOLEDs have low transparency. Ag layers exhibit three-dimensional (3D) island growth as a result of the poor wettability of Ag on the substrate. This 3D island growth mode causes a significant delay in the formation of continuous Ag film. A discrete Ag film that consist of islands strongly absorbs and scatters light due to localized surface plasmon resonance. Third, the enhancement in the efficiency of TOLEDs is still required. The majority of the light generated in the organic layers is confined in the ITO (nITO = 1.9) anode and glass (nglass = 1.5) 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 (waveguide mode). Here, we demonstrate an innovative method to make transparent and high performance OLEDs on flexible substrates using only ultra-thin Ag electrodes as both the top and bottom transparent electrodes (Fig. 1a). 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 (> 70 %) and sheet resistance (< 15 Ω/□). 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 finite-difference time-domain (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 44.8 % (from 40.5 cd/A to 58.68 cd/A at 12.5 mA/cm2). 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.