Abstract

We present highly power-efficient flexible organic light-emitting diodes by combining nanostructured metallic transparent conductor on plastic substrates, leading to a power efficiency over 160 lm/W with angular color stability. Flexible organic light-emitting diodes (OLEDs) hold great promise for future bendable display and curved lighting applications. One key challenge of high-performance flexible OLEDs is to develop new flexible transparent conductive electrodes (TCEs) with superior mechanical, electrical and optical properties. Indium-tin-oxide (ITO) electrode is the most widely used TCE in optoelectronic devices due to its excellent electrical conductivity and light transmission, but the high-temperature fabrication and the brittle nature under repeated bending condition hinder its application on flexible plastic substrates. An additional drawback of using ITO in flexible OLEDs on plastic substrates is the limited light outcoupling efficiency due to the severe trapping loss of the internally emitted photons. Various materials and structures have been proposed to function as flexible TCEs. Metal-dielectric composite electrode (MDCE) has been regarded as an effective TCE for flexible devices in terms of mechanical flexibility, electrical conductivity, optical transparency, and large-area film uniformity. Whilst MDCE may be an ideal candidate to replace ITO, particularly in flexible devices on plastic substrates, two key challenges should be overcome. First, forming an ultrathin metal film (≤ 10 nm) in a MDCE for improving the optical transmission is extremely difficult due to the dewetting problem with an isolated granular morphology, causing the adverse degradation in optical transmittance and electrical conductance with particle plasmon absorption and severe disconnection. Second, an optical microcavity effect is inevitable with the use of a planar MDCE structure, leading to the spectral and angular dependence of the emission characteristics. Herein, we demonstrate a new strategy to achieve a powerful transparent conductive electrode on plastic substrate that combines a quasi-random nanostructured optical coupling layer and an ultrathin metal alloy conduction layer [1,2]. The optimum electrical conductivity, optical manipulation capability, and high tolerance to mechanical bending are realized in this composite electrode, which is favorable for the fabrication of ITO-free flexible OLEDs with state-of-the-art performance on low-refractive-index plastic substrate. The angularly and spectrally independent boost in light outcoupling of white emission is obtained by minimizing the waveguide mode, metallic electrode-related microcavity effect and surface plasmonic loss due to the integrated quasi-random outcoupling structure in the composite electrode[3-5]. The resulting white flexible OLED exhibits the high enhancement in efficiency, e.g., external quantum efficiency ~70% and power efficiency over 160 lm/W. In addition, this composite electrode has a scalable manufacturing potential in large-area flexible electronic systems. Fig. 1. Device structure and performance of flexible white OLEDs. (a-c) Comparison of different transparent electrodes: (a) Optical properties. Inset is the schematic illustration of flexible OLED structure with NMDCE on plastic substrate. (b) Sheet resistance as a function of nominal film thickness. (c) Mechanical stability under continuous bending test. (d-f) Performance characteristics of white flexible OLEDs: (d) Current density and luminance as a function of driving voltage. (e) Current and power efficiencies as a function of luminance. (f) Normalized EL spectra. Inset is the photograph of white-emission flexible OLED. (g-i) Simulations of normalized cross-section intensity field distributions (at 520 nm) with propagation of OLEDs on (g) ITO, (h) MDCE and (i) NMDCE using FDTD method.

© 2016 Optical Society of America