Research Article www.acsami.org
Efficient Color-Stable Inverted White Organic Light-Emitting Diodes with Outcoupling-Enhanced ZnO Layer Xin-Dong Zhao, Yan-Qing Li,* Heng-Yang Xiang, Yi-Bo Zhang, Jing-De Chen, Lu-Hai Xu, and Jian-Xin Tang* Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China S Supporting Information *
ABSTRACT: Inverted organic light-emitting diode (OLED) has attracted extensive attention due to the demand in active-matrix OLED display panels as its geometry enables the direct connection with n-channel transistor backplane on the substrate. One key challenge of high-performance inverted OLED is an efficient electron-injection layer with superior electrical and optical properties to match the indium tin oxide cathode on substrate. We here propose a synergistic electroninjection architecture using surface modification of ZnO layer to simultaneously promote electron injection into organic emitter and enhance out-coupling of waveguided light. An efficient inverted white OLED is realized by introducing the nanoimprinted aperiodic nanostructure of ZnO for broadband and angleindependent light out-coupling and inserting an n-type doped interlayer for energy level tuning and injection barrier lowering. As a result, the optimized inverted white OLEDs have an external quantum efficiency of 42.4% and a power efficiency of 85.4 lm W1−, which are accompanied by the superiority of angular color stability over the visible wavelength range. Our results may inspire a promising approach to fabricate high-efficiency inverted OLEDs for largescale display panels. KEYWORDS: inverted organic light-emitting diodes, white OLED, ZnO, light outcoupling, energy level tuning
1. INTRODUCTION Organic light-emitting diodes (OLEDs) have been extensively studied over recent decades for the applications of large-area solid-state lighting and full-color flat-panel displays due to their lightweight, fast response time, wide color gamut, low power consumption, and large viewing angle.1−5 With the rapid development of widely adopted active-matrix architectures in high-quality displays, inverted OLEDs using the bottom cathode on substrates have gained increasing attention since this device structure can render feasible use of generally applied n-channel thin film transistors (i.e., amorphous silicon or transparent oxide semiconductors) in the active-matrix OLED pixel circuitry.6−8 However, it remains challenging for inverted OLEDs to get an equal status with conventional devices because of the poor electron-injection behavior from the transparent indium tin oxide (ITO) cathode into the organic emitter and serious optical confinement of the ITO/organic waveguide mode during light out-coupling process.9−13 Further improvement in the performance of inverted OLEDs is thus necessary for practical applications. To achieve highly efficient inverted OLEDs, it is crucial to select an appropriate electron-injection layer (EIL) for balancing the electron−hole recombination and retaining a low operating voltage by reducing the energy loss during electron-photon conversion. Several EILs have been proposed to modify the ITO cathode, including organic layers doped with alkali metal © 2017 American Chemical Society
compounds (e.g., lithium carbonate (Li2CO3), cesium carbonate (Cs2CO3), or cesium fluoride (CsF)),11−14 and n-type metal oxides (e.g., zinc oxide (ZnO), titanium oxide (TiO2), hafnium oxide (HfO2) or zirconium oxide (ZrO2)).15−22 Among these promising candidates, ZnO has many attractive features such as relatively high electron mobility, environmental stability, and high transparency. Particularly, sol−gel-derived ZnO film is widely used as an efficient EIL in inverted organic optoelectronic devices since a uniform ZnO layer can obtained in a mild environment at relatively low annealing temperature (
