Simple-structured efficient white organic light emitting diode via solution process

Highlights

• Solution-processed simple-structured organic light emitting diode

• Low cost energy-saving display and lighting panels

• Highly-efficient, bright and chromaticity stable white organic light emitting diodes (OLEDs)

• This work will greatly reduce the complexity of the white OLED fabrication process and promote its commercialization.

Abstract

High-efficiency white emission is crucial to the design of energy-saving display and lighting panels, whereas solution-process feasibility is highly desirable for large area-size and cost-effective roll-to-roll manufacturing. In this study, we demonstrate highly-efficient, bright and chromaticity stable white organic light emitting diodes (OLEDs) with solution-processed single emissive layer. The resultant best white OLED shows excellent electroluminescence performance with forward-viewing external quantum efficiency, current efficiency and power efficiency of 22.7%, 48.8 cd A^− 1 and 27.8 lm W^− 1 at 100 cd m^− 2, respectively, with a maximum luminance of 19,590 cd m^− 2. Furthermore, we also observed an increment of 112% in the power efficiency, 86.9% in the current efficiency and a decrement of 39.2% in the external quantum efficiency at 100 cd m^− 2 as the doping concentration of blue dye was increased from 10 wt% to 25 wt% in the devices. The better efficiency performance may be attributed to the effective exciton-confining device architecture and low-energy barrier for electrons to inject from the hole-blocking electron-transport layer to the host layer.

Introduction

After the demonstration of first succinct organic light-emitting diode (OLED) with two-layer structure in 1987 by C.W. Tang [1], flat-panel displays and lighting applications based on OLED technology have grown dramatically because of their attractive features such as simple fabrication process, ultra-thin structure, wide viewing angles, impressive color rendering, light-weight, and high compatibility with flexible substrates [2], [3]. In particular, white light-emitting OLEDs are known to be an ideal light source, which not only acts as a backlight for OLED display [4] but also can be utilized as an area light source for decorative and general lightings, or lighting in galleries, hospitals, and museums, because OLEDs do not emit ultraviolet radiation [5].

Typically, OLEDs are fabricated using either vacuum evaporation or solution process. Current white OLEDs consist of two or more multiple layers of different materials in precise optoelectrical design [6]. Such multilayer structures allow the separation of the charge-injecting, charge transporting and light-emitting functions to the different layers, thus leading to a marked increase in efficiency and lifetime [7], which can be easily achieved stepwise in vacuum evaporation process. However, the vacuum deposition technique is faced with many shortcomings such as inefficient utilization of materials and high energy consumption, which remains a great challenge for the mass-production of cost-effective white OLEDs [8], [9]. However, solution-processed is deemed more superior in enabling flexible, large area size roll-to-roll production, and consequently more cost-effective [10], [11]. In the solution based fabrication process, OLED devices generally exhibit efficiency much lower than their dry-processed counterparts, especially at high luminance. Markedly improving the efficiency of solution-processed OLEDs is hence crucial.

To achieve high device efficiency, three major approaches can be applied, namely, (i) design and synthesis of electroluminescent (EL) active materials, (ii) design and employment of efficiency-effective device architectures, and (iii) incorporation of internal and external light out-coupling technologies. From a device structure perspective, it includes the employment of low interfacial resistance P − I − N structures [12], low carrier-injection barriers [13], balanced carrier injection [14], carrier and exciton confinement [15], stepwise emissive layers [16], carrier modulation layers [17], structures enabling exciton to generate on host or on both host and guest [18], structures facilitating host-to-guest energy transfer [19], and co-host structures [20]. Mostly all are applicable only for vacuum deposition technique. Only a few approaches can be utilized in the solution process unless any certain special techniques are adopted.

Therefore, it is highly desirable to develop a newly effective device that can render various channels to harvest all the excitons together with a reduced efficiency roll-off. In this work, we demonstrate the feasibility of fabricating a single emissive layer white OLED via solution process using two white light complementary color approach to avoid device complexity and to reduce cost. The resultant white OLED shows at 100 cd m^− 2 an external quantum efficiency of 22.7%, current efficiency 48.8 cd A^− 1 and power efficiency 27.8 lm W^− 1 with a maximum luminance of 19,590 cd m^− 2 without using any light out-coupling techniques.