Simple-structured efficient white organic light emitting diode via solution process
Graphical abstract
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.
Section snippets
Device fabrication
All the devices are composed of a 125 nm indium tin oxide (ITO) anode layer, a 35 nm poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) hole injection layer (HIL), following a 30 nm single emissive layer (EML), a 32 nm 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene (TPBi) electron transporting layer (ETL), a 0.8 nm lithium fluoride (LiF) electron injection layer (EIL), and a 150 nm aluminum (Al) cathode layer. Among, all the layers are deposited via spin-coating, while the ETL, EIL,
Results and discussion
We fabricated solution-processed single emissive layer white OLEDs W1 to W4 by incorporating x wt% (x = 10, 15, 20, and 25) blue emitter, iridium (III) bis[(4,6-difluorophenyl)pyridinato-N,C2′]-picolinate (FIrpic) and a 1 wt% yellow emitter, iridium (III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2′)acetyla-cetonate (PO-01) into 4, 4′-N, N′-dicarbazole-biphenyl (CBP) with the following device structure: ITO/PEDOT: PSS/1 wt% PO-01 and x wt% Flrpic doped in CBP/TPBi/LiF/Al.
Table 1 summarizes the
Conclusion
In summary, we have fabricated a solution-proceed feasible, efficient and highly chromaticity stable single layered white organic light emitting diode. The resultants best device shows a maximum luminance of more than 19,000 cd m− 2 and with a 27.8 lm W− 1 power efficiency, 48.8 cd A− 1 current efficiency and 22.7% external quantum efficiency at 100 cd m− 2. Furthermore, we also observed that the power efficiency and current efficiency of fabricated white OLEDs increases from 13.1 lm W− 1 to 27.8 lm W− 1 and 26.0
Acknowledgement
The authors are grateful for the financial support in part by Ministry of Economic Affairs through grant MEA 104-EC-17-A-07-S3-012 and Ministry of Science and Technology through grants MOST 104-2119-M-007-012, 103-2923-E-007-003-MY3, and 105-2119-M-007-012.
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