Phosphorescent white organic light-emitting diodes with stable white color depending on luminance
Graphical abstract
Introduction
Organic light-emitting diodes (OLEDs) with double-layer structures were first developed by Tang et al. [1], and OLED technologies have rapidly evolved because of their many advantages compared with other displays, such as fast response time, high contrast ratio, low thickness, and superior form factor [[2], [3], [4], [5], [6], [7]]. OLED display panels have been utilized in many premium mobile phones and TVs. In addition, transparent, flexible, and foldable OLED displays have been commercialized.
White OLEDs can be utilized as OLED display panels as well as solid-state lighting sources. The efficiency and lifetime of white OLEDs have dramatically improved. Huang et al. recently reported a white OLED with an efficiency of 150 lm/W for solid-state lighting sources [8]. White OLEDs with color filters have been used in commercialized OLED TVs [9]. There are many kinds of OLED structures that can realize white emission. For example, white OLEDs can be fabricated using a tandem structure consisting of two or three OLED units with charge generation layers [10]. This structure has high efficiency and operational stability, but requires a high driving voltage and complex device structure. White OLEDs can also be implemented by stacking emitting layers (EMLs) with different colors, which is called a multiple EML structure [11]. This structure has a low driving voltage and simple structure compared with the tandem structure. However, the color coordinate and correlated color temperature (CCT) of white emission can easily change depending on the driving voltage because of the shift in the recombination zone, energy transfer rate, and saturation of lightly doped lumophores. Changes in white color and CCT can cause color distortion in display panels and lighting sources. Therefore, a delicate structure design is required for stable white color emission with the multiple EML structure.
In this work, we fabricated white OLEDs with blue and red phosphorescent EMLs. The devices show very stable white colors, regardless of luminance. For example, the International Commission on Illumination (CIE) 1931 color coordinate difference of the device is (0.008, 0.007) when the luminance of the device is increased from approximately 265 cd/m2 to 9156 cd/m2. Furthermore, we used transient photoluminescence (PL) and electroluminescence (EL) techniques to investigate the emission mechanism in this device.
Section snippets
Material and methods
Two types of substrates, quartz glass and patterned indium tin oxide (ITO) glass, were used for measuring the PL and device fabrication, respectively. The substrates were cleaned using isopropyl alcohol, acetone, methanol, and deionized water sequentially for 20 min each in an ultrasonic bath. The cleaned substrates were dried in a vacuum oven at 120 °C for 1 h.
Fig. 1 shows the white OLED device structure, schematic energy level diagram, and chemical structures of the materials used. Three
Results and discussion
Fig. 3(a) shows the normalized EL spectra of the devices at the same current density. The EL spectra of devices A and B have nearly the same shape. They are typical two-wavelength white EL spectra, with separate peaks at 470 nm and 624 nm contributed by FIrpic and Ir(piq)2(acac), respectively. This result indicates that the main recombination of blue emission is the blue EML adjacent to the ETL. The red emission of device B at 624 nm is approximately 10.2% higher than that of device A. The
Conclusions
We fabricated simple and color-stable two-wavelength phosphorescent white OLEDs without interlayers in the EML. The CIE 1931 color coordinate difference of the device is (0.008, 0.007) when the luminance of the device is increased from approximately 265 cd/m2 to 9156 cd/m2. In other words, the color difference of the device is approximately 2.14 JNCD, which is regarded as visually indistinguishable in practice. In addition, we investigated the emission mechanism in this device using PL,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported herein.
Acknowledgements
This work was supported by Sookmyung Women's University Research Grants (1-2003-2001) and in part by an Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government (20ZB1200). I would like to thank Professor Changhee Lee for his guidance during this research.
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