Light-emitting electrochemical cells (LECs) and organic light emitting diodes (OLEDs) are frequently used in screens and displays. They are bright, thin, and efficient, and so are especially popular for mobile devices. LECs are solid-state devices that generate light from an electric current, a phenomenon called electroluminescence (EL), and are usually composed of a material containing mobile ions sandwiched between two metal electrodes. In general, LECs have several advantages over OLEDs, such as a simple single-layer configuration, solution-processing, and low operation voltages with air-stable electrodes.1 However, saturated deep-blue emission, which is essential for full-color displays, cannot be easily obtained from commonly used LEC materials, which include cationic transition metal complexes (CTMCs) and conducting polymers.
Iridium-based CMTCs can cover a large color range to achieve full-color displays and white light emissions, but to date, the development of efficient saturated blue-emitting ionic iridium complexes has lagged behind those of other colors. Previous complexes with large optical band gaps have mainly exhibited emissions in the bluish-green region. The difficulty in color-tuning toward the deep-blue region through molecular design of iridium-based CTMCs is largely due to intrinsically narrower energy gaps in such cationic complexes, relative to neutral complexes. LECs based on polymers such as polyfluorene (PF) suffer from significant green emission due to interchain aggregation, which deteriorates blue emission in these devices.2 To avoid the intrinsic tendency of aggregation that is widely observed for PF derivatives, we selected members of the terfluorene family—low-molecular-weight analogues of PFs—to create saturated blue-emitting LECs. We have used an ionic terfluorene (referred to as Compound 1: see Figure 1 for the structure) to achieve saturated deep-blue EL from LEC devices.3
Figure 1. Molecular structure of Compound 1, an ionic terfluorene derivative.
Figure 2. Electroluminescence (EL) spectra of Device I under 3.8V and Device II under 3.4V. Photoluminescence (PL) spectra of the emissive layers are presented for comparison. Inset: standard color-space coordinates (CIE 1931, NTSC) of the EL and PL spectra. BMIM+PF6−: 1-Butyl-3-methylimidazolium hexafluorophosphate.
Figure 3. Brightness (solid symbols) and current density (open symbols) plotted with respect to time under a constant bias voltage of (a) 3.4–4.2V for Device I and (b) 3.2–3.6V for Device II.
Figure 4. EQE (solid symbols) and power efficiency (open symbols) plotted with respect to time under a constant bias voltage of (a) 3.4–4.2V for Device I and (b) 3.2–3.6V for Device II.
Figure 2 compares the EL spectra of two LEC devices with the PL spectra of their emissive layers. In Device I, the emissive layer is a spun film of the ionic terfluorene derrivative. In Device II we added 10wt.% 1-Butyl-3-methylimidazolium hexafluorophosphate, BMIM·PF6, to the film to enhance ionic conductivity and accelerate device response. The similarity of the EL spectra of both devices indicates that the addition of BMIM·PF6 did not alter the EL of Compound 1. The LEC devices based on Compound 1 exhibited saturated deep-blue EL emissions. The International Commission on Illumination (CIE) standard color-space coordinates for Devices I and Device II were (0.151, 0.122) and (0.159, 0.115), respectively. The inset to Figure 2 reveals that the CIE coordinates of the EL spectra for both LEC devices approached the blue apex of the National Television System Committee (NTSC) color gamut—indeed, they are the bluest EL emissions ever reported for blue LECs. Thus, the ionic terfluorene derivative Compound 1 is a promising candidate for use as a deep-blue emitting material for LECs.
Figure 3 presents the time-dependent brightness and current densities of Devices I and II when operated under various bias voltages. Under the same bias voltage, Device II required a significantly shorter time for its brightness to reach the maximum value compared with Device I (e.g., 30 and 161min, respectively, at 3.4V). This result indicates that the additional mobile ions provided by the electrolyte BMIM·PF6 increased the rate of ion accumulation near the electrodes, which leads to accelerated formation of doped regions. Corresponding time-dependent external quantum efficiencies (EQEs) and power efficiencies of the same device are shown in Figure 4. (The EQE is the number of photons generated by an injected electron.) Immediately after a forward bias was applied, the EQE was rather low because of unbalanced carrier injection. During the formation of the doped regions near the electrodes, the balance of the carrier injection improved and, accordingly, the EQE of the device increased rapidly. The peak EQE and peak power efficiency for Device I under 3.4V were 1.04% and 0.63lm/W, respectively. For Device II under 3.2V, they were 1.14% and 1.24lm/W, respectively.
In summary, LECs offer many advantages over OLEDs, which are increasingly used for digital screens in devices such as TVs and mobile phones. Until now, researchers have struggled to create an LEC that could produce the saturated deep blue light necessary for full color screens. We successfully demonstrated the bluest EL emissions ever obtained, using an LEC emission layer based on an ionic small-molecule terfluorene derivative. We also confirmed that this compound is a promising candidate for use in saturated deep-blue solid-state LECs. Further studies to improve response time, stability, and efficiency of the LEC device will be needed to achieve the properties required for market-ready applications.
The authors gratefully acknowledge the financial support from National Science Council of Taiwan.
Chih-Teng Liao, Hai-Ching Su
National Chiao Tung University
Hsiao-Fan Chen, Ken-Tsung Wong
National Taiwan University
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