Enhanced carrier balance by organic salt doping in single-layer polymer light-emitting devices

We have showed that the doping of an organic salt into a PVK-based polymer emissive layer could enhance the carrier balance greatly to result in higher luminance and luminous efficiency. It is found out that the salt-doped devices show the similar operating characteristics of frozen-junction light-emitting electrochemical cells (LECs). With the salt doping of 0.6 wt.% and an appropriate salt activation process, the fabricated PVK-based polymer light-emitting diodes (PLEDs) shows the luminous efficiency of 15 cd/A at the highest luminance of 55,000 cd/m² even without an electron-injecting LiF layer. Due to the enhanced carrier balance, the luminous efficiency is found to be maintained from the turn-on voltage to the voltage for the maximum luminance, which means a linear relationship between luminance and current density.     

Solution-processable tandem solid-state light-emitting electrochemical cells

Compared to organic light-emitting diodes (OLEDs), solid-state light-emitting electrochemical cells (LECs) exhibit simple single-layered structure and low operating voltages due to in situ electrochemical doped layers. However, device efficiencies of LECs are usually lower than those of sophisticatedly designed OLEDs. Furthermore, device efficiencies and lifetimes of LECs degrade significantly as brightness increases. In this work, we demonstrate tandem LECs to obtain nearly doubled light outputs (μW cm⁻²) in comparison with single-layered LECs under similar current densities. Since the output EL emission is modified by microcavity effect of the device structure, the EL spectra of tandem LECs exhibit EL emission peak at ca. 625 nm while the EL spectra of single-layered LECs center at ca. 660 nm. Better spectral overlap between the EL spectrum of tandem LECs and the luminosity function results in further enhanced candela values, rendering a tripled brightness (cd m⁻²). The device efficiencies can be optimized by adjusting the thickness of the connecting layer between the two emitting units of the tandem devices. The peak external quantum efficiency achieved in tandem LECs is up to 5.83%, which is higher than twice of that obtained in single-layered LECs due to improved carrier balance. When single-layered and tandem LECs are biased under higher voltages to reach similarly higher brightness, tandem LECs show higher device efficiencies and longer lifetimes simultaneously. These results indicate that device efficiencies and lifetimes of LECs can be improved by employing a tandem device structure.     

UV light-emitting electrochemical cells based on an ionic 2,2′-bifluorene derivative

UV light-emitting electrochemical cells (LECs) were, for the first time, achieved by the ionic 2,2′-bifluorene derivative, 1, which was synthesized through covalent tethering of methylimidazolium moieties as pendent groups. LEC devices incorporating ionic bifluorene 1 without (Device I) and with (Device III) the presence of poly(methyl methacrylate) (PMMA) exhibited UV EL emissions centered at 388 and 386 nm with maximum external quantum efficiencies and power efficiencies of 1.06% and 7.44 mW W⁻¹ for Device III and 0.15% and 1.06 mW W⁻¹ for Device I, respectively. Transmission electron microscopy (TEM) images showed that 1 tends to form nanospheres due to amphiphilic nature. The presence of PMMA unified the size of nanospheres which greatly reduced the void area in films, suppressing the current leakage and enhancing the device efficiency. Furthermore, thicker thickness of the emissive layer of LECs increases the distance between carrier recombination zone and electrodes to avoid exciton quenching. Thus, a sevenfold increase in device efficiency was obtained in thicker UV LECs containing PMMA (Device III) as compared to thinner UV LECs based on neat films of 1 (Device I). The EL emissions in the UV region are successfully achieved by LECs based on 1, which are so far the shortest emission wavelength achieved in LECs.     

Optimizing carrier balance of a red quantum-dot light-emitting electrochemical cell with a carrier injection layer of cationic Ir(III) complex

Solid-state light-emitting electrochemical cells (LECs) with promising features of solution processability, low-voltage operation and compatibility with inert cathode metals have shown great potential in display and lighting applications in recent years. Among the reported emissive materials for LECs, ionic transition metal complexes (iTMCs) have relatively higher electroluminescence (EL) efficiencies due to their phosphorescent property. However, the red iTMCs generally exhibit moderate color saturation and low emission efficiency, limiting their display applications. To improve color saturation and device efficiency of red LECs, efficient quantum dots (QDs) with narrow emission bandwidth are good alternative emissive materials. In this work, efficient and saturated red QD LECs employing iTMC carrier injection layers to provide in situ electrochemical doping are demonstrated. The thicknesses of iTMC and red-QD layers are systematically adjusted to achieve the best carrier balance. In the optimized device, the iTMC carrier injection layer facilitates hole injection into the red-QD layer while electrons are injected from the cathode into the red-QD layer directly since the electron injection barrier is low. The Commission Internationale de I'Eclairage (CIE) coordinates of the EL spectra approach the red standard point of National Television System Committee (NTSC). High external quantum efficiency and current efficiency reaching 9.7% and 16.1 cd A⁻¹, respectively. These results confirm superior carrier balance in such a simple iTMC/QD bilayer device structure. Furthermore, compared with iTMC LECs, less degree of device efficiency roll-off upon increasing device current is observed in QD LECs since a shorter excited-state lifetime of fluorescent QDs reduces the probability of collision exciton quenching. Saturated and efficient red EL with mitigated efficiency roll-off from red-QD LECs employing iTMC carrier injection layers confirms that they are good candidates of saturated light sources for displays.     

Improving color saturation of blue light-emitting electrochemical cells by plasmonic filters

In consideration of the advantages of light-emitting electrochemical cells (LECs), it is desired to develop the saturated blue LECs for LEC display, which is hindered by the features of broad emission spectrum and emission peak not short enough. In this study, we demonstrated a novel method to improve blue saturation of the sky-blue LECs by engineering its emission spectrum through the plasmonic filters. These plasmonic filters composed of randomly distributed silver nanoparticles (Ag-NPs) can absorb the green and red emission tail of the sky-blue LECs due to localized surface plasmon resonance (LSPR). The LSPR wavelengths of Ag-NPs are tuned by manipulating the effective refractive index of materials around Ag-NPs through the accurate control of the TiO₂ thickness using atomic layer deposition technique. By integrating with the plasmonic filters, the CIE1931 coordinate of the blue LECs can approach to (0.14, 0.22), which is comparable to or even better than the reported bluest values of blue LECs. Combination with the green and red LECs, the color gamut increases from 34% (without filters) to 54% of National Television System Committee (NTSC) color gamut, corresponding to 1.6 times enhancement. In addition, the blue LECs integrated with plasmonic filters still have better efficiency than those of the reported bluest LECs.     

Flexible light-emitting electrochemical cells on muscovite mica substrates

Novel silicon-gallium-doped zinc oxide (SGZO) coated muscovite mica substrates are proposed for flexible light-emitting electrochemical cells (LECs). Inorganic muscovite mica substrates show advantages of good thermal stability, mechanical durability, and resistance to oxygen and moisture, which are promising substrates for flexible light-emitting devices. The SGZO film exhibits high transmittance and low sheet resistance through annealing treatment. Furthermore, the sheet resistance of SGZO film on mica substrate only increases by 20% after 10⁴ bending cycles with a curvature radius of 10 mm. Blue, orange, red, and white LECs based on ion transition metal complexes are tested on the proposed substrates and the device performance of the LECs on mica/SGZO is comparable with that of the LECs on reference glass/SGZO. Less than 20% reduction in device efficiency of the mica/SGZO-based LECs are measured after 10⁴ bending cycles. These results confirm that the SGZO coated mica substrates are good potential candidates for applications in flexible light-emitting devices.     

Flexible light-emitting electrochemical cells with single-walled carbon nanotube anodes

In this work, we demonstrate flexible solution processed light emitting electrochemical cells (LECs) which use single-walled carbon nanotubes (SWCNTs) films as the substrate. The SWCNTs were synthesized by an integrated aerosol method and dry-transferred on the plastic substrates at room temperature. The addition of a screen printed poly (3,4-ethylene dioxythiophene) doped with poly (styrene sulfonate) (PEDOT:PSS) film onto the nanostructured electrode further homogenizes the surface and enlarges the work function, enhancing the hole injection into the active layer. By using an efficient phosphorescent ionic transition metal complex (iTMC) as the active material, efficacies up to 9 cd/A have been obtained. These values are among the highest reported so far for light-emitting diodes employing CNTs as transparent electrode.     

Dynamically tuning the correlated color temperature of white light-emitting electrochemical cells with electrochromic filters

Recently, white solid-state light-emitting electrochemical cells (LECs) have drawn great attention since they exhibit advantages such as low-voltage operation, compatibility with solution processes and employing inert cathode metals. Since different correlated color temperatures (CCTs) of background illumination are necessary for various lighting applications, a real-time tunable CCT of white LECs would be highly desired in modern smart lighting systems. In this work, a widely and dynamically tuning CCT (>10000 K) of white LEC is demonstrated by employing an electrochromic device (ECD) as a real-time controllable color filter. By increasing the applied bias on the ECD to attenuate more the red parts of white EL from the white LEC, the LEC-based white light source becomes more bluish and, in consequence, shows higher CCT. This proposed LEC-based white light source with the characteristics of wide CCT range and real-time tunability is suitable for most lighting applications and modern smart lighting systems.     

Flexible blue-green and white light-emitting electrochemical cells based on cationic iridium complex

In this paper, we demonstrated a simple solution fabrication route to realize flexible single layer light-emitting electrochemical cells (LEC) by employing cationic iridium complexes. The flexible LEC show the efficiency as high as 10.71 cd/A at 5 V and 9.8 cd/A at 7 V for blue-green and white electroluminescence, respectively. Bending test was also performed for the as-fabricated flexible LEC, and they exhibited an excellent light-emitting stability during the multiple mechanical bending at a 10 mm curvature radius.     

Improving device efficiencies of solid-state white light-emitting electrochemical cells by adjusting the emissive-layer thickness

Exciton quenching in the recombination zone close to electrochemically doped regions would be one of the bottlenecks for improving device efficiencies of solid-state white light-emitting electrochemical cells (LECs). To further enhance device efficiencies of white LECs for practical applications, we adjust the emissive-layer thickness to reduce exciton quenching. In white LECs with properly thickened emissive-layer thickness, the recombination zone can be situated near the center of the emissive layer, rendering mitigated exciton quenching and thus enhanced device efficiencies. High external quantum efficiencies and power efficiencies of optimized devices reach ca. 11% and 20 lm/W, respectively, which are among the highest reported for white LECs. These results confirm that tailoring the thickness of the emissive layer to avoid exciton quenching would be a feasible approach to improve device efficiencies of white LECs.     

Enhancing device efficiencies of solid-state near-infrared light-emitting electrochemical cells by employing a tandem device structure

Compared to near-infrared (NIR) organic light-emitting devices, solid-state NIR light-emitting electrochemical cells (LECs) could possess several superior advantages such as simple device structure, low operating voltages and balanced carrier injection. However, intrinsically lower luminescent efficiencies of NIR dyes and self-quenching of excitons in neat-film emissive layers limit device efficiencies of NIR LECs. In this work, we demonstrate a tandem device structure to enhance device efficiencies of phosphorescent sensitized fluorescent NIR LECs. The emissive layers, which are composed of a phosphorescent host and a fluorescent guest to harvest both singlet and triplet excitons of host, are connected vertically via a thin transporting layer, rendering multiplied light outputs. Output electroluminescence (EL) spectra of the tandem NIR LECs are shown to change as the thickness of emissive layer varies due to altered microcavity effect. By fitting the output EL spectra to the simulated model concerning microcavity effect, the stabilized recombination zones of the thicker tandem devices are estimated to be located away from the doped layers. Therefore, exciton quenching near doped layers mitigates and longer device lifetimes can be achieved in the thicker tandem devices. The peak external quantum efficiencies obtained in these tandem NIR LECs were up to 2.75%, which is over tripled enhancement as compare to previously reported NIR LECs based on the same NIR dye. These efficiencies are among the highest reported for NIR LECs and confirm that phosphorescent sensitized fluoresce combined with a tandem device structure would be useful for realizing highly efficient NIR LECs.     

Accelerated lifetime test based on general electrical principles for light-emitting electrochemical cells

We report the use of a straightforward alternative accelerated lifetime test (ALT) method, derived from general electrical principles, for estimating how encapsulated light-emitting electrochemical cells (LECs) operate in ambient conditions. The LECs that we tested were made with poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), poly(ethylene oxide) (PEO), trifluoromethanesulfonate (KCF₃SO₃) and PEDOT:PSS. These LECs were fabricated by using a common method that is described in published investigations of operational lifetime. The method we developed used only a single data point originating from a high level of current density, and could predict the operational lifetimes of the encapsulated LECs to within a margin of error of less than 4% in this system.     

Enhancing extracted electroluminescence from light-emitting electrochemical cells by employing high-refractive-index substrates

Solid-state light-emitting electrochemical cells (LECs) show several advantages over conventional organic light-emitting devices (OLEDs) such as simple device structure compatible with solution processes, low operation voltage and capability of utilizing inert cathode metals. However, device performance of LECs must be improved, e.g. enhancing light extraction, to meet the requirements for practical applications. Among the optical modes trapped in LECs, light trapped in substrate mode is easier to be extracted, e.g., by simply roughing the output surface. Therefore, increasing the percentage of substrate mode is beneficial in improving light extraction. In this work, the contributions of optical modes in LECs employing substrates with various refractive indices are analyzed. Higher-refractive-index substrates are shown to trap more light in the substrates. Smaller refractive index difference between higher-refractive-index substrate and indium tin oxide (ITO) layer also increases the cutoff spectral range of light waveguided in ITO layer. Furthermore, light intensity in surface plasmon mode significantly reduces as the refractive index of the substrate increases. Reducing the percentage of surface plasmon mode facilitates light extraction since it requires more complicated methods for outcoupling light in this mode. With commercially available unpolished sapphire substrates, light output of LECs is enhanced by 56%. When a scattering layer was inserted between ITO and sapphire substrate, more light in substrate mode can be extracted and 71% enhancement in light output is realized. High external quantum efficiency up to 5.5% is consequently obtained in LECs based on a ruthenium complex. Such device efficiency is among the highest reported values for red-emitting LECs and thus confirms that utilizing higher-refractive-index substrates would offer a simple and feasible approach to improve light output of LECs. In comparison to OLEDs, increased EL trapped in substrates of LECs mainly comes from surface plasmon mode rather than waveguide mode.     

Efficient and color-stable solid-state white light-emitting electrochemical cells employing red color conversion layers

We report efficient and color-stable white light-emitting electrochemical cells (LECs) by combining single-layered blue-emitting LECs with red-emitting color conversion layers (CCLs) on the inverse side of the glass substrate. By judicious choosing of the red-emitting dye doped in CCLs, good spectral overlap between the absorption spectrum of the red-emitting dye and the emission spectrum of the blue-emitting emissive material results in efficient energy transfer and thus sufficient down-converted red emission at low doping concentrations of the red-emitting dye in the CCLs. Low doping concentration is beneficial in reducing self-quenching of the red-emitting dye, rendering efficient red emission. Electroluminescent (EL) measurements show that the peak external quantum efficiency and the peak power efficiency of the white LECs employing red CCLs reach 5.93% and 15.34 lm W⁻¹, respectively, which are among the highest reported for white LECs. Furthermore, these devices exhibit bias-insensitive white EL spectra, which are required for practical applications, due to nondoped emissive layers. These results reveal that single-layered blue-emitting LECs combined with red-emitting CCLs are one of the potential candidates for efficient and color-stable white light-emitting devices.     

Synthesis and photophysical characterization of an ionic fluorene derivative for blue light-emitting electrochemical cells

A highly fluorescent an ionic fluorene derivative 1 was synthesized and its photophysical, electrochemical and electroluminescence characteristics were investigated. Deep blue emissions were observed for compound 1 in solid as well as in dilute solutions. The synthesized compound shows high fluorescence quantum yield around 77% indicates that compound 1 can perform its role as efficient ionic emitter in LEC devices. Light-emitting electrochemical cell (LEC) devices were fabricated incorporating compound 1 without (device I) and with (device II) ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM·PF₆). Devices I and II exhibited blue electroluminescence maximum centered at 455 and 454 nm with CIE coordinates of (0.15, 0.21) and (0.16, 0.22), respectively. Maximum luminance and current efficiency of 1105 cd m⁻² and 0.14 cd A⁻¹ respectively, has achieved for device I while that of device II resulted in 1247 cd m⁻² and 0.14 cd A⁻¹ respectively.     

The role of the polymer solid electrolyte molecular weight in light-emitting electrochemical cells

Different molecular weights (M_w) of poly (methyl methacrylate) (PMMA) were used as the base for the polymer solid electrolyte (PSE) in light-emitting electrochemical cells (LECs). The rheological properties of the LECs formulations are influenced by the M_w of PMMA. The M_w of PMMA also influences the PSE ionic conductivity and therefore affects the threshold voltage of the devices. Furthermore, partial segregation of the two polymers is observed, which correlates directly to the PMMA M_w. The device with the best performance was prepared with a PMMA M_w of 350,000 and exhibited an effective maximum luminance ～3000 cd m^?2.     

Extracting evolution of recombination zone position in sandwiched solid-state light-emitting electrochemical cells by employing microcavity effect

Techniques of probing for time-dependent evolution of recombination zone position in sandwiched light-emitting electrochemical cells (LECs) would be highly desired since they can provide direct experimental evidence to confirm altered carrier balance when device parameters are adjusted. However, direct imaging of recombination zones in thin emissive layers of sandwiched LECs could not be obtained easily. In this work, we propose an alternative way to extract evolution of recombination zone position in sandwiched LECs by utilizing microcavity effect. Recombination zone positions can be estimated by fitting the measured electroluminescence spectra to simulated output spectra based on microcavity effect and properly adjusted emissive zone positions. With this tool, effects of modified carrier transport and carrier injection on performance of LECs are studied and significantly altered carrier balance can be measured, revealing that microcavity effect is useful in tracing evolution of recombination zone position in sandwiched LECs.     

Stable blue-green light-emitting electrochemical cells based on a cationic iridium complex with phenylpyrazole as the cyclometalated ligands

A novel blue-green light-emitting cationic iridium complex [Ir(ppz)₂pzpy]PF₆ (complex 1), where ppz is phenylpyrazole and pzpy is 2-(1-phenyl-1H-pyrazol-3-yl)pyridine, is developed. Its crystal, photophysical and electrochemical properties are characterized through experiments and quantum chemical calculations. The novel complex exhibits enlarged energy gap and blue-shifted emission spectrum at 77 K compared to a reference complex, [Ir(ppy)₂pzpy]PF₆ (complex 2), with phenylpyridine (ppy) as the cyclometalated ligands. Moreover, light-emitting electrochemical cells (LECs) fabricated with complex 1 show both improved color purity with CIE (Commission Internationale de L’Eclairage) coordinates of (0.25, 0.39) and elongated lifespan (t_1/2 = 218 min, E_tot = 0.55 mJ) compared to LECs based on complex 2.     

Heteroleptic iridium (III) complexes with three different ligands: Unusual triplet emitters for light-emitting electrochemical cells

Two cationic iridium (III) complexes [Ir(dfppy)(tpy)(bpy)](PF₆) and [Ir(dfppy)(tpy)(phen)](PF₆) bearing three different ligands were tested as triplet emitters for Light-Emitting Electrochemical Cells (LECs). These two phosphorescent materials only constitute the third and fourth examples of triple heteroleptic cationic iridium complexes to be tested in electroluminescent devices. LECs fabricated with this almost unknown class of iridium complex furnished green-emitting devices. Parallel to investigations devoted to electroluminescent properties, photophysical and electrochemical properties of the two new complexes were examined. Density functional theory calculations were also performed to provide insight into the electronic structure of the two emitters.     

Synthesis of red-emitting cationic Ir (III) complex and its application in white light-emitting electrochemical cells

Construction of intramolecular π-π packing between cyclometalated and ancillary ligand have been proved to be a feasible way to design cationic Ir(III) phosphors for stable light-emitting electrochemical cells (LECs). Employing blue-green (C1) and yellow-emitting (C2) Ir(III) complexes as emitters, in which such π-π interactions between ligands occur, the stable and efficient LECs with peak current efficiency of 21.9 cd/A and 20.3 cd/A, respectively, were realized. To achieve the white electroluminescence, herein, a new red-emitting cationic Ir (III) complex (C3) using 2-(5-phenyl-2-phenyl-2H-1,2,4-triazol-3-yl)pyridine as ancillary ligand was designed and synthesized. A warm white light emitting, by doping C3 into the blue-green emitting C1, exhibits the current efficiency of 10.1 cd/A. In addition, the obtained LECs are more stable than those of previous reports due to presence of intrinsic super-cage structures in the systems. The results indicate that the efficient white LECs hold promising applications in the practical solid state lighting.