Bottom-emitting Devices Research Articles

Blue light-emitting diodes based on colloidal quantum dots with reduced surface-bulk coupling

To industrialize printed full-color displays based on quantum-dot light-emitting diodes, one must explore the degradation mechanism and improve the operational stability of blue electroluminescence. Here, we report that although state-of-the-art blue quantum dots, with monotonically-graded core/shell/shell structures, feature near-unity photoluminescence quantum efficiency and efficient charge injection, the significant surface-bulk coupling at the quantum-dot level, revealed by the abnormal dipolar excited state, magnifies the impact of surface localized charges and limits operational lifetimes. Inspired by this, we propose blue quantum dots with a large core and an intermediate shell featuring nonmonotonically-graded energy levels. This strategy significantly reduces surface-bulk coupling and tunes emission wavelength without compromising charge injection. Using these quantum dots, we fabricate bottom-emitting devices with emission colors varying from near-Rec.2020-standard blue to sky blue. At an initial luminance of 1000 cd m−2, these devices exhibit T95 operational lifetimes ranging from 75 to 227 h, significantly surpassing the existing records.

Influence of Light-Matter Interaction on Efficiency of Quantum-Dot Light-Emitting Diodes.

Light-matter interaction can affect the radiative decay rate of excitons and thus the emission quantum yield (QY) of quantum dots (QDs). In this work, light-matter interaction and outcoupling efficiency of QD light-emitting diodes with different structures are investigated experimentally and theoretically. We show that the external quantum efficiency (EQE) of top-emitting devices is higher than that of the bottom-emitting devices, which is mainly due to the stronger light-matter interaction and higher light outcoupling efficiency of the top-emitting structures. In addition, we show that the QY enhancement induced by light-matter interaction is more significant for low-QY QDs. Our results suggest that top-emitting structures are more powerful for improving the EQE of devices built with low-QY QDs.

Ultrapure Blue Phosphorescent Organic Light-Emitting Diodes Employing a Twisted Pt(II) Complex.

Described herein is a stable complex, Pt(mpzpyOczpy-mesi), embodying efficient, narrow blue emission. The highly twisted structure of the complex improves the stability and efficiency of photo- and electroluminescence by reducing the intermolecular interactions. The complex in solution shows high photoluminescence efficiency (>95%) and radiative decay rate (Kr = 2.9 × 105 s-1) with a narrow emission spectrum. The bottom-emitting phosphorescent device, BE1, exhibits durable deep blue emission with CIE coordinates of (0.145, 0.166) and 5.2 h of LT50 at an initial luminance of 685 cd/m2. Top-emitting devices, TE1 and TE2, achieve ultrapure blue color with CIEx,y values of (0.141, 0.068) and (0.140, 0.071), respectively. TE4 shows high brightness of 3405 cd m-2 at 50 mA m-2, EQE of 10.2% at 1000 cd/m2, and almost negligible color deviation around (0.135, 0.096) at viewing angles of 0°-60°.

Impact of pixel surface topography onto thin-film encapsulated top-emitting organic light-emitting diodes performances

Two different designs of top-emitting green OLEDs (Organic Light-Emitting Diodes) have been studied. The first one presents a planar OLED architecture. The second one presents an OLED having a topographic surface, so as to simulate a pixel partitioning of a display using an electrically insulating, 200 nm-thick, resist. It has been observed that the topography has a large impact on OLED performances. Studying devices using an ALD (Atomic Layer Deposition)-deposited Al2O3 barrier film shows that topographic OLEDs have a lower stability under storage in 65 °C/85% RH conditions compared to planar ones, with a difference in ageing models between the two devices. As the ALD deposition technology has a high conformity, which implies that ALD-deposited Al2O3 barrier films should be as good on topographic devices as on planar ones, we inferred that the topographic OLED Achilles’ heel lies rather in the OLED structure rather than in the Al2O3 encapsulation itself. Thus, topographic and planar unencapsulated OLEDs (without Al2O3) were studied: interestingly, it has been observed that planar OLEDs can live several weeks, while topographic OLEDs show a very short shelf lifetime (in laboratory atmosphere, at 21 °C/50% RH), of only a couple of hours. It will be shown that the topographic OLED surface tends to reduce the thickness of the PVD (Physical vapour Deposition)-deposited layers in the OLED, as this is expected for a non-conformal deposition PVD technique, on tapered angle regions of the resist (pixel edges). While this thickness variation would not be critical for thick electrodes, as for instance for bottom-emitting devices made on glass substrates, this thickness reduction turns out to be a critical point for the ultrathin, 15 nm, silver cathode, used as semi-transparent electrode in this top-emitting architecture and will therefore be discussed in the framework of using OLED top-emitting architectures in (micro)display technology.

Photon management in solution-processed organic light-emitting diodes: a review of light outcoupling micro- and nanostructures

To allow a greater acceptance in the display and lighting markets, organic light-emitting diode (OLED) technology is currently the subject of intensive research efforts aimed at manufacturing cost-effective devices with higher efficiencies. In this regard, strategies matured in the field of photonics and nanophotonics can be applied for photon management purposes to improve the outcoupling of the generated light and to control the emission pattern. In this review, we report on the recent experimental and numerical advances to pursue those goals by highlighting the example of bottom-emitting devices. The cases of periodical micro- and nanostructures, as well as of stochastic ensembles that can be easily implemented using printing techniques, are covered herein. It is shown that beyond the sole optical properties, such additional elements can simultaneously improve the electrical characteristics of solution-processed OLEDs, and thus enable an optimization of the devices at different levels.

ITO-free large-area top-emission organic light-emitting diode by blade coating

Large-area top-emitting organic light-emitting diodes (TEOLEDs) with multi-layer structure are successfully demonstrated using the solution-processable blade coating on ITO-free substrate. The semitransparent cathode of TEOLED is composed of lithium fluoride (LiF), aluminum (Al) and silver (Ag). The composition of 3nm Al and 10nm Ag has a transmittance of 56% and a sheet resistance of 11Ω/□. It is applied to the green phosphorescence device with an emissive area of 2cm by 2.5cm. The maximum current efficiency is 25.2cd/A with high light-emission uniformity within 10% variation. The large-area TEOLEDs show comparable current efficiency as the small-area devices with an emissive area of 2mm by 2mm (having the same device structure) and better efficiency than traditional large-area bottom-emitting devices. Cesium fluoride (CsF) and n-doped electron transport layer are applied to improve electron injection. At 6V, the luminance is raised from 141cd/m2 to 502cd/m2 and 304cd/m2, respectively. In n-doped device, a simple Al/Ag cathode is used without LiF.

Influences of wide-angle and multi-beam interference on the chromaticity and efficiency of top-emitting white organic light-emitting diodes

Wide-angle interference (WI) and multi-beam interference (MI) in microcavity are analyzed separately to improve chromaticity and efficiency of the top-emitting white organic light-emitting diodes (TWOLEDs). A classic electromagnetic theory is used to calculate the resonance intensities of WI and MI in top-emitting organic light-emitting diodes (TOLEDs) with influence factors (e.g., electrodes and exciton locations) being considered. The role of WI on the performances of TOLEDs is revealed through using δ-doping technology and comparing blue and red EML positions in top-emitting and bottom-emitting devices. The blue light intensity significantly increases and the chromaticity of TWOLEDs is further improved with the use of enhanced WI (the blue emitting layer moving towards the reflective electrode) in the case of a weak MI. In addition, the effect of the thicknesses of light output layer and carrier transport layers on WI and MI are also investigated. Apart from the microcavity effect, other factors, e.g., carrier balance and carrier recombination regions are considered to obtain TWOLEDs with high efficiency and improved chromaticity near white light equal-energy point.

Application of three-coordinate copper(I) complexes with halide ligands in organic light-emitting diodes that exhibit delayed fluorescence.

A series of three-coordinate copper(I) complexes (L(Me))CuX [X = Cl (1), Br (2), I (3)], (L(Et))CuBr (4), and (L(iPr))CuBr (5) [L(Me) = 1,2-bis[bis(2-methylphenyl)phosphino]benzene, L(Et) = 1,2-bis[bis(2-ethylphenyl)phosphino]benzene, and L(iPr) = 1,2-bis[bis(2-isopropylphenyl)phosphino]benzene] exhibit efficient blue-green emission in the solid state at ambient temperature with peak wavelengths between 473 and 517 nm. The emission quantum yields were 0.38-0.95. The emission lifetimes were measured in the temperature range of 77-295 K using a nanosecond laser technique. The temperature dependence of the emission lifetimes was explained using a model with two excited states: a singlet and a triplet state. The small energy gaps (<830 cm(-1)) between the two states suggest that efficient emission from 1-5 was thermally activated delayed fluorescence (TADF). Alkyl substituents at ortho positions of peripheral phenyl groups were found to have little effect on the electronic excited states. Because the origin of the emission of complexes 2, 4, and 5 was thought to be a (σ + Br)→π* transition, photoluminescence characteristics of these complexes were dominated by the diphosphine ligands. Complexes 2, 4, and 5 had similar emission properties. Complexes 1-5 had efficient green TADF in amorphous films at 293 K with maximum emission wavelengths of 508-520 nm and quantum yields of 0.61-0.71. Organic light-emitting devices that contained complexes 1-5 and exhibited TADF exhibit bright green luminescence with current efficiencies of 55.6-69.4 cd A(-1) and maximum external quantum efficiencies of 18.6-22.5%.

Highly efficient green organic light-emitting diodes containing luminescent tetrahedral copper(i) complexes

A series of highly emissive sublimable copper(I) complexes with tetrahedral geometries, Cu(dppb)(pz2Bph2) 1, Cu(dppb-F)(pz2Bph2) 2, and Cu(dppb-CF3)(pz2Bph2) 3 [dppb = 1,2-bis(diphenylphosphino)benzene, dppb-F = 1,2-bis[bis(3,5-difluorophenyl)phosphino]benzene, and dppb-CF3 = 1,2-bis[bis[3,5-bis(trifluoromethyl)phenyl]phosphino]benzene, pz2Bph2− = diphenyl-bis(pyrazol-1-yl)borate], were synthesized and investigated as luminescent guest molecules in prototype organic light-emitting diodes. Thermogravimetric analysis of 1–3 under vacuum revealed that introduction of F or CF3 substituents to the meta positions of the four peripheral phenyl groups in the dppb skeleton increased the ability of the copper(I) complexes to be sublimed. 1–3 exhibited strong green emission in amorphous films at 293 K with maximum emission wavelengths of 523–544 nm, quantum yields of 0.50–0.68, and decay times of 3.6–8.2 μs. Molecular orbital calculations indicated that the origin of green phosphorescence from 1–3 was a mixture of σ → π* and π → π* transitions. Conventional bottom-emitting devices with three-layer structures containing 1–3 produced bright green luminescence with maximum external quantum efficiencies of 11.9, 16.0, and 17.7% for 1, 2 and 3, respectively.

Transparent oxide/metal/oxide trilayer electrode for use in top-emitting organic light-emitting diodes

The most commonly used transparent electrode, indium-tin oxide (ITO), is costly and requires methods of deposition that are highly destructive to organic materials when it is deposited on top of the organic layers in top-emitting organic light-emitting devices (OLEDs). Here we have employed a trilayer electrode structure consisting of a thin layer of metal sandwiched between two MoO3 layers, which can be deposited through vacuum thermal evaporation without much damage to the organic active layers. Such MoO3/Au/MoO3 trilayer electrodes have a maximum transmittance of nearly 90% at 600 nm and a sheet resistance of <10 ohms per square (Ω/sq) with a 10-nm thick Au intermediate layer. Using these trilayers as the top transparent anode, we have fabricated top-emitting OLEDs based on either a fluorescent or phosphorescent emitter, and observed nearly identical emission spectra and similar external quantum efficiencies as compared to the more conventional bottom-emitting OLEDs based on the commercial ITO anode. The power efficiency of the top-emitting devices is 20% to 30% lower than the bottom-emitting devices due to the somewhat inferior charge injection in the top-emitting devices. The performance and emission characteristics of these devices indicate that this trilayer structure is a promising candidate as a transparent anode in top-emitting OLEDs.

Study of the interference effects in an optical cavity for organic light-emitting diode applications

The interference effects generated in a bottom-emitting electroluminescent device fabricated on a polymer underlayer introduced with the aim of improving the anode roughness have been studied. The analysis of the interference fringes at different detection angles and the spatial coherence demonstrates that this phenomenon is due to multiple internal reflections that propagate in the polymer layer. This effect can be eliminated by modifying the polymer thickness and the incidence angle of the electromagnetic radiation at the anode-polymer interface. Inkjet etching technology is adopted for microcavities-shaped polymer structuring to destroy the resonator effect of the optical cavity.

Transparent organic light-emitting diodes based on Cs2CO3:Ag/Ag composite cathode

The transparent organic light-emitting diodes (TOLEDs) based on Cs2CO3:Ag/Ag composite cathode are reported in this paper. They show higher efficiency and better stability than the control devices with the Cs2CO3/Ag cathode. The devices are highly transparent, and show total efficiencies from both sides of the devices comparable with conventional bottom-emitting devices. The electroluminescence (EL) spectrum from the cathode side of the device is closer to the photoluminescence (PL) spectrum of the light-emitting material than that from the anode side. The effect of the cathode Ag thickness on the device performance has been investigated. It is observed that a critical Ag thickness of 15 nm is required to form a good Ag film with high optical transparency and electrical conductivity.

Performance Comparison of Top- and Bottom-emitting Long Wave Infrared Light Emitting Diode Devices

Electroluminescence in the range of 5-10 μm with peak emission at 8 μm from an Sb-based type II interband quantum cascade structure is reported. We used both top- and flip-chip-mount bottom-emitting long wave infrared (LWIR) light emitting diode (LED) devices. An increase in light emission was observed by thinning the GaSb substrate material for both top- and bottom-emitting devices. For a similar GaSb substrate thickness, a flip-chip-mount bottom-emitting LED device had higher light emission intensity than a top-emitting device. Higher emission intensity for the bottom-emitting device was attributed to better cooling and to reflection of light from the anode metal surface in the LED mesa area.

High-Efficiency and High-Operational-Stability Top-Emitting Organic Light-Emitting Diodes Using Antioxidant Buffer Layer

We have succeeded in developing top-emitting organic light-emitting diodes (OLEDs) with high efficiency and high operational stability using an antioxidant buffer layer. V2O5 was selected as the antioxidant buffer layer to suppress the degradation of the organic materials of OLEDs caused by active oxygen during buffer layer deposition and the damage caused by the bombardment of high-energy particles during transparent electrode sputtering. The top-emitting device with a V2O5 buffer layer had the same current density–voltage (J–V) characteristics as the bottom-emitting device with the same material system. The quantum efficiency of the top-emitting device was about 5%, the same as that of the bottom-emitting device. The time required for luminance to drop to 90% of the initial value of the top-emitting device was over 700 h, which is longer than that of the bottom-emitting device (300 h). From X-ray photoelectron spectroscopy (XPS) analysis of the interface between organic materials and V2O5, it was determined that there was no degradation of organic materials under V2O5.

Realization of high efficiency microcavity top-emitting organic light-emitting diodes with highly saturated colors and negligible angular dependence

An alternative way to optimize the emission characteristics of a microcavity top-emitting organic light-emitting diode (TOLED) based on a simple device structure is demonstrated via combining a comprehensive theoretical analysis in the microcavity effects with the experimental modification in the carrier injection of both electrodes. It can be seen that the resulting TOLED exhibits much higher efficiencies and a more saturated color than those of the corresponding conventional bottom-emitting device, as well as hardly detectable color shift with viewing angles. Such a strategy may be more feasible in practical application for active-matrix organic light-emitting diode displays.

Microcavity top-emitting organic light-emitting devices integrated with diffusers for simultaneous enhancement of efficiencies and viewing characteristics

We show that integrating diffuser films with microcavity top-emitting organic light-emitting devices (OLEDs) provide a convenient approach for simultaneously achieving large quantum- efficiency enhancement (1.38 times compared to top-emitting devices without diffusers and 2.1 times compared to conventional bottom-emitting devices) and improving viewing characteristics (more saturated/stable colors over angles, emission patterns more Lambertian, pixel sharpness remained) of top-emitting microcavity OLEDs for display applications. Most importantly, the fabrication of the diffuser films is simple and effective. These features may make it attractive for use in enhancing OLED performances in various applications.

Effective hole transport layer structure for top-emitting organic light emitting devices based on laser transfer patterning

The design and preparation of top-emitting organic light emitting devices with thick hole transport layers that effectively smooth the substrate were performed. Bilayer transfer with laser thermal patterning method was used to simplify the fabrication of hole transport layers with various thicknesses, with the aim of optimizing the cavity effect in the top-emitting devices. By carrying out optical simulations and experiments, the optimal thicknesses of the hole transport layers for our structure of blue, green and red devices were found to be 140 nm, 160 nm and 230 nm, respectively. We have briefly illustrated the underlying device physics of top-emission in terms of the resonance and microcavity effect. The resulting top-emitting devices with thick hole transport layers exhibit better colour purity than bottom-emitting devices and satisfactory luminous efficiency. The relatively high operating voltages can be further improved by the use of transport materials with enhanced charge mobility.

Chemical structure of the bilayer Ag∕Li2O cathode interface in organic light emitting diodes

The chemical structure of the interface between Ag with Li2O and tri(8-hydroxyquinoline) aluminum (Alq) was investigated by using in situ characterization of x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Li2O on Ag had lower barrier height than LiF on Ag. XPS and UPS results show the interaction between Li2O and Alq leads to gap state formation in highest occupied molecular orbital of Alq. Efficient bottom-emitting devices employing such a cathode scheme have been demonstrated.

High-Performance Top-Emitting White Organic Light-Emitting Devices

High-performance top-emitting white organic light-emitting device (WOLED), using Al as the reflective bottom anode and LiF/Ca/Au as the semitransparent top cathode, has been fabricated. The WOLED consisted of three emissive layers with red, green, blue phosphorescent emitters, respectively. Its microcavity structure was optimized by varying the thicknesses of the organic functional layers and the dielectric capping layer of the semitransparent top cathode using an optical model. The WOLED showed a maximum external quantum efficiency of 9.1% and a maximum power efficiency of 12.8 lm/W. As well, broadband emission spectra with very weak angle dependence were achieved. The emission in normal direction has a Commission Internationale de L'Eclairage coordinates of (x,y)=(0.36,0.43). The characteristics of this top-emitting WOLED are competitive with those of bottom-emitting devices.

Highly efficient top-emitting organic light-emitting diodes with self-assembed monolayer-modified Ag as anodes

A series of p-substituted benzylmercaptans was used to form self-assembled monolayers (SAMs) on silver surface to serve as the anode in the fabrication of top-emitting organic light-emitting diodes. SAMs with electron-withdrawing substituents such as cyano and trifluoromethyl groups led to enhanced hole injection and electroluminescent performance, whereas the opposite is true for electron-donating groups. Direct quantitative correlation between the molecular dipole moment/work function or work function/hole injection does not exist. A high current efficiency of ∼12cd∕A was obtained, which is two and half times that of the conventional bottom-emitting device of the same layer structure.