Visible Electroluminescence Research Articles

Enhanced electroluminescence from silicon-based light-emitting devices with Mg0.4Zn0.6O/erbium-doped ZnO heterostructures by using ITO/MoO3 combined anode

We have realized the erbium (Er)-related visible and near-infrared (NIR) electroluminescence (EL) from the Mg0.4Zn0.6O/ZnO:Er/n+-Si heterostructured light-emitting device (LED) using the semi-transparent Au film as the anode, where the impact-excitation of Er3+ ions is enabled by hot holes that are generated in the Mg0.4Zn0.6O acceleration layer. However, it remains a challenge to simultaneously achieve more efficient injection of holes into Mg0.4Zn0.6O layer and stronger light emanation for the Mg0.4Zn0.6O/ZnO:Er/n+-Si heterostructured LED. Addressing this issue, we report on the substitution of an ITO/MoO3 combined anode for the Au anode in the aforementioned LED to enhance the EL. Through the optimization of MoO3 film thickness, the substitution of ITO/MoO3 combined anode for Au anode leads to the enhanced EL from the Mg0.4Zn0.6O/ZnO:Er/n+-Si heterostructured LED with a factor of more than 5 in the visible region and with a factor of more than 13 in the NIR region. The higher transmittances and larger refraction indices in both visible and NIR regions and the better hole-injection capability of the ITO/MoO3 combined anode, with respect to those of Au anode, are responsible for the significantly enhanced EL as mentioned above. This work sheds light on the application of ITO/MoO3 combined anode into all-inorganic LEDs.

Ultraviolet electroluminescence with electrically tunable colour from epitaxial n-(0001)ZnO/p-(0001)GaN light-emitting diodes

Pulsed laser deposition technique is used to grow unintentionally n-type (0001)ZnO layers with high crystalline and morphological qualities on p-type (0001)GaN/sapphire templates. Electroluminescent devices are fabricated on these p–n heterojunctions. Oxygen pressure during growth has been found to influence strongly the crystalline and defect properties of the grown layers, which affect not only the current–voltage characteristics but also the emission properties of the electroluminescent devices. It has been observed that the electroluminescence (EL) spectra have both defects related visible and band-edge related ultraviolet (UV) transition features stemming from both GaN and ZnO sides. The study reveals that UV to visible EL intensity ratio maximizes at an optimum oxygen pressure. The relative contribution of EL originating from ZnO and GaN sides can be tuned by the applied bias, as the bias can control the depletion width and hence the carrier interdiffusion across the junction. This finding thus offers a scope to electrically tune the colour of the emitted light in these devices.

Interfacing Lanthanide Metal-Organic Frameworks with ZnO Nanowires for Alternating Current Electroluminescence.

Alternating current electroluminescence (ACEL) devices are attractive candidates in cost-effective lighting, sensing, and flexible displays due to their uniform luminescence, stable performance, and outstanding deformability. However, ACEL devices have suffered from limited options for the light-emitting layer, which presents a significant constraint in the progress of utilizing ACEL. Herein, a new class of ACEL phosphors based on lanthanide metal-organic frameworks (Ln-MOFs) is devised. A synthesis of lanthanide-benzenetricarboxylate (Ln-BTC) thin film on a brass grid substrate seeded with ZnO nanowires (NWs) as anchors is developed. The as-synthesized Ln-BTC thin film is employed as the emissive layer and shows visible electroluminescence driven by alternating current (2.9V µm-1 , 1kHz) for the first time. Mechanistic investigations reveal that the Ln-based ACEL stems from impact excitation by accelerated electrons from ZnO NWs. Fine-tuning of the ACEL color is also demonstrated by controlling the Ln-MOF compositions and introducing an extra ZnS emitting layer. The advances in these optical materials expand the application of ACEL devices in anti-counterfeiting.

Electroluminescence from Silicon-Based Light-Emitting Devices with Erbium-Doped ZnO Films: Strong Enhancement Effect of Titanium Codoping.

We report on the visible and near-infrared electroluminescence (EL) from the light-emitting device (LED) based on the erbium (Er)-doped ZnO (ZnO:Er)/SiO2/n+-Si heterostructure, wherein an ∼10 nm thick SiO2 intermediate layer serves as the energy plateau for producing hot electrons, which come from n+-Si via the trap-assisted tunneling mechanism. These hot electrons excite the doped Er3+ ions by inelastic collision, enabling the Er-related EL from the aforementioned LED. More importantly, by means of codoping the appropriate content of titanium (Ti) into the ZnO:Er film, the aforementioned Er-related emissions can be significantly enhanced. The density functional theory calculations indicate that the Ti-codoping improves rather than degrades the symmetry of the crystal field around the optically active Er3+ ions, hence not increasing the intra-4f transition probabilities of Er3+ ions. However, it is found that Ti-codoping nearly eliminates the segregation of Er3+ ions near the ZnO/SiO2 interface. Moreover, Ti-codoping is derived to result in a number of Zn vacancies, which provide the sites for incorporating Er3+ ions in the ZnO matrix. For the above two reasons, the Ti-codoping promotes the incorporation of optically active Er3+ ions into the ZnO matrix, thus enhancing the EL from the aforementioned LED.

Enhancement and tuning of the defect-induced electroluminescence of ZnO mesoporous layers in the visible range

We show a way to pattern the visible electroluminescence of solution-processed mesoporous ZnO layers. Our approach consists in locally changing the nanoscale morphology of the coated ZnO layers by patterning the underlying surface with thin metallic patches. Above the metal, the ZnO film is organized in clusters that enhance its defect-induced electroluminescence. The resulting emission occurs over a large continuum of wavelengths in the visible and near-infrared range. This broad emission continuum is filtered by thin film interferences that develop within the device, making it possible to fabricate LEDs with different colours by adjusting the thickness of their transparent electrode. When the metallic patterns used to change the morphology of the ZnO layer reach sub-micron dimensions, additional plasmonic effects arise, providing extra degrees of freedom to tune the colour and polarization of the emitted photons.

Electroluminescence from light-emitting device with erbium-doped TiO2 film sputtered onp+-Si substrate: Enhancement effect of codoping zirconium

In our previous report, the erbium (Er)-related visible and near-infrared (NIR, ∼1.54 μm) electroluminescence (EL) from the light-emitting device (LED) with Er-doped TiO2 (TiO2:Er) film sputtered on the heavily boron-doped p-type silicon (p+-Si) substrate thus forming the so-called TiO2:Er/p+-Si heterostructure was realized. However, it is still a challenge to enhance the Er-related emissions from the above-structured LED. In this work, we have adopted the strategy of codoping zirconium (Zr) into TiO2 host to significantly enhance the Er-related visible and NIR EL from the TiO2:Er/p+-Si heterostructured LED. It is revealed that the oversize Zr4+ ions substantially substitute for Ti4+ ions in TiO2 lattice. Such substitution results in somewhat distorted crystal field of Er3+ ions incorporated into TiO2 host, which increases the intra-4f transition probabilities of Er3+ ions thus enhancing the characteristic visible and NIR emissions.

Reverse‐Bias Electroluminescence in Er‐Doped β‐Ga2O3 Schottky Barrier Diodes Manufactured by Pulsed Laser Deposition

Strong electroluminescence (EL) of reverse‐biased Er‐doped β‐Ga2O3 Schottky barrier diodes is demonstrated. The devices are prepared by pulsed laser deposition featuring co‐doping of n‐type dopant Si and isovalent Er, while Schottky contacts are formed by Pt‐sputtering. The diodes display a rectification ratio of more than nine orders of magnitude at ±3 V in the virgin state, but under a reverse bias that yields a leakage current density of 0.2–0.4 A cm−2, clearly visible multiband EL emerges. The EL is homogeneously distributed across the diode area, and the peak wavelengths compare well with the reported transition for Er3+.

Electroluminescence from the light-emitting devices with erbium-doped SrTiO3 films on oxidized silicon substrate

We report on the visible and near-infrared (NIR) electroluminescence (EL) from the light-emitting devices (LEDs) with erbium-doped SrTiO3 (SrTiO3:Er) films on oxidized silicon substrate, namely, with the structure of SrTiO3:Er/SiO2/Si in abbreviation. It is found that the 900 °C-annealed SrTiO3:Er films are most desirable for the SrTiO3:Er/SiO2/Si structured LEDs. In the EL-enabling voltage range, the electron transportation through the SiO2 layer of SrTiO3:Er/SiO2/Si structured LED is revealed to be compliant with the trap-assisted tunneling (TAT) mechanism. Based on the energy band diagram of the SrTiO3:Er/SiO2/Si structured LED under the EL-enabling forward bias where the negative voltage is connected with Si substrate, it is believed that a number of conduction electrons in Si can tunnel into the conduction band of SiO2 layer via the aforementioned TAT mechanism and then jump down to the conduction band of SrTiO3 host. Because the conduction band offset of SiO2 and SrTiO3 host is larger than 3 eV, the conduction electrons reaching the SrTiO3 host gain considerably high energies so that they become the so-called hot electrons. The Er3+ ions incorporated into the SrTiO3 host are impact-excited by the hot electrons, ultimately emanating the characteristic visible and NIR light.

Yb/Er coordinatively doping in bilayer WSe2 for fascinating up-conversion luminescence

Abstract Benefited from the controllable synthesis of large-scale transition metal dichalcogenides (TMDCs), abundant physical properties of TMDC van der waals heterostructures, such as moire-exciton, electrostatic gating effects, interlayer magnetism, are widely reported. In optoelectronic categories, the applications of TMDC van der waals heterostructures involve interlayer exciton lasers, single photon emitters, and light-emitting diodes (LEDs). However, the employment of TMDCs in visible light emitter is still challenging due to their narrow band gap. The up-converted luminescence from TMDCs could be an effective way to achieve this goal, which has been reported rarely. In this work, the rare earth Yb/Er co-doped bilayer WSe2 of atomic thickness is designed and synthesized using precursor ultrasonic mixing and molten-salt assisted chemical vapor deposition technology. A fascinating up-converted luminescence has been realized on the Yb–Er co-doped bilayer WSe2 excited by 980 nm laser. The up-conversion luminescence peak at 550 nm originates from Er3+ energy level transition 4S3/2→4I15/2. Furthermore, a modulated visible electroluminescence is probed from the LED with architecture of Cr/Au electrodes on Yb–Er co-doped bilayer WSe2/monolayer MoSe2 van der waals heterostructure. Unveiling up-conversion luminescence in TMDCs opens a novel way of utility in photomedicine, infrared thermometer, and infrared photography, etc.

Effects of residual solvent in printed phosphorescent emissive thin films as, the origin of limited efficiency in organic light emitting diodes

Using printed organic materials to fabricate organic light emitting diodes (OLEDs) is a low-cost process and one of the most innovative deposition technologies. Nozzle jet printing is a facile method with the advantages of high speed and large area fabrication. However, the printing process leaves residual solvent in the deposited organic films. How that remaining solvent affects the performance of the OLEDs, and the fundamental mechanism of that effect, is still unclear. In this study, we investigated the effect of residual solvent in a jet printed phosphorescent emissive layer. Gas chromatography-mass spectrography (GC–MS) was used to measure the amount of residual solvent, which was dependent on the film drying environment. The printed phosphorescent films exhibited weak intersystem crossing and decreased coupling between the vibrational states of the tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3) in measurements of ultraviolet–visible absorption (UV–vis Abs), photoluminescence (PL), and electroluminescence (EL). The deterioration of the photophysical characteristics of the phosphorescence was attributed to the polaronic effect of the residual solvent molecules. The dipoles of the solvent molecules influenced the orbitals of surrounding organic molecules, and those perturbations induced nonradiative relaxation of the excitons. There was no shift in the peaks, but the intensities of the photophysical measurements decreased, indicating that the deterioration in phosphorescence was not chemical degradation. Finally, when the amount of residual solvent increased, the current density and the efficiency of the fabricated OLED decreased. The residual solvent also resulted in efficiency roll-off at high luminance in the devices.

Low power consumption light emitting device containing TiO2:Er3+ thin film prepared by sol-gel method.

Er3+ ions doped titanium dioxide (TiO2) thin films have been prepared by sol-gel method. The photoluminescence both in visible light range (510-580 nm and 640-690 nm) and near infrared light range (1400-1700nm) have been observed. The photoluminescence excitation spectra demonstrate that energy transfer from wide band-gap TiO2 to Er3+ ions causes the infrared light emission. It is also found that the post annealing temperature can influence the luminescence intensity significantly. Based on sol-gel prepared TiO2:Er3+ thin films, we fabricate light emitting device containing ITO/TiO2:Er3+/SiO2/n+-Si/Al structure. Both the visible and near infrared electroluminescence (EL) can be detected under the operation voltage as low as 5.6 V and the working current of 0.66 mA, which shows the lower power consumption compared with the conventional EL devices.

Electroluminescence from silicon-based light-emitting device with erbium-doped TiO<sub>2</sub> films: Enhancement effect of ytterbium codoping

In the past years, light-emitting devices (LEDs) based on erbium (Er)-doped insulators or wide-bandgap semiconductors have received intensive attention because the intra-4f transition (<sup>4</sup>I<sub>13/2</sub>→<sup>4</sup>I<sub>15/2</sub>) of Er<sup>3+</sup> ions at ～ 1540 nm has potential applications in the optical interconnection for silicon-based circuits. The LEDs with rare-earth (RE)-doped SiO<i><sub>x</sub></i> (<i>x</i> ≤ 2) or SiN<i><sub>x</sub></i> (<i>x</i> ≤ 4/3) films have been well investigated as the silicon-compatible emitters. However, they suffer difficulty in injecting current and easing fatigue. In this context, the LEDs with RE-doped oxide semiconductors have been extensively investigated out of research interest in recent years. Among the oxide semiconductors, TiO<sub>2</sub> is a desirable host for RE-doping because it is transparent for visible and infrared light, and cost-effective, and has considerably high RE solubility. In our previous work (Zhu C, Lü C Y, Gao Z F, Wang C X, Li D S, Ma X Y, Yang D R 2015 <i>Appl. Phys. Lett.</i> <b>107</b> 131103), we have realized erbium (Er)-related visible and near-infrared (～ 1540 nm) electroluminescence (EL) from the LED with a structure of ITO/TiO<sub>2</sub>:Er/SiO<sub>2</sub>/n<sup>+</sup>-Si, in which TiO<sub>2</sub>:Er refers to the Er-doped TiO<sub>2</sub> film as the light-emitting layer. In this work, we co-dope ytterbium (Yb) into the TiO<sub>2</sub>:Er film in the aforementioned LED to significantly enhance the Er-related visible and near-infrared EL. It is revealed that a certain amount of Yb co-doping enables the TiO<sub>2</sub>:Er film to transform its crystal phase from anatase to rutile. Such a phase transformation reduces the symmetry of crystal field surrounding the Er<sup>3+</sup> ions incorporated into the TiO<sub>2</sub> host. Moreover, the substitution of over-sized Yb<sup>3+</sup> ions for Ti<sup>4+</sup> ions in the TiO<sub>2</sub> host leads to the distortion of the crystal field around the Er<sup>3+</sup> ions. The aforementioned symmetry-reduction and distortion of the crystal field increase the probabilities of the intra-4f transitions of Er<sup>3+</sup> ions. Due to the aforementioned reason, the Yb co-doping into the TiO<sub>2</sub>:Er film remarkably enhances the EL from the corresponding LED. It is believed that the strategy of Yb-codoping can be adopted to enhance the EL from the LEDs with other RE-doped TiO<sub>2</sub> films.

Photoluminescence and Electroluminescence Properties of La2Ti2O7:Pr

La2Ti2O7:Pr powder was prepared by a solid-state reaction method. Photoluminescence due to f-f transitions of Pr3+was induced by band-gap excitation of La2Ti2O7. La2Ti2O7:Pr thin film was prepared by sol-gel and spin-coating methods. The thin film showed visible electroluminescence due to f-f transition of Pr3+by applying ac voltages.

Synthesis, structures and spectroscopy of three new lanthanide β-diketonate complexes with 4,4′-dimethyl-2,2′-bipyridine. Near-infrared electroluminescence of ytterbium(III) complex in OLED

Three new lanthanide(III) β-diketonate complexes [Ln(btfa)3(4,4′-dmbpy)] (Ln = Yb(III), Gd(III) and Nd(III); btfa = anionic 4,4,4-trifluoro-1-phenyl-1,3-butanedione and 4,4′-dmbpy = 4,4′-dimethyl-2,2′-bipyridine) were synthesized and their characterization, including elemental analysis, FTIR spectroscopy and thermal analysis (TG/DTA), is reported. The crystal description based on powder X-ray diffraction reveals that Gd(III) and Yb(III) compounds are isostructural and that lanthanide ion is eight-coordinated by oxygen and nitrogen atoms to give new tris- β-diketonates [Ln(btfa)3(4,4′-dmbpy)]. The cell parameters of Nd(III) complex are close to Gd(III) and Yb(III) ones. Phosphorescence data of Gd(III) complex shows that the triplet states (T1) of the ligands have higher energy than the emitting states of Yb(III) and Nd(III), indicating the possibility of intramolecular energy transfer to these metal ions, which exhibit near-infrared (NIR) emission. We have used the Yb(III) complex as an emitting layer (EML) in a near infrared organic light emitting diode (NIR-OLED) with the structure: CuPc(15 nm)/XD-03(40 nm)/Yb(60 nm)/BCP(15 nm)/Alq3(10 nm)/Al(120 nm). OLEDs exhibit both visible electroluminescence and a NIR electroluminescence at 980 nm from the 2F5/2 → 2F7/2 transition of the Yb(III).

Single crystalline ZnO radial homojunction light-emitting diodes fabricated by metalorganic chemical vapour deposition

ZnO radial p–n junction architecture has the potential for forward-leap of light-emitting diode (LED) technology in terms of higher efficacy and economical production. We report on ZnO radial p–n junction-based light emitting diodes prepared by full metalorganic chemical vapour deposition (MOCVD) with hydrogen-assisted p-type doping approach. The p-type ZnO(P) thin films were prepared by MOCVD with the precursors of dimethylzinc, tert-butanol, and tertiarybutylphosphine. Controlling the precursor flow for dopant results in the systematic change of doping concentration, Hall mobility, and electrical conductivity. Moreover, the approach of hydrogen-assisted phosphorous doping in ZnO expands the understanding of doping behaviour in ZnO. Ultraviolet and visible electroluminescence of ZnO radial p–n junction was demonstrated through a combination of position-controlled nano/microwire and crystalline p-type ZnO(P) radial shell growth on the wires. The reported research opens a pathway of realisation of production-compatible ZnO p–n junction LEDs.

Influence of the gate and dielectric thickness on the electro-optical performance of SRO-based LECs: Resistive switching, IR and deep UV emission

This work presents the charge transport and electroluminescent properties of silicon rich oxide (SRO) films with ~41at% silicon content and different thicknesses (tSRO=24, 53 and 80nm), as dielectric layer in metal-oxide-semiconductor (MOS) capacitive structures. SRO films were thermally annealed at high temperature to induce the silicon nanoparticles (Si-nps) formation. Light emitting capacitors (LECs) with SRO thickness about 80nm behave like a normal MOS structure with a current level at about 10−10 to 10–12A (equipment sensitivity) at low electric fields. However, a high conduction state (HCS) and then a resistance switching (RS) behavior to a low conduction state is observed when the thickness of the SRO film reduces to (and below) 53nm. When the SRO thickness reduces to 24nm, the electric field required to obtain RS behavior increases. The presence of the HCS and RS is related with silicon diffusion from the polysilicon film, used as gate electrode, towards the SRO bulk. The silicon content within the SRO film increases by the Si diffusion as the thickness reduces allowing the formation of preferential conductive paths. Once RS is obtained, LECs emit a broad visible and near infrared (IR) electroluminescence (EL) spectra with several maximum peaks at ~455, 490, 540, 650 and 800nm. For SRO thickness ≥ 53nm, the most intense EL peak is the placed at 650nm. It becomes narrower and stronger when the thickness increases to 80nm as compared to the other ones making a uniform red EL. Nevertheless, when the SRO thickness decreases to 24nm, an additional IR EL peak above 1000nm is observed. This IR band is slightly stronger than that observed at 650nm and results from the silicon substrate emission enhanced by the high current flow through thin SRO films. By other hand, two narrow spectral lines of low intensity in the deep ultraviolet region (253 and 288nm) are observed when the thickness is above 53nm, which are related to the resonant coherent fluorescence effect. The electrical behavior and optical power emission and its relation with the electroluminescent behavior of LECs are analyzed.

Two-Dimensional Organic Tin Halide Perovskites with Tunable Visible Emission and Their Use in Light-Emitting Devices

Hybrid organic lead trihalide perovskites continue to generate significant interest for use in optoelectronic devices such as solar cells and light-emitting devices. However, the toxicity of lead is considered one of the main obstacles to the commercialization of this technology. Although challenging, the replacement of lead by tin is currently the most promising alternative. Herein, we explore a class of low-dimensional, lead-free perovskite materials (2D (PEA)2SnIxBr4–x, where PEA ≡ C6H5CH2CH2NH3+) with tunable optical properties in the visible region of the spectrum. Specifically, we show that 2D (PEA)2SnI4 perovskite exhibits superior photoluminescence properties to conventional 3D CH3NH3SnI3 and that (PEA)2SnI4 can act as a sensitizer on mesoporous TiO2. We go on to demonstrate visible (∼630 nm) electroluminescence from a device employing a (PEA)2SnI4 emitter sandwiched between ITO/PEDOT:PSS and F8/LiF/Al as hole and electron injection electrodes, respectively. These devices reach a luminance of 0.15...

Mechanisms of visible electroluminescence in diode structures on the basis of porous silicon: A review

The main mechanisms of the visible electroluminescence (EL) of porous silicon are reviewed. Characteristics of photoluminescence and EL of diode structures based on porous silicon are compared. Metals having a smaller value of the electron work function (3.6 eV, Mg) than do Al and Au are proposed as the material for making contacts in such diode structures to increase the efficiency of their EL in the visible region of the spectrum. The main problems and prospects of light-emitting devices based on porous silicon are formulated.

Multicolor light-emitting devices with Tb2O3 on silicon

Great efforts have been devoted to achieving efficient Si-based light-emitting devices. Here we report new light-emitting devices fabricated with Tb2O3 on Si substrates. Intense green electroluminescence was observed, with a turn-on voltage of about 8 V. The green emission is attributed to the characteristic transitions of Tb3+ ions in Tb2O3. The electroluminescence mechanisms of the Tb2O3 light-emitting devices are discussed. In addition, visible and near infrared electroluminescence was observed in rare-earth (Eu3+, Sm3+ and Yb3+) doped Tb2O3 light-emitting devices.

Lasing with Pumping Levels of Si Nanocrystals on Silicon Wafer.

It is reported that the silicon nanocrystals (NCs) are fabricated by using self-assembly growth method with the annealing and the electron beam irradiation processes in the pulsed laser depositing, on which the visible lasing with higher gain (over 130 cm−1) and the enhanced emission in optical telecommunication window are measured in photoluminescence (PL). It is interesting that the enhanced visible electroluminescence (EL) on silicon nanocrystals (Si-NCs) is obviously observed by the naked eyes, and the light-emitting diode (LED) of the Si-NCs with external quantum efficiency of 20% is made on silicon chip in our laboratory. A four-level system is built for emission model in nanosilicon, in which the PL and EL measurement and transmission electron microscope (TEM) analysis demonstrate that the pumping levels with shorter lifetime from the rising energy of the Si quantum dots due to the quantum confinement effect occur, and the electronic localized states with longer lifetime owing to impurities bonding on Si-NCs surface are formed in the crystallized process to produce the inversion of population for lasing, where the optical gain is generated.