Weak Electroluminescence Research Articles

Synergistic Enhancement of Near-Infrared Emission in CsPbCl3 Host via Co-Doping with Yb3+ and Nd3+ for Perovskite Light Emitting Diodes.

Perovskite nanocrystals (PeNCs) have emerged as a promising class of luminescent materials offering size and composition-tunable luminescence with high efficiency and color purity in the visible range. PeNCs doped with Yb3+ ions, known for their near-infrared (NIR) emission properties, have gained significant attention due to their potential applications. However, these materials still face challenges with weak NIR electroluminescence (EL) emission and low external quantum efficiency (EQE), primarily due to undesired resonance energy transfer (RET) occurring between the host and Yb3+ ions, which adversely affects their emission efficiency and device performance. Herein, we report the synergistic enhancement of NIR emission in a CsPbCl3 host through co-doping with Yb3+/Nd3+ ions for perovskite LEDs (PeLEDs). The co-doping of Yb3+/Nd3+ ions in a CsPbCl3 host resulted in enhanced NIR emission above 1000 nm, which is highly desirable for NIR optoelectronic applications. This cooperative energy transfer between Yb3+ and Nd3+ can enhance the overall efficiency of energy conversion. Furthermore, the PeLEDs incorporating the co-doped CsPbCl3/Yb3+/Nd3+ PeNCs as an emitting layer exhibited significantly enhanced NIR EL compared to the single doped PeLEDs. The optimized co-doped PeLEDs showed improved device performance, including increased EQE of 6.2% at 1035 nm wavelength and low turn-on voltage. Our findings highlight the potential of co-doping with Yb3+ and Nd3+ ions as a strategy for achieving synergistic enhancement of NIR emission in CsPbCl3 perovskite materials, which could pave the way for the development of highly efficient perovskite LEDs for NIR optoelectronic applications.

Position Influence of Sputtered Zn1-xMgxO/Zn1-xMgxO:Al Layers in Flexible and Cd-Free Cu(In,Ga)(S,Se)2 Solar Cells.

Flexible, Cd-free, and all-dry process Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells on stainless steel (SUS) substrates are fabricated, and their structure consists of SUS/glass (SiO2)/Mo/CIGSSe absorber/sputtered Zn0.84Mg0.16O/sputtered Zn1-xMgxO:Al transparent conductive oxide (TCO). The effect of the sample position during the sputtering of Zn0.84Mg0.16O buffer and Zn1-xMgxO:Al TCO layers of the solar cells is examined to avoid intense plasma exposure. The sample position plays a vital role in improving the cell performance. Namely, the sample position close to the material targets of the sputtering system causes severe exposure of the sample to the intense plasma, giving rise to low and nonuniform local external quantum efficiency (EQE) with very weak electroluminescence (EL) imaging, thereby reducing photovoltaic performance. On the other hand, the deviation of the sample position from material targets helps to avoid the intense plasma, thus resulting in high and uniform local EQE with bright EL imaging as well as reducing carrier recombination rates (or carrier lifetimes) throughout the solar cells. Ultimately, the conversion efficiency of flexible, Cd-free, and all-dry process CIGSSe solar cells is enhanced to 16.5% under the optimized sample position deviation from material targets to avoid intense plasma exposure.

Impact of SiN passivation film stress on electroluminescence characteristics of AlGaN/GaN high-electron-mobility transistors

This letter reports an impact of SiN film stress on electroluminescence (EL) of AlGaN/GaN HEMTs. The EL color of HEMTs turned from high-intensity white to low-intensity red, as the SiN film stress was increased from −24.2 (compressive) to +11.5 MPa (tensile). The weak reddish EL from the HEMT with a film stress of +11.5 MPa turned bright whitish when a drain-to-source voltage (V ds) was increased. A transient I d response after biasing stress revealed that severe current collapse emerged in the HEMT with a film stress of −24.2 MPa. In addition, the current collapse in HEMTs increased with the increased V ds.

Photoluminescence and Electroluminescence Properties of (Ca,Sr)TiO<sub>3</sub>:Er

Powder samples of (Ca,Sr)TiO3:Er were prepared by a solid-state reaction method. Photoluminescence due to f-f transitions of Er3+ was not induced by band-gap excitation of (Ca,Sr)TiO3 but by f-f transitions of Er3+. An electroluminescent device in which thin films of Ca0.6Sr0.4TiO3:Er and SnO2:Sb are stacked alternately was prepared by sol-gel and spin-coating methods. Very weak electroluminescence due to f-f transition of Er3+ was observed in the device.

High-Resolution Light-Emitting Diode Array Based on an Ordered ZnO Nanowire/SiGe Heterojunction

A high-resolution light-emitting diode (LED) array based on an ordered n-type ZnO nanowire/p-type SiGe heterojunction is proposed. The pixel pitch of the LED array is about 15 μm, corresponding to a pixel density of 1693 dpi. The fabrication and characterizations of the developed LED device are analyzed in detail. The results show that the electroluminescence (EL) emission of this LED device is mainly in the infrared range, which is dominated by the band gap of the SiGe alloy. Furthermore, a weak EL emission in the visible range can also be observed, and the enhanced light emission is demonstrated by the externally applied compressive strain using a patterned sapphire mold. In addition, the feasibility of implementing GeSn alloys in the developed LED structure is discussed.

Blue and green electroluminescence from CdSe nanocrystal quantum-dot-quantum-wells

CdS/CdSe/ZnS quantum dot quantum well (QDQW) nanocrystals were synthesized using the successive ion layer adsorption and reaction technique, and their optical properties were tuned by bandgap and strain engineering. 3-monolayer (ML) CdSe QWs emitted blue photoluminescence at 467 nm with a spectral full-width-at-half-maximum of ∼30 nm. With a 3 ML ZnS cladding layer, which also acts as a passivating and strain-compensating layer, the QDQWs acquired a ∼35% quantum yield of the QW emission. Blue and green electroluminescence (EL) was obtained from QDQW light-emitting devices with 3–4.5 ML CdSe QWs. It was found that as the peak blueshifted, the overall EL was increasingly dominated by defect state emission due to poor hole injection into the QDQWs. The weak EL was also attributed to strong field-induced charge separation resulting from the unique QDQW geometry, weakening the oscillator strength of optical transitions.

Electroluminescence from Cr3+ in Perovskite Thin-Film Phosphors Using LaAlO3 and LaGaO3 as Hosts

We observed multicolor and near-infrared electroluminescence (EL) from perovskite oxide thin-film phosphors using Cr- and Ce-co-doped LaGaO3 and LaAlO3. These phosphor thin films were prepared by varying either Cr or Ce content using a combinatorial r.f. magnetron sputtering deposition method. Intense deep-red and weak yellow-green EL emissions were observed from thin-film electroluminescent (TFEL) devices fabricated using posannealed LaGaO3:Ce,Cr thin films. Intense near-infrared emission and yellow-green EL emissions were observed from TFEL devices fabricated using posannealed LaAlO3:Ce,Cr thin films. The highest EL intensity was obtained in TFEL devices using postannealed LaGaO3:Ce,Cr and LaAlO3:Ce,Cr phosphor thin films with Cr content approximetly 3 and 1at.%, respectively. All the observed emission peaks in EL from the LaGaO3:Ce,Cr and/or the LaAlO3:Ce,Cr phosphor thin films were assigned to either the emission originating from the transition in the Ce3+ and Cr3+ ions.

Surface-Assisted Luminescence: The PL Yellow Band and the EL of n-GaN Devices

Although everybody should know that measurements are never performed directly on materials but on devices, this is not generally true. Devices are physical systems able to exchange energy and thus subject to the laws of physics, which determine the information they provide. Hence, we should not overlook device effects in measurements as we do by assuming naively that photoluminescence (PL) is bulk emission free from surface effects. By replacing this unjustified assumption with a proper model for GaN surface devices, their yellow band PL becomes surface-assisted luminescence that allows for the prediction of the weak electroluminescence recently observed in n-GaN devices when holes are brought to their surfaces.

Origin of strong white electroluminescence from dense Si nanodots embedded in silicon nitride.

Strong white electroluminescence (EL) from SiN-based devices containing Si nanodots with a density of more than 4.6×10(12)cm(2) was investigated. The white EL illustrates enhanced light emission with increasing applied voltage and can be divided into two components, a dominant peak at ~710 nm and weak one at ~550 nm, which are close to those of the PL spectra optically pumped by the 325 and 488 nm lines, respectively. Based on the PL characteristics, we propose that the dominant EL band arises from the band-to-band recombination in the dense Si nanodots where quantum confinement plays a decisive role in the light emission, whereas the weak EL band originates from the radiative Si dangling bond (K0) centers in the silicon nitride matrix.

Properties of ZnO whiskers under CO2-laser irradiation

Needlelike ZnO single crystals (whiskers) 0.3–0.8 mm long and 1–10 μm in diameter with a resistivity from 3 × 102 to 1 Ω cm have been grown under cw CO2-laser irradiation. The whiskers exhibit weak electroluminescence caused by injection from contacts with participation of intrinsic defects, which affect the electric field distribution.

Electroluminescence behavior of ZnO/Si heterojunctions: Energy band alignment and interfacial microstructure

n-ZnO/p-Si heterojunction light-emitting diodes (LEDs) show weak defect-related electroluminescence (EL). In order to analyze the origin of the weak EL, the energy band alignment and interfacial microstructure of ZnO/Si heterojunction are investigated by x-ray photoelectron spectroscopy. The valence band offset (VBO) is determined to be 3.15±0.15 eV and conduction band offset is −0.90±0.15 eV, showing a type-II band alignment. The higher VBO means a high potential barrier for holes injected from Si into ZnO, and hence, charge carrier recombination takes place mainly on the Si side rather than the ZnO layer. It is also found that a 2.1 nm thick SiOx interfacial layer is formed at the ZnO/Si interface. The unavoidable SiOx interfacial layer provides to a large number of nonradiative centers at the ZnO/Si interface and gives rise to poor crystallinity in the ZnO films. The weak EL from the n-ZnO/p-Si LEDs can be ascribed to the high ZnO/Si VBO and existence of the SiOx interfacial layer.

Photonic applications of Silicon nanostructures

This presentation highlights of some scientific insights on the possibilities of photonic applications of silicon nanostructures (NSs) one of the most fertile research field in nano-crystallite physics that has innumerable possibilities of device applications. Nanostructured silicon is generic name used for porous Si (p-Si) as well as Si nanocrystals (NC-Si) having length scale of the order of few nanometer. The emission of a very bright photo-luminescence (PL) band and relatively weak electro-luminescence (EL) from low-dimensional silicon has opened up new avenue in recent years. It is important from a fundamental physics viewpoint because of the potential application of Si wires and dots in opto-electronics devices and information technology. Nanostructuring silicon is an effective way to turn silicon into a photonic material. It is observed that low-dimensional (one and two dimensions) silicon shows light amplification, photon confinement, photon trapping as well as non-linear optical effects. There is strong evidence of light localization and gas sensing properties of such NSs. Future nano-technology would replace electrical with optical interconnects that has appealing potentialities for higher-speed performance and immunity to signal cross talk. A varieties of applications includes LD, LED, solar cells, sensors, photonic band gap devices and Fibonacci quasi-crystals, to cite a few.

Electroluminescence from Charge Transfer States in Polymer Solar Cells

In this article we report the weak but omnipresent electroluminescence (EL) from several types of organic polymer:fullerene bulk heterojunction solar cells biased in the forward direction. The light emitted from blends of some commonly used polymers and the fullerene molecule is significantly different from that of any of the pure materials comprising the blend. The lower energy of the blend EL is found to correlate with both the voltage onset of emission and the open-circuit voltage of the photovoltaic cell under solar illumination. We accordingly interpret the emission to originate from interfacial charge transfer state recombination and emphasize EL as a very valuable tool to characterize the charge transfer state present in donor/acceptor organic photovoltaic (OPV) cells.

전자수송층이 청색 인광 OLED의 전기 및 광학적 특성에 미치는 영향

We have developed blue-emitting phosphorescent organic light emitting diodes (OLEDs) using 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and tris (8-quinolinolato)aluminum (<TEX>$Alq_3$</TEX>) electron transport layers. As blue dopant and host materials, bis[(4,6-di-fluorophenyl)-pyridinate-N,C2']picolinate (FIrpic) and N,N'-dicarbazolyl-3,5-benzene (mCP) were used, respectively. The driving voltage, current efficiency and emission characteristics of devices were investigated. While the driving voltage was about <TEX>$1{\sim}2$</TEX> V lower in the device with an <TEX>$Alq_3$</TEX> layer, the current efficiency was about 66 % higher in the device with BCP electron transport layer. the blue phosphorescent OLED with BCP layer exhibited higher purity of color, resulting from a relatively weak electroluminescence intensity at 500 nm.

Rare-earth metal 8-hydroxyquinolinate complexes as materials for organic light-emitting diodes

Rare-earth metal 8-hydroxyquinolinates (q) were studied as emission layers for organic light-emitting diodes. Compounds Dyq3, Tbq3, and Smq3 possess weak electroluminescence due to the emission of the ligand, and the electroluminescence of Hoq3, Tmq3, and Ybq3 is due to electronic transitions of the lanthanide ion. The best electroluminescence characteristics were demonstrated by Scq3 and Yq3. The power efficiency of the organic light emitter based on ITO/TPD/Scq3/Yb (ITO is tin-alloyed indium oxide, TPD is N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine) is 2.6 lm W−1 at a luminance of the device of 300 cd m−2.

White-light sources based on composites of GaN nanocrystals with conducting polymers and nanophosphors

Optical properties of nanocomposites of GaN nanocrystals, 2–4 nm in diameter, with conjugated polymers such as poly(2-methoxy,5-(2-ethylhexoxy)-1,4-phenylenevinylene (MEHPPV) as well as with phosphors nanoparticles have been investigated for use as white light sources. Photoluminescence spectra of the nanocomposites with different weight ratios of the two components are reported. Weak electroluminescence has been observed in the case of MEHPPV-GaN nanocomposites. The GaN–nanophosphor composites exhibit a high quality white light with good CIE coordinates along with an excellent colour rendering index.

Electroluminescent and carrier transport mechanisms of MgxZn1−xO∕Si heterojunctions

Mg x Zn 1 − x O (x≈0.19) films were, respectively, deposited by reactive sputtering on the heavily arsenic-doped (n+), lightly phosphorus-doped (n−), heavily boron-doped (p+), and lightly boron-doped (p−) silicon substrates. The sputtered MgxZn1−xO films were n-type with an electron concentration of 4.1×1017cm−3. Moreover, the energy-band gap of the MgxZn1−xO films was determined to be ∼3.67eV. The MgxZn1−xO∕n−-Si and MgxZn1−xO∕p−-Si heterojunctions showed rectifying behavior to different degrees while the MgxZn1−xO∕n+-Si and MgxZn1−xO∕p+-Si heterojunctions did not possess rectifying function. The MgxZn1−xO∕n+-Si heterojunction exhibited UV near-band-edge and visible defect-related electroluminescence (EL) under the forward bias but only visible EL under reverse bias; while the MgxZn1−xO∕n−-Si heterojunction only emitted relatively weak UV and visible EL under the forward bias. On the other hand, the MgxZn1−xO∕p+-Si heterojunction was electroluminescent in the visible region under the forward bias while in both the UV and visible regions under the reverse bias; and the MgxZn1−xO∕p−-Si heterojunction did not exhibit detectable EL under either forward or reverse bias. The mechanisms of the EL and carrier transport for the above-mentioned heterojunctions have been tentatively explained in terms of their energy-band structures.

NANOSILICON FOR PHOTONIC APPLICATIONS

This presentation is a short review of some scientific insights on the possibilities of photonic applications of nanostructured silicon ( NS – Si ), porous Si ( p - Si ) and Si nanocrystals ( NC – Si ), one of the most interesting problems in nano-crystallite physics. The emission mechanism of a very bright photo-luminescence (PL) band and relatively weak electro-luminescence (EL) are presently the main issue. The basic question lies in whether the emission is an extrinsic or intrinsic property of nanocrystals. It is important from a fundamental physics viewpoint because of the potential application of Si wires and quantum dots in optoelectronic devices and information technology. Nanostructuring silicon is an effective way to turn silicon into a photonic material. It is observed that low-dimensional (one and two dimensions) silicon shows light amplification, photon confinement, photon trapping as well as non-linear optical effects. There is strong evidence of light localization and gas sensing properties of such nanostructures. Future nano-technology would replace electrical with optical interconnects, which has appealing potential for higher-speed performance and immunity to signal cross talk.

Surface electroluminescence phenomena correlated with trapping parameters of insulating polymers

Electroluminescence (EL) phenomena are closely linked to the space charge and degradation in insulating polymers, and dominated by the luminescence and trap centers. EL emission has been promising in defining the onset of electrical aging and in the investigation of dissipation mechanisms. Generally, polymeric degradation reveals the increment of the density of luminescence and trap centers, so a fundamental study is proposed to correlate the EL emission of insulating polymers and their trapping parameters. A sensitive photon counting system is constructed to detect the weak EL. The time- and phase-resolved EL characteristics from different polymers (LDPE, PP and PTFE) are investigated with a planar electrode configuration under stepped ac voltage in vacuum. In succession, each sample is charged with exposing to multi-needle corona discharge, and then its surface potential decay is continuously recorded at a constant temperature. Based on the isothermal relaxation current theory, the energy level and density of both electron and hole trap distribution in the surface layer of each polymer is obtained. It is preliminarily concluded that EL phenomena are strongly affected by the trap properties, and for different polymers, its EL intensity is in direct contrast to its surface trap density, and this can be qualitatively explained by the trapping and detrapping sequence of charge carriers in trap centers with different energy level.

Photoluminescence and electroluminescence properties of ZnO films on p-type silicon wafers

A simplified n-ZnO/p-Si heterojunction has been prepared by growing n-type ZnO rods on p-type silicon wafer through the chemical vapour deposition method. The reflectance spectrum of the sample shows an independent absorption peak at 384 nm, which may be originated from the bound states at the junction. In the photoluminescence spectrum a new emission band is shown at 393 nm, besides the bandedge emission at 380 nm. The electroluminescence spectrum of the n-ZnO/p-Si heterojunction shows a stable yellow luminescence band centred at 560 nm_ which can be attributed to the emission from trapped states. Another kind of discrete ZnO rod has also been prepared on such silicon wafer and is encapsulated with carbonated polystyrene for electroluminescence detection. This composite structure shows a weak ultraviolet electroluminescence band at 395 nm and a yellow electroluminescence band. These data prove that surface modification which blocks the transverse movement of carriers between neighbouring nanorods plays important roles in the ultraviolet emission of ZnO nanorods. These findings are vital for future display device design.