Electroluminescence Wavelength Research Articles

Optical and Thermal Performances of (Ga,In)N/GaN Light Emitting Diodes Transferred on a Flexible Tape

Blue (Ga,In)N-based light-emitting diodes (LEDs) grown on a Si(111) substrate by metal-organic vapor phase epitaxy are transferred on a flexible tape after the Si substrate removal. Their optical and thermal behaviors are measured and compared to those of regular LEDs on Si. The light output power of the flexible LEDs is increased due to a higher light extraction efficiency related to the removal of the absorbing Si substrate. However, the maximum output power is limited by thermal effects due to the lower thermal conductivity of the flexible tape. Monitoring the electroluminescence wavelength of the flexible LEDs allows determining their acceptable operating range. The maximum flexible LED luminance is 5×10 5 cd/m 2 .

Low-Turn-On-Voltage, High-Brightness, and Deep-Red Light-Emitting Electrochemical Cell Based on a New Blend of [Ru(bpy)3]2+ and Zn-Diphenylcarbazone.

Deep red light-emitting electrochemical cells were prepared based on a blend of [Ru(bpy)3]2+, a cationic complex, and a neutral Zn(II)-complex based on diphenylcarbazone ligands, named Zn(DPCO). The crystal structure of the Zn(DPCO)2 (bpy)] molecule revealed that the DPCO ligand has been deprotonated to form DPCO– and coordinated to the Zn center metal through the C=O and N=N moieties of DPCO. From the cyclic voltammetry results and controlled potential coulometry data of the diphenylcarbazide (DPC) ligand, it is possible to establish that DPC is oxidized in an irreversible process at +0.77 V, giving DPCO and later oxidized at a higher potential (+1.32 V) to produce diphenylcarbadiazone (DPCDO). A detailed assignment of UV–vis spectra futures to determine the origin of ground- and excited-state transitions was achieved by time-dependent density functional theory calculations, which showed good agreement with the experimental results. Using a simple device architecture, we obtained deep red electroluminescence (EL) with high brightness (740 cd m–2) and luminous efficiency of 0.39 cd/A at a low turn-on voltage of 2.5 V. The favorable configuration of the cell consists of only a blend of complexes of indium tin oxide as the anode electrode and molten alloy cathode (Ga/In) without any polymer as the transporting layer. The comparison between [Ru(bpy)3]2+ and [Ru(bpy)3]2+/Zn(DPCO) demonstrates a red shift in the EL wavelength from 625 to 700 nm in the presence of Zn(DPCO), revealing the importance of using blends for future systems.

UVA and UVB light emitting diodes with AlyGa1−yN quantum dot active regions covering the 305–335 nm range

Ultra-violet (UV) light emitting diodes (LEDs) using III-N quantum dot (QD) active regions have been fabricated by molecular beam epitaxy on (0001)-oriented sapphire substrates. By using the epitaxial compressive stress between the QD material and the template/barrier layers, leading to a 2D–3D growth mode transition, self-assembled AlyGa1−yN QDs with a nominal Al composition of 10% and 20% have been fabricated on Al0.6Ga0.4N. Atomic force microscopy and transmission electron microscopy measurements show high QD densities, ranging between 2 × 1011–5 × 1011 cm−2, and height and diameter distributions between 1.5–3 nm and 5–20 nm. LED structures including two different AlyGa1−yN/Al0.6Ga0.4N (0001) QD active regions have then been fabricated and processed using a standard planar geometry and squared mesa structures. Current–voltage characteristics and electroluminescence (EL) measurements have been performed at room temperature. In particular, the EL properties are investigated in terms of spectral range and wavelength shift as a function of the injection current density. Typically, an emission between 325 and 335 nm is obtained for Al0.1Ga0.9N QDs and between 305 and 320 nm for Al0.2Ga0.8N QDs. The LED characteristics (EL wavelength and broadening) are then correlated to the QD structural properties and the results, supported by calculations, show the main influence of the QD height dispersion and composition fluctuations. Finally, the light output intensity variation as a function of the injection current density has also been investigated and is discussed in terms of injection and recombination mechanisms in the devices.

Synthesis, photophysical and electrochemical properties of a blue emitter with binaphthalene and carbazole units

A blue emitter, 3,3′-(2,2′-dimethoxy-[1,1′-binaphthalene]-6,6′-diyl)bis(9-benzyl-9H-carbazole), was synthesized by Suzuki coupling reaction. The photophysical properties of the emitter in solution were firstly investigated by UV–Vis absorption and fluorescence emission techniques. The results indicate that the emitter has excellent optical and electron transfer properties. The maximum absorption and emission peaks of the emitter are 302 nm and 406 nm with 67.4% fluorescence quantum yield in chloroform, respectively. Thermal stability study reveals that the emitter has a good thermal stability (Td > 330 °C, Tg > 160 °C). Electrochemical Redox properties of the emitters were measured by cyclic voltammetry, and the energy gaps of highest occupied molecular orbital and the lowest unoccupied molecular orbital levels are in good agreement with the results of theoretical calculation. Furthermore, the multilayer electrochemcial device with the emitter was fabricated and its properties were explored. The wavelength of electroluminescence for the device with this emitter locates at 428 nm. These results indicate the emitter as a deep blue-emitting material has promising application in organic light-emitting diode devices.

Monolithic Inorganic ZnO/GaN Semiconductors Heterojunction White Light-Emitting Diodes.

Monolithic light-emitting diodes (LEDs) that can generate white color at the one-chip level without the wavelength conversion through packaged phosphors or chip integration for photon recycling are of particular importance to produce compact, cost-competitive, and smart lighting sources. In this study, monolithic white LEDs were developed based on ZnO/GaN semiconductor heterojunctions. The electroluminescence (EL) wavelength of the ZnO/GaN heterojunction could be tuned by a post-thermal annealing process, causing the generation of an interfacial Ga2O3 layer. Ultraviolet, violet-bluish, and greenish-yellow broad bands were observed from n-ZnO/p-GaN without an interfacial layer, whereas a strong greenish-yellow band emission was the only one observed from that with an interfacial layer. By controlled integration of ZnO/GaN heterojunctions with different postannealing conditions, monolithic white LED was demonstrated with color coordinates in the range (0.3534, 0.3710)-(0.4197, 0.4080) and color temperatures of 4778-3349 K in the Commission Internationale de l'Eclairage 1931 chromaticity diagram. Furthermore, the monolithic white LED produced approximately 2.1 times higher optical output power than a conventional ZnO/GaN heterojunction due to the carrier confinement effect at the Ga2O3/n-ZnO interface.

Highly efficient deep-red emitting methyl substituted thiophenylquinoline based Ir(III) complexes for solution-processed organic light-emitting diodes

ABSTRACTHerein, we synthesized two new methyl substituted thiophenyl-quinoline based heteroleptic Ir(III) complexes as guest molecules for deep-red emitting solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). Their thermal stabilities, photophysical properties and electroluminescence (EL) properties are systematically investigated. (MTPQ)2Ir(pic) and (MTPQ)2Ir(acac) showed photoluminescence emission of 614 and 629 nm and a photoluminescence quantum yield of 21% and 15%, respectively. Solution-processed deep-red emitting PhOLEDs were fabricated using standard structure with PEDOT:PSS and TPBi as hole and electron transport layers, respectively and, a mixed host of TCTA:TPBi (1:1) doped with the guest Ir(III) molecules. The emission electroluminescence wavelength was slightly red shifted by 11 nm for both (MTPQ)2Ir(pic) (625 nm) and (MTPQ)2Ir(acac) (640 nm). A maximum external quantum efficiency of 8.46% and maximum current efficiency of 7.37 cd/A was achieved for (MTPQ)2Ir(pic) with a deep-red CIE coordinates of (0.679, 0.318).

Hybrid electroluminescent device based on MEH-PPV and ZnO

Hybird organic/inorganic electroluminescent device based on the structure of glass/ITO/PEDOT:PSS/MEH-PPV/ZnO/ZnO submicrorods/Al has been manufactured. Spin coating has been used to deposit both PEDOT:PSS and MEH-PPV. Two-step process has been used to synthesis ZnO submicrorods, namely, spin coating and chemical bath deposition. Changing the dimensions of the ZnO submicrorods in this layer structure has been investigated to improve the performance of the organic/inorganic electroluminescence device. Such layer structure provides electroluminescence with narrow emission bands due to a high gain with this structure. X-ray diffraction patterns and scanning electron microscope images show that ZnO submicrorods have hexagon structure. Current-voltage curve for the structure has been reported. Electroluminescence curves (electroluminescence intensity versus wavelength) at different bias voltages have been presented and these results show narrowing in full width at half maximum in the spectra at high current density compared to photoluminescence excitation. The narrowing in the spectrum has been explained.

Photoluminescence and electroluminescence from Ge/strained GeSn/Ge quantum wells

Ge/strained GeSn/Ge quantum wells are grown on a 300 mm Si substrate by chemical vapor deposition. The direct bandgap emission from strained GeSn is observed in the photoluminescence spectra and is enhanced by Al2O3/SiO2 passivation due to the field effect. The electroluminescence of the direct bandgap emission of strained GeSn is also observed from the Ni/Al2O3/GeSn metal-insulator-semiconductor tunneling diodes. Electroluminescence is a good indicator of GeSn material quality, since defects in GeSn layers degrade the electroluminescence intensity significantly. At the accumulation bias, the holes in the Ni gate electrode tunnel to the strained n-type GeSn layer through the ultrathin Al2O3 and recombine radiatively with electrons. The emission wavelength of photoluminescence and electroluminescence can be tuned by the Sn content.

Enhanced Exciton Binding Energy of ZnO by Long-Distance Perturbation of Doped Be Atoms.

The excitonic effect in semiconductors is sensitive to dopants. Origins of dopant-induced large variation in the exciton binding energy (E(b)) is not well understood and has never been systematically studied. We choose ZnO as a typical high-E(b) material, which is very promising in low-threshold lasing. To the best of our knowledge, its shortest wavelength electroluminescence lasing was realized by ZnO/BeZnO multiple quantum wells (MQWs). However, this exciting result is shadowed by a controversial E(b) enhancement claimed. In this Letter, we reveal that the claimed E(b) is sensible if we take Be-induced E(b) variation into account. Detailed first-principle investigation of the interaction between dopant atoms and the lattice shows that the enhancement mainly comes from the long-distance perturbation of doped Be atoms rather than the local effect of doping atoms. This is a joint work of experiment and calculation, which from the angle of methology paves the way for understanding and predicting the E(b) variation induced by doping.

Wavelength-tunable infrared light emitting diode based on ordered ZnO nanowire/Si1–x Ge x alloy heterojunction

A novel infrared light emitting diode (LED) based on an ordered p-n heterojunction built of a p-Si1–xGex alloy and n-ZnO nanowires has been developed. The electroluminescence (EL) emission of this LED is in the infrared range, which is dominated by the band gap of Si1–xGex alloy. The EL wavelength variation of the LED shows a red shift, which increases with increasing mole fraction of Ge. With Ge mole fractions of 0.18, 0.23 and 0.29, the average EL wavelengths are around 1,144, 1,162 and 1,185 nm, respectively. The observed magnitudes of the red shifts are consistent with theoretical calculations. Therefore, by modulating the mole fraction of Ge in the Si1–xGex alloy, we can adjust the band gap of the SiGe film and tune the emission wavelength of the fabricated LED. Such an IR LED device may have great potential applications in optical communication, environmental monitoring and biological and medical analyses.

Plasma induced by pulsed laser and fabrication of silicon nanostructures**Project supported by the National Natural Science Foundation of China (Grant Nos. 11264007 and 61465003).

It is interesting that in preparing process of nanosilicon by pulsed laser, the periodic diffraction pattern from plasmonic lattice structure in the Purcell cavity due to interaction between plasmons and photons is observed. This kind of plasmonic lattice structure confined in the cavity may be similar to the Wigner crystal structure. Emission manipulation on Si nanostructures fabricated by the plasmonic wave induced from pulsed laser is studied by using photoluminescence spectroscopy. The electronic localized states and surface bonding are characterized by several emission bands peaked near 600 nm and 700 nm on samples prepared in oxygen or nitrogen environment. The electroluminescence wavelength is measured in the telecom window on silicon film coated by ytterbium. The enhanced emission originates from surface localized states in band gap due to broken symmetry from some bonds on surface bulges produced by plasmonic wave in the cavity.

Organic solution-processible electroluminescent molecular glasses for non-doped standard red OLEDs with electrically stable chromaticity

Organic light-emitting molecular glasses (OEMGs) are synthesized through the introduction of nonplanar donor and branched aliphatic chain into electroluminescent emitters. The target OEMGs are characterized by 1H NMR, 13C NMR, IR, UV–vis and fluorescent spectra as well as elemental analysis, TG and DSC. The results indicated that the optical, electrochemical and electroluminescent properties of OEMGs are adjusted successfully by the replacement of electron-donating group. The non-doped OLED device with a standard red electroluminescent emission is achieved by spin-coating the THF solution of OEMG with a triphenylamine moiety. This non-doped red OLED device takes on an electrically stable electroluminescent performance, including the stable maximum electroluminescent wavelength of 640nm, the stable luminous efficiency of 2.4cd/A and the stable CIE1931 coordinate of (x, y)=(0.64, 0.35), which is basically in accord with the CIE1931 coordinate (x, y)=(0.64, 0.33) of standard red light in PAL system.

Interfacial Emission Adjustment in ZnO Quantum Dots/p-GaN Heterojunction Light-Emitting Diodes

Ultraviolet (UV) light-emitting diodes (LEDs) were made by using ZnO quantum dots (QDs) as the emission layer. ZnO QDs with the diameter of 5 nm were fabricated by using a simple sintering method. By using p-GaN as the hole injection layer, a ZnO QDs/p-GaN heterojunction LED was constructed. Trap-controlled SCLC behavior of QDs led the LED to emit light mainly from the QDs layer, and the direct physical contact between ZnO QDs and GaN could effectively reduce the interfacial emission. As the result, a UV LED with the electroluminescence wavelength of 382 nm has been achieved.

High brightness turquoise light-emitting diodes based on ZnO microwires

ZnO microwire clusters with green emission have been fabricated and the luminescence mechanism and relationship of the point defect and emission were studied. A turquoise ZnO microwire/n-GaN heterojunction LED with an electroluminescence wavelength of 490 nm was constructed.

White Light Emitting Polymers from a Luminogen with Local Polarity Induced Enhanced Emission

Aggregation induced enhanced emission (AIEE) is considered as an important tool to circumvent the aggregation caused quenching (ACQ) effect in organic light emitting diodes (OLEDs). Charge trapping and surplus long wavelength electroluminescence is a cause of concern in single polymer based white OLEDs. However, the potential of luminogens with AIEE property as a credible tool to offset the above problems in white light emitting single polymer is not properly explored. In this study design, synthesis and spectral characterization of a polymerizable luminogen, (2Z,2′Z)-6,6′-(2,7-dibromo-9H-fluorene-9,9-diyl)bis(hexane-6,1-diyl)bis(2-cyano-3-(10-hexyl-10H-phenothiazin-3-yl)acrylate(FCPA) with AIEE property and its copolymers is presented. Lippert-Mataga studies showed that reduced local polarity caused by aliphatic chains in condensed state of FCPA resulted in AIEE property. The copolymers P(FCPA-1) and P(FCPA-0.5) with 1% and 0.5% FCPA moieties showed white electroluminescence and enhanced thin film photol...

A dual‐character InGaN/GaN multiple quantum well device for electroluminescence and photovoltaic absorption of near‐mutually exclusive wavelengths

AbstractWe report on the dual‐character of an InGaN/GaN multiple quantum well (MQW) device structure epitaxially grown by MOCVD on a 150 mm Si‐on‐insulator (SOI) wafer. The fabricated mesa device exhibits photovoltaic (PV) response mainly for λ < 500 nm and strong electroluminescence (EL) for 470 nm < λ < 750 nm with a major amber component about 580 nm < λ < 620 nm. Multiple, high amplitude oscillations are observed in the luminescence spectra and verified by theoretical modelling as strong Fabry‐Perot interference effects. HR‐XRD ω ‐2θ (0002) scan estimates a mean In composition of ∼22% in the MQWs, consistent with the PV absorption edge and weak photoluminescence (PL) emission when excited by a 518 nm laser at 20K. Under 405 nm laser excitation at low temperatures, PL emission for 600 nm < λ < 740 nm is greatly enhanced. Asymmetric broadening of the spectral peak to 570 nm < λ < 740 nm occurs when the excitation energy is increased with a 325 nm laser, and is attributed to radiative transitions associated with MQW interface defects. This is supported by the (105) reciprocal space map which shows slight relaxation of the MQWs. The strong but broad long‐wavelength emission, is ascribed to the MQW interface defects and highly recombinative In‐rich clusters. These hardly contribute to PV response and are responsible for the protracted Stokes shift that constitutes the apparent near‐mutual exclusion of EL and PV absorption wavelengths. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

InGaN quantum dot green light-emitting diodes with negligible blue shift of electroluminescence peak wavelength

InGaN quantum dot (QD) light-emitting diodes (LEDs) were grown by metalorganic vapor phase epitaxy using an interruption method. As the injection current density increased from 2 to 88 A/cm2, the peak electroluminescence (EL) wavelength of the LED remained almost constant at around 527 nm. The negligible blue shift indicates that the quantum-confined Stark effect induced by piezoelectric polarization is suppressed well in InGaN QDs because of strain relaxation. Temperature-dependent EL spectra measurements indicate that the capture of electrons by QDs needs further improvement because of severe electron overflow to the p-type region. In addition, the peak EL wavelength is found to be abnormally longer than the photoluminescence wavelength.

Key role of ancillary ligands in imparting blue shift in electroluminescence wavelength in ruthenium polypyridyl light-emitting diodes

Ancillary ligand substitution proves to be an effective way to produce the blue shift of electroluminescence peak wavelength.

Uni-axial external stress effect on green InGaN/GaN multi-quantum-well light-emitting diodes

The influence of uni-axial external stress on green InGaN/GaN multi-quantum-well (MQW) light-emitting diodes (LEDs) is evaluated. LEDs with external tensile stress show an improvement in light output power up to 35% at an injection current of 20 mA. The maximum output power can be obtained by considering the residual mechanical stress induced by the wafer bowing. In contrast, when the LEDs were exposed to an external compressive stress, the light output power was reduced by ∼7% at an injection current of 20 mA. Moreover, when the compressive strain developed in InGaN/GaN MQW active region is relaxed, the peak wavelength of electroluminescence was blue-shifted. The results confirmed that applying external tensile stress effectively compensates for the compressive strain and alters the piezoelectric field in the InGaN/GaN active region, and hence increases the probability of radiative recombination.

Stress Distribution in GaN Films grown on Patterned Si (111) Substrates and Its Effect on LED Performance

Crack free GaN films were grown on 1200×1200μm2 patterned Si (111) substrates and 36 light emitting diodes (LEDs) were fabricated in each pattern unit. Spatial distribution of the tensile stress in the pattern units and its influence on the LED performance are studied by micro-Raman and electroluminescence (EL). The Raman shift of the GaN E2 mode shows that the tensile stress is the maximum at the center, partially relaxed at the edge, and further relaxed at the corner. With the stress relaxation, the EL wavelength has a significant blue shift and the luminous intensity shows a great enhancement.