EL Emission Research Articles

Vertical InGaN Light-Emitting Diode with Hybrid Distributed Bragg Reflectors.

A vertical-type InGaN light-emitting diode with a resonant cavity was demonstrated with a 9 μm aperture size and a short cavity formed by hybrid distributed Bragg reflectors (DBRs). The approach involved designing epitaxial structures and utilizing an electrochemical etching process to convert heavily doped n-type gallium nitride (n+-GaN) layers into porous GaN layers as a porous-GaN DBR structure. Thirteen pairs of the conductive porous-GaN:Si/GaN:Si DBR structure provided a vertical current path in a vertical-type light-emitting diodes (LED) structure. The LED epitaxial layers were separated from sapphire for membrane-type LED structures through a laser lift-off process. During the free-standing membrane fabrication process, the dielectric DBR deposited on ITO/p-GaN:Mg layers was inverted from top to bottom, thereby establishing the concept of higher reflectivity for the bottom DBR compared to the porous-GaN DBR. The physical cavity length was reduced from about 2.3 μm for the LED membrane to 0.74 μm for the membrane-type LED with the embedded porous-GaN DBR structure. The divergent angles and line width of EL emission light were reduced from 124°/31.7 nm to 44°/3.3 nm due to the resonant cavity effect. The membrane-type LED structures with hybrid DBRs consisted of small divergent angles, narrow line width, and vertical current injection properties that have potential for directional emission light sources and vertical-cavity surface-emitting diode laser applications.

Sterically Wrapped Oxygen‐Bridged Boron Derivatives for High‐Performance Blue Organic Light‐Emitting Diodes

AbstractNear ultraviolet (NUV) organic light‐emitting diodes (OLEDs) have great advantages in the field of light excitation sources, chemical and biological sensors, etc. However, the molecular design of NUV emitters still faces the challenges of spectral broadening and red‐shift. In this study, attaching weak oxygen‐bridged triarylboron acceptor to weak carbazolyl donor is able to address intractable problems by weakening donor–acceptor (D–A) charge transfer. Simultaneously, sterically wrapped and modified oxygen‐bridged triarylboron acceptor by bulk substituents (multiple carbazolyl) can suppress intermolecular π–π stacking through steric hindrance effect. These design strategies are able to obtain short‐wavelength, narrow‐band emission, and great color purity. As a result, carbazolyl derivatives xCz‐BO‐based (x = 2, 3, 4, 5) emitters show EL emission peaks from 406 to 422 nm in the NUV light region and small full‐width at half‐maximums (FWHMs) of 32–41 nm over a wide range of dopant concentrations (10–40 wt.%). The most violet Commission International de I'Eclairage (CIE) color coordinate is (0.16, 0.03). Moreover, 5Cz‐BO can be employed as host for vacuum‐/solution‐processed blue OLEDs and these devices exhibit high performance with EQEs of over 20%.

In-situ fabricated amorphous silicon quantum dots embedded in silicon nitride matrix: Photoluminescence control and electroluminescence device fabrication

Herein, we report that high-quality SiN:H films embedded with Si QDs were in-situ fabricated and prepared by the reactive facing target sputtering, which relies on tuning N2 and Ar mixture gas flow ratio. The size of Si QDs in SiN:H films can be tuned from 3.87 nm to 1.40 nm, which exhibit color-tunable photoluminescence from red to blue in the visible region. The microstructure evolution and optical properties of SiN:H films embedded with Si QDs were analyzed by FTIR, Raman, PL, and TRPL measurements, respectively. Considering their unique emission properties, SiN:H films embedded with Si QDs were especially utilized as the emitting layer for fabricating high-performance EL SiN-based LEDs. The broad EL emission properties of the devices were explored by both experimental observations and a theoretical model. This work offers a novel idea to prepare high-quality SiN:H films embedded with Si QDs and a deep understanding of their optoelectronic properties.

Efficient Room-Temperature Operation of a Quantum Dot Spin-Polarized Light-Emitting Diode under High-Bias Conditions

Spin-polarized light-emitting diodes (spin LEDs) are essential for the optical transfer of carrier-spin information in spin-based electronic circuits. Efficient operation under high-bias conditions is required for high-speed transfer of spin information and strong light emission. However, achieving high-electroluminescence (EL) circular polarization is hampered by electric-field-induced spin relaxation during drift transport in undoped $\mathrm{Ga}\mathrm{As}$ barriers. Herein, we demonstrate the efficient room-temperature operation of a spin LED using ($\mathrm{In},\mathrm{Ga})\mathrm{As}$ quantum dots (QDs) tunnel coupled with a 5-nm-thick $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ quantum well (QW). A high-EL circular polarization of approximately 7% is achieved, even under high-bias conditions, in comparison with the lower value of approximately 3% for the conventional QD spin LED without $\mathrm{Ga}(\mathrm{N},\mathrm{As})$. This is realized by an increase of spin polarization after injection into the QDs by utilizing remote spin filtering of QD electrons via an adjacent tunnel-coupled $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ QW. These results demonstrate that the tunnel-coupled structure of ($\mathrm{In},\mathrm{Ga})\mathrm{As}$ QDs and $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ QW is a promising active layer of spin LEDs for achieving both strong EL emission and high-EL circular polarization at room temperature.

(Digital Presentation) Strong Room-Temperature EL Emissions from Strained Sige/Ge-on-Si (111) LEDs

Introduction In recent years, encryption technology has become more and more important, and the method to use the circular polarized light in optical communications has been proposed. Spin LEDs are expected as on-chip circular polarized light generating devices, and we have been focusing on a Ge as an active material because it is capable of being monolithically integrated on a Si substrate and emitting light in the telecommunications wavelength band. Particularly, the (111) surface orientation of Ge allows for the epitaxial growth of high-quality ferromagnetic materials on Ge, making it possible to inject spin-polarized electrons from ferromagnetic materials into Ge (111) [1]. Furthermore, it has been proven that the use of strained-SiGe can increase the spin lifetime because of its ability to suppress intervalley scattering [2]. In this study, we fabricate strained-SiGe/Ge-on-Si (111) LEDs and obtain very strong EL emissions at room temperature. Experimental The sample was prepared using a solid source molecular beam epitaxy (MBE). First, a high-quality tensile-strained Ge layer was grown on a Si (111) substrate using two-step growth method, where a Ge buffer layer of 40 nm was grown on a p-type Si (111) substrate at a temperature of 350℃ and a Ge layer of 500 nm was grown at 700℃. Secondly, a Si0.15Ge0.85 layer of 40 nm and a P-doped Ge layer of 500 nm were grown at lower temperature of 350℃ to prevent relaxation of the SiGe. After the growth of the p-i-n structure, a P delta-doping layer of 2×1014 cm-2 was inserted to obtain a low resistivity Ohmic contact. Before P delta-doping, an ultra-thin Si (UT-Si) layer of 2 monolayers was inserted in order to avoid surface segregation of P atoms [3]. Finally, 7 nm thick Ge was grown for the capping. The fabricated multilayers were defined into a mesa diode by photolithography and reactive ion etching. AuSb and AuGa were deposited as top and back contacts, respectively. Results and Discussions Current-voltage output characteristics were evaluated and good diode characteristics were obtained. Figure 1 shows the room temperature EL spectra for the Si0.15Ge0.85/Ge LEDs with various injected currents. It was found that strong EL emissions covering the communication wavelength band of 1500-1600 nm was obtained. The EL peak intensity increases with the injected current and it is drastically enhanced above 350 mA as shown in inset of Fig. 1. We consider that recombination efficiency can be improved by the strained Si0.15Ge0.85 layer. While exact light emission wavelengths cannot be known with the current experiments due to the detector upper limit, characterizations in the longer wavelength range are now under investigation. Moreover, we expect further enhancements of EL intensity by introducing Si0.15Ge0.85/Ge multiple quantum wells in the active layer. It is also expected that by changing compositions of Si and Ge in SiGe, we can control the emission wavelength range, indicating that the SiGe/Ge LEDs are very attractive for application to on-chip circular polarized LEDs.This work was supported in part by Grant-in-Aid for Scientific Research (19H02175, 19H05616, 20K21009) from MEXT, Japan.[1] K. Hamaya et al., J. Phys. D: Appl. Phys. 51, 393001 (2018).[2] T. Naito et al., Phys. Rev. Appl. 13, 054025 (2020).[3] M. Yamada et al., Appl. Phys. Lett. 107. 132101 (2015). Figure 1

(Digital Presentation) Fabrication of Thick SiGe／Ge Multiple Quantum Wells on Ge-on-Si and Their Optical Properties

Recently, silicon photonics has been attracting increased attention toward the realization of on-chip optical interconnection, which requires high-efficiency light-emitting devices to be integrated on Si substrates. Although Ge is an indirect-band-gap semiconductor like Si, it is expected that strong direct-transition luminescence can be obtained from tensile-strain induced Ge-on-Si. We have previously reported that highly efficient room temperature EL emission can be obtained from strained Ge-on-Si p-i-n diodes[1]. Further increase in luminescence intensity and control of emission wavelengths can be made possible by introducing a quantum well structure in the active layer. Although the fabrication of high-quality strained SiGe layers on Ge is essential for the fabrication of quantum well structures, critical thickness limits the total growth thickness and numbers of strained layers, which makes device applications difficult. We have succeeded in fabricating a strained SiGe layer that is much thicker than the critical thickness by using a patterning method, and this result is very promising for the fabrication of SiGe/Ge multi-quantum well structure[2]. It also makes it possible to increase the number of layers of MQW structure. In this work, we fabricate relatively thick strained SiGe/Ge MQW structures on Ge-on-Si by utilizing the patterning method, and evaluate their crystallinity and optical properties.In experiments, the crystal growth was carried out with solid source molecular beam epitaxy. The Ge-on-Si was fabricated using the so-called two-step method. 40 and 500 nm thick Ge layers were subsequently grown on a Si(100) or Si(111) substrate at 350 and 600℃, respectively, followed by annealing at 800℃ for 10 min. Subsequently we carried out the patterning of the Ge-on-Si(111) substrate by photolithography process. 50nm thick Ge buffer layer ware grown on a Ge-on-Si(100) or Ge-on-Si(111), 6nm thick Si0.1Ge0.9 barrier layer and 10nm thick Ge well layer ware grown with 10-20 cycles at 350℃.Laser microscopic measurements showed no clear roughness on the surface. X ray diffraction measurements show periodic peaks originated from the multi layers of SiGe/Ge, which indicates that the sample has high crystallinity and abrupt interface between the Ge and SiGe layers. TEM observation indicated that no defect was found inside the crystal in the MQW structure. Photoluminescence (PL) measurements were carried out at room temperature and a PL peak originated from the quantum-confinement in the SiGe/Ge MQW was obtained. This PL peak wavelength was also found to shift depending on the Ge concentration of the SiGe. PL intensity of the peak was increased compared with Ge-on-Si without the SiGe/Ge MQW overgrowth. From these results, it is demonstrated that high quality SiGe/Ge multiple quantum structures can be fabricated on Ge-on-Si by utilizing the patterning method and strong room temperature PL was obtained from the MQW, indicating that SiGe/Ge MQW is promising for the applications to light-emitting devices integrated on the Si platform.[1] K.Yamada et al. Appl. Phys. Express 14 045504(2021) [2] Y.Wagatsuma et al. Appl. Phys. Express 14 025502(2021) Figure 1

(Digital Presentation) Strain Engineering of Heteroepitaxial SiGe/Ge on Si with Various Crystal Orientations

In recent years, Ge has emerged as one of the most promising materials toward next-generation high-performance and low-power-consumption optoelectronic devices. Moreover, SiGe/Ge heterostructures, consisting of strained SiGe with high Ge contents and Ge, have high potential for various applications. In terms of crystal orientations, there are various attractive choices for Ge and Ge-rich SiGe differently from Si. Particularly, the (111) orientation is very attractive for spintronics applications owing to capability of lattice matched epitaxial growth of high-quality ferromagnetic materials on the Ge(111) and SiGe(111). To create the SiGe/Ge heterostructures on the Si substrate, high quality Ge virtual substrates firstly have to be formed on the Si. The so-called Ge-on-Si is simple but very important templates for various applications, such as Ge-MOSFETs, LEDs, LDs, photo detectors, micro structures and Ge-on-Insulator (GOI) wafers etc. Especially the tensile strain induced in the Ge due to the thermal mismatch between the Ge and Si plays very important roles. For example, very efficient light emission can be obtained through strain-induced Γ-valley shift and resultant increase in the direct transition probabilities. We fabricated both Ge-on-Si(100) and Ge-on-Si(111) p-i-n LEDs and strong room-temperature EL emissions were obtained [1]. Epitaxial growth of the high quality strained SiGe on the Ge-on-Si is the most important for strain engineering of SiGe/Ge heterostrucutures. As the growth of the strained layer is limited by the critical thickness, we systematically investigated the critical thickness of the strained SiGe and clarified that it depends on the crystal orientations [2]. It was also shown that the critical thickness is markedly reduced when grown on the Ge-on-Si compared to the Ge substrate due to dislocations in the Ge-on-Si. We proposed to grow the SiGe on the Ge-on-Si on which the mesa patterning is performed. As a result, the critical thickness is drastically increased owing to the suppression of defect generation and propagation [3]. This patterning method enables us to increase the strained layer thickness and highly widen potential applicabilities of the strained SiGe/Ge heterostructures. One of the attractive applications is strained SiGe/Ge multi quantum wells (MQWs). We obtained strong roome temperature light emissions from the relatively thick MQWs formed on the Ge-on-Si (111) and Si(100) with patterning. Based on the Ge-on-Si, the larger tensile strain can be induced by the fabrication of microbridge structures, where the freestanding Ge is realized and elastic tensile stress is applied to the narrow microbridge area via the local strain relaxation of large pad areas sandwiching the bridge. In many reports, standard Si(100) substrates have been used with the bridge direction set to be <001> direction, whereas the Γ-valley shift via the tensile strain is expected to be the largest with the <111> directional uniaxial strain [4]. We attempt to fabricate the Ge microbridge along the <111> direction on the Ge-on-Si(110) substrate and very strong room-temperature PL was obtained. Moreover, when microbridge side walls are formed vertical, the generated light is well reflected by the side walls and very strong resonant light emissions are observed. Technically, the selective wet-etching of the underlying Si of the Ge-on-Si is a key process of importance to fabricate completely freestanding structures. As the crystal orientation highly affects the etching rate, a choice of the orientation is very important. Additionally, we are attempting other fabrication methods. One is making many holes in the pad areas, which enables to enhance the under-etching. Use of Ge-on-SOI or Ge-on-Insulator (GOI) substrates is another way to facilitate the under-etching as the buried oxide can be easily removed by the fast etching. Moreover, we show recent results on SiGe/Ge heterostructure imbedded microbridge, where the light emission wavelength becomes controllable and emission efficiency can be enhanced owing to the strain and band engineering.[1] K. Yamada et al. Appl. Phys. Express 14, 045504 (2021). [2] M. M. Alam, Appl. Phys. Express 12, 081005 (2019). [3] Y. Wagatsuma et al. Appl. Phys. Express 14, 025502 (2021). [4] H. Tahini et al. J. Phys. Condens. Matter 24, 195802 (2012).

Effective passivation of perovkiste grain boundaries by a carboxylated polythoiphene for bright and stable Pure-Red perovskite light emitting diodes

High luminance and operative stability are essential parameters for realizing the commercialization of perovskite-based light-emitting diodes (PLEDs). However, the optoelectronic properties of PLEDs are still limited owing to the un-optimized charge transport behavior of perovskite films arising from severe defect densities and imperfect lattice structure at grain boundaries. To retrieve the morphological imperfection, we herein suggest a carboxylated polythiophene (P3CT) as a novel passivating agent of perovskite films in PLEDs. The carboxylic acid groups of P3CT enable Lewis acid-base interactions with the defective surface of perovskites such as under-coordinated Pb2+ sites or halide deficiencies, leading to efficient defect passivation and stabilization of the perovskite film surface. The P3CT-passivation successfully fabricates pinhole-free perovskite films with diminished defects at grain boundaries, bringing enhanced optoelectronic properties of red electroluminescence EL emission with a peak current efficiency and external quantum efficiency (EQE) values up to 11.15 % and 4 %, respectively. Moreover, the operative stability of the passivated PLEDs is markedly improved due to the stabilization and increased hydrophobicity of perovskite films by the inclusion of P3CT. The present findings provide a new perspective for developing stable red-PLEDs with efficient luminescence.

Tuning optical properties of CsPbBr3 by mixing Nd3+ trivalent lanthanide halide cations for blue light emitting devices

In recent years, significant progress has been made in the red and green perovskite quantum dots (PQDs) based light-emitting devices. However, a scarcity of blue-emitting devices that are extremely efficient precludes their research and development for optoelectronic applications. Taking advantage of tunable bandgaps of PQDs over the entire visible spectrum, herein we tune optical properties of CSPbBr3 by mixing Nd3+ trivalent lanthanide halide cations for blue light-emitting devices. The CsPbBr3 PQDs doped with Nd3+ trivalent lanthanide halide cations emitted strong photoemission from green into the blue region. By adjusting their doping concentration, a tunable wavelength from (515 nm) to (450 nm) was achieved with FWHM from (37.83 nm) to (16.6 nm). We simultaneously observed PL linewidth broadening thermal quenching of PL and the blue shift of the optical bandgap from temperature-dependent PL studies. The Nd3+ cations into CsPbBr3 PQDs more efficiently reduced non-radiative recombination. As a result of the efficient removal of defects from PQDs, the photoluminescence quantum yield (PLQY) has been significantly increased to 91% in the blue-emitting region. Significantly, Nd3+ PQDs exhibit excellent long-term stability against the external environment, including water, temperature, and ultraviolet light irradiation. Moreover, we successfully transformed Nd3+ doped PQDs into highly fluorescent nanocomposites. Incorporating these findings, we fabricate and test a stable blue light-emitting LED with EL emission at (462 nm), (475 nm), and successfully produce white light emission from Nd3+ doped nanocomposites with a CIE at (0.32, 0.34), respectively. The findings imply that low-cost Nd3+ doped perovskites may be attractive as light converters in LCDs with a broad color gamut.

Monomeric carbazolylcyanobenzenes as thermally activated delayed fluorescence emitters: effect of substitution position on photoluminescent and electroluminescent properties

In this study, two new donor-acceptor type thermally activated delayed fluorescence materials using carbazole as a donor and cyano substituted benzene as an acceptor were synthesized. Structural parameters, molecular orbital densities, photophysical characteristics were determined. By changing the substitution position of the cyano groups on the phenyl rings, the relationship between structure and properties was also be disclosed. These materials were tested as emissive species for the fabrication of organic light-emitting diodes. The blue devices showed EL emission peaks at 464 and 488 nm with the CIE coordinates of (0.17, 0.18) and (0.21, 0.31), respectively.

Identification of multi-color emission from coaxial GaInN/GaN multiple-quantum-shell nanowire LEDs.

Multi-color emission from coaxial GaInN/GaN multiple-quantum-shell (MQS) nanowire-based light-emitting diodes (LEDs) was identified. In this study, MQS nanowire samples for LED processes were selectively grown on patterned commercial GaN/sapphire substrates using metal–organic chemical vapor deposition. Three electroluminescence (EL) emission peaks (440, 540, and 630 nm) were observed, which were primarily attributed to the nonpolar m-planes, semipolar r-planes, and the polar c-plane tips of nanowire arrays. A modified epitaxial growth sequence with improved crystalline quality for MQSs was used to effectively narrow the EL emission peaks. Specifically, nanowire-based LEDs manifested a clear redshift from 430 nm to 520 nm upon insertion of AlGaN spacers after the growth of each GaInN quantum well. This demonstrates the feasibility of lengthening the EL emission wavelength since an AlGaN spacer can suppress In decomposition of the GaInN quantum wells during ramping up the growth temperature for GaN barriers. EL spectra showed stable emission peaks as a function of the injection current, verifying the critical feature of the non-polarization of GaN/GaInN MQSs on nanowires. In addition, by comparing EL and photoluminescence spectra, the yellow-red emission linked to the In-fluctuation and point defects in the c-plane MQS was verified by varying the activation annealing time and lowering the growth temperature of the GaInN quantum wells. Therefore, optimization of MQS nanowire growth with a high quality of c-planes is considered critical for improving the luminous efficiency of nanowire-based micro-LEDs/white LEDs.

Green multi-resonance thermally activated delayed fluorescence emitters containing phenoxazine units with highly efficient electroluminescence

Two phenoxazine fused MR-TADF materials based on a B/N framework show high PLQYs and small FWHMs in toluene solutions, while their OLEDs exhibit EQEs of up to 21.3% with small efficiency roll-offs and narrow EL emissions.

Spectrally Stable and Efficient Pure Red CsPbI3 Quantum Dot Light-Emitting Diodes Enabled by Sequential Ligand Post-Treatment Strategy.

Metal halide perovskites are promising semiconductors for next-generation light-emitting diodes (LEDs) due to their high luminance, excellent color purity, and handily tunable band gap. However, it remains a great challenge to develop perovskite LEDs (PeLEDs) with pure red emission at the wavelength of 630 nm. Herein, we report a spectrally stable and efficient pure red PeLED by employing sequential ligand post-treated CsPbI3 quantum dots (QDs). The synthesized CsPbI3 QDs with a size of ∼5 nm are treated in sequential steps using the ligands of 1-hydroxy-3-phenylpropan-2-aminium iodide (HPAI) and tributylsulfonium iodide (TBSI), respectively. The CsPbI3 QD films exhibit improved optoelectronic properties, which enables the fabrication of a pure red PeLED with a peak external quantum efficiency (EQE) of 6.4% and a stable EL emission centered at the wavelength of 630 nm. Our reported sequential ligand post-treatment strategy opens a new route to improve the stability and efficiency of PeLEDs based on QDs.

White-emission from ZnS:Eu incorporated in AC-driven electroluminescent devices via ultrasonic spray pyrolysis

In this work, white-emitting-alternating-current-thin film electroluminescent (w-ACTFEL) devices are demonstrated using europium-doped zinc sulfide (ZnS:Eu) and zirconium oxide (ZrO2) as the emissive and dielectric layers, respectively. These films were deposited by the ultrasonic spray pyrolysis technique on antimony-doped tin oxide glass substrates, forming a standard metal-insulator-semiconductor-insulator-metal (MISIM) architecture. 10 kHz sinusoidal voltages activated the white-EL of the devices. The colorimetric characteristics were investigated for three amplitudes of the applied voltage. The emission of the devices is made up of wide and narrow bands with peaks corresponding to violet, blue, green and red light, which together produce the resulting white light. According to the colorimetric analysis, this white light is close to the standard D65 CIE illuminant with a minimal dominant blue component. The variation in voltage amplitude induces small changes in the visual characteristics of the EL emission. The white-EL emission of these MISIM devices is attributed to the electron-impact excitation and subsequent relaxation of the excited levels of Eu2+ and Eu3+ impurities, and defect levels in the sublayer regions adjacent to the ZrO2–ZnS:Eu interfaces.

Ultraviolet Light-Emitting Diodes Using Aluminium Indium Nitride Nanowire Structures

Ultraviolet (UV) light-emitting diodes (LEDs) have various applications in the field of biochemistry, medicine, water/air purification, etc. To achieve efficient UV LEDs, researchers have essentially focused on the use of AlGaN alloys in the active region. However, the practical applications of AlGaN based UV LEDs are severely limited due to poor external quantum efficiency (EQE). Some reasons behind the low EQE in AlGaN based UV LEDs are efficiency droop, mostly due to electron leakage into the p-region, poor light extraction efficiency (LEE) which is mainly due to the total internal reflection and the absorption of UV light in the p-GaN contact layer. Identifying and developing the potential of alternative UV materials will be critical to make further progress in the development of deep UV emitters. In this regard, AlxIn1-xN alloy has not been widely studied even though it holds great potential applications in UV and visible light-emitting devices [1]. In this study, we have performed the epitaxial growth, electrical and optical properties of spontaneously grown AlInN nanowires on Si (111) substrate by molecular beam epitaxy (MBE) and demonstrated a high crystalline quality of AlInN nanowire LED structures with stable and strong EL emission at ~299 nm peak wavelength.The AlInN nanowire LED structure, shown in Fig. 1(a), was spontaneously grown on n-Si (111) substrates under nitrogen rich conditions by a Veeco GEN II MBE system equipped with a RF plasma-assisted nitrogen source. The growth conditions for GaN nanowire segments include a growth temperature of ~ 770 °C, nitrogen flow rate of 1 sccm, and forward plasma power of ~ 400 W. The device active region consists of ~ 100 nm n-Al0.78In0.22N, 40 nm undoped Al0.75In0.25N well and 100 nm p-Al0.78In0.22N layers. Growth temperature of the active region was at ~ 670 ̶ 700 °C to increase the In incorporation in AlInN segments and the nitrogen flow rate was kept at 2.5 sccm, while the plasma power was fixed at 400 W. After being grown by MBE, the nanowire LED samples were fabricated using standard lithography method as reported elsewhere [2]. The device characterization was performed on LED with an area size of 500×500 µm2. Photoluminescence (PL) spectrum of the grown nanowire LEDs exhibits strong emission in the UV wavelength range as shown in Fig. 1(b). The peak emission at ~ 299 nm and ~ 280 nm is responsible for the emission from the AlInN quantum well and barriers, respectively. The AlInN nanowire UV LEDs have strong EL intensity with the peak wavelength at ~ 299 nm, as shown in Fig. 1(c). This work is being developed toward the achievement of high power AlInN nanowire UV LEDs, and the result will be presented. Reference R. T. Velpula, B. Jain, M. R. Philip et al., Sci. Rep., 10(1) 1-10 (2020).R. T. Velpula, B. Jain, H. Q. T. Bui et al., Optical Materials Express, 10(2) 472-483 (2020) Figure 1

Tuning electroluminescence performance in Pr-doped piezoelectric bulk ceramics and composites

Alternating current electroluminescence (ACEL) shows great potential in lighting, display and intelligent skin. Herein, the ACEL properties of Pr3+-doped Ba0·85Ca0·15Ti0·90Zr0·10O3 ceramic and its PDMS-based composite have been investigated. Intense red EL emission was obtained in the ceramic sample whereas blue EL emission of Pr3+ was observed for the first time in the composite counterpart. The red EL emission should be attributed to the impact of hot electrons driven by the large piezoelectric electric field. Owing to the cross-relaxations through the 4f5d levels and Pr-to-metal charge transfer state, the 3PJ emissions were completely quenched, and thus leading to an enhancement in red emission. However, external E field induced a large local piezoelectric deformation of the ceramic particles embedded in the PDMS matrix, which in turn caused a bending of CB and then a downwards shift of the 4f5d levels from the CB. Hence the cross-relaxations were hindered, and the blue EL emission was observed in the composites. The results would attract attention of functional materials studies and expand our understanding of such facile structure and oxide EL devices to facilitate their use in integral part of flexible device systems.

Deep-blue fluorescent emitter based on a 9,9-dioctylfluorene bridge with a hybridized local and charge-transfer excited state for organic light-emitting devices with EQE exceeding 8%

The doped device of TPA-DFCP exhibits excellent deep-blue emission with the EL emission peak at 436 nm and the CIE coordinates at (0.153, 0.077), and excellent EL performance with a maximum external quantum efficiency (EQE) of 8.30%.

The design, synthesis and performance of thermally activated delayed fluorescence macromolecules

The design, synthesis and performance of thermally activated delayed fluorescence macromolecules are summarized, and the typical solution-processed polymeric and dendritic emitters are also organized herein as a function of EL emission color.

Aggregation-state engineering and emission switching in D–A–D′ AIEgens featuring dual emission, MCL and white electroluminescence

D–A–D′ AIEgens exhibit dual emission involving a blue TADF (monomer) and an orange TADF/RTP (dimer), which are switchable via aggregation-state engineering, and demonstrate high contrast MCL as well as white EL emission with high CRI.

Uniform and bright light emission from a 3D organic light-emitting device fabricated on a bi-convex lens by a vortex-flow-assisted solution-coating method

We herein present the results of a study on the novel fabrication process of uniform and homogeneous semiconducting polymer layers, in this case hole-injecting and fluorescent light-emitting layers that were produced by a simple solution-coating process for 3D conformal organic light-emitting diodes (3D OLEDs) on curvilinear surfaces. The solution-coating process used was a newly developed method of vortex-flow-assisted solution-coating with the support of spinning of the coating solution. It is shown that the vortex-flow-assisted spin-coating process can produce high-quality thin films at nanoscale thicknesses by controlling the liquid surface of the coating solutions, which can easily be adjusted by changing the spinning speed, even on complex curvilinear surfaces, i.e., a quasi-omnidirectional coating. This excellent film-forming ability without any serious film defects is mainly due to the reduction of line tension among the solution, air, and the substrate at the contact line due to vortex flows of the coating solution on the substrate during the vortex-spin-coating process. As a proof of concept, we present vortex-spin-coated 3D OLEDs fabricated on bi-convex lens substrates which exhibit excellent device performance with high brightness and current efficiency levels comparable to those of a conventional spin-coated 2D planar OLED on a flat substrate. It is also shown that the EL emission from the 3D OLED on the bi-convex lens substrate exhibits a diffusive Lambertian radiation pattern. The results here demonstrate that the vortex-flow-assisted spin-coating process is a promising approach for producing efficient and reliable next-generation OLEDs for 3D conformal opto-electronics.