Low-cost Light-emitting Diodes Research Articles

Optical Amplification at 637 and 1067 nm Based on Organic Molecule AQ(PhDPA)2 and NdIII Complex Codoped Polymer Waveguides (Small Methods 4/2023)

Front Cover In article number 2201366, Hui Xu, Dan Zhang, and co-workers demonstrate an innovative strategy of realizing dual-wavelength simultaneous amplification in thermally activated delayed fluorescence molecule and NdIII complex co-doped polymer waveguides. The molecular energy transfer mechanism makes it possible for the low-cost light-emitting diodes to replace the traditional lasers as the pump sources of the optical waveguide amplifiers.

Enhanced Photoredox Activity of BiVO4/Prussian Blue Nanocomposites for Efficient Pollutant Removal from Aqueous Media under Low-Cost LEDs Illumination

Bismuth vanadate (BiVO4, BV) is a widely explored photocatalyst for photo(electro)chemical applications, but its full photocatalytic potential is hindered by the fast recombination and low mobility of photogenerated charge carriers. Herein, we propose the photodeposition of different amounts of Prussian blue (PB) cocatalysts on the surface of monoclinic BV to obtain BV-PB composite photocatalysts with increased photoactivity. The as-prepared BV and BV-PB composites were characterized by an array of analytic techniques such scanning eletron microscopy (SEM), transmission eletron microscopy (TEM), X-day diffraction (XRD), and spectroscopic techniques including Fourier-transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS), electrochemical impedance spectroscopy (EIS), photoluminescence (PL), and Raman spectroscopy. The addition of PB not only increases the absorption of visible light, as indicated by DRS, but also improves the charge carriers’ transfer across the photocatalysts/solution interface and hence reduces electron-hole (e−-h+) recombination, as confirmed by EIS and PL measurements. Resultantly, the BV-PB composite photocatalysts with optimum PB loading exhibited enhanced Cr(VI) photoreduction efficiency as compared to pristine BV under visible light illumination from low-power blue light-emitting diodes (LEDs), thanks to the cocatalyst role of PB which mediates the transfer of photoexcited conduction band (CB) electrons from BV to Cr(VI) species in solution. Moreover, as compared to pristine BV and BV + H2O2, a drastic increase in the methylene blue (MB) photo-oxidation efficiency was observed for BV-PB in the presence of a minute quantity of H2O2 due to a synergic effect between the photocatalytic and Fenton-like processes. While pure BV photodegraded around 70% of MB dye within 120 min, the BV-PB/H2O2 and BV/H2O2 system could degrade almost 100% of the dye within 20 min (kobs. = 0.375 min−1) and 40 min (kobs. = 0.055 min−1), respectively. The practical approach employed in this work may pioneer new prospects for synthesizing new BV-based photocatalytic systems with low production costs and high photoredox efficiencies.

Efficient single-component white light emitting diodes enabled by lanthanide ions doped lead halide perovskites via controlling Förster energy transfer and specific defect clearance

Currently, a major challenge for metal-halide perovskite light emitting diodes (LEDs) is to achieve stable and efficient white light emission due to halide ion segregation. Herein, we report a promising method to fabricate white perovskite LEDs using lanthanide (Ln3+) ions doped CsPbCl3 perovskite nanocrystals (PeNCs). First, K+ ions are doped into the lattice to tune the perovskite bandgap by partially substituting Cs+ ions, which are well matched to the transition energy of some Ln3+ ions from the ground state to the excited state, thereby greatly improving the Förster energy transfer efficiency from excitons to Ln3+ ions. Then, creatine phosphate (CP), a phospholipid widely found in organisms, serves as a tightly binding surface-capping multi-functional ligand which regulates the film formation and enhances the optical and electrical properties of PeNC film. Consequently, the Eu3+ doped PeNCs based-white LEDs show a peak luminance of 1678 cd m-2 and a maximum external quantum efficiency (EQE) of 5.4%, demonstrating excellent performance among existing white PeNC LEDs from a single chip. Furthermore, the method of bandgap modulation and the defect passivation were generalized to other Ln3+ ions doped perovskite LEDs and successfully obtained improved electroluminescence (EL). This work demonstrates the comprehensive and universal strategies in the realization of highly efficient and stable white LEDs via single-component Ln3+ ions doped PeNCs, which provides an optimal solution for the development of low-cost and simple white perovskite LEDs.

(Invited) InGaN/AlGaN Red-Emitting Nanowire LEDs for Monolithic Micro-LED Displays

High efficiency, high color rendition and low-cost light-emitting diodes (LEDs) with low power consumption, long lifetime and high reliability are highly expected for general lighting illumination, smartphones, smartwatches, virtual reality (VR), augmented reality (AR) headsets, and micro-display applications. Nonetheless, the achievement of deep green to red-emitting LEDs using conventional III-nitride quantum well heterostructures has been difficult, due to the presence of large densities of dislocations, strong polarization fields, poor hole transport, and carrier delocalization [1]. The external quantum efficiency (EQE) of the blue and green InGaN LEDs surpassed 80% and 53%, respectively. However, due to the aforementioned issues, the current InGaN red LEDs with high Indium composition exhibit extremely low EQE which is less than 3% [2]. In this regard, LEDs using nanowire structures offer dramatically reduced strain-induced polarization fields and dislocation densities, providing ideal material structure for high efficiency full-color and even white light emission without using phosphor-converters. In this study, we have successfully designed, and fabricated high efficiency red-emitting nanowire heterostructures and demonstrated micro-LEDs with stable and strong emission at ~645 nm. Moreover, the micro-LEDs have high internal quantum efficiency of >40%.The InGaN/AlGaN nanowire micro-LED structures are grown by RF plasma-assisted molecular beam epitaxy (MBE) under nitrogen-rich condition. The LED structure consists of a 250 nm n-GaN nanowire template, 10 couples of 3 nm AlGaN quantum barrier (QB)/ 3 nm InGaN quantum well (QW) served as the active region, and a 200 nm p-GaN. During the epitaxial growth of AlGaN barriers, an AlGaN shell spontaneously forms, enabling a unique InGaN/AlGaN core-shell structure [3]. The emission color of the micro-LEDs can be defined by controlling the ratio of Ga/In flux and the substrate temperature during the MBE growth process. Detailed growth conditions and the device fabrication can be found elsewhere [3-5]. The nanowires are uniformly arranged on Si substrates, as illustrated in Figure 1(a). Figure 1(b) shows the schematic structure of the fabricated micro-LEDs. Strong red emissions were measured from the InGaN/AlGaN core-shell LEDs, as shown in Figure 1(c). At injection current of 400mA, the peak emission wavelength is at ~645nm. The red-emitting micro-LEDs exhibit stable emissions with a blue-shift of only ~ 4nm under injection current from 50 mA to 400 mA, attributed to the significantly reduced quantum-confined Stark effect (QCSE) in the nanowire structures. Moreover, full color micro-LEDs with device size from 10x10 µm2 to 100x100 µm2 have been fabricated using similar approach. Such high efficiency, high color rendering properties, and low power consumption micro-LEDs are promising candidates for emerging AR/VR devices and micro-LED display applications.

Vegetation detection based on spectral information and development of a low-cost vegetation sensor for selective spraying.

Real-time site-specific selective spraying for crop protection against weeds, pests and diseases requires fast and precise detection of targets. This study investigated several aspects of vegetation target detection based on spectral information and developed a low-cost vegetation sensor. Specific objectives included: (1) compare vegetation-detection performance using reflected and fluorescence data; (2) evaluate the effect of light source and bandwidth, and select the optimal light source and waveband; and (3) develop a low-cost vegetation sensor and test its performance under different light conditions. The main outcomes of this study are as follows. (1) The fluorescence excited by blue and red light-emitting diodes (LEDs) was an effective attribute with which to differentiate vegetation from non-vegetation and provided results comparative with reflected spectroscopy. (2) A blue LED could excite strong fluorescence and was recommended as the light source. (3) Under illumination by blue and red LEDs, bandwidth did not have any obvious effect on classification accuracy within the studied bandwidths from 0nm (single wavelength) to 120 nm. (4) A low-cost vegetation sensor was developed and tested, with 100% detection accuracy in both dark and outdoor environments. This study suggests that single-waveband fluorescence spectroscopy is an effective approach to detect vegetation targets and low-cost LEDs can be used for illumination. Light source modulation with a sinusoidal signal is an effective way to resist the influence of environmental light. Overall, the findings of this study contribute to an improved understanding of developing low-cost and effective vegetation sensors for selective spraying from theory to application. © 2022 Society of Chemical Industry.

Chirality Transfer from Chiral Mesoporous Silica to Perovskite CsPbBr 3 Nanocrystals: The Role of Chiral Confinement

Chirality Transfer from Chiral Mesoporous Silica to Perovskite CsPbBr ₃ Nanocrystals: The Role of Chiral Confinement

Benefits of using TiO2 Quantum Dots in producing low-cost and high-quality white LEDs

Quantum dots (QDs) is considered as a potential material for the improvement of light-emitting diodes (LEDs). However, different from the traditional phosphor materials, they have unique scattering and absorption properties affected by their several nanometers sizes, which makes their application in the production of LED confronts more challenges. In addition to this, the influence of QDs on QDs-converted LEDs (QCLEDs) is barely studied. In order to propose solutions for those problems, in this article, we carried out experimental and theoretical investigation of the effect of TiO 2 QDs’ scattering and absorption on the optical performance of QCLEDs by comparing their properties with the traditional yttrium aluminum garnet phosphors’. The results showed that the strong absorption (reabsorption) of QDs is the cause of low radiant efficacy and stability of QCLEDs, and their weak scattering ability results in a low color homogeneity. For achieving high efficiency and stability white LEDs, we highly suggest using a low QD concentration to get reductions in the reabsorption loss and the total internal reflection loss. With 0.05 concentration of TiO 2 NPs, the white LEDs can achieve a high CCT of ∼ 7500 K and a high CRI of ∼ 85 simultaneously. Moreover, this TiO 2 concentration can also increase the luminous intensity by ∼ 31%. Therefore, we believe that the study can propose a potential approach to the production of low-cost CDs-based LEDs with high performance in a near future.

Optical gain based on NaYF4: Er3+, Yb3+ nanoparticles-doped polymer waveguide under convenient LED pumping

The relative optical gains at three near infrared wavelengths (1550 nm, 1064 nm, and 980 nm) were achieved in NaYF4: Er3+, Yb3+ nanoparticle-doped SU-8 waveguides when using two low-cost light-emitting diodes (LEDs) instead of traditional 980 nm semiconductor laser as pump source. The polymer waveguides were fabricated by one-step photolithography process. The fluorescence bands around 1550 nm and 1000 nm wavelengths due to the 4I13/2 → 4I15/2 transition of Er3+ ions and 2F5/2 → 2F7/2 transition of Yb3+ ions were observed under the excitation of 405 nm and 520 nm LEDs. By using the vertical top pumping mode of LEDs, the relative gains of 4.2 dB, 1.7 dB, and 2.1 dB at 1550 nm, 1064 nm, and 980 nm wavelengths were achieved, respectively, on a 10-mm-long waveguide.

Practical Implementation of an Indoor Robot Localization and Identification System using an Array of Anchor Nodes

This paper proposes a low-cost Light Emitting Diodes (LED) system with a novel arrangement that allows an indoor multi-robot localization. The proposed system uses only a matrix of low-cost LED installed uniformly on the ground of an environment and low-cost Light Dependent Resistor (LDR), each equipped on bottom of the robot for detection. The matrix of LEDs which are driven by a modified binary search algorithm are used as active beacons. The robot localizes itself based on the signals it receives from a group of neighbor LEDs. The minimum bounded circle algorithm is used to draw a virtual circle from the information collected from the neighbor LEDs and the center of this circle represents the robot’s location. The propose system is practically implemented on an environment with (16*16) matrix of LEDs. The experimental results show good performance in the localization process.

Carrierless amplitude and phase modulation in wireless visible light communication systems.

Visible light communications (VLCs) have attracted considerable interest in recent years owing to the potential to simultaneously achieve data transmission and illumination using low-cost light-emitting diodes (LEDs). However, the high-speed capability of such links is typically limited by the low bandwidth of LEDs. As a result, spectrally efficient advanced modulation formats have been considered for use in VLC links in order to mitigate this issue and enable higher data rates. Carrierless amplitude and phase (CAP) modulation is one such spectrally efficient scheme that has attracted significant interest in recent years owing to its good potential and practical implementation. In this paper, we introduce the basic features of CAP modulation and review its use in the context of indoor VLC systems. We describe some of its attributes and inherent limitations, present related advances aiming to improve its performance and potential and report on recent experimental demonstrations of LED-based VLC links employing CAP modulation. This article is part of the theme issue 'Optical wireless communication'.

Stable and bright formamidinium-based perovskite light-emitting diodes with high energy conversion efficiency

Solution-processable perovskites show highly emissive and good charge transport, making them attractive for low-cost light-emitting diodes (LEDs) with high energy conversion efficiencies. Despite recent advances in device efficiency, the stability of perovskite LEDs is still a major obstacle. Here, we demonstrate stable and bright perovskite LEDs with high energy conversion efficiencies by optimizing formamidinium lead iodide films. Our LEDs show an energy conversion efficiency of 10.7%, and an external quantum efficiency of 14.2% without outcoupling enhancement through controlling the concentration of the precursor solutions. The device shows low efficiency droop, i.e. 8.3% energy conversion efficiency and 14.0% external quantum efficiency at a current density of 300 mA cm−2, making the device more efficient than state-of-the-art organic and quantum-dot LEDs at high current densities. Furthermore, the half-lifetime of device with benzylamine treatment is 23.7 hr under a current density of 100 mA cm−2, comparable to the lifetime of near-infrared organic LEDs.

Enhanced surface and optical properties of colloidal silver nano-particles on GaN-based light-emitting diodes by a localized surface plasmon resonance effect using a low-cost metal-assisted chemical etching method

This study examined the characteristics of localized surface plasmon resonance (LSPR)-enhanced GaN-based light-emitting diodes (LEDs) fabricated using colloidal silver nanoparticles (cAg NPs). cAg NPs were deposited on a p-GaN surface using a low-cost metal-assisted chemical etching (MAC-etch) method. MAC-etching can increase the light extraction efficiency (LEE) by producing a roughened p-GaN surface and LSPR produces high internal quantum efficiency (IQE) through an interaction of cAg NPs with the emitted light. Field-emission scanning electron microscopy and atomic force microscopy of the surface revealed an etched p-GaN surface with cAg NPs and a roughness of ∼40-45 nm. In the MAC-etch samples, XRD showed that the cAg NPs had (111) and (200) preferred orientations. In addition, the calculated grain size of the 60 min cAg NPs was 50% of the grain size of the 30 min sample. The optical characteristics of the samples were measured by photoluminescence (PL) spectroscopy, ultraviolet/visible (UV/Vis) spectroscopy, and electroluminescence (EL) spectroscopy. The MAC-etch samples showed a higher PL intensity than the normal-LEDs. The UV/Vis absorbance spectra showed that the cAg NPs on the p-GaN surface generated LSPR. The EL intensity of the MAC-etch sample for 30 min increased by approximately 25%. The results of this investigation herald the development of MAC-etching for the low-cost GaN-based LED applications that are commercially viable large-scale.

Ultra-Low Cost High-Density Two-Dimensional Visible-Light Optical Interconnects

Visible light communications have attracted considerable interest in recent years owing to the ability of low-cost light emitting diodes (LEDs) to act both as illumination sources and data transmitters with moderate data transmission rates. In this paper, we propose the formation of ultra-low cost visible-light integrated optical links by interfacing dense micro-pixelated LED arrays with matching multi-layered multimode polymer waveguide arrays. The combination of these two optical technologies can offer relatively high aggregate data densities ≥ 0.5 Tb/s/mm2 using very low cost components that can be directly interfaced with CMOS electronics and integrated onto standard printed circuit boards. Here, we present the basic system design and report the first proof-of-principle demonstration of such a visible light system employing 4×4 μLED arrays on a pitch matching four-layered waveguide array samples. Different interconnection topologies and light coupling schemes are investigated and their performance in terms of loss and crosstalk is compared. Data transmission of 2.5 Gb/s with a bit error rate within the forward-error correction threshold of 3.8×10−3 is achieved over a single μLED-waveguide channel using PAM-4 modulation and equalization. The results presented here demonstrate the potential of such ultra-low cost visible-light optical interconnects.

Bright and Blue Nanocrystal Emitters

The discovery and development of blue light emitting diodes (LEDs) enabled energy-efficient white light emission that continues to impact global energy consumption today. Alternative materials that can outcompete commercial blue LEDs based on epitaxial InGaN could have a large impact on the lighting industry and in curbing anthropogenic climate change. Recently in Joule, Congreve and colleagues show that metal halide perovskite nanocrystals—greatly improved by Mn incorporation—have promise for low-cost, efficient, and bright blue LEDs.

Structural characteristics of a non-polar ZnS layer on a ZnO buffer layer formed on a sapphire substrate by mist chemical vapor deposition

ZnS is attractive as a material for low-cost light-emitting diodes. In this study, a non-polar ZnS layer was epitaxially grown on a sapphire substrate by inserting a ZnO buffer layer between ZnS and sapphire. The ZnS and ZnO layers were grown by a mist chemical vapor deposition system with a simple setup operated under atmospheric pressure. The sample was characterized by high-resolution X-ray diffraction measurements including 2θ/ω scans, rocking curves, and reciprocal space mapping. The results showed that an m-plane wurtzite ZnS layer grew epitaxially on an m-plane wurtzite ZnO buffer layer formed on the m-plane sapphire substrate to provide a ZnS/ZnO/sapphire structure.

Polystyrene based perovskite light emitting diode

Hybrid organic–inorganic semiconductors (HOIS) based on lead halides exhibit unique optoelectronic properties even at room temperature, immediately assigning these for potential applications as optical and active device materials, where most common applications have been light emitting diodes (LED), solar cell devices and X-ray detectors. In this work, a low cost and simple prototype single layer LED emitting green light under low voltage, is presented. The main advantage in this work's LED is that the active layer is based on a blend of low dimensional (LD) perovskites entrapped in a polystyrene film, never reported before. The HOIS blend is formed by the admixture of two-dimensional (2D) (C6H5CH2CH2NH3)2PbBr4, one dimensional (1D) CH3NH3CdBr3, while the blend's entrapment is afforded by mixing it with dissolved polystyrene powder. It is found that final composite contains a blend of quasi two-dimensional (quasi-2D) perovskites, that exhibits green luminescence and electroluminescence, assisted by energy funneling phenomena toward the blend's moiety with the smallest energy band gap. The addition of Cd++ is reported as a paradigm for several effects; its addition is not inhibiting the electroluminescence, while also polystyrene successfully prevents even cadmium halide's degradation by moisture. Moreover, successful doping of the LED active material portrays possibilities for enhancing perovskite-based devices, as it has been shown that cation dopants enhance the photoluminescence or transfer energy toward the dopant's light emission energy bands.

Investigation of the Emission Spectral Properties of Carbon Dots in Packaged LEDs Using TiO2 Nanoparticles

We examined light-emitting diodes (LEDs) packaged with silicone encapsulation of carbon dots (CDs) that is doped using TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles (TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs). The results reveal that the adjustable emission spectra in a wide wavelength range (peak wavelength varies from ~430 to ~600 nm) can be obtained by controlling the amount of TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs for CDs embedded in the silicone encapsulant, and that the various colors of LEDs can be produced using CDs at a concentration of 10 wt%. Moreover, the luminous intensity can be increased by 31% at a TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs concentration of 0.05 wt%. These optical characteristics of CDs-based LEDs emerge from the enhanced light scattering ability of TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NPs, which increases the light reabsorption and extraction probability. Therefore, the proposed method has great potential for achieving low-cost and high-performance CDs-based LEDs for particular applications.

Mn2−x Y x (MoO4)3 Phosphor Excited by UV GaN-Based Light-Emitting Diode for White Emission

One option for low-cost white light-emitting diodes (LEDs) is the combination of a near-ultraviolet (UV) LED chip (382 nm) and a single phosphor. Such Mn2−xYx(MoO4)3 single phosphors have been fabricated by a simple solid-state reaction route and their emission color tuned by controlling the Mn doping amount. The chromaticity coordinates of the white light emitted by the UV GaN LED with the MnY(MoO4)3 phosphor were x = 0.5204 and y = 0.4050 [correlated color temperature (CCT) = 7958 K].

Efficient Blue Electroluminescence Using Quantum-Confined Two-Dimensional Perovskites.

Solution-processed hybrid organic-inorganic lead halide perovskites are emerging as one of the most promising candidates for low-cost light-emitting diodes (LEDs). However, due to a small exciton binding energy, it is not yet possible to achieve an efficient electroluminescence within the blue wavelength region at room temperature, as is necessary for full-spectrum light sources. Here, we demonstrate efficient blue LEDs based on the colloidal, quantum-confined 2D perovskites, with precisely controlled stacking down to one-unit-cell thickness (n = 1). A variety of low-k organic host compounds are used to disperse the 2D perovskites, effectively creating a matrix of the dielectric quantum wells, which significantly boosts the exciton binding energy by the dielectric confinement effect. Through the Förster resonance energy transfer, the excitons down-convert and recombine radiatively in the 2D perovskites. We report room-temperature pure green (n = 7-10), sky blue (n = 5), pure blue (n = 3), and deep blue (n = 1) electroluminescence, with record-high external quantum efficiencies in the green-to-blue wavelength region.

High-performance InGaN/GaN MQW LEDs with Al-doped ZnO transparent conductive layers grown by MOCVD using H2O as an oxidizer**Project supported by the National Natural Science Foundation of China (Grant Nos. 61204091, 61404177, 51402366, and U1201254) and the Science and Technology Planning Project of Guangdong Province, China (Grant No. 2015B010132006).

In this study, the high performance of InGaN/GaN multiple quantum well light-emitting diodes (LEDs) with Al-doped ZnO (AZO) transparent conductive layers (TCLs) has been demonstrated. The AZO-TCLs were fabricated on the n+-InGaN contact layer by metal organic chemical vapor deposition (MOCVD) using H2O as an oxidizer at temperatures as low as 400 °C without any post-deposition annealing. It shows a high transparency (98%), low resistivity (510−4 Ω·cm), and an epitaxial-like excellent interface on p-GaN with an n+-InGaN contact layer. A forward voltage of 2.82 V @ 20 mA was obtained. Most importantly, the power efficiencies can be markedly improved by 53.8%@20 mA current injection and 39.6%@350 mA current injection compared with conventional LEDs with indium tin oxide TCL (LED-III), and by 28.8%@20 mA current injection and 4.92%@350 mA current injection compared with LEDs with AZO-TCL prepared by MOCVD using O2 as an oxidizer (LED-II), respectively. The results indicate that the AZO-TCL grown by MOCVD using H2O as an oxidizer is a promising TCL for a low-cost and high-efficiency GaN-based LED application.