Efficient metal halide perovskite light-emitting diodes with significantly improved light extraction on nanophotonic substrates.

Metal halide perovskite has emerged as a promising material for light-emitting diodes. In the past, the performance of devices has been improved mainly by optimizing the active and charge injection layers. However, the large refractive index difference among different materials limits the overall light extraction. Herein, we fabricate efficient methylammonium lead bromide light-emitting diodes on nanophotonic substrates with an optimal device external quantum efficiency of 17.5% which is around twice of the record for the planar device based on this material system. Furthermore, optical modelling shows that a high light extraction efficiency of 73.6% can be achieved as a result of a two-step light extraction process involving nanodome light couplers and nanowire optical antennas on the nanophotonic substrate. These results suggest that utilization of nanophotonic structures can be an effective approach to achieve high performance perovskite light-emitting diodes

Luminous Intensity for Traffic Signals: A Scientific Basis for Performance Specifications

Humnan factors experiments on visual responses to simulated traffic signals using incandescent lamps and light-emitting diodes are described

Temperature compensation of light-emitting diodes

Circuit which includes a thermistor-resistor combination to compensate for temperature fluctuations by supplying input voltage to light-emitting diode, maintains constant light output. Similar circuits can be used for temperature-induced variations in photodiode applications

High yield and ultrafast sources of electrically triggered entangled-photon pairs based on strain-tunable quantum dots

Triggered sources of entangled photon pairs are key components in most quantum communication protocols. For practical quantum applications, electrical triggering would allow the realization of compact and deterministic sources of entangled photons. Entangled-light-emitting-diodes based on semiconductor quantum dots are among the most promising sources that can potentially address this task. However, entangled-light-emitting-diodes are plagued by a source of randomness, which results in a very low probability of finding quantum dots with sufficiently small fine structure splitting for entangled-photon generation (∼10−2). Here we introduce strain-tunable entangled-light-emitting-diodes that exploit piezoelectric-induced strains to tune quantum dots for entangled-photon generation. We demonstrate that up to 30% of the quantum dots in strain-tunable entangled-light-emitting-diodes emit polarization-entangled photons. An entanglement fidelity as high as 0.83 is achieved with fast temporal post selection. Driven at high speed, that is 400 MHz, strain-tunable entangled-light-emitting-diodes emerge as promising devices for high data-rate quantum applications

Direct measurement of the magnetic field effects on carrier mobilities and recombination in tri-(8-hydroxyquinoline)-aluminum based light-emitting diodes

The magnetic field effects on the carrier mobilities and recombination in tri-(8-hydroxyquinoline)-aluminum (Alq3) based light-emitting diodes have been measured by the method of transient electroluminescence. It is confirmed that the magnetic field has no effect on the electron and hole mobilities in Alq3 layers and can decrease the electron-hole recombination coefficient. The results imply that the dominant mechanism for the magnetic field effects in Alq3 based light-emitting diodes is the interconversion between singlet e-h pairs and triplet e-h pairs modulated by the magnetic field when the driving voltage is larger than the onset voltage of the electroluminescence.Comment: 14 pages, 4 figures,The revised version submitted to applied physics letter

Solid State Lighting: A Summarization of Advancements

Solid State Lighting is a rapidly growing new technology in the field of lighting. By utilizing the concepts of solid-state physics and electronics, it generates light. Light emitting diodes and organic light emitting diodes pose several advantages over the current lighting technology but they still require development and research for using them to their full potential. In this paper the characteristics, sources of uncertainty, and market status of light emitting diode are reviewed to provide more suitable research directions for advancement in the field of solid-state lighting. Challenges faced by Light emitting diodes for maintaining color and visual comfort are also illustrated. Failure modes and environmental impact of light emitting diodes are also analysed. Quantum dot based solid state lightening is also presented to study the chromatic characteristics.  Some critical factors of concern for broader application of light emitting diodes and additional enhancements in electrical, optical, temperature characteristic, high power output and color furnishing capabilities are also demonstrated in paper. Light emitting diodes wattage output and efficiency are also discussed for practical viability of solid state devices in emerging fields. The extension lead of current LED technology in evolving applications are considered as accumulation of numerous technologies such as wireless, communication, sensors and control engineering. Undoubtedly, LED engineering is contemporary and the price maybe unreasonable. Nevertheless, it will find its usage in very nearly all applications and the initiation of new techniques that might lessen the cost

Contact Injection into Polymer Light-Emitting Diodes

The variation of current I with voltage V for poly(phenylene vinylene) and other polymer light-emitting diodes has been attributed to carriers tunneling into broad conduction and valence bands. In actuality the electrons and holes tunnel into polaron levels and transport is by hopping among these levels. We show that for small injection the I-V characteristic is determined mainly by the image force, for large injection by space charge effects, but in both cases the strong variation of mobility with field due to disorder plays an important role.Comment: 9 pages, two Postscript figures are aviable upon reques

Improved Charge Injection and Transport of Light-Emitting Diodes Based on Two-Dimensional Materials

Light-emitting diodes (LEDs) are considered to be the most promising energy-saving technology for future lighting and display. Two-dimensional (2D) materials, a class of materials comprised of monolayer or few layers of atoms (or unit cells), have attracted much attention in recent years, due to their unique physical and chemical properties. Here, we summarize the recent advances on the applications of 2D materials for improving the performance of LEDs, including organic light emitting diodes (OLEDs), quantum dot light emitting diodes (QLEDs) and perovskite light emitting diodes (PeLEDs), using organic films, quantum dots and perovskite films as emission layers (EMLs), respectively. Two dimensional materials, including graphene and its derivatives and transition metal dichalcogenides (TMDs), can be employed as interlayers and dopant in composite functional layers for high-efficiency LEDs, suggesting the extensive application in LEDs. The functions of 2D materials used in LEDs include the improved work function, effective electron blocking, suppressed exciton quenching and reduced surface roughness. The potential application of 2D materials in PeLEDs is also presented and analyzed

Perovskite light-emitting diodes

Abstract. The usage of artificial lighting and displays is increasing all the time. Energy efficient and affordable light-emitting materials are being developed widely. Light-emitting diodes (LEDs) are energy efficient, and LED technology and displays based on LEDs have developed greatly in the recent years. In the year 2014, the first research article related to a perovskite LED (PeLED) that is functional in room temperature was released. The light-emitting material in PeLED has ABX3 stoichiometry and perovskite structure. The most common A-cations used in perovskites in PeLEDs are methylammonium, formamidinium and cesium (Cs+). The most common B-cation is lead (Pb2+) and X-anion is a halide or a mixture of halides. Lead halide perovskites are interesting light-emitting materials, since they can be readily solution processed with inexpensive methods. The emission of lead halide perovskites can be tuned across the whole visible spectrum by changing the composition, and the emission colors are bright due to narrow emission. There are multiple challenges in PeLED development. Especially the efficiency of blue PeLEDs needs to be improved. Furthermore, there are challenges related to health and stability. The perovskite used in PeLEDs contains lead, which is poisonous, and perovskites are often solution processed from toxic solvents such as dimethylformamide. The biggest challenges in perovskite stability are sensitivity to moisture, oxygen, illumination and heat. Moreover, the stability is affected by mechanical stress, reactions caused by electric bias and reactions between materials used in PeLEDs. In this thesis, the most common perovskite materials, PeLED architectures, and their characterizations are introduced. Challenges in PeLED development are discussed, especially stability. Perovskite stability can be increased by e.g. perovskite substitution, additives, morphology control and optimization of PeLED structure.Tiivistelmä. Keinovalaistuksen ja näyttöjen määrä kasvaa maailmassa jatkuvasti. Energiatehokkaita ja edullisia valoa emittoivia materiaaleja kehitetään paljon. Valoa emittoivat diodit (light-emitting diode, LED) ovat energiatehokkaita, ja erilaiset LED teknologiat ja niihin perustuvat näytöt ovatkin kehittyneet lähivuosina nopeasti. Vuonna 2014 julkaistiin ensimmäinen tutkimus huoneenlämmössä toimivasta perovskiitti LED:sta (PeLED). PeLED:n valoa emittoivalla materiaalilla on ABX3 koostumus ja perovskiitin kiderakenne. PeLED:ssa käytettävissä perovskiiteissa yleisimmät A-kationit ovat metyyliammonium, formamidinium ja cesium (Cs+). Yleisin B-kationi on lyijy (Pb2+) ja X-anionina käytetään halidia tai niiden seosta (Cl−, Br−, tai I−). Lyijyhalidi-perovskiitit ovat erityisen kiinnostavia materiaaleja, sillä niitä voidaan valmistaa liuoksista edullisesti ja helposti. Lyijyhalidi-perovskiittien emissiota voidaan säätää koko näkyvän valon aallonpituusalueella muuttamalla niiden koostumusta, ja niiden kapea emissiospektri mahdollistaa kirkkaat värit. PeLED:en kehityksessä on vielä lukuisia haasteita. Erityisesti sinisten PeLED:en hyötysuhde vaatii vielä kehittämistä. Lisäksi haasteina on mm. terveysriskit ja stabiilisuus. PeLED:t sisältävät myrkyllistä lyijyä ja niiden valmistuksessa käytetään myrkyllisiä liuottimia, kuten dimetyyliformamidia. Suurimmat haasteet stabiilisuudessa ovat herkkyys kosteudelle, hapelle, valolle ja lämmölle. Lisäksi stabiilisuuteen vaikuttaa mekaaninen rasitus, sähkövirran aiheuttamat reaktiot ja PeLED:ssa käytettyjen materiaalien keskinäiset reaktiot. Tässä tutkielmassa esitellään yleisesti PeLED:ssa käytettäviä perovskiittimateriaaleja, yleisiä PeLED:en rakenteita ja niiden karakterisointia. Lisäksi tutkielmassa perehdytään PeLED:en kehityksen haasteisiin, erityisesti stabiilisuuteen. Perovskiittien stabiilisuutta voidaan parantaa esimerkiksi vaihtamalla perovskiitin koostumusta, käyttämällä lisäaineita, muokkaamalla perovskiittikerroksen morfologiaa ja optimoimalla PeLED:n rakenne