Structural,electronic, and optical properties of oligoquinolines for light‐emitting diodes

The purpose of this work is to provide an in‐depth investigation of the electronic and optical properties of a series of n‐type conjugated oligomers, including 4‐phenyl‐6‐(4‐phenylquinolin‐6‐yl)quinoline (B1), 6,6′‐bis(2,4‐diphenylquinoline) (B1PPQ), 6,6′‐bis(2‐(4‐tert‐butylphenyl)‐4‐phenylquinoline) (BtBPQ), 6,6′‐bis(2‐p‐biphenyl)‐4‐phenylquinoline) (B2PPQ), and 4‐(6‐(2‐(4‐aminophenyl)‐4‐phenylquinolin‐6‐yl)‐4‐phenylquinolin‐2‐yl)benzenamine (BNPPQ). The geometric and electronic structures of the oligomers in the ground state were investigated using density functional theory (DFT) and the ab initio HF, whereas the lowest singlet excited states were optimized with ab initio CIS. To assign the absorption and emission peaks observed in the experiment, we computed the energies of the lowest singlet excited states with time‐dependent (TD) DFT (TD‐DFT). All DFT calculations were performed using the B3LYP functional and the 6‐31G basis set. The results show that the HOMOs, LUMOs, energies gaps, ionization potentials and electron affinities for each molecular are significantly affected by varying the aryl substituents, which favor the hole injection into OLEDs. The absorption and emission spectra exhibit red shifts to some extent [the absorption spectra: 335.85 (B1)< 370.63 (B1PPQ) < 376.77 (BtBPQ)< 388.67 (B2PPQ)< 412.93 nm (BNPPQ); the emission spectra: 391.48 (B1)< 430.11 (B1PPQ)< 435.86 (BtBPQ)< 444.57 (B2PPQ)< 463.28nm (BNPPQ)]. The radiative lifetimes (τ) of each oligomers are calculated as well. Because of introducing the cooperation with the electron donators such as the amidocyanogen in the common 4‐phenyl‐6‐(4‐phenylquinolin‐6‐yl)quinoline core for BNPPQ, which results in improving the hole‐creating ability. Copyright © 2008 John Wiley & Sons, Ltd.     

Measuring and structuring the spatial coherence length of organic light‐emitting diodes

The spatial coherence of organic light‐emitting diodes (OLEDs) is an important parameter that has gained little attention to date. Here, we present a method for making quantitative measurements of the spatial coherence of OLEDs using a Young's double‐slit experiment. The usefulness of the method is demonstrated by making measurements on a range of OLEDs with different emitters (iridium and europium complexes) and architectures (bottom and top emitting) and the fringe visibility is further manipulated by gratings embedded in external diffractive optical elements. Based on the experiments and simulation of the results, we quantitatively determine the spatial coherence lengths of several OLEDs and find them to be a few micrometers. A 60% increase in the spatial coherence length was observed when using a narrow bandwidth emitter and a metal‐coated grating.

     

Development of organic light‐emitting diodes for electro‐optical integrated devices

Organic light‐emitting diodes (OLEDs) are discussed for electro‐optical integrated devices that are used for optical signal transmission. Organic optical devices including polymeric optical fibers are used for optical communication applications to realize polymeric electro‐optical integrated devices. The OLEDs were fabricated by vacuum process, i.e. the organic molecular beam deposition (OMBD) technique or a solution process on a polymeric or a glass substrate, for comparison. Optical signals faster than 100 MHz have been created by applying pulsed voltage directly to the OLED utilizing rubrene doped in 8‐hydoxyquinolinum aluminum (Alq₃), as an emissive layer. OLEDs fabricated by solution process utilizing rubrene doped in carrier‐transporting materials have also discussed. OLEDs utilizing polymeric materials by solution process are also fabricated and discussed. Moving‐picture signals are transmitted utilizing both vacuum‐ and solution‐processed OLEDs, respectively.     

High color rendering index and chromatic stability of top‐emitting white organic light‐emitting diodes capped with a supplementary green color conversion layer

We have demonstrated high color rendering index (CRI) and chromatic stability of top‐emitting white organic light‐emitting diodes (TEWOLEDs) by capping a supplementary green color conversion layer (CCL) on the cathode of single blue emitter based TEWOLEDs. By employing CCLs, blue photon energy is absorbed and converted to green emission efficiently, resulting in an improvement of CRI from 72 to 86. We attribute the increased high CRI to the broadening of the out‐coupled spectrum and an appropriate intensity match among the three primary colors. Meanwhile, the TEWOLEDs show extremely high chromatic stability. The results indicate that this method provides a new avenue to improve the CRI of TEWOLEDs.

     

A step towards the electrophotonic nose: integrating 1D photonic crystals with organic light‐emitting diodes and photodetectors

An innovative integrated sensing platform for the detection of various chemical analytes via translating the photonic stop‐band shift of a one‐dimensional photonic crystal (PC) into an electrical current change is proposed. The miniaturized sensing platform features an organic light‐emitting diode (OLED) as a light source and an organic photodetector (OPD) as a light sensor and allows for the detection of ethanol vapor concentrations down to ≈ 10 parts per million (ppm) in nitrogen, which corresponds to a stop‐band shift of ≈ 27 pm. The resolution of the proposed platform exceeds the capabilities of most commercial spectrometers and by far the human eye, while, at the same time, such a sensor is less expensive and less power consuming than a spectrometer. The presented setup is generic and can detect optical changes in the transmission of PCs, which can be induced by both vapor adsorption or by a liquid analyte, as demonstrated with a microfluidic setup.     

Concentration quenching effect of organic light‐emitting devices using DCM1‐doped tetraphenylgermole

We examine the concentration quenching of a 4‐(dicyanomethylene)2‐methyl‐6‐(p‐dimethylaminostyryl)‐4H‐pyran (DCM1)‐doped 1,1‐bis(2‐phenylethynyl)‐2,3,4,5‐tetraphenylgermole (HPAG)‐based light‐emitting diode. Originally, HPAG emits in the ~500‐nm (green) region, which can be converted to a red‐emission material by using DCM1 doping. As the DCM1 concentration increased from 1 to 10 wt%, the electroluminescence peak positions are red‐shifted from 604 to 644 nm, respectively. Increasing doping density not only shows the red‐shift but also shows decreasing luminance efficiency. Förster energy transfer between the HPAG host material and the DCM1 guest material is responsible for the strong red‐emission behavior. The calculated Förster radius (4.0 nm) for excellent Förster energy transfer characteristics with increasing doping concentration of DCM1 is consistent with experimental results. The maximum luminance efficiency was 6.64 cd/A at 11.0 mA/cm². The HPAG germole compound shows excellent red‐emission host–guest system properties for red organic light‐emitting device applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Bi‐directional organic light‐emitting diodes with nanoparticle‐enhanced light outcoupling

An effective method is presented for enhancing the outcoupling efficiency of translucent/bi‐directional organic light‐emitting diodes (TL/BD‐OLEDs) with a bottom indium tin oxide (ITO) anode and a top cathode comprised of a thin Ag layer covered with an organic capping layer. Upon insertion of a nanoparticle (NP)‐based scattering layer (NPSL) between the substrate and the ITO anode, the TL/BD‐OLEDs exhibit significantly enhanced external quantum efficiency (EQE) in both emission directions. Furthermore, the NPSL improves the color stability of the TL/BD‐OLEDs over a wide range of viewing angles. Simulations based on geometrical and statistical optics are performed to elucidate the mechanism by which the efficiency is enhanced and to establish strategies for further optimization. Simulations performed on the scattering layers with varying NP volume percentage reveal that the bottom‐side emission is governed by competition between waveguide‐mode extraction and backward scattering by NPs in the film, while the top‐side emission is largely dominated by the latter. Optimized bi‐directional OLEDs achieve a 1.64‐fold enhanced EQE compared to reference devices without NPSL.     

Density functional theory and time‐dependent density functional theory study on a series of iridium complexes with tetraphenylimidodiphosphinate ligand

A series of heteroleptic cyclometalated Ir(III) complexes for organic light‐emitting diodes (OLEDs) application have been investigated theoretically to explore their electronic structures and spectroscopic properties. The geometries, the electronic structures, the lowest‐lying singlet absorptions and triplet emissions of Ir(dfppy)₂(tpip), Ir(tfmppy)₂(tpip), and theoretically designed models of Ir(ppy)₂(tpip) were investigated with the density functional theory (DFT)‐based approaches, where ppy = 2‐phenylpyridine, dfppy = 4,6‐difluorophenylpyridine, tfmppy = 4‐trifluoromethylphenylpyridine, and tpip = tetraphenylimidodiphosphinate. Their structures in the ground and their excited states have been optimized at the DFT/Becke 3‐parameter Lee Yang Parr (B3LYP)/Los Alamos National Laboratory 2‐double‐z (LANL2DZ) and time‐dependent DFT/B3LYP/LANL2DZ levels, and the lowest absorptions and emissions were evaluated at B3LYP and M062X level of theory, respectively. Furthermore, the energy transfer mechanism together with the advantage of low efficiency roll‐off for these complexes also can be analyzed here. Copyright © 2013 John Wiley & Sons, Ltd.     

Multi‐peak and chromaticity‐stable top‐emitting white organic light‐emitting diodes using single blue emitter

We have fabricated multi‐peak and chromaticity‐stable top‐emitting white organic light‐emitting diodes (TEWOLEDs) using single blue emitter. Besides the intrinsic emission of blue emitter, the additional emission can be well realized by simply adjusting the thickness of hole transporting layer (HTL), thus modifying the optical cavity length to obtain different resonant wavelengths. The detailed variation process for multi‐peak spectra with the increase of HTL thickness is studied, which provides a guidance for the design of microcavity TEWOLEDs.

     

Improved efficiencies of hybrid organic light‐emitting diodes using efficient electron injection layer

Hybrid organic‐inorganic light‐emitting diodes were developed with pristine ZnO (2.0 wt%) and Cu‐doped ZnO (2.0 wt%) as electron injection layer and iridium(III)‐bis‐2‐(4‐fluorophenyl)‐1‐(naphthalen‐1‐yl)‐1H‐phenanthro[9,10‐d]imidazole (acetylacetonate) [Ir(fpnpi)₂ (acac)] as green emissive layer (521 nm). The pristine ZnO and Cu‐doped ZnO are deposited at indium tin oxide cathode and emissive layer interface. The electroluminescent performances increased by electron injection layer–Cu‐doped ZnO compared with ZnO‐based device because Cu‐doped ZnO injects electron efficiently result in balanced h⁺ ? e^? recombination in emissive layer than ZnO‐based device. The Cu‐doped ZnO (2.0 %) device shows luminance (L) of 10 982 cd/m² at 23.0 V (ZnO, 1450 cd/m² at 23.0 V).     

A facile method for flexible GaN‐based light‐emitting diodes

Flexible GaN‐based light‐emitting diodes (LEDs) on polyethylene terephthalate (PET) substrates are demonstrated. The process uses commercial LEDs on patterned sapphire substrates, laser lift‐off (LLO), wet etching for additional surface roughening, and mounting of the freestanding LED on a PET substrate. Electrical and optical properties from the free‐standing LLO‐LEDs mounted on the flexible PET substrates were characterized. The process is scalable to large wafer diameters. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of UV excited white light‐emitting diodes for optimization of luminous efficiency and color rendering index

Wavelength down‐converted white‐light sources excited by near ultraviolet light‐emitting diodes require specific phosphor properties in order to generate high‐quality white light (namely, light with good color rendition and stability of color coordinate). Simulation and experimental results are discussed, with particular emphasis on the spectral distribution property of red phosphor required to realize high values of luminous efficiency and color rendition. A peak wavelength of 610 nm and an FWHM of 80 nm for the spectral power distribution were proposed, and co‐doped phosphate materials were synthesized successfully. This can contribute to a white‐light source with a luminous efficiency of 45 lm/W and color rendering index greater than 90 at a color temperature of 5600 K and an operational current of 20 mA. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The photonic perspective of organic light‐emitting transistors

This article will give an overview of the current state‐of‐the‐art of OLETs from the point of view of their photonic characteristics. In particular, the different device structures realized, the materials used and the strategies implemented to integrate optical resonators and waveguiding structures into light‐emitting field‐effect transistors will be reviewed and the main findings discussed. (Picture: Courtesy of E. T. C. srl)     

Color tunable light‐emitting diodes with modified pulse‐width modulation

In this study, a color tunable light source, operated by a modified pulse width modulation method, is investigated. By utilizing this method along with anti‐parallel connected discrete light‐emitting diodes (LEDs) and two electrical terminals, a wide range of the chromaticity coordinates is attained and varied by electrical control. Using the combination of a blue LED and a phosphor‐converted yellow LED (blue LED plus yellow phosphor), the chromaticity range is varied by electrical control from pure blue to pure yellow. In addition, using the modified pulse‐width modulation method and a combination of white and red LEDs, white light with correlated color temperatures ranging from 5000 K to 2000 K is demonstrated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simplified hybrid white organic light‐emitting diodes with efficiency/efficiency roll‐off/color rendering index/color‐stability trade‐off

A high‐performance hybrid white organic light‐emitting diode (WOLED) based on a simple structure has been developed. The resulting device exhibits a maximum total current efficiency and power efficiency of 35.7 cd/A and 30.6 lm/W, respectively. Even at a high luminance of 1000 cd/m², a current efficiency of 32.0 cd/A and a power efficiency of 19.4 lm/W are obtained, suggesting that the device exhibits a low efficiency roll‐off. Besides, the device shows excellent color‐stability during a wide range of luminance and a high color rendering index (CRI) of 83 is obtained. Moreover, the origin of the superior properties is explored comprehensively. Such achieved results demonstrate that high efficiency, low efficiency roll‐off, stable color and high CRI can be simultaneously realized in a simplified hybrid WOLEDs. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Efficiency droop in light‐emitting diodes: Challenges and countermeasures

Efficiency droop, i.e. the loss of efficiency at high operating current, afflicts nitride‐based light‐emitting diodes (LEDs). The droop phenomenon is currently the subject of intense research, as it retards the advancement of solid‐state lighting which is just starting to supplant fluorescent as well as incandescent lighting. Although the technical community does not yet have consented to a single cause of droop, this article provides a summary of the present state of droop research, reviews currently discussed droop mechanisms, and presents a recently developed theoretical model for the efficiency droop. In the theoretical model, carrier leakage out of the active region caused by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, is discussed in detail. The model is in agreement with the droop's key behaviors not only for GaInN LEDs but also for AlGaInP LEDs.     

Comparison between blue lasers and light‐emitting diodes for future solid‐state lighting

Solid‐state lighting (SSL) is now the most efficient source of high color quality white light ever created. Nevertheless, the blue InGaN light‐emitting diodes (LEDs) that are the light engine of SSL still have significant performance limitations. Foremost among these is the decrease in efficiency at high input current densities widely known as “efficiency droop.” Efficiency droop limits input power densities, contrary to the desire to produce more photons per unit LED chip area and to make SSL more affordable. Pending a solution to efficiency droop, an alternative device could be a blue laser diode (LD). LDs, operated in stimulated emission, can have high efficiencies at much higher input power densities than LEDs can. In this article, LEDs and LDs for future SSL are explored by comparing: their current state‐of‐the‐art input‐power‐density‐dependent power‐conversion efficiencies; potential improvements both in their peak power‐conversion efficiencies and in the input power densities at which those efficiencies peak; and their economics for practical SSL.     

Impact of surface recombination on efficiency of III‐nitride light‐emitting diodes

The paper considers surface recombination at the free active region surface as the mechanism of carrier losses which has not yet been discussed with regard to III‐nitride LEDs despite of its evident importance for AlGaInP‐based light emitters. Using advanced thin‐film and triangular volumetric chip designs reported in literature as prototypes, we have demonstrated by simulation a noticeable impact of surface recombination on the wall‐plug efficiency of InGaN‐based LEDs. Various types of LEDs whose efficiency may be especially affected by surface recombination are discussed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

White light‐emitting diodes: History,progress, and future

About twenty years ago, in the autumn of 1996, the first white light‐emitting diodes (LEDs) were offered for sale. These then‐new devices ushered in a new era in lighting by displacing lower‐efficiency conventional light sources including Edison's venerable incandescent lamp as well as the Hg‐discharge‐based fluorescent lamp. We review the history of the conception, improvement, and commercialization of the white LED. Early models of white LEDs already exceeded the efficiency of low‐wattage incandescent lamps, and extraordinary progress has been made during the last 20 years. The review also includes a discussion of advances in blue LED chips, device architecture, light extraction, and phosphors. Finally, we offer a brief outlook on opportunities provided by smart LED technology.

     

Superior color rendering with a phosphor‐converted blue‐cyan monolithic light‐emitting diode

The future generation of modern illumination should not only be cheap and highly efficient, but also demonstrate high quality of light, light which allows better color differentiation and fidelity. Here we are presenting a novel approach to create a white solid‐state light source providing ultimate color rendition necessary for a number of applications. The proposed semi‐hybrid device combines a monolithic blue‐cyan light emitting diode (MBC LED) with a green‐red phosphor mixture. It has shown a superior color rendering index (CRI), 98.6, at correlated color temperature of around 3400 K. The MBC LED epi‐structure did not suffer from the efficiency reduction typical for monolithic multi‐color emitters and was implemented in the two most popular chip designs: “epi‐up” and “flip‐chip”. Redistribution of the blue and cyan band amplitudes in the white‐light emission spectrum, using the operating current, is found to be an effective tool for fine tuning the color characteristics.