Electroluminescence Emission Spectra Research Articles

Optical, Photophysical, and Electroemission Characterization of Blue Emissive Polymers as Active Layer for OLEDs.

Polymers containing π-conjugated segments are a diverse group of large molecules with semiconducting and emissive properties, with strong potential for use as active layers in Organic Light-Emitting Diodes (OLEDs). Stable blue-emitting materials, which are utilized as emissive layers in solution-processed OLED devices, are essential for their commercialization. Achieving balanced charge injection is challenging due to the wide bandgap between the HOMO and LUMO energy levels. This study examines the optical and photophysical characteristics of blue-emitting polymers to contribute to the understanding of the fundamental mechanisms of color purity and its stability during the operation of OLED devices. The investigated materials are a novel synthesized lab scale polymer, namely poly[(2,7-di(p-acetoxystyryl)-9-(2-ethylhexyl)-9H-carbazole-4,4'-diphenylsulfone)-co-poly(2,6-diphenylpyrydine-4,4'-diphenylsulfone] (CzCop), as well as three commercially supplied materials, namely Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO), poly[9,9-bis(2'-ethylhexyl) fluorene-2,7-diyl] (PBEHF), and poly (9,9-n-dihexyl-2,7-fluorene-alt-9-phenyl-3,6-carbazole) (F6PC). The materials were compared to evaluate their properties using Spectroscopic Ellipsometry, Photoluminescence, and Atomic Force Microscopy (AFM). Additionally, the electrical characteristics of the OLED devices were investigated, as well as the stability of the electroluminescence emission spectrum during the device's operation. Finally, the determined optical properties, combined with their photo- and electro-emission characteristics, provided significant insights into the color stability and selectivity of each material.

Spectrally Stable Deep-Blue Light-Emitting Diodes Based on Layer-Transferred Single-Crystalline Ruddlesden-Popper Halide Perovskites.

A novel approach to producing high-color-purity blue-light-emitting diodes based on single-crystalline Ruddlesden-Popper perovskites (RPPs) is reported. The utilization of a pure bromide composition eliminates any possibility of halide segregation, which can otherwise lead to undesired shifts in the emission wavelength or irreversible degradation of the spectral line width. Phase-pure PEA2MAPb2Br7 single crystals with a lateral size exceeding 1 cm2 can be synthesized using the inverse temperature crystallization method. To prepare RPP layers with a thickness of less than 50 nm, we employ a thinning process of the initially thick bulk crystals, followed by a dry-transfer process to place them onto a hole transport layer and an indium-tin-oxide-coated glass substrate. By utilizing polydimethylsiloxane as a handling layer, deformations of the bulk RPP crystal and exfoliated RPP layer, as well as the formation of defects such as pinholes, can be effectively suppressed. Subsequent depositions of an electron transport layer and a metal contact complete the fabrication of electroluminescence (EL) devices. The EL devices utilizing the single-crystalline RPP demonstrate excellent spectral stability across a broad range of the applied bias voltage spanning from 4.5 to 10 V, exhibiting a significantly narrow line width of 14 nm at an emission wavelength of 440 nm that can potentially cover 99.3% of the Rec. 2020 color gamut. The sharp EL emission spectrum can be effectively preserved, avoiding any broadening of the line width, by suppressing Joule heating throughout the device operation, in addition to the intrinsic stability of single-crystalline RPPs.

Resonant cavity enhanced InAs/GaAsSb SLS LEDs with a narrow spectral linewidth and a high-spectral intensity operating at 4.6 μm

We investigated the design, growth, fabrication, and characterization of InAs/GaAsSb SLS resonant cavity light emitting diodes (RCLEDs) grown on InAs by molecular beam epitaxy. The structure consists of a 1λ-thick micro-cavity positioned between two lattice-matched AlAsSb/GaAsSb distributed Bragg reflector mirrors (DBRs). A 44-pair InAs/GaAsSb SLS active region is placed at the antinode of the electric field intensity in the center of the cavity. Electroluminescence emission spectra were recorded at room-temperature. Due to the resonant cavity effect, 400 μm-diameter SLS RCLEDs exhibited emission spectra peaked at 4.587 μm with a narrow spectral linewidth of 52 nm. A high-spectral intensity of >3 mW cm−2 nm−1 was achieved for the 400 μm SLS RCLED using 1% duty cycle to avoid Joule heating. Furthermore, temperature dependence of the emission spectra of the RCLED showed excellent temperature stability, with a rate of 0.34 nm/K. Compared to existing mid-infrared 5-stage InAs/GaAsSb SLS ICLEDs operating at ∼4.5 μm, the (400 μm-diameter) InAs/GaAsSb SLS RCLEDs exhibited 10.5× brighter spectral intensity, 14× narrower spectral linewidth, and 8× improvement in the temperature stability. Owing to these attractive features, our SLS RCLEDs could be used to develop the next generation CO gas instruments and active imaging.

Realization of plant growth lighting and temperature detecting based on novel Bi3+, Sm3+ and Mn4+ doped Ca2GdNbO6 double perovskite phosphors

The Bi3+ or Sm3+ ions are introduced into Mn4+-activated Ca2GdNbO6 (CGN) double perovskite phosphors to achieve highly efficient plant growth lighting and temperature measurement. Here, the energy band and density of states for CGN are computed using density functional theory (DFT) in detail. The emission bands of CGN: Bi3+/Mn4+ consist of both the blue-light band (3P1→1S0 transition of Bi3+ ions) and red-light band (2E→4A2 transition of Mn4+ ions), which solves the issue of the lack of blue light in Mn4+-activated phosphors used in plant-growth LEDs. The electroluminescence emission spectrum of the LEDs assembled with CGN: Sm3+/Mn4+ shows a wide emission band (550–800 nm), which coincides perfectly with the absorption (550–825 nm) of Phytochrome (PR and PFR). Moreover, non-contact thermometers are designed by luminescence intensity ratio technique (LIRMn/Bi or LIRMn/Sm) with excellent absolute (Sa = 0.074 K-1@448 K or 0.036 K-1@373 K) and relative sensitivity (Sr = 2.13 %K−1@498 K or 1.11 %K−1@448 K). The results reveal that as-prepared phosphors have great potential in promoting indoor plant cultivate and optical temperature sensing.

Electro‐optical Properties of ZnSe Doped with Gd and (Ag,Gd) Phosphors

AbstractTo understand the position of donor‐ acceptor level and nature of electronic state of activators and co‐ activators, the photo luminescence (PL) and electroluminescence (EL) emission spectra of Znse: Gd and Zn Se: (Ag, Gd) phosphors, have been recorded at room temperature. The method of preparation of phosphors is described. Voltage and frequency dependence of electroluminescent has also b been described. The light output varies as been studied for a wide range of frequencies. It is observed that the EL emission spectra of these phosphors are shifted towards lower wavelength sides on increasing the field frequency.

Oxygen Plasma Surface Treatment onto ITO Surface for OLEDs Based on Europium Complex

In this work, Organic Light-Emitting Diodes (OLEDs) based on Europium (III) complex were studied, especially those with an oxygen plasma surface treatment onto indium tin oxide (ITO) transparent electrode. An OLED with the same thin-film structure but with untreated ITO was fabricated for comparison purposes. Current density-voltage characteristics for treated devices demonstrated an increase from 0.4 to 3.3 mA/cm² and a decrease of the turn-on voltage from 28 to 22 V. Additionally, improved hole injection through the transparent electrode impacted on optical response, as luminous efficiency increased from 26 to 44 mcd/A with the advantage of no significant disturb on the europium typical electroluminescence emission spectrum. Emission peak was observed at a wavelength of approximately 614 nm, which is defined by the transition associated with spectroscopic terms (5D0→7D2) and CIE chromaticity coordinates of (0.50;0.36).

In Situ-Fabricated Perovskite Nanocrystals for Deep-Blue Light-Emitting Diodes.

Efficient and stable deep-blue emission from perovskite light-emitting diodes (LEDs) is required for their application in lighting and displays. However, this is difficult to achieve due to the phase segregation issue of mixed halide perovskites and the challenge of synthesizing high-quality single-halide deep-blue perovskite nanocrystals through a traditional method. Here, we show that an antisolvent treatment can facilitate the in situ formation of perovskite nanocrystals using a facile spin-coating method. We find that the dropping time of the antisolvent can significantly affect the constitution of nanocrystal perovskite films. With a delay in the start time of the antisolvent treatment, small single-halide perovskite nanocrystals can be achieved, exhibiting efficient deep-blue emission. The LED device shows a stable electroluminescence (EL) peak at 465 nm, with a peak external quantum efficiency and a peak current efficiency of 2.4% and 2.5 cd A-1, respectively. This work provides a facile approach to changing the size of perovskite nanocrystals, thus effectively tuning their EL emission spectra.

High-speed electroluminescence from semiconducting carbon nanotube films

High-speed light emitters integrated on silicon chips can enable novel architectures for silicon-based optoelectronics, such as on-chip optical interconnects and silicon photonics. However, conventional light sources based on compound semiconductors face major challenges for their integration with the silicon-based platforms because of the difficulty of their direct growth on a silicon substrate. Here, we report high-speed, ultra-small-size on-chip electroluminescence (EL) emitters based on semiconducting single-walled carbon nanotube (SWNT) thin films. The peaks of the EL emission spectra are about 0.15-eV redshifted from the peaks of the absorption and photoluminescence emission spectra, which probably suggest emission from trions. High-speed responses of ∼100 ps were experimentally observed from the EL emitters, which indicate the possibility of several-GHz modulation. The pulsed light generation was also obtained by applying the pulse voltage. These high-speed and ultra-small-size EL emitters can enable novel on-chip optoelectronic devices for highly integrated optoelectronics and silicon photonics.

Graphene/ZnO Nanowire/p-GaN Vertical Junction for a High-Performance Nanoscale Light Source.

We report on a high-brightness ultraviolet (UV) nanoscale light source. The light emission diodes are constructed with graphene/ZnO nanowire/p-GaN vertical junctions, which exhibit strong UV electroluminescence (EL) emissions centered at a wavelength of 397 nm at one end of the ZnO nanowire. Compared to the horizontal heterojunction, the vertical junction based on the ZnO nanowire increases the interface area of the heterojunction along with a high-quality interface, thus making the device robust under a large excitation current. In this structure, transparent flexible graphene is used as the top electrode, which can effectively improve performance by increasing the carrier injection area. Moreover, by analyzing the relationship between the integrated light intensity and applied bias, a superlinear dependency with a slope of 3.99 is observed, which means high electrical-to-optical conversion efficiency. Three electron–hole irradiation recombination processes are distinguished according to the EL emission spectra.

Symmetrical Bi-Heterojunction Alternating Current Ultraviolet Light-Emitting Diode

The development of bidirectional light emitting diodes (LEDs) is significant to avoid stroboscopic effects and save energy for illumination and display applications. However, the existing bidirectional LEDs present different electroluminescent (EL) emission spectra and intensity at forward and reverse bias conditions which causes stroboscopic effects. Herein, A symmetrical bi-heterojunctional n-ZnO/n-GaN/n-ZnO LED was fabricated by bonding n-ZnO microwires on a n-GaN layer. The LEDs generate a stable UV light output under both forward and reverse bias conditions. The EL intensities of LEDs under forward and reverse bias conditions are approximately equal because of symmetrical characters between two heterojunctions. Our results provide a bi-directionbiased bi-heterojunction ultraviolet LED avoiding stroboscopic effects.

Electroluminescence enhancement in mid-infrared InAsSb resonant cavity light emitting diodes for CO2 detection

In this work, we demonstrated a mid-infrared resonant cavity light emitting diode (RCLED) operating near 4.2 μm at room temperature, grown lattice-matched on a GaSb substrate by molecular beam epitaxy, suitable for CO2 gas detection. The device consists of a 1λ-thick microcavity containing an InAs0.90Sb0.1 active region sandwiched between two high contrast, lattice–matched AlAs0.08Sb0.92/GaSb distributed Bragg reflector (DBR) mirrors. The electroluminescence emission spectra of the RCLED were recorded over the temperature range from 20 to 300 K and compared with a reference LED without DBR mirrors. The RCLED exhibits a strong emission enhancement due to resonant cavity effects. At room temperature, the peak emission and the integrated peak emission were found to be increased by a factor of ∼70 and ∼11, respectively, while the total integrated emission enhancement was ∼×33. This is the highest resonant cavity enhancement ever reported for a mid-infrared LED operating at this wavelength. Furthermore, the RCLED also exhibits a superior temperature stability of ∼0.35 nm/K and a significantly narrower (10×) spectral linewidth. High spectral brightness and temperature stable emission entirely within the fundamental absorption band are attractive characteristics for the development of next generation CO2 gas sensor instrumentation.

Experimentally study electron overflow from quantum wells into p-side GaN and its effect on the efficiency droop in InGaN/GaN blue-light-emitting-diodes

We experimentally study the electron overflow in InGaN/GaN blue light emitting diodes (LEDs) by measuring the electroluminescence (EL) emission spectra from p-type InGaN/GaN superlattices (SLs) adjacent to the quantum well (QW) active region. The integrated EL intensity ratio of the SL peak to quantum well peak was calculated to reveal the dependence of electron overflow from active region on current density. Our experimental results indicate that the electron overflow from the active region at both low and high current densities, and no saturation is observed even at the maximum current density (280 A/cm2) applied. It is found that both direct electron leakage and Auger assisted leakage contribute to the electron overflow. Further analyses reveal that at low current densities (less than 150 A/cm2) direct electron leakage dominates the mechanism for electron overflow, and simultaneously for efficiency droop, while electron overflow is mainly caused by Auger assisted leakage at high current density. These results identify the main mechanisms for efficiency droop of LEDs at different current density regimes.

Broadband light sources based on InAs/InGaAs metamorphic quantum dots

We propose a design for a semiconductor structure emitting broadband light in the infrared, based on InAs quantum dots (QDs) embedded into a metamorphic step-graded InxGa1−xAs buffer. We developed a model to calculate the metamorphic QD energy levels based on the realistic QD parameters and on the strain-dependent material properties; we validated the results of simulations by comparison with the experimental values. On this basis, we designed a p-i-n heterostructure with a graded index profile toward the realization of an electrically pumped guided wave device. This has been done by adding layers where QDs are embedded in InxAlyGa1−x−yAs layers, to obtain a symmetric structure from a band profile point of view. To assess the room temperature electro-luminescence emission spectrum under realistic electrical injection conditions, we performed device-level simulations based on a coupled drift-diffusion and QD rate equation model. On the basis of the device simulation results, we conclude that the present proposal is a viable option to realize broadband light-emitting devices.

Study on blue organic light-emitting diodes doped with 4,4’‐bis (9‐ethyl‐3carbazovinylene)‐1,1’‐biphenyl in various host materials

We have fabricated efficient blue organic light-emitting devices (OLEDs) with 4,4′-bis (9-ethyl-3carbazovinylene)-1,1′-biphenyl(BCzVBi) as the fluorescent emitter doped into 4,4′-bis(carbazol-9-yl)biphenyl(CBP) and 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi), respectively, and the results show that luminance and luminous efficiency are greatly enhanced in the doped devices. Particularly, the optimized blue CBP-host device with a well-designed structure has a significantly higher luminous efficiency of 4.45cd/A. The energy level structure of the BCzVBi molecules is obtained, which yields useful information on the light emission processes. We carry out a spectroscopic analysis based on Gaussian multi-peak fit for the electroluminescence (EL) emission spectra and present a theoretical explanation of the energy transfer mechanism in the host–guest system. These are expected to provide an effective strategy in enhancing high-efficiency blue OLEDs.

CGL 층으로 MoOx를 사용한 다중 적층구조 OLED의 발광 특성

We studied emission characteristics of blue fluorescent multi-tandem OLEDs using Al/MoO x as charge generation layer(CGL). Threshold voltage for 2, 3, 4, and 5 units tandem OLEDs was 8, 11, 14 and 18 V, respectively. The threshold voltage in multi-tandem OLEDs was lower than multiple of 4 V for the single OLED. Maximum current efficiency and maximum quantum efficiency of single OLED were 7.6 cd/A and 5.5%. Maximum current efficiency for 2, 3, 4, and 5 units tandem OLEDs was 22.6, 31.4, 41.2, and 46.6 cd/A, respectively. Maximum quantum efficiency for 2, 3, 4, and 5 units tandem OLEDs was 11.8, 15.8, 21.8, and 25.6%, respectively. The maximum current efficiency and maximum quantum efficiency in multi-tandem OLEDs were higher than multiple of those for the single OLED. The intensity for 508 nm peak was changed and the peak wavelength was red shift by increase of tandem unit in electroluminescent emission spectra. These phenomena can be caused by micro-cavity effect with increasing of organic layer thickness.

Nanothermometer Based on Resonant Tunneling Diodes: From Cryogenic to Room Temperatures.

Sensor miniaturization together with broadening temperature sensing range are fundamental challenges in nanothermometry. By exploiting a large temperature-dependent screening effect observed in a resonant tunneling diode in sequence with a GaInNAs/GaAs quantum well, we present a low dimensional, wide range, and high sensitive nanothermometer. This sensor shows a large threshold voltage shift of the bistable switching of more than 4.5 V for a temperature raise from 4.5 to 295 K, with a linear voltage-temperature response of 19.2 mV K(-1), and a temperature uncertainty in the millikelvin (mK) range. Also, when we monitor the electroluminescence emission spectrum, an optical read-out control of the thermometer is provided. The combination of electrical and optical read-outs together with the sensor architecture excel the device as a thermometer with the capability of noninvasive temperature sensing, high local resolution, and sensitivity.

Defect-induced color-tunable monolithic GaN-based light-emitting diodes

We have demonstrated defect-induced color-tunable monolithic GaN-based vertical light-emitting diodes (VLEDs). With Ag nanorod arrays embedded in p-GaN, large numbers of Ga vacancies (VGa) were produced during the thermal bonding process in VLED fabrication. VGa-related donor–acceptor pair (DAP) transitions in p-GaN resulted in red emission in photoluminescence (PL) measurements as well as a broad electroluminescence (EL) emission spectrum extending from green to red. In combination with high-emission-efficiency blue InGaN/GaN multiple quantum wells (MQWs), the emission color of VLEDs can be changed from red to white by increasing the injection current.

Defect Analysis of Monolithic GaInP/GaInAs/Ge Triple-Junction Solar Cells by Luminescence Technology

The luminescence of solar cell gives unique signatures of the bandgap and cell composition. Electroluminescence (EL) and photoluminescence (PL) of the monolithic GaInP/GaInAs/Ge triple-junction solar cells are investigated to probe the quality of each subcell. The band-to-band emissions from GaInP, GaInAs, and Ge junctions are observed at the wavelengths of 0.7, 0.9, and 1.8 μm, respectively. The emission from tunneling junction and defects are also characterized. By adjusting the excitation wavelength, PL spectra can probe individual subcell. EL spectra can measure spectral uniformity and defect level of individual subcell with selected filter. PL and EL mapping with selected filter corresponding to relative emission wavelength range can help to determine the spatial uniformity, defect, and hot spots of individual layer. The PL and EL emission spectra are corresponding to absorption edge and temperature coefficient. PL complementary to EQE measurement can give a quick look of the current matching for cell design.

Emission Spectral Control of a Silicon Light Emitting Diode Fabricated by Dressed-Photon-Phonon Assisted Annealing Using a Short Pulse Pair

We fabricated a high-efficiency infrared light emitting diode (LED) via dressed-photon-phonon (DPP) assisted annealing of a p-n homojunctioned bulk Si crystal. The center wavelength in the electroluminescence (EL) spectrum of this LED was determined by the wavelength of a CW laser used in the DPP-assisted annealing. We have proposed a novel method of controlling the EL spectral shape by additionally using a pulsed light source in order to control the number of phonons for the DPP-assisted annealing. In this method, the Si crystal is irradiated with a pair of pulses having an arrival time difference between them. The number of coherent phonons created is increased (reduced) by tuning (detuning) this time difference. A Si-LED was subjected to DPP-assisted annealing using a 1.3 μm (hν=0.94 eV) CW laser and a mode-locked pulsed laser with a pulse width of 17 fs. When the number of phonons was increased, the EL emission spectrum broadened toward the high-energy side by 200 meV or more. The broadening towards the low-energy side was reduced to 120 meV.

Observation of the energy transfer sequence in an organic host–guest system of a luminescent polymer and a phosphorescent molecule

We used steady state optical spectroscopies such as photoluminescence and photoinduced absorption (PA), and magnetic-field PA (MPA) for studying the energy transfer dynamics in films and organic light emitting diodes (OLED) based on host–guest blends with different guest concentrations of the fluorescent polymer poly-[2-methoxy, 5-(2′-ethyl-hexyloxy)phenylene vinylene] (MEHPPV-host), and phosphorescent molecule PtII-tetraphenyltetrabenzoporphyrin [Pt(tpbp); guest]. We show that the energy transfer process between the excited states of the host polymer and guest molecule takes a ‘ping-pong’ type sequence, because the lowest guest triplet exciton energy, ET(guest), lies higher than that of the host, ET(host). Upon photon excitation the photogenerated singlet excitons in the host polymer chains first undergo a Förster resonant energy transfer process to the guest singlet manifold, which subsequently reaches ET(guest) by intersystem crossing. Because ET(guest)>ET(host) there is a subsequent Dexter type energy transfer from ET(guest) to ET(host). This energy transfer sequence has profound influence on the photoluminescence and electroluminescence emission spectra in both films and OLED devices based on the MEHPPV-Pt(tpbp) system.