Exciton Confinement Effect Research Articles

Sub-Second Long Lifetime Triplet Exciton Reservoir for Highly Efficient and Stable Organic Light-Emitting Diode.

In organic light-emitting diode (OLED), achieving high efficiency requires effective triplet exciton confinement by carrier-transporting materials, which typically have higher triplet energy (ET) than the emitter, leading to poor stability. Here, an electron-transporting material (ETM), whose ET is 0.32 eV lower than that of the emitter is reported. In devices, it surprisingly exhibits strong confinement effect and generates excellent efficiency. Additionally, the device operational lifetime is 4.9 times longer than the device with a standard ETM, 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl) phenyl (whose ET 0.36 eV is higher than the emitter). This anomalous finding is ascribed to the exceptionally long triplet state lifetime (≈0.2 s) of the ETM. It is named as long-lifetime triplet exciton reservoir effect. The systematic analysis reveals that the long triplet lifetime of ETM can compensate the requirement for high ET with the help of endothermic energy transfer. Such combination of low ET and long lifetime provides equivalent exciton confinement effect and high molecular stability simultaneously. It offers a novel molecular design paradigm for breaking the dilemma between high efficiency and prolonged operational lifetime in OLEDs.

Energy-well band structure for enhanced exciton confinement effect in perovskite light-emitting diodes via spray-coating

Perovskites possess bright emission and high color purity, but constructing strong charge confinement is requisite for application in efficient light-emitting diodes. Here, we demonstrate a breakthrough to enhance the exciton confinement effect by introducing a Cs4PbBr6 layer on a CsPbBr3 perovskite film through the spray-coating deposition. The impact of the Cs4PbBr6 layer is thoroughly investigated and indicates that the Cs4PbBr6 layer results in a confined energy-wall band structure and a stronger exciton binding energy. Accordingly, the CsPbBr3 film with Cs4PbBr6 wall exhibits higher photoluminescence (PL) and electroluminescence (EL) from the perovskite light-emitting diodes (PeLEDs). We further optimize the device architecture in an energy-well band structure by sandwiching the CsPbBr3 between the Cs4PbBr6 buried layer and overlayer. As a result, the PeLED with Cs4PbBr6 well exhibits ∼18 nm of full width at half maximum (FWHM) in the emission spectrum, 25,738 cd m−2 of maximum luminance, 65.94 cd A−1 of maximum current efficiency (CE), 15.08% of maximum external quantum efficiency (EQE).

Structural, elastic, electronic, optical and vibrational properties of single-layered, bilayered and bulk molybdenite MoS2-2H.

In recent years, transition metal dichalcogenides have received great attention since they can be prepared as two-dimensional semiconductors, presenting heterodesmic structures incorporating strong in-plane covalent bonds and weak out-of-plane interactions, with an easy cleavage/exfoliation in single or multiple layers. In this context, molybdenite, the mineralogical name of molybdenum disulfide, MoS2, has drawn much attention because of its very promising physical properties for optoelectronic applications, in particular a band gap that can be tailored with the material's thickness, optical absorption in the visible region and strong light-matter interactions due to the planar exciton confinement effect. Despite this wide interest and the numerous experimental and theoretical articles in the literature, these report on just one or two specific features of bulk and layered MoS2 and sometimes provide conflicting results. For these reasons, presented here is a thorough theoretical analysis of the different aspects of bulk, monolayer and bilayer MoS2 within the density functional theory (DFT) framework and with the DFT-D3 correction to account for long-range interactions. The crystal chemistry, stiffness, and electronic, dielectric/optical and phonon properties of single-layered, bilayered and bulk molybdenite have been investigated, to obtain a consistent and detailed set of data and to assess the variations and cross correlation from the bulk to single- and double-layer units. The simulations show the indirect-direct transition of the band gap (K-K' in the first Brillouin zone) from the bulk to the single-layer structure, which however reverts to an indirect transition when a bilayer is considered. In general, the optical properties are in good agreement with previous experimental measurements using spectroscopic ellipsometry and reflectivity, and with preliminary theoretical simulations.

Monolayer MoS2 for nanoscale photonics

Abstract Transition metal dichalcogenides are two-dimensional semiconductors with strong in-plane covalent and weak out-of-plane interactions, resulting in exfoliation into monolayers with atomically thin thickness. This creates a new era for the exploration of two-dimensional physics and device applications. Among them, MoS2 is stable in air and easily available from molybdenite, showing tunable band-gaps in the visible and near-infrared waveband and strong light-matter interactions due to the planar exciton confinement effect. In the single-layer limit, monolayer MoS2 exhibits direct band-gaps and bound excitons, which are fundamentally intriguing for achieving the nanophotonic and optoelectronic applications. In this review, we start from the characterization of monolayer MoS2 in our group and understand the exciton modes, then explore thermal excitons and band renormalization in monolayer MoS2. For nanophotonic applications, the recent progress of nanoscale laser source, exciton-plasmon coupling, photoluminescence manipulation, and the MoS2 integration with nanowires or metasurfaces are overviewed. Because of the benefits brought by the unique electronic and mechanical properties, we also introduce the state of the art of the optoelectronic applications, including photoelectric memory, excitonic transistor, flexible photodetector, and solar cell. The critical applications focused on in this review indicate that MoS2 is a promising material for nanophotonics and optoelectronics.

Effects of Charge Balance and Exciton Confinement on the Operational Lifetime of Blue Phosphorescent Organic Light-Emitting Diodes

For 15 years, the rapid degradation of luminance over time has rendered blue phosphorescent light emitting diodes (PHOLEDs) too short-lived for many practical applications. To date, studies have concentrated on the degradation of the emitting materials, yet not the many other parts of these devices. The authors develop a method to measure charge and exciton leakage directly over time, and find that these effects are $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t$ responsible for PHOLED degradation---an important revelation that shifts our attention toward the actual sources of failure.

Effect of substrates and thickness on optical properties in atomic layer deposition grown ZnO thin films

Atomic Layer Deposition technique was used to grow high quality, very low roughness, crystalline, Zinc Oxide (ZnO) thin films on silicon (Si) and fused quartz (SiO2) substrates to study the optical properties. Spectroscopic ellipsometry results of ZnO/Si system, staggered type-II quantum well, demonstrate that there is a significant drop in the magnitudes of both the real and imaginary parts of complex dielectric constants and in near-band gap absorption along with a blue shift of the absorption edge with decreasing film thickness at and below ∼20nm. Conversely, UV–vis absorption spectroscopy of ZnO/SiO2, thin type-I quantum well, consisting of a narrower-band gap semiconductor grown on a wider-band gap (insulator) substrate, shows the similar thickness dependent blue-shift of the absorption edge but with an increase in the magnitude of near-band gap absorption with decreasing film thickness. Thickness dependent blue shift, energy vs. 1/d2, in two different systems, ZnO/Si and ZnO/SiO2, show a difference in their slopes. The observed phenomena can be consistently explained by the corresponding exciton (or carrier/s) deconfinement and confinement effects at the ZnO/Si and ZnO/SiO2 interface respectively, where Tanguy-Elliott amplitude pre-factor plays the key role through the electron-hole overlap factor at the interface.

Photoluminescence and Confinement of Excitons in Disordered Porous Films

The exciton confinement effect in quantum dots at the surface of SiO2 spheres and the percolation phase transition in films based on a mixture of pure SiO2 spheres and spheres covered by CdS quantum dots (SiO2/CdS nanoparticles) are studied. It is found that, due to the high surface energy of spheres, the quantum dots deposited onto their surface are distorted, which modifies the exciton confinement effect: the effect is retained only in one direction, the direction normal to the surface of the spheres. As a result, the energy of the exciton ground state exhibits a complex dependence on both the quantum-dot radius and sphere size. In the optical spectra of films based on this mixture, the clustering of small-sized nanoparticles and then, at a critical concentration of nanoparticles of ~60%, the formation of a percolation cluster are detected for the first time. The critical concentration is twice higher than the corresponding quantity given by the model of geometrical “colored percolation”, which is a consequence of interaction between submicrometer nanoparticles. The relation between the basic parameters of the percolation transition, such as the film porosity, coordination number, and the quantity defining the number of particles in the percolation cluster, is obtained and analyzed.

Correction: Corrigendum: Exciton and core-level electron confinement effects in transparent ZnO thin films

The excitonic light emission of ZnO films have been investigated by means of photoluminescence measurements in ultraviolet-visible region. Exciton confinement effects have been observed in thin ZnO coatings with thickness below 20 nm. This is enhanced by a rise of the intensity and a blue shift of the photoluminescence peak after extraction of the adsorbed species upon annealing in air. It is found experimentally that the free exciton energy (determined by the photoluminescence peak) is inversely proportional to the square of the thickness while core-level binding energy is inversely proportional to the thickness. These findings correlate very well with the theory of kinetic and potential confinements.

Photon antibunching in single-walled carbon nanotubes at telecommunication wavelengths and room temperature

We investigated the photoluminescence of individual air-suspended single-walled carbon nanotubes (SWNTs) from 6 to 300 K. Time-resolved and antibunching measurements over the telecommunication wavelength range were performed using a superconducting single-photon detector. We detected moderate temperature independent antibunching behavior over the whole temperature range studied. To investigate the exciton dynamics, which is responsible for the antibunching behavior, we measured excitation-power and temperature dependence of the photoluminescence spectra and lifetime decay curves. These measurements suggested an exciton confinement effect that is likely caused by high-dielectric amorphous carbon surrounding the SWNTs. These results indicate that SWNTs are good candidates for light sources in quantum communication technologies operating in the telecommunication wavelength range and at room temperature.

Effect of broad recombination zone in multiple quantum well structures on lifetime and efficiency of blue organic light-emitting diodes

Blue phosphorescent organic light-emitting diodes with multiple quantum well (MQW) structures (from one to four quantum wells) within an emitting layer (EML) are fabricated with charge control layers (CCLs) to control carrier movement. The distributed recombination zone and balanced charge carrier injection within EML are achieved through the MQW structure with CCLs. Remarkably, the half-decay lifetime of a blue device with three quantum wells, measured at an initial luminance of 500 cd/m2, is 3.5 times longer than that using a conventional structure. Additionally, the device’s efficiency improved. These results are explained with the effects of triplet exciton confinement and triplet–triplet annihilation within each EML.

Exciton and core-level electron confinement effects in transparent ZnO thin films

The excitonic light emission of ZnO films have been investigated by means of photoluminescence measurements in ultraviolet-visible region. Exciton confinement effects have been observed in thin ZnO coatings with thickness below 20 nm. This is enhanced by a rise of the intensity and a blue shift of the photoluminescence peak after extraction of the adsorbed species upon annealing in air. It is found experimentally that the free exciton energy (determined by the photoluminescence peak) is inversely proportional to the square of the thickness while core-level binding energy is inversely proportional to the thickness. These findings correlate very well with the theory of kinetic and potential confinements.

Fluorinated Carbazole Derivatives as Wide-Energy-Gap Host Material for Blue Phosphorescent Organic Light-Emitting Diodes

2,7-Difluo-carbazole and 2,4,5,7-tetrafluoro-carbazole were synthesized as new building blocks of wide-energy-gap host material for phosphorescent organic light-emitting diodes (PHOLEDs). These fluorinated positions in the carbazole ring were determined on the basis of density functional theory calculation results. Spectroscopic analyses supported the hypothesis that poly(N-vinyl-2,7-difluoro-carbazole) (2,7-F-PVK) with the fluorinated pendant group possessed a wide energy gap, leading to the exciton energy confinement on the blue phosphorescent dopant as well as nonsubstituted poly(N-vinyl-carbazole) (PVK). 2,7-F-PVK was used in solution-processed blue PHOLED to achieve 27 cd/A at 760 cd/m2, which is 1.8 times higher than that of nonsubstituted PVK. We assumed that the replacement of nonsubstituted PVK with 2,7-F-PVK improved the charge balance in the emission layer, while keeping the exciton confinement effect. The fluorination of the carbazole ring is a useful molecular design strategy for wide-energy-gap host material.

Crossover from strong to weak exciton confinement and third-harmonic generation on one-dimensional quantum dots

We study the effects of exciton confinement on the nonlinear optical susceptibility of one-dimensional quantum dots. We use a direct numerical diagonalization to obtain the eigenenergies and eigenstates of the discretized Hamiltonian representing an electron–hole pair confined by a semiparabolic potential and interacting with each other via a Coulomb potential. Density matrix perturbation theory is used to compute the nonlinear optical susceptibilities due to third-harmonic generation and the corresponding nonlinear corrections to the refractive index and absorption coefficient. These quantities are analyzed as a function of ratio between the confinement length L and the exciton Bohr radius a0. The Coulomb potential degrades the uniformity of the level separation. We show that this effect promotes the emergence of multiple resonance peaks in the third-harmonic generation spectrum. In the weak confinement regime β=L/a0≫1, the third-order susceptibility is shown to decay as 1/β8 due to the prevalence of the hydrogenoid character of the exciton eigenstates.

Effect of triplet multiple quantum well structures on the performance of blue phosphorescent organic light-emitting diodes

We investigate multiple quantum well [MQW] structures with charge control layers [CCLs] to produce highly efficient blue phosphorescent organic light-emitting diodes [PHOLEDs]. Four types of devices from one to four quantum wells are fabricated following the number of CCLs which are mixed p- and n-type materials, maintaining the thickness of the emitting layer [EML]. Remarkably, such PHOLED with an optimized triplet MQW structure achieves maximum luminous and external quantum efficiency values of 19.95 cd/A and 10.05%, respectively. We attribute this improvement to the efficient triplet exciton confinement effect and the suppression of triplet-triplet annihilation which occurs within each EML. It also shows a reduction in the turn-on voltage from 3.5 V (reference device) to 2.5 V by the bipolar property of the CCLs.

Embedded Silicon Nanocrystals Studied by Photoluminescence and Raman Spectroscopies: Exciton and Phonon Confinement Effects

The optical and vibrational properties of silicon nanocrystals are studied in two systems elaborated by evaporation. The first one is constituted by a thick SiO layers. The second one is a multilayered sample made by successive evaporations of SiO and SiO2 layers with controlled thicknesses. The luminescence and Raman spectra are fitted by phenomenological exciton and phonon confinement models accounting for the size distribution of the embedded nanocrystals. The coherence between the two models and experimental data is demonstrated and gives support to the notion of exciton and phonon confinement effect in silicon nanocrystals embedded within silica matrix.

Enhanced efficiency and reduced roll-off in nondoped phosphorescent organic light-emitting devices with triplet multiple quantum well structures

Highly efficient nondoped phosphorescent organic light-emitting devices (NPOLEDs) with triplet multiple quantum well structures are fabricated by using 4,4′-N,N′-dicarbazole-biphenyl and an iridium(III) complex as the potential barrier layer and the potential well layer/light-emitting layer, respectively. Remarkably, such NPOLED with an optimized device configuration achieves reduced current efficiency roll-off, which slightly decreases from its peak value of 31.5 cd/A at 19.8 mA/cm2 to 29.2 cd/A at 100 mA/cm2. We attribute this improvement to the efficient triplet exciton confinement effect and the suppression of triplet-triplet annihilation which occurs via single-step long range (Förster-type) energy transfer between excited molecules.

High-Efficiency Blue Emitting Phosphorescent OLEDs

In this paper, we will show our latest results on high-efficiency blue phosphorescent organic light-emitting diodes (PHOLEDs). Effects of triplet exciton confinement, exciton energy transfer and charge trapping, and charge balance on iridium(III)bis [(4,6-di-fluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic)-based blue PHOLEDs will be presented. By optimizing the aforementioned device parameters, a high-efficiency blue PHOLED with 59 cd/A (48 lm/W at 100 cd/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) was demonstrated.

Variation of ZnO shell thickness and its effects on the characteristics of coaxialnanowires

We have changed the thickness of the ZnO shell layers inSiOx/ZnO core–shell nanowires by increasing the atomic layer deposition (ALD) cycle, using diethylzinc (DEZn)and H2O as precursors of Zn and O, respectively. The samples were characterized by means of x-raydiffraction (XRD), scanning electron microscopy, transmission electron microscopy, andphotoluminescence (PL) spectroscopy. While the thickness of the ZnO shell layers increasedalmost linearly with increase of the ALD cycle, their tubular morphologies weremaintained. The variation in the thickness of the ZnO shell layers was accompaniedby changes in the relative intensities of the characteristic ultraviolet (UV) andvisible emission bands and by a shift of the UV peak position. On the basis of theobserved relationship between the XRD shell grain size and the PL shift, we suggestthat the PL shift is attributable to an exciton confinement effect. The findingsin this study can be applied to a wide range of materials, and are expected tocontribute to the development of potential applications of coaxial nanowires.

Investigation of spectrum blue shift in organic quantum well electroluminescent devices

In this paper, the organic quantum well devices, which are similar to the type-II quantum well of inorganic semiconductor, have been fabricated, in which the NPB (N,N’-Di-[(1-naphthalenyl)-N,N’-diphenyl]-(1,1’-biphenyl)-4,4’-diamine) and Alq3 (Tris-(8-quinolinolato)aluminum) act as the potential barrier layer and the potential well layer, respectively. In the electroluminescence, the blue shift of spectrum with the decreasing of well width is observed for the device with different well width, and this is interpreted by combination of quantum size effect and exciton confinement effect. The blue shift of spectrum with increasing applied voltage is observed for the same device, and this is interpreted in terms of polarization effect and quantum size effect.

Luminescence properties of type-II quantum well light-emitting diodes formed with NPB and Alq3

The organic quantum well devices which are similar to the type-II quantum well of inorganic semiconductor have been fabricated. In the electroluminescence, the blue shift of spectrum with increasing applied voltage is observed, which is interpreted by exciton confinement effect and polarization effect, and the generation of exciton, including carrier injection and energy transfer, is discussed. This energy transfer from barrier to well is studied by photoluminescence and is interpreted in terms of Förster energy transfer. The electromodulation of photoluminescence demonstrates the quenching mainly comes from the dissociation of exciton in NPB and that in Alq3 is very stable.