Quantum confinement of carriers in heterostructured GaAs/GaP quantum wires

We present theoretical calculations on the confinement of electrons and holes in a heterostructured GaAs/GaP quantum wires within the effective mass approximation. Energy levels (ground state and excited states), as well as electron-hole recombination energies for some transitions, are calculated for different combinations of wire radius, well widths, and interface thickness.     

Antagonistic responses between magnetoconductance and magnetoelectroluminescence in polymer light-emitting diodes

Antagonistic responses between magnetoconductance (MC) and magnetoelectroluminescence (MEL) in the polymer light-emitting diodes with an interfacial layer between Al cathode and active layer are simultaneously measured. As the interfacial layer (tetraoctylammonium bromide) is used, the significant increase in the number of injected negative polarons and the blocking of positive polarons promote the triplets-(free polaron) reaction and provide a good explanation for the reason that electroluminescence (EL) efficiency is maximal in the trap free space charge limited current regime at high bias. By fitting of MC and MEL curves using Lorentzian and non-Lorentzian empirical equations, three magnetic field dependent mechanisms, which are the intersystem crossing between singlet/triplet polaron pairs, the triplets-(free polaron) reaction, and the triplets-(trapped polaron) reaction are elucidated. The distribution of the three components is tunable by varying the applied electric field, which primarily modulates the triplets-(free polaron) reaction rate. The results pave a new route toward understanding the mechanism of organic spintronics for developing of multifunctional devices.     

Origin of the radiative emission in blue-green light emitting diodes based on GaN/InGaN heterostructures

Phase separation effects induced by spinodal decomposition taking place in cubic In_xGa_1−xN epitaxial layers were investigated by means of resonant Raman scattering (RRS) and X-ray diffractometry (XRD) experiments. The alloy epilayers were grown by radio-frequency plasma-assisted molecular beam epitaxy on GaAs (001) substrates. Ab initio theoretical calculation of the alloy phase diagram predicts the formation of In-rich phases in the layers which is confirmed by the RRS and XRD experiments. Photoluminescence observed at room temperature and 30 K from the layers shows light emission in the blue-green region of the spectrum. RRS experiments demonstrated that the observed emission is directly linked to the In-rich separated phases (quantum dots) in the alloy. The results support the model that the origin of light emission in nitride-based light emitting diodes and laser diodes is related to quantum confinement effects taking place in quantum dots formed in the InGaN layers, active media of the devices.     

Highly efficient all-solution processed blue quantum dot light-emitting diodes based on balanced charge injection achieved by double hole transport layers

Solution-processed blue quantum dot light-emitting diodes (QLEDs) suffer from low device efficiency, whereas the balance of electron and hole injection is critical for obtaining high efficiency. Herein, synergistical double hole transport layers (D-HTLs) are employed, which use poly(9-vinylcarbazole) (PVK) stacked on poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB). The fabrication of D-HTLs is achieved by using dimethyl formamide (DMF) as the solvent for PVK, with which the underlying TFB layer almost remains unwashed and undamaged during the spin-coating process of PVK layer. TFB/PVK D-HTLs form the stepwise energy level for hole injection, which reduces the hole injection barrier and favors the carrier balance in the emission layer (EML). The optimized blue QLED with TFB/PVK D-HTLs shows a maximum external quantum efficiency (EQE) of 13.7%, which is 3-fold enhancement compared to that of the control device with single TFB HTL. The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge injection balance for the stepwise D-HTLs based QLEDs. This work manifests the positive effect on performance boost by selecting appropriate solvents towards stepwise D-HTLs formation and paves the way to fabricate highly efficient all-solution processed light emitting diodes.     

Solution-processed quantum dot light-emitting diodes with PANI:PSS hole-transport interlayers

For solution-processed quantum dot light-emitting devices (QD-LEDs), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/poly(N-vinylcarbozole) (PEDOT:PSS/PVK) bilayers have been widely used as the hole injection/transport layer. The high work function of the hole transport layer is crucial for high electroluminescence efficiency with balanced electron/hole charge injection. Herein, we report improvement of the performance of QD-LEDs by inserting a polyaniline (PANI)-poly (p-styrenesulfonic acid) (PSS) (PANI:PSS) hole-transport layer between the PVK and PEDOT:PSS layers. The insertion of the PANI:PSS layer significantly shifted the electronic energy levels of the PVK layers to lower values, which reduced the energy barrier of holes traveling to the QD layer by 0.22 eV. The QD-LEDs with PANI:PSS interlayer exhibited superior electric and electroluminescent characteristics. The hole-only devices with PANI:PSS interlayer also presented high hole injection and transport capability. Ultraviolet photoelectron spectroscopy (UPS) was used to investigate the electronic energy level alignment of the QD-LEDs with/without the PANI:PSS interlayer. The device performance results of QD-LEDs and hole-only devices indicated enhanced electric and electroluminescent characteristics for the PANI:PSS-inserted QD-LEDs with high hole conduction capability, in agreement with UPS findings.     

Efficient larger size white quantum dots light emitting diodes using blade coating at ambient conditions

Here, we propose a facile strategy to realize all-solution-processed highly efficient full-color-enabling white emitting quantum dot light-emitting diodes (QLEDs) at ambient conditions by using low-cost blade coating technique, which was also compatible with the roll to roll fabrication process for large size production. Firstly, by using red quantum dots (QDs) as the representative to optimize the QDs films by blade coating, the QDs films exhibit excellent morphology and well-ordered self-assembly structure. Then, the trichromatic white QLEDs based on mixed red, green and blue quantum dots were obtained with Commission Internationale De I'Eclairage (CIE) coordinates ranging from (0.42, 0.41) to (0.31, 0.33) within the white region of CIE 1931 when driving voltage vary from 5 v to 8 v. The device enjoys excellent optoelectronic performance including a maximum luminance of 11,465 cd/m², a maximum current efficiency (η_A) of 9.2 cd/A and an external quantum efficiency (EQE) of 3.7%. In addition, 3 × 8 cm² white QLEDs with bright and homogenous light emission fabricated by blade coating are demonstrated. Our strategy for fabricating large-area white QLEDs indicate promising applications in the low-cost solid-state lighting and flat-panel displays.     

Controlling charge balance using non-conjugated polymer interlayer in quantum dot light-emitting diodes

The balance of electron–hole charge carriers in quantum dot (QD) light-emitting diodes (QLEDs) is an important factor to achieve high efficiency. However, poor interfacial properties between QDs and their adjacent layers are likely to deteriorate the electron–hole charge balance, resulting in the poor performance of a QLED. In this paper, we report an enhanced efficiency in red-emitting inverted QLEDs by modifying the interface properties between QDs and ZnO electron transport layer (ETL) using a thin layer of non-conjugated polymer, poly(4-vinylpyridine) (PVPy). Based on the precise control of the electrical properties with PVPy, the maximum efficiency of the QLED is enhanced by 30% compared to the device without a PVPy layer. In particular, the efficiency at low current density region is significantly increased. We investigate the effect of the PVPy interlayer on the performance of QLEDs and find that this thin layer not only shifts the energy levels of the underlying ZnO ETL, but also effectively blocks the leakage current at the ETL/QD interface.     

Correlation between interlayer thickness and device performance in blue phosphorescent organic light emitting diodes with a quantum well structure

We systematically examined the effects of interlayer (ITL) thickness variation in an emission layer (EML) on electrical and optical characteristics of blue phosphorescent organic light-emitting diodes. The EML consisted of a quantum well structure using a hole transport material 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as an ITL. This ITL facilitated the confinement of charge carriers in the recombination zone (RZ), adjusted the charge carrier balance in the EML, and prevented the triplet exciton loss to adjacent transport layers. The thickness variation in the ITL greatly influenced the size and location of the RZ and the exciton density (ED), which is related to charge balance and exciton diffusion in the EML. A micro-cavity effect around 500 nm and the corresponding redshift/blueshift in the electroluminescent spectrum arose from different ITL thicknesses. Remarkably, the device having a 5-nm-thick TAPC ITL showed better current and power efficiencies than those of any other devices because of the rearrangement of the locations of excitons and ED through control of the hole/electron charge density.     

High morphology stability and ambipolar transporting host for use in blue phosphorescent single-layer organic light-emitting diodes

The authors report a small molecule host of 2,7-bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF) doped with 8 wt% iridium(III)-bis[(4,6-difluorophenyl)pyridinato-N,C^2′]picolinate (FIrpic) for use in efficient and single-layer blue phosphorescent organic light-emitting diodes (PHOLEDs) exhibiting a maximum external quantum efficiency of ∼20.3% at brightness of 100 cd/m². The high performance of such single layer PHOLEDs is attributed to the POAPF host’s high morphological stability, suitable triplet energy level, and equal charge carrier mobilities of hole and electron to form the broad carrier recombination zone in the emitting layer, thus reducing the triplet-triplet annihilation and resulting in a slight efficiency roll off of 0.5% from the brightness of 1 and 1000 cd/m². This work also systematically investigated the arrangement of the POAPF:FIrpic recombination zone for optimizing the performance of the single layer PHOLED.     

On the physics of semiconductor quantum dots for applications in lasers and quantum optics

The progression of carrier confinement from quantum wells to quantum dots has received considerable interests because of the potential to improve the semiconductor laser performance at the underlying physics level and to explore quantum optical phenomena in semiconductors. Associated with the transition from quantum wells to quantum dots is a switch from a solid-state-like quasi-continuous density of states to an atom-like system with discrete states. As discussed in this paper, the transition changes the role of the carrier interaction processes that directly influence optical properties. Our goals in this review are two-fold. One is to identify and describe the physics that allows new applications and determines intrinsic limitations for applications in light emitters. We will analyze the use of quantum dots in conventional laser devices and in microcavity emitters, where cavity quantum electrodynamics can alter spontaneous emission and generate nonclassical light for applications in quantum information technologies. A second goal is to promote a new connection between physics and technology. This paper demonstrates how a first-principles theory may be applied to guide important technological decisions by predicting the performances of various active materials under a broad set of experimental conditions.     

Improved electron injection from silver electrode for all solution-processed polymer light-emitting diodes with Cs2CO3:conjugated polyelectrolyte blended interfacial layer

This study demonstrates the incorporation of a Cs₂CO₃:conjugated polyelectrolyte blended interfacial layer between the emissive layer and a silver (Ag) cathode, for realizing all-solution processed polymer light-emitting diodes. For a device with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the emissive layer, this approach improves the maximum luminance of approximately 80,000 cd/m² and maximum current efficiency of 10.6 cd/A. It is clarified that the interfacial layer prevents Ag nanoparticles from penetrating into the emissive layer, resulting in yellow–green emission from F8BT. We also demonstrate the possibility of all-solution processed polymer light-emitting diodes utilizing solution-processed Cs₂CO₃:conjugated polyelectrolyte interfacial layer and Ag nano-ink.     

Hot electron effects on the operation of potential well barrier diodes

A study has just been carried out on hot electron effects in GaAs/Al_0.3Ga_0.7As potential well barrier (PWB) diodes using both Monte Carlo (MC) and drift-diffusion (DD) models of charge transport. We show the operation and behaviour of the diode in terms of electric field, mean electron velocity and potential, mean energy of electrons and Γ-valley population. The MC model predicts lower currents flowing through the diode due to back scattering at anode (collector) and carrier heating at higher bias. At a bias of 1.0 V, the current density obtained from experimental result, MC and DD simulation models are 1.35, 1.12 and 1.77 μA/μm² respectively. The reduction in current over conventional model, is compensated to a certain extent because less charge settles in the potential well and so the barrier is slightly reduced. The DD model results in higher currents under the same bias and conditions. However, at very low bias specifically, up to 0.3 V without any carrier heating effects, the DD and MC models look pretty similar as experimental results. The significant differences observed in the I–V characteristics of the DD and MC models at higher biases confirm the importance of energy transport when considering these devices.     

Surface initiated oxidative crosslinking of a polymeric hole transport material for improved efficiency and lifetime in soluble organic light-emitting diodes

A surface initiated oxidative coupling method was developed as a crosslinking approach of hole transport materials for solution processed organic light-emitting diodes. The surface initiated crosslinking method was better than bulk oxidative crosslinking method in terms of quantum efficiency and lifetime of the organic light-emitting diodes by suppressing exciton quenching by the hole transport layer. Doubled efficiency and quadrupled lifetime were obtained using the new crosslinking approach without any chemical modification of the hole transport material.     

Inelastic light scattering from electronic excitations in quantum dots

A review is given of progress in our understanding of the electronic excitations in semiconductor quantum dots as studied by inelastic light scattering spectroscopy. Such experiments have revealed the characteristics of single particle, charge density, and spin density excitations of many-electron dots in zero and applied magnetic fields. Theoretical calculations, which reveal an electronic shell structure, are in general accord with the experimental results. Although the behaviour in a magnetic field is extremely complex, it is now feasible to study collective excitations from a range of strongly correlated ground states using this technique.     

Coherence and localization in AC-driven quantum dots

We investigate the effects of an AC field on the dynamics of interacting electrons in quantum dots (QDs). We first consider a double QD containing two electrons. By mapping the system to an effective lattice model and employing Floquet theory we analyze the dynamics of the two-electron wavefunctions in the singlet space. This system presents a richer phenomenology than the case of a single electron in a double QD, displaying two distinct regimes of behaviour depending on whether the applied field strength or the inter-electron Coulomb repulsion is dominant. We then study the dynamics of two electrons in a 2D square QD in the limit of very low density, where the Coulomb interaction causes the electrons to become highly localized, forming a ‘Wigner molecule’. Using the same techniques we investigate how the AC field drives the dynamics of the Wigner states, and show how the parameters of the effective model may be measured in experiment.     

Nonequivalent heterointerfaces in AlGaAs/GaAs double barrier resonant tunnelling diodes grown by metalorganic vapour phase epitaxy

The peak/valley current ratio, of AlGaAs/GaAs double barrier resonant tunnelling diodes at room temperature is found to be very sensitive to the quality of the GaAs/AlGaAs interface, while the AlGaAs/GaAs interface is much less critical.<>     

An antimonene modified hole extraction layer for high efficiency PEDOT:PSS-free nonfullerene organic solar cells

In this paper, a bilayer hole extraction layer (HEL) with solution-processed molybdenum trioxide (MoO₃) and two-dimensional (2D) material of antimonene was developed to achieve high performance nonfullerene organic solar cells (NF–OSCs). The application of antimonene facilitates effective charge extraction and lowered recombination loss, achieving improved photovoltaic performance. By inserting the antimonene layer, power conversion efficiency (PCE) of devices with MoO₃ HEL was increased from 8.92% to 11.30% in OSCs with non-fullerene systems of PBDB-T-2F:IT-4F, which was even much higher than that of the devices with PEDOT:PSS HEL (10.59%). Results make it clear that the solution-processed bilayer MoO₃/antimonene HEL shows great potential for application in high performance PEDOT:PSS-free NF–OSCs.     

Effect of quantum dot shape on dynamical dephasing suppression in exciton qubits under applied electric field

We investigate the efficiency of periodic dynamical decoupling of an exciton qubit confined in a self-assembled quantum dot in the presence of an applied electric field. The shape of the quantum dot is found to have a large effect on the excitonic dephasing. It is shown that dynamical suppression of dephasing, through a simple series of equally spaced bit flips, is most efficient for quantum dots that are close to spherical. In addition, compared to the no field case, the presence of an electric field increases the efficiency of the decoupling technique as the quantum dot becomes more oblate. Our calculations show that dephasing can be significantly suppressed in GaAs/AlAs quantum dots suitable for quantum information processing.     

Transient electroluminescence in heavy metal complex-based phosphorescent organic light-emitting diodes

In phosphorescent organic light-emitting diodes (PHOLEDs), both the rise time and decay time decrease with increasing amplitude of the applied voltage pulse. The rise time τ_r of the transient electroluminescence (TEL) increases linearly with increasing value of the ratio of voltage V to the current j, that is, with V/j. Using the equations for the dynamics of charge carriers an expression is derived for the rise time τ_r of the TEL in OLEDs. It is shown that τ_r should increase with increasing values of the ratio (V/j), dielectric constant ε, and area of cross-section of the emission layer, however, it should decrease with the thickness of emission layer. For higher values of the applied voltage nonlinearity occurs in the τ_r versus V/j plot because the increase in mobility of carriers at high electric field causes increase in the current flowing through the OLEDs. In fact, the rise time of TEL is related to the product of capacitance and effective resistance of the OLED. Considering the rate of generation and decay of radiative triplet excitons in the emission layer, an expression is derived for the decay time of TEL in PHOLEDs and it is shown that, for higher values of the time-constant of OLED, the decay time should be equal to the time-constant, however, for lower values of the time-constant, the decay time should be equal to the lifetime of radiative triplet excitons in the emission layer. A good agreement is found between the theoretical and experimental results.