P-type Hole Injection Layer Research Articles

Balanced Resistivity in n-AlGaN Layer to Increase the Current Uniformity for AlGaN-Based DUV LEDs

In this work, by sandwiching a p-AlGaN layer into the n-AlGaN layer, we generate an energy barrier in the n-AlGaN layer to improve the current spreading effect of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). The abrupt energy barrier can balance the resistivity in the n-AlGaN layer, and this leads to the better lateral current distribution in the p-type hole injection layer. Numerical and experimental results show that the homogenized current distribution enables the enhanced external quantum efficiency (EQE) for the proposed DUV LED. In addition, the better conductivity modulation arising from the more homogenized current distribution decreases the forward voltage and suppresses the self-heating effect.

Impact of an AlGaN thin insertion layer in the p-GaN region on InGaN/GaN vertical-cavity surface-emitting laser diodes

In this work, we propose a p-GaN/p-AlxGa1-xN/p-GaN structure in p-type hole injection layer for InGaN/GaN vertical-cavity surface-emitting laser diodes (VCSEL), and this design can efficiently improve the hole injection into the multiple quantum wells (MQWs). In the p-GaN/p-AlxGa1-xN/p-GaN structure, the polarization induced charges generated at the heterojunction interface can produce the polarization induced electric field, which can accelerate holes. Meanwhile, the p-AlxGa1-xN layer also favors the current transport into the cavity center that can better enhance the device performance, i.e., the threshold current can be reduced and the lasing power can be enhanced. Moreover, we also find that the device performance is sensitive to the structural designs for the p-GaN/p-AlxGa1-xN/p-GaN junctions. Thus, the parametric investigations have also been conducted in this work.

Interfacial charge transfer study of hexacarbonitrile-based intermediate connecter in blue tandem organic light-emitting diodes

The charge generation process of hexaazatriphenylene hexacarbonitrile-based intermediate connectors (ICs) in blue tandem organic light-emitting diodes (OLEDs) is investigated via photoelectron spectroscopic and device studies. Though 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) is well-known to be a p-type hole-injection layer, our photoelectron spectroscopic results indicate that HATCN becomes the n-type when it is sandwiched between Li-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) and N,N″-diphenyl-N,N″-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) layers, and the charge generation and separation at the Li:BPhen/HATCN/NPB interfaces become energetically favorable, as depicted by their energy level alignments. This explains the superior device performance when Li:BPhen/HATCN/NPB is used as a connecting unit in tandem devices.

Enhancing the lateral current injection by modulating the doping type in the p-type hole injection layer for InGaN/GaN vertical cavity surface emitting lasers.

In this report, we propose GaN-based vertical cavity surface emitting lasers with a p-GaN/n-GaN/p-GaN (PNP-GaN) structured current spreading layer. The PNP-GaN current spreading layer can generate the energy band barrier in the valence band because of the modulated doping type, which is able to favor the current spreading into the aperture. By using the PNP-GaN current spreading layer, the thickness for the optically absorptive ITO current spreading layer can be reduced to decrease internal loss and then enhance the lasing power. Furthermore, we investigate the impact of the doping concentration, the thickness and the position for the inserted n-GaN layer on the lateral hole confinement capability, the lasing power, and the optimization strategy. Our investigations also report that the optimized PNP-GaN structure will suppress the thermal droop of the lasing power for our proposed VCSELs.

On the Impact of Electron Leakage on the Efficiency Droop for AlGaN Based Deep Ultraviolet Light Emitting Diodes

In this work, we have investigated the origin of efficiency droop for AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). We find that the efficiency droop is likely to be caused by the electron leakage for DUV LEDs studied in this work. The electron leakage arises from the unbalanced electron and hole injection efficiencies. The correlation between the efficiency droop and the electron leakage can be numerically calculated by manipulating the conduction band barrier height of the p-AlGaN electron blocking layer (p-EBL) for the proposed DUV LEDs. For the purpose of demonstrating that the efficiency droop for DUV LEDs can be experimentally decreased by reducing the electron leakage, a p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GaN/In0.15Ga0.85N/n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GaN tunnel junction on DUV LED is grown by using metal organic chemical vapor deposition (MOCVD) technology. The tunnel junction helps to enhance the hole injection, thus decreasing the electron leakage and the efficiency droop. Moreover, the parasitic emission in the p-type hole injection layer is no longer observed thanks to the decreased electron leakage level.

Modulating the Layer Resistivity by Band-Engineering to Improve the Current Spreading for DUV LEDs

In this letter, we propose to enhance the hole injection efficiency by modulating the layer resistivity in the n-AlGaN layer for 280 nm AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs). The layer resistivity for the n-AlGaN layer is controlled by generating the energy barriers, which is enabled by locally engineering the energy band gap for the n-AlGaN layer, such that a thin n-AlGaN layer with high Al composition is inserted before growing the subsequent multiple quantum wells (MQWs). As a result, such inserted n-AlGaN layer is able to tune the current flow path, i.e., improving the current spreading effect in the p-type hole injection layer. The improved current spreading effect favors the promoted hole injection into the active region. We numerically and experimentally obtain the improved external quantum efficiency, the optical power, and the wall-plug efficiency, thanks to the better current spreading and the correspondingly enhanced hole injection capability.

On the Hole Injection for III-Nitride Based Deep Ultraviolet Light-Emitting Diodes.

The hole injection is one of the bottlenecks that strongly hinder the quantum efficiency and the optical power for deep ultraviolet light-emitting diodes (DUV LEDs) with the emission wavelength smaller than 360 nm. The hole injection efficiency for DUV LEDs is co-affected by the p-type ohmic contact, the p-type hole injection layer, the p-type electron blocking layer and the multiple quantum wells. In this report, we review a large diversity of advances that are currently adopted to increase the hole injection efficiency for DUV LEDs. Moreover, by disclosing the underlying device physics, the design strategies that we can follow have also been suggested to improve the hole injection for DUV LEDs.

On the electric-field reservoir for III-nitride based deep ultraviolet light-emitting diodes.

The drift velocity for holes is strongly influenced by the electric field in the p-type hole injection layer for III-nitride based deep ultraviolet light-emitting diodes (DUV LEDs). In this work, we propose an electric-field reservoir (EFR) consisting of a p-AlxGa1-xN/p-GaN architecture to facilitate the hole injection and improve the internal quantum efficiency (IQE). The p-AlxGa1-xN layer in the EFR can well reserve the electric field that can moderately adjust the drift velocity and the kinetic energy for holes. As a result, we are able to enhance the thermionic emission for holes to cross over the p-EBL with a high Al composition provided that the composition in the p-AlxGa1-xN layer is properly optimized to avoid a complete hole depletion therein.

Preparation, characterization, and photoswitching/light-emitting behaviors of coronene nanowires

Coronene nanowires were prepared through the reprecipitation method. The as-prepared one-dimensional (1D) nanostructures were characterized by UV-vis, fluorescence spectra, X-ray diffraction (XRD), optical microscopy, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). We found that coronene nanowires in aqueous solution emitted strong green light instead of blue light for coronene molecules in THF solution. Moreover, the thin film of coronene nanowires on rGO/SiO2/Si electrode produced a strong photocurrent response upon irradiation. In addition, a heterojunction light emitting diode (LED) device with the structure of quartz/ITO/p-coronene nanowires/n-SiC/Ti (10 nm)/Au (120 nm) has been fabricated. The strong electroluminescence (EL) emission centered at ∼430 nm was detected with a forward bias at 20 V. Our result showed that the use of organic nanowires as the p-type hole injection layer could produce diodes with performance better than those with only inorganic thin-film structures.

Improved hole injection and transport of organic light-emitting devices with an efficient p-doped hole-injection layer

A 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine thin film doped with Fe3O4 has been demonstrated an efficient p-type hole-injection layer (HIL) in organic light-emitting devices (OLEDs). The tris-(8-hydroxyquinoline) aluminum-based OLEDs with the p-type HIL exhibit a very low turn-on voltage of 2.4 V and a high luminance of 29 360 cd/m2 at 8 V, while it is 3 V and 6005 cd/m2, respectively, for the nondoped devices. The improvement in the device performance is clarified as arising from the improved hole injection and transport by the results of ultraviolet/visible/near-infrared absorption, x-ray photoelectron spectra and current density-voltage characteristics of hole-only devices.

Enhancement of efficiency of white organic light-emitting diodes with p-type hole injection layer

This study elucidates the optoelectronic properties of high-efficiency p–i–n white organic light-emitting diodes (WOLEDs) with molybdenum oxide (MoO x )-doped 4,4′,4″-tris[2-naphthyl(phenyl)amino] triphenylamine (2-TNATA) as a p-doping hole injection layer ( p-HIL). Devices with 10 wt.% MoO x doping in 2-TNATA had electroluminescent (EL) efficiencies of 7.71 cd/A and 4.32 lm/W, respectively, at a driving voltage of 5.65 V and current density of 20 mA/cm 2. The X-ray photoelectron spectroscopy (XPS) and UV–vis–NIR absorption spectra of MoO x -doped 2-TNATA layers revealed that MoO x -doped 2-TNATA p-HIL had an improvement on hole conductivity. Such the improvement is caused by the formation of the charge transfer (CT) complex (MoO x −/2-TNATA +) that is generated by doping MoO x into 2-TNATA, markedly increasing the number of hole carriers, improving the balance of the electrons and holes in the recombination zone. Additionally, annealing improves the morphological stability of the p-HIL and extends the lifetime of the p–i–n WOLED.