Injection Barrier Research Articles

Energy-Efficient Stable Hyperfluorescence Organic Light-Emitting Diodes with Improved Color Purities and Ultrahigh Power Efficiencies Based on Low-Polar Sensitizing Systems.

Multi-resonance (MR) molecules with thermally activated delayed fluorescence (TADF) are emerging as promising candidates for high-definition displays because of their narrow emission spectra. However, the electroluminescence (EL) efficiencies and spectra of MR-TADF molecules are highly sensitive to hosts and sensitizers when applied to organic light-emitting diodes (OLEDs), and the highly polar environments in devices often lead to significantly broadened EL spectra. In this study, a proof-of-concept TADF sensitizer (BTDMAC-XT) with low polarity, high steric hindrance, and concentration-quenching free feature is constructed, which acts as a good emitter in doped and non-doped OLEDs with high external quantum efficiencies (ηext s) of 26.7% and 29.3%, respectively. By combining BTDMAC-XT with conventional low-polarity hosts, low-polarity sensitizing systems with a small carrier injection barrier and full exciton utilization are constructed for the MR-TADF molecule BN2. Hyperfluorescence (HF) OLEDs employing the low-polar sensitizing systems successfully improve the color quality of BN2 and afford an excellent ηext of 34.4%, a record-high power efficiency of 166.3lmW-1 and a long operational lifetime (LT50 =40309h) at an initial luminance of 100cdm-2 . These results provide instructive guidance for the sensitizer design and device optimization for energy-efficient and stable HF-OLEDs with high-quality light.

Energy‐Level Regulation and Low‐Dimensional Phase Rearrangement via a Multifunctional Spacer Group toward Efficient Sky‐Blue Quasi‐2D Perovskite Light‐Emitting Diodes

AbstractQuasi‐2D perovskite materials have great potential for achieving high‐performance blue perovskite light‐emitting diodes (PeLEDs). Major challenges lie in the need to minimize the energy level mismatch between the blue‐emitting and the charge‐transport materials for better carrier injection, and to suppress low‐dimensional phases in quasi‐2D systems for improved radiation recombination. Here, this work introduces a subtly functionalized compound, methoxylated phenethylammonium bromide (y‐MeO‐PEABr), into the sky‐blue quasi‐2D perovskite. This approach induces an upward shift of the energy levels of the perovskite and reduces the holes injection barrier, thereby achieving a better charge balance. In addition, the introduction of large y‐MeO‐PEA+ cations effectively suppresses the formation of undesirable phases in quasi‐2D perovskites, leading to a more concentrated phase distribution and accelerated energy transfer. Specifically, the sky‐blue quasi‐2D PeLED based on 2‐MeO‐PEABr exhibits optimal device performance, obtaining a maximum external quantum efficiency of 10.85% at 486 nm. This work suggests that this method may provide a guide for designing novel organic spacer cations to obtain high‐performance blue quasi‐2D PeLEDs.

Efficient Near-Infrared Organic Photodetectors with Spectral Response up to 1600 nm for Accurate Alcohol Concentration Detection

The development of near-infrared organic photodetectors (NIR-OPDs) in 1000-1700 nm is essential for medical monitoring, food quality inspection, machine vision, and biomedical imaging. However, when solving the high dark current density (JD) in bulk-heterojunction (BHJ) NIR-OPDs based on narrow-bandgap systems, it is often accompanied by photocurrent loss, which is a great challenge in achieving high-performance NIR-OPDs. Here, an ideal hybrid pseudo-PHJ (planar-heterojunction)/BHJ structure is proposed to overcome this challenge, which is introducing the N2200 layer between the cathode and BHJ. The introduction of the N2200 raises the external charge injection barrier and reduces the trap density, thus achieving significant suppression of JD (6.22 × 10-7 A cm-2 at -0.2 V bias, about 2 orders of magnitude lower compared to the BHJ NIR-OPDs). Meanwhile, the hybrid structure combines the advantages of PHJ and BHJ, thus maintaining a high photocurrent, resulting in responsivity and detectivity of 18.71 mA W-1 and 4.19 × 1010 Jones, respectively, at 1400 nm at -0.2 V bias, which is superior to the performance of BHJ NIR-OPDs. And the hybrid structured NIR-OPDs are proven to rapidly quantify the alcohol content of mixtures to within 2% accuracy, which exhibits great potential for application in the food and pharmaceutical industries.

The effect of terminal group of hole injection self-assembled monolayers on the performance of optoelectronic devices on ITO anodes

We demonstrate the effect on the performance of conventional (Indium tin oxide (ITO)/N,Nj-Di (1- naphthyl)-N,Nj-diphenyl-(1,1-biphenyl)-4,4-diamine (NPB)/Tris-(8-hydroxyquinoline)aluminum (Alq3)/LiF/Al) optoelectronic devices when the ITO surface is modified with various types of self-assembled monolayer (SAM)s such as 3-(trimethoxysilyl)propylamine (NH2SAM), Trimethoxy (propyl)silane CH3SAM, and Trimethoxy (3,3,3-trifluoropropyl)silane (F3SAM). The head functional group and chain configuration are trimethoxysilylpropyl (-(CH3O)3Si(CH2)2) and the tail groups distinguished SAMs (-NH2, -CH3, and -CF3). The contact angles of NH2SAM, CH3SAM, and F3SAM deposited on ITO and ITO were 70.7°, 81.42°, 68.23°, and 30.85°, respectively. The SAM-modified ITO's surface morphology Rpv and Rq parameters were reduced from 31.02 nm to 2.0 nm and 1.18 nm–0.26 nm. The work functions values of ITO, ITO/NH2SAM, ITO/CH3SAM and ITO/F3SAM measured by UPS were 4.99eV, 4.39, 5.09 and 5.69eV, respectively. The maximum luminance of the SAM-modified device increased to 166-fold, and the current density 105-fold, compared to the unmodified device when the operating voltage is set to 11 V. Current and power efficiency increased by 1.9 and 2-fold, respectively. The hole injection conditions are enhanced and the hole injection barrier height between ITO/NPB is reduced.

Transition between resistive switching modes in asymmetric HfO2-based structures

We claim that the evolution of conductive filaments (CF) in bipolar resistive switching (BRS) and complementary resistive switching (CRS) is similar in HfO2-based structures with one active layer. The difference in the observed I-V characteristics is explained by the presence of a non-Ohmic contact at the interface with the injection electrode, with the higher barrier at the BRS. In order to describe the transition between BRS and CRS in the asymmetric structure, we form Pt/HfO2(2 nm)/HfOXNY (4 nm)/TiN structure. The active layer of the structure and part of the active electrode were formed via plasma-enhanced atomic layer deposition. Top Pt-electrode was formed via sputtering throughout a shadow mask and area of top electrodes was equal to 0.0360 ± 0.0015 mm2. Pulse and DC measurements of I-V characteristics were carried out. A self-compliance RS with a current lower than 10 mA was found. The primary conduction mechanism in the low-resistance state for both RS modes was estimated as space-charge limited current (SCLC). For BRS, the unusual transition from trap-limited conduction to charge-carrier saturation (I ∼ V1.14) is found to be at 0.35 V in positive bias polarity. Charge-carrier saturation was not found at negative bias polarity. We attribute this to the higher injection barrier on the active electrode side (TiN). We attribute the increase in the barrier height to the accumulation of positive vacancies near the electrode interface.

Enhanced performance of flexible quantum dot light-emitting diodes using a low-temperature processed PTAA hole transport layer

Low-temperature processing is important for improving the stability and performance of flexible quantum dot light-emitting diodes (QLEDs). In this study, QLEDs were fabricated using poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) as a suitable hole transport layer (HTL) material owing to its low-temperature processability and vanadium oxide as the low-temperature solution-processable hole injection layer material. The maximum luminance and highest current efficiency of the QLEDs on a glass substrate with an optimal PTAA HTL was 8.9 × 104 Cd/m2 and 15.9 Cd/A, respectively, which was comparable to that of conventional devices. The QLEDs on a flexible substrate showed a maximum luminance of 5.4 × 104 Cd/m2 and highest current efficiency of 5.1 Cd/A. X-ray and ultraviolet photoelectron spectroscopies were used to investigate the chemical state and interfacial electronic structure according to the materials and the state changes of the HTL, respectively. The interfacial electronic structure showed that PTAA exhibited a better hole transport ability owing to its low hole injection barrier ({Phi }_{{text{h}}}). Moreover, QLEDs with a PTAA HTL could operate as photosensors under reverse bias conditions. These results indicate that the low-temperature-processed PTAA HTL is suitable for improving the performance of flexible QLEDs.

High temperature energy storage and release properties of polyimide nanocomposites simulated by considering charge trapping effects

AbstractDielectric energy storage capacitors with excellent high temperature resistance are essential in fields such as aerospace and pulse power. However, common high‐temperature resistant polymers such as polyimide (PI) and polyether sulfone have low energy storage densities and energy efficiencies at high temperature, which are greatly limited in practical applications. The polymer nanocomposites prepared by doping modification can regulate the charge injection and transport process, and improve the high‐temperature energy storage performance. However, the quantitative relationship between charge injection and charge trapping and the energy storage performance of linear polymer nanocomposites still needs further study. An energy storage and release model considering the charge trapping effects is constructed by the authors. We simulate the high‐temperature energy storage properties of polyimide nanocomposite dielectrics (PI PNCs) with different charge injection barriers and trap parameters at 150°C. A triangular voltage is applied to the electrodes at both sides of the PI PNCs, the electric displacement‐electric field loop is simulated, and the discharged energy densities and energy efficiencies are calculated. The simulation results are consistent with the experimental results. Increasing the charge injection barrier, deep trap energy and deep trap density can effectively reduce the charge injection and the carrier mobility, thereby improving the discharged energy densities and energy efficiencies of dielectric capacitors. In the case of low charge injection barrier (1.3 eV), with the increase of deep trap energy (0.7–1.5 eV) and deep trap density (1 × 1021–1 × 1025 m−3), the discharged energy density changes from 0.20 to 1.44 Jcm−3, the energy efficiency changes from 9.0% to 99.9%, and the high‐temperature energy storage performance improves significantly. This research provides theoretical and model support for the improvement of the high‐temperature energy storage performance of nanocomposites.

A Mixed Organic-Inorganic Interlayer With Tunable Electrical Properties Enabling Stable and Efficient Perovskite Light-Emitting Diodes

The operational stability of perovskite light-emitting diodes (PeLEDs) is subjected to undesired Joule heating induced by the charge imbalance and the current leakage. The hole injection efficiency is inferior to that of electrons in the electron-dominated PeLEDs, owing to the low hole mobility of hole transport materials and a relatively large hole injection barrier. Herein, we inserted a mixed interlayer of 4,7-diphenyl-1,10-phenanthroline and cesium carbonate (Bphen:Cs2CO3) (~10 nm) between the electron transport layer (ETL) TPBi and the metal cathode to improve the charge imbalance by optimizing the electron injection and suppressing the current leakage. Compared to TPBi, the Bphen interlayer builds a larger electron injection barrier from the cathode to perovskite layer owing to its shallower lowest unoccupied molecular orbital (LUMO), leading to inefficient electron injection. The Cs2CO3, an n-type dopant, was co-evaporated with Bphen to increase the conductivity and thus balance the electron and hole transport. The resultant quasi-2D PeLED achieves an external quantum efficiency of 17.62% and an operational lifetime of over 1900 min at an initial luminance of 100 cd <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{2}}$ </tex-math></inline-formula> .

Passivation engineering via silica‐encapsulated quantum dots for highly sensitive photodetection

AbstractOrganometal halide perovskites are promising semiconducting materials for photodetectors because of their favorable optoelectrical properties. Although nanoscale perovskite materials such as quantum dots (QDs) show novel behavior, they have intrinsic stability issues. In this study, an effectively silane barrier‐capped quantum dot (QD@APDEMS) is thinly applied onto a bulk perovskite photosensitive layer for use in photodetectors. QD@APDEMS is synthesized with a silane ligand with hydrophobic CH3‐terminal groups, resulting in excellent dispersibility and durability to enable effective coating. The introduction of the QD@APDEMS layer results in the formation of a low‐defect perovskite film with enlarged grains. This is attributed to the grain boundary interconnection effect via interaction between the functional groups of QD@APDEMS and uncoordinated Pb2+ in grain boundaries. By passivating the grain boundaries, where various trap sites are distributed, hole charge‐carrier injection and shunt leakage can be suppressed. Also, from the energy point of view, the deep highest occupied molecular orbital (HOMO) level of QD@APDEMS can work as a hole charge injection barrier. Improved charge dynamics (generation, transfer, and recombination properties) and reduced trap density of QD@APDEMS are demonstrated. When this perovskite film is used in a photodetector, the device performance (especially the detectivity) stands out among existing perovskites evaluated for energy sensing device applications.

Large-Area Monolayer n-Type Molecular Semiconductors with Improved Thermal Stability and Charge Injection

We fabricated monolayer n-type two-dimensional crystalline semiconducting films with millimeter-sized areas and remarkable morphological uniformity using an antisolvent-confined spin-coating method. The antisolvent can cause a downstream Marangoni flow, which improves the film morphologies. The deposited crystalline monolayer films exhibit excellent thermal stabilities after annealing, which reveals the annealing-induced enhancement of crystallinity. The transistors based on the n-type monolayer crystalline films show linear output characteristics and superior electron mobilities. The improved charge injection between monolayer films and Au electrodes results from the energy level shift as the films decrease to the monolayer, which leads to a lower injection barrier. This work demonstrates a promising method for fabricating air-stable, low-cost, high-performance, and large-area organic electronics.

Optimized TFB-based perovskite quantum dot light emitting diode

Perovskite quantum dots (PQDs) are semiconducting nanocrystals that have been exploited for fabricating light-emitting diodes because of their exciton confinement, high quantum yield, and adjustable color. Therefore, PQD-based Light-emitting diodes (PQDLEDs) have attracted huge attention because of their significant external quantum efficiencies. In our research, to prohibit fluorescence quenching dots which is caused by the connection of the PQDs and hole injection material into the emissive layer and hole injection layer, poly [(9,9- dioctylfluorenyl-2,7-diyl)-co-(4,4’-(N-(pbutylphenyl)) diphenylamine)] (TFB) is utilized which can decrease the hole injection barrier. To reach the TFB's optimal thickness, we control both the precursor solution concentration and the rotational speed during the spin coating process. Finally, we find out that the optimal TFB precursor solution concentration and rotational speed are 8.0 mg ml−1, and 4500 rpm, respectively, which results in 35 nm thickness. The optimized PQDLED turn-on voltage, maximum brightness, maximum external quantum efficiency (EQE) (vs luminance), and maximum current efficiency were 2.2 V, 4522 cd m−2, ∼0.15%, and ∼0.5 cd A−1, respectively.

Charge transport mechanism of cross‐linked polyethylene/silicone rubber composite insulation by energy band theory

AbstractThe composite insulation composed of cross‐linked polyethylene (XLPE) and silicone rubber (SiR) is common in high voltage direct current cable accessory installation. However, the space charge accumulation, especially the interfacial charge accumulation of XLPE/SiR, poses a serious threat to the safe operation of cable accessories, and its charge transport mechanism is still unclear, especially at the micro‐scale. In order to investigate the charge transport mechanism of XLPE/SiR, simultaneous measurement of space charge and relaxation current is performed on XLPE/SiR at various electric fields with different polarities, and the electronic energy levels of XLPE and SiR are determined by quantum chemical calculation (QCC). The results of QCC show that both the hole traps and the electron traps in XLPE are mainly shallow traps. As for SiR, the hole traps are shallow traps, while the electron traps are deep traps. The results of simultaneous measurement show that the interfacial charge accumulation and the composite conductivity of XLPE/SiR are significantly different under different polarities, that is, there is an obvious polarity effect. Based on the results of QCC, the electronic energy levels of XLPE/SiR system are constructed considering the interface between XLPE and SiR, as well as the interfaces between the materials and the electrodes. On this basis, the charge transport mechanism of XLPE/SiR is discussed in detail, and the polarity effect is well explained, which is believed to be related to the differences in the charge injection barrier and the interfacial barrier under different polarities, as well as the electron/hole traps in XLPE and SiR.

C8‐BTBT‐C8Thin‐Film Transistors Based on Micro‐Contact Printed PEDOT:PSS/MWCNT Electrodes

AbstractAdvances in organic materials manufacturing have enabled the creation of electronic devices using solution‐processing techniques by employing soluble materials with high conductivity grade. In this exploratory study, the use of micro‐contact for poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymer ink deposition as high‐quality structured electrodes for organic field‐effect transistors (OFETs) in top‐contact geometry is demonstrated. The optimized OFET's solution‐processed fabrication is a promising strategy to be realized in the simple, cost‐effective roll‐to‐roll manufacturing processes. The electrical performance of the fabricated devices is comparable to transistors with gold electrodes prepared via vacuum deposition, and even exceeding the values of the charge carriers’ mobilities and featuring lower contact resistance (Rc), due to lower charge‐carrier injection barrier for carbon‐based organic electrodes. An addition of multi‐walled carbon nanotubes to the PEDOT:PSS decreasesRceven further, changing the work function for better energy alignment with semiconductor materials.

Effect of Cyano Substitution on Non‐Fullerene Acceptor for Near‐Infrared Organic Photodetectors above 1000 nm (Adv. Funct. Mater. 8/2023)

Non-Fullerene Acceptors In article number 2211486, Seo-Jin Ko, Jong H. Kim, Sung Cheol Yoon, and co-workers present a new CN-substituted non-fullerene acceptor COTCN2, which exhibits a deeper highest occupied molecular orbital energy level and a narrower optical bandgap (< 1.10 eV). As a result, the near-infrared organic photodetector (NIR OPD) based on PTB7-Th:COTCN2 provides high non-detectability beyond 1000 nm with dark hole injection well suppressed due to the larger charge injection barrier. Additionally, as a practical implementation of NIR OPD, photoplethysmography sensors are successfully demonstrated.

The possibility of gallium oxide (β-Ga2O3) heterojunction bipolar transistors

Bipolar junction transistors have not been viable with β-Ga2O3 due to its poor hole mobility and unavailability of shallow acceptors. Many p-type oxides form high-quality heterojunction diodes (low ideality factor and high breakdown voltage) with β-Ga2O3. We propose using these heterojunctions to make a β-Ga2O3 heterojunction bipolar transistor (HBT). Cu2O/β-Ga2O3 heterojunction is especially promising because of the relatively high electron diffusion length (∼μ m) in Cu2O, a low electron injection barrier at the Cu2O-Ga2O3 interface, and breakdown voltages of >1000 V. Using Silvaco TCAD, we simulate a β-Ga2O3 heterojunction bipolar transistor with a Cu2O base and estimate the power figure of merit (PFOM). We find that the low bandgap of Cu2O severely limits the performance of these HBTs. Reports of Cu2O-Ga2O3 diodes with extremely high breakdown voltage are probably due to heavily doped Cu2O or interface defects, but these effects do not translate to the HBT. For HBTs with PFOM better than the state-of-the-art β-Ga2O3 unipolar transistors, we need alternative p-type oxides with a bandgap E g > 3.4 eV and electron diffusion length >0.4 μ m. We discuss the possible candidates. Using an empirical model for the critical avalanche breakdown field, we estimate the maximum PFOM for possible β-Ga2O3 HBTs.

Surface-Ligand-Modified CdSe/CdS Nanorods for High-Performance Light-Emitting Diodes.

Colloidal nanocrystals (NCs) play an important role in the field of optoelectronic devices such as photovoltaic cells, photodetectors, and light-emitting diodes (LEDs). The properties of NC films are strongly affected by ligands attached to them, which constitute a barrier for charge transport between adjacent NCs. Therefore, the method of surface modification by ligand exchange has been used to improve the electrical conductivity of NC films. However, surface modification to NCs in LEDs can also affect emission characteristics. Among NCs, nanorods have unique properties, such as suppression of nonradiative Auger recombination and linearly polarized light emission. In this work, CdSe/CdS nanorods (NRs) were prepared by the hot injection method. To increase the charge transport into CdSe/CdS NRs, we adopted ligand modification to CdSe/CdS NRs. Using this technique, we could shorten the injection barrier length between CdSe/CdS NRs and adjacent layers. It leads to a more balanced charge injection of electron/hole and a greatly increased current efficiency of CdSe/CdS NR-LEDs. In the NR-LEDs, the ligand exchange boosted the electroluminance, reaching a sixfold increase from 848 cd/m2 of native surfactants to 5600 cd/m2 of the exchanged n-octanoic acid ligands at 12 V. The improvement of CdSe/CdS NR-LED performance is closely correlated to the efficient control of charge balance via ligand modification strategy, which is expected to be indispensable to the future NR-LED-based optoelectronic system.

Electro-Optic Properties of High-Efficiency Organic Electronics with the Addition of An Interlayer

This study investigated the electro-optic properties of organic light-emitting diode (OLED) devices with the addition of an interlayer. A device with the perfluorododecyl-1H,1H,2H,2H-triethoxysilane–perfluorotetradecyl 1H,1H,2H,2H-triethoxysilane mixture (hybrid solution) membrane applied to the hole injection layer (HIL) using a self-assembled monolayer (SAM), along with N,N’-Bis (naphtha-len-1-yl)-N,N’-bis(phenyl)-2,2’-dimethylbenzidine (α-NPD) applied to the hole transport layer (HTL), was fabricated and subsequently assessed. Compared to the device without an HIL, the device with HIL using SAM showed a luminance of 1630.16 (Cd/m2) at the maximum cut-off voltage of 11 V, and the efficiency was increased to ≥200% from the initial turn-on voltage of 3 V to the maximum cut-off voltage. These results suggest that adding an interlayer and the consequent carrier ladder effect led to more efficient injection and transport, possibly lowering the hole injection barrier and allowing excellent power efficiency.

On the mechanism to suppress dark current via blending with an all-inorganic perovskite precursor in colloidal quantum dot photodetectors

The heightened injection barrier for charge carriers after blending PbS CQDs with CsPbBr3 plays the dominant role in suppressing the dark current.

Facile light exposure of zinc oxide via interfacial engineering for boosting responsivity and detectivity in organic photodetectors

A light-exposed ZnO interface is filled with oxygen vacancies to limit surface defects, a smooth morphology is formed at an optimized time and a larger hole injection barrier is formed to suppress the dark current of an organic photodetector.

Quantification of an effective charge injection barrier in non-fullerene-based organic photodetectors

Studies on the origin of dark current in organic photodetectors reveal that the charge injection from the contact to the active layer via subgap traps is the dominant source of dark current once the effective injection is correctly described.