Charge Injection Research Articles

Achieving efficient and stable deep-blue tandem organic light-emitting diodes with high color purity via novel charge generator layer

Efficient and stable deep blue OLEDs with high color purity are desirable for OLEDs based displays. However, the development of deep blue OLEDs is lagging far behind green and red OLEDs for lots of years due to their higher-energy exciton, which reduces the final device performance and increases the consumption of electrical energy. Here, we designed a novel charge generator layer (CGL): Liq/Al/MoO3/SimCP2 with improved charge injection capability than traditional CGL. Based on this CGL, we successfully fabricated tandem deep blue OLEDs using thermally actively delayed fluorescence materials SYBN as guest and triplet–triplet annihilation material NPADN as host, which exhibits high external quantum efficiency of 17.7 %, current efficiency of 13.3 cd/A, power efficiency of 7.1 lm/W and stable LT95 device lifetime over 1200 h at initial luminescence of 1000 cd/m2. Simultaneously, it’s convenient to adjust the micro-cavity effect through increasing Spiro-BFA-2 thickness to reduce the full width at half maximum (FWHM) of EL spectra from 26 nm to 21 nm, realizing small CIEy value of 0.061 at a mild emission wavelength of 461 nm while maintaining a low driving voltage of 8.6 V at 1000 cd/m2. We believe that this work is significant to promote the development of deep blue OLEDs.

Controllable Ultrathin Nickel Nanoislands With Dense Discrete Space Charge Regions: Steering Hole Extraction for High-Performance Underwater Multispectral Weak-Light Photodetection.

Undersea optical communication (UOC) is vital for ocean exploration and military applications. In the dim-light underwater environment, photodetectors must maximize photon utilization by minimizing optical losses and carrier recombination. This can be achieved by integrating ultrathin metal nanostructures with photocatalysts to form Schottky junctions, which enhance charge separation and injection while mitigating metal-induced light shading. The strategic design of discrete metal nanostructures providing numerous high-depth space charge regions (SCRs) without overlap offers a promising approach to optimize hole transport paths and further suppress recombination. Here, a facile phase-separation lithography technique is explored to fabricate tunable ultrathin Ni nanoislands atop n-Si, yielding high-performance photoelectrochemical photodetectors (PEC PDs) tailored for underwater weak-light environments. This results indicate that key determinant of hole extraction behavior is the relationship between the spacing distance of adjacent Ni nanostructures (ds) and twice the SCR radius (Ws). PEC PDs with optimized 8nm ultrathin Ni nanostructures featuring closely but non-overlapping SCRs, exhibit a 55-fold increase in photoresponsivity (2.2mAW-1) and a 128-fold enhancement in detection sensitivity (3.2 × 1011 Jones) at 0V over Ni film, revealing the exceptional stability. Furthermore, this approach enables effective detection across UV-vis-near infrared spectrum, supporting reliable multispectral UOC and underwater imaging capabilities.

Giant Charge Separation-Driving Force Together with Ultrafluent Charge Transfer in TiO2/ZnFe-LDH Photoelectrode via Ferroelectric Interface Engineering.

Hydrogen fuel production using photoelectrochemical (PEC) water-splitting technology is incredibly noteworthy as it provides a sustainable and clean method to alleviate the energy environmental crisis. A highly rapid electron shuttle at the semiconductor/WOC (water oxidation cocatalyst) interface is vital to improve the bulk charge transfer and surface reaction kinetics of the photoelectrode in the PEC water-splitting system. Yet, the inevitably inferior interface transition tends to plague the performance enhancement on account of the collision of hole-electron transport across the semi/cat interface. Herein, we address these critical challenges via inserting ferroelectric layer BTO (BaTiO3) into the semi/cat interface. The embedded polarization electric field induced by ferroelectric BTO remarkably pumped hole transfer at the semiconductor/WOC (TiO2/ZnFe-LDH) interface and selectively tailored the electronic structure of LDH surface-active sites, leading to overwhelmingly improved surface hole transfer kinetics at LDH/electrolyte interface, which minish the electron-hole recombination in bulk and on the surface of TiO2. The TiO2 nanorods encapsulated by ferroelectric-assisted ZnFe-LDH achieve 105% charge separation efficiency improvement and 53.8% charge injection efficiency enhancement compared with pure TiO2. This finding offers a strategic design for electrocatalytic-assisted photoelectrode systems by ferroelectric-pumped charge extraction and transfer at the semiconductor/WOC interface.

Evaluation of degradation phenomena in the electric potential distribution inside organic light-emitting diodes by electron holography

Understanding the intrinsic degradation processes of organic light-emitting diodes is necessary to improve their lifetimes. This intrinsic degradation is typically caused by carrier injection at the interface between the hole transport layer (HTL) and the emissive layer (EML). However, revealing the charge behavior in this local region with a high spatial resolution remains challenging. Thus, this study employed electron holography, a transmission electron microscopy (TEM) technique, to measure the nanometer scale potential distribution inside an OLED composed of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminum (Alq3) that was degraded via continuous voltage application. The α-NPD and Alq3 functioned as the HTL and EML, respectively. The degraded OLED was found to exhibit several potential distributions, depending on the local positions from which the TEM samples were lifted out of the same bulk sample. The distributions included (i) formation of a potential valley at the α-NPD/Alq3 interface, (ii) disappearance of electric fields within the organic layers, and (iii) similar distribution to original before degradation. We suggest that the degradation was caused by charge accumulation, cationization of Alq3, and local failures. Thus, this study revealed the influence of electric degradation at the nanometer scale because of charge injection to the α-NPD/Alq3 interface. Electron holographic degradation analysis near the HTL/EML interface is expected to aid in the development of design guidelines for preventing device degradation and thus extend device lifetime.

Effective Steady‐State Recombination Decay Times in Comparison to Time‐Resolved Photoluminescence Decay Times in Halide Perovskite Solar Cells

One of the key topics in perovskite solar cells is the reduction of charge carrier recombination, with the aim of increasing power conversion efficiency. The recombination lifetime is a commonly used tool, as it directly affects the current–voltage curve via the diffusion length. The lifetime is often estimated using time‐domain measurement methods such as time‐resolved photoluminescence. However, two obstacles emerge when applying the transiently measured decay times to the steady‐state theory. In general, the decay time depends on the charge carrier concentration, and it is often not clear under which conditions the transient measurement must be conducted to be comparable with the steady‐state performance of the device. Furthermore, diffusion and capacitive effects due to charge injection and extraction can influence transient techniques and cause the measured decay time to deviate from the sought‐after recombination lifetime. Voltage‐dependent steady‐state photoluminescence measurements can be used to estimate the internal voltage during device operation and allow the extraction of collection efficiencies and effective steady‐state decay times that are independent of transport and capacitive effects. Here, the differences between the steady‐state and transient decay times are identified and discussed, and the losses in the current–voltage curve caused by extraction issues are quantified.

Monolithically Integrated Quantum Dots in a 22‐nm Fully Depleted Silicon‐on‐Insulator Process Operating at 3 K

ABSTRACTQuantum computers comprising large‐scale arrays of qubits will enable complex algorithms to be executed to provide a quantum advantage for practical applications. A prerequisite for this milestone is a power‐efficient qubit control and detection system operating at cryogenic temperatures. Implementing such systems in complementary metal‐oxide‐semiconductor (CMOS) technology offers clear advantages in terms of scalability. Here, we present a fully integrated quantum dot array in which silicon quantum wells are co‐located with control and detection circuitry on the same die in a commercial 22‐nm fully depleted silicon‐on‐insulator (FDSOI) process. Our system comprises a two‐dimensional quantum dot array, integrated with 8 detectors and 32 injectors, operating at 3 K inside a cryo‐cooler. The power consumption of the control and detection circuitry is 2.5 mW per qubit without body biasing. The design utilizes 0.8‐V nominal devices. The setup allows us to verify discrete charge injection control and detection at the quantum dot array and demonstrate the feasibility of this architecture for scaling up the existing quantum core to hundreds and thousands of physical qubits.

On the mechanism for work function change of gold electrodes by ultrathin polyethyleneimine (PEI) films: Effect of molecular order.

Polyethyleneimine (PEI) is a widely used cationic polyelectrolyte. In organic electronics, it is a universal surface modifier for shifting the electrode work function (Φ) and improving charge injection into electronic devices. This effect may depend on the conformation and dipolar order of the PEI ultrathin film, but their detailed experimental evaluation has not yet been reported. Thus, we used sum-frequency generation (SFG) spectroscopy to probe the net orientation of polar groups of PEI films on glass and gold. The films were fabricated by spin-coating from alcoholic solutions or by dip-coating from aqueous solutions of various pH values, with both branched (b-PEI) and linear (l-PEI) structures. The obtained SFG spectra and atomic force microscopy (AFM) images indicated that the conformational ordering of the PEI layers increases over the period of 14 days after fabrication, being slightly more pronounced for l-PEI vs b-PEI, and for dip-coating vs spin coating fabrication. Furthermore, both the pH of the dip-coating solutions and the substrate nature influence the final morphology and order of the adsorbed films. On glass, they are optimized at an intermediate pH 5, while on gold, the greatest homogeneity is observed at pH 2 and the largest dipolar order is observed at pH 10. The pH dependence of changes in the work function of gold by PEI (|ΔΦ|) suggests that the electronic contribution is dominant. Nevertheless, the evolution of the PEI dipolar ordering was accompanied by small variations of |ΔΦ|, suggesting that it does have a significant contribution, especially at conditions for which the electronic contribution is reduced.

Interface Energy‐Level Reorganization for Efficient Perovskite γ‐Ray Detectors

AbstractMetal halide perovskites are promising candidates for gamma‐ray (γ‐ray) spectrum detectors. However, achieving high‐resolution energy spectra in single‐photon pulse‐height analysis mode remains challenging, due to the inevitable leakage currents degrade the recognizable fingerprint energies which is critical for resolving γ‐ray spectroscopy. We demonstrate under high bias voltage, a deficient contact barrier can lead to excessive surface charge injection, thereby increasing leakage current from electrodes to perovskites. Hence, we conceive to employ surface ligand engineering on perovskite single crystals to manipulate energy levels to suppress leakage current. In particular, anchoring a strong dipole ligand onto the perovskite induced surface charge‐density displacement, leading to a downward band bending and heightened the corresponding contact barrier. Consequently, the strategy minimized the detectors’ leakage current by an order of magnitude, to as low as 44 nA cm−2 at −100 V. The resulting detectors show a significant improvement in energy resolution, 3.9 % for 22Na 511 keV γ‐rays has been achieved at room temperature. The resulting detector further resolves each fingerprint energy for 152Eu γ‐spectrum, representing one of the best γ‐rays perovskite detectors reported to date. Moreover, the detectors exhibited stabilized energy resolution without any degradation under a continuous electric field (1,000 V cm−1) for over 300 minutes, representing the longest longevity reported to date.

Degradation Analysis of Exciplex-Based Organic Light-Emitting Devices Using Carbazole-Based Materials.

A spectral shift and new emission bands in the green and red regions have been observed in deep blue exciplex-based organic light-emitting diodes (OLEDs) using carbazole-based materials, namely, tris(4-carbazoyl-9-ylphenyl)amine (TCTA). To deeply understand the origin of these new bands, single-layer and bilayer TCTA-based OLEDs subjected to electrical and optical (ultraviolet (UV)) stresses were investigated by using various optical, electrical, morphological, and chemical measurements. The results showed that the stress-induced emission bands primarily originate from morphological changes rather than chemical changes. The accumulation of excitons in the TCTA layer induces molecular aggregation, leading to the formation of electrically active electronic states, namely, electroplexes and electromers, which lead to the appearance of additional emission bands in green and red regions. Impedance spectroscopy measurements on single-layer OLEDs complemented this study. The results showed that TCTA degradation affects charge injection and transport. It was concluded that the stress-induced emission bands are caused by aggregate domain formation and are closely linked to the formation of electrically active defects, which act as trap states for charge carriers in the TCTA band gap.

Investigation of the impact of internal acceptor and π-linker in 2, 5-di(2-thienyl)pyrrole based organic dye photosensitizer for DSSCs

The effect of π-linkers and internal acceptor group on the optoelectronic characteristics of D-π-A and D-A′-π-A dyes for dye-sensitized solar cells (DSSC) were examined. The DFT was used for estimating geometries and charge transport parameters, and the TD-DFT for calculating electronic excitations of these dyes. HOMO, LUMO, and HOMO-LUMO energy gap were also calculated for appreciating the suitable electron transfer, dye regeneration, and charge injection. For achieving the improved performance of dyes in DSSC, injection driving force (ΔGinj), short circuit current (JSC), the open circuit voltage (VOC), light harvesting efficiency (LHE), dye regeneration energy (ΔGreg), and Power conversion efficiency were also determined. For suitable Power conversion efficiency of the DSSCs, electron affinity (EA), hole and electron extraction potential, Ionization potential (IP), Total density of states, and Electrostatic potential (ESP) were also calculated. The result shows that the internal acceptor and π-linkers affect the photovoltaic parameters and a small variations in the dye architecture increase the efficiency of DSSCs. The dyes containing diphenyl π-linkers showed maximum power conversion efficiency for solar cell.

Minimizing Efficiency Roll-Off in Organic Emitters via Enhancing Radiative Process and Reducing Binding Energy: A Theory Insight.

Organic solid-state lasers have received increasing attention due to their great potential for realizing organic continuous-wave or electrically driven lasers. Moreover, they exhibit significant promise for optoelectronic devices due to their chemically tunable optoelectronic properties and cost-effective self-assembly traits. Recently, a great progress has been made in organic solid-state lasers via spatially separated charge injection and lasing. However, making directly electrically driven organic semiconductor lasers is very challenging. It is difficult because of a number of excitonic losses caused by the spin-forbidden nature as well as serious efficiency roll-off at a high current density. Here, a multifunction gain material, functioning both as a thermally activated delayed fluorescence (TADF) emitter with exceptional optical gain and as a source of phosphorescence, was theoretically investigated. The new molecule we designed exhibits a reduction of triplet accumulation through an effective exciton radiative process (5-fold boost in figure of merit) and significantly decreased exciton binding energy (dipole moment from 5.77 to 14.03 D), which benefit amplified spontaneous emission and lasing emission. Our work provides theoretical insights into organic solid-state lasers and may contribute to the development of new and efficient laser-gaining molecules.

Tailoring the Tunneling‐Effect‐Boosted Interfacial Charge Trapping via Effective Conjugation Length

AbstractHighly efficient charge injection and charge trapping stability of the tunneling layer are of desirable and practical importance to transistor memory applications. However, both of which can be contradictory to the nature properties of its material. It is herein demonstrated that lowering the energy mismatch of the tunneling layer by employing a longer conjugation length of the polymer can improve the charge injection efficiency, albeit the trapped charges will be easily diminished and finally losing its memory characteristics, and vice versa. To further elaborate and verify this concept, both materials are blended with distinct nature of properties as the tunneling layer in pentacene‐based transistor devices. As the results, the device using 300 nm SiO2 with optimum blending ratio displays a broad memory window of ≈77.6 V which is superior to the non‐blended tunneling layer carrying appropriate energy levels and band gap energy, not to mention, revealing fast operation time (≈1 s), low driving voltage (≈20 V), long retention (>104 s), and high switching stability over 50 cycles with on–off ratio of >104. Most importantly, this finding shed insight into the design of tunneling materials for advancing organic transistor memory technologies based on tunneling‐effect‐boosted interfacial charge trapping.

Scalable all polymer dielectrics with self-assembled nanoscale multiboundary exhibiting superior high temperature capacitive performance

Polymers are key dielectric materials for energy storage capacitors in advanced electronics and electric power systems due to their high breakdown strengths, low loss, great reliability, lightweight, and low cost. However, their electric and dielectric performance deteriorates at elevated temperatures, making them unable to meet the rising demand for harsh-environment electronics such as electric vehicles, renewable energy, and electrified transportation. Here, we present an all-polymer nanostructured dielectric material that achieves a discharged energy density of 7.1 J/cm³ with a charge-discharge efficiency of 90% at 150°C, outperforming the existing dielectric polymers and representing more than a twofold improvement in discharged energy density compared with polyetherimide. The self-assembled nano-scale multiboundaries effectively impede the charge injection and excitation, leading to more than one order of magnitude lower leakage current density than the pristine polymer matrix PEI at high electric fields and elevated temperature. In addition, the film processing is simple, straightforward, and low cost, thus this all-polymer nanostructured dielectric material strategy is suitable for the mass production of dielectric polymer films for high-temperature capacitive energy storage.

Boosting Photoelectrochemical Water Oxidation Performance of Ti–Fe2O3 Photoanode via NH2–MIL–53(FeCo) as a Cocatalyst

α–Fe2O3 is a promising photoanode that can be used for solar‐driven photoelectrochemical (PEC) water splitting. However, due to low carrier separation efficiency and slow water oxidation kinetics, the PEC performance of Ti–Fe2O3 is still severely hindered. Here, a novel inorganic–organic hybrid composite photoanode is prepared by depositing NH2–materials of institute Lavoisier (MIL)–53(FeCo) cocatalyst on the surface of Ti–Fe2O3 using solvothermal method. In the results, it is shown that the Ti–Fe2O3/NH2–MIL–53(FeCo) photoanode has a high photocurrent density (3.0 mA cm−2 at 1.23 V vs reversible hydrogen electrode (RHE), which is 5.1 times that of bare Ti–Fe2O3, and the initial potential of water oxidation also undergoes a negative change of about 100 mV. The separation efficiency of Ti–Fe2O3/NH2–MIL–53(FeCo) is significantly enhanced, and the charge injection efficiency increases from 17.6% to 68% at 1.23 V versus RHE. In the further research, it is suggested that the excellent PEC performance of Ti–Fe2O3/NH2–MIL–53(FeCo) may be attributed to an increase in carrier density, prolonged carrier recombination time, and an increase in exposed reactive sites. In this study, a new understanding for the design of Ti–Fe2O3 photoanodes with superior PEC performance based on metal organic framework modification is provided.

Molecular Probing Coupled with Density Functional Theory Calculation to Reveal the Influence of Fe Doping on Fe-NiOOH Electrode for High Current Density of Water Splitting.

Fe-doped NiOOH electrocatalysts have attracted wide interest for the exceptional oxygen evolution reaction (OER) performance, but the precise role of Fe doping on the improved intrinsic activity remains unclear. Herein, the molecular probe technique combined with density functional theory calculation is used to reveal the influence of the Fe atom on the rate-determining step of the OER reaction, where the pre-catalyst of hierarchical self-supporting NiFe layered double hydroxide [LDH] nanosheets equipped on nickel foam (NiFe LDH/NF) is generated via a facile and industrially well-matched one-pot corrosion method. The physical characterization results reveal the reconstruction of NiFe LDH into Fe-doped NiOOH for promoted OER, which has a lower OH* adsorption energy with fast subsequent steps that help in obtaining an improved charge injection efficiency compared to NiOOH. In addition, more exposed electroactive species and facile delivery of mass/electron inside the catalytic procedure actually have a high-quality contribution to the outstanding catalytic activity. Therefore, the NiFe LDH36/NF electrocatalyst provides high catalytic activities of 241 and 320mV at 10mA cm-2 toward the OER and overall water-splitting in 1m KOH. This work provides a promising avenue for the rational design of durable self-supporting electrodes toward large-scale water splitting.

Influence of interface voltage on charge build-up under high voltage

Abstract The interface voltage appearing when two materials are brought into contact was measured with a high sensitive pressure-wave-propagation measurement setup in various low density and high density polyethylene samples coated either with aluminum or gold electrodes. The samples were then tested under high voltage (40 kV/mm) for 24 h to assess the correlation between interface voltage and charge injection at 22 °C. Aluminum electrodes induce a positive interface voltage toward the insulator favoring electron injection at the cathode while gold electrodes induce a negative interface voltage toward the insulator favoring hole injection at the anode. After the injections due to interface voltage, charges diffuse more easily in low density polyethylene that triggers a subsequent injection due to heterocharges buildup.

Modulation of Charge Transport Layer for Perovskite Light-Emitting Diodes.

Perovskite light-emitting diodes (Pero-LEDs) have garnered significant attention due to their exceptional emission characteristics, including narrow full width at half maximum, high color purity, and tunable emission colors. Recent efficiency and operational stability advancements have positioned Pero-LEDs as a promising next-generation display technology. Extensive research and review articles on the compositional engineering and defect passivation of perovskite layers have substantially contributed to the development of multi-color and high-efficiency Pero-LEDs. However, the crucial aspect of charge transport layer (CTL) modulation in Pero-LEDs remains relatively underexplored. CTL modulation not only impacts the charge carrier transport efficiency and injection balance but also plays a critical role in passivating the perovskite surface, blocking ion migration, enhancing perovskite crystallinity, and improving light extraction efficiency. Therefore, optimizing CTLs is pivotal for further enhancing Pero-LED performance. Herein, this review discusses the roles of CTLs in Pero-LEDs and categorizes both reported and potential CTL materials. Then, various CTL optimization strategies are presented, alongside an analysis of the selection criteria for CTLs in high-performance Pero-LEDs. Finally, a summary and outlook on the potential of CTL modulation to further advance Pero-LED performances are provided.

Electrostatic charge injection for reusing face masks: Mechanisms, performance, and a household alternative

AbstractThe COVID‐19 pandemic underscores the effectiveness of face masks in combating respiratory infectious diseases and the importance of adequate supply. However, the widespread use of disposable masks has led to severe environmental pollution. In this study, we propose a two‐step strategy for mask reuse, aimed at both mitigating mask waste pollution and improving mask availability in future epidemic outbreaks. Our strategy involves disinfection and corona charging processes, enabling surgical masks to maintain a filtration efficiency of 88.7% even after five cycles of reuse. We highlight the crucial role of volume charges over surface charges in maintaining filtration performance stability and durability, and we visualize the underlying mechanisms using energy band diagrams and potential well models. Additionally, we introduce a simple household solution for simultaneously drying and charging, making it accessible for widespread use. Our research offers a viable strategy for promoting environmental sustainability and alleviating mask supply pressures during significant public health crises.image

Manufacturing strategies for highly sensitive and self-powered piezoelectric and triboelectric tactile sensors

Abstract Piezoelectric and triboelectric effects are of growing interest for facilitating high-sensitivity and self-powered tactile sensor applications. The working principles of piezoelectric and triboelectric nanogenerators provide strategies for enhancing output voltage signals to achieve high sensitivity. Increasing the piezoelectric constant and surface triboelectric charge density are key factors in this enhancement. Methods such as annealing processes, doping techniques, grain orientation controls, crystallinity controls, and composite structures can effectively enhance the piezoelectric constant. For increasing triboelectric output, surface plasma treatment, charge injection, microstructuring, control of dielectric constant, and structural modification are effective methods. The fabrication methods present significant opportunities in tactile sensor applications. This review article summarizes the overall piezoelectric and triboelectric fabrication processes from materials to device aspects. It highlights applications in pressure, touch, bending, texture, distance, and material recognition sensors. The conclusion section addresses challenges and research opportunities, such as limited flexibility, stretchability, decoupling from multi-stimuli, multifunctional sensors, and data processing.

Intrinsic charge manipulation for solution‐processed organic photodetectors with high sensitivity and fast response

AbstractCharge manipulation is crucial in optoelectronic devices. The unoptimized interfacial charge injection/extraction in solution‐processed bulk‐heterojunction (BHJ) organic photodetectors (OPDs) presents significant challenges in achieving high detectivity and fast response speed. Here, we first develop an approach for intrinsic charge manipulation induced by molecularly engineered donors to block electron injection and facilitate hole extraction between the indium tin oxide (ITO) transparent anode and the photoactive layer. By utilizing a polymer donor with 3,4‐ethylenedioxythiophene (EDOT) as the conjugated side chain, a polymer‐rich layer forms spontaneously on the ITO substrate due to the increased oxygen interactions between ITO and EDOT. This results in electron‐blocking‐layer (EBL)‐free devices with lower dark current and noise without a reduction in responsivity compared to control devices. As a result, the EBL‐free devices exhibit a peak specific detectivity of 2.36 × 1013 Jones at 950 nm and achieve a −3 dB bandwidth of 30 MHz under −1 V. Enhanced stability is also observed compared to the devices with poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). This work demonstrates a new method to intrinsically manipulate charge injection in BHJ photoactive layers, enabling the fabrication of solution‐processed EBL‐free OPDs with high sensitivity, rapid response, and good stability.