Charge Trap States Research Articles

Hydrazide Molecular Configuration Induced Phase Regulation and Defect Passivation Enable Efficient Quasi‐2D Blue Perovskite Light‐Emitting Diodes

AbstractPerovskite light‐emitting diodes (PeLEDs) have demonstrated significant potential in the display sector, attributed to their wide color gamut, narrow emission spectra, and cost‐effectiveness. Despite the rapid advancements in red and green PeLEDs, the attainment of high brightness and high external quantum efficiency (EQE) for blue PeLEDs remains a considerable challenge, which substantially limits their practical applications in white lighting and optical communication. In this study, a method for the passivation of blue quasi‐2D perovskites using hydrazide derivatives with varying alkyl chain lengths is presented. Density functional theory analysis indicates that hydrazide derivatives can effectively adsorb onto halogen vacancies, thereby reducing charge trapping states associated with undercoordinated Pb2+. Experimental results demonstrate that the optimal hydrazide derivative can better eliminate non‐radiative recombination loss, suppress ions migration, and regulate phase distribution, facilitating smoother energy transfer. Through this approach, stable sky‐blue PeLEDs are achieved with an EQE of 14.5% and a maximum luminance of 2659 cd m−2. This work offers a systematic understanding of hydrazide additive design, which will further enhance the performance and stability of blue quasi‐2D PeLEDs.

Cu‐Implanted MXene/TiO2 Photoanodes for Efficient Quantum Dot‐Sensitized Solar Cells

Noble metal‐doped TiO2‐based photoanode for quantum dot‐sensitized solar cells (QDSSCs) has gained significant importance in enhancing performance by increasing the light absorption and subsequently minimizing the number of recombinations due to the formation of new charge trap states. In this work, Cu ions are implanted in MXene/TiO2‐based photoanode at different fluence rates (5 × 1012, 5 × 1013, 5 × 1014, and 5 × 1015 ions cm−2). The X‐ray photoelectron spectroscopy investigations reveal the doping mechanism as at lower fluence, Cu+ ions are present, but as the fluence increased the Cu2+ ions dominate. The field emission scanning electron microscopy and energy‐dispersive X‐ray analysis are used to find the surface morphology and the elemental composition of the implanted samples. The implantation of Cu ions creates new impurity states between the energy bands, thereby enhancing light absorption capabilities and suppressing charge recombinations of the photoanode, which is confirmed by UV‐Vis and photoluminescence spectroscopy. Afterward, Cu‐implanted photoanodes are employed to fabricate QDSSC devices, and the QDSSC based on photoanode implanted at 5 × 1014 ions cm−2 fluence (Cu_3) demonstrates the highest power conversion efficiency of 3.86%, which is 34.9% higher than pristine unimplanted photoanode. This enhancement is attributed to the inhibition of the charge recombinations at the photoanode/electrolyte interface and enhanced light harvesting capability of the photoanode.

Solution-Based Processing of Two-Dimensional Materials for High-Performance and Wafer-Scale Electronics

The integration of two-dimensional (2D) materials into practical electronics has been hindered by the lack of scalable fabrication methods. To overcome the limitation, I will introduce a general approach for wafer-scale electronics based on 2D nanomaterial inks, which is fabricated by molecular intercalation-assisted exfoliation method, enabling highly uniform, ultrathin, phase-pure, and large lateral size 2D nanoflakes. This approach enables the printing of large-area thin films characterized by effective van der Waals (vdW) sheet-to-sheet contacts, which promote a clean interface and minimal charge trapping states. We demonstrate the versatility of the printed thin films by integrating them with various electrical functionalities—dielectric, semiconducting, and metallic—resulting in vdW thin-film devices that exhibit exceptional performance over large scales. These devices not only enhance current electronics but also expand potential applications to include photodetectors, diode, and logic gates, demonstrating their broad utility in advanced electronic systems.

Enhancing Carrier Behavior via Controlled Molecular Film Formation Engineering Leads to Significant Improvement in Electroluminescence.

The quality of organic thin films critically influences carrier dynamics in organic semiconductors. In neat/doped films, even tiny voids can be penetrated by water or oxygen molecules to create charge-trap states called water/oxygen-induced traps that significantly hinder carrier mobility. While the water/oxygen-induced traps in non-doped films and crystalline states have been investigated comprehensively, there is a lack of thorough examination regarding their properties in the doped state. Therefore, there is a high demand for a molecular design strategy that effectively modulates the molecular stacking behavior in doped films and practical devices and enhances the quality of these films. Herein, we proposed a versatile molecular design principle that enables the formation of "nano-cluster" structures on both the surface and interior of doped films in target molecule 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1'-(4-fluorophenyl)-10H-spiro[acridine-9,9'-xanthene] (DspiroO-F-TRZ), which is modified with a fluorophenyl group. These "nano-cluster" structures exhibit more uniform shapes within doped films and effectively reduce defective state densities while enhancing carrier injection and transport properties, ultimately improving device performance. Notably, TADF-OLED based on DspiroO-F-TRZ demonstrates nearly twice as much efficiency as its control counterpart due to contributions from 'nano-cluster' structure enhancements toward improved electroluminescence performance.

Optimizing charge transport and band-offset in silicon heterojunction solar cells: impact of TiO2 contact deposition temperature

Carrier selective contacts are a primary requirement for fabricating silicon heterojunction solar cells (SHSCs). TiO2 is a prominent carrier selective contact in SHSCs owing to its excellent optoelectronic features such as suitable band offset, work function, and cost-effectiveness. Herein, we fabricated simple SHSCs in an Al/TiO2/p-Si/Ti/Au device configuration. Ultrathin 3 nm TiO2 layers were deposited onto a p-type silicon substrate using the atomic layer deposition method. The deposition temperature of TiO2 layers varied from 100 °C to 250 °C. X-ray photoelectron spectroscopic studies suggest that deposition temperature highly affects the chemical states of TiO2 and reduces the formation of defective state densities at the Fermi energy. The optical band gap values of TiO2 layers are also altered from 3.13 eV to 3.27 eV when the deposition temperature increases. The work function tuning from −5.13 eV to −4.83 eV has also been observed in TiO2 layers, suggesting the variation in Fermi level tuning, which arises due to changes in carrier concentrations at higher temperatures. Several device parameters, such as ideality factor, trap density, reverse saturation current density, barrier height, etc, have been quantified to comprehend the effects of deposition temperature on photovoltaic device performance. The results suggest that the deposition temperature significantly influences the charge transport and device performance. At an optimum temperature, a significant reduction in charge carrier recombination and trap state density has been observed, which helps to improve power conversion efficiency.

(Invited) 14.8% Quantum Efficient Gallium Phosphide Photocatalyst for Hydrogen Evolution

Gallium phosphide is a well-established photoelectrode material for H2 or O2 evolution from water, but particle-based GaP photocatalysts for H2 evolution are very rare. To understand the reasons, we investigated the photocatalytic H2 evolution reaction (HER) of suspended n-type GaP particles with iodide, sulfite, ferricyanide, ferrous ion, and hydrosulfide as sacrificial electron donors, and using Pt, RhyCr2-yO3, and Ni2P HER cocatalysts. A record AQE of 14.8% at 525 nm was achieved after removing charge trapping states from the GaP surface, adding a Ni2P cocatalyst to reduce the proton reduction overpotential, lowering the Schottky-barriers at the GaP-cocatalyst and GaP-liquid interfaces, and adjusting the electrochemical potential of the electron donor. The work showcases the main factors that control charge separation in suspended photocatalysts, and it explains why most known HER photocatalysts in the literature are based on n-type and not p-type semiconductors. Figure 1

Hole transport mechanism at high temperatures in p-GaN/AlGaN/GaN heterostructure

This Letter reports an investigation of hole transport in p-GaN/AlGaN/GaN heterostructures through experimental and theoretical analyses under varied conditions. Highly non-linear current–voltage (I–V) characteristics, obtained via the linear transmission line method measurements, are utilized for this study. At low bias voltage, the transport can be ascribed to the Schottky nature of the contact, while at high bias, the conduction is observed to be governed by space-charge limited current (SCLC). The Schottky characteristics (Schottky barrier height and non-ideality factor) and the SCLC exponent were analyzed for devices with varying contact spacings and at different high temperatures. The SCLC exponent, m, is in the range of 2≤m≤4 depending on the applied voltage range, revealing the existence of the trap states in the channel region. The findings of this work indicate that the charge injection, field-induced ionization, and trap states in the p-GaN channel are critical factors in the current transport of p-GaN/AlGaN/GaN heterostructure.

Controllable memory window in two-dimensional hybrid van der Waals heterostructured devices

Van der Waals (vdW) heterostructures based on inorganic layered materials have been demonstrated as potential candidates for a variety of electronic applications due to their flexibility in energy band engineering. However, the presence of unstable charge-trapping states in atomically thin two-dimensional (2D) materials may limit the performance of devices. Here, we aim to conduct a systematic investigation on hybrid heterostructured memory devices that consist of 2D layered organic and inorganic materials. The objective is to explore the potential of these devices in offering efficient charge-trapping states. Molybdenum disulfide (MoS2) is employed as a channel, while N, N′-Dimethyl-3,4,9,10-perylenedicarboximide (Me-PTCDI) serves as the charge-trapping layer to store charges from MoS2. The hysteresis window of these heterostructured devices can be effectively modified within a range of 13–70 V by manipulating both the thickness of the organic layer and the gate voltages. The largest hysteresis window is found in a combination of a few-layer Me-PTCDI (12.6 nm) and MoS2 (6 nm), showing a high on/off current ratio (>104) and a long retention time (104 s). Furthermore, the endurance test, which lasts for over 1000 cycles, demonstrates an exceptional level of stability and reliability. In addition, multilevel memory effects can be observed when gate pulses with different widths and amplitudes are applied. These 2D hybrid heterostructured devices have the capability to broaden the scope of material systems and present substantial potential for functional neuromorphic applications.

Plasma-enhanced Ni atom-modulated Co-OH to promote surface charge transfer of CuWO4 photoanode for efficient solar hydrogen evolution

The high charge recombination and slow transfer kinetics of photoanode remains to be great challenges in photo-electrochemistry (PEC). While, the semiconductor (CuWO4)/layered double-metal hydroxides (LDHs)/plasma coupling strategy is identified as one of the effective methods to enhance the PEC behaviors. The LDHs act as hole-conducting layers to promote interfacial catalytic reactions of PEC and release charges from the CuWO4 photoanode, which effectively enhances charge separation and boosts kinetic processes. Metallic Ag nanoparticles enhances light-absorbing capacity of the photoanode, as well as improves charge separation and kinetics of the photoanode utilizing the surface plasmon resonance (SPR) effect for increasing carrier groups and facilitatingcharge separation. The study shows that the CuWO4/NiCo-LDH/Ag photoanode prepared via as-mentioned coupling strategy exhibits a potential photocurrent density of 2.84 mA·cm−2 at 1.23 V vs. RHE, which is 9.2 times higher than that of the pristine CuWO4 (0.31 mA·cm-2) in phosphate buffered electrolyte. Moreover, it is determined that a special charge-trapping states (surface state) exists during the charge transfer process, which plays an indispensable role for the charge storage and transfer. Density Functional Theory (DFT) calculations validate that the introduction of NiCo-LDH and Ag facilitates charge transfer, whereas the modulation of the rate-determining steps (RDS) boosts the overall efficiency of the reaction. This study presents a guiding design approach for regulating PEC behavior of the semiconductor/LDHs/plasma strategy.

14.8% Quantum Efficient Gallium Phosphide Photocatalyst for Hydrogen Evolution.

Gallium phosphide is an established photoelectrode material for H2 or O2 evolution from water, but particle-based GaP photocatalysts for H2 evolution are very rare. To understand the reasons, we investigated the photocatalytic H2 evolution reaction (HER) of suspended n-type GaP particles with iodide, sulfite, ferricyanide, ferrous ion, and hydrosulfide as sacrificial electron donors, and using Pt, RhyCr2-yO3, and Ni2P HER cocatalysts. A record apparent quantum efficiency of 14.8% at 525 nm was achieved after removing gallium and oxide charge trapping states from the GaP surface, adding a Ni2P cocatalyst to reduce the proton reduction overpotential, lowering the Schottky-barrier at the GaP-cocatalyst interface, adjusting the polarity of the depletion layer at the GaP-liquid interface, and optimizing the electrochemical potential of the electron donor. The work not only showcases the main factors that control charge separation in suspended photocatalysts, but it also explains why most known HER photocatalysts in the literature are based on n-type and not p-type semiconductors.

High Temperature Resistant Solar‐Blind Ultraviolet Photosensor for Neuromorphic Computing and Cryptography

AbstractHigh‐temperature resistant solar‐blind optoelectronic synapse has a significant demand such as aerospace and fire warning, which integrates sensing and processing functions to realize complex functions like learning, recognition, and memory. However, developing such device remains a tremendous challenge. Herein, a two‐terminal GaOX solar‐blind optoelectronic synapse with high‐temperature working ability is proposed, and it is applied to neuromorphic computing and cryptography. Benefiting from the high internal gain, the device can detect the light intensity of nW cm−2, displaying one of the best figures‐of‐merit in solar‐blind photodetectors. Furthermore, the device possesses remarkable image sensing and memorization ability because of its ultrasensitive light detection ability and prominent synapse performance resulting from large charge trapping states density. Simultaneously, the device shows undamped photodetection and synaptic performances even at 610 K, reflecting a high‐temperature endurance and a desired property for practical applications under harsh environment. Moreover, by constructing an artificial neural network, high‐precision recognition of handwritten digits are realized under 610 K. A photosynaptic array with 12 × 12 pixels is constructed, and it is applied in cryptography that enables simultaneous sensing and encryption in the same devices. This work is expected to drive the progress of Ga2O3 in harsh environment.

Critical Review on Crystal Orientation Engineering of Antimony Chalcogenide Thin Film for Solar Cell Applications.

The emerging antimony chalcogenide (Sb2 (Sx Se1-x )3 , 0 ≤ x ≤ 1) semiconductors are featured as quasi-1D structures comprising (Sb4 S(e)6 )n ribbons, this structural characteristic generates facet-dependent properties such as directional charge transfer and trap states. In terms of carrier transport, proper control over the crystal nucleation and growth conditions can promote preferentially oriented growth of favorable crystal planes, thus enabling efficient electron transport along (Sb4 S(e)6 )n ribbons. Furthermore, an in-depth understanding of the origin and impact of the crystal orientation of Sb2 (Sx Se1-x )3 films on the performance of corresponding photovoltaic devices is expected to lead to a breakthrough in power conversion efficiency. In fact, there are many studies on the orientation control of Sb2 (Sx Se1-x )3 colloidal nanomaterials. However, the synthesis of Sb2 (Sx Se1-x )3 thin films with controlled facets has recently been a focus in optoelectronic device applications. This work summarizes methodologies that are applied in the fabrication of preferentially oriented Sb2 (Sx Se1-x )3 films, including treatment strategies developed for crystal orientation engineering in each process. The mechanisms in the orientation control are thoroughly analyzed. An outlook on perspectives for the future development of Sb2 (Sx Se1-x )3 solar cells based on recent research and issues on orientation control is finally provided.

Analysis and Classification of Charge Trapping States in β-Ga2O3 Mosfets Using I-V and C-V in Dark and Illumination Conditions

Beta-gallium oxide (β-Ga2O3) is a promising ultra-wide bandgap semiconductor for high power applications. Its breakdown electric field of 8 MV cm-1, a Baliga figure of merit 3-10 times larger than that of SiC and GaN, and the ability to be grown from the melt make it highly attractive for the semiconductor industry. Significant improvements have been made in β-Ga2O3 device performance for both high power and high frequency operation. Currently, aluminum oxide (Al2O3) is the preferred gate dielectric for β-Ga2O3 transistors due to its electron and hole barriers, and similar atomic structure. Process optimizations to improve the interface quality has been explored by chemical treatments1 to the β-Ga2O3 surface before dielectric deposition, during the deposition of Al2O3, and with post-deposition annealing. Therefore, there is a desire to understand and classify defects at and near the Al2O3/β-Ga2O3 interface. Here, we report on a study of the Al2O3/β-Ga2O3 interface and border traps in planar depletion-mode β-Ga2O3 MOSFETs using DC I-V and C-V measurements in dark and under illumination with wavelengths from 730 nm (1.7 eV) to 230 nm (5.4 eV). The MOSFETs are fabricated on a 50 nm Si-doped epitaxial layer with a target doping concentration of 2.4 x 1018 cm-3 grown on a (010) semi-insulating β-Ga2O3 substrate. A Ti/Al/Ni/Au metal stack is deposited and annealed to form the Ohmic contacts. Plasma-assisted atomic layer deposition (PE-ALD) is used to deposit the 20 nm Al2O3 gate dielectric. The fabricated transistor structures have a LG of 1.94 µm, LS of 0.5 µm, and LD varying from 0.5 µm, 5.5 µm, and 10.5 µm. Measurements were performed at room temperature. Most FETs exhibited a threshold voltage of approximately -4 V, high linearity, and ION/IOFF ratios between 107 – 109. Using the conductance method, we extract an interface trap state density, Dit, of 3 x 1011 eV-1 cm-2 up to 0.44 eV below the conduction band. Illumination is used to obtain deep trap concentration and behavior. We observe little to no change in IV and CV curves for wavelengths above 365 nm but see a significant voltage shift in the CV curves for illumination wavelengths between 365 nm (3.4 eV) to 230 nm (5.4 eV), indicating significant activated traps from 3.4 eV below the conduction band. We will present the analysis of photo-assisted CV and stress-CV curves at various wavelengths to obtain densities of fixed, fast, and slow trap states and discuss their impact on device performance.

Synergistic effect of carbon-deficient and Rh2O3-decorated on carbon nitride: Robust photocatalyst for bisphenol a degradation and the mechanism insight

The sluggish charge transfer and fast recombination of charge carriers is a determining factor limiting photocatalytic efficiency of carbon nitride. Herein, carbon vacancies and Rh2O3 nanoparticles were sequentially introduced into CN to accelerate photo-generated electron and hole transfer simultaneously, thereby improving the photocatalytic performance of CN to achieve the complete removal of Bisphenol A (BPA) in 15 min. Mechanism analyses reveal that the modification of the carbon vacancies regulates the active charge carriers, excitons and trap states, thus enhances the carrier separation capabilities and light absorption. Meanwhile, the in situ photodeposited Rh2O3 nanoparticles, as a hole-transfer mediator, maximally suppress recombination of photogenerated charges. These synergistic effects result in promoting the separation and transfer of charge carriers and thus improving photocatalytic performances. This strategy can serve as guideline for the reasonable design of efficient photocatalysts for phenolic wastewater remediation.

Charge-Carrier Dynamics of Evaporated Bismuth-Based Chalcogenide Thin Films Probed with Time-Resolved Microwave Conductivity.

Chalcogenide-based semiconductors are emerging as a set of highly promising candidates for optoelectronic devices, owing to their low toxicity, cost-effectiveness, exceptional stability, and tunable optoelectronic properties. Nonetheless, the limited understanding of charge recombination mechanisms and trap states of these materials is impeding their further development. To fill this gap, we conducted a comprehensive study of bismuth-based chalcogenide thin films and systematically investigated the influence of post-treatments via time-resolved microwave conductivity and temperature-dependent photoluminescence. The key finding in this work is that post-treatment with Bi could effectively enhance the crystallinity and charge-carrier mobility. However, the carrier density also increased significantly after the Bi treatment. On the contrary, post-treatment of evaporated Bi2S3 thin films with sulfur could effectively increase the carrier lifetime and mobility by passivating the trap states on the grain boundaries, which is also consistent with the enhanced radiative recombination efficiency.

Enhanced Open-Circuit Voltage and Improved Stability with 3-Guanidinoproponic Acid as the Passivation Agent in Blade-Coated Inverted Perovskite Solar Cells

The formation of high-density charge trap states along the surface and grain boundaries of the perovskite absorber is a major cause of recombination that limits the efficiency and quickly degrades the long-term stability of perovskite solar cells (PSCs). To improve the PSC performance, we investigated the use of amino acids with Lewis acid–base functional groups, namely, 2-guanidinoacetic acid, l-aspartic acid, l-glutamic acid, and 3-guanidinoproponaic acid (3gpa), as passivating dopants for blade-coated perovskite films. The best photovoltaic performance was achieved with 4 mol% 3gpa doping, which effectively passivated both positively and negatively charged defects, resulting in a 12.9% improvement in the open-circuit voltage and a high PCE of 19.83%. Additionally, the improved carrier lifetime and reduced defect density led to improved long-term stability in blade-coated PSCs.

Tetrachromatic vision-inspired neuromorphic sensors with ultraweak ultraviolet detection

Sensing and recognizing invisible ultraviolet (UV) light is vital for exploiting advanced artificial visual perception system. However, due to the uncertainty of the natural environment, the UV signal is very hard to be detected and perceived. Here, inspired by the tetrachromatic visual system, we report a controllable UV-ultrasensitive neuromorphic vision sensor (NeuVS) that uses organic phototransistors (OPTs) as the working unit to integrate sensing, memory and processing functions. Benefiting from asymmetric molecular structure and unique UV absorption of the active layer, the as fabricated UV-ultrasensitive NeuVS can detect 370 nm UV-light with the illumination intensity as low as 31 nW cm−2, exhibiting one of the best optical figures of merit in UV-sensitive neuromorphic vision sensors. Furthermore, the NeuVS array exbibits good image sensing and memorization capability due to its ultrasensitive optical detection and large density of charge trapping states. In addition, the wavelength-selective response and multi-level optical memory properties are utilized to construct an artificial neural network for extract and identify the invisible UV information. The NeuVS array can perform static and dynamic image recognition from the original color image by filtering red, green and blue noise, and significantly improve the recognition accuracy from 46 to 90%.

Cation exchange strategy to construct nanopatterned Zn:NiOx electrode with highly conductive interface for efficient inverted perovskite solar cells

Inorganic hole-transporting materials (HTMs) with excellent hole extraction and transfer properties are highly desirable for inverted perovskite solar cells (PSCs). However, the widely employed oxide HTMs (e.g., NiOx nanocrystal) often suffer from the low p-type conductivity and severe trap-assisted recombination at HTMs/perovskite interface, which significantly limits the final performance of PSCs. Herein, the facile template-assisted cation exchange strategy is developed to fabricate nanopatterned Zn:NiOx as efficient HTMs for PSCs, for the first time. Promisingly, by virtue of the advantageous 1D nanoscale architecture and synergistic substitutional Zn doping, high conductivity, high hole mobility and low interfacial charge trap states density have been achieved by such nanopatterned Zn:NiOx HTMs. These features endow the highly conductive interface for PSCs to accelerate hole extraction and minimize recombination loss. As a consequence, PSCs with nanopatterned Zn:NiOx deliver a champion efficiency of ≈20 % with open-circuit voltage of as high as 1.14 V. This work demonstrates and paves a new insight and avenue for the rational design of efficient oxide HTMs.

Charge Trap States of SiC Power TrenchMOS Transistor under Repetitive Unclamped Inductive Switching Stress.

Silicon carbide (SiC) has been envisioned as an almost ideal material for power electronic devices; however, device reliability is still a great challenge. Here we investigate the reliability of commercial 1.2-kV 4H-SiC MOSFETs under repetitive unclamped inductive switching (UIS). The stress invoked degradation of the device characteristics, including the output and transfer characteristics, drain leakage current, and capacitance characteristics. Besides the shift of steady-state electrical characteristics, a significant change in switching times points out the charge trapping phenomenon. Transient capacitance spectroscopy was applied to investigate charge traps in the virgin device as well as after UIS stress. The intrinsic traps due to metal impurities or Z1,2 transitions were recognized in the virgin device. The UIS stress caused suppression of the second stage of the Z1,2  transition, and only the first stage, Z10, was observed. Hence, the UIS stress is causing the reduction of multiple charging of carbon vacancies in SiC-based devices.

Evidence of Hot-Hole Induced Degradation in Lateral NDRIFT MOSFET: Characterization and TCAD Analysis

The role of secondary hot holes in the degradation of n-channel lateral drift MOSFETs, possibly leading to gate oxide breakdown, is discussed. Trapping of positive charges in the gate and field oxides and the formation of interface states under hot-carrier stress (HCS) have been successfully monitored by means of direct current current-voltage (DCIV) measurements and through gate-drain capacitance measurements. Experimental data have been complemented with Technology Computer Aided Design (TCAD) analysis. A degradation model including both hot-electrons and secondary hot-holes induced degradation capable of interpreting and reproducing measured data is proposed. Simulations allow to gain insights in the kinetics laws governing charge trapping and interface states generation along the silicon/oxide interface. The drift kinetics of the ON-state resistance have been also characterized and reproduced by simulations.