Trap States Research Articles

Group‐VA Doped ZnO Injection Layer for Bright and Efficient Perovskite Light‐Emitting Diodes

AbstractHeteroatom doping of the ZnO electron injection layer (EIL) is widely applied to the fabrication of high‐performance quantum‐dot and perovskite light‐emitting diodes (PeLEDs), while group‐VA atomic dopant is rarely studied. Here, VA bismuth (Bi) with strong metallicity is screened as a substitution dopant to regulate the colloid size, surface terminal, and defect density of the ZnO precursor. The as‐fabricated Bi‐doped ZnO (ZnBiO) EIL shows enhanced conductivity, passivated trap states, more n‐type nature, and appropriate band alignment with FAPbI3 (FA: formamidinium), contributing to smooth injection paths with minimal transport losses. Promoted by inhibited hydroxyl terminations of ZnBiO, the FAPbI3 crystals grown into flat nanocylinder domains with higher crystallinity and area‐thickness ratio, leading to a champion PeLED efficiency of 22.3%, a high near‐infrared radiance of 684 W sr−1 m−2, and nearly an order of magnitude extended operational stability, which shows great promise of ZnBiO as the optimal EIL for various LED application scenarios beyond classical ZnMgO.

Enhancing negative thermal quenching in green-emitting perovskite microspheres via shallow trap state modulation

AbstractFor luminescent materials, negative thermal quenching (NTQ), characterized by an increase in the luminescent intensity with temperature, has a large potential in lighting and display technologies. However, leveraging NTQ in metal halide perovskites is challenging, and the mechanism is not well understood. Herein, by utilizing low-temperature photoluminescence, persistent luminescence and thermoluminescence, the origins of NTQ in CsPbBr3 microspheres are systematically studied, which pertain to the liberation of carriers from shallow trap states. Experimental and theoretical investigations reveal that the energy of these shallow defect states is approximately 0.135 eV beneath the conduction band. A rapid thermal treatment increases the density of these shallow traps and amplifies the NTQ effect, resulting in an enhancement of room-temperature photoluminescence by more than 60% compared to that at 150 K. The process also reduces the threshold for amplified spontaneous emission to about 45 W/cm2. Our findings not only provide a deeper understanding of the NTQ phenomenon in CsPbBr3 microspheres but also open new avenues for enhancing the performance of perovskite optoelectronic devices through energy state regulation.

Electrostatic Field Regulate Interfacial Hole Transfer for Photocatalytic Valorization of Biopolyols

AbstractPhotocatalysis has shown power in the valorization of biomass while controlling the selectivity is a long‐standing goal and challenge, especially for the complex and reactive bio‐polyols. The selectivity control of photocatalytic biopolyols reforming to formic acid (FA) or CO is realized via engineering the electrostatic field on TiO2 semiconductor. The electrostatic field is generated by surface modification with anion adsorbates, which can alter the trap states of photoexcited holes and regulate interfacial hole transfer to the surface‐bound species, thereby strongly affecting photocatalytic activity. Taking formic acid dehydration as an example, a shallow trap of photoexcited holes on pristine TiO2 favors the dehydration of FA to CO, while a deep trap of photoexcited holes after introduced anion adsorbates makes FA stable. Based on this result, the selectivity is successfully tuned in the photocatalytic oxidation of biopolyols via controlling electrostatic field. A wide range of biopolyols can be selectively converted into FA or CO. This work presents an effective strategy to manipulate reaction pathways via generating electric field.

Fe−N Co‐Doped BiVO4 Photoanode with Record Photocurrent for Water Oxidation

AbstractBismuth vanadate (BVO) ranks among the most promising photoanodes for photoelectrochemical (PEC) water splitting. Nonetheless, slow charge separation and transport, besides the sluggish water oxidation kinetics, are key barriers to its photoefficiency. Here, we present a co‐doping strategy that significantly improves the charge separation performance of BVO photoanodes. We found that, under standard one sun illumination, the Fe−N co‐doped BVO photoanode (Fe−N−BVO) by N‐coordinated Fe precursor reaches a record photocurrent density of 7.01 mA cm−2 at 1.23 V vs RHE after modified a surface co‐catalyst (FeNiOOH), and exhibits an outstanding stability. By contrast, much lower photocurrent density is obtained for the N‐doped, Fe‐doped and Fe/N‐doped BVO photoanode with separated N and Fe precursors. The detailed experimental characterizations show that the high activity of the Fe−N co‐doped BVO photoanode is attributed to the enhanced photo‐induced bulk charge separation, as well as the accelerated surface water oxidation kinetics. XPS, EXAFS and DFT calculations clearly show that, instead of formation of deep trapping state in the individually doped BVO, the co‐doping of Fe−N into BVO generates Fe‐based electronic states just below the bottom of conduction band and N‐derived states just above the top of valence band. Such modulations in electronic structure enable the efficient trap of the electrons and holes to enhance the separation of photo‐induced carriers, but hinder the charge recombination originated from the deep trapping sites.

Elucidating the Interplay between Symmetry Distortions in Passivated MAPbI3 and the Rashba Splitting Effect.

Hybrid organic-inorganic perovskites play a critical role in modern optoelectronic applications, particularly as single photon sources due to their unusual bright ground state. However, the presence of trap states resulting from surface dangling bonds hinders their widespread commercial application. This work uses density functional theory (DFT) to study the effects of various passivating ligands and their binding sites on Rashba splitting, a phenomenon directly linked to the bright ground state. Our results predict that X2- and X4-type ligands that adsorb at acidic oxygen binding sites and zwitterionic binding sites efficiently eliminate trap states introduced by surface iodine vacancies. Furthermore, our results show that distortions from the nominally symmetric cubic structure of the perovskite predominantly determine the presence and magnitude of the Rashba splitting. Specifically, the loss of more symmetry elements consistently leads to Rashba splitting in both the valence band (VB) and the conduction band (CB) with small Rashba splitting coefficients. Conversely, although inversion symmetry breaking alone fails to guarantee the presence of pure Rashba splitting in both the VB and the CB, it significantly increases the degree of splitting. The adsorption of ligands not only mitigates trap states but also plays a critical role in altering the local symmetry, thus influencing Rashba splitting. DFT predicts a distinct Rashba-Dresselhaus splitting in the CB with X2 ligands, causing the largest splitting. The presence of local electric fields causes consistent Rashba splitting of the VB across all studied systems except for the X4 zwitterionic passivated systems (sulfobetaine and lecithin). Electric fields are predicted to cause significant splitting of the CB, particularly for MAPbI3 and SH passivated MAPbI3 surfaces that possess freely rotating ligand binding sites. This study reveals that the wavelength, tunability of Rashba splitting through an applied electric field, and nature of Rashba-Dresselhaus splitting are influenced by the characteristics of the ligand binding site. On the other hand, pure Rashba splitting is predicted to exhibit a greater susceptibility to symmetry distortion than to specific ligand binding sites. These findings elucidate how surface passivating ligands and symmetry distortions influence Rashba splitting, shaping the optoelectronic properties of perovskite nanocrystals.

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.

Improved Thermal Dissipation in a MoS2 Field-Effect Transistor by Hybrid High-k Dielectric Layers.

Transition metal dichalcogenides like MoS2 have been considered as crucial channel materials beyond silicon to continuously advance transistor scaling down owing to their two-dimensional structure and exceptional electrical properties. However, the undesirable interface morphology and vibrational phonon frequency mismatch between MoS2 and the dielectric layer induce low thermal boundary conductance, resulting in overheating issues and impeding electrical performance improvement in the MoS2 field-effect transistors. Here, we employed hybrid high-k dielectric layers of Al2O3/HfO2 to simultaneously reduce the interfacial thermal resistance and improve device electrical performance. The enhanced contact, greater vibrational phonon overlapping region, and stronger interfacial bonding force between the top Al2O3 layer and MoS2 promote the heat removal efficiency across the interface to the substrate. Under the same input power density, the temperature profile of the MoS2 transistor on the Al2O3/HfO2 has been largely reduced compared to that of the device on HfO2, with a maximum reduction of 49.5 °C. In addition, the field-effect mobility and current of MoS2 devices on the Al2O3/HfO2 high-k dielectric layers have been significantly improved, attributed to the depressed electron scattering and trap states at the interface. The design of the hybrid high-k dielectric layers provides an efficient solution to simultaneously improve the thermal and electrical performance of the two-dimensional devices.

Collective dynamics of swarmalators driven by a mobile pacemaker.

Swarmalators, namely, oscillators with intrinsic frequencies that are able to self-propel to move in space, may undergo collective spatial swarming and meanwhile phase synchronous dynamics. In this paper, a swarmalator model driven by an external mobile pacemaker is proposed to explore the swarming dynamics in the presence of the competition between the external organization of the moving pacemaker and the intrinsic self-organization among oscillators. It is unveiled that the swarmalator system may exhibit a wealth of novel spatiotemporal patterns including the spindle state, the ripple state, and the trapping state. Transitions among these patterns and the mechanisms are studied with the help of different order parameters. The phase diagrams present systematic scenarios of various possible collective swarming dynamics and the transitions among them. The present study indicates that one may manipulate the formation and switching of the organized collective states by adjusting the external driving force, which is expected to shed light on applications of swarming performance control in natural and artificial groups of active agents.

Performance enhancement of inverted CsPbI2Br perovskite solar cells via butylammonium cation additive modification

Defects in all inorganic perovskite films currently limit the efficiency and long-term stability of perovskite solar cells (PSCs). In this study, we utilized the butylammonium cation as an additive to enhance the performance of PSCs. By incorporating butylammonium iodide, modifications in crystallization were achieved, resulting in high-quality CsPbI2Br perovskite films with larger grain sizes. As a result, the inverted PSCs with 1 mol percent additive exhibited an enhanced power conversion efficiency (PCE) of up to 13.0%, compared to the control PCE of 10.8%. The improvement can be primarily attributed to reduced trap states and suppressed charge recombination within the inverted PSCs. Furthermore, the hydrophobic nature of the additive noticeably improved the long-term stability of CsPbI2Br PSCs.

An Approach to Extract the Trap States via the Dynamic Ron Method With Substrate Voltage Applied During the Recovery Time

An Approach to Extract the Trap States via the Dynamic R_on Method With Substrate Voltage Applied During the Recovery Time

High-Temperature Characterization of AlGaN Channel High Electron Mobility Transistor Based on Silicon Substrate

In this paper, it is demonstrated that the AlGaN high electron mobility transistor (HEMT) based on silicon wafer exhibits excellent high-temperature performance. First, the output characteristics show that the ratio of on-resistance (RON) only reaches 1.55 when the working temperature increases from 25 °C to 150 °C. This increase in RON is caused by a reduction in optical phonon scattering-limited mobility (μOP) in the AlGaN material. Moreover, the device also displays great high-performance stability in that the variation of the threshold voltage (ΔVTH) is only 0.1 V, and the off-state leakage current (ID,off-state) is simply increased from 2.87 × 10−5 to 1.85 × 10−4 mA/mm, under the operating temperature variation from 25 °C to 200 °C. It is found that the two trap states are induced at high temperatures, and the trap state densities (DT) of 4.09 × 1012~5.95 × 1012 and 7.58 × 1012~1.53 × 1013 cm−2 eV−1 are located at ET in a range of 0.46~0.48 eV and 0.57~0.61 eV, respectively, which lead to the slight performance degeneration of AlGaN HEMT. Therefore, this work provides experimental and theoretical evidence of AlGaN HEMT for high-temperature applications, pushing the development of ultra-wide gap semiconductors greatly.

High-Performance CuInS2 Quantum Dot Sensitized Solar Cells Through I-/MPA Dual-Ligands Passivation.

High-efficiency quantum dot sensitized solar cells (QDSSCs) can be received by increasing quantum dot (QD) loading and mitigating QD surface trap states. Herein, the surface state of CuInS2 QDs is optimized through an I-/MPA dual-ligands passivation strategy. The steric hindrance and electrostatic repulsion between QDs can be effectively reduced, thereby enabling an increased QD loading capacity. Meanwhile, the I-/MPA dual-ligands passivation strategy can further lower the surface trap density, leading to substantially enhanced charge transfer efficiency of the solar cells. Interestingly, various iodized salts, including TBAI, MAI, and KI, are proved to possess comparable property, underscoring the versatility and broad applicability of this I-/MPA dual-ligands passivation strategy. Eventually, CuInS2 QDSSCs based on the NH4I/MPA dual-ligands exhibit a noteworthy enhancement in photovoltaic conversion efficiency, surpassing the benchmark of 5.71 % to reach 7.03 %.

As‐Doped Polycrystalline CdSeTe: Localized Defects, Carrier Mobility and Lifetimes, and Impact on High‐Efficiency Solar Cells

AbstractThe efficiency potential for single‐junction photovoltaics (PV) is described by the detailed balance model, which requires the elimination of nonradiative recombination and perfect minority carrier collection. Improvements in GaAs, Si, and perovskite PV follow this model. It might be more complex for CdTe, a leading thin‐film PV technology. While lifetime, passivation, and doping goals for 25% efficient CdTe solar cells are largely reached, voltage is ≈20% below the detailed balance limit. Why is that? In Se‐alloyed CdSexTe1‐x (Se is required for >20% efficiency) additional losses can occur due to electrostatic and bandgap fluctuations and due to electronic trap states. To understand mechanisms limiting CdSeTe solar cell performance and to suggest improvements, carrier dynamics, and transport in CdSexTe1‐x with variation in Se composition and as doping is analyzed. It is shown that trapping, likely due to anion‐site defects and their complexes, is correlated with low charge carrier mobility of 0.1–0.6 cm2 (Vs)−1. Even with 1000 ns charge carrier lifetimes, carrier diffusion length is less than the absorber thickness, reducing efficiency to ≈23%. Device simulations are used to analyze the performance of CdSexTe1‐x solar cells; thermodynamic models are not sufficient for absorbers with electronic disorder and trapping.

Kinetic Control over Social and Narcissistic Self-Sorting from Multicomponent Mixtures in Seed-Initiated Supramolecular Polymerization by Fine-Tuning of Steric Effects.

Supramolecular polymers offer an intriguing possibility to transfer molecular properties from the nano- to the mesoscale. Towards this achievement, seed-initiated supramolecular polymerization has emerged as a powerful tool, as it prevents unlimited growth and enables size control of the assembly outcome. However, the potential application of the seeding method in the context of complex supramolecular systems is hitherto unclear. Herein we demonstrate that minute differences in molecular design in direct proximity to intermolecular recognition sites govern the molecular packing and in turn dictate the efficacy of seeded polymerization processes. We introduce a stepwise increase in steric demand in the central amino acid residue of a diamide system, which gradually increases the rotational displacement within the aggregated state. This fine-tuning of the molecular packing directly affects the propensity of the different aggregates to act as seeds for the other supramolecular synthons. In turn this allows us to selectively target specific trapped monomer states in binary mixtures for social or narcissistic seeded polymerization.

Kinetic Control over Social and Narcissistic Self‐Sorting from Multicomponent Mixtures in Seed‐Initiated Supramolecular Polymerization by Fine‐Tuning of Steric Effects

Supramolecular polymers offer an intriguing possibility to transfer molecular properties from the nano‐ to the mesoscale. Towards this achievement, seed‐initiated supramolecular polymerization has emerged as a powerful tool, as it prevents unlimited growth and enables size control of the assembly outcome. However, the potential application of the seeding method in the context of complex supramolecular systems is hitherto unclear. Herein we demonstrate that minute differences in molecular design in direct proximity to intermolecular recognition sites govern the molecular packing and in turn dictate the efficacy of seeded polymerization processes. We introduce a stepwise increase in steric demand in the central amino acid residue of a diamide system, which gradually increases the rotational displacement within the aggregated state. This fine‐tuning of the molecular packing directly affects the propensity of the different aggregates to act as seeds for the other supramolecular synthons. In turn this allows us to selectively target specific trapped monomer states in binary mixtures for social or narcissistic seeded polymerization.

Defect induced persistence photoconductivity in spray pyrolyzed ZnO thin films: Impact of Sm3+doping

The persistent photoconductivity of the rare earth ion doped ZnO is an emerging field. The present work focuses on the effect of samarium (Sm3+) ion doping on the persistence photo response behaviour ZnO host matrix. The Zn1-xSmxO (x = 0.0 to 0.10) thin films were deposited on chemically cleaned glass substrates using spray pyrolysis technique. The films showed polycrystalline nature and hexagonal wurtzite structure without any impurity peaks. The Zn0.93Sm0.07O films showed maximum crystallite size of 24.2 nm and minimum dislocation density. The gradual change in the thickness of the fibrous nature was observed with the addition of Sm3+ ions into ZnO lattice. Energy dispersive analysis of x-ray and X-ray photoelectron spectroscopy confirmed the presence of elements and its oxidation states in the deposited film respectively. The area under the curve of deconvoluted photoluminescence spectra of deposits confirmed the decrease in the various defect percentage with increase in the doping concentration of Sm3+ ions. The photoluminescence spectra of Zn0.93Sm0.07O films showed maximum near band edge emission and minimum defects. The Zn0.93Sm0.07O films showed higher carrier concentration of 1 × 1017 cm−3, mobility 32 cm2/Vs and lower resistivity of 1 × 102 Ω cm due to improved film quality. The Zn0.93Sm0.07O films exhibited the current value under dark and ultraviolet light illumination was in the range of 10−6 A and 10−4 A respectively. The maximum photocurrent was noticed at 375 nm which corresponds to the bandgap of the deposited films. The Zn0.93Sm0.07O films showed faster photo response (5 s and 131 s of response and recovery time) due to the presence of minimum trap states. Hence the Zn0.93Sm0.07O films can be used in the fabrication of light dependent resistors and ultraviolet sensors.

Decomposition of mixed DMPC-aescin vesicles to bicelles is linked to the lipid's main phase transition: A direct evidence by using chain-deuterated lipid

This work investigates the conversion of bicelles into larger sheets or closed vesicles upon dilution and temperature increase for a system composed of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and the saponin aescin. Due to its peculiar amphiphilic character, aescin is able to decompose DMPC bilayers into smaller, rim-stabilized bicelles. Aspects of the transition process are analyzed in an aescin content- and temperature-dependent manner by photon correlation spectroscopy (PCS), turbidimetry and small-angle neutron scattering (SANS). Both the conversion of bicelles into vesicles induced by temperature increase and the decomposition process upon cooling are presumably related to the main phase transition temperature Tm of DMPC. Therefore, not only conventional DMPC, but also chain-deuterated d54-DMPC was used due to its significantly lower Tm-value compared to the conventional DMPC. It will be demonstrated that the reconversion of vesicle structures (present at low aescin content) into bicelles shows a strong hysteresis effect whereas this is not observed for the reconversion at high aescin amounts, at which for high temperature still bicelle structures are present. The results indicate formation of a trapped state, correlated with the lipid's Tm and the decomposition of vesicles into bicelles is only possible if the lipid membrane entirely adopts the rigid phase state.

Charge Carrier Dynamics at the Perovskite Interface with Self-Assembled Monolayers.

Self-assembled monolayers (SAMs) deposited on the hole-collecting electrodes of p-i-n perovskite solar cells effectively replace bulky hole transporting layers. However, the mechanism by which monolayers control the electronic processes and how they depend on the properties of the monolayer molecules remain poorly understood. In this study, we developed a simplified perovskite solar cell imitator with blocked electron extraction to investigate the photocurrent dynamics between the perovskite and the hole-collecting ITO electrode. We investigated the photoluminescence and photovoltage dynamics under short laser pulse excitation and addressed the influence of bulky and monomolecular hole transport layers. Our findings reveal that the photovoltage dynamics is significantly affected by the properties of the transport and perovskite layers, which in turn depend on the methods of sample preparation and exploration. Photocurrent dynamics is determined by several processes, including charge carrier displacement in the local electric field, hole transport to ITO, trapping of holes in interface trap states, and electron-hole recombination at the interface. We propose a model that takes into account molecular dipole moments and their ionization potentials to partially explain the different influences of different monolayers on the hole extraction and interfacial recombination rates. Additionally, the photovoltage dynamics also strongly depends on the illumination of the sample and shows memory effects that persist over minutes and hours and are attributed to the redistribution of ions.

Antithermal-Quenching All-Inorganic Perovskite Crystals with Green Ultralong Persistent Luminescence for Advanced Anticounterfeiting Applications.

Long persistent luminescence (PersL) materials have revolutionized many fields of optoelectronics and photonics due to their applications in anticounterfeiting, information encryption, and in vivo bioimaging. Here, we reported a novel PersL crystal prepared by the heterovalent doping of Sb3+ into perovskite tetragonal phase RbCdCl3, comparing with the pristine non-perovskite orthorhombic phase analogue without PersL property. Surprisingly, under the UV light irradiation, the title crystals concurrently exhibit green ultralong PersL (>2400 s), high photoluminescence quantum yield (49.1%), and antithermal quenching in the range from 148 to 328 K. It was revealed by experimental results and theoretical analyses that green ultralong PersL and antithermal quenching of perovskite-phase RbCdCl3/Sb3+ crystals originate from the electron transition between the 5s2 level of the dopant Sb3+ and the electronic defect-induced trap states. Enlightened by the excellent optical properties, the tetragonal perovskite-phase RbCdCl3/Sb3+ PersL materials show promising application prospects in anticounterfeiting and encryption of information.

Simultaneous Structural and Electronic Engineering on Bi- and Rh-co-doped SrTiO3 for Promoting Photocatalytic Water Splitting.

Activating metal ion-doped oxides as visible-light-responsive photocatalysts requires intricate structural and electronic engineering, a task with inherent challenges. In this study, we employed a solid (template)-molten (dopants) reaction to synthesize Bi- and Rh-codoped SrTiO3 (SrTiO3 : Bi,Rh) particles. Our investigation reveals that SrTiO3 : Bi,Rh manifests as single-crystalline particles in a core (undoped)/shell (doped) structure. Furthermore, it exhibits a well-stabilized Rh3+ energy state for visible-light response without introducing undesirable trapping states. This precisely engineered structure and electronic configuration promoted the generation of high-concentration and long-lived free electrons, as well as facilitated their transfer to cocatalysts for H2 evolution. Impressively, SrTiO3 : Bi,Rh achieved an exceptional apparent quantum yield (AQY) of 18.9 % at 420 nm, setting a new benchmark among Rh-doped-based SrTiO3 materials. Furthermore, when integrated into an all-solid-state Z-Scheme system with Mo-doped BiVO4 and reduced graphene oxide, SrTiO3 : Bi,Rh enabled water splitting with an AQY of 7.1 % at 420 nm. This work underscores the significance of simultaneous structural and electronic engineering and introduces the solid-molten reaction as a viable approach for this purpose.