Distribution Of Trap States Research Articles

Unveiling full-dimensional distribution of trap states toward highly efficient perovskite photovoltaics

To gain a deep understanding and address key issues in perovskite photovoltaics, such as power conversion efficiency (PCE) and long-term stability, defect passivation and analysis of the device performance are required. Here, we proposed a non-contact characterization technique named scanning photocurrent measurement system (SPMS) for device surface detection, having conducted signal analysis and method adjustments based on perovskite photovoltaic devices. This technique enables the monitoring of minority carriers in the device, allowing for the investigation of carrier behavior based on photocurrent signals. By integrating SPMS with thermal conductance spectroscopy (TAS) and drive-level capacitance profiling (DLCP), we further simulated a spatial three-dimensional (3D) distribution of trap states in the device and analyzed the distribution of trap states matched with energy space. Through extensive case studies, we have validated the universality and accuracy of this method. The integration of trap state characterization techniques provides strong support for targeted defect passivation and performance evaluation of perovskite photovoltaic devices, yielding a highly efficient perovskite solar cell with PCE as high as 25.74%.

Photoexcited Polaron Relaxation as a Structurally Sensitive Reporter of Charge Trapping in a Conducting Polymer

AbstractConjugated polymers (CPs) play a central role in electronic applications due to their easily tuned electronic and ionic conductivities via chemical or electrochemical doping. Although doping improves charge conduction by introducing high densities of carriers into the CP, the accompanying structural changes and their impact on carrier mobility remain elusive. Methods capable of probing carrier distributions and their dependence on polymer morphology are needed to better understand how to improve conductivity. Here, a transient absorption (TA) spectroscopy approach is demonstrated, capable of directly probing mobile and trapped carriers in doped CPs and that is also sensitive to polymer nanostructure by using a model polythiophene system with tuned crystallinity. Exciting polarons in the polymer films produces distinct photoinduced absorption signals in the near‐infrared spectrum that decay during the picosecond timescale in the form of biphasic, stretched exponential kinetics, which reflect a distribution of mobile (free) and trapped polarons. The kinetic analysis provides evidence for mobile polarons irrespective of polymer film crystallinity, whereas polarons located in impure amorphous phases with reduced chain ordering exist within a deeper distribution of trap states. Altogether, these observations suggest a stronger correlation of carrier trapping with local chain ordering (planarity or aggregation) rather than polymer crystallinity.

Repetitively pulsed streamer discharge with laser-induced surface trapped electron desorption to exploit residual charges in situ

Positive streamer behaviors under repetitive pulses are predominantly dependent on the availability of free electrons. If surface residual electrons stored from previous discharges could be intentionally released and involved into the next discharge, an alternative control freedom is provided apart from voltage waveform tailoring methods that mainly attract or repel gaseous residual charges. Evolutions of repetitively pulsed surface streamers in compressed (0.2 MPa) air were investigated after low-photon-energy pulsed visible (532 nm) and infrared (1064 nm) laser irradiations. Pulse-sequence and temporally resolved diagnostics were implemented to investigate effects of laser parameters (irradiation moment, wavelength, energy) and gas composition. A 2D surface streamer fluid simulation was performed to qualitatively unveil impacts of localized plasma patches. The surface streamer morphology and emission light are significantly and repeatably affected by the laser irradiation before the streamer inception, while, variations totally disappear without the solid surface. The secondary streamer is prolonged accompanied by a higher flashover probability after the pulsed laser irradiation in compressed air. Intriguingly, influences of the infrared laser persist for tens of microseconds before the next voltage pulse. Residual charge dynamics under the laser irradiation are analyzed, where the additional increase of O2− of low electron bound energy is emphasized. The laser induced surface trapped electron desorption is achieved through the direct or the step-wise process, dependent on the laser energy and the surface trap state distribution.

Forward leakage currents in GaN p-i-n diodes

Excessive forward leakage currents in GaN p-i-n diodes were investigated. Traditional diffusion mechanism dominates at VF > 2 V. The effective band gap is derived to be ∼2.21 eV, which is much lower than 3.4 eV and attributed to a band fluctuation caused by dislocations; At 1.35 V < VF < 2 V, a trap-assisted tunneling process becomes important, whose ideality factor is still larger than 4.1 at T = 400 K; Two distinct power-law relationships were observed at lower biases, separated at VF = 0.8 V, whose exponents are extracted to be ∼8 and ∼4, respectively. The behavior is in a good agreement with the space-charge-limited model, featuring an exponentially decaying distribution of trap states below the conduction band.

Versatility of thermoluminescence materials in thermal history sensing: a review

While several devices can monitor temperature, measuring non-volatile thermal history is far more challenging, especially for sensors placed in hazardous settings like fires and explosions. Herein, the utilisation of thermoluminescent microparticles in thermal history sensors is discussed. These transparent dielectric microparticles can store charge carriers for very extended periods due to their wide distribution of trap states. In its most basic form, an Arrhenius expression is strongly temperature-dependent for determining the population of the trap states. When exposed to high temperatures for a brief period, a particle with filled traps will preferentially lose carriers in shallow traps. To ascertain the temperature and timing of the thermal event, this depopulation creates a mark on the particle luminescence. By being exposed to deep ultraviolet, X-ray, beta, or gamma radiation, which fills the traps with charge carriers, particles are prepared many months before a test, if required. No matter if they are embedded in garbage or other inert materials, one or more particles can nevertheless be made to emit light. Laboratory tests using microheaters and high-energy explosives in the field are used to test and analyse the technology. It is demonstrated that a variety of high-explosive conditions may be used to accurately detect temperatures in the 200 ∘ C to 500 ∘ C range using thermoluminescent materials like LiF:Mg,Ti, MgB4O7:Dy,Li, and CaSO4:Ce,Tb. The article will provide the utilisation of different luminescent materials for sensing thermal history and analysing the particle luminescence mechanism through different thermal models for developing improved thermal history sensing technologies. The thermal history reconstruction techniques are also discussed to predict the thermal history. Therefore, the article will pave the way to understanding the luminescent properties of thermoluminescent microparticles for developing state-of-the-art thermal history sensors for measuring non-volatile thermal history and temperature in extreme environments such as fires and explosions.

Suppression of dielectric surface flashover induced by strong electromagnetic field at multiple spatial scales based on above/sub-surface discharge development mechanisms

Spacecraft charging and electrostatic discharging (ESD) are prone to occur in harsh space environments. In particular, in the case of coupling strong electromagnetic field (EMF), ESD damages may occur at a low charging potential, posing a serious threat to on-orbit spacecraft missions. To investigate the mechanism and the pertinent suppression method for vacuum surface discharge induced by EMF, a specially-designed platform for EMF-induced surface discharge was set up. Surface structures with various spatial scales were created separately by using different surface engineering strategies, including direct fluorination, mechanical polishing, and 3D-printed grooving. The resulting surface physicochemical characteristics of the samples were examined. Furthermore, the surface discharge characteristics for different methods induced by strong EMF were systematically analyzed, considering the surface trap state distribution and secondary electron yield (SEY). The findings indicate that the proposed surface treatment methods demonstrate varying levels of improvement in mitigating EMF-induced discharge. Direct fluorination was found to produce lower SEY and to accelerate surface charge dissipation due to an elevated shallow trap density, making it favorable for suppressing the EMF-induced discharge. In addition, suitable surface roughness and groove size can effectively impede the development of the multipactor, thereby preventing EMF-induced discharge. This research is expected to provide valuable insights into the protection design of EMF-induced discharge on spacecraft.

Characterizing Spatial and Energetic Distributions of Trap States Toward Highly Efficient Perovskite Photovoltaics.

Due to their greater opt electric performance, perovskite photovoltaics (PVs) present huge potential to be commercialized. Perovskite PV's high theoretical efficiency expands the available development area. The passivation of defects in perovskite films is crucial for approaching the theoretical limit. In addition to creating efficient passivation techniques, it is essential to direct the passivation approach by getting precise and real-time information on the trap states through measurements. Therefore, it is necessary to establish quantitative characterization methods for the trap states in energy and 3D spaces.The authorscover the characterization of the spatial and energy distributions of trap states in this article with an eye toward high-efficiency perovskite photovoltaics. After going over the strategies that have been created for characterizing and evaluating trap states,the authorswill concentrate on how to direct the creative development of characterization techniques for trap states assessment and highlight the opportunities and challenges of future development. The 3D space and energy distribution mappings of trap states are anticipated to be realized. The review will give key guiding importance for further approaching the theoretical efficiency of perovskite photovoltaics, offering some future research direction and technological assistance for the development of appropriate targeted passivation technologies.

Space charge limited current in 4H-SiC Schottky diodes in the presence of stacking faults

The presence of crystallographic defects can induce notable effects on the mechanisms ruling the current transport in metal/semiconductor contacts. In this context, in this Letter, the impact of stacking faults (SFs) on the characteristics of 4H-SiC Schottky diodes was investigated under both forward and reverse bias. In particular, in the presence of SFs under the contact, while no significant effect on the ideality factor and barrier height was observed under forward bias, an anomalous increase in the leakage current occurred under reverse bias. The observed behavior of the leakage current could be explained by a space-charge limited current model, consistent with the presence of a distribution of trapping states in the gap of 4H-SiC. An increase in the reverse bias above 30 V leads to a complete trap filling. The weak temperature-dependence of the leakage current observed in this regime suggests the coexistence with a tunneling of the carriers through the barrier. The results can be useful to understand unexpected failures in 4H-SiC Schottky diodes.

Influence of Adsorption of para‐Aminothiophenol on Trap States and Recombination Kinetics at Plasmonic Ag‐TiO2 Heterostructure Interfaces

AbstractThe variation of trap states and the involving kinetics on the Ag‐TiO2 interfaces after the adsorption of reactant molecules were explored with the help of the combination of spectroscopic, electrochemical and photoelectrochemical methods by using para‐aminothiophenol (PATP) as a probe molecule. The results of photoluminescence spectroscopy and chronoamperometry revealed the distribution of trap states of Ag‐TiO2 heterostructures and the influence of PATP's adsorption. Kinetic measurements of open‐circuit photovoltage showed that the adsorption of PATP affects not only the electron lifetime (τn) but also the recombination mechanism of charge‐carrier on Ag‐TiO2 heterostructures. The analysis of the obtained effective recombination orders indicated that the charge‐carrier recombination on the Ag‐TiO2 electrode followed second‐order kinetics, while the one of the Ag‐TiO2/PATP electrode evolved from first‐order kinetics at first (at higher Voc) to second‐order kinetics (at lower Voc).

Effect of synthesis temperature on the charge transport properties of poly(3-methylthiophene) devices

Low temperature-dependent I-V measurement was adopted to investigate an insight into the effect of synthesis temperature on the field and temperature-dependent charge transport properties of electrochemically polymerized poly(3-methylthiophene) devices. Devices having different doping levels were fabricated at two different temperatures, one at room temperature (25oC) and another at a lower temperature (5oC). The transport mechanism in the devices were studied in the temperature range 40 K to 300 K. Our studies revealed that trap controlled space charge limited conduction (TCSCLC) mechanism dominated the transport mechanism at higher temperature(T>200K) and changed to field dependent mobility model at lower temperature(T<200K). It was observed that the synthesis temperature influenced the transport parameters. Devices prepared at low temperature showed an increase in resistivity and a corresponding decrease in the charge carrier mobility compared to those prepared at high temperature. Moreover, the distribution of the trap states in the devices was also affected by the synthesis temperature, with devices fabricated at low temperature exhibiting a wider trap state distribution.

Accessing Deep Traps Distribution in FeS2‐Doped Organic Photovoltaics

Although doping has the potential to improve the performance of organic photovoltaic cells (OPVc), doping effects on charge transport, recombination, and energetic disorder are still obscure. Doping has two opposing effects: on the one hand, dopant ions create more trap centers, while free dopant‐induced charges fill deep states, potentially providing better performance. The optimum amount of dopants can considerably improve the performance of OPVc. Herein, the energetic distribution of trap states in P3HT: PC71BM‐based OPVc doped with iron pyrite nanocubes (NCs) is reported. Using the reverse bias transient photocurrent (TPC) measurement, the energetic trap distributions with different doping conditions are studied. The photovoltaic characteristics and TPC phenomena of the OPVc greatly improve through doping. Variations in trap distributions with doping levels are analyzed to interpret the obtained trap density of states profiles. The light‐dependent current–voltage characteristics help to identify the presence of a less trap‐assisted recombination process in the optimum device. This study highlights the mechanism for performance improvement in devices with optimal doping of iron pyrite NCs.

Influence of Two- and Three-Dimensional Engineering on the Trap State Distribution and Photophysical Properties of Lead Halide Perovskite Polycrystals

Constructing a two- and three-dimensional (2D/3D) heterojunction structure on the surface of a 3D perovskite film, termed 2D/3D engineering, is effective in elevating the stability of perovskite polycrystal-based photovoltaic and photoelectronic devices; however, it remains controversial whether this protocol is favorable or detrimental to the device performance. Here, we prepare a series of 2D/3D perovskite films by post-treating the perovskite polycrystalline film with different concentrations of phenethylammonium iodide (PEAI). Systematic spectroscopy and electrochemical studies illustrate that PEAI can penetrate the 3D perovskite network and eliminate the intrinsic trap states of perovskite polycrystals, while the 2D perovskite nanosheets enriched on the top of the polycrystalline film may introduce additional trap states, which manipulate the photoluminescence performance and dynamics of the as-prepared perovskite films in an opposite manner. Based on this finding, the strategy of optimizing the photophysical properties of the host 3D perovskite through 2D/3D engineering is elaborated, paving the way for fabricating high-performance and high-stability perovskite polycrystalline films.

Terahertz properties of hydrogenated Wadsley phase vanadium oxides

In the present work, VO2(B), V6O13 and α-V2O5 polycrystalline films have been successfully fabricated by DC reactive magnetron sputtering, and large-area uniform hydrogenated VO2(B) and V6O13 samples were realized by a proton-electron co-doping method. The analysis of the X-ray and Raman spectrum data revealed that hydrogenated V6O13 film undergoes a significant change of micro strain in comparison to the VO2(B) sample. Terahertz properties of VO2(B) and V6O13 samples were characterized and their complex terahertz conductance successfully simulated by the Drude-Smith model. The temperature dependence of the conductivity of hydrogenated samples at the long-wavelength limit can be described by Mott’s small polaron theorem. It also suggests that the discrepancy in the distribution of trap states induced by the proton-electron co-doping method plays a dominant role. These results have a specific significance for understanding the fundamental properties of hydrogenated oxides and their optical applications in long wavelengths.

Characterizing Density and Spatial Distribution of Trap States in Ta3N5 Thin Films for Rational Defect Passivation.

Tantalum nitride (Ta3N5) has gained significant attention as a potential photoanode material, yet it has been challenged by material quality issues. Defect-induced trap states are detrimental to the performance of any semiconductor material. Beyond influencing the performance of Ta3N5 films, defects can also accelerate the degradation in water during desired electrochemical applications. Defect passivation has provided an enormous boost to the development of many semiconductor materials but is currently in its infancy for Ta3N5. This is in part due to a lack of experimental understanding regarding the spatial and energetic distribution of trap states throughout Ta3N5 thin films. Here, we employ drive-level capacitance profiling (DLCP) to experimentally resolve the spatial and energetic distribution of trap states throughout Ta3N5 thin films. The density of deeper energetic traps is found to reach ∼2.5 to 6 × 1022 cm-3 at the interfaces of neat Ta3N5 thin films, over an order of magnitude greater than the bulk. In addition to the spatial profile of deep trap states, we report neat Ta3N5 thin films to be highly n-type in nature, owning a free carrier density of ∼9.74 × 1017 cm-3. This information, coupled with the present understanding of native oxide layers on Ta3N5, has facilitated the rational design of a targeted passivation strategy that simultaneously provides a means for catalyst immobilization. Loading catalyst via silatrane moieties suppresses the density of defects at the surface of Ta3N5 thin films by two orders of magnitude, while also reducing the free carrier density of films by over one order of magnitude, effectively dedoping the films to ∼2.40 × 1016 cm-3. The surface passivation of Ta3N5 films translates to suppressed defect-induced trapping and recombination of photoexcited carriers, as determined through absorption, photoluminescence, and transient photovoltage. This illustrates how developing a deeper understanding of the distribution and influence of defects in Ta3N5 thin films has the potential to guide future works and ultimately accelerate the integration and development of high-performance Ta3N5 thin film devices.

A contactless method to study carrier kinetics in SnS thin films

Contactless methods to study laser-induced carrier kinetics present important advantages for materials characterization without necessity for contact fabrication. For example, mobility and lifetime of photogenerated carriers are studied by the transient grating method and by the transient photoluminescence technique. In this report we will mainly discuss the Microwave Transient Reflection (MWTR) technique in the nanosecond time regime applied to thin films of undoped and Ga-doped tin sulphide, SnS.As described in detail in previous publications, the SnS thin films were produced by Chemical Bath Deposition (CBD) from appropriate precursor solutions, and characterized with respect to morphological, structural, and optoelectronic properties by optical transmission, SEM and AFM, XRD, XPS, and RBS. We also add a detailed analysis of the optical properties from Spectral Ellipsometry (SE).The kinetics of photoinduced carriers after high-power laser pulses was monitored for a series of SnS samples after different anneal procedures with temperatures up to 600 °C. Overall power law decays observed in the MWTR signals is discussed in relation to trap state distributions by applying a simple recombination and reemission model. The results are compared to conventional time-resolved photocurrent measurements, TPC, which show similar power law decays.

Trap state-assisted electron injection in blue quantum dot light-emitting diode

We report trap state-assisted electron injection in a blue quantum dot light-emitting diode (QLED) in this work. By replacing an electron transport layer and a quantum dot emission layer, we identify trap states are indeed on blue quantum dots. We also analyze the equivalent circuit model and the density of trap state distribution by impedance spectroscopy. Furthermore, the trap states induce charge transfer in the blue QLED and lower the device efficiency, suggesting the competition between electron injection and trapping in a working device. Our work shows a distinct electron injection mechanism in blue QLEDs that has not been shown in red and green QLEDs.

Potassium Salt Coordination Induced Ion Migration Inhibition and Defect Passivation for High-Efficiency Perovskite Solar Cells.

The disordered distribution of trap states and ion migration limit the commercial application of perovskite solar cells (PSCs). Herein, we apply an oxamic acid potassium salt (OAPS) as a bifunctional additive of perovskite film. The Lewis base group C=O of OAPS can interact with the uncoordinated Pb2+ caused by the I site substitution by Pb and the dangling bonds of the perovskite, which is beneficial to reduce the nonradiative recombination loss. In addition, the countercation K+ of OAPS is confirmed to occupy the perovskite lattice interstitial sites and result in lattice expansion, inhibiting the formation of iodide Frenkel defects and I- ion migration. As a result, the synergistic effect achieves enhanced power conversion efficiency (PCE) from 19.98 to 23.02%, with a fill factor reaching up to 81.90% and suppressed current-voltage hysteresis. The device also presents improved stability, maintaining 93% of the initial PCE after 2000 h of storage.

Hydrogen effects at sputtered Tb-doped AlNxOy:H / c-Si(p) interfaces: A transient surface photovoltage spectroscopy study

In the present work, we studied the interface of terbium doped aluminum oxynitride (Tb-doped AlNxOy:H) deposited under different hydrogen flows with p-type doped crystalline silicon by applying transient surface photovoltage spectroscopy. We observed strong accumulation with concomitant passivation of boron acceptors in the crystalline silicon and defect generation near the interface. With increasing hydrogen flows, the net negative charge in the Tb-doped AlNxOy:H layer decreased, surface photovoltage signals related to defects increased, surface photovoltage transients decayed faster, and the slowest relaxation of charge carriers separated in space changed from trap limited to hopping transport via an exponential distribution of trap states in energy.

Observations of optically and thermally stimulated luminescence from aluminosilicate glasses

Analysis of thermoluminescence (TL) glow curves from aluminosilicate glass reveals that the glow curve in the region ∼400 K–∼600 K can be partly described by thermally excited tunneling from a defect excited state to the recombination center, plus a TL signal caused by thermal excitation directly to the delocalized band from a distribution of trapping states. Mathematical fitting reveals a range of trap depths and frequency factors for the tunneling which depend upon pre-heat temperature in a manner consistent with that expected from a tunneling model. Optical bleaching removes the TL signal from this region of the glow curve while at the same time producing an optically stimulated luminescence (OSL) signal. The OSL decay curve can be described by optically excited tunneling, plus a term consistent with ionization of trapped electrons directly to the delocalized band. Analysis of the dependence of the optical cross-section for tunneling indicates that the excited state resides within the band-tails states for the glass. Fading of the OSL signal is consistent with ground-state tunneling directly to the recombination site, while fading of the TL signal is consistent of ground-state tunneling plus thermal emptying of the shallower parts of the distribution of trapping states.

(Invited) Combining Machine Learning, DFT, EFM, and Modeling to Design Nanodielectric Behavior

Polymer Nanodielectrics are class of materials with intriguing combinations of properties. Predicting and designing the properties, however, is complex due to the number of parameters controlling the properties. This makes it difficult to compare results across groups, validate models, and develop a design methodology. This presentation will share a recent approach to developing a data driven design methodology grounded in physics-based models and experimental calibration. We combine finite element modeling of dielectric constant and loss functions with a Monte Carlo multi-scale simulation of carrier hopping to predict break down strength. Filler dispersion, filler geometry, isotropy and interface properties are explicitly taken into to account to compute objective functions for ideal nanodielectric insulators. Further, we have used machine learning to develop an EFM technique for directly measuring the dielectric constant of the nanoscale interfacial region as critical input to our models. Ultimately, we calculate the Pareto frontier with respect to nanocomposite constitute properties and geometry to optimize properties.The finite element approach can be used to forward predict properties based on the properties of the interfacial region and filler dispersion, or as an inverse tool to calculate interface properties or develop optimized filler morphologies. The breakdown strength model critically depends on the energy distribution of trap states that inhibit space charge motion. We have developed an ab initio approach to determine the trap states at amorphous interfaces, and have used that to do a systematic analyses of trap distributions in composites with functionalized particle interfaces. The models all use 3D particle distributions based on 2D imaging (TEM) of the composites and publicly available tools for binarization and characterization of filler morphology. Finally, we use a design of experiment approach (Latin Hybercube Design) to sample the complete experimental design space, and using calibrated models and morphologies, create a data set spanning the full set of parameters. We use the data from the DOE to train a Gaussian Process metamodel and a genetic algorithm to determine which designs fulfill the design parameters. The talk will present several case studies as examples.The data for this work was accessed from a new data resource, MaterialsMine, using FAIR (Findable, Accessible, Interoperable, and Reusable) principles. MaterialsMine is an open source data repository, and resource. It includes unique visualization tools as well as modeling and characterization resources.