Deep Trap Formation Research Articles

Boost of solar water splitting on SrTiO3 by designing V-ions center for localizing defect charge to suppress deep trap

The deep trap such as Ti3+ in SrTiO3 has a great influence on its photocatalytic performance in overall water splitting, acting as a recombination center of carriers. The applied defect engineering is focused on decreasing the Ti3+ concentration for enhancing solar water splitting mainly by doping low-valence metal ions, while the high-valence metal ions are noneffective. In this work, we doped trace V ions into SrTiO3 accompanied by the Al3+ from the Al crucible, promoting the photocatalytic activity in overall water splitting based on the photocatalysis evaluation. A series of advanced techniques are used to characterize the chemical and physical property changes in SrTiO3 aroused by V ions. Combined with the density functional theory, we found that trace V ions can cooperate with Al3+ to localize the defect charge around it, suppressing the formation of deep trap and increasing the amount of potential delocalized charge at shallow state, responsible for the boost of activity. This work provides another effective method to modulate SrTiO3-based photocatalysts in defect engineering.

Trojans That Flip the Black Phase: Impurity-Driven Stabilization and Spontaneous Strain Suppression in γ-CsPbI3 Perovskite

The technological progress and widespread adoption of all-organic CsPbI3 perovskite devices is hampered by its thermodynamic instability at room temperature. Because of its inherent tolerance toward deep trap formation, there has been no shortage to exploring which dopants can improve the phase stability. While the relative size of the dopant is important, an assessment of the literature suggests that its relative size and impact on crystal volume do not always reveal what will beneficially shift the phase transition temperature. In this perspective, we analyze the changes in crystal symmetry of CsPbI3 perovskite as it transforms from a thermodynamically stable high-temperature cubic (α) structure into its distorted low-temperature tetragonal (β) and unstable orthorhombic (γ) perovskite structures. Quantified assessment of the symmetry-adapted strains which are introduced due to changes in temperature and composition show that the stability of γ-CsPbI3 is best rationalized from the point of view of crystal symmetry. In particular, improved thermal-phase stability is directly traced to the suppression of spontaneous strain formation and increased crystal symmetry at room temperature.

Surface band bending and carrier dynamics in colloidal quantum dot solids.

Band bending in colloidal quantum dot (CQD) solids has become important in driving charge carriers through devices. This is typically a result of band alignments at junctions in the device. Whether band bending is intrinsic to CQD solids, i.e. is band bending present at the surface-vacuum interface, has previously been unanswered. Here we use photoemission surface photovoltage measurements to show that depletion regions are present at the surface of n and p-type CQD solids with various ligand treatments (EDT, MPA, PbI2, MAI/PbI2). Using laser-pump photoemission-probe time-resolved measurements, we show that the timescale of carrier dynamics in the surface of CQD solids can vary over at least 6 orders of magnitude, with the fastest dynamics on the order of microseconds in PbS-MAI/PbI2 solids and on the order of seconds for PbS-MPA and PbS-PbI2. By investigating the surface chemistry of the solids, we find a correlation between the carrier dynamics timescales and the presence of oxygen contaminants, which we suggest are responsible for the slower dynamics due to deep trap formation.

The impact of γ-irradiation on Cd-B2o3 glass doped WO3: New evidences by TL and ESR spectroscopy

A well resolved Thermoluminescence (TL) glow peak at ~168 °C was observed in Cd-borate glass doped WO3. The maximum TL intensity was observed for 1.5 mol% of WO3+ doped samples and thereafter decreases. The variation of gamma exposure (0.025→2 kGy) on TL glow curves of Cd-borate glass showed linear and sub-linear behavior supported by track interaction model (TIM). Here γ-irradiated sample shows the formation of shallow trap (surface trapping) beside the formation of deep trapping. The composition dependence of the g ≈ 1.94 ESR absorption line intensity shows a decrease with increasing WO3 content, which is attributed to a fall in the ratio of the number of unpaired pentavalent ions W5+ ions to the number of W6+ ions. The effect of gamma irradiation doses on TL and ESR studies show high response with dose which indicates that these glass samples may be useful for TL dosimetry.

Investigation of surface strain by digital image correlation and charge trap characteristics of epoxy alumina nanocomposites

Epoxy alumina nanocomposites were fabricated through shear mixing of alumina nanoparticles in to epoxy resin under optimum process conditions for its potential application as insulator in transformer. The fundamental insight in to the influence of ageing of nanocomposite insulator due to its continuous exposure to UV irradiation and water in terms of their charge trap characteristics was explored. The surface potential decay and strain variation of nanocomposite samples were measured by using Electrostatic voltmeter and Digital Image Correlation technique (DIC), respectively. Diffusion of water into epoxy alumina nanocomposite is less compared with pure epoxy resin. It has been observed that surface potential decay rate and trap depth have shown increasing and decreasing trend, respectively, with the addition of alumina nanoparticles in to epoxy resin. The UV and water aged nanocomposites exhibited a decreasing trend of surface potential decay rate and the governing reasons could be due to surface damage causing increase in deep trap formation. The tensile strength and stiffness of the samples have increased with the addition of alumina nanoparticles in to the epoxy resin. It could be due to the better bonding of alumina particles with epoxy resin. The surface strain induced in to the sample during tensile loading and the surface potential decay rate of nanocomposite has shown a direct correlation, as observed in the present work.

Effect of gamma irradiation on space charge and charge trap characteristics of epoxy–MgO nanocomposites

Insulating materials used in power apparatus required to be space charge free. Epoxy–magnesium oxide (MgO) nanocomposites were prepared and gamma rays were irradiated for different dosages to understand the dielectric parameters variation, space charge characteristics, surface potential variations, and for getting charge trap characteristics of the material. It is realised that permittivity increases and then reduces when the irradiation level is increased. Tan (δ) marginally increased with increasing dosage of gamma irradiation. Space charge accumulation reduces with increase in wt% of MgO in epoxy resin. 3 and 5 wt% MgO added samples showed stable space charge accumulation and fast decay during depoling, irrespective of irradiation level. Surface potential decay time reduces with irradiated specimens. After irradiation, epoxy nanocomposites showed both shallow trap formation and deep trap formation and found that shallow trap density is high compared to the deep trap density. All the irradiated specimens showed a decrease in the trap energy level compared to the virgin specimens.

Methylammonium-chloride post-treatment on perovskite surface and its correlation to photovoltaic performance in the aspect of electronic traps

Herein, MACl post-treatment is utilized for MAPbI3 and (Cs0.05FA0.79MA0.16)Pb(I0.84Br0.16)3 (MA, methylammonium; FA, formamidinium). Photoluminescence of both perovskite films exhibits a peak shift to the higher emission energy as well as a slower decay of recombination by the MACl treatment, suggesting the passivation of defects in band tail states. Photovoltaic performance is further correlated with the change of deep electronic traps in the bandgap, which is characterized through capacitance analyses of solar cells. Degradation of MAPbI3 cells by the MACl treatment is attributed to the additional deep-trap formation, and the improvement of photovoltaic parameters for triple-cation perovskites is correlated with the reduction of deep traps with trap-level shifts toward the band edge. The reactive characters of MAPbI3 against post-treatment conditions seem to be responsible for the deep-trap formation as evidenced by the microstructural/morphological results, and consequently, the improved resistance to trap the formation of triple-cation perovskites provides viability for further performance enhancement by surface passivation.

Modulation of the dielectric breakdown strength in polyimide nanocomposites by deep traps tailoring in interphase regions

From a novel empirical power law between the DC breakdown field and DC conductivity, an analysis of the trapping characteristics in polyimide/boron nitride (PI/BN) nanocomposites is presented based on space-charge-limited current (SCLC) measurements. The impact of the total trap density H t enhancement (increasing up to 1.1×1017 cm−3) induced by the BN nanoparticles on the space charge attenuation, the DC conductivity reduction and the DC breakdown field increase is clearly reported. This work proves that an efficient control of the trapping parameters allows huge enhancement of the insulating properties of PI nanocomposites. High temperature thermal breakdown can be delayed by free carrier reduction through deep trap formation caused by nanofiller addition.

Experimental demonstration of deep traps in silica-based polyethylene nanocomposites by combined isothermal surface potential decay and pulsed electro-acoustic measurements

The ability to suppress space charge accumulation at high electric fields makes nanocomposites attract significant research interest as potential insulation materials in high-voltage direct current cable development. At present, the deep trap introduced by nanoparticles is frequently applied to be responsible for the observed space charge suppression in nanocomposites. However, the experimental results that support deep-trap formation have not been rigorously examined. We therefore propose herein a simple and more direct approach based on isothermal surface potential decay combined with pulsed electro-acoustic measurements to verify the presence of deep traps in silica-based blend polyethylene nanocomposites. The results indicate that the deep traps are indeed introduced by filling nanosilica and the space charge suppression observed in the nanocomposite with a low loading ratio is caused by deeply trapped charges in the sub-surface region of specimens.

Kinetic and TL glow curve analysis of UV-, β- and γ-irradiated natural limestone collected from Chunkatta mines

ABSTRACTHerein the manuscript reports kinetics and Thermoluminescence (TL) glow curve analysis of UV-, β- and γ-irradiated natural limestone samples collected from Chunkatta mines of Chhattisgarh basin. The collected samples were annealed at 100°C for 1 h for preheat treatment before irradiation after that the samples were irradiated by UV, β and γ, respectively. After irradiation the 1 mg weigh samples perform for TL glow curve analysis. Collected limestone was characterized by X-ray diffraction analysis technique and crystallite size was calculated using Scherer’s formula. UV-irradiated sample shows the high-temperature peak centered at 250°C, for β-irradiated sample showed well resolved broad TL glow curve at 404 K which is fitted using computerized glow curve deconvolution (CGCD) technique. Similarly, for γ-irradiation the well resolved three distinct peaks at 209, 288 and 388°C showed the formation of deep trapping for γ-irradiation. Dose rate effect for UV- and γ-irradiation reported in details as well as the trap formation were described by trap depth model. Kinetic parameters for UV-, β- and γ-irradiation were calculated using peak shape method by CGCD technique. The study was determined the trap level formation and application related to TL dose response in collected limestone sample.

Contribution of oxygen-vacancy defect-types in enhanced CO2 sensing of nanoparticulate Zn-doped SnO2 films

In spray deposited nanocrystalline SnO2 films, large number of defects involving vacancies and interstitials exist. Doping SnO2 lightly with zinc introduces new Sn2+ states and in fact more oxygen vacancies for charge compensation, which promotes the increase in concentration of chemisorbed oxygen. As the deposition temperature is higher, the XRD and HRTEM results exhibit a clear preferred (110) orientation. The Zn2+ doping in SnO2 introduces new surface and structural oxygen defects, as apparent from Raman and Photoluminescence analysis. The doping alters the polarization of atoms, as seen from the widening in Raman lines. Decreased UV emission peaks (~ 333nm) can be attributed to enhanced crystallization and the incited oxygen vacancy (OV) defects. A dominant blue-green luminescence peak centered at 495nm reveals the formation of deep trap, due to in-plane OVs. The red-shift in PL broad yellow-shoulder at 556nm features an increase of deep oxygen defect energy levels occurring on the surface of the nanocrystal films. On doping, involvement of the defect composition and the contribution of different kinds of OVs play important roles in enhancing CO2 sensing response of 94.4% to 500ppm. In 2.4wt% Zn doping at 310°C, the in-plane OVs are found to dominate over bridging OVs.

TL glow curve analysis and kinetics of UV, β and γ irradiated YBO3:Eu3+ and Y2O3:Eu3+ phosphors

This paper reports on the thermoluminescence (TL) properties of Eu3+ doped in two different host matrix phosphors (Y2O3 and YBO3). The phosphors were prepared by a high temperature solid state reaction method. The method is suitable for a large scale production. The samples were characterized by X-ray diffraction and Rietveld refinement. The different concentration of Eu3+ samples were also examined by TL glow curve analysis for UV, β and γ irradiation. The Y2O3 samples showed well resolved broad peaks that covered the temperature range from 50 to 350 °C and the peak temperature was found at 157 °C for the γ irradiation. For the UV irradiated samples a well resolved peak at 121 °C was found for the Y2O3:Eu3+ doped phosphors. The β irradiated samples showed a resolved peak at 182 °C. A different behaviour was observed for the YBO3:Eu3+ doped phosphors which showed both higher temperature peaks and lower temperature peaks for UV, β and γ irradiation. Here UV irradiated sample showed the formation of shallow trap (surface trapping) and the γ irradiated sample showed the formation of deep trapping. The estimation of the trap formation was evaluated by the calculation of the trapping parameters. The trapping parameters such as activation energy, order of kinetics and frequency factor were calculated by the peak shape method using the computerized glow curve deconvolution technique. Here most of the peak showed second order of kinetics. The effect of γ, β and UV dose exposure on the TL studies was also examined and it showed a linear response with dose which indicated that the samples might be useful for TL dosimetry. The possible mechanics and information about traps formation are presented in detail.

Enhanced lifetime of organic photovoltaic diodes utilizing a ternary blend including an insulating polymer

We report on the lifetime of unencapsulated organic photovoltaic diodes (OPVs) based on a ternary blend of poly(3-hexylthiophene) (P3HT), phenyl-C61-butyric acid methyl ester (PCBM) and a soft insulating polymer, poly(methyl methacrylate) (PMMA) as compared to reference binary P3HT:PCBM OPVs. The performance of ternary devices was shown to decay more slowly than that of their binary counterparts to an extent that depends on the relative humidity (RH). The power conversion efficiency of ternary OPVs when stored in a low humidity environment (1% RH) decayed to 80% of their initial value after 200h, almost double that of the reference binary OPVs. AFM measurements suggest that the PMMA forms pillars within the P3HT:PCBM matrix. It is proposed that the PMMA absorbs water in the active layer, and in doing so, slows the rate of deep trap formation that would otherwise lead to enhanced Shockley-Read-Hall recombination.

Study of formation of deep trapping mechanism by UV, beta and gamma irradiated Eu3+ activated SrY2O4 and Y4Al2O9 phosphors

This paper reports the thermoluminescence properties of Eu3+ doped different host matrix phosphors (SrY2O4 and Y4Al2O9). The phosphor is prepared by high temperature solid state reaction method. The method is suitable for large scale production and fixed concentration of boric acid using as a flux. The prepared samples were characterized by X-ray diffraction technique and the crystallite size calculated by Scherer's formula. The prepared phosphor characterized by Scanning Electron Microscopic (SEM), Fourier Transform Infrared (FTIR), Energy Dispersive X-ray analysis (EDX), thermoluminescence (TL) and Transmission Electron Microscopic (TEM) techniques. The prepared phosphors for different concentration of Eu3+ ions were examined by TL glow curve for UV, beta and gamma irradiation. The UV 254nm source used for UV irradiation, Sr90 source was used for beta irradiation and Co60 source used for gamma irradiation. SrY2O4:Eu3+and Y4Al2O9:Eu3+ phosphors which shows both higher temperature peaks and lower temperature peaks for UV, beta and gamma irradiation. Here UV irradiated sample shows the formation of shallow trap (surface trapping) and the gamma irradiated sample shows the formation of deep trapping. The estimation of trap formation was evaluated by knowledge of trapping parameters. The trapping parameters such as activation energy, order of kinetics and frequency factor were calculated by peak shape method. Here most of the peak shows second order of kinetics. The effect of gamma, beta and UV exposure on TL studies was also examined and it shows linear response with dose which indicate that the samples may be useful for TL dosimetry. Formation of deep trapping mechanism by UV, beta and gamma irradiated Eu3+ activated SrY2O4 and Y4Al2O9 phosphors is discussed in this paper.

UV and gamma ray induced thermoluminescence properties of cubic Gd2O3:Er3+ phosphor

This paper reports the thermoluminescence properties of Er3+ doped gadolinium oxide nanophosphor. The phosphor is prepared by high temperature solid state reaction method. The method is suitable for large scale production. Starting materials used for sample preparation were Gd2O3, Er2O3 (0.5–2.5mol%) and fixed concentration of boric acid using as a flux. The prepared samples were characterized by X-ray diffraction technique and the particle size calculated by Scherer's formula. The surface morphology of prepared phosphor is determined by scanning electron microscopic (SEM) technique. Functional group analysis was done by Fourier transform infra-red spectroscopy (FTIR) analysis. The elemental analysis of prepared sample was determined by energy dispersive X-ray analysis (EDX) and the exact particle size of prepared phosphor for the different concentration of dopant (Er3+) was evaluated by transmission electron microscopy (TEM) technique. The prepared phosphors for different concentration of Er3+ were examined by thermoluminescence (TL) glow curve for UV and gamma irradiation. The UV 254 nm source was used for UV irradiation and Co60 source was used for gamma irradiation. The samples show well resolved broad peak covered the temperature range 50–250 °C and the peak temperature found at 126 °C for UV irradiation and higher temperature peak at 214 °C for gamma irradiation. The effect of heating rate on TL studies was presented for optimized sample. Here UV irradiated sample shows the formation of shallow trap (surface trapping) and the gamma irradiated sample shows the formation of deep trapping. The estimation of trap formation was evaluated by knowledge of trapping parameters. The trapping parameters such as activation energy, order of kinetics and frequency factor were calculated by peak shape method. Here most of the peak shows second order of kinetics. The effect of gamma and UV exposure on TL studies was also examined and it shows linear and sublinear response with dose which indicates that the sample may be useful for TL dosimetry.

De-excitation process in BaMgAl10O17:Eu phosphor evaluated from vacuum–UV-excited photoluminescence spectroscopy and X-ray absorption fine structure

Luminescence properties based on the valence variation of Eu ions in BaMgAl10O17:Eu (BAM) is investigated using the conversion electron yield- and fluorescence yield-X-ray absorption fine structure, and temperature dependence of PL spectroscopy to clarify the de-excitation process in BAM under UV and vacuum–UV (VUV) excitation. The baking temperature dependence of the fraction of Eu2+ ions shows three thresholds at about 600, 1000, and 1200 °C. Even though the fraction of Eu2+ in the 1200-°C-baked sample decreases to almost zero, a blue luminescence from Eu2+ still remains under VUV excitation and its intensity increases with increasing temperature. The internal quantum efficiency (iQE) of a raw sample under VUV excitation is more than 0.84. This value decreases with increasing baking temperature because of the deep trap formation accompanied with the valence change of Eu ions from divalent to trivalent. The iQE of the 1200-°C-baked sample remains 0.10, which suggests a long diffusion length of excitons in the BAM crystal after VUV excitation.

Ambient-Processed Colloidal Quantum Dot Solar Cells via Individual Pre-Encapsulation of Nanoparticles

We report colloidal quantum dot solar cells fabricated under ambient atmosphere with an active area of 2.9 mm(2) that exhibit 3.6% solar power conversion efficiency. The devices are based on PbS tuned via the quantum size effect to have a first excitonic peak at 950 nm. Because the formation of native oxides and sulfates on PbS leads to p-type doping and deep trap formation and because such dopants and traps dramatically influence device performance, prior reports of colloidal quantum dot solar cells have insisted on processing under an inert atmosphere. Here we report a novel ligand strategy in which we first encapsulate the quantum dots in the solution phase with the aid of a strongly bound N-2,4,6-trimethylphenyl-N-methyldithiocarbamate ligand. This allows us to carry out film formation and all subsequent device fabrication under an air atmosphere.