Density Of Surface Trap States Research Articles

Reduced Surface Trap States of PbS Quantum Dots by Acetonitrile Treatment for Efficient SnO2-Based PbS Quantum Dot Solar Cells.

The solution-phase ligand-exchange strategy offers a simple pathway to prepare PbS quantum dots (QDs) and their corresponding solar cells. However, the production of high-quality PbS QDs with reduced surface trap state density for efficient PbS QD solar cells (QDSCs) still faces challenges. As the hydroxyl group (-OH) has been demonstrated to be the primary source of the surface trap states on PbS QDs in the general oleic acid method, here, we present an effective and facile strategy for reducing the surface -OH content of PbS QDs by using acetonitrile (ACN) as precipitant to wash the surface of QDs, which significantly decreases the trap state density and enables the preparation of superior PbS QDs. The resulting solar cell with an ITO/SnO2/n-PbS/p-PbS/Au structure obtained an improved photoelectric conversion efficiency (PCE) from 8.53 to 10.49% with an enhanced air storage stability, realizing a high PCE for SnO2-based PbS QDSCs.

Tailoring Organic/Inorganic Interface Trap States of Metal Oxide/Polyimide toward Improved Vacuum Surface Insulation.

High-voltage and high-power devices are indispensable in spacecraft for outer space explorations, whose operations require aerospace materials with adequate vacuum surface insulation performance. Despite persistent attempts to fabricate such materials, current efforts are restricted to trial-and-error methods and a universal design guideline is missing. The present work proposes to improve the vacuum surface insulation by tailoring the surface trap state density and energy level of the metal oxides with varied bandgaps, using coating on a polyimide (PI) substrate, aiming for a more systematical workflow for the insulation material design. First-principle calculations and trap diagnostics are employed to evaluate the material properties and reveal the interplay between trap states and the flashover threshold, supported by dedicated analyses of the flashover voltage, secondary electron emission (SEE) from insulators, and surface charging behaviors. Experimental results suggest that the coated PI (i.e., CuO@PI, SrO@PI, MgO@PI, and Al2O3@PI) can effectively increase the trap density and alter the trap energy levels. Elevated trap density is demonstrated to always yield lower SEE. In addition, increasing shallow trap density accelerates surface charge dissipation, which is favorable for improving surface insulation. CuO@PI exhibits the most remarkable increase in shallow trap density, and accordingly, the highest flashover voltage is 42.5% higher than that of pristine PI. This study reveals the critical role played by surface trap states in flashover mitigation and offers a novel strategy to optimize the surface insulation of materials.

Polymer‐Wrapped Perovskite Single Crystal for Surface Passivation and Robust Stability

AbstractMetal halide perovskites (MHPs) emerging as a promising semiconducting material have shown superiority in optoelectronic applications. Currently, polycrystalline perovskite thin films are dominantly employed; while owing to the existence of grain boundary, the polycrystalline films typically have charge transport issues. To overcome these limitations, a perovskite single crystal (SC) is synthesized and applied owing to the more superior bulk charge transport properties compared with its polycrystalline counterpart. Nevertheless, due to the defective nature of the crystal surface, the surfaces of single crystals remain the bottleneck that not only induces stability issues but also causes interfacial charge carrier recombination. In this work, triple cation perovskite single crystals, FA0.8MA0.15Cs0.05PbI3 (denoted as FAMACs) are synthesized, and a facile surface modification strategy is reported for single crystals by polymer monolayer coupling, which shows high effectiveness for improving stability and reducing surface trap state density for single crystals. The polymer‐wrapped single crystals exhibit robust stability and a well‐passivated surface, which shows an order of magnitude lower trap density compared with those of unmodified single crystals and achieves a high responsivity of 5.069 A W−1 when used in photodetector applications.

Mechanism of surface passivation of methylammonium lead tribromide single crystals by benzylamine

Hybrid organic-inorganic perovskites are semiconductors that have great potential for optoelectronic applications such as light-emitting diodes, photodetectors, and solar cells. In such devices, the surface plays a crucial role in the performance and stability, as it strongly influences the recombination rate of excited charge carriers. It is reported that molecular ligands such as benzylamine are capable of reducing the surface trap state density in thin films. In this work, we aim to clarify the mechanisms that govern the surface passivation of hybrid perovskites by benzylamine. We developed a versatile approach to investigate the influence of benzylamine passivation on the well-defined surface of freshly cleaved hybrid perovskite crystals. We show that benzylamine permanently passivates surface trap states in these single crystals, resulting in enhanced photoluminescence and charge carrier lifetimes. Additionally, we show that exposure to benzylamine leads to the replacement of the methylammonium cations by benzylammonium, thereby creating a thermodynamically more stable two-dimensional (2D) perovskite (BA)2PbBr4 on the surface of the three-dimensional crystal. This conversion to a 2D perovskite drives an anisotropic etching of the crystal surface, with the {100} planes being most prone to etching. Initially, square etching pits appear spread over the surface. As time elapses, these etching pits broaden and merge to yield large flat terraces that are oriented normally to the cleaving plane when they form. A thorough understanding of the mechanisms governing the surface passivation is crucially important to optimize and design novel passivation schemes, with the ultimate goal of further advancing the efficiency of optoelectronic devices based on hybrid perovskites.

Energetics of Nonradiative Surface Trap States in Nanoparticles Monitored by Time-of-Flight Photoconduction Measurements on Nanoparticle-Polymer Blends.

Nanoparticles (NPs) have had increasingly successful applications including in emissive or photovoltaic devices; however, trap states associated with the surface of NPs often drastically reduce the efficiency of devices and are difficult to detect spectroscopically. We show the applicability of photoconduction as the means of detecting and quantifying trap states in NPs. We performed time-of-flight (ToF) photoconduction measurements, using semiconducting poly[bis(4-phenyl)(4-butyphenyl)amine] (P-TPD) doped with either core/shell CdSeS/CdS quantum dots (QDs) or perovskite CsPbBr3 NPs, both of which are carefully designed to be energetically matched. In the case of the QDs, a drop in the hole mobility from ∼10-3 to ∼10-4 cm2 V-1 s-1 was observed when compared to a control sample, suggesting the presence of a hole trapping. These trap states were found to be around -5.0 to -4.9 eV from the vacuum level. The presence of the trap states was further supported by a coincident reduction in the photoluminescence (PL), quantum yield (QY), and lifetime of the core/shell QDs after purification. Using the measured reductions in the PL, QY, and lifetime, the surface trap state density was estimated to increase by between 20 and 40%, most likely due to a ligand detachment. In the case of the perovskite NP-doped samples, a mobility of ∼10-3 cm2 V-1 s-1 was measured. Thus, doping with perovskite NPs did not generate any obvious hole trapping from the P-TPD matrix. The absence of a trapping may be related to the reduced surface-to-volume ratio and NP number density of the perovskite NPs compared to the core/shell QDs, since the perovskite NPs are approximately 10 times larger in radius than that of the core/shell QDs. Our results suggest that to minimize the presence of trap states with a view to improving device performance, large-size perovskite NPs appear to be better than small-size QDs.

Charge/energy transfer dynamics in CuO quantum dots attached to photoresponsive azobenzene ligand

A series of stable CuO quantum dots (QDs) in aqueous alkaline medium are prepared and protected against rapid oxidation by capping with mercaptoacetic acid (MAA). The MAA not only regulates the size of the CuO QDs but it also acts as pendent group for further functionalization of QDs. A specific concentration of MAA improves the crystallinity and increases the fluorescence lifetime by decreasing the density of surface trap states. The MAA capped CuO QDs are attached to (E)-5-((4-chlorophenyl) diazenyl)-2-hydroxybenzaldehyde (CPDHB) and upon UV irradiation at 300 nm the CPDHB experiences a reversible trans- to cis-isomerization. The absorption band of the cis-isomer of CPDHB overlaps with the fluorescing band of the CuO QDs, as a result the fluorescence of the QDs is strongly quenched. The photoluminescenc (PL) quenching is attributed due to the photoinjected electron transfer from the conduction band (CB) of the CuO QDs to the lowest unoccupied molecular orbital (LUMO) of the cis-isomer of CPDHB. Förster resonance energy transfer (FRET) is also observed but its contribution is minor. The fluorescence quenching of CuO QDs in QDs - CPDHB assembly upon ultra-violet excitation and reversible trans-cis interconversion of CPDHB may potentially suggests to design fluorescent probes for bioimaging and to develop data and energy storage devices.

Cryogenically probing the surface trap states of single nanowires passivated with self-assembled molecular monolayers.

Optoelectronically probing the trap state density of single nanoscale devices is a powerful in situ nondestructive technique that is of significance for developing high gain photoconductors by surface engineering. However, the previously demonstrated optoelectronic methods are based on the exponential transient photoresponse assumption and only trap states in a very narrow bandgap region can be probed. In this Letter, we demonstrate a cryogenic technique that is capable of measuring the density of surface trap states in the full half bandgap without the exponential transient photoresponse assumption. The technique is applied to an array of silicon nanowire photoconductors that are fabricated on silicon-on-insulator (SOI) wafer by the top-down approach. Diethyl 1-propylphosphonate (DPP) and hexadecane molecular monolayers are self-assembled on silicon nanowire surfaces as the passivation layer in comparison with dry oxide passivation. The surface trap state density of the dry oxide passivated nanowires exponentially increases from the bandgap center, reaching a peak of ∼5 × 1013 cm-2 eV-1 at 50 meV below the conduction band. The defect state density is significantly suppressed after DPP and hexadecane molecules are grafted onto the nanowire surfaces via covalent bonds. The experimental observations are consistent with the density functional theory calculations.

Atomically thin cesium lead bromide perovskite quantum wires with high luminescence.

We report a room-temperature colloidal synthesis of few-unit-cell-thick CsPbBr3 QWs with lengths over a hundred nanometers. The surfactant-directed oriented attachment growth mechanism was proposed to explain the formation of such CsPbBr3 QWs. Owing to the strong quantum confinement effect, the photoluminescence (PL) emission peak of few-unit-cell-thick CsPbBr3 QWs blue-shifted to 430 nm. The ensemble PL quantum yield (PLQY) of the few-unit-cell-thick CsPbBr3 QWs increased to 21.13% through a simple heat-treatment process. The improvement of PLQY was ascribed to the reduction of the density of surface trap states and defect states induced by the heat-treatment process. Notably, the dependence of the bandgap on the diameter with different numbers of unit cells was presented for the first time in 1-D CsPbBr3 QWs on the basis of the produced few-unit-cell-thick CsPbBr3 QWs.

Trap-Induced Dispersive Transport and Dielectric Loss in PbS Nanoparticle Films

Abstract In this work, we investigate the electrical and dielectric response of lead sulfide (PbS) nanoparticle (NP) films with impedance spectroscopy. In particular, the influence of the ligand passivation on the surface trap state density of PbS NPs is demonstrated by comparing two different types of ligands: ethane-1,2-dithiol (EDT) and 3-sulfanylpropanoic acid (MPA). We observe that the MPA treatment passivates the PbS surface more efficiently than EDT. By analyzing the dielectric loss spectra, we are able to visualize shallow trap states in the bulk of PbS-EDT films and correlate this with the dispersive response observed in the impedance spectra. Evidence of deep trap states is revealed for both PbS-EDT and PbS-MPA diodes. Under illumination, the PbS-MPA and PbS-EDT films demonstrate almost identical trap profiles, showing solely the deep trap state densities. We conclude that the deep traps are related to the stoichiometry of the PbS NPs.

Uncovering the density of nanowire surface trap states hidden in the transient photoconductance.

The gain of nanoscale photoconductors is closely correlated with surface trap states. Mapping out the density of surface trap states in the semiconductor bandgap is crucial for engineering the performance of nanoscale photoconductors. Traditional capacitive techniques for the measurement of surface trap states are not readily applicable to nanoscale devices. Here, we demonstrate a simple technique to extract the information on the density of surface trap states hidden in the transient photoconductance that is widely observed. With this method, we found that the density of surface trap states of a single silicon nanowire is ∼10(12) cm(-2) eV(-1) around the middle of the upper half bandgap.

Influence of indoline dye and coadsorbate molecules on photovoltaic performance and recombination in dye-sensitized solar cells based on electrodeposited ZnO

A detailed photoelectrochemical analysis of dye-sensitized solar cells based on electrodeposited porous ZnO, the indoline dye D149, cholic acid as optional coadsorbate, and a liquid iodide/triiodide electrolyte was performed. The effects of changes in the amount and chemical environment of the dye molecules on overall cell performance, distribution of trap states and recombination were studied. Short-circuit photocurrent, open-circuit photovoltage and cell efficiency improved with increasing amount of dye within the range investigated, while fill factors were lowered. The decline of the fill factor could be attenuated by coadsorbing cholic acid. Electrochemical impedance spectroscopy (EIS) was used to study the chemical capacitance, recombination resistance and series resistance in cells with different D149 loadings in the presence or absence of cholic acid. For increasing dye loading, the recombination parameter β and, in consequence, the recombination resistance around the maximum power point decreased, which explained the drop in the fill factors. In cells containing cholic acid, the conduction band edge was shifted upwards, the density of surface trap states was reduced, and the observed decrease of β was less pronounced compared to cells without coadsorbate. The charge transport in the electrolyte and the kinetics of charge transfer at the counter electrode were slowed down in cells with high dye content. A diode model accounting for non-linear recombination served to quantitatively link dye- and coadsorbate-induced changes in the voltage-dependence of the recombination resistance to the fill factor variation. Recombination with oxidized D149 molecules in aggregates and weak interaction between D149 and ZnO are discussed as physical origins of the observed trends.

Charge Transport through Electrospun SnO2 Nanoflowers and Nanofibers: Role of Surface Trap Density on Electron Transport Dynamics

A larger amount of tin precursor was dispersed in electrospun polyvinyl acetate fibers than that required for SnO2 fiber formation upon annealing, thereby creating a constraint such that all nuclei formed during annealing could not be accommodated within the fiber, which leads to enhanced reaction kinetics and formation of highly crystalline−cum−higher surface area SnO2 flowers. The flowers are shown to have a lower density of surface trap states than fibers by combining absorption spectra and open circuit voltage decay (OCVD) measurements. Charge transport through the SnO2 flowers in the presence of the iodide/triiodide electrolyte was studied by OCVD, electrochemical impedance spectroscopy, and transient photodecay techniques. The study shows that the flowers are characterized by higher chemical capacitance, higher recombination resistance, and lower transport resistance compared with fibers. Photocurrent transients were used to extract the effective electron diffusion coefficient and mobility which were an order of magnitude higher for the flowers than that for the fibers. The flowers are also shown to have an enhanced Fermi energy, on account of which as well as higher electron mobility, dye-sensitized solar cells fabricated using the SnO2 flowers gave VOC ∼700 mV and one of the highest photoelectric conversion efficiencies achieved using pure SnO2.

On‐Nanowire Band‐Graded Si:Ge Photodetectors

An on-nanowire (on-NW) band-graded photodetector that pertains to the on-nanowire composition gradation from pure Si to pure Ge, Si1–xGex (0 ≤ x ≤ 1), is reported. The spectral onset of interband photocarrier generation and photocurrent amplitude are on-NW de-multiflexed over the continuously varying energy band gap and surface trap state density in an individually addressable manner.

Investigation of Trap States in AlInN/AlN/GaN Heterostructures by Frequency-Dependent Admittance Analysis

We present a systematic study on the admittance characterization of surface trap states in unpassivated and SiN x -passivated Al0.83In0.17N/AlN/GaN heterostructures. C–V and G/ω–V measurements were carried out in the frequency range of 1 kHz to 1 MHz, and an equivalent circuit model was used to analyze the experimental data. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming models in which traps are located at the metal–AlInN surface. The density (D t) and time constant (τ t) of the surface trap states have been determined as a function of energy separation from the conduction-band edge (E c − E t). The D st and τ st values of the surface trap states for the unpassivated samples were found to be $$ D_{\rm{st}} \cong (4 - 13) \times 10^{12} $$ $$ {\hbox{eV}}^{ - 1} {\hbox{cm}}^{ - 2} $$ and τ st ≈ 3 μs to 7 μs, respectively. For the passivated sample, D st decreased to $$ 1.5 \times 10^{12} $$ $$ {\hbox{eV}}^{ - 1} {\hbox{cm}}^{ - 2} $$ and τ st to 1.8 μs to 2 μs. The density of surface trap states in Al0.83In0.17N/AlN/GaN heterostructures decreased by approximately one order of magnitude with SiN x passivation, indicating that the SiN x insulator layer between the metal contact and the surface of the Al0.83In0.17N layer can passivate surface states.

The Role of Band Bending in Affecting the Surface Recombination Velocities for Si(111) in Contact with Aqueous Acidic Electrolytes

The role of band bending in affecting surface recombination velocity measurements has been evaluated by combining barrier height data with charge-carrier lifetime measurements for Si(111) surfaces in contact with a variety of acidic aqueous electrolytes. Charge-carrier lifetimes and thus surface recombination velocities have been measured by contactless radio frequency photoconductivity decay techniques for long bulk lifetime n-Si(111) samples in contact with 11 M (40% by weight) NH4F(aq), buffered (pH = 5) HF(aq), 27 M (48% by weight) HF(aq), or concentrated 18 M H2SO4. Regardless of the sample history or surface condition, long charge-carrier lifetimes were observed for n-Si(111) surfaces in contact with 11 M NH4F(aq) or buffered HF(aq). On the basis of previous barrier height measurements, this behavior is consistent with the formation of an electrolyte-induced surface accumulation layer that reduces the rate of steady-state surface recombination even in the presence of a significant density of surface trap sites. A straightforward evaluation of the surface trap state density from the measured surface recombination velocities, S, is thus precluded for such Si/liquid contacts. In contrast, a wide range of S values, depending on the history of the sample and the state of the surface, were observed for n-Si(111) surfaces in contact with 27 M HF(aq). These results in conjunction with previously measured barrier height data indicate that the charge-carrier lifetimes measured for n-Si(111) in contact with 27 M HF(aq) can be directly correlated with the surface condition and the effective surface-state trap density. These conclusions were confirmed by measurements of the apparent S values of n-Si(111) surfaces in contact with various solutions in the presence of the known deep trap, Cu. For Si(111)/HF(aq) contacts, very high (≥920 ± 270 cm s-1) surface recombination velocities were observed when 0.16 mM (10 ppm) Cu2+ was in the solution and/or adsorbed onto the Si(111) surface as Cu0 deposits, whereas low (100 ± 75 or 225 ± 20 cm s-1) apparent surface recombination velocities were measured for Cu-contaminated Si(111) samples in contact with 0.16 mM (10 ppm) Cu2+-containing 11 M NH4F(aq) or BHF(aq) solutions, respectively.

Passivation and Secondary Functionalization of Allyl-Terminated Si(111) Surfaces

Synthesis of passivated silicon surfaces with tunable properties requires formation of a monolayer that includes a synthetically useful functional group, such as an alkene. Thus, Si(111) surfaces have been chemically and electrically passivated by attachment of an allyl monolayer. The structure of the monolayer was confirmed using infrared spectroscopy. The allyl-functionalized surface exhibited resistance to oxidation and had a low density of surface trap states. Metal-catalyzed reactions, in particular, Heck coupling and ruthenium-catalyzed olefin cross-metathesis, allowed attachment of small molecules despite the steric constraints of the dense surface-bound layer. Allyl-terminated silicon surfaces thus offer a means of attaching a variety of chemical moieties to a silicon surface through a short linking group, enabling applications in energy conversion, catalysis, and sensing.

Effect of surface treatments on the electrical and optical properties of CdZnTe single crystal

The effects of surface treatments on the electrical and optical properties of CdZnTe (CZT) crystals were studied experimentally. The IR transmittance of CZT after cutting is only 15%, after chemically polishing with Br-MeOH it increased to 63%. The reflection of surface roughness and absorption of surface-damaged layer should be responsible for the variation. After aging and passivation,CZT possesses low IR transmittance. After fine polishing, the surface oxide layer was removed and the IR transmittance increased. CZT wafers with high roughness possess low surface sheet resistance, and the Te enrichment introduced by etching with Br-MeOH results in high surface leakage current. The effects of etching and passivation on surface trap state have been analyzed with photoluminescence spectra. Etching can deprive effectively the surface damage layer, and passivation can minimize the surface trap state density.

Surface passivation and electrical properties of p-CdZnTe crystal

The electrical properties of Au/p-CdZnTe contacts with different surface treatments, especially passivation treatment, are investigated in this paper. After the passivation, a TeO2 oxide layer with a thickness of 3.1 nm on the CdZnTe surface was identified by XPS analysis. Meanwhile, photoluminescence (PL) spectra confirmed that the passivation treatment minimized the surface trap state density and decreased the deep-level defects related to recombination of Cd vacancies. Current–voltage and capacitance–voltage characteristics were measured. It was shown that the passivation treatment could increase the barrier height of the Au/p-CdZnTe contact and decrease the leakage current.

Multigram scale synthesis and characterization of monodisperse tetragonal zirconia nanocrystals.

A new and simple method has been developed to synthesize large quantities of highly monodisperse tetragonal zirconia nanocrystals. In this synthesis, a nonhydrolytic sol-gel reaction between zirconium(IV) isopropoxide and zirconium(IV) chloride at 340 degrees C generated 4 nm sized zirconia nanoparticles. A high-resolution transmission electron microscopic (HRTEM) image showed that the particles have a uniform particle size distribution and that they are highly crystalline. These monodisperse nanoparticles were synthesized without any size selection process. X-ray diffraction studies combined with Rietveld refinement revealed that the ZrO(2) nanocrystals are the high-temperature tetragonal phase, and very close to a cubic phase. When zirconium(IV) bromide is used as a precursor instead of zirconium chloride, zirconia nanoparticles with an average size of 2.9 nm were obtained. The UV-visible absorption spectrum of 4 nm sized zirconia nanoparticles exhibited a strong absorption starting at around 270 nm. A fluorescence spectrum with excitation at 300 nm showed a broad fluorescence band centered around 370 nm. FTIR spectra showed indication of TOPO binding on the ZrO(2) nanoparticle surface. These optical studies also suggest that the nanoparticles are of high quality in terms of narrow particle size distribution and relatively low density of surface trap states.

Synthesis, Optical Spectroscopy and Ultrafast Electron Dynamics of PbS Nanoparticles with Different Surface Capping

Lead sulfide, PbS, nanoparticles have been synthesized using a number of surface capping agents including poly(vinyl-alcohol) (PVA), poly(vinyl-pyrrolidone) PVP, gelatin, DNA, polystyrene (PS), and poly(methyl-methacrylate) (PMMA). The electronic absorption spectra and particle shapes have been found to depend on the capping molecules used. An excitonic feature at 580 nm was observed for capping with PVA and DNA, while no such excitonic feature was observed for PVP, PS or PMMA. A weak excitonic feature was observed for gelatin. The particle shape varied from cubic, needle to spherical as controlled by the capping agents. For the DNA-capped PbS nanocrystals, HRTEM demonstrated the presence of oval crystals with a diameter of 3−8 nm. Powder X-ray diffraction of the PbS-DNA nanocrystals showed the characteristic peaks for PbS at 2.97, 3.43, and 2.10 Å. The XRD suggested the size of the nanoparticles to be approximately 4 nm. The dynamics of photoinduced electrons in PbS nanoparticles have been determined using femtosecond laser spectroscopy. For all the samples studied the electronic relaxation has been found to be very similar and follow a double exponential decay with time constants of 1.2 and 45 ps. The fast decay can be attributed to trapping from the conduction band to shallow traps or from shallow traps to deep traps while the slower decay is most likely due to electron−hole recombination mediated by a high density of surface trap states that lie within the band gap. The decay profiles are independent of particle size, shape, surface capping, probe wavelength, and excitation intensity. The results seem to indicate a high density of surface states, consistent with no detectable fluorescence signal at room temperature.