Surface Trap States Research Articles

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 %.

In-situ synthesis of Fe phosphate layer on the porous hematite nanocube for efficient photoelectrochemical water splitting

Hematite (α-Fe2O3) is considered as an ideal photoanode material in photoelectrochemical (PEC) water splitting system due to its suitable band gap, excellent stability and abundant reserves. However, the actual PEC water oxidation performance of α-Fe2O3 is greatly limited by its severe charge recombination and sluggish water oxidation kinetics. Here, an in-situ generated Fe phosphate (Fe-Pi) coating strategy is applied on the surface of porous hematite nanocube (Fe2O3 NC) to realize PEC water oxidation effectively. The obtained Fe-Pi/Fe2O3 NC photoanode exhibits a remarkable PEC property with a photocurrent density of 2.7mAcm-2 at 1.23V vs. RHE, which was about 5.7-fold enlarged compared to the pristine nanorod-like hematite (Fe2O3 NR) photoanode. Detailed studies have shown that the porous Fe2O3 NC possesses an enhanced ability in light absorption and charge separation than the traditional Fe2O3 NR photoanode. In addition, the in-situ loading of Fe-Pi layer as a co-catalyst can not only effectively passivate the surface trapping states of Fe2O3 NC and suppress interfacial charge recombination, but also quickly extract the holes to participate in the water oxidation reaction. Our study suggests that the further surface modification based on morphological engineering is an effective strategy to improve the PEC performance of hematite.

Ions-induced Assembly of Perovskite Nanocomposites for Highly Efficient Light-Emitting Diodes with EQE Exceeding 30.

Metal halide perovskites, a cost-effective class of semiconductos, hold great promise for display technologies that demand high-efficiency, color-pure light-emitting diodes (LEDs). Early research on three-dimensional (3D) perovskites showed low radiative efficiencies due to modest exciton binding energies. To inprove luminescence, reducing dimensionality or grain size has been a common approach. However, dividing the perovskite lattice into smaller units may hinder carrier transport, compromising electrical performance. Moreover, the increased surface area introduce additional surface trap states, leading to greater non-radiative recombination. Here, an ions-induced growth method is employed to assembe lattice-anchored perovskite nanocomposites for efficient LEDs with high color purity. This approach enables the nanocomposite thin films, composed of 3D CsPbBr3 and its variant of zero-dimensional (0D) Cs4PbBr6, to feature significant low trap-assisted nonradiative recombination, enhanced light out-coupling with a corrugated surface, and well-balanced charge carrier transport. Based on the resultant 3D/0D perovskite nanocomposites, the perovskite LEDs (PeLEDs) achieving an remarkable external quantum efficiency of 31.0% at the emission peak of 521 nm with a narrow full width at half-maximum of only 18 nm. This sets a new benchmark for color purity in high performance PeLED research, highlighting the significant advantage of this approach.

Enhancing broadband absorption and photocurrent generation in carbon dots via P3HT integration

The growing interest in carbon dots (CDs) arises from their diverse applications and unique properties. This study addresses challenges in CDs for photodetector (PD) applications, specifically surface defects and trap states hindering efficient charge transport. CDs/P3HT composites were prepared to overcome these issues by incorporating CDs in a poly(3-hexylthiophene) (P3HT) matrix. Broad absorption in spectroscopic characterization revealed its utility in fabricating a broadband PD. The CDs/P3HT PD displays a remarkable broadband photoresponse, spanning both UV and visible regions. The CDs and P3HT are effectively combined via non-covalent π-π interactions constituted by their conjugated systems. The π-π interaction increases electron delocalization and facilitates efficient charge transfer due to band alignment at the junction interface. Hence, fabricated CDs/P3HT PD demonstrated enhanced photocurrent compared to pure CDs, exhibiting high responsivity of 6.12 × 10−3 AW−1 and detectivity of 0.69 × 109 Jones. This study highlights the potential of CD/P3HT composites for broadband photodetector applications with enhanced photoelectric conversion.

Amino‐Arsine and Amino‐Phosphine Based Synthesis of InAs@InP@ZnSe core@shell@shell Quantum Dots

AbstractA colloidal synthesis protocol is demonstrated for InAs@InP core@shell quantum dots (QDs) with a tunable InP shell thickness (ranging from 3 to 8 monolayers), utilizing tris(diethylamino)‐arsine and ‐phosphine. Structural analysis reveals that the InP shell preferentially grows onto the tetrahedral InAs cores along the <‐1‐1‐1> directions, forming tetrapodal‐shaped InAs@InP QDs. Growth of the InP shell causes a red shift in the absorption spectrum of the QDs. This is explained by considering that electrons are delocalized throughout the whole core@shell QDs, while holes preferentially leak along the <‐1‐1‐1> directions, as indicated by the density functional theory calculations. This means such heterostructures cannot be described as type‐I or quasi type‐II, contrary to earlier assumptions. The overlap of carrier wavefunctions throughout the entire InAs@InP QD structure results in no significant reduction of the Auger recombination rate, which remains as fast as in InAs QDs. However, the InP shell enhances photoluminescence (PL) efficiency (up to ≈13%) by passivating surface trap states of the InAs QDs (mainly located close to the top of the valence band). The overgrowth of a ZnSe shell endows the QDs with a high PL efficiency (≈55%) and good stability upon air exposure (≈80% PL intensity retention after 14 days).

Green light all the way: Triple modification synergistic modification effect to enhance the photoelectrochemical water oxidation performance of BiVO4 photoanode

The recombination of photogenerated electron-hole pairs of the photoanode seriously impairs the application of bismuth vanadate (BiVO4) in photoelectrochemical water splitting. To address this issue, we prepared a Yb:BiVO4/Co3O4/FeOOH composite photoanode by employing drop-casting and soaking methods to attach Co3O4/FeOOH cocatalysts to the surface of ytterbium-doped BiVO4. The prepared Yb:BiVO4/Co3O4/FeOOH photoanode demonstrates a high photocurrent density of 4.89 mA cm−2 at 1.23 V versus the reversible hydrogen electrode (RHE), which is 5.1 times that of bare BiVO4 (0.95 mA cm−2). Detailed characterization and testing demonstrated that Yb doping narrows the band gap and significantly enhances the carrier density. Furthermore, Co3O4 serves as a hole transfer layer to expedite hole migration and diminish recombination, while FeOOH offers additional active sites and minimizes surface trap states, thus boosting stability. The synergistic effects of Yb doping and Co3O4/FeOOH cocatalyst significantly improved the reaction kinetics and overall performance of PEC water oxidation. This work provides a strategy for designing efficient photoanodes for PEC water oxidation.

Unveiling the critical role of crystallinity in photogenerated hole transfer behavior of TiO2 with an aminoxyl radical mediator

Integrating aminoxyl radical mediators (ARMs) with titanium dioxide (TiO2) based photoelectrochemical (PEC) processes presents a promising approach for achieving mild and sustainable chemical transformations. However, a comprehensive understanding of the interaction between ARMs and PEC processes remains elusive. This study investigates the PEC behavior of photoelectrodes fabricated from single- and polycrystalline TiO2 in the presence of a representative ARM, 4-acetamido-2,2,6,6-tetramethylpiperidine 1-oxyl radical, in acetonitrile. Interestingly, we observe contrasting PEC behavior: photocurrent enhancement for single-crystalline TiO2 and photocurrent suppression for polycrystalline TiO2. We attribute this difference to subtle variations in surface properties arising from the material’s crystallinity. Photoelectrochemical impedance spectroscopy (PEIS) is employed to gain deeper insights into the influence of surface defects on charge transfer and recombination mechanisms. The PEIS results suggest that an anomalous hole transfer mechanism mediated by surface trap states governs the PEC performance of polycrystalline TiO2.

Dual Functionality of Surface States in Dictating Photocurrent Generation of Au Nanocluster-Sensitized TiO2 Electrodes.

Gold nanoclusters (Au NCs), composed of only a few atoms, exhibit molecule-like behavior due to their distinct electronic structures arising from quantum confinement effects. Unlike their plasmonic nanoparticle counterparts, these nonplasmonic Au NCs possess unique properties with significant potential for photosensitizer applications. While traditional and NC-based electrodes share architectural similarities, the photoelectrochemical (PEC) behavior of the latter diverges significantly. Sensitizing TiO2 with Au NCs introduces additional surface trap states. In contrast to conventional photosensitizers, where surface states typically have a negligible impact on hole transfer, these trap states actively mediate the charge transfer process in Au NC-sensitized TiO2 electrodes. In this study, we employed impedance spectroscopy to elucidate the role of surface trap states in photocurrent generation. Our investigation revealed that these states are critical in determining PEC performance, presenting a dichotomy: they facilitate charge transfer (enhancing PEC performance) while simultaneously promoting carrier recombination (limiting efficiency). We demonstrated that the judicious control of otherwise deleterious surface trap states can significantly boost photocurrent. Our findings highlight that the dual nature of surface trap states demands a comprehensive investigation to fully understand their intricate impact on PEC performance.

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.

High‐Efficiency Single‐Photon Upconversion Photoluminescence of Perovskite Quantum Dots via Intragap State Regulation

AbstractLead halide perovskite quantum dots (PQDs) have shown remarkable single‐photon upconversion photoluminescence (SPUC‐PL) bringing them a brilliant perspective as the active media for biological imaging, optiag, and upconversion photovoltaics. However, the mechanism of SPUC in PQDs is still in dispute, and the specific strategy for optimizing PQDs toward a higher SPUC performance has yet to be established, which impedes the application of PQDs‐based SPUC‐PL in practice. In this work it is revealed that the PL quantum yield is declined by surface trap states, while the SPUC efficiency is promoted by Urbach tail states. These two types of intragap states can be chemically manipulated by engineering the surface ligand and the growth kinetics of perovskite nanocrystals, bringing on the improvement of light emission efficiency and the promotion of subgap low‐energy photon absorption, respectively. As a proof of concept, utilizing the synergistic effect of the proposed chemical manipulation method, the SPUC‐PL efficiency of PQDs is boosted by more than 40%, which gives rise to a spectral window of optical refrigeration gain as broad as ≈130 meV.

Unveiling heterogeneity of hysteresis in perovskite thin films

The phenomenon of current–voltage hysteresis observed in perovskite-based optoelectronic devices is a critical issue that complicates the accurate assessment of device parameters, thereby impacting performance and applicability. Despite extensive research efforts aimed at deciphering the origins of hysteresis, its underlying causes remain a subject of considerable debate. By employing nanoscale investigations to elucidate the relationship between hysteresis and morphological characteristics, this study offers a detailed exploration of photocurrent–voltage hysteresis at the nanoscale within perovskite optoelectronic devices. Through the meticulous analysis of localized I–V curve arrays, our research identifies two principal hysteresis descriptors, uncovering a predominantly inverted hysteresis pattern in 87% of the locations examined. This pattern is primarily attributed to the energetic barrier encountered at the interface between the probe and the perovskite material. Our findings underscore the pronounced heterogeneity and grain-dependent variability inherent in hysteresis behavior, evidenced by an average Hysteresis Index value of 0.24. The investigation suggests that the localized hysteresis phenomena cannot be exclusively attributed to either photocharge collection processes or organic cation migration at grain boundaries. Instead, it appears significantly influenced by localized surface trap states, which play a pivotal role in modulating electron and hole current dynamics. By identifying the key factors contributing to hysteresis, such as localized surface trap states and their influence on electron and hole current dynamics, our findings pave the way for targeted strategies to mitigate these effects. This includes the development of novel materials and device architectures designed to minimize energy barriers and enhance charge carrier mobility, thereby improving device performance and longevity. This breakthrough in understanding the microscale mechanisms of hysteresis underscores the critical importance of surface/interface defect trap passivation in mitigating hysteretic effects, offering new pathways for enhancing the performance of perovskite solar cells.

Achieving Low Cost and High Performance Flexible CsPbIBr2 Perovskite Photodetectors Arrays with Imaging System via Dual Interfacial Optimization and Structural Design

AbstractAll‐inorganic metal halide perovskites are competitive candidate as next‐generation flexible photodetectors (PDs) materials due to the remarkable photoelectric properties, enhanced heat stability, and mechanical flexibility. However, constructing low‐cost flexible perovskites PDs with high photodetection performance, good operation stability and mechanical robustness, and manufacturing corresponding flexible PDs array have been a challenge. Herein, the PbCl2 and Zinc(II) Dibenzyldithiocarbamate (Zn(DBDTC)2) are employed to modify bottom and top interface of CsPbIBr2 perovskite layer, respectively, to passivate surface trap state, enhance hydrophobicity and resist phase segregation, and improve interface contact. Consequently, the self‐powered champion PDs with low‐cost Bi2Te3 electrode reach a responsivity of 0.38 A W−1 and a maximum detectivity of 3.44 × 1013 Jones at 570 nm, and maintain the outstanding long‐term and operational stability without additional encapsulation. Meanwhile, the HTL‐free flexible PDs based on low‐temperature CsPbIBr2 perovskite layer exhibit impressive photodetection performance and mechanical robustness in bending test. More importantly, the flexible PDs array (5 × 5 pixels) and corresponding imaging system are fabricated with uniform and clear imaging results under flatting and bending state. This research is of great significance for the application of stable and high‐performance inorganic perovskites PDs and flexible array imaging systems.

Uncovering Energetic Positions of Surface Trap States in α-Fe2O3 Treated with Cobalt Phosphate (Co-Pi) Using Charge Carrier-Selective Heterodyne Transient Grating Technique.

Near-field heterodyne transient grating (NF-HD-TG) responses of hematite (α-Fe2O3) treated with cobalt phosphate (Co-Pi) were measured with the burn lasers inducing the depletion of the response by the removal of the trapped charge carriers in the target state, which is called charge carrier-selective heterodyne transient grating (CS-HD-TG) spectroscopic technique. We found that two distinct trap states co-existed in Co-Pi loaded on the surface of α-Fe2O3. One of them named r-SS2, of which potential was similar to that of r-SS1 in the surface of α-Fe2O3, acted as a recombination centre but could increase the lifetime of the trapped holes by the charge separation. We also revealed that the energetic position of the other (i-SS2), which has been regarded as the intermediate state for oxygen evolution reaction with low overpotential, was higher than that of i-SS1 in α-Fe2O3 but lower than those of r-SS1 and r-SS2.

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.

Surface repair of wide-bandgap perovskites for high-performance all-perovskite tandem solar cells

Wide-bandgap (WBG) perovskite solar cells (PSCs) play a fundamental role in perovskite-based tandem solar cells. However, the efficiency of WBG PSCs is limited by significant open-circuit voltage losses, which are primarily caused by surface defects. In this study, we present a novel method for modifying surfaces using the multifunctional S-ethylisothiourea hydrobromide (SEBr), which can passivate both Pb-I and FA-I terminated surfaces. Moreover, the SEBr upshifted the Fermi level at the perovskite interface, thereby promoting carrier collection. This proposed method was effective for both 1.67 and 1.77 eV WBG PSCs, achieving power conversion efficiencies (PCEs) of 22.47% and 19.90%, respectively, with VOC values of 1.28 and 1.33 V, along with improved film and device stability. With this advancement, we were able to fabricate monolithic all-perovskite tandem solar cells with a champion PCE of 27.10%. This research offers valuable insights for passivating the surface trap states of WBG perovskite through rational multifunctional molecular engineering.

Top-Down Exfoliation Process Constructing 2D/3D Heterojunction toward Ultrapure Blue Perovskite Light-Emitting Diodes.

3D perovskites with low energy disorder and high ambipolar charge mobility represent a promising solution for efficient and bright light-emitting diodes. However, the challenges of regulating the nanocrystal size to trigger the quantum confinement effect and control the surface trap states to reduce charge loss hinder the applications of 3D perovskites in blue perovskite light-emitting diodes (PeLEDs). In this study, we present a top-down exfoliation method to obtain blue 3D perovskite films with clipped nanocrystals and tunable bandgaps by employing methyl cyanide (MeCN) for post-treatment. In this method, the MeCN solvent exfoliates the surface components of the 3D perovskite grains through a partial dissolution process. Moreover, the dissolved precursor can be further utilized to construct an ingenious 2D/3D heterostructure by incorporating an organic spacer into the MeCN solvent, contributing to efficient defect passivation and improved energy transfer. Consequently, efficient PeLEDs featuring ultrapure blue emission at 478 nm achieve a record external quantum efficiency of 12.3% among their 3D counterparts. This work emphasizes the significance of inducing the quantum confinement effect in 3D perovskites for efficient blue PeLEDs and provides a viable scheme for the in situ regulation of perovskite crystals.

Selective Photoelectrochemical Oxidation of Glycerol to Glyceric Acid on (002) Facets Exposed WO3 Nanosheets

AbstractGlycerol is a byproduct of biodiesel production. Selective photoelectrochemical oxidation of glycerol to high value‐added chemicals offers an economical and sustainable approach to transform renewable feedstock as well as store green energy at the same time. In this work, we synthesized monoclinic WO3 nanosheets with exposed (002) facets, which could selectively oxidize glycerol to glyceric acid (GLYA) with a photocurrent density of 1.7 mA cm−2, a 73 % GLYA selectivity and a 39 % GLYA Faradaic efficiency at 0.9 V vs. reversible hydrogen electrode (RHE) under AM 1.5G illumination (100 mW cm−2). Compared to (200) facets exposed WO3, a combination of experiments and theoretical calculations indicates that the superior performance of selective glycerol oxidation mainly originates from the better charge separation and prolonged carrier lifetime resulted from the plenty of surface trapping states, lower energy barrier of the glycerol‐to‐GLYA reaction pathway, more abundant active sites and stronger oxidative ability of photogenerated holes on the (002) facets exposed WO3. Our findings show great potential to significantly contribute to the sustainable and environmentally friendly chemical processes via designing high performance photoelectrochemical cell via facet engineering for renewable feedstock transformation.

Selective Photoelectrochemical Oxidation of Glycerol to Glyceric Acid on (002) Facets Exposed WO3 Nanosheets.

Glycerol is a byproduct of biodiesel production. Selective photoelectrochemical oxidation of glycerol to high value-added chemicals offers an economical and sustainable approach to transform renewable feedstock as well as store green energy at the same time. In this work, we synthesized monoclinic WO3 nanosheets with exposed (002) facets, which could selectively oxidize glycerol to glyceric acid (GLYA) with a photocurrent density of 1.7 mA cm-2 , a 73 % GLYA selectivity and a 39 % GLYA Faradaic efficiency at 0.9 V vs. reversible hydrogen electrode (RHE) under AM 1.5G illumination (100 mW cm-2 ). Compared to (200) facets exposed WO3 , a combination of experiments and theoretical calculations indicates that the superior performance of selective glycerol oxidation mainly originates from the better charge separation and prolonged carrier lifetime resulted from the plenty of surface trapping states, lower energy barrier of the glycerol-to-GLYA reaction pathway, more abundant active sites and stronger oxidative ability of photogenerated holes on the (002) facets exposed WO3 . Our findings show great potential to significantly contribute to the sustainable and environmentally friendly chemical processes via designing high performance photoelectrochemical cell via facet engineering for renewable feedstock transformation.

Resonance Plasmonic Coupling: Selective Enhancement of Band Edge Emission over Trap State Emission of CdSe Quantum Dots

The photoluminescence properties of quantum dots (QDs) are often enhanced by eliminating surface trap states through chemical methods. Alternatively, a physical approach is presented here for improving photoluminescence purity in...

Profound influence of surface trap states on the utilization of charge carriers in CdS photoanodes

In CdS photoanodes, the decoration of surface sulfur vacancies by CTAB passivates the electron trapping process, resulting in improved photoelectrochemical water oxidation performance.