Slow Carrier Research Articles

Disentangling the Optoelectronic Behavior of Lead Iodide Governed by Two-Dimensional Electron Confinement.

We present a joint experimental and theoretical study for complete spectroscopic characterization and optoelectronic properties of lead iodide. Experimentally, we combine X-ray diffraction experiments to elucidate the structure with photoelectron spectroscopy to explore its electronic structure. Computationally, simulations are performed in the frame of density functional theory. We show that PbI2 presents a two-dimensional layered structure and exhibits a large transient photocurrent effect under visible light illumination, which are compatible with the surface photovoltage scenario. The transient photocurrent has an extremely long lifetime: when the sample is lightened with visible light, it shows very long relaxation times and, consequently, huge charge carrier diffusion lengths. We explain this anomalous behavior with the slow carrier mobility of holes and electrons caused by the 2D electron confinement in the layered material. Our results can be used as a simple model for understanding the optoelectronic properties of more complex 2D hybrid perovskites.

Suppressing Nonradiative Recombination through Dielectric Screening of Defects in Crystalline Carbon Nitride for Enhanced Photocatalytic Activity.

Abundant defect-induced nonradiative recombination greatly reduces the charge separation efficiency in photocatalysts. Dielectric screening of defects has been proven to be an effective strategy to improve the charge separation efficiency; however, it has been rarely reported in photocatalysis. Here, a developed calcium poly(heptazine imide) (CaPHI) is utilized as a model photocatalyst to explore the dielectric screening of defects. Through embedding potassium ions in CaPHI, the dipole moment and polarity of the PHI structure are increased, thus enhancing the dielectric constant and enabling the dielectric screening of defects. In addition, compared to the original CaPHI, the optimized Ca/KPHI exhibits a 79.3% reduction in defect capture cross-section, and a decrease in the nonradiative recombination rate from 0.6224 to 0.1452 ns-1, thus achieving an apparent quantum efficiency of 51.4% for H2 production at 420 nm. This proposed dielectric screening strategy effectively addresses the issue of slow carrier transport and separation caused by defect-induced nonradiative recombination in photocatalysts.

In-situ ‘X’ doped ‘GCN’ photocatalyst enables selective aldehyde synthesis via photo-oxidation of benzyl alcohol under ambient conditions

Photo-oxidative selective conversion of benzyl alcohols using graphitic carbon nitride (GCN) is a sustainable strategy for conventional metal-doped heterogeneous catalytic oxidation systems. Non-metal doped graphitic carbon nitride (a subclass of GCN) enhances its properties as an efficient photocatalyst for oxidation. These heteroatom non-metal dopants alter the optical energy gap and chemical and electronic properties of GCN, making it ideal for creating cost-effective, practical utility. So, herein, we designed heteroatoms (X = B, S, C) doped GCN photocatalysts derived from melamine (M) with different heteroatom ingredients and compared their activity in detail. Among these, the photo-oxidation of benzyl alcohol under blue LED by S-doped GCN (S-GCN) photocatalyst exhibits utmost higher photocatalytic activity than other heteroatoms doped GCN photocatalyst due to suitable band gap and slow photoinduced charge carriers recombination. The suitable energy gap with higher chemical stability is accountable for the substantial increase in the photocatalytic efficiency of S-GCN in the solar light responsive integrating reactions of oxidation of benzyl alcohol in aerobic conditions. Attaining an exceptional >99 % selectivity towards benzaldehyde (up to 5 cycles without significant loss of activity) marks a remarkable milestone. This opens up new possibilities for using these simple non-metal-doped photocatalysts in fine chemical synthesis.

Construction of a 2D/2D Vs-ZnIn2S4/Zn-TCPP heterojunction for effective charge transfer to enhance photocatalytic hydrogen evolution

The integration of inorganic and organic semiconductor components in composites has tremendous potential in photocatalysis, owing to the ultrafast photogenerated exciton dissociation and slow carrier recombination kinetics at their interfaces, which can result in enhanced photocatalytic performances. By exploiting the synergy among morphology modulation, sulfur vacancy (Vs) engineering, and close interface control, a novel inorganic–organic Vs-ZnIn2S4/ tetrakis (4-carboxyphenyl) zinc porphyrin (Zn-TCPP) composite for photocatalytic hydrogen evolution (PHE) was successfully synthesized via microwave-assisted solvothermal and self-assembly methods. The cocatalyst-free PHE rate of Vs-ZnIn2S4/Zn-TCPP (ZZT-20 %) reaches 8.997 mmol h−1 g−1 under visible light irradiation, which is 10.9, 2.2, and 67.1 times that of ZnIn2S4, Vs-ZnIn2S4, and Zn-TCPP, respectively. Moreover, the apparent quantum yield of ZZT-20 % under 420 nm monochromatic light (1.11 %) clearly exceeds that of Vs-ZnIn2S4. Testing experiments and DFT calculations revealed that the significantly enhanced PHE activity can be mainly attributed to the Z-scheme charge transfer mechanism, which greatly improves the visible light response range and redox potentials. Remarkably, Vs sites act as electron traps, inducing the vertical transport of electrons within the bulk phase and their accumulation at the surface Zn–S bonds. Simultaneously, van der Waals forces and weak hydrogen bonds between Vs-ZnIn2S4 and the Zn-TCPP interface cause the rapid migration of photogenerated electrons to Zn-TCPP. This work not only reveals new insights into the structure–property relationships of inorganic–organic Vs-ZnIn2S4 and Zn-TCPP composites, but also provides an appropriate strategy for cleaner water resourcelization and sustainable solar energy conversion.

Unveiling Mechanistic Insight into Accelerating Oxygen Molecule Activation by Oxygen Defects in Co3O4-x/g-C3N4 p-n Heterojunction for Efficient Photo-Assisted Uranium Extraction from Seawater.

Photo-assisted uranium extraction from seawater (UES) is regarded as an efficient technique for uranium resource recovery, yet it currently faces many challenges, such as issues like biofouling resistance, low charge separation efficiency, slow carrier transfer, and a lack of active sites. Based on addressing the above challenges, a novel oxygen-deficient Co3O4-x/g-C3N4 p-n heterojunction is developed for efficient photo-assisted uranium extraction from seawater. Relying on the defect-coupling heterojunction synergistic effect, the redistribution of molecular charge density formed the built-in electric field as revealed by DFT calculations, significantly enhancing the separation efficiency of carriers and accelerating their migration rate. Notably, oxygen vacancies served as capture sites for oxygen, effectively promoting the generation of reactive oxygen species (ROS), thereby significantly improving the photo-assisted uranium extraction performance and antibacterial activity. Thus, under simulated sunlight irradiation with no sacrificial reagent added, Co3O4-x/g-C3N4 extracted a high uranium extraction amount of 1.08mgg-1 from 25 L of natural seawater after 7 days, which is superior to most reported carbon nitride-based photocatalysts. This study elaborates on the important role of surface defects and inerface engineering strategies in enhancing photocatalytic performance, providing a new approach to the development and design of uranium extraction material from seawater.

Charge redistribution of a spatially differentiated ferroelectric Bi4Ti3O12 single crystal for photocatalytic overall water splitting

Artificial photosynthesis is a promising approach to produce clean fuels via renewable solar energy. However, it is practically constrained by two issues of slow photogenerated carrier migration and rapid electron/hole recombination. It is also a challenge to achieve a 2:1 ratio of H2 and O2 for overall water splitting. Here we report a rational design of spatially differentiated two-dimensional Bi4Ti3O12 nanosheets to enhance overall water splitting. Such a spatially differentiated structure overcomes the limitation of charge transfer across different crystal planes in a single crystal semiconductor. The experimental results show a redistribution of charge within a crystal plane. The resulting photocatalyst produces 40.3 μmol h–1 of hydrogen and 20.1 μmol h–1 of oxygen at a near stoichiometric ratio of 2:1 and a solar-to-hydrogen efficiency of 0.1% under simulated solar light.

An Analytical Model for Describing Transient Photocurrents in Bias‐Assisted Charge Extraction for Low‐Mobility Organic Solar Cells

Bias‐assisted charge extraction (BACE) is a powerful technique for measuring the carrier density in organic solar cells under operational conditions and to deduce information on the charge recombination properties. Hereby, the carrier density in the active layer device is determined by integrating the transient extraction current, while nongeminate recombination during the charge extraction processes is generally neglected. This assumption becomes questionable for low mobilities, for example, at low temperatures and in case of high energetic disorder. Herein, the extraction process in BACE measurement is investigated by incorporating both drift and bimolecular recombination of charges during charge extraction into a unified framework. An explicit analytical model is proposed to describe the time dependence of the transient photocurrent in BACE measurement and excellent agreement is found with drift‐diffusion simulations. It is shown that global fitting of transient photocurrents measured at various collection biases with this analytical model allows to determine the carrier density in the device with high accuracy. Furthermore, the analytical model is demonstrated to provide accurate mobilities for both the fast and slow carriers within the device, rendering it a valuable supplement to other mobility measurement techniques such as resistance‐dependent photovoltage and space–charge‐limited current.

Ultrafast nonlinear optical response of layered violet phosphorus for femtosecond noise-like pulse generation

As a new member of two-dimensional (2D) phosphorene, 2D layered violet phosphorus (VP) has unique optoelectronic properties and good environmental stability, showing its huge advantages in optoelectronic applications. In this paper, the ultrafast nonlinear optical (NLO) properties of layered VP nanosheets at 1 µm band were explored, which exhibit an obvious saturable absorption response with a modulation depth of ∼1.97%. Meanwhile, the fast and slow carrier lifetimes of VP nanosheets at 1µm band were also determined as 295.9 fs and 2.36 ps, respectively, which are much shorter than that of most reported 2D materials. The excellent saturable absorption response combined with ultrashort carrier lifetimes indicate the prospect of layered VP nanosheets as a fast saturable absorber (SA) for ultrafast laser modulation. Then we demonstrated a Yb-doped fiber laser based on the VP-deposited taper-shaped fiber (TSF) SA, which delivers stable Q-switched mode-locked (QSML) pulses, dual-wavelength mode-locked pulses and 404-fs noise-like pulses. This work fully demonstrates the great potential of 2D VP materials for 1 µm ultrashort laser pulse generation.

Reducing hot carrier cooling rate in metal halide perovskites through lead vacancies: time-domain ab initio analysis

Pb vacancies slow down hot carrier cooling dynamics in MAPbI3 perovskites by introducing intraband states that can trap holes and by decreasing the deformation of the [PbI6]4− octahedron, which leads to weakened electron–phonon coupling.

Stimulating Phonon Bottleneck Effect in Organic Semiconductors by Charge-Transfer-Mediated J-Aggregation.

Hot carriers rapidly lose kinetic energies on a subpicosecond time scale, posing significant limitations on semiconductors' photon-conversion efficiencies. To slow the hot carrier cooling, the phonon bottleneck effect is constructed prevalently in quantum-confined structures with discrete energy levels. However, the maximum energy separation (ΔEES) between the energy levels is in a range of several hundred meV, leading to unsatisfactory cooling time. To address this, we design a novel organic semiconductor capable of forming intermolecular charge transfer (CT) in J-aggregates, where the lowest singlet excited state (S1) splits into two states due to the significant interplay between the Coulomb interaction and intermolecular CT coupling. The ΔEES between the two states can be adjusted up to 1.02 eV, and an extremely slow carrier cooling process of ∼72.3 ps was observed by femtosecond transient absorption spectroscopy. Moreover, the phonon bottleneck effect was identified in organic materials for the first time, and CT-mediated J-aggregation with short-range interactions was found to be the key to achieving large ΔEES. The significantly prolonged carrier cooling time, compared to <100 fs in the isolated molecule (10-6 M), highlights the potential of organic molecules with diversified aggregation structures in achieving long-lived hot carriers. These findings provide valuable insights into the intrinsic photophysics of electron-phonon scattering in organic semiconductors.

Hydrothermal synthesis of porous CdS/Bi2S3/BiOIO3 nanocomposites for solar-driven photocatalytic degradation of rhodamine B

A heterojunction structure incorporating three photocatalysts, corresponding to a ternary composite CdS/Bi2S3/BiOIO3, was proposed to address the issues of poor stability, low efficiency in utilizing visible light, and poor performance in degrading refractory organic pollutants for conventional photocatalysts. Therefore, the interconnected CdS/Bi2S3/BiOIO3 structure facilitated the transfer of electrons from Bi2S3 to BiOIO3 by reducing the band gap, as well as the transfer of h+ from BiOIO3 to Bi2S3. As a result, slow carrier recombination and a decreasing band gap were found, resulting in the good performance of RhB degradation. As a medium, CdS could improve the e- and h+ transmission efficiency between Bi2S3 and BiOIO3. According to performance measurements, after excluding the influence of adsorption, the photocatalytic activity of CdS/Bi2S3/BiOIO3 reached 94.22 % after 30 min and remained stable after five cycles. A quenching experiment illustrating the importance of superoxide and hydroxyl radicals in RhB degradation, which might be applied to other refractory organic contaminants. DRS studies demonstrated that the composite combines the benefits of CdS and Bi2S3, with better light absorption across a broader wavelength range. According to the photocatalytic mechanism, the formation of a compact, three-component heterostructure at the interface increases light absorption while decreasing electron-hole recombination. The results of corresponding novels may yield valuable insights for the development of an effective approach in the design of a high-performance photocatalysis system for the degradation of pollutants.

Chlorella protein hydrogel preparation and its application in loading and slow release of poly(lysine)

This article is dedicated to the preparation of a novel Chlorella protein hydrogel that can be used as a slow release carrier. Chlorella proteins and alginate were used as cross-linking agent at the present of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide/N-hydroxysuccinimide to obtain hydrogel with different 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide/N-hydroxysuccinimide concentration. Textural results showed that hydrogel with high N-hydroxysuccinimide concentration possessed good hardness, springiness, gumminess and chewiness, espasically those hydrogel with 23% N-hydroxysuccinimide concentration. Scanning electron microscopy results revealed that the prepared hydrogel with higher N-hydroxysuccinimide concentration had better stable spatial porous structures. Swelling analyses indicated the prepared hydrogel exhibited pH sensitivity, which were beneficial for the slow-release. Then the hydrogels with 23% N-hydroxysuccinimide were used to be poly(lysine) slow-release carriers. Moreover, this hydrogel exhibited a prolonged and efficient period of poly(lysine) slow release at pH 1.2 (30%) and 7.4 (95%) within 200 h, which were further proved by good antibacterial activity against Escherichia coli and Staphylococcus aureus. Finally, based on the above results, we simulated the mechanism of hydrogel formation and poly(lysine) loading and slow release. The results reveal that the prepared Chlorella protein hydrogels are beneficial for use as slow-release carriers.

Thickness-Dependent nonlinear optical absorption of InSe/Graphene van der Waals heterostructures

InSe/Graphene van der Waals heterostructures emerge as promising candidates for optoelectronic applications. In this work, we systematically examine the saturable absorption properties of InSe/Graphene heterostructures with varying thickness of InSe. We establish a positive correlation between the third-order nonlinear absorption coefficient and heterostructure thickness. Furthermore, we uncover the interplay of carrier dynamics within the heterostructures, revealing prolonged interlayer transfer times and slowed carrier relaxation as InSe thickness increases. These insights shed light on the potential of InSe/Graphene heterostructures for advanced optoelectronic devices, fostering further advancements in this field.

A single-shot pump-probe technique for measuring ultrafast absorption dynamics in a single-beam basis

We demonstrate a single-shot pump-probe technique in a single-beam basis. A transmissive annular echelon is used to spatially encode a single pulse, and generates a pump pulse and a probe pulse train with approximately 8 ps delay time and 40 fs time interval. Using this technique, we studied the ultrafast absorption dynamics of ZnSe at a wavelength of 515 nm. And our results indicate the ZnSe has an ultrafast two-photon absorption process and a slow carrier absorption process at 515 nm. In addition, with the help of a neutral density filter (ND), we also investigated the effect of probe intensity on the experimental results.

Heterogeneous doping via charge carrier transport improves Photoelectrochemical H2O oxidative H2O2 synthesis

Photoelectrochemical (PEC) water splitting into H2O2 and H2 has attracted significant attention due to its low cost and sustainability. However, slow charge carrier transport and water oxidation kinetics limit the selectivity for H2O2 production and solar conversion efficiency. Herein, we propose a heterogeneous doping approach that combines surface gradient doping with bulk doping to improve the charge carrier transport in the photoelectrode. Inducing gradient Nb and homogeneous Mo doping into BiVO4 photoanode (G-Nb/Mo:BVO) can promote charge separation and carrier transfer, leading to the high selectivity for H2O2 generation and suppression of O2 production. Consequently, the heterogeneously doped G-Nb/Mo:BVO photoanode presents an average Faraday efficiency (FE) of over 80% for H2O2 production in a wide potential of 0.6–1.8 VRHE with the maximum FE of 83.7% at 1.2 VRHE under AM 1.5G illumination. More importantly, H2O2 production rate can reach 1.23 μmol min−1 cm−2 at 1.23 VRHE, representing the best H2O2 production rate reported for the photoelectrodes. Density functional theory calculations prove that Mo and Nb co-doping increases the reactivity of BiVO4 and improves 2e− water oxidation reaction activity and selectivity. This work demonstrates that heterogeneous doping provides a cost-effective strategy to break performance trade-offs by improving charge carrier separation and transport in light absorbers, and modulating the selectivity and activity of water oxidation reaction to generate H2O2, which can be extended to other photocatalytic reactions.

Is carrier mobility a limiting factor for charge transfer in TiO[formula omitted]/Si devices? A study by transient reflectance spectroscopy

TiO2 coatings are often deposited over silicon-based devices for surface passivation and corrosion protection. However, the charge transfer (CT) across the TiO2/Si interface is critical as it may instigate potential losses and recombination of charge carriers in optoelectronic devices. Therefore, to investigate the CT across the TiO2/Si interface, transient reflectance (TR) spectroscopy was employed as a contact-free method to evaluate the impact of interfacial SiOx, heat-treatments, and other phenomena on the CT. Thin-film interference model was adapted to separate signals for Si and TiO2 and to estimate the number of transferred carriers. Charge transfer velocity was found to be 5.2 × 104 cm s−1 for TiO2 heat-treated at 300 °C, and even faster for amorphous TiO2 if the interfacial SiOx layer was removed using HF before TiO2 deposition. However, the interface is easily oversaturated because of slow carrier diffusion in TiO2 away from the TiO2/Si interface. This inhibits CT, which could become an issue for heavily concentrated solar devices. Also, increasing the heat-treatment temperature from 300 °C to 550 °C has only little impact on the CT time but leads to reduced carrier lifetime of ¡3 ns in TiO2 due to back recombination via the interfacial SiOx, which is detrimental to TiO2/Si device performance.

Observable Hole-State Kinetics and Its Implications for Optical Gain in Hole-Engineered Quantum Dots

Disentangling carrier kinetics is of both fundamental and technological importance for optoelectronics, while the hole-state dynamics in Cd chalcogenide quantum dots (QDs) remains far from understood. Herein, we take the CdZnS-based alloyed QDs as the model system to examine the hole-state dynamics by transient absorption (TA) spectroscopy. The spectroscopic signature of the hole states could be directly manifested from the TA spectra, even in low-quality CdZnS QDs, which arises from the distinctive features of alloyed QDs. By completely passivating the hole traps, the resulting CdZnS/ZnS QDs feature a significant hole contribution of 17% to the TA bleach and associated bleach buildup kinetics. These merits endow the CdZnS-based QDs with excellent optical gain properties, where an ultralong gain lifetime (1.1 ns) and a broad gain bandwidth (>300 meV) have been observed thanks to the positive trion-suppressed Auger recombination and the retarded hot hole cooling under high excitation intensities. Such a slow hot carrier cooling (200 ps) in the alloyed QDs further indicates their potential in other optoelectronic devices, such as the hot carrier solar concentrators. These findings clarify the characters of the valence band hole and offer insights into the manipulation of hole states for optimizing device performances.

Harvesting Plasmonic Near-Infrared Photons by Hot Hole Transfer in Nonstoichiometric-Semiconductor Plasmonic Heterojunctions

Infrared light is an untapped energy source that accounts for half of solar energy. Thus, harvesting near-infrared photons for different applications needs further development for efficient utilization of the whole solar spectrum. Herein, we report harvesting infrared photons by hot hole transfer from a nonstoichiometric plasmonic semiconductor (Cu2–xSe) to an adjacent semiconductor (CdSe) in the heterostructure (HNCs). We found a decrease in transient signal intensity and hole–phonon scattering time in Cu2–xSe/CdSe HNCs compared to the pure Cu2–xSe nanocrystals, attributed to plasmon-induced hot hole transfer. A similar kind of TA kinetics has been observed for 800 and 930 nm laser pulse excitation for Cu2–xSe/CdSe HNCs. The spectroscopic results reveal anomalous carrier populations and slow carrier recovery dynamics at the 1P state of the CdSe phase in Cu2–xSe/CdSe HNCs, further establishing the plasmonic hot hole transfer mechanism. The estimated plasmon-induced hot hole transfer time from the nonstoichiometric semiconductor Cu2–xSe NC system is determined and found to be ∼177 fs. Fabricated plasmonic thin-film devices exhibit an excellent conductivity (0.15 A at 2 V) and photoconductivity (0.2 A at 2 V) under near-infrared light illustration (λlight = 1200 nm) in a plasmonic HNC system. These results reveal a novel approach for harvesting light photons from the visible to near-infrared region of the solar spectrum and provide a new avenue for developing hot carrier-based device applications of plasmonic semiconductor-based nanomaterials.

Insight into Hot Carrier Kinetics of CsPbBr3/ZnO Heterostructures for Photodetector Application

Understanding the hot carrier kinetics in heterostructures of inorganic halide perovskite nanocrystals (PNCs) and extraction of the excess energy of hot-carriers is crucial for their future emergence in optoelectronic devices. Herein, CsPbBr3/ZnO heterostructures have been prepared via the most adopted in situ hot-injection method, and ZnO nanoparticles were obtained by a simple chemical process. We have investigated the carrier transfer dynamics through steady-state and ultrafast optical measurements. Steady-state measurements (i.e., photoluminescence and time-resolved photoluminescence decay) reveal that electron transfer from PNCs to surface-attached ZnO nanoparticles is the most probable transition process that happened in heterostructured systems. Furthermore, deep insights into the charge transfer kinetics were investigated by performing ultrafast transient absorption (UTA) spectroscopy at two varying pump fluences (i.e., 0.40 and 0.20 mW) with an excitation wavelength of 450 nm. UTA analysis reveals a significant slow hot carrier cooling at 0.40 mW fluence (from 670 fs to 1.56 ps) and 0.20 mW (from 480 fs to 1.23 ps) in CsPbBr3/ZnO heterostructure systems, suggesting reduced exciton recombination and high charge-carrier transfer through the heterostructure interface. The efficient charge extraction in CsPbBr3/ZnO results in a measurable change in photodetector responsivity and detectivity. This study provides an understanding of the hot carrier kinetics of metal halide perovskites CsPbBr3/ZnO for optoelectronic device applications.

Anomalous indirect carrier relaxation in direct band gap atomically thin gallium telluride

We report ultrafast studies on atomically thin Gallium telluride, a 2D metal monochalcogenide that has appeared to display superior photodetection properties in visible frequencies. Pump photon energy-dependent spectroscopic studies reveal that photoinduced carriers in this direct band-gap material undergo indirect relaxation within $\ensuremath{\sim}30$ ps of photoexcitation, which is at least an order slower than that of most 2D materials. Despite the direct band-gap nature, slow and indirect carrier relaxation places this layered material as a prime candidate in the multitude of atomically thin semiconductor-based photodetectors and highlights the potential for prospective optoelectronic applications.