Transport Of Photoinduced Carriers Research Articles

Competitive behavior between the piezoelectric and piezoresistive effects in monolayer WS2 photodetector

The strain induced piezoelectric and piezoresistive effects have been regarded as promising methods to regulate the photoelectric properties of two-dimensional transition metal dichalcogenides. However, the distinction between the influence of piezoelectric and piezoresistive effects on devices is ambiguous. Here, piezo-phototronic photodetectors based on monolayer WS2 were fabricated to investigate the competitive behavior of the piezoelectric and piezoresistive effects. We have shown that the piezoresistive effect dominates the photocurrent enhancement through narrowing the bandgap under small strain. With increasing strain, the influence of piezoelectric effect became more and more obvious, and it dominated the photo-induced carrier transport behavior through polarization charges accumulated at the metal–semiconductor contact interface when the strain exceeded 0.78%. Under the strain condition, the modulation of strain on photocurrent reached as high as 1400%.

Microstructures and photovoltaic effect of KNbO3–SrCo0.5Hf0.5O3−δ ferroelectric semiconductors

AbstractPolarization is one of the unique properties of ferroelectric materials; yet the polarization mechanism for enhancing ferroelectric photovoltaic performance is rarely been investigated, particularly in terms of bandgap variation. In this work, the effect of high‐field polarization on the enhanced photovoltaic performance of a ferroelectric ceramic, 0.98KNbO3–0.02SrCo0.5Hf0.5O3−δ (KNSCH2), was explored in terms of bandgap variation. The bandgap of the KNSCH2 sample shrank after polarization because of the increase in potential energy band overlap and the upward shift of the valence band due to increased oxygen‐vacancy defects. The polarization optimized the energy band structure of KNSCH2, promoting the separation and transport of photoinduced carriers and thus further enhancing its photovoltaic performance. The KNSCH2 sample shows a twofold enhancement in Jsc after 60 kV/cm polarization. The degree of the lattice distortion of KNSNH2 increased following polarization, causing a minute increase in its cell asymmetry. The reasons for the bandgap narrowing and the creation of sub‐bandgaps in the KNSCH samples were also investigated. This work opened new doors to understanding the mechanisms underlying the polarization‐enhanced photovoltaic performance of ferroelectric materials.

Enhanced charge transport from Pd-doping in CsPbBr3 quantum dots for efficient photoelectrocatalytic water splitting

The Pd-CsPbBr3 QDs accelerate the separation and transport of photo-induced carriers and exhibit a superior photocurrent of about 2.07 mA cm−2 at 1.23 VRHE.

Design and synthesis of α-Fe2O3/MIL-53(Fe) composite as a photo-Fenton catalyst for efficient degradation of tetracycline hydrochloride

In this study, the α-Fe2O3/MIL-53(Fe) composite was successfully designed and developed through a solvothermal method as a photo-Fenton catalyst and for the degradation of tetracycline hydrochloride (TC-HCl). The results show that the prepared composite exhibits superb performance for the degradation of TC-HCl ascribing to the synergistic effect between MIL-53(Fe) and α-Fe2O3. And the formation of heterojunction structure is beneficial to reducing the band gap, enhancing the absorption of visible light, and improving the separation and transportation of photo-induced carriers. Moreover, this heterojunction structure can be formed without annealing treatment. The effects of pH, H2O2 consumption and catalyst dosage on the degradation of TC-HCl are also investigated. The optimum conditions for target pollutant degradation were investigated according to the response surface methodology (RSM) result based on a central composite design (CCD). A possible mechanism of degradation of TC-HCl is proposed according to the main active radical that existed in the reaction process. This work provides a novelty perspective and in-depth and independent insight for the photo-Fenton mechanism of heterojunction structure modified MOFs material.

Constructing Heterogeneous Photocatalysts Based on Carbon Nitride Nanosheets and Graphene Quantum Dots for Highly Efficient Photocatalytic Hydrogen Generation.

Although graphitic carbon nitride nanosheets (CNs) with atomic thickness are considered as promising materials for hydrogen production, the wide band gap (3.06 eV) and rapid recombination of the photogenerated electron–hole pairs impede their applications. To address the above challenges, we synergized atomically thin CNs and graphene quantum dots (GQDs), which were fabricated as 2D/0D Van der Waals heterojunctions, for H2 generation in this study. The experimental characterizations indicated that the addition of GQDs to the π-conjugated system of CNs can expand the visible light absorption band. Additionally, the surface photovoltage spectroscopy (SPV) confirmed that introducing GQDs into CNs can facilitate the transport of photoinduced carriers in the melon chain, thus suppressing the recombination of charge carriers in body. As a result, the H2 production activity of the Van der Waals heterojunctions was 9.62 times higher than CNs. This study provides an effective strategy for designing metal-free Van der Waals hetero-structured photocatalysts with high photocatalytic activity.

Dimensional Engineering Enables 1.31 V Open‐Circuit Voltage for Efficient and Stable Wide‐Bandgap Halide Perovskite Solar Cells

Wide‐bandgap (WBG) perovskite solar cells (PSCs) are important ingredients for tandem solar cells and play a crucial role in next‐generation multijunction photovoltaics. Yet, the severe open‐circuit voltage loss (Vloss) and stability have not been solved. Herein, a dimensionally graded 2D/3D heterostructure is fabricated by in situ fabricating a 2D FPEA2PbI4 capping layer on the surface of the 3D WBG perovskite film. Through this 2D/3D dimensionally graded design, an enhanced build‐in potential promotes the oriented transport of photoinduced carriers and reduces the nonradiative recombination, leading to an ultrahigh open‐circuit voltage of 1.31 V with a minimum Vloss of 0.43 V in a 1.74 eV WBG perovskite system and a desirable efficiency of 18.06%. A longer photoluminescence lifetime and decreased trap density indicate the reduced trap‐assisted nonradiative recombination. Moreover, such a 2D/3D heterostructure exhibits enhanced stability under moisture and heat. This passivation strategy offers an effective approach to achieving high open‐circuit voltage WBG PSCs by facile in situ dimensional engineering, which may pave a general way to step forward in achieving high‐performance and stable WBG PSCs.

Environmental effects on layer-dependent dynamics of Dirac fermions in quasicrystalline bilayer graphene

The carrier dynamics in various types of epitaxial graphene layers on SiC substrates was investigated by means of time- and angle-resolved photoemission spectroscopy (TARPES). Layer-dependent electron doping was observed in the Dirac bands of quasicrystalline bilayer graphene after optical pumping, leading to generation of transient voltage between the upper and lower layers. The amount of photoinduced carrier transport depends on the distance from the substrate. A comparison of the TARPES results of single-layer graphene between flat and stepped SiC substrates experimentally indicates that a source of the doping carriers likely originates from interface step states. The dynamic model is described based on the electronic structure, calculated using density functional theory.

Photoinduced carrier transport mechanism in pn- and nn-GaN/GaON heterojunctions

The unexpected high persistent photoconductivity effect in Ga2O3 material hinders the application of deep-ultraviolet photodetectors, while GaON can effectively avoid the effect caused by oxygen vacancies through anion engineering. As the behaviors of the carrier’s transport are crucial and essential to the photoelectric conversion processes, analysis of the carrier transport mechanism is helpful to propose feasible and effective strategies for high-performance photodetectors. In this work, GaN/GaON pn- and nn-heterojunctions with various thicknesses of GaON thin films were obtained by oxidizing the n-GaN and p-GaN films, and their photoinduced carrier transport mechanism has been comprehensively investigated. At a low bias, as the electric field is limited in the GaON layer and only the carriers generated in the GaON layer can be collected by the electrodes for both GaN/GaON pn- and nn-heterojunctions, the current increases linearly with an increase in the voltage. At a high bias, the electric field can affect the GaN/GaON heterojunction interface. For the GaN/GaON nn-heterojunction, the current continues to increase with increasing voltage as a small potential barrier is created between GaON and n-GaN to separate and transport the photogenerated carriers. However, for the GaN/GaON pn-heterojunction, the current increases slowly and then rapidly with an increase in the high voltage, because the electric field is not strong enough to help the carriers cross the potential barrier caused by the reverse GaN/GaON pn-heterojunction first and then overcome the barrier with a higher voltage.

Hierarchical Double-Shelled CoP Nanocages for Efficient Visible-Light-Driven CO2 Reduction.

Visible-light-driven photocatalytic CO2 reduction is considered an appealing strategy to mitigate the energy crisis and environmental issues, whereas the reactivity is limited due to the difficulties in activation of inert CO2 molecule and efficient transportation of photoinduced carriers. Herein, we report the design of novel Fe doped CoP hierarchical double-shelled nanocages (Fe-CoP HDSNC) via a MOF-templated approach for highly efficient visible-light-driven CO2 reduction. The unique hierarchical double-shelled hollow architectures can greatly shorten the charge transfer distances and also expose abundant reactive sites. Moreover, Fe atoms doping is able to reduce the CO2 activation energy barrier through stabilizing the *COOH intermediates and promote the CO desorption by destabilizing the CO* adduct. As expected, the Fe-CoP HDSNC achieves an unprecedented catalytic efficiency in visible-light-driven CO2 reduction with an up to 3.25% apparent quantum yield and 90.3% CO selectivity, superior to most of the state-of-the-art photocatalysts under comparable conditions. More importantly, the Fe-CoP HDSNC is also highly effective under diluted CO2 atmosphere, suggesting the practicability of the present photocatalytic system.

Ultrafast dynamics of photoconductivity in lead sulfide nanocrystals in terahertz region

The dynamics of carrier relaxation and charge transport of photoinduced carriers in Lead Sulfide nanocrystals with quantum confinement, synthesized by chemical bath deposition method, have been simultaneously determined using time-resolved terahertz spectroscopy with sub-picosecond time resolution. We use both Drude-Smith and Drude Lorenz model to successfully reproduce the photoconductivity spectra, which manifest that localized free-carriers not bounded excitons are responsible to the THz transport in PbS nanocrystals. The pump fluence dependence of carrier dynamics reveals the process of Auger recombination through the interaction between free carriers and excitons.

Low-crystalline bimetallic metal-organic frameworks as an excellent platform for photo-Fenton degradation of organic contaminants: Intensified synergism between hetero-metal nodes

In this study, the low-crystalline bimetallic metal-organic frameworks (MOFs) of L-MIL-53(Fe, Mn) and L-MIL-53(Fe, Cu) (MIL: Material of Institute Lavoisier) were successfully developed as photo-Fenton catalysts, exhibiting significantly enhanced degradation performance (15.2 and 2.49 times increase in reaction rate constant, respectively) for ciprofloxacin (CIP) in contrast to their crystalline counterparts. That were mainly attributed to the intensified synergism between hetero-metal nodes. Specifically, taking L-MIL-53(Fe, Mn) as an example, the formed electron-rich Fe sites along with the strengthened roles of Mn for enlarged specific surface area, enhanced visible-light absorption and separation and transportation of photo-induced carriers in the low-crystalline state, are responsible for acceleration of Fe(II)/Fe(III) and Mn(II)/Mn(III) cycles, beneficial for H2O2 activation. Moreover, increased metal coordinately unsaturated sites (CUSs) are favorable for more H2O2 coordination. This work gives a deep insight into the heterogeneous photo-Fenton mechanism of the bimetallic MOFs in their low-crystalline state.

A reduced-dimensional polar hybrid perovskite for self-powered broad-spectrum photodetection.

Polar hybrid perovskites have been explored for self-powered photodetection benefitting from prominent transport of photo-induced carriers and the bulk photovoltaic effect (BPVE). However, these self-powered photodetection ranges are relatively narrow depending on their intrinsic wide bandgaps (>2.08 eV), and the realization of broad-spectrum self-powered photodetection is still a difficult task. Herein, we successfully obtained a polar multilayered perovskite, (I-BA)2(MA)2Pb3I10 (IMP, MA+ = methylammonium and I-BA+ = 4-iodobutylammonium), via rational dimension reduction of CH3NH3PbI3. It features the narrowest bandgap of 1.71 eV in a BPV material. As a consequence, the integration of narrow bandgap and BPVE causes the self-powered photodetection to extend to 724 nm for IMP, and a repeatable photovoltaic current reaching 1.0 μA cm−2 is acquired with a high “on/off” ratio of ∼103 and photodetectivity (∼109 Jones) at zero bias. This innovative research provides a foothold for adjusting the physical properties of hybrid perovskites and will expand their potential for self-powered broad-spectrum detection.

Effects of annealing process on the structural and photodetection properties of new thin-film solar-blind UV sensor based on Si-photonics technology

In this paper, a new heterojunction structure based on ultrathin-film ITO sputtered on non-hydrogenated amorphous-silicon (a-Si) is developed for high-detectivity solar-blind UV-photodetector (UV-PD) based on silicon (Si) photonics technology. A strategic combination of Particle Swarm Optimization (PSO) and numerical analysis is used to find out the best design offering superior optoelectronic performance. The optimized design is then elaborated using RF magnetron sputtering technique. A comprehensive investigation of the device structural and optoelectronic properties was carried out, incorporating the influence of heat treatment at temperature values ranging from 300°C to 600°C. X-Ray Diffraction (XRD) measurements indicate that the crystallinity of the sputtered layers was enhanced by increasing the annealing temperature. Significantly, photoelectrical characterization showed that the annealed ITO/a-Si UV-PD exhibits high detectivity exceeding 1013 Jones with a highly improved UV-to-Vis rejection ratio of 5.7 × 103. The ITO/a-Si heterojunction generated a built-in potential, enabling effective separation and transport of photo-induced carriers, thereby reducing recombination losses. Therefore, by well optimizing the proposed heterostructure and the annealing conditions, we were able to elaborate new highly detective, thin-film solar-blind UV-PD based on Si-photonics platform, which can be a promising alternative for future high-performance and cost-effective optoelectronic systems.

Construction of Embedded Heterostructured SrZrO3/Flower-like MoS2 with Enhanced Dye Photodegradation under Solar-Simulated Light Illumination.

SrZrO3/flower-like MoS2 composites with an embedded heterostructure were synthesized via a simple two-step hydrothermal method and their performance was evaluated by photodegradation of methylene blue (MB) under solar-simulated light irradiation. The sandwiched flower-like MoS2 with a high Mo(VI) ratio was adopted as the matrix, and SrZrO3 was grown between the MoS2 layers, forming an intense contact interface, which promotes the efficient separation and transport of photoinduced carriers. The enhanced photocatalytic degradation of 99.7% after 80 min of irradiation is exhibited over the MS5 sample (5 wt % SrZrO3 loading amount on the MoS2 matrix). Moreover, the ratio of Mo(VI) and the superoxide radical plays a crucial role in the photodegradation of MB, and the higher the ratio the better the performance. This work provided a strategy to design a new kind of photocatalyst for photocatalysis and indicated that MoS2 is preferably adopted as a matrix rather than as a loading component.

The favourable synergistic operation of photocatalysis and catalytic oxygen reduction reaction by a novel heterogeneous CoFe2O4-TiO2 nanocomposite

The improved photocatalytic activity of TiO2 with the addition of spinel CoFe2O4 as an effective catalyst for O2 reduction reaction (ORR) is studied and proved in this study. CoFe2O4-TiO2 heterostructure nanoparticles are prepared by a reflux condensation and two-step hydrolysis-hydrothermal method. The addition of trace CoFe2O4 onto TiO2 can improve its affinity to O2, generate a faster transportation of photo-induced carriers, but hardly affect band structure. With an appropriate amount of CoFe2O4 to TiO2, a remarkable acceleration occurs in photocatalytic decolorization of dye wastewater, and the calculated apparent kinetic constant (kapp) of the composite (0.020 min−1) is 2.4 and 5.6 times of pure TiO2 (0.0085 min−1) and CoFe2O4 (0.0036 min−1), respectively. Quenching experiments with different radical scavengers show that O2–∙ is the key oxidative radical in all photocatalytical reactions. Meanwhile, the rapid growth of O2–∙ production by CoFe2O4-TiO2 is proved to be a major cause for photocatalytic performance improvement. Furthermore, an improved electrochemical ORR catalytic ability by CoFe2O4-TiO2 is verified and the 2-plus-2 electron route generates the same product (H2O2) as the photocatalytic process. Therefore, the synergistic operation of photocatalytic and catalytic ORR process by CoFe2O4-TiO2 resulted in a rapid degradation of dye wastewater.

Enhanced self-powered photoresponse in perovskite films with in situ induced p–n homojunction by Ar+ bombardment

In the past ten years, hybrid perovskite materials have attracted extensive research attention due to their outstanding physical properties. Further improvement in the photoresponsive performance is limited by the recombination of charge carriers in the perovskite films. Here, by controlling the stoichiometry of precursors, p-type perovskite films are obtained. The transformation of p-type to n-type in the surface layer of CH3NH3PbI3 film is dramatically achieved by Ar+ bombardment. An n/p-type perovskite homojunction is successfully in situ induced, whose built-in electric field could promote the oriented transport of photo-induced carriers. This perovskite homojunction exhibits an increased Ilight/Idark ratio by almost one order of magnitude at room temperature under 532 nm light irradiation. The Ar+ bombardment method has generic appeal, and its key attributes-in situ induction of p–n homojunction-improve the unicity of preparation for growing perovskite homojunction and make this method potentially suitable for future multifunctional perovskite-based photoelectric devices.

Template-free synthesis of tetragonal graphitic carbon nitride microtubes doped by sodium chloride for enhanced photocatalytic H2 performance under visible light irradiation

Element doping and nanostructure forming remain the effective means to improve the visible-light driven photocatalytic performance for graphitic carbon nitride (CN). Herein, Na/Cl doped g-C3N4 with tetragonal microtube structure is successfully prepared by a hydrothermal method using melamine and sodium chloride as precursors. Nanostructure formation increases the specific surface area of the 3wt% NaCl-CN photocatalyst to 76.8 m2/g from 25.8 m2/g of CN. The Na/Cl doping reduces the band gap and effectively improves separation and transport of photoinduced carriers. The photocatalytic hydrogen evolution rate of 3wt% NaCl-CN reaches 4.95 μmol/h, which is about 4.3 times as high as that of CN. Not only does the more specific surface area increase, and the visible light harvesting increase, but also the photoinduced carriers become more effectively separated and transported, facilitating the photocatalytic performance by the synergistic effect of element doping and nanostructure forming.

New media for efficient light energy conversion: Hollow multi-shelled structures (HoMSs)

Solar energy is one of the most promising alternative of fossil fuels and supplement of current energy consumption structure in the future to tackle increasingly serious energy crisis and environmental problems. The capture and utilization of solar/light energy have focused on four main categories, namely photo-thermal conversion, photo-electrical conversion, photo-chemical conversion and photoluminescence. They all involve with several major steps, which is the capture of photons, the absorption and conversion of photons, and the physical/chemical process on the surfaces/interfaces. In order to achieve high light energy conversion efficiency, to design excellent photo-functional materials as the media is the key point. Generally, photo-functional materials should process the corresponding advantages: (1) Good light harvesting ability; (2) efficient conversion of photons; (3) favored transportation of photo induced carriers; (4) accelerated reactions on the surface. Recently, hollow multi-shelled structures (HoMSs) material has shown its great potential for an advanced photo-functional material. As a class of special hierarchical micro/nanostructures, HoMSs material has more than one shell with corresponding internal cavities, and the multiple shells are arranged in order from outside to inside. Till now, HoMSs material has raised hot research topics in a wide range of fields including sensors, catalysts, adsorbents, drug cargos and so on. Specifically, theoretical and experimental results have discovered that the unique characteristics or features of HoMSs are extremely suitable for photo-related process, which gives many outstanding advantages in the field of light energy conversion. Excellent works have been published by groups across the world. It is about time to summarize recent works on using HoMSs as media for light conversion which converting light to other forms of energy. Starting from the merits of HoMSs photofunctional materials in the process of light energy conversion, we will discuss the efficient light capture ability, enhanced photogenerated charge separation ability, and flexible/adjustable shell-wall composition of different HoMSs in details. Sufficient comparisons of HoMSs with other materials will be made, which will clearly demonstrate how they effectively improve the overall conversion efficiency of the materials. Then we surveyed papers on different applications in the fields of photocatalysis, solar cell, photoluminescence, and so on. We also provide personal prospects for the opportunities and challenges in future development of the HoMSs photo-functional materials.

Phosphotungstic anion-paired quinoline salt for heterogeneous photocatalytic hydroxylation of benzene to phenol with air

Heterogeneous catalysts are preferred in production of chemicals due to the facile production separation and catalyst reusage. However, it is still one challenge to construct efficient heterogeneous catalyst for target reaction. Herein, functional polyoxometalates paired ionic salts (IL-POMs) were prepared through pairing quinoline cations with Keggin-type phosphotungstic (PW) anions. Long alkyl chain in the organic cations enabled the solid nature of these hybrids and endowed them as the recyclable heterogeneous photocatalysts. They exhibited high yield (20.9%) and selectivity (> 99%) in the benzene hydroxylation to phenol by using atmospheric air as the sole oxidant. The activity of the IL-POM hybrid is even higher than the quinoline salt precursor, thanks to the unique redox property of PW anions that promotes the generation, separation and transport of photoinduced carriers. After reaction, the catalyst was facilely reused with stable activity. The catalyst was also active and recoverable in the oxidation of benzyl alcohol to benzaldehyde with atmospheric air.

Single-crystalline melem (C6N10H6) nanorods: a novel stable molecular crystal photocatalyst with modulated charge potentials and dynamics

Here we reported single-crystalline molecular crystal melem nanorods with modulated charge potentials and dynamics due to the high-crystallinity and low-dimensionality facilitating transport of photo-induced carriers.