Charge Carriers In Bulk Research Articles

Tailoring Carrier Dynamics of BiVO4 Photoanode via Dual Incorporation of Au and Co(OH)x Cooperative Modification for Photoelectrochemical Water Splitting

AbstractPhotoelectrochemical solar to hydrogen production is a promising way to achieve carbon neutrality, but severe charge recombination in photoanodes limits the conversion efficiency. Herein, Au nanoparticles and Co(OH)x co‐sensitized bismuth vanadate (BiVO4) to construct AuCo(OH)x/BiVO4 photoanode for significantly enhancing the performance of photoelectrochemical water splitting. This process significantly improves the bulk charge carrier separation efficiency, the surface kinetics of water oxidation, and the electron density of BiVO4 photoanode through Au surface plasmon resonance (SPR) and Co(OH)x oxygen evolution catalysts effect. Additionally, the enhancement of the *O and the *OOH generation accelerate the oxygen evolution reaction kinetics. Consequently, the constructed AuCo(OH)x/BiVO4 photoanode demonstrates an excellent photocurrent of 6.2 mA cm−2 at 1.23 V versus reversible hydrogen electrode and a stable continuous output within 42 h. This work contributes to developing high‐efficiency and high‐stability photoanodes for solar H2 production through SPR effect and oxygen evolution catalysts.

Construction of "Metal Defect/Oxygen Defect Junction" in ZnFe2O4-NiCo2O4 Heterostructures for Enhancing Electrocatalytic Oxygen Evolution.

Defect engineering is a promising approach to improve the conductivity and increase the active sites of transition metal oxides used as catalysts for the oxygen evolution reaction (OER). However, when metal defects and oxygen defects coexist closely within the same crystal, their compensating charges can diminish the benefits of both defect structures on the catalyst's local electronic structure. To address this limitation, a novel strategy that employs the heterostructure interface of ZnFe2O4-NiCo2O4 to spatially separate the metal defects from the oxygen defects is proposed. This configuration positions the two types of defects on opposite sides of the heterojunction interface, creating a unique structure termed the "metal-defect/oxygen-defect junction". Physical characterization and simulations reveal that this configuration enhances electron transfer at the heterostructure interface, increases the oxidation state of Fe on the catalyst surface, and boosts bulk charge carrier concentration. These improvements enhance active site performance, facilitating hydroxyl adsorption and deprotonation, thereby reducing the overpotential required for the OER.

Analytical Model for Current-Voltage Characteristics in Perovskite Solar Cells Incorporating Bulk and Surface Recombination.

The effects of surface recombination on the steady-state carrier profiles and photocurrent in perovskite solar cells are investigated in this paper. The continuity equations for both holes and electrons are solved considering carrier drift and diffusion under the exponential carrier generation profile in the perovskite layer and considering both bulk and interface carrier recombination. An analytical expression for the solar-induced photocurrent is derived. The rate of carrier recombination at the interfaces has a very significant effect on the carrier profile, photocurrent, and, hence, on the charge collection efficiency. The external current density is calculated considering the dark current and nominal solar spectrum-induced photocurrent. The proposed model is fitted and verified with published experimental results from various publications. The fittings of the model with experimental results provide information about the interface and bulk charge carrier transport parameters.

Full-Space Electric Field in Mo-Decorated Zn2In2S5 Polarization Photocatalyst for Oriented Charge Flow and Efficient Hydrogen Production.

Integration of photocatalytic hydrogen (H2) evolution with oxidative organic synthesis presents a highly attractive strategy for the simultaneous production of clean H2 fuel and high-value chemicals. However, the sluggish dynamics of photogenerated charge carriers across the photocatalysts result in low photoconversion efficiency, hindering the wide applications of such a technology. Herein, this work overcomes this limitation by inducing the full-space electric field via charge polarization engineering on a Mo cluster-decorated Zn2In2S5 (Mo-Zn2In2S5) photocatalyst. Specifically, this full-space electric field arises from a cascade of the bulk electric field (BEF) and local surface electric field (LSEF), triggering the oriented migration of photogenerated electrons from [Zn-S] regions to [In-S] regions and eventually to Mo cluster sites, ensuring efficient separation of bulk and surface charge carriers. Moreover, the surface Mo clusters induce a tip enhancement effect to optimize charge transfer behavior by augmenting electrons and proton concentration around the active sites on the basal plane of Zn2In2S5. Notably, the optimized Mo1.5-Zn2In2S5 catalyst achieves exceptional H2 and benzaldehyde production rates of 34.35 and 45.31mmol gcat -1 h-1, respectively, outperforming pristine ZnIn2S4 by 3.83- and 4.15-fold. These findings mark a significant stride in steering charge flow for enhanced photocatalytic performance.

High carrier mobility along the [111] orientation in Cu2O photoelectrodes

Solar fuels offer a promising approach to provide sustainable fuels by harnessing sunlight1,2. Following a decade of advancement, Cu2O photocathodes are capable of delivering a performance comparable to that of photoelectrodes with established photovoltaic materials3–5. However, considerable bulk charge carrier recombination that is poorly understood still limits further advances in performance6. Here we demonstrate performance of Cu2O photocathodes beyond the state-of-the-art by exploiting a new conceptual understanding of carrier recombination and transport in single-crystal Cu2O thin films. Using ambient liquid-phase epitaxy, we present a new method to grow single-crystal Cu2O samples with three crystal orientations. Broadband femtosecond transient reflection spectroscopy measurements were used to quantify anisotropic optoelectronic properties, through which the carrier mobility along the [111] direction was found to be an order of magnitude higher than those along other orientations. Driven by these findings, we developed a polycrystalline Cu2O photocathode with an extraordinarily pure (111) orientation and (111) terminating facets using a simple and low-cost method, which delivers 7 mA cm−2 current density (more than 70% improvement compared to that of state-of-the-art electrodeposited devices) at 0.5 V versus a reversible hydrogen electrode under air mass 1.5 G illumination, and stable operation over at least 120 h.

Accelerating Surface Reaction Kinetics and Enhancing Bulk as well as Surface Charge Carrier Dynamics in Al/ Al2O3‐Coated BiVO4 Photoanode

AbstractPhotoelectrochemical (PEC) hydrogen evolution from water splitting of semiconductor photoanodes is intricately linked to their light‐absorbing capacity, surface reaction kinetics, bulk charge carrier separation efficiency, and surface charge carrier injection efficiency. Harnessing the surface plasmon resonance effect of metals emerges as a promising approach to enhance PEC performance of semiconductor photoanodes. This work presents the first application of non‐noble metal Al nanoparticles to sensitize a BiVO4 nanoporous film (NPF) photoanode. This process significantly expedites the surface kinetics of water oxidation, and improves both bulk charge carrier separation and surface carrier injection efficiencies of BiVO4 NPF photoanode. Additionally, the surface‐formed Al2O3 film effectively preserves the stability of photoanode, leading to a substantial improvement in PEC hydrogen evolution of BiVO4 NPF photoanode in a three‐electrode PEC cell with sulfite scavenger. The constructed BiVO4/Al NPF heterojunction photoanode exhibits a hydrogen evolution rate of 73.9 µmol cm⁻2 h⁻¹ at 1.23 V versus reversible hydrogen electrode, 1.68 times of BiVO4 photoanode. Additionally, BiVO4/Al NPF photoanode exhibits excellent stability under long‐time light irradiation of 8 h. This study provides valuable insights into the design of nonnoble‐metal‐based plasmonic photoanodes for efficient hydrogen generation through PEC cell from the perspectives of surface reaction kinetics, charge carrier injection, and separation.

Collaborative role of co-doping and surface modifying for adjusting surface state of hematite to release the photoelectrochemical activity

Inhibition of bulk and interfacial charge carrier recombination is an urgent solution to release furthest the photoelectrochemical potential abilities of hematite photoelectrode. Herein, the Zr-Y dual metal co-doping engineering and Co/Fe dual metal-organic frameworks cocatalyst layer were employed for modifying hematite photoanode. The novel CoFe-MOF/Zr-Y:Fe2O3 nanoarray composite photoanode exhibits higher PEC activity, the photocurrent density at 1.23 V vs.RHE is increased by 4.7 times than that of the pristine α-Fe2O3. The gain in bulk charge separation efficiency was mainly attributed to the synergistic effect between the two doping ions to improve the electrical conductivity. Wherein, Zr4+ doping endows more free electrons in the bulk α-Fe2O3 and Y3+ introduction causes polaron mobility enhancement. For another, co-doping could reduce the recombination surface state, but form the intermediate surface state to promote hole transfer, thus effectively inhibiting the severe recombination of photogenerated electrons and holes. Furthermore, the CoFe-MOF cocatalyst coating with abundant active sites not only expedites the oxygen evolution reaction kinetics process, but also passivates the surface hole trapping state more thoroughly, resulting in efficient interfacial hole transfer and utilization. This work affords a new collaborative role of co-doping and surface modifying effective avenue for adjusting surface state of hematite to release the photoelectrochemical activity.

Defect Anchoring [S–Ni–P] Interfacial Channel Regulating Charge Migration for Efficient Photoelectrochemical Water Splitting

AbstractRegulating bulk charge carrier transfer and surface catalytic reaction kinetics is thought a big challenge to photoelectrochemical (PEC) water splitting. Herein, the dual sites of CoNiP are delicately introduced into ZnIn2S4 (RZIS‐CoNiP) nanosheet arrays via a defect anchoring method. The paving [S─Ni─P] interfacial bond like a “bridge” can greatly reduce the phase resistance, improve the charge separation and migration, and promote the surface oxygen evolution reaction (OER) reaction. As expected, the optimized RZIS‐CoNiP photoanode achieved a maximum photocurrent density of 4.77 mA cm−2 at 1.23 V versus reversible hydrogen electrode (RHE) in neutral electrolyte solution without the presence of any sacrificial agents, which is ≈12 times higher than that of the pristine ZnIn2S4 under AM 1.5G illumination. And the amount of oxygen evolution for the RZIS‐CoNiP photoanode is as high as 21.9 µmol in 3 h. Transient spectroscopy measurements and density functional theory (DFT) calculations in situ discovered the mechanism of defect anchoring [S─Ni─P] bond on regulating charge transfer and surface reaction processes. This work provides a feasible anchoring interface route through defect engineering to regulate charge carrier transfer for PEC water splitting.

Charge Carrier Trapping in the Surface Region of BiVO4 Photoanodes

Photoelectrochemical water splitting is a promising method for converting solar energy into chemical energy stored in molecular hydrogen and oxygen. However, the efficiency of this process is heavily dependent on the properties of the photoelectrode materials used. Metal oxide semiconductors, such as BiVO4, are commonly used as photoanode materials, but they have poor charge transport properties and surface catalytic activity, which can limit their efficiency.To address these limitations, catalysts are often incorporated onto the surface of the photoanode to improve its surface activity. However, the overall efficiency of the photoelectrochemical process is also influenced by charged defect sites present both in the bulk and on the surface of the semiconductor. These defect sites can trap charges and hamper the kinetics of the process in the millisecond time scale. To address this issue, covalently coordinated catalyst layers and surface doping can be used to eliminate these trapped charges. Moreover, the bulk charge carriers are not entirely decoupled from the surface processes and can be influenced by surface treatments. Therefore, a multi-folded approach that considers both the surface and bulk properties of the photoanode, as well as the role of the catalysts, is necessary to improve the overall efficiency of photoelectrochemical water splitting.We have investigated interfacial charge trapping and transfer processes through a CoOx catalyst layer deposited on BiVO4 by atomic layer deposition (ALD) and cobalt incorporated into the subsurface region. We have shown, by the combination of activity studies and spectroscopy investigations, that oxygen vacancies modified by cobalt incorporation have a remarkable impact on the water oxidation activity and charge injection efficiency. We further demonstrated by transient photocurrent measurements performed by fast LED light pulses (within ms time scale) that the CoOx catalyst layer reduced the number of trapped electrons and improved the rate of charge utilization. Cobalt ions in the subsurface region on the other did not act as catalytic centers but they eliminated the trapping process. We have also confirmed these observations by transient absorption spectroscopy investigations and showed that cobalt incorporation had a much bigger effect on the charge recombination lifetime.

Low-Vacuum Catalyst-Free Physical Vapor Deposition and Magnetotransport Properties of Ultrathin Bi2Se3 Nanoribbons.

In this work, a simple catalyst-free physical vapor deposition method is optimized by adjusting source material pressure and evaporation time for the reliable obtaining of freestanding nanoribbons with thicknesses below 15 nm. The optimum synthesis temperature, time and pressure were determined for an increased yield of ultrathin Bi2Se3 nanoribbons with thicknesses of 8-15 nm. Physical and electrical characterization of the synthesized Bi2Se3 nanoribbons with thicknesses below 15 nm revealed no degradation of properties of the nanoribbons, as well as the absence of the contribution of trivial bulk charge carriers to the total conductance of the nanoribbons.

Surface conductivity in SmB6

We present a study of low temperature electron transport (resistivity, magnetoresistance and Hall coefficient) in SmB6 single crystals having different polar (100) and nonpolar (110) and (111) surfaces after mechanical polishing and chemical etching. The estimation of effective parameters for surface and bulk charge carriers allows us to conclude that surface conductivity is very sensitive to the method of surface treatment. The most pronounced change is observed for the polar (100) surface, for which the related Hall concentration of charge carriers at 2 K decreases more than by 2 orders of magnitude and the Hall mobility increases by a factor of 15 after etching these faces in diluted nitric acid. We suggest that the strong dependence of surface properties on the type of treatment may result from both topological protection, which is influenced by intrinsic defects or surface reconstruction, and band bending effects, which modulate the properties of the surface conduction layer in case of polar faces.

Density-Dependence of Surface Transport in Tellurium-Enriched Nanograined Bulk Bi2 Te3.

Three-dimensional topological insulators (3D TI) exhibit conventional parabolic bulk bands and protected Dirac surface states. A thorough investigation of the different transport channels provided by the bulk and surface carriers using macroscopic samples may provide a path toward accessing superior surface transport properties. Bi2 Te3 materials make promising 3D TI models; however, due to their complicated defect chemistry, these materials have a high number of charge carriers in the bulk that dominate the transport, even as nanograined structures. To partially control the bulk charge carrier density, herein the synthesis of Te-enriched Bi2 Te3 nanoparticles is reported. The resulting nanoparticles are compacted into nanograined pellets of varying porosity to tailor the surface-to-volume ratio, thereby emphasizing the surface transport channels. The nanograined pellets are characterized by a combination of resistivity, Hall- and magneto-conductance measurements together with (THz) time-domain reflectivity measurements. Using the Hikami-Larkin-Nagaoka (HLN) model, a characteristic coherence length of ≈200 nm is reported that is considerably larger than the diameter of the nanograins. The different contributions from the bulk and surface carriers are disentangled by THz spectroscopy, thus emphasizing the dominant role of the surface carriers. The results strongly suggest that the surface transport carriers have overcome the hindrance imposed by nanoparticle boundaries.

Comprehensive Model for Charge Carrier Recombination in Czochralski‐Grown Silicon Due to Oxygen Precipitation in Industrial Solar Cell Manufacturing

Oxygen precipitates are among the most detrimental oxygen‐related silicon bulk defects formed during solar cell manufacturing. These defects are formed only during high‐temperature processes, impeding an identification of prone materials during incoming inspection. Moreover, the prediction of oxygen‐precipitate‐related bulk charge carrier recombination currently requires advanced numerical simulation. This work presents an easily implementable model to predict the bulk carrier lifetime limit, using the temperature–time profile of a high‐temperature process as well as the material properties as the input data. In addition to published analytical descriptions of oxygen precipitation, an empirical description of the retarded growth of small precipitates is included. Furthermore, the time‐lag in nucleation is explicitly considered, which is, to our knowledge, not implemented in oxygen precipitation modeling so far. The calibration of the two free parameters of the model is achieved using the experimental data of 19 different thermal process combinations performed using a single material. This results in a good agreement not only for the material used for calibration but also for other silicon materials. A validation based on passivated emitter and rear cells as well as on test structures confirms the ability of the model to predict bulk carrier lifetimes after solar cell processing.

Effect of gallium doping on structural and transport properties of the topological insulator Bi2Se3 grown by molecular beam epitaxy

Topological insulators possess non-conductive bulk and present surface states, henceforth, they are electrically conductive along their boundaries. Bismuth selenide (Bi2Se3) is one of the most promising topological insulators. However, a major drawback is its n-type nature arising from its natural doping, which makes the transport in the bulk dominant. This effect can be overcome by shifting the chemical potential into the bandgap, turning the transport of the surface states to be more pronounced than the bulk counterpart. In this work, Bi2Se3 was grown by molecular beam epitaxy and doped with 0.8, 2, 7, and 14 at. % of Ga, with the aim of shifting the chemical potential into the bandgap. The structural, morphological, and electronic properties of the Ga doped Bi2Se3 are studied. Raman and x-ray diffraction measurements confirmed the incorporation of the dopants into the crystal structure. Transport and magnetoresistance measurements in the temperature range of 1.5–300 K show that Ga-doped Bi2Se3 is n-type with a bulk charge carrier concentration of 1019 cm−3. Remarkably, magnetotransport of the weak antilocalization effect measurements confirms the existence of surface states up to a doping percentage of 2 at. % of Ga and coherence length values between 50 and 800 nm, which envisages the possibility of topological superconductivity in this material.

Dual bulk and interface engineering with ionic liquid for enhanced performance of ambient-processed inverted CsPbI3 perovskite solar cells

All-inorganic cesium lead iodide (CsPbI3) perovskites with superior thermal stability are attractive candidates for perovskite solar cells (PSCs). Fabricating such inorganic PSCs in the ambient atmosphere is desirable for practical production, however, the challenge remains in inhibiting the phase transition of CsPbI3 in ambient air. Herein, we demonstrate a dual bulk and interface engineering using ionic liquid to stabilize CsPbI3 perovskite structure, thus enhancing the performance of ambient-processed inverted CsPbI3 PSCs. Such dual bulk and interface engineering is found effective not only in suppressing the bulk and interfacial charge carrier recombination and enhancing charge carrier transport and extraction, but also in protecting CsPbI3 crystal structure by leaving hydrophobic alkyl chains coverage at the boundary and surface to prevent phase transition caused by moisture from ambient air. The optimized device fully processed in the open air with relative humidity up to 55% exhibits remarkably enhanced efficiency and stability over the control device, with the efficiency increasing from 8.6% to 13.21%, and 92% efficiency maintaining after storage for 1680 h, which outperforms the control device with only 82% retaining after 648 h storage. We thus believe this work can provide an efficient alternative for the low-cost fabrication of ambient-processible PSCs.

Structural and Morphological Modifications Induced by Fe Ion Implantation in Sb2Te3 Thin Films

AbstractRecently, considerable interest has been initiated by the influence of magnetic dopants as well as the generation of internal strain on the transport properties of topological insulators. The controlled bulk charge carriers in topological insulators help in removing barriers for surface‐dominated transport. In this work, the impact of Fe‐ion implantation on the structural and optical properties of Sb2Te3 thin films is presented. The implantation is done with low energy Fe4– ions at three different fluences (1 × 1013, 1 × 1014, and 5 × 1014 ions cm−2). The structural studies carried out using X‐ray diffraction measurements show that the thin films exhibit rhombohedral structures with an R‐3m space group. A monotonic decrease in the lattice parameter “c” is also observed with increasing ion fluence confirms the contraction in lattice after implantation with ions of smaller radii. The optical studies indicate the structural strain induces by implantation. The incorporated Fe ion affects the grain size, as studied by Atomic Force Microscopy (AFM) measurements. The suppression of grain boundaries towards the highest fluence point is towards a possibility of surface reconstruction.

Ultrafast bipolar conductivity driven by intense single-cycle terahertz pulses in a topological insulator Bi2Se3

We present the measurement of ultrafast terahertz (THz) conductivity of a 50 nm thick topological insulator Bi2Se3 at low temperature 5 K by using a THz pump and THz probe spectroscopy scheme. The THz conductivity driven by a single-cycle intense THz pump exhibits a bipolar lineshape and frequency-dependent relaxation dynamics due to the separated surface and bulk charge carriers with distinctly different scattering rates and relaxation times. We also demonstrate THz pump field-dependent excitation of the surface and bulk spectral weights, which suggests one of the optimal THz excitation conditions for generating the most charge carriers from the topology-protected surface state relative to the bulk. This also allows THz control of surface and bulk transport channels in a selective way.

A Physics-Based Analytical Model for Current–Voltage Characteristics of Perovskite Solar Cells Incorporating Bulk Recombination

A physics-based compact analytical model for studying the current–voltage characteristics of perovskite solar cells has been proposed by considering the external voltage-dependent carrier transport, exponential photon absorption, and bulk charge carrier recombination. The explicit analytical expressions for both the forward dark and photocurrents in perovskite solar cells are derived. The current in the external circuit is calculated considering the actual solar spectrum. The mathematical models are verified and useful physical parameters are extracted by comparing the model calculations with the published experimental results on various perovskite solar cells. The proposed model shows excellent agreement with the experimental results. The power conversion efficiency can be improved further by enhancing the carrier transport in the perovskite layer. The improvement in charge carrier transport enhances the fill factor and hence the power conversion efficiency.

Fe, Cu co-doped BiOBr with improved photocatalytic ability of pollutants degradation

Designing efficient photocatalysts is essential for improving photocatalytic performance. Separation efficiency of photogenerated carriers is one of the key factors affecting photocatalytic performance. Combining the construction of two-dimensional materials with elemental doping can further improve the separation of photogenerated carriers. As expected, the performance of the samples produced by Fe and Cu co-doping was superior to that of pure BiOBr and mono-doped samples, with significant improvements in the photocatalytic degradation of both rhodamine B and tetracycline. The dopant of excess metal element atoms can form impurity energy levels in the forbidden band that effectively facilitate the separation and migration of photogenerated carriers. Meanwhile, the thin-layer two-dimensional structure can effectively shorten the bulk charge carrier transmission distance and improve the transmission efficiency. This study not only provides a simple, green and low-cost method for combining iron and copper into composites materials at same time but also confirmed that the photocatalytic performance of BiOBr was strongly influenced by different Fe and Cu contents.

Fe2O3 hierarchical tubular structure decorated with cobalt phosphide (CoP) nanoparticles for efficient photoelectrochemical water splitting

• Fabrication of large surface area and high crystallinity CoP/SnO 2 :Fe 2 O 3 HTS for PEC application. • The role of SnO 2 and CoP on the oxidation reaction kinetics in the photoanode system was proved. • The highest photocurrent density of 3.54 mA cm −2 at 1.23 V RHE among the Fe 2 O 3 -based photoanodes. Severe charge recombination and poor water oxidation kinetics limit the performance of hematite-based (Fe 2 O 3 ) photoanodes far from their theoretical levels and restrict their applicability for photoelectrochemical (PEC) water splitting devices. In this study, a hierarchical tubular structure (HTS) CoP/SnO 2 :Fe 2 O 3 with a large surface area and well-defined crystalline structures was fabricated via a facile template-engaged redox etching method. The SnO 2 nano-layer served as a dopant source to improve the bulk charge carrier transport efficiency along the Fe 2 O 3 HTS growth axis; therefore, the achieved photocurrent could be up to 2.0 mA cm −2 at 1.23 V RHE (V vs. reversible hydrogen electrode). The CoP nanoparticles (NPs) were introduced as a high-speed hole collector from the light-absorber layer as well as a catalyst to enhance charge carrier injection efficiency on the photoelectrode surface. As a result, CoP/SnO 2 :Fe 2 O 3 shows the highest photocurrent density of 3.50 mA cm −2 (at 1.23 V RHE ) with a negative shift in the onset potential of 180 mV.