Lower Charge Recombination Research Articles

Constructing magnetic separable BiOBr/MnFe2O4 as efficient Z-scheme nanocomposite for visible light-driven degradation of palm oil mill effluent and inactivation of bacteria

Z-scheme BiOBr/MnFe2O4 nanocomposite was successfully prepared by a surfactant-free hydrothermal method. A series of characterizations revealed that the MnFe2O4 nanoparticles were intimately decorated on the BiOBr nanosheets. The as-prepared BiOBr/MnFe2O4 displayed lower charge recombination and higher hydroxyl radical formation rates compared to BiOBr and MnFe2O4. Visible light photocatalytic experimental results showed that the BiOBr/MnFe2O4 possessed excellent performances towards the palm oil mill effluent degradation and Escherichia coli inactivation. The high photoactivity of BiOBr/MnFe2O4 was attributed to the successful establishment of Z-scheme heterojunction between BiOBr and MnFe2O4. Moreover, the BiOBr/MnFe2O4 can easily be recovered using an external magnet and exhibited good repeatability of photoactivity.

Bridge effect on the charge transfer and optoelectronic properties of triphenylamine-based organic dye sensitized solar cells: theoretical approach

In this work, a series of six organic dyes-sensitized solar cells (DSSCs) combining various π-bridges with a fixed donor (triphenylamine) and a fixed electron acceptor (cyanoacrylic acid), namely D1-6, were studied. The geometrical structure, electronic and optical properties of these dyes have been investigated with the density functional theory and TD-BHandHLYP hybrid functional (time-dependent Becke-Half-and-Half-Lee–Yang–Parr’s) methods. The effects of π-bridging of the dyes have shown that the rings with a sulfur atom reduce the energy gaps and provide a redshift of the absorption spectra. Similarly, we focus on the description of the ground and excited state properties. On the other hand, the pyrrole group improves the open-circuit voltage (VOC) and the light-harvesting efficiency parameters leading to greater power conversion efficiency. Furthermore, the results revealed lowest total reorganization energy λ (λ+ and λ−) for the dye D3 with pyrrole linkage, which reflects its most favorable charge-transport properties, implying a lower charge recombination rate, faster charge injection and dye regeneration processes. Therefore, this study would provide a new path to design novel conjugated organic molecules as dyes for high-performance DSSCs.

Position engineering of cyanoacrylic-acid anchoring group in a dye for DSSC applications

This study provides insights into the molecular tailoring of DSSC dyes via synthesis/investigation of three new organic dyes with the position engineering of the acceptor moiety. Differences in photophysical, electrochemical and photovoltaic properties are found and interpreted in detail. The excitation-spectrum study shows that the population of the intramolecular charge transfer (ICT) is incident-wavelength dependent; and the ICT excitons originate not only from direct ICT band transition but also from π-π* energy transfer, and the degree of the latter depends much on the position of the anchoring group. The transient photoluminescence discloses that the non-injection quenching of the ortho dye is the most severe possibly due to the instability of the excited state resulting from the hurdling of electron delocalization by the large steric hinderance. Large steric hinderance in the ortho dye also results in the least dye loading on TiO2 photoanode. Due to broader/stronger absorption, higher dye loading, high injection efficiency and low charge recombination rate, the para-substituted dye DSSC device achieves the best performance with power conversion efficiency of 6.63%; while the ortho dye based device performs the worst with significant lower of photocurrent and photovoltage compared to the para dye. The dye dependent VOC is explained by combination of dipole moment and dye loading. Particularly, the employing of excitation spectrum as tools to investigate the intrinsic photophysical complicity of dyes with D-π-A structure, which is scarcely reported, is proved to be an effective way.

Noble Metal-Free TiO2-Coated Carbon Nitride Layers for Enhanced Visible Light-Driven Photocatalysis.

Composites of g-C3N4/TiO2 were one-step prepared using electron impact with dielectric barrier discharge (DBD) plasma as the electron source. Due to the low operation temperature, TiO2 by the plasma method shows higher specific surface area and smaller particle size than that prepared via conventional calcination. Most interestingly, electron impact produces more oxygen vacancy on TiO2, which facilitates the recombination and formation of heterostructure of g-C3N4/TiO2. The composites have higher light absorption capacity and lower charge recombination efficiency. g-C3N4/TiO2 by plasma can produce hydrogen at a rate of 219.9 μmol·g−1·h−1 and completely degrade Rhodamine B (20mg·L−1) in two hours. Its hydrogen production rates were 3 and 1.5 times higher than that by calcination and pure g-C3N4, respectively. Electron impact, ozone and oxygen radical also play key roles in plasma preparation. Plasma has unique advantages in metal oxides defect engineering and the preparation of heterostructured composites with prospective applications as photocatalysts for pollutant degradation and water splitting.

Carbon quantum dots promote charge transfer of Ce0.7Zr0.3O2@Bi2MoO6 heterojunction for efficient photodegradation of RhB in visible region

Novel visible light-driven photocatalysts with high separation efficiency of the photo-induced charges urgently need to be developed. In this work, a Ce0.7Zr0.3O2@Bi2MoO6 core-shell heterostructure photocatalyst and carbon quantum dots (CQDs) were fabricated via the hydrothermal method. Establishing a close surfacial contact between the CQD particles and this type-II heterojunction yielded the new CQDs/Ce0.7Zr0.3O2@Bi2MoO6 optical catalytic material. Degradation of the Rhodamine B dye under visible-light irradiation was used to evaluate its catalytic performance. CQDs/Ce0.7Zr0.3O2@Bi2MoO6 exhibited superior and stable activity, which can be attributed to the enrichment of surface oxygen vacancies, more easily driven by visible light, the excellent optical and electronic properties and a lower charge recombination. Various experiments demonstrated that the type-II charge transfer process facilitates the separation of electron-hole pairs at the Ce0.7Zr0.3O2@Bi2MoO6 interface. CQDs can act as cocatalysts to further promote efficient charge separation and transfer in such structure. Moreover, this work presents a mechanism and major active species underlying the reaction on photocatalytic system.

Introducing Trifluoromethyl to Strengthen Hydrogen Bond for High Efficiency Organic Solar Cells.

Nowadays, the ternary strategy has become a common way to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). The intermolecular interaction between the third component and donor or acceptor plays a key role in achieving a high performance. However, hydrogen bond as a strong intermolecular interaction is rarely considered in ternary OSCs. In this work, we introduce trifluoromethyl on a newly synthesized small molecular DTBO to strength hydrogen bonds between DTBO and IEICO-4F. Due to the existence of hydrogen bonds has a strong impact on electrostatic potential (ESP) and benefits π-π stacking in the active layer, the ternary OSCs show superior charge extraction and low charge recombination. In DTBO, PTB7-Th and IEICO-4F based ternary devices, the PCE increases from 11.02 to 12.48%, and short-circuit current density (JSC) increases from 24.94 to 26.43 mA/cm2 compared with typical binary devices. Moreover, the addition of DTBO can realize an energy transfer from DTBO to PTB7-Th and broaden the absorption spectrum of blend films. Grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns show that the π-π stacking distance of IEICO-4F decreased after adding 10 wt% DTBO. The effect of the hydrogen bond is also achieved in the PM6: Y6 system, showing 16.64% efficiency by comparison to the 15.49% efficiency of binary system. This work demonstrates that introduce trifluoromethyl to enhance hydrogen bond which improve π-π stacking can achieve higher performance in OSCs.

Enhanced Photocatalytic CO2 Reduction and Water Splitting Over a Boron-Rich Alloy Boron Suboxide (B6O) via Cr or Co Doping

Boron-rich alloy boron suboxide (B6O) as a novel metal-free photocatalyst was recently explored for photocatalytic CO2 reduction and water splitting. However, to broaden the potential applications of B6O in the field, further improving its activity and understanding its fundamental chemical properties are essential. Herein, transition metal doping (Cr or Co) is designed to engineer the band structure of B6O via a solid state reaction. The valence band edge of Cr or Co doped B6O is proposed to be more positive than that of pristine B6O while the original conduction band of B6O is not much influenced by doping, contributing to the better light harvesting ability and lower charge recombination rate of doped B6O. The photocatalytic and photoelectrochemical (PEC) performances of doped B6O were therefore remarkably enhanced. The apparent quantum yields (AQY) of 20 wt% Cr or Co doped B6O are much higher than that of pristine B6O. In addition, the 20 wt% Cr doped B6O exhibits the best electrical conductivity and photocatalytic performance compared with the Co doped B6O and pristine B6O. The present work demonstrates a facile and effective strategy to engineer the band structure and reduce the charge migration resistance of B6O.

Molecular engineering of metal-free organic sensitizers with polycyclic benzenoid hydrocarbon donor for DSSC applications: The effect of the conjugate mode

Optimizing the molecular structure to adjust the properties of photosensitizer is one of the crucial means to improve the power conversion efficiency of dye-sensitized solar cells (DSSCs). In this paper, four aromatic fused rings were selected as electron donor units to construct photosensitizers PBT–series. The effects of structural differences in the donor moiety on the photophysical and photochemical properties of the molecules were investigated in detail. The results show that the devices based on the dyes with triphenylene or fluoranthene as electron donor can yield higher short-circuit photocurrent densities (Jsc) and open-circuit voltages (Voc) than the devices based on the reference dyes with pyrene or phenanthrene as electron donor, and the power conversion efficiency (PCE) is increased by 37% and 23%, respectively. PBT–2 and PBT–4 sensitizers have better light absorption properties due to stronger intramolecular charge transfer (ICT) charge transitions, which helps to increase the photocurrent of devices, while they also exhibit significantly lower charge recombination rates. Eventually, PBT–2-based devices showed the best power conversion efficiency of 7.67% under the standard global AM 1.5 solar conditions.

Ni/Si-Codoped TiO2 Nanostructure Photoanode for Enhanced Photoelectrochemical Water Splitting.

We synthesized Ni/Si-codoped TiO2 nanostructures for photoelectrochemical (PEC) water splitting, by electrochemical anodization of Ti-1Ni-5Si alloy foils in ethylene glycol/glycerol solutions containing a small amount of water. The effects of annealing temperature on PEC properties of Ni/Si-codoped TiO2 photoanode were investigated. We found that the Ni/Si-codoped TiO2 photoanode annealed at 700 °C had an anatase-rutile mixed phase and exhibited the highest photocurrent density of 1.15 mA/cm2 at 0 V (vs. Ag/AgCl), corresponding to a photoconversion efficiency of 0.70%, which was superior to Ni-doped and Si-doped TiO2. This improvement in PEC water splitting could be attributed to the extended light absorption, faster charge transfer, possibly lower charge recombination, and longer lifetime.

Synthesis and solar cell application of a Ti (metal)-organic framework/carbon nanotube composite

A composite of MIL-125 (MIL = Material from Institute Lavoisier, a Ti based metal-organic framework) and single walled carbon nanotubes (SWCNT) has been synthesized via one-pot solvothermal reaction method. The successful formation of the MIL-125/SWCNT composite has been verified with electron microscopy technique. A thin film of the above composite material was used to modify a FTO/TiO2 slide and this photoanode then sensitized with MK-2 dye (2-Cyano-3-[5″′-(9-ethyl-9H-carbazol-3-yl)-3′, 3″, 3″′,4-tetra-n-hexyl-[2, 2′,5′, 2″, 5″, 2″′]-quater thiophen-5-yl] acrylic acid). A dye sensitized solar cell has been assembled using the above photosensitized photoanode. Due to porous nature of MOF, the MIL-125/SWCNT composite based photoanode has contributed toward a good dye absorption and lower charge recombination, which resulted favorable values of short circuit current density (JSC = 17.6 mA cm−2) and open circuit voltage (VOC = 603 mV). The power conversion efficiency of the MIL-125/SWCNT thin layer modified solar cell is estimated as 5.26%.

MIL-53(Fe)-derived Fe2O3 with oxygen vacancy as Fenton-like photocatalysts for the elimination of toxic organics in wastewater

Abstract A series of Fe2O3 nanomaterials with oxygen vacancies were synthesized by pyrolysis of MIL-53(Fe) at different temperatures. Compared with commodity Fe2O3 and Fe2O3 prepared directly from FeCI2·6H2O, 500-Fe2O3 was prepared by MIL-53(Fe) pyrolysis at 500 °C and had a smaller particle size, more active sites and a higher oxygen vacancies content. Electrochemical analysis indicated that 500-Fe2O3 had a stronger photoelectric response, lower electrochemical impedance and lower charge recombination rate. The presence of oxygen vacancies increased the photocurrent density and promoted electron transfer and 500-Fe2O3 had better activation properties for persulfate. Activity experiments, X-ray diffraction and Fourier transform infrared showed that 500-Fe2O3 had a wide range of pH adaptation and good catalytic stability. Free-radical trapping experiments and electron paramagnetic resonance analysis confirmed that the main active species in the reaction system was ·O2- and holes. This study provides a viable solution for improving the performance of the Fe2O3 catalyst.

Enhanced piezoelectric-effect-assisted photoelectrochemical performance in ZnO modified with dual cocatalysts

Promising strategies are of great significance for designing photoelectrodes with high charge separation efficiency and low charge recombination rate in photoelectrochemical (PEC) water splitting. In this paper, we firstly propose a method of combining piezoelectric effect with photoelectrochemical cell by utilizing of direct band gap metal-oxide semiconductor ZnO as the photoanode. After introducing piezoelectric effect resulted from ultrasonic vibrations, the current density value reaches 0.45 mA·cm−2 at 1.23 VRHE by the production of strain-induced charge and the enhancement of the electric field, 1.7 times higher than that of ZnO without ultrasonic vibrations (0.27 mA·cm−2 at 1.23 VRHE). The deposition of spatially separated dual cocatalysts, of which the bottom Pt act as electron collection and transport layer while the outmost Co-Pi serve as hole-transfer layer, further facilitates the separation of charge (strain-induced and photogenerated) and accelerates the reaction kinetics. The utilization of dual cocatalysts efficiently improves the performance of photoanodes and leads to a current density of 0.80 mA·cm−2 at 1.23 VRHE, 1.8 and 3.0 times higher than the values of ZnO with and without ultrasonic vibrations. The strategy provides a promising method and idea for the manufacture of practical and high-performance electrodes.

Effect of annealing temperature on internal absorption, charge recombination and internal quantum efficiency of HC(NH2)2PbI3 perovskite solar cells

Abstract Despite the rapid development of perovskite solar cells (PeSCs) in photovoltaic field, clear understanding about fundamental mechanisms of PeSCs is still sparse. For PeSCs, the thermal annealing temperature of perovskite layer has a great influence on the physical processes of PeSCs. Here, the effect of annealing temperature at 100 °C, 120 °C, 145 °C and 150 °C for the perovskite layer on the internal absorption, charge recombination and internal quantum efficiency of formamidinium lead triiodide (HC(NH2)2PbI3, or FAPbI3) PeSCs was investigated by transfer matrix method (TMM), photo-induced charge extraction by linearly increasing voltage (photo-CELIV) and photocurrent density (Jph) versus effective voltage (Veff) (Jph-Veff) measurements. We obtained the internal absorption (Q) spectra of PeSCs by TMM, and estimated the internal quantum efficiency (IQE) spectrum of PeSC devices from Q spectra and external quantum efficiency (EQE) spectrum. It was found that the PeSC with FAPbI3 film annealed at 145 °C has relatively higher power conversion efficiency (PCE) and IQE. Furthermore, the analysis of photo-CELIV and Jph-Veff characteristics demonstrated that the PeSC with FAPbI3 film annealed at 145 °C possesses relatively low charge recombination and high charge collection. In a word, there is less power loss of PeSCs when the annealing temperature for the FAPbI3 layer is 145 °C because of the better internal absorption, suppressed charge recombination and higher charge collection.

Study on the construction of YMnO3/CeO2 composite photocatalyst heterostructure and photocatalytic degradation of methyl red

The present investigation reports, the novel chemical synthesis of the CeO2, YMnO3 and YMnO3/CeO2 oxides using sonochemistry method and their physicochemical characterization. Phase structure and functional group analysis indicates that the CeO2, YMnO3 and YMnO3/CeO2 oxides has good crystallization with a polycrystalline structure and only a few NO3− impurities. Scanning electron microscope (SEM) reveals that a nanoscale ball shape with a size range of 30–100 nm consist of the CeO2, YMnO3 and YMnO3/CeO2 oxides. The optical absorption band gap of the CeO2, YMnO3 and YMnO3/CeO2 oxides was estimated to be 3.03, 1.03 and 2.51 eV, respectively. Photoluminescence, electrochemical and photocatalytic experiments indicates that the YMnO3/CeO2 oxides exhibits high photocatalytic activity due to the strong absorption capacity, fluorescence quenching, low interfacial resistance, high photocurrent density and low charge recombination rate.

Ultrafine Cu2ZnSnS4 quantum dots functionalized TiO2 nanotube arrays for potential optoelectronic applications

Tremendous progress has been made in power conversion efficiency (PCE) of thin-film photovoltaics over the past few years, yet most current high-efficient photoactive layer usually contains rare or toxic elements accompanied by expensive and complicated vacuum processes, which increases the cost and limits the scope of the applications in the long run. Here we present a synergistic effect of quantum effect of an earth-abundant and low toxic ultrafine Cu2ZnSnS4 (CZTS) quantum dots (QDs) and low charge recombination in one dimensional TiO2 nanotube arrays for optoelectronic devices. By ligands exchange, the as-obtained ultrafine CZTS QDs have been robustly anchored to a highly ordered TiO2 nanotube arrays (TNAs) to be served as a function layer in a simple QDs sensitized solar cells. Such ultrafine CZTS QDs based solar cells exhibit significant enhancement up to 457% in PCE compared to that of CZTS QDs with larger size. The CZTS QDs functionalized TNAs has also shown excellent charge transport capability with lower recombination rate than QDs sensitized TiO2 nanoparticles and it is expected to be used as a low-cost environment-friendly function layer for various potential optoelectronic applications.

Enhanced performance of carbon-based planar CsPbBr3 perovskite solar cells with room-temperature sputtered Nb2O5 electron transport layer

Inorganic CsPbBr3 perovskite solar cell (PSC) has attracted much attention owing to its outstanding air and thermal stability and low cost manufacture process. Crystalline TiO2 (c-TiO2) has been widely used as electron-transporting layer (ETL) material for inorganic CsPbBr3 PSC. However, c-TiO2 requires high-temperature (>450 °C) fabrication process which impedes the application of flexible inorganic CsPbBr3 PSC and its low electron mobility further limits the performance enhancement. Herein, we prepared novel amorphous Nb2O5 (a-Nb2O5) ETL through a facile room-temperature sputtering method for inorganic planar CsPbBr3 PSC. The PSC with a-Nb2O5 ETL has gained a champion efficiency of 5.74%, which is higher than that of the PSC (5.12% or 4.67%) based on crystalline Nb2O5 (c-Nb2O5) ETL or c-TiO2 ETL by high-temperature (500 °C) annealing. The improved photovoltaic characteristic for CsPbBr3 PSC with a-Nb2O5 ETL may be ascribed to its suitable work function, high optical transmittance, low charge recombination at the a-Nb2O5/CsPbBr3 interface and the superior crystallinity of CsPbBr3 film deposited on a-Nb2O5 ETL. Moreover, the a-Nb2O5-based CsPbBr3 PSC without encapsulation exhibits a good long-term stability in ambient atmosphere. This work offers a new research direction for preparing high-performance inorganic PSC.

Interfacial Engineering at the 2D/3D Heterojunction for High-Performance Perovskite Solar Cells.

Perovskite solar cells based on two-dimensional/three-dimensional (2D/3D) hierarchical structure have attracted significant attention in recent years due to their promising photovoltaic performance and stability. However, obtaining a detailed understanding of interfacial mechanism at the 2D/3D heterojunction, for example, the ligand-chemistry-dependent nature of the 2D/3D heterojunction and its influence on charge collection and the final photovoltaic outcome, is not yet fully developed. Here we demonstrate the underlying 3D phase templates growth of quantum wells (QWs) within a 2D capping layer, which is further influenced by the fluorination of spacers and compositional engineering in terms of thickness distribution and orientation. Better QW alignment and faster dynamics of charge transfer at the 2D/3D heterojunction result in higher charge mobility and lower charge recombination loss, largely explaining the significant improvements in charge collection and open-circuit voltage (VOC) in complete solar cells. As a result, 2D/3D solar cells with a power-conversion efficiency of 21.15% were achieved, significantly higher than the 3D counterpart (19.02%). This work provides key missing information on how interfacial engineering influences the desirable electronic properties of the 2D/3D hierarchical films and device performance via ligand chemistry and compositional engineering in the QW layer.

Synergistic effect and enhanced electrical properties of TiO2/SnO2/ZnO nanostructures as electron extraction layer for solar cell application

The present work rationalizes the significance of smart combination of the metal oxide semiconductor (MOS) as electron extraction layer for photovoltaic applications. A synergistic effect of multi-metal oxides, at the nanoscale domain can yield an improved optical and electrical property when compared to their individual phases. In this paper, we report the preparation of a new ternary metal oxide composed of TiO2, SnO2 and ZnO by solvothermal method. The physicochemical properties of the samples were determined by an array of spectroscopic and microscopic techniques. XRD analysis showed the formation of the SnO2 and ZnO in the TiO2 sample. This was further confirmed by Raman and XPS analysis. Optical studies (PL and TRPL) revealed lower charge recombination of the ternary metal oxides. TEM images show the presence of SnO2 and ZnO in the TiO2 lattice. Intergrowth of SnO2 and ZnO in the TiO2 increased the surface area thereby reducing the electron – hole recombination in the ternary metal oxide. Hall measurements display higher carrier concentration and lower resistivity of the metal oxide when compared with pure TiO2. The results suggest that the blend of TiO2, SnO2 and ZnO can serve as a better electron extraction layer to promote charge separation and suppress charge recombination. The ternary metal oxide exhibits promising properties and can enhance the performance when used as an electron extraction layer for solar cell applications.

TTA as a potential hole transport layer for application in conventional polymer solar cells

Abstract Hole transport layers (HTLs) play a vital role in organic solar cells (OSCs). In this work, a derivative of tetrathiafulvalene with four carboxyl groups TTA was introduced as a novel HTL to fabricate OSC with high performance. Displaying a better energy level match between HTL and active layers, the TTA based devices show a peak power conversion efficiency of 9.09%, which is comparable to the devices based on PEDOT:PSS. The favorable surface morphology recorded via atomic force microscopy, low series loss and charge recombination indicated by electrochemical impedance spectroscopy, synchronously verify the potential of TTA for application in OSCs as a valid kind of HTLs.

Doping-Induced Rapid Decoherence Suppresses Charge Recombination in Mono/Divalent Cation Mixed Perovskites from Nonadiabatic Molecular Dynamics Simulation.

Experiment shows that solar cells based on FA0.75Cs0.25Pb0.5Sn0.5I3 carry a lower charge recombination rate and higher power conversion efficiency than those of FAPbI3 despite the fact that the former has a smaller band gap. However, the underlying mechanism remains unclear. Using nonadiabatic (NA) molecular dynamics, we demonstrate that low-frequency vibrations drive electron-hole recombination in pristine FAPbI3 occurring in about 1 ns, showing excellent agreement with experiment. Cs/Sn substitution to FA/Pb not only narrows its band gap by 0.3 eV but also delocalizes the electron wave function significantly, leading to enhancement of NA coupling. Importantly, doping accelerates quantum decoherence caused by increased atomic fluctuations. As a result, rapid decoherence prevails a small band gap and strong NA coupling, slowing charge recombination and extending the charge carriers' lifetime to several nanoseconds. Our study reveals the importance of quantum coherence on quantum dynamics in perovskite materials and suggests a rational strategy to design high-performance perovskite solar cells.