Transient Photovoltage Techniques Research Articles

PH-dependent assembly of tungsten oxide three-dimensional architectures and their application in photocatalysis.

In this work, tungsten oxide (WO3) with three-dimensional flower-like and wheel-like architectures, based on the spontaneous aggregation of one-dimensional nanorods, were successfully fabricated by adjusting the pH of the precursor solution. The influence of pH on the morphologies of WO3 was systematically studied, and the different WO3 architectures were used to photocatalytically degrade rhodamine B. The kinetic features of photoinduced charges of as-prepared WO3 have been investigated by surface photovoltage spectroscopy and transient photovoltage techniques in detail. WO3 with wheel-like and flower-like structures possess the higher charge separation efficiency and the lower recombination rate of photoinduced charges, resulting in higher photocatalytic activity for the degradation of RhB.

Study on dynamics of photoexcited charge injection and trapping in CdS quantum dots sensitized TiO2 nanowire array film electrodes

The photoexcited electrons transfer dynamics of the CdS quantum dots (QDs) deposited in TiO2 nanowire array films are studied using surface photovoltage (SPV) and transient photovoltage (TPV) techniques. By comparing the SPV results with different thicknesses of QDs layers, we can separate the dynamic characteristics of photoexcited electrons injection and trapping. It is found that the TPV signals of photoexcited electrons trapped in the CdS QDs occur at timescales of about 2 × 10−8 s, which is faster than that of the photoexcited electrons injected from CdS into TiO2. More than 90 nm of the thickness of the CdS QDs layer will seriously affect the photoexcited electrons transfer and injection.

D-π-A Porphyrin Employing an Indoline Donor Group for High Efficiency Dye-Sensitized Solar Cells

Dye-sensitized solar cell (DSC) devices were fabricated using a novel donor-(π bridge)-acceptor (D-π-A) porphyrin sensitizer, VC-70, in which an indoline is linked directly to the porphyrin core and functions as the donor group. The best efficiencies of VC-70 and reference YD2-o-C8 devices were found to be 7.31 and 7.60%, respectively, and AMG 1.5 illumination and device properties were fully characterized using transient absorption, charge extraction, and transient photovoltage techniques. A notable effect on TiO2 conduction band energetics and electron lifetime was observed following light soaking of VC-70 devices under AMG 1.5 illumination. Upon cosensitization of VC-70 with the organic dye D205 an improved efficiency of 8.10% was obtained.

Enhanced photoelectrochemical performance and charge transfer properties in self-organized NiOx-doped TiO2 nanotubes

In this present work, 5 wt% NiOx-doped TiO2 nanotubes with 8μm thickness were fabricated by anodization from Ti-Ni alloy. After the characterization of SEM, XRD, XPS and electrochemical measurements, the doped nanotube structure shows a strongly enhanced electrochemical performances in conductivity, donor concentration and OH− ion intercalation ability. In order to better understand the effects of NiOx doping on mechanism of photoinduced carriers’ transfer, the photovoltaic performances were investigated by Transient Photovoltage (TPV) and Surface Photovoltage (SPV) techniques. It is interesting to find that the doped one exhibits a strong photoresponse in visible region and high photovoltage intensity. These results indicated that the incorporation of NiOx promoted the charge separation and inhibited recombination of photoinduced carriers in TiO2 nanotube.

Relation of Nanostructure and Recombination Dynamics in a Low‐Temperature Solution‐Processed CuInS2 Nanocrystalline Solar Cell

The understanding and control of nanostructures with regard to transport and recombination mechanisms is of key importance in the optimization of the power conversion efficiency (PCE) of solar cells based on inorganic nanocrystals. Here, the transport properties of solution‐processed solar cells are investigated using photo‐CELIV (photogenerated charge carrier extraction by linearly increasing voltage) and transient photovoltage techniques; the solar cells are prepared by an in‐situ formation of CuInS2 nanocrystals (CIS NCs) at the low temperature of 270 °C. Structural and morphological analyses reveal the presence of a metastable CuIn5S8 phase and a disordered morphology in the CuInS2 nanocrytalline films consisting of polycrystalline grains at the nanoscale range. Consistent with the disordered morphology of the CIS NC thin films, the CIS NC devices are characterized by a low carrier mobility. The carrier density dynamic indicates that the recombination kinetics in these devices follows the dispersive bimolecular recombination model and does not fully behave in a diffusion‐controlled manner, as expected by Langevin‐type recombination. The mobility–lifetime product of the charge carriers properly explains the performance of the thin (200 nm) CIS NC solar cell with a high fill‐factor of 64% and a PCE of over 3.5%.

Noble-metal-free CuS/CdS composites for photocatalytic H2 evolution and its photogenerated charge transfer properties

CuS/CdS composites have been successfully prepared by a simple hydrothermal and cation exchange method. Even without noble-metal cocatalyst, the prepared CuS/CdS composites exhibited enhanced photocatalytic H2 evolution activity. CuS content had a great influence on photocatalytic activity and an optimum amount of CuS was determined to be ca. 3 mol%, at which the CuS/CdS displayed the highest photocatalytic activity, giving an H2 evolution rate of 332 μmol g−1 h−1, exceeding that of pure CdS by 3.5 times. The results of SPV (surface photovoltage) and SPC (surface photocurrent) revealed that photogenerated electrons were captured by CuS loaded. TPV (transient photovoltage techniques) indicated that photogenerated charges lifetime in CdS, was prolonged with CuS loaded. Those are the main reasons for the improvement of photocatalytic H2 evolution.

Enhancement of photocatalytic H2 evolution on Zn0.8Cd0.2S loaded with CuS as cocatalyst and its photogenerated charge transfer properties

CuS/Zn(0.8)Cd(0.2)S composites have been successfully prepared by simple hydrothermal and cation exchange method. The Cu species loaded on Zn(0.8)Cd(0.2)S, together with the intimate contact formed between CuS and Zn(0.8)Cd(0.2)S, was clearly demonstrated with X-ray photoelectron spectroscopy and transmission electron microscopy. The optimized CuS/Zn(0.8)Cd(0.2)S photocatalyst has a high H2 evolution rate of 2792 μmol g(-1) h(-1) at CuS content of 3 wt% and the apparent quantum efficiency of 36.7% at 420 nm. The photophysical mechanism of the photocatalytic activity was investigated with the help of surface photovoltage spectroscopy (SPS) and transient photovoltage (TPV) techniques. The results revealed that photogenerated charge separation efficiency in Zn(0.8)Cd(0.2)S was enhanced and the photogenerated electrons were trapped by the loaded CuS, which benefits photo-reduction. Those were the reasons for significant enhancement in the photocatalytic H2 evolution from water splitting.

Effect of Polymer Crystallinity in P3HT:PCBM Solar Cells on Band Gap Trap States and Apparent Recombination Order

AbstractThe non‐geminate recombination of charge carriers in polymer‐fullerene solar cells has been modeled in the last few years with a trap‐assisted recombination model, which states that the apparent recombination order depends on the concentration of trapped charges tailing into the band gap. Higher concentrations of trapped charges lead to higher apparent recombination orders. In this work, the mass fractionfof highly crystalline nanofibrillar P3HT to the total P3HT content in P3HT:PCBM solar cells is consistently varied, controlling the temperature of a nanofibers‐P3HT casting dispersion. A systematic study of the apparent recombination order, measured with a transient photovoltage technique, as a function offis presented. A correlation is shown between the apparent recombination order, the P3HT crystallinity, and the trap concentration in the band gap measured with an admittance spectroscopy technique.

Facile fabrication of efficient AgBr–TiO2 nanoheterostructured photocatalyst for degrading pollutants and its photogenerated charge transfer mechanism

A simple microemulsion-like chemical precipitation method has been successfully developed to construct effectively-contacted AgBr–TiO2 composite. The key of this method is the dual roles of Br− in the synthetic process, as linkers between cetyltrimethyl ammonium cation surfactants and nanocrystalline anatase TiO2 in the acidic condition, and as bromine sources to directly produce nanocrystalline AgBr on the surfaces of TiO2 by chemical precipitation. It is well demonstrated that the as-constructed AgBr–TiO2 nanoheterostructured composites display effective photogenerated charge transfer between AgBr and TiO2, favorable to improve charge separation, by means of the surface photovoltage technique in different atmospheres at the aid of outer electric fields, especially for the transient surface photovoltage technique in air. And also, the Br− in crystal lattice of AgBr could effectively capture photogenerated holes under illumination. These factors are well responsible for the enhanced activity for photocatalytic degradation of liquid phase aqueous phenol solution and gas phase acetaldehyde under either UV–visible or visible irradiation, and the stability of AgBr in the photocatalytic processes.

Hierarchical Composite of Ag/AgBr Nanoparticles Supported on Bi2MoO6 Hollow Spheres for Enhanced Visible‐Light Photocatalytic Performance

AbstractThe fabrication of multicomponent composite systems may provide benefits in terms of charge separation and the retardation of charge pair recombination. For this purpose, a hierarchical Ag/AgBr/Bi2MoO6 composite was fabricated by using prepared hierarchical floriated Bi2MoO6 hollow spheres as a supporting material. The interleaved nanoflakes assembled in the hierarchical floriated Bi2MoO6 hollow spheres contributed to the stable deposition of Ag/AgBr nanoparticles and the formation of effective nanojunctions, which led to the low recombination rates of the photoinduced electron–hole pairs. The presence of double visible‐light‐active components in the Ag/AgBr/Bi2MoO6 nanojunction system further broadened the visible‐light photoresponse range. As a result, the hierarchical Ag/AgBr/Bi2MoO6 composite exhibited a higher photocatalytic activity than both photocatalysts containing single visible‐light‐active components and crushed Ag/AgBr/Bi2MoO6 nanoparticles for the degradation of alizarin red S (ARS) and phenol. X‐ray photoelectron spectroscopy (XPS) analyses indicated that both AgBr and Ag0 components coexist in the system, and the as‐prepared composite is relatively stable. The improved photoinduced charge‐transfer properties of the hierarchical Ag/AgBr/Bi2MoO6 composite were investigated by using the transient photovoltage (TPV) technique.

Photoelectric properties of ZnO/Ag2S heterostructure and its photoelectric ethanol sensing characteristics

The photoelectric properties of ZnO microspheres, ZnO/Ag2S heterogeneous microspheres and Ag2S hollow microspheres were investigated systematically by surface photovoltage, transient photovoltage and surface photocurrent techniques. The ZnO/Ag2S heterostructure shows superior photoelectric properties in visible-light region compared with pure Ag2S. Transient photovoltage results reveal the separation processes of photo-generated charge carriers in the samples. The photoelectric ethanol sensing property induced by visible light for the ZnO/Ag2S heterostructure has been found, which should be valuable for the practical application of semiconductor gas sensors at room temperature.

Effect of BiVO4 Crystalline Phases on the Photoinduced Carriers Behavior and Photocatalytic Activity

A series of different crystalline phases BiVO4 photocatalysts (tetragonal, monoclinic, and monoclinic/tetragonal heterophase) have been prepared by a coprecipitation and molten salt method. High-resolution transmission electron microscopy (HRTEM) results show that an interface of intimate contact is formed in monoclinic/tetragonal heterophase and monoclinic phase is mainly on the surface of nanoparticles. Surface photovoltage (SPV) and transient photovoltage (TPV) techniques are used to further investigate the transfer process of photoinduced charge carriers. The results show that the behavior of photoinduced charges markedly depend on the crystalline phases of BiVO4 samples, and the presence of interface in monoclinic/tetragonal heterophase provides a spatial condition for charge transfer, promotes the separation of photoinduced electron-hole pairs, and changes the migration direction of photoinduced carriers. The relationship between behavior of photoinduced charge carriers and photocatalytic activity was discussed in detail, which would provide a greater insight into the intrinsic reasons of the enhancement in photocatalytic activity.

Enhancement of visible-light-driven photoresponse of Mn/ZnO system: photogenerated charge transfer properties and photocatalytic activity

A visible-light-active ZnO photocatalyst system in the presence of manganese ions (Mn/ZnO) was prepared via a simple and rapid approach. XRD, XPS, Raman scattering and UV-Vis DRS confirmed the manganese exists in multivalent forms (Mn(3+)/Mn(2+)) in the ZnO lattice, furthermore, ZnO light absorption is extended to the visible region. The photocatalytic activities of the catalysts were evaluated by measuring the photodegrading efficiency of 2,4-dichlorophenol (DCP) under visible light irradiation. With an optimal molar ratio of 5% in Mn/ZnO the highest rate photodegradation was achieved under the experimental conditions. We have characterized the separation and transfer behavior of the photogenerated charges in the visible region by means of surface photovoltage (SPV), surface photocurrent (SPC) and transient photovoltage (TPV) techniques. Based on the comprehensive investigation of the photovoltaic properties of Mn/ZnO photocatalyst, we illustrate the behavior of photogenerated charges have distinct effects on the photocatalytic activity. It is demonstrated that the incorporation of multivalent Mn in ZnO promoted the separation of photogenerated charges, inhibited the recombination of photogenerated carriers, and thus prolonged the charges lifetime to participate in the photocatalytic reaction, resulting in highly efficient photocatalytic activity, which is attributed to the formation of a strong electronic interaction between the multivalent Mn and ZnO.

Synthesis and photo activity of flower-like anatase TiO 2 with {001} facets exposed

Developing photocatalysts with specific morphology promises good opportunities to discover the geometry-dependent properties. Herein, flower-like anatase TiO 2 assemblies with dominant {001} facets exposed were successfully synthesized via a simple, economical hydrothermal route with titanium sulfate and hydrofluoric acid. Their surface morphology and structure were investigated by scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, X-ray diffraction, and Brunauer–Emmet–Teller N 2 gas adsorption–desorption isotherms. The optical property and the photo-induced charge carriers of the flower-like TiO 2 were studied by UV–vis diffuse reflectance spectroscopy and transient photovoltage technique. The flower-like TiO 2 particles exhibited a good photocatalytic activity in degrading rhodamine B.

Photogenerated charge carrier recombination processes in CdS/P3OT solar cells: effect of structural and optoelectronic properties of CdS films

Research and development activities in organic solar cells have been intensified in the last two decades, and the reported energy conversion efficiency in small cell samples is rapidly increased. However, the relation between cell performance and material preparation conditions is not fully understood. In this work charge carrier recombination processes in hybrid poly-3-octylthiophene (P3OT)/cadmium sulfide (CdS) photovoltaic cells were analyzed as a function of structural and optoelectronic properties of chemical bath deposited CdS thin films. The temperature of the bath solution varied between 60 and 80 °C, and the deposition time from 1 to 3 h. Charge carrier recombination times in CdS films were measured with photoconductance decay technique, whereas the same time in P3OT films was estimated by Time-of-Flight method. Charge carrier recombination rates at CdS/P3OT interface were determined by transient photovoltage technique. It is found that CdS films grown at lower solution temperature (60 °C) give a higher charge carrier recombination rate at CdS/P3OT interface and larger short-circuit current density and energy conversion efficiency values in the corresponding solar cells, in comparison with the 80 °C deposited ones. This improvement could come from the reduction of charge carrier trap density inside grains as well as at grain boundaries in lower temperature deposited CdS films.

Investigation of Photocatalytic Activities over Bi2WO6/ZnWO4 Composite under UV Light and Its Photoinduced Charge Transfer Properties

Bi(2)WO(6)/ZnWO(4) composite photocatalysts have been successfully synthesized by a facile hydrothermal process. The catalysts were characterized by powder X-ray diffraction (XRD), transmission electron microcopy (TEM), and UV-vis diffuse reflectance spectrum (DRS). The results show that Bi(2)WO(6) nanoparticles grow on the primary ZnWO(4) nanorods. The Bi(2)WO(6)/ZnWO(4) composites have better UV light photocatalytic activities compared to single ZnWO(4) nanorods. Furthermore, the photoinduced charge transfer properties of Bi(2)WO(6)/ZnWO(4) composites were investigated by means of transient photovoltage (TPV) technique in detail. The interconnected interface of Bi(2)WO(6)/ZnWO(4) composites led to the low recombination ratios of photoinduced electron-hole pairs and enhanced photocatalytic activities.

Surface charge transfer properties of high-performance Ag-decorated ZnO photocatalysts

Silver-decorated ZnO (Ag/ZnO) chain-like nanoparticle aggregates were prepared by a facile nonaqueous sol–gel method. The influence of Ag decoration on the surface photo-induced charge transfer behaviour of ZnO was investigated by means of surface photovoltage spectroscopy, electric field-induced surface photovoltage spectroscopy and transient photovoltage techniques. The photovoltage spectra revealed that an electronic interaction between Ag and ZnO formed. Ag acted as electron acceptors and effectively inhibited the charge recombination in ZnO with UV light illumination, while under the illumination of visible light a redshift was observed due to the appearance of a plasmon-induced band resulting from the formation of Ag nanoparticle clusters. These were responsible for the significant enhancement in the photocatalytic activity of the Ag/ZnO sample. The results also suggested that the high-performance visible light photocatalytic activity could be manipulated by external biases.

A comparative study on plate-like and flower-like ZnO nanocrystals surface photovoltage property and photocatalytic activity

ZnO nanostructures materials with two-dimensional (2D) plate-like and three-dimensional (3D) flower-like morphologies have been synthesized via facile hydrothermal process. Scanning electron microscopy (SEM), high resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD) have been used to characterize the samples. We have compared the surface photoelectric properties of flower-like ZnO with that of plate-like ZnO by means of surface photovoltage (SPV), field-induced surface photovoltage (FISPV), transient photovoltage (TPV) and surface photocurrent (SPC) techniques. Based on the photovoltage responses, the influence of the morphology on transfer characteristics of photogenerated charge carriers was discussed. The synthesized flower-like ZnO exhibited more efficient separation and transfer properties of photogenerated charge carriers than that of plate-like ZnO in the UV region due to the special flower-like hierarchical nanostructures. We have also evaluated the photocatalytic activity of the ZnO samples by photodegradation efficiency of RhB. The experiments demonstrated that RhB in aqueous solution was more efficiently photodegraded using flower-like ZnO than plate-like ZnO. The results also show the consistency between the carrier dynamics of photogenerated electron–hole pairs and photocatalytic activity.

Macroporous V2O5−BiVO4 Composites: Effect of Heterojunction on the Behavior of Photogenerated Charges

Macroporous V2O5−BiVO4 composites with a heterojunction structure have been successfully synthesized under the assistance of colloidal carbon spheres. X-ray diffraction, Raman, and X-ray photoelectron spectroscopies reveal that the as-prepared composites are composed of monoclinic BiVO4 and orthorhombic V2O5. The behavior of photogenerated charges in V2O5, BiVO4, and the V2O5−BiVO4 composite have been investigated through surface photovoltage spectroscopy (SPS) and transient photovoltage (TPV) techniques. It is demonstrated that the formation of heterojunction structure in the V2O5−BiVO4 composite plays an important role in the kinetic behaviors (including separation, transport, and recombination) of photogenerated charges. The heterojunction greatly increases the separation extent and the lifetime of the photogenerated charges in the composites.

Carrier recombination losses in inverted polymer: Fullerene solar cells with ZnO hole-blocking layer from transient photovoltage and impedance spectroscopy techniques

In this study, full coincidence between impedance spectroscopy and transient photovoltage techniques in measuring recombination kinetics of photogenerated charge carriers in inverted polymer:fullerene organic solar cells with ZnO hole-blocking layer is reported. Carrier lifetime exhibits values at illumination intensities near 1 sun within the microseconds time scale. Photogenerated charge carrier density attains values within 1015–1016 cm−3. Decay kinetics is analyzed by means of a bimolecular recombination law with a recombination coefficient slightly dependent on the charge density, which lies within the order of k ∼ 10−12 cm3 s−1. It is also demonstrated that inverted-processed cells exhibit capacitance, recombination resistance, and lifetime parameters comparable to those extracted from regular cells, despite the great differences between the contact structures of these kinds of devices.