Retardation Of Recombination Research Articles

Photoresponse enhancing in nanostructured WO3 films by slight change in heating ambient

Providing solar hydrogen as a clean energy resource is one of the human challenges for future. Controlling oxygen vacancies as well as surface morphology in metal oxide semiconductors enables developing PEC H2 production in some understood ways. Here, the influence of simple change in annealing atmosphere, air and pure oxygen, on photoresponse of nanocrystalline WO3 has been studied completely. Results revealed that such slight change in annealing procedure increases effective surface interface and donor density by 77 and 72%, respectively. These effects and also retarding recombination of photogenerated electro-hole pair resulted in photocurrent enhancement under solar like illumination more than three times.

A Gallium Oxide-Graphene Oxide Hybrid Composite for Enhanced Photocatalytic Reaction.

Hybrid composites (HCs) made up of gallium oxide (GaO) and graphene oxide (GO) were investigated with the intent of enhancing a photocatalytic reaction under ultraviolet (UV) radiation. The material properties of both GaO and GO were preserved, even after the formation of the HCs. The incorporation of the GO into the GaO significantly enhanced the photocatalytic reaction, as indicated by the amount of methylene blue (MB) degradation. The improvements in the reaction were discussed in terms of increased surface area and the retarded recombination of generated charged carriers.

Promising photovoltaic application of multi-walled carbon nanotubes in perovskites solar cells for retarding recombination

With MWCNTs incorporating spiro-OMeTAD, it exhibits a Jsc of 22.13 mA cm−2 and PCE of 10.42% and the optical absorption performance, charge transfer and recombination performance were investigated in order to infer the origin of increasing Jsc and PCE.

Peripheral Hole Acceptor Moieties on an Organic Dye Improve Dye-Sensitized Solar Cell Performance.

Investigation of charge transfer dynamics in dye‐sensitized solar cells is of fundamental interest and the control of these dynamics is a key factor for developing more efficient solar cell devices. One possibility for attenuating losses through recombination between injected electrons and oxidized dye molecules is to move the positive charge further away from the metal oxide surface. For this purpose, a metal‐free dye named E6 is developed, in which the chromophore core is tethered to two external triphenylamine (TPA) units. After photoinduced electron injection into TiO2, the remaining hole is rapidly transferred to a peripheral TPA unit. Electron–hole recombination is slowed down by 30% compared to a reference dye without peripheral TPA units. Furthermore, it is found that the added TPA moieties improve the electron blocking effect of the dye, retarding recombination of electrons from TiO2 to the cobalt‐based electrolyte.

Enhanced photocatalytic activity of {1 0 1}-oriented bipyramidal TiO2 agglomerates through interparticle charge transfer

Uniformly packed dense TiO2 bipyramid agglomerates (BPA) with the dominantly exposed {101} facets were designed and synthesized by using a simple solvothermal process. From scanning electron microscopy (SEM) image, BPA consists of uniformly and densely packed nanoparticles forming secondary mesopores. BPA structure was intentionally crushed by ball-milling to obtain c-BPA, the photoactivity of which was compared with BPA. The photocatalytic activities of as-prepared BPA and c-BPA were investigated for hydrogen production and 4-chlorophenol degradation and compared with P25 as a reference TiO2. The BPA showed much higher photocatalytic activities than those of c-BPA and P25 (randomly aggregated commercial TiO2 nanoparticles). The marked efficiency improvement in BPA can be ascribed to the noble nanostructural features: (i) the agglomerated bipyramid (BP) units suppressing the electron-hole recombination through efficient interparticle charge transfer, (ii) the well-developed mesopores allowing the efficient diffusion of reactants and products, (iii) the highly {101}-oriented BP units that help efficient charge separation, and (iv) a good crystallinity of the BP units. These superior properties of BPA were also confirmed in their photoelectrochemical behaviors: the electrode of BPA exhibited the highest photocurrent under UV light, the slowest decay of open-circuit potential after turning off the light, and the lowest charge transfer resistance from the electrochemical impedance spectroscopic (EIS) measurement, which implies the retarded recombination of charge pairs and efficient interparticle charge transfer on BPA. The controlled agglomerated structure of the nano-sized TiO2 can enhance the photocatalytic and photoelectrochemical properties through efficient interparticle charge transfer.

Understanding the role of the dye/oxide interface via SnO2-based MK-2 dye-sensitized solar cells.

To understand the role of the dye/oxide interface, a model system using a nanocrystalline SnO2 and 3-hexyl thiophene based MK-2 dye is proposed. A thin interfacial TiO2 blocking layer (IBL) is introduced in between SnO2 and MK-2 and its effects on photocurrent-voltage, electron transport-recombination, and density of states (DOS) are systematically investigated. Compared to the bare SnO2 film, the insertion of IBL leads to a 14-fold improvement in the power conversion efficiency (PCE) despite little change in the dye adsorption amount, which is due to the 7-fold and 2-fold increase in the photocurrent density and voltage, respectively. The charge collection efficiency is substantially improved from 38% to 96% mainly due to the increase in the electron lifetime. The IBL is also found to enhance the dye regeneration efficiency as confirmed by the 15-fold faster dye bleaching recovery dynamics. The recombination resistance increases and the DOS decreases after surface modification of SnO2, which is responsible for the doubly increased voltage. This study suggests that the interfacial layer between the oxide and the dye plays a crucial role in retarding recombination, improving charge collection efficiency, increasing diffusion length, accelerating dye regeneration and narrowing the density of states.

Molecular Design and Photovoltaic Performance of Organic Dyes Containing Phenothiazine for Dye-Sensitized Solar Cells

We synthesized novel organic photosensitizers based on fluorine-substituted phenothiazine with thiophene bridge units in the chromophore for application in dye-sensitized solar cells (DSSCs). Furthermore, organic dyes with different acceptors exhibited higher molar extinction coefficients, and better light absorption at longer wavelengths. The photovoltaic properties of organic dyes composed of different acceptors in their chromophores were measured to identify their effects on the DSSC performance. The organic dye, PFSCN2 containing multi-cyanoacrylic acid as the electron acceptor, showed a power conversion efficiency of 4.67% under AM 1.5 illumination (100 mW/cm2). The retarded recombination kinetics from TiO2 electrode to electrolyte enhanced the electron life time of the organic dye, PFSCN2 in the photoanode of the DSSC. This was confirmed with impedance analysis.

Surface chemical and photocatalytic consequences of Ca-doping of BiFeO3 as probed by XPS and H2O2 decomposition studies

Pure and Ca-doped Bi1−xCaxFeO3 samples were prepared with x=0.0–0.2, adopting a sol–gel method. Previously reported studies performed on similarly composed and prepared samples revealed that Ca-doping, above solubility limit (namely at ≥10%-Ca), results in phase separation and formation of BiFeO3/α(γ)-Fe2O3 nanocomposite particles. Hetero p/n nanojunctions thus established were considered to help separating photo-generated electron–hole pairs and, therefore, explain consequent promotion of photo-Fenton catalytic activity of BiFeO3 towards methylene blue degradation in presence of H2O2 additive. However, the encompassed decomposition of H2O2 was not addressed. To bridge this gap of knowledge, the present investigation was designed to assess Ca-doping-effected surface chemical modifications and gauge its impact on the heterogeneous photo-/thermo-catalytic activity of BiFeO3 towards H2O2 decomposition, by means of X-ray photoelectron spectroscopy (XPS) and H2O2 decomposition gravimetry. XPS results revealed generation of high binding energy Bi 4f and Fe 2p states, as well as enhancement of the surface basicity, upon doping to 10%-Ca. These surface chemical consequences are rendered hardly detectable upon further increase of the dopant magnitude to 20%-Ca. In parallel, the H2O2 decomposition activity of the ferrite, under natural visible light, is enhanced to optimize upon Ca-doping at 10%, and, then, decreased on further doping to 20%. H2O2 decomposition experiments carried out in absence of light indicate that the doping promoting impact is reflected essentially in the photocatalytic activity. Accordingly, the observed surface chemical consequences of Ca-doping are considered to consolidate the p/n nanojunctions consequently established in the material bulk, by retarding recombination of visible light generated electron–hole pairs, thus enhancing the heterogeneous photocatalytic activity of BiFeO3.

Visible light photocatalysis of fullerol-complexed TiO2 enhanced by Nb doping

Visible light photocatalysis by TiO2 nanoparticles modified with both fullerol complexation and Nb-doping (fullerol/Nb–TiO2) demonstrated an enhanced performance. Nb-doped TiO2 (Nb-TiO2) was firstly prepared by a conventional sol–gel method, and subsequently fullerol was adsorbed on the surface of Nb–TiO2. The physicochemical and optical properties of as-prepared fullerol/Nb-TiO2 were analyzed by various spectroscopic methods (TEM, EELS, XPS, and DRS). The adsorption of fullerol on Nb–TiO2 surface increased the visible light absorption through a surface-complex charge-transfer (SCCT) mechanism. Nb-doping enhanced the charge transport and induced the Ti cation vacancies that retarded the recombination of photo-generated charge pairs by trapping the electrons injected from the HOMO level of fullerol. Due to the advantage of simultaneous modification of fullerol and Nb-doping, the visible light photoactivity of fullerol/Nb–TiO2 was more enhanced than either Nb–TiO2 or fullerol/TiO2. The photocatalytic activities of fullerol/Nb–TiO2 for the reduction of chromate (CrVI), the oxidation of iodide, and the degradation of 4-chlorophenol were all higher than bare TiO2 and singly modified TiO2 (i.e., Nb–TiO2 and fullerol/TiO2) under visible light (λ>420nm). A similar result was also confirmed for their photoelectrochemical behavior: the electrode made of fullerol/Nb–TiO2 exhibited an enhanced photocurrent under visible light. On the other hand, the decay of open-circuit potential of the fullerol/Nb–TiO2 electrode after turning off the visible light was markedly slower than either that of Nb–TiO2 or fullerol/TiO2, which implies the retarded recombination of photo-generated charge pairs on fullerol/Nb–TiO2. In addition, the electrochemical impedance spectroscopic (EIS) data supported that the charge transfer resistance is lower with the fullerol/Nb–TiO2 than either Nb–TiO2 or fullerol/TiO2. This specific combination of the bulk (Nb-doping) and surface (fullerol complexation) modifications of titanium dioxide might be extended to other cases of bulk+surface combined modifications.

Montmorillonite as bifunctional buffer layer material for hybrid perovskite solar cells with protection from corrosion and retarding recombination

Perovskite corrosion by TBP was proved, and montmorillonite (MMT) was used as dual-functional buffer layer to prevent corrosion and retard electron recombination.

Graphene oxide as dual functional interface modifier for improving wettability and retarding recombination in hybrid perovskite solar cells

GO as an amphiphilic modifier enhances the selective contact between perovskite and the HTL to improve the photovoltaic performance of perovskite-sensitized solar cells.

Prompting electron transport in mesoporous semiconductor electrode by simple film compression

Tuning materials packing in the photoanode films through a simple cold isostatic pressing (CIP) post-treatment allows highly efficient charge transport and a dramatically retarded recombination that are witnessed by electrochemical impedance spectroscopy (EIS) studies. The pressing post-treatment produces densely packed TiO 2 nanoparticle film with fewer relatively large vacancies and better inter-contacts between neighbouring nanoparticles while sustaining the specific surface area and pore size distribution of the mesoporous film. Characterisations of the sensitised photoanodes reveal that the prompted electron transport greatly contributes to a significant enhancement on overall solar energy conversion efficiency (PCE) by up to 11.7%. The demonstrated advantages from the simple but innovative approach are not limited to dye-sensitised solar cell applications and can benefit broad range developments for highly efficient devices, sensors and reactors using mesoporous semiconductor nanoparticle electrode films.

Polyoxometalate/TiO2 Interfacial Layer with the Function of Accelerating Electron Transfer and Retarding Recombination for Dye-Sensitized Solar Cells

A new H3PW12O40 (PW12)-based interfacial layer for dye-sensitized solar cells (DSSCs) has been fabricated by the LBL method. The features of the interfacial layer were tested by IR and UV–vis spectroscopies and AFM. The cells were systemically tested by the photocurrent–voltage (J–V) curve, dark-current measurement, open-circuit voltage decay (OCVD), and monochromatic incident photon-to-photocurrent conversion efficiency (IPCE) techniques. The PW12-based interfacial layer was found to accelerate electron transfer and retard recombination, eventually leading to an efficient increase in energy conversion efficiency. The investigations indicate that the energy conversion efficiency of a (PW12/TiO2)3-based DSSC is significantly enhanced by 54% at 100 mW cm–2 compared with a DSSC with no treatment and by 20% compared with a TiCl4-treatment DSSC. Polyoxometalate was first introduced into the interfacial layer in modifying the photoanode to accelerate electron transfer and retard recombination to improve DSSC efficiency in this work.

Time-resolved infrared spectroscopic investigation of roles of valence states of Cr in (La,Cr)-doped SrTiO3 photocatalysts

The kinetics of photogenerated electrons in SrTiO 3 (La,Cr) pretreated with either H 2 or O 2 were studied using time-resolved infrared spectroscopy. The X-ray photoelectron and Raman spectra showed that the Cr cations in the sample reduced with H 2 were all in Cr 3+ , whereas those oxidized with O 2 were in mixed of Cr 3+ and Cr 6+ . Electrons excited with 355 and 532 nm light pulses showed the absorption of mid-IR light, and this was traced as a function of the time delay in a microsecond domain. The time-resolved results revealed that the decay rate of the photoinduced electrons with Cr 3+ was slower than that with Cr 6+ , implying that trivalent Cr contributed more to retarding recombination of photoinduced electrons and holes, and enhanced photocatalytic H 2 production activity. The Cr valence state plays an important role in H 2 production under irradiation in the visible light region. Electrons are quickly trapped by Cr 6+ ions and cannot contribute to H 2 production.

Role of Interparticle Charge Transfers in Agglomerated Photocatalyst Nanoparticles: Demonstration in Aqueous Suspension of Dye-Sensitized TiO2.

The interparticle charge transfer within the agglomerates of TiO2 nanoparticles in slurries markedly enhanced the dye-sensitized production of H2 under visible light. By purposely decoupling the light absorbing part of Dye/TiO2 from the active catalytic center of Pt/TiO2, the role of bare TiO2 nanoparticles working as a mediator that connects the above two parts in the agglomerates was investigated systematically. The presence of mediator in the agglomerate facilitated the charge separation and the electron transfer from Dye/TiO2 to Pt/TiO2 through multiple grain boundaries and subsequently produced more hydrogen. The dye-sensitized reduction of Cr(VI) to Cr(III) was also enhanced when Dye/TiO2 nanoparticles were agglomerated with bare TiO2 nanoparticles. The charge recombination between the oxidized dye and the injected electron was retarded in the presence of bare TiO2 nanoparticles, and this retarded recombination on Dye/TiO2 was confirmed by using transient laser spectroscopy. This phenomenon can be rationalized in terms of an interparticle Fermi level gradient within the agglomerates, which drives the charge separation.

Structure and Photocatalysis of TiO<sub>2</sub>/ZnO Double-Layer Film Prepared by Pulsed Laser Deposition

Nanocrystalline double-layer film of TiO2/ZnO was prepared by pulsed laser deposition on quartz substrate and their microstructure, optical property and photocatalysis were investigated. The first/bottom ZnO layer grew on its [001] direction of the wurtzite structure, while the second/top TiO2 layer consisted of nanorods grown on the [001] direction of its anatase structure. The TiO2/ZnO film showed improved photoabsorption in visible light in the UV­visible absorption spectrum; and showed weaken UV emission and enhanced deep level emission in the photoluminescence spectrum. By the degradation of methyl orange and methylene blue, the TiO2/ZnO film exhibited evident enhanced photodegradation efficiency in a wide wavelength range. That can be explained by the enhanced wavelength response in visible range, more effective use of irradiation light and the retarded recombination of electron­hole pairs in the double layer films. [doi:10.2320/matertrans.M2011345]

Photovoltaic performance enhancement of dye-sensitized solar cells by formation of blocking layers via molecular electrostatic effect

The improvement in photovoltaic performance of dye-sensitized solar cells was found after aging in the dark and was analyzed by linear sweep voltammetry and electrochemical impedance spectroscopy. The promotion was found to arise from the formation of blocking layers on the surface of nanocrystalline TiO 2, resulting most likely from the intermolecular electrostatic action between the 4- tert-butylpyridine and the 1,2-dimethyl-3-propylimidazolium ions. These blocking layers can retard the interfacial reaction of the electron with I 3 − ions without deteriorating the rate of regeneration of the oxidized dye molecules. By virtue of the blocking layers, the retarding recombination of electrons with the I 3 − ions significantly increased the electron lifetime and enlarged the electron diffusion length, resulting in a higher open-circuit voltage and an improvement in charge collection efficiency, and eventually an enhancement of the energy conversion efficiency.

Molecular Design and Photovoltaic Performances of Organic Dyes Containing Triphenylamine for Dye-Sensitized Solar Cell

Organic dyes containing multi-acceptors/anchors in a chromophore were synthesized for use in a dye-sensitized solar cell. The photovoltaic properties of organic dyes composed of different acceptor in their chromophores were measured to identify the effects on the DSSC performance. The organic dye, 4 containing multi-cyanoacrylic acid as the electron acceptor showed a power conversion efficiency of 4.7% under AM 1.5 illumination (100 mW cm−2) in an photo active area of 0.24 cm2, short circuit current density of 12.9 mA cm−2, open circuit photo voltage of 0.62 V and a fill-factor of 60%. The retarded recombination kinetics from TiO2 electrode to electrolyte enhanced the electron life time of organic dye, 4 in the photoanode in DSSC that was well confirmed with the impedance analysis.

Effect of Conducting Ability of Electrolytes on the Photovoltaic Performance of Quasi-Solid State Dye-Sensitized Solar Cells

DSSCs based on quasi-solid type of electrolytes were prepared to investigate the influence of ionic mobility and recombination kinetics in electrolytes on DSSC performances. The improved ionic mobility of electrolyte in PMMA system resulted in the enhancement of Jsc value in DSSC which was well confirmed with the incident photon-to-current spectra and the impedance analysis. Furthermore, the retarded recombination rate from TiO2 electrode to electrolyte in PMMA system enhanced the electron life time of DSSC in the photoanode. DSSCs based on quasi-solid type of electrolytes composed of PMMA showed a power conversion efficiency of 3.36% underAM 1.5 illumination (100 mWcm−2) in an photo active area of 0.24 cm2, short circuit current density of 7.69 mAcm−2, open circuit photo voltage of 0.69 Vandafill-factor of 64%.

Charge separation in water molecule clusters under thermal fluctuations: 2. Ionization-recombination equilibrium

The dependence of the mean force potential of interaction between H3O+ and OH− ions in water molecule clusters is calculated by the Monte Carlo method in a grand canonical statistical ensemble at temperature of 273 K within a broad range of vapor pressures corresponding to natural atmospheric conditions. A high (up to 200k BT) barrier, which resulted in the retardation of recombination in clusters, as well as the accumulation of nonrecombined ion pairs, is revealed based on the dependence of the mean force potential on the interionic distance. Using parameters calculated by the Monte Carlo method, we solved the kinetic equation for the stationary regime of the production and recombination of ion pairs and evaluated the relative concentration of ion pairs and relaxation time, which are consistent with experimental values and explain the enhanced sensitivity of electric properties of water vapors to the sources of ionizing radiation observed in experiments.