Enhanced Photocurrent Generation Research Articles

Enhanced Photocurrent Generation of Binary Self-Assembled Monolayers of Di-(3-aminopropyl)-Viologen and Methylviologen on Indium Tin Oxide

The binary self-assembled monolayers (SAMs) of di-(3-aminopropyl)-viologen (DAPV) and methylviologen (MV) molecules on indium tin oxide (ITO) were prepared by dipping the DAPV SAMs/ITO substrates into MV solution. The DAPV-MV SAM films were characterized by UV-vis. absorption spectroscopy, Rutherford backscattering spectroscopy, and cyclic voltammetry. Optical band gap, lowest unoccupied molecular orbital, and highest occupied molecular orbital of DAPV-MV SAMs were measured to be 1.6, -4.3, and -5.9 eV, respectively. We found that although DAPV SAMs have a quantum yield of 0.11%, the binary SAM films have a good quantum yield of 2.30%, which was 20 times higher than that of DAPV SAMs on ITO. This result may be due to the higher adsorption property of the binary SAMs for the light in visible range compared to that of DAPV SAMs. From this study, we demonstrated that the photocurrent generation systems with a high quantum yield can be obtained by the functional binary SAMs.

Preparation of CdS/Graphene Composites and Photocatalytic Hydrogen Generation from Water under Visible Light Irradiation

CdS/graphene composite photocatalysts were prepared by photocatalytically reducing graphene oxide with CdS nanoparticles in an aqueous ethanol solution. The structure and photoelectrical properties of the resulted materials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and transient photocurrent measurements. The photocatalytic activities of the CdS/graphene composites for hydrogen evolution were evaluated under visible light irradiation (λ≥420 nm). The results show that the graphene oxide can be efficiently reduced by the photogenerated electrons of CdS and thus CdS/graphene composite is formed and it shows strong interactions between CdS and graphene. Compared with CdS, the enhanced photocurrent generation and photocatalytic activity toward hydrogen evolution for the CdS/ graphene composite photocatalysts could be attributed to the ability of graphene to capture and transport electrons, and to promote charge separation.

Enhanced photo-conversion efficiency of CdSe–ZnS core–shell quantum dots with Aunanoparticles on TiO2electrodes

This study demonstrates that configurations that include gold nanoparticles (Au NPs) as the primary layer attached to a TiO2 electrode with quantum dots (QDs) as the secondary layer have superior photoelectrochemical properties. We found that all Au–QD nanoparticle hybrid systems reveal enhanced photocurrent generation as compared to only a QD–nanoparticle interface. The enhanced Jsc is attributed to sensitization that results indirectly from quantum dots becoming attached to Au NPs by intramolecular charge transfers. As a result of the improved performance, the overall energy conversion efficiency was increased by 100% as compared to that of a reference cell without Au NPs at 100 mW cm−2. When (CdSe)ZnS is prepared from Au-coated TiO2, incident photon-to-current efficiency values approaching 20% can be achieved by (CdSe)ZnS devices. In the electrochemical impedance spectroscopy results, the intermediate frequency region of the Au–QD cells was more significantly reduced compared to that in bare QD cells due to the enhanced charge separation that occurs in the Au–QD structure. Mott–Schottky (C−2–ϕ) analysis shows that the lowest acceptor and donor densities having a positive effect on the efficiency could be found in (CdSe)ZnS–Au/TiO2 cells. Therefore, (CdSe)ZnS–Au/TiO2 has a very thin depletion layer that is restricted to the surface of Au/TiO2. Consequently, this system can be employed to enhance the effective efficiency in the design of the QD-sensitized solar cells.

Enhanced photocurrent generation from UV-laser-synthesized-single-wall-carbon-nanotubes/n-silicon hybrid planar devices

We report on the significant generation of photocurrent (PC) from planar devices built from the drop casting of UV-laser-synthesized single-wall-carbon-nanotubes (SWCNTs) onto n-Si substrate. These SWCNTs/n-Si hybrid devices are shown to generate PC with external quantum efficiencies (EQE) reaching up to ∼10%. Their EQE has been optimized by controlling the amount of deposited SWCNTs, and is shown to be significantly enhanced over all the spectral range with a pronounced boost (up to ∼25× times) around 460 nm. The extension of the photoresponse of these devices toward UV correlates well with the absorbance of SWCNTs.

Enhanced Photocurrent Generation in Nanostructured Chromophore/Carbon Nanotube Hybrid Layer-by-Layer Multilayers

In this paper, we demonstrate photocurrent generation from nanostructured layer-by-layer (LbL) ultrathin films consisting of chromophores and single-walled carbon nanotubes (SWNT). We fabricated 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetr (p-toluenesulfonate) (TMPyP)-SWNT/sodium copper chlorophyllin (SCC)-SWNT LbL film from noncovalently adsorbed composites. SWNT were dissolved in water-soluble cationic TMPyP and anionic SCC, and the resulting solutions were used for electrostatic LbL multilayer fabrication. The solubility of SWNT in water was studied by UV−vis absorption spectroscopy. The composites were highly dispersed owing to the π−π interactions. The fluorescence spectroscopy measurements showed efficient quenching of TMPyP and SCC fluorescence, which was due to the interaction with SWNT. In situ surface plasmon resonance spectroscopy during the LbL multilayer fabrication indicated a stepwise increase in reflectivity, implying the successive formation of nanostructured hybrid ultrathin films. Cyclic voltammetry revealed that the electroactivity of the hybrid film was enhanced by the incorporation of SWNT. The composite LbL film electrode exhibited an enhancement of photocurrent compared to a TMPyP/SCC (no SWNT) film electrode, suggesting efficient charge separation and electron transfer in the system.

Photoelectrochemical properties of donor-acceptor nanocomposite films composed of porphyrin-functionalized cup-shaped nanocarbon materials

Porphyrin-functionalized cup-shaped nanocarbons (CNC-H2P) have been assembled onto nanostructured SnO2 films using an electrophoretic deposition method to examine the photoelectrochemical properties. The obtained CNC-H2P nanohybrid films were examined by a series of steady-state and time-resolved spectroscopic measurements and photoelectrochemical measurements. The resulting nanohybrid film afforded drastic enhancement in the photoelectrochemical performance as well as broader photoresponse in the visible region as compared with the reference CNC system without porphyrins. The enhancement of photocurrent generation may be caused by the efficient electron injection from the long-lived charge-separated state of CNC-H2P upon photoexcitation. This feature makes cup-shaped nanocarbon materials a useful candidate for developing efficient photoelectrochemical and photovoltaic cells.

Synthesis and Characterization of Silver Nanoparticles and Titanium Oxide Nanofibers: Toward Multifibrous Nanocomposites

A new method was investigated to produce new multiscale fibrous nanocomposites comprised of titanium oxide (TiO2) nanofibers and silver (Ag) nanoparticles (NPs). The process involved electrospinning TiO2 precursor solution containing colloidal solution of Ag NPs, and organic solvent (dimethyl-n′n-formamide) to fabricate a porous, nonwoven, free-standing nanofiber mesh. Postprocess heating of the electrospun nanofibers entailed calcination in air environment at 500°C for 3 h. Microemulsion processing was used to generate NPs of Ag in a monodispersed distribution throughout the colloidal solution. X-ray diffraction data were consistent with the anatase phase of TiO2, while transmission electron microscopy and hydrogen desorption measurements revealed a very porous microstructure. It was demonstrated that NP colloidal stability is solvent dependent. It is anticipated that incorporation of metal particles in nanofibers will lead to enhanced photocurrent generation, when used in functional devices.

Enormous enhancement in photocurrent generation using electrochemically fabricated goldnanostructures

A self-assembled monolayer of porphyrins fabricated on the surface of an electrochemically deposited gold nanostructure exhibits enormous enhancement of photocurrent due to porphyrin excitation, especially in the near-infrared region, where localized surface plasmon resonance was responsible.

Got TiO2 Nanotubes? Lithium Ion Intercalation Can Boost Their Photoelectrochemical Performance

Cations such as H(+) and Li(+) are intercalated into TiO(2) nanotube arrays by subjecting them to short-term electrochemical pulses at controlled potentials (<-1.0 V vs Ag/AgCl). The intercalation of these small cations has a profound effect toward enhancing photocurrent generation under UV light irradiation. A nearly three-fold increase in the photoconversion efficiency (IPCE) was observed upon intercalation of Li(+) ions into TiO(2) nanotube arrays. The intercalation process is visualized by the color change from gray to blue. Spectroelectrochemical measurements were carried out to monitor the absorption changes at different applied potentials. The analysis of the V(oc) decay following termination of UV light shows a significant decrease in the rate of recombination of accumulated electrons upon Li(+) ion intercalation.

Porphyrin-modified electrodes for solar energy conversion

This mini review presents our recent developments in porphyrin-modified electrodes for solar energy conversion. Various porphyrins have been assembled on nanostructured semiconducting electrodes to achieve efficient photocurrent generation. First, porphyrins have been organized with fullerenes onto semiconducting electrodes to elucidate the relationship between the molecular structures, film structures, and photoelectrochemical properties of the modified electrodes. Formation of hole and electron-transporting highways in the porphyrin/fullerene composite film led to the remarkable enhancement of photocurrent generation. Second, porphyrin-modified single-walled carbon nanotubes have also been assembled onto semiconducting electrodes. The degree of chemical functionalization by the bulky porphyrins was found to have a large impact on the photoelectrochemical properties. Third, asymmetrically π-elongated porphyrins have been successfully employed in dye-sensitized solar cells to improve the cell performance as well as the light-harvesting properties. The power conversion efficiency of the fused porphyrin cell was improved by 50% compared to the reference cell using the corresponding unfused porphyrin. Finally, carboxyquinoxalino derivatives of zinc porphyrin have been further developed to extend the concept of asymmetrically π-elongated porphyrins. The maximum power conversion efficiency of 5.2% was obtained by using 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)-6′-carboxyquinoxalino[2,3-b]porphyrinatozinc(II).

Enhanced Photocurrent Generation in Self‐Assembled Monolayers Formed at Plasmonic Gold Nanostructures

AbstractThe effect of localized surface plasmon resonance (SPR) appearing at the surfaces of gold nanoparticles (AuPs) was successfully applied for the enhancement of photocurrents from porphyrin (Po) immobilized on AuPs. The electrode consisting of AuPs with different sizes (∼15 or ∼35 nm) was prepared using the AuP film formed at the liquid/liquid interface, and then Po was self‐assembled on the gold surface. The SPR effect for the AuP film was verified from the attenuated total reflection SPR and absorption measurements. Photocurrents from the modified electrodes were compared with the corresponding planar electrode. Appreciable enhancement of photocurrents was observed.

Enhanced Photocurrent Generation by Electron Hopping through Regularly-Arranged Chromophores in a Helical Peptide Monolayer

Novel 310-helical peptides having a disulfide group at the N-terminal and carrying naphthyl groups at the side chains in a linear arrangement were synthesized, those are 9mer, 18mer, and 27mer peptides with three, six, and nine naphthyl groups, respectively. Spectroscopic measurements of the peptide solutions showed that there was no interaction between the neighboring naphthyl groups. 310-Helical conformation was confirmed by circular dichroism spectroscopy and computational molecular modeling. The peptides were self-assembled on gold by an Au-S linkage, and the defect sites in the self-assembled monolayers (SAMs) were filled up with dodecanethiol. Cyclic voltammetry, ellipsometry, and infrared reflection-absorption spectroscopy studies on the SAMs showed that the peptides formed the densely-packed monolayers with vertical orientation. Upon photoexcitation of the naphthyl groups in an aqueous solution containing electron donors, the monolayer generated an anodic photocurrent by photoinduced electron transfer from the electron donor via the naphthyl groups to gold. The longest 27mer peptide generated the largest photocurrent among the peptides. Theoretical simulations successfully demonstrated that the helix scaffold with many naphthyl groups in a linear arrangement was effective to suppress quenching of the excited naphthyl group by gold due to the elongation of the helix length, and to promote electron hopping among the naphthyl groups enabling long-range electron transfer from the distant naphthyl group to gold, eventually realizing an efficient photocurrent generation molecular system.

Enhancement of photocurrent generation and open circuit voltage in dye-sensitized solar cells using Li+ trapping species in the gel electrolyte

The influence of the addition of 12-crown-4 ether in a gel polymer electrolyte based to a PEO copolymer and its application in dye sensitized solar cells were investigated. Introduction of these Li+ trapping species brought beneficial contributions to both V(oc) and J(sc) values, increasing the device's performance.

Photoelectrochemistry of Mesoporous NiO Electrodes in Iodide/Triiodide Electrolytes

Mesoporous NiO electrodes were used in photoelectrochemical cells using iodide/triiodide-based redox electrolytes. Cathodic photocurrents were found upon visible light excitation. These photocurrents were enhanced when coumarin 343 dye was adsorbed at the NiO surface or when a unanchored dye (coumarin 337) was added to the electrolyte. The proposed reaction for photocurrent generation is as follows: in the absence of dyes, excitation of triiodide leads to the generation of diiodide radicals, which subsequently accept electrons from the NiO electrode. Enhanced photocurrent generation in the presence of dyes is possibly due to energy transfer from excited dye molecules to triiodide. Charge transport studies of C343-sensitized NiO suggest that hole transport in mesoporous NiO is due to hopping of positive charge between Ni-surface atoms.

A Dyadic Sensitizer for Dye Solar Cells with High Energy‐Transfer Efficiency in the Device

A new bichromophoric dyad based on an alkyl-functionalized aminonaphthalimide as energy-donor chromophore and [Ru(dcbpy)2(acac)]Cl (dcbpy=4,4'-dicarboxybipyridine, acac=acetylacetonato) as energy acceptor and sensitizing chromophore is synthesized. Efficient quenching of the donor-chromophore emission is observed in solution, presumably due to resonant energy transfer. This dyad is then used as a sensitizer in a dye solar cell. By comparing the spectral properties of transparent dye solar cells sensitized with the dyad and [Ru(dcbpy)2(acac)]Cl, it is possible to demonstrate that photons absorbed by the donor moiety also contribute significantly to the generation of current. Instead of using acceptor luminescence as a probe, enhanced photocurrent generation is employed to estimate the energy-transfer efficiency. Fitting theoretical to experimental external quantum efficiency functions gives a value for the energy-transfer efficiency of 85 %. Evaluation of the maximum output power of dye solar cells sensitized with the dyad and [Ru(dcbpy)2(acac)]Cl showed, under selective illumination at the absorption maximum of the donor chromophore, that the introduction of the energy-donor moiety leads to a significant increase in the monochromatic maximum output power under blue illumination. This result demonstrates the usefulness of energy transfer for the generation of current in dye-sensitized solar cells.

Kinetic competition in liquid electrolyte and solid-state cyanine dye sensitized solar cells

The photovoltaic performance of liquid electrolyte and solid-state dye sensitized solar cells, employing a squarilium methoxy cyanide dye, are evaluated in terms of interfacial electron transfer kinetics. Dye adsorption to the metal oxide film resulted in a mixed population of aggregated and monomeric sensitizer dyes. Emission quenching data, coupled with transient absorption studies, indicate that efficient electron injection was only achieved by the monomeric dyes, with the aggregated dye population having an injection yield an order of magnitude lower. In liquid electrolyte devices, transient absorption studies indicate that photocurrent generation is further limited by slow kinetics of the regeneration of monomeric dye cations by the iodide/iodine redox couple. The regeneration dynamics are observed to be too slow (≫ 100 µs) to compete effectively with the recombination of injected electrons with dye cations. In contrast, for solid-state devices employing the organic hole conductor spiro-OMeTAD, the regeneration dynamics are fast enough (≪ 1 µs) to compete effectively with this recombination reaction, resulting in enhanced photocurrent generation.

Enhancement of Photocurrent Generation by Honeycomb Structures in Organic Thin Films

A light-emitting poly (distyryldimethylbenzene-co-triethylene glycol) rod-coil block copolymer was used to fabricate films with three-dimensionally ordered honeycomb structures by the breath-figure method. Photocurrent generation and photovoltaic performance are studied, and the dependence of photocurrent on applied electric field is investigated. Introducing the ordered porous structure significantly improves the photoelectric conversion behavior, because porous structures not only enhance the light-harvesting efficiency but also benefit charge separation and charge transfer. This phenomenon may have great prospects for enhancing the photovoltaic behavior of organic thin-film devices.

Enhanced Photoelectrochemistry in CdS/Au Nanoparticle Bilayers

AbstractThree different configurations of Au‐nanoparticle/CdS‐nanoparticle arrays are organized on Au/quartz electrodes for enhanced photocurrent generation. In one configuration, Au‐nanoparticles are covalently linked to the electrode and the CdS‐nanoparticles are covalently linked to the bare Au‐nanoparticle assembly. The resulting photocurrent, φ = 7.5 %, is ca. 9‐fold higher than the photocurrent originating from a CdS‐nanoparticle layer that lacks the Au‐nanoparticles, φ = 0.8 %. The enhanced photocurrent in the Au/CdS nanoparticle array is attributed to effective charge separation of the electron–hole pair by the injection of conduction‐band electrons from the CdS‐ to the Au‐nanoparticles. Two other configurations involving electrostatically stabilized bipyridinium‐crosslinked Au/CdS or CdS/Au nanoparticle arrays were assembled on the Au/quartz crystal. The photocurrent quantum yields in the two systems are φ = 10 % and φ = 5 %, respectively. The photocurrents in control systems that include electrostatically bridged Au/CdS or CdS/Au nanoparticles by oligocationic units that lack electron‐acceptor units are substantially lower than the values observed in the analogous bipyridinium‐bridged systems. The enhanced photocurrents in the bipyridinium‐crosslinked systems is attributed to the stepwise electron transfer of conduction‐band electrons to the Au‐nanoparticles by the bipyridinium relay bridge, a process that stabilizes the electron–hole pair against recombination and leads to effective charge separation.

Enhancement of Photocurrent Generation by ITO Electrodes Modified Chemically with Self-Assembled Monolayers of Porphyrin–Fullerene Dyads

Porphyrin–fullerene linked systems (see Figure), which form self-assembled monolayers on ITO electrodes, exhibit enhanced photocurrent generation: This is due to a two-step electron transfer protocol—first intramolecularly from 1P to C60, then intermolecularly from the resulting C60• – to the electron carriers.

Enhanced photocurrent generation and photooxidation of benzene sulfonate in a continuous flow reactor using hybrid TiO2 thin films immobilized on OTE electrodes

Abstract The enhancement of photocurrent and photodegradation of organic pollutants was investigated using hybrid TiO 2 thin films prepared by a combined paste/sol–gel technique to immobilize the TiO 2 particles onto optically transparent electrodes (OTEs); they were connected in series in a continuous flow, bench-top, two-compartment photoreactor. The benzene sulfonate (BS) of the model substrate surfactant used was photodegraded in aqueous media to test the efficacy of the TiO 2 /OTE electrode preparation and the construction of the photoreactor. Results indicate that the hybrid TiO 2 film/OTE electrodes were more efficient than either the pasted TiO 2 /OTE electrode or the sol–gel TiO 2 /OTE electrode for the photodegradation of BS, as evidenced by the evolution of carbon dioxide and the photocurrent generated. The extent of ring opening and generated photocurrent during the photodegradation of BS with the hybrid (HY) electrode was correlated to the pasted TiO 2 electrode and to the number of dip-coatings in the sol–gel. High photocurrents were generated in the cylindrical large photoreactor under continuous flow of the aqueous BS solution. Optimization of experimental factors for optimal efficiency of the setup was addressed with regard to the preparation of the HY electrodes, and to their size and location in the photoreactor.