Photocurrent Action Spectra Research Articles

ChemInform Abstract: Structure and Photoelectrochemical Properties of Semiconducting Rhenium Cluster Chalcogenides: Re6X8Br2 (X: S, Se).

Abstract Single crystals of Re 6 Se 8 Br 2 and Re 6 S 8 Br 2 were synthesized by chemical vapour transport reactions. The latter was crystallized. X-ray structure analysis reveals that Re 6 S 8 Br 2 is isostructural to Re 6 Se 8 Br 2 . Both semiconducting cluster materials were photoelectrochemically characterized and showed p-type behaviour. From the photocurrent action spectra, their estimated band gaps are 0.84 eV and 1.65 eV for the selenide and sulphide, respectively, with a probably indirect optical transition.

Reaction of RuII Diazafluorenone Compound with Nanocrystalline TiO2 Thin Film

The Ru(II) compounds [Ru(bpy)(2)(mcbH)](2+) and [Ru(bpy)(2)(dafo)](2+), bpy is 2,2'-bipyridine where mcbH is 3-(CO(2)H)-2,2'-bipyridine and dafo is 4,5-diazafluoren-9-one, were synthesized, characterized, and anchored to nanocrystalline mesoporous TiO(2) thin films for excited state and interfacial electron transfer studies. X-ray crystallographic studies of [Ru(bpy)(2)(mcbH)](PF(6))(Cl) revealed a long Ru-N distance to the unsubstituted pyridine ligand of mcbH. Reaction of [Ru(bpy)(2)(dafo)](2+) with TiO(2) thin films resulted in interfacial chemistry. The IR, (1)H NMR, UV-vis, and photoluminescence spectral data indicated a room-temperature ring-opening reaction of the dafo ligand of [Ru(bpy)(2)(dafo)](2+) that ultimately yielded a carboxylate group in the 3-position of bipyridine anchored to TiO(2). Comparative reactions of [Ru(bpy)(2)(mcbH)](2+) with TiO(2) were performed and support this conclusion. In regenerative photoelectrochemical solar cells with 0.5 M LiI/0.05 M I(2) in acetonitrile, photocurrent action spectra were observed for both sensitized materials. The incident photon-to-current efficiency (IPCE) was significantly lower for Ru(bpy)(2)(dafo)/TiO(2), behavior attributed to a lower excited-state injection yield.

Transport and Interfacial Transfer of Electrons in Dye-Sensitized Solar Cells Utilizing a Co(dbbip)2 Redox Shuttle

The transport and interfacial transfer of electrons in dye-sensitized solar cells utilizing the Co(dbbip)2 (dbbip = 2,6-bis(1′-butylbenzimidazol-2′-yl)pyridine) redox couple as an alternative to the conventional I3−/I− couple have been investigated using intensity modulated photocurrent and photovoltage spectroscopy (IMPS/IMVS) combined with in situ near IR absorption spectroscopy. Attempts to use impedance spectroscopy to determine the electron diffusion length were unsuccessful due to overlap of the cathode and electron transport impedances. Values of the electron diffusion length in the range 5−8 μm were derived by IMPS/IMVS as well as by analysis of the ratio of the normalized photocurrent action spectra measured for illumination through the counter electrode and through the TiO2 electrode. These values indicate that loss of electrons by electron transfer to the Co(III) species will be important for TiO2 films thicker than about 5 μm, unless steps are taken to passivate the surface to retard back electron transfer.

Observation of catalytic properties of CdS–ZnO composite nanorods synthesized by aqueous chemical growth technique

We have synthesized the nanorods of CdS–ZnO composite by chemical method and have investigated their structure, optical and electrical properties by suitable techniques. The X-rays diffraction results prove the co-existence of CdS and ZnO in usual hexagonal structure and no peak corresponding to any intermediate phase has been identified. The above observation has been confirmed by micro-Raman study. The photo-current action spectra of CdS–ZnO nanorods show a significant enhancement of the photo-current due to visible sensitization by surface CdS layer. In an attempt to explore the catalytic activity of CdS–ZnO, the photodegradation of 3,4-dihydroxy benzoic acid has been investigated under visible irradiation from a halogen lamp. The concentration of aqueous solution of 3,4-dihydroxy benzoic acid decreases with increase in the irradiation time which is attributed to photodegradation of the benzoic acid. A possible mechanism for the redox reactions responsible for degradation of the benzoic acid has been suggested.

Gel probe photocurrent measurement of cuprous oxide films

We describe a new method and gel probe for measuring photocurrent action spectra of cuprous oxide thin films, without the need for assembling solar cells or using a liquid electrolyte cell. Thus, external quantum efficiency of photoactive films can be measured directly without coating a transparent conducting oxide layer or using metal evaporation for contacts. With an aqueous gel, we observe dissolution of the CuO and Cu 2O layers, causing photocurrent decay. Replacement of the aqueous gel electrolyte with a non-aqueous gel electrolyte significantly stabilized photocurrents. A mechanism is proposed to account for all the electrochemical observations.

Electrochemical and Photoelectrochemical Investigation of New Electrostatic Self-Assembled Films Based on Prussian Blue and a Binuclear Ru(II) Complex

New hybrid multilayer films containing Prussian blue (PB) and a binuclear Ru(II) complex were successfully fabricated using the electrostatic layer-by-layer self-assembly technique. The process was carefully monitored by ultraviolet-visible (UV-vis) spectroscopy. A linear increase in the UV-vis absorbance with the number of deposited layers indicated that the film deposition was uniform and reproducible. The electrochemical and photoelectrochemical properties of these hybrid multilayer films were investigated in an aqueous Na2SO4 solution. PB and the binuclear Ru(II) complex assembled in the film were redox active with surface confined characteristics. The surface concentration and molecular area of RuL in the film were calculated according to a UV-vis absorption spectrum and cyclic voltammetry, respectively. The photocurrent generated by switching on the light irradiation of the film increased linearly as the deposited bilayers increased. The photocurrent action spectrum was in agreement with metal-to-ligand charge-transfer (MLCT) band of RuL in the absorption spectrum, which indicated that the photocurrent was generated based on MLCT excitation of the RuL in the (PB/RuL) film.

Improved photovoltaic properties of pentacene/ N, N′-Dioctyl-3,4,9,10-perylenedicarboximide-based organic heterojunctions with thermal annealing

We have studied the effect of thermal annealing on the photovoltaic properties of an organic solar cell based on an efficient donor/acceptor combination of pentacene / N, N′-Dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C 8) discrete heterojunctions. As deposited samples showed a maximum power conversion efficiency about 1.12% under standard A.M 1.5 illumination, whereas 1.60% power conversion efficiency has been achieved after annealing the samples at 100 °C for 5 min. External quantum efficiency (EQE) is also found to be increased from∼60% to∼70% after heat treatment. Photocurrent action spectra and the absorption profile of this donor/acceptor combination confirm the efficient light harvesting capability of the device throughout the visible region of the solar spectrum. Morphology and crystallinity of the active layers improved significantly after annealing the samples, resulting in an almost two times enhancement in the short-circuit current density and improved overall cell performances.

Photovoltaic properties of bisazomethine dye thin films

Photovoltaic properties of bisazomethine dye (DE2) in vacuum-deposited thin films were examined for use in Schottky-type photovoltaic cells (ITO/DE2/Al). We measured dark and photo current–voltage characteristics and determined photocurrent action spectra. We confirmed the p-type semiconductor property of DE2 thin films evidenced by the formation of the Schottky barrier at the DE2/Al interface. We further found a relatively high incident photon to electron conversion efficiency comparable to that of metal phthalocyanine thin films in devices having the same structure. These results demonstrate that DE2 thin film is a candidate for a photocarrier-generating material in organic solar cells.

Fabrication of dye-sensitized solar cells using natural dye for food pigment: Monascus yellow

A food pigment (Monascus yellow) extracted from Monascus fermentations (red yeast rice) has been studied as a novel sensitizing dye for dye-sensitized solar cells (DSCs). The photocurrent action spectrum was in agreement with the absorption spectrum of the dye adsorbed on a nanocrystalline-TiO2 electrode. The DSC fabrication process has been optimized in terms of the rinsing solvent used after dye adsorption and the dye-uptake duration. After optimizing nanocrystalline-TiO2 electrodes and pH conditions, the resulting maximal photovoltaic characteristics were an open-circuit voltage of 0.57 V, a short-circuit current of 6.1 mA cm−2, and a fill factor of 0.66, yielding an energy conversion efficiency of 2.3%.

Influence of the electrolyte cation in organic dye-sensitized solar cells: lithium versus dimethylimidazolium

We investigate the influence of electrolyte cations such as lithium and dimethylimidazolium on the spectroscopic and electrical characteristics of dye-sensitized solar cells based on a metal-free chromophore C218. An evident bathochromic effect of lithium with respect to dimethylimidazolium is noticed for the C218 dye-coated nanocrystalline titania film via measuring electronic absorption and photocurrent action spectra. In comparison with dimethylimidazolium, the use of lithium as the electrolyte cation evokes a downward shift of the excited-state redox potential of the C218 sensitizer by 120 mV, and that of the conduction band edge of the nanocrystalline TiO2 film by 390 meV. The resultant remarkable variation in the energy alignment at the titania/dye interface brings on dissimilar electron injection yields as revealed by transient emission measurements. Modulating the titania surface states distribution with lithium relative to dimethylimidazolium induces a slightly declining electron diffusion coefficient in the mesoporous titania film. However, the adsorption of lithium cations on titania strongly retards the interfacial charge recombination compared to dimethylimidazolium, contributing to an over one order of magnitude of enhancement of electron diffusion length. The improvement of electron diffusion length has a noticeable effect on the charge collection yield, which can also be observed by measuring monochromatic incident photon-to-electron conversion efficiencies. Further interface engineering is needed to enhance both the photovoltage and photocurrent, making use of the full power of metal-free organic dyes.

Mechanism of aerobic visible light formic acid oxidation catalyzed by poly(tri-s-triazine) modified titania

Visible light aerial oxidation of formic acid catalyzed by N/C-modified titania (TiO(2)-N,C) is investigated by wavelength-dependent photocatalytic and photoelectrochemical experiments in the presence of oxygen, tetranitromethane, and methylviologen as electron acceptors. The title reaction is shown to proceed both through oxidative and reductive primary processes. Contrary to the urea-derived (TiO(2)-N,C), so-called "N-doped" titania (TiO(2)-N) as prepared from ammonia is inactive. In accord with photocurrent action spectra of corresponding powder electrodes, this different activity of the two photocatalysts is traced back to the different chemical nature of the reactive holes localized at the modifier. Hole stabilization by delocalization within an extended poly(tri-s-triazine) network of TiO(2)-N,C is proposed to render recombination with conduction band electrons less probable than in TiO(2)-N.

Implementation of redox gradients in hydrogen bonded complexes containing N,N-dimethylaniline, flavin and fullerene derivatives

A variety of supramolecular complexes were formed by associating an electron donor-substituted flavin dyad (10-[4′-(N,N-dimethylamino)phenyl]isoalloxazine: DMA–Fl) and a family of fullerene derivatives that contain single and double hydrogen bond receptors (SRC60 and DRC60). The stoichiometry of the corresponding complexes, that is, DMA–Fl and SRC60 or DRC60, were examined by Job's plot analysis of the absorption changes linked to DMA–Fl at 470 nm. To this end, 1 : 1 and 1 : 2 complex stoichiometries were determined for DMA–Fl-SRC60 and (DMA–Fl)2-DRC60, respectively. Molecular aggregates in the form of remarkably stable clusters were formed by the fast injection method. Insights into these clusters came from transmission electron microscopy (TEM) measurements, which revealed network configurations for (DMA–Fl-SRC60)n with diameters in the range between 200 and 500 nm, while uniform nanoparticles of about 40 nm diameter were discernible for [(DMA–Fl)2-DRC60]n. The highly colored composite clusters were assembled onto an optically transparent electrode covered with nanostructured SnO2 films by the electrophoretic deposition technique. Important are the photocurrent action spectra providing maximum IPCE values that are twice as high for [(DMA–Fl)2-DRC60]n than for (DMA–Fl-SRC60)n. To complement these studies the dynamics of electron transfer were investigated by femtosecond laser flash photolysis in the supramolecular clusters (i.e., (DMA–Fl-SRC60)n and [(DMA–Fl)2-DRC60]n). As a matter of fact, for the first time we were able to demonstrate the unidirectional electron transfer from the terminal electron donor (N,N-dimethylaniline) moiety to the terminal electron acceptor (fullerene) moiety.

Influence of Lithium Ion Concentration on Electron Injection, Transport, and Recombination in Dye-Sensitized Solar Cells

Varying Li+ concentration in the electrolyte of dye-sensitized solar cells equipped with compact TiO2 blocking layers is found to alter the mean slopes of semilogarithmic open-circuit photovoltage−intensity and dark current−voltage plots. Almost identical values of ideality factor or transfer coefficient are required to fit data in the dark and under illumination for each Li+ concentration. It is found that cell characteristics become progressively more “ideal” as Li+ concentration is increased, with a transfer coefficient of ca.1 for 1 M Li+ in the electrolyte. We find that trends in photovoltage−intensity data are well fitted using a model which assumes that electron transfer to acceptor species in the electrolyte occurs from both the conduction band of the TiO2 and an exponential distribution of band gap surface states. Changes in the mean ideality factor and linearity of semilogarithmic photovoltage−intensity plots can be rationalized by considering the variation in overlap between occupied donor states (conduction band and surface states) with electron acceptor states in the electrolyte, as the conduction band edge is shifted positive by increasing Li+ concentration. In accordance with previous studies, this positive shift in conduction band edge is also found to cause a dramatic increase in the photocurrent generation efficiency of the cells, especially in the long-wavelength region of the photocurrent action spectrum. It is argued that this improvement in photocurrent is predominantly due to an increase in wavelength-dependent electron injection efficiency, as opposed to an increase in electron collection efficiency.

Spectrally selected photodiodes for colorimetric application

We demonstrate that it is possible to fine-tune the photocurrent action spectrum of an organic photodiode, based onto a prototype semiconducting polymer, simply varying the active layer thickness and properly exploiting the filter effect of the optical absorption. The theory adopted to model the experimental data represents a useful tool to predict with accuracy the position and the spectral shape of the photovoltaic current. Based on this result, we apply our method in the colorimetric field, where a simple way to tune the spectral response of the detector unit is highly desired: we realize an organic color sensor device without any optical filtering.

Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer Blends

AbstractHere, studies on the evolution of photophysics and device performance with annealing of blends of poly(3‐hexylthiophene) with the two polyfluorene copolymers poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthien‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2′′‐diyl) (F8TBT) and poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) are reported. In blends with F8TBT, P3HT is found to reorganize at low annealing temperatures (100 °C or below), evidenced by a redshift of both absorption and photoluminescence (PL), and by a decrease in PL lifetime. Annealing to 140 °C, however, is found to optimize device performance, accompanied by an increase in PL efficiency and lifetime. Grazing‐incidence small‐angle X‐ray scattering is also performed to study the evolution in film nanomorphology with annealing, with the 140 °C‐annealed film showing enhanced phase separation. It is concluded that reorganization of P3HT alone is not sufficient to optimize device performance but must also be accompanied by a coarsening of the morphology to promote charge separation. The shape of the photocurrent action spectra of P3HT:F8TBT devices is also studied, aided by optical modeling of the absorption spectrum of the blend in a device structure. Changes in the shape of the photocurrent action spectra with annealing are observed, and these are attributed to changes in the relative contribution of each polymer to photocurrent as morphology and polymer conformation evolve. In particular, in as‐spun films from xylene, photocurrent is preferentially generated from ordered P3HT segments attributed to the increased charge separation efficiency in ordered P3HT compared to disordered P3HT. For optimized devices, photocurrent is efficiently generated from both P3HT and F8TBT. In contrast to blends with F8TBT, P3HT is only found to reorganize in blends with F8BT at annealing temperatures of over 200 °C. The low efficiency of the P3HT:F8BT system can then be attributed to poor charge generation and separation efficiencies that result from the failure of P3HT to reorganize.

Charge transport at hybrid bulk heterojunction based on CdS nanopillar arrays embedded in a conducting polymer

Charge transport in hybrid devices based on a quasiordered heterojunction consisting of a CdS film with nanopillarlike nanostructures embedded in a poly[2-methoxy-5-(20ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) layer was investigated. Temperature-dependent photoluminescence spectra indicate that there exist two kinds of defects (sulfur vacancy and sulfur interstitial) in this CdS film, which act as electron and hole traps, respectively. Current (I)-voltage (V) characteristics in dark show that charge recombination is the main mechanism at the hybrid interface when Ohmic contacts are formed at CdS/indium tin oxide and MEH-PPV/Au interfaces. Photocurrent action spectra and I-V performance under illumination indicate that the recombination was enhanced at the interface due to the participation of the defects. In view of these realities, the light intensity dependence of the photocurrent and the photovoltage of the hybrid devices could be understood by a quantitative model including trap-assistant recombination.

Enhanced photoelectrocatalytic activity of FTO/WO3/BiVO4 electrode modified with gold nanoparticles for water oxidation under visible light irradiation

Abstract Gold nanoparticles were successfully deposited on FTO/WO3/BiVO4 electrode surface by means of electrolysis of AuCl4− ions. The composite films were characterized by SEM, XPS and XRD techniques. An increase in photocurrent and a negative shift of onset potential for water oxidation were observed upon modification of the electrode surface with the Au particles. The electrochemical impedance spectroscopy was used to confirm the acceleration of charge transfer process by Au deposition at the electrode surface. The photocurrent action spectrum did not correlate with the plasmonic absorbance of Au nanoparticles at 560 nm, suggesting that the Au nanoparticles increased charge separation without undergoing a plasmon resonance effect under visible light irradiation.

Organic thin-film solar cells with a Cu anode: Improvement of the photovoltaic properties on aging in air

The photovoltaic properties of a Ga-doped ZnO (GZO)/3,4,9,10-perylene-tetracarboxyl-bis-benzimidazole (PTCBI)/Zn-phthalocyanine (ZnPc)/Cu heterojunction cell (cell A) and a GZO/ZnPc/Cu Schottky-barrier cell (cell B) were investigated. The energy conversion efficiency η of cell A was only 0.02% immediately after the device preparation but improved to 0.46% after aging for 24 days in air. To elucidate the mechanism of this aging effect, photocurrent action and electro-absorption spectra were measured for cell B. The results reveal that a Schottky barrier exists at the ZnPc/Cu interface which blocks the transport of photogenerated holes to the Cu electrode, and the barrier height is reduced by a white-light illumination of the device after aging. The change in barrier height is attributed to the formation of electron traps at the surface of the ZnPc layer on aging that trap photogenerated electrons.

TiO 2 nanoparticles incorporated with CuInS 2 clusters: preparation and photocatalytic activity for degradation of 4-nitrophenol

TiO 2 nanoparticles incorporated with CuInS 2 clusters were prepared in a solvothermal process and characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersion X-ray analysis (EDX). Compared with pure TiO 2 nanoparticles, the TiO 2 nanoparticles incorporated with CuInS 2 clusters display higher photocatalytic activity with 99.9% of degradation ratio of 4-nitrophenol after 2 h irradiation. In order to investigate the effect of the CuInS 2 clusters on the photocatalytic activity of TiO 2 nanoparticles, diffuse reflectance UV–Vis spectra (DRS), photoluminescence (PL) spectra, and photocurrent action spectra were measured. The results indicate that the enhanced photocatalytic activity is probably due to the interface between TiO 2 and CuInS 2 as a trap of the photogenerated electrons to decrease the recombination of electrons and holes.

Plasmon-Induced Enhancement in Analytical Performance Based on Gold Nanoparticles Deposited on TiO2 Film

This paper demonstrates a novel approach for developing the analytical performance of electrochemical biosensors in which hydrogen peroxide (H(2)O(2)) is selected as a model target, based on surface plasmon resonance of gold nanoparticles (Au NPs) deposited onto a TiO(2) nanoneedle film. Direct electron transfer of cytochrome c (cyt. c) is realized at Au NPs deposited onto a TiO(2) nanoneedle film (Au/TiO(2) film), and both anodic and cathodic currents of the redox reaction at the Au/TiO(2) film upon visible-light irradiation are amplified. Meanwhile, in the presence of oxidized or reduced states of cyt. c, cathodic or anodic photocurrents are generated respectively by the Au/TiO(2) film, suggesting that the amplified anodic and cathodic currents are ascribed to the visible-light excitation. The photocurrent action spectrum obtained at the Au/TiO(2) film in the presence of cyt. c is in a good agreement with the surface plasmon absorption spectrum of Au NPs deposited onto the TiO(2) film, and maximum photocurrent is also consistent with the plasmon absorption peak of Au NPs themselves. It indicates that the enhanced photocurrents generated by visible-light irradiation are attributed to the surface plasmon resonance of Au NPs. On the other hand, experimental results reveal that cyt. c is stably immobilized onto the Au/TiO(2) film and maintains inherent enzymatic activity toward H(2)O(2) even under continuous visible-light illumination. The amplified redox currents of cyt. c produced by surface plasmon resonance of Au NPs, combined with the stability and enzymatic activity of cyt. c confined on the Au/TiO(2) film even after continuous visible-light illumination, subsequently provide the enhanced analytical performance in determination of H(2)O(2). The sensitivity of the present biosensor for H(2)O(2) is 4-fold larger than that obtained without visible-light irradiation, the detection limit is achieved to be 4.5 x 10(-8) M and the dynamic detection linear range extends from 1 x 10(-7) M to 1.2 x 10(-2) M.