Dye-sensitized Semiconductor Research Articles

ELECTRON INJECTION AT DYE-SENSITIZED SEMICONDUCTOR ELECTRODES

Electron injection at dye-sensitized semiconductors is reviewed. Particular emphasis is placed on theoretical and photoelectrochemical studies of dye-sensitized planar and single-crystal electrodes. The accepted mechanism of electron injection, which was derived from these classical studies, is introduced. Selected photoelectrochemical studies of dye-sensitized nanocrystalline semiconductors are reviewed; emphasis is given to factors that influence the efficiencies of electron injection and charge recombination. The development of quasi-solid-state nanocrystalline dye-sensitized solar cells is also discussed. Recent time-resolved spectroscopic studies of electron injection and charge recombination are reviewed. These studies have led to a better understanding of electron injection mechanisms, and have revealed the limitations of the classical models.

Ab Initio Nonadiabatic Molecular Dynamics of the Ultrafast Electron Injection across the Alizarin−TiO2 Interface

The observed 6-fs photoinduced electron transfer (ET) from the alizarin chromophore into the TiO2 surface is investigated by ab initio nonadiabatic (NA) molecular dynamics in real time and at the atomistic level of detail. The system derives from the dye-sensitized semiconductor Grätzel cell and addresses the problems of an organic/inorganic interface that are commonly encountered in photovoltaics, photochemistry, and molecular electronics. In contrast to the typical Grätzel cell systems, where molecular donors are in resonance with a high density of semiconductor acceptor states, TiO2 sensitized with alizarin presents a novel case in which the molecular photoexcited state is at the edge of the conduction band (CB). The high level ab initio analysis of the optical absorption spectrum supports this observation. Thermal fluctuations of atomic coordinates are particularly important both in generating a nonuniform distribution of photoexcited states and in driving the ET process. The NA simulation resolves the controversy regarding the origin of the ultrafast ET by showing that although ultrafast transfer is possible with the NA mechanism, it proceeds mostly adiabatically in the alizarin-TiO2 system. The simulation indicates that the electron is injected into a localized surface state within 8 fs and spreads into the bulk on a 100-fs or longer time scale. The molecular architecture seen in the alizarin-TiO2 system permits efficient electron injection into the edge of the CB by an adiabatic mechanism without the energy loss associated with injection high into the CB by a NA process.

Photopolymerization of methyl methacrylate using dye-sensitized semiconductor based photocatalyst

Photopolymerization of methyl methacrylate (MMA) was carried out using semiconductor (CdS) as a photocatalyst under visible light exposure. The influences of additives like eosin (dye) and triethylamine on the photochemical properties of semiconductor (CdS) particle and photopolymerization of MMA were studied. A 2–3% conversion was achieved on using only CdS as photocatalyst. On using an electron donor (Et 3N) along with CdS however a conversion of up to 14% was reached. The molecular weight in this case however was low. On carrying with eosin dye along with CdS, up to 22% yield of the polymer was obtained. The polymer so obtained was characterized by GPC. Polydispersity was found to be in the range of 6–6.5 for the polymer obtained by using dye as sensitized semiconductor catalyst and 4–4.5 for the polymer obtained using only dye as catalyst. The free radical polymerization of MMA has been established with EPR spectra.

Enzyme-assisted Reforming of Glucose to Hydrogen in a Photoelectrochemical Cell¶

Hydrogen gas has been produced by reforming glucose in a hybrid photoelectrochemical cell that couples a dye-sensitized nanoparticulate wide band gap semiconductor photoanode to the enzyme-based oxidation of glucose. A layer of porphyrin sensitizer is adsorbed to a TiO2 nanoparticulate aggregate sintered to a conducting glass substrate to form the photoanode. Excitation of the porphyrin results in electron injection into the TiO2, and migration to a microporous platinum cathode where hydrogen is produced by hydrogen ion reduction. The oxidized sensitizer dye is reduced by NADH, regenerating the dye and poising the NAD /NADH redox couple oxidizing. The NAD is recycled to NADH by the enzyme glucose dehydrogenase, which obtains the necessary electrons from oxidation of glucose. The reforming of glucose produces gluconolactone, which hydrolyzes to gluconate; the electrochemical potential necessary to overcome thermodynamic and kinetic barriers to hydrogen production by NADH is provided by light. The quantum yield of hydrogen is ∼2.5%.

Enzyme Assisted Reforming of Glucose to Hydrogen in a Photoelectrochemical Cell

Hydrogen gas has been produced by reforming glucose in a hybrid photoelectrochemical cell that couples a dye-sensitized nanoparticulate wide band gap semiconductor photoanode to the enzyme-based oxidation of glucose. A layer of porphyrin sensitizer is adsorbed to a TiO2 nanoparticulate aggregate sintered to a conducting glass substrate to form the photoanode. Excitation of the porphyrin results in electron injection into the TiO2, and migration to a microporous platinum cathode where hydrogen is produced by hydrogen ion reduction. The oxidized sensitizer dye is reduced by NADH, regenerating the dye and poising the NAD+/NADH redox couple oxidizing. The NAD+ is recycled to NADH by the enzyme glucose dehydrogenase, which obtains the necessary electrons from oxidation of glucose. The reforming of glucose produces gluconolactone, which hydrolyzes to gluconate; the electrochemical potential necessary to overcome thermodynamic and kinetic barriers to hydrogen production by NADH is provided by light. The quantum yield of hydrogen is approximately 2.5%.

The photocapacitor: An efficient self-charging capacitor for direct storage of solar energy

A light-driven self-charging capacitor was fabricated as an efficient solar energy storage device. The device, which we name the photocapacitor, achieves in situ storage of visible light energy as an electrical power at high quantum conversion efficiency. The photocapacitor was constructed on a multilayered photoelectrode comprising dye-sensitized semiconductor nanoparticles/hole-trapping layer/activated carbon particles in contact with an organic electrolyte solution, in which photogenerated charges are stored at the electric double layer. Repeated charge-discharge cycles with a charging voltage of >0.45V yielded a capacitance of 0.69Fcm−2.

Porphyrin-sensitized nanoparticulate TiO2 as the photoanode of a hybrid photoelectrochemical biofuel cell.

Porphyrin-sensitized nanoparticulate TiO(2) on conducting glass has been investigated as a photoanode material for a new cell that converts light energy into electricity. The cell is a hybrid of a dye-sensitized nanoparticulate semiconductor photoelectrochemical solar cell, and a biofuel cell that oxidizes glucose. Porphyrin molecules excited by light inject electrons into the photoanode, from where they enter the external circuit. The resulting porphyrin radical cations are reduced by NADH in aqueous buffer, ultimately regenerating the photoanode and producing NAD(+). Glucose dehydrogenase oxidizes glucose, and in the process recycles NAD(+) back to NADH. The photoanode is coupled with a suitable cathode to make a functioning cell (Hg/Hg(2)SO(4) was employed for evaluation purposes). The cell produces 1.1 V at open circuit and has a fill factor of 0.61. These values are both significantly higher than those for a previously reported cell of a similar type based on an SnO(2) electrode.

Kinetics and mechanism of electron injection and charge recombination in dye-sensitized nanocrystalline semiconductors

Kinetics and mechanism of electron injection and charge recombination in dye-sensitized nanocrystalline semiconductors

Photoinduced intra-molecular electron transfer in aniline-containing Ru(II) polypyridine complexes

The photoinduced intra-molecular electron transfer in three aniline-containing Ru(II) polypyridine complexes, [Ru(bpy) 2(bpy-bmam)](PF 6) 2 ( 2), Ru(dcbpy)(bpy-bmam)(NCS) 2 ( 3), and Ru(dcbpy)(bpy-mam)(NCS) 2 ( 4) (bpy-bmam=4,4′-bis( N-methyl-anilinomethyl)-2,2′-bipyridine, bpy-mam=4-methyl-4′-( N-methyl-anilinomethyl)-2,2′-bipyridine, dcbpy=4,4′-dicarboxyl-2,2′-bipyridine, bpy=2,2′-bipyridine), was investigated using steady-state and time-resolved absorption and emission spectra, and photoinduced inter-molecular electron transfer between [Ru(bpy) 2(dmbpy)](PF 6) 2 ( 1) (dmbpy=4,4′-dimethyl-2,2′-bipyridine) and N, N-dimethylaniline was also studied for comparison. While inter-molecular reductive quenching of 1’s emission by aniline was less efficient, nearly half emission was quenched intra-molecularly in 2. Upon photoinduced electron injection onto the conduction band of TiO 2 from 3 or 4 anchored on the TiO 2 surface, intra-molecular electron transfer from aniline moiety to Ru(III) orbital occurred rapidly with rate constants of 3.4×10 7 and 2.6×10 7 s −1, respectively, restricting the interfacial recombination of injected electrons with Ru(III) and as a result improving the photoelectrochemical behavior of 3 or 4 when used in dye-sensitized nanocrystalline semiconductor solar cells.

Current Density versus Potential Characteristics of Dye-Sensitized Nanostructured Semiconductor Photoelectrodes. 2. Simulations

The impact of changes in various parameters on the steady-state current density−potential (J−E) characteristics of dye-sensitized nanostructured semiconductor photoelectrodes has been evaluated through a series of simulations. The model parameters can be divided into three classes, designated as type I, type II, and type III, respectively. Type I parameters primarily affect the open-circuit potential without changing the overall shape of the J−E curves. Type II parameters primarily affect the limiting quantum yield for photocurrent production. Rate constants for injection and quenching of the excited state of the dye by the contacting phase are type II parameters, whereas the rate constant for the direct electrolyte reduction reaction, the reduced equilibrium constant for iodine formation, and the rate constant for recombination are type I parameters. The rate constant for regeneration affects both the shape of the J−E curves and the limiting quantum yield for photocurrent flow, and it is therefore designated as a type III parameter. Variation of the diffusion coefficient parameter for electrons in the semiconducting membrane produces a mixed response having both type I and II characteristics. Comparisons between simulations and experimental data have delineated the factors that should be manipulated to increase the energy conversion efficiency of photoelectrochemical cells that utilize nanostructured TiO_2 electrodes.

Current Density versus Potential Characteristics of Dye-Sensitized Nanostructured Semiconductor Photoelectrodes. 1. Analytical Expressions

A closed-form analytical model is developed to describe the steady-state current density−potential (J−E) characteristics of dye-sensitized nanostructured semiconductor photoelectrodes. The basic components of the model are a set of differential equations that describe the generation, recombination, and transport of charge carriers in mesoporous semiconductor electrode systems. Charge-carrier transport is treated as a diffusion process, and semiclassical Marcus theory is used to describe the kinetics at the interfaces between the semiconductor and the contacting phase as well as the kinetics at the interfaces with adsorbed dye. The model relates explicitly, within a single formalism, the rate constants for charge transfer of the mesoporous membrane electrode system to conventional intramolecular and intermolecular electron-transfer rate constant expressions and to interfacial electron-transfer processes at planar metal or semiconductor electrodes. The near-equilibrium situation is considered by including t...

Photophysics and Electron Dynamics in Dye-Sensitized Semiconductor Film Studied by Time-Resolved Mid-IR Spectroscopy

Photophysics and electron dynamics in dye-sensitized semiconductor film were studied by transient mid-IR spectroscopy, in particular on the nano- to millisecond time scale. As sensitizers, a Ru com...

Mechanism of Charge Recombination in Dye-Sensitized Nanocrystalline Semiconductors: Random Flight Model

The trap-filling effect on the kinetics of charge recombination in dye-sensitized TiO2 nanoparticles is discussed on the basis of the random flight model. The model assumes that electrons move betw...

Dye-sensitized composite semiconductor nanostructures

Understanding of the charge transport and recombination mechanisms of dye-sensitized solar cells based on semiconductor nanostructures is essential for the improvement of their performance. A great deal of information on these systems have been obtained from studies on a single material (mostly TiO 2 and to a lesser extent ZnO and SnO 2). We have conducted extensive measurements on composite dye-sensitized nanosturctures and found that the composite systems possess unusual properties. Dye-sensitized photoelectrochemical cells made from nanocrystalline films of some materials (e.g., SnO 2) yield comparatively small open-circuit voltages and energy and quantum conversion efficiencies, despite excellent dye-semiconductor interaction. However, on deposition of ultra-thin shells of insulators or high band gap semiconductors on the crystallites, a dramatic increase in the above parameters is observed. Outer shells were found to have insignificant or in most cases a negative effect on TiO 2 films. We explain the above findings on the basis of vast differences in the leakage rates of trapped electrons in different materials which is sensitive to the effective electron mass. Electrons injected to the conduction band in dye-sensitization enter into shallow traps from which they get thermally reemitted to the conduction band. The building up of the electron quasi-fermi level and transport depends on this process. The spread of the hydrogenic wave function of a trapped electron increases inverse exponentially with the effective mass so that the electron leakage and their recombination with acceptors ‘outside’ become severe when the crystallite size is comparable to the Bohr radius of the trapped electron. Such recombinations are effectively suppressed by deposition of thin films on the crystallites. Excited dye molecules anchored to the outer shell injects electrons to the conduction band via tunneling.

Dye-Sensitized Nanocrystalline TiO2 Solar Cells Based on Ruthenium(II) Phenanthroline Complex Photosensitizers

We have synthesized four carboxylated Ru(II) phenanthroline complexes with different numbers of carboxyl groups, cis-bis(4,7-dicarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(dcphen)2(NCS)2), cis-bis(4-monocarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(mcphen)2(NCS)2), cis-(4,7-dicarboxy-1,10-phenanthroline)(1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(dcphen)(phen)(NCS)2), and cis-(4-monocarboxy-1,10-phenanthroline)(1,10-phenanthroline)dithiocyanato ruthenium(II) (Ru(mcphen)(phen)(NCS)2), as photosensitizers for oxide semiconductor solar cells. We have studied photovoltaic properties of dye-sensitized nanocrystalline semiconductor solar cells based on Ru phenanthroline complexes and an iodine redox electrolyte. The photovoltaic performance of the TiO2 solar cell sensitized by Ru(dcphen)2(NCS)2(TBA)2 exceeded that of ZnO, SnO2, and In2O3 solar cells. A solar energy to electricity conversion efficiency (η) of 6.6% was obtained under the standard AM 1.5 irradiation (100 mW cm...

Significant effects of the distance between the cyanine dye skeleton and the semiconductor surface on the photoelectrochemical properties of dye-sensitized porous semiconductor electrodes

The incident photon-to-current conversion efficiency (IPCE) of a porous TiO2 electrode sensitized by cyanine dyes increased with decreasing distance between the skeleton of the dye and the TiO2 surface. The photocurrent of oxide semiconductor electrodes sensitized by a cyanine dye increased with the positive shift of the conduction band potential of the oxide semiconductor in the following order: Nb2O5<TiO2<ZnO<SnO2. The SnO2 semiconductor cell showed the best light-to-electric conversion efficiency among the four semiconductors.

Highly efficient photon-to-electron conversion with mercurochrome-sensitized nanoporous oxide semiconductor solar cells

Dye-sensitized solar cells based on nanoporous oxide semiconductor thin films such as TiO 2, Nb 2O 5, ZnO, SnO 2, and In 2O 3 with mercurochrome as the sensitizer were investigated. Photovoltaic performance of the solar cell depended remarkably on the semiconductor materials. Mercurochrome can convert visible light in the range of 400–600 nm to electrons. A high incident photon-to-current efficiency (IPCE), 69%, was obtained at 510 nm for a mercurochrome-sensitized ZnO solar cell with an I −/I 3 − redox electrolyte. The solar energy conversion efficiency under AM1.5 (99 mW cm −2) reached 2.5% with a short-circuit photocurrent density ( J sc) of 7.44 mA cm −2, a open-circuit photovoltage ( V oc) of 0.52 V, and a fill factor (ff) of 0.64. The J sc for the cell increased with increasing thickness of semiconductor thin films due to increasing amount of dye, while the V oc decreased due to increasing of loss of injected electrons due to recombination and the rate constant for reverse reaction. Dependence of photovoltaic performance of mercurochrome-sensitized solar cells on semiconductor particles, light intensity, and irradiation time were also investigated. High performance of mercurochrome-sensitized ZnO solar cells indicate that the combination of dye and semiconductor is very important for highly efficient dye-sensitized solar cells and mercurochrome is one of the best sensitizers for nanoporous ZnO photoelectrode. In addition, a possibility of organic dye-sensitized oxide semiconductor solar cells has been proposed as well as one using metal complexes.

The possibility of ballistic electron transport in dye-sensitized semiconductor nanocrystalline particle aggregates

A dye-sensitized photoelectrochemical cell made from a nano-porous composite film consisting of tin(IV) and zinc oxides generates exceptionally high photocurrents at an optimum mixing ratio of the two oxides. It is suggested that this phenomenon originates from ballistic transport of electrons injected by the excited dye molecules along the interconnected chains of tin(IV) oxide nanocrystallites to a zinc oxide crystallite.

Band Edge Movement and Recombination Kinetics in Dye-Sensitized Nanocrystalline TiO2 Solar Cells: A Study by Intensity Modulated Photovoltage Spectroscopy

The charge-recombination kinetics and band edge movement in dye-sensitized nanocrystalline TiO2 solar cells are investigated by intensity modulated photovoltage spectroscopy (IMVS). A theoretical model of IMVS for dye-sensitized nanocrystalline semiconductor electrodes is developed, and analytical expressions for the frequency dependence of the photovoltage response at open circuit are derived. The model considers charge trapping/detrapping and electron transfer from the conduction band and surface states of the semiconductor to redox species at the solid/solution interface. IMVS is shown to be valuable in elucidating the contributions of band edge shift and recombination kinetics to changes of the open-circuit photovoltage (Voc) resulting from surface modifications of the semiconductor. IMVS measurements indicate that surface treatment of [RuL2(NCS)2] (L = 2,2‘-bipyridyl-4,4‘-dicarboxylic acid)-sensitized TiO2 electrodes with 4-tert-butylpyridine or ammonia leads to a significant band edge shift concomit...

Ultrafast Studies of Imaging Processes

Interfacial electron transfer is the most fundamental process driving all imaging technologies. The advent of ultrafast lasers has led to the development of novel experimental techniques to probe such dynamics. This article presents five time domain spectroscopies that allow direct measurement of different segments of the electron trajectory across a heterogeneous interface. Electro-optic sampling measures field-assisted transport of carriers to the surface. Time-resolved two-photon photoemission enables measurement of electron relaxation at surfaces. Timecorrelated single-photon counting, transient grating, and transient absorption techniques are implemented to determine electron transfer rates at interfaces. With these real-time approaches, the primary photophysical and photochemical processes at semiconductor/liquid interfaces and dye-sensitized semiconductors can be studied directly. The new information forthcoming from such studies is that electron transfer processes can be extremely fast at surfaces, in a range approaching adiabatic coupling conditions between the delocalized bond states and discrete molecular donor or acceptor states. This observation leads to a new conceptual framework for understanding photoinduced interfacial charge transfer processes.