Photovoltage Generation Research Articles

Surface photovoltage studies ofInxGa1−xAsandInxGa1−xAs1−yNyquantum well structures

Surface photovoltage in GaAs-based type I strained quantum well structures is discussed. First, a model for the photovoltage generation in our samples is presented. Then surface photovoltage spectra of two structures, GaAs/InGaAs/GaAs and GaAs/InGaAsN/GaAs, are analyzed. The samples show four steplike main features in the spectrum. The InGaAs sample shows also excitonic peaks associated with some of the steps. To explain the spectra, a comparison with calculations and photoluminescence measurements is made. For photon energies under the barrier band gap, the photovoltage is generated due to the optical absorption in the quantum well. To account for the magnitude of the photovoltage in this case, carriers must escape out of the quantum well. We propose and discuss three such carrier excitation-escape mechanisms. We discuss the role of an optical transition involving confined and extended states in the surface photovoltage spectra. By analyzing the temperature dependence of the spectra we come to the conclusion that the transitions involving extended states play an essential role in the photovoltage generation in our quantum well (QW) samples. This allows the determination of the band alignment of QW structures. We find that by adding 1,7% N in ${\mathrm{In}}_{0.35}{\mathrm{Ga}}_{0.65}\mathrm{As}$ strained quantum wells, the energies of the valence band maximum and conduction band minimum are lowered by 43 and 140 meV, respectively, at room temperature.

Cl − Concentration Dependence of Photovoltage Generation by Halorhodopsin from Halobacterium salinarum

The photovoltage generation by halorhodopsin from Halobacterium salinarum (shR) was examined by adsorbing shR-containing membranes onto a thin polymer film. The photovoltage consisted of two major components: one with a sub-millisecond range time constant and the other with a millisecond range time constant with different amplitudes, as previously reported. These components exhibited different Cl − concentration dependencies (0.1–9 M). We found that the time constant for the fast component was relatively independent of the Cl − concentration, whereas the time constant for the slow component increased sigmoidally at higher Cl − concentrations. The fast and the slow processes were attributed to charge (Cl −) movements within the protein and related to Cl − ejection, respectively. The laser photolysis studies of shR-membrane suspensions revealed that they corresponded to the formation and the decay of the N intermediate. The photovoltage amplitude of the slow component exhibited a distorted bell-shaped Cl − concentration dependence, and the Cl − concentration dependence of its time constant suggested a weak and highly cooperative Cl −-binding site(s) on the cytoplasmic side (apparent K D of ∼5 M and Hill coefficient ≥5). The Cl − concentration dependence of the photovoltage amplitude and the time constant for the slow process suggested a competition between spontaneous relaxation and ion translocation. The time constant for the relaxation was estimated to be >100 ms.

Micelle induced generation of photovoltage using Fluorescein dye

Photovoltage generation using Fluorescein dye in aqueous and aqueous-micellar medium at different pH has been studied in photogalvanic cell. The photopotential and photocurrent generated were maximum in non-ionic micellar medium. The observed solar energy efficiency in non-ionic micellar medium, diethylene glycol hexadecyl ether (Brij-52) was 0.06% and maximum power of the cell was 15.5 μw. The output of the cell was sensitive to surfactant nature. The rate constants of the reactions during photovoltage growth and decay were calculated. The thermodynamic properties of the cell were also discussed. A comparison of the micelle induced photovoltage generation of anionic dye Fluorescein with cationic dye Safranine T in different micellar media and at different pH has been presented.

Temperature dependence for the photovoltaic device parameters of polymer-fullerene solar cells under operating conditions

We report on the temperature dependence of various photovoltaic device parameters of solar cells, fabricated from interpenetrating networks of conjugated polymers with fullerenes, in the wide temperature range of their possible operating conditions (25–60 °C). The open-circuit voltage was found to decrease linearly with increasing temperature. For the short-circuit current, we observed a monotonic increase with increasing temperature, followed by a saturation region. The rate of this increase (coupled to a corresponding increase for the fill factor) was found to overtake the corresponding rate of decrease in voltage, resulting in an overall increase of the energy conversion efficiency. The efficiency was observed to reach a maximum value in the approximate range 47–60 °C. The results are discussed with respect to possible mechanisms for photovoltage generation and charge carrier transport in the conjugated polymer-fullerene composite, and in particular, thermally activated charge carrier mobility.

Potential Distribution and Photovoltage Origin in Nanostructured TiO2 Sensitization Solar Cells: An Interference Reflection Study

A well-defined modulation reflection spectrum due to a multiple interference process is originated in the TiO2 dye sensitizated solar cell (DSSC). Experimental evidence is shown that the interference process leading to the reflection spectrum takes place at the n-conducting F:SnO2(FTO) layer of the FTO/TiO2 back contact. Moreover, the interference reflectance spectrum is influenced by the applied potential and illumination bias and disappears when FTO is metallized with 10 A of platinum. These results show that FTO/TiO2 cannot be considered as an ohmic but as a rectifying contact where the FTO behaves as a highly doped n-type semiconductor which absorbs an important part of the equilibrium contact potential in the dark. On the basis of our experimental results a new insight on the role of the dark equilibrium contact potential at the FTO/TiO2 interface in the processes of electric charge separation and photovoltage generation is given. Evidence is shown that the theoretically maximum attainable photovolta...

Calcium-activated potassium current clamps the dark potential of vertebrate rods.

Vertebrate photoreceptors respond to light with a graded hyperpolarization from a membrane potential in the dark of approximately -35 mV. The present work investigates the physiological role of the Ca2+-activated K+ current in the photovoltage generation in mechanically isolated rods from salamander retina. Membrane current or voltage in isolated rods was recorded from light- and dark-adapted rods under voltage- or current-clamp conditions, respectively. In light-adapted rods of the salamander, selective blockade of Ca2+-activated K+ channels by means of charybdotoxin depolarized the plasma membrane of current-clamped rods by approximately 30 mV, from a resting potential of approximately -35 mV. A similar depolarization was observed if external Ca2+ (1 mM) was substituted with Ba2+ or Sr2+. Under control conditions, the injection of currents of increasing amplitude (up to -100 pA, to mimic the current entering the rod outer segment) could not depolarize the membrane potential beyond a saturating value of approximately -20 mV. However, in the presence of charybdotoxin, rods depolarized up to +20 mV. In experiments with dark-adapted current-clamped rods, charybdotoxin perfusion lead to transient depolarizations up to 0 mV and steady-state depolarizations of approximately 5 mV above the dark resting potential. Finally, the recovery phase of the voltage response to a flash of light in the presence of charybdotoxin showed a transient overshoot of the membrane potential. It was concluded that Ca2+-activated K+ current is necessary for clamping the rod photovoltage to values close to the dark potential, thus allowing faithful single photon detection and correct synaptic transmission.

Photophysics of thionine dye in aqueous and liposome media in presence of different reducing agents

The photoelectrochemical and spectral (both absorption and fluorescence) studies of thionine, a cationic phenothiazine dye, have been carried out in aqueous and phosphatidylcholine liposome media in the presence of different reducing agents, such as I −, Br −, Cl − and Fe 2+. The results show that the photovoltage generation from photoelectrochemical studies and Stern-Volmer quenching constant studied by fluorescence quenching support the photoinduced electron transfer from the reducing agent to the singlet excited thionine dye. Moreover, a good correlation between photovoltages/Stern-Volmer quenching constants vs. reduction potentials of the reducing agents also confirms the above electron transfer in the photoexcited state.

Photophysical studies of 3,3′ dioctadecyloxacarbocyanine dye in model biological membranes and different solvents

The absorption and fluorescence spectra of 3,3′-dioctadecyloxacarbocyanine [DiOC 18(3)], a cationic oxacarbocyanine dye have been studied in aqueous and nonaqueous media containing egg phosphatidylcholine (PC) as well as in different solvents of diverse nature. The results show the evidence of complex formation of the dye in the ground and in the excited states with PC. The excited state interaction of the dye with PC suggests the electron transfer from PC to dye and this is supported by photovoltage generation in a photoelectrochemical cell consisting of dye and PC in aqueous medium. An attempt has been made to determine the polarity of the microenvironment of the dye in PC liposome or PC reverse micelle from the spectral studies of the dye in different solvents of known polarity.

Photoelectrochemistry of azure dyes in phospholipid liposome

The systems consisting of azure (Az) dyes in presence of sonicated aqueous dispersion of phospholipid generate photovoltage when studied in a photoelectrochemical cell. A possible mechanism of photovoltage generation suggests the photoinduced electron transfer from phospholipid to Az dye in liposome. The electrode kinetics of the photoinduced redox reactions in the cell have also been studied to confirm the mechanism of photoinduced electron transfer as well as the interaction between Az dye and phospholipid.

Extremely nonlinear photosensitive nematic liquid crystal film

Optically induced nematic liquid crystal (NLC) axis reorientations result in extraordinarily large nonlinear refractive index changes. We discuss the origins of these nonlinearities, and recently, obtained results of experimental studies of photovoltage generation, dynamic grating diffraction, and image conversion using dye-doped NLC films in all-optical configurations are presented. These processes are characterized by unprecedented low threshold laser powers. For example, we have demonstrated incoherent–coherent image conversion with laser intensity on the order of 70 microWatts/cm 2.

Time-resolved measurements of photovoltage generation by bacteriorhodopsin and halorhodopsin adsorbed on a thin polymer film.

We constructed a time-resolved photovoltage measurement system and examined the photovoltage kinetics of wild-type bacteriorhodopsin, its D96N mutant, and halorhodopsins from Halobacterium salinarum and Natronobacterium pharaonis. Upon illumination with a laser flash, wild-type bacteriorhodopsin showed photovoltage generation with fast (10-100 micros range) and slow (ms range) components while D96N lacked the latter, as reported previously [Holz, M., Drachev, L.A., Mogi, T., Otto, H., Kaulen, A.D., Heyn, M.P., Skulachev, V.P., and Khorana, H.G. (1989) Proc. Natl. Acad. Sci. USA 86, 2167-2171]. In contrast, photovoltage generation in halorhodopsins from H. salinarum and N. pharaonis was significant only in the ms time range. On the basis of the photovoltage kinetics and photocycle, we conclude that major charge (chloride) movements within halorhodopsin occur during the formation and decay of the N intermediate in the ms range. These observations are discussed in terms of the "Energization-Relaxation Channel Model" [Muneyuki, E., Ikematsu, M., and Yoshida, M. (1996) J. Phys. Chem. 100, 19687-19691].

Role of surface states and adsorbates in time-resolved photocurrent measurements and photovoltage generation at phthalocyaninatozinc(II)-photocathodes

The role of surface states in the photoelectrochemistry of phthalocyaninatozinc(II) (PcZn) has been investigated by time-resolved photocurrent measurements in the millisecond range and by photovoltage measurements. Plots of the photovoltage against the redox potential of the electrolyte yielded slopes a with −1< a< 0 indicating partial Fermi-level pinning due to the presence of surface states. The charging and discharging of the surface states could be seen clearly in the photocurrent transients. The amount of charging and discharging was found to be determined by the recombination rate as a function of the applied electrode potential and by the charge–transfer rate, which was studied in dependence upon the nature and the concentration of the redox couple in the electrolyte (ferri/ferrocyanide, hydroquinone/benzoquinone) in comparison to oxygen- saturated and deaerated KCl electrolytes. The charge–transfer rate was found to be limited by an adsorption step of the electroactive species. The number of surface states was found to increase with exposure to air. This is interpreted as a modification of the PcZn surface due to reaction with oxygen. The surface state densities as derived from the extent of the Fermi-level pinning corresponded well with the discharges seen in the photocurrent transients and show that a considerable fraction of the PcZn surface molecules represents surface states after longer exposure to air.

1PB036 ハロロドプシンの光電圧発生と光サイクルのCl^-濃度依存性

1PB036 ハロロドプシンの光電圧発生と光サイクルのCl^-濃度依存性

Photoinduced phase transformation in Y-Ba-Cu-O ceramics

Investigations of the photoinduced phenomena in Y-Ba-Cu-O ceramics have been performed using time-resolved measurements of the photovoltage, photocurrent, photoresistivity and optical second harmonic generation. A time delay shift between the non-linear optical response and photocurrent was revealed. Application of external magnetic fields of up to 1 T leads to an essential decrease of the second harmonic generation response. Different approaches for explaining the observed data are discussed.

Photovoltage evidence that Glu-204 is the intermediate proton donor rather than the terminal proton release group in bacteriorhodopsin

Electrogenic events in the E204Q bacteriorhodopsin mutant have been studied. A two-fold decrease in the magnitude of microsecond photovoltage generation coupled to M intermediate formation in the E204Q mutant is shown. This means that deprotonation of E204 is an electrogenic process and its electrogenicity is comparable to that of the proton transfer from the Schiff base to D85. pH dependence of the electrogenicity of M intermediate formation in the wild-type bacteriorhodopsin reveals only one component corresponding to the protonation of D85 in the bacteriorhodopsin ground state and transition of the purple neutral form into the blue acid form. Thus, the p K of E204 in the M state is close to the p K of D85 in the bacteriorhodopsin ground state (<3) and far below the p K of the terminal proton release group (∼6). It is concluded that E204 functions as the intermediate proton donor rather than the terminal proton release group in the bacteriorhodopsin proton pump.

Photoelectric Effect of Tetrasulfonated Gallium Phthalocyanine on a Nanostructured TiO2 Electrode

The photoelectric conversion and surface photovoltage spectrum of tetrasulfonated gallium phthalocyanine ( GaTsPc ) on a titanium dioxide ( TiO 2) nanostructured electrode are reported. As a comparison, the surface photovoltage spectrum of the sublimed film of GaTsPc deposited on an indium tin oxide conducting glass held at 10 ± 0.1°C is also reported. The GaTsPc molecule exists mainly as a dimer on the nanostructured titanium dioxide electrode and in the sublimed film. The absorption of the GaTsPc dimer on the nanostructured titanium dioxide electrode does not convert into a photocurrent or a surface photovoltage while that of the GaTsPc dimer in the sublimed film contributes to the generation of a surface photovoltage, which is due to the different orientation of the GaTsPc molecule aggregation.

Generation of light‐induced electrical potential from ion exchange membranes containing 4,4′‐bipyridine moiety. II. Effect of species of anion exchange membranes on photovoltage

Various anion exchange membranes containing the 4,4'-bipyridine moiety as anion exchange groups were prepared from membranous copolymers of chloromethylstyrene and divinylbenzene and membranes of chloromethylated polysulfone and 4,4'-bipyridine. After evaluating the electrochemical properties of the obtained anion exchange membranes, the effect of membrane species on the generation of a photovoltage was examined by irradiation using a xenon lamp. The membranes swelled with ethylene glycol were clamped between two ITO electrodes and sealed by adhesive. The generated photovoltage and photocurrent from about a 120 μm thick membrane were about 80 mV and 400 nA, respectively, in a 200K Ω load resistance, though dependent of membrane species. The voltage decreased with increasing crosslinking by the divinylbenzene in the copolymer membranes. The effect of counter ion species on the voltage was examined and a chloride ion form of membrane showed the highest photovoltage. The membranes with different thicknesses, which were prepared from polysulfone derivatives, were evaluated and the voltage decreased with decreasing thickness. Even a porous membrane from polysulfone derivatives showed a photovoltage though a porous membrane in which a methyl viologen ethylene glycol solution had been impregnated did not have a stable voltage. Also, the anion exchange membrane containing the benzyl trimethylammonium moiety, which is the conventional anion exchange groups, did not show a high and stable photovoltage upon photoirradiation.

Generation of Light-Induced Electrical Potential from Ion Exchange Membranes Containing 4,4′-Bipyridine Moiety

Generation of photovoltage and photocurrent from ion exchange membranes containing a viologen moiety was examined, cation exchange membranes ion-exchanged with methyl viologen and anion exchange membranes to which a viologen moiety was bonded. After the membrane, swelled with ethylene glycol, had been clamped between two ITO electrodes and sealed, it was irradiated with a xenon lamp. In the case of the cation exchange membranes ion-exchanged with methyl viologen, 155.3 mV of photovoltage was observed immediately after photoirradiation, and the voltage decreased and attained almost a constant value. The photovoltage of anion exchange membranes with the viologen moiety increased very slowly (maximum 81 mV, 405 nA; load resistance 200 KΩ) after beginning the irradiation. However, when the light was irradiated again on the membrane after interruption of the irradiation, almost the same photovoltage was generated immediately after the irradiation. Though the anion exchange membrane showed absorbance only at 320 nm in the UV-VIS spectrum, wavelengths between 300 and 400 nm were active to reduce the viologen moiety of the membrane. This might be due to a polymer effect. On the other hand, the electrical resistance between the ITO electrodes decreased upon photoirradiation because of radical formation. In order to accelerate generation of the voltage, an oxidative agent (ferric ions) or a reductive agent (triethanolamine) was added to the system. The photovoltage was generated immediately after irradiation in both cases. Ferric ions act as an electron acceptor and triethanolamine forms cation radicals in the membrane before the irradiation.

Photovoltage generation of Si/C60 heterojunction

Photovoltage generation was observed for C60/Si heterojunction fabricated by the deposition of C60 thin film on Si substrate. Four types of Si substrates (two for p-type and two for n-type) were used. All junctions showed rectifying behaviors and photovoltage generation under illumination of Ar-ion laser. The highest value of the photovoltage was 0.40 V. From the saturated photovoltage values for four junctions, the Fermi level of C60 thin film was estimated to be located about 4.7 eV below the vacuum level.

Photo-effect in phospholipid liposome containing riboflavin

The photovoltage developed in a photoelectrochemical cell consisting of riboflavin (RFN), a neutral dye and phospholipid (PL) liposome in aqueous solution has been found to correlate with the Stern-Volmer constant ( K sv ) or quenching-rate constant ( K q) of RFN-PL studied by fluorescence spectra. A possible mechanism of photovoltage generation suggests the photo-induced electron transfer from PL to RFN in liposome through the excited state molecular interaction.