Photovoltaic Media Research Articles

СИНТЕЗ ТА ФОТОВОЛЬТАЇЧНІ ВЛАСТИВОСТІ СИМЕТРИЧНИХ БІС-АЗОМЕТИНІВ З АКЦЕПТОРНИМИ ЗАМІСНИКАМИ У 4,4'-ПОЛОЖЕННІ

Two new symmetric azomethine dyes capable of photoinduced isomerization were synthesized by condensing symmetric bis-aldehyde (obtained by the reaction of epichlorohydrin with 4-oxybenzaldehyde) with 4-nitroaniline and 4-chloroaniline, respectively. The yield of the target products decreases with the transition from nitro-substituted azomethine to chlorine-substituted. This is due to the greater basicity of the starting amines with nitro-substitution when increasing the acceptor force of the substituent complicates the course of the reaction. Azomethines are characterized by absorption with a maximum at 400–410 nm, which makes them sensitive to radiation with a blue component of the spectrum. The photoelectric properties of azomethines upon irradiation were investigated by the method of measuring the surface potential with the help of a Kelvin dynamic probe. The maximum value of the electric potential of the photosensitive films free surface during irradiation with white LED at I = 60 W/m2 is about 270 mV in the case of azomethine with a nitro group in the 4,4' position and about 125 mV in the case of azomethine with chlorine as a substituent. That is, the magnitude of the electric potential of the free surface decreases approximately twice during the transition from nitro substituent to chlorine. This may be explained by the fact that the photoinduced changes in azomethine with a higher acceptor substituent flow more quickly and with greater efficiency. But at the same time, the reverse changes when turning off the light are just as fast. For chlorine substituted azomethine samples, the reverse process proceeds rather slowly, which may indicate greater stability over time of the photochemically modified form in case of chlorine substitution compared to the nitro-substituted analogue. Thus, the synthesized azomethines can be used in the development of new photovoltaic media and recording media for optical information recording.

Photoinitiated [corrected] charge separation in a hybrid titanium dioxide metalloporphyrin peptide material.

In natural systems, electron flow is mediated by proteins that spatially organize donor and acceptor molecules with great precision. Achieving this guided, directional flow of information is a desirable feature in photovoltaic media. Here, we design self-assembled peptide materials that organize multiple electronic components capable of performing photoinduced charge separation. Two peptides, c16-AHL3K3-CO2H and c16-AHL3K9-CO2H, self-assemble into fibres and provide a scaffold capable of binding a metalloporphyrin via histidine axial ligation and mineralize titanium dioxide (TiO2) on the lysine-rich surface of the resulting fibrous structures. Electron paramagnetic resonance studies of this self-assembled material under continuous light excitation demonstrate charge separation induced by excitation of the metalloporphyrin and mediated by the peptide assembly structure. This approach to dye-sensitized semiconducting materials offers a means to spatially control the dye molecule with respect to the semiconducting material through careful, strategic peptide design.

Optimization of radiative heat transfer in hyperbolic metamaterials for thermophotovoltaic applications

Using our recently developed method we analyze the radiative heat transfer in micron-thick multilayer stacks of metamaterials with hyperbolic dispersion. The metamaterials are especially designed for prospective thermophotovoltaic systems. We show that the huge transfer of near-infrared thermal radiation across micron layers of metamaterials is achievable and can be optimized. We suggest an approach to the optimal design of such metamaterials taking into account high temperatures of the emitting medium and the heating of the photovoltaic medium by the low-frequency part of the radiation spectrum. We show that both huge values and frequency selectivity are achievable for the radiative heat transfer in hyperbolic multilayer stacks.

Light-induced waveguide by a finite self-trapped vortex beam in a photorefractivemedium

We report the formation of a 2D waveguide induced by a self-trapped vortexbeam in a photorefractive–photovoltaic medium. Demonstrations are performedby monitoring the self-trapping dynamic of an annular vortex beam in aLiNbO3:Fe crystal. Control of the experimental observations is consistent with the underlying physicsrevealed by a numerical model.

Nanocoax solar cells based on aligned multiwalled carbon nanotube arrays

AbstractWe fabricated and studied solar cells based on a distributed nanocoax architecture by depositing amorphous silicon as photovoltaic medium on arrays of aligned multiwalled carbon nanotubes. These inexpensive cells demonstrate an initial efficiency of 6.1% that can be further enhanced by increasing the nanocoax density per unit area and improving the amorphous silicon quality.

Bacteriorhodopsin-based photo-electrochemical cell

A simple solution-based electrochemical cell has been constructed and successfully employed in the detection of the photoelectric response upon photoexcitation of bacteriorhodopsin (bR) without external bias. Commercially-available indium tin oxide (ITO) glasses served as the optical windows and electrodes. Small amounts of bR suspensions (∼100 μL) were utilized as the photovoltaic medium to generate the proton gradient between two half-cells separated by a molecular porous membrane. Continuous broadband visible light ( λ > 380 nm) and a short-pulse 532-nm laser were employed for the photoexcitation of bR. Upon the modulated cw broadband irradiation, an instantaneous rise and decay of the current was observed. Our observations of the pH-dependent photocurrent are consistent with previous reports in a bR thin film configuration, which also showed a polarity inversion at pH 5–6. This is due to the change of the priority of the proton release and proton uptake in the photocycle of bR. Studies on the ionic strength effect were also carried out at different KCl concentrations, which resulted in the acceleration of the rise and decay of the photoelectric response. This was accompanied by a decrease in the stationary photocurrent at higher KCl concentrations in the broadband excitation experiments. The solution-based electrochemical cell uses aqueous medium, which is required for the completion of the bR proton pumping function. Due to the generation of the stationary current, it is advantageous to convert solar energy into electricity without the need of film-based photovoltaic devices with external bias.

Dynamics of second-harmonic generation in a photovoltaic photorefractive quadratic medium

We investigate second-harmonic generation (SHG) in a photorefractive photovoltaic medium such as lithium niobate. Our numerical model reveals the complex dynamics of the parametric process during the buildup of the index modification due to the photorefractive (PR) nonlinearity. We investigate a condition in which no external field is applied to the crystal, resulting in a defocusing nonlinearity, as well as the case in which an external bias is applied, producing a self-focusing effect that can enhance the conversion efficiency of the parametric process. We also find the conditions for the initial phase matching and for the background illumination leading to a stable self-confined propagation of the second-harmonic generated light. The developed numerical model shows that as a general case SHG in a self-focusing PR medium results in mode beating inside the generated waveguide, as experimentally observed.

Photorefractive hologram writing with high modulation depth in photovoltaic media under different boundary conditions

Photorefractive recording response to sinusoidal intensity pattern in photovoltaic media under different boundary conditions, namely, open-circuit, crystal-resistor-circuit, short-circuit and crystal-voltage supply-circuit, are investigated by solving steady-state Kukhtarev equations numerically. In high modulation depth m limit and when photovoltaic field becomes sufficiently large, we find that both the magnitude and imaginary part of fundamental space-charge field E 1 as a function of m may transit from superlinear increment to sublinear increment for short-circuit condition. The role of photovoltaic field in holographic storage and two-wave mixing is also discussed in comparison with an earlier experimental literature [G. Cook et al., Opt. Commun. 192 (2001) 393]. We further generalize Gu et al.’s remark on photovoltaic field and dc space-charge field, which is for small m only [C. Gu et al., J. Appl. Phys. 69 (1991) 1167], to whole range of m in the limit donor density much larger than ionized donor density that the effect of photovoltaic field cannot be cancelled or considered as an externally applied field for an arbitrary non-zero m.

Interaction between a dark spot and a two-dimensional nonlinear photonic lattice with fully incoherent white light

We study experimentally the interaction of a dark spot with a nonlinear photonic lattice with fully incoherent white light emitted from an incandescent bulb in the self-defocussing photovoltaic media when the dark spot is aimed at different positions of lattices with different lattice spacing. In this case a host of novel phenomena is demonstrated, including dark spot induced lattice dislocation-deformation, the annihilation of the dark spot and so on. Results demonstrate that the interaction between incoherent dark spot and photonic lattice is always attraction and the large-spacing photonic lattice is analogous to the continuous medium.

Interaction between a dark stripe and a two-dimensional photonic lattice with fully incoherent white light

Two-dimensional nonlinear photonic lattices are created with fully incoherent white light emitted from an incandescent lamp in the self-defocusing photovoltaic media. We study experimentally the interaction of a dark stripe with such a nonlinear photonic lattice and observe a novel type of composite state. The composite state is generated by coupling of localized stripe and the periodic pixels nearest neighboring to the stripe.

Effect of spatial phase modulation on propagation of photovoltaic spatial solitons

Based on the technology of spatial phase modulation, a simple scheme to steer optical Gaussian beams in photovoltaic media is presented. Numerical simulations show that we can realize the self-deflection, soliton like propagation, divergence and splitting of Gaussian beams in photovoltaic media. The relationship between the intensity profile of the incident beam and the modulation parameters is also discussed.

Existence and stability of rigid photovoltaic solitons in an open-circuit amplifying or absorbing photovoltaic medium.

A different type of photovoltaic soliton is predicted for open-circuit photovoltaic media with a gain provided by two-wave mixing. Such solitons are a result of a double balance, i.e., loss is balanced by gain and diffraction is balanced by nonlinearity that is due to both, photovoltaic effects and two-wave mixing. Exact bright and dark analytical solutions are obtained under the condition that the signal beam is much weaker than the pump beam. Such solitons have fixed amplitude and width for the fixed values of the system parameters and hence are named rigid photovoltaic solitons. When the pump beam is switched to a background illumination, rigid photovoltaic solitons can become previously observed photovoltaic solitons. Numerical simulations show that rigid photovoltaic solitons are stable against small perturbations.

Y junctions arising from dark-soliton propagation in photovoltaic media

We report the observation of planar Y-junction waveguide splitters that are due to the bulk photovoltaic effect. The junctions are generated by multiple dark-soliton propagation in LiNbO(3) by use of low power levels (20 mW) and average intensities of 10 W/cm(2) at 488 nm. The junctions persist in the dark and can be used to divide input beams of less photorefractive sensitivity. We describe some possibilities for generating other waveguide structures in bulk and thin-film media.

Direct autocorrelation measurements of mid-infrared picosecond pulses by quantum-well devices

We report the direct autocorrelation of ultrashort mid-infrared laser pulses, using an unbiased GaAsy/GaAlAs quantum-well device as a nonlinear photovoltaic medium. Using this method, we autocorrelate ultrashort (3-ps) low-energy (~500-nJ) pulses of 10.6-microm light. This approach should permit the autocorrelation of subpicosecond infrared pulses.

Dynamics of grating formation in photovoltaic media

The Kukhtarev equations are solved taking into account the photovoltaic effect and different boundary conditions. In the case of open circuit, the voltage across the crystal is found to vary with a time scale similar to the photorefractive time constant. This effect explains the dynamic behavior observed experimentally.

Vectorial interactions in photovoltaic media

Abstract Photorefraction of doped noncentrosymmetric crystals, caused by spatially-oscillating photovoltaic currents, makes possible the holographic recording by two orthogonally polarized waves, ordinary and extraordinary, and allows several vectorial multiwave interactions. The present paper shows how a new information on the photovoltaic effect can be extracted from the study of vectorial interactions.