Incident Photon-to-current Efficiency Values Research Articles

New insights into enhancement of bio-hydrogen production through encapsulated microalgae with alginate under visible light irradiation

The production of green hydrogen is a promising alternative to fossil fuels. The current study focuses on the design of microalgae as a catalyst in bioelectrochemical systems for the generation of biohydrogen. Furthermore, the abovementioned target could be achieved by optimizing different parameters, including strains of microalgae, different optical filters, and their shapes. Synechocystis sp. PAK13 (Ba9), Micractinium sp. YACCYB33 (R4), and Desmodesmus intermedius (Sh42) were used and designed as free cells and immobilized microalgae for evaluating their performance for hydrogen production. Alginate was applied for immobilization not only for protecting the immobilized microalgae from stress but also for inhibiting the agglomeration of microalgae and improving stability. The amount of studied immobilized microalgae was 0.01 g/5 ml algae-dissolved in 10 ml alginate gel at 28 °C, 12 h of light (light intensity 30.4 μmol m−2 s−1), and 12 h of darkness with continual aeration (air bump in every strain flask) at pH = 7.2 ± 0.2 in 0.05 %wuxal buffer which has 3.7 ionic strength. Different modalities, including FTIR, UV, and SEM, were performed for the description of selected microalgae. The surface morphology of Ba9 with alginate composite (immobilized Ba9) appeared as a stacked layer with high homogeneity, which facilitates hydrogen production from water. The conversion efficiencies of the immobilized microalgae were evaluated by incident photon-to-current efficiency (IPCE). Under optical filters, the optimum IPCE value was ∼ 7 % at 460 nm for immobilized Ba9. Also, its number of hydrogen moles was calculated to be 16.03 mmol h−1 cm−2 under optical filters. The electrochemical stability of immobilized Ba9 was evaluated through repetitive 100 cycles as a short-term stability test, and the curve of chrono-amperometry after 30 min in 0.05 %wuxal at a constant potential of 0.9 V for 30 min of all studied samples confirmed the high stability of all sample and the immobilized Ba9 has superior activity than others.

Surface plasmon resonance effects of silver nanoparticles in graphene-based dye-sensitized solar cells

Localized surface plasmon resonance (LSPR) has been applied in photovoltaic devices to improve their photoelectric conversion efficiency. In this study, the mechanism of LSPR in dye-sensitized solar cells (DSSCs) was determined. Specifically, silver nanoparticles were formed by evaporating silver in electro-beam equipment and then annealing it in a tube furnace. The sizes of the nanoparticles varied with the changes in their thicknesses during evaporation and annealing. A DSSC-based graphene was designed to consist of different sizes of metal nanoparticles assembled on a cathode electrode. The photon electric performance of the DSSCs, which depended on Ag nanoparticles, was analyzed in detail, with a particular focus on nanoparticle size. Compared with the DSSC without Ag nanoparticles, the DSSC with LSPR exhibited excellent electric current density and incident photon-to-current efficiency (IPCE) performance due to the LSPR effect. The DSSC assembled with 10 nm-thick Ag film and annealed to form nanoparticles exhibited a high IPCE of 70.03%. The IPCE value of this DSSC was 45.15% higher than that of the pure graphene-based DSSC (31.62%). However, Ag nanoparticles increased to a certain degree and became aggregated and concatenated, thereby decreasing the LSPR effect on DSSCs. Therefore, LSPR plays an important role in the photon-electrical performance of DSSCs.

An Inorganic–Organic Hybrid Polymer Cocatalyst for Photoelectrochemical Water Oxidation with Dual Functions of Accelerating Kinetics and Improving Charge Transfer

Oxygen evolution cocatalysts (OECs) play important roles in improving the efficiency of photocatalysts in solar water splitting. Inorganic–organic hybrid polymers (IOHPs), which have good electroly...

Exploiting Direct Current Plasma Electrolytic Oxidation to Boost Photoelectrocatalysis

In this study, we report an investigation of the photoelectrochemical activity of TiO2 films formed by DC plasma electrolytic oxidation (PEO) at a variable potential in a sulfuric acid electrolyte at 0 and 25 °C. The surface morphology was mainly determined by the oxide-forming potential. X-Ray Diffraction and Raman analyses showed that the relative amount of the anatase and rutile phases varied from 100% anatase at low potential (110–130 V) to 100% rutile at high potential (180–200 V), while mixed-phase oxide films formed at intermediate potential. Correspondingly, the band gap of the TiO2 films decreased from about 3.20 eV (pure anatase) to 2.94 eV (pure rutile) and was red-shifted about 0.1 eV by reducing the electrolyte temperature from 25 °C to 0 °C. Glow-Discharge Optical Emission Spectroscopy (GD-OES) and X-ray Photoelectron Spectroscopy (XPS) analyses evidenced S-containing species located preferentially close to the TiO2/Ti interface. The photoelectrochemical activity was assessed by measuring the incident photon-to-current efficiency (IPCE) under Ultraviolet C (UV-C) irradiation, which showed a non-gaussian normal trend as a function of the PEO cell potential, with maximum values exceeding 80%. Photoelectrocatalytic activity was assessed by decolorization of model solutions containing methylene blue. Photoanodes having higher IPCE values showed faster decolorization kinetics.

Investigation of Strain Effects on Photoelectrochemical Performance of Flexible ZnO Electrodes

In this report, the growth of zinc oxide (ZnO) nanocrystals with various morphologies, nanoflower, nanosheet, and nanorod, on flexible stainless steel (SS) foils to be utilized as photoanodes in photoelectrochemical (PEC) solar cells has been presented. It has been aimed to provide flexibility and adaptability for the next generation systems with the incorporation of SS foils as electrode into PEC cells. Therefore, physical deformation tests have been applied to the prepared ZnO thin film photoanodes. These thin films have been thoroughly characterized before and after straining for better understanding the relationship between the morphology, straining effect and photoelectrochemical efficiency. We observed a notable increase in the maximum incident photon-to-current efficiency (IPCE) and durability of all ZnO photoelectrodes after straining process. The increase in IPCE values by 1.5 and 2.5 folds at 370 nm has been observed for nanoflower and nanorod morphologies, respectively after being strained. The maximum IPCE of 69% has been calculated for the ZnO nanorod structures after straining. Bending of the SS electrodes resulted in the more oriented nanorod arrays compared to its flat counterpart, which improved both the light absorption and also the photo-conversion efficiency drastically. The finite-difference time-domain simulations have also been carried out to examine the optical properties of flat and bent ZnO electrodes. Finally, it has been concluded that SS photoanodes bearing ZnO semiconducting material with nanoflower and nanorod morphologies are very promising candidates for the solar hydrogen generator systems in terms of efficiency, durability, flexibility, and lightness in weight.

Fabrication of Pure Sb2S3 and Fe (2.5%): Sb2S3 Thin Films and Investigation Their Properties

Pure Sb2S3 and Fe (2.5%): Sb2S3 thin films were synthesized on Zn2SnO4 coated with FTO conductive glasses using chemical bath deposition (CBD) technique. The X-ray diffraction (XRD) patterns obtained show that both thin films have an orthorhombic structure. Although the crystal structure of the two thin films was the same, the crystalline size of Fe (2.5%): Sb2S3 thin film (51.15 nm) was found to be smaller than that of pure Sb2S3 (52.89 nm). The effect of Fe-doped metal on crystal size of Sb2S3 was observed with this result. Another important observation is that the energy band gap of Fe (2.5%): Sb2S3 thin film (2.00 eV) is larger than that of pure Sb2S3 (1.89 eV). The photovoltaic properties of the synthesized thin films were examined by applying both incident photon-to-current efficiency (IPCE) and current density (J)–voltage (V) measurements. The obtained IPCE(%) values at 600 nm for pure Sb2S3 and Fe (2.5%): Sb2S3 thin films are 30.29 and 49.06, respectively. Using the J–V curves, the calculated η (%) values for pure Sb2S3 and Fe (2.5%): Sb2S3 thin films are 3.95 and 5.44, respectively. Based on the data obtained from both measurements, it was observed that the Fe dopant significantly enhance the performance of the Sb2S3-based solar cell devices.

Incident photon-to-current efficiency of thermally treated SWCNTs-based nanocomposite for dye-sensitized solar cell

This study focuses on incident photon-to-current efficiency (IPCE) performance of In2O3-SWCNTs for dye-sensitized solar cell (DSSC) application. The thin films were prepared by sol-gel method using spin-coating technique annealed at 400, 450, 500, 550, and 600 °C. Morphology transition of In2O3 from spherical to cubic and then octahedral structure occurred as the annealing temperature rises. The photoanode annealed at 450 °C (cubic structure) provides a stable phase of cubic structure with large surface area and optimum thickness for effective dye adsorption. However, the IPCE value does not solely depends on the dye adsorption of photoanodes (light harvesting efficiency (LHE)) but the electron injection efficiency (ηinj) and the collection efficiency (ηcoll). Smaller energy bandgap of photoanodes favors the injected electrons with higher driving force to the conduction band (CB) of the photoanode, which in turn increases the ηinj from the LUMO of dye to the In2O3-SWCNTs CB. Besides that, the absence of single-walled carbon nanotubes (SWCNTs) above 500 °C caused the energy bandgap to increase and leads to lower driving force of injected electrons. In addition, SWCNTs are capable of absorbing visible light faster than other materials. Therefore, the cubic structure-based photoanode (450 °C) exhibited better electron transport with larger driving force on injected electron (ηinj) that decreased the electron recombination rate and increased electron lifetime and subsequently obtained larger charge collection efficiency (ηcoll) of almost 99%. Consequently, the IPCE performance of DSSC was enhanced.

Fabrication of TiO2–BiOCl double-layer nanostructure arrays for photoelectrochemical water splitting

A novel TiO2–BiOCl double-layer nanostructure array (rutile nanowires@anatase/BiOCl nanosheets) was successfully designed and prepared. The obtained samples were systematically characterized by scanning electron microscopy, X-Ray diffraction, the UV–Vis diffuse reflectance spectra, Raman spectroscopy, photocurrent versus potential (I–V) curves, and the incident photon to current efficiency (IPCE) curves. According to the photoelectrochemical data, the introduction of the BiOCl nanosheet on the TiO2 nanowire film could markedly enhance the photocurrent and IPCE value, due to the formation of TiO2/BiOCl heterostructures on the FTO substrate. Furthermore, the sample exhibits the best photoelectrochemical performance when the depositing cycle of BiOCl is 60 times.

Nanoforest Nb2O5 Photoanodes for Dye-Sensitized Solar Cells by Pulsed Laser Deposition

Vertically aligned bundles of Nb(2)O(5) nanocrystals were fabricated by pulsed laser deposition (PLD) and tested as a photoanode material in dye-sensitized solar cells (DSSC). They were characterized using scanning and transmission electron microscopies, optical absorption spectroscopy (UV-vis), and incident-photon-to-current efficiency (IPCE) experiments. The background gas composition and the thickness of the films were varied to determine the influence of those parameters in the photoanode behavior. An optimal background pressure of oxygen during deposition was found to produce a photoanode structure that both achieves high dye loading and enhanced photoelectrochemical performance. For optimal structures, IPCE values up to 40% and APCE values around 90% were obtained with the N(3) dye and I(3)(-)/I(-) couple in acetonitrile with open circuit voltage of 0.71 V and 2.41% power conversion efficiency.

Detailed Investigations of ZnO Photoelectrodes Preparation for Dye Sensitized Solar Cells

Wurtzite ZnO hexagonal nanopyramids were successfully synthesized in the liquid phase from homogeneous methanolic solutions of zinc acetate and tetramethylammonium hydroxide at an excess of zinc ions. The formation and properties of the nanocrystals were examined as a function of synthesis conditions. No significant influence of the [Zn(2+)]/[OH(-)] ratio was noticed on the final particle size, in spite of increased amounts of OH(-) ions, which tend to accelerate the particle nucleation and growth. Nevertheless, the reactant concentration ratio influences the surface properties of the ZnO nanocrystals. Mesoporous ZnO films were prepared by doctor blading ethanolic pastes containing ZnO nanoparticles and ethyl cellulose onto FTO conductive glass substrate followed by calcination. Additionally, the influence of a plasticizer (triacetin)-used during the paste preparation-on the film quality was investigated. A higher content of ZnO nanoparticles and plasticizer in the pastes improved the film quality. Four different temperatures (i.e., 400, 425, 450, and 475 °C) were used for the film calcination and their influence on the structural properties of the films was characterized. In principle, increasing the calcination temperature goes hand in hand with an increase of particle size, as well as the pore diameter and reduction of the surface area. Suitable mesoporous films were employed as photoanodes in dye sensitized solar cells (DSSCs). In order to assess the effect of the varied parameters on complete DSSC devices-using cis-diisothiocyanato-bis(2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium(II)bis(tetrabutylammonium (N719) as a sensitizer-incident photon to current efficiency (IPCE) and current voltage measurements were carried out. The IPCE measurements confirmed photoinduced electron injection from the dye, reaching IPCE values up to 76%. Furthermore, current-voltage characteristics of complete cells emphasized the importance of the proper preparation methods and temperatures. These features are important assets for the preparation of nanocrystalline ZnO based photoelectrodes and for improving the DSSC performance.

Good Solvent Effects of C70 Cluster Formations and Their Electron-Transporting and Photoelectrochemical Properties

Good solvent effects of C(70) cluster formations and their electron-transporting and photoelectrochemical properties have been systematically examined for the first time. Nano-to-micrometer scale assemblies of C(70) with different morphologies were prepared by rapidly injecting poor solvent (i.e., acetonitrile) into a solution of C(70) dissolved in various good solvents (i.e., benzene, toluene, chlorobenzene, etc). The cluster morphology engineering was successfully achieved by changing the good solvent, yielding the spherical, rodlike, or platelike clusters in the mixed solvents. The clusters of C(70) were electrophoretically deposited onto a nanostructured SnO(2) electrode to examine the photoelectrochemical properties under the white light or monochromatic light illumination. The maximum incident photon-to-current efficiency (IPCE) varied from 0.8 to 10% depending on the combinations of the poor-good solvents. The differences in the IPCE values are discussed in terms of the surface area, thickness, and electron mobility of the deposited cluster films. The electron mobility is found to be the most predominant factor for the IPCE, indicating the importance of the electron-transporting process in the overall photocurrent generation. In addition, the electron mobility is closely correlated with the underlying molecular alignment and the resultant cluster structure. Thus, these results will provide basic clue for the design of C(70)-based molecular devices including the organic photovoltaics.

Comparison of Cluster Formation, Film Structure, Microwave Conductivity, and Photoelectrochemical Properties of Composites Consisting of Single-Walled Carbon Nanotubes with C60, C70, and C84

The cluster formation, electrophoretically deposited film structures, microwave conductivity, and photoelectrochemical properties of the composites consisting of single-walled carbon nanotube (SWNT) with C60, C70, or C84 have been systematically compared for the first time. In an ortho-dichlorobenzene (ODCB)−acetonitrile mixture, the higher fullerenes (i.e., C70 and C84) were found to form single composite clusters exclusively with highly soluble SWNT bearing bulky swallow-tailed substituents (f-SWNT). These are in marked contrast with the unselective formation of three different clusters in the C60−f-SWNT composites. Electrophoretic deposition of the composite clusters yielded the corresponding films on nanostructured SnO2 electrodes. The microwave conductivity measurements revealed the occurrence of electron transfer from C70 to f-SWNT, followed by electron transportation through f-SWNT, in addition to electron hopping through C70 molecule arrays due to the alignment of C70 on the sidewalls of f-SWNT in the C70−f-SWNT film. The C70−f-SWNT photoelectrochemical device exhibited higher incident photon-to-current efficiency (IPCE) value (26% at 400 nm) than the C60−f-SWNT device (18%). The higher IPCE value results from selective formation of the single composite film, in which the SWNT network is covered with C70 molecules, and the high electron mobility (2.4 cm2 V−1 s−1) through the C70−SWNT network. In contrast, the C84−f-SWNT photoelectrochemical device revealed poor photocurrent generation (4.8%) owing to the inefficient electron injection from C84 radical anion (C84/C84•− ≈ 0.0 V vs NHE) to the SnO2 electrode (ECB = 0 V vs NHE) directly or indirectly despite the exclusive formation of the single composite clusters. Thus, the results obtained here will provide valuable information on the design of molecular devices based on carbon nanotubes and fullerenes.

Photoactive WO3 and Se-WO3 thin films for photoelectrochemical oxidation of organic compounds

Thin films of bare and Se-containing tungsten trioxide (WO3) on AISI304-type stainless steel were prepared by electrochemical deposition using peroxy-tungstate solutions. The obtained films were characterized by X-ray diffraction, photoelectron spectroscopy, scanning electron microscopy, thermal and photovoltammetry analyses. The oxidation of methylene blue, phenol, and methanol was used to evaluate the photoelectrocatalytic activity of the prepared films. It has been established that the incident photon-to-current efficiency (IPCE) in 0.1-M K2SO4 decreases as the concentration of methylene blue and phenol increases. On the contrary, the IPCE values increase with the increase in initial concentration of methanol. The bulk electrolysis experiments revealed that the prepared films are stable and can be used for photoelectrochemical oxidation of methanol.

Photoconductivity of Porphyrin Nanochannels Composed of Diprotonated Porphyrin Dications with Saddle Distortion and Electron Donors

Supramolecular architecture named as porphyrin nanochannels (PNCs), including tetrathiafulvalene (TTF) and p-aminophenol as electron-donating guests in the inner space, was prepared with the hydrochloride salt of dodecaphenylporphyrin ([H4DPP]Cl2) by self-assembly based on intermolecular π−π interactions. The crystal structure of the TTF-included PNC (PNC−TTF) was determined by X-ray crystallography. Intermolecular π−π interaction was recognized among peripheral phenyl groups of the porphyrin, mainly in the direction of the crystallographic c axis to form a column structure. Photoinduced electron transfer from the guest molecules to [H4DPP]Cl2 occurred to give the electron-transfer state involving cation radicals of the guest molecules and one-electron reduced [H4DPP]Cl2, {[H4DPP+•]Cl2}−, via the photoexcited singlet state of [H4DPP]Cl2 in PNC. The reactions were examined by solid-state femtosecond laser flash photolysis and ESR measurements to determine the rate constants of electron transfer and electronic structures of the cation radicals included in the cavity, respectively. A single crystal (0.87 × 0.23 × 0.10 mm3) of PNC−TTF exhibited photoconductivity upon photoirradiation at 633 nm with a He−Ne laser (5 mW), and the photocurrent was 0.7 nA at electrical field strength of 3.5 × 104 V cm−1. The photocurrent showed direction dependence toward the crystallographic c axis. This indicates that the intermolecular π−π interaction is the main conduction pathway. Various PNC supramolecules including TTF and other electron-donating guest molecules were also employed to construct photoelectrochemical cells with use of SnO2 transparent electrodes. Short-circuit photocurrent measurements were made on the cells with the OTE/SnO2/PNC−guest photoanodes, and they exhibited clear photoresponse upon photoirradiation. The photocurrents increase with increasing the rate constants of the photoinduced electron transfer from the guest molecules to [H4DPP]Cl2, exhibiting saturation behavior. The performance of the cell with the OTE/SnO2/PNC−TTF electrode exhibited the maximum IPCE (incident photon-to-current efficiency) value of 10.1% at 460 nm, which corresponded to the absorption maximum of the Soret band of [H4DPP]Cl2 on the electrode.

Naphthyl-Fused π-Elongated Porphyrins for Dye-Sensitized TiO2 Cells

Novel unsymmetrically π-elongated porphyrins, in which the naphthyl moiety is fused to the porphyrin core at the naphthyl bridge with a carboxyl group (fused-Zn-1) or at the opposite side of the phenyl bridge with a carboxyl group (fused-Zn-2), have been synthesized to improve the light-harvesting abilities in porphyrin-sensitized solar cells. As the results of π-elongation with low symmetry, Soret and Q bands of fused-Zn-1 and fused-Zn-2 were red-shifted and broadened, and the intensity of Q-band relative to that of Soret band was enhanced. The fused-Zn-1 and fused-Zn-2-sensitized TiO2 solar cells showed the power conversion efficiencies (η) of 4.1% and 1.1%, respectively, under standard AM 1.5 conditions. The η value of the fused-Zn-1 cell was improved by 50% compared to the reference cell using unfused porphyrin (Zn-1). The fused-Zn-1-sensitized cell revealed high IPCE (incident photon-to-current efficiency) values of up to 55%, extending the response of photocurrent generation close to 800 nm. Thus, the improved photocurrent generation of the fused-Zn-1-sensitized cell relative to the Zn-1-sensitized reference cell is responsible for the remarkable difference in the η values. The η value of the fused-Zn-2 cell was much lower than that of the fused-Zn-1 cell. DFT calculations disclosed that there are significant electron densities on the carboxyl group in the LUMO of fused-Zn-1, whereas there are little electron densities on the carboxyl group in the LUMO of fused-Zn-2. Accordingly, the larger electronic coupling between the porphyrin and the TiO2 surface in the fused-Zn-1-sensitized cell may be responsible for the high cell performance, due to the efficient electron injection from the porphyrin excited singlet state to the conduction band of the TiO2 electrodes. To further improve the cell performance, 5-(4-carboxylphenyl)-10,15,20-tetrakis-(2,4,6-trimethylphenyl)porphyrinatozinc(II) (Zn-3), possessing different light-harvesting properties, was coadsorbed with fused-Zn-1 onto an TiO2 electrode. Under the optimized conditions, the cosensitized cell yielded maximal IPCE value of 86%, short circuit photocurrent density of 11.7 mA cm−2, open-circuit voltage of 0.67 V, fill factor of 0.64, and η of 5.0% under standard AM 1.5 conditions.

An experimental study on the molecular organization and exciton diffusion in a bilayer of a porphyrin and poly(3-hexylthiophene)

The exciton root-mean-square displacement (ΛD) in regioregular poly(3-hexylthiophene) (P3HT) deposited onto meso-tetrakis (n-methyl-4-pyridyl) porphyrin tetrachloride (H2TMPyP) has been determined from the photovoltaic response of a device based on these materials in a bilayer configuration. Excitons formed on illumination that reach the interface between H2TMPyP and P3HT can undergo interfacial charge separation by electron injection into the H2TMPyP and hole injection into the P3HT. The incident photon to current efficiency (IPCE) exceeds 20% over a broad wavelength regime. The theoretical analysis of the IPCE values gives a value for ΛD in H2TMPyP that amounts to 14 nm, while for P3HT a value of 18 nm is obtained. The latter value exceeds literature values reported for P3HT by almost a factor of 3. X-ray diffraction analysis shows that in the studied bilayer the P3HT backbones are aligned parallel to the interface with H2TMPyP. In contrast, in the case of P3HT deposited onto TiO2, for which ΛD has been reported to equal only 7 nm, hardly any organization of the P3HT backbones is observed. The excitonic coupling between P3HT backbones deposited onto H2TMPyP is as high as 125 cm−1, a factor of 3 larger than the excitonic coupling between the disordered P3HT backbones that amounts to 47 cm−1. The difference illustrates the importance of controlling the molecular organization for the realization of efficient energy transfer in organic optoelectronics.

Effect of Fullerene on Photocurrent Performance of 6‐O‐Porphyrin‐2,3‐di‐O‐stearoylcellulose Langmuir‐Blodgett Films

AbstractThin films consisting of 6‐O‐porphyrin‐2,3‐di‐O‐stearoylcellulose (H2PCS) and fullerene (C60) were fabricated for anodic photocurrent generation systems by the Langmuir‐Blodgett (LB) technique. The π‐complexation between the porphyrin moiety and C60 in the LB films was investigated by means of the surface pressure (π)‐area (A) isotherms, UV‐vis, and fluorescence spectroscopy. The photocurrent density generated from the H2PCS–C60 LB monolayer films exhibited an increase with increasing the C60 proportion and reached a maximum at a mixing ratio of 1:2, yielding a quantum yield of 12.5% and an IPCE (incident photon‐to‐current efficiency) of 0.50% at a bias potential of +100 mV vs. SCE. Furthermore, the LB five‐layer films could give rise to the IPCE value of 1.5% at +100 mV without significant decline of the quantum yields, which was due to the function of C60 as an electron carrier to improve the interlayer electron transfer through each layer. These results have demonstrated a promising method for preparing the donor–acceptor systems using cellulose as a scaffold in the LB films.magnified image

Tuning the optical and photoelectrochemical properties of surface-modified TiO2

Surface-modification of TiO(2) is found to be a powerful tool for manipulating the fundamental optical and photoelectrochemical properties of TiO(2). High surface area nanocrystalline TiO(2) was modified by urea pyrolysis products at different temperatures between 300 degrees C and 500 degrees C. Modification occurs through incorporation of nitrogen species containing carbon into the surface structure of titania. The N1s XPS binding energies are 399-400 eV and decrease with increasing modification temperature whereby the Ti2p(3/2) peak is also shifted to lower binding energies by about 0.5 eV. With increasing modification temperature the optical bandgap of surface-modified TiO(2) continuously decreases down to approximately 2.1 eV and the quasi-Fermi level of electrons at pH 7 is gradually shifted from -0.6 V to -0.3 V vs. NHE. The surface-modified materials show enhanced sub-bandgap absorption (Urbach tail) and exhibit photocurrents in the visible down to 750 nm. The maximum incident photon-to-current efficiency (IPCE) was observed for the materials modified at 350 degrees C and 400 degrees C (IPCE approximately 14% at 400 nm, and IPCE approximately 1% at 550 nm, respectively). The efficiency of photocurrent generation is limited by surface recombination, which leads to a significant decrease in IPCE values and significantly changes the shape of the IPCE spectra in dependence on the optical bandgap.

Effects of Porphyrin Substituents on Film Structure and Photoelectrochemical Properties of Porphyrin/Fullerene Composite Clusters Electrophoretically Deposited on Nanostructured SnO2 Electrodes

We have systematically examined the substituent effects of meso-tetraphenylporphyrins on film structures and the photoelectrochemical properties of the composite clusters of free-base porphyrin and C(60) electrophoretically deposited on nanostructured SnO(2) electrodes. The photocurrent generation efficiency was found to correlate with the complexation ability of the porphyrin for C(60). Basically, the incident photon-to-current efficiency (IPCE) value was increased with increasing relative amounts of the porphyrin versus C(60) in the films. The unique molecular arrangement of the porphyrin with the simple, specific substituents (i.e., methoxy groups at the meta-positions of the meso-phenyl rings of tetraphenylporphyrins (3,5-OMeTPP; TPP=tetraphenylporphyrin)) and C(60) on SnO(2) electrodes resulted in the largest IPCE value (ca. 60 %) among this type of photoelectrochemical device. The rapid formation of the composite clusters and microcrystals from the combination of 3,5-OMeTPP and C(60) in a mixed solvent is unique as the association is accelerated by intermolecular interactions (i.e., hydrogen-bonding and CH-pi interactions) between the methoxy groups of the porphyrins and the porphyrin/C(60), in addition to the pi-pi interactions between the porphyrins/C(60) and C(60) molecules. Both the films and single crystals composed of the porphyrin and C(60) exhibited remarkably high electron mobility (7x10(-2) and 0.4 cm(2) V(-1) s(-1)), which is comparable to the value for highly efficient bulk heterojunction solar cells. Our experimental results have successfully demonstrated the importance of nanostructured electron- and hole-transporting pathways in bulk heterojunction solar cells. Such a finding will provide basic and valuable information on the design of molecular photovoltaics at the molecular level.

Photovoltaic conversion system using carotenoid-chlorophyll assembled TiO 2 film electrode

Zn chlorophyll- a derivative, Zn chlorin-e 6 (ZnChl-e 6) and carotenoid derivative, crocetin (Cro) adsorbed onto a nanocrystalline TiO 2 film (ZnChl-e 6-Cro/TiO 2) electrode was prepared and the photovoltaic properties of ZnChl-e 6-Cro/TiO 2 electrode were studied. Incident photon to current efficiency (IPCE) value at 640 nm in photocurrent action spectrum of ZnChl-e 6-Cro/TiO 2 electrode was ca. 2.50%. In contrast, IPCE value at 640 nm in ZnChl-e 6/TiO 2 electrode was ca. 1.27%. Thus, the IPCE value increases by co-adsorbed crocetin and ZnChl-e 6 onto TiO 2 film electrode. From the results of photocurrent responses with light intensity of 100 mW cm −2 irradiation, generated photocurrents in the ZnChl-e 6-Cro/TiO 2 and ZnChl-e 6/TiO 2 electrodes are 380 and 180 μA cm −2, respectively. Photocurrent increases using ZnChl-e 6-Cro/TiO 2 electrode compared with that of ZnChl-e 6/TiO 2 electrode.