Teruhisa Komura

High-performance carbon counter electrode for dye-sensitized solar cells

Here, we reported that a new carbon electrode prepared with an activated carbon was superior to a Pt sputtered electrode as the counter electrode ofdye-sensitized solar cells. The photovoltaic performance was largely influenced by the roughness factor of carbon electrode. The open-circuit voltage increased by about 60 mV using the carbon counter electrode compared to the Pt counter electrode because of positive shift of the formal potential for I � =I �

Charge-transporting properties of electropolymerized phenosafranin in aqueous media

The redox properties of polyphenosafranin were investigated at its different oxidation and protonation levels by potential scan voltammetry and electrochemical impedance spectroscopy. The polymer comprised phenosafranin units linked via a secondary amino nitrogen. A two-electron, two-proton process is proposed as its main redox reaction; its reduction yields the corresponding 5-hydro-phenazine form. In an isolated electroactive site, the polymer resembled redox polymers rather than conducting polymers. Low-frequency capacitance versus potential curves showed their maxima near the formal potential. Since the low-frequency capacitance increased linearly with the film thickness, it was identified with the redox capacitance. The width of the Warburg region showed a minimum at the half-reduced state of the polymer. This relationship indicated that electron transport was a key factor controlling the rate of charge transport in the polymer. The potential of the minimum Warburg width shifted to more negative values as the solution pH increased, with the minimum increasing exponentially with the pH. These observations lead to a conclusion that electron transport occurs via a process of sequential electron self-exchange between neighboring redox sites. This electron hopping is accompanied by proton transfer via intermolecular acid/base reactions.

Redox and ionic-binding switched fluorescence of phenosafranine and thionine included in Nafion® films

Abstract This report describes electrochemical and optical properties of azine-loaded Nafion® film electrodes that generate redox-activated optical signal. The apparent charge diffusion coefficient in the film decreased abruptly as the solution pH increased from 0.4 to 3, probably because deprotonation of the dyes lowered the rate of intermolecular electron hopping. Reversible switching of the fluorescence of phenosafranine was performed by electrochemical oxidation/reduction cycles. Both of the redox couple were isolable and stable; the reduced form was non-fluorescent, whereas the oxidized form was strongly fluorescent. Since the photonic output depended on chemical binding, the electrode system could operate as a dual-mode switching device. The second protonation of the dye molecule changed the fluorescence intensity sufficiently for chemical signaling of the photonic output, because the fully protonated oxidized-form was non-fluorescent; however, a p K a1 of 0.8 was too low to change the pH rapidly. 1,1′-ferrocenedimethanol quenched the fluorescence of the oxidized form efficiently to cause the ‘switching off’ of fluorescence. In the presence of the guest, however, the dye-loaded film did not recover the fluorescent state at any potential, because the cationic dye formed a nonfluorescent complex with the guest incorporated into the film.

Dependence of redox-kinetic parameters at poly(o-phenylenediamine)-modified electrodes upon the oxidation and protonation levels of the polymer

Abstract The charge-transfer resistance, the redox capacitance and the diffusion coefficient of charge carriers at poly( o -phenylenediamine)-modified electrodes were determined by electrochemical impedance spectroscopy. Impedance results are interpreted in terms of the diffusion–migration transport of both electrons and protons through the film. Their coupled diffusion coefficients obtained at different potentials show a maximum near the formal potential of the polymer. This relationship between the oxidation level and the coupled diffusion coefficient indicates that the rate of charge transport within the film may be controlled by interchain electron-hopping. An exponential decrease in the coupled diffusion coefficient with increasing solution pH implies that the electron-hopping process is coupled with intermolecular proton exchange. Deprotonation of the polymer can result in a decrease in the homogeneous electron-transfer rate constant.

Charge-transport processes at poly-o-aminophenol film electrodes: electron hopping accompanied by proton exchange

The kinetics of charge transport in a poly-o-aminophenol-coated electrode was investigated by our measuring its ac response at the varying oxidation and protonation levels of the polymer. Impedance data are interpreted in terms of the diffusion-migration transport of both electrons and protons through the film. An observed change in coupled diffusion coefficients with the oxidation level corresponds with the potential dependence expected in the extreme case of electron-transport control, where electrons are transported by interchain electron hopping. A proportional relationship between the coupled diffusion coefficient and the proton concentration in solution implies that homogeneous electron transfer in the film is accompanied with proton transfer from the reduced sites to the oxidized sites of the polymer. The change in diffusion coefficient with pH can be attributed to a decrease in homogeneous electron-transfer rate constant with deprotonation of the polymer.

Impedance study of the charge transport at poly-o-phenylenediamine film electrodes

Abstract The measured low-frequency capacitance of the polymer film was proportional to the film thickness and showed a maximum at the formal potential of the polymer. We analyzed the relation between the capacitance and potential by taking into account the contribution of an interaction between electroactive sites. The Warburg coefficient and the width of a linear 45 ° region in an impedance diagram showed each their minima near the formal potential. These impedance data were interpreted in terms of the diffusion-migration transport of both electron and anion through the film. The results of the impedance analysis suggested that electron transport was ensured by interchain electron hopping. From the combinations of the measured kinetic parameters, we inferred that one of the two charge carriers moved much faster than the other. The obtained diffusion coefficient showed a maximum in the vicinity of a voltammetric current peak; this corresponded with the potential dependence of a coupled diffusion coefficient expected in the extreme case of electron-transport control.

Organic solid-state solar cells with a mixture of monomeric porphyrins for light-harvesting and regioregular polythiophene for charge transport

Abstract An enhanced photocurrent was observed for a Schottky-barrier cell with a mixed solid consisting of metal-free porphyrin (H 2 tpp) and regioregular polythiophene (PTh) compared to similar cells with pure metal-free porphyrin or pure PTh. This is because produced holes efficiently transport in the high-mobility PTh solid soon after a photoinduced electron-transfer from the PTh molecule to the photon harvesting H 2 tpp molecule.

Photoelectrochemical properties of rhodamine-C18 sensitized p-CuSCN photoelectrochemical cell (PEC)

Abstract Surface states in p-type CuI thiocyanate (CuSCN) were detected from I−V characteristics, diffuse reflectance spectra, and photocurrent action spectra. The p-CuSCN films are sensitized by rhodamine with octadecyl-alkyl chain, and the sensitized photocurrent is observed with the visible light illumination. In spite of the surface states in p-CuSCN, the maximum photocurrent quantum efficiency (gfmax) at λ = 560 nm, in 1 × 10−4 M KI + I2 solution, pH = 6, reached ∼ 8.6%, where the surface dye concentration of photocathode Cu/p-CuSCN/Dye was 1.1 × 1014 molecules cm−2. Photocathodes were biased at −0.25 V versus AgCl/Ag to give a zero dark current. From the variation of φ values with the reduction potential of electron acceptors, the cathodic sensitization mechanism presented is further confirmed.

Voltammetric and impedance study of the binding of ferrocene derivatives to a sulfonated calix[6]arene host in aqueous solutions

Abstract This report describes the effect of host–guest complexation on the thermodynamic and kinetic parameters of ferrocene redox reactions in aqueous media. The formal potentials of the ferrocene/ferrocenium couples examined shifted to less positive values with increasing calixarene concentration, because of the formation of a 1:1 inclusion compound. The order of binding constants was cationic ferrocene≫non-charged ferrocene>anionic ferrocene. The oxidized form with a higher positive charge was bound more strongly to the host than was the corresponding reduced form. The result was contrary to that observed for β-cyclodextrin. The inclusion compounds formed by sulfonated calix[6]arene were greatly stabilized by an electrostatic interaction between the negatively charged host and cationic guest. Fast-scan voltammetric behavior suggested that inclusion-ejection processes practically attain equilibrium in the scan-rate range below 2 V s −1 . Randles plots for the Faradaic impedance of the electrode reaction virtually comprised two nearly parallel straight lines with a small separation over the frequency range of 10 2 −10 −3 Hz, indicating negligible chemical reaction impedance. As the host concentration increased, the mean diffusion coefficient of the (ferrocenylmethyl)trimethylammonium cation decreased inversely proportionally from 6.0×10 −6 cm 2 s −1 of the free ion to (2.1±0.2)×10 −6 cm 2 s −1 of the corresponding inclusion complex. The result of the impedance analysis suggests that the included guest exchanges no electrons directly with the electrode.

Enhanced quantum yield in porphyrin/electron-donor double-layer solar cells

When the layer of 3-carboxymethyl-5-[(3-ethyl-2(3H)-benzothiazolylidine)ethylidene (MC(COOH)) is inserted into the Au/Zntpyp interface in Al/Zntpyp/Au sandwich-type solar cell (Zntpyp: 5,10,15,20-tetra(3-pyridyl)porphyrinatozinc), the photovoltaic properties are remarkably improved. For the Al/Zntpyp(thickness 10 nm)/MC(COOH)(20 nm)/Au cell, a short-circuit photocurrent (Jsc) of 0.93 μ Acm−2, open-circuit photovoltage (Voc) of 0.71 V, fill factor (ff) of 0.41, and energy conversion yield (η) of 3.6% are obtained when illuminated at the Al/Zntpyp interface with 455 nm monochromatic light of 7.5 μ Wcm−2 intensity. A rapid electron-transfer from the donor MC(COOH) to photogenerated holes in Zntpyp suppresses the charge recombination of the photogenerated carriers. The energetically well-arranged valence band levels eventually enhance the η value about 9 times compared with the Al/Zntpyp/Au cell. Further the Al/HD(9 nm)/MC(COOH)(20 nm)/Au cell using a longer-lived sensitizer (HD) instead of Zntpyp gives a Jsc value of 2.36 μ Acm−2, Voc value of 0.69 V, ff value of 0.34, and η value of 4.8% when illuminated with 445 nm monochromatic light of 11.7 μ Wcm−2 intensity at the Al/HD interface, where HD represents a heterodimer consisting of 5,10,15-tri(4-chlorophenyl)-20-(3-pyridyl)porphyrin(H2pyp3p(Cl)) and 5,10,15,20-tetra(2,5-dimethoxyphenyl)porphyrinatozinc(Zntpp(OMe)2).