Satoshi Makuta

Dye Regeneration Kinetics in Dye-Sensitized Solar Cells

The ideal driving force for dye regeneration is an important parameter for the design of efficient dye-sensitized solar cells. Here, nanosecond laser transient absorption spectroscopy was used to measure the rates of regeneration of six organic carbazole-based dyes by nine ferrocene derivatives whose redox potentials vary by 0.85 V, resulting in 54 different driving-force conditions. It was found that the reaction follows the behavior expected for the Marcus normal region for driving forces below 29 kJ mol(-1) (ΔE = 0.30 V). Driving forces of 29-101 kJ mol(-1) (ΔE = 0.30-1.05 V) resulted in similar reaction rates, indicating that dye regeneration is diffusion controlled. Quantitative dye regeneration (theoretical regeneration yield 99.9%) can be achieved with a driving force of 20-25 kJ mol(-1) (ΔE ≈ 0.20-0.25 V).

Application of the Tris(acetylacetonato)iron(III)/(II) Redox Couple in p‐Type Dye‐Sensitized Solar Cells

An electrolyte based on the tris(acetylacetonato)iron(III)/(II) redox couple ([Fe(acac)3](0/1-)) was developed for p-type dye-sensitized solar cells (DSSCs). Introduction of a NiO blocking layer on the working electrode and the use of chenodeoxycholic acid in the electrolyte enhanced device performance by improving the photocurrent. Devices containing [Fe(acac)3](0/1-) and a perylene-thiophene-triphenylamine sensitizer (PMI-6T-TPA) have the highest reported short-circuit current (J(SC)=7.65 mA cm(-2)), and energy conversion efficiency (2.51%) for p-type DSSCs coupled with a fill factor of 0.51 and an open-circuit voltage V(OC)=645 mV. Measurement of the kinetics of dye regeneration by the redox mediator revealed that the process is diffusion limited as the dye-regeneration rate constant (1.7×10(8) M(-1) s(-1)) is very close to the maximum theoretical rate constant of 3.3×10(8) M(-1) s(-1). Consequently, a very high dye-regeneration yield (>99%) could be calculated for these devices.

Organic/inorganic hybrid electrochromic devices based on photoelectrochemically formed polypyrrole/TiO2 nanohybrid films

Electrochromic devices employing photoelectrochemically fabricated polypyrrole/TiO2 nanohybrid films have been demonstrated for the first time. The nanohybrid film was successfully formed photoelectrochemically by exciting nanocrystalline TiO2 with UV light irradiation in the presence of pyrrole molecules in an electrochemical cell. The nanohybrid film exhibited electrochromic colour changes similar to the flat polypyrrole film. The fastest polypyrrole reduction response of 0.4 ms (evaluated by an absorbance change from 0 to 90%) was achieved. The electrochromic responses of the hybrid films were controlled by relative potential energy levels between the TiO2 conduction band edge and the polypyrrole redox potential, adjusted by the solution pH, and homogeneity/thickness of a polypyrrole layer on the surface of a TiO2 film. Based on the experimental results, the dual electron transport mechanisms influencing the electrochromic behaviours are suggested: Path 1, electron transport along the polypyrrole layer; and Path 2, electron transport through the nanocrystalline TiO2 conduction band and electron transfer reactions at the hybrid interface. Finally, the pattern electrochromic reactions can now be implemented as the direct application to display devices.

Semiconductor Quantum Dot Sensitized Solar Cells Based on Ferricyanide/Ferrocyanide Redox Electrolyte Reaching an Open Circuit Photovoltage of 0.8 V

Semiconductor quantum dot sensitized solar cells (QDSSCs) have rapidly been developed, and their efficiency has recently exceeded 9%. Their performances have mainly been achieved by focusing on improving short circuit photocurrent employing polysulfide electrolytes. However, the increase of open circuit photovoltage (VOC) cannot be expected with QDSSCs based on the polysulfide electrolytes owing to their relatively negative redox potential (around -0.65 V vs Ag/AgCl). Here, we demonstrate enhancement of the open circuit voltage by employing an alternative electrolyte, ferricyanide/ferrocyanide redox couple. The solar cell performance was optimized by investigating the influence of ferricyanide and ferrocyanide concentration on their interfacial charge transfer and transport kinetics. The optimized ferricyanide/ferrocyanide species concentrations (0.01/0.2 M) result in solar energy conversion efficiency of 2% with VOC of 0.8 V. Since the potential difference between the TiO2 conduction band edge at pH 7 and the electrolyte redox potential is about 0.79 V, although the conduction band edge shifts negatively under the negative bias application into the TiO2 electrode, the solar cell with the optimized electrolyte composition has nearly reached the theoretical maximum voltage. This study suggests a promising method to optimize an electrolyte composition for maximizing solar energy conversion efficiency.

Photo-induced electron transfer reactions at semiconductor quantum dot interfaces

Cadmium sulfide (CdS) quantum dots with a series of electron donor and acceptors were employed to investigate thermodynamic and kinetic influences on photo-induced electron transfer reactions at CdS surface. Although the potential energy levels of all electron donor and acceptors are located within the QD band gap, the QD photoluminescence (PL) behavior is dependent upon the type of quencher. PL decreased into half, when one methyl viologen or benzyl viologen molecule per one QD was added into the QD solution, implying that the molecule attaches to the QD surface. In contrast, the dynamic quenching behavior was observed when thionine or o-tolidine was employed as a quencher. PL quenching efficiency decreased, when the distance between the QD surface and the quencher was increased by capping the QD with butylamine. Therefore, the PL quenching is mainly controlled kinetically rather than thermodynamically.

Preparation and Characteristic Control of Conducting Polymer/Metal Oxide Nano‐Hybrid Films for Solar Energy Conversion

We review our recent investigation of photoinduced polymerization of thiophene inside TiO2 nanopores to form nanostructured polythiophene/TiO2 heterojunction films and the related kinetic studies. The nanohybrid film possesses dense heterojunction and electronic connection within the TiO2 nanoporous domain. Photo-polymerization proceeded in 3 stages, (i) photoexcitation of bithiophene covalently attached to the TiO2 surface, (ii) an electron injection reaction from the surface attached thiophene to the TiO2 and (iii) an electron transfer from a thiophene reactant in an electrolyte to the surface attached oxidized bithiophene. The nanohybrid film was applied to a sensitized-type photoelectrochemical solar cell, substantiating direct application of the nanohybrid film to electronic devices. Despite increase in light harvesting efficiency, wavelength dependent incident photon-to-current conversion efficiency decreased with the light irradiated polymerization time. In order to identify a factor controlling photocurrent efficiency, kinetic studies at TiO2/bithiophene/electrolyte interfaces were conducted, and their parameters to the solar cell functions were related. Comparison of emission studies between the bithiophene adsorbed TiO2 and Al 2O3 revealed the electron injection from the excited bithiophene into the TiO2 with the efficiency of nearly 100 %. The charge recombination between the bithiophene cation and the electron in the TiO2 appeared to be fast with a half decay time of 70 ?s in comparison to the ruthenium dye sensitized TiO2 film (?1 ms). The bithiophene regeneration kinetics was slightly faster, clarifying the inferior photocurrent performance.