Yoshinori Nishikitani

Electrochemical properties of non-conjugated electrochromic polymers derived from aromatic amine derivatives

Abstract The electrochemical and optical properties of two types of non-conjugated electrochromic polymers derived from aromatic amine derivatives (DDP-A, DDB-P) are presented. DDP-A is synthesized by the polymerization of N , N ′-dimethyl- N , N ′-diphenyl-1,4-phenylenediamine (DDP) and acetaldehyde (A), and DDB-P is polymerized with N , N ′-dimethyl- N , N ′-diphenylbenzidine (DDB) and propionaldehyde (P). Both DDP-A and DDB-P have a band gap in the ultraviolet region, and are colorless and transparent in neutral states. At one-electron oxidation states, DDP-A + absorbs light mainly in a visible region, whereas DDB-P + absorbs light mainly in a near-infrared (NIR) region. DDP-A + and DDB-P + are nitrogen-centered π-bridged mixed-valence compounds showing intervalence-charge transfer bands. The electrochemical and optical properties of DDP-A and DDB-P depend on the nitrogen–nitrogen distance correlating the electronic coupling of nitrogen redox centers. From the standpoint of energy saving, DDB-P is very interesting because it absorbs light in an NIR region. Finally, focusing on the smart window application, the optical properties of the solid-state electrochromic cell fabricated with DDP-A and heptyl viologen are examined. The cell was confirmed to be colored blue by applying a potential of about 1.0 V, and bleached at 0 V.

Electrophoretically Deposited TiO2 Nanotube Light-Scattering Layers of Dye-Sensitized Solar Cells

We report herein the enhanced light-to-electricity conversion efficiency of a dye-sensitized solar cell using a new bilayer structure of the TiO2 electrode. The bilayer structure consists of a light-absorbing TiO2 nanoparticle layer together with a light-scattering TiO2 nanotube layer formed upon. High aspect-ratio TiO2 nanotubes (α-TNTs) with a diameter of 20 nm were prepared via anodization of a Ti sheet in a perchloric acid solution, and the α-TNT layer was electrophoretically deposited onto the sintered TiO2 nanoparticle layer. The light-scattering property of the α-TNT layer was comparable to that of the commonly used TiO2 sub-micron nanoparticle layer. The α-TNT layer provided a large surface area for dye-adsorption as well as an efficient transport pathway for photo-generated carriers. These effects allowed a higher incident photon-to-current efficiency of the bilayer TiO2 structure with the nanotube light-scattering layer over the whole spectral range relative to that with a sub-micron nanoparticle layer.

Triarylamine-Functionalized Ruthenium Dyes for Efficient Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) based on mesoporous nanocrystalline TiO2 films have been attracting intensive interest for scientific and industrial applications because of their high photo-to-electricity conversion efficiency and low production cost . Although numerous sensitizers, including metal-free organic dyes and nonruthenium metal dyes, have been employed, the best energy conversion efficiency of up to 11% was achieved by using ruthenium dyes, such as N3, N719, and the black dye [2] in standard global air mass 1.5 sunlight. To raise further the efficiency of these cells, much effort has been directed toward the development of highly efficient solar cells based on ruthenium dyes. There are several basic requirements guiding the molecular engineering of an efficient sensitizer. The LUMO of the dye must be sufficiently high in energy for efficient charge injection into the TiO2, and the HOMO must be sufficiently low in energy for efficient regeneration of the oxidized dye by the hole-transport material (HTM) . Finally, light excitation should be associated with vectorial electron transfer from the excited dye to the TiO2 conduction band. On the other hand, the triarylamine molecules have aroused great interest for their excellent hole-transport capability , and organic dyes including triarylamine moiety acting as an electron donor exhibit high efficiencies in the conversion of solar energy into electricity .

Short-circuit current density behavior of dye-sensitized solar cells

Short-circuit current densities (Jscs) were measured on two kinds of dye-sensitized solar cells (DSCs). The dependences of Jsc on the diffusion coefficient of I3- and the cell gap are discussed using relevant model equations for the DSC. In the lower diffusion coefficient region, the DSC operates under an I3- diffusion-limiting condition and, under this condition, narrowing the cell gap is a simple and effective way to increase the Jsc. The Jsc of a gel-type polymeric-electrolyte-based DSC with a cell gap of 20 µm is verified to be equal to that of a liquid electrolyte-based DSC.

TiO2 Nanotube Layers on Ti Substrates for High Efficiency Flexible Dye-Sensitized Solar Cells

We report herein the promising application of TiO2 nanotubes formed on a Ti substrate to a flexible dye-sensitized solar cell illuminated from the back-side. We found that anodization of a Ti substrate in an extremely dilute perchloric acid solution yielded the formation of a high aspect-ratio TiO2 nanotube layer on the Ti substrate. The nanotube layer has the equivalent surface area for dye-adsorption to a TiO2 nanoparticle layer. This novel one-dimensional TiO2 material provided efficient carrier generation interfaces as well as an efficient transport pathway for the carriers, evidenced by the enhanced photocurrent and photovoltage of the cell.

Thermal and optical behavior of electrochromic windows fabricated with carbon-based counterelectrode

We proposed a carbon-based counterelectrode for electrochromic windows (ECWs) and fabricated a new solid-state ECW consisting of an indium tin oxide electrode (ITO, IN2O3:SN)/a WO3 film/a polymeric solid electrolyte (PSE)/a carbon-based counterelectrode. The carbon-based counterelectrode is a series of arrays of carbon material dots formed on an ITO substrate and is virtually transparent in a visible region, Those carbon dots play a part in the formation of an electric double layer in an electrochromic reaction of the ECW. The electric double layer capacitance of the counterelectrode increases linearly as a function of carbon-dot covering percentage on the ITO substrate. Maximum differential optical density of the ECW increases with the covering percentage of the carbon dots up to a point and levels off for further increase in the covering percentage. The response time of coloration decreases with temperatures, which is caused chiefly by the temperature dependence of an ionic conductivity of the PSE. The behavior of ECWs is explained well with a simple equivalent-circuit comprising two capacitors corresponding to the WO3, film and the carbon-based counterelectrode, an electric resistor of the PSE and a power source connected in series.

Anodic Formation of Titania Nanotubes with Ultrahigh Aspect Ratio

We report on the anodic formation of titania nanotubes with a uniform diameter of about 20 nm and an aspect ratio over 1000:1 in an HClO 4 solution. The as-prepared product was obtained as an amorphous structure, which can be crystallized to a pure anatase structure with hollow characteristics remaining. The cross-sectional scanning electron microscopy images of interfaces between the Ti substrate and titania indicate that the gradual structural transformation of titania induced by concentrated ion species via localized electrochemical reactions is the underlying mechanism for the formation of nanotubes.

White polymer light-emitting electrochemical cells using emission from exciplexes with long intermolecular distances formed between polyfluorene and π-conjugated amine molecules

The authors present white polymer light-emitting electrochemical cells (PLECs) fabricated with polymer blend films of poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and π-conjugated triphenylamine molecules. The PLECs have bulk heterojunction structures composed of van der Waals interfaces between the PFD segments and the amine molecules. White-light electroluminescence (EL) can be achieved via light-mixing of the blue exciton emission from PFD and long-wavelength exciplex emission from excited complexes consisting of PFD segments (acceptors (As)) and the amine molecules (donors (Ds)). Precise control of the distances between the PFD and the amine molecules, affected through proper choice of the concentrations of PFD, amine molecules, and polymeric solid electrolytes, is critical to realizing white emission. White PLECs can be fabricated with PFD and amine molecules whose highest occupied molecular orbital (HOMO) levels range from −5.3 eV to −5.0 eV. Meanwhile, PLECs fabricated with amine molecules whose ...

Band edge disorder producing Urbach's tail and enhanced photochromic efficiency caused by density of states inside band gap in tungsten oxide films

The band gap of WO3 and its disorder at the band edge were evaluated, assuming Urbachs rule, and flat-band potentials (V fb s) were also measured on WO3 films with high photochromic efficiency (HPE) and low photochromic efficiency (LPE). The main results are as follows: 1) the optical absorption of LPE- WO3 sets on at higher photon energy than that of HPE- WO3, 2) HPE- WO3 has the density of states deeper in the band gap than LPE- WO3 does, and 3) V fb of HPE- WO3 is higher than that of HPE- WO3. These results indicate that the band edge disorder of WO3, which is closely related to Urbachs tail, determines the Fermi level position, and therefore makes V fb high; they also imply that a high V fb dissociates optically excited electrons and holes to make colors center resulting in photochromism at a high efficiency.

Nanostructured Organic Bulk Heterojunction Solar Cells

Publisher Summary This chapter deals with the nanostructured organic bulk heterojunction solar cells. More than three decades of research on organic solar cells based on π-conjugated materials has led to steadily increasing efficiency of solar cells. This chapter focuses on small molecular organic materials and their solar cells comprise three sections: (1) photophysical properties of organic p-conjugated materials; (2) characteristics of simple bulk heterojunction solar cells; and (3) characteristics of hybrid-type heterojunction solar cells. The hybrid bulk heterojunction (HH) solar cell was shown to have a high conversion efficiency compared to the heterojunction (PH) and bulk heterojunction (BH) solar cells. Moreover, there is a possibility that applying anti-reflecting coatings to the tandem cell could increase efficiency to over 6%. This chapter explains the design of the BH solar cell structure that has been discussed so far by focusing on the exciton diffusion and charge carrier collection efficiencies only. The product of the four key factors, ηA × ηED ×ηCT × ηCC, determines the conversion efficiency of an organic solar cell. Therefore, all four quantum efficiencies must be optimized to achieve conversion efficiency in excess of 10–15%. New donor and acceptor materials must be tailored to have optimum energy levels of highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO and LUMO) so that the materials can absorb a broader solar-spectral region and have lower exciton-binding energy. This continuous and fundamental research will mark a new epoch in organic solar cells, which are expected to offer low-cost solar energy conversion and be environmental friendly.