Takashi Sagawa

Vertically aligned ZnO nanorods doped with lithium for polymer solar cells: defect related photovoltaic properties

The nanorod arrays of ZnO incorporated with lithium atoms show specific crystallinity, photoluminescence and absorption properties, which are promising for the improvement of photovoltaic performance of hybrid solar cells based on ZnO/poly(3-hexylthiophene). Li ions can be incorporated into ZnO crystals during the hydrothermal growth of the nanorods. The presence of Li in ZnO crystal was confirmed through X-ray diffraction analysis and by the photoluminescence spectra obtained. The difference in photovoltaic properties brought about by Li doping was determined from concentrations of the precursor solution. It was determined that appropriate Li doping improves both the short circuit current density (Jsc) and open circuit voltage (Voc). The quenching of photoluminescence of Li-doped ZnO nanorods/P3HT films indicates effective charge transfer at the interface due to oxygen-enrichment of the surface, corresponding to the enhancement of Jsc. The improvement of Voc was due to the suppression of the charge injection from the electrode brought about by the increase in barrier height at the ITO/ZnO interface as the work function of the ZnO nanorods was reduced after Li ion doping. However, further substitution of Li to Zn (LiZn) leads to increased reverse current densities of minority carriers decreasing Voc after the maximum value at 5 atom% incorporation. The maximum power conversion efficiency of 0.37% was obtained at 5 atom% doping, although improvements in photovoltaic performances through Li doping were still seen up to 20 atom% doping.

Improvement of Dye-Sensitized Solar Cell Through TiCl4-Treated TiO2 Nanotube Arrays

Titania nanotube arrays have been fabricated on a fluoride-doped tin oxide substrate by liquid phase deposition using ZnO nanorod arrays as a template and have been applied as the electrode for dye-sensitized solar cells (DSCs). The performance of DSCs based on Ti0 2 nanotube electrodes can be improved by treating the TiO 2 nanotube arrays with titanium tetrachloride (TiCl 4 ). Both the short-circuit current density and the conversion efficiency increased almost 2 times after the TiCl 4 treatment. The TiCl 4 treatment not only increased the amount of absorbed dyes but also enhanced the electron transport in the TiO 2 films. TiCl 4 -treated TiO 2 nanotube arrays with 4 μm thickness showed a short-circuit current density of 8.37 mA/cm 2 , an open-circuit voltage of 0.80 V, a fill factor of 0.67, and an overall conversion efficiency (η) of 4.53%.

Control of self organization in conjugated polymer fibers.

We propose new strategy to facilitate the fabrication of conjugated polymer fiber with higher oriented structures, which focused on electrospinning of a blend solution of regioregular poly(3-hexylthiophene) (rr-P3HT) and poly(vinyl pyrrolidone) (PVP). SEM observation revealed that the blend system forms homogeneous composite nanofibers. This system exhibits the specific feature of strong interchain contribution of P3HT from UV-vis absorption, fluorescence spectroscopic, XRD, and photoelectron spectrometric (for HOMO levels) investigations. We also demonstrate the removal of the PVP component from the P3HT/PVP composite fibers through the selective extraction and such strong interchain stacking of pristine P3HT fiber mat can be remarkably maintained.

Versatile chiroptics of peptide-induced assemblies of metalloporphyrins

Zinc porphyrin functionalized with double long-chain alkylated L-glutamide (GTPP-Zn) was synthesized for the first time, and its self-assembling behaviour was investigated in nonpolar organic solvents. The uniqueness of this functionalized porphyrin is characterized by its drastic colour change from dark green to purple via the formation of chirally stacked structures through selective axial coordination on zinc with pyridine derivatives. In this paper, we report the versatility of the GTPP-Zn assembly process as a stimuli-responsive chiroptical switching system and describe the remarkable ligand-specific induction of secondary chirality accompanied by aggregation morphological change.

Improvement of Power Conversion Efficiency in Organic Photovoltaics by Slow Cooling in Annealing Treatment

We investigated the effect of cooling rate in the annealing treatment of bulk heterojunction organic photovoltaic devices using blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the active layer. Slow cooling after thermal annealing altered the surface morphology and optical properties of the film as compared with those of fast cooling, which resulted in higher power conversion efficiency (PCE). The incident photon-to-current conversion efficiency (IPCE) was also improved in the case of slow cooling. The highest PCE, 3.4% under air mass 1.5 simulated solar illumination (100 mW/cm2), was achieved.

Effect of Heat-Treatment on Electron Transport Process in TiO2 Nanotube Arrays Prepared Through Liquid Phase Deposition for Dye-Sensitized Solar Cells

Nanotube arrays of TiO 2 were synthesized by using a template of ZnO nanorod arrays on fluorine-doped SnO 2 transparent conducting oxide glass substrate by liquid phase deposition method. Dye N719-sensitized photoelectrochemical cells comprising TiO 2 nanotube arrays were fabricated and characterized. With an increase in the calcination temperature, the anatase content of the crystallized film increased and the conduction band of TiO 2 shifted toward a negative potential, resulting in an increase in the cell performance in terms of the short-circuit photocurrent density and open-circuit voltage.

Charge Transporting Properties and Output Characteristics in Polythiophene:Fullerene Derivative Solar Cells

The carrier mobility (µ) and lifetime (τ) of polymer solar cells based on the active layer with various compositions of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been investigated through photoinduced charge carrier extraction by linearly increasing voltage measurement. Increase in the PCBM content resulted in the increase in the mobility up to one order of magnitude and the highest performance was attained at the composition of P3HT:PCBM (3:2), which corresponds to the bending point of mobility and lifetime. Furthermore, the mobility-lifetime product µτ was almost constant with various weight ratios of P3HT and PCBM. This result implies that the composition does not affect the charge correction efficiency before recombination. A semiconducting thin-film optics simulator was applied for prediction of the photovoltaic performance. Theoretical and experimental cell performance results were in good agreement with each other.

Fabrication and Characterizations of Poly(3-hexylthiophene) Nanofibers

Conductive nanofibers with the average diameters in the range of 60 nm - 2 μm were fabricated by electrospinning of a mixture of poly(3-hexylthiophene) (P3HT) and polyvinylpyrrolidone (PVP) in a mixed solvent of chlorobenzene and methanol. Beaded fibers and/or uniform, smooth-surface fibers were successfully fabricated. The average diameter of the as-spun fibers decreased and the color of as-spun fibers changed with decreasing the concentration of P3HT or PVP. After the removal of PVP from as-spun fibers by Soxhlet extraction, pure P3HT fibers were obtained as a spindle-like with groove-like morphological appearance which may be widely applicable for some specific applications, such as photovoltaic cells, thin film transistors, and light emitting diodes.

Poly(3-hexylthiophene) Nanofibers Fabricated by Electrospinning and Their Optical Properties

Beaded fibers and/or uniform, smooth-surface fibers of conductive polymers with the average diameters ranging in nanometers to sub-micrometers were fabricated by electrospinning of a mixture of poly(3-hexylthiophene) (P3HT) and polyvinylpyrrolidone (PVP) in a mixed solvent of chlorobenzene and methanol. After the removal of PVP from as-spun fibers by Soxhlet extraction, pure P3HT fibers were obtained as a spindle-like with groove-like morphological appearance which may be widely applicable for some specific applications, such as photovoltaic cells, thin film transistors, and light emitting diodes. Optical properties, including UV absorption and photoluminescence (PL) of fibers were investigated. As-spun fibers showed relatively higher conjugation length and different chain distribution, in comparison with the cast film.