Formation of single-walled carbon nanotube thin films enriched with semiconducting nanotubes and their application in photoelectrochemical devices

Single-walled carbon nanotube (SWCNT) thin films, containing a high-density of semiconducting nanotubes, were obtained by a gel-centrifugation method. The agarose gel concentration and centrifugation force were optimized to achieve high semiconducting and metallic nanotube separation efficiency at 0.1 wt% agarose gel and 18,000g. The thickness of SWCNT films can be precisely controlled from 65 to 260 nm with adjustable transparency. These SWCNT films were applied in photoelectrochemical devices. Photocurrents generated by semiconducting SWCNT enriched films are 15-35% higher than those by unsorted SWCNT films. This is because of reducing exciton recombination channels as a result of the removal of metallic nanotubes. Thinner films generate higher photocurrents because charge carriers have less chances going in metallic nanotubes for recombination, before they can reach electrodes. Developing more scalable and selective methods for high purity semiconducting SWCNTs is important to further improve the photocurrent generation efficiency by using SWCNT-based photoelectrochemical devices.

Dispersion of carbon nanotubes by photo- and thermal-responsive polymers containing azobenzene unit in the backbone

Photo- and thermal-responsive polymers containing azobenzene units in the main chain have been utilized as removable dispersants for single-walled carbon nanotubes (SWNTs) in organic solvents. Intermolecular interactions between SWNTs and the polymers are reversibly controllable by tuning the trans-cis composition.

Large pi-aromatic molecules as potential sensitizers for highly efficient dye-sensitized solar cells

Recently, dye-sensitized solar cells have attracted much attention relevant to global environmental issues. Thus far, ruthenium(II) bipyridyl complexes have proven to be the most efficient TiO(2) sensitizers in dye-sensitized solar cells. However, a gradual increment in the highest power conversion efficiency has been recognized in the past decade. More importantly, considering that ruthenium is a rare metal, novel dyes without metal or using inexpensive metal are desirable for highly efficient dye-sensitized solar cells. Large pi-aromatic molecules, such as porphyrins, phthalocyanines, and perylenes, are important classes of potential sensitizers for highly efficient dye-sensitized solar cells, owing to their photostability and high light-harvesting capabilities that can allow applications in thinner, low-cost dye-sensitized solar cells. Porphyrins possess an intense Soret band at 400 nm and moderate Q bands at 600 nm. Nevertheless, the poor light-harvesting properties relative to the ruthenium complexes have limited the cell performance of porphyrin-sensitized TiO(2) cells. Elongation of the pi conjugation and loss of symmetry in porphyrins cause broadening and a red shift of the absorption bands together with an increasing intensity of the Q bands relative to that of the Soret band. On the basis of the strategy, the cell performance of porphyrin-sensitized solar cells has been improved intensively by the enhanced light absorption. Actually, some push-pull-type porphyrins have disclosed a remarkably high power conversion efficiency (6-7%) that was close to that of the ruthenium complexes. Phthalocyanines exhibit strong absorption around 300 and 700 nm and redox features that are similar to porphyrins. Moreover, phthalocyanines are transparent over a large region of the visible spectrum, thereby enabling the possibility of using them as "photovoltaic windows". However, the cell performance was poor, owing to strong aggregation and lack of directionality in the excited state. Novel unsymmetrical zinc phthalocyanine sensitizers with "push" and "pull" groups have made it possible to reduce the aggregation on a TiO(2) surface, tune the level of the excited state, and strengthen the electronic coupling between the phthalocyanine core and the TiO(2) surface. As a result, the power conversion efficiency of up to 3.5% has been achieved. Perylenes are well-known as chemically, thermally, and photophysically stable dyes and have been used in various optical devices and applications. Nevertheless, the power conversion efficiency remained low compared to other organic dyes. The origin of such limited cell performance is the poor electron-donating abilities of the perylenes, which makes it difficult to inject electrons from the excited singlet state of the perylenes to the conduction band of the TiO(2) electrode efficiently. Strongly electron-donating perylene carboxylic acid derivatives with amine substituents at their perylene core have allowed us to increase the power conversion efficiency of up to approximately 7% in perylene-sensitized solar cells. The efficiency of large pi-aromatic molecule-sensitized solar cells could be improved significantly if the dyes with larger red and near-infrared absorption could be developed.

Substituent effects of porphyrins on structures and photophysical properties of amphiphilic porphyrin aggregates

Substituent effects of porphyrin on the structures and photophysical properties of the J-aggregates of protonated 5-(4-alkoxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin have been examined for the first time. Selective formation of the porphyrin J-aggregate was attained when suitable length of the alkoxy group was employed for the amphiphilic porphyrin. Namely, a regular leaflike structure was observed for the J-aggregates of protonated 5-(4-octyloxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin, which was consistent with the results obtained by using the UV-visible absorption and dynamic light-scattering measurements. A bilayer structure in which the hydrophobic alkoxyl groups facing inside the bilayer are interdigitated to each other, whereas the hydrophilic porphyrin moieties are exposed outside, was proposed to explain the unique porphyrin J-aggregate. Fast energy migration and efficient quenching by defect site in the J-aggregates were suggested to rationalize the short lifetimes of the excited J-aggregates.

meso-3,5-Bis(trifluoromethyl)phenyl-substituted expanded porphyrins: synthesis, characterization, and optical, electrochemical, and photophysical properties

Trifluoroacetic acid-catalyzed condensation of pyrrole with electron-deficient and sterically hindered 3,5-bis(trifluoromethyl)benzaldehyde results in the unexpected production of a series of meso-3,5-bis(trifluoromethyl)phenyl-substituted expanded porphyrins including [22]sapphyrin 2, N-fused [22]pentaphyrin 3, [26]hexaphyrin 4, and intact [32]heptaphyrin 5 together with the conventional 5,10,15,20-tetrakis(3,5-bis(trifluoromethyl)phenyl)porphyrin 1. These expanded porphyrins are characterized by mass spectrometry, (1)H NMR spectroscopy, UV/Vis/NIR absorption spectroscopy, and fluorescence spectroscopy. The optical and electrochemical measurements reveal a decrease in the HOMO-LUMO gap with increasing size of the conjugated macrocycles, and in accordance with the trend, the deactivation of the excited singlet state to the ground state is enhanced.

Light harvesting and energy transfer in multiporphyrin-modified CdSe nanoparticles

Multiporphyrin-modified CdSe nanoparticles (CdSe-H2P) were prepared to elucidate the interaction between chromophores and luminescent semiconducting nanoparticles in the excited and ground states. The CdSe-H2P nanoparticles were obtained by place-exchange reactions of hexadecylamine-thiophenol-modified CdSe nanoparticles with porphyrin alkanethiols in toluene. The number of porphyrin molecules on the surface of a single CdSe nanoparticle increased with increasing reaction time to reach a saturated maximum of 21. The porphyrins as well as the core in CdSe-H2P can absorb UV/Vis radiation. Steady-state emission and emission-lifetime measurements reveal efficient energy transfer from the CdSe excited state to the porphyrins in the CdSe-H2P nanoparticles. The resulting porphyrin excited singlet state is not quenched by the CdSe core. These unique properties are in sharp contrast with those of multiporphyrin-modified metal and silica nanoparticles. Thus, semiconducting nanoparticle-multiporphyrin composites are highly promising as novel artificial photosynthetic materials.

Synthesis and photophysical properties of electron-rich perylenediimide-fullerene dyad

An electron-rich perylenediimide-C60 dyad has been prepared to explore a new type of donor-acceptor system. Time-resolved absorption measurements in benzonitrile revealed unambiguous evidence for the formation of a charge-separated state consisting of perylene diimide radical cation and C60 radical anion via photoinduced electron transfer, showing a new class of artificial photosynthetic models in terms of charge separation.

Electron-Donating Perylene Tetracarboxylic Acids for Dye-Sensitized Solar Cells

Novel perylene imide derivatives with both electron-donating and bulky substituents have been synthesized for dye-sensitized solar cells. The power conversion efficiency reached 2.6%, which is the highest value among perylene-sensitized TiO2 solar cells.

Novel unsymmetrically π-elongated porphyrin for dye-sensitized TiO2cells

A novel naphthyl-fused zinc porphyrin carboxylic acid has been synthesized and employed successfully in a dye-sensitized TiO2 solar cell, with a power conversion efficiency of 4.1%, which is improved by 50% relative to the unfused porphyrin reference cell.