Masahiko Saito

Thiophene–Thiazolothiazole Copolymers: Significant Impact of Side Chain Composition on Backbone Orientation and Solar Cell Performances

The backbone orientation in the thiophene-thiazolothiazole (TzTz) copolymer system can be altered by tuning of the alky side chain composition. We highlight that the orientation significantly impact their solar cell efficiency in particular when using thicker active layers.

Drastic Change of Molecular Orientation in a Thiazolothiazole Copolymer by Molecular-Weight Control and Blending with PC61BM Leads to High Efficiencies in Solar Cells

A thiazolothiazole-thiophene copolymer is examined as the active material in bulk heterojunction (BHJ) solar cells. By optimizing the molecular weight, the polymer-based cells exhibit power conversion efficiencies as high as 5.7%. The increase in molecular weight improves the orientational order, and blending with phenyl-C61-butyric acid methyl ester (PC61BM) changes the orientational motif from edge-on to face-on, which accounts for the trend in photovoltaic performances. These results might give new insight into the structure-property relationships in BHJ solar cells.

Carbonyl Sulfide Hydrolase from Thiobacillus thioparus Strain THI115 Is One of the β-Carbonic Anhydrase Family Enzymes

Carbonyl sulfide (COS) is an atmospheric trace gas leading to sulfate aerosol formation, thereby participating in the global radiation balance and ozone chemistry, but its biological sinks are not well understood. Thiobacillus thioparus strain THI115 can grow on thiocyanate (SCN(-)) as its sole energy source. Previously, we showed that SCN(-) is first converted to COS by thiocyanate hydrolase in T. thioparus strain THI115. In the present work, we purified, characterized, and determined the crystal structure of carbonyl sulfide hydrolase (COSase), which is responsible for the degradation of COS to H2S and CO2, the second step of SCN(-) assimilation. COSase is a homotetramer composed of a 23.4 kDa subunit containing a zinc ion in its catalytic site. The amino acid sequence of COSase is homologous to the β-class carbonic anhydrases (β-CAs). Although the crystal structure including the catalytic site resembles those of the β-CAs, CO2 hydration activity of COSase is negligible compared to those of the β-CAs. The α5 helix and the extra loop (Gly150-Pro158) near the N-terminus of the α6 helix narrow the substrate pathway, which could be responsible for the substrate specificity. The k(cat)/K(m) value, 9.6 × 10(5) s(-1) M(-1), is comparable to those of the β-CAs. COSase hydrolyzes COS over a wide concentration range, including the ambient level, in vitro and in vivo. COSase and its structurally related enzymes are distributed in the clade D in the phylogenetic tree of β-CAs, suggesting that COSase and its related enzymes are one of the catalysts responsible for the global sink of COS.

Very Small Bandgap π-Conjugated Polymers with Extended Thienoquinoids

The introduction of quinoidal character to π-conjugated polymers is one of the effective approaches to reducing the bandgap. Here we synthesized new π-conjugated polymers (PBTD4T and PBDTD4T) incorporating thienoquinoids 2,2-bithiophene-5,5-dione (BTD) and benzo[1,2-b:4,5-b]dithiophene-2,6-dione (BDTD) as strong electron-deficient (acceptor) units. PBTD4T showed a deep LUMO energy level of -3.77 eV and a small bandgap of 1.28 eV, which are similar to those of the analog using thieno[3,2-b]thiophene-2,5-dione (TTD) (PTTD4T). PBDTD4T had a much deeper LUMO energy level of -4.04 eV and a significantly smaller bandgap of 0.88 eV compared to those of the other two polymers. Interestingly, PBDTD4T showed high transparency in the visible region. The very small bandgap of PBDTD4T can be rationalized by the enhanced contribution of the resonance backbone structure in which the p-benzoquinodimethane skeleton in the BDTD unit plays a crucial role. PBTD4T and PBDTD4T exhibited ambipolar charge transport with more balanced mobilities between the hole and the electron than PTTD4T. We believe that the very small bandgap, i.e., the high near-infrared activity, as well as the well-balanced ambipolar property of the π-conjugated polymers based on these units would be of particular interest in the fabrication of next-generation organic devices.

Highly Efficient and Stable Solar Cells Based on Thiazolothiazole and Naphthobisthiadiazole Copolymers

A critical issue in polymer-based solar cells (PSCs) is to improve the power conversion efficiency (PCE) as well as the stability. Here, we describe the development of new semiconducting polymers consisting of thiophene, thiazolothiazole and naphthobisthiadiazole in the polymer backbone. The polymers had good solubility and thus solution-processability, appropriate electronic structure with narrow band gaps of ~1.57u2009eV and low-lying HOMO energy levels of ~−5.40u2009eV, and highly ordered structure with the favorable face-on backbone orientation. Solar cells based on the polymers and PC71BM exhibited quite high PCEs of up to 9%. More interestingly, the cells also demonstrated excellent stability as they showed negligible degradation of PCE when stored at 85˚C for 500u2009hours in the dark under nitrogen atmosphere. These results indicate that the newly developed polymers are promising materials for PSCs in the practical use.

Dithienylthienothiophenebisimide, a Versatile Electron-Deficient Unit for Semiconducting Polymers.

A novel electron-deficient building unit, dithienylthienothiophenebisimide, and its polymers (PTBIs) are reported. Organic photovoltaic (OPV) cells based on PTBIs as p-type material exhibit 8.0% efficiencies with open-circuit voltages higher than 1 V. Interestingly, PTBIs also function as n-type material in OPVs depending on the molecular structure. These polymers also exhibit p-channel, n-channel, and ambipolar behaviors in field-effect transistors.

Degradation of ambient carbonyl sulfide by Mycobacterium spp. in soil

The ability to degrade carbonyl sulfide (COS) was confirmed in seven bacterial strains that were isolated from soil, without the addition of COS. Comparative 16S rRNA gene sequence analysis indicated that these isolates belonged to the genera Mycobacterium, Williamsia and Cupriavidus. For example, Mycobacterium sp. strain THI401, grown on PYG agar medium, was able to degrade an initial level of 30 parts per million by volume COS within 1 h, while 60 % of the initial COS was decreased by abiotic conversion in 30 h. Considering natural COS flux between soil and the atmosphere, COS degradation by these bacteria was confirmed at an ambient level of 500 parts per trillion by volume (p.p.t.v.), using sterilized soil to cultivate the bacterium. Autoclave sterilization of soil resulted in a small amount of COS emission, while Mycobacterium spp. degraded COS at a faster rate than it was emitted from the soil, and reduced the COS mixing ratio to a level that was lower than the ambient level: THI401 degraded COS from an initial level of 530 p.p.t.v. to a level of 330 p.p.t.v. in 30 h. These results provide experimental evidence of microbial activity in soil as a sink for atmospheric COS.

Small band gap polymers incorporating a strong acceptor, thieno[3,2-b]thiophene-2,5-dione, with p-channel and ambipolar charge transport characteristics

We present new donor–acceptor semiconducting polymers based on a strong acceptor unit, thieno[3,2-b]thiophene-2,5-dione (TTD). The polymers exhibit a deep LUMO energy level of around −4 eV while preserving a relatively low-lying HOMO energy level of below −5 eV and a quite small optical band gap of 1.2 eV. Interestingly, bottom-gate-top-contact transistor devices based on the polymers demonstrate p-channel behavior with high hole-mobilites of 1.38 cm2 V−1 s−1, whereas top-gate-bottom-contact devices show ambipolar behavior with hole and electron mobilities of ∼0.12 and ∼0.20 cm2 V−1 s−1, respectively. These results indicate the great potential of TTD to be used as the building unit for high-performance semiconducting polymers.

Impact of side chain placement on thermal stability of solar cells in thiophene–thiazolothiazole polymers

Stability of polymer-based bulk-heterojunction solar cells (PSCs) is an even more crucial factor than power conversion efficiency for commercialization. Here, the thermal stability of PSCs is studied using a thiophene–thiazolothiazole semiconducting polymer system. Interestingly, it was found that the cells that used polymers with a less regular side chain placement (iPTzBT) showed higher thermal stability than the cells that used polymers with a more regular placement (PTzBT). The difference can be correlated with the change in the polymer structural order upon thermally annealing the polymer/fullerene blend films. Whereas the structural order of iPTzBT is unchanged after annealing, that of PTzBT is decreased, although the crystallinity is lower in iPTzBT than in PTzBT. These results clearly demonstrate that careful design of the side chain placement in the semiconducting polymers is an important factor for improving the thermal stability of PSCs.

Amide-bridged terphenyl and dithienylbenzene units for semiconducting polymers

We here describe the synthesis, characterization, and structures of new semiconducting polymers (PIQP2T and PTPQ2T) based on amide-bridged terphenyl (IQP) and dithienylbenzene (TPQ), and their performances in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). The polymers are found to have relatively wide band gaps of >2.0 eV and deep HOMO energy levels of −5.4 eV. Interestingly both the HOMO and LUMO energy levels similarly shift upward and downward, respectively, by the π-extension from the monomer unit to the polymer, which can be ascribed to the delocalized HOMO and LUMO along the molecular frameworks. This suggests that IQP and TPQ can be viewed as electron-neutral building units. Both polymers had similar ordering structures in the thin film despite the fact that the IQP is more sterically hindered at the end of the moiety than TPQ. This is probably due to the strong intermolecular interactions originating in the amide group. The polymers exhibited similar hole mobilities of 0.03–0.04 cm2 V−1 s−1 in the OFET devices. Although the PCEs were modest, the OPV devices based on these polymers showed a quite high VOC of 0.94 V.