Makoto Karakawa

Transparent Conductive Nanofiber Paper for Foldable Solar Cells.

Optically transparent nanofiber paper containing silver nanowires showed high electrical conductivity and maintained the high transparency, and low weight of the original transparent nanofiber paper. We demonstrated some procedures of optically transparent and electrically conductive cellulose nanofiber paper for lightweight and portable electronic devices. The nanofiber paper enhanced high conductivity without any post treatments such as heating or mechanical pressing, when cellulose nanofiber dispersions were dropped on a silver nanowire thin layer. The transparent conductive nanofiber paper showed high electrical durability in repeated folding tests, due to dual advantages of the hydrophilic affinity between cellulose and silver nanowires, and the entanglement between cellulose nanofibers and silver nanowires. Their optical transparency and electrical conductivity were as high as those of ITO glass. Therefore, using this conductive transparent paper, organic solar cells were produced that achieved a power conversion of 3.2%, which was as high as that of ITO-based solar cells.

High-Performance Poly(3-hexylthiophene) Field-Effect Transistors Fabricated by a Slide-Coating Method

Highly oriented thin-films of poly(3-hexylthiophene) (P3HT) have been successfully fabricated by slide-coating method, in which the chloroform solution is sandwiched between a Si wafer and a slide glass, and followed by slow drawing of the Si wafer with respect to the slide glass. The performance of this organic field-effect transistor (OFET) based on the P3HT film exhibit high-performance of up to 0.056 cm2 V-1 s-1 in FET mobility. This simple solution process is an effective method to fabricate the well-ordered structure of the P3HT film and its OFETs.

Electron-Transporting Oligothiophenes Containing Dicyanomethylene-Substituted Cyclopenta[b]thiophene: Chemical Tuning for Air Stability in OFETs

We have synthesized new electron-transporting oligothiophenes containing dicyanomethylene-substituted cyclopenta[b]thiophene as an active material for the fabrication of solution-processable n-type organic field-effect transistors (OFETs). The influence of the number of dicyanomethylene groups as well as the position of hexyl groups was investigated in detail by performing photophysical and electrochemical measurements. Results revealed that the optical energy gaps and the lowest unoccupied molecular orbital (LUMO) energy levels can be controlled by changing the number of dicyanomethylene groups. In contrast, the position of hexyl groups has little influence on molecular electronic properties. X-ray diffraction and atomic force microscopy measurements revealed that spin-coated thin films of the new compounds had a crystalline structure. OFETs based on these compounds were evaluated in vacuum and air-exposed conditions, and the electron mobility of up to 0.016 cm(2) V(-1) s(-1) was achieved. Furthermore, we demonstrated that the air stability of the OFETs depends on the LUMO energy level of the compounds.

Air-stable n-type organic field-effect transistors based on solution-processable, electronegative oligomers containing dicyanomethylene-substituted cyclopenta[b]thiophene.

Solution-processable, electronegative, π-conjugated systems containing dicyanomethylene-substituted cyclopenta[b]thiophene were synthesized as potential active materials for air-stable n-type organic field-effect transistors (OFETs). Electrochemical measurements revealed that these compounds exhibited electrochemical stability and that the lowest unoccupied molecular orbital (LUMO) had an energy level less than -4.0 eV. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements were performed, and the value of intradomain electron mobility was determined to be as high as 0.1 cm(2) V(-1) s(-1) . The OFETs were fabricated by spin-coating thin films of the compounds as an active layer. The electron mobility of the OFETs was 3.5×10(-3) cm(2) V(-1) s(-1) in vacuum. Furthermore, electron mobility of the same order of magnitude and stable characteristics were obtained under air-exposed conditions. X-ray diffraction measurements of the spin-coated thin films revealed the difference of molecular arrangements depending on the inner conjugated units. Atomic force microscopy measurements of crystalline-structured films exhibited the formation of grains. The accomplishment of air-stability was attributed to the combined effect of the low-lying LUMO energy level and the molecular arrangements in the solid state, avoiding both the quenching of electron carriers and the intrusion of oxygen and/or moisture.

Narrow-optical-gap π-conjugated small molecules based on terminal isoindigo and thienoisoindigo acceptor units for photovoltaic application

Two acceptor–donor type conjugated small molecules based on isoindigo and thienoisoindigo acceptor units are synthesized. The small molecules showed broad and low-energy light absorption properties. On application of the small molecules in organic photovoltaic cells, thienoisoindigo-based small molecule in combination with PC71BM showed moderate photovoltaic efficiency with a PCE of 1.51% with an open-circuit voltage of 0.72 V under AM 1.5 solar irradiation.

N-phenyl[60]fulleropyrrolidines: alternative acceptor materials to PC61BM for high performance organic photovoltaic cells

A series of novel soluble [60]fulleropyrrolidine derivatives bearing relatively simple substituents like [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) is synthesized under Prato reaction conditions. The photoabsorption and electrochemical properties of the fulleropyrrolidines are investigated to elucidate their molecular-level electronic properties similar to those of PC61BM. The investigations of the fulleropyrrolidines as electron acceptors based on bulk-heterojunction type organic photovoltaic (OPV) devices fabricated using P3HT as a donor show clear differences in performance depending on the substituents, and the devices based on the N-phenyl[60]fulleropyrrolidine derivatives exhibit good power conversion efficiencies (PCEs) comparable to, or even higher than, that of the standard PC61BM-based device. Finally, the OPV devices based on a low-bandgap donor polymer (PTB7) with the N-phenyl[60]fulleropyrrolidines show high PCEs up to 7.3%, which is the highest class performance among [60]fullerene-based OPV devices, indicating that N-phenyl[60]fulleropyrrolidine derivatives are a promising alternative to PC61BM for OPV acceptor materials.

Pyradinodithiazole: An Electron-Accepting Monomer Unit for Hole-Transporting and Electron-Transporting Conjugated Copolymers.

Pyradinodithiazole (PDTz) was designed as a new electron-accepting unit. The physical property measurements indicated that the PDTz unit has stronger electron-accepting characteristics than thiazolothiazole and benzodithiazole. A donor-acceptor copolymer containing PDTz as an acceptor unit was synthesized for hole-transporting semiconductors in organic photovoltaics (OPV). Furthermore, an acceptor-acceptor copolymer containing PDTz has also been developed for electron-transporting OPV materials. These copolymer-based blend films showed expected photovoltaic characteristics in individual OPV devices.

Low band-gap donor–acceptor copolymers based on dioxocyclopenta[c]thiophene derivatives as acceptor units: synthesis, properties, and photovoltaic performances

New donor–acceptor type copolymers containing dihexyldioxocyclopenta[c]thiophene or (dihexylmethylidene)dioxocyclopenta[c]thiophene as acceptor units have been designed and synthesized for the application as hole-transporting (p-type) organic semiconducting materials in organic photovoltaics (OPVs). The investigation of photophysical and physicochemical properties revealed that these copolymers featured low optical band gaps (1.56–1.73 eV) and low-lying HOMO energy levels (−5.40 to −5.02 eV). Bulk heterojunction OPV devices based on these copolymers and [6,6]-phenyl-C61-butyric acid methyl ester as active layers showed moderate power conversion efficiencies (PCEs) of between 1.36 and 2.68% under air mass 1.5 simulated solar illumination. Space-charge-limited current measurements and atomic force microscopy measurements of the blend films revealed that both charge-transporting characteristics and film morphologies have significant influences on the photovoltaic performances. OPV devices based on the copolymers with [6,6]-phenyl-C71-butyric acid methyl ester showed a PCE of up to 5.17% with a short circuit current of 10.1 mA cm−2, an open circuit voltage of 0.80 V and a fill factor of 0.64.

Branched polythiophene as a new amorphous semiconducting polymer for an organic field-effect transistor

We have designed and synthesized new polymer semiconductors containing branched oligothiophene units. These polymers form amorphous films with high thermal stability owing to strong interactions between oligothiophene side chains. On thin-film devices fabricated by spin coating of these polymers as a semiconducting layer, the field-effect mobility increased from 10−6 to 10−4 cm2 V−1 s−1 with increasing side-chain length. The UV–vis absorption, photoluminescence and electrochemical properties of the polymers in solutions were also affected by the length of the side chains.

Three-dimensional π-conjugated compounds as non-fullerene acceptors in organic photovoltaics: the influence of acceptor unit orientation at phase interfaces on photocurrent generation efficiency

Recently, organic photovoltaics (OPVs) using electron-accepting π-conjugated systems as non-fullerene acceptors have been extensively studied. The fine-tuning of donor–acceptor (D–A) interfaces in bulk-heterojunction (BHJ) structures is crucial for accomplishing high power conversion efficiencies (PCEs); however, the rational design of non-fullerene acceptors for control over the film morphology is still unclear. To investigate the influence of structural modification on D–A interfaces, we synthesized a series of three-dimensional (3D) π-conjugated acceptors that contain perylene bis(dicarboximide) (PDI) units. These compounds showed little difference in the molecular properties. However, OPVs containing a blend of the poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) donor and an acceptor showed different PCEs, ranging from 0.02 to 2.02%, and originating from the differences in the short-circuit current densities (JSC). By investigating the blended film properties, we found that the degree of charge-separation mainly influences the photovoltaic characteristics of the OPVs. Furthermore, the JSC of OPVs and the London dispersion (γd) components of the surface free energy of the 3D acceptors are correlated. Consequently, increasing the interfacial exposure of the π-conjugated framework increases the value of γd orienting the PDI π-planes toward the D–A interfaces, which is desirable for the efficient charge separation into free carriers. This study highlights the importance of γd for the molecular design of non-fullerene acceptors for BHJ-type OPVs.