Seiichiro Izawa

Dominant Effects of First Monolayer Energetics at Donor/Acceptor Interfaces on Organic Photovoltaics

Energy levels of the first monolayer are manipulated at donor/acceptor interfaces in planar heterojunction organic photovoltaics by using molecular self-organization. A cascade energy landscape allows thermal-activation-free charge generation by photoirradiation, destabilizes the energy of the interfacial charge-transfer state, and suppresses bimolecular charge recombination, resulting in a higher open-circuit voltage and fill factor.

Electrochemical Detection of Circadian Redox Rhythm in Cyanobacterial Cells via Extracellular Electron Transfer

Recent research on cellular circadian rhythms suggests that the coupling of transcription-translation feedback loops and intracellular redox oscillations is essential for robust circadian timekeeping. For clarification of the molecular mechanism underlying the circadian rhythm, methods that allow for the dynamic and simultaneous detection of transcription/translation and redox oscillations in living cells are needed. Herein, we report that the cyanobacterial circadian redox rhythm can be electrochemically detected based on extracellular electron transfer (EET), a process in which intracellular electrons are exchanged with an extracellular electrode. As the EET-based method is non-destructive, concurrent detection with transcription/translation rhythm using bioluminescent reporter strains becomes possible. An EET pathway that electrochemically connected the intracellular region of cyanobacterial cells with an extracellular electrode was constructed via a newly synthesized electron mediator with cell membrane permeability. In the presence of the mediator, the open circuit potential of the culture medium exhibited temperature-compensated rhythm with approximately 24 h periodicity. Importantly, such circadian rhythm of the open circuit potential was not observed in the absence of the electron mediator, indicating that the EET process conveys the dynamic information regarding the intracellular redox state to the extracellular electrode. These findings represent the first direct demonstration of the intracellular circadian redox rhythm of cyanobacterial cells.

Interface-induced crystallization and nanostructure formation of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in polymer blend films and its application in photovoltaics

Crystallization of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in thin films and in blend films with various polymers was investigated by X-ray diffraction. Thermal annealing induced the crystallization of PCBM in the blend films only through direct contact with a crystallized pure PCBM layer beneath, suggesting that an epitaxial crystallite growth occurred from the bottom interface. The morphology of the crystals depended strongly on the mixing ratio and the crystal structure of the bottom layer, and nanorod-like PCBM crystallites with widths in the range of 100–150 nm and lengths in the range of 150–500 nm were observed. A bulk-heterojunction (BHJ) organic solar cell utilizing the PCBM crystallite as the acceptor showed the highest VOC of 0.83 V for a PTB7:PCBM device to date. These findings offer the ways to use the crystallized PCBM with the controlled nanostructures as the electron conducting materials in organic and hybrid perovskite photovoltaics.

Drawing organic photovoltaics using paint marker pens

Active layers for organic photovoltaic devices (OPVs) were prepared by hand drawing with paint marker pens containing solutions of the materials. Although the pen-coated organic films were visually non-uniform with quite high surface roughness, OPV devices using these films exhibited similar or slightly better performances than those using spin-coated films. As such, the pen-coating technique represents an easily accessible, inexpensive, and highly material-efficient method for fabricating OPVs.

Parts-per-Million-Level Doping Effects in Organic Semiconductor Films and Organic Single Crystals

Controlling the pn-type behavior of a semiconductor such as silicon by adding an extremely small quantity of an impurity (doping) is a central part of inorganic semiconductor electronics since the 20th century. Recent progress in the doping of organic semiconductors strongly suggests the advent of a new era of doped organic semiconductors. Here, the principles and effects of doping at the level of parts per million (ppm) in organic semiconductor films and single crystals are described, including descriptions of complete pn-control, doping sensitization, ppm doping using an extremely low-speed deposition technique reaching 10-9 nm s-1 , and emerging ppm-level doping effects, such as trap filling, majority carriers, homojunction formation, and decreased mobility, as well as ppm-level doping effects in organic single crystals measured by the Hall effect, which shows a doping efficiency of 24%. The Wannier excitonic doping of organic single crystals possessing band conduction and the defect science of organic single crystals related to carrier trapping and scattering are introduced as a new scientific field. The dawn of organic single-crystal electronics is also discussed.