Kenichiro Takagi

High-performance and electrically stable solution-processed polymer field-effect transistors with a top-gate configuration

The electrical performance of organic field-effect transistors (organic FETs or OFETs) based on the soluble semiconductor regioregular poly(3-hexylthiophene) (P3HT) with a top-gate configuration is investigated. P3HT FETs with the top-gate configuration show that field-effect mobility is independent of substrate surface energy and are comparable to bottom-gate P3HT FETs with hydrophobic surface passivations on SiO2 gate insulators. Gate bias stress experiments reveal a high electrical stability with negligible hysteresis and small threshold voltage shifts in top-gate devices with different polymer gate insulators, which is comparable or superior to that of hydrogenated amorphous silicon thin film transistors.

Characterization of transport properties of organic semiconductors using impedance spectroscopy

Abstract Methods for the determination of drift mobility, localized-state distribution and deep trapping lifetime in organic semiconductors using impedance spectroscopy are reviewed. The theoretical basis is single-injection space-charge-limited current under small sinusoidal voltage perturbation. Major advantages of impedance spectroscopy are: full automatic measurements and simultaneous measurements of these physical quantities. Information on these physical quantities are essential for the understanding of transport properties in organic semiconductors and for the design of organic devices such as organic light-emitting diodes and organic solar cells using a device simulator. The determination of drift mobility, localized-state distribution and deep trapping lifetime from impedance spectra is demonstrated in a molecularly doped polymer. A molecularly doped polymer is a prototypical organic semiconductor and is a good example for the demonstration of simultaneous determination of these quantities. The methods presented here are applicable to insulating semiconductors and thus to inorganic disordered semiconductors such as hydrogenated amorphous silicon and amorphous oxide semiconductors, as well.

Determination of deep trapping lifetime in organic semiconductors using impedance spectroscopy

A method for determining deep trapping lifetime in semiconductors using an impedance spectroscopy is proposed. A unique feature of the method is the simultaneous determination of the drift mobility and deep trapping lifetime in thin-film electronic devices. The validity of the proposed method is examined by numerical calculation. Simultaneous determinations of the drift mobility and deep trapping lifetime using this method are demonstrated in prototypical hole transporting organic semiconductors.

Electrical characterization of thieno[3,4-b]thiophene and benzodithiophene copolymer using field-effect transistor configuration

A thieno[3,4-b]thiophene and benzodithiophene copolymer PTB7, which has received much attention as a donor material of organic solar cells, is electrically characterized using a field-effect transistor (FET) configuration. The hole mobility of PTB7 FETs is ~2.0 × 10−3 cm2V−1s−1, which is independent of annealing temperature, while the threshold voltage shifts towards a negative value with increasing annealing temperature. This is due to the increase in the density of deep trapping states with thermal annealing. The threshold voltage also shifts towards a negative value after aging, indicating that additional deep trapping states are generated in PTB7 thin films even at room temperature.