Yasutaka Kuzumoto

Threshold voltage control of bottom-contact n-channel organic thin-film transistors using modified drain/source electrodes

Bottom-contact n-channel C60 thin-film transistors (TFTs) with drain/source electrodes modified by benzenethiol derivatives have been fabricated to investigate the influence of the modification on the transistor characteristics. Modification using methylbenzenethiol, aminobenzenethiol, and (dimethylamino)benzenethiol having electron-donating groups causes threshold voltages to shift to low voltages. In addition, the modification provides no significant decrease in saturation mobilities. A C60 TFT with (dimethylamino)benzenethiol-modified electrodes has a low threshold voltage of 5.1 V as compared to that of 16.8 V for a TFT with nonmodified electrodes. The threshold-voltage shift is probably because the modification reduces electron-injection barrier height and improves electron injection into organic semiconductors.

High-Frequency Organic Complementary Ring Oscillator Operating up to 200 kHz

Organic complementary circuits consisting of bottom-contact p-channel pentacene and n-channel C60 thin-film transistors (TFTs) have been fabricated to evaluate their dynamic properties. Modified drain and source electrodes were used to balance the threshold voltages of the pentacene and C60 TFTs. The balanced threshold voltage allowed use of equal-size channel dimensions for both channel-type TFTs in the circuits. The signal delay per stage of a five-stage ring oscillator was consistent with the mobilities of the individual TFTs. The oscillation frequency increased with supply voltage up to 200 kHz, which is the highest frequency in organic five-stage complementary ring oscillators.

Work function of gold surfaces modified using substituted benzenethiols: Reaction time dependence and thermal stability

The work function of Au surfaces modified with various substituted benzenethiols has been systematically investigated for application to the design of organic electronic devices. The work function was found to vary in the range of 4.37 to 5.48 eV depending on the substituted benzenethiol used, which included pentafluorobenzenethiol, 4-fluorobenzenethiol, 4-methylbenzenethiol, 4-aminobenzenethiol, and 4-(dimethylamino)benzenethiol. Subsequent thermal annealing of the modified Au films above 373 K changed the work function back to that of an unmodified Au surface. Meanwhile, thermal desorption spectroscopy revealed species desorbing from the modified Au surfaces, indicating cleavage of the C–S bond as well as the S–Au bond.

High conductance bottom-contact pentacene thin-film transistors with gold-nickel adhesion layers

Bottom-contact pentacene thin-film transistors (TFTs) with different electrode types have been fabricated to investigate influence of the adhesion layers and surface modification on the transistor characteristics. Gold-nickel alloy or Ti was used for the adhesion layer; the drain/source electrodes were modified with pentafluorobenzenethiol (PFBT) or nonmodified. The TFTs with AuNi layers and PFBT-modified electrodes exhibit channel-length independent saturation mobility. The electrode-type TFT with a channel length of 2.2 μm has a saturation mobility of 0.73 cm2/V s and a transconductance of 229 μS/mm. The high performance is attributed to the low contact resistance of 408 Ω cm.

High-performance fullerene C60 thin-film transistors operating at low voltages

Low-voltage operation of fullerene C60 thin-film transistors (TFTs) has been realized using zirconium-silicon oxide (ZSO) as a gate insulator. The gate insulator consisted of triple layers of SiO2∕ZSO∕SiO2 deposited by rf sputtering. The C60 TFTs with the insulators operated at a low voltage of 5V. The surface of the insulator was treated with hexamethyldisilazane (HMDS) or octadecyltrimethoxysilane (ODS). The C60 TFT with the ODS-treated insulator exhibited higher performance than that with the HMDS-treated insulator, and it had a field-effect mobility of 1.46cm2∕Vs, threshold voltage of 1.9V, and a current on/off ratio of 2×106.

Surface properties of substituted-benzenethiol monolayers on gold and silver: Work function, wettability, and surface tension

The surface properties, including work function and wettability, of Au and Ag surfaces modified with various substituted benzenethiols have been investigated. Whereas the work functions of the modified Au surfaces ranged from 4.42 to 5.48 eV, those of the modified Ag surfaces ranged from 3.99 to 5.77 eV. The highest work function of 5.77 eV was obtained on the Ag surface modified with pentafluorobenzenethiol, and the lowest work function of 3.99 eV was obtained on the Ag surface modified with 4-methylbenzenethiol. The water contact angle on modified Au surfaces was found to be in a wide range from 30.9 to 88.3°. The water contact angle on the Au surface modified with a substituted benzenethiol was close to that on the Ag surface modified with the same benzenethiol. Furthermore, the tension of the modified Au surfaces was estimated from their contact angles of water and ethylene glycol.

Structural and electrical properties of fluorinated copper phthalocyanine toward organic photovoltaics: Post-annealing effect under pressure

The morphology and current–voltage characteristics of organic films with copper phthalocyanine (CuPc) and hexadecafluoro CuPc (F16CuPc) prepared under different conditions have been investigated. Substrate heating improved the current–voltage characteristics of CuPc single-layer devices. Also, substrate heating from room temperature suppressed breakdown at low voltages in F16CuPc devices. In addition, the post-annealing effects under pressure on the current–voltage characteristics of CuPc/F16CuPc devices were investigated. Although a CuPc/F16CuPc device prepared at a substrate temperature of 120 °C exhibited a reverse rectifying property and provided no photocurrent, a CuPc/F16CuPc device post-annealed at 300 °C under pressure showed a normal rectifying property and worked as a photovoltaic cell.

Partially fluorinated copper phthalocyanine toward band engineering for high-efficiency organic photovoltaics

The electronic structures of copper phthalocyanine (CuPc) and fluorinated CuPc, FxCuPc (x = 4, 8, 12, and 16), have been investigated by density functional theory. The HOMO and LUMO energies systematically decrease with an increase in the number of fluorine atoms. The degree of the decrease depends on the position of the substitution of hydrogen with fluorine. The HOMO (LUMO) energies vary in the range of −5.33 to −6.82 eV (−3.12 to −4.65 eV). The UV–visible absorption spectra and photoelectron ionization energies of the deposited FxCuPc (x = 0, 8, and 16) thin films are compared with the calculation results. The calculated bandgap energies and HOMO levels are consistent with those obtained from the experimental results.

Annealing Effect on Field-Effect Mobilities in Bottom-Contact Alkylated Dinaphthothienothiophene Transistors

Effect of post annealing on the current characteristics of alkylated dinaphthothienothiophen (DNTT) thin-film transistors (TFTs) has been investigated. The annealing at 80 C for long time dramatically improved the field-effect mobilities of the TFTs. The mobility of 3.3 cm/Vs was obtained in a bottom-contact alkylated DTNTT TFT.

Structural and Electrical Properties of Fluorinated Copper Phthalocyanine for Organic Photovoltaics

We have investigated influence of substrate temperature and post annealing on morphology and current-voltage characteristics of organic films with copper phthalocyanine series, FxCuPc (x = 0, 8, 16). The substrate heating at 120 C improved rectifying properties for CuPc and F16CuPc. In addition, the substrate heating from room temperature suppressed surface roughness. These results lead to improve current-voltage characteristics for F8CuPc and F16CuPc films.