Masatoshi Kitamura

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.

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.

Operational stability in pentacene thin-film transistors with threshold voltages tuned by oxygen plasma treatment

Pentacene-based organic thin-film transistors (TFTs) having a SiO2 gate dielectric treated with oxygen plasma have been investigated for control of the threshold voltage. The threshold voltage changed in the wide range from −15 to 80 V, depending on plasma treatment time, AC power for plasma generation, and gate dielectric thickness. The threshold voltage change was attributed to negative charges induced on and/or near the surface of the gate dielectric. The threshold voltage change on the order of 1 V was particularly proportional to plasma treatment time. The predictable change enables the control of threshold voltage in this range. In addition, the effect of gate bias stress on threshold voltage was examined. The results suggested that gate bias stress does not negate the threshold voltage change induced by plasma treatment.

Pentacene Thin-Film Transistors with Controlled Threshold Voltages and Their Application to Pseudo CMOS Inverters

Threshold voltage control in pentacene thin-film transistors (TFTs) has been demonstrated by oxygen plasma treatment to the surface of the SiO2 gate dielectric. The threshold voltage linearly shifts with increase in plasma treatment time. Pseudo CMOS inverters consisting of the TFTs with controlled threshold voltages successfully operated at 10 V.

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 uf0b0C 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.

Thermal Stability of Short Channel, High-Mobility Organic Thin-Film Transistors having Bottom-Contact Configuration

Thermal stability of alkylated-DNTT thin-film transistors (TFTs) has been investigated. Although the properties of the TFTs degraded even for storage at room temperature, the annealing at 120 uf0b0C after the degradation improved the properties toward those at early stage. The annealed short-channel TFTs exhibited mobilities of 1.9 to 2.6 cm/Vs.