Shuichi Nagamatsu

Solution-processed n-type organic thin-film transistors with high field-effect mobility

We report the performance of solution-processed n-type organic thin-film transistors (OTFTs) based on C60 derivatives. Long-chain alkyl-substituted C60, C60-fused N-methylpyrrolidine-meta-C12 phenyl (C60MC12), was used as a semiconducting layer. The C60MC12-thin-film transistor shows high electron mobility of 0.067cm2∕Vs in saturation regime. From the result of x-ray diffraction analysis, the C60MC12 active layer forms highly ordered crystalline film. We found that self-assemble ability of long alkyl chains plays an important role for fabrication of highly ordered crystalline film, leading to achievement of high electron mobility in solution-processed n-type OTFTs.

Highly polarized polymer light-emitting diodes utilizing friction-transferred poly(9,9-dioctylfluorene) thin films

Polarized polymer light-emitting diodes (PLEDs) have been constructed utilizing friction-transferred poly(9,9-dioctylfluorene) (PFO) thin films. The friction transfer technique allows oriented PFO to be deposited directly onto an indium tin oxide anode without an alignment layer such as polyimide. Polarized absorption and photoluminescence spectra revealed that the polymer backbones are highly aligned in the friction direction. We fabricated PLEDs consisting of friction-transferred PFO as an emissive layer, vacuum-deposited bathocuproine as an electron transport and hole-blocking layer, and a vacuum-deposited LiF∕Al cathode. Highly polarized blue emission with an integrated polarization ratio of 31 and a luminance of up to 300cd∕m2 was observed from the PLEDs.

Regioregularity vs Regiorandomness: Effect on Photocarrier Transport in Poly(3-hexylthiophene)

The time-of-flight technique has been used to study transports of photocarriers in regioregular and regiorandom poly(3-hexylthiophene) (P3HT) films by fabricating indium-tin-oxide/P3HT/Al sandwich cell. It has been found that regularity in the coupling of monomers, head-to-tail (HT) has a marked effect on electrical properties and photocarrier transport in P3HT. Both regular and random P3HT films exhibit a dispersive photocurrent response with the positive sign of photocarrier (hole). The room temperature hole mobility of regioregular P3HT is estimated to be 1.21 ×10-3 cm2/Vs and is one order of magnitude larger than that of the random one. The mobility in regioregular P3HT is almost independent of the applied field while that of random P3HT shows a positive field dependence at the field larger than 1.0 ×105 V/cm.

Highly efficient polarized polymer light-emitting diodes utilizing oriented films of β-phase poly(9,9-dioctylfluorene)

Uniaxially oriented films of β-phase poly(9,9-dioctylfluorene) (PFO) were realized by a friction-transfer technique followed by thermal annealing and vapor treatments. Absorption and photoluminescence (PL) spectra show the characteristics of β-phase: an additional absorption peak at 433nm and redshifted PL peaks compared with those of the usual nematic (N) phase. We fabricated polarized polymer light-emitting diodes utilizing oriented films of β-phase PFO. Highly polarized β-phase emission with an integrated polarization ratio of 51 was observed from the devices. The efficiency of the devices based on β-phase reached 2.0cd∕A, which is two times higher than that based on N-phase.

Polymer field-effect transistors by a drawing method

We demonstrated the polymer field-effect transistors (FETs) utilizing regioregular poly(3-alkylthiophene)s (P3AT) films prepared by a drawing method. The P3AT film exhibited large optical dichroic ratio, which originated in the polymer backbones aligned to the drawing direction. In-plane anisotropy and enhancement of FET characteristics have been observed that are caused by molecular alignment. In the case of poly(3-dodecylthiophene), the hole mobility along the drawing direction was enhanced by a factor of 25 compared with that of spin-coated film.

Organic field-effect transistors by a wet-transferring method

Organic field-effect transistors (OFETs) were prepared from an epitaxially grown film fabricated by a wet-transferring process. 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) was grown by thermal evaporation on the (001) surface of potassium bromide (KBr) single crystals. When the film was grown at room temperature, the planar molecules were aligned orthogonally on the crystal surfaces along the [110] direction with edge-on orientation to the surface normal direction. The epitaxy film was transferred to on SiO2/Si surface immediately after removing the KBr on the water surface to product the OFETs. The calculated μFET of the OFET for the wet-transferred vertically aligned film were 1.3×10−4 and 2.2×10−4 cm2 V−1 s−1 at the linear and saturation regions, respectively, at Vg=−50 V at an ION/IOFF (on/off ratios of source–drain current) of 104∼105.

Photocarrier Mobility in Regioregular Poly(3-hexylthiophene) Studied by the Time of Flight Method

The carrier mobility in the regioregular poly(3-hexylthiophene)(PHT) film has been studied by means of the time of flight (TOF) method using a sandwich-type cell of indium tin oxide(ITO)/PHT (cast film)/Al. The response of the photocurrent exhibits a dispersive character; however, the clear kink associated with the transit time of the carrier is observed. The drift mobility of holes is determined to be 4 ×10-4 cm2/Vs and almost independent of the field > 105 V/cm at room temperature. This is the largest TOF mobility in poly(3-alkylthiophene) reported to date. It is also found that the activation energy of the mobility is 88 meV and almost independent of the field. The results are discussed taking various transport models into consideration.

Enhancement of Transport Characteristics in Poly(3-hexylthiophene) Films Deposited with Floating Film Transfer Method

A new solution-processed technique of floating film transfer method (FTM) is proposed for depositing organic thin film semiconductor. Organic field-effect transistors (OFETs) have been fabricated to evaluate the transport characteristics in poly(3-hexylthiophene) (P3HT) films deposited with FTM and spin-coating methodology. An enhancement of hole mobility was found in the FTM film by one order of magnitude as compared to that in spin-coated films. Absorption spectra as well as X-ray diffraction profiles represent well-grown lamella structures of P3HT with FTM transverse to the substrate, representing the advantage of this method suitable for the carrier transport.

Mechanism of Photocarrier Generation and Transport in Poly(3-Alkylthiophene) Films

Carrier mobilities in poly(3-alkylthiophene) (PAT) films (alkyl = 6, 12 and 18), synthesized by chemical oxidation of 3-alkylthiophenes with FeCl3, have been studied using the time of flight (TOF) method. The response of photocurrents indicates the hole as the carrier sign and the highly dispersive transport particularly in the films substituted with long alkyl chains. Using regioregular poly(3-dodecylthiophene) (PAT12) and indium tin oxide (ITO) in the ITO/PAT12/Al sandwich cell, the depletion layer width at the Al/PAT12 interface was estimated to be 60 nm. It has been proposed that photocarriers are effectively generated at the interface and the high internal field helps in the separation of photocarriers. The drift mobilities in PAT12 and PAT18 are 8 ×10-6 cm2/V.s and 3 ×10-6 cm2/V.s, respectively, at the bias field, E = 1.0 MV/cm. They depend on the field as exp (βE1/2/kT), indicating the Poole-Frenkel mechanism for the carrier transport. On the other hand, the mobility of PAT6 is 1.1 ×10-4 cm2/V.s at E = 0.1 MV/cm, shows very weak positive field dependence. The mechanisms of photocarrier generation and transport are discussed taking the polymer structure into consideration and are compared with the results of regioregular PAT films.

Macroscopic self ordering of solution processible poly(3,3″′-dialkylquaterthiophene) by floating film transfer method

Ordering and alignment of π-conjugated polymer chains are highly desirable for high performance and long life organic electronic devices. We report here self assembly of ordered and aligned solution processible Poly(3,3″′-dialkylquaterthiophene) (PQT-12) polymer at macroscopic level using Floating Film Transfer Method (FTM). PQT-12 polymer film is formed over solution of ethylene glycol and glycerol at different temperatures viz. 22, 26, 33, and 38 °C. PQT-12 films formed by FTM technique are further characterized for optical and morphological properties. UV-vis absorption (for polarize and unpolarize light) and surface topography/phase imaging are carried out by using UV-vis spectrometer and atomic force microscope (AFM), respectively. UV-vis spectra show the polymer chains alignment perpendicular to the film propagation direction and it is well supported by AFM images. The effect of temperature on ordering and alignment of PQT-12 shows 33 °C as an optimum temperature for alignment of polymer chains (a l...