Kazuhiro Marumoto

2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion

Doping is one of the most important methods to control charge carrier concentration in semiconductors. Ideally, the introduction of dopants should not perturb the ordered microstructure of the semiconducting host. In some systems, such as modulation-doped inorganic semiconductors or molecular charge transfer crystals, this can be achieved by spatially separating the dopants from the charge transport pathways. However, in conducting polymers, dopants tend to be randomly distributed within the conjugated polymer, and as a result the transport properties are strongly affected by the resulting structural and electronic disorder. Here, we show that in the highly ordered lamellar microstructure of a regioregular thiophene-based conjugated polymer, a small-molecule p-type dopant can be incorporated by solid state diffusion into the layers of solubilizing side chains without disrupting the conjugated layers. In contrast to more disordered systems, this allows us to observe coherent, free-electron-like charge transport properties, including a nearly ideal Hall effect in a wide temperature range, a positive magnetoconductance due to weak localization and the Pauli paramagnetic spin susceptibility.

Electron Spin Resonance of Field-Induced Polarons in Regioregular Poly(3-alkylthiophene) Using Metal–Insulator–Semiconductor Diode Structures

We report electron spin resonance (ESR) studies on field-induced charge carriers in regioregular poly(3-hexylthiophene) (RR-P3HT) and regioregular poly(3-octylthiophene) (RR-P3OT) for metal–insulator–semiconductor (MIS) diode structures with Al 2 O 3 as an insulating layer. The field-induced ESR signals ( g ∼2.002) were clearly observed; their intensities monotonically increased as the absolute value of the gate bias increased in the accumulation mode with a saturation behavior at higher voltages. The ESR signals were consistent with those of the photogenerated positive polarons in RR-P3HT and RR-P3OT, demonstrating that the carriers are polarons. The transient responses of the field-induced ESR intensity show fast and slow components. The self-organized lamellar molecular orientation was confirmed by the anisotropic ESR signals due to π-electrons, which is semiquantitatively reproduced by an ESR simulation.

Quadrimolecular recombination kinetics of photogenerated charge carriers in regioregular poly(3-alkylthiophene)/fullerene composites

The recombination kinetics of photogenerated charge carriers in composites of regioregular poly(3-alkylthiophene)s and C60 have been investigated by a transient response of light-induced electron spin resonance (LESR). The dependence of the prompt decay component on excitation-light intensity Iex shows that quadrimolecular recombination (QR) is dominant at higher Iex while bimolecular recombination is relatively important at lower Iex. The time decay of the LESR intensity is also well explained by the QR model. The temperature dependence of the QR rate constant clearly shows a crossover from hopping to tunneling behavior with decrease of temperature; the activation energy for the hopping of 0.10 eV is obtained.

Spin distributions and excitation spectra of optically generated polarons in poly(p-phenylenevinylene) derivatives

Abstract Light-induced electron spin resonance (LESR) measurements have been performed on two dialkoxy derivatives (MEH-PPV and CN-PPV) of poly(p-phenylenevinylene) where cyano groups are attached at the vinylic carbon sites in CN-PPV. The transient response and excitation spectra of the LESR signals indicate that the observed spins are optically generated polarons. A clear reduction of ESR linewidth has been observed in CN-PPV compared with MEH-PPV, providing direct evidence that the polaron spin density resides predominantly on the vinylic carbons. Linewidth analysis based on a theoretical model for the spin distribution yields an extension of the polaron over approximately four phenyl rings.

Direct Observation of Hole Accumulation in Polymer Solar Cells During Device Operation using Light‐Induced Electron Spin Resonance

Organic thin-fi lm solar cells are a promising alternative source of electrical energy because of their printable and fl exible device structure, light weight, and low-cost production. [ 1–3 ] Solar cells have been investigated using a variety of methods with the aim of improving their performance. [ 4–10 ] There has been a signifi cant amount of interest in the high power conversion effi ciency (PCE) of 10% that was recently obtained in organic thin-fi lm solar cells due to their potential practical applications. [ 11 , 12 ] In addition to device performance, the durability of solar cells is an important problem for the practical use of solar cells. For durability studies, degradation of solar cells due to extrinsic factors such as oxygen and water has been reported, where materials and device structures were irreversibly degraded. [ 13–15 ]

Electrical Conduction of Regioregular and Regiorandom Poly(3-hexylthiophene) Doped with Iodine

The electrical conduction of regioregular poly(3-hexylthiophene) (RR-P3HT) and regiorandom poly(3-hexylthiophene) (RRa-P3HT) doped with iodine is studied as a function of doping level. The electrical conductivity shows the thermally-activated behavior at low doping levels below 10% for RR-P3HT and 18% for RRa-P3HT. At high doping levels higher than 10% for RR-P3HT and 20% for RRa-P3HT, the electrical conductivity shows the three-dimensional variable-range hopping. The change in the conduction mechanism is explained qualitatively in terms of the growth of the density of states at the Fermi level and the dimensional crossover of the conduction path. The difference in the doping levels where the change occurs in RR-P3HT and RRa-P3HT is understood in terms of the carrier localization length, that is, the spatial extent of polaronic excitations in these polymers.

Direct observation of the charge carrier concentration in organic field-effect transistors by electron spin resonance

Charge carrier concentration in operating field-effect transistor (FET) of regioregular poly(3-hexylthiophene) has been directly determined by electron spin resonance (ESR). ESR signals of field-induced polarons are observed around g=2.003 under the application of negative gate-source voltage (Vgs). Upon applying drain-source voltage (Vds), ESR intensity decreases linearly in the low Vds region, reaching to about 50% of the initial intensity at the pinch-off point (Vds≅Vgs). For larger absolute values of Vds, it becomes nearly Vds independent. These behaviors are well explained by the change in the carrier concentration obtained by the FET theory using gradual channel approximation.

Light-induced ESR studies of polarons in regioregular poly(3-alkylthiophene)-fullerene composites

Light-induced electron spin resonance (LESR) has been employed to study photo-generated positive and negative polarons on poly(3-alkylthiophene) (PAT) and fullerene (C 60 ) in regioregular PAT-C 60 composites, respectively, using variable photo-excitation energy up to 4.1 eV. A remarkable enhancement of prompt LESR signal is found in excitation spectrum at around 1.8 eV, which is consistent with the enhancement of the photoconductivity, while persistent LESR signal is weakly dependent on photo-excitation energy due to small numbers of deep trap sites. In addition, light-induced electron-nuclear double resonance (LENDOR) measurements have been successfully performed on the composites. Two different LENDOR signals with broad and narrow linewidth are observed due to PAT and C 60 polarons, respectively.

Electron Spin Resonance Observations of Field-Induced Polarons in Regioregular Poly(3-octylthiophene) Metal-Insulator-Semiconductor Diode Structures

Electron spin resonance (ESR) measurements are performed on field-induced carriers in regioregular poly(3-octylthiophene) (RR-P3OT) using metal–insulator–semiconductor (MIS) diode structures with RR-P3OT and Al 2 O 3 as the active semiconductor and insulating layers, respectively. Clear ESR signals ( g ∼2.002) are observed, which increase as the absolute value of the gate bias increases in the accumulation mode. The ESR signal is consistent with that of the photogenerated positive polarons in RR-P3OT detected by the light-induced ESR of RR-P3OT/C 60 composites, providing direct evidence that the field-induced carriers are polarons. Moreover, the molecular orientation of RR-P3OTs in the diode structures, which is consistent with the self-organized lamellae reported, is confirmed by the anisotropy of the ESR signal.

Electron Spin Resonance Observation of Gate-induced Charge Carriers in Organic Field-effect Devices Fabricated on Silicon Substrates

Electron spin resonance (ESR) measurements have been performed on metal–insulator–semiconductor (MIS) diode structures of regioregular poly(3-hexylthiophene), RR-P3HT, fabricated on silicon substrates with SiO2 layers as gate insulators. The conductivity of substrates was chosen so that it does not significantly lower the quality factor of the ESR cavity. Clear ESR signals due to field-induced polarons have been observed at g-values of around 2.002, consistent with those observed in the MIS devices of RR-P3HT fabricated on Al2O3 gate insulators. Carrier spins tend to saturate above the charge concentration of about 0.3%, suggesting the conversion of polarons with spin 1/2 to spinless bipolarons for higher carrier concentrations. The angular dependence of ESR signals exhibits distinct anisotropy, reflecting the fact that the surface of SiO2 is flatter than that of Al2O3. These results demonstrate that ESR can be performed on organic field-effect devices fabricated on silicon substrates.