In situ determination of the energy dependence of the high-frequency mobility in polymers

The high-frequency mobility in disordered systems is governed by transport properties on mesoscopic length scales, which makes it a sensitive probe for the amount of local order. Here we present a method to measure the energy dependence of the high frequency mobility by combining an electrochemically gated transistor with in-situ quasi-optical measurements in the sub-terahertz domain. We apply this method to poly([2-methoxy-5-(3',7'-dimethylocyloxy)]-p-phenylene vinylene) (OC_1C_10-PPV) and find a mobility at least as high as 0.1 cm^2V^-1s^-1.Comment: 3 pages (incl. 3 figures) in Appl. Phys. Let

Bias-dependent Contact Resistance in Rubrene Single-Crystal Field-Effect Transistors

We report a systematic study of the bias-dependent contact resistance in rubrene single-crystal field-effect transistors with Ni, Co, Cu, Au, and Pt electrodes. We show that the reproducibility in the values of contact resistance strongly depends on the metal, ranging from a factor of two for Ni to more than three orders of magnitude for Au. Surprisingly, FETs with Ni, Co, and Cu contacts exhibits an unexpected reproducibility of the bias-dependent differential conductance of the contacts, once this has been normalized to the value measured at zero bias. This reproducibility may enable the study of microscopic carrier injection processes into organic semiconductors.Comment: 4 pages, 4 figure

Doping, density of states and conductivity in polypyrrole and poly(p-phenylene vinylene)

The evolution of the density of states (DOS) and conductivity as function of well controlled doping levels in OC_1C_10-poly(p-phenylene vinylene) [OC_1C_10-PPV] doped by FeCl_3 and PF_6, and PF_6 doped polypyrrole (PPy-PF_6 have been investigated. At a doping level as high as 0.2 holes per monomer, the former one remains non-metallic, while the latter crosses the metal-insulator transition. In both systems a similar almost linear increase in DOS as function of charges per unit volume c* has been observed from the electrochemical gated transistor data. In PPy-PF_6, when compared to doped OC_1C_10-PPV, the energy states filled at low doping are closer to the vacuum level; by the higher c* at high doping more energy states are available, which apparently enables the conduction to change to metallic. Although both systems on the insulating side show log(sigma) proportional to T^-1/4 as in variable range hopping, for highly doped PPy-PF_6 the usual interpretation of the hopping parameters leads to seemingly too high values for the density of states.Comment: 4 pages (incl. 6 figures) in Phys. Rev.

Current saturation and Coulomb interactions in organic single-crystal transistors

Electronic transport through rubrene single-crystal field effect transistors (FETs) is investigated experimentally in the high carrier density regime (n ~ 0.1 carrier/molecule). In this regime, we find that the current does not increase linearly with the density of charge carriers, and tends to saturate. At the same time, the activation energy for transport unexpectedly increases with increasing n. We perform a theoretical analysis in terms of a well-defined microscopic model for interacting Frohlich polarons, that quantitatively accounts for our experimental observations. This work is particularly significant for our understanding of electronic transport through organic FETs.Comment: Extended version with 1 additional figure and an appendix explaining the consistency of the theoretical calculatio

Wide energy-window view on the density of states and hole mobility of poly(p-phenylene vinylene)

Using an electrochemically gated transistor, we achieved controlled and reversible doping of poly(p-phenylene vinylene) in a large concentration range. Our data open a wide energy-window view on the density of states (DOS) and show, for the first time, that the core of the DOS function is Gaussian, while the low-energy tail has a more complex structure. The hole mobility increases by more than four orders of magnitude when the electrochemical potential is scanned through the DOS.Comment: 4 pages, 4 figure

Tunable Frohlich Polarons in Organic Single-Crystal Transistors

In organic field effect transistors (FETs), charges move near the surface of an organic semiconductor, at the interface with a dielectric. In the past, the nature of the microscopic motion of charge carriers -that determines the device performance- has been related to the quality of the organic semiconductor. Recently, it has been appreciated that also the nearby dielectric has an unexpectedly strong influence. The mechanisms responsible for this influence are not understood. To investigate these mechanisms we have studied transport through organic single crystal FETs with different gate insulators. We find that the temperature dependence of the mobility evolves from metallic-like to insulating-like with increasing the dielectric constant of the insulator. The phenomenon is accounted for by a two-dimensional Frohlich polaron model that quantitatively describes our observations and shows that increasing the dielectric polarizability results in a crossover from the weak to the strong polaronic coupling regime

mTOR Controls Mitochondrial Dynamics and Cell Survival via MTFP1

The mechanisms that link environmental and intracellular stimuli to mitochondrial functions, including fission and fusion, ATP production, metabolite biogenesis and apoptosis, are not well understood. Here, we demonstrate that the nutrient-sensing mechanistic/mammalian target of rapamycin complex 1 (mTORC1) stimulates translation of mitochondrial fission process 1 (MTFP1) protein to control mitochondrial fission and apoptosis. Expression of MTFP1 is coupled to pro-fission phosphorylation and mitochondrial recruitment of the fission GTPase, dynamin-related protein 1 (DRP1). Potent active-site mTOR inhibitors engender mitochondrial hyperfusion due to the diminished translation of MTFP1 mediated by the translation initiation factor 4E (eIF4E)-binding proteins (4E-BPs). Uncoupling MTFP1 levels from the mTORC1/4EBP pathway upon mTOR inhibition blocks the hyperfusion response and leads to apoptosis by converting mTOR inhibitor action from cytostatic to cytotoxic. These data provide direct evidence for the survival function of mitochondrial hyperfusion upon mTOR inhibition by employing MTFP1 as a critical effector of mTORC1 to govern cell fate decisions

High Electron Mobility in Vacuum and Ambient for PDIF-CN2 Single-Crystal Transistors

We have investigated the electron mobility on field-effect transistors based on PDIF-CN$_{2}$ single crystals. The family of the small molecules PDI8-CN$_{2}$ has been chosen for the promising results obtained for vapour-deposited thin film FETs. We used as gate dielectric a layer of PMMA (spinned on top of the SiO$_{2}$), to reduce the possibility of electron trapping by hydroxyl groups present at surface of the oxide. For these devices we obtained a room temperature mobility of 6 cm$^{2}$/Vs in vacuum and 3 cm$^{2}$/Vs in air. Our measurements demonstrate the possibility to obtain n-type OFETs with performances comparable to those of p-type devices.Comment: published online in JAC