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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.

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.

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.

We have investigated the contact resistance of rubrene single-crystal field-effect transistors (FETs) with Nickel electrodes by performing scaling experiments on devices with channel length ranging from 200 nm up to 300 $\mu$m. We find that the contact resistance can be as low as 100 $\Omega$cm with narrowly spread fluctuations. For comparison, we have also performed scaling experiments on similar Gold-contacted devices, and found that the reproducibility of FETs with Nickel electrodes is largely superior. These results indicate that Nickel is a very promising electrode material for the reproducible fabrication of low resistance contacts in organic FETs.

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.

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.

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.

Dye doping is a promising way to increase the spectral purity of polymer light-emitting diodes (LEDs). Here we analyze the frequency and field dependence of the complex admittance of Al-Ba-PPV-PEDOT-ITO LEDs with and without dye. We compare the charge carrier mobilities of pristine and dye-doped double-carrier and hole-only (Au replacing Al-Ba) devices. Dye doping is shown to significantly influence the electron mobilities while the hole mobilities are left unchanged and thereby changing the carrier balance in a double carrier device towards that of a hole only device. The minimum in the LED capacitance as function of voltage appears to be an excellent probe for the electron trapping phenomenon underlying the reduction of the mobility.