Exponential behavior of the Ohmic transport in organic films

Corneliu N. Colesniuc, Rudro R. Biswas, Samuel A. Hevia, Alexander V. Balatsky, and Ivan K. Schuller

Phys. Rev. B 83, 085414 – Published 18 February 2011

Abstract

An exponential dependence of conductance on thickness and temperature was found in the low-voltage, Ohmic regime of copper (CuPc) and cobalt (CoPc) phthalocyanine, sandwiched between palladium and gold electrodes unlike ever claimed in organic materials. To assure cleanliness and integrity of the electrode-phthalocyanine interface, the devices were prepared in situ, using organic molecular beam deposition with a floating shadow mask. The dc transport measurements in a wide temperature and thickness range show that (i) the low-voltage J-V curve is linear, with current increasing sharply at higher voltages, (ii) the low-voltage conductance increases exponentially with temperature, and (iii) it decreases exponentially with film thickness. A comparison with conventional models fails to explain all the data with a single set of parameters. On the other hand, a model outlined here, which incorporates tunneling between localized states with thermally induced overlap, agrees with the data and decouples the contributions to conductance from the electrode-film interface and the bulk of the film.

Received 3 October 2010

DOI:https://doi.org/10.1103/PhysRevB.83.085414

Authors & Affiliations

Corneliu N. Colesniuc^1,*, Rudro R. Biswas², Samuel A. Hevia^1,3, Alexander V. Balatsky⁴, and Ivan K. Schuller¹

¹Department of Physics and Center for Advanced Nanotechnology, University of California San Diego, La Jolla, California 92093, USA
²Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
³Facultad de Física, Universidad Católica de Chile, Casilla 306, Santiago, Chile 6904411
⁴Theoretical Division and Center for Integrated Nano-Technologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

^*ccoles@physics.ucsd.edu

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Vol. 83, Iss. 8 — 15 February 2011

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Images

Figure 1
(a) Logarithmic plots of current density as a function of applied voltage at 300 K for Pd/CuPc/Pd devices with different organic layer thickness. Positive bias was applied on the bottom electrode. (b) Distribution of the low-voltage logarithmic J-V slopes for all measurements. The continuous line is a Lorentzian fit of the data. The blue circles include the slope values corresponding to the data shown in (a).Reuse & Permissions
Figure 2
(a) Conductance per unit area as function of the organic layer thickness for Pd/CuPc/Pd devices. Black triangles: devices grown on different samples; the decrease is very sharp for thicknesses up to 100 nm, but becomes much slower for thicknesses larger than 100 nm. Red circles: devices grown on the same sample show a similar behavior. (b) Thickness dependence of conductance at different temperatures. The straight lines are fits to the data. The drop in conductance becomes steeper at lower temperatures.Reuse & Permissions
Figure 3
Thickness dependence of conductance per unit area for devices with palladium and gold electrodes and CuPc and CoPc as the organic layer. The behavior is similar for all samples: The decrease is very sharp for thicknesses up to 100 nm, but becomes much slower for thicknesses larger than 100 nm.Reuse & Permissions
Figure 4
(a) Plot of conductance per unit area vs temperature for selected data sets for a nearest-neighbor hopping model with an activated conductivity. No single activation energy can be obtained for the whole temperature range. (b) Plot of the same data assuming a variable-range hopping model. Data can not be fitted with a single straight line across the whole temperature range. (c) Semilogarithmic plot of conductance per unit area vs temperature. All devices show a similar exponential dependence, suggesting a universal behavior for the whole temperature range.Reuse & Permissions
Figure 5
(a) Molecular stacks grown on a palladium electrode. The carriers hop from one molecule to another whenever a favorable path forms. (b) Slopes of the linear fits from Fig. 2b as function of temperature. The straight line is a fit of the data. The slope decreases when the temperature increases in agreement with Eq. (4).Reuse & Permissions
Figure 6
(a) Fit of the data for the Pd/CuPc/Pd devices using the functional dependence derived in Eq. (3). The model (solid lines) can fit the experimental values of the conductance (dots) for all temperatures, and for thicknesses up to 100 nm. Data and corresponding fit for each thickness have been shifted along the $y$ axis for clarity; the data and fit values corresponding to the 21 nm device were not changed; all the other data sets and fit values, starting with the 25 nm device, were multiplied in consecutive order by 0.1, 0.01, and so on. (b) Above 100 nm, the model no longer fits the data. Data and corresponding fit values for the 102 nm device were not changed, while for the thicker devices, they were shifted along the $y$ axis as described in (a).Reuse & Permissions

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