Models for transport properties of polymer thin films

Abstract

We have observed that thin films of different undoped non‐conjugated polymers exhibit high conductivity in metal–polymer–metal (M‐P‐M) structures. Below the critical temperature, Tc of the superconductivity of the contacts, a supercurrent in superconductor–polymer–superconductor (S‐P‐S) structures was obtained. Investigations of the current‐voltage characteristics as a function of contact diameter at different locations on the polymer showed that only in some small channels does a high conductivity appear, whereas at other places a reversible on/off switching was obtained. We postulate that the conductivity effect is connected with the phenomenon of electrification. The electrons or holes are supplied by the electrodes which are in contact with the polymer. These injected charge carriers accumulate in localised states on different locations in the polymer matrix. As a result, a large inner electrical field appears in the polymer giving rise to local changes between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In the high field region the forbidden gap can vanish, resulting in small channels with a high “metallic‐like” conductivity of the polymer. At present, there exists no theory explaining all the peculiarities of the electrical transport observed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The dependence of polymer conductivity on the work function of metallic electrodes

“…The electrification process initiated by the contacts can charge some particular regions of the polymer which then produce a high local electric field. In the region of high electric field, the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) can be modified and can even vanish, or give rise to overlap [6] of the LUMO and the HOMO. In this case, an "electric field-induced metallic conductivity" appears in the polymer.…”

Metallic conductivity in a polyamidine film

“…In accordance with the model [3] the main reason for metallic conductivity in dielectric polyamidine films is the collapse of the HOMO/ LUMO gap of macromolecules (here HOMO -is the highest occupied molecular orbital and LUMO -is the lowest unoccupied molecular orbital) in a strong internal electrical field inside the polymer. (Fig.…”

“…We attribute the high (metallic) conductivity (Fig. 5) in the polymer film to the vanishing of the HOMO/LUMO gap [3] in separated macromolecules or even in some part of macromolecules which extends from one electrode to another in strong electrical field. This is due to the electrification phenomenon randomly distributed inside the polymer film.…”

“…All these findings agree well with the assumption of the accumulation of charge inside the polymer film from the metallic electrodes due to the electrification phenomenon. The metallic conductivity in polymer films is attributed to the vanishing of the HOMO/LUMO gap [3] in macromolecules in a strong electrical field randomly distributed inside the polymer film due to the electrification effect. …”

Sub-electron charge relaxation via 2D hopping conductors

“…For relatively low fields, E ≪ E 0 , these formulas describe the so-called "high-field" variable range hopping [27,28,29,30,31].…”