Polymer Degradation Effect On The Organic Magnetoresistance

Eindhoven University of Technology Department of Applied Physics Group Physics of Nanostructures (FNA)

The effect of polymer degradation on the organic magnetoconductance

I.D.C. Pérez, August 2012

Supervisors: ir. M. Cox prof. dr. B. Koopmans

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Abstract
This investigation studies polymers which were usually characterized as insulators or nonconductive, until in 1977 Heeger et.al. (29) discovered they can conduct electrical charges. These polymers, commonly dubbed as plastics, can be influenced by the presence of an external magnetic field. This fact opens the way to a new research field called organic spintronics. The change in the electrical resistance of organic materials when a magnetic field is applied on it is called organic magnetoresistance (OMAR). OMAR canbe as large as ~25% magnetoresistance when an external magnetic field in the order of 10 mT is applied, at room temperature (33). The objective of this work is to investigate what is the effect of polymer degradation on OMAR. Degradation is a relevant topic for the industry of technology since it must be considered when materials are evaluated as candidates for making new devices. It isexpected that the results of this work will contribute to a better understanding of the underlying mechanism of OMAR, whose models are based on spin-based reactions of the charge carriers, but are not known with certainty. To degrade the samples, these were placed in an atmosphere with oxygen, using an environment with pure oxygen (inside a vacuum chamber) and the lab environment. All the resultsshowed that polymer degradation leads to a reduction of the magnetoresistance. It is considered that the degradation creates traps that block the free movement of charges, reducing the magnetoresistance. It was found that samples degrade much faster in the lab environment, deducing that not only oxygen is influencing the OMAR effect, other factors present in the lab environment are having an impacton the devices; it is thought that these factors can be water and light. It was observed that the process of degradation is reversible when samples are degraded using short periods of time, but it becomes irreversible after long periods of exposition.

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Contents
1 Introduction…………………………………………….……………………….……………………….………….…………..4

1.I) Organicspintronics……………………….……………………….……………………….……………………….……….4 1.II) The objective of this internship……………………….……………………….………………………..…………..5

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Theory……………………….……………………….……………………….……………………………………………….……7

2.I) Conduction in organic semiconductors………………………………………………..…………………..…......7 2.I.1) Charge carriers and charge transport……………………….……………………………………………....7 2.I.2) Transport through hopping……………………….……………………….………………………….………....8 2. I.3) How to induce a current through an organicsemiconductor……………………….…………..9 2. I.4) The formation of traps……………………….……………………….……………………….…….…..………11 2.II) Charge interactions in organic materials……………………….……………………….………………....……12 2.II.a) Polarons……………………….……………………….……………………….……………………….……….……12 2.II.b) Electron-hole pairs……………………….……………………….……………………….…………………..…..12 2.II.c) Excitons……………………….……………………….……………………….……………………….………..…..…12 2.III) Organic Magneto-Resistance (OMAR)……………………….……………………….……………………….…13 2.IV) Spin interactions that describe the origin of OMAR……………………….………………………….…….14 2.IV.1) Bipolaron model……………………….……………………….……………………….……………….…………..14 2.IV.2) Electron-hole pair model……………………….……………………….………..……………….……..……..15 2.IV.3) Trion model……………………………………….……………………….…………………..…….……………..….16 2.V) Fitting functions……………………….……………………….……………………….……………..……………………..18 2.V.1)Lorentzian and non-Lorentzian fits……………………….……………………….………………………….18 2.V.2) Alternative fitting function……………………….……………………………………………………………….18 2.VI) Modeling of the MC(V) curve……………………….……………………….……….………………………………..19 2.VII) The presence of oxygen……………………….……………………….……………………….…………..…………..20

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Experimental...