Hall Effect and Magnetoresistance in Undoped Poly(3 Hexylthiophene)
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HALL EFFECT AND MAGNETORESISTANCE IN UNDOPED POLY(3-HEXYLTHIOPHENE). Azar Assadi, Christer Svensson and Magnus Willander. Dept. of Physics and Measurement Technology, Link6ping University, S-581 83 Linkbping, Sweden.
ABSTRACT. We report studies of transport properties of thin films of undoped poly(3-hexylthiophene) (P3HT) fabricated by spinning a polymer solution onto oxidised silicon or glass. The film thicknesseswere on the order of 1000
A. We
studied
magnetoresistance, the effect of magnetic field on field effect mobility and Hall effect. Transverse magnetoresistance was measured on films on glass. A positive anomalous magnetoresistance with a saturation value of about 0.1 % was found. Transverse magnetoresistance was also studied by measuring the change of a field effect mobility with magnetic field. This measurement was performed using the field effect transistor structure. 2 An initial mobility of 6.5 X 10-5 cm /Vs was reduced by about 15 % in a transverse
magnetic field. We also carried out Hall effect measurements on films on glass using a Van der Pauw 2 contact configuration. The measured Hall mobility was 2.17 x 10-5 cm /Vs. Finally we measured the temperature dependence of the Hall mobility and found it follow the 1 4 exp(T- / ) law of variable range hopping.
INTRODUCTION Many conjugated polymers show high electrical conductivity, which is associated with the high degree of delocalisation of the nt electrons. The detailed mechanism of charge transport is however not well understood. Presently, the general assumption is that transport occurs through a hopping mechanism, similar to what is believed to be the transport mechanism in amorphous inorganic semiconductors [ 1]. In the case of poly(3-hexylthiophene) the charge carriers are assumed to be polarons (or possibly also bipolarons)[ 2
1,[3 ],
so that hopping is expected to occur between polaron or bipolaron
states. Mat. Res. Soc. Symp. Proc. Vol. 173. @1990 Materials Research Society
404
As magnetoresistance and Hall effect have been successfully used to provide information about the transport mechanism in amorphous and crystalline semiconductors, we have performed such experiments on P3HT films. The results of these investigations can then be compared to previous results from field effect measurements [4 , [ 5] and hopefully contribute further to our knowledge of the transport mechanisms in semiconductor P3HT.
EXPERIMENTAL The P3HT used was synthesised by Neste Oy as reported previosly [ 6 ]. The polymer was dissolved in chloroform (20 mg/ml) and the solution was spin coated onto the substrates. This method of preparation of the polymer has been used for all samples. The film thickness measured by a Dektak 3030 surface profile measuring system was 700-1000A. The substrates used were either glass substrates (for magnetoresistance and Hall effect measurements) or heavily boron doped and oxidised silicon wafers ( axis)(for field effect measurements). The silicon wafer, acting as a gate, was covered with thermally grown oxide film (3000 A thick
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