International Journal of Scientific and Research Publications, Volume 4, Issue 8, August 2014 1 ISSN 2250-3153 www.ijsrp.org Measurement of Bulk Resistance of Conducting Polymer Films in Presence of Rectifying Contacts Bindu S, M S Suresh Dept. of Electronics & Communication, B N M Institute of Technology, PB No: 7087, 12th main road, 27th cross, Banashankari 2nd stage, Bangalore, India Abstract- Measurement of bulk resistance/resistivity of conducting polymers is very common and an important requirement. Conducting polymers are semiconductors and four probe measurements are needed to avoid errors due to contact resistance and spreading resistances. However, in a device it is convenient to make two probe measurements rather than four probe measurement. It has been shown here that the bulk resistance of a thick film can be measured in the presence of rectifying contacts, using two probe method by measuring the impedance of the film beyond a critical frequency. The metal- film contact has been modeled as a diode to explain the behavior. Index Terms- Critical frequency, Conducting polymer, Four probe measurement, PEDOT-PSS, Strain gauge, Two probe measurement. I. INTRODUCTION emiconducting polymers have emerged as novel electronic materials for use in variety of devices [1]. Use of conducting polymers as strain gauge material to measure force, strain or pressure is a recent application that exploits the piezoresistive property of certain conducting polymers [2-4]. Advantages of using conducting polymers as strain gauge materials are high gauge factor and flexibility [5-6] and also compatibility with biological environment [7]. In strain gauges change in the bulk resistance of the polymer film due to strain has to be measured. Since, conducting polymers are semiconductors, metal- semiconductor contacts form rectifying contacts rather than ohmic contact. In order to measure the change in the bulk resistance of the film it is necessary to use four probe measurements [8] or form ohmic contacts with the film. Thus measurement of bulk resistance of conducting polymer films poses several challenges. However, for strain gauge applications it is convenient to use two probe measurements rather than four probes. This paper proposes a simple two probe method of measuring the bulk resistance of polymer films using AC technique and provides a method for estimation of frequency(critical frequency) at which measurements are to be made. Comparison of two probe AC and four probe DC measurements for PEDOT-PSS (poly 3,4ethylenedioxythiophene – polystyrenesulfonic acid) films pristine and doped with DMSO (Dimethyl sulfoxide) is made to prove the efficacy of the method. A phenomenological model is presented to explain the results. The AC method allows use of any metal contact such as copper or tin rather than expensive gold or ruthenium. II. EXPERIMENTAL DETAILS PEDOT-PSS was procured from H.C. Stark, which is dispersion in water, with a weight ratio of 1:6. Film of PEDOT- PSS is drop cast on a kapton sheet of 120μm thickness. Before casting the film on kapton substrate the substrate is ultrasonicated in acetone, triple distilled water and isopropyl alcohol for 30 minutes. The film cast is annealed (at 50-55 deg. C) for about 24 hours [9] so that water evaporates and a thin polymer film of about 20μm thick, 15mm length and 5mm width is formed. Four contacts (1 to 4) on to the film are made by roller pressure contacts, using tinned copper rollers of diameter 0.61 mm with a distance of 5mm between them. Pressure on the contacts is applied using a C clamp which holds the polymer film between two rigid Perspex sheets. Two sets of measurements are done; one, four probe measurements with DC excitation and another with two probes with AC excitation. These are compared to show that two probe AC method indeed gives the same value as the DC four probe method. For making four probe DC measurements, circuit connections are made as shown in Fig 1 where RL is a standard resistor of known value. The voltage across RL (VO) gives the current through the film. Voltages at contacts1, 2, 3 and 4 are measured to calculate the voltage drop across contacts 1-2, 2-3, 3-4 and consequently resistance between those contacts. Fluke 179 digital multimeter is used for DC voltage measurements. Measurements are made by applying different DC voltages in the range 1 to 30V. AC measurements was made to determine Impedance between two terminals( 2 and 3) by applying AC voltage as shown in Fig.2 and measuring the voltage across RL to get current and the voltage across the film V23 (V2 –V3). Measurements are made by applying a fixed AC voltage of 5V peak and varying S