Investigations in the conducting polymer poly(3,4-

ethylenedioxythiophene) doped with poly(4-styrenesulfonate) in a field-effect transistor configuration

James Needham Ohio State Physics REU 2002

University of Memphis Aug. 22, 2002

Supervisor: Dr. Arthur J. Epstein

Mentor: Dr. Oliver Waldmann

Abstract

The π-conjugated backbone conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is chemically doped to the conducting regime (σ ~ 30 S/cm) with poly(4-styrenesulfonate) (PSS). Construction of an all polymer FET is described and the time dynamics of a simple polymer channel device using aluminum as the gating material is discussed. On/off ratios approaching 103 are achieved but with relatively slow switching speeds. The Hall effect was used in an attempt to measure the Hall mobility of the charge carriers but no discernible Hall voltage (Vh) was detected.

Introduction

The intriguing optic and electronic properties of π-conjugated backbone conducting polymers have captivated much interest in the scientific community. Organic semiconductors, light emitting diodes, flexible display units and transistors are only some of the recent developments in the field of conducting polymers1,2. Organic conducting polymers are of particular interest in that they offer many electronic benefits over their inorganic counterparts including: control of Fermi levels by doping3, ease of manufacture, inexpensive production and material flexibility. Particular to this paper is the unexpected field-effect behavior observed with the polymer PEDOT/PSS doped to the conducting regime with conductivity on the order of 1000 times greater than that of the typical semiconductor. The insulated gate FET is one of the most fundamental components of electronic circuitry and so it is of particular interest when such an organic conductor displays such semiconducting behavior. As this research paper is intended for a typically undergraduate audience, more emphasis will be given to a background in a typical FET set-up and instrumentation than is normally required. As a result this paper will consist of four parts: a brief background in which the polymer structure and FET set-up are discussed, measurement instrumentation, device construction and testing and, finally, Hall effect experimentation. Background PEDOT contains alternating carbon bonds with overlapping pz orbitals4 (the so-called π-conjugated backbone structure). PEDOT is chemically “p-doped” to the conducting regime (σ ~ 30 S/cm) with PSS.

(a) PEDOT chemical structure

(b) PSS chemical structure Fig. 1 The chemical structures of (a)

PEDOT and (b) PSS 5 The solution of PEDOT/PSS is placed on

the FET device using spin coating techniques which will be described in a later section. The solution is dried to form the source and drain electrodes for the FET. The dielectric and gating material vary.

The typical inorganic FET device is used to either enhance or deplete the ‘source/drain’ current by application of an electric field via the ‘gate’. In typical semiconductors, the effect takes advantage of the presence of electrons or holes in the channel and the applied electric field can be seen as either increasing or decreasing the number of charge carriers present.6

The following picture shows a typical FET under investigation. The gate voltage (Vg) is applied to the conducting gate which is separated from the channel by an insulating dielectric. The ‘leakage current’ is any (conventional) current that actually passes from the gate to the source through the dielectric. The source/drain voltage (Vs/d) generates the source/drain current which is enhanced or depleted upon application of a given Vg.

Ammeter

Ammeter

Vs/d

Vg

Gate

Drain

Source

Dielectric

Fig. 2 Typical FET experimental Set-Up

Instrumentation

The first portion of the project concerned the instrumentation involved in taking the current measurements for both the source/drain current (Is/d) and the leakage current (Ig, the current through the gate). Two Keithley 487 picoammeters were used to measure both Is/d and Ig. It was necessary to measure the leakage current with picoamp precision in order to determine whether or not the observed field-effect could be explained by the amount of charge that leaked through a given device into the channel.

Typically measuring with picoamp precision is not a problem with the picoammeter, however when both picoammeters are used simultaneously it becomes relatively easy to introduce unwanted noise in the leakage current measurements due to interference between the picoammeters and the required extra cabling. Significant amount of time was spent locating the sources of noise (e.g., unnecessary or ‘noisy’ cables, ground loops, different grounds coming from voltage sources, etc.) and eliminating this noise as much as possible. Different ammeter configurations were employed with the picoammeters located on either side of the voltage sources in an attempt to reduce noise – i.e., typically one would connect the ammeter ‘hi’ input directly to the load output with ‘lo’ connected to the ground/negative voltage terminal in order to obtain the most precise current measurements. However, it was found that the most precise measurements are obtained as shown in figure 2 – the atypical FET set-up – only with the leads to the ammeters reversed; having the ‘hi’ input to the ammeter connected to voltage source ‘lo’ and input ‘hi’ connected to the source. As a result, the read currents would be the negative of the true conventional current.

g062602_1b Leakage current vs. Time

Time (sec)38 40 42 44 46 48

I g pA

-10

-8

-6

-4

-2

0

2

4

Fig. 3 The sensitivity of the leakage current measurements was improved to better than 1 pA.

Figure 3 shows the results of the changes made: 40-60 pA jumps were reduced to less than 1 pA. The large jumps followed by an exponential rise/decay correspond to the switching on/off of the gate voltage. The information provided by these graphs is solely for signal noise comparison only.

Device Construction and Testing The polymer PEDOT/PSS was provided by the Bayer Corporation. 20 ml of PEDOT/PSS is mixed with ethyleneglycol, approximately 20% ethyleneglycol by weight, and one drop of dodecylbenzenesulfonic acid (DBSA) is added to the solution. The ethyleneglycol helps control the conductivity7 and the DBSA provides surface adhesion for the spin coating techniques employed. The solution will, henceforward, be called BED1 for simplicity.

An All-Polymer Construction

The general idea for the production of the all-polymer FET device is straightforward, and it remains the basis for the rest of the construction techniques used in this paper. The following describes the MacDiarmid line-patterning technique – so called after its developer, Alan G. MacDiarmid – which is easily used in developing all-polymer devices. The devices are printed out using a PET transparency and an office laser printer. This includes the source/drain electrodes and the

channel. The general shape that is printed out on the transparency is shown in figure 4. The conducting polymer will be distributed on such a figure to produce the active layer. Fig. 4 The basic shape of an allFET. The source and drain are separatechannel, all of which are made of the sampolymer. Once the device is printed on the transparcoat on the BED1. Various methods mayemployed to dry the polymer including usair gun, baking, or simply air drying. After the device has dried, ultrasin toluene removes the unwanted ink andany conduction due to the presence of theone may spin coat an insulating polymer the dielectric. Marking off the area needegate and spin coating on a conducting pothe gating material (e.g., BED1 is conducresults in an all-polymer FET. (a) (b)

Fig. 5 A top (a) and side (b) view of an polymer insulated gate FET.

Experiment and Results

Okuzaki, Ishihara and Ashizawa have

recently (July, 2002) been able to produce an all polymer PEDOT/PSS device with on/off ratios of 103 and switching speeds on the order of milliseconds7. Okuzaki et. al used the line-

Drain

Source

~ 1cm

PEDOT

Gate

Transpare

Dielectric

Gate

Dielectric/Ins

Transparency

BED1 (~2000 Angs. thick)

~ 1cm

-polymer d by a thin e

ency, spin be ing a hot

onication eliminates ink. Now to act as d for a

lymer for ting)

all-

patterning technique on annealed transparencies with an annealing step for the insulating PVP to reduce the surface roughness and to eliminate ‘pinholes.’ Until these results, the best MOSFET’s made with PEDOT/PSS as the conducting channel had switching speeds on the order of minutes.

Several devices were constructed in the attempt to reproduce the above results or to make improvements. The all-polymer construction techniques were used as described above. The polymer dielectric and conducting gate were replaced by an insulating copper wire (diameter ~100 µm). This was placed over the channel to act as both the gate and dielectric, still providing the necessary electric field. However, device performance and reliability interfered with any quantitative measurements so the method of construction was changed in order to further investigate the behavior of the conducting polymer (PEDOT/PSS) acting as the channel.

For ease of experimentation a glass substrate was used instead of the annealed PET transparency and tape was used to define the source/drain and gate. This allowed for a smoother substrate – decreasing the overall surface roughness of the BED1 and allowing for better overall device performance. The BED1 was applied using spin coating techniques (in air) to a thickness of approximately 2000 angstroms. The channel width varied from 1-2mm with a gate width of 2-3mm. The devices were allowed to dry overnight in a glove box. PVP (poly(4-vinylphenol)) was used as the insulating dielectric material. Aluminum was evaporated to act as the gate. Colloidal silver paste was used to make electrical contact with the source/drain and gate electrodes.

Fig. 6 Top view of three FET

devices constructed on a glass substrate. Gate width ~1-2mm.

~ 2.5 cm

PVP Dielectric layer on BED1

Silver Paste

Glass

Aluminum gate

BED1

ncy

ulator

Time Dynamics

The time dynamics of the a`bove glass substrate devices are evaluated in this section. A source/drain voltage is applied to the device and held constant. A gate voltage is then switched on and varied as can be seen in the following figure.

Time Dynamics of Device bp081302_2c_3

Time (s)200 400 600 800 1000 1200 1400 1600

I (S

/D) (

µΑ)

-120

-100

-80

-60

-40

-20

0

Gat

e Vo

ltage

(V)

-6

-4

-2

0

2

4

6

I (S/D) vs. TimeTime vs Gate Voltage

Fig. 7 The dynamical behavior of a MOSFET using the conducting polymer PEDOT/PSS as the active layer. In the above graph, the green represents the alternating gate voltage and red the source/drain current. Devices with on/off ratios approaching 103 are readily achieved. There are several things to note from this graph, however. First, we note that the magnitude of the source/drain current undergoes depletion upon application of a positive gate voltage, similar to the traditional ‘p-doped’ behavior of inorganic FET’s. But perhaps the most prominent feature is the slow source/drain switching time. Typical organic FET’s may have switching speeds on the order of milliseconds, whereas the typical device constructed in the above manner has a 90% switching time on the order of 2 minutes. Also of particular interest is the ‘recovery’ of the device, meaning once a positive gate voltage is applied, causing the aforementioned depletion, to recover to the initial source/drain current it is necessary to apply a negatively biased gate voltage.

Hall Effect Measurements

Motivation

With relatively quick switching speeds, Okuzaki produced the quintessential I-V curves for the PEDOT/PSS devices in both enhancement and depletion modes. In inorganic semiconductor theory it is possible to calculate the mobility of the charge carriers from the I-V curves by determining the so called threshold voltage. This method has been used to reasonably estimate the mobility of charge carriers in organic materials acting as the semiconducting channel in field-effect transistors6. However, the calculated mobility for the PEDOT/PSS charge carriers resulted in µFET ~ 170 cm2/Vs. This calculated mobility is almost 100 times greater than the mobility of the best known organic conductor8, hence it seemed necessary to attempt to directly determine the mobility of the charge carriers. The mobility of the charge carriers is typically readily and directly measured for inorganic semiconductors by using the Hall effect.

The Expected Hall Effect

Given the mobility of the charge carriers along with the conductivity and thickness of the sample it is possible to calculate the expected Hall voltage. However, some assumptions must be made: it is assumed that the charge carriers are electronic – this is plausible in that the reported (Okuzaki) switching time is on the order of milliseconds. Sample uniformity is also assumed – this may become a problem with the polymer, particularly along the edges, due to spin coating effects. This problem is addressed further in the experiment portion of this paper.

In basic semiconductor theory9:

Eqn. (1) µσ =HR ,

Eqn. (2) pe

RH1=

Where HR is the magnitude of the Hall coefficient, σ the conductivity of the semiconductor, µ the mobility of the charge carriers, p the density of the charge carriers, and e the electronic charge. Also,

Eqn. (3) zxH

H BIt

RV =

Where t is the thickness, Ix the transverse current and Bz the perpendicular applied magnetic field. Using a constant current source of 100µA (the max available for the sample), a thickness of 2000 Å, a max magnetic field of 0.7T and a mobility of 170 cm2/Vs one calculates an induced Hall voltage on the order of a few millivolts. See the figure 8, below, for reference:

Fig. 8 The Hall effect10

Experiment and Results The Van der Pauw method of applying

currents and reading adjacent voltages determined the conductivity of the sample10 (2 samples were measured). The Van der Pauw method determines the sheet resistivity. Finding the thickness of the sample allows one to determine the conductivity. No apparatus existed in the laboratory for making the Hall effect experiment besides a magnet with a maximum field of 0.7 T. A device holder was constructed by simply using a thin copper bar which held the device and connecting wires at one end and could be secured outside the magnet at the other end. Four insulated copper wires ran up to the sample to make the necessary connections for the Hall measurements. All experiments were performed in air at room temperature. To ensure the validity of the experimentation, tests were first performed on an n-doped silicon wafer. The sample was cut into a square, 2.5 cm on a side and contacts were made using colloidal silver paste. Conductivity measurements were made using the standard Van der Pauw method with the four probes attached to the corners of the sample. The measurements and results along with comparisons to some expected values can be seen in Appendix 1.

A sample of BED1 was prepared by the usual spin coating techniques on a square glass substrate. The edges of the sample were removed with tape and a razor blade to ensure there were no ‘build-up’ effects along the edges of the square substrate and to increase sample uniformity. Contacts were made using colloidal silver paste. The current was applied through opposite corners of the sample and the Hall voltage read across the remaining corners.

Silver Paste

~2 cm

Fig. 9 The BED1 sample used in detecting the Hall effect The Van der Pauw method found a conductivity for the BED1 samples of σ = 37 ± 4 S/cm. The thickness, t = 1200 ±150 Å was determined using a profiler. Using a constant current source and a nanovoltmeter, a sensitivity of ±10µV was obtained with no further instrumentation. A current of 100.00 µA was sent through the sample (R~5kΩ) and the applied magnetic field was varied from 0 to ±7000 Gauss. No detectable magneto-induced Hall voltage appeared across the electrodes to within ±10µV, contrary to what could be expected if the calculated mobility of 170 cm2/Vs was accurate. A lock-in amplifier was then used to further increase the sensitivity to within ±1µV, but no detectable Hall voltage was induced with an applied magnetic field of ±2000 Gauss. The Hall voltage is then certainly less than 10µV with an applied field of ±0.7T. With such results one may calculate (with t = 1200 Å, Ix = 100.00 µA)

sVcm

H ⋅≤≈2

6.0µµ

And a carrier charge density of 320103 −⋅≥ cmp Both of which are reasonable results. The mobility is on the order of the mobility of other organic

BED1

conductors and the carrier charge density is close to that which can be expected from the doping of the PEDOT8.

Summary This project consisted of three major parts: instrumentation, device construction and mobility measurements using the Hall effect. Leakage current measurement precision was increased by almost 2 orders of magnitude. Device construction was found to be inconsistent but certain methodologies, such as those described in this paper, seemed to work far better than others. No Hall voltage could be detected, and although the experimental set-up was admittedly crude the sensitivity of the Hall voltage measurements was at least on the order of ±10µV – indicative of a charge carrier mobility less than 1 cm2/V·s and a carrier concentration greater than 1020 cm-3.

Appendix 1

Hall effect measurements of an n-doped Si wafer at room temperature

The current is sent through leads 1-3 and the voltage is measured across leads 2-4. See the figure 10 below.

Induced Hall voltage vs. magnetic field for Si wafer sample

Magnetic Field, Gauss

-8000 -6000 -4000 -2000 0 2000 4000 6000 8000

Volta

ge(V

) mag

nitu

de a

cros

s V 2-

4

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Voltage vs. Magnetic Field

One can approximate the magnitude of the induced Hall voltage at 7000 Gauss as (2.035 – 1.705) = .330mV. The thickness of the Si wafer was measured with a micrometer and found to be ~400µm. Using the Van der Pauw technique for determining conductivity and the equations given in the paper, the charge carrier density and mobility may be determined.

n ~ 3.3 * 1015 and µ ~ 2200 cm2/Vs

Magnetic Field (Gauss)

Current (microAmps)

V2-4 (mV)

0 -50.00 -1.705 0 50.00 1.705

3500 -50.00 -1.868 3500 50.00 1.865

-3500 -50.00 -1.544 -3500 50.00 1.54 7000 -50.00 -2.034 7000 50.00 2.037

-7000 -50.00 -1.385 -7000 50.00 1.384

Fig. 10 The conductivity of the Si wafer was measured using the Van der Pauw technique. The current was sent through the square Si sample from opposite corners (1-3) and the induced Hall voltage was read across the remaining corners(2-4).

1 2 3 4

which are reasonable results for the n-doped Silicon wafer. See the following for comparisons. (http://www.ioffe.rssi.ru/SVA/NSM/Semicond/Si/Figs/13.gif)

Endnotes 1 Chin-Tsou Kuo, Wen-Hong Chiou, Field-effect transistor with polyaniline thin film as semiconductor. Synth. Metals 93, 155-160 (1997). 2 Emsley, John, Science Watch®, July/August 2000, Vol. 11, No. 4. http://www.sciencewatch.com/july-aug2000/sw_july-aug2000_page7.htm 3 A. Tsumura , H. Koezuka, and T. Ando, Macromolecular electronic device: Field-effect transistor with polythiophene thin film. Appl. Phys. Lett. 49, 18 (1986). 4 Arthur J. Epstein. http://www.physics.ohio-state.edu/~ppl/metallic.html 5 Nanoscale Electronics. http://www.emf.net 6 Edward Hughes, Electrical Technology, Seventh Ed., Pearson Education Lim., England, 1997 p. 423-430. 7 H. Okuzaki, M. Ishihara, S. Ashizawa, Yamanashi University. ICSM July, 2002. 8 Arthur J. Epstein, Interview, Aug. 2002. 9 Chihiro Hamaguchi, Basic Semiconductor Physics. Springer, New York, 2001 p. 261-263. 10 National Institute of Standards and Technology, http://www.eeel.nist.gov/812/effe.htm