PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Soluble polyacetylenes derived from
the ring-opening metathesis
polymerization of substituted
cyclooctatetraenes: electrochemical
characterization and Schottky barrier
devices
Thomas H. Jozefiak, Michael J. Sailor, Eric J. Ginsburg,
Christopher B. Gorman, Nathan Saul Lewis, et al.
Thomas H. Jozefiak, Michael J. Sailor, Eric J. Ginsburg, Christopher B.
Gorman, Nathan Saul Lewis, Robert H. Grubbs, "Soluble polyacetylenes
derived from the ring-opening metathesis polymerization of substituted
cyclooctatetraenes: electrochemical characterization and Schottky barrier
devices," Proc. SPIE 1436, Photochemistry and Photoelectrochemistry of
Organic and Inorganic Molecular Thin Films, (1 March 1991); doi:
10.1117/12.45109
Event: Optics, Electro-Optics, and Laser Applications in Science and
Engineering, 1991, Los Angeles, CA, United States
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/19/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
Invited Paper
Solublepolyacetylenes derived from the ring-opening metathesis polymerization (ROMP) of substituted cyclooctatetraenes:
electrochemical characterization and Schottky barrier devices
Thomas H. Jozefiak, Michael J. Sailor, Eric J. Ginsburg, Christopher B. Gor.man,
Nathan S. Lewis*, Robert H. Grubbs*
Contribution #8376from The Arthur A. Noyes Laboratory of Chemical Physics and
Arnold and Mabel Beckman Laboratory ofCheinical Synthesis, The Division ofChemistry and Chemical Engineering,
California Institute ofTechnology, Pasadena, California 91125, USA
ABSTRACT
Recent developments in ring-opening metathesis polymerization (ROMP) have enabled the synthesis of poly-
cyclooctatetraene (poly-COT), a material which is isosiructural to polyacetylene. This liquid-phase polymerization method
allows facile construction of interfaces, films, and devices with polyacetylene-like materials. The ROMP method also
allows the preparation of soluble, yet highly conjugated polyacetylene analogs from substituted cyclooctatet.raenes (R-COT).
The redox characteristics of R-COT polymers were investigated at elecirodes modified with thin polymer films.
Voltammetric methods were used to characterize the redox response, band gap, electrochemical doping, and cis-trans
isomerization properties of these polyenes. We have applied poly-COT technology to the fabrication of Schottky diodes and
photoelectrochemical cells, by forming poly-COT films on semiconductor surfaces. The resultant semiconductor/organic-
metal interfaces behave more ideally than semiconductor contacts with conventional metals, in that changes in the work
function of the conducting polymer exert a large and predictable effect on the electrical properties of the resulting Schottky
diodes. Transparent films of the solution-processible polymer poly-trimethylsilyl-cyclooctatetraene (poly-TMS-COT) have
been cast onto n-silicon substrates and doped with iodine to form surface barrier solar cells. These devices produce
photovoltages that are much larger than can be obtained from n-silicon contacts with conventional metals.
1. INTRODUCTION
Within the last 15 years, a large volume of work1 exploring the synthesis and properties of polyacetylene has
generated tremendous interest within the scientific community. As a one-dimensional semiconductor capable of either n- or
p-type doping to high metallic conductivities, polyacetylene is a material with many potential applications2 in diverse fields
such as sensors, solar energy conversion, and lightweight batteries. However, the use of polyacetylene in many of these
applications is hindered by an inherent lack of processability that is found in nearly all highly conjugated, and necessarily
rigid, organic conducting polymers. Polyacetylene is a brittle, insoluble, and unprocessable material, classically formed by
the exposure of acetylene gas to catalyst-treated surfaces. The polymer films which result have a fibrillar morphology and
are semi-crystalline. Both the lack of processability of this material and its method of synthesis have hindered many
potential applications where uniformly thin, mechanically strong films are required for casting onto a variety of substrates.
8
/ SPIE Vol. 1436 Photochemistry and Photoelectrochemistry of Organic and Inorganic Molecular Thin Films (1991)
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/19/2018
Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
Considerable effort has been directed toward the preparation of soluble prepolymers from which polyacetylene may be
obtained after processing.3
The synthesis of polyacetylene from the transition-metal catalyzed ring-opening metathesis polymerization
(ROMP) of cyclooctatetraene, reported by Grubbs in 1988, represents a liquid-phase route to polyacetylene. The resulting
polymer, poly-cyclooctatetraene (poly-COT), has many of the same properties as polyacetylene formed by the classical
(Shirakawa) polymerization of acetylene over Ziegler..Natta type catalysts. However, the liquid-phase ROMP method of
synthesis is more amenable to the preparation of thin films and the fabrication of devices. More importantly, this method
can be generalized to the polymerization of substituted cyclooctatetraenes.57 Polymers from substituted cyclooctatetraenes
(poly-R-COT), bear a substituent group every eight carbons on the average. Thus, the ROMP procedure provides
substituted polyacetylenes not accessable from other monomers. It has been found that the appropriate choice of substituent
results in a polymer which is soluble in common organic solvents (tetrahydrofuran, CH2C12) while retaining the high
conjugation length required for doping to the metallic state.
As prepared, COT polymers have a high content of cis double bonds. In the case of the soluble poly-R-COT
materials, a photochemical means of cis-irans isomerization has been developed,5 which results in a mostly trans polyene.
This serves as an alternative to the thermal isomerization method commonly employed for insoluble polyacetylene films
(see Scheme 1).
1h;F
Scheme 1. Synthesis of trans-poly-RCOT from a substituted cyclooctatetraene.
In this report, we describe the electrochemical characterization of poly-R-COT materials, demonstrating the ease
with which surfaces (such as electrodes) can be modified. As a potential application of this technology, we also describe the
properties of Schottky barrier diodes formed by the interface between n-silicon and a metallically doped coating of poly-COT.
Further, we describe Schouky barrier solar cells formed from thin transparent layers of poly-trimethylsilyl-COT deposited
from solvent onto n-silicon. We have previously reported on the unique features of these semiconductor/organic-metal
interfaces and found them to be superior to traditional semiconductorfmorganic-metal devices.
Substituted cyclooctatetraenes were polymerized as previously described5 by mixing the liquid monomer with the
tungsten carbene catalyst8 W(CHAr')(NAr)[OCMe(CF3)J2(THF) where Ar =
C6H5
and Ar' =
o-MeOC6H.
The polymer
films were prepared and manipulated in a nitrogen filled dry-box. All electrochemical experiments were performed in the dry-
box. All single-crystal silicon samples were ohmically contacted with gallium-indium eutectic, mounted in epoxy to cover
the back contact and edges, and etched with 48% hydrofluoric acid before deposition of the polymer. n-Si samples were
oriented along the (100) plane, and the p-Si samples were oriented in the (111) plane.
SPIE
Vol. 1436 Photochemistry and Photoelectrochemistry of Organic and Inorganic Molecular Thin Films (1991)! 9
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/19/2018
Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
2 VOLTAMMETRIC STUDIES OF R-COT POLYMERS
2. 1 Cyclic voltammetry and redox potentials
We have reported that, like polyacetylene, poly-R-COT films can be doped p-type with iodine vapour, or n-type
with a tetrahydrofuran solution of potassium benzophenone. These redox processes have now been studied electrochemically.
Voltammetry experiments were performed using carbon electrodes modified with thin (<0.2 pm) films of R-COT polymers.
Using a microliter syringe, a small amount (0.5-3.0 il) of a tetrahydrofuran polymer solution (0.3 mg/mi) was placed on the
surface of a glassy carbon electrode, and the solvent was evaporated. Voltammetry was performed using a standard three-
electrode configuration in a single compartment cell. The polymer modified electrode was placed in a voltammetry cell along
with a platinum counter electrode and a Ag/Ag+ reference electrode. The solvent/supponing-electrolyte was acetonitrile/O. 1M
TBABF4 (tetra-n-butylammonium tetraflluoroborate). Though soluble in nonpolar solvents such as tetrahydrofuran,
dichloromethane and toluene, the R-COT polymers were found to be insoluble in more polar solvents such as acetonitrile.
The voltammograms obtained show reversible couples for both anodic and cathodic redox processes (see Figure 1). The
anodic couple was characterized by sharp peaks with small peak separation (<100 mV). The cathodic couple generally shows
slightly broader waves and a peak shape which is more dependent on scan rate and film thickness. The small voltammetric
peak separations seen for these materials may be attributed to their amorphous morphology. Solvent cast films of R-COT
polymers have been shown by microscopy7b to be unusually smooth in contrast to the fibrillar or pitted morphology
frequently found for conducting polymers. Table 1 lists the oxidation and reduction potentials for a series of R-COT
polymers. Also listed is the energy difference between the oxidation and reduction couples. Although this is not a rigorous
measure of the band gap, these two values can be expected to show similar behavior.
The data of Table 1 reveal some interesting trends. Particularly noteworthy is the effect of substitution on oxidation
potential. Relative to alkyl-substitution, the trimethylsilyl (TMS) polymer is more difficult to oxidize by 130 mY, and the
tert-butoxy polymer is more easily oxidized by 130 mY. The TMS group is known as a it-acceptor,9 while the tert-butoxy
group is an electron donating substituent. There is also an observable effect of steric bulk on conjugation length. Within
the series of butyl substituted polymers, n-butyl shows the smallest voltammetric "band-gap", while the bulky tert-butyl
substituted polymer is so severely twisted that it cannot be doped anodically or cathodically within the available
electrochemical window. This finding is supported by spectroscopic measurements in THF solution, where increasing
conjugation length is indicated by a lower energy r-it' transition.
The inability to measure the relative amounts of capacitive and faradaic charge in the electrochemical doping of a
polymer film complicates the coulombic determination of stoichiometry.1° With this point in mind, coulometric analysis
of the electrochemical doping was attempted. In this experiment, a polymer modified electrode was swept through its anodic
or cathodic couple within limits that closely bracketed the peak region of the voltammetric wave. This portion of the wave
should represent predominantly faradaic charging. The charge passed was measured for the anodic sweep of the oxidation
couple (p-doping), and the cathodic sweep of the reduction couple (n-doping). This measurement was carried out on the
10
/ SPIE Vol. 1436 Photochemistry and Photoelectrochemistry of Organic and Inorganic Molecular Thin Films (1991)
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/19/2018
Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
second cycle in order to avoid a 'break-in" capacitance usually seen in the first cycle. The background charge passed in
cycling the clean electrode was measured and subtracted.
For poly-sec-butyl-COT, oxidative doping from 0 to ÷0.4 V (all potentials are vs SCE) required 0.16 coul/mg, and
reduction from -1.3 to -1.8 V required 0.16 coul/mg. These data imply stoichiometries of: (sec-butyl-COT)3.76[BF4J and
(sec-butyl-CO1)37[TBAJ, or a chain-carbon/dopant molar ratio [(CH)XIJ of y=O.03. This result is dependent on potential
and assumes no redox induced decomposition or capacitive charging. Under these conditions, a conjugation length of 3.76
COT units per soliton, or a soliton length of 30 carbon atoms is suggested as an upper limit.
Volts vs. Ag/Ag+
+0.4
Figure 1. Representative cyclic voltammogram of trans-poly-sec-butyl-COT thin (<0.5 .tm) film at a glassy carbon
electrode, acetonitrile/O.1M TBABF4, 50 mV/sec. Arrowhead indicates scan direction.
R
EO'rgj (V)
E°'ox (V)
E°'ox-E°'red(V)
tert-butyl
- - -C
..C .C
432
TMS
-1.44
+0.37 1.81
538
sec-butyl
-1.55
+0.25
1.80
556
n-butyl
-1.54
+0.23
1.77
- -
tert-butoxy
-1.49
+0.10
1.59 595
Table 1. Voltammetric potentials vs. SCE taken as (Ep1+Epc)/2. (a) spectra measured in dilute
ThF solution. (b) trans polymer is insoluble. (c) cannot be doped between -2.5V and ÷2.8V
SPIE Vol. 1436 Photochemistry and Photoelectrochemistry of Organic and/norganic Molecular Thin Films (1991) I 11
I 50
tA
l,,,,J!,,,I,,,,I,!,,IJ'luII,u,
—2.0
—1.6 —1.2
—0.8
—0.4
0.0
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/19/2018
Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
