Synthesis and optical and transport properties of a phenyl-substituted polythiophene

Article abstract of DOI:10.1002/pola.24701

The synthesis and characterization of a novel polythiophene substituted with a 2′-pentyloxy-5′-(1″′-oxooctyl) phenyl group (PPOPT) is reported. The bulk transport properties of thin films of PPOPT are investigated by admittance spectroscopy. The dramatic

Full text of DOI:10.1002/pola.24701

Synthesis and Optical and Transport Properties of a
Phenyl-Substituted Polythiophene
SIRAYE E. DEBEBE,1 DESTA A. GEDEFAW,1 WENDIMAGEGN MAMMO,1 TEKETEL YOHANNES,1 FRANCESCA TINTI,2
ALBERTO ZANELLI,2 VALERIA FATTORI,2 NADIA CAMAIONI2
1Department of Chemistry, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia
2Istituto per la Sintesi Organica e la Fotoreattivita`, Consiglio Nazionale delle Ricerche, via P. Gobetti 101, I-40129 Bologna, Italy
Received 22 February 2011; accepted 1 April 2011
DOI: 10.1002/pola.24701
Published online 27 April 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The synthesis and characterization of a novel poly-
thiophene substituted with a 20-pentyloxy-50-(1000-oxooctyl) phe-
nyl group (PPOPT) is reported. The bulk transport properties of
thin films of PPOPT are investigated by admittance spectros-
copy. The dramatic effect of the phenyl side chain on the mo-
bility of positive carriers in films of PPOPT is described. The
photophysics of PPOPT in both solution and thin film is also
investigated and correlated to substituent-driven intrachain
C
V
and interchain arrangements.
2011 Wiley Periodicals, Inc. J
Polym Sci Part A: Polym Chem 49: 2693–2699, 2011
KEYWORDS: admittance spectroscopy; charge transport; conju-
gated polymers; phenyl substitution; photophysics; polythio-
phene
INTRODUCTION An intensive research activity on p-conju-
gated polymers has been stimulated by the prospect of low-
cost fabrication of plastic electronic devices, since their
discovery in 1977. A notable feature of conjugated polymers
lies in the versatility of their molecular structure which
affords wide space to prepare new materials with improved
properties.1 Polythiophene and its derivatives constitute one
of the most promising class of conjugated polymers and
have been widely used in solution-processed polymer solar
cells2 and field-effect transistors,3 as well as in the fabrica-
tion of light-emitting diodes.4
thoroughly investigated. In this article, we report the synthe-
sis and characterization of a novel phenyl-substituted poly-
thiophene, poly[3-(20-pentyloxy-50-(1000-oxooctyl) phenyl)
thiophene] (PPOPT). In addition to the optical and electro-
chemical characterization, the investigation of bulk transport
properties of positive carriers in thin films of PPOPT is
described.
EXPERIMENTAL
Synthesis of the Polymer
Synthesis of 1-Bromo-2-pentyloxybenzene (2)
To tune their electronic properties to meet the request for
different applications, chemical modifications of polythiopene
derivatives are performed. One method to tailor the elec-
tronic properties is the introduction of substituents on the
thiophene rings.5 Substituents can influence the electrical,
electrochemical, and optical properties of the resulting poly-
mers, other than modifying their processability in organic
solvents. Depending on their nature, size and position, sub-
stituents can affect the interaction between polymer chains
or induce a distortion of the polymer backbone, resulting in
the variation of the p conjugation of the aromatic system.
To a mixture of o-bromophenol (5 g, 0.03 moles) in 30 mL
dimethylformamide, anhydrous potassium carbonate (4.14 g,
0.03 moles) was added. The mixture was stirred, and 1-bro-
mopentane (4.53 g, 0.03 moles) was added gradually with
heating over an oil bath. The mixture was heated overnight,
cooled to room temperature, and filtered by suction. The fil-
trate was acidified with 2 M HCl and extracted with diethyl
ether. The ether extract was washed with 1 M NaOH and
water and was dried over anhydrous Na2SO4. The ether
was removed by rotary evaporator to afford 1-bromo-2-pen-
tyloxybenzene (2) as a yellowish oil (6 g, 82.3%).
1H NMR (CDCl3, 400 MHz): d 7.45 (dd, J ¼ 8, 1.36 Hz, 1H),
7.15 (m, 1H), 6.8 (dd, J ¼ 8, 1.36 Hz, 1H), 6.7 (m, 1H), 3.9 (t,
2H), 1.75 (m, 2H), 1.45 (m, 2H), 1.35 (m, 2H), 0.85 (t, 3H).
13C NMR (CDCl3, 100 MHz): d 156.0, 133.7, 128.8, 122.0,
113.7, 112.7, 69.6, 29.3, 28.6, 22.9, 14.5.
Phenyl-substituted polythiophenes have already been
reported in the literature, showing how the phenyl substitu-
ents affect the electrochemistry,6 the light emission proper-
ties,7 as well as the photoinduced charge transfer behavior.8
However, the effect of the phenyl substitution on the trans-
port properties of this class of polythiophenes has not been
Correspondence to: W. Mammo (E-mail: wmammo@chem.aau.edu.et) or N. Camaioni (E-mail: camaioni@isof.cnr.it)
C
V
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2693–2699 (2011) 2011 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Synthesis of 1-Bromo-2-pentyloxy–5-(100-oxooctyl)
28.6, 25.1, 23.0, 22.8, 14.5, 14.4. IR mmax (cmꢁ1): 2950, 2875,
benzene (3)
1700, 1600, 1500, 1290, 1080, 800–500.
In a two-necked round-bottomed flask, 1-bromo-2-pentyloxy-
benzene (2) (6 g, 0.025 moles), aluminum chloride (3.34 g),
and carbon disulfide (25 mL) were mixed. A condenser was
attached to one of the necks of the flask and to the other, a
dropping funnel containing octanoyl chloride (4.1 g, 0.025
moles) was added. Both the condenser and the dropping fun-
nel were protected with calcium chloride guard tubes. The
octanoyl chloride was added drop by drop for 30 min with
continuous stirring of the contents of the flask. The mixture
was refluxed for 6 h until the evolution of hydrogen chloride
gas almost ceased. The solvent was removed by distillation,
and the remaining mixture was poured into a beaker con-
taining 13 g of crushed ice and concentrated HCl (8 mL).
The mixture was stirred and extracted with diethyl ether.
The ether extract was washed with water (twice), 10%
NaOH solution (once) and water (twice) and dried over an-
hydrous sodium sulfate. The ether was removed by rotary
evaporator to yield 3 as an orange colored solid (6 g, 65%).
mp 33.3–35.4 ꢀC. 1H NMR (CDCl3, 400 MHz): d 8 (d,
J ¼ 1.26 Hz, 1H), 7.81 (dd, J ¼ 8.86, 1.26 Hz, 1H), 6.84 (d,
J ¼ 8.86 Hz, 1H), 4.1 (t, 2H), 2.85 (t, 2H), 1.8–1.0 (m, 16H),
1.1–0.8 (m, 3H), 0.8 (t, 3H). 13C NMR (CDCl3, 100 MHz): d
198.0, 159.4, 133.8, 131.2, 129.4, 112.6, 112.2, 69.7, 38.5,
32.1, 29.7, 29.5, 29.0, 28.5, 24.8, 23.0, 22.7, 14.4, 14.3. Infra-
red (IR) mmax (cmꢁ1): 2950, 2875, 1700, 1600, 1500, 1290,
1080, 800–500.
Synthesis of Poly[3-(20-pentyloxy-50-(1000-oxooctyl)
phenyl)thiophene] (5)
Compound 4 (0.5 g, 1.45 mmol) was dissolved in chloroform
(15 mL) and a slurry of ferric chloride (1.32 g) in chloro-
form (15 mL) was added to it for 90 min, and the mixture
was stirred for an extra 3.5 h. The reaction mixture was
poured into methanol, and the red precipitate was collected
by suction filtration. The solid material was dissolved in
chloroform and was washed with concentrated aqueous am-
monia solution (six times), with 0.05 M EDTA (twice) and
finally with distilled water (twice) and filtered using a sin-
tered glass funnel. The chloroform solution was concentrated
to a small volume and poured into methanol and the poly-
mer that precipitated was collected by suction filtration.
Polymer 5 was obtained as red powder (270 mg).
mp > 250 ꢀC. Mn ¼ 50,500, Mw ¼ 92,000. 1H NMR (CDCl3,
400 MHz) (partial data): d 7.9 (d, 1H), 7.7 (d, 1H), 6.8 (d,
1H), 6.5 (s, 1H), 3.8 (t, 2H), 2.75 (t, 2H), 0.5–2.0 (unresolved,
aliphatic protons); IR mmax (cmꢁ1): 2950, 2875, 1700, 1600,
1500, 1290, 1100, 900–675.
Optical and Electrochemical Characterization
UV–Vis absorption spectra were recorded with a Perkin
Elmer 950 spectrophotometer. Photoluminescence (PL) spec-
tra, in solution and in thin film on quartz substrate, were
obtained by using a Spex Fluorolog II 1681 spectrofluorome-
ter. Quantum yields of PL for solutions were obtained as rel-
ative values using Ru(bipy)3Cl2 in aerated water solution as
the reference, and an Edinburgh fluorometer equipped with
a Labsphere integrating sphere was used to measure abso-
lute quantum yields of PL of solid films.9
Synthesis of 3-[20-Pentyloxy-50-(1000-oxooctyl)phenyl]
thiophene (4)
In a one-necked round-bottomed flask kept under nitrogen
atmosphere, Pd(PPh3)4 (0.25 g), 1-bromo-2-pentyloxy–5-(100-
oxooctyl)benzene (3) (4.03 g, 0.01 mol), and 1,2-dimethoxy
ethane (50 mL) were added, and the mixture was stirred for
10 min. 3-Thiopheneboronic acid (1.40 g, 0.01 mol) was then
added followed by the addition of 1 M sodium bicarbonate
(40 mL). The mixture was refluxed monitoring the progress
of the reaction by thin layer chromatography. After 5 h, the
reaction mixture was cooled, and the 1,2-dimethoxy ethane
was removed by rotary evaporator. Water (100 mL) was
added to the residue, and the mixture was extracted with
diethyl ether five times. The ether extract was washed with
distilled water, dried over anhydrous Na2SO4, and the ether
was removed to afford dark-brown oil (5.57 g), which solidi-
fied on standing. The crude product was chromatographed
over a short column of silica gel using petroleum ether as
eluent. Compound 4 (2.10 g, 51.7%) was obtained in pure
form as white needle-like crystal after recrystallization from
acetone–methanol.
Cyclic voltammetries of thin films of PPOPT, spin coated onto
indium tin oxide (ITO)-coated glass substrate (15 ꢂ 15
mm2) from a chloroform solution, were recorded by an
AMEL electrochemical system model 5000. A three-electrode
system was used, consisting of a platinum wire as auxiliary
electrode, aqueous saturated calomel (SCE), whose potential
resulted ꢁ0.392 V versus ferrocene/ferricenium, as internal
reference electrode,10 and ITO as working electrode. The
three electrodes were placed in an electrochemical cell con-
sisting of three compartments separated by glass frits. A
0.1 mol Lꢁ1 tetrabutylammonium perchlorate (C4H9)4NClO4
(Fluka puriss., crystallized from CH3OH and vacuum dried)
in acetonitrile, CH3CN (Merck Uvasol, stored under argon
pressure and over molecular sieves 3 Å activated at 400 ꢀC
for 4 h) was used as supporting electrolyte. Argon gas was
bubbled for 20 min in the working compartment before the
measurement. During the measurement, argon gas was
flushed into the working electrode compartment.
mp 61.3–63.1ꢀC. 1H NMR (CDCl3, 400 MHz): d 8.05 (d, J ¼
1.95 Hz, 1H), 7.88 (dd, J ¼ 7.8, 1.95 Hz, 1H), 7.58 (dd,
J ¼ 3.20, 1.0 Hz, 1H), 7.4 (dd, J ¼ 5.6, 1.0 Hz, 1H), 7.25 (dd,
J ¼ 7.80, 2.34 Hz, 1H), 6.88 (d, J ¼ 7.80 Hz, 1H), 4.0 (t, 2H),
2.85 (t, 2H), 1.9–1.0 (m, 16H), 0.9 (m, 6H). 13C NMR (CDCl3,
100 MHz): 199.6, 160.1, 137.9, 130.4, 130.3, 129.4, 128.8,
125.4, 124.9, 124.1, 111.9, 69.2, 38.7, 32.1, 29.8, 29.6, 29.2,
Characterization of Transport Properties
The devices used for admittance spectroscopy measurements
were prepared in the sandwiched structure ITO/PEDOT:PSS/
PPOPT/Al, where PEDOT:PSS is poly(3,4-ethylenedioxythio-
phene)/polystyrene sulfonic acid (CLEVIOS P VP AI 4083,
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SCHEME 1 Scheme of the synthesis of poly[3-(20-pentyloxy-50-(1000-oxooctyl) phenyl)thiophene] (PPOPT).
H.C. Starck). ITO/glass substrates were first cleaned in deter-
gent and water, then ultrasonicated in acetone and isopropyl
alcohol for 15 min each. The layer of PEDOT:PSS (around 40
nm) was spin coated at 4000 rpm onto ITO/glass substrates,
then baked in an oven at 120 ꢀC for 10 min. The polymer
layer was spin coated at 850 rpm from a chloroform solution
(22 g Lꢁ1) and thermally annealed at 120 ꢀC for 25 min in
argon atmosphere before the deposition of top electrode.
The thickness of the polymer layer, measured with a Tencor
AlphaStep profilometer, was 260 nm. The top Al electrode
(70 nm) was thermally evaporated at a base pressure of 4 ꢂ
10ꢁ6 mbar through a shadow mask giving an active device
area of 0.25 cm2.
The 1H NMR spectrum of PPOPT (not shown here) revealed
a singlet peak in the aromatic region at d 6.5, which is attrib-
utable to the thiophene ring proton. It can, therefore, be sug-
gested that the polymer is regioregular,11 and the coupling
between the thiophene repeating units is mainly in a head-
to-tail fashion. However, the substitution at C-3 of the poly-
thiophene backbone could cause twisting of the phenyl rings
out of plane, which could affect the packing of the polymer
chain in the solid state.12
The optical properties of PPOPT were investigated by UV–Vis
absorption and PL spectroscopy. The normalized spectra, in
a diluted chloroform solution (10ꢁ5 mol Lꢁ1) and in thin
film spin coated onto quartz substrate, are shown in Figure
1. The polymer exhibits absorption maxima at 487 nm and
at 501 nm, in solution and in thin film, respectively, leading
to a red shift of 14 nm in the solid state. An optical energy
gap of 2.0 eV was estimated from the absorption onset of
the film. In addition, while the absorption spectrum in solu-
tion is structureless, the appearance of a relatively weak
shoulder at ꢃ580 nm is visible in the solid state. This fea-
ture, along with the moderate red shift of the spectrum
when going from solution to thin film, is an indication of
moderate interactions between the polymer chains in the
solid state13 or a larger overlap of p orbitals on the polymer
backbone due to an increased coplanarity of the thiophene
rings in the solid state induced by alignment of the alkyl
chains.14 The narrowing of the PL band in the solid state
(full width at half-maximum is 0.25 eV in film and 0.36 eV
in solution) seems to support a well-ordered solid state
induced by the substituents on the phenyl ring.15
The electrical characterization of the devices was carried out
at room temperature under dynamic vacuum (5 ꢂ 10ꢁ5
mbar). Admittance was measured by using a Solartron 1255
Frequency Response Analyzer with a Solartron 1294 dielec-
tric interface. The amplitude of the ac modulation voltage
was 50 mV; the forward dc bias was varied in the range of
0–10 V with a voltage step of 1 V, and a frequency sweep
range of 1–5 ꢂ 105 Hz was used.
RESULTS AND DISCUSSION
Scheme 1 shows the synthetic route to PPOPT (5). Thus, 2-
bromophenol (1) was O-alkylated with 1-bromopentane to
afford 2 in high yield. The Friedel–Crafts acylation of 2 with
octanoyl chloride went smoothly in CS2 to provide 3. Com-
pound 3 was then subjected to a Suzuki coupling reaction
with freshly prepared 3-thiopheneboronic acid in the pres-
ence of Pd(PPh3)4 to give monomer 4, which was subse-
quently polymerized using FeCl3 in chloroform to afford 5 as
a red powder. PPOPT is more soluble in organic solvents
such as chloroform, chlorobenzene, and dichlorobenzene,
leading to good film-forming properties when spin coated
from the related solutions.
Stokes shifts are low and similar in solution and in film
(0.38 eV in solution and 0.35 eV in film), suggesting a mod-
erate distortion of the excited state with respect to the
ground state.16 Quantum yield of PL in solution (0.11) is rel-
atively high pointing out a good planarity of the polymer
backbone, accordingly, quantum yield of PL in solid state is
PHENYL-SUBSTITUTED POLYTHIOPHENE, DEBEBE ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
related to structural differences in the thin film due to swel-
ling by the solvent or the exciton binding energy for conju-
gated polymers.19
Admittance spectroscopy20 was used to investigate the bulk
average hole mobility in thin film of PPOPT. In an admittance
experiment, the charge relaxation driven by a small har-
monic voltage modulation is probed and by superimposing a
forward dc bias Vdc to the harmonic voltage, free carriers
can be injected into a sample having a diode structure. Devi-
ces with the sandwich structure ITO/PEDOT:PSS/PPOPT/Al
were prepared for this purpose. Under forward bias condi-
tions (ITO positively biased), ITO/PEDOT:PSS electrode is
able to inject holes into the HOMO level of the polymer,
while an energy barrier for electron injection of around 1 eV
is expected at the Al/PPOPT interface (Fig. 3). So, the alumi-
num top contact acts as an electron-blocking contact in the
investigated hole-only devices under forward bias.
On injecting charge carriers, by applying a dc bias Vdc to the
sample, dramatic changes are observed for admittance Y,
given by:
YðxÞ ¼ GðxÞ þ ixCðxÞ
(1)
where G is the conductance, C the capacitance, i is the imagi-
nary unit, and x ¼ 2pf is the angular frequency of the har-
monic voltage.
FIGURE 1 Normalized UV–Vis absorption and PL spectra of
PPOPT in chloroform solution (a) and in spin-coated film onto
quartz substrate (b). The excitation wavelengths were 445 and
500 nm, in solution and in film, respectively.
As expected, conductance increases by increasing Vdc, as
shown in Figure 4(a) for the sample ITO/PEDOT:PSS/PPOPT/
Al; however, charge injection also affects capacitance spectra.
Although, at zero bias (no injection) capacitance is nearly fre-
quency independent, under high-injection conditions and for
one-carrier devices, C usually shows a light minimum in the
intermediate frequency range. As an example, the capacitance
spectrum obtained for an ITO/PEDOT:PSS/PPOPT/Al sample
at Vdc ¼ 5 V is shown in Figure 4(b) and compared with that
at Vdc ¼ 0. The positive contribution to C in the low
high (0.11) and this must be due to a well-ordered polymer
backbone that is separated from neighboring chains, making
the interchain interactions small. This is achieved with
attachment of two side groups on the opposite ortho and
meta positions of the phenyl ring which will force the phenyl
out of the backbone plane, and thereby separate the polymer
chains from each other.15,17
Electrochemical cyclic voltammetry measurements were car-
ried out on PPOPT films spin coated onto ITO/glass sub-
strate, used as working electrode. As shown in Figure 2, a
quasi-reversible oxidation wave, with the onset potential of
0.81 V versus SCE, and a quasi-reversible reduction wave,
with the onset potential of ꢁ1.54 V versus SCE, were
observed. If compared to a similar phenyl-substitued poly-
thiophene, poly(3-[20-pentyloxy-50-(octyl)phenyl]thiophene),6
the relevant shift of the reduction wave toward less negative
potentials (of around 0.5 V) could be attributed to the conju-
gation of ketone in meta position of the phenyl side group.
The energy levels of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital were
calculated to be ꢁ5.49 and ꢁ3.14 eV, respectively, from the
onset oxidation and reduction potentials, assuming the SCE
level at ꢁ4.68 eV.18 As often observed for conjugated poly-
mers, the electrochemical energy gap was found to be
slightly higher than the optical energy gap. This could be
FIGURE 2 Cyclic voltammogram (fourth scan) of a PPOPT film,
spin coated onto ITO/glass electrode, in (C4H9)NClO4/acetoni-
trile supporting electrolyte at a scan rate of 100 mV sꢁ1
.
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FIGURE 3 Energy diagram and struc-
ture of hole-only devices used in this
study.
FIGURE 4 (a) Conductance spectra for an ITO/PEDOT:PSS/
PPOPT/Al device, for dc bias (Vdc) values in the range of 0–10 V
and with a step of 2 V. The arrow indicates the direction of
increasing Vdc. (b) Capacitance spectra for an ITO/PEDOT:PSS/
PPOPT/Al device at dc bias of 0 and 5 V.
FIGURE 5 Intermediate-frequency range capacitance (a) and
negative differential susceptance spectra (b) for an ITO/
PEDOT:PSS/PPOPT/Al device at different dc bias (values shown
in the figure).
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
rable with that measured for regioregular poly(3-hexylthio-
phene) (P3HT) in the same experimental conditions. This
indicates that the substitution of the solubilizing alkyl chain
of P3HT with the phenyl–alkyl group of POPT leads to a simi-
lar organization of polymer chains in the solid state, profita-
ble for charge transport. Differently, the 20-pentyloxy-50-(1000-
oxooctyl) phenyl substituent in PPOPT likely prevents interac-
tions between adjacent polymer chains due to its steric effect,
resulting in a strong negative effect for charge transport.
The rough linear trend of the plot displayed in Figure 6 indi-
cates that hole mobility in PPOPT thin films follows the
Poole-Frenkel expression24
pffiffiffi
l ¼ l0 expðc EÞ
(4)
FIGURE 6 Square-root field-dependence of the average hole
mobility for an ITO/PEDOT:PSS/PPOPT/Al device. The line rep-
resents the linear fit to the experimental data.
where l0 denotes the mobility at zero field and c is the pa-
rameter describing how strong is the field dependence. The
parameters for the Poole–Frenkel fit to mobility data of
Figure 6 are l0 ¼ 1.77 ꢂ 10ꢁ8 cm2 Vꢁ1 sꢁ1 and c ¼ 6.0 ꢂ
10ꢁ3 (V cmꢁ1 ꢁ1/2. The value of c is rather high, indicating a
)
strong dependence of hole mobility on electric field.
frequency range, with respect to zero-bias conditions, is often
observed for disordered materials and has been attributed to
trapping and subsequent detrapping of charge carriers.20,21
The minima in the capacitance spectra are better displayed in
Figure 5(a) and the figure also shows their shift toward
higher frequency as the dc bias increases. The average transit
times tt of charge carriers can be easily evaluated from the
negative differential subsceptance ꢁDB, obtained from capac-
itance through the following expression:
CONCLUSIONS
A novel phenyl-substituted polythiophene has been synthe-
sized and its electrochemical and optical properties have
been investigated. PPOPT shows an optical energy gap of 2.0
eV and its relatively high quantum yield of PL in the solid
state (0.11), equal to that measured in diluted solution, indi-
cates small interactions between polymer chains. This is
attributed to the long alkyl chain of the 20-pentyloxy-50-(1000-
oxooctyl) phenyl substituent which causes twisting of the
phenyl ring out of the plane and leads to poor electronic
interaction of the thiophene chains.
ꢁ DB ¼ ꢁxðC ꢁ CgÞ
(2)
where Cg, the geometrical capacitance of the sample, is given
by the nearly frequency-independent capacitance value at Vdc
¼ 0 [Fig. 4(b)]. It has been demonstrated that tt is related to
the frequency fmax at which ꢁDB exhibits its maximum value
through fmax ¼ ktꢁt 1, where for the empirical coefficient k, a
value of 0.54 is usually assumed.22 Peaks in ꢁDB plots ver-
sus frequencies were clearly observed for PPOPT-based devi-
ces for Vdc of at least 5 V [Fig. 5(b)]. The peaks increase and
shift toward higher frequencies as Vdc increases. The average
hole mobility l was calculated by using
This picture is confirmed by the investigation of the bulk
transport properties of PPOPT. Hole mobility in thin films of
PPOPT was found to be moderate (of the order of 10ꢁ7 cm2
Vꢁ1 sꢁ1 for an electric field of 105 V cmꢁ1) and strongly field
activated, as indicated by the rather high value [6.0 ꢂ 10ꢁ3
(V cm )
ꢁ1 ꢁ1/2] of the Poole–Frenkel parameter describing the
field dependence of mobility. This study demonstrates the
importance of substituents for the p conjugation of the aro-
matic system of polymer materials and how dramatically
they can affect the electronic properties.
d2
ðVdc ꢁ VbiÞtt Ett
d
l ¼
¼
(3)
S.E. Debebe acknowledges the ‘‘Programme for Training and
Research in Italian Laboratories (TRIL),’’ The Abdus Salam Inter-
national Centre for Theoretical Physics (ICTP), for financial sup-
port (budget code 421.FITU.21.G). This work was partially
supported by ENI S.p.A. and by the CNR PM.P04.010 MACOL.
where d is the thickness of the polymer layer, Vbi is the built-
in potential, based on the difference of the work functions of
PEDOT:PSS and Al in the samples investigated in this study,
and E is the applied electric field. The values of l, calculated
by assuming Vbi ¼ 1 V, are displayed in Figure 6 as a func-
tion of the square root of E. Mobilities between 2.1 ꢂ 10ꢁ7
and 5.6 ꢂ 10ꢁ7 cm2 Vꢁ1 sꢁ1 were obtained at room tempera-
ture for thin film of PPOPT in the electric field range of 1.9
ꢂ 105–3.5 ꢂ 105 V cmꢁ1. These values are orders of magni-
tude lower than that recently reported for another phenyl-
substituted polythiophene,23 poly[3-(4-n-octyl)-phenylthio-
phene] (POPT).12 Indeed, a bulk hole mobility of 1.0 ꢂ 10ꢁ4
cm2 Vꢁ1 sꢁ1 has been demonstrated for POPT films, compa-
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