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Photorefractivity of Polymeric Composite Sensitized with P3HT Bull. Korean Chem. Soc. 2010, Vol. 31, No. 1 41
DOI 10.5012/bkcs.2010.31.01.041
Photorefractive Performance of Poly[methyl-3-(9-c
a
rbazolyl) propylsiloxane]
Based Composites Sensitized with Poly(3-hexylthiophene) in a 0.2-1wt % Range
Jin-Woo Oh* and Nakjoong Kim*
Department of Chemistry, Hanyang University, Seoul 133-791, Korea
*E-mail: daybreak0929@hanmail.net (JWO); kimnj@hanyang.ac.kr (NJK)
Received February 17, 2009, Accepted April 22, 2009
In this work, we report on the characterization of six low-Tg poly[methyl-3-(9-carbazolyl) propylsiloxane] based
photorefractive (PR) composites sensitized with poly(3-hexylthiophene) (P3HT) in different concentrations,
ranging from 0.2 to 1 wt %. At 632.8 nm, photoconductivity, space charge field, refractive index modulation, and
grating buildup time were measured versus external electric field. The photoconductivity was strongly dependent on
the visible light absorption and mobility. The magnitude of space charge field was affected by the conductivity
contrast σph/(σph + σd). The refractive index modulation increased with the magnitude of space charge field and the
PR grating buildup speed increased with the photoconductivity.
Key Words: Poly(3-hexylthiophene), Space charge field, Refractive index modulation, PR grating buildup
speed
Tabl e 1 . Composition, absorption coefficient (α),
p
hotoconductivity (σph), space charge field (
E
sc), refractive index modulation (∆n), and
grating buildup time (τ) of samples 1-6.
# PSX-Cz DB-IP-DC P3HT α (cm-1)aσph
(pS/cm)a,b Esc
(V/µm)a,b ∆n
(10-3)a,b
τ
(sec)a,b
1 69.8 30 0.2 109 0.362 4.61 1.73 5.40
2 69.6 30 0.4 127 0.47 5.45 2.08 3.10
3 69.4 30 0.5 136 0.527 5.50 2.20 2.15
4 69.2 30 0.6 145 0.58 4.36 1.83 1.35
5 69.2 30 0.8 163 0.815 3.52 1.30 0.75
6 69.0 30 1.0 181 1.084 1.00 0.83 0.33
aMeasured at λ = 632.8 nm; bMeasured at E0 = 40 V/µm.
Introduction
Since the first polymeric photorefractive (PR) composite1
was reported in 1991, several polymeric composites with high
PR performance were developed2-5 that compete with and in
some aspects even surpass the performance level of the best
currently known inorganic materials.6 Therefore, polymeric
PR composites are today considered a highly promising class
of new materials for optical applications. In polymeric PR
composites, the photorefractivity depends on both the space
charge field (Esc) and the reorientation of the chromophore by
Esc. A primary step in the formation of a space charge field is
the creation of free electron-hole pairs via the absorption of a
spatially modulated light intensity. In order to provide photo-
sensitivity at the wavelengths of a commercially available
low-cost He-Ne laser (632.8 nm), the photoconducting poly-
mer matrix is often doped with a small concentration of an
electron deficient molecule as a sensitizer.7 The sensitizer
assists the PR material in generating photo-charge, which is
governed by competition between the recombination of a
charge carrier with its parent countercharge, which is termed
geminate recombination, and electron-hole pair dissociation.
To date, several classes of organic molecules have been used
as sensitizers in organic PR materials. The choice of a sensi-
tizer is often determined by the transport molecule (photocon-
ductor), and the best performance is obtained by optimizing
the charge-transfer properties between a chosen sensitizer and
its parent photoconductor.8 The intermolecular interaction
between the photoconductor and sensitizer leads to a new
absorption band that does not appear in the spectrum of either
component alone. Hence, spectral sensitivity can be achieved
in the visible and the near infrared part of the spectrum using
charge-transfer (CT) complexes.8,9 After generating free elec-
tron-hole pairs in the CT complex, the holes can migrate from
one site to another with the aid of a strong electric field and
become trapped in the dark region of the illumination pattern.7
As a result of the trapping process, the trapped charges form a
spatially varying space charge field that can be translated into
a variation in the refractive index through the Pockels effect
using a nonlinear optical chromophore.7
In this work, we used a poly(3-hexylthiophene) (P3HT) as
a sensitizer to achieve a high performance of polymeric PR
composite. P3HT has been used recently in the fabrication of
electronic devices, since it has good stability, reasonably high
hole mobility, and a field effect mobility.10-12 By measurements
of photoconductivities, space charge fields, refractive index
modulations, and PR grating buildup times as a function of
electric field, and sensitizer concentration, we have analyzed
42 Bull. Korean Chem. Soc. 2010, Vol. 31, No. 1 Jin-Woo Oh and Nakjoong Kim
(a)
N
Si On
(b)
N
NC
CN
(c)
S
n
S
Figure 1. The chemical structure of the components in the PR com-
posite: (a) PSX-Cz, (b) DB-IP-DC, and (c) P3HT.
some of the fundamental properties of P3HT-sensitized poly-
[methyl-3-(9-carbazoly) propylsiloxane] based composites.
Increasing the P3HT concentration changes the rate at which
the mobile charges are generated in the material through
increased absorption.7 As a result, a faster grating buildup would
be expected with increasing sensitizer concentration.
Experimental Section
Materials and Sample Fabri cations. In this work, six low Tg
photorefractive composites were prepared by doping the
optically anisotropic chromophore, 2-[3-((E)-2-(dibutylami-
no)-1-ethenyl)-5,5-dimethyl-2-cyclohexenyliden] malononi-
trile (DB-IP-DC), into photoconducting polymer matrix, poly-
[methyl-3-(9-carbazolyl) propylsiloxane] (PSX-Cz) sensi-
tized by poly(3-hexylthiophene) (P3HT). PSX-Cz and DB-
IP-DC were synthesized using previously described me-
thods.13,14 P3HT obtained from Aldrich was used after purifi-
cation. Figure 1 shows the chemical structures of the materials.
The composition of polymeric composite was PSX-Cz : DB-
IP-DC : P3HT = 70-x : 30 : x by wt %. Table 1 shows the com-
position of our samples used in this study. For sample pre-
paration, the mixtures (total 100 mg) were dissolved in 400
mg of dichloromethane and the solution was filtered through
a 0.2 µm membrane. The PR composites were cast on a
patterned indium tin oxide (ITO) glass substrate, dried slowly
for 12 h at ambient temperature, then heated in an oven to 90
oC for 24 h to completely remove the residual solvent. The
composites were then softened on a hot plate at 100 oC, and
next sandwiched between ITO glasses with Teflon film spacer
of 100 µm to yield a film with a uniform thickness.13
Me asure ments . The conductivity (σ) of the PR samples was
measured at a wavelength of 632.8 nm, using a simple dc
current method.15 The current flowing through the sample was
measured by the Keithley 6485 during illumination to be 20
mW/cm2. The conductivity was calculated using the equation
E
J
=
σ
(1)
where J is the current density, which is determined experi-
mentally, and E is the magnitude of the externally applied
electric field.
The magnitude of the space charge field was measured
using the following method, reported in Ref. 16. In this
method, the chromophore group, which had previously been
aligned along the external electric field, was reoriented by the
newly formed space charge field. A change in the birefringence
was induced by the reorientation and was closely associated
with the space charge field. Using numerical analysis based
on the oriented gas model and the index ellipsoid method, the
magnitude of the space charge field can be determined from
the birefringence change.
The diffraction efficiency of photorefractive grating was
determined by degenerated four-wave mixing (DFWM) ex-
periments. Photorefractive grating was formed by the irradia-
tion of two s-polarized beams with an intensity of 20 mW/cm2
and a spot size of 6 mm in order to minimize the beam cou-
pling between the writing beams, which causes the variation
of photorefractive grating throughout the sample. Then the
recorded photorefractive grating was read out by a p-polari-
zed counter-propagating probe beam with an intensity of 0.06
mW/cm2 and a spot size of 1.5 mm. Two coherent laser beams
with λ = 632.8 nm were irradiated on the composite in the
tilted geometry at an incident angle of θ = 30o and 60o with
respect to the composite’s normal axis. The magnitude of the
diffraction efficiency (η) was determined from the measured
transmitted and diffracted intensities of the reading beam,14
using the relation
η = IR,diffracted / (IR,diffra cted + IR,transmitted)(2)
The PR grating buildup times of the photorefractive
composite were calculated by fitting the evolution of the
growth of the diffraction signal.17
Results and Discussion
The absorption spectra of six composites are shown in
Figure 2. All the photosensitizers were found to form a CT
complex with PSX-Cz in the solid state.14 In the 550 - 800 nm
wavelength range, the absorbance of composite increased
with increasing the P3HT concentration. For sample 6 the
absorbance was one and a half times more than in sample 1. In
sample 1-6, where the equilibrium concentration of CT com-
plex increases with the available amount of sensitizer, the
absorption tails extend further into the long wavelength with
increasing sensitizer concentration. The absorption coeffi-
cients (α) for samples 1-6 (at 632.8 nm of 100 µm thickness)
were 109, 127, 136, 145, 163, and 181 cm-1, respectively.
The photoconductivity (σph) is related to the number
density (p) of free charges produced by light absorption and
the charge mobility (µ) by18
σph =peµ =
µ
ν
τφα
e
h
Iq
φαIqτ
hν
e
µ
(3)
where p is the density of mobile charge carriers (holes), e is
the fundamental electric charge, µ is the mobility of holes, φ is
photo-charge generation efficiency, Iq is the optical intensity,
τ is the life time of charge, hν is the energy of photon. From
σ
Photorefractivity of Polymeric Composite Sensitized with P3HT Bull. Korean Chem. Soc. 2010, Vol. 31, No. 1 43
1
2
3
4
5
6
600 650 700 750 800
Wavelength (nm)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Figure 2. Visible spectrum of PR samples 1 (closed square), 2 (open
square), 3 (closed circle), 4 (open circle), 5 (closed triangle), and 6
(open triangle).
1
2
3
4
5
6
10 20 30 40 50 60
Electric field (V/µm)
2.4
2.0
1.6
1.2
0.8
0.4
0.0
Figure 3. Field dependence of the photoconductivity measured in
samples 1 (closed square), 2 (open square), 3 (closed circle), 4 (open
circle), 5 (closed triangle), and 6 (open triangle). The line is a guide
to the eye.
1
2
3
4
5
6
10 20 30 40 50 60
Electric field (V/µm)
1.0
0.8
0.6
0.4
0.2
0.0
Figure 4. Field dependence of the dark conductivity measured in
samples 1 (closed square), 2 (open square), 3 (closed circle), 4 (open
circle), 5 (closed triangle), and 6 (open triangle). The line is a guide
to the eye.
0.2 0.4 0.6 0. 8 1.0
0
2
4
6
P3HT content
Esc
1
2
3
4
5
6
0 10 20 30 40 50 60
Electric field (V/µm)
15
12
9
6
3
0
0.2 0.4 0.6 0.8 1.0
P3HT content
6
4
2
0
Figure 5. Field dependence of the space charge field measured in
samples 1 (closed square), 2 (open square), 3 (closed circle), 4 (open
circle), 5 (closed triangle), and 6 (open triangle). Inset shows P3HT
concentration dependence of the space charge field in six samples.
The line is a guide to the eye.
equation (3) we can confirm that the photoconductivity is
governed by the charge generation and transport.19 All other
things being equal, the inclusion of a photosensitizer results in
an increase in σph and, because σph is an integral component of
the PR effect, in an enhanced PR performance. The photocon-
ductivity as a function of the applied electric field is presented
in Figure 3. The photoconductivity was calculated as the
difference between the total conductivity in the presence of
light and dark conductivity in the absence of light. In Figure 3
the photoconductivity increased considerably with an increase
in the electric field. This nonlinear dependence on the electric
field is due to the electric field’s dependencies on both the
quantum efficiency of charge generation and the hole mobility.
As shown in Figure 3, the photoconductivity increased with
increasing the P3HT concentration. The lager absorption and
the faster mobility led to a significant increase in the photo-
conductivity. Figure 4 shows the dark conductivity as a func-
tion of the applied electric field. The dark conductivity (σd) in-
creased with increasing P3HT concentration. The above results
will be explained in detail in the space charge field section.
We measured the space charge field of samples 1-6 as a
function of the applied electric field. For clarity, Figure 5
shows the experimental data of the composites. Esc increased
linearly with an increasing electric field. The electric field
dependence of the Esc resulted from electric field-assisted
separation of the charge from the electron-hole pair with a
high energy distribution.14 As shown in Figure 5, the Esc of the
six samples was in the following order: sample 6 < sample 5 <
sample 4 < sample 1 < sample 2 < sample 3. The space charge
field can qualitatively be attributed to the σph/(σph + σd) ratio
associated with PR sample. This so because the magnitude of
the space charge field, |Esc|, is correlated with the ratio σph/(σph +
σd) as predicted by the equation20
44 Bull. Korean Chem. Soc. 2010, Vol. 31, No. 1 Jin-Woo Oh and Nakjoong Kim
0.4 0.6 0.8 1.0
2
4
6
Esc
σph/(σph+σd)
1
2
3
4
5
6
0 10 20 30 40
Electric field (V/µm)
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
0.4 0.6 0.8 1.0
σph/(σph+σd)
6
4
2
Figure 6. The ratio σph / (σph + σdark) as a function of field in samples
1 (closed square), 2 (open square), 3 (closed circle), 4 (open circle),
5 (closed triangle), and 6 (open triangle). Inset shows σph / (σph +
σdark) dependence of the space charge field in six samples. The line is
a guide to the eye.
0153045
0.0
0.5
1.0
1.5
2.0
∆nBR (10-3)
Eext
1
2
3
4
5
6
-10 0 10 20 30 40 50
Electric field (V/µm)
1.0
0.8
0.6
0.4
0.2
0.0
Eext
Figure 7. Field dependence of the steady-state transmittance mea-
sured in samples 1 (closed square), 2 (open square), 3 (closed circle),
4 (open circle), 5 (closed triangle), and 6 (open triangle). Inset shows
field dependence of the birefringence measured in six samples. The
line is a guide to the eye.
0246
0.5
1.0
1.5
2.0
2.5
Esc (V/µm)
∆n (10-3)
1
2
3
4
5
6
10 20 30 40 50 60
Electric field (V/µm)
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
Esc (V
/
µm)
Figure 8 . Field dependence of the refractive index modulation ampli-
tude ∆n measured in samples 1 (closed square), 2 (open square), 3
(closed circle), 4 (open circle), 5 (closed triangle), and 6 (open
triangle). Inset shows the correlation between the space charge field
(Esc) and the refractive index modulation amplitude (∆n) at 40 V/µm
in six samples. The fits are according to n = a·Eb (Ref. 14).
λ
0
22
0
ph q
SC
ph d q
EE
Em
EE
σ
σσ
=⋅
++
(4)
where m is the modulation depth, and E0 is the projection of
applied electric fields along the grating wave vector. The
saturation field Eq = eΛNT/[2πε0ε], where Λ is the grating cons-
tant, and NT is the PR trap density. Equation (4) assumes that
the charge drift in the electric field dominates the diffusion of
charges.19 Figure 6 shows the conductivity contrast σph/(σph +
σd) of samples 1-6. The tendency of σph /(σph + σd) has reserved
at 3 wt % of P3HT concentration where the space charge field
has a maximum value. Below 3 wt %, the σph/(σph + σd) in-
creases with increasing P3HT content due to the large absorp-
tion and the fast mobility. Above 3 wt %, however, the σph/(σph +
σd) decreases with the P3HT content. This is due to the increase
in hole detrapping by high dark conductivity. As shown in the
inset in Figure 6, the space charge field increased linearly with
the σph/(σph + σd).
Differential scanning calorimetry experiments conducted
on composites 1-6 yielded a broad range of glass transition
temperatures between 20 and 30 oC. The ability of the chromo-
phores to reorient in the composite was further confirmed
using a transient ellipsometry technique.21 The transmittance
(T) as a function of the applied electric field for six samples is
shown in Figure 7. The birefringence (∆nBR) of samples was
determined from the variation of the transmitted intensity
through crossed polarizers upon the external electric field, as
described by the following equation:
∆= BR
nlT
λ
π
2
sin2
(5)
where is wavelength, and l is the distance of the light path.
These results from the transmission ellipsometer experiments
can be used to predict both the steady-state holographic con-
trast of the PR composite, and to quantify the rotational free-
dom of the chromophores within the sample. The birefringence
of our samples strongly depended on the chromophore content.
When describing chromophore alignment in the electric field,
it is conventional to apply the oriented gas model,22 This model
assumes be freely rotating, noninteracting molecules. In the
low-field limit, the electric field-induced change in birefrin-
gence is found using the following equation.23,34
2
22
00
0
13 2
245 3 5
BR m
nNf NffE
nkT kT
ζκ
δα
ε
∞∞
∆= +
(6)
Here, N is the chromophore concentration, f0, f∞ are local field
factors, is the polarizability anisotropy, κ is the molecular
first hyperpolarizability, and is the dipole moment. Accor-
s
c
Eσph E0 Eq
E
0 + Eq
2
2
σph + σd
δ
α
ζ
ζ
l∆nBR
∆nBR
Photorefractivity of Polymeric Composite Sensitized with P3HT Bull. Korean Chem. Soc. 2010, Vol. 31, No. 1 45
20 30 40 50 60
Electric field (V/µm)
60
50
40
30
20
10
0
σph
Figure 9 . Field dependence of the grating buildup time (τ) in samples
1 (closed square), 2 (open square), 3 (closed circle), 4 (open circle),
5 (closed triangle), and 6 (open triangle). Inset shows the correlation
between the photoconductivity (σph) and the PR grating
b
uildup
speed (1/τ) at 40 V/µm in six samples. The line is a guide to the eye.
ding to Equation (6), the steady-state electric-field induced
∆nBR is dependent on the chromophore concentration. As shown
in the inset of Figure 7, the ∆nBR of six samples shows similar
tendencies, since they have same content of chromophore.
The steady-state diffraction efficiencies of sample 1-6 were
measured as a function of the applied electric field. The
experiments were carried out at Tg to take full advantage of the
birefringence contribution to the index modulation ampli-
tude.14 Figure 8 shows the refractive index modulation ampli-
tudes, ∆n, calculated using Kogelnik’s expression for diffrac-
tion efficiency in thick transmission holograms,25
]
coscos
)cos(
[sin
21
12
2
θθλ
θθπ
η
−∆
=nd (7)
where d is the composite thickness, and θ1 and θ2 are the
internal angles of incidence of the two writing beams, respec-
tively. The refractive index modulation of six samples was in
the following order: sample 6 < sample 5 < sample 1 < sample
4 < sample 2 < sample 3. The index contrast of the photore-
fractive grating is given by the combination of the field-
induced orientational birefringence factor and the space
charge field as in equation (8),
)()( EEEfn sc
∝∆ (8)
where f(E) is an orientational birefringence factor, which is a
function of the internal angles of incidence and the coefficients
defined in Equation (6).23 Since the orientational birefringence
factors were fixed in all samples, the refractive index modula-
tion is only dependent on the magnitude of the space charge
field. The inset in Figure 8 shows the relation between the
magnitude of space charge field and the refractive index mo-
dulation. The refractive index modulation directly increased
with the space charge field which was dependent upon the
ratio σph/(σph + σd).
The field dependence of photorefractive grating buildup
time was also analyzed. The PR grating buildup rate is very
important for real applications such as real-imaging and real-
data processing.26 The buildup time of the photorefractive
composites was evaluated from the buildup of the beam inten-
sity of the DFWM measurement.16 The time constants, τ1 and
τ2, were calculated by fitting the evolution of the growth of the
gain, g(t), with the following biexponential function,27
)}/exp(1{)}/exp(1{)( 2211
ττ
tatatg −−+−−= (9)
where τ1 and τ2 are the fast and slow time constants,
respectively. Figure 9 shows the PR grating buildup times (τ1)
as a function of the external electric field. An elevated external
electric field caused faster grating formation in all composites.
This was expected since the charge mobility as well as the
photo-charge generation efficiency both increased with the
electric field.14 As shown in the inset in Figure 9, for samples
1-6 the PR grating buildup speed (τ-1) increased linearly with
the photoconductivity. Unlike the diffraction efficiency,
which is dependent upon the ratio σph/(σph + σd), the PR
grating buildup speed is proportional to σph, as given by the
equation
ε
σ
τ
ph
=
−1
(10)
where ε is the dielectric constant of the composite.28 Increa-
sing the absorption coefficient with P3HT leads to a larger
photoconductivity and a faster grating buildup through a
faster photo-charge generation.
Conclusions
In this work, we investigated the steady-state and the
dynamic properties of six PSX-Cz based composites, con-
taining different amounts of P3HT. The photoconductivity,
which was strongly dependent on the visible light absorption
and mobility, increased with increasing the P3HT concen-
tration. The magnitude of space charge field was affected by
the conductivity contrast σph/(σph + σd). The refractive index
modulation increased with the magnitude of space charge
field and the PR grating buildup speed increased with the
photoconductivity. It would be very interesting to continue
the investigation toward the improvement of the performance
of PR polymers.
Acknowledgments. This work was supported by the National
Research Foundation (NRF) grant funded by the Korea govern-
ment (MEST) through the Active Polymer Center for Pattern
Integration (No. R11-2007-050-00000-0) and by the Research
fund of HYU (HYU-2008-T).
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