Triphenylamine photoconductive polymers for high performance photorefractive devices

Article abstract of DOI:10.1016/j.jphotochem.2014.06.008

Photorefractive performances of the composites using two kinds of photoconductive triphenylamine-based polymer have been compared and investigated. One polymer is poly(4-(diphenylamino)benzyl acrylate) (PDAA). The other is newly synthesized one of photoco

Full text of DOI:10.1016/j.jphotochem.2014.06.008

Journal of Photochemistry and Photobiology A: Chemistry 291 (2014) 26–33
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Triphenylamine photoconductive polymers for high performance
photorefractive devices
Ha Ngoc Giang, Kenji Kinashi, Wataru Sakai, Naoto Tsutsumi∗
Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, 1 Hashigami-cho, Matsugasaki, Sakyo, Kyoto 606-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 April 2014
Received in revised form 10 June 2014
Accepted 13 June 2014
Available online 5 July 2014
Photorefractive performances of the composites using two kinds of photoconductive triphenylamine-
based polymer have been compared and investigated. One polymer is poly(4-(diphenylamino)benzyl
acrylate) (PDAA). The other is newly synthesized one of photoconductive acrylate polymer with
methoxy substituted triphenylamine pendant, poly(4-((4-methoxyphenyl)(phenyl)amino)benzyl acry-
late) (PMPAA). The methoxy substituent in PMPAA does not only shift the highest occupied molecular
orbital (HOMO) level of the polymer, but also effectively enhances the chromophore orientation. Larger
phase shift is confirmed by using the modified photoconductive polymer of PMPAA. The plasticizer of
(4-(diphenylamino)phenyl)methanol (TPAOH) (IP = −5.64 eV) works as an effective trap in the PDAA
(IP = −5.69 eV)-based composite, resulting in higher diffraction efficiency. Diffraction efficiency of 70%
and fast response time of 25 ms (dominant) is measured at 532 nm under the moderate electric field of
45 V/␮m.
Keywords:
Composite
Photoconductor
Photorefractive
Polymer
Triphenylamine
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
addition, the use of the high HOMO level polymer for PR compos-
ites could avoid the possibility of accumulation of trap caused by a
Photorefractive (PR) effect has received much attention because
of its updatable holographic property. PR materials can be applied
to varied applications include holography, data storage, optical
phase conjugation, image amplification, novelty filtering and edge
enhancement [1,2]. PR polymers and composites have been consid-
ered as an alternative to the inorganic PR crystals due to the benefit
from their organic properties such as low cost, ease in processabil-
ity, etc. A great development in material design, PR performances
and applications has been reported [3–6]. Typical components for
PR effect include a photoconductive polymer, a non-linear optical
(NLO) chromophore, a plasticizer and a sensitizer. The photocon-
ductive polymer plays a main role to provide a charge transport
medium. Poly(vinyl carbazole) (PVK) has been successfully applied
as a polymer matrix for PR composite [7]. However, PVK has some
drawbacks such as low hole mobility and high glass transition tem-
perature (Tg) of >200 ◦C. The low hole mobility limits speed of
space-charge field formation and consequently leads to slow PR
response time. A photoconductive polymer with higher level of the
highest occupied molecular orbital (HOMO), i.e. more readily ion-
ized, is preferred to obtain an appropriate photoconductivity. In
chromophore with HOMO level which lies above the charge trans-
port manifold [8]. In the case of a high Tg polymer such as PVK, a
plasticizer is added to the composite to decrease the Tg and enhance
the chromophore re-orientation process which plays as an essen-
tial factor for the high PR performances [9]. To make the composite
less inert, the photoconductive plasticizer ethyl carbazole (ECZ)
is widely used with PVK and even with bistriarylamine polymer
[10–16].
Recently, photoconductive polymers based on triphenylamine
have been utilized for fast response PR application because of
their fast hole mobility. A vinyl type of triphenylamine poly-
mer, poly((4-diphenylamino)styrene) (PDAS), for PR composite has
been reported [17]. However, high intensity of writing beams (up
to 3 W/cm2) was applied for the PDAS-based PR composite. The
polymer of poly(4-(diphenylamino)benzyl acrylate) (PDAA) with
acrylate structure following by a relatively low Tg of 75 ◦C has been
applied to PR composites successfully with a moderate intensity
of writing beams [18]. To further improvement and investiga-
tion of the PR performances, the side-chain type triphenylamine
PDAA is modified with a methoxy group at para-position. In this
study, we report a novel photoconductive polymer of poly(4-((4-
methoxyphenyl)(phenyl)amino)benzyl acrylate) (PMPAA) and PR
performances of the PMPAA-based composite in comparison of the
PDAA-based composite. Chromophore orientation and trap density
in both composites shall be discussed.
∗
Corresponding author. Tel.: +81 757247810.
E-mail addresses: giangngocha@gmail.com (H.N. Giang), tsutsumi@kit.ac.jp
(N. Tsutsumi).
http://dx.doi.org/10.1016/j.jphotochem.2014.06.008
1010-6030/© 2014 Elsevier B.V. All rights reserved.

H.N. Giang et al. / Journal of Photochemistry and Photobiology A: Chemistry 291 (2014) 26–33
27
Scheme 1. Synthetic procedure of PMPAA and PDAA.
2. Experimental
saturated aqueous NH4Cl solution (30 mL) was then added. The
obtained mixture was extracted with dichloromethane, and the
resulting organic layer was dried over anhydrous MgSO4 and the
solvent was removed totally by a rotary evaporator to give the
relative alcohol.
(4-((4-Methoxyphenyl)(phenyl)amino)phenyl)methanol (2).
Yield: 90%. TOF/MS (ESI) m/z: found 298.09 (M+Na+). 1H NMR
(300 MHz, CDCl3, ı): 7.32–6.82 (m, 14H, ArH), 4.55 (d, 2H,
CH2 OH), 3.8 (s, 3H, ArOCH3), 1.59 (s, 1H, CH2OH).
(4-(Diphenylamino)phenyl)methanol (5). Yield: 95%. TOF/MS
(ESI) m/z: found 298.09 (M + Na+). 1H NMR (300 MHz, CDCl3, ı):
7.27–6.94 (m, 14H, ArH), 4.55 (d, 2H, CH2 OH), 1.84 (s, 1H,
CH2OH).
2.1. Synthesis
The synthetic procedure for PDAA and PMPAA is summarized in
Scheme 1. All synthesized products were characterized by means of
mass spectroscopy (MS) (MicroTOF, BrukerDaltonics Co.), 1H NMR
spectroscopy (AV-300, BrukerBioSpin Co., 300 MHz, tetramethylsi-
lane – TMS – as an internal standard). Weight-average molecular
weight (Mw) and number average molecular weight (Mn) were
determined by a gel permeation chromatography (GPC) using a
Shodex GPC KF-805 + KF-803 column (Showa Denko K. K) and
tetrahydrofuran (THF) as an eluent. The molecular weight of the
product was calibrated by polystyrene standards. Glass transition
temperature (Tg) was determined using a DSC 2920 (TA Instru-
ments Co.) at a heating rate of 10 ◦C min−1
.
2.1.3. 4-((4-Methoxyphenyl)(phenyl)amino)benzyl acrylate (3)
and 4-(diphenylamino)benzyl acrylate (6)
A solution of (2) or (5), triethylamine (2 equiv.) and a catalytic
amount of dimethylaminopyridine (DMAP) in dichloromethane
(50 mL) was cooled to 0 ◦C by an ice bath. To the mixture, acryloyl
chloride (1.5 equiv.) was added slowly. The mixture was warmed
up to r.t. and left stirring for 24 h. Water (100 mL) was then
added to the mixture. The resulting solution was extracted with
dichloromethane. The mixture was washed with saturated aqueous
NaHCO3, then with water. The organic layer was dried over anhy-
drous MgSO4 and the solvent was removed by a rotary evaporator.
The crude product was purified using silica gel column chromatog-
raphy using ethylacetate:hexane (3:1) to give acrylate monomer.
2.1.1. 4-((4-Methoxyphenyl)(phenyl)amino)benzaldehyde (1)
POCl3 (6 mL, 64.17 mmol) was carefully added to a solution
of 4-methoxy-N,N-diphenylaniline (10 g, 36.32 mmol), DMF (5 mL,
64.85 mmol) and 50 mL of 1,2-dichloroethane. The mixture was
refluxed for 4 h, then cooled to room temperature (r.t.), and poured
into a saturated aqueous sodium acetate solution (100 mL). The
product was extracted with dichloromethane. The organic layer
was dried over anhydrous MgSO4. The solvent was evaporated by a
rotary evaporator. The residue was purified by silica gel column
chromatography using ethyl acetate: n-hexane (2:3) to give (1)
(7.6 g, Yield: 69%). TOF/MS (ESI): found 326.1 (M+Na+). 1H NMR
(300 MHz, CDCl3, ı): 9.79 (s, 1H, ArCHO), 7.68–6.89 (m, 14H, ArH),
3.83 (s, 3H, ArOCH3).
4-((4-Methoxyphenyl)(phenyl)amino)benzyl
acrylate
(3):
Yield: 50%. 1H NMR (300 MHz, CDCl3, ı): 6.81–7.25 (m, 14H, ArH),
6.44 (d, 1H, CHcisHtrans CH COO), 6.16 (q, 1H, CH2 CH COO),
5.84 (d, 1H, CHcisHtrans CH COO), 5.12 (s, 2H, COO-CH2-Ar), 3.8
(s, 3H, ArOCH3).
2.1.2. 4-((4-Methoxyphenyl(phenyl)amino)phenyl)methanol (2)
and (4-(Diphenylamino)phenyl)-methanol (TPAOH) (5)
Aldehyde ((1) or (4)) was added to a solution of NaBH4 (1 equiv.)
in dichloromethane (20 mL) and methanol (5 mL). The mixture was
stirring for 2 h at r.t. and water (25 mL) was then added and stirred
continuously for 24 h. The mixture was concentrated in vacuo and
4-(Diphenylamino)benzyl acrylate (6):Yield: 69%. 1H NMR
(300 MHz, CDCl3, ı): 6.9–7.2 (m, 14H, ArH), 6.45 (d, 1H,
CHcisHtrans CH COO), 6.16 (q, 1H, CH2 CH COO), 5.84 (d, 1H,
CHcisHtrans CH COO), 5.1 (s, 2H, COO CH2 Ar).

28
H.N. Giang et al. / Journal of Photochemistry and Photobiology A: Chemistry 291 (2014) 26–33
angles of 40◦ and 55◦. A weak intensity p-polarized reading (probe)
beam which counter-propagated to one of the writing beams was
diffracted by the refractive index grating in the sample film. The
diffracted and transmitted signals were detected by photodiode
detectors. From the intensity of the transmitted beam (It) and the
diffracted beam (Id), the diffraction efficiency (ꢀ) was calculated
with Eq. (1):
Id
ꢀ% =
× 100
(1)
It + Id
The PR response time for the composite was measured using the
same geometry. The diffraction efficiency growth was fitted with
bi-exponential function Eq. (2):
ꢀ
ꢁ
ꢂ
ꢁ
ꢂꢃ
−t
−t
ꢀ = ꢀ0 1 − m exp
− (1 − m) exp
(2)
ꢁ1
ꢁ2
where ꢁ1, ꢁ2 are time constants with weighing factor of m
(0 ≤ m ≤ 1) and (1 − m), respectively.
2.2.3. Two-beam coupling measurement (TBC)
PR optical gain values of the sample were measured using a
two-beam coupling (TBC) technique. The same geometric configu-
ration to DFWM technique in which two equal intensity p-polarized
beams that crossed at the thin film sample was used, except with-
out a probe beam. The intensities of two beams that transmitted
through the sample film were detected using photodiode detectors
to evaluate the optical gain using Eq. (3):
Fig. 1. Chemical structures of photoconductive matrix (PDAA or PMPAA), NLO chro-
mophore (7DCST), plasticizer (TPAOH), sensitizer (PCBM).
2.1.4. Polymerization of poly(4-(diphenylamino)benzyl acrylate)
(PDAA) and poly(4-((4-methoxyphenyl)(phenyl)amino)benzyl
acrylate) (PMPAA)
Polymers were synthesized by radical polymerization. Mixture
of monomer, AIBN (0.1%) and THF was degassed carefully by freeze-
pump-thaw method. The mixture was heated up to 60 ◦C for 24 h.
After reaction, the obtained solution was diluted with THF (10 mL)
and precipitated into MeOH (80 mL). The obtained polymer powder
was dried in vacuo for 24 h to give white polymer powder of PDAA
(or PMPAA).
ꢁ
ꢂ
1
d
IA(IB =/ 0)
IA(IB = 0)
IB(IA =/ 0)
IB(IA = 0)
ꢂ =
cos ꢃA · ln
− cos ꢃB · ln
(3)
where d is the thickness of the sample film; ꢃA, ꢃB are the internal
angles of two beams with respect to the normal; IA and IB are the
intensities of two beams after the sample film.
2.3. Characterization of electro-optic response of the composite
PDAA: monomer/THF (2.3 g/2 mL); Yield: 95%; Mw = 23,000
(degree of polymerization (DP) = 56); Mw/Mn = 1.25; Tg = 75 ◦C.
PMPAA: monomer/THF (2.4 g/3.5 mL); Yield: 96%; Mw = 21,000
(DP = 48); Mw/Mn = 1.23; Tg = 74 ◦C.
A Mach–Zehnder (MZ) interferometer was employed using a
modulated poling field to observe the electro-optic response of the
PR composite. The PR composite was placed in one arm of the inter-
ferometer with an applied voltage device. A laser at 830 nm is used
in MZ interferometer because most of the investigated PR com-
posites have small or neglected absorption at this wavelength. A
constant poling field of 40 V/␮m was applied, and a stepwise mod-
ulating voltage with frequency of 1 Hz and amplitude of 4 V/␮m
was superposed onto this.
2.2. Photorefractive characterization
2.2.1. PR sample preparation
The polymer of PDAA or PMPAA was mixed with NLO chro-
mophore 2-(4-(azepan-1-yl)benzylidene)malononitrile (7DCST),
plasticizer (4-(diphenylamino)phenyl)methanol (TPAOH) and sen-
sitizer phenyl-C61-butyric acid methyl ester (PCBM) in THF. The
chemical structural formulae and abbreviations of the components
in the PR composite are shown in Fig. 1. The concentration of
7DCST was fixed at 30 wt.% in all samples while the concentration
of the photoconductive matrix (PDAA or PMPAA) and the plasti-
cizer (TPAOH) were varied. The mixture was left stirring overnight.
Then, the mixture was cast on a hot plate at 70 ◦C for 24 h and fol-
lowed by drying in vacuum oven at 60 ◦C for 24 h. The obtained
composite was sandwiched between two indium–tin-oxide (ITO)
coated glasses as the composite was melted at 150 ◦C. Thickness of
the composite film was controlled by using Teflon spacers.
2.4. Photocurrent measurement
A typical experimental run for photocurrent measurement
included the following: the electric field (40 V/␮m) was turned on
for 10 min in dark condition. Then, a laser at 640 nm (900 mW/cm2)
as an exciting source was turned on, the current was monitored in
10 s.
3. Results and Discussion
PDAA and PMPAA were synthesized with a comparable molec-
ular weight, degree of polymerization and polydispersity (see
Section 2). Because of that, the dependence of molecular weight
on PR effect can be neglected when PR properties of the two kinds
of composite were compared. Due to a similar polymer backbone
structure of acrylate type, both polymers PDAA and PMPAA possess
very close Tg values of 75 ◦C and 74 ◦C, respectively. The ioniza-
tion potential (IP) values of PDAA and PMPAA are −5.69 eV and
−5.57 eV, respectively. A modification by a strong electron donat-
ing group ( OCH3) at para-position has slightly shifted the HOMO
2.2.2. Degenerate four-wave mixing (DFWM)
Diffraction efficiency of the PR sample was measured using
a degenerate four-wave mixing (DFWM) technique. The holo-
graphic gratings were written in the sample by two s-polarized
beams of He–Ne laser (at 633 nm, 10 mW, Melles-Griot, USA,
250 mW/cm2) or DPSS laser (SambaTM, 532 nm, 25 mW, Cobolt,
Sweden, 650 mW/cm2) intersected in the sample at the incidence

H.N. Giang et al. / Journal of Photochemistry and Photobiology A: Chemistry 291 (2014) 26–33
29
Table 1
Photorefractive parameters of the PDAA-based composites.
PDAA/7DCST/TPAOH/PCBM
Thickness (␮m)
ꢀc (%)
Response timec
Optical gainc
ꢄn
Phase shift
Absorption
о
ꢂ (cm−1
)
˚ (
)
coefficient ˛
(cm−1
)
ꢁ1 = 80 ms
ꢁ2 = 1834 ms
m = 0.305
55/30/14/1a
50/30/19/1a
45/30/24/1a
1.9 × 10−4
3.9 × 10−4
9.4 × 10−4
1.7 × 10−3
97.5
92.5
87.5
87.5
1
7
10.8
15.4
24
ꢁ = 1299 ms
ꢁ1 = 136 ms
ꢁ2 = 1741 ms
m = 0.303
4
21
72
8
ꢁ = 1254 ms
ꢁ1 = 64 ms
ꢁ2 = 761 ms
m = 0.706
ꢁ = 266 ms
19
70
22.65
19
ꢁ1 = 25 ms
ꢁ2 = 273 ms
m = 0.791
ꢁ = 77 ms
-
n-
-
n-
-
n-
45/30/24/1b
-n- no data.
a
Writing beams are at 633 nm.
Writing beams are at 532 nm.
Measured at 45 V/␮m. ꢁ = mꢁ1 + (1 − m)ꢁ2
b
c
.
level of the photoconductive polymer. Instead of the popular plas-
ticizer ethyl carbazole (ECZ), the new plasticizer of TPAOH shown
in Fig. 1 has been applied to the PR composite. The main purpose
of the TPAOH plasticizer is to increase the concentration of the
photoconductive triphenylamine in the composite which can assist
to have a better photoconductivity. In addition, the use of TPAOH
has been found to improve the shelf life time of the PR compos-
ite. Two samples: PDAA/7DCST/ECZ and PDAA/7DCST/TPAOH with
a weight ratio of 45/30/25 have been prepared. Fig. 2 shows the pic-
ture of PR samples right after preparation and after 24 h storage at
room temperature. As can be seen, the sample with ECZ showed
a severe re-crystallization of NLO chromophore and a totally
non-transparent sample was observed after 24 h. In some cases,
in the sample with ECZ, the sign of re-crystallization appeared only
after a few hours. While in the sample with TPAOH, the trans-
parency was maintained for not only 24 h but also one month. All
of the PR samples which were used in the present study showed no
sign of degradation caused by the chromophore re-crystallization
during the investigation time.
Tables 1 and 2 summarize PR properties for the PDAA-based
and the PMPAA-based composites, respectively. Diffraction effi-
ciency growth at 45 V/␮m as a function of time with 633 nm He:Ne
laser writing beams is shown in Fig. 3. In the composites with 14
and 19 wt.% of the TPAOH plasticizer, the PMPAA-based composite
shows a significant enhancement over the PDAA-based composite.
However, this tendency is reversed at the composition with 24 wt.%
TPAOH as the diffraction efficiency of the PDAA-based composite
is two times larger than that of the PMPAA-based composite. The
generation of PR grating can be divided into two steps: formation
of a spatially non-uniformed space-charge field and a refractive
index change under the overall electric field which is a superpo-
sition of the space-charge field and the externally applied field
[12,19]. Hence, the steady state of diffraction efficiency in PR mate-
rials is determined by two factors: the strength of space-charge
field and the magnitude of the refractive index change under an
electric field. To explain the DFWM results, both of the two factors
have to be considered. The ability of refractive index change under
electric field can be estimated among PR samples by comparing MZ
interferometer signals.
Fig. 4 is the MZ interferometer results of the relative compos-
ites. In MZ interferometer, the signal represents the phase shift
variation between two arms of the interferometer. In this case,
the MZ signal is directly related to refractive index change in the
PR sample under electric field. In a low Tg polymeric PR materi-
als, refractive index change under electric field that modulates at a
low frequency of 1 Hz originates from orientational birefringence
(BR) and electro-optic (EO) contributions [9]. Both of the contrib-
utions depend on the ability of the chromophore to be oriented
under the external electric field in the different matrixes as well
as the concentration and the type of NLO chromophore itself. In
the present case, the same NLO chromophore (7DCST) at the fix
concentration (30 wt.%) was used. It means that the obtained MZ
results under a same electric field strongly reflect the orientation
of chromophore. As can be observed in Fig. 4, in the composition
Fig. 2. Photorefractive composites with using different plasticizers. (a)
PDAA/7DCST/ECZ (45/30/25) – right after preparation; (b) PDAA/7DCST/TPAOH
(45/30/25) – right after preparation; (c) PDAA/7DCST/ECZ (45/30/25) after 24 h; (d)
PDAA/7DCST/TPAOH (45/30/25) – after 24 h.

30
H.N. Giang et al. / Journal of Photochemistry and Photobiology A: Chemistry 291 (2014) 26–33
Table 2
Photorefractive parameters of the PMPAA-based composites.
PMPAA/7DCST/TPAOH/PCBM
Thickness (␮m)
ꢀc (%)
Response timec
Optical gainc
(cm−1
ꢂ
ꢄn
Phase shift
о
˚ ( )
Absorption
coefficient
)
˛ (cm−1
)
ꢁ1 = 96 ms
ꢁ2 = 1271 ms
m = 0.492
55/30/14/1a
50/30/19/1a
45/30/24/1a
5.8 × 10−4
6.4 × 10−4
7.2 × 10−4
1.2 × 10−3
92.25
92
8
46
24.1
28.7
27.4
47
ꢁ = 696 ms
ꢁ1 = 81 ms
ꢁ2 = 898 ms
m = 0.631
ꢁ = 382 ms
10
11
38
61
66
33
58
ꢁ1 = 55 ms
ꢁ2 = 647 ms
m = 0.729
ꢁ = 214 ms
87.5
87.5
ꢁ1 = 35 ms
ꢁ2 = 457 ms
m = 0.757
ꢁ = 137 ms
-
n-
-
n-
-
n-
45/30/24/1b
-n- no data.
a
Writing beams are at 633 nm.
Writing beams are at 532 nm.
Measured at 45 V/␮m. ꢁ = mꢁ1 + (1 − m)ꢁ2
b
c
.
with 14 and 19 wt.% of plasticizer TPAOH, MZ signals detected in the
PMPAA-based composites are significantly larger than those in the
PDAA-based composites. The result indicates a dramatic enhance-
ment in chromophore orientation in the PMPAA-based composites.
However, in the composition with 24 wt.% of plasticizer, MZ signals
of the two kinds of composite are overlapped to each other. The MZ
signals in the PDAA-based composite increase with decreasing the
PDAA content whereas the MZ signals in the PMPAA-based com-
posite are almost the same irrespective of the PMPAA content. By
using 14 wt.% of the plasticizer TPAOH, the chromophore alignment
in the PDAA-based composite is seriously restricted at room tem-
perature. An increase in the plasticizer concentration which leads
to an decrease in Tg improves this situation and with 24 wt.% of
TPAOH, the same chromophore alignment ability comparing to the
PMPAA-based composites can be reached in the PDAA-based com-
posite. For the PMPAA-based composite, only 14% plasticizer of
TPAOH is enough to have a low Tg for chromophore alignment. A
good chromophore orientation at a low plasticizer concentration is
preferred to maximize the photoconductive matrix concentration.
In this meaning, PMPAA is a better matrix for the chromophore
Fig. 3. Transient DFWM (633 nm, 45 V ␮m−1) results of the composites. The red triangle: PMPAA-based composites; the black circle: PDAA-based composites. Polymer (PDAA
or PMPAA)/7DCST/TPAOH/PCBM: (a) 55/30/14/1, (b) 50/30/19/1, and (c) 45/30/24/1.
Fig. 4. Mach–Zehnder interferometer signals of the composites. The red triangle: PMPAA-based composites; the black circle: PDAA-based composites. Polymer (PDAA or
PMPAA)/7DCST/TPAOH/PCBM: (a) 55/30/14/1, (b) 50/30/19/1, and (c) 45/30/24/1.

H.N. Giang et al. / Journal of Photochemistry and Photobiology A: Chemistry 291 (2014) 26–33
31
Fig. 5. Photocurrent (640 nm, 900 mW/cm2, 40 V/␮m) results of the composites. The red line: PMPAA-based composites; the black line: PDAA-based composites. Polymer
(PDAA or PMPAA)/7DCST/TPAOH/PCBM. (a) 55/30/14/1, (b) 50/30/19/1, and (c) 45/30/24/1.
orientation and PR performances. While the thermal properties of
PDAA and PMPAA are similar with a very close Tg of 75 and 74 ◦C,
respectively (see Section 2), the orientation of the chromophore
in the two kinds of composite is very different at 14 and 19 wt.%
of TPAOH. The modification in PMPAA from the original polymer
PDAA is only an addition of methoxy group (OCH3) while the main
structure is kept unchanged. There is a possibility that the bulk
group OCH3 might provide better environment for chromophore
orientation in PMPAA-based composite, resulting in better MZ sig-
nals. Following this explanation, modifying PDAA with methoxy
group at para-position does not only create a more readily ionized
photoconductive polymer with a higher HOMO level but also sig-
nificantly improve the chromophore orientation in the matrix. In
the composition with 14 and 19 wt.% TPAOH, the difference in MZ
signals (Fig. 4(a and b)) is correlated well with the enhancement
in diffraction efficiency detected in the PMPAA-based compos-
ite. Because of that, the results of DFWM (Fig. 3(a and b)) can be
explained well by the difference in the chromophore orientation
in the two kinds of composites. However, with 24 wt.% TPAOH and
45 wt.% polymer concentration, the PDAA-based composites shows
larger diffraction efficiency whereas the MZ responses in two kinds
of composite are the same. In this case, the MZ interferometer
results or chromophore orientation property can no longer explain
what happened in the DFWM results. However, an important fact
which can be concluded from the MZ measurement is the ability
of changing refractive index under a specific electric field of these
composites is the same at this composition. Since the same external
electric field of 45 V/␮m was applied for every samples, the differ-
ence in diffraction efficiency results can only be explained by the
strength of the internal space-charge field.
The formation of the space-charge field is a complicated pro-
cess which includes photo-generation of charge carriers, migration
of the more mobile charges and trapping process in the dark area
[19,20]. Photocurrent measurement can give a closer look of elec-
tronic processes which occur inside the PR samples. Fig. 5 shows
the photocurrent results for the PDAA and PMPAA-based compos-
ites. As can be seen, a significantly large photocurrent comparing
to dark current is measured in all composites. Except the samples
with 14 wt.% of TPAOH, the PR composites using PMPAA shows
larger photocurrent. Improving the photo-generation efficiency by
the modification of methoxy (OCH3) as an electron donating group
at para-position which has increased the HOMO level of the poly-
mer matrix can be used to explain this phenomenon. A similar
result of increasing photo-generation efficiency by changing IP val-
ues has been reported by Hendrickx et al. [21] However, the largest
photocurrent was detected in the PDAA-based composite with the
lowest TPAOH ratio and 55 wt.% of polymer. The reason of for this
phenomenon has not been made clear. It is worth noting that the
MZ signal with this sample is significantly low comparing to others.
There is a possibility that with this composition, the PR compos-
ite is still in a glassy state in which the non-uniform hopping site
paths are formed and thus large photocurrent was measured. Dur-
ing 10 s when the light illuminated the sample, a gradual decay of
photocurrent was observed. The filling of deep trap which leads
to an increase of recombination center has been attributed to this
phenomenon [20,22]. At 19 and 24 wt.% TPAOH (Fig. 5(b and c)),
photocurrent curves in the PDAA-based composites show a larger
decay than those in the PMPAA-based composites. The result indi-
cates that a larger trap density has been induced in the PDAA-based
composite than in the PMPAA-based composite. The values of trap
density can be estimated by Kukhtarev model which was described
in the previous papers [10,23,24]. Table 3 shows the values of the
trap-limited space-charge field Eq and number density of traps Nt.
As can be observed, the values of Eq and trap density in the
PDAA-based composites are larger than those in the PMPAA-based
composites. This result is consistent with the result obtained in
photocurrent measurement. The difference in trap density of the
two kinds of composite might relate to the relative position of
photoconductive polymer’s HOMO level to the others. The IP values
of PMPAA, PDAA, 7DCST, PCBM and TPAOH are −5.57, −5.69, −5.9,
−6.2 and −5.64 eV, respectively. The IP values represent the HOMO
levels of each component. The PMPAA’s HOMO level is higher than
that of all the other components whereas the PDAA’s HOMO level is
lower than that of the plasticizer TPAOH. The slightly higher HOMO
level of TPAOH than PDAA induces the energetically distributed
higher potential site which works as a trap site in the charge
transport manifold. Namely, the plasticizer TPAOH could act as an
effective trap in the PDAA-based composite. It is known that larger
trap density leads to larger space-charge field. By using 24 wt.%
TPAOH and 45 wt.% polymer concentration, the chromophore
orientation of the two composites is the same, and thus higher
diffraction efficiency in the PDAA-based composites as shown in
Fig. 3(c) is ascribed to the larger space-charge field created by the
larger amount of trap. Tsujimura et al. has reported a significant
small diffraction efficiency (less than 2%) with triphenylamine
plasticizer due to the low trap density comparing to using ECZ
plasticizer [17]. By an optimization of chromophore orientation
and trap density by using TPAOH plasticizer, high diffraction
efficiency could be detected.
As can be observed in Tables 1 and 2, it is clearly shown that
diffraction efficiency for the PMPAA-based composites is not
changed so much irrespective of the composition of components.
Whereas the PDAA-based composites gave rise to the strong depen-
dence of diffraction efficiency on the composition of components.
The behavior relates to the good chromophore orientation and the
low trap density by using PMPAA as the photoconductive matrix.

32
H.N. Giang et al. / Journal of Photochemistry and Photobiology A: Chemistry 291 (2014) 26–33
Table 3
Trap-limited space-charge field and number density of trap calculated by Kukhtarev model.
Polymer/7DCST/TPAOH/PCBM
PDAA
PMPAA
Trap-limited
space-charge field
Eq (V/␮m)
Number density of
Trap-limited
space-charge field
Eq (V/␮m)
Number density of
trap NT (cm−3
)
trap NT (cm−3
)
55/30/14/1
50/30/19/1
45/30/24/1
106
73
48
3.1 × 1016
2.1 × 1016
1.4 × 1016
45
36
38
1.3 × 1016
1.09 × 1016
1.16 × 1016
As a good matrix for the chromophore orientation, a large MZ
signal could be detected even at a small amount of 14 wt.% of
TPAOH plasticizer (Fig. 4a). However, the low trap density which
leads to the small space-charge field limits the diffraction effi-
ciency. The growth of diffraction efficiency was fitted well with
bi-exponential function and the dominant response time with its
weigh factor was reported. The fastest response time was detected
with the largest plasticizer concentration. Diffraction efficiency
and response time with 532 nm laser as the writing beams for the
composition with 24 wt.% TPAOH and 45 wt.% polymer concen-
tration were also listed in Tables 1 and 2. At the same externally
applied electric field of 45 V/␮m, the diffraction efficiency values
in both composites were increased dramatically comparing to
the results in which 633 nm writing beams have been used. The
diffraction efficiency originates from changes in refractive index
modulation (ꢄn) which can be calculated from Kogelnik’s coupled-
wave theory [25]. As can be seen in Tables 1 and 2, the magnitude
of refractive index modulation in which the 532 nm writing beams
were used is larger than that when the 633 nm writing beams were
used. A strong space-charge field caused by improving charge
generation with 532 nm laser has been assigned to explain this
phenomenon [18]. The PMPAA-based composite reaches 38%
diffraction efficiency with a fast response time of 35 ms (dominant,
m = 0.757). Due to larger trap density, the PDAA-based composite
shows 70% diffraction efficiency with a fast time constant of 25 ms
(dominant, m = 0.791). In comparison to the previous studies
[17,18], the results indicate that high diffraction efficiency and
fast response time can be obtained in this type of material with
a moderate laser intensity and applied field. Although better PR
properties were achieved in the PDAA-based composite with the
appropriate amount of the plasticizer TPAOH, a careful considera-
tion has to be taken into account. The IP value of TPAOH (−5.64 eV)
is higher than that of PDAA (−5.69 eV). The present of a component
with a little higher HOMO level than charge transport manifold can
lead to the accumulation of the compensating traps. Prolonging
optical exposure during PR operation can create excessive traps
which reduce the photoconductivity and thus suppress the PR
performances. The phenomenon is related to the pre-illumination
effect which was found in PVK due to its energy level comparing
to NLO chromophore [19,20].
Tables 1 and 2. The performance of optical gain follows the same
tendency to the diffraction efficiency results. Except the compo-
sition with 14 wt.% TPAOH, the net gain values (ꢂ –˛) which is
favorable to utilize the material for TBC applications were achieved.
Interestingly, the calculated phase shift values detected in the
PMPAA-based composites are larger than that in the PDAA-based
composites for all of investigated samples (Tables 1 and 2). The dif-
ference in trap density between both composites can be applied to
explain this phenomenon. A photoconductive matrix with higher
HOMO level than the other PR components meets less probability to
trap the mobile charges. Thus, the electrical charges, the holes, can
migrate a longer distance after free carrier formation from the ion
pair. As a result, the larger phase shift was induced by using PMPAA.
4. Conclusions
PR performances of the two kinds of triphenylamine-based com-
posite have been investigated. The chemical modification by a
methoxy group at para-position to triphenylamine has not only
raised HOMO level but also provided a better environment for
molecular alignment and orientation. The chromophore orien-
tation was effectively enhanced by using the modified polymer
of PMPAA. Larger phase shift values were obtained in the all
PMPAA-based composites and net gain was detected in the present
composites. Photoconductive plasticizer TPAOH (IP = −5.64 eV)
worked as an effective trap for the charge transport manifold in
the PDAA (IP = −5.69 eV)-based composite, which led to larger trap
density than that in the PMPAA-based composite. High diffraction
efficiency of 70% with fast response time of 25 ms (dominant) could
be detected in the PDAA-based composite at the moderate laser
intensity and electric field of 45 V/␮m.
Acknowledgements
Authors thank to Prof. Kawabe, Chitose Inst. Sci. Tech. for mea-
suring IP; Dr. Takafumi Sassa and Dr. Takahashi Fujihara (RIKEN,
The Institute of Physical and Chemical Research, Japan) for pro-
viding apparatus and technical advices in MZ interferometer and
photocurrent measurement. This work is partly supported by the
program for Strategic Promotion of Innovative Research and Devel-
opment (s-Innovation), Japan Science and Technology Agency (JST).
One of the most important properties of PR material is the asym-
metric energy transfer between two beams. The space-charge field
formed by a non-uniform illumination induces a refractive index
grating which is shifted a spatial angle of ˚ comparing to the opti-
cal interference pattern. This non-local grating leads to an energy
transfer between two beams. The existence of the non-local grating
which is confirmed by TBC signal and phase shift is a fingerprint
for PR effect. p-Polarization beams were used as writing beams
because it is relatively easier to observe the effect. The asymmet-
ric energy transfer with p-polarization beams is significantly larger
than that using s-polarization [26]. The direction of applied electric
field was chosen to reduce beam fanning. Values of phase shift were
calculated by coupled-wave theory from the values of diffraction
efficiency and optical gain [10,24,25]. The results of optical gain,
absorption coefficient and phase shift values are summarized in
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