Carrier mobility of photochromic diarylethene amorphous films

Article abstract of DOI:10.1016/j.orgel.2014.06.032

Photochromic diarylethenes (DAEs) have gained attention as attractive current switching materials by light irradiation in the organic electronics field. We investigated the hole mobility of amorphous films consisting of three types of DAEs using a space-charge-limited current method and a better chemical structure to achieve high mobility. The hole mobility of open-ring (colorless) DAE having benzothiophene rings substituted with triphenylamine (TPA) as an aryl group was 2 × 10⁴ times (2 × 10^-6 cm²/V s) larger than that of DAE containing thiophene rings without TPA. When the DAE film was irradiated with ultraviolet (UV) light, the hole mobility decreased temporarily at 4% of closed-ring (colored) isomers and then increased to two-three times of the initial colorless state at 85% of the closed-ring isomers. The temporary decrease in the hole mobility originated in the hole trapping effect of the closed-ring molecules in a matrix consisting of open-ring isomers.

Full text of DOI:10.1016/j.orgel.2014.06.032

Organic Electronics 15 (2014) 2264–2269
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Carrier mobility of photochromic diarylethene amorphous films
⇑
Naoya Matsui, Tsuyoshi Tsujioka
Osaka Kyoiku University, Asahigaoka 4-698-1, Kashiwara, Osaka 582-8582, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Photochromic diarylethenes (DAEs) have gained attention as attractive current switching
materials by light irradiation in the organic electronics field. We investigated the hole
mobility of amorphous films consisting of three types of DAEs using a space-charge-limited
current method and a better chemical structure to achieve high mobility. The hole mobility
of open-ring (colorless) DAE having benzothiophene rings substituted with triphenylamine
(TPA) as an aryl group was 2 ꢀ 104 times (2 ꢀ 10ꢁ6 cm2/V s) larger than that of DAE con-
taining thiophene rings without TPA. When the DAE film was irradiated with ultraviolet
(UV) light, the hole mobility decreased temporarily at 4% of closed-ring (colored) isomers
and then increased to two-three times of the initial colorless state at 85% of the closed-ring
isomers. The temporary decrease in the hole mobility originated in the hole trapping effect
of the closed-ring molecules in a matrix consisting of open-ring isomers.
Received 17 April 2014
Received in revised form 25 May 2014
Accepted 29 June 2014
Available online 9 July 2014
Keywords:
Photochromism
Diarylethene
Hole mobility
Photo isomerization
Amorphous film
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
transistors (OFETs) [7–9], a light energy harvesting device
[10], and electrically recordable organic semiconductor
Organic electronic devices, such as light emitting
devices, thin film transistors and photovoltaic devices are
of great interest to the academic and industrial research
community because of their potential for application to
ubiquitous electronic devices with low cost, light weight,
and flexibility [1,2]. Photochromic materials have attracted
considerable attention as photo- or electrically controlled
current switching materials in the organic electronics field
memory [11–13].
Carrier mobility is an important property in organic
electronics, but few studies on DAE mobility have been
carried out using an OFET technique [8,9]. The improve-
ment of carrier mobility is essential to achieve improved
performance of electrical devices with DAEs. There are var-
ious methods to measure carrier mobility, for example, the
OFET technique, time of flight (TOF) technique and space-
charge-limited current (SCLC) method. The OFET and TOF
methods have been widely used for the mobility measure-
ment of organic materials [14–17]. These methods, how-
ever, have some difficulty measuring the mobility of
DAEs; the OFET method is not suitable for measuring the
very low hole mobility of DAE films and the TOF method
induces photoisomerization in the DAE films. In this paper,
we report the hole mobility measurement of several kinds
of DAE amorphous films using the SCLC method and its
change based on photoisomerization in order to better
understand the chemical structure of DAEs for obtaining
improved performance devices.
[3]. The reversible change of a
p-conjugation molecular
system between the colorless state and colored state based
on isomerization induces different electronic properties,
including energy (highest occupied molecular orbital,
(HOMO) and/or lowest unoccupied molecular orbital,
(LUMO)) levels [4,5]. The electrical function of photochro-
mic diarylethenes (DAEs) has been applied to various
devices, including a photoprogrammable organic light-
emitting diode [6], optically switchable organic field-effect
⇑
Corresponding author. Tel./fax: +81 72 978 3633.
E-mail address: tsujioka@cc.osaka-kyoiku.ac.jp (T. Tsujioka).
http://dx.doi.org/10.1016/j.orgel.2014.06.032
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

N. Matsui, T. Tsujioka / Organic Electronics 15 (2014) 2264–2269
2265
Hole-only devices with a colorless DAE layer were pre-
pared as follows: DAE (thickness: 1.5
m), 4,40-Bis(N-car-
2. Colorless DAE films
l
bazolyl)-1,10-biphenyl (CBP, HTL4, thickness: 5 nm,
Ip = 6.0 eV), Tris(4-carbazoyl-9-ylphenyl)amine (TCTA,
HTL3, thickness: 5 nm, Ip = 5.7 eV), N,N7-Di[(1-naphthyl)-
N,N0-diphenyl]-1,10-biphenyl)-4,40diamin (NPB, HTL2,
thickness: 5 nm, Ip = 5.5 eV), and Copper(II)-phthalocya-
nine (CuPc, HTL1, thickness: 5 nm, Ip = 5.2 eV) were depos-
ited in turn on an indium tin oxide (ITO) substrate by a
conventional vacuum evaporation method. Au was then
deposited on the top organic layer. When the voltage was
applied to the Au electrode in positive, only holes were
injected from the Au electrode into the organic layers; it
is difficult to inject electrons from the ITO electrode
because of the large potential barrier.
First, for investigating the influence of the basic chemi-
cal DAE structure and functional side groups, we tested
three types of DAE, as shown in Fig. 1. An open-ring (color-
less) DAE molecule is converted to the closed-ring (col-
ored) isomer by UV irradiation and the closed-ring
isomer is reverted to the open-ring isomer by visible irra-
diation. DAE1 has thiophene rings as an aryl group, which
was first reported as a photo-controlled current switching
molecule [18]. DAE2 also has thiophene rings, but these are
substituted with triphenylamine (TPA) [19], which is a typ-
ical hole-transporting base. DAE3 has benzothiophene
rings modified with TPA [20]. DAEs with TPA are expected
to show better hole mobility compared with that of DAE
without TPA groups.
The open-ring state of DAEs has a large ionization
potential (Ip) of over 6.2 eV [3,18]. The potential barrier
for hole injection from the anode into the DAE layer, in
general, is large, and electrical contact between the DAE
layer and the anode is not ohmic. The key requirement of
the SCLC method is the ohmic contact [21] and, therefore,
multi-hole-injection layers (HILs) should be introduced
between the Au anode and the DAE layer to obtain ohmic
contact, as shown in Fig. 2. The gradual potential levels
reduce the hole injection barrier and generate ohmic con-
tact. Consequently, the accumulated carriers create a
space-charge field and the current is an SCLC.
Fig. 3a shows the current-electric field (J–E) characteris-
tics of each colorless DAE device. All the devices demon-
strate that J is proportional to E2 in the high electric field
region (>300 kV/cm). These results indicate that the
injected current is described by the SCLC mechanism,
9
J ¼ ee0
8
E2
L
l
;
ð1Þ
where L is the thickness of the organic layer,
ity and
l
is the mobil-
e
and e0 are the relative and vacuum permittivity,
respectively [22].
From the results in Fig. 3a, the DAE hole mobility was
derived using Eq. (1), as shown in Fig. 3b. The mobility of
UV
Vis
DAE1
UV
Vis
DAE2
UV
Vis
DAE3
Fig. 1. Photochromic reaction of DAEs.

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N. Matsui, T. Tsujioka / Organic Electronics 15 (2014) 2264–2269
-4.0
Anode
Au
ITO
-5.0
-6.0
-4.7
-4.7
DAE (closed-ring)
-5.7~-5.8
DAE (open-ring)
>-6.2
+
Fig. 2. Energy band diagram of DAE samples.
10-2
ITO substrate
10-3
10-4
10-5
J
E2.4
J
J
E2.3
10-6
10-7
10-8
10-9
DAE3
DAE2
E2.2
DAE1
10
100
1000
Electric field [kV/cm]
UV lamp
Fig. 3a. J–E characteristics of colorless samples.
DAE Source
Fig. 4a. Vacuum evaporation with UV irradiation.
10-5
10-6
DAE3
colorless film
3. Colored DAE films
10-7
Obtaining colored DAE films is not simple and the
mobility measurement of the colored films requires a more
complicated process. In general, colored DAE films (in the
photostationary state (PSS)) are generated by sufficient
UV irradiation, but these PSS films are in a mixed state of
open-ring and colored-ring isomers for a variety of reasons,
including reverse photoreaction at the UV wavelength
region and/or incomplete conversion to the closed-ring
state in the solid film state. Furthermore, since our samples
DAE2
colorless film
10-8
10-9
10-10
DAE1
colorless film
0
200
400
600
800
1000
Square root of electric field [(V/cm)1/2
]
consist of a thick DAE layer (ꢂ1.5
lm), UV irradiation to
the DAE layer is not able to isomerize the DAE layer uni-
formly in depth; a PSS is generated only on the surface
region. We, therefore, prepared the colored samples by
UV irradiation during DAE evaporation onto the substrate
(k = 254 nm, light power: 0.3 mW/cm2, deposition rate:
0.5 nm/s), as shown in Fig. 4a. The samples prepared by
this method consisted of uniform-depth isomerized (in
PSS) thick DAE films.
Fig. 3b. Hole mobility of colorless films.
DAE1, DAE2 and DAE3 was calculated to be 9.2 ꢀ 10ꢁ11
cm2/V s at 350 kV/cm, 1.7 ꢀ 10ꢁ8 (cm2/V s) at 350 kV/cm
and 2.3 ꢀ 10ꢁ6 (cm2/V s) at 700 kV/cm, respectively. (The
relative permittivity
e of DAE1 is 2.9 and that of DAE2
Fig. 4b shows the absorption spectra of the DAE1, DAE2,
and DAE3 films obtained by the above method, reflecting
the isomerization ratio in the DAE films. For the reference,
the PSS in toluene solution (solution-PSS) is also displayed.
The isomerization ratios to the closed-ring isomer were
estimated to be 85% for DAE1, 10% for DAE2 and 6% for
DAE3, for which the isomerization ratios coincided with
those of the PSS in the thin DAE-film states (film-PSS).
and DAE3 is 3.0. The vacuum permittivity e0 is 8.85 ꢀ
10ꢁ12 (F/m2).) The hole mobility of the colorless DAE2,
which was modified with TPA, was 2 ꢀ 102 times greater
than that of DAE1. Furthermore, the hole mobility of the
colorless DAE3, which has benzothiophene rings modified
with TPA, was 100 times greater than that of DAE2. A dra-
matic difference in mobility depending on the chemical
structure was observed.

N. Matsui, T. Tsujioka / Organic Electronics 15 (2014) 2264–2269
2267
(a)
(b)
(c)
DAE1
DAE2
DAE3
700
400 500
600
400 500
600
700
400 500
600
700
Wavelength [nm]
Wavelength [nm]
Wavelength [nm]
Fig. 4b. Absorption spectra of (a) DAE1, (b) DAE2 and (c) DAE3 obtained by vacuum evaporation with UV irradiation. The dotted lines, solid lines and
dashed lines indicate the colorless state, colored film state (film-PSS) obtained by vacuum evaporation with UV irradiation, and solution-PSS in toluene
solution state (for the reference), respectively.
Although it is ideally right that current characteristics
and mobilities for the colored DAE films should be com-
pared in the same and high isomerization ratios, however,
the isomerization ratio in the film-PSS is dramatically
reduced compared with the solution-PSS, and the ratio in
the film-PSS is strongly depending on DAE species. The
DAE1 film showed the high isomerization ratio exception-
ally in the film-PSS, but those for DAE2 and 3 were consid-
erably low. The many origins of the reduced isomerization
ratio in the film-PSS have been reported: high reverse reac-
tion quantum yield upon UV irradiation, presence of balky
substitutes, conformation of aryl groups and/or energy
transfer from excited molecules. In this field, it is one of
important research subjects even now. Not only the mobil-
ity but also the isomerization ratio in the film-PSS, there-
fore, are important factors as performance of organic
semiconductor devices with a photochromic film. In this
study, therefore, we compare the current characteristics
and mobilities of colored DAE films not in the same isom-
erization ratio but in their film-PSS.
is SCLC. On the other hand, J of DAE2 was not proportional
to E2; the DAE2 showed the thermal emission-type cur-
rent-characteristic that was caused by the difference in
ionization potential depending on DAE species.
Fig. 5b shows the hole mobility of the colored DAE1 and
DAE3 films derived from Fig. 5a. The hole mobilities of col-
ored DAE1 and DAE3 were 2.6 ꢀ 10ꢁ10 (cm2/V s) (at
350 kV/cm) and 9.7 ꢀ 10ꢁ8 (cm2/V s) (at 700 kV/cm),
respectively. The hole mobility of the colored DAE1 film
was two-three times that of the colorless DAE1 film, which
was caused by the extended
p-conjugated molecular sys-
tem of the closed-ring isomer. However, the mobility of
the colored DAE3 film, in which the isomerization ratio
was 6%, decreased from that of the colorless film. This
decrease in mobility suggests that closed-ring isomers act
as hole traps in a colorless matrix consisting of open-ring
isomers [23,24].
To investigate whether or not the mobility decrease in
the DAE film is a common phenomenon, we studied the
isomerization ratio dependence of hole mobility using
DAE1, which can achieve various isomerization ratios from
0% to 85% in the amorphous solid film state (see Fig. 4b).
DAE1 samples with various closed-ring isomer isomeriza-
tion ratios were prepared by controlling the DAE1 deposi-
tion rate with constant UV light irradiation (k = 254 nm,
light power: 0.3 mW/cm2, deposition rate: 20, 5, 1 or 0.5
(nm/s)). Fig. 6a shows the absorption spectra and reflects
the isomerization ratios of each sample. For reference,
the PSS in the toluene solution state is also displayed in
the solid line. The isomerization ratios to the closed-ring
isomer were estimated to be 0% (colorless state), 4% (depo-
sition rate: 25 nm/s), 17% (5 nm/s), 40% (1 nm/s), and 85%
(0.5 nm/s).
Fig. 5a shows the J–E characteristics of the colored DAE
samples. J was proportional to E2 in the high electric field
region for DAE1 and DAE2, which means that the current
J
E2.0
10-4
10-5
DAE3
DAE1
10-6
J
E2.2
10-7
10-8
Fig. 6b shows the isomerization ratio dependence of
hole mobility for the DAE1 amorphous films. The vertical
axis represents the hole mobility at 350 kV/cm and the
horizontal represents the isomerization ratio. The hole
mobility of the colorless state (0%) was 9.2 ꢀ 10ꢁ11
(cm2/V s), which decreased temporarily to 1.6 ꢀ 10ꢁ11
(cm2/V s) at 4% and then increased to 2.4 ꢀ 10ꢁ10 cm2/V s
at 85% according to isomerization. This result reproduces
DAE2
10-9
10-10
10
100
1000
Electric field [kV/cm]
Fig. 5a. J–E characteristics of colored (film-PSS) samples.

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N. Matsui, T. Tsujioka / Organic Electronics 15 (2014) 2264–2269
10-5
10-6
10-7
DAE3
colored film (6%)
10-8
10-9
DAE1
colored film (85%)
10-10
0
200
400
600
1000
800
Square root of electric field [(V/cm)1/2
]
Fig. 5b. Hole mobility of colored (film-PSS) DAE1 and DAE3 films.
PSS in toluene by UV irradiation
Colored film deposited at 0.5 nm/s
4. Conclusion
Colored film deposited at 1 nm/s
Colored film deposited at 5 nm/s
Colored film deposited at 25 nm/s
The hole mobility of several DAEs was estimated using
the SCLC method. DAE with benzothiophene rings substi-
tuted with TPA (DAE3) showed relatively high hole mobil-
ity in the colorless amorphous state. We studied the
relation between the isomerization ratio and the hole
mobility, and found that mobility decreased temporarily
at a low isomerization ratio (<closed-ring isomer 20%)
and then increased to two-three times (85%) that of the
colorless state (0%) according to the isomerization reaction.
This decrease in mobility was caused by a hole-trapping
effect on the colored isomers in the colorless matrix.
Obtaining a large change in the mobility based on photo-
isomerization for organic electronics applications requires
a large isomerization ratio in the film-PSS. Our results pro-
vide a significant perspective in the application of photo-
chromic materials to organic electronics and photonics.
Colorless state
300
400
500
600
700
800
Wavelength [nm]
Fig. 6a. Absorption spectra corresponding to DAE1 films obtained by
deposition at various deposition rates.
[10-10
]
2.5
5. Experimental
2.0
1.5
1.0
5.1. Substrate cleaning
ITO substrates (Sigma–Aldrich: 576352-25PAK) were
washed by ultrasonication in ethanol (Sigma–Aldrich: 09-
0770-3-3L-J, Purity: >99.5%) and acetone (Sigma–Aldrich:
01-0460-3-3L-J, Purity: >99.5 %) in turn for 10 min. The
substrates were then cleaned using a UV-ozone cleaner.
0.5
0
0
20
40
60
80
100
Isomerization ratio to closed-ring isomer [%]
5.2. Vacuum evaporation
Fig. 6b. Isomerization ratio dependence of hole mobility of DAE1 film (at
350 (kV/cm)).
Thermal evaporation of organic or inorganic materials
was carried out under 1 ꢀ 10ꢁ5 Torr at room temperature
(20 °C). The deposition thicknesses were monitored and
controlled by a quartz thickness monitor during evapora-
tion. The deposition rates of CBP, TCTA, NPB and CuPc were
0.1 nm/s and the thickness was 5 nm. The deposition rate
of DAE was 2 nm/s for the samples in Fig. 3 and thickness
the decrease of the mobility for the colored DAE3 film with
a low isomerization ratio and means that the origin of low-
ered mobility at a low isomerization ratio is the hole-trap-
ping effect. The lowered mobility for the colored (film-PSS)
DAE3 film attribute to the same origin. This result indicates
that the mobility increase according to coloring reaction
requires a high isomerization ratio in the film-PSS.
was 1.5 lm.

N. Matsui, T. Tsujioka / Organic Electronics 15 (2014) 2264–2269
2269
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diated the substrate during DAE deposition. The DAE
deposition rate was 0.5 nm/s (Figs. 4b and 5) or varied
from 0.5 to 25 nm/s (Fig. 6). The DAE layer thickness was
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5.5. Absorption spectra
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Figs. 4b and 6a were measured as follows: DAE toluene
solutions were obtained by dissolving the thick DAE films
having various isomerization ratios (including the film-
PSS sample), which were formed using the vacuum evapo-
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This research was partially supported by JSPS KAKENHI
Grant Number 23350066.
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