Ambipolar organic phototransistors based on 6,6′-dibromoindigo

Article abstract of DOI:10.1039/c8ra02346h

Ambipolar organic phototransistors were fabricated using a natural pigment 6,6′-dibromoindigo (6-BrIG) as the active channel. These phototransistors yielded significantly enhanced currents upon light illumination with photoresponsivities and external quantum efficiencies as high as 10.3 A W^-1 and 2437% for the n-channel, and 55.4 mA W^-1 and 13.1% for the p-channel, respectively. In addition, simple inverter complementary circuits were fabricated by integrating two ambipolar phototransistors. Channel current was dependent on light intensity and voltage bias. This study provides a basis for an in-depth understanding of the optoelectronic characteristics of 6-BrIG, and introduces this material as an ecofriendly candidate for optoelectronic applications.

Full text of DOI:10.1039/c8ra02346h

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0
Ambipolar organic phototransistors based on 6,6 -
dibromoindigo†
Cite this: RSC Adv., 2018, 8, 14747
a
b
a
b
a
Hyoeun Kim, Gyoungsik Kim, Inho Song, Jungho Lee, Hanum Abdullah,
b
a
Changduk Yang * and Joon Hak Oh
*
0
Ambipolar organic phototransistors were fabricated using a natural pigment 6,6 -dibromoindigo (6-BrIG) as
the active channel. These phototransistors yielded significantly enhanced currents upon light illumination
ꢀ
1
with photoresponsivities and external quantum efficiencies as high as 10.3 A W and 2437% for the n-
ꢀ1
channel, and 55.4 mA W
and 13.1% for the p-channel, respectively. In addition, simple inverter
Received 17th March 2018
Accepted 11th April 2018
complementary circuits were fabricated by integrating two ambipolar phototransistors. Channel current
was dependent on light intensity and voltage bias. This study provides a basis for an in-depth
understanding of the optoelectronic characteristics of 6-BrIG, and introduces this material as an
ecofriendly candidate for optoelectronic applications.
DOI: 10.1039/c8ra02346h
rsc.li/rsc-advances
advantages, research on ambipolar material-based photo-
transistors is insufficient. Therefore, new photoconductive
1
. Introduction
Organic p-conjugated materials have received much atten- ambipolar materials must be developed to improve our
tion due to their tunable optical and electrical properties, understanding of their photophysical properties and suit-
suitability for exible device fabrication, and large-area ability for practical applications.
1
–3
4,5
processability.
Organic eld-effect transistors (OFETs),
In this work, we investigated the use of a natural pigment,
6
0
organic light-emitting diodes (OLEDs), organic photo- 6,6 -dibromoindigo (6-BrIG or tyrian purple), as an ambipolar
transistors (OPTs),
organic memories,
7
–9
10,11
organic photovoltaics (OPVs),
semiconductor for use in phototransistors. 6-BrIG is a well-
1
2
13
and organic sensors
have been known chromophore and is the oldest known purple dye,
successfully developed. Phototransistors, which allow one to extracted from sea snails. Interestingly, this material has
control channel conductance via light absorption, are several unique properties that match well with the require-
2
8–30
particularly applicable to electronic devices such as light ments of electronic devices.
6-BrIG boasts (i) a small
1
4
sensors and optical switches. Although many research bandgap energy of 1.8 eV for balanced ambipolar charge
groups have reported phototransistors with unipolar char- injection, (ii) a relatively deep-lying lowest unoccupied
acteristics,
few have explored ambipolar photo- molecular orbital (LUMO) level of ꢀ3.7 eV for air-stable
transistors, which have mostly been demonstrated using electron transport, (iii) strong intra- and intermolecular
1
5–19
2
0
21,22
23
blend, heterojunction,
or complementary logic circuits
hydrogen bonding between its amine and carbonyl groups;
made by sequentially processing two p- and n-type semi- this induces planarity and short p–p stacking distances,
conductors. Phototransistors based on ambipolar materials, which are benecial for efficient charge transport, and (iv)
2
9
which are single component systems that can function as non-toxicity and biocompatibility. Although ambipolar
both p- and n-type semiconductors, have also been demon- OFETs and inverter circuits based on 6-BrIG have been
2
4–27
31–33
strated.
cost-effective and boast several advantages including large- in transistors.
Single component, ambipolar transistors are demonstrated,
early studies have been conned to its use
area applicability and simple fabrication. Despite their
The small bandgap and broad absorption band (350–
7
00 nm) of 6-BrIG imply facile excitation from the highest
occupied molecular orbital (HOMO) to the LUMO upon
exposure to visible light. We therefore introduced 6-BrIG into
phototransistors as an ambipolar organic semiconductor.
Ambipolar charge transport was observed in the dark and was
signicantly enhanced upon exposure to light. This indicated
a
Department of Chemical Engineering, Pohang University of Science and Technology
(
POSTECH), Pohang, Gyeongbuk 790-784, South Korea. E-mail: joonhoh@postech.
ac.kr
b
Department of Energy Engineering, School of Energy and Chemical Engineering,
Perovtronics Research Center, Low Dimensional Carbon Materials Center, Ulsan
National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan that 6-BrIG functioned as a photoactive layer. OPTs based on
ꢀ
1
4
†
1
4919, South Korea. E-mail: yang@unist.ac.kr
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra02346h
6-BrIG exhibited photoresponsivity as high as 10.3 A W and
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remarkable external quantum efficiency of 2437% under light ltration and washed several times with water and cold ethanol.
illumination.
The resulting purple solid was dried under vacuum and
combined to give a purple solid as 6,6 -dibromoindigo (0.55 g,
0
7
5%). NMR peaks were not detected due to limited solubility in
2
. Experimental
ꢀ
1
various solvents. IR nmax (solid)/cm : 3385 (N–H), 1633 (C]O),
610 (C]C). GC-DIP/MS (m/z) calcd: 420.05; found: 419.9. EA:
-BrIG was synthesized in accordance with previously published anal. calcd for C H Br N O : C, 45.75; H, 1.92; N, 6.67; found:
2.1 Instruments
1
6
1
6
8
2 2 2
34,35
1
procedures.
H NMR spectra were recorded on a Varian C, 45.50; H, 1.71; N, 6.51.
VNRS 400 MHz (Varian USA) spectrophotometer using CDCl as
3
the solvent and tetramethylsilane (TMS) as the internal stan-
dard. UV-vis spectra were acquired on a Cary 5000 spectropho-
tometer. Cyclic voltammetry (CV) was performed on an AMETEK
VersaSTAT 3 with a three-electrode cell in a nitrogen-purged
2
.5 Device fabrication
Devices were fabricated with a bottom-gate top-contact cong-
uration. Commercially available, thermally grown, 300 nm-thick
SiO
device substrate. Si functioned as both the substrate and the
gate electrode, and SiO acted as the gate dielectric with
a capacitance per unit area of 10 nF cm . The SiO
modied with an n-octadecyltrimethoxysilane (OTS) self-
assembled monolayer according to a reported method. The
OTS-treated wafers were rinsed with toluene, acetone and iso-
propyl alcohol, and dried under a stream of N
2
on highly n-doped Si (100) (<0.004 U cm) was used as the
solution
of
0.1
M
tetra-n-butylammonium
hexa-

uorophosphate (n-Bu
4
NPF
6
) in acetonitrile at a scan rate of
ꢀ
1
+
2
0.1 V s at room temperature. A platinum wire, Ag/Ag (0.01 M
ꢀ
2
2
surface was
of AgNO in acetonitrile) electrode, and 6-BrIG deposited onto
3
ITO-coated glass were used as the counter electrode, reference
36
+
electrode, and working electrode, respectively. The Ag/Ag
reference electrode was calibrated using
a
ferrocene/
2
. For the channel
ferrocenium redox couple as an external standard, whose
oxidation potential was set at ꢀ4.8 eV with respect to the zero
vacuum level. The HOMO energy level was obtained from the
layer, 6-BrIG was deposited by thermal evaporation under high
ꢀ
6
ꢁ
ꢀ1
˚
vacuum (<5.0 ꢂ 10 torr, Tsubstrate ¼ 80 C, rate ¼ 0.1–0.3 A s
,
onset
onset
and lm thickness ¼ 30 nm). Thermal annealing was carried
equation HOMO ¼ ꢀ(Eox
ꢀ Eferrocene + 4.8) eV. The LUMO
ꢁ
opt
g
out at 100 C for 30 min in an N
2
atmosphere. Au top-contact
level was obtained from the equation LUMO ¼ HOMO + E eV.
electrodes (40 nm) were thermally evaporated through
a shadow mask with a channel width–length ratio (W/L) of 17.5.
2.2 Synthesis of 6-bromo-3-iodoindole (1)
Iodine (2.59 g, 10.2 mmol) was added to a solution of 6-bro-
moindole (2 g, 10.2 mmol) and sodium hydroxide (0.41 g, 10.2
mmol) in methanol (100 mL) and an aqueous potassium iodide
2
.6 Measurement of OFETs and OPTs
Electrical performance was recorded in a N
2
-lled glovebox
using a Keithley 4200 semiconductor parametric analyzer due to
their low air-stability. Optoelectronic properties were measured
in a vacuum chamber using monochromic and polychromic
light sources.
(1.69 g, 10.2 mmol) solution (10 mL). The reaction mixture was
stirred at room temperature for 3 h and water was added. The
resulting precipitate was collected by ltration, washed with
water, and dried under reduced pressure. The product 6-bromo-
3-iodoindole was used for the following reaction without
purication.
3
. Results and discussion
3.1 Synthesis and characterization
2.3 Synthesis of 3-acetoxy-6-bromoindole (2)
The synthetic procedures to obtain intermediates and the target
Silver acetate (1.56 g, 9.32 mmol) was added to a solution of 6-
bromo-3-iodoindole (2 g, 6.21 mmol) in glacial acetic acid (50
mL). The reaction mixture was stirred at 90 C for 2 h and the
molecule (6-BrIG) are outlined in Scheme 1, which were
34,35
ꢁ
prepared according to the literatures.
Briey, 3-acetoxy-6-
bromoindole (2) was synthesized by sequential reactions, i.e.,
iodination of 6-bromoindole and nucleophilic substitution with
silver acetate, followed by hydrolysis with a sodium hydroxide
solution in ethanol, affording 6-BrIG in a 25.7% overall yield.
Detailed synthetic routes and characterization can be found in
the Experimental section and ESI.† The synthesized 6-BrIG
showed limited solubility in common organic solvents, mainly
due to strong intramolecular hydrogen bonding. Thus, 6-BrIG
was difficult to characterize by NMR. Therefore, the structural
mixture was cooled to room temperature and ltered. The crude
product was puried by column chromatography (silica gel,
dichloromethane) to afford 0.88 g (56%) of 3-acetoxy-6-
1
bromoindole. H NMR (400 MHz, CDCl
3
) d (ppm) 7.89 (br,
1
H), 7.47 (d, 1H, J ¼ 1.6 Hz), 7.42 (d, 1H, J ¼ 8.4 Hz), 7.32 (d, 1H,
J ¼ 2.8 Hz), 7.25 (dd, 1H, J ¼ 8.4 Hz, J ¼ 1.6 Hz), 2.36 (s, 3H).
1
2
GC-DIP/MS (m/z) calcd: 254.08; found: 254.7. EA: anal. calcd for
BrNO : C, 47.27; H, 3.17; N, 5.51; found: C, 47.11; H,
.31; N, 5.37.
C
3
10
H
8
2
0
2
.4 Synthesis of 6,6 -dibromoindigo (6-BrIG)
1
M sodium hydroxide solution (10 mL) was added to a solution
of 3-acetoxy-6-bromoindole (0.88 g, 3.47 mmol) in ethanol (30
mL). Water was added to the mixture aer stirring at room
temperature for 3 h. The resulting precipitate was collected by Scheme 1 Synthetic routes to 6-BrIG.
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characteristics and elemental composition of the synthesized 6- Table 1 UV-vis absorption and electrochemical properties of the 6-
BrIG
BrIG were veried by mass spectrometry. Both were consistent
with calculated values. In addition, the infrared absorption
lm
opta
b
c
l
max (nm)
E
g
(eV)
E
HOMO (eV)
ELUMO (eV)
spectrum of 6-BrIG contained stretching vibrations at 3385,
ꢀ
1
1
633, and 1610 cm , corresponding to N–H, C]O, and C]C 6-BrIG
523
1.82
ꢀ5.49
ꢀ3.67
bonds, respectively. Thermogravimetric analyses revealed
a high thermal stability with a decomposition temperature of
a
opt
g
Calculated from the absorption band edge of the thin lm, E
¼
6
b
1
240/ledge
.
Vacuum deposited thin lm in 0.1 M CH CN/n-Bu NPF ,
3 4
ꢀ
1
ꢁ
3
46 C, which corresponded to a 5% weight loss (see Fig. S1 and versus ferrocene/ferrocenium at 0.1 V s . Energy level of HOMO
estimated from the onset oxidation potentials, assuming the absolute
S2 in ESI†).
The UV-vis absorption spectrum of a 6-BrIG thin lm is
shown in Fig. 1a and the corresponding spectral data are
summarized in Table 1. Interestingly, despite its small struc-
ture, 6-BrIG exhibited a broad absorption band from 350–
energy level of ferrocene/ferrocenium to be 4.8 eV below vacuum.
c
opt
LUMO (eV) ¼ EHOMO + E
E
g
.
light. In our work, 6-BrIG lm thickness was optimal at 30 nm
and the lms were annealed at 100 C.
The surface morphology of 6-BrIG lms was investigated via
tapping-mode atomic force microscopy (AFM) before and aer
annealing at 100 C for 30 min (Fig. 2). Thin lms of 6-BrIG
consisted of large, crystalline, needle-like grains with an average
length of 1.16 mm. Aer annealing, a relatively more uniform
and atter surface was formed, with a root-mean-square (rms)
roughness of 8.56 nm, much lower than that of the as-cast lm
13.3 nm). This result indicates an enhancement of intermo-
lecular packing and charge transport aer the thermal
annealing.
700 nm, which can be attributed to a combination of the H-
ꢁ
chromophore and a complex set of intramolecular interac-
37–39
tions induced by the indigo core,
and an absorption
maximum (lmax) at 523 nm. The estimated optical bandgap
ꢁ
opt
g
(E
) of 6-BrIG was 1.82 eV.
Cyclic voltammetry (CV) was conducted with a working elec-
trode consisting of thermally evaporated 6-BrIG on indium tin
oxide (ITO)-coated glass. An n-Bu NPF solution was used as the
electrolyte and ferrocene was employed as an external standard
Fig. 1b). The HOMO level was determined as the rst onset
4
6
(
(
potential of the oxidation peak at 0.86 V, but no reduction peak
was observed in the potential range. The HOMO level of 6-BrIG,
OPT performances. Fig. 3a, b and S3† show the IDS–VGS
curves measured in the dark and under illumination at VDS
onset
onset
calculated using the equation EHOMO ¼ ꢀ(E
ꢀ E
+
ferrocene
ox
¼
4
.8) eV, was ꢀ5.49 eV and the LUMO level, calculated with
8
0 V and ꢀ80 V, respectively. Photoelectronic properties were
opt
g
E
, was ꢀ3.67 eV. These energy levels are commensurate with
investigated using green monochromic (l ¼ 523 nm, P
green
¼
ambipolar charge transport in OFETs.
ꢀ2
ꢀ2
5
30 mW cm ) and white polychromic (Pwhite ¼ 5.4 mW cm
)
Characterizations of the thin lms. A schematic device
structure with a bottom-gate top-contact conguration is shown
in Fig. 1c. 6-BrIG thin lms were prepared by thermal evapo-
light sources, respectively. Typical ambipolar performance with
dominant electron transport was observed from 6-BrIG; OFET
2 2
ration onto OTS-pretreated SiO /Si substrates; the SiO layer
thickness was 300 nm. Film morphology and the thickness of
the semiconducting layer are key factors in the optimization of
phototransistor devices. Well-packed microstructural grains are
important for efficient charge transport, while the thickness of
the semiconducting layer is vital for sufficient light absorption.
If the lm is too thick, light may not be able to penetrate the
nanometer-thin channel due to internal ltering. Light is
absorbed into the non-accumulated bulk semiconductor, which
40–42
acts as an optical lter, before reaching the channel.
Conversely, if the semiconducting layer is too thin, grains will
not be sufficiently connected and will not absorb sufficient
Fig. 1 (a) UV-vis absorption spectrum of 6-BrIG film. (b) Cyclic vol- Fig. 2 Tapping-mode AFM height (left) and phase (right) images (5 mm
tammogram of 6-BrIG film evaporated on ITO. (c) Schematic device ꢂ 5 mm) of 6-BrIG films on OTS-treated SiO
2
/Si substrates: (a) as-cast
ꢁ
structure of the phototransistor.
and (b) annealed film at 100 C.
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Table 2 OPT performance of 6-BrIG under light illumination
n-channel p-channel
R
max
h
max
R
(A W
max
h
max
ꢀ1
ꢀ1
(A W
)
P
max
(%)
)
P
max
(%)
Mono chromic
Poly chromic
10.3
1.05
364
532
2437
—
0.0554
0.0137
214
574
13.1
—
where Ilight is the drain current under illumination, Idark is the
drain current in the dark, and Pinc is the incident illumination
power on the active channel of the device, respectively. The OPT
ꢀ
1
of 6-BrIG shows a maximum R of 10.3 A W in the n-channel
ꢀ
1
(
(
0.0554 A W in p-channel) and P of 532 in the n-channel
574 in p-channel) under green and white light irradiation,
respectively (Fig. 3c and d and Table 2). This implies that R is
closely related to the absorption range of the photoactive
material, 6-BrIG (see UV-vis absorption spectra in Fig. 1a),
whereas P is more dependent on the incident optical power. For
Fig. 3 Transfer characteristics, photoresponsivity (R), and photocur-
rent/dark-current ratio (P) of 6-BrIG film in the dark and under illu-
ꢀ2
example, P
(5.4 mW cm ) was ten-fold higher than Pgreen
white
ꢀ
2
15
mination: (a and c) n-dominant operation, sweep from ꢀ80 to 80 V (0.53 mW cm ).
and (b and d) p-dominant operation, sweep from 80 to ꢀ80 V. The
circle, triangle, and square symbols represent the dark, polychromic,
and green monochromic light, respectively.
In addition, the external quantum efficiency (EQE, h) of OPTs
can be dened as the ratio of the number of photogenerated
carriers to the number of incident photons in the OPT channel,
calculated as follows:
characteristics in the saturation region were extracted using the
following eqn (1):
h ¼ {(Ilight ꢀ Idark)hc}/(ePinclpeak
)
(4)
where h represents Planck's constant, c is the speed of light, e is
the fundamental unit of charge, and lpeak is the peak wave-
length of incident light, respectively. Only a monochromic light
source can be used for these measurements. The n-channel
2
I
DS ¼ 1/2(W/L)mC
i
(VGS ꢀ VTH
)
(1)
where W and L are the channel width and length, respectively, m
is the eld-effect mobility, C is the capacitance per unit area of
i
exhibited a high h
green light irradiation, while the p-channel showed an hp of
3% under the same conditions. Thus, the electron photocur-
n
value up to 2437% at |VGS| ¼ 80 V under
the dielectric layer, and VTH is the threshold voltage. The elec-
trical characteristics in the dark and under illumination are
summarized in Table S1.†
Under light irradiation, off-currents signicantly increased
in both electron- and hole-enhancement modes due to the
creation and dissociation of photogenerated excitons. There-
fore, the on/off ratios under illumination became smaller, from
1
rent was much larger than the hole photocurrent, indicating
that 6-BrIG is a strongly n-dominant ambipolar material. We
also tested thickness-dependent phototransistor performances
to nd the optimal lm thickness. When the lm is too thin (15
nm), grains are not sufficiently connected, which precludes
device operation (Fig. S4†). On the other hand, when the lm is
thick (60 nm), light is absorbed into the bulk active layer before
reaching the channel, reducing phototransistor performances
7
5
6
5
3
1
0 to 10 –10 for n-channel transport, and from 10 to 10 for p-
channel transport. In addition, mobilities were enhanced and
the threshold voltages shied negatively in the n-channel (or
positively in the p-channel), meaning the easier turn-on of the
device. This can be attributed to a decrease in the density of trap
(Fig. S5†). Therefore, we concluded that the optimized thickness
of semiconductor lm is around 30 nm in phototransistor
applications.
Characterization of photocurrent response. To investigate
the real-time photocurrent response of 6-BrIG phototransistors,
4
3,44
sites by photogenerated charge carriers.
The amount of
electrons (or holes) increases owing to the active charges
coming from the additional photogenerated charge carriers, not
only from gate-induced effects.
Photoresponsivity (R) and the photocurrent/dark-current
ratio (P) are quantitative parameters for the sensitivity of
OPTs. These values were calculated by the following eqn (2) and
I
DS was measured as a function of time using a pulsed exposure
with green light at time intervals of 30 s, as shown in Fig. 4. The
rise and decay times for light-on and -off modes were estimated
to be in the range of few seconds (Fig. S6†). In both n- and p-
dominant operation, signicant increases in photocurrent
(
3):
were observed with increasing VGS
.
R ¼ (Ilight ꢀ Idark)/Pinc
P ¼ (Ilight ꢀ Idark)/Idark
(2)
(3)
In particular, the normalized IDS was about 2 at VGS ¼ 0 V but
increased dramatically to nearly 30 at VGS ¼ 20 V in the n-
channel. Likewise, a two-fold increase in the normalized IDS
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4. Conclusions
This report describes the rst fabrication and characterization
of ambipolar phototransistors based on 6-BrIG. The small
optical bandgap of 6-BrIG allowed for excitation with visible
light. OFETs made with 6-BrIG exhibited typical ambipolar
performance with both high photoresponsivity (R
n
¼ 10.3 A
ꢀ
1
W
) and excellent external quantum efficiency (h ¼ 2437%) in
n
n-channel mode. However, 6-BrIG is an n-dominant ambipolar
material and p-channel mode yielded relatively low gures of
ꢀ
1
Fig. 4 Photocurrent responses of the 6-BrIG film under green light merit (R
exposure with a time interval of 30 s: (a) n-dominant operation at VDS photocurrent response was reversible and variable with voltage
p
¼ 0.0554 A W
and h
p
¼ 13.1%). In addition,
¼
20 V and (b) p-dominant operation at VDS ¼ ꢀ80 V.
bias and light intensity. These ndings open new possibilities
for the low-cost fabrication of organic optical sensors with
tunable polarity and photoresponsivity.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant (2017R1E1A1A01074090 and
Fig. 5 (a) Circuit diagram of the inverter. (b) Voltage transfer (VOUT
–
2015R1A2A1A10053397) and by Nano Material Technology
V
IN) characteristic curve measured in the dark and under green light
ꢀ2
Development Program (2017M3A7B8063825) through the NRF
funded by the Ministry of Science and ICT (MSIT), Korea.
with illumination intensities between 115–530 mW cm
.
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4
ꢀ2
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(
i) At low V , V
decreases with increasing light intensity
OUT
IN
7
8
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depends on both voltage bias and light intensity. In such
ambipolar phototransistors and complementary circuits, both
light illumination and gate bias can be used to modulate the
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