Facile synthesis of organo-soluble surface-grafted all-single-layer graphene oxide as hole-injecting buffer material in organic light-emitting diodes

Article abstract of DOI:10.1039/c1jm00044f

Graphene oxide (GO) was modified by surface grafting and the product P-GO exhibited a surprisingly good solubility in common solvents (as high as 1.8 mg mL^-1 in DMF) without sonication and can be spin coated to form large single-layer sheets on a substrate. In contrast to GO, all-single-layer P-GO sheets are readily formed in solution, which are highly useful for separation from other multi-layer sheets and as precursors to all-single-layer graphene. The unique property of single-layer P-GO made it a promising functional material, such as hole-injecting buffer (HIB) material in organic light-emitting devices. The devices using P-GO as a hole-injecting buffer layer on the ITO electrode perform better than those with PEDOT:PSS.

Full text of DOI:10.1039/c1jm00044f

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PAPER
Facile synthesis of organo-soluble surface-grafted all-single-layer graphene
oxide as hole-injecting buffer material in organic light-emitting diodes†
Ze Zhong, Yanfeng Dai, Dongge Ma and Zhi Yuan Wang*
Received 5th January 2011, Accepted 16th February 2011
DOI: 10.1039/c1jm00044f
Graphene oxide (GO) was modified by surface grafting and the product P-GO exhibited a surprisingly
ꢀ1
good solubility in common solvents (as high as 1.8 mg mL in DMF) without sonication and can be
spin coated to form large single-layer sheets on a substrate. In contrast to GO, all-single-layer P-GO
sheets are readily formed in solution, which are highly useful for separation from other multi-layer
sheets and as precursors to all-single-layer graphene. The unique property of single-layer P-GO made it
a promising functional material, such as hole-injecting buffer (HIB) material in organic light-emitting
devices. The devices using P-GO as a hole-injecting buffer layer on the ITO electrode perform better
than those with PEDOT:PSS.
1
. Introduction
injection barrier and improve the interface morphology.
In comparison, solution-processable organic hole-injecting
buffer material that can replace PEDOT:PSS is desirable but
scarce.
The injection of charge carriers from electrodes into organic
layers is an extremely important process in organic devices, such
as organic light emitting diodes (OLEDs) and photovoltaic
devices. In the devices using indium tin oxide (ITO) as an anode,
a large hole-injection barrier is often found at the ITO/organic
interface because the work function of ITO is in the range of 4.1–
The water-soluble graphene oxide (GO) is often used as
11–14
a precursor for graphene,
and also shows a great potential
15
for use in optoelectronic devices such as organic solar cells.
However, a small amount of water that is tightly adsorbed on
GO can adversely affect the performance of OLEDs and other
organic optoelectronic devices, such as the formation of dark
spots and pixel shrinkage over time due to intrusion of water
and air. In addition, the water-soluble GO is made with the
layer numbers beyond control and tends to form multilayered
sheets, which may hamper its ability as an anodic buffer
4
.7 eV, which is significantly lower than the typical ionization
1
energy of organic materials. Electrically doped charge transport
layers may be used to enhance the injection of charge carriers
2
from an ITO anode into organic layers, such as the use of
3
a strong electron acceptor F4-TCNQ in the hole transport layer
and the use of poly(3,4-ethylenedioxythiophene):poly-
4
(
styrenesulfonate) (PEDOT:PSS) on top of the ITO electrode.
material since similar to graphene the property of GO is also
16–18
The PEDOT:PSS dispersion is considered today to be the most
promising and most widely used hole-injecting buffer (HIB)
material in OLED research and development. Furthermore, the
PEDOT:PSS layer acts as a physical barrier against the many
defect sites known to be present in ITO. However, due to its high
acidity, PEDOT:PSS can gradually erode the ITO electrode over
time, making it necessary to explore other organic HIB materials.
Among the reported anodic buffer layers, a large number
dependent on the layer number of the sheets.
Therefore,
structural modification of GO is needed to improve its solubility
in anhydrous organic solvents and make it available as all-
single-layer sheets with the desirable and fairly consistent elec-
trical property for use in OLEDs and other organic devices.
Accordingly, by reducing interaction between GO sheets,
surface grafting with long-chain molecules is deemed necessary
and appropriate in order to obtain single-layer GO sheets with
a good solubility in organic solvents.
5
6
7
of transition metal oxides, such as V
9
CuO , NiO, and Fe O ,
x 3 4
2
O
5
,
MoO
x 3
, WO ,
8
10
have been used to lower the hole-
State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences and
Graduate School of Chinese Academy of Sciences, Changchun, 130022,
China. E-mail: wwjoy@ciac.jl.cn; Fax: +86 431 852901; Tel: +86 431
2
. Experimental
Materials
85262901
†
Electronic
supplementary
information
(ESI)
available:
4-Aminophenol, Aliquatꢀ 336 and 1-chlorooctane were received
from Aladdin Reagent Company. All the solvents were
purchased from Beijing Chemical Company and were used
without purification.
Pyrolysis-GC/MS of P-GO; IR spectra of graphite oxide and P-GO;
Raman spectrum of P-GO; TEM image of P-GO sheets; XRD
spectrum of graphite oxide; AFM images of chemically reduced
graphene. See DOI: 10.1039/c1jm00044f
6
040 | J. Mater. Chem., 2011, 21, 6040–6045
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1
9,20
Synthesis of 4-(octoxy)aniline
with acetone (2 times), detergent, deionized water (2 times), and
ethanol (2 times) in sequence, followed by oxygen-plasma
cleaning. The thermal evaporation of organic materials was
To a 100 mL flask were added 4-aminophenol (20 mmol, 2.18 g),
-chlorooctane (20 mmol, 2.98 g), NaOH (40 mmol, 2.76 g) and
1
ꢀ
4
carried out at a chamber pressure of 10 Pa. The thickness of
each layer was determined by a quartz thickness monitor.
PEDOT:PSS was spin-coated on ITO-coated glass at the speed
Aliquatꢀ 336 (1.0 mmol, 0.404 g). The mixture was stirred in Ar
at 60 C for 15 h. The dark brown mixture was diluted with
ꢁ
dichloromethane, filtered and then washed with deionized water
ꢁ
of 3000 rpm for 60 s and then dried at 120 C in vacuum for 1 h.
2 4
three times. The organic phase was dried over Na SO , concen-
P-GO was spin-coated on ITO-coated glass at the speed of
ꢁ
trated in vacuum to give dark brown oily residue. The crude
product was purified by chromatography (silica gel, ethyl
3
000 rpm for 30 s and then dried at 80 C for 1 h. The effective
2
size of the light-emitting diode was 16 mm .
ꢁ
acetate/petroleum ether (60–90 C) ¼ 1/3 as the eluent). The
Current–voltage–power measurements were performed
simultaneously using a Keithley 2400 Source meter and a New-
port 1830-C Optical meter equipped with a Newport 818-UV
silicon photodiode, respectively. All the measurements were
carried out under ambient atmosphere at room temperature.
1
product was a light brown solid (4.90 g, 77% yield). H NMR
(
300 MHz, CDCl
3
): d 6.63–6.76 (m, 4H), 3.87 (t, J ¼ 6.61 Hz,
2
H), 1.71–1.76 (m, 2H), 1.20–1.40 (m, 12H), 0.84–0.90 (m, 3H).
21,22
Synthesis of 4-(octoxy)benzenediazonium tetrafluoroborate
A flask charged with 4-(octoxy)aniline (10 mmol, 3.20 g) and HCl
solution (1 : 1, 6 mL) was heated and then cooled in an ice-bath
Characterization
ꢀ
1
XRD measurements were performed using a Philips PW1700 X-
ray diffractometer. TEM images were recorded on a JEM-1011
transmission electron microscope. AFM images were obtained
using SPI3800N probe station and SPA400SPM. Infrared
spectra were collected on a Shimadzu IRAffinity-1 in an atten-
uated total reflectance (ATR) mode. Thermogravimetry
measurements were recorded using a Perkin-Elmer-TGA 7.
Raman spectra were collected on a Horiba LabRAM HR 800
Raman spectrometer. Pyrolysis-GC/MS data were obtained
using a CDS 5000 with an Agilent 5975 GC/MS.
for 10 min. A solution of NaNO (2 mol L , 0.70 g of NaNO )
2
2
was added dropwise. The mixture was then stirred for 1 h, fol-
lowed by addition of the saturated solution of NaBF (1.7 g). The
4
resulting mixture was stirred for another 30 min in an ice-bath,
filtered, and washed with deionized water and cold methanol to
1
give light powder (2.27 g, 71% yield). H NMR (300 MHz,
CDCl
3
) d 8.51–8.59 (d, 2H), 7.16–7.23 (d, 2H), 4.15–4.23 (t, 2H),
1
.23–1.51 (10H), 0.84–0.94 (m, 3H).
Synthesis of graphite oxide
Graphite oxide was synthesized using a modified procedure of
23
3. Results and discussion
Hummers and Offeman. Graphite (1.0 g, Alfa Alsar, 325 mesh)
and NaNO (1.5 g) and concentrated sulfuric acid (25 mL) were
charged to a 250 mL flask that was placed in an ice bath, followed
24–27
3
Graphene oxide can be readily modified by either covalent
28–31
or
non-covalent
bonding methods. By covalent bonding
4
by addition of KMnO (4.5 g) slowly. The mixture was allowed
through the carboxylic acid groups via the esterification or
acylation reactions, the degree of functionalization is dependent
on the number of the carboxylic acid groups available at the
edges of GO sheets and is too low to impart a meaningful solu-
bility to the modified GO. In addition to functionalization on the
edges, a single-layer GO sheet has two surfaces accessible to
chemical modification. The surface grafting with a large number
of long-chain molecules can be expected to effectively diminish
the van der Waals force between the GO sheets, which should
improve the solubility in organic solvents. For bi-layer and
multi-layer GO sheets, since only the edges and both sides are
available for functionalization, the resulting products would
have a lower solubility than single-layer GO sheets.
ꢁ
to warm up to 35 C for 30 min and then stirred at room
temperature for another 2 h. The brownish grey paste was then
diluted with deionized water (500 mL) with caution. The
suspension was then further diluted to a total volume of 1.5 L
with deionized water and H O (5 mL, 30%) and then filtered.
2
2
The brown cake was dispersed in a solution of 3 wt% H SO /0.5
2
4
2 2
wt% H O (200 mL) and then filtered. This washing process was
repeated for 5 times and the product was washed with deionized
water repeatedly until the filtrate was pH 7. After drying
ꢁ
in vacuum at 50 C for 5 h, GO was obtained as brown powder
(
1.6 g).
Preparation of P-GO
In our work, freshly prepared GO was simply treated with 4-
octoxyphenyldiazonium tetrafluoroborate at room temperature
for 10 to 60 min to yield the phenylated graphene oxide (P-GO),
similar to a reaction of graphene or expanded graphite with
ꢀ1
Aqueous solution of as-prepared GO (1 mg mL , 60 mL) was
sonicated and then reacted with 4-octoxyphenyldiazonium
tetrafluoroborate at room temperature for a given time (e.g., 10–
32
aryldiazonium salts. The solubility of P-GO is very much
dependent on the degree of functionalization or reaction time.
The higher the solubility is, the higher the degree of grafting or
functionalization of GO should be. The number of octoxyphenyl
groups that can be introduced into GO seems to reach
a maximum after 60 minutes of the reaction, which gives rise to
6
0 min). The suspension was filtered through a PTFE membrane
(
0.45 mm), copiously washed with acetone and DMF, and dried
ꢁ
in vacuum at 60 C for 5 h to yield P-GO (70–90 mg).
Device fabrication and measurement
ꢀ
1
The OLEDs were fabricated by thermal vacuum deposition on
prepatterned ITO-coated glass substrates with sheet resistances
of 10 ohms per square. The substrate was ultrasonically cleaned
P-GO (sample C) with the highest solubility of 1.8 mg mL in
DMF ever reported to date. At the reaction time of 10 min and
30 min, the two different P-GO samples (A and B) were obtained
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ꢀ1
with a solubility of 0.6 mg mL in DMF for sample A and
ꢀ1
1
.0 mg mL for sample B, respectively.
It was found that all the P-GO samples can be dissolved in
DMF with only gentle stirring at room temperature, without aid
of sonication or surfactant and ionic liquid, and the solution is
stable for more than 2 months without any precipitation. It is
known that sonication over a long time may break up and crumb
GO sheets to small particles, which would adversely affect the
device performance. Thus, high solubility of P-GO offers a better
chance to obtain and maintain the large-size GO sheets. To
ensure the reproducibility, the starting GO was prepared under
the same conditions from batch to batch. As a result, the P-GO
produced in each batch showed similar characteristics and
properties.
The success of the grafting reaction or the presence of the
octoxyphenyl groups in P-GO was first confirmed by thermog-
ravimetric analysis. Thermogravimetry (TG) in nitrogen atmo-
ꢁ
sphere was performed up to 800 C on P-GO samples (A and B)
and the parent graphite oxide for comparison. The weight loss of
ꢁ
graphite oxide is ꢂ57% at 800 C (trace C, Fig. 1), similar to the
24,33,34
reported value,
and is less than the overall weight loss of
P-GO samples A (62%) and B (68%). The extra weight loss from
P-GO is clearly attributed to the presence of the octoxyphenyl
groups, since it is reasonable to believe no residual octoxyphenyl
groups are still attached to GO at this high temperature.
Accordingly, P-GO samples A and B contain ꢂ13% and ꢂ34% of
ꢁ
the octoxyphenyl groups. The initial weight loss below 150 C is
due to adsorbed water or DMF in three samples. P-GO was
ꢁ
further characterized by pyrolysis-GC/MS at 250 C, which
confirms the presence of the octoxyphenyl groups in P-GO
(
Fig. S1 in the ESI†).
Fig. 2 (A) Survey XPS spectrum and (B) deconvoluted C1s spectrum
The P-GO samples were then characterized by IR in
(
lines) of P-GO sample B.
comparison with graphite oxide (Fig. S2 in the ESI†). The new
ꢀ1
peaks at 1248 and 1105 cm are assigned to the ether moiety of
the octoxyphenyl groups. The Raman spectrum (633 nm laser
excitation) of P-GO sample B (Fig. S3 in the ESI†) displays two
band at ꢂ2640 cmꢀ is indicative of the sp network in P-GO. The
1
2
peaks shift to lower frequency because of lower incident laser
35–39
ꢀ
1
excitation energy and the substrate effect.
bands at 1327 and 1573 cm corresponding to the D (disorder
band) and G (in-phase vibration) bands, respectively. The D-
band indicates a large disorder caused by unreduced oxygen
groups in GO and introduction of the aryl groups. The weak 2D
Fig. 3 AFM images of P-GO (sample B) films spin-coated on freshly
cleaved mica. (A) 20 ꢃ 20 mm, (B) 5 ꢃ 5 mm, (C) height profile of P-GO
sheets in (B).
Fig. 1 Thermogravimetric traces of P-GO sample A (trace A), sample B
(trace B) and graphite oxide (trace C) in nitrogen.
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042 | J. Mater. Chem., 2011, 21, 6040–6045
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Finally, the composition of P-GO was analyzed by X-ray
photoelectron spectroscopy (XPS). As shown in Fig. 2, the high
intensity of O1s is due to the residual oxygen from GO together
with oxygen from the octoxyphenyl moiety. The deconvoluted
C1s spectrum (Fig. 2B) shows various types of carbons, which
was imaged by AFM using a tapping mode (Fig. 3). The height of
all the films is between 0.8 and 1.2 nm and falls in an acceptable
16,41,42
height range for modified single-layer graphene.
Therefore,
the spin-coated P-GO films are all-single-layer sheets. The AFM
images further indicate that the size of the all-single-layer P-GO
40
represents various functional groups in P-GO.
a
sheets is up to 1 mm and the average R of the P-GO film over an
2
The films of P-GO readily form on various substrates (e.g.,
glass, mica and ITO) by spin-coating of its solution in DMF. The
topography of P-GO film (sample B) cast from the DMF solu-
area of 20 ꢃ 20 mm is ca. 0.81 nm. The morphology of the P-GO
sheets was further revealed by transmission electron microscopy
(TEM) to be highly transparent, largely stacked and wrinkled
(Fig. S4 in the ESI†). The XRD spectrum of graphite oxide
ꢀ
1
tion with a concentration of 1.0 mg mL on freshly cleaved mica
Fig. 4 Structure of the OLED device. (A) The C–V–L characteristics of OLED with the P-GO layer (sample B) as a hole-injecting buffer layer. (B)
Power efficiency. (C) Current efficiency. (D) Luminance.
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ꢁ
(
Fig. S5 in the ESI†) displays a peak at 10.2 and the interlayer
Table 1 Characteristics of the OLEDs using PEDOT or P-GO
ꢀ
spacing is calculated to be 8.66 A. However, there is no peak or
layered structures for P-GO, which implies the single-layer P-GO
sheets. Therefore, the surface grafting imparts high solubility and
low interaction to graphene oxide, which in turn promotes the
formation of large-area and single-layer sheets.
Current Power
Operating efficiency/ efficiency/
HIB
Device layer
ꢀ1
ꢀ1
Thickness voltage/V
cd A
lm W
a
b
c
1
2
P-GO
P-GO
P-GO
1 nm
nm
1 nm
2 nm
1 nm
8.1–8.3
7.7–7.9
2.5–2.7
2.6–2.8
4.3–4.5
2.8–3.0
21
23
23
20
20
15
6
6
14
13
11
11
2
Since single-layer P-GO sheets are readily formed, subsequent
reduction is expected to afford single-layer graphene sheets.
Thus, the spin-coated P-GO films on freshly cleaved mica were
reduced with hydrazine hydrate vapor. The change in the
chemical structure after reduction is evident by the changes in the
3
4
PEDOT 40 nm
a
b
c
ꢀ1
ꢀ1
ꢀ1
Sample and concentration of P-GO (in DMF): sample A, 0.6 mg mL
Sample and concentration of P-GO (in DMF): sample B, 1.0 mg mL
Sample and concentration of P-GO (in DMF): sample C, 1.8 mg mL
.
.
.
ꢀ1
IR spectra (1000–1500 cm , Fig. S2†). By AFM analysis, the
average height of as-prepared graphene sheets was in a range of
0
.6–0.9 nm (Fig. S6 in the ESI†), which significantly decreased
from the original height of 0.8–1.2 nm for the P-GO films,
strongly suggesting the formation of single-layer graphene.
2
P-GO is an insulator because of the disrupted sp conjugation
adequate degree of surface grafting and the use of water-free
organic solvent in the fabrication process. For the effect of P-GO
thickness, the operating voltage increased and both current and
power efficiency went down for the device with a 2 nm thick HIB
layer in comparison with 1 nm thickness of P-GO (Table 1). The
increase in operating voltage is due to an increase in serial
resistance caused by an increase in thickness, which has been
2
of the graphene lattice. However, the residual sp clusters in P-
GO can still allow for hole or electron transport to occur at the
Fermi level by hopping. Taking a fairly large resistance of P-GO
into account, it is conceivable to use P-GO as a HIB layer in
OLEDs. Accordingly, the green light-emitting diodes were
fabricated using DMF-soluble P-GO samples, in order to
investigate its usefulness as an alternative to PEDOT and the
structural effect that the P-GO sheets may have due to a different
degree of surface grafting on the device performance. The four
OLED devices having three P-GO samples with different solu-
15
observed and explained before. The solubility or degree of
surface grafting of P-GO is another factor that can affect the
device performance, due to the arrangement of and space
between P-GO sheets. For P-GO sample B, there are some
overlaps and small gaps as seen in Fig. 3, which would allow
efficient hole hopping through and between P-GO sheets.
ꢀ1
bilities (0.6, 1.0, and 1.8 mg mL in DMF) and PEDOT:PSS as
a HIB were fabricated, all with a device configuration of ITO/
ꢀ1
However, for sample A with a lower solubility (0.6 mg mL ), the
P-GO sheets are deposited too much apart; while the P-GO
sheets are mostly stacked for sample C with a higher solubility
HIB/NPB (70 nm)/(Alq
37.5 nm)/LiF (1 nm)/Al (100 nm) (Fig. 4), where NPB is 1,4-
bis[(1-naphthylphenyl)amino)]biphenyl, Alq is tris(8-hydrox-
yquinolinato)aluminium and C545T is 10-(2-benzothiazolyl)-
,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)benzopyr-
3 3
) : C545T (1 : 0.8 wt%, 37.5 nm)/Alq
(
ꢀ1
(1.8 mg mL ) (Fig. S7 in the ESI†). The large gaps between the
P-GO sheets on the ITO electrode make hole injection/transport
less efficient and more ITO defect sites left uncovered; while the
sheet stacking is likely to increase the resistance, which will
adversely affect the device performance. Evidently, using P-GO
sample A as a HIB, the device with a 2 nm thick P-GO layer
performs better than the one with a 1 nm thick P-GO layer,
because the 2 nm thick layer consists of two layers of P-GO
sheets and has fewer gaps.
3
2
opyrano(6,7-8-I,j)quinolizin-11-one.
Since all the variables except for the HIB layer are fixed, the
device performance can be correlated to the type of HIB used and
its layer thickness in the OLEDs. The PEDOT layer was
deposited by spin coating with a thickness of 40 nm as usual. For
P-GO, the layer thickness was controlled to be 1 nm and 2 nm.
Since a single-layer P-GO sheet has a thickness between 0.8 and
1
.2 nm or an average of ꢂ1 nm, the 1 nm or 2 nm thick HIB layer
in the device corresponds to one layer or two layers of P-GO
sheets, respectively. It should also be noted that the in-plane gaps
and stacks between the P-GO sheets deposited on the electrode
depend on the solubility of each of P-GO samples and its
concentration in DMF, which in turn can affect the device
performance (Fig. 3). Under the same deposition conditions, the
saturated DMF solution of sample C led to a film consisting of
stacked P-GO sheets without any gaps (Fig. S7 in the ESI†).
The characteristics of the device with P-GO (sample B) are
shown in Fig. 4, and the characteristics of all the devices are
summarized in Table 1. The devices with 1 nm (round symbols)
and 2 nm (triangle symbols) P-GO layers (sample B) work
noticeably better than the one with PEDOT (square symbols) in
terms of operating voltage, current efficiency and power effi-
ciency (Fig. 4). The improvement in the device performance can
be attributed to the unique hole-transport and electron-blocking
properties of single-layer, large-area P-GO sheets having an
4. Conclusions
In summary, we have used a simple surface-grafting method for
the preparation of the organo-soluble all-single-layer graphene
oxide. With a high solubility in organic solvent, the modified GO
is able to form single-layer large sheets that can either be used as
an alternative hole-injecting buffer to PEDOT:PSS in optoelec-
tronic devices or as a precursor to single-layer graphene. The
OLED device having a single-layer graphene oxide performs
better than the one with PEDOT.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21074132, 20920102032 and 20834001).
6
044 | J. Mater. Chem., 2011, 21, 6040–6045
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