Synthesis of Poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] Copolymer and Its Potential as Hole-Transporting Material to Solid-State Dye-Sensitized Solar Cells

Article abstract of DOI:10.1080/15421406.2019.1597528

In this study, thermally stable and solution-processable conjugated poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer containing alternating carbazole and triarylamine moieties in the polymer backbone was synthesized in a good yield by Pd-catalyzed polycondensation. The polymer exhibited maximum UV-vis absorption peak at 309 nm and maximum photoluminescence peak at 452 nm. From the optical and electrochemical characterization, the band gap, HOMO, and LUMO energy levels of the polymer were measured to be 2.91 eV, -5.19 eV, and -2.28 eV, respectively. The polymer was used to examine the hole-transporting performance to a solid-state dye-sensitized solar cell (ssDSSC). The ssDSSC fabricated with poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer showed a PCE of 1.50% with J_sc = 3.76 mA/cm², V_oc = 538 mV, and FF = 74.3%, implying the potential of the synthesized polymer as a hole-transport material (HTM) to ssDSSC.

Full text of DOI:10.1080/15421406.2019.1597528

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Synthesis of Poly[N-(2-ethylhexyl)-3,6-
carbazole-alt-aniline] Copolymer and Its Potential
as Hole-Transporting Material to Solid-State Dye-
Sensitized Solar Cells
Van Nga Nguyen & Younghwan Kwon
To cite this article: Van Nga Nguyen & Younghwan Kwon (2019) Synthesis of Poly[N-(2-
ethylhexyl)-3,6-carbazole-alt-aniline] Copolymer and Its Potential as Hole-Transporting Material to
Solid-State Dye-Sensitized Solar Cells, Molecular Crystals and Liquid Crystals, 678:1, 53-61, DOI:
10.1080/15421406.2019.1597528
To link to this article: https://doi.org/10.1080/15421406.2019.1597528
Published online: 29 Jul 2019.
Submit your article to this journal
Article views: 2
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=gmcl20

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
019, VOL. 678, NO. 1, 53–61
2
https://doi.org/10.1080/15421406.2019.1597528
Synthesis of Poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline]
Copolymer and Its Potential as Hole-Transporting Material
to Solid-State Dye-Sensitized Solar Cells
Van Nga Nguyen and Younghwan Kwon
Department of Chemical Engineering, Daegu University, Gyeongsan, Gyeongbuk 38453, Republic
of Korea
ABSTRACT
KEY WORDS
carbazole; hole transport;
ssDSSC; triarylamine
In this study, thermally stable and solution-processable conjugated
poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer containing
alternating carbazole and triarylamine moieties in the polymer back-
bone was synthesized in a good yield by Pd-catalyzed polycondensa-
tion. The polymer exhibited maximum UV-vis absorption peak at
3
09nm and maximum photoluminescence peak at 452nm. From the
optical and electrochemical characterization, the band gap, HOMO,
and LUMO energy levels of the polymer were measured to be
2
.91 eV, -5.19 eV, and -2.28 eV, respectively. The polymer was used to
examine the hole-transporting performance to a solid-state dye-sen-
sitized solar cell (ssDSSC). The ssDSSC fabricated with poly[N-(2-ethyl-
hexyl)-3,6-carbazole-alt-aniline] copolymer showed a PCE of 1.50%
2
with Jsc ¼ 3.76 mA/cm , V ¼ 538mV, and FF ¼ 74.3%, implying
oc
the potential of the synthesized polymer as a hole-transport material
(HTM) to ssDSSC.
1. Introduction
Dye-sensitized solar cells (DSSCs), generally composed of a photoelectrode, a liquid
electrolyte, and a Pt counter electrode, are considered to be a promising candidate as a
large-area and low-cost renewable energy source, due to their various advantages, such
as reasonable photovoltaic efficiency, low production cost, brief fabrication process,
semi-transparency, and flexibility [1–5]. Power conversion efficiency (PCE) of 14.3%
has been obtained with a cobalt (II/III) complex redox liquid electrolyte [6]. Despite the
fact that considerable progress has been made in the field of DSSCs, the risk of leakage
of volatile liquid electrolyte limit significantly the commercialization of DSSCs.
Significant efforts have been made on solid-state dye-sensitized solar cells (ssDSSCs)
to replace former liquid electrolyte system, as well as the development of new HTMs
with low cost, facile synthesis and high charge-carrier mobility has been of high priority
[7]. Many organic HTMs were originally developed for use in xerography and then in
organic light-emitting diodes (OLEDs). One of the most common classes of materials
0
0
0
0
was arylamines, such as N,N -diphenyl-N,N -bis(3-methylphenyl)-1,1 -biphenyl-4,4 -
0
0
diamine (TPD) and N,N -diphenyl-N,N -bis(a-naphthyl)benzidine (NPB) [8, 9].
CONTACT Younghwan Kwon
ß 2019 Taylor & Francis Group, LLC
y_kwon@daegu.ac.kr

5
4
V. N. NGUYEN AND Y. KWON
Moreover, spiro-type arylamines showed higher PCE of 7.2% when employed as HTMs
in ssDSSC [10]. Carbazole-based materials have attracted much attention as another
type of HTMs. Up to now, many different types of carbazole-structured materials
have been intensively utilized in OLEDs because of their good hole-transporting
function [11]. The versatile chemical modification of carbazole derivatives was also
exploited in the molecular design of new types of HTMs in OLEDs.
Herein, we report on the synthesis of a solution-processable conjugated copolymer
composed of alternating carbazole and triarylamine moieties, because both of carbazole
and triarylamine groups are known to have hole-transporting performance as men-
tioned previously. And then, the synthesized copolymer is applied on the preliminary
investigation of its potential as a HTM to ssDSSCs.
2
. Experiment
Materials
-Ethylhexylbromide (97%), 3,6-dibromocarbazole (97%), aniline (99.5%), sodium-tert-
butoxide (NaO-t-Bu, 97%), tris(dibenzylidene-acetone)dipalladium(0) (Pd (dba) ), TiCl ,
2
2
3
4
and tri-tert-butylphosphine (P(t-Bu) , 90%) were purchased from Sigma-Aldrich Chemical
3
Co. Fluorine-doped tin oxide (FTO; sheet resistance ꢀ 7 X/square) glass (TCO22-7),
TiO paste for the photoelectrode (Ti-nanooxide T/SP), TiO paste for the scattering layer
2
2
(
(
Ti-nanooxide R/SP), N719 dye (Ruthenizer 535-bisTBA), and hot-melt adhesive
SX1170-60PF, Surlyn) were obtained from Solaronix. Platinum paste (PT-1, Dyesol-
Timo) was selected as the source for the Pt counter-electrode. Lithium bistrifluoro
methane sulfonamide (LiTFSI), 1-methyl-3-propylimidazolium iodide (MPII), and 4-tert-
butylpyridine (TBP) used as dopants were brought from Sigma-Aldrich Chemical Co. All
other chemicals used in the study were reagent grade and used as received.
Synthesis of N-(2-Ethylhexyl)-3,6-dibromocarbazole
A mixture of 3,6-dibromocarbazole (18.46 mmol) and K CO (37 mmol) in DMF
2
3
ꢁ
(
(
20 ml) was refluxed in N2 atmosphere and heated to 80 C. 2-Ethylhexylbromide
28 mmol) was added dropwise. After complete addition, the reaction was continued for
16 hrs. The product was isolated by diethylether/water extraction. The organic layer was
dried with MgSO , then filtered and concentrated. The viscous liquid N-(2-ethylhexyl)-
4
3
,6-dibromocarbazole was obtained by column chromatography (eluent: hexane). Yield
1
13
5
6%. Figure 1 shows H and C NMR spectra of N-2-ethylhexyl-3,6-dibromocarbazole.
1
H NMR (300 MHz in CDCl , d): 8.11 (d, 2H, Ar-H), 7.48 (d, 2H, Ar-H), 7.22 (d, 2H,
3
Ar-H), 4.18 (m, 2H, -N-CH -), 1.23 ꢀ 1.44 (m, 9H, -CH-, -CH -), 0.91 (m, 6H, -CH ).
2
2
3
1
3
C NMR (60 MHz in CDCl , d): 139.7, 129.0, 123.4, 123.2, 112.1, 110.7, 47.6, 39.5,
3
31.1, 29.0, 24.6, 23.3, 14.3, 11.1.
Polymerization of poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer
The polymerization was carried out via Pd-catalyzed polycondensation reaction. A mix-
ture of N-(2-ethylhexyl)-3,6-dibromocarbazole (2.58 mmol) and aniline (2.58 mmol) was

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
55
1
13
Figure 1. H and C NMR spectra of N-2-ethylhexyl-3,6-dibromocarbazole.
dissolved in toluene (25 ml). NaO-t-Bu (7.74 mmol), Pd (dba) (0.065 mmol), and P(t-
2
3
Bu) (0.39 mmol) were added to the solution at room temperature. The reaction mixture
3
ꢁ
was stirred at 100 C for 48 hrs. After cooling to room temperature, the mixture was
quenched by adding aqueous ammonia (50 ml) and the product was extracted with
CHCl . The organic fraction was concentrated and reprecipitated from CHCl /methanol
3
3
several times.
ssDSSC fabrication
The FTO glasses were cleaned in a detergent solution using a sonication for 20min, and
then thoroughly rinsed with deionized water and ethanol. After the ultrasonic cleaning,
ꢁ
the FTO glasses were immersed in a 40-mM TiCl solution at 70 C for 30min, and then
4
washed with deionized water and ethanol. One active TiO layer, which formed on the
2
FTO glass, was prepared via doctor-blade coating with the TiO paste. An additional
2
TiO layer consisting of particles with around 400 nm diameter was deposited on the
2
ꢁ
active TiO layer and then calcinated at 500 C for 60hr to produce the scattering layer.
2
ꢁ
The TiO films were again treated with a 40mM TiCl solution and annealed at 500 C
2
4
for 60 hr; thus, TiO /FTO electrodes with the scattering layer were prepared. Finally, the
2
TiO /FTO photoelectrodes were immersed into 0.5 mM of N719 dye solution (aceto-
2
nitrile/tert-butyl alcohol ¼ 1/1, v/v) for 12hrs to obtain working electrodes.
To prepare the counter-electrode, two holes were formed in the FTO glass with a
drill, and then cleaned with the method described above. A Pt layer was formed on the
FTO glass via the doctor-blade method using Pt paste, followed by a calcination process

5
6
V. N. NGUYEN AND Y. KWON
ꢁ
at 400 C for 30 min. The thermally treated Pt counter-electrodes were placed on the
photoelectrodes and sealed with a 25-lm-thick sealing material. The solution of the pol-
ymer:dopant (3:1 v:v) was introduced into the cells through one of the two small holes
2
drilled on the counter-electrodes to produce ssDSSCs with a 25 mm active area, and
ꢁ
dried under vacuum at 50 C for 1 hr. This process was repeated 3 times. Hole-only
device [12] was fabricated with a configuration of ITO/PEDOT:PSS (30 nm)/HTM
(100 nm)/Au (70 nm), where Au with a high work function was used as the cathode to
keep the injection of electrons away from the cathode.
Instruments
1
13
H and C NMR spectra were recorded using a Varian Unity Plus 300 spectrometer, and
chemical shifts were recorded in ppm. Number and weight average molecular weights
were measured by Waters gel permeation chromatography (GPC) equipped with Styragel
ꢁ
HR 5E column using THF as an eluent against polystyrene standards at 40 C. Thermal
gravimetric analysis (TGA) was performed under N atmosphere at a heating rate of
2
ꢁ
2
0 C/min with a TG/DTA320 thermal analyzer. The decomposition temperature (T )
d
was taken as the temperature at which a 5% weight loss occurred. UV-visible absorption
spectra were recorded on a Shimadzu UV-2100 spectrophotometer, and the photolumi-
nescence (PL) spectra were made on a Shimadzu RF-5301PC fluorimeter. The ionization
potential (I ) was measured by photoelectron spectroscopy (Riken Keiki AC-2). The
p
photocurrent-voltage measurement was performed using a CompactStat (Ivium
Technologies B.V.) potensiostat and a PEC-L01 solar simulator system equipped with a
1
50 W xenon arc lamp (Peccell Technologies, Inc.). The light intensity was adjusted to 1
2
sun (100 mW/cm ) with a silicon photodiode (model PEC-SI01, Peccell Technologies,
Inc.). The active area of the dye-adsorbed TiO films was estimated using a digital micro-
2
scope camera (OLYMPUS SZ61) with image analysis software.
3. Result and discussion
Synthesis and characterization of poly[N-(2-ethylhexyl)-3,6-carbazole-alt-anil-
ine] copolymer
Synthetic route to the carbazole/triarylamine-based alternating poly[N-(2-ethylhexyl)-
3,6-carbazole-alt-aniline] copolymer prepared by Pd-catalyzed polycondensation is pre-
sented in Figure 2. The alternating copolymer was synthesized in high yields. According
to its structure, poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer possessed
basic structure of alternating carbazole/triarylamine polymers, in which triarylamine
moiety was linked to 3,6-disubstituted carbazole groups. N-2-Ethylhexyl group on the
carbazole in the main chain of the alternating copolymer was introduced to impart high
solubility in common organic solvent, which was required for a solution process.
Therefore, the copolymer exhibited excellent solubility in common organic solvents
such as toluene, benzene, chloroform, TCE, and THF. In addition, the polymer showed
good film forming quality from a spin coating process.
Results on physical properties of the synthesized alternating copolymer such as
molecular weight, molecular weight distribution, and decomposition temperature are

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
57
Figure 2. Reaction scheme to poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer.
Table 1. Physical and Optical Properties of Poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer
k
max,UV (nm)
kmax,PL (nm)
Bandgap
energy (eV)
HOMO
(eV)
LUMO
(eV)
o
a
b
c
d
M
w
(g/mole)
M
w
/M
n
T
d
( C)
solution
309
film
310
solution
452
film
453
1
,790
1.40
442
2.91
ꢂ5.19
ꢂ2.28
a
temperature at 5 wt% loss based on the initial weight
b
calculated from the crosspoint of UV-Vis and PL spectra
measured by a RIKEN Keiki AC-2
estimated from the HOMO and band gap
c
d
summarized in Table 1. As shown in Figure 3, the polymer showed molecular weight
M ) of 1,790 g/mole with a molecular weight distribution of 1.40. Thermal properties
(
w
of the copolymer were determined by using TGA and DSC measurements. The polymer
ꢁ
showed good thermal stability with decomposition temperature (T ) of 442 C at 5 wt%
d
loss under a nitrogen atmosphere, as presented in Figure 4. Over the decomposition
temperature, the weight loss curves of the polymer fell rapidly, indicating the decom-
position of the backbone of the polymer. In DSC measurement, no melting temperature
ꢁ
and obvious glass transition temperature could be observed below 400 C, suggesting
the polymer was virtually amorphous polymeric material.
Optical properties such as UV-visible absorption maximum (kmax,UV) and PL emis-
sion maximum (kmax,PL) are also summarized in Table 1. As presented in Figure 5, the
polymer presented maximum PL emission peak at 442 nm with narrow bandwidth.
From photophysical study, band gap, HOMO, and LUMO energy levels of the polymer
were measured as follows; 2.91 eV, -5.19 eV, and -2.28 eV, respectively. In this paper, we
preliminary study on the feasibility of the synthesized alternating copolymer for hole-
transporting layer in ssDSSC.

5
8
V. N. NGUYEN AND Y. KWON
0
5
10
15
20
25
30
35
40
Elution volume (mL)
Figure 3. GPC chromatogram of poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer.
1
00
80
60
40
20
0
0
100 200 300 400 500 600 700 800
o
Temperature
(
C
)
Figure 4. TGA thermogram of poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer.
Performance of ssDSSC with polymers as a HTM
Space charge limited current (SCLC) measurement was used to evaluate charge mobility
under steady state conditions of current in organic layers [12–14]. A hole mobility study
of the device was conducted using the SCLC method. The current density using the
SCLC model is given by Eq. (1)
rffiffiffiffi
ln JSCLCL3 0:89
V
L
ln 9
¼
pffiffiffiffiffi
þ
e e l
r 0
(1)
0
V2
8
E
0
where e is the electric constant (assumed to be 3, which is a typical value for conju-
r
gated polymers) of the polymer, e is the permittivity of free space, lh0 (or l ) is the
0
e0
zero-field hole (or electron) mobility, L is the film thickness, E is the characteristic
0
field, and V ¼ Vappl – (V þ V ); where V
is the applied voltage to the device, V is
r
r
bi
appl

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
59
UV absorption
PL emission
3
00
400
500
600
700
800
Wavelength (nm)
Figure 5. UV-visible absorption and PL emission spectrum of poly[N-(2-ethylhexyl)-3,6-carbazole-alt-
aniline] copolymer.
-
-
-
-
-
39
40
41
42
43
1
00
200
300
400
500
0
.5
0.5
(V/L) (V/cm)
3
2
0.5
Figure 6. ln(JL /V ) vs. (V/L) plot constructed from J-V characteristics of hole-only device with a
configuration of ITO/PEDOT:PSS (30nm)/HTM (100 nm)/Au (70 nm).
the voltage drop due to series resistance across the electrodes, and Vbi is the built-
in voltage.
The hole mobility of the device was calculated from Eq. (1) using the J-V curve of
the hole-only device with the configuration of ITO/PEDOT:PSS (30 nm)/HTM
(100 nm)/Au (70 nm), which could prevent the electron injection from the cathode due
to the large mismatch between the LUMO energy level (2.18 eV) of the copolymer and
the work function (5.10 eV) of the Au cathode. In the graph (Figure 6) of the logarithm
3
2
of JL /V vs. the square root of V/L, the intercept of the line gives the hole mobility.

6
0
V. N. NGUYEN AND Y. KWON
Figure 7. Schematic ssDSSC device structure with a cross-sectional view.
1
0
8
6
4
2
0
0
.0
0.2
0.4
0.6
0.8
Voltage (V)
Figure 8. J-V characteristics of ssDSSC with poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer
as a HTM.
The hole mobility of the device with the poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline]
ꢂ5
2
copolymer was calculated to be 0.7 ꢃ 10 cm /Vs.
The ssDSSC device was fabricated with a structure as shown in Figure 7. Figure 8
shows the current density (J) and voltage (V) curve of the ssDSSC fabricated with
poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer as a HTM. The ssDSSC
2
showed a PCE of 1.50% with Jsc ¼ 3.76 mA/cm , V ¼ 538 mV, and FF ¼ 74.3%,
oc
implying the potential of the synthesized polymer as the HTM to ssDSSC.
4
. Conclusions
In conclusion, we have succeeded in synthesizing the conjugated poly[N-(2-ethylhexyl)-
,6-carbazole-alt-aniline] copolymer by Pd-catalyzed polycondensation reaction.
3
Common structural characteristics of the copolymer were alternating units of a carba-
zole and a hole-transporting triarylamine in the polymer backbone. The synthesized
copolymer was soluble in common organic solvents, and showed good film forming
and high thermal stability. From the optical and electrochemical characterization, the
polymer exhibited kmax,UV at 309 nm and kmax,PL at 452 nm with wide band gap
(2.91 eV) energy, HOMO (-5.19 eV) and LUMO (-2.28 eV) levels. The synthesized

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
61
conjugated poly[N-(2-ethylhexyl)-3,6-carbazole-alt-aniline] copolymer was applied to
ssDSSC as the HTM. The ssDSSC device showed a performance with Jsc ¼ 3.76 mA/
2
cm , V ¼ 538 mV, and FF ¼ 74.3%, leading to a PCE of 1.50%.
oc
Acknowledgement
This research work was supported in part by Daegu University Research Grant 2017.
References
[
[
1] O’Regan, B. & Gr €a tzel, M. (1991). Nature, 353, 737.
2] Hagfeldt, A., Boschloo, G., Sun, L., Kloo, L., & Pettersson, H. (2010). Chem. Rev., 110,
6595.
[3] Gong, J., Liang, J., & Sumathy, K. (2012). Renew. Sust. Energ. Rev., 16, 5848.
[4] Hawash, Z., Ono, L. K., & Qi, Y. (2018). Adv. Mater. Interfaces, 5, 1700623.
[5] Liu, P., Xu, B., Hua, Y., Cheng, M., Aitola, K., Sveinbjornsson, K., Zhang, J., Boschloo, G.,
Sun, L., & Kloo, L. (2017). J. Power Sources, 344, 11.
[
[
[
[
6] Kakiage, K., Aoyama, Y., Yano, T., Oya, K., Fujisawa, J., & Hanaya, M. (2015). Chem.
Commun., 51, 15894.
7] Xu, B., Sheibani, E., Liu, P., Zhang, J., Tian, H., Vlachopoulos, N., Boschloo, G., Kloo, L.,
Hagfeldt, A., & Sun, L. (2014). Adv. Mater., 26, 6629.
8] Bellmann, E., Shaheen, S. E., Grubbs, R. H., Marder, S. R., Kippelen, B. N., &
Peyghambarian, B. (1999). Chem. Mater., 11, 399.
9] Redecker, M., Bradley, D. D. C., Inbasekaran, M., Wu, W. W., & Woo, E. P. (1999). Adv.
Mater., 11, 241.
[
[
[
10] Burschka, J., Dualeh, A., Kessler, F., Baranoff, E., Cevey-Ha, N.-L., Yi, C., Nazeeruddin,
M. K., & Gr €a tzel, M. (2011). J. Am. Chem. Soc., 133, 18042.
11] Puckyte, G., Schmaltz, B., Tomkeviciene, A., Degbia, M., Grazulevicius, J. V., Melhem, H.,
Boucle, J., & Tran-Van, F. (2013). J. Power Sources, 233, 86.
12] Chivase, D., Chiguvare, Z., Knipper, M., Parisi, J., Dyakonov, V., & Hummelen, J. C.
(
2003). J. Appl. Phys., 93, 3376.
[
[
13] Shrotriya, V., Yao, Y., Li, G., & Yang, Y. (2006). Appl. Phys. Lett., 89, 063505.
14] Jeong, S., Han, Y. S., Kwon, Y., Choi, M.-S., Cho, G., Kim, K.-S., & Kim, Y. (2010). Syn.
Met., 160, 2109.