Thiophene-fused carbazole derivative dyes for high-performance dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2021.132124

Two novel dyes that are similar in chemical structure, except for different donor units, AJ301and AJ303 were synthesized, characterized and applied as sensitizers in dye-sensitized solar cells (DSSCs). Both dyes exhibited a wide absorption of visible sunlight. The introduction of fused rings on the donor unit of AJ303 presented an appropriate energy level, less recombination and longer electron lifetime to achieve a power conversion efficiency (PCE) of 10.2%, far above that achieved for AJ301 of 6.2% with a [Co(bpy)₃]^2+/3+-based electrolyte under standard AM1.5G solar irradiation (100 mW cm^?2). The DSSCs based on AJ303 and AJ301 with [Cu(tmby)₂]^2+/+-based electrolyte showed a lower PCE of 8.2% and 5.4%, respectively. Therefore, the results indicated that the introduction of a fused-ring in the donor group is a meaningful synthetic strategy to improve the photovoltaic performance.

Full text of DOI:10.1016/j.tet.2021.132124

Tetrahedron 88 (2021) 132124
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Thiophene-fused carbazole derivative dyes for high-performance dye-
sensitized solar cells
,
Jincheng An a, Xichuan Yang a *, Zhifeng Tian a, Bin Cai a, Li Zhang a, Ze Yu a, Xiuna Wang a,
,
c
Anders Hagfeldt b, Licheng Sun a
a State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUTꢀKTH Joint Education and Research Center on Molecular Devices, Dalian
University of Technology (DUT), Dalian, 116024, China
b
ꢀ
ꢀ ꢀ
Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne, CH-1015, Lausanne,
Switzerland
c Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, 100 44, Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel dyes that are similar in chemical structure, except for different donor units, AJ301and AJ303
were synthesized, characterized and applied as sensitizers in dye-sensitized solar cells (DSSCs). Both
dyes exhibited a wide absorption of visible sunlight. The introduction of fused rings on the donor unit of
AJ303 presented an appropriate energy level, less recombination and longer electron lifetime to achieve
a power conversion efficiency (PCE) of 10.2%, far above that achieved for AJ301 of 6.2% with a
[Co(bpy)3]2þ/3þ-based electrolyte under standard AM1.5G solar irradiation (100 mW cmꢀ2). The DSSCs
based on AJ303 and AJ301 with [Cu(tmby)2]2þ/þ-based electrolyte showed a lower PCE of 8.2% and 5.4%,
respectively. Therefore, the results indicated that the introduction of a fused-ring in the donor group is a
meaningful synthetic strategy to improve the photovoltaic performance.
Received 19 February 2021
Received in revised form
25 March 2021
Accepted 27 March 2021
Available online 8 April 2021
Keywords:
Novel carbazole derivative donor
Dye-sensitized solar cell
Efficient photovoltaics
© 2021 Elsevier Ltd. All rights reserved.
D-p-A-structured organic sensitizers
Cobalt electrolyte
1. Introduction
12%, which delays the rate for back electron transfer occurring at
the TiO2/electrolyte interface and leads to
a high voltage
Dye-sensitized solar cells (DSSCs), a third-generation photo-
voltaic technology, have attracted much attention, owing to low-
cost fabrication and stability under operating conditions, to delay
the exhaustion of fossil fuels. The DSSCs mainly consist of a
photosensitizer, electrolyte and counter electrode (CE) [1e3]. Since
approaching 1.0 V [14,15]. Meanwhile, metal-free organic dyes have
gradually attracted more attention owing to the high cost and low
yield of metal complexes, which normally give a donor (D)e
-bridge ( )eaccepter (A) structure. Wang et al. synthesized a series
of dyes, where N-annulated perylene was employed as a donor
p
p
€
group. These were applied as sensitizers in DSSCs with
a
the first report by Gratzel and co-workers of a power conversion
[Co(bpy)3]2þ/3þ redox couple to achieve a high PCE of 10%e13%
[16e18]. Yang et al. reported that for a sensitizer based on triaza-
truxene (TAT), named ZL003, the devices sensitized by ZL003 with
efficiency (PCE) of 7.1% [4], various types of dyes have emerged and
been applied as photosensitizers for dye-sensitized solar cells,
which have boosted the PCE to 14.2% on account of its extremely
vital role [5e9]. Metal complex dyes, which contain ruthenium
polypyridyl and zinc porphyrin complexes. The Ru-dye use of
precious metal is one of the biggest weaknesses, although zinc
porphyrin complexes present a high molar extinction coefficient (ε)
within their specific feature IR absorption region, they have in
common is a lower synthetic yield [10e13]. Yella et al. reported a
well-designed dye YD2-o-C8 to achieve an efficiency exceeding
a [Co(bpy)3]2þ 3þ
/
redox couple exhibited the highest PCE for a
€
single dye reported to date of 13.6% [19]. Gratzel and co-workers
designed and synthesized a series of dyes based on indacenodi-
thiophene (IDT), including named L350. The DSSCs based on L350
achieved a PCE of 11.2% with a high open-circuit voltage (>1.1 V) in
conjunction with a copper redox shuttle [20]. Recent work reported
by Anders Hagfeldt showed that a higher PCE performance can be
attributed to the copper redox shuttle considering the more posi-
tive redox potential compared with the traditional Co (II/III) (~0.56
vs NHE) and Iꢀ/Iꢀ3 (~0.4 vs NHE) electrolytes [21e24]. Therefore,
* Corresponding author.
E-mail address: yangxc@dlut.edu.cn (X. Yang).
https://doi.org/10.1016/j.tet.2021.132124
0040-4020/© 2021 Elsevier Ltd. All rights reserved.

J. An, X. Yang, Z. Tian et al.
Tetrahedron 88 (2021) 132124
Fig. 1. Molecular structure of AJ301 and AJ303.
more innovative work on the development of new photosensitizers
by adjustment to a suitable energy level (tuning mainly the HOMO
values) to further enhance the power conversion efficiency is
required.
2. Experimental
2.1. Basic synthesis process
Carbazole ramification is an excellent method to obtain a rigid
conjugated electron-rich aromatic ring, which can then be used in
organic thin-film transistors (OTFT), organic-light-emitting diodes
(OLEDs) and hole-transport materials (HTM) [25e28]. However,
carbazole-ramified photosensitizers have been applied in DSSCs,
but there are only a few reports where the PCE is high with a
classical redox electrolyte [29e32]. The traditional Iꢀ/I3ꢀ electrolyte
can perfectly and widely match a number of dyes to regenerate the
oxidized dyes, owing to the low redox potential, but this results in a
limit to obtain a high photovoltage. Until now, very few dyes with
carbazole ramification have been applied to DSSCs devices with
The synthetic routes towards AJ301 and AJ303 are illustrated in
Figure 1, 2, 3, 4, 5, 6, 7 and 8 (see Fig. 1). The two dyes included a
donor moiety, a
p -bridge and an accepter moiety, which were
linked by using Suzuki cross-coupling reactions. The acceptor-side
was synthesized according to the process given in a previous report
[40]. Cost-effective and readily available 2-bromo-9H-carbazole (1)
and 3-(bromomethyl) heptane with a branched alkyl chain were
used as starting materials to synthesize 2-bromo-9-octyl-9H-
carbazole (3) in a yield that reached approximately 90%. 9-Octyl-2-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole
(4)
was prepared by n-butyllithium and isopropoxyboronic acid pina-
col ester at ꢀ78 ꢁC. 2,3-dibromothiophene was combined with (4)
to generate 2,2’-(thiophene-2,3-diyl)bis(9-octyl-9H-carbazole) by
using Suzuki cross-coupling reactions. Thereafter, electron-rich (5)
two different redox shuttles, Co(bpy)23þ/3þ and Cu(tmby)þ2 /2þ
.
Fortunately, carbazole ramification dyes can successfully solve this
problem to be ready for commercialization in the future.
Recently, the introduction of aromatic-fused structures was
applied to achieve higher PCE values for DSSCs [33e36] and
perovskite solar cells [37e39] owing to the synthesis being simple
and easy. To the best of our knowledge, the incorporation of
substituted thiophene-fused carbazole cores is still rare. Herein,
two novel organic dyes, 2,2’-(thiophene-2,3-diyl) bis(9-(2-
ethylhexyl)-9H-carbazole) (TPC), employed as a donor unit and
named AJ301 with a twisty skeleton, and 11-((R)-2-ethylhexyl)-5-
carried out a lithium-halogen exchange with n-butyllithium
at ꢀ20 ꢁC and was converted into its corresponding boric acid ester
(6). The main roles of 1-bromo-4-hexylbenzene in the central core
were to eliminate the thiophene active site and decrease the
stacking. The subsequent anhydrous iron (III) chloride oxidative
cyclization formed a -extended electron-rich donor intermediate
p e p
p
8-(4-hexylphenyl)-5,11-dioctyl-5,11-dihydrothieno[20,3’:5,6]benzo
[1,2-b:4,3-b’]dicarbazole (9) at mild synthetic conditions. The N
atom at the para-position of the fused carbazole was brominated
with N-bromosuccinimide (NBS), and the para bromo group of (10)
was converted to a para boryl group by a palladium-catalyzed re-
action. The electron-donating moiety (6 or 11) was further coupled
(2-ethylhexyl)-8-(4-hexylphenyl)-5,11-dihydrothieno
[20,3’:5,6]
benzo[1,2-b:4,3-b’] dicarbazole (DBD), employed as a donor unit
and named AJ303 with a coplanar and rigid skeleton, are investi-
gated. 4,7-bis(4-hexylthiophen-2-yl) benzo[c] [1,2,5] thiadiazole
(BTBT) as a
benzoic acid as an accepter unit, were both appointed to the two
dyes. We also introduced alkyl chains into the donor unit and
p
-bridge, used to better harvest visible sunlight, and
to the p-bridgeeaccepter to afford the esterified dyes by the Suzuki
coupling reaction. The two novel dyes were then successfully ob-
tained with high yields through basic hydrolysis. Detailed synthetic
procedures are given in the Supporting Information.
p
-bridge to increase the solubility in solvents and reduce intermo-
lecular packing. The effect on the optical and electrochemical
properties as well as the application as a sensitizer in devices, with
the two dyes as different donor units, were investigated. The ideal
match of the HOMO energy level with a cobalt electrolyte was the
perfect combination for the electron-donating ability of carbazole
with a cobalt redox shuttle, which achieved a PCE of 10.2% and 6.2%
for AJ303 and AJ301 in Fig. 1, respectively, and, combined with a
copper redox shuttle, achieved a PCE of 8.2% and 5.4%, respectively.
Additionally, the fusion of thiophene used in the donor units may
boost the photovoltaic performances.
2.2. Solar cell fabrication
For the preparation of the TiO2 photoanode, the glass substrates
of fluorine-doped tin oxide (FTO) were rinsed with water and
sonicated, and then were cleaned sequentially in a dishwashing
liquid solution, distilled water, ethanol and acetone for 20 min.
Finally, the FTO glass underwent UV-ozone treatment for 30 min to
remove the residual organics. The clean FTO glass was treated two
times by a TiCl4 solution (40 mM) at 70 ꢁC for 35 min, which formed
2

J. An, X. Yang, Z. Tian et al.
Tetrahedron 88 (2021) 132124
Fig. 2. Synthetic routes towards organic dyes AJ301 and AJ303.
a homogeneous and tight thin TiO2 film underlayer to reduce the
backward electron transfer in the device. Each of the formed TiO2
films were then sintered at 500 ꢁC for 40 min. On the cooled sin-
tered TiO2 glass was printed three layers of 18NR-T as the adsorp-
tion layer and one layer of TPP200 as the light-scattering layer.
Every layer was solidified with the TiO2 paste before printing the
next layer, which was heated at 135 ꢁC for 5 min in a dryer, followed
by sintering the complete TiO2 layer again at 500 ꢁC for 40 min. The
complete working electrode was prepared by immersion in a TiCl4
(40 mM) deionized water solution at 70 ꢁC for 40 min, which
increased the surface area of the TiO2 particles and tuned the
conduction band (CB) of TiO2. The electrodes were washed with
water, followed by sintering at 500 ꢁC for 40 min. After cooling to
70 ꢁC, the TiO2 photoanode was immersed in a solution of the dye
3

J. An, X. Yang, Z. Tian et al.
Tetrahedron 88 (2021) 132124
Fig. 3. Uvevis spectra of AJ301 and AJ303 measured (a) in dichloromethane and (b) on a TiO2 film.
Table 1
Optical and electrochemical parameters of AJ301 and AJ303.
εa (Mꢀ1cmꢀ1
)
lmaxb (nm)
HOMOc (V)
Eopt d (eV)
LUMO
(V)
Eg (DFT)f
(eV)
HOMO (DFT)f
(eV)
LUMO (DFT)f
(eV)
a
Dyes
lmax (nm)
g
e
AJ301
AJ303
504
502
27,600
22,300
518
511
1.09
1.03
1.94
1.93
ꢀ0.85
ꢀ0.93
2.36
2.33
0.80
0.65
ꢀ1.56
ꢀ1.68
a
The UVevis absorption spectra were measured in dichloromethane (1 ꢂ 10ꢀ5 M).
The dyes were absorbed on TiO2 film.
b
c
d
e
f
Oxidation potentials of dyes were measured at a scan rate of 40 mV/s (calibrated with Fc/Fcþ as an external reference and converted to NHE by addition of 0.63 V).
The band gap (Eoptg) was obtained using equation 1240/
l, where l is the tangent line of the TiO2 film in the UVevis absorption spectra with the X-axis.
The LUMO energy level was calculated with the equation LUMO ¼ HOMO e Eg.
The computational parameters by DFT.
The corresponding data are found in Table 1, 2, 3 and 4. As shown in
Fig. 3a, AJ301 and AJ303 exhibited their maximum absorption at
lmax ¼ 504 nm (ε ¼ 27,600 mol Lꢀ1 cmꢀ1) and lmax ¼ 502
(ε ¼ 22,300 mol Lꢀ1 cmꢀ1) in dichloromethane, respectively. The
high ε values and appropriate band gap energies (Eoptg) will be a
great benefit to obtain sufficiently light capture capacity. The two
dyes shared a similar absorption tendency when dissolved in
CH2Cl2, owing to the similar chemical structure except their
different donor unit, which indicated that the incorporation of the
TPC and DBD donors into the dye backbone did not impact the light
capture. Both dyes exhibited an intense absorption between 300
and 400 nm, as well as between 450 and 600 nm. A slight red-shift
of 14 nm for AJ301 and 9 nm for AJ303 of the maximum absorption
is shown in Fig. 3b when the dyes were attached to the TiO2 film.
Fig. 4. Energy level diagram of AJ301 and AJ303.
3.1.1. Electrochemical properties
(2 ꢂ 10ꢀ4 M in CH2Cl2) and chenodeoxycholic acid (CDCA, 2 mM)
Differential pulse voltammetry (DPV) was performed with 0.1 M
TBAPF6 as a support electrolyte dissolved in CH2Cl2, Ag/AgNO3 as
reference electrode with ferrocene as an external standard, glassy
carbon as a working electrode and Pt wire as a counter electrode.
The oxidation potential of ferrocene, AJ301 and AJ303 were 0.16 V,
0.62 V and 0.56 V, respectively. The results are shown in Fig. S1.
AJ301 (1.09 V vs normal hydrogen electrode, NHE) exhibited a more
positive highest occupied molecular orbital (HOMO) energy level
compared with AJ303 (1.03 V vs NHE), which implied a weaker
donor-ability of TPC than DBD. An ample potential gap was ob-
tained between the HOMO energy level for both dyes with
Co(bpy)23þ/3þ (0.56 vs NHE) and Cu(tmby)þ2 /2þ (0.87 V vs NHE),
for 12 h to absorb the dye. A 25-mm-thick Surlyn film was used to
encapsulate the platinum (Pt) counter electrode and the TiO2
photoanode with the adsorbed dyes at 125 ꢁC for 30s. The elec-
trolyte was injected from predrilled holes in the counter electrodes
under a negative pressure by using a water pump. The devices were
measured use a mask area of 0.16 cm2. The electrolyte composition
was 0.25 M [Co(II) (bpy)3] (TFSI)2, 0.05 M [Co(III) (bpy)3] (TFSI)3,
0.1 M LiTFSI and 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile.
The concentrations of the copper electrolytes used were 0.2 M Cu(I)
(tmby)2(TFSI)2, 0.04 M Cu(II) (tmby)2TFSI, 0.1 M LiTFSI and 0.5 M 4-
tert-butylpyridine (TBP) in acetonitrile.
which ensured efficient dye regeneration in the DSSCs. Eopt was
g
3. Results and discussion
calculated using the equation Eopt ¼ 1240/
l, where l is the inter-
g
section of the tangent of the absorption on the TiO2 film with the x-
axis, which was 1.94 V for AJ301 and 1.93 V for AJ303. The minor
3.1. Optical properties
difference for Eopt was attributed to the influence of the donor
g
Uvevis absorption spectra of AJ301 and AJ303 measured in
CH2Cl2 solution and absorbed on TiO2 film are shown in Fig. 3 (a, b).
units TPC and DBD, which is normally negligible since Eopt is
g
largely dominated by the same p-bridgeeaccepter unit. The lowest
4

J. An, X. Yang, Z. Tian et al.
Tetrahedron 88 (2021) 132124
Table 2
Frontier molecular orbitals of the HOMO and LUMO for the dyes AJ301 and AJ303 that were calculated using DFT.
AJ301
AJ303
HOMO-1
HOMO
LUMO
LUMO þ 1
Fig. 5. Optimized geometries of dyes AJ301 and AJ303.
unoccupied molecular orbital (LUMO) energy levels of AJ301 and
vs NHE, calculated by
3.2. Theoretical model calculations
AJ303 were ꢀ0.85
V
and ꢀ0.93
V
LUMO ¼ HOMO - Eoptg, which meant sufficient driving force for
electron injection from the excited sensitizers to the CB of TiO2. The
calculated HOMO values are 0.80 V and 0.65 V, the calculated LUMO
values are ꢀ1.56 V and-1.68 V. The calculated energy level accord
with the variation trend of the measured values. The energy level
diagram for the two dyes is shown in Fig. 4 and the corresponding
data can be found in Table 1.
To gain a clear understanding of the influence of TBP and DBD
donors on the geometric configuration and electron distribution,
density functional theory (DFT) was applied for calculations at the
B3LYP/6-31G*(d, p) level. The calculated transition energies are
shown in Table 2. For the HOMO orbitals of the two dyes in the
ground state, the electron density was mainly focused on the TPC
and DBD donors. The LUMO orbitals were mainly distributed on the
p
-bridge groups. The LUMOþ1 orbitals were predominantly
5

J. An, X. Yang, Z. Tian et al.
Tetrahedron 88 (2021) 132124
Fig. 6. (a) J e V curves of the corresponding DSSCs measured under AM 1.5 G illumination (100 mW cmꢀ2). (b) IPCE spectra for DSSCs based on dyes AJ301 and AJ303 with a
[Co(bpy)3]2þ/3þ redox shuttle.
Table 3
conversion efficiency (IPCE) was also measured to further investi-
gate the performance difference of the devices based on the two
dyes. As shown in Fig. 6b (the copper electrolyte based on device for
Photovoltaic performance parameters of the AJ301 and AJ303 dyes measured under
AM1.5 G irradiation.
Dyes
Electrolyte Jsc (mA cmꢀ2
)
Voc (mV)
FF (%)
PCE (%)
IPCE in Fig. S2), AJ303 maintained a relatively high conversion ef-
ficiency over all of the visible region compared with AJ301, which
corresponded to the order of the photocurrent, where AJ303
(15.8 mA cmꢀ2) > AJ301 (10.3 mA cmꢀ2). This was attributed to the
different energy gaps between LUMO and the CB level of the TiO2
semiconductor sensitized by dyes. The integrated photocurrent,
10.6 mA cmꢀ2 for AJ301 and 14.5 mA cmꢀ2 for AJ303, was calcu-
lated and agreed-well with the J - V measurements. The efficiency
distribution of repeatable devices shown in Fig. S3a. The best per-
forming dye of AJ303 exhibited IPCE values above 70% in the range
of 400e680 nm, which revealed a higher intensity IPCE response
compared with the AJ301 dye, and resulted in an improved
photocurrent and photovoltaic efficiency.
Therefore, it should be noted that the surface coverage of dyes
on the TiO2 film is also an important factor for the IPCE values. The
higher IPCE values can be ascribed to the increased dye loading to
obtain a better light-harvesting capacity. We measured the absor-
bance of dye loading on a TiO2 electrode, which was immersed into
a THF/H2O (V/V ¼ 1:1) solution containing 0.1 M NaOH. The dye
loading amount of AJ303 (2.32 ꢂ 10ꢀ7 mol cmꢀ2) was higher than
that of AJ301 (9.91 ꢂ 10ꢀ8 mol cmꢀ2), as shown in Fig. S4. The
higher dye loading values were attributed to the presence of a
smaller steric hindrance of the thiophene-fused electron-donating
group in the AJ303 dye[41,42].[]The relatively high Jsc of AJ303
compared with that of the AJ301-based DSSC device was not sur-
prising because the two dyes had a similar absorption range in the
visible region and ε, so the major factor was a higher light capture
ability to increase the light-harvesting ability.
AJ301 Co2þ/Co3þ 10.3 ( 0.59)
AJ303 Co2þ/Co3þ 15.8 ( 0.49)
807 ( 9.2)
914 ( 4.7)
951( 5.7)
74.6 ( 0.62) 6.2( 0.26)
70.4 ( 0.61) 10.2( 0.25)
77.8 ( 0.63) 5.4( 0.17)
AJ301 Cuþ/Cu2þ
AJ303 Cuþ/Cu2þ
7.3 ( 0.28)
11.3 ( 0.31)
1007( 4.9) 71.9 ( 0.64) 8.2( 0.36)
distributed on the accepter and slightly distributed on the p-bridge
framework. The result indicates that the excited state dye was
effective for intramolecular charge transfer from the donor to
acceptor group.
For the two dyes, a significant difference of the twisted dihedral
angles was clearly observed between the two donor units and the
neighboring p-bridge-acceptors in Fig. 5. Compared with AJ301, the
DBD donor of AJ303 exhibited a larger dihedral angle of 54.1ꢁ be-
tween the conjugated carbazole unit and the hexylthiophene of the
-bridge group, because the backbone coplanarity of the donor
p
group of AJ301 was introduced in the dye. However, for AJ301,
owing to the strong steric effect between adjacent flexible carba-
zole groups on the thiophene with large twisted dihedral angles of
48.1ꢁ and 41.1ꢁ, the dihedral angle between the strong electron-
donating ability of thiophene and adjacent hexylthiophene of the
p
-bridge group exhibited excellent backbone coplanarity with the
smallest twisted dihedral angles of 47.5ꢁ. This result explains the
small difference between the UVevis absorption spectra.
3.3. Device performance
The photocurrent densityevoltage (J - V) curves measured un-
der standard AM1.5G solar irradiation (100 mW cmꢀ2) of the de-
vices based on AJ301 and AJ303 are shown in Fig. 6a. The
corresponding photovoltaic parameters in Table 3. The devices
based on AJ301 showed a lower PCE of 6.2% with a short-circuit
photocurrent density (Jsc) of 10.3 mA cmꢀ2, an open-circuit pho-
tovoltage (Voc) of 870 mV and a fill factor (FF) of 74.6%, in
conjunction with a cobalt redox shuttle. The DSSCs based on AJ303
exhibited a better PCE of 10.2%, (Jsc ¼ 15.8 mA cmꢀ2, Voc ¼ 914 m V,
FF ¼ 70.4%). AJ301 and AJ303 were also used to fabricate DSSC
devices with a copper redox shuttle. For the excellent AJ303, a PCE
of 8.2% was obtained, with a Jsc of 11.3 mA cmꢀ2, Voc of 1007 mV and
FF of 71.9%, which was 5.4% higher than for AJ301. A lower Voc was
obtained owing to the more negative potential for Co (bpy)32þ/3þ
3.4. Electrochemical impedance spectroscopy
To better research the different donor structure with perfor-
mance relationships, electrochemical impedance spectroscopy
(EIS) was performed to understand the interfacial charge-transfer
mechanisms in the devices. The devices were operated at
different bias voltages in the range of 0.75e0.95 V with an interval
of 50 mV in a dark room. The parameters of the frequency were the
set range of 0.1 Hze100 kHz and the amplitude applied at 10 mV.
The chemical capacitance was directly related to the Voc values,
which can present the electronic storage ability to effect the shift of
CB of TiO2. The electron recombination resistance (Rrec) and
transport resistance (Rtr) were present at the TiO2/dye/electrolyte
interface and electrolyte/Pt interface. The Bode plots are shown and
Nyquist plots of the devices with a forward bias of ꢀ0.90 V are
shown in Fig. 7a and Fig. S5. Two semicircles were visible, where
compared with Cu(tmby)2þ/2þ
.
More flexible alkyl chains and fused rings were introduced to
DBD to decrease the intermolecular packing and recombination for
a higher photovoltaic performance. The incident photo-to-electron
6

J. An, X. Yang, Z. Tian et al.
Tetrahedron 88 (2021) 132124
Fig. 7. EIS analysis data with a [Co(bpy)3]2þ/3þ redox shuttle. (a) Bode phase plots of the dyes. (b) Chemical capacitance Cm. (c) Recombination resistance Rrec. (d) Electron lifetime
t.
Table 4
more effective suppression of electron recombination owing to the
bulky hexylphenyl moieties. Photovoltaic devices based on copper
electrolyte have similar charge transfer mechanism, as shown in
Fig. S6.
EIS parameters a with a [Co(bpy)3]2þ/3þ redox shuttle for DSSCs sensitized by AJ301
and AJ303.
Dyes
Rtr
(U
)
Rrec
(
U
)
Cm (mF)
t
(ms)
hcc(%)
AJ301
AJ303
4.68
5.01
31.38
50.33
0.165
0.143
5.19
7.21
87.0
90.9
3.5. The stability of devices
a
Calculated value from EIS data measured at a forward bias of ꢀ0.90 V under dark
conditions. Rtr, transport resistance; Rrec, charge recombination resistance; Cm,
The stability of devices are important for practical application.
We measured the AJ303 devices based on [Co(bpy)3]2þ/3þ redox
shuttle at LED light for 360 h. The parameters of change rule of
device were depicted in Fig. 8. The PCE values have fallen by
roughly 16% compared with initial value, which shows a relatively
stable property under the 1 sun illumination.
chemical capacitance; t, electron lifetime; hcc, charge-collection efficiency.
the small semicircle was attributed to Rtr corresponding to a high
frequency in the Bode plots and the large semicircles depicted the
impedances of Rrec at the middle frequency. The Rtr represent the
FTO/Pt interface, which have similar resistance for Pt counter
4. Conclusion
electrode [43]. A larger semicircle in theory means a larger Rrec
,
which indicates a slower electron recombination rate. Comparing
AJ301 with AJ303, the large semicircle radius increased in the order
We designed and synthesized two carbazole sensitizers con-
taining a thiophene group with the use of different redox couples to
improve the photovoltaic efficiency of DSSC. The dye of the TPC
donor and the dye of the DBD donor using a cobalt-based electro-
lyte exhibited PCE of 6.2% and 10.2%, and a copper-based electrolyte
exhibited PCE of 5.4% and 8.2%. The higher Jsc values of AJ303 than
AJ301 dyes were ascribed to more dye loading and a higher charge-
collection efficiency. The DBD donor showed a better rigidity and
planarity of the conjugated skeleton to reduce steric hindrance.
Therefore, the fused carbazole concept was applied to the intro-
duction of a thiophene-fused structure on the donor to improve
high-performance DSSCs. The sensitizer AJ303 exhibited a blocking
effect of hexylbenzene group in the donor, which effectively sup-
pressed the recombination process with an electrolyte, as shown by
EIS and photovoltage transient studies. The results indicated the
enhanced Voc values were attributed to the longer electron lifetimes
by introducing the bulkiness of an alkyl chain around the
thiophene-fused donor side. These results provide an enlightened
of AJ303 (50.33
in good agreement with the Voc tendencies of AJ303 > AJ301 in
Table 4. The electron lifetime ( ) was calculated by using Rrec
values showed
U ) > AJ301 (31.38 U) under dark room, which was
t
(
t
¼ Rrec ꢂ Cm), as shown in Fig. 7d. As a result, the
t
increase tendencies of AJ303 > AJ301, which indicted a longer
electron lifetime and meant that a lower charge recombination rate
led to the TiO2 CB shifting-up, thereby resulting in enhanced Voc
and PCE values. A longer
explanation for AJ303 having a higher Voc. The charge-collection
efficiency (hcc) of the dye-based devices was calculated by hcc
ꢀ1. The hcc values of AJ301 and AJ303 were 87.0% and
t and larger Rrec provided a reasonable
¼
(1þRtr/Rrec
)
90.9%, respectively, where AJ303 clearly reflected that the injected
electrons rate was more efficient than that of AJ301 to obtain a good
photovoltaic performance. Although, the DBD donor showed a
better planarity of the conjugated skeleton, the higher Voc values of
AJ303 than AJ301 dyes using the Co2þ/3þredox mediator suggested
7

J. An, X. Yang, Z. Tian et al.
Tetrahedron 88 (2021) 132124
7587e7589.
[8] K. Kakiage, Y. Aoyama, T. Yano, K. Oya, M.J. Fujisawa, M. Hanaya, Chem.
Commun. 51 (2015) 15894e15897.
[9] M. Freitag, J. Teuscher, Y. Saygili, X. Zhang, F. Giordano, P. Liska, J. Hua,
€
S.M. Zakeeruddin, J-E.Moser M. Gratzel, A. Hagfeldt, Nat. Photonics 11 (2017)
372e379.
[10] S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B.F.E. Curchod, N. Ashari-
Astani, I. Tavernelli, U. Rothlisberger, M.K. Nazeeruddin, Nat. Chem. 6 (2014)
242e247.
[11] A. Yella, C.-L. Mai, S.M. Zakeeruddin, S.-N. Chang, C.-H. Hsieh, C.-Y. Yeh,
€
M. Gratzel, Angew. Chem. Int. Ed. 53 (2014) 2973e2977.
[12] S. Kotteswaran, P. Ramasamy, New J. Chem. 45 (2021) 2453e2462.
[13] F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Wang, P. Humphry-Baker,
€
S.M. Zakeeruddin, M. Gratzel, J. Am. Chem. Soc. 130 (2008) 10720e10728.
[14] A. Yella, H.-W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W.-
€
G. Diau, C.-Y. Yeh, S.M. Zakeeruddin, M. Gratzel, Science 334 (2011) 629e634.
[15] N. Zhou, K. Prabakaran, B. Lee, S.H. Chang, B. Harutyunyan, P. Guo, M.R. Butler,
A. Timalsina, M.J. Bedzyk, M.A. Ratner, S. Vegiraju, S. Yau, C.-G. Wu,
R.P.H. Chang, A, M.-C. Facchetti Chen, T.J. Marks, J. Am. Chem. Soc. 137 (2015)
4414e4423.
[16] Z. Yao, H. Wu, Y. Ren, Y. Guo, P. Wang, Energy Environ. Sci.
8 (2015)
Fig. 8. The stability of AJ303 with a [Co(bpy)3]2þ/3þ redox shuttle by normalization.
1438e1442.
[17] Z. Yao, M. Zhang, H. Wu, L. Yang, R. Li, P. Wang, J. Am. Chem. Soc. 137 (2015)
3799e3802.
[18] Z. Yao, H. Wu, Y. Li, J. Wang, J. Zhang, M. Zhang, Y. Guo, P. Wang, Energy
Environ. Sci. 8 (2015) 3192e3197.
strategy for designing the fusion of substituted new efficient
[19] L. Zhang, X. Yang, W. Wang, G.G. Gurzadyan, J. Li, X. Li, J. An, Z. Yu, H. Wang,
B. Cai, A. Hagfeldt, L. Sun, ACS Energy Lett. 4 (2019) 943e951.
[20] Y. Liu, Y. Cao, W. Zhang, M. Stojanovic, D.M. Ibrahim, Y. Saygili, H. Anders,
organic dyes for high-performance DSSCs.
Notes
€
S.M. Zakeeruddin, M. Gratzel, Chem. Int. Ed. 57 (2018) 14125e14128.
[21] S. Yasemin, S. Magnus, P. Norman, Gi Fabrizio, C. Yiming, B.M.-G. Ana,
€
M.Z. Shaik, V. Nick, P. Michele, B. Gerrit, K. Ladislav, M. Jacques-E, M. Gratzel,
The authors declare no competing financial interest.
Declaration of competing interest
H. Anders, F. Marina, J. Am. Chem. Soc. 138 (2016) 15087e15096.
[22] J. Li, X. Yang, Z. Yu, G.G. Gurzadyan, M. Cheng, F. Zhang, J. Cong, W. Wang,
L. Sun, RSC Adv. 7 (2017) 4611e4615.
[23] W. Zhang, X. Yang, J. An, L. Zhang, B. Cai, K. Yang, Energy Technol. 8 (2020)
2000032.
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
[24] A.K.K. Kyaw, H. Tantang, T. Wu, L. Ke, J. Wei, H.V. Demir, Q. Zhang, X.W. Sun,
J. Phys. D Appl. Phys. 45 (2012) 165103.
[25] R.S. Mai, X.Y. Wu, Y. Jiang, Y.Y. Meng, B.Q. Liu, X.W. Hu, J. Roncali, G.F. Zhou,
J.M. Liu, K. Kempa, J.W. Gao, J. Mater. Chem. 7 (2019) 1539e1547.
[26] Y.L. Wu, Y.N. Li, S. Gardner, B.S. Ong, J. Am. Chem. Soc. 127 (2005) 614e618.
[27] Y.N. Li, Y.L. Wu, S. Gardner, B.S. Ong, Adv. Mater. 17 (2005) 849e853.
[28] X.-H. Zhang, Z.-S. Wang, Y. Cui, N. Koumura, A. Furube, K. Hara, J. Phys. Chem.
C 113 (2009) 13409e13415.
[29] S. Cai, X. Hu, G. Tian, H. Zhou, W. Chen, J. Huang, X. Li, J. Su, Tetrahedron 70
(2014) 8122e8128.
[30] S. Cai, G. Tian, X. Li, J. Su, H. Tian, J. Mater. Chem. 1 (2013) 11295e11305.
[31] G. Tian, S. Cai, X. Li, H. Ågren, Q. Wang, J. Huang, J. Su, J. Mater. Chem. 3 (2015)
3777e3784.
Acknowledgements
The authors acknowledge the financial support by the National
Natural Science Foundation of China (51661135021, 21606039 and
U1710117), the Swiss National Science Foundation (IZLCZ2_170177),
the Swedish Foundation for Strategic Research (SSF), the Swedish
Energy Agency, as well as the Knut and Alice Wallenberg
Foundation.
[32] X. Qian, L. Shao, H. Li, R. Yan, X. Wang, L. Hou, J. Power Sources 319 (2016)
39e47.
[33] Z. Yao, M. Zhang, R. Li, L. Yang, Y. Qiao, P. Wang, Angew. Chem. Int. Ed. 54
(2015) 5994e5998.
Appendix A. Supplementary data
[34] Y. Ren, J. Liu, A. Zheng, X. Dong, P. Wang, Adv. Sci. 4 (2017) 1700099.
€
[35] Y. Ren, D. Sun, H.N. Tsao, Y. Yuan, S.M. Zakeeruddin, P. Wang, M. Gratzel, J. Am.
Chem. Soc. 140 (2018) 2405e2408.
[36] H. Wu, J. Zhang, Y. Ren, Y. Zhang, Y. Yuan, Z. Shen, S. Li, P. Wang, ACS Appl.
Energy Mater. 3 (2020) 4549e4558.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2021.132124.
ꢀ
[37] W. Zimmermann, J. Urieta-Mora, P. Gratia, J. Arago, G. Grancini, A. Molina-
ontoria, Adv. Energy Mater. 7 (2016) 1601674.
[38] P.Y. Gu, N. Wang, A. Wu, Z. Wang, M. Tian, Z. Fu, X.W. Sun, Q. Zhang, Chem.
Asian J. 11 (2016) 2135e2138.
[39] N. Wang, K. Zhao, T. Ding, W. Liu, A.S. Ahmed, Z. Wang, M. Tian, X.W. Sun,
Q. Zhang, Adv. Energy Mater. 7 (2017) 1700522.
[40] J. An, X. Yang, W. Wang, L. Zhang, B. Cai, Sol. Energy 185 (2019) 124e130.
[41] Y. Kurumisawa, T. Higashino, S. Nimura, Y. Tsuji, H. Iiyama, H. Imahori, J. Am.
Chem. Soc. 141 (2019) 9910e9919.
References
[1] J. Mogensen, H. Michaels, R. Roy, L. Broløs, M. D. Kilde, M. Freitag, M. B.
Nielsen. Eur. J. Org Chem. 2020 (2020), 6127 - 6134.
[2] H. Wu, X. Xie, Y. Mei, Y. Ren, Z. Shen, S. Li, P. Wang, ACS Photonics 6 (2019)
1216e1225.
[3] H.S. Jung, J.-K. Lee, J. Phys, Chem. Lett. 4 (2013) 1682e1693.
[4] B. O’Regan, Nature 353 (1991) 737e739.
[5] J.-M. Ji, H. Zhou, Y.K. Eom, C.H. Kim, H.K. Kim, Adv. Energy Mater. 10 (2020)
2070070.
[42] A.J. Huckaba, A. Yella, P. Brogdon, J. Scott Murphy, M.
K Nazeeruddin,
€
M. Gratzel, J.H. Delcamp, Chem. Commun. 52 (2016) 8424e8427.
[43] M.B. Desta, N.S. Vinh, C.H. Pavan Kumar, S. Chaurasia, W.-T. Wu, J.T. Lin, T.-
C. Wei, E.W.-G. Diau, J. Mater. Chem. 6 (2018) 13778e13789.
[6] S.H. Kang, M.J. Jeong, Y.K. Eom, I.T. Choi, S.M. Kwon, Y. Yoo, J. Kim, J. Kwon,
J.H. Park, H.K. Kim, Adv. Energy Mater. 7 (2017) 1602117.
[7] J. Liu, Y. Numata, C. Qin, A. Islam, X. Yang, L. Han, Chem. Commun. 49 (2013)
8