Effect of photo-induced charge separated state lifetimes in donor-acceptor1-acceptor2 organic ambipolar semiconductors on their photovoltaic performances

Article abstract of DOI:10.1016/j.dyepig.2016.12.066

A newly ambipolar organic semiconductor with styrene based indoline derivative (YD) as the electron donor (D), s-triazine group (TRC) as the electron acceptor (A₁), and fullerene derivative (NMF) as the second electron acceptor (A₂)

Full text of DOI:10.1016/j.dyepig.2016.12.066

Dyes and Pigments 139 (2017) 601e610
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Effect of photo-induced charge separated state lifetimes in donor-
acceptor1-acceptor2 organic ambipolar semiconductors on their
photovoltaic performances
Tianyang Wang a b , Haiya Sun , Linli Zhang , Nathan D. Colley ,
,
,
*
a
,
b
a
,
b
c
Chelsea N. Bridgmohan , Dongzhi Liu *,*W* enping Hu , Wei Li a
c
a,
d
b,
e
, d
,
a,
b
,
d
,
**
a, b, c,
Xueqin Zhou
a
, Lichang Wang
School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300354, China
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China
Department of Chemistry and Biochemistry and the Materials Technology Center, Southern Illinois University Carbondale, IL 62901, United States
Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin, 300354, China
School of Science, Tianjin University, Tianjin, 300354, China
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 December 2016
Received in revised form
A newly ambipolar organic semiconductor with styrene based indoline derivative (YD) as the electron
donor (D), s-triazine group (TRC) as the electron acceptor (A
second electron acceptor (A ) has been synthesized and characterized, which had a charge-separated
state of 7.9 s, corresponding to a 37 fold increase in comparison with the 215 ns lifetime of the D-A
precursor. Here we compared photovoltaic properties of YD-TRC-NMF with those of other four D-TRC-A
1
), and fullerene derivative (NMF) as the
2
28 December 2016
m
1
2
Accepted 28 December 2016
Available online 30 December 2016
systems, which replaced indoline with triphenylamine (MTPA) and fullerene with perylene bisimide or
anthraquinone (AEAQ), indicating that long-lived CS states are playing an important role in promoting
photoelectric conversion in YD-TRC-AEAQ (or NMF) and MTPA-TRC-AEAQ, in which the short-circuit
current and photoelectric conversion efficiency were significantly improved in comparison with YD-
Keywords:
Donor-acceptor1-acceptor2 system
s-triazine
Fullerene
TRC and MTPA-TRC, respectively, due to triplet states of A
2
being higher than charge-separated states.
Photo-induced electron transfer
Single-layer organic solar cell
Photovoltaic property
This proves that a general D-A
photovoltaics.
1 2
-A architecture can become a reasonable materials design strategy for
©
2016 Elsevier Ltd. All rights reserved.
1
. Introduction
absorption and are believed to be the key for the subsequent
chemical reactions [1e3]. Great efforts have been made to design
dyes in which long-lived CS states are attained through photo-
induced electron transfer [4e9]. In several of the artificial photo-
synthetic complexes, photo-induced CS states have reached the
lifetimes of 380 ms to 2 h that are comparable to, or even longer
than those in nature photosynthetic systems [5,7]. It is recognized
that materials with long-lived CS states may improve the perfor-
mance of organic solar cells (OSCs) [10e13]. Actually, generation of
CS states with suitable lifetimes is critical in the formation of
photocurrents for solar cells [14e17], that means the study of
photoinduced charge separation processes and investigation of
their essential role in improving the efficiency of solar cell devices
should be required in depth. However, only a few publications have
focused on the application of long-lived CS organic compounds to
OSCs. It is important to perform systematic studies to establish a
relationship between the lifetimes of CS states and the photovoltaic
Natural photosynthesis, the most efficient process of converting
sunlight into electrical or chemical energy, is of great interest to
researchers focused on energy conversion and utilization. In a
natural photosynthetic system, long-lived charge-separated (CS)
states are formed via multistep electron transfers upon light
*
Corresponding author. School of Chemical Engineering and Technology, Tianjin
University, Tianjin, 300354, China.
* Corresponding author. School of Chemical Engineering and Technology, Tianjin
University, Tianjin, 300354, China.
** Corresponding author. Department of Chemistry and Biochemistry and the
*
*
Materials Technology Center, Southern Illinois University Carbondale, IL 62901,
United States.
E-mail addresses: tianyangwang@tju.edu.cn (T. Wang), zhouxueqin@tju.edu.cn
(
X. Zhou), lwang@chem.siu.edu (L. Wang).
http://dx.doi.org/10.1016/j.dyepig.2016.12.066
143-7208/© 2016 Elsevier Ltd. All rights reserved.
0

602
T. Wang et al. / Dyes and Pigments 139 (2017) 601e610
performance of OSCs. This contains two levels of work: 1) Design
and construction of organic materials which had long-lived CS
states; 2) Select the appropriate structure of OSCs device to
investigate the effect of long-lived CS states of the material
effectively.
generate the CS state with lifetime of 7.9
single layer OSCs devices with all of the above D-A
with their D-A precursors. The photovoltaic properties were dis-
cussed in detail and were related to their charge separation pro-
cesses and lifetime of CS state.
m
s. Then, we fabricated
1
-A systems
2
1
Photo-induced CS were discovered in organic systems contain-
ing donor (D) and acceptor (A) moieties via photo-induced intra-
molecular electron transfer. This result suggests that D and A
determine the driving force of electron transfer [18e20] while the
distance, spatial orientation, and flexibility between D and A have a
marked impact on the rate of photo-induced electron transfer and
the efficiency of generating CS species and lifetimes of CS states.
Photo-induced CS states can also be created by photo-induced hole
transfer when the acceptor moiety is initially photo-excited [19].
That means that ambipolar organic semiconductors, which include
both photo-induced electron and hole transfers, can form CS states
effectively because of the synergistic combination of above two
2
. Experimental
2.1. Synthesis
Structures of the key compounds being synthesized and studied
in this work are provided in Chart 1. While the other compounds
were synthesized and studied previously [29,43e45], YD-TRC-NMF
was first reported here and its synthetic routes were shown in
Schemes 1 and 2. Other compounds discussed in this paper e.g. 4,4’-
dimethyl-4 -styryl-triphenylamine (MTPA) [29] and 4-(4-
methylphenyl)-7-styryl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]
indole (YD) [44](ESI, Chart S1), D-A systems 4,4’-dimethyl-4”-(4-
0
0
transfers [21e28]. Using a D-A
ambipolar organic semiconductor MTPA-TRC-AEAQ using s-triazine
TRC) as the first acceptor to connect the donor styrene based tri-
1 2
-A strategy, we reported an
(
(
(
4,6-dichloro-1,3,5-triazin-2-ylamino)styryl)triphenylamine
MTPA-TRC) [29] and 4-(4-methylphenyl)-7-(4-(4,6-dichloro-1,3,5-
phenylamine (MTPA) with anthraquinone (AEAQ) as the second
acceptor, which had been proven to be a good linker to promote the
photo-induced electron transfer and increase the light-stability for
MTPA-TRC-AEAQ [29,30]. Upon the absorption of photons, the CS
states in the organic semiconductor MTPA-TRC-AEAQ was elon-
gated to 650 ns, more than eight-fold compared to those in the D-
triazin-2-ylamino)styryl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]
indole (YD-TRC) [43,44], D-A -A systems 4,4’-dimethyl-4”-(4-(4-
chloro-6-(2-(9,10-dioxoanthracen-1-ylamino)ethylamino)-1,3,5-
triazin-2-ylamino)styryl)triphenylamine (MTPA-TRC-AEAQ) [29],
1
2
4
,4’-dimethyl-4”-(4-(4-Chloro-6-(N-(1-hexylheptyl)-N’-(4-amino)
phenyl-perylene-3,4,9,10-tetracarboxylbisimide)-1,3,5-triazin-2-
ylamino)styryl)triphenylamine (MTPA-TRC-PDI) [45], 4,4’-
A
1
precursor MTPA-TRC (80 ns). MTPA-TRC exhibits similarly low
photovoltaic characteristics in the single layer OSCs with common
organic semiconductors such as phthalocyane, poly(-
dimethyl-4”-(4-(4-Chloro-6-(N-(1-hexylheptyl)-N’-(4-amino)
phenyl-1,7-di(4-tert-butylphenoxyl)-perylene-3,4,9,10-
phenylenevinylene) and triarylamine derivatives [12,31e33], of
which the efficiency has been improved to greater than 6% and can
even exceed 10% by the combined application of other semi-
conductors and the proper design of solar cell structures
tetracarboxylbisimide)-1,3,5-triazin-2-ylamino)styryl)triphenyl-
amine (MTPA-TRC-PBI) [45] and 4-(4-methylphenyl)-7-(4-(4-
chloro-6-(2-(9,10-dioxoanthracen-1-ylamino)ethylamino)-1,3,5-
triazin-2-ylamino)styryl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]
indole (YD-TRC-AEAQ) [44] were prepared according to the litera-
ture. The synthetic pathways of 4-(4-methylphenyl)-7-(4-(4-
chloro-6-phenylamino-1,3,5-triazin-2-ylamino)styryl)-
[12,34e39]. Importantly, the single layer OSCs constructed using
MTPA-TRC-AEAQ were found to be significantly more efficient than
those using MTPA-TRC [29], indicating that single layer OSCs had
been proven to be the highly effective cell structure reflecting the
effect of long-lived CS states of organic materials directly. That was
why we chose single-layer OSC structure in this paper, although its
photoelectric conversion efficiency was relatively low in various
literature [12,31e33,38,40e42].
Encouraged by these results, we obtained a new ambipolar
organic semiconductor YD-TRC-AEAQ by substitution of MTPA for
an indoline derivative (YD) which has been reported to be a more
1
,2,3,3a,4,8b-hexahydrocyclopenta[b]indole
methyl-2-(4-nitrophenyl)fulleropyrrolidine (NMFn), N-methyl-2-
4-aminophenyl)fulleropyrrolidine (NMFa), N-methyl-2-(4-(4,6-
dichloro-1,3,5-triazin-2-ylamino)phenyl)fulleropyrrolidine
NMFt), 4-(4-methylphenyl)-7-(4-(4-chloro-6-(N-methyl-2-(4-
(YD-TRC-BA),
N-
(
(
phenyl)-fulleropyrrolidine)ylamino-1,3,5-triazin-2-ylamino)
styryl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (YD-TRC-NMF)
were illustrated in Schemes 1 and 2. All reagents and solvents were
reagent grade and further purified by the standard methods if
necessary. All synthetic procedures were carried out under an at-
mosphere of dry nitrogen unless otherwise indicated.
effective donor [29,43,44]. The lifetime of CS states reached 1.14 ms
for YD-TRC-AEAQ [44]. Further studies suggest that charge sepa-
ration efficiency and lifetime of CS states may be controlled by
changing the donor and second acceptor of MTPA-TRC-AEAQ
[29,43e45]. The aforementioned ambipolar organic semi-
conductors have cascade energy levels and similar photophysical
processes to generate photo-induced CS states. Therefore, it is
possible and significant to relate their CS states to the photovoltaic
performance of single layer OSC devices, and reveals the advantage
2.2. Synthesis of YD-TRC-BA
of introducing the acceptor2 to D-A
architecture in the research perspective of photovoltaic
characteristics.
1
system to form the D-A
1
-A
2
YD-TRC (77.2 mg, 0.15 mmol) was dissolved in anhydrous THF
(20 mL) and then phenylamine (14.0 mg, 0.15 mmol) was added
into the solution. The mixture was stirred at 40 C for 6 h. After-
ꢀ
To systematically study the solar cell performance of these
promising materials and to establish correlation between the life-
time of charge separation and cell efficiency, more systems are
required. Therefore, we have synthesized a new ambipolar organic
semiconductor, YD-TRC-NMF, introducing a fullerene derivative
wards, the solvent was removed by rotary evaporation and the
residue was purified by column chromatography on silica gel using
dichloromethane as the eluent. An orange solid was yielded
þ
(21.6 mg, 25.2%). HRMS-ESI (m/z): 571.2373 [MþH] (calcd for
þ
1
C
d
35
H32ClN
6
:
m/z
¼
3
571.2371). HNMR (500 MHz, CDCl ):
(
NMF) as the second acceptor (Figs. S1eS2 of Electronic Supple-
mentary Information (ESI)) to the reported D-A system YD-TRC
43,44]. We investigated its photophysical properties by absorp-
tion and emission spectra, which showed that YD-TRC-NMF can
7.55e7.47 (m, 7H), 7.34 (s, 1H), 7.18 (m, 7H), 7.03 (d, J ¼ 16 Hz, 1H),
1
6.89 (dd, J ¼ 16,8.5 Hz, 2H), 4.81 (m, 1H), 3.85 (m, 1H), 2.35 (s, 3H),
[
2.12e1.49 (m, 6H). Elem. Anal.: Found: C, 73.52; H, 5.40; N, 14.84%
6
(Calc. for C35H31ClN : C, 73.61; H, 5.47; N, 14.72%)

T. Wang et al. / Dyes and Pigments 139 (2017) 601e610
603
Chart 1. Structures of key compounds: MTPA-TRC, MTPA-TRC-AEAQ, MTPA-TRC-PDI, MTPA-TRC-PBI, YD-TRC, YD-TRC-AEAQ and YD-TRC-NMF.
Scheme 1. Synthesis scheme of NMFn, NMFa and NMFt.
2.3. Synthesis of NMFn
chromatography on silica gel using toluene as the eluent. A light
brown solid was yielded (164 mg, 54.8%). MALDI-TOF-MS (m/z):
þ
þ
1
C
60 (240 mg, 0.33 mmol) was dissolved in the toluene (70 mL)
898.07[M] (calcd for C69
H
10
N
2
O
2
:m/z ¼ 898.0737). H NMR
and then p-nitrobenzaldehyde (53.5 mg, 0.35 mmol) and sarcosine
41.5 mg,0.46 mmol) were added into the solution. The mixture was
stirred at 120 C for 6 h. Afterwards, the solvent was removed by
(500 MHz, C ):
7
D
8
d
8.72 (d, J ¼ 8.5 Hz, 2H), 7.94 (d, J ¼ 8.5 Hz, 2H),
(
5.33 (s, 1H), 5.25 (d, J ¼ 9.5 Hz, 1H), 4.57 (d, J ¼ 9.5 Hz, 1H), 3.19 (s,
ꢀ
3H). Elem. Anal.: Found: C, 92.38; H, 1.21; N, 3.19% (Calc. for
rotary evaporation and the residue was purified by column
69 10 2 2
C H N O : C, 92.20; H, 1.12; N, 3.12%).

604
T. Wang et al. / Dyes and Pigments 139 (2017) 601e610
Scheme 2. Synthesis scheme of YD-TRC-BA and YD-TRC-NMF.
2.4. Synthesis of NMFa
Afterwards the solvent was removed by rotary evaporation and the
residue was purified by column chromatography on silica gel using
NMFn (100 mg, 0.11 mmol) was dissolved in the toluene (45 mL)
and ethanol (45 mL). The mixture was stirred at 25 C for 1 h.
dichloromethane as the eluent. An olive-green solid was yielded
ꢀ
þ
(30.3 mg, 15%). MALDI-TOF-MS (m/z): 1345.30[M] (calcd for
þ
1
SnCl$2H
2
O (124 mg, 0.55 mmol) and 2 mL concentrated HCl were
C
98
H36ClN
7
: m/z ¼ 1345.2715). H NMR (500 MHz, CDCl
3
): d 7.56 (d,
ꢀ
added to the solution. The mixture was stirred at 50 C for 7 h.
Afterwards, the solvent was removed by rotorary evaporation and
the residue was purified by column chromatography on silica gel
using toluene/dichloromethane (1:1 v/v) as the eluent. A light
brown solid was yielded (164 mg, 54.8%). MALDI-TOF-MS (m/z):
J ¼ 8 Hz, 3H), 7.35 (d, J ¼ 8 Hz, 2H), 7.28 (d, J ¼ 8 Hz, 3H), 7.16 (dd,
J ¼ 8.5,8.5 Hz, 6H), 7.10 (d, J ¼ 8 Hz, 2H), 6.98 (d, J ¼ 16 Hz, 1H), 6.84
(dd, J ¼ 16,8 Hz, 2H), 5.07e4.89 (m, 3H), 4.76 (m, 1H), 3.79 (m, 1H),
2.83 (s, 3H), 2.33 (s, 3H), 2.11e1.73 (m, 6H).
þ
þ
1
8
(
4
1
68.10[M] (calcd for C69
500 MHz, CDCl /CS ):
H
12
N
2
: m/z ¼ 868.0995). H NMR
2.7. Assembly of single-layer organic solar cells
3
2
d
7.33 (d, J ¼ 8 Hz, 2H), 6.68 (d, J ¼ 8 Hz, 2H),
.83 (s, 2H), 4.29 (d, J ¼ 6.5 Hz, 1H), 4.25 (m, 1H), 4.07 (d, J ¼ 6.5 Hz,
The single-layer organic solar cells were fabricated by placing
H), 2.81 (s, 3H).
MTPA-TRC, MTPA-TRC-AEAQ, MTPA-TRC-PDI, MTPA-TRC-PBI, YD-
TRC, YD-TRC-AEAQ and YD-TRC-NMF in between FTO and Au
electrodes. The FTO-coated glass substrate was first cleaned with
detergent, ultrasonicated in water, washed with acetone and iso-
2
.5. Synthesis of NMFt
ꢀ
Cyanuric chloride (18.3 g, 0.10 mmol) was first dissolved in
anhydrous THF (20 mL) and then the solution was cooled down to
propyl alcohol, and then dried overnight in an oven at 50 C under a
ꢂ6
vacuum of about 3 ꢁ 10 Torr. A film of MTPA-TRC (MTPA-TRC-
ꢀ
0
C. NMFa (86.9 mg, 0.10 mmol) was added into the solution,
AEAQ, MTPA-TRC-PDI, MTPA-TRC-PBI, YD-TRC, YD-TRC-AEAQ and
YD-TRC-NMF) was prepared on the surface of FTO layer by spin
coating from tetrahydrofuran solutions (10 mg/mL) and then a
clean Au substrate of 45 nm was covered onto this film with vac-
uum evaporation. The fabricated solar cells were subsequently
ꢀ
followed by a stirring at 0 C for 10 min. Then, the solution was
warmed to room temperature while stirring for 30 min. Afterwards,
the solvent was removed by rotary evaporation and the residue was
purified by column chromatography on silica gel using petroleum
toluene/dichloromethane (1:1 v/v) as the eluent to yield a yellow
ꢂ6
placed in a vacuum atmosphere of about 3 ꢁ 10 Torr for 3 h to
þ
solid (0.21 g, 97.5%). MALDI-TOF-MS (m/z): 1015.04[M] (calcd for
remove the trace amounts of the solvent. All these procedures were
carried out under an atmosphere of dry nitrogen. The active area of
þ
1
C
d
72
H11Cl
2
N
5
: m/z ¼ 1015.0386). H NMR (500 MHz, CDCl
3 2
/CS ):
2
9.43 (s, 1H), 7.61 (d, J ¼ 8 Hz, 2H), 7.44 (d, J ¼ 8 Hz, 2H), 4.29 (d,
devices was 0.04 cm and the thickness of the film was about
J ¼ 6.5 Hz, 1H), 4.25 (m, 1H), 4.07 (d, J ¼ 6.5 Hz, 1H), 2.81 (s, 3H).
100 nm.
2.6. Synthesis of YD-TRC-NMF
2.8. Characterization
YD-TRC (77.2 mg, 0.15 mmol) was dissolved in the anhydrous
1H NMR, ESI mass and MALDI-TOF mass spectra were obtained
on the VARIAN INOVA 500 MHz spectrometer, Thermo Fisher LCQ
Deca XP MAX mass spectrometer and Bruker Autoflextof/tofIII mass
THF (20 mL) and then NMFa (130.3 m g, 0.15 mmol) were added
ꢀ
into the solution. The mixture was stirred at 80 C for 24 h.

T. Wang et al. / Dyes and Pigments 139 (2017) 601e610
605
spectrometer, respectively. The testing temperature was set to
electron in YD module can migrate to the TRC module followed by
electron transfer to NMF module. Based on the experimental
observation and computational results (Figs. S4eS5 of ESI), YD-
ꢀ
2
5
C. The UVevis absorption and fluorescence spectra were
measured using a UVevis spectrophotometer (ThermoSpectronic,
Helios Gamma spectrometer) and a fluorescence spectrophotom-
eter (Varian CARY ECLIPSE), respectively. Geometry optimizations,
molecular orbitals and the corresponding energies of key com-
pounds using B3LYP/6-31G(d,p) in toluene were carried out. The
electrochemical properties were measured using a BAS 100 W
electrochemical analyzer by cyclic voltammetry. Emission fluores-
cence decay spectra of the samples were done with picosecond
diode lasers (Horiba Jobin Yvon Instruments) using Time-
Correlated Single Photon Counting (TC-SPC). Nanosecond tran-
sient absorption measurements were performed on a LP-920 laser
flash photolysis setup (Edinburgh). Some details of computations,
cyclic voltammetry, emission fluorescence decay spectra and
transient absorption spectra were exhibited in ESI. The current
density-Voltage (J-V) characteristic of obtained solar cells was
measured using solar simulator (Zolix) under AM 1.5 G (100 mW/
1 2
TRC-NMF has a typical D-A -A architecture with cascade energy
levels and should lead to the consecutive electron transfers. Addi-
tionally, the lower HOMO level of NMF module in comparison to
that of YD module shows a possible hole transfer process from the
excited NMF to YD module, thus making the triad ambipolar.
3.2. YD-TRC-NMF as D-A -A architecture has the amazing long-
1
2
lived CS state
As shown in Fig. 1, the absorption spectrum of the mixture of
YD-TRC and NMFt is identical with the sum of the spectra of YD-TRC
and NMFt, indicating that in the ground-state, the intermolecular
non-bonded interactions between both modules are very weak.
When a second acceptor NMF is attached to YD-TRC to form YD-
TRC-NMF, the wavelength maxima (386 nm) of YD module is be-
tween those of YD (374 nm) and YD-TRC (402 nm), and the
absorbance of NMF module is overlapped by that of YD module.
This implies the existence of certain electronic and steric effects
between both modules in YD-TRC-NMF, but these effects did not
change the nature of various transitions as evidenced by similar
shapes of the corresponding peaks.
2
cm ), which was adjusted using a calibrated Si solar cell (National
Institute of Metrology), and recorded with a computer-controlled
digital source meter (Kethily 2400).
3
. Results and discussion
The emission spectrum of the mixture of YD-TRC and NMFt is
identical to the sum of the spectra of YD-TRC and NMFt (Fig. 2). The
3
.1. Determination of YD-TRC-NMF as D-A
1
-A
2
architecture
-A architecture,
To determine whether YD-TRC-NMF has a D-A
1
2
cyclic voltammetry was used to measure the electrochemical
curves of YD-TRC-NMF and its monomeric component parts. The
electrochemical measurements are shown in Fig. S3 of ESI. For re-
ported D-A system YD-TRC [44], there were one reversible oxida-
þ/0
tion wave (E1/2 , 0.28 V) ascribed to YD module and one irreversible
0/ꢂ
reduction wave (E1/2 , ꢂ0.19 V) attributed to TRC module. Once NMF
module has been attached to construct YD-TRC-NMF, the respective
electrochemical curve shows one reversible oxidation wave and
þ/0
three reversible reduction waves (Fig. S3 of SI): The E1/2 (0.3 V)
corresponds to the oxidation potential of YD module (0.28 V) while
the E 01 /2 , E1/2 and E1/2 (ꢂ0.93, ꢂ1.32 and ꢂ1.86 V) pertain to the
reduction potentials of fullerene module NMFt (ꢂ0.92, ꢂ1.30
and ꢂ1.84 V). The reduction wave of TRC module in YD-TRC-NMF is
covered by that of NMF module. Here we synthesized YD-TRC-BA so
that the LUMO energy of TRC module can be evaluated by the
reduction potential of YD-TRC-BA, and this method has been used
/ꢂ ꢂ/2ꢂ
2ꢂ/3ꢂ
in the reported D-A
AEAQ [29,44].
1 2
-A systems MTPA-TRC-AEAQ and YD-TRC-
As shown in Table 1, for YD-TRC-NMF, the HOMO and LUMOþ1
levels are attributed to YD module, and HOMO-1 and LUMO levels
pertain to NMF module, the LUMO level of TRC module is between
that of YD module and NMF module in YD-TRC-NMF. The excited
Fig. 1. Absorption of YD (olive), YD-TRC (cyan), NMFt (red), the equimolar mixture of
YD-TRC and NMFt (black), and YD-TRC-NMF (blue) in toluene. (Concentration:
ꢂ6
ꢂ1
5
ꢁ 10 mol L ). The data of YD and YD-TRC were taken from Ref. [44]. (For inter-
pretation of the references to colour in this figure legend, the reader is referred to the
web version of this article.)
Table 1
þ
Electrochemical data obtained vs Ag/Ag in dichloromethane and frontier orbital energies.
2
ꢂ/3ꢂ,b
ꢂ/2ꢂ,b
0/ꢂ,b
1/2
þ/0,b
c
c
Compounds
E
1/2
E
1/2
E
E
1/2
E
HOMO
E
LUMO
E
HOMO-1
E
LUMOþ1
(
V)
(V)
(V)
(V)
(eV)
(eV)
(eV)
(eV)
a
e
YD
e
e
e
0.24
0.28
e
ꢂ5.17
ꢂ5.21
ꢂ6.49
e
ꢂ2.53
ꢂ3.74
ꢂ4.01
ꢂ3.76
ꢂ4.00
e
e
YD-TRCa
NMFt
YD-TRC-BA
YD-TRC-NMF
e
e
ꢂ1.19
ꢂ0.92
ꢂ1.17
ꢂ0.93
e
ꢂ2.65
e
e
e
d
ꢂ1.84
e
ꢂ1.30
e
e
e
e
e
d
ꢂ1.86
ꢂ1.32
0.30
ꢂ5.23
ꢂ6.48
ꢂ2.67
a
The data were taken from Ref. [44] for comparison purposes.
V vs Ag/Ag .
b
c
þ
þ/0
0/ꢂ
E
E
E
HOMO ¼ -E1/2 -4.93eV; ELUMO ¼ -E1/2 -4.93eV.
d
e
NMF
NMF
HOMO(EHOMO-1) ¼ ELUMO-Egop , the optical band gap (Egop , the value is 2.48eV) was estimated from the onset of the absorption band of NMF module.
YD
YD
LUMO(ELUMOþ1) ¼ EHOMO þ Egop , the optical band gap (Egop , the valuesare2.64 eV in compound YD, and 2.56 eV in compound YD-TRC and YD-TRC-NMF) was estimated
from the onset of the ICT absorption band of YD module in toluene.

606
T. Wang et al. / Dyes and Pigments 139 (2017) 601e610
As shown in Fig. 2a, the emission intensity of YD module in YD-
TRC and YD-TRC-NMF decreased by 96% and 98% respectively,
compared to the reference compound YD when they are excited at
380 nm, where the YD module has a strong absorption but NMF
module has a weak absorption. The fluorescence maximum from
the YD module of YD-TRC-NMF (451 nm) is similar to that of YD-
TRC (452 nm), but red-shifted from that of compound YD
(422 nm). Meanwhile, the fluorescence of YD-TRC and YD-TRC-NMF
at 460 nm follows the bi-exponential decay equation as shown in
Table 2 and Fig. S6 of ESI. The fast decay components, which were
measured as 0.82 ns for YD-TRC and 1.02 ns for YD-TRC-NMF, are
assigned to the photo-induced electron transfer from YD to TRC
module and consecutive electron transfer from YD to TRC module
followed by an electron transfer to NMF module, respectively,
whereas the slow decay component of YD-TRC-ABF (4.29 ns) is
attributed to the salvation relaxation of YD singlet, and is clearly
shorter than that in YD-TRC (4.62 ns). This is due to the intra-
molecular photoinduced Forster energy transfer which has been
2
shown in similar D-TRC-A systems [29,44], where the energy
transfer is possible from the excited YD to the ABF module as the
fluorescence emission spectrum of YD partially overlaps with the
absorption spectrum of ABF.
When YD-TRC-NMF is excited at 470 nm (Fig. 2a), NMF module
shows mild absorption, while little absorption was found for YD
module. Therefore, only emissions from the NMF module were
found for the triad and the fluorescence was quenched by about 42%
with respect to the NMF module of compound NMFt. The fluores-
cence at 717 nm from NMF module decays bi-exponentially with
lifetimes of 0.36 and 1.65 ns (Table 2 and Fig. S7 of SI). The fast
component (0.36 ns) is attributed to the photo-induced hole transfer
.þ
from the excited NMF to YD module to form the CS state YD -TRC-
.ꢂ
NMF , and the slow component (1.65 ns) is attributed to the singlet-
triplet conversion, which is similar as the value of NMFt (1.64 ns).
To determine the lifetimes of the CS states in D-A
1
system YD-
TRC and D-A -A system YD-TRC-NMF, nanosecond transient ab-
1
2
sorption measurements were performed. Fig. 3a shows the char-
þ
acteristics of YD absorption of YD-TRC in the range 400e700 nm
with the negative
.ꢂ
states [46e48]. Fig. 3b shows the formation of CS state YD -TRC
with an estimated lifetime of 215 ns. [44]. When the second
acceptor NMF is attached to YD-TRC, significantly different tran-
sient absorption spectra and kinetics of YD-TRC-NMF have been
found. As shown in Fig. 3c, nanosecond transient absorption
DOD corresponding to the bleach of ground
.þ
Fig. 2. Fluorescence emission spectra excited at 380 nm(a) and 470 nm (b) of YD
olive), YD-TRC (cyan), NMFt (red), the equimolar mixture of YD-TRC and NMFt (black),
and YD-TRC-NMF (blue) in toluene (Concentration: 5 ꢁ 10 mol L ). (For interpre-
tation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
(
ꢂ6
ꢂ1
spectrum of YD-TRC-NMF reveals a wide peak in the 400 nm-
þ
fluorescence from the mixture still follows a bi-exponential decay
equation with similar lifetimes to those of pure YD-TRC and NMFt
6
00 nm range attributed to the YD absorption and the peak at
3
*
710 nm corresponds to triplets C60 absorption. However, the ab-
ꢂ
(Table 2 and Fig. S6 of ESI). This illustrates little change in the
sorption of anions C60 , which was reported at 1020 nm is not
photophysical processes induced by the intermolecular interaction.
observed in the spectra due to the instrument limitation [49,50].
The transient absorption at 710 nm follows a bi-exponential decay
equation as shown in Fig. 3d, yielding two lifetimes of 7.9 ms and
24.9 ms indicating that there is overlap between YD and C60 ab-
þ
3 *
Table 2
Emission lifetimes and fractions (in parentheses) of YD, YD-TRC, NMFt, the equi-
molar mixture of YD-TRC and NMFt, and YD-TRC-NMF in toluene by fitting transient
spectra with exponential decay equations.
sorption around 710 nm. The fast component (7.9
m
s) is attributed
þ
to YD and is confirmed by the single-order decay equation of
transient absorption at 550 nm, which remains unaffected by the
introduction of the oxygen (Fig. S7 of ESI). The slow component
Sample
Emission lifetime, t/ns (fraction %)
3
*
l
l
ex ¼ 366 nm
em ¼ 460 nm
l
l
ex ¼ 457 nm
em ¼ 717 nm
(24.9
Thus, the 37-fold elongation of the lifetime of CS states in YD-
TRC-NMF (7.9 s) over YD-TRC (215 ns) is determined in toluene,
ms) is ascribed to C60.
YDa
1.26
0.82(66.8)
e
e
m
YD-TRCa
and the formation of outstandingly long-lived CS state of YD-TRC-
NMF can be explained that the energy level of CS state was lower
than that of acceptor2 (NMF) triplet state (Fig. 4). This principle has
been shown in previous studies by others [51e57] ourselves
[43,44]. Additionally, the reported D-A -A system YD-TRC-AEAQ
1 2
also showed the consecutive electron transfer occurs from excited
4.62(33.2)
NMFt
YD-TRC and NMFt
e
1.64
1.63
0.81(67.1)
4
.65(32.9)
1.02(74.1)
.29(25.9)
YD-TRC-NMF
0.36(75.5)
1.65(24.5)
4
a
The data were taken from Ref. [44].
YD to TRC and then to AEAQ, resulting in a fivefold increase in the

T. Wang et al. / Dyes and Pigments 139 (2017) 601e610
607
Fig. 3. Nanosecond transient absorption spectra (a) and kinetics at 540 nm (b) of YD-TRC and nanosecond transient absorption spectra (c) and kinetics at 710 nm (d) of YD-TRC-NMF
ꢂ5
ꢂ1
in toluene following excitation with 410 nm, 8 ns laser pulses (Concentration: 1 ꢁ 10 mol L ). The red lines are the fitting curves by the single-order (b) and bi-exponential (d)
exponential decay equation. The data of YD-TRC (a & b) was taken from Ref. [44]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
lifetime of charge separation of YD-TRC-AEAQ (1.14
parison with the same D-A precursor YD-TRC (215 ns) [44]. This
indicates that NMF as the acceptor2 should be more effective than
AEAQ in forming the long-lived lifetime of CS states in D-A -A
system. As shown in Fig. 4 a total of about 94% of excited YD singlets
convert into the charge separated state in YD-TRC-ABF, indicating
m
s) in com-
than the work function of Au (ꢂ5.1 eV) (Table S1 of ESI). Fig. 5
shows device structure and the current density-voltage (J-V)
curves of the seven compounds in these solar cells under AM 1.5 G
illumination. The photovoltaic performances in cells and overall
generation efficiency from excited donor singlets and lifetimes of
CS states in toluene of corresponding compounds are given in
Table 3.
1
1
2
that D-A
1 2 1
-A architecture through D-A precursor can be used as an
important design principle to achieve CS states effectively.
For ambipolar organic semiconductors, we could not detect any
photocurrents with devices of MTPA-TRC-PDI, and MTPA-TRC-PBI
exhibits a very low short circuit current Jsc and power conversion
efficiency ƞ similar to MTPA-TRC. These correspond to their rela-
tively short lifetime of CS states due to efficient conversions from
3.3. Using long-lived CS states to improve efficiency of single layer
organic solar cells with D-A -A architectured materials
1
2
.
þ
.ꢂ
.þ
.ꢂ
the CS states MTPA -TRC-PDI and MTPA -TRC-PBI to the triplet
Using TRC as an acceptor, we have obtained two D-A type
organic semiconductors, MTPA-TRC and YD-TRC, and five D-A -A
type ambipolar organic semiconductors, MTPA-TRC-PDI, MPA-TRC-
PBI, MTPA-TRC-AEAQ, YD-TRC-AEAQ and YD-TRC-NMF. Here, we
have employed these dyes to assemble single layer organic solar
cells, FTO/Dye/Au. Consecutive photo-induced electron transfer
3
*
3
*
states MTPA-TRC- PDI and MTPA-TRC- PBI , respectively [45].
Intriguingly, device of MTPA-TRC-AEAQ shows a clearly increased
1
2
Jsc and ƞ, agreeing well with its 8-time-longer lifetime of CS states,
in comparison with those of MTPA-TRC. This suggests that the
longer lifetime of CS states for the dyes benefits their photovoltaic
performance.
As the donor was replaced by YD module, YD-TRC presented
similarly low photovoltaic characteristics as observed in MTPA-TRC,
yet its lifetime of CS states was extended to 215 ns, about 2.5 times
occurred from D to TRC to A
hole transfer occurred from A
which is allowed in these devices because LUMO of A
than the conduction band of FTO (ꢂ4.4 eV) and HOMO of D is lower
2
and then to FTO, and photo-induced
to D directly and then to Au,
is higher
2
2

608
T. Wang et al. / Dyes and Pigments 139 (2017) 601e610
than that of MTPA-TRC. The photovoltaic characteristics (Jsc and ƞ)
are significantly improved in devices of YD-TRC-AEAQ and YD-TRC-
NMF. The Jsc and ƞ are clearly higher with YD-TRC-NMF than YD-
TRC-AEAQ, though the absorption ability in visible light and over-
all generation efficiency from excited YD singlets of CS states
(Table S1 of ESI and Table 3) for YD-TRC-AEAQ, which are higher
than those for YD-TRC-NMF. This could be explained by the longer
CS states in YD-TRC-NMF.
However, although the lifetime of CS states has been prolonged
to 1.14
MTPA-TRC-AEAQ. Only YD-TRC-NMF with a CS state lifetime of 7.9
s shows comparable photovoltaic properties with MTPA-TRC-
ms, YD-TRC-AEAQ shows a relatively low Jsc and ƞ than
m
AEAQ. This may be due to the lower visible light absorption abil-
ity of YD module when compared to the MTPA module, as well as
the fact that the visible light absorption of the second acceptor in
YD-TRC-AEAQ or YD-TRC-ABF is clearly lower than that in MTPA-
TRC-AEAQ (Table S1 of ESI). The results from Table 3 indicate that,
2
as the Jsc is below 3 mA/cm , the Jsc and ƞ may increase obviously
Fig. 4. Energy level diagram and photophysical processes of YD-TRC-NMF upon exci-
tation in toluene with singlet energies of various compounds from the fluorescence
spectra and the triplet energies of NMF from literature data [49]. The energy of charge
and couple with the improvement of CS state lifetime of ambipolar
organic semiconductors, revealing a positive effect of long-lived CS
separated state YD -TRC-NMFꢃ
ꢃ
þ
ꢂ
(e
CR ¼ e(EoxꢂEred), and Eox is the first one-electron oxidation potential of the donor
YD) module, while Ered refers to the first one-electron reduction potential of the
D
G
)
was estimated by the equation
states. However, when the Jsc is beyond 3 mA/cm , the elongated
2
CR
eDG
(
lifetime of CS states yield a limited effect on the photovoltaic per-
formance of single layer organic solar cells. This means that the
charge transfer at the interfaces between organic semiconductors
and inorganic electrode significantly influences the formation of
currents. Moreover, The far better photovoltaic characteristics of D-
acceptor2 (NMF) module.
2
TRC-A than D-TRC suggests potential applications in OSCs, if better
structural designing of OSCs and extension of absorption spectrum
range of organic optoelectronic material.
4. Conclusions
In this paper, we have synthesized and characterized a new D-
A -A system: YD-TRC-NMF. Sequential electron transfers from
1 2
excited YD to TRC then to NMF module and the hole transfer from
the excited NMF to YD module were revealed in YD-TRC-NMF upon
light absorption.
The lifetime of CS state of YD-TRC-NMF is 7.9
ms, a 37-fold
elongation over YD-TRC as 215 ns. The formation of outstandingly
long-lived CS state of YD-TRC-NMF was explained by noting that
the energy level of CS state was lower than that of acceptor2 (NMF)
triplet state. This result absolutely confirms the rationale behind
using D-A
1 2 1
-A architecture via D-A precursor to obtain the long-
lived CS state.
The photovoltaic tests indicate potential applications of YD-TRC-
NMF (or AEAQ) and MTPA-TRC-AEAQ in solar cells. In these D-TRC-
A systems, the triplet states of acceptor2 are higher than CS states,
2
and the photovoltaic characteristics (Jsc and ƞ) are significantly
improved in comparison with those of their D-TRC precursors.
Long-lived CS states can play an important role in promoting
Fig. 5. Device structure and J-V curves of obtained single layer organic photovoltaic
cells FTO/Dye/Au based on MTPA-TRC (magenta), MTPA-TRC-PBI (green), MTPA-TRC-
AEAQ (wine), YD-TRC (cyan), YD-TRC-AEAQ (gray) and YD-TRC-NMF (blue). (For
interpretation of the references to colour in this figure legend, the reader is referred to
the web version of this article.)
1 2
photoelectric conversion in devices of D-A -A architectured
photovoltaic materials. However, the positive effect of CS states will
be limited as the interfacial charge transfer between organic
semiconductors and inorganic electrode, which is the ultimate
factor that determines device performance. This sheds a light on
Table 3
Photovoltaic performances of single layer organic photovoltaic cells FTO/Dye/Au
under AM 1.5 G illumination, and overall generation efficiency (4) from excited
donor singlets and lifetimes (t) of CS states of respective dyes in toluene.
1 2
the design of novel D-A -A architectures for organic photovoltaics
and other optoelectronic device.
Dyes
V
oc/V
J
sc/mA/cm2
FF/%
ƞ/%
4/%a
t
/nsa
MTPA-TRC
0.50
e
0.29
e
32.6
e
0.04
e
40
e
80
e
Acknowledgments
MTPA-TRC-PDI
MTPA-TRC-PBI
MTPA-TRC-AEAQ
YD-TRC
YD-TRC-AEAQ
YD-TRC-NMF
0.59
0.55
0.50
0.49
0.57
0.25
3.59
0.25
3.08
3.68
30.0
39.0
36.5
39.7
34.6
0.04
0.77
0.04
0.60
0.73
77
84
67
98
94
75
This work was supported by the National Nature Science
Foundation of China (Grant No. 21576195 and 21506151) and
Tianjin science and technology innovation platform program (No.
650
215
1140
7900
14TXGCCX00017). Lichang Wang acknowledges the support by the
a
The data except YD-TRC-NMF were taken from Refs. [44,45].
Tianjin 1000 talent program for her stay at Tianjin University.

T. Wang et al. / Dyes and Pigments 139 (2017) 601e610
609
Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.12.066.
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