Conjugated Oligothiophene Derivatives Based on Bithiophene with Unsaturated Bonds as Building Blocks for Solution-Processed Bulk Heterojunction Organic Solar Cells

Article abstract of DOI:10.1002/asia.201601281

A new building block ATVTA that uses stiff carbon–carbon triple bonds (A) on 1,2-di(2-thienyl)-ethene (TVT) has been developed. Oligothiophene derivatives S-01 with a TVT unit, S-02 with a 5,5′-diethynyl-2,2′-dithienyl (AT2) unit and S-03 with ATVTA were synthesized to compare their effects in a systematic study. Due to the better π-conjugation extension of the TVT unit, S-01 exhibits the most red-shifted absorption profile among them, whereas S-02 possesses the deepest HOMO level. While the HOMO level of S-03 is down-shifted by 0.02 eV relative to that of S-01, the alkyne linkages can effectively down-shift the HOMO level. By replacing the terminal units of S-03 with stronger electron acceptors, S-04 and S-05 exhibited broader absorption profiles and lower HOMO levels than those of S-03. Organic solar cells based on these molecules were fabricated and an S-03:PC₆₀BM (1:1, w/w) based device afforded the highest V_oc value of 0.96 V and a power conversion efficiency (PCE) of 2.19 %.

Full text of DOI:10.1002/asia.201601281

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Conjugated Oligothiophene Derivatives Based on Bithiophene
with Unsaturated Bonds as Building Block for Solution-Processed
Bulk Heterojunction Organic Solar Cells
Authors: Chaohua Cui, Yue Wu, Man-Sing Cheung, Cheuk-Lam Ho,
Qingchen Dong, Zhenyang Lin, Yongfang Li, and Wai Yeung
Wong
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To be cited as: Chem. Asian J. 10.1002/asia.201601281
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10.1002/asia.201601281
Chemistry - An Asian Journal
Chem. Asian J.
Conjugated Oligothiophene Derivatives Based on Bithiophene with
Unsaturated Bonds as Building Block for Solution-Processed Bulk
Heterojunction Organic Solar Cells
Chaohua Cui,*[a, b] Yue Wu,[b] Man-Sing Cheung,[c] Cheuk-Lam Ho,[a] Qingchen
Dong,[d] Zhenyang Lin,[c] Yongfang Li[b] and Wai-Yeung Wong*[a,e]
[a] Institute of Molecular Functional Materials, Department of Chemistry and Institute
of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon
Tong, Hong Kong, P.R.China. E-mail: rwywong@hkbu.edu.hk
[b] Laboratory of Advanced Optoelectronic Materials, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou
215123, P.R. China. E-mail: cuichaohua@suda.edu.cn
[c] Department of Chemistry, The Hong Kong University of Science and Technology,
Clearwater Bay, Kowloon, Hong Kong, P.R. China.
[d] MOE Key Laboratory for Interface Science and Engineering in Advanced
Materials and Research Center of Advanced Materials Science and Technology,
Taiyuan University of Technology, 79 Yingze West Street, Taiyuan 030024, P.R.
China.
[e] Department of Applied Biology and Chemical Technology, The Hong Kong
1
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10.1002/asia.201601281
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Polytechnic University, Hung Hom, Hong Kong, P.R. China. E-mail:
wai-yeung.wong@polyu.edu.hk
2
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Abstract
A new building block ATVTA which uses stiff carbon-carbon triple bonds (A) on
1,2-di(2-thienyl)-ethene (TVT) is developed. For comparing the effect of the three
building blocks, the oligothiophene derivatives S-01 with TVT unit, S-02 with
5,5’-diethynyl-2,2’-dithienyl (AT2) unit and S-03 with ATVTA were synthesized. Due
to the better π-conjugation extension of TVT unit, S-01 exhibits the most red-shifted
absorption profile among them and S-02 possesses the deepest HOMO level. While
the HOMO level of S-03 is down-shifted by 0.02 eV relative to that of S-01, the
alkyne linkages can effectively down-shift the HOMO level. By replacing the
terminal units of S-03 with the stronger electron acceptors, S-04 and S-05 were
prepared which exhibited broader absorption profiles and lower HOMO levels than
that of S-03. Organic solar cells based on these molecules were fabricated and
S-03:PC60BM (1:1, w/w) based device afforded the highest Voc value of 0.96 V and a
PCE of 2.19%.
Keywords: Alkyne; Ethene; Oligothiophene; Organic solar cell; Synthesis
3
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Introduction
Compared with solar cells based on inorganic materials, organic solar cells (OSCs)
have attracted significant attention due to their attractive features, such as low-cost,
light weight, easy fabrication and high mechanical flexibility.[1-4] Bulk heterojunction
(BHJ) solar cells are composed of a photoactive blend layer of electron-rich donor
and electron-deficient acceptor sandwiched between an indium-tin oxide (ITO)
positive electrode and a low work function metal negative electrode.[5] Nowadays,
high power conversion efficiency (PCE) is still the main concern of the studies on the
OSCs. Materials innovation is one of the most effective approaches currently that can
drive the performance of OSCs.[6-17] The key issues of donor materials design include
engineering the energy levels and bandgap to achieve high open-circuit voltage (Voc)
and short-circuit current density (Jsc), hence enhancing planarity to obtain high carrier
mobility.[18, 19]
It is well known that the π-conjugated backbone is the most important component
in determining most of the OSC-related physicochemical properties of the conjugated
donors. Thiophenes are considered as promising π-conjugated building blocks for
extending π-conjugated backbone due to their remarkable electronic and redox
properties as well as the prominent supramolecular behavior on the solid surface or in
the bulk.[20] In addition, thiophenes can be selectively modified by incorporation of
the thiophene units into oligomers which render an effective conjugation along the
main chain.[21-23] Oligothiophenes possess extensive π-electron delocalization along
the molecular backbone with well-known excellent charge transporting properties,
4
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10.1002/asia.201601281
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high polarizability, and tunable optical and electrochemical properties.[24-26] Dipolar
push-pull chromophores with highly polarizable π-electron systems involving a donor
group and an acceptor group can serve as excellent donor materials in OSCs. They
possess unique features such as the well-defined structures which make them
synthetically reproducible, widen the light absorbing region to cover the low-energy
spectrum, and favor the self-organization of the oligomers into ordered structures and
close packing with increasing π-π orbital overlap due to the strong intermolecular
interactions.[27-29] Enlargement of the π systems is regarded as an efficient way to
broaden the absorption spectra of donor materials to narrow their optical bandgaps,
which is desirable for enhancing the Jsc value.[30-33] For example, the essential
structural feature of polymeric semiconductors in OSC applications is a π-conjugated
backbone composed of linked unsaturated units, resulting in extended π orbitals along
the polymer chain, and thus enabling proper charge transport and optical
absorption.[23, 34-36] 1,2-Di-(2-thienyl)-ethene (TVT) is a promising building block to
extend π orbitals of the semiconducting materials.[23, 37] Nevertheless, due to its
electron-rich property, the HOMO energy levels of the resulting donor materials
would probably be pushed up at the same time by introducing the TVT unit into the
backbone, which will decrease the Voc value. Here, we develop a new building block
ATVTA (Scheme 1) which uses stiff carbon-carbon triple bonds as the linkage on
TVT unit to tackle this issue. Introducing alkyne unit as the linkage is more
accommodating than the alkene because of its reduced steric and conformational
constraints.[38, 39] We employed ATVTA as a building block of oligothiophene to
5
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10.1002/asia.201601281
Chemistry - An Asian Journal
synthesize S-03 (Scheme 1). The corresponding oligothiophenes S-01 with TVT unit
and S-02 with with 5,5’-diethynyl-2,2’-dithienyl (AT2) unit were also synthesized for
comparing the effect of the three building blocks (i.e. TVT, AT2 and ATVTA). The
three small molecules show excellent solution processability, thermal stability and
broad and strong absorption bands in the visible and near-infrared regions. Due to the
better π-conjugation extension of TVT unit, S-01 exhibits the most red-shifted
absorption profile among the three molecules (S-01 to S-03), and S-03 shows slightly
red-shifted absorption spectrum relative to that of S-02. On the other hand, S-02
possesses the deepest HOMO energy level among the three molecules. Since the
HOMO energy level of S-03 is down-shifted by 0.02 eV as compared to that of S-01,
the alkyne linkages are shown to down-shift the HOMO energy level of the resulting
molecule. Using the simple spin-coating fabrication process to fabricate BHJ OSCs,
the S-01 and S-02 based devices exhibited similar Voc of 0.85 and 0.86 V, respectively,
while S-03 based device afforded a very high Voc value of 0.96 V, leading to a PCE of
2.19%. We replaced the terminal unit of S-03 by the strong electron-withdrawing
groups in S-04 and S-05 in order to further examine the effect of ATVTA building
block. S-04 and S-05 exhibited broader absorption profiles and lower HOMO energy
levels than that of S-03, which can be attributed to the stronger terminal acceptor units
of S-04 and S-05. The OSC devices based on S-04 and S-05 also demonstrated high
Voc values of 0.98 and 0.99 V, respectively. The results indicate that ATVTA unit is a
promising building block for extending π-conjugated backbone without pushing up
the HOMO energy levels, which guarantee the high Voc value.
6
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10.1002/asia.201601281
Chemistry - An Asian Journal
S
S
S
S
S
S
TVT
ATVTA
AT2
C H
C H
6
13
6
13
CN
O
S
S
O
S
C H
8
17
S
S
S
O
C H
8
17
O
C H
6
13
CN
C H
6
13
S-01
C H
6
13
CN
C H
6
13
O
S
S
C H
8
17
O
O
S
S
O
S
S
C H
8
17
C H
6
13
CN
C H
6
13
S-02
C H
6
13
CN
C H
6
13
O
S
S
C H
8
17
S
O
S
O
S
S
C H
8
17
C H
O
6
13
CN
S-03
C H
6
13
C H
6
13
CN
C H
6
13
S
S
NC
S
S
S
S
CN
C H
6
13
NC
C H
6 13
S-04
S
N
C H
6
13
O
N
C H
6
13
O
S
S
O
S
S
N
S
S
S
N
C H
6
13
O
C H
6
13
S-05
Scheme 1 Structures of S-01, S-02, S-03, S-04 and S-05.
Results and Discussion
Syntheses and Characterization
The general synthetic routes towards all of the new compounds are outlined in
Scheme 2. The key intermediate compound 4 was synthesized from Stille coupling
between compound 2 and tributyl(4-hexylthiophen-2-yl)stannane, followed by
bromination. This compound then underwent palladium-catalyzed Sonogashira
7
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10.1002/asia.201601281
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coupling reaction with trimethylsilylacetylene (TMSA) and the terminal alkyne was
obtained by proto-desilylation approach using basic methanol solution to give the
ethynyl precursor 6. Bis-stannylated compound 9 was prepared in quantitative yield
by forming dilithiated species followed by quenching with tributyltin chloride. The
aldehydes 11, 12 and 13 were then synthesized by Stille or Sonogashira coupling
between compounds 4 and 9, compounds 6 and 10, as well as compounds 6 and 8,
respectively. The target molecules S-01, S-02 and S-03 were prepared by
Knoevenagel condensation of aldehydes with octyl cyanoacetate in good yields. S-04
and S-05 were obtained by Knoevenagel condensation between an
aldehyde-containing compound 13 and malononitrile and 1,3-diethyl-2-thiobarbituric
acid, respectively.
All the small molecules exhibit excellent solubility in common organic solvents
such as chloroform, toluene, dichloromethane and chlorobenzene. Their structures
1
were unequivocally characterized by the techniques such as mass spectrometry, H
and 13C NMR spectroscopy. The thermal stabilities of S-01 to S-05 were investigated
by thermogravimetric analysis (TGA) and the temperatures with 5% weight-loss (Td)
o
o
o
o
o
are 355 C, 327 C, 322 C, 405 C and 373 C, respectively. The stabilities of the
three compounds are adequate for applications in OSCs and other optoelectronic
devices. Figure 1 depicts the thermograms of S-01 to S-05.
8
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C6H13
C6H13
C6H13
Br
C6H13
Br
POCl3 / DMF
90 oC, 12 h
Bu3Sn
CHCl3 / AcOH
S
OHC
NBS, 0.5 h
Toluene, Pd(PPh3)4
110 oC, 24 h
S
S
S
1
2
C6H13
C6H13
S
CHCl3 / AcOH
NBS, 12 h
CH2Cl2 / Et3N, CuI, Pd(PPh3)4
TMSA, 60 oC, overnight
S
OHC
S
Br
OHC
S
C6H13
4
C6H13
3
C6H13
S
C6H13
CH2Cl2 / MeOH
OHC
S
S
TMS
OHC
S
K CO ,
6 h
3
2
C6H13
5
C6H13
6
CHCl3 / AcOH
NIS, 2 h
Zn / TiCl4
THF
S
S
I
I
OHC
S
S
S
8
7
S
SnBu3
THF, n-BuLi
Bu3Sn
S
Bu3SnCl, 24 h
9
CHCl3 / AcOH
NIS, 2 h
S
S
I
I
S
S
10
9
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10.1002/asia.201601281
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C6H13
S
C6H13
Toluene,
Pd(PPh3)4
S
S
OHC
+
CHO
4
9
S
S
S
110 oC, 24 h
C6H13
C6H13
11
CHCl3 / Et3N
Octyl cyanoacetate
40 h
C6H13
S
C6H13
CN
O
C8H17
S
O
O
S
S
O
S
S
C8H17
CN
C6H13
C6H13
S-01
C6H13
C6H13
THF / Et3N
S
S
OHC
S
S
+
6
10
CHO
Pd(PPh3)4, CuI
50 oC, 24 h
S
S
C6H13
C6H13
12
CHCl3 / Et3N
Octyl cyanoacetate
40 h
C6H13
S
CN
O
C6H13
C8H17
S
O
O
S
S
O
S
S
C8H17
C6H13
CN
S
C6H13
S-02
C6H13
C6H13
THF / Et3N
S
S
OHC
+
6
8
S
Pd(PPh3)4, CuI
50 oC, 24 h
CHO
S
S
C6H13
13
C6H13
Scheme 2 Synthetic routes for S-01, S-02, S-03, S-04 and S-05.
10
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S-01
S-02
S-03
S-04
S-05
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
100
200
300
400
500
600
Temperature (oC)
Figure 1 TGA plots of S-01to S-05 with a heating rate of 20 oC min–1 under nitrogen
atmosphere.
Optical and Electrochemical Properties
Optical and redox properties of the compounds are important parameters for their
application in OSCs. Figure 2 shows the ultraviolet-visible (UV-vis) absorption
spectra of S-01, S-02, S-03, S-04 and S-05 in CH2Cl2 and in solid state. All of them
exhibit strong absorption in the visible region, and the absorption coefficients of S-01,
S-02, S-03, S-04 and S-05 are 1.1 × 105, 8.0 × 104, 6.3 × 104, 5.9 × 104 and 4.6 × 104
M–1 cm–1, respectively. The well-defined absorption peaks with a vibronic shoulder in
the long wavelength range (590-630 nm) of S-01 to S-03 imply the existence of
ordered aggregation and strong π-π stacking. S-01 film displays considerably broad
and strong absorption profile with the absorption edge at 726 nm. By comparing S-01
with S-03, the introduction of two carbon-carbon triple bonds into the system (S-03)
11
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10.1002/asia.201601281
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makes its absorption edge more blue-shifted (a blue-shift of 55 nm) than that of S-01.
The absorption edge of S-02 is also blue-shifted as compared with S-01. It appears
that the carbon-carbon double bond acts as a better linker than the carbon-carbon
triple-bond for conjugation extension to red-shift the absorption wavelength. Due to
the strong electron accepting ability of the terminal units, S-04 and S-05 showed
red-shifted absorption spectra than S-03, and S-05 has the most red-shifted absorption
band edge at ca. 723 nm with the corresponding band gap of 1.72 eV. The optical data
of the compounds are summarized in Table 1 for a clear comparison.
S-01
1.0
0.8
0.6
0.4
0.2
0.0
S-02
S-03
S-04
S-05
(a)
300
400
500
600
700
Wavelength (nm)
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S-01
S-02
S-03
S-04
S-05
1.0
0.8
0.6
0.4
0.2
0.0
(b)
300
400
500
600
700
800
Wavelength (nm)
Figure 2 Absorption spectra of S-01 to S-05 in (a) CH2Cl2 and (b) thin film.
Figure 3 shows the cyclic voltammograms of S-01, S-02, S-03 S-04 and S-05 films
on glassy carbon electrode in 0.1 mol L–1 Bu4NPF6 acetonitrile solution. The onset
reduction potentials (Ered) of S-01, S-02 and S-03 are –1.17 V, –1.23 V and –1.16 V
vs. Ag/Ag+, and the onset oxidation potentials (Eox) are 0.54 V, 0.64 V and 0.46 V vs.
Ag/Ag+, respectively. Due to the stronger electron acceptance ability of their terminal
groups, S-04 and S-05 show higher oxidation (0.67 and 0.68 V) potential than S-03.
The HOMO and LUMO levels are calculated from the onset potentials of oxidation
and reduction and by assuming the energy level of ferrocene/ferrocenium (Fc/Fc+) to
be –4.8 eV below the vacuum level.[40] The formal potential of Fc/Fc+ was measured
as 0.07 V against Ag/Ag+. The HOMO and LUMO energy levels of S-01 to S-05
were calculated according to the following equations:
HOMO = –e(Eox + 4.73) (eV)
LUMO = –e(Ered + 4.73) (eV)
where the units of Eox and Ered are V vs. Ag/Ag+.
13
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The HOMO energy levels of S-01, S-02 and S-03 are –5.27, –5.37 and –5.29 eV,
respectively. Among S-01, S-02 and S-03, S-02 shows the lowest HOMO energy
level, while S-03 shows a slightly lower HOMO energy level than S-01. Their
calculated LUMO energy levels are –3.54, –3.55 and –3.58 eV, respectively. The
LUMO energy levels of S-04 and S-05 are –3.69 and –3.44 eV, and the HOMO
energy levels are relatively low at –5.40 and –5.41 eV, respectively.
S-01
S-02
S-03
S-04
S-05
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential
(
V vs. Ag/Ag+
)
Figure 3 Cyclic voltammograms of the films for S-01, S-02, S-03, S-04 and S-05 on
glassy carbon electrode in 0.1 mol L–1 Bu4NPF6 acetonitrile solution at a scan rate of
100 mV s–1.
Table 1 Optical and electrochemical properties of S-01, S-02, S-03, S-04 and S-05.
λ
edge [nm] ε [M–1 cm–1] Egopt [eV]a HOMO [eV] b
LUMO [eV] b
–3.56
S-01
726
1.1 × 105
1.71
–5.27
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682
671
679
723
8.0 × 104
6.3 × 104
5.9 × 104
4.6 × 104
1.82
1.85
1.83
1.72
–5.37
–5.29
–5.40
–5.41
–3.55
S-02
S-03
S-04
S-05
–3.57
–3.69
–3.44
opt
aEgopt, estimated from the optical absorption band edge of the thin film, Eg
1240/λedge. bHOMO = –e(Eox + 4.73) (eV); LUMO = –e(Ered + 4.73) (eV).
=
Theoretical Calculations
To provide further insight into the fundamental aspects of molecular architecture,
density functional theory (DFT) calculations for S-01 to S-05 were carried out. The
molecular geometries, surface plots and HOMO and LUMO energy levels are
displayed in Figure 4, and the results are shown in Table 2. It can be seen that
although discrepancies exist between the calculation and experimental results, the
trends of variation in the HOMO and LUMO energy levels are similar. From the
results, the HOMO and LUMO are evenly distributed on the whole π-conjugated
systems of the molecules.
The estimated bandgaps of S-01, S-02 and S-03 obtained from DFT calculations
were found to be 2.04 eV, 2.19 eV and 2.09 eV, respectively. According to the
calculation results, the introduction of carbon-carbon triple bonds as linkage increases
the HOMO-LUMO gaps. In other words, carbon-carbon triple bond is not as a good
linkage as the carbon-carbon double bond for extending π-conjugation. S-04 and S-05
show lower estimated bandgaps at 2.02 eV and 1.96 eV, respectively.
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Figure 4 HOMO and LUMO energy levels and the frontier molecular orbitals
obtained from DFT calculations on S-01 to S-05.
Table 2 TDDFT calculation results of S-01 to S-05.
The largest coefficient in The largest coefficient in
the CI expansion of the T1 the CI expansion of the S1
The oscillator strength
(f) of the S0→S1
transition
Compound
statea (S0→S1 excitation
energy)
statea (S0T1 excitation
energy)
2.8506
2.9374
3.2557
2.6983
2.9291
S-01
S-02
S-03
S-04
S-05
HL: 0.71 (1133.0 nm)
HL: 0.66 (986.2 nm)
HL: 0.67 (1080.0 nm)
HL: 0.67 (1109.4 nm)
HL: 0.59 (1089.3 nm)
HL: 0.65 (681.1 nm)
HL: 0.66 (637.5 nm)
HL: 0.66 (667.7 nm)
HL: 0.66 (685.7 nm)
HL: 0.66 (716.2 nm)
aHL represents the HOMO to LUMO transition. CI stands for configuration interaction.
Photovoltaic Properties
Photovoltaic performance characteristics of the OSCs are summarized in Table 3.
The optimized donor/acceptor ratio of S-01 to S-03 based devices is 1:1 w/w, while
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the optimized donor/acceptor ratio of S-04 and S-05 based devices is 1:3 w/w. Figure
5 shows the J-V curves of the OSCs based on S-01:PC60BM (1:1, w/w), S-02:PC60BM
(1:1, w/w), S-03:PC60BM (1:1, w/w), S-04:PC60BM (1:3, w/w) and S-05:PC60BM
(1:3, w/w) under the illumination of AM 1.5 G at 100 mW cm–2. The PCE of the OSC
based on S-01:PC60BM (1:1, w/w) is 2.13%, with Voc = 0.85 V, Jsc = 4.26 mA cm–2
and FF = 0.589. Much lower Jsc (2.88 mA cm–2) and FF (0.455) were measured from
S-02:PC70BM (1:1, w/w) based OSC, leading to an overall decrease of PCE to 1.13%.
By using ATVTA as the building block for π-conjugation extension in S-03, an
elevated Voc value of 0.96 V was achieved from S-03:PC60BM (1:1, w/w) based
device, with a higher PCE of 2.19%, a Jsc of 4.57 mA cm–2 and a FF of 0.50. It is well
known that Voc is tightly correlated with the energy level difference between the
HOMO of the donor and the LUMO of the acceptor. Furthermore, the morphology of
active layer has also been demonstrated to have a noticeable effect on the Voc.[41] Thus,
the more well-defined morphology probably is a factor that leads to higher Voc of S-03
based device than that of S-01 and S-02 based devices in this case.
Satisfactory Voc values were also achieved from the S-04 or S-05 based devices.
Device based on S-04:PC60BM (1:3, w/w) exhibited a PCE of 1.02%, with a high Voc
of 0.98 V. The device based on S-05:PC60BM (1:3, w/w) has a slightly higher Voc of
0.98 V, with a enhanced Jsc of 4.48 mA cm–2, leading to a PCE of 1.22%. The results
indicate that the new unit ATVTA is a promising building block to guarantee the
low-lying energy levels while extending π-conjugation of the molecules.
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Table 3 The photovoltaic performance of the OSCs based on S-01, S-02, S-03, S-04
or S-05 as donor, PC60BM as acceptor, under the illumination of AM 1.5 G at 100
mW cm–2.
Active layer
Voc [V]
0.85
0.86
0.96
0.98
0.99
Jsc [mA cm–2]
4.26
FF
PCE [%]
2.13
S-01:PC60BM (1:1)
S-02:PC60BM (1:1)
S-03:PC60BM (1:1)
S-04:PC60BM (1:3)
S-05:PC60BM (1:3)
0.589
0.455
0.50
2.88
1.13
4.57
2.19
3.40
0.308
0.276
1.02
4.48
1.22
6
4
S-01:PC60BM = 1:1
S-02:PC60BM = 1:1
S-03:PC60BM = 1:1
S-04:PC60BM = 1:3
S-05:PC60BM = 1:3
2
0
-2
-4
-6
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage (V)
Figure 5 The J-V curve of the OSCs based on S-01:PC60BM (1:1, w/w),
S-02:PC60BM (1:1, w/w), S-03:PC60BM (1:1, w/w), S-04:PC60BM (1:3, w/w) and
S-05:PC60BM (1:3, w/w) under the illumination of AM 1.5 G at 100 mW cm–2.
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The external quantum efficiency (EQE) of the OSCs based on S-01:PC60BM (1:1,
w/w), S-02:PC60BM (1:1, w/w), S-03:PC60BM (1:1, w/w), S-04:PC60BM (1:3, w/w)
and S-05:PC60BM (1:3, w/w) are shown in Figure 6. The Jsc values derived from
integration of the EQE curve are rather consistent (less than 5% error) with the values
obtained by J-V measurements. The maximum EQE value of the OSC based on
S-03:PC60BM (1:1, w/w) reached 35% at 543 nm. The maximum EQE values of the
S-03, S-04 and S-05 based devices are ~28% at around 480 nm.
S-01
S-02
S-03
S-04
35
30
25
S-05
20
15
10
5
0
300
400
500
600
700
Wavelength (nm)
Figure 6 The EQE of the OSCs based on S-01:PC60BM (1:1, w/w), S-02:PC60BM
(1:1, w/w), S-03:PC60BM (1:1, w/w), S-04:PC60BM (1:3, w/w) and S-05:PC60BM
(1:3, w/w).
Morphology Characterization
A nanoscale phase separation and bicontinuous donor/acceptor (D/A)
interpenetrating network in the active layer is desirable for high efficiency charge
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separation and transportation to get high photovoltaic performance. To better
understand the morphology effect on the photovoltaic performance of the devices, the
morphology of the active layer of S-01:PC60BM (1:1, w/w), S-02:PC60BM (1:1, w/w)
and S-03:PC60BM (1:1, w/w) was studies by transmission electron microscopy (TEM).
As shown in Figure 7, the blend films of S-01:PC60BM (1:1, w/w) and S-03:PC60BM
(1:1, w/w) gave a similar and better bicontinuous D/A interpenetrating network and
relatively well-developed fibrillar structure, which should favor high efficiency
exciton dissociation and charge transportation. While the blend film of S-02:PC60BM
(1:1, w/w) exhibited a larger domain size and a less defined phase separation, it would
result in the overall lower photovoltaic performance of the S-02 based OSCs.
Figure 7 TEM images of the blend films of (a) S-01:PC60BM (1:1, w/w), (b)
S-02:PC60BM (1:1, w/w) and (c) S-03:PC60BM (1:1, w/w).
SCLC Mobilities
It is well known that a balanced charge-carrier transport is desirable for high FF.[42]
In order to investigate charge transport properties of the S-01:PC60BM (1:1, w/w),
S-02:PC60BM (1:1, w/w) and S-03:PC60BM (1:1, w/w) blend films, both the electron
(µe) and hole mobilities (µh) were measured by using the space-charge-limited current
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(SCLC) method.[43] The corresponding blend film dependency of the hole and the
electron mobilities are shown in Figure 8. The detailed hole and electron mobilities of
the blend films of S-01:PC60BM (1:1, w/w), S-02:PC60BM (1:1, w/w) and
S-03:PC60BM (1:1, w/w) are summarized in Table 4. The same order of magnitude for
the hole mobilities as well as electron mobilities of the blend films based on the three
molecules was obtained, respectively. The blend film of S-01:PC60BM exhibited more
balanced µh and µe than those of S-02:PC60BM and S-03:PC60BM, resulting in the
higher FF value of the corresponding OSCs obtained.
105
(a)
104
103
S-01
S-02
102
S-03
Fitting curves
101
100
0
1
2
3
4
5
Voltage (V)
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103
102
101
100
(b)
S-01
S-02
S-03
Fitting curves
0
1
2
3
4
Voltage (V)
Fig. 8 The current-voltage plots of (a) the electron-only devices and (b) hole-only
devices.
Table 4 Hole and electron mobilities of the blend films of S-01:PC60BM (1:1, w/w),
S-02:PC60BM (1:1, w/w) and S-03:PC60BM (1:1, w/w)
Blend films
μe (cm2 v–1 s–1)
4.67 × 10–4
5.53 × 10–4
1.40 × 10–4
μh (cm2 v–1 s–1)
1.34 × 10–3
4.31 × 10–3
1.02 × 10–3
μh /μe
2.87
7.79
7.28
S-01:PC60BM (1:1)
S-02:PC60BM (1:1)
S-03:PC60BM (1:1)
Conclusion
A new building block ATVTA which utilizes stiff carbon-carbon triple bonds as the
linkage on TVT unit is developed. We have designed and synthesized three small
molecules S-01, S-02 and S-03 with TVT, AT2 and ATVTA as the building unit,
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respectively, to investigate their physicochemical and photovoltaic properties. S-01,
S-02 and S-03 possess good solubility in common organic solvents, broad visible
absorption at 350–650 nm and low-lying HOMO energy levels at –5.27 eV, –5.37 eV
and –5.29 eV, respectively. The OSCs based on S-01:PC60BM (1:1, w/w) and
S-02:PC60BM (1:1, w/w) exhibited a high Voc of 0.85 V and 0.86 V, respectively.
Especially, device based on S-03 with ATVTA building block showed a higher Voc of
0.96 V, with a PCE of 2.19%. The result indicates that ATVTA unit is a promising
building block for extending π conjugation of the molecules without pushing up their
HOMO energy levels. We changed the terminal unit of S-03 by the strong
electron-withdrawing groups to give S-04 and S-05. The devices based on S-04 and
S-05 exhibited a slightly higher Voc of 0.98 V and 0.99 V, respectively. This present
work demonstrates that the ATVTA building unit offers a good avenue toward
designing new photovoltaic small molecule-based donor materials with high Voc for
OSCs.
Experimental Section
Physical Measurements
NMR spectra were measured in CDCl3 on Bruker AV 400 MHz FT-NMR
1
spectrometer and chemical shifts are quoted relative to tetramethylsilane for H and
13C nuclei. The 1H and 13C NMR spectra of S-01 to S-05 are shown in the Supporting
Information, and the NMR data are consistent with the proposed structures.
Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TGA-7. The
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positive-ion fast atom bombardment (FAB) mass spectra were recorded in
m-nitrobenzyl alcohol matrices on a Finngin-MAT SSQ710 mass spectrometer and
high resolution (HR) MALDI-TOF (matrix-assisted laser desorption
ionization/time-of-flight) spectra were obtained by a Autoflex Bruker MALDI-TOF
mass spectrometer. UV-vis absorption spectra were obtained on a Hitachi U-3010
spectrometer. Cyclic voltammetry was performed on a Zahner IM6e electrochemical
workstation with a three-electrode system in a solution of 0.1 M Bu4NPF6 acetonitrile
solution at a scan rate of 100 mV s–1. Glassy carbon coated with the relevant
compound as a thin film was used as the working electrode. A Pt wire was used as the
counter electrode and Ag/Ag+ was used as the reference electrode. The geometries of
the compounds were optimized using density functional theory (DFT) with B3LYP
functional and 6-31G (d, p) basis set. In particular, the HOMO and LUMO energy
levels and related electron distributions were calculated. The alkyl chains were
replaced by methyl groups to save the computational time within a reasonable
simplification.
Device Fabrication and Characterization of OSCs
OSCs were fabricated with ITO glass as a positive electrode, Ca/Al as a negative
electrode and the blend films of the S-01 to S-05/PC60BM between them as a
photosensitive layer. The ITO glass was pre-cleaned and modified by a thin layer of
PEDOT:PSS which was spin-cast from a PEDOT:PSS aqueous solution on the ITO
substrate, and the thickness of the PEDOT:PSS layer is about 30 nm. The
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photosensitive layer, consisting of the donor material and fullerene derivative, was
obtained by dissolving the mixture in the chloroform solvent and spin-coated on the
ITO/PEDOT:PSS electrode. Finally, the Ca/Al cathode was deposited on the active
layer by vacuum evaporation under 1 × 10–4 Pa. The effective area of the device is 4
mm2. The characterization of the current density-voltage (J-V) curve was done under
an inert nitrogen atmosphere and the EQEs of the devices without encapsulation were
measured in air.
Mobility Measurements
The hole and electron mobilities were calculated by using the space-charge-limited
current (SCLC) method based on the following equation.43c
9
3
J   00V 2 exp(0.89 V / E0L) / L
8
where  is the dielectric constant of the polymers, 0 is the permittivity of the vacuum,
µ0 is the zero-field mobility, E0 is the characteristic field, J is the current density, and
L is the thickness of the film.
Syntheses
2-Bromo-3-hexylthiophene (1)
A solution of 3-hexylthiophene (5 g) in chloroform and acetic acid (100 mL, 1:1, v/v)
o
was stirred at 0 C, and N-bromosuccinimide (5.56 g) was added in small portions.
After being stirred for half an hour at room temperature, the reaction mixture was
poured into water and extracted with CH2Cl2. The organic layer was washed with
water three times and dried over Na2SO4. After the removal of solvent, the crude
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product was purified by column chromatography on silica gel using hexane as eluent
to afford compound 1 (7 g, yield: 95%) as a colorless oil.
1H NMR (400 MHz, CDCl3), δ (ppm): 7.18−7.17 (d, J = 4 Hz, 1H, Ar), 6.80−6.78 (d,
J = 8 Hz, 1H, Ar), 2.58−2.54 (t, J = 16 Hz, 2H, alkyl), 1.59−1.54 (m, 2H, alkyl),
13
1.35−1.27 (m, 6H, alkyl), 0.90−0.87 (m, 3H, alkyl). C NMR (100 MHz, CDCl3), δ
(ppm): 141.99, 128.25, 128.13, 108.82 (Ar), 31.66, 29.74, 29.43, 28.93, 22.63, 14.11
(alkyl). MALDI-TOF m/z 246.1 [M−1]+.
5-Bromo-4-hexylthiophene-2-carbaldehyde (2)
Dry DMF (3 mL) was added dropwise to POCl3 (4 mL) and the mixture was stirred at
0
oC under argon. After stirring at 0 oC for half an hour, 4 g of
2-bromo-3-hexylthiophene (1) (16.2 mmol) in 1,2-dichloroethane (30 mL) was added
to the mixture which was stirred at 90 oC for 12 h. Then the mixture was poured into
ice water (200 mL), neutralized with NaHCO3 and then extracted with CH2Cl2. The
organic layer was washed with water three times and dried over Na2SO4. After the
removal of solvent, the residue was chromatographed on silica gel using a mixture of
hexane and dichloromethane (5:1) as eluent to afford the target product as a light
yellow oil (3 g, yield: 67%).
1H NMR (400 MHz, CDCl3), δ (ppm): 9.75 (s, 1H, CHO), 7.46 (s, 1H, Ar), 2.61–2.57
(t, J = 16 Hz, 2H, alkyl), 1.62–1.56 (m, 2H, alkyl), 1.36–1.29 (m, 6H, alkyl),
0.90–0.87 (m, 3H, alkyl). 13C NMR (100 MHz, CDCl3), δ (ppm): 181.85 (CHO),
143.99, 142.90, 136.81, 108.82 (Ar), 31.66, 29.74, 29.43, 28.93, 22.63, 14.11 (alkyl).
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MALDI-TOF: m/z 276.0 [M+1]+.
3,4'-Dihexyl-(2,2'-bithiophene)-5-carbaldehyde (3)
5-Bromo-4-hexylthiophene-2-carbaldehyde (2) (3 g, 10.8 mmol) and
2-(tributylstannyl)-4-octylthiophene (5.7 g, 13 mmol) were charged into a flask with
50 mL toluene, the solution was flushed with nitrogen for 10 min, and 100 mg of
Pd(PPh3)4 was added. The solution was flushed again for 10 min. After being stirred
o
at 110 C for 24 h under nitrogen protection, the reaction mixture was poured into
water (100 mL) and extracted with CH2Cl2. The organic layer was washed with water
and dried over Na2SO4. After the removal of solvent, the crude product was purified
by column chromatography on silica gel using a mixture of hexane and
dichloromethane (5:1) as eluent to afford compound 3 (3.4 g, yield: 85%) as a light
yellow oil.
1H NMR (400 MHz, CDCl3), δ (ppm): 9.82 (s, 1H, CHO), 7.57 (s, 1H, Ar), 6.96 (s,
1H, Ar), 2.76–2.72 (t, 2H, alkyl), 2.59–2.55 (t, 2H, alkyl), 1.69–1.56 (m, 4H, alkyl),
1.42–1.26 (m, 12H, alkyl), 0.91–0.87 (m, 6H, alkyl). 13C NMR (100 MHz, CDCl3), δ
(ppm): 182.49 (CHO), 144.11, 141.74, 140.11, 138.97, 134.61, 128.82, 122.17 (Ar),
31.68, 31.62, 30.38, 30.29, 29.30, 29.16, 29.04, 28.89, 22.69, 22.63, 14.14, 14.10
(alkyl). MALDI-TOF: m/z 362.2 [M]+.
5'-Bromo-3,4'-dihexyl-(2,2'-bithiophene)-5-carbaldehyde (4)
A solution of 3,4'-dihexyl-(2,2'-bithiophene)-5-carbaldehyde (3) (3.4 g 9.4 mmol) in
27
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chloroform and acetic acid (100 mL, 1:1, v/v) was stirred at 0 oC,
N-bromosuccinimide (1.84 g, 10.3 mmol) was added in small portions. After being
stirred for 12 h at room temperature, the reaction mixture was poured into water and
extracted with CH2Cl2. The organic layer was washed with water three times and
dried over Na2SO4. After the removal of solvent, the crude product was purified by
column chromatography on silica gel using hexane as eluent to afford compound 4
(3.7 g, yield: 90%) as a yellow oil.
1H NMR (400 MHz, CDCl3), δ (ppm): 9.82 (s, 1H, CHO), 7.57 (s, 1H, Ar), 6.96 (s,
1H, Ar), 2.76–2.72 (t, 2H, alkyl), 2.59–2.55 (t, 2H, alkyl), 1.69–1.56 (m, 4H, alkyl),
1.42–1.26 (m, 12H, alkyl), 0.91–0.87 (m, 6H, alkyl). 13C NMR (100 MHz, CDCl3), δ
(ppm): 182.52 (CHO), 142.97, 140.52, 140.49, 140.43, 138.82, 134.35, 128.27,
111.22 (Ar), 31.61, 31.52, 30.31, 29.28, 29.12, 28.90, 22.69, 22.62, 14.16, 14.12
(alkyl). MALDI-TOF: m/z 442.0 [M+1]+.
3,4'-Dihexyl-5'-((trimethylsilyl)ethynyl)-(2,2'-bithiophene)-5-carbaldehyde (5)
To a solution (50 mL) of compound 4 (2 g, 4.5 mmol) in NEt3 and CH2Cl2 (1:1, v/v)
was added a catalytic amount of CuI and Pd(PPh3)4 (0.2 g) under nitrogen. After
stirring for 30 min at room temperature, trimethylsilylacetylene (1.76 g, 18 mmol)
o
was added and then the mixture was stirred overnight at 60 C. The reaction mixture
was dried and the crude product was purified by column chromatography on silica gel
using hexane and dichloromethane (1:1, v/v) as eluent to give compound 5 (1.5 g,
yield: 78%) as a yellow oil. 1H NMR (400 MHz, CDCl3), δ (ppm): 9.81 (s, 1H, CHO),
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7.56 (s, 1H, Ar), 7.01 (s, 1H, Ar), 2.79–2.75 (t, J = 16 Hz, 2H, alkyl), 2.70–2.66 (m,
4H, alkyl), 1.42–1.26 (m, 12H, alkyl), 0.91–0.86 (m, 6H, alkyl), 0. 26 (s, 9H, alkyl).
13C NMR (100 MHz, CDCl3), δ (ppm): 182.48 (H, alkyl), CHO), 149.65, 140.89,
140.53, 140.44, 138.88, 134.96, 128.17, 119.01 (Ar), 85.38, 76.07 (C≡C), 31.60,
30.26, 30.09, 29.46, 29.43, 29.15, 28.93, 22.69, 22.61, 14.15, 14.11, 14.08, 3.40
(alkyl). MALDI-TOF: m/z 458.2 [M]+.
5'-Ethynyl-3,4'-dihexyl-(2,2'-bithiophene)-5-carbaldehyde (6)
Compound 5 (1.5 g, 3.4 mmol) and K2CO3 (0.5 g, 3.7 mmol) were dissolved in a
solvent mixture of MeOH and CH2Cl2 (1:1, v/v). The mixture was stirred for 6 h at
room temperature. It was poured into water and extracted with CH2Cl2. The organic
layer was washed with water three times and then dried over Na2SO4. After the
removal of solvent, the crude product was purified by column chromatography on
silica gel using hexane and dichloromethane (1:1, v/v) as eluent to afford compound 6
(0.78 g, yield: 60%) as a yellow oil.
1H NMR (400 MHz, CDCl3), δ (ppm): 9.82 (s, 1H, CHO), 7.57 (s, 1H, Ar), 7.03 (s,
1H, Ar), 3.56 (s, 1H, C≡CH), 2.79–2.75 (t, J = 16 Hz, 2H, alkyl), 2.72–2.68 (t, J = 16
Hz, 2H, alkyl), 1.69–1.60 (m, 4H, alkyl), 1.42–1.26 (m, 12H, alkyl), 0.91–0.86 (m,
13
6H, alkyl). C NMR (100 MHz, CDCl3), δ (ppm): 182.48 (CHO), 149.65, 140.89,
140.53, 140.44, 138.88, 134.96, 128.17, 119.01 (Ar), 85.38, 76.07 (C≡C). 31.71,
31.55, 30.21, 29.48, 29.13, 28.70, 22.67, 22.62, 14.16, 14.12 (alkyl), MALDI-TOF:
m/z 386.1 [M]+.
29
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