Indacenodithiophene core-based small molecules with tunable side chains for solution-processed bulk heterojunction solar cells

Article abstract of DOI:10.1039/c3ta14659f

Two indacenodithiophene (IDT) core-based small molecules with different side chains of bulky 4-hexylphenyl and flexible n-dodecyl with the same number of carbon atom, namely SM1 and SM2, respectively, were designed and synthesized as the donor materials in organic solar cells (OSCs). The impacts of the different side chains combined with the IDT core on the optical absorption, electrochemical property, hole mobility, film morphology, and solar cell performance were studied thoroughly. The two compounds possess a broad absorption covering the wavelength range of 450-700 nm and relatively low HOMO energy levels of -5.46 and -5.52 eV. The power conversion efficiency (PCE) of the OSCs based on SM2 as the donor material and PC₆₁BM as the acceptor material (1:2, w/w) is 2.33%. In contrast, a PCE of 4.72% was achieved for the device based on SM1 as the donor and PC₇₁BM as the acceptor (1:2, w/w) without any treatment such as thermal annealing or the utilization of a solvent additive.

Full text of DOI:10.1039/c3ta14659f

Journal of
Materials Chemistry A
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PAPER
Indacenodithiophene core-based small molecules
with tunable side chains for solution-processed
bulk heterojunction solar cells
a
Cite this: J. Mater. Chem. A, 2014, 2,
4004
Xin Liu,a QingDuan Li,a Yunchuan Li,a Xiong Gong,ab Shi-Jian Su and Yong Caoa
*
Two indacenodithiophene (IDT) core-based small molecules with different side chains of bulky 4-
hexylphenyl and flexible n-dodecyl with the same number of carbon atom, namely SM1 and SM2,
respectively, were designed and synthesized as the donor materials in organic solar cells (OSCs). The
impacts of the different side chains combined with the IDT core on the optical absorption,
electrochemical property, hole mobility, film morphology, and solar cell performance were studied
thoroughly. The two compounds possess a broad absorption covering the wavelength range of 450–
700 nm and relatively low HOMO energy levels of ꢀ5.46 and ꢀ5.52 eV. The power conversion efficiency
(PCE) of the OSCs based on SM2 as the donor material and PC61BM as the acceptor material (1 : 2, w/w)
is 2.33%. In contrast, a PCE of 4.72% was achieved for the device based on SM1 as the donor and
PC71BM as the acceptor (1 : 2, w/w) without any treatment such as thermal annealing or the utilization of
a solvent additive.
Received 12th November 2013
Accepted 18th December 2013
DOI: 10.1039/c3ta14659f
www.rsc.org/MaterialsA
for the polymer-based BHJ solar cells could be applied for
SMSCs.13 The active materials, especially the donor materials,
Introduction
are still the most important factor for obtaining high PCEs of
SMSCs.13,14 To address this issue, it is believed that several
requirements should be considered systematically to design
molecular structures for high-performance and solution-pro-
cessed SMSCs, that is, (1) an excellent lm-formation ability, (2)
a wide and efficient absorption, (3) matched energy levels with
acceptors, (4) a planar structure for high hole mobility, (5) a
good solubility and chemical and thermal stability.15 It is
notable that the design of small molecules with a good lm-
formation ability is a prerequisite for high-performance solu-
tion-processed BHJ devices, since it has been generally difficult
for small molecules to form a comparably good quality lm as
polymers, especially for those molecules with relatively low
molecular weights and rigid planar structures owing to their
intrinsic aggregation.16,17
To date, there are three typical types of solution-processed
organic molecular photovoltaic donor materials reported in the
literatures, that is, (1) triphenylamine (TPA)-based mole-
cules,18–25 (2) hyperbranched molecules,26–30 and (3) planar and
linear structured molecules based on thiophene oligomers31–35
or other donor and acceptor units.36–41 Although the TPA-based
molecules and hyperbranched molecules possess a good solu-
bility, their photovoltaic properties are limited owing to their
weak intermolecular interaction when blended with the
acceptor like 6,6-phenyl C-butyric acid methyl ester (PC61BM),
which results in a lower hole mobility and poorer ll factor (FF)
for the BHJ OSCs based on these molecules as the donor.
Currently, the maximum PCE of the BHJ OSCs based on these
Solution-processed bulk-heterojunction (BHJ) organic solar
cells (OSCs) have been considered a green and effective tech-
nology to harvest sun-light and generate electricity because of
the potential for low cost manufacturing of large area solar cells
through the roll-to-roll coating technique on various
substrates.1–6 In the past few years, polymer-based BHJ solar
cells have been dominant, and a power conversion efficiency
(PCE) of over 9% has been acquired7 for single junction devices.
Meanwhile, tremendous progress has also been made by
designing novel small molecules as the donor in the BHJ
architecture, and a PCE over 9%8 was most recently achieved for
single junction devices. Compared to the polymer-based OSCs
(PSCs), small-molecule-based OSCs (SMSCs) possess numerous
advantages, including a relatively simple synthesis and puri-
cation process, monodispersity, a well-dened structure, no
end-group contaminants, less batch-to-batch variation,9,10
a
versatile molecular structure, easier band structure control,11,12
a high charge carrier mobility, etc. Even so, solution-processed
SMSCs have not been investigated as extensively as the PSCs,
and their overall efficiencies are still behind those of their
polymer counterparts. Currently, many techniques and lessons
aState Key Laboratory of Luminescent Materials and Devices and Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, P. R. China. E-mail: mssjsu@scut.edu.cn; Fax: +86 20
87110606; Tel: +86 20 87114346
bDepartment of Polymer Engineering, The University of Akron, 250 South Forge Street,
Akron, Ohio 44325-0301, USA
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types of molecules is only 4.3%.23 In contrast, the planar and properties in a small molecular platform because of the well-
linear molecules embody prospective photovoltaic perfor- dened molecular structure. However, there are few publica-
mances with a PCE surpassing 6%,34,38,39,41 which is benetted tions that report on the effects of the side chain variation on the
from their stronger intermolecular interactions resulting in a SMSCs.
higher hole mobility and a higher FF of the BHJ OSCs.
In this article, two small molecules with the same conjugated
Recently, the indacenodithiophene (IDT) unit has been backbone in an acceptor-p-donor-p-acceptor (A-p-D-p-A)
emerging as an attractive donor building block for the conju- framework comprised of an IDT core as the donor, two BT units
gated polymer used in high performance OSCs, because the IDT as the acceptor, and thiophene p bridges were designed and
unit possesses many desirable features for incorporation into synthesized as the platform for investigating the effect of the
semiconducting materials. The three aromatic rings are in different side chains. 4-Hexylphenyls and n-dodecyls are
conjugation, and a bridging atom xes coplanarity between the attached on the IDT core to give 7,70-(5,50-(4,4,9,9-tetrakis(4-
adjacent rings, thus maximizing the p orbital overlap as well as hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b0]dithiophene-2,
reducing the conformational energetic disorder.42 As a result, 7-diyl)bis(thiophene-5,2-diyl))bis(4-(50-dodecyl-[2,20-bithiophen]-
the IDT-based polymers generally show a high and stable eld- 5-yl)benzo[c]1,2,5 thiadiazole) (SM1) and 7,70-(5,50-(4,4,9,9-tetra-
effect hole mobility, and a hole mobility as high as 1 cm2 Vꢀ1 sꢀ1 dodecyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b0]dithiophene-2,7-
from an alternating polymer of IDT and benzothiadiazole (BT) diyl)bis(thiophene-5,2-diyl))bis(4-(50-dodecyl-[2,20-bithiophen]-
units has been reported by Zhang et al.43 The polymers 5-yl)benzo[c]1,2,5 thiadiazole) (SM2), respectively (Scheme 1).
combining IDT as the donor unit and various electron-decient n-Dodecyl-substituted thiophene terminals are incorporated to
moieties as the acceptor unit were extensively investigated in extend the p-conjugated structure and to improve the solubility
the previous article reports. For example, Jen and co-workers44 and lm-formation property. As one of the most extensively
have reported two donor–acceptor (D–A) copolymers consisting exploited acceptor units in linear donor–acceptor type mole-
of a 4-hexylphenyl-substituted IDT donor unit and two qui- cules, two BT acceptor units are linked with the IDT core
noxaline derivatives as the acceptor unit, and PCEs of 6.24% through thiophene p bridges, leading to intramolecular charge
and 5.69% were achieved for the devices based on PIDT-phanQ/ transfer and thus broad optical absorption. Rigid and bulky
PC71BM and PIDT-diphQ/PC71BM, respectively. Li and co- aromatic 4-hexylphenyls and exible aliphatic n-dodecyls with
workers45 have reported four D–A copolymers consisting of a the same number of carbon atoms are attached onto the IDT
tetradodecyl-substituted IDT donor unit and different acceptor unit on both sides, which can facilitate solubility and regulate
units, including bis(thiophen-2-yl)-bithiazole, bis(thiophen-2- intermolecular interactions between the molecular chains for
yl)thiazolothiazole, bis(thiophen-2-yl)-tetrazine and bis(thio- tuning their solution processability and lm morphology. The
phen-2-yl)-benzothiadiazole.
planarity of the small molecule is desirable to promote intra-
In addition, the bridging position of IDT offers the oppor- molecular p-delocalization and intermolecular p–p stacking,
tunity to attach substituents, typically aliphatic hydrocarbon both are benecial for a high charge carrier mobility.48 Both
chains, which can aid solubility and impact the intermolecular materials show a good solubility in common organic solvents,
interactions. Aside from the various acceptor units, the role of such as chloroform, dichloromethane, tetrahydrofuran (THF)
the side chains on a conjugated polymer is also of importance and toluene, and can be readily solution-processed to form
because it can be used to tune its crystallinity in the solid state smooth and pinhole-free lms upon spin-coating. Small
as well as its miscibility with other materials in thin blend lms. molecular BHJ devices were fabricated using these materials as
For example, McCulloch and co-workers have reported a PCE of the donor and PC61BM or PC71BM as the acceptor. A PCE of
6.5% from an alternating polymer consisting of 2-ethylhexyl- 4.72% was achieved with an open circuit voltage (Voc) of 0.87 V, a
substituted IDT as the donor unit and BT as the acceptor unit.46 short circuit current density (Jsc) of 9.85 mA cmꢀ2 and a FF of
In comparison,
a polymer incorporating 4-hexylphenyl-
substituted IDT as the donor unit, thiophene as the bridge, and
BT as the acceptor unit gives a PCE of 4.4% and a hole mobility
of 3.4 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1
.
47 A PCE of 6.17% and a hole mobility
of 2.24 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1 were achieved for the PSCs based on
an alternating polymer consisting of tetradodecyl-substituted
IDT as the donor unit and bis(thiophen-2-yl)-benzothiadiazole
as the acceptor unit.45 It is well known that polymer materials
generally suffer from batch-to-batch variations, broad molec-
ular weight distributions, end-group contamination, or difficult
purication methods, which may be signicant problems for
understanding their molecular structure–property relation-
ships. Moreover, although the acceptor units for these polymers
are different which may result in the difference in device
performance, the effect of the different side chains should not
be omitted. Accordingly, it is more meaningful to study the
relationships between the side chain structure and the Scheme 1 The molecular structures of SM1 and SM2.
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0.55 for the device based on SM1 and PC71BM. In contrast, the
device based on SM2 and PC71BM displays a much lower PCE of
1.52%, and it is even lower than that of the device with PC61BM
as the acceptor (2.33%).
Experimental
Measurement and characterization
1H NMR and 13C NMR spectra were recorded on a Bruker AV-
300 spectrometer operating at 300 and 75 MHz, respectively,
in a deuterated chloroform solution at room temperature.
Differential scanning calorimetry (DSC) measurements were
performed on a Netzsch DSC 209 under an N2 ow at a
heating and cooling rate of 10 ꢂC minꢀ1. Thermogravimetric
analyses (TGA) were performed on a Netzsch TG 209 under an
N2 ow at a heating rate of 10 ꢂC minꢀ1. UV-vis absorption
spectra were recorded on a HP 8453 spectrophotometer.
Cyclic voltammetry (CV) was performed on a CHI600D elec-
trochemical workstation with a platinum working electrode
and a Pt wire counter electrode at a scanning rate of 100 mV
Scheme 2 The synthetic routes of SM1 and SM2. Reagents and
conditions: (i) NBS, CHCl3, N2, room temperature, overnight; (ii) NBS,
THF/DMF, N2, room temperature, 3 h; (iii) Pd(PPh3)4, toluene, N2, reflux
at 110 ꢂC, two days; (iv) LDA, THF, N2, ꢀ20 ꢂC for 15 min; (CH3)3SnCl,
ꢀ20 ꢂC then room temperature for 2 h.
sꢀ1 against
a
Ag/Ag+ (0.1
M
of AgNO3 in acetonitrile)
reference electrode with
a
nitrogen-saturated anhydrous
acetonitrile solution of 0.1 mol Lꢀ1 tetrabutylammonium
hexauorophosphate. The thin solid lms used for the
absorption spectral measurement were prepared by spin-
coating their chloroform solutions on quartz substrates. Atom
force microscopy (AFM) measurements were carried out by
using a Digita Instrumental DI Multimode Nanoscope IIIa in
the tapping mode. The processing conditions to make the
blend lms for the morphology study were the same as those
for the fabrication of the solar cell devices.
(ppm) 0.87 (t, J ¼ 6.6 Hz, 12H), 1.29 (m, 24H), 1.57 (m, 8H), 2.56
(t, J ¼ 8.0 Hz, 8H), 6.99 (s, 2H), 7.07–7.09 (m, 16H), 7.32 (s, 2H).
13C NMR (300 MHz, CDCl3): d (ppm) 157.0, 154.6, 143.2, 142.5,
141.4, 136.3, 129.4, 129.1, 128.5, 124.3, 118.7, 62.8, 35.7, 31.9,
31.5, 29.3, 22.8, 15.4. MS (MALDI-TOF) m/z: calculated for
C
64H72Br2S2, 1065.19; found, 1064.32.
Compound 4 (ref. 43) To a solution of compound 3 (4.0 g,
4.25 mmol) in THF/N,N-dimethylformamide (DMF) (2 : 1, v/v,
100 ml) NBS (1.66 g, 9.35 mmol) was added. This mixture was
Synthesis
4,4,9,9-Tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5, stirred for 3 h in the absence of light at room temperature and
6-b0]dithiophene (1),49 4,4,9,9-tetradodecyl-4,9-dihydro-s-indaceno- then poured into water. The precipitate was collected and
[1,2-b:5,6-b0]dithiophene (3),43 4-(5-bromothiophen-2-yl)-7-(thi- washed with water and then recrystallized from acetone to give a
1
ophen-2-yl)benzo[c]1,2,5 thiadiazole (5)50 and tributyl(5-dodecyl- pale yellow solid (3.96 g, 85%). H NMR (CDCl3, 300 MHz): d
thiophen-2-yl)stannane (6)51 were prepared according to the (ppm) 7.17 (s, 2H), 6.96 (s, 2H), 1.89–1.95 (m, 4H), 1.78–1.88 (m,
reported methods. The IDT-based small molecules, SM1 and 4H), 1.00–1.53 (m, 68H), 0.74–0.88 (m, 24H); 13C NMR (300
SM2, were synthesized by a Stille cross-coupling reaction MHz, CDCl3): d (ppm) 154.2, 152.2, 141.9, 135.6, 124.9,
(Scheme 2). All reactions and manipulations were carried out in 113.1, 112.5, 55.0, 39.1, 32.1, 29.9, 29.8, 29.7, 29.5, 29.4, 24.2,
an inert atmosphere with the use of standard Schlenk tech- 22.8, 14.3.
niques. THF was distilled from sodium before use. The other
Compound 7. 6.46 g (17.0 mmol) of 5 and 104 mg (0.09
reagents and solvents, unless otherwise specied, were mmol) tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4)
purchased from commercial suppliers and used without further were placed in a dried round bottom ask under protective gas.
purication.
50 ml of dry toluene was added, and the solution was then
Compound 2 (ref. 49) 907 mg (1 mmol) of compound 1 and heated to 110 ꢂC. 9.76 g (18.0 mmol) of 6 was then added to the
392 mg (2.2 mmol) of N-bromosuccinimide (NBS) were placed in reaction mixture and the solution was stirred for two days at
a 100 ml two-necked bottle. 30 ml of chloroform was added to 110 ꢂC. The crude product was obtained by extraction with
the bottle. Then, the whole bottle was wrapped in Al foil and chloroform, and the organic phase was subsequently washed
immersed into an ice bath. The mixed solution was stirred in an with a diluted hydrochloric acid, water and sodium carbonate
N2 atmosphere overnight. The organic phase was extracted by solution. The organic phase was dried over magnesium
chloroform and washed with saturated sodium chloride solu- sulphate and evaporated, and the residue was recrystallized
tion and then dried over anhydrous MgSO4 and ltered. The from ethanol. Further purication was performed by column
ltrate was concentrated on a rotary evaporator to give a pale chromatography on silica with an eluent of hexane–dichloro-
yellow solid 2 (852 mg, 80%). 1H NMR (CDCl3, 300 MHz): d methane (1 : 1, v/v) to give the product as a red solid (6.1 g,
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65%). 1H NMR (CDCl3, 300 MHz): d (ppm) 8.10–8.12 (dd, J ¼ 3.9 (182 mg, 45%). 1H NMR (CDCl3, 300 MHz): d (ppm) 8.05–8.03 (d,
Hz, 1H, Ar-H), 8.01–8.03 (d, J ¼ 3.9 Hz, 1H, Ar-H), 7.84–7.87 (d, 4H), 7.90–7.82 (m, 3H), 7.18–7.12 (m, 8H), 7.15–7.08 (m, 2H),
J ¼ 7.5 Hz, 1H, Ar-H), 7.80–7.83 (d, J ¼ 7.5 Hz, 1H, Ar-H), 7.44– 7.00–6.94 (s, 1H), 6.74–6.72 (m, 2H), 2.85–2.80 (t, 4H), 2.61–2.53
7.46 (dd, J ¼ 5.1, 0.9 Hz, 1H, Ar-H), 7.19–7.22 (dd, J ¼ 5.1, 3.9 Hz, (t, 8H), 1.78–1.73 (m, 4H), 1.40–1.10 (m, 108H), 0.91–0.82 (m,
1H, Ar-H), 7.17–7.19 (d, J ¼ 3.9 Hz, Ar-H), 7.10–7.11 (d, J ¼ 3.9 26H). 13C NMR (300 MHz, CDCl3): d (ppm) 154.2, 152.7, 152.6,
Hz, 1H, Ar-H), 6.71–6.73 (d, J ¼ 3.9 Hz, 1H, Ar-H), 2.79–2.84 (t, 152.2, 146.2, 141.9, 139.60, 139.55, 137.5, 135.6, 134.8, 128.5,
J ¼ 7.5 Hz, 2H, CH2), 1.65–1.75 (m, 2H, CH2), 1.25–1.42 (m, 18H, 128.2, 127.6, 126.9, 125.91, 125.85, 125.76, 125.1, 124.9, 123.9,
CH2), 0.88–0.96 (m, 3H, CH3). 13C NMR (300 MHz, CDCl3): d 113.1, 112.5, 55.0, 39.1, 32.1, 31.78, 31.76, 30.4, 29.9, 29.8, 29.7,
(ppm) 152.7, 152.6, 146.2, 139.60, 139.55, 137.5, 134.8, 128.5, 29.5, 29.4, 28.0, 23.2, 22.6, 13.3. MS (MALDI-TOF) m/z: calcu-
128.2, 127.6, 126.9, 125.91, 125.85, 125.76, 125.1, 123.9, 31.78, lated for C124H170N4S10, 2036.07; found, 2035.76.
31.76, 30.4, 29.0, 22.8, 14.3. MS (MALDI-TOF) m/z: calculated for
C
30H34N2S4, 550.86; found, 550.15.
Device fabrication and characterization
Compound 8. To a solution of compound 7 (551 mg, 1 mmol)
in dry THF (50 ml) in an N2 atmosphere at ꢀ20 ꢂC a lith-
iumdiisopropylamide (LDA) solution (1 M in THF/hexane, 1.1
ml) was added dropwise. The mixture was then stirred at ꢀ20 ꢂC
for 15 min. Trimethyltin chloride (1.2 ml, 1.2 mmol) was dis-
solved in THF and added in small portions. The reaction was
stirred at ꢀ20 ꢂC for 10 min before being warmed to room
temperature and then stirred for 2 h. The resulting mixture was
poured into water and extracted with ether. The organic phase
was then washed with brine (60 ml ꢁ 3) and water (60 ml ꢁ 3).
Further purication was performed by recrystallization from
ethanol to give the product as a red solid. 1H NMR (CDCl3, 300
MHz): d (ppm) 8.21–8.18 (d, 1H), 8.05–8.02 (d, 1H), 7.89–7.82
(dd, 2H), 7.31–7.28 (d, 1H), 7.21–7.18 (d, 1H), 7.12–7.08 (d, 1H),
6.72–6.70 (d, 1H), 2.85–2.78 (t, 2H), 1.73–1.69 (m, 2H), 1.35–1.21
(m, 18H), 0.90–0.82 (t, 3H), 0.45–0.32 (t, 9H). MS (MALDI-TOF)
m/z: calculated for C33H42N2S4Sn, 713.67; found, 712.13.
The photovoltaic devices were fabricated with a structure of
glass/ITO/PEDOT:PSS/donor:acceptor/PFN/Al. Indium tin oxide
(ITO)-coated glass substrates were cleaned by sonication in
detergent, deionized water, acetone and isopropyl alcohol, and
dried in a nitrogen stream, followed by an oxygen plasma
treatment. A 40 nm-thin layer of PEDOT:PSS (Baytron P VP AI
4083, ltered at 0.45 mm) was spin-coated (3000 rpm) onto the
ITO surface. Aer being baked at 150 ꢂC for 20 min, the
substrates were transferred into a nitrogen-lled glove box.
Subsequently, a 90–100 nm-thin active layer was spin-casted
from different blend ratios (w/w) of donor (SM1 or SM2) and
acceptor (PC71BM or PC61BM) in a chloroform solution (10 mg
mlꢀ1) at 1500 rpm for 20 s on the ITO/PEDOT:PSS substrate
without further special treatment. The active layer thickness
was measured using a Dektak 150 prolometer. A 5 nm-thin
PFN layer was spin-coated from its solution in methanol.
Finally, a 90 nm-thin Al layer was deposited on top of the PFN
layer under a high vacuum (3 ꢁ 10ꢀ6 torr). The effective device
area was patterned to be 0.15 cm2 by using a shadow mask. The
PCEs were measured in an AM 1.5G solar simulator (Oriel
model 91192) under ambient conditions. The power of the sun
simulator was calibrated before the testing using a standard
silicon solar cell, giving a value of 100 mW cmꢀ1 in the test. The
current density–voltage (J–V) characteristics were recorded with
a Keithley 2400 source meter. The spectral response was
measured with a commercial photomodulation spectroscopic
setup (Oriel). A calibrated Si photodiode was used to determine
the photosensitivity. The external quantum efficiency (EQE) of
the devices was measured on a Hypermonolight System (Bun-
koh-Keiki SM-250).
Compound SM1. Compound 2 (213 mg, 0.2 mmol) and
compound 8 (357 mg, 0.5 mmol) were placed in a dried round
bottom ask under protective gas. 10 ml of dry toluene was
added to the ask which then was sealed and degassed with
argon for 15 min. Then Pd(PPh3)4 (10 mg) was added and the
solution was heated to 110 ꢂC. The reaction mixture was stirred
for two days at 110 ꢂC. The crude product was obtained by
extraction with chloroform, and the organic phase was subse-
quently washed with a diluted hydrochloric acid, water and
sodium carbonate solution. The organic phase was dried over
magnesium sulfate and ltered. The ltrate was concentrated
on a rotary evaporator. Aer the removal of solvent, the crude
product was puried by silica gel using a mixture solvent of
hexane–dichloromethane (2 : 1, v/v) as an eluent. The crude
solid was recrystallized from CH2Cl2 several times to afford SM1
The hole mobilities were measured with the hole-only
devices by using the space charge limited current (SCLC)
method with a device conguration of ITO/PEDOT:PSS/SM1 or
SM2 (90 nm)/MoO3 (10 nm)/Al. The device fabrication and
mobility measurements were conducted in a nitrogen-lled
glove box. The mobility was determined by tting the dark
current to the model of a single carrier SCLC according to the
Mott–Gurney law, which is described as
1
as a black solid (150 mg, 37%). H NMR (CDCl3, 300 MHz): d
(ppm) 8.05–8.03 (d, 2H), 8.01–7.98 (d, 2H), 7.84–7.82 (m, 4H),
7.43–7.41 (m, 2H), 7.25–7.18 (m, 16H), 7.12–7.08 (m, 6H), 7.00–
6.98 (m, 2H), 6.73–6.71 (d, 2H), 2.85–2.80 (t, 4H), 2.61–2.53 (t,
8H), 1.78–1.73 (m, 4H), 1.59–1.52 (m, 8H), 1.40–1.20 (m, 60H),
0.91–0.82 (m, 18H). 13C NMR (300 MHz, CDCl3): d (ppm) 154.2,
152.7, 152.6, 152.2, 146.2, 141.9, 139.60, 139.55, 137.5, 135.6,
134.8, 128.5, 128.2, 127.6, 126.9, 125.91, 125.85, 125.76, 125.1,
124.9, 123.9, 113.1, 112.5, 55.0, 39.1, 32.1, 31.78, 31.76, 30.4,
29.9, 29.8, 29.7, 29.5, 29.4, 29.0, 24.2, 22.8, 14.3. MS (MALDI-
9
V2
h L3
J ¼ 303rm
(1)
8
TOF) m/z: calculated for C124H138N4S10, 2003.82; found, 2003.52. where J is the current density, 30 is the permittivity of free space,
Compound SM2. SM2 was synthesized and puried in a 3r is the relative permittivity of the material, mh is the hole
similar manner to that of SM1 and was obtained as a dark solid mobility, L is the lm thickness of the active layer, and V, the
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internal voltage, is dened as V ¼ Vappl ꢀ Vr ꢀ Vbi, where Vappl is and broad absorption bands in the wavelength range from 300
the applied voltage to the device, Vr is the voltage drop due to to 700 nm with clear vibronic peaks at 360, 440 and 570 nm. The
contact resistance and series resistance across the electrodes, absorption bands at 360 and 440 nm should be commonly
and Vbi is the built-in voltage due to the relative work-function ascribed to the p–p* transition of the conjugated backbone,
difference of the two electrodes.
and the absorption band at the longer wavelength region could
be attributed to the intramolecular charge transfer (ICT) inter-
action between their donor moieties and acceptor groups.52
As shown in Fig. 1b, the absorption spectra of the thin solid
lms display fewer vibronic peaks in comparison with the cor-
responding solutions, and the absorption band at a longer
wavelength is red-shied and broadened with a slight shoulder
at ꢃ620 nm. It seems that the relative intensity of the shoulder
of the absorption spectrum for SM2 is stronger than that for
SM1, giving a further red-shied and broader absorption band,
and it might be attributed to the presence of stronger inter-
molecular interactions in the solid state. Compared to the
absorption peak at 565 nm for the SM1 solution, the SM2
solution has a ꢃ9 nm red-shied band at 574 nm. Similarly, a
red-shied absorption maximum was also observed at 587 nm
for the SM2 lm, compared with the 576 nm for the SM1 lm.
Such a difference of these two molecules in the optical
absorption should be attributed to the different side chains.
The rigid and bulky 4-hexylphenyl substituents seem to offer
SM1 a low tendency of self-aggregation compared with the
exible n-dodecyl side chains due to the steric hindrance of the
side phenyls. The optical band gaps (Eogpt) of SM1 and SM2 were
estimated to be 1.66 and 1.65 eV, respectively, from the onset of
the lm absorption spectra (Table 1). In addition, SM1 exhibits
higher absorption coefficients of 6.0 ꢁ 104 Mꢀ1 cmꢀ1 (565 nm)
and 5.0 ꢁ 104 cmꢀ1 (576 nm) in chloroform solutions and in
thin solid lms, respectively, compared with SM2 (3.0 ꢁ 104
Mꢀ1 cmꢀ1 (574 nm) and 3.2 ꢁ 104 cmꢀ1 (587 nm)) (Table 1),
indicating a stronger solar light absorption ability which is
preferable to light-harvesting.
Results and discussion
Thermal stability and optical absorption
The thermal property of the developed materials was investi-
gated by TGA and DSC. TGA suggests that SM1 and SM2 show a
good thermal stability with a decomposition temperature (Td)
greater than 350 and 321 ꢂC, respectively, in an N2 atmosphere,
indicating that they can effectively resist the thermal degrada-
tion at the operating temperatures in the resultant solar cells.
SM1 has a higher decomposition temperature due to its bulky
and rigid 4-hexylphenyl side chains, while SM2 contains exible
aliphatic n-dodecyl side chains. The DSC traces for these two
materials do not show any peaks from room temperature to
ꢂ
250 C, indicating that no phase transition was found in this
temperature range.
To study the relationships between the chemical structure
and the photophysical property, the UV-vis absorption spectra
of SM1 and SM2 in diluted chloroform solutions (10ꢀ5 M) and
in thin solid lms (90 nm) prepared by spin-coating were
recorded as shown in Fig. 1. Beneted from their D–A molecular
structures with thiophene p-bridges, the absorption spectra of
the compounds in diluted chloroform solutions exhibit strong
Electrochemical properties and energy levels
In order to insightfully understand the relationships between
the chemical structure and the electronic structure of the
resulting materials and consequently provide key parameters
for the design of small-molecule-based BHJ solar cells, CV
experiments were conducted to measure the HOMO and
LUMO energy levels of SM1 and SM2. The potentials were
calibrated with the redox couple of ferrocene/ferrocenium (Fc/
Fc+) under the same experimental conditions. In the oxida-
tion curves shown in Fig. 2, both the CV curves of SM1 and
SM2 in acetonitrile solution show one irreversible p-doping
process. The onset oxidation potentials (Eox) versus Ag/AgNO3
are 0.98 and 0.92 V for SM1 and SM2, respectively. As
summarized in Table 1, the HOMO energy levels of SM1 and
SM2 are estimated to be ꢀ5.52 and ꢀ5.46 eV, respectively,
according to an equation of EHOMO ¼ ꢀe(Eox + 4.54) (eV).53
Because the reliable onset reduction potentials of these
materials are hardly gained, their LUMO energy levels are
thus estimated from the HOMO energy levels and the
Eogpt based on the relation of ELUMO ¼ EHOMO + Eogpt. The
LUMO energy levels are calculated to be ꢀ3.86 and ꢀ3.81 eV
Fig. 1 The UV-vis absorption spectra of SM1 (solid lines) and SM2
(dashed lines) (a) in chloroform solutions and (b) in thin solid films.
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Table 1 The optical and electrochemical properties of SM1 and SM2
Solution
Film
lmax
3
3
Compounds
(nm)
(Mꢀ1 cmꢀ1
)
lmax (nm)
(cmꢀ1
)
Eogpt
HOMO
LUMO
SM1
SM2
565
574
6.0 ꢁ 104
3.0 ꢁ 104
576
587
5.0 ꢁ 104
3.2 ꢁ 104
1.66
1.65
ꢀ5.52
ꢀ5.46
ꢀ3.86
ꢀ3.81
possess delocalized p conjugation, which may be advantageous
to the charge transport.
Photovoltaic performance
To evaluate the photovoltaic properties of SM1 and SM2, BHJ
solar cells using the small molecules as the donor material were
fabricated with the conventional solution spin-coating process.
The device structure is ITO/PEDOT:PSS/photoactive layer
(90–100 nm)/PFN/Al. The device optimizations were conducted
by varying the acceptor and the weight ratios of the donor and
the acceptor. Aside from PC61BM, PC71BM was also selected to
be as an electron acceptor material due to its stronger light-
harvesting ability in the visible region from 440 to 530 nm
which can complement the absorption valley of the small
molecules.56 In addition, a slightly lower (about 0.1 eV) LUMO
energy level of PC71BM offers a larger driving force for charge
transfer in the resulting SMSCs.57 Typical current density–
voltage (J–V) characterizations of the fabricated devices under 1
sun illumination (AM 1.5, 100 mW cmꢀ2) are displayed in Fig. 4,
and the device performances are summarized in Table 2.
From Fig. 4 and Table 2, one can see the characteristically
high Voc of the devices based on SM1 and SM2 (>0.8 V) because
of their relatively lower-lying HOMO energy levels, which is
predominantly determined by the backbone donor moiety. In
addition, the Voc of the devices based on SM1 is 0.03–0.05 V
higher than that of the devices based on SM2 with the same
Fig. 2 Cyclic voltammograms of the SM1 (solid line) and SM2 (dashed
line) films on the Pd/C electrode in an anhydrous acetonitrile solution
of 0.1 mol Lꢀ1 Bu4NPF6 with a scan rate of 100 mV sꢀ1. The arrows
indicate the onset oxidation potentials (Eox).
for SM1 and SM2, respectively, as listed in Table 1. Because
the Voc is proportional to the energy difference between the
HOMO energy level of the electron donor and the LUMO
energy level of the electron acceptor in the active layer of the
BHJ solar cells,54 such low-lying HOMO energy levels are
expected to increase the open circuit voltages in the solar cell
devices. It has generally been accepted that the HOMO and
LUMO energy levels of the D–A type conjugated molecules are
mainly governed by those of the D and A units, respectively.
Considering the same conjugated main chain structure, their
frontier energy levels are almost the same or change little
when different side chain groups are introduced.
Hole mobility
It is generally accepted that the charge carrier transporting
abilities of the donor materials can tremendously inuence the
Jsc and FF of the resulting BHJ solar cells and consequently the
overall device performance.55 The hole mobility of the pristine
SM1 and SM2 lms was measured by the SCLC method with a
device conguration of ITO/PEDOT:PSS/SM1 or SM2 (90 nm)/
MoO3/Al. As plotted in Fig. 3, the J1/2–V plots of the devices are
consistent with the corresponding single charge SCLC tting
curves. The SM1 and SM2 lms show hole mobilities of 2.05 ꢁ
10ꢀ4 and 5.49 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1, respectively. The relatively
greater hole mobility of SM1 with 4-hexylphenyl side chains may
Fig. 3 The J1/2–V plots of the devices with a configuration of ITO/
PEDOT:PSS (40 nm)/SM1 or SM2 (90 nm)/MoO3 (10 nm)/Al. The solid
lines are the fitting curves according to the single charge SCLC
be attributed to the fact that the aromatic phenyl side groups method.
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PC71BM concentration (D : A ¼ 1 : 3, w/w) induced a lower PCE
for both SM1 and SM2, possibly due to a reduction in the total
light absorption at the same lm thickness and undesirable
phase segregation, which is critical to the electron–hole charge
balance. It is necessary to note that no additive or thermal
annealing was utilized for the current device fabrication, and
this is an advantage for simplifying the device fabrication and
improving the repeatability of the device performance.
To further understand the device performance, the EQE
spectra of the fabricated devices were measured as shown in
Fig. 5. From the curves, it is observed that all the EQE spectra
cover a broad wavelength range from 400 to 700 nm and show a
maximum EQE value of 57% at 560 nm, 35% at 470 nm, 31% at
435 nm, and 29% at 467 nm for the SMSCs based on SM1:PC71BM
(1 : 2, w/w), SM2:PC61BM (1 : 2, w/w), SM1:PC61BM (1 : 2, w/w),
and SM2:PC71BM (1 : 2, w/w), respectively. It can be clearly seen
that the performance of the above devices is well consistent with
the results of the EQE measurements. In addition, the calculated
Jsc values obtained by the integration of the EQE data for the SM1-
and SM2-based devices showed a 3–6% mismatch compared with
the Jsc values obtained from the J–V measurements. Obviously,
the EQE of the devices based on SM1:PC71BM is about 2 times
higher than that of the devices based on the blend with PC61BM,
Fig. 4 Current density–voltage characteristics of the OSC devices in a
structure of ITO/PEDOT:PSS (40 nm)/photoactive layer (90–100 nm)/
PFN (5 nm)/Al based on an active layer of SM1 or SM2/PC61BM or
PC71BM (1 : 2, w/w) under the illumination of AM 1.5G, 100 mW cmꢀ2
.
acceptor material and their weight ratios due to its further
lower-lying HOMO energy level. For the conventional devices
with PC61BM as the acceptor material, the active layer of
SM1:PC61BM with a weight ratio of 1 : 2 exhibited a high Voc of
0.90 V, a Jsc of 4.95 mA cmꢀ2, a FF of 0.36, and a PCE of 1.59%. In
contrast, the active layer based on SM2:PC61BM with the same
weight ratio gave a Voc of 0.87 V, a Jsc of 5.00 mA cmꢀ2, a FF of
0.54, and a PCE of 2.33%. By using PC71BM instead of PC61BM
as the acceptor material, the photovoltaic devices based on
SM1:PC71BM (1 : 2, w/w) with a thickness of 100 nm showed a
PCE value up to 4.72% with a Voc of 0.87 V, a signicantly
improved Jsc of 9.85 mA cmꢀ2, and an FF of 0.55. In comparison,
the devices based on SM2:PC71BM (1 : 2, w/w) exhibited a
slightly lower Voc of 0.82 V, a very low Jsc of 4.27 mA cmꢀ2, an FF
of 0.43, and a PCE of only 1.52%, which is even lower than the
devices with PC61BM as the acceptor. Obviously, compared with
the SM1/PC61BM devices, the SM1/PC71BM devices showed a 2
times higher Jsc and an enhanced FF, which may be attributed
to the stronger light absorption from PC71BM and a suitable
morphology. Nevertheless, for the SM2 blended with PC71BM,
the device performance was lower than that blended with
PC61BM. It could be largely attributed to the lower FF which may
be due to an undesirable morphology. A further increase of the
Fig. 5 The EQE spectra of the OSC devices in a structure of ITO/
PEDOT:PSS (40 nm)/photoactive layer (90–100 nm)/PFN (5 nm)/Al
based on an active layer of SM1 or SM2/PC61BM or PC71BM (1 : 2, w/w).
Table 2 A summary of the device performances under the illumination of AM 1.5, 100 mW cmꢀ2 for the devices in a structure of ITO/PEDOT:PSS
(40 nm)/photoactive layer (90–100 nm)/PFN (5 nm)/Al. At least 10 solar cell devices were fabricated for each composition
Active layer
Ratio (w/w)
Voc (V)
Jsc (mA cmꢀ2
)
FF (%)
PCEmax(PCEave) (%)
SM1/PC61BM
SM2/PC61BM
SM1/PC71BM
SM2/PC71BM
SM1/PC71BM
SM2/PC71BM
1 : 2
1 : 2
1 : 2
1 : 2
1 : 3
1 : 3
0.90
0.87
0.87
0.82
0.86
0.83
4.95
5.00
9.85
4.27
10.16
3.04
35.7
53.6
55.1
43.4
51.0
42.5
1.59(1.36)
2.33(2.09)
4.72(4.50)
1.52(1.34)
4.46(4.20)
1.07(0.95)
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and it can be attributed to the stronger light absorption of efficient percolation channels for charge transport, thus
PC71BM and the appropriate lm morphology as proven by the improving the exciton separation and carrier collection efficiency
following AFM images. For the devices based on SM2, the EQE of and leading to a high Jsc and FF. In comparison, the lms blended
the devices based on the blend with PC61BM is higher than that of with SM2 exhibit a larger roughness, which indicates an unfav-
the devices based on the blend with PC71BM due to the unfav- ourable lm morphology that is a major disadvantage for charge
ourable lm morphology. Such a difference in the EQE spectra separation from the donor to the acceptor, thus leading to lower
agrees well with the observed different Jsc of the solar cell devices. Jsc and FF. From a structural point of view, the intermolecular
At the same time, the relatively higher hole mobility of SM1 may interactionsamongtheSM2moleculeswithexiblealiphaticside
also contribute to the higher EQE of the corresponding SMSCs.
chains should be stronger than the SM1 molecules with aromatic
4-hexylphenyl side chains for their stronger self-aggregation
tendency, which may greatly increase the phase separation in the
blended thin lms with the acceptors, as can be observed in the
corresponding AFM images. In contrast, there might be p–p
intermolecular interactions between the phenyl side groups of
the SM1 molecules and PC61BM or PC71BM that may improve
their miscibility with each other, leading to a desirable lm
morphology for exciton separation and charge transport.
Film morphology
In order to deeply understand the photovoltaic properties of the
resulting small molecular materials, the active layer morphology
was studied by AFM in the tapping-mode. Similar to the OSC
devices, the lms for the AFM measurements were also prepared
by spin-coating their chloroform solutions on top of the
PEDOT:PSS layer, which was spin-coated on the ITO glass
substrates. As shown by the topography images presented in
Fig. 6, it can be obviously seen that the different side chains of
these two small molecules result in substantial morphology
variation in the condensed state. The root-mean-square (rms)
roughness of the pure SM1, pure SM2, SM1/PC61BM (1 : 2, w/w),
SM2/PC61BM (1 : 2, w/w), SM1/PC71BM (1 : 2, w/w), and SM2/
PC71BM (1 : 2, w/w) lms are 0.418, 3.753, 0.455, 3.149, 0.439, and
5.715 nm, respectively. The surfaces of the SM1-based lms are
quite smooth and uniform. The smaller rms roughness of the
SM1-based blend lms with PC61BM or PC71BM indicates that
SM1 has a good miscibility with both PC61BM and PC71BM to give
a favourable donor–acceptor interpenetrating network (IPN). The
well-dened IPN structure ensures large D–A interfaces and
Conclusions
Two IDT-core-containing molecules were successfully synthe-
sized and characterized for use in solution-processed small
molecular BHJ solar cells. These two molecules employ the
same conjugated backbone but different side chains of rigid
and bulky aromatic 4-hexylphenyls and exible aliphatic n-
dodecyls with the same number of carbon atoms. Such a
structural difference in the side chains combined with the
central IDT moiety leads to different intermolecular interac-
tions, self-aggregation tendency, lm morphology, charge
transport ability, and thus device performance, as evidenced by
the results from thermogravimetric analysis, optical absorption,
cyclic voltammetry, the single carrier space charge limited
current, and AFM topography images. Compared with the n-
dodecyl side chains, the introduction of 4-hexylphenyl side
chains onto the IDT unit signicantly improves the solar light
absorption and gives a favourable blend lm morphology for
exciton separation and charge transport, leading to a high PCE
of 4.72% under the illumination of AM. 1.5, 100 mW cmꢀ2. The
current results indicate the potential impact of the aromatic or
aliphatic side chains of the donor molecules with the same
backbone on the performance of the SMSCs to guide the next
molecular design for highly efficient OSCs.
Acknowledgements
The authors greatly appreciate the nancial support from the
Ministry of Science and Technology (2014CB643501 and
2011AA03A110), the National Natural Science Foundation of
China (51073057, 51010003 and 91233116), the Ministry of
Education (NCET-11-0159), the Guangdong Natural Science
Foundation (S2012030006232), and the Fundamental Research
Funds for the Central Universities (2013ZG0007).
Notes and references
Fig. 6 Tapping mode AFM topography images (5 ꢁ 5 mm2) of (a) pure
SM1, (b) pure SM2, (c) SM1:PC61BM (1 : 2, w/w), (d) SM2:PC61BM (1 : 2,
w/w), (e) SM1:PC71BM (1 : 2, w/w), and (f) SM2:PC71BM (1 : 2, w/w).
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