Linkage effects of linear D-π-A-π-D type diketopyrrolopyrrole- triphenylamine based solution-processable organic small molecule photovoltaic materials

Article abstract of DOI:10.1039/c3tc32214a

Four novel D-π-A-π-D-type small molecules (SMs) were synthesized and their π-linkage effects were investigated. Among them, SM2 with vinylene as the π-linkage exhibited better molecular coplanarity, reaching a relatively higher power conversion efficiency (PCE) of 3.76%. SM3 and SM4 with acetylene and acrylonitrile as π-linkages exhibited relatively higher open-circuit voltages (V_OC) of 0.93 V and 0.90 V, respectively, owing to their deep-lying HOMO levels. These results gave an important guide for developing new materials in solution-processed small molecule solar cells.

Full text of DOI:10.1039/c3tc32214a

Journal of
Materials Chemistry C
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PAPER
Linkage effects of linear D–p–A–p–D type
diketopyrrolopyrrole-triphenylamine based
solution-processable organic small molecule
photovoltaic materials†
Cite this: J. Mater. Chem. C, 2014, 2,
4019
Changyan Ji,a Lunxiang Yin,a Lihui Wang,a Tao Jia,b Shixiang Meng,a Yingji Suna
a
*
and Yanqin Li
Four novel D–p–A–p–D-type small molecules (SMs) were synthesized and their p-linkage effects were
investigated. Among them, SM2 with vinylene as the p-linkage exhibited better molecular coplanarity,
reaching a relatively higher power conversion efficiency (PCE) of 3.76%. SM3 and SM4 with acetylene
and acrylonitrile as p-linkages exhibited relatively higher open-circuit voltages (VOC) of 0.93 V and 0.90
V, respectively, owing to their deep-lying HOMO levels. These results gave an important guide for
developing new materials in solution-processed small molecule solar cells.
Received 11th November 2013
Accepted 5th March 2014
DOI: 10.1039/c3tc32214a
www.rsc.org/MaterialsC
other hand, the benzene rings inserted in small molecule
materials usually sacrice the relatively lower band gap for their
Introduction
larger aromatic resonance energy, compared with thiophene
rings. The most creative solution is to reduce the aromaticity as
well as dilute the effect of the benzene rings. This could be
realized by introducing multiple-bond linkage between the
benzene ring and the end-capping group, such as double bond
and triple bond.1a,7,9 Moreover, it has been demonstrated that
inserting vinylene into D–A type molecular architecture will
enhance the conjugation degree and coplanarity of the OSMs,
leading to the increase of ICT and short-circuit current (JSC).10
Furthermore, using acetylene or acrylonitrile groups as the p-
linkage group could result in an increased oxidation potential,
hence increasing the VOC. Zeng et al. have reported that a
solution-processed OSM solar cell based on acrylonitrile linkage
exhibits an impressive VOC of 1.04 V and a PCE of 3.85%.11
Although there are several reports about p-linkage of OSMs, it
seems to lack systematic investigation. Therefore further study
of the p-linkage effect on novel OSMs will be critical for deeply
understanding the relationship between the molecular struc-
ture and property.
In this work, a series of novel compounds, SM1, SM2, SM3
and SM4, have been synthesized successfully, consisting of
diketopyrrolopyrrole (DPP) as the electron-withdrawing group
and triphenylamine (TPA) as the electron-donating unit. In
addition, four linkage units, single bond, vinylene, acetylene
and acrylonitrile, were introduced to form a D–p–A–p–D type
structure. To investigate the p-linkage effect of synthesized
compounds, we reported the detailed discussions on theoretical
calculations, photophysical and electrochemical properties,
hole mobility as well as PV performance. The chemical struc-
tures of the synthesized PV materials are shown in Fig. 1a. DPP
Bulk heterojunction (BHJ) organic solar cells (OSCs) have
developed rapidly during the past decade due to their advan-
tages of easy-fabrication, light-weight, exible and cost-effective
device characteristics.1 Though solution-processed polymer-
based BHJ OSCs have received much attention with PCEs over
9%,2 much effort has been made on the low band gap solution-
processed small molecular OSCs, due to their potential of
reproducibility, easy purication and good photovoltaic (PV)
efficiency reproduction.3 To date, organic small molecules
(OSMs) exhibit outstanding PV performance with PCEs over
8%,4 in particular, an encouraging PCE of 9.02% has been
reported by Heeger et al.5 However, small molecules based BHJ
OSCs still require a better material design.
In recent years, much attention has been paid to the linear
donor–acceptor–donor (D–A–D) type organic PV materials.6
Because this design makes it possible to introduce monomer
units with quinoidal character into the conjugated system,
which can not only efficiently reduce the band gap of the
material, but also tune the electrochemical properties through
intramolecular charge transfer (ICT) between donor and
acceptor units within molecules.7 To date, the design of small
molecular materials containing both fused thiophene and
benzene rings can efficiently lower the band gap, broaden the
absorption range and improve the device performance.8 On the
aSchool of Chemistry, Dalian University of Technology, Dalian, China. E-mail:
liyanqin@dlut.edu.cn; Fax: +86-411-84986040; Tel: +86-411-84986040
bState Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc32214a
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Fig. 1 (a) Chemical structures of the synthesized compounds and (b) DFT theoretically calculated HOMO and LUMO energy levels and elec-
tronic-density distribution.
was chosen to be the central acceptor for its exceptional prop-
erties with broad absorption and strong uorescence perfor-
mance, as well as for possessing excellent photochemical and
thermal stability.12 The purpose of using triphenylamine (TPA)
as the end-cap group was due to its good hole-transporting
capability and solubility.13 Though both DPP and TPA have been
regarded as excellent units for organic PV materials, the highest
PCE value of the OSM donor based on the DPP-TPA unit
reported so far is still below 1.81%.14 In this paper, the solution-
processed OSCs based on SM1, SM2, SM3 and SM4 exhibit
PCEs of 2.74%, 3.76%, 3.10% and 2.92%, respectively. These
results strongly suggest that it is an effective approach to
improve PCE of DPP-TPA based OSMs by selecting an appro-
priate p-linkage group.
Measurements and characterization
1H-NMR and 13C-NMR spectra were obtained using a Bruker
AVANCE II 400 MHz spectrometer with CDCl3 as the solvent and
TMS as the internal standard. High-resolution mass spectra
(HRMS) were collected using a MALDI Micro MX spectrometer.
UV-vis spectral data were recorded on an Agilent Cary 5000
spectrophotometer at room temperature in chloroform solution
and in thin lm. Cyclic voltammetry (CV) was carried out on a
CHI 610D electrochemical workstation from CH Instruments,
Inc. On electrochemical analyses, the redox properties of these
molecules were measured in Bu4NBF4/dichloromethane solu-
tions. The glass-carbon electrode served as the working elec-
trode, and the Ag/Ag+ electrode (Ag in 0.1 M AgNO3 solution
of MeCN) and platinum wire were chosen as the reference
electrode and the count electrode, respectively. A ferrocene–
ferrocenium (Fc/Fc+) couple was selected as the internal stan-
dard. The current density–voltage (J–V) curves were recorded
using a computer-controlled Keithley 2400 Source Measure Unit
Experimental section
Reagents and materials
All reagents were obtained commercially and used without
further purication, unless otherwise noted. Toluene was dried
by distillation from metallic sodium/benzophenone under a
nitrogen atmosphere before use. All reactions were carried out
in an air atmosphere unless otherwise noted.
under AM 1.5 G illumination with an intensity of 100 mW cmꢁ2
.
The incident photon-to-current efficiency (IPCE) spectra were
measured with a SM-25 photoelectric conversion analyzer.
Atomic force microscopy (AFM) images of blend lms were
taken using a Nanoscope IIIa Dimension 3100.
Fabrication of a photovoltaic device
General procedures of Suzuki coupling reaction for the target
molecules
BHJ OSCs were fabricated with a conventional device congu-
ration of ITO/PEDOT : PSS/SMs : PC61BM (1 : 2, w:w)/Al.
The ITO substrate was pre-cleaned in water, acetone, toluene A synthetic route is shown in Scheme 1. To a solution of
and isopropyl alcohol for 10 min, respectively. Then a thin compound 5 (0.2 mmol ratio), D (compound D1–D4) (0.5 mmol
layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfo- ratio), K2CO3 (4 mmol ratio), and Pd(PPh3)4 (0.02 mmol ratio) in
nate) (PEDOT : PSS from Baytron P VP AI 4083) was spin-coated dry toluene (10 mL), degassed deionized water (4 mL) and
(4000 rpm, 60 s) onto the ITO glass. Aer being baked at 140 ꢀC ethanol (2 mL) were added using a syringe under a nitrogen
ꢀ
for 15 minutes, the substrate was transferred into a nitrogen- atmosphere, then the mixture was reuxed at 110 C for 24 h.
lled glove box. Subsequently, the active layer was spin-coated Aer being cooled to room temperature, the mixture was
at 1500 rpm for 60 s with the blend solution of SMs and PC61BM poured into water (50 mL) and the organic layer was separated.
(total concentration of 9 mg mLꢁ1 in chloroform) on top of the The aqueous layer was extracted with dichloromethane (3 ꢂ
ITO/PEDOT : PSS substrate. Finally, a 100 nm aluminium elec- 20 mL) and the combined organic layers were dried over
trode was deposited by thermal evaporation through a shadow anhydrous Na2SO4. The organic solvent was evaporated and the
mask, yielding six individual devices with 5 mm2 nominal area. crude product was puried by silica column chromatography
4020 | J. Mater. Chem. C, 2014, 2, 4019–4026
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Scheme 1 The synthetic route of SM1, SM2, SM3 and SM4.
eluting with petroleum ether/dichloromethane to afford the 125.97, 124.64, 124.19, 123.35, 123.19, 46.01, 39.31, 30.41,
pure product.
28.62, 23.73, 23.15, 14.12, 10.63. MALDI-TOF HRMS: 1215.5588
[M+] (calcd for C82H78N4O2S2: 1215.5644).
Synthesis of compound SM1
Synthesis of compound SM3
Compound 5 (136 mg, 0.2 mmol), D1 (224 mg, 0.5 mmol),
K2CO3 (552 mg, 4 mmol), and Pd(PPh3)4 (23 mg, 0.02 mmol) Compound 5 (136 mg, 0.2 mmol), D3 (236 mg, 0.5 mmol),
were used to afford compound SM1 as a dark purple solid (174 K2CO3 (552 mg, 4 mmol), and Pd(PPh3)4 (23 mg, 0.02 mmol)
mg, 75%). M.p.: 249–252 ꢀC; 1H-NMR (400 MHz, CDCl3): d 9.02 were used to afford compound SM3 as a dark purple solid (189
(s, 2H), 7.64 (d, J ¼ 3.8 Hz, 8H), 7.49 (m, 6H), 7.28–7.30 (m, 8H), mg, 78%). M.p.: 280–281 ꢀC; 1H-NMR (400 MHz, CDCl3): d 8.98
7.14–7.15 (m, 12H), 7.05 (t, J ¼ 7.3 Hz, 4H), 4.09 (s, 4H), 1.97 (s, (d, J ¼ 4.1 Hz, 2H), 7.62 (d, J ¼ 8.3 Hz, 4H), 7.52 (d, J ¼ 8.3 Hz,
2H), 1.39–1.25 (m, 16H), 0.95–0.86 (m, 12H). 13C-NMR (100 4H), 7.46 (d, J ¼ 4.1 Hz, 2H), 7.38 (d, J ¼ 8.6 Hz, 4H), 7.28 (m,
MHz, CDCl3, ppm): d 161.71, 149.44, 147.71, 147.57, 141.03, 8H), 7.12 (d, J ¼ 7.8 Hz, 8H), 7.07 (t, J ¼ 7.3 Hz, 4H), 7.01 (d, J ¼
139.78, 136.99, 133.68, 131.58, 129.57, 129.36, 128.74, 127.55, 8.6 Hz, 4H), 4.13–4.01 (m, 4H), 1.94 (s, 2H), 1.40–1.25 (m, 16H),
126.44, 124.67, 124.32, 123.60, 123.21, 108.29, 46.07, 39.42, 0.94–0.86 (m, 12H). 13C-NMR (100 MHz, CDCl3, ppm): d 161.66,
30.46, 28.66, 23.79, 23.15, 14.10, 10.65. MALDI-TOF HRMS: 148.88, 148.18, 147.18, 139.68, 136.9, 132.69, 132.55, 132.13,
1163.5297 [M+] (calcd for C78H74N4O2S2: 1163.5331).
129.48, 129.21, 125.83, 125.13, 124.77, 124.02, 123.71, 122.20,
115.73, 108.40, 91.92, 88.52, 46.05, 39.36, 30.46, 28.69, 23.78,
23.20, 14.16, 10.66. MALDI-TOF HRMS: 1211.5380[M+] (calcd for
Synthesis of compound SM2
C
82H74N4O2S2: 1211.5311).
Compound 5 (136 mg, 0.2 mmol), D2 (237 mg, 0.5 mmol),
K2CO3 (552 mg, 4 mmol), and Pd(PPh3)4 (23 mg, 0.02 mmol)
were used to afford compound SM2 as a dark purple solid (175
Synthesis of compound SM4
1
ꢀ
mg, 72%). Mp.: 231–234 C; H-NMR (400 MHz, CDCl3): d 8.99 Compound 5 (136 mg, 0.2 mmol), D4 (249 mg, 0.5 mmol),
(s, 2H), 7.64 (d, J ¼ 7.6 Hz, 4H), 7.51 (d, J ¼ 7.6 Hz, 4H), 7.46 (s, K2CO3 (552 mg, 4 mmol), and Pd(PPh3)4 (23 mg, 0.02 mmol)
2H), 7.39 (d, J ¼ 8.4 Hz, 4H), 7.27–7.29 (m, 10H), 7.11 (d, J ¼ 4.0 were used to afford compound SM4 as a dark purple solid (190
Hz, 8H), 6.96–7.06 (m, 10H), 4.08 (s, 4H), 1.95 (s, 2H), 1.38–1.29 mg, 75%). M.p.: 290–292 ꢀC; 1H-NMR (400 MHz, CDCl3): d 8.97
(m, 16H), 0.94–0.87 (m, 12H). 13C-NMR (100 MHz, CDCl3, ppm): (d, J ¼ 5.3 Hz, 2H), 7.79 (d, J ¼ 8.7 Hz, 4H), 7.69 (m, 6H), 7.49 (d,
d 161.59, 147.67, 147.46, 139.67, 139.63, 138.24, 137.04, 131.88, J ¼ 3.7 Hz, 4H), 7.45 (s, 2H), 7.32 (t, J ¼ 7.9 Hz, 8H), 7.13–7.17
130.91, 129.32, 129.00, 128.68, 127.52, 126.92, 126.28, 126.25, (m, 12H), 7.04 (d, J ¼ 8.7 Hz, 4H), 4.08 (m, 4H), 1.95 (s, 2H),
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1.38–1.25 (m, 16H), 0.91–0.87 (m, 12H). 13C NMR (100 MHz, bis(pinacolato)diborane.11,18 Finally, SM1, SM2, SM3 and SM4
CDCl3, ppm): d 161.64, 150.20, 148.50, 146.49, 141.63, 139.67, were prepared by the Suzuki coupling reaction, using Pd(PPh3)4 as
136.94, 135.30, 133.14, 130.88, 129.61, 129.25, 126.43, 126.23, the catalyst and 2 M K2CO3 as the base in toluene at 110 ꢀC under
126.13, 125.83, 124.86, 124.55, 120.65, 118.50, 108.44, 106.60, a nitrogen atmosphere with yields of 75%, 72%, 78% and 75%,
46.02, 39.31, 30.38, 28.57, 23.72, 23.13, 14.12, 10.62. MALDI- respectively. The chemical structures of these synthesized
TOF HRMS: 1265.5537[M+] (calcd for C84H76N6O2S2: 1265.5549). compounds were identied by HRMS, 1H-NMR and 13C-NMR
spectra. The obtained OSMs are readily dissolved in common
organic solvents, such as chloroform, dichloromethane, chloro-
benzene and dichlorobenzene, owing to the alkyl-substituted DPP
as the central unit and TPA groups as ending units. The thermal
Results and discussion
Theoretical calculation
stabilities of four OSMs were investigated by thermogravimetric
analysis (TGA) under a nitrogen atmosphere at a heating rate of 10
ꢀC minꢁ1, as shown in Fig. 2, and the thermal data of the mate-
rials are summarized in Table 1. The onset decomposition
temperaꢀtures (Td, atꢀ5% weight loss) of four SMs are 411 ꢀC, 415
ꢀC, 411 C and 409 C, respectively. The results show that these
materials are fairly stable for their applications in PV devices.
To examine the ground-state geometries, electronic-density
distribution and energy levels of SMs, the density functional
theory (DFT) calculations were performed using the Gaussian 09
soware at the Becke's three-parameter gradient-corrected func-
tional (B3LYP) with a polarized 6-31G(d) basis.15 The theoretically
calculated energy levels and electronic-density distribution are
shown in Fig. 1b. The calculation results demonstrated that the
electronic density of the HOMO was distributed over the whole
molecules, whereas that of the LUMO was located predominantly
on the DPP unit, indicating that an ICT process occurred from
the TPA group to the central DPP unit. The DFT theoretically
calculated energy levels and the band gap (Eg) are summarized in
Table 1. Despite some marginal error in the calculated values,
there was a consistent changing law between the theoretically
calculated energy levels and the electrochemical experiment data.
The theoretically calculated results gave an important guide for
designing new materials.
Optical properties
The UV-visible absorption spectra of the synthesized SMs in
chloroform solution and thin-lm state are shown in Fig. 3, and
the correlated optical parameters are summarized in Table 1,
including the absorption peak wavelength (lmax) and the optical
band gap (Eogpt). As shown in Fig. 3, these compounds exhibited
broad absorption covering 300–900 nm wavelengths in the solid
sol
max
state, owing to their D–p–A type molecular structure. The l
in the long wavelength region were located at 618, 627, 621 and
622 nm for SM1, SM2, SM3 and SM4, respectively, corre-
sponding to the ICT transition between the DPP and the TPA
Synthesis and thermal stability
The synthetic route of four SMs is shown in Scheme 1. The
synthetic details of S1, S2 and S3 are shown in the ESI.† The
DPP-based acceptor moiety, compound 5, was synthesized in
three steps starting from commercially available 2-thio-
phenecarbonitrile following the procedures reported in the
literature.16 Compound 1 was synthesized in a yield of 65% by
Pd(PPh3)4 catalyzed Suzuki reaction of S1 and 4-bromoiodo-
benzene. The Heck reaction of N,N-diphenyl-4-iodoaniline and
4-bromostyrene took place to give compound 2 in a yield of
47%, with Pd(OAc)2 as the catalyst, TBAB as the phase transfer
catalyst and KOAc as the base in DMF. Compound 3 was
obtained in a yield of 93% by Pd(PPh3)2Cl2 catalyzed Sonoga-
shira coupling reaction between S2 and 4-bromoiodobenzene.17
Compound 4 was obtained by Knoevenagel condensation of S3
and 4-bromobenzonitrile, using NaOH as the base in EtOH with
a yield of 89%. Subsequently, the donor units, compounds D1,
D2, D3 and D4, were synthesized by the Miyaura borylation
Fig. 2 TGA plots of SM1, SM2, SM3 and SM4 at a heating rate of 10 ꢀC
reaction from bromo-substituted donating groups and minꢁ1 under a nitrogen atmosphere.
Table 1 The photophysical, electrochemical and theoretically calculated data of SM1, SM2, SM3 and SM4
sol
max
lm
max
Compound
l
nm
l
nm
Eogpt eV
Td ꢀC
HOMOec eV
LUMOec eV
Eegc eV
HOMODFT eV
LUMODFT eV
EDg FT eV
SM1
SM2
SM3
SM4
618
627
621
622
643
661
653
666
1.72
1.67
1.70
1.67
411
415
411
409
ꢁ5.10
ꢁ5.07
ꢁ5.14
ꢁ5.12
ꢁ3.35
ꢁ3.37
ꢁ3.40
ꢁ3.41
1.75
1.70
1.74
1.71
ꢁ4.70
ꢁ4.61
ꢁ4.70
ꢁ4.76
ꢁ2.51
ꢁ2.55
ꢁ2.59
ꢁ2.65
2.19
2.06
2.11
2.11
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ferrocene
LUMOec ¼ ꢁ(Ered ꢁ E1/2
+ 4.8) eV
Eegc ¼ LUMOec ꢁ HOMOec
ferrocene
where the unit of potential is relative to Ag/Ag+. E1/2
¼
0.05 eV versus Ag/Ag+.19 The related electrochemical data
(HOMOec, LUMOec and Egec) are listed in Table 1. The HOMOec
energy levels were ꢁ5.10 eV, ꢁ5.07 eV, ꢁ5.14 eV and ꢁ5.12 eV,
respectively, and the LUMOec energy levels were ꢁ3.35 eV, ꢁ3.37
eV, ꢁ3.40 eV and ꢁ3.41 eV, respectively. These results clearly
demonstrated that the synthesized OSM donors possessed
appropriate energy levels to satisfy the requirement of solution-
processable OSCs, with PC61BM as the acceptor. In comparison
with P3HT, the synthesized four compounds gave rise to rela-
tively lower HOMO energy levels. It is well known that the deep-
lying HOMO level is desirable for obtaining high VOC since the
value of VOC is determined by the energy difference between
the HOMO level of the donor material and the LUMO level of
the acceptor material.1a
Fig. 3 Normalized UV-vis absorption spectra of SM1, SM2, SM3 and
SM4 in chloroform and in thin film.
unit. In comparison with the absorption spectra in solution, the
lmlmax were red-shied by 25, 34, 32 and 44 nm for SM1, SM2, SM3
and SM4, respectively, indicating a strong intermolecular
interaction in the solid state. As a result, their optical band gaps
Hole-mobility measurement
(Eogpt) corresponding to their absorption onsets were 1.72 eV, Hole mobility is a key parameter for both material design and
1.67 eV, 1.70 eV and 1.67 eV, respectively, in agreement with device fabrication. To determine the hole mobilities of synthesized
their electrochemical band gaps.
compounds, the current density–voltage (J–V) characteristic of
each material is measured and the space-charge-limited currents
(SCLC) model is tted according to the Mott–Gurney law:20
Electrochemical characteristics
J ¼ (9/8)303rmh(V2/L3)
The CV measurements were carried out to evaluate the HOMO
and LUMO energy levels of the synthesized SMs. Fig. 4 displays
the CV curves in anhydrous CH2Cl2 solutions of 0.1 M NBu4BF4
under a nitrogen atmosphere with ferrocene–ferrocenium
(Fc/Fc+) couple as an internal reference. The HOMOec, LUMOec
energy levels and the electrochemical band gaps (Eegc) could be
obtained from onset oxidation potentials (Eox) and onset
reduction potentials (Ered) of the CV curves according to the
following equations:
where J is the current density, 3r is the relative permittivity and
assumed as approximately 3.0, 30 is the permittivity of free
space, mh is the hole mobility, V is the effective voltage, and L is
the thickness of the active layer.
The J–V characteristics of hole-only devices with a structure
of ITO/PEDOT : PSS/SMs : PC61BM/Au were measured in the
dark, as shown in Fig. 5a. By tting the J–V curves in a double
logarithmic scale to a SCLC model, the hole mobility can be
derived, and the results are plotted in Fig. 5b. The hole mobility
values of SM1, SM2, SM3 and SM4 were 6.17 ꢂ 10ꢁ5, 2.68 ꢂ
10ꢁ4, 1.20 ꢂ 10ꢁ4 and 4.46 ꢂ 10ꢁ5 cm2 Vꢁ1 sꢁ1, respectively,
and the data are summarized in Table 2. The charge mobility in
the BHJ lm might be closely related to the charge stability, the
morphology of the active layer as well as the intermolecular
interaction, leading to the increase of the transporting property
of the carrier. It was interesting to note that SM2 exhibited
about 2 times and 6 times higher hole mobility than those of
SM3 and SM4, respectively, which were consistent with their PV
performance data (JSC and PCEs). Nevertheless, the hole
mobility of SM1 is slightly higher than that of SM4, probably
due to the better morphology of the SM1 based active layer, as
shown in AFM images (ESI, Fig. S9†).
HOMOec ¼ ꢁ(Eox ꢁ E1/2
+ 4.8) eV
ferrocene
Photovoltaic properties
The PV properties of the synthesized SMs were investigated by
fabricating solution-processable BHJ devices in a conventional
structure of ITO/PEDOT : PSS/SMs : PC61BM/Al, and the
Fig. 4 Cyclic voltammogram of SM1, SM2, SM3 and SM4 in 0.1 M
NBu4BF4/CH2Cl2 solution under N2 at a scan rate of 100 mV sꢁ1
.
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Fig. 5 (a) J–V characteristics of hole-only devices with a structure of ITO/PEDOT : PSS/SMs : PC61BM/Au in the dark and (b) J–V curves of the
hole-only devices in a double logarithmic scale, the solid lines are fits of the data points to a SCLC model.
Table 2 The hole mobility and photovoltaic performance (with standard deviations) of SM1, SM2, SM3 and SM4
Compound
mha cm2 Vꢁ1 sꢁ1 nm
JSCb mA cmꢁ2
VOCb V
FFb
PCEb %
SM1
SM2
SM3
SM4
(6.17 ꢃ 0.78) ꢂ 10ꢁ5
(2.68 ꢃ 0.43) ꢂ 10ꢁ4
(1.20 ꢃ 0.35) ꢂ 10ꢁ4
(4.46 ꢃ 0.90) ꢂ 10ꢁ5
8.62 ꢃ 0.20
11.90 ꢃ 0.23
10.30 ꢃ 0.10
9.73 ꢃ 0.20
0.88 ꢃ 0.01
0.84 ꢃ 0.01
0.93 ꢃ 0.00
0.90 ꢃ 0.01
0.36 ꢃ 0.01
0.38 ꢃ 0.01
0.32 ꢃ 0.00
0.33 ꢃ 0.01
2.74 ꢃ 0.16
3.76 ꢃ 0.10
3.10 ꢃ 0.03
2.92 ꢃ 0.10
Hole-mobility (mh): error analysis over 8 devices. b PV performance: error analysis over 16 devices.
a
corresponding device structure is shown in Fig. 6a. In addition, dihedral angle between the thiophene and benzene rings, q3 is
the related J–V characteristics of these devices are described in the dihedral angle between the benzene ring and p-linkage, and
Fig. 6c and the corresponding PV data are summarized in Table q4 is the dihedral angle between the p-linkage and TPA unit.
2, including the VOC, JSC, ll factor (FF) and PCEs. The PV The differences of dihedral angles q1 and q2 were less than 2ꢀ
devices based on SM1, SM2, SM3 and SM4 gave PCEs of 2.74%, for all SMs, illustrating that thiophene and benzene rings were
3.76%, 3.10% and 2.92%, respectively. Among them, the best PV just benecial for extending the conjugation length. The dihe-
performance of SM2 could be attributed to the vinylene linkage, dral angle q3 in SM1 was 34.03ꢀ, indicating that the coplanarity
increasing the rigidity and coplanarity of the molecular back- decreases with the introduction of TPA to the benzene ring,
bone, and facilitating intermolecular interaction, which was while the torsion angles were weakened in SM2, SM3 and SM4.
consistent with its high hole mobility. The relatively higher VOC There was a slight increase in the dihedral angle q4 of SM2 and
values of SM3 and SM4 were proportional to their deep-lying SM3 compared with q3 of them, because the vinylene and
HOMO energy levels caused by the acetylene and acrylonitrile acetylene as p-linkages could stiffen the molecular structure
linkage groups with the electron-withdrawing effect, increasing and eliminate torsion between benzene and TPA. Compared
the oxidation potential. The JSC values of SM2, SM3 and SM4 were with the dihedral angles of SM2, a larger q3 and a smaller q4 of
11.90, 10.30 and 9.73 mA cmꢁ2, respectively. The descending of SM4 were observed, indicating that acrylonitrile in the D–A–D
JSC was in agreement with their hole mobility. In contrast, the structure leads to a slightly twisted structure, thereby forming a
relatively lower JSC of SM1 was probably due to the poor coplanar relatively lower JSC. Obviously, in comparison with SM1, the
structure. Bearing in mind that the better FF value indicated relatively higher PCEs of devices based on the other three
good morphology of the active layer, it was interesting to note materials could be attributed to their better coplanar structure
that SM1 and SM2 exhibited a relatively higher FF, which was and strong intermolecular interaction. In addition, the copla-
consistent with their good bicontinuous interpenetrating narity and intermolecular interaction lead to more red-shied
network, as shown in the AFM image (ESI, Fig. S9†). As a result, absorption spectra of SM2, SM3 and SM4 in lms.
the p-linkage units signicantly affected the coplanarity of the
OSMs and intermolecular charge transfer.7b,21
The incident photon-to-current efficiency (IPCE) spectra of
the devices were measured as well. As shown in Fig. 6d, all
In order to deeply understand the relationship between the devices based on these SMs exhibited signicant photon-to-
molecular structure and PV property, the dihedral angles of SMs current responses in the range of 300–800 nm with the
were discussed. Fig. 6b shows the dihedral angles (q) of SMs maximum IPCE of 38%, 45%, 43% and 41% at around 350 nm,
optimized at the B3LYP/6-31G (d) level. The q1 is the dihedral and 31%, 38%, 27% and 30% at around 600 nm, respectively,
angle between the DPP unit and the thiophene ring, q2 is the which were consistent with their UV-vis absorption proles. By
4024 | J. Mater. Chem. C, 2014, 2, 4019–4026
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Journal of Materials Chemistry C
Fig. 6 (a) The device structure; (b) dihedral angles of SM1, SM2, SM3 and SM4 optimized at the B3LYP/6-31G (d) level; (c) schematic J–V curves
of PV applications based on SMs : PC61BM at a 1 : 2 ratio (w:w) under an illumination of AM 1.5G (100 mW cmꢁ2); and (d) IPCE spectra of the
devices.
introducing vinylene linkage in SM2, the IPCE spectra became (2011J21DW001), the SRF for ROCS-SEM (no. 201001438, no.
broader and higher, which was attributed to its high hole 201001439), the Fundamental Funds for the Central Universi-
mobility and strong intermolecular interaction, in agreement ties (DUT11LK20), and the RFDP (no. 20090041120017) for
with its best PCE performance.
nancial support. We are very grateful for the useful discussion
with Prof. Yue Wang and Prof. Wenjing Tian.
Conclusions
Notes and references
In summary, we have successfully designed and synthesized
four novel DPP-TPA based D–p–A–p–D type compounds by
introducing single bond, vinylene, acetylene and acrylonitrile
groups as linkages, respectively. The best PCE performance of
SM2 could be attributed to the vinylene linkage, increasing the
coplanarity of the molecular structure and facilitating the
intermolecular interaction, which is consistent with its high
hole mobility. The relatively higher VOC values of SM3 and SM4
were due to the deep-lying HOMO levels caused by the electron-
withdrawing effect of acetylene and acrylonitrile linkages. As a
conclusion, we explored an effective approach to improve the PV
performance of DPP-TPA based OSM donors by selecting an
appropriate p-linkage. These results gave an important guide
for developing new materials in solution-processed small
molecule solar cells. Further experiments on device optimiza-
tion are underway in our laboratory.
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