Diketopyrrolopyrrole-based acceptors with multi-arms for organic solar cells

Article abstract of DOI:10.1039/c8ra03792b

Three small molecules SBF-1DPPDCV, SBF-2DPPDCV and SBF-4DPPDCV consisting of a spirobifluorene (SBF) unit as the core and one, two, and four diketopyrrolopyrrole dicyanovinyl (DPPDCV) units as the arms have been designed and synthesized for solution-processed bulk-heterojunction (BHJ) solar cells. The UV-Vis absorption and cyclic voltammetry measurement of these compounds showed that all these compounds have an intense absorption band over 300-750 nm with a LUMO energy level at around -3.87 eV. When pairing with PTB7-Th as the donor, devices fabricated based on PTB7-Th?:?SBF-4DPPDCV blends showed a decent PCE of 3.85%, which is the highest power conversion efficiency (PCE) amongst the three DPP acceptor fabricated devices without extra treatment. Devices with SBF-1DPPDCV and SBF-2DPPDCV acceptors showed lower PCEs of 0.26% for SBF-1DPPDCV and 0.98% for SBF-2DPPDCV respectively. The three dimensional (3D) structure of SBF-4DPPDCV facilitates the formation of a 3D charge-transport network and thus enables a rational electron-transport ability (1.04 × 10^-4 cm² V^-1 s^-1), which further leads to a higher J_sc (10.71 mA cm^-2). These findings suggest that multi-arm acceptors present better performance than one-arm or two-arm molecules for organic solar cells.

Full text of DOI:10.1039/c8ra03792b

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Diketopyrrolopyrrole-based acceptors with multi-
arms for organic solar cells†
Cite this: RSC Adv., 2018, 8, 25031
Po Sun,abc Xiangzhi Li,ab Yulong Wang,b Haiquan Shan,b Jiaju Xu,b Changmei Liu,a
a
Cong Zhang,a Fei Chen,a Zongxiang Xu,
b Zhi-kuan Chen
a and Wei Huang
*
*
*
Three small molecules SBF-1DPPDCV, SBF-2DPPDCV and SBF-4DPPDCV consisting of a spirobifluorene
(SBF) unit as the core and one, two, and four diketopyrrolopyrrole dicyanovinyl (DPPDCV) units as the arms
have been designed and synthesized for solution-processed bulk-heterojunction (BHJ) solar cells. The UV-
Vis absorption and cyclic voltammetry measurement of these compounds showed that all these
compounds have an intense absorption band over 300–750 nm with a LUMO energy level at around
ꢀ3.87 eV. When pairing with PTB7-Th as the donor, devices fabricated based on PTB7-Th : SBF-4DPPDCV
blends showed a decent PCE of 3.85%, which is the highest power conversion efficiency (PCE) amongst
the three DPP acceptor fabricated devices without extra treatment. Devices with SBF-1DPPDCV and SBF-
2DPPDCV acceptors showed lower PCEs of 0.26% for SBF-1DPPDCV and 0.98% for SBF-2DPPDCV
respectively. The three dimensional (3D) structure of SBF-4DPPDCV facilitates the formation of a 3D
Received 3rd May 2018
Accepted 28th June 2018
charge-transport network and thus enables a rational electron-transport ability (1.04 ꢁ 10ꢀ4 cm2 Vꢀ1
s
ꢀ1),
DOI: 10.1039/c8ra03792b
which further leads to a higher Jsc (10.71 mA cmꢀ2). These findings suggest that multi-arm acceptors
rsc.li/rsc-advances
present better performance than one-arm or two-arm molecules for organic solar cells.
years, some new non-fullerene acceptors that showed much
better performance than fullerenes have been found.9,10 The
1 Introduction
power conversion efficiency of fullerene-free OPVs using
a polymer donor PBDB-T-2Cl paired with a small molecular
acceptor IT-4F blend has surpassed 14% by far.11
The last decade has witnessed rapid progress in solution pro-
cessed bulk heterojunction organic photovoltaics (OPVs) boos-
ted by the design and synthesis of novel donor materials, owing
to their potential advantages such as low cost, light weight, and
exibility.1–3 To date, a power conversion efficiency (PCE) of
>10% has been achieved for fullerene-based OPVs.4–8
Fullerene-derivatives, such as PC61BM and PC71BM, have
been used to play a dominant role as electron acceptors in the
OPV research community. The high electron mobility as well as
the isotropic charge-transporting characteristics of fullerene-
derivatives made them superior to organic acceptors. Despite
the signicant success of fullerene-derivative acceptors in OPVs,
their further improvement is limited due to their poor light-
harvesting capability for solar light, low chemical/energetic
tailorability, and the tedious synthesis and purication
process, especially for industrial scale applications. In the past
Currently, non-fullerene acceptor materials, including small
molecules and polymers, have strong electron decient units such
as naphthalene diimide (NDI),12,13 perylene diimide (PDI),14–16
diketopyrrolopyrrole (DPP),17–20 or other building blocks with
electron withdrawing moieties.21–23 Small molecular acceptors are
normally believed to be more superior than polymeric acceptors in
terms of their purity and small batch to batch variance on
synthesis, which is important for industrial scale applications.24,25
However, most of the small molecule acceptors do not have 3D
ball-shape molecular structure thus may not form a 3D charge-
transporting network as readily as fullerenes do.26–28 For these
reasons, a potentially promising approach is to develop small
molecular materials with quasi-3D or 3D molecular structures.14
Zhan and co-workers have demonstrated a novel, nonplanar, star-
shaped PDI acceptor S(TPA-PDI) with a triphenylamine (TPA) core,
which exhibited a quasi-3D structure.29 Yan's group designed
a unique 3D molecular TPE-PDI4 with the highly twisted nature of
the tetraphenylethylene (TPE) core, which gave a PCE of 5.53% for
non-fullerene based OPVs.14 Ma and Cho reported SBF-PDI4 con-
sisting of SBF-core and tetra-PDI subunits that constructed a 3D
geometry.30,31 Chen's group synthesized a series of materials based
on DPP, amongst of them SF-4DPP4 and SF-DPPB4 with spirobi-
uorene (SBF) core as acceptor materials showed good
aKey Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials
(IAM), National Jiangsu Synergistic Innovation Center for Advanced Materials,
Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China. E-mail:
iamzkchen@njtech.edu.cn; iamwhuang@njtech.edu.cn
bDepartment of Chemistry, South University of Science and Technology of China,
Shenzhen, P. R. China. E-mail: xu.zx@sustc.edu.cn
cAnalysis and Testing Central Facility, Engineering Research Institute, Anhui University
of Technology, Maanshan 243002, P. R. China. E-mail: sunpoo@ahut.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra03792b
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performance when blended with P3HT, the PCEs of them were
3.63% and 5.16% respectively aer annealing.17,32 All of the above
mentioned molecules have been found to exhibit a 3D molecular
structure, as well as weak intermolecular interactions and weak
molecular aggregation in the solid lms and can easily form
smooth lms.
Aiming in further proving the nature of 3D dendrimeric
acceptors, we report three small molecules consisting of SBF
core attached with various numbers of DPP derivatives as arms
to identify the effect of the 3D structures on acceptor perfor-
mance in solar cells. Herein, SBF with an orthogonal molecular
structure offers these new acceptor materials a perfect 3D
geometry.17,33–35 DPP derivative was selected to build up the
arms because of its great light-harvesting capability as well as
the excellent charge carrier transport behavior.33,36–39 To further
lower the energy levels, strong electron accepting dicyanovinyl
(DCV) group was incorporated as well.40–44 Based on these
considerations, SBF-1DPPDCV functionalized with one arm,
SBF-2DPPDCV with two arms, and SBF-4DPPDCV with four
arms were designed (Fig. 1),14,41,45,46 respectively. PTB7-Th was
used as the donor for device fabrication and evaluation of the
three acceptor molecules. The performance of devices based on
PTB7-Th : acceptor blends suggested that 4-armed molecule,
SBF-4DPPDCV, exhibited more superior performance than the
other two materials, and a PCE as high as 3.85% was obtained,
which is the highest PCE amongst the devices using DPP based
acceptors without additives and annealing treatment.17,47
Scheme
4DPPDCV.
1
Synthesis of SBF-1DPPDCV, SBF-2DPPDCV and SBF-
NMR, 13C NMR (see Fig. S1–S12†), matrix-assisted laser desorp-
tion ionization time of ight (MALDI-TOF) mass spectroscopy
(MS), and elemental analysis.
All of the molecules exhibited good solubility in common
organic solvents such as chloroform and o-dichlorobenzene at
room temperature because of the introduction of alkyl substit-
uents to the primary structures.
2 Results and discussion
2.1 Synthesis and characterization
The synthetic routes of the materials studied in this work are
illustrated in Scheme 1. We synthesized SBF-2B and SBF-4B as
the cores, and the electron withdrawing DPP arms functionalized
with DCV as end capping groups. Then SBF-1DPPDCV, SBF-
2DPPDCV and SBF-4DPPDCV were synthesized by Suzuki
coupling reaction between DPPDCV arms and SBF cores. The
2.2 Thermal properties
As shown in Fig. 2a, thermogravimetric analysis (TGA) under
nitrogen indicates that decomposition temperatures (Td,
dened at the temperature at which 5% of the mass is lost) were
236 ꢂC, 374 ꢂC, and 383 ꢂC for SBF-1DPPDCV, SBF-2DPPDCV
and SBF-4DPPDCV, respectively. The higher Td values observed
for SBF-2DPPDCV and SBF-4DPPDCV indicated that the struc-
tures were more thermally stable.
1
chemical structures and purities were fully characterized by H
The differential scanning calorimetry (DSC) analysis of the
three compounds are illustrated in Fig. 2b. During the rst
Fig. 1 Chemical structures of SBF-1DPPDCV, SBF-2DPPDCV, SBF- Fig.
2
(a) Thermogravimetric Analysis (TGA) and (b) Differential
4DPPDCV, PTB7-Th and PDIN.
Scanning Calorimetry (DSC) of the compounds.
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Table 1 Photophysical and electrochemical properties of the studied compounds
lmax [nm]
ox
onset
red
onset
Compounds
Solution
Film
E
[V]
E
[V]
HOMO [eV]a
LUMO [eV]a
Band gap [eV]
SBF-1DPPDCV
SBF-2DPPDCV
SBF-4DPPDCV
685
700
700
702
717
717
1.12
1.06
1.08
ꢀ0.51
ꢀ0.55
ꢀ0.54
ꢀ5.52
ꢀ5.46
ꢀ5.48
ꢀ3.89
ꢀ3.85
ꢀ3.86
1.59b, 1.63c
1.55b, 1.61c
1.56b, 1.62c
a
ox
Calculated based on the respective onsets of oxidation and reduction potentials of the compounds via the equations: HOMO ¼ ꢀ(E
+ 4.4) eV,
onset
red
onset
LUMO ¼ ꢀ(E
+ 4.4) eV. b Based on the onset absorption data of thin lms. c Based on the redox potentials.
heating scans, SBF-1DPPDCV showed weak melting peak at spectrum of SBF-1DPPDCV showed a peak value at ꢃ685 nm
138.5 ꢂC, SBF-2DPPDCV and SBF-4DPPDCV exhibiꢂted sharp while those of SBF-2DPPDCV and SBF-4DPPDCV were at
ꢂ
melting endothermic transitions at 143 C and 227 C respec- 700 nm. The slightly red shied spectra of SBF-2DPPDCV and
tively, which revealed certain degree of molecular order within SBF-4DPPDCV compared with that of SBF-1DPPDCV is due to
the bulk materials. A broad shoulder peak was present in the their larger p-conjugation on the DPP backbone. Similar
onset of the melting curve for SBF-4DPPDCV, indicating the proles in the absorption spectra of SBF-2DPPDCV and SBF-
possible presence of conformational polymorphs.9 While no 4DPPDCV could be attributed to their similar chemical struc-
obvious recrystallization was observed upon cooling and ture of the DPP and the limited electron coupling between the
a featureless second heating was obtained, the materials then two perpendicular DPP in SBF-4DPPDCV.50 Signicant red-
appeared to become kinetically trapped in the amorphous shis in the absorption spectra were observed for all of the
phase aer melting, indicating a poorly crystalline property.9,48 three acceptors in their thin-lms compared with those in
diluted solution, suggested the increase of molecular aggrega-
tion in the solid state. The optical bandgaps of SBF-1DPPDCV,
SBF-2DPPDCV and SBF-4DPPDCV determined from the
2.3 Optical and electrochemical properties
Fig. 3a and b depicts the absorption spectra of three DPP
acceptors in dichloromethane solutions and thin lms. In
solution, SBF-1DPPDCV exhibited absorption maxima at
absorption onset of the absorption spectra of these compounds
were estimated to be 1.59 eV, 1.55 eV and 1.56 eV, respectively
(Table 1).
685 nm with the extinction coefficient of 1.5 ꢁ 104 Mꢀ1 cmꢀ1
,
The electrochemical behaviors of the acceptor materials
were investigated by cyclic voltammetry (CV) in anhydrous
dichloromethane solutions, as shown in Fig. 3c. The lowest
unoccupied molecular orbital (LUMO) and highest occupied
molecular orbital (HOMO) of newly synthesized materials can
be determined by their reduction and oxidation onset potentials
respectively. The onset potentials were determined from the
intersection of two tangents drawn as the rising current and
baseline charging current of the CV traces. LUMO values were
SBF-2DPPDCV and SBF-4DPPDCV exhibited absorption maxima
at 700 nm with the extinction coefficients of 2.1 ꢁ 104 Mꢀ1 cmꢀ1
and 1.4 ꢁ 105 Mꢀ1 cmꢀ1, respectively. The three compounds in
dichloromethane solution exhibits similar absorption transi-
tion bands between 500–750 nm that can be ascribed to the
intramolecular charge transfer (ICT) transitions between
central SBF and terminal acceptor units.49 The absorption
calculated (similar as HOMO value determination) from the
red
onset
corresponding reduction onsets using LUMO ¼ –(E
+
4.4) eV. LUMO values were found to be ꢀ3.89 eV, ꢀ3.85 eV and
ꢀ3.86 eV for SBF-1DPPDCV, SBF-2DPPDCV, and SBF-4DPPDCV,
respectively (Table 1), which is close to PC60BM energy level.
Moreover, similar to PC60BM,14,51 SBF-4DPPDCV exhibits 3D
electron transportation characteristics. These acceptors show
similar LUMO levels that can match well with the donor PTB7-
Th and the interfacial material PDIN (Fig. 3d).
2.4 Theoretical calculations
The molecular geometry was optimized by density functional
theory (DFT) using the B3LYP hybrid functional with basis set 6-
31G(d). Quantum chemical calculation was performed with the
Gaussian 09 package. In the calculations, the alkyl side chains
of the molecules were replaced by methyl groups to reduce
machine time. Fig. 4 illustrates the optimized ground-state
geometries of the three acceptors. In the top view of three
molecular, the two uorene planes of the SBF-core were tilted at
Fig. 3 (a) Normalized absorption spectra of the compounds in diluted
dichloromethane and (b) normalized absorption spectra of thin films
on glass; (c) cyclic voltammogram for the compounds in dichloro-
methane solution. (d) HOMO/LUMO energies for all compounds in the
devices.
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poorer photovoltaic performances with PCE of 0.98%, Voc of
0.74 V, Jsc of 3.22 mA cmꢀ2 and FF of 41.37% for SBF-1DPPDCV,
and PCE of 0.26%, Voc of 0.74 V, Jsc of 1.09 mA cmꢀ2, and FF of
31.52% for SBF-2DPPDCV, respectively. The three devices
exhibited the same Voc as the three acceptors showed similar
LUMO energy levels. But the ll factor and short circuit current
of SBF-4DPPDCV were much higher than SBF-1DPPDCV and
SBF-2DPPDCV.
The external quantum efficiency (EQE) of the blended lms
with a PTB7-Th/acceptor weight ratio of 1.5 : 1 without any post-
treatment is shown in Fig. 5b. All of the blended lms showed
broad EQE spectra in the range from 300 nm to 800 nm, with
the EQE maximum peaks at 750 nm. While the peak intensity of
spectra increases from 5% for SBF-1DPPDCV to 55% for SBF-
4DPPDCV, which matched well with the increase of Jsc. Since
the absorption spectra of PTB7-Th and DPP-based acceptors are
very similar and have strong overlap in the region of 300 nm
ꢀ800 nm, it was hard to differentiate their respective contri-
butions to the photocurrent.
The electron and hole mobilities of the devices were estimated
from space-charge-limited current (SCLC) measurements.52 For
hole-only and electron-only devices, the structures of ITO/
PEDOT:PSS/PTB7-Th : acceptors (1.5 : 1, w/w)/Au and Al/PTB7-
Th : acceptors (1.5 : 1, w/w)/Al were used, respectively (Fig. 5c and
d). The estimated hole and electron mobilities are summarized in
Table 2. The PTB7-Th : SBF-1DPPDCV blend lms displayed
a hole mobility (mh) and an electron mobility (me) of 6.97 ꢁ 10ꢀ5
cm2 Vꢀ1 sꢀ1 and 6.14ꢁ10ꢀ6 cm2 Vꢀ1 sꢀ1, respectively (mh/me ¼
11.4). The PTB7-Th : SBF-2DPPDCV blend lms exhibited a better
balanced charge transport (mh ¼ 1.04 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1, me ¼
3.66 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1; mh/me ¼ 2.84). Among the three devices,
PTB7-Th : SBF-4DPPDCV exhibits the highest and the most
balanced charge mobility with mh of 1.06 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1, me
of 1.04 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1, mh/me ratio of 1.01. It suggests that
variation of the 3D conformation of the non-fullerene acceptor
material does signicantly tune the bulk charge transport prop-
erties of the BHJ devices, and the higher and more balanced
charge transport is benecial to Jsc and PCE enhancement.10,19
Fig. 4 Front and top views of SBF-1DPPDCV, SBF-2DPPDCV and SBF-
4DPPDCV obtained by Tinker and optimized at B3LYP/6-31G(d) level.
an angle of 90ꢂ, so the four arms SBF-4DPPDCV tended to form
a cross three-dimensional structure, while for SBF-1DPPDCV
and SBF-2DPPDCV, the DPPDCV arms are only attached to one
planar uorene, and they does not form an obvious three-
dimensional structure.
2.5 Device performances
The solution-processed BHJ solar cell devices were fabricated in
a conventional conguration of ITO glass/PEDOT:PSS/PTB7-
Th : acceptor/PDIN/Al and tested under an AM 1.5G simulated
solar light at 100 mW cmꢀ2. Here, PEDOT:PSS and PDIN were
used as hole transport layer and cathode interlayer respec-
tively.52 The blend lms at a donor/acceptor weight ratio of
1.5 : 1 gave the best solar cell performance without additives
and annealing treatment, as shown in Fig. 5a and Table 2. The
device with SBF-4DPPDCV as acceptor afforded the best
performance (Voc ¼ 0.74 V, Jsc ¼ 10.71 mA cmꢀ2, FF ¼ 48.69%
and PCE ¼ 3.85%). For the purpose of comparison, SBF-
1DPPDCV and SBF-2DPPDCV under the same conditions gave
2.6 Film morphology
Fig. 6 shows the atomic force microscope (AFM) images of the
blend thin lms of PTB7-Th : acceptors at weight ratios of
1.5 : 1. The root-mean-square (RMS) roughness of the three
compounds was found to be 1.66 nm, 1.41 nm and 1.10 nm for
the blend lms with SBF-1DPPDCV, SBF-2DPPDCV, and SBF-
4DPPDCV, respectively. The lms of PTB7-Th blended with SBF-
1DPPDCV at a weight ratio of 1.5 : 1 produced a signicantly
larger domain size (60–80 nm). For SBF-2DPPDCV, AFM image
shows a domain size between 30 nm and 40 nm, while it was
ꢃ20–30 nm for SBF-4DPPDCV and continuous interpenetrating
networks for charge transportation were formed, which could
be the reason of the improvement of electron and hole mobil-
ities. The continuous mesh structure induced by the 4-arm
cross structures of SBF can be benecial to efficient exciton
dissociation and charge transportation in the thin lms.
Fig. 5 (a) J–V curves of OPVs based on PTB7-Th : acceptors under
the illumination of AM 1.5G, 100 mW cmꢀ2; (b) EQE spectra of PTB7-
Th : acceptors PSCs; plots of ln(JL3/V2) vs. (V/L)0.5 extracted from (c)
ITO/PEDOT:PSS/PTB7-Th : acceptors/Au hole-only devices and (d)
Al/PTB7-Th : acceptors/Al electron-only devices.
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Table 2 The device data of OPVs based on PTB7-Th : acceptors under the illumination of AM 1.5G, 100 mW cmꢀ2
Donor : acceptor
w/w
Voc [V]
Jsc [mA cmꢀ2
]
FF [%]
PCEa [%]
mhb [cm2 Vꢀ1 sꢀ1
]
me
mh/me
PTB7-Th : SBF-1DPPDCV
PTB7-Th : SBF-2DPPDCV
PTB7-Th : SBF-4DPPDCV
1.5 : 1
1.5 : 1
1.5 : 1
0.74
0.74
0.74
1.09
3.22
10.71
31.52
41.37
48.69
0.26 (0.23)
0.98 (0.92)
3.85 (3.79)
6.97 ꢁ 10ꢀ5
1.04 ꢁ 10ꢀ4
1.06 ꢁ 10ꢀ4
6.14 ꢁ 10ꢀ6
3.66 ꢁ 10ꢀ5
1.04 ꢁ 10ꢀ4
11.4
2.84
1.01
The best and average (in brackets, over 12 devices) PCEs. b Hole and electron mobilities measured by SCLC method.
a
3.2 Synthesis
3 Experimental
3.2.1 Synthesis of 2,20,7,70-tetrakis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9,90-spirobi[uorene] (SBF-4B). A solu-
tion of 2,20,7,70-tetrabromo-9,90-spirobi[uorene] (1.60 g, 2.50
mmol), bis(pinacolato)diboron (3.80 g, 15.0 mmol), Pd(dppf)Cl2
(0.40 g, 0.50 mmol), and KOAc (2.90 g, 30.0 mmol) in THF (150
3.1 General
Most of the organic and inorganic chemicals in this work were
obtained from Aldrich, Alfa Aesar, and TCI and used without
further purication. 1H and 13C NMR spectra were collected on
a Bruker 400 MHz spectrometer. Chemical shis for 1H and 13
C
ꢂ
mL) was stirred at 80 C under N2 for three days. The reaction
NMR were referenced to residual signals from CDCl3 (1H NMR
d ¼ 7.26 ppm and 13C NMR d ¼ 77.23 ppm). Matrix-assisted
laser desorption-ionization time of ight mass spectrometry
(MALDI-TOF MS) was performed on a Bruker Autoex II.
Samples were prepared by diluting the molecules in CH2Cl2
using 2,5-dihydroxybenzoic acid as the matrix. Elemental
analyses were conducted on a Flash EA 1112 elemental analyzer.
Solution and thin-lm UV-Vis absorption spectra were recorded
on a UV-1750 UV-VIS spectrophotometer. Electrochemical
measurements were carried out under nitrogen in deoxygenated
was quenched by adding DI water, and the resulting mixture
was washed with ethyl acetate (50 mL ꢁ 3). Then the organic
layers were washed with brine, dried over with Na2SO4, and
dried in vacuum box. Aer purication by silica gel column
chromatography using CH2Cl2/hexane (v/v, 1/1), white solid was
obtained in a yield of 79% (1.66 g). 1H NMR (400 MHz, CDCl3).
d (ppm) 7.87 (m, 8H), 7.08 (s, 4H), 1.23 (s, 48H). MALD-TOF MS:
calcd for C49H60B4O8: 820.47, found 820.25.
3.2.2 Synthesis of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-9,90-spirobi[uorene] (SBF-2B). The synthetic
route of SBF-2B was similar to the synthesis of SBF-4B. White
0.1
M
solutions
of
tetra-n-butylammonium
hexa-
uorophosphate in dry dichloromethane using a CHI 660E
electrochemical workstation, a glassy-carbon working electrode,
a platinum-wire auxiliary electrode, and an Ag wire anodised
with AgCl as a pseudo-reference electrode. The cyclic voltam-
metry was performed using a scan rate of 100 mV sꢀ1. The
potentials were referenced to Ag/AgCl by use of internal ferro-
cene. TGA measurements were performed on a TA Discovery
1
solids were obtained (yield 50%). H NMR (400 MHz, CDCl3).
d (ppm) 7.72 (m, 4H), 7.19 (m, 2H), 7.09 (s, 2H), 6.97 (s, 2H), 6.91
(m, 2H), 6.49 (d, J ¼ 7.4, 2H), 0.96 (s, 24H). MALD-TOF MS: calcd
for C37H38B2O4: 568.30, found 568.33.
3.2.3 Synthesis
of
3,6-di(thiophen-2-yl)pyrrolo[3,4-c]
pyrrole-1,4(2H, 5H)-dione (DPP). Potassium tert-butoxide
(20.0 g, 180 mmol) was added to a round-bottom ask with
argon protection. A solution of tert-amyl alcohol (125 mL) and 2-
thiophenecarbonitrile (16.4 g, 150 mmol) was injected in one
TGA analyzer under nitrogen with a heating rate of 10 ꢂC minꢀ1
.
DSC measurements were performed on a TA Discovery DSC
analyzer under nitrogen with a heating rate of 10 ꢂC minꢀ1. The
surface morphologies of the thin lms were examined using
atomic force microscopy (AFM, MFP-3D Classic, tapping mode).
ꢂ
portion. The mixture was heated to 105 C, and a solution of
dimethyl succinate (7.30 g, 50.0 mmol) in t-amyl alcohol (40 mL)
was introduced dropwise in 1 h. The reaction was stirred and
heated at 105 ꢂC for 1 h, and then the methanol byproduct was
distilled off, and the reaction mixture was kept stirring at 105ꢂꢂC
for 2 h. The reaction mixture was then cooled down to 65 C,
diluted with 50 mL of methanol, neutralized with acetic acid,
and reuxed for 10 min. The resulting suspension was ltered,
and the black lter cake was washed with hot methanol and
water twice, and then dried in vacuum box to afford a purple
black solid in a yield of 88.2% (13.2 g).
3.2.4 Synthesis of 2,5-dibutyloctyl-3,6-dithiophen-2-ylpyr-
rolo[3,4-c]pyrrole-1,4-dione (DPP-C12). DPP (13.0 g, 43.3 mmol)
and anhydrous potassium carbonate (24.0 g, 173 mmol) were
dissolved in N,N-dimethylformamide (250 mL) in a two-necked
round ask, and heated to 145 ꢂC under argon protection. 2-
Butyloctyl bromide (49.8 g, 200 mmol) was introduced in one
portion by syringe. Aer stirring at 145 ꢂC for 12 h, the solution
was allowed to cool down to room temperature, and then
500 mL ice water was poured into. The suspension was ltered,
and the solid collected on the lter paper was washed with water
Fig. 6 AFM height (a, b and c) and phase (d, e and f) images of PTB7-Th
blend with SBF-1DPPDCV (a and d), SBF-2DPPDCV (b and e) and SBF-
4DPPDCV (c and f) at a weight ratio of 1.5 : 1. The scale bar in all images
is 500 nm.
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and methanol successively, and then dried in vacuum box. The and CH2Cl2 (60 mL) was put in a 250 mL two-necked round
crude product was puried by silica gel column chromatog- bottom ask, and stirred at room temperature for 1 h. The
raphy using dichloromethane as eluent to give a deep purple resulting mixture was ltered, and residual solvent was
1
solid (15.2 g, yield 67%). H NMR (400 MHz, CDCl3): d (ppm) removed by rotary evaporation. The crude product was dis-
8.86 (d, J ¼ 3.4 Hz, 2H), 7.63 (d, J ¼ 4.8 Hz, 2H), 7.28 (s, 2H), 4.03 solved in 2 mL CH2Cl2 and reprecipitated in methanol. The
(d, J ¼ 7.6 Hz, 4H), 1.91 (m, 2H), 1.43–1.25 (m, 32H), 0.84 (m, resulting suspension was ltered and dried in vacuum box to
1
12H). MALD-TOF MS: calcd for C38H56N2O2S2: 634.40, found gave a dark blue solid powder. Yield: 69.62% (0.55 g). H NMR
635.32.
(400 MHz, CDCl3): d(ppm) 9.02 (d, J ¼ 4.3 Hz, 1H), 8.84 (d, J ¼
3.2.5 Synthesis of 5-(2,5-bis(2-butyloctyl)-3,6-dioxo-4-(thio- 4.2, 1H), 7.87 (s, 1H), 7.80 (d, J ¼ 4.3, 1H), 7.26 (d, J ¼ 3.9, 1H),
phen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)thiophene- 3.98 (dd, J ¼ 33.6, 7.3 Hz, 4H), 1.85–1.95 (m, 2H), 1.2–1.4 (m,
2-carbaldehyde (DPPCHO). POCl3 (7.66 g, 50.0 mmol) was 32H), 0.8–1.0 (m, 12H). MALD-TOF MS: calcd for C42H55BrN4-
added dropwise into a mixture of N,N-dimethylformamide O2S2: 790.29, found 790.05.
(3.65 g, 50.0 mmol) and 1,2-dichloroethane (5 mL) at 0 ꢂC. The
3.2.8 Synthesis of SBF-4DPPDCV. A mixture of SBF-4B
mixture was stirred at room temperature until the orange Vils- (82.0 mg, 0.10 mmol), DPPDCV-Br (396 mg, 0.50 mmol),
meier reagent was obtained. The above Vilsmeier reagent was aqueous 2 M K2CO3 (2.5 mL, 5.00 mmol), Aliquat 336 (1 drop),
added dropwise into the solution of 3,6-di(thiophen-2-yl)pyrrolo and toluene (8 mL) was carefully degassed for 20 min before and
[3,4-c]pyrrole-1,4(2H,5H)-Dione (DPP-C12) (1.85 g, 2.90 mmol) aer the addition of Pd(PPh3)4 (47.0 mg, 0.04 mmol). Then the
in 1,2-dichloroethane (50 mL). The mixture was stirred and resulting mixture was heated to 85 ꢂC and stirred under
ꢂ
heated at 85 C for overnight, and then cooled down to room a nitrogen atmosphere for 36 h. Aer cooling down to room
temperature. Aer the introduction of a saturated NaHCO3 temperature, water and CH2Cl2 were added, the organic layer
aqueous solution, the mixture was kept stirring for 2 h. The was separated, the aqueous layer was extracted with CH2Cl2,
product was extracted with CH2Cl2 (20 mL ꢁ 3), and the organic and the combined organic layers were dried over anhydrous
components were collected and washed successively with DI Na2SO4. Aer removal of the solvent under reduced pressure,
water and brine, and then dried over MgSO4. The crude product the residue was puried by column chromatography on silica
was puried by silica gel column chromatography using gel using CH2Cl2/petroleum ether (v/v, 1/3) as eluent to afford
CH2Cl2/petroleum ether (v/v, 1/1) as eluent to afford the purple a purple black solid (113 mg, 36%).1H NMR (400 MHz, CDCl3):
1
solid in a yield of 47.5% (0.91 g). H NMR (400 MHz, CDCl3): d(ppm) 9.00 (m, 4H), 8.06 (d, J ¼ 8.0 Hz, 4H), 7.86 (d, J ¼ 8.2 Hz,
d (ppm) 10.03 (s, 1H), 9.04 (dd, J ¼ 3.9, 1.1 Hz, 1H), 8.92 (d, J ¼ 4H), 7.80 (s, 4H), 7.75 (d, J ¼ 4.2 Hz, 4H), 7.39 (d, J ¼ 4.0 Hz, 4H),
4.2 Hz, 1H), 7.87 (d, J ¼ 4.2 Hz, 1H), 7.73 (dd, J ¼ 5.0, 1.1 Hz, 7.35–7.27 (m, 4H), 7.16 (s, 4H), 4.02 (s, 16H), 1.95–1.80 (m, 8H),
1H), 7.33 (dd, J ¼ 5.0, 4.0 Hz, 1H), 4.07 (m, 4H), 1.85–1.95 (m, 1.40–1.10 (m, 128H), 1.00–0.50 (m, 48H). 13C NMR (100 MHz,
2H), 1.2–1.4 (m, 32H), 0.8–1.0 (m, 12H). MALD-TOF MS: calcd CDCl3): d(ppm) 161.80, 160.92, 151.42, 149.39, 149.19, 142.82,
for C39H56N2O3S2: 664.37, found 663.13.
3.2.6 Synthesis of 5-(4-(5-bromothiophen-2-yl)-2,5-bis(2- 130.52, 128.43, 127.71, 125.32, 121.61, 121.49, 113.87, 113.11,
butyloctyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl) 112.37, 108.78, 78.84, 46.62, 46.49, 38.36, 37.89, 32.97, 31.71,
142.05, 139.04, 138.48, 137.56, 136.15, 134.67, 133.40, 132.55,
thiophene-2-carbaldehyde (DPPCHO-Br). A solution of N-bro- 31.69, 31.13, 30.88, 30.60, 29.73, 29.62, 28.38, 28.23, 26.13,
mosuccinimide (NBS) (0.15 g, 0.86 mmol) in CHCl3 (30 mL) was 26.03, 24.83, 23.02, 22.99, 22.61, 14.19, 14.16, 14.12, 14.07.
added dropwise into a solution of 5-(2,5-bis(2-butyloctyl)-3,6- MALDI-TOF-MS: calcd for C193H232N16O8S8: 3157.60; Found:
dioxo-4- (thiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1- 3159.03. Elemental analysis calcd for C193H232N16O8S8: C 73.35,
yl)thiophene-2-carbaldehyde (DPPCHO) (0.57 g, 0.86 mmol) in H 7.40, N 7.09%; found C 72.92, H 7.44, N 6.70%.
ꢂ
CHCl3 (30 mL) with stirring at 0 C. The mixture was kept stir-
3.2.9 Synthesis of SBF-1DPPDCV and SBF-2DPPDCV. The
ring at 0 ꢂC for overnight and then quenched with 20 mL DI synthetic route of SBF-1DPPDCV and SBF-2DPPDCV was similar
water. The aqueous phase was collected and extracted with to the synthesis of SBF-4DPPDCV. And they were puried by
CH2Cl2 (20 mL ꢁ3). The combined organic phase was washed silica gel chromatography (petroleum ether:CH2Cl2 ¼ 1 : 1 for
with brine, and then dried over anhydrous Na2SO4. Aer SBF-1DPPDCV and 1 : 3 for SBF-2DPPDCV, respectively).
removal of the solvent under reduced pressure, the residue was
SBF-1DPPDCV (black solid, yield 50%). 1H NMR (400 MHz,
puried by silica gel column chromatography using CH2Cl2/ CDCl3): d(ppm) 9.01 (m, 2H), 7.88 (m, 4H), 7.82 (s, 1H), 7.79 (d, J
petroleum ether (v/v, 2/1) as eluent to afford purple red solid ¼ 4.1 Hz, 1H), 7.71 (d, J ¼ 8.0 Hz, 1H), 7.40 (m, 3H), 7.30 (d, J ¼
1
(331 mg, 52%). H NMR (400 MHz, CDCl3): d (ppm) 10.03 (s, 4.4 Hz, 1H), 7.14 (m, 3H), 7.03 (s, 1H), 6.81–6.70 (m, 3H), 4.02
1H), 9.03 (d, J ¼ 4.7, 1H), 8.92 (d, J ¼ 4.1 Hz, 1H), 7.87 (d, J ¼ (m, 4H), 1.95–1.80 (m, 2H), 1.40–1.10 (m, 32H), 1.00–0.50 (m,
4.2 Hz, 1H), 7.73 (d, J ¼ 5.9 Hz, 1H), 7.33 (m, 1H), 4.06 (m, 4H), 12H). 13C NMR (100 MHz, CDCl3): d(ppm) 162.27, 160.04,
1.85–2.00 (m, 2H), 1.2–1.4 (m, 32H), 0.8–1.0 (m, 12H). MALD- 152.21, 149.75, 149.09, 149.01, 147.86, 143.99, 143.17, 143.14,
TOF MS: calcd for C39H55BrN2O3S2: 742.28, found 742.13.
3.2.7 Synthesis of 2-((5-(4-(5-bromothiophen-2-yl)-2,5- 134.41, 133.13, 132.45, 130.41, 130.16, 129.96, 128.16, 128.12,
bis(2-butyloctyl)-3,6-dioxo- 2,3,5,6-tetrahydropyrrolo[3,4-c] 128.08, 127.99, 127.96, 126.43, 124.97, 124.04, 121.59, 120.74,
141.59, 140.22, 140.05, 138.76, 138.56, 136.03, 134.51, 134.46,
pyrrol-1-yl)thiophen-2-yl)methylene)malononitrile (DPPDCV- 120.35, 120.19, 114.21, 113.44, 113.20, 113.11, 108.49, 104.22,
Br). A mixture of DPPCHO-Br (0.74 g, 1.00 mmol), malononitrile 78.49, 46.31, 45.96, 38.31, 37.44, 31.83, 31.68, 31.02, 30.77,
(0.26 g, 3.98 mmol), and aluminum oxide (0.77 g, 9.30 mmol), 30.61, 29.92, 29.72, 29.61, 28.30, 26.09, 24.80, 23.23, 23.10,
25036 | RSC Adv., 2018, 8, 25031–25039
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22.81, 14.36, 14.32, 14.24, 14.18. MALDI-TOF-MS: calcd for tting the J-V curves in the near quadratic regions according to
C
67H70N4O2S2: 1027.44; found: 1026.78. Elemental analysis the modied Mott–Gurney equation:53
!
calcd for C67H70N4O2S2: C 78.32, H 6.87, N 5.45%; found C
77.74, H 6.96, N 5.38%.
pffiffiffiffi
9
J ¼ 330m
8
V2
L3
V
pffiffiffi
0:89b
(1)
SBF-2DPPDCV (black solid, yield 40%). 1H NMR (400 MHz,
CDCl3): d(ppm) 9.01 (dd, J ¼ 6.6, 4.3 Hz, 4H), 7.94 (d, J ¼ 2.9 Hz,
2H), 7.92 (d, J ¼ 3.1 Hz, 2H), 7.83 (s, 1H), 7.78 (d, J ¼ 4.4 Hz, 2H),
7.74 (dd, J ¼ 8.0, 1.7 Hz, 2H), 7.45 (t, J ¼ 7.5 Hz, 2H), 7.33 (d, J ¼
4.2 Hz, 2H), 7.17 (t, J ¼ 7.5 Hz, 2H), 7.01 (s, 2H), 6.82 (d, J ¼
7.2 Hz, 2H), 4.02 (dd, J ¼ 13.1, 7.8 Hz, 8H), 1.87 (m, 4H), 1.40–
1.10 (m, 64H), 0.90–0.75 (m, 24H). 13C NMR (100 MHz, CDCl3):
d(ppm) 161.85, 160.89, 151.85, 150.62, 149.21, 147.42, 146.32,
143.16, 142.02, 141.84, 139.24, 138.89, 138.55, 137.46, 135.98,
134.60, 133.05, 128.34, 128.20, 126.66, 125.24, 124.06, 121.56,
121.54, 120.46, 113.84, 113.06, 112.44, 108.60, 78.72, 46.55,
46.42, 38.30, 37.76, 31.72, 31.08, 30.90, 30.83, 30.63, 29.63,
28.34, 28.26, 26.10, 26.06, 23.04, 22.98, 22.62, 14.10, 14.09,
L
where J is the current density, 30 is the permittivity of free space,
3 is the relative permittivity, m is the zero-eld mobility, V is the
applied voltage, L is the thickness of active layer, and b is the
eld-activation factor.
4 Conclusions
In summary, three conjugated molecules consisting of a spi-
robiuorene (SBF) core attached with different numbers of DPP
arms were designed and synthesized, and utilized as non-
fullerene acceptors for OPVs. The blend thin lms of SBF-
4DPPDCV with PTB7-Th as the donor at a weight ratio of 1.5 : 1
yielded the highest PCE of 3.85%, while the PCEs of SBF-
1DPPDCV and SBF-2DPPDCV devices achieved PCEs of only
0.26% and 0.98% respectively. The superior performance of
SBF-4DPPDCV to the others could be attributed to the multiple
electronic transmission property of the materials as well as
better phase separation morphology with a continuous mesh
structure in the thin lms blended with PTB7-Th.
14.04, 14.00. MALDI-TOF-MS: calcd for
1736.86; found: 1738.04 (M + 1). Elemental analysis calcd for
109H124N8O4S4: C 75.31, H 7.19, N 6.45%; found C 75.12, H
C109H124N8O4S4:
C
7.28, N 6.16%.
3.3 Photovoltaic cells
OSCs were fabricated with the structure ITO/PEDOT: PSS/PTB7-
Th : acceptors/PDIN/Al. The patterned ITO glass was precleaned
in an ultrasonic bath of acetone and isopropyl alcohol, and
treated in an ultraviolet-ozone chamber (Novascan Company,
USA) for 20 min. A thin layer (30 nm) of PEDOT:PSS (Baytron P,
now Clevios VP AI 4083, from H. C. Starck, Leverkusen, Ger-
Conflicts of interest
There are no conicts to declare.
ꢂ
many) was spin-coated onto the ITO glass and baked at 150 C
for 15 min. A solution (total of 30 mg mLꢀ1) of PTB7-
Th : acceptors blend was subsequently spin-coated (2000 rpm)
on the PEDOT:PSS layer to form a photosensitive layer
(ca.100 nm thick). The thickness of the photosensitive layer was
measured using an Ambios Technology (Santa Cruz, CA) XP-2
prolometer. The methanol solution (0.2% acetic acid) of
PDIN at a concentration of 1.5 mg mLꢀ1 was deposited on the
active layer at 3000 rpm for 30 s, giving a PDIN layer ca. 13 nm
thick. An aluminium (ca. 100 nm) layer was subsequently
evaporated onto the surface of the PDIN layer under vacuum (ca.
10ꢀ6 Pa) to form the negative electrode. The active area of the
device was 0.1 cm2. The J–V curve was measured with
a computer controlled Keithley 236 Source Measure Unit (Zolix
ss150 Solar Simulator). A xenon lamp coupled with AM1.5 solar
spectrum lters was used as the light source, and the optical
power at the sample was 100 mW cmꢀ2. The light intensity of
the solar simulator was calibrated using a standard silicon solar
cell. The EQE spectrum was measured using a Solar Cell QE/
EQE Measurement System (Zolix Solar cell scan 100) consist-
ing of a model SR830 DSP lock-in amplier coupled with
a WDG3 monochromator and 500 W xenon lamp.
Acknowledgements
The authors would like to thank the nancial support from
National Basic Research Program of China (Fundamental
Studies of Perovskite Solar Cells 2015CB932200), National
Natural Science Foundation of China (No. 61605075, NSFC-PSF
51661145021), Natural Science Foundation of Jiangsu Province
(BK20170985), and the Special Funds for the Development of
Strategic
Emerging
Industries
in
Shenzhen
(JCJY20170818154457845).
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