New conjugated molecules with two and three dithienyldiketopyrrolopyrrole (DPP) moieties substituted at meta positions of benzene toward p- and n-type organic photovoltaic materials

Article abstract of DOI:10.1002/asia.201400089

Two conjugated molecules, TADPP3 and TADPP2-TT, are reported, in which three and two dithienyldiketopyrrolopyrrole (DPP) moieties, respectively, are substituted at the meta positions of benzene. Based on cyclic voltammetry and absorption data, TADPP3 and TADPP2-TT possess similar HOMO and LUMO energies of about -5.2 and -3.4 eV, respectively. Thin films of TADPP3 and TADPP2-TT exhibit p-type semiconducting behavior with hole mobilities of 2.36×10 ^-3 and 3.76×10^-4 cm² V^-1 s^-1 after thermal annealing. Molecules TADPP3 and TADPP2-TT were utilized as p-type photovoltaic materials to fabricate organic solar cells after blending with phenyl C₇₁ butyric acid methyl ester (PC ₇₁BM) and phenyl C₆₁ butyric acid methyl ester (PC ₆₁BM). The relatively low J_SC and fill factor values can be attributed to poor film morphologies based on AFM and XRD studies. A solar cell with a thin film of TADPP3 with PC₇₁BM in a weight ratio of 1:2 exhibits a high open-circuit voltage (V_OC) of 0.99 V and a power conversion efficiency (PCE) of 2.47 %. Interestingly, TADPP3 can also be employed as an n-type photovoltaic material. The blended thin film of TADPP3 with P3HT in a weight ratio of 1:2 gave a high V_OC of 1.11 V and a PCE of 1.08 % after thermal annealing. Star-shaped small molecules: Two conjugated molecules, TADPP3 and TADPP2-TT, have been developed and applied in solution-processed organic solar cells. Molecule TADPP3 can be applied as both a donor and an acceptor. The resulting solar cells have a high open-circuit voltage (>0.9 V) and the power conversion efficiency reaches 2.47 %.

Full text of DOI:10.1002/asia.201400089

FULL PAPER
DOI: 10.1002/asia.201400089
New Conjugated Molecules with Two and Three
Dithienyldiketopyrrolopyrrole (DPP) Moieties Substituted at meta Positions
of Benzene toward p- and n-Type Organic Photovoltaic Materials
Chenmin Yu, Chang He, Yang Yang, Zhengxu Cai, Hewei Luo, Wenqiang Li, Qian Peng,
Guanxin Zhang, Zitong Liu,* and Deqing Zhang*[a]
Abstract: Two conjugated molecules,
TADPP3 and TADPP2-TT, are report-
ed, in which three and two dithienyldi-
ketopyrrolopyrrole (DPP) moieties, re-
spectively, are substituted at the meta
positions of benzene. Based on cyclic
voltammetry and absorption data,
TADPP3 and TADPP2-TT possess
similar HOMO and LUMO energies of
about À5.2 and À3.4 eV, respectively.
Thin films of TADPP3 and TADPP2-
TT exhibit p-type semiconducting be-
havior with hole mobilities of 2.36ꢀ
10À3 and 3.76ꢀ10À4 cm2 VÀ1 sÀ1 after
thermal annealing. Molecules TADPP3
and TADPP2-TT were utilized as p-
type photovoltaic materials to fabricate
organic solar cells after blending with
phenyl C71 butyric acid methyl ester
(PC71BM) and phenyl C61 butyric acid
methyl ester (PC61BM). The relatively
low JSC and fill factor values can be at-
tributed to poor film morphologies
based on AFM and XRD studies. A
solar cell with a thin film of TADPP3
with PC71BM in a weight ratio of 1:2
exhibits
a high open-circuit voltage
(VOC) of 0.99 V and a power conversion
efficiency (PCE) of 2.47%. Interesting-
ly, TADPP3 can also be employed as
an n-type photovoltaic material. The
blended thin film of TADPP3 with
P3HT in a weight ratio of 1:2 gave
a high VOC of 1.11 V and a PCE of
1.08% after thermal annealing.
Keywords: donor–acceptor
systems
· electrochemistry · ful-
lerenes · organic solar cells
Introduction
rification methods, and batch-to-batch variations.[9–14] These
problems can be detrimental for practical applications of
conjugated polymers in OSCs.
Diketopyrrolopyrrole (DPP) is a planar and polar electron-
acceptor moiety with strong absorptions in the visible
region.[1,2] Thus, DPP has been intensively investigated to
construct electron donor–acceptor (D-A) molecules and
polymers for optoelectronic materials, in particular, for or-
ganic photovoltaic materials.[3–8] A number of DPP-contain-
ing D-A conjugated polymers were reported.[6–8] Janssen and
co-workers[7c] described DPP-containing low band gap poly-
In comparison, small conjugated molecules possess advan-
tages in terms of ease of synthesis and purification, which
can greatly improve the reproducibility of device fabrica-
tion.[9–14] In addition, small molecules tend to self-assemble
into ordered domains and their intermolecular arrangements
can be clarified by crystal-structure analysis. In fact, various
D-A molecules were investigated for photovoltaic materi-
als.[15–18] Nguyen and co-workers reported a series of D-A-D
molecules with DPP as the electron-accepting moieties.
mers with oligothiophene or thienylthienoACHTNUTRGNEUNG[3,2-b]thiophene
as electron donors. These conjugated polymers with relative-
ly high carrier mobilities were successfully utilized as elec-
tron donors for organic solar cells (OSCs) with remarkably
high power conversion efficiencies (PCEs, above 7%). How-
ever, conjugated polymers suffer from broad molecular-
weight distributions, end-group contamination, difficult pu-
Solar cells with
a blend of thin films of compound
1 (Scheme 1), in which the DPP moiety is flanked with two
benzofuran moieties and phenyl C71 butyric acid methyl
ester (PC71BM) exhibited a PCE higher than 4.4%.[16b]
Marks and co-workers reported A-D-A compound 2, in
which the donor is flanked by DPP moieties, and the combi-
nation of 2 (Scheme 1) with phenyl C61 butyric acid methyl
ester (PC61BM) led to solar cells with a PCE of 4.06%.[16c]
A PCE of 5.29% was achieved with A-D-A compound 3
(Scheme 1) and PC71BM as the active layer after optimiza-
tion of the thin-film morphology.[16e]
The results demonstrate that small A-D-A and D-A-D
molecules are promising for photovoltaic materials of high
performance. Nevertheless, small molecules exhibit lower
performances compared with the respective conjugated
polymers.[6–8,16–18] This may be because conjugated polymers
[a] C. Yu, Dr. C. He, Y. Yang, Z. Cai, H. Luo, W. Li, Dr. Q. Peng,
Dr. G. Zhang, Dr. Z. Liu, Prof. D. Zhang
Beijing National Laboratory for Molecular Sciences
Organic Solids Laboratory, Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190 (P.R. China)
Fax : (+86)10-62562693
E-mail: drzitong@gmail.com
dqzhang@iccas.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201400089.
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Deqing Zhang, Zitong Liu et al.
ceived increasing attention for
developing new organic photo-
voltaic materials. Zhan and co-
workers described compound 4
(Scheme 1), with three dithien-
yl-DPP moieties linked to tri-
phenylamine, which was em-
ployed as an electron acceptor
to fabricate a solar cell with
a PCE of 1.2% after blending
with poly(3-hexylthiophene-2,5-
diyl) (P3HT).[18c]
Herein, we report two conju-
gated molecules (TADPP3 and
TADPP2-TT; Scheme 2) with
two and three dithienyl-DPP
moieties substituted at the meta
positions of benzene toward
new photovoltaic materials. The
molecular design rationale is
based on the following consid-
Scheme 1. Chemical structures of representative DPP-containing conjugated molecules 1–4.
erations:1) DPP
moieties
flanked with thiophenes show
form thin films and heterojunctions with appropriate elec-
tron acceptors more easily. Conjugated molecules with
multi-A-D-A or D-A-D moieties may improve the thin-film
formation abilities and, at the same time, their chemical
structures are definite; moreover, the frontier orbital levels
and intermolecular interactions can be finely tuned. There-
fore, conjugated molecules with such frameworks have re-
strong absorptions in the visible region and also relatively
high carrier mobilities; 2) the 1,3,5-triethynyl benzene ring
as the central core is planar and rigid, and substitutions at
the 1,3,5-positions are favorable for intermolecular interac-
tions;[19] 3) electron donors, such as 2,2’:5’,2’’-terthiophene in
TADPP2-TT, can be incorporated to finely tune the
HOMO/LUMO levels and intermolecular interactions;[20]
4) furthermore, these compounds exhibit 2D structures,
which may be beneficial for forming multiple carrier-trans-
porting pathways.[21] The results reveal that both TADPP3
and TADPP2-TT can function as electron donors (as a p-
type photovoltaic material) and the respective solar cells
after blending with PC71BM exhibit PCEs of 2.47 and
Abstract in Chinese:
1.11%, respectively, with an open-circuit voltage (VOC
)
higher than 0.90 V. Furthermore, TADPP3 can also be uti-
lized as an electron acceptor (as a n-type photovoltaic mate-
rial), and the PCE of the solar cell with thin films TADPP3
and P3HT reaches 1.08% with a high VOC of 1.11 V after
thermal annealing. These results demonstrate that such 2D
conjugated frameworks of multiple D-A-D moieties is
promising for solution-processed OSCs.
Results and Discussion
Synthesis, Characterization, and HOMO/LUMO Energies
The synthesis of TADPP3 starts from 5 (Scheme 2), which
was prepared according to the reported procedure.[16c] As
shown in Scheme 2, the Sonogashira coupling of 5 with
1,3,5-triethynylbenzene gave TADPP3 in 72% yield. Com-
pound TADPP2-TT was synthesized in three steps. First,
1,3,5-triethynylbenzene was allowed to react with nBuLi at
À788C, followed by the addition of TIPSCl, leading to 6 in
85% yield. Then, the Sonogashira coupling of 6 with 5 gave
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Deqing Zhang, Zitong Liu et al.
are thermally stable under
3008C based on the thermogra-
vimetric analysis (TGA) data
shown in Figure S1a in the Sup-
porting Information. The differ-
ential scanning calorimetry
(DSC) traces of both TADPP3
and TADPP2-TT display no
clear phase transition below
2508C (see Figure S1b and c in
the Supporting Information).
Figure 1 shows the cyclic vol-
tammograms of TADPP3 and
TADPP2-TT. Three reversible
oxidation waves and one reduc-
tion wave were observed for
TADPP3. Similarly, TADPP2-
TT also displays three reversi-
ble oxidation waves and one re-
versible reduction wave. Based
on the respective onset oxida-
tion and reduction potentials of
the two compounds, their
HOMO and LUMO energies,
as well as band gaps, were esti-
mated based on the following
equations:
LUMO=
À(4.41+Ered1onset) eV, HOMO=
À(Eox1onset +4.41) eV
(see
Table 1). Interestingly, both
compounds possess similar
HOMO (ca. À5.2 eV) and
LUMO (ca. À3.4 eV) energies.
This is indeed in agreement
Scheme 2. Chemical structures of TADPP3 and TADPP2-TT and their synthetic routes. Reagents and condi- with the absorption data and
tions: a) [Pd
(TIPSCl), À788C, 85%; c) 5, [Pd
furan (THF); e) 5-bromo-2,2’:5’,2’’-terthiophene, [Pd
ACHTUNGTRENNUNG
theoretical calculation results
(see below). Figure 1 also
shows the respective HOMO
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
and LUMO levels of TADPP3,
TADPP2-TT, P3HT, PC61BM,
7 in 75% yield. Finally, TADPP2-TT was obtained by re-
moval of TIPS in 7 in the presence of K2CO3, followed by
Sonogashira coupling with 5-bromo-2,2’:5’,2’’-terthiophene.
The chemical structures of both compounds were character-
ized by NMR spectroscopy and MS data and their purities
were confirmed with elemental analysis. Both compounds
and PC71BM. Notably, the respective LUMO and HOMO
energy differences between both TADPP3/TADPP2-TT and
P3HT/PC71BM (PC61BM) are large enough to guarantee ef-
ficient exciton dissociation. Judging from the energy levels,
both compounds are suitable as either electron donors or ac-
ceptors in OSCs.[20a] Moreover, the energy differences be-
Table 1. Absorption and electrochemical data, HOMO/LUMO energies, and band gaps of TADPP3 and TADPP2-TT.
lmax
/
[nm][a]
lmax
/
[nm]
film
Eox1onset [V][c]
Ered1onset [V][c]
LUMO [eV]
HOMO [eV]
Eg [eV]
solution
Exptl[d] (Calcd)[e]
Exptl[d] (Calcd)[e]
TADPP3
584 (102000)[b]
547, 376
613, 564, 402, 376
614, 564, 401, 385
0.83
0.82
À0.98
À1.01
À3.43 (À2.71)
À3.40 (À2.67)
À5.24 (À4.91)
À5.23 (À4.88)
1.81[f] (1.83)[g]
1.83[f] (1.85)[g]
TADPP2-TT
583 (55000)[b]
545, 385
[a] Measured in CHCl3 (1.0ꢀ10À5 m). [b] Molar extinction coefficient (emax, cm2 mÀ1). [c] In CH2Cl2 containing Bu4NPF6 (0.1m) at a scan rate of 50 mVsÀ1
.
[d] Estimated from the following equations: LUMO=À(4.41+Ered1onset) eV, HOMO=À(Eox1onset +4.41) eV. [e] Based on DFT calculations. [f] Based on
redox potentials. [g] Based on absorption spectra data.
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Deqing Zhang, Zitong Liu et al.
thin films of TADPP3 and TADPP2-TT are redshifted.
Both thin films of TADPP3 and TADPP2-TT show strong
absorptions at lꢀ610 nm, which are extended to l=
670 nm. As a result, they possess rather similar optical band
gaps (ca. 1.8 eV) based on the respective onset absorptions.
This is in agreement with the band gaps based on cyclic vol-
tammetric data (see Table 1). However, TADPP2-TT shows
stronger absorption at lꢀ380 nm than that of TADPP3.
This is probably owing to the presence of the 2,2’:5’,2’’-ter-
thiophene moiety in TADPP2-TT. By considering that
P3HT and PC71BM absorb strongly in the region of l=400–
600 and 200–350 nm, respectively, blend films of either
TADPP3 or TADPP2-TT with P3HT or PC71BM are ex-
pected to absorb strongly in the region of l=300–750 nm.
The chemical structures of TADPP3 and TADPP2-TT
were investigated with calculations based on DFT. The alkyl
chains were replaced by methyl groups. As depicted in
Figure 3, the HOMO and LUMO orbitals of TADPP3 are
Figure 1. a) Cyclic voltammograms of TADPP3 and TADPP2-TT (1.0ꢀ
10À3 m) in CH2Cl2 at a scan rate of 50 mVsÀ1, with platinum as the work-
ing and counter electrodes, an Ag/AgCl electrode (saturated KCl) as the
reference electrode, and nBu4NPF6 (0.1m) as the supporting electrolyte.
b) HOMO/LUMO energy levels of different conjugated molecules and
polymers.
tween the HOMO levels of the respective donors and the
LUMO levels of the respective acceptors are higher than
1.3 eV; thus high VOC values are expected for solar cells with
TADPP3 or TADPP2-TT as donors or acceptors after being
combined with PC71BM (PC61BM) or P3HT as the acceptor
or donor.
Figure 2 shows the absorption spectra of solutions and
thin films of TADPP3 and TADPP2-TT. The solution of
TADPP3 exhibits three broad absorptions at around l=376
Figure 2. Normalized UV/Vis absorption spectra of TADPP3 and
TADPP2-TT in CHCl3 (1.0ꢀ10À5 m) and their thin films.
Figure 3. LUMO and HOMO orbitals of TADPP3 and TADPP2-TT ob-
tained by DFT calculations; the alkyl chains were replaced with methyl
groups.
(emax =53000), 547 (87000), and 584 nm (102000 cm2 mÀ1). A
solution of TADPP2-TT also shows strong absorptions in
this region at around 385 (emax =57000), 545 (50000), and
l=583 nm (55000 cm2 mÀ1). It is clear that TADPP3 shows
stronger absorptions above l=440 nm than those of
TADPP2-TT based on the molar absorption coefficients.
Thus, it is expected that TADPP3 will show better photovol-
taic performance than that of TADPP2-TT (see below).
Compared with those in solution, the absorption spectra of
delocalized over the three dithienyl-DPP moieties and the
central 1,3,5-triethynylbenzene core. For TADPP2-TT,
HOMO and LUMO orbitals are mainly localized on two
DPP moieties; however, the central core contributes slightly
to the HOMO/LUMO orbitals, and the 2,2’:5’,2’’-terthio-
phene moiety makes no contribution to either the HOMO
or LUMO orbitals. The calculated HOMO/LUMO energies
of TADPP3 are close to those of TADPP2-TT (see
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Deqing Zhang, Zitong Liu et al.
Table 2. OFET performance data for thin films of TADPP3 and TADPP2-TT.
Table 1). These results reveal
that the interaction between
2,2’:5’,2’’-terthiophene and the
[a]
T [8C]
mh [cm2 VÀ1 sÀ1
]
Ion/off
Vth [V]
80
(2.18/1.53)ꢀ10À3
(2.36/2.06)ꢀ10À3
(2.07/1.96)ꢀ10À3
(1.24/1.00)ꢀ10À4
(3.76/2.49)ꢀ10À4
(2.00/1.88)ꢀ10À4
106
106
106
104
104
104
À10 to À5
À35 to À15
À40 to À10
À22 to À8
À17 to À11
À15 to À5
dithienyl-DPP
TADPP2-TT are negligible;
this is probably because
moieties
in TADPP3
100
120
80
100
120
TADPP2-TT
2,2’:5’,2’’-terthiophene is not
fully conjugated with the cen-
tral benzene core. This agrees
well with the observations
based on absorption and cyclic
[a] The mobilities are provided in “highest/average” form; the performance data were obtained based on more
than 10 different OFETs.
voltammetry data (see Table 1). The solvent effects were
not included in the theoretical calculations, and thus, the cal-
culated HOMO/LUMO energies of TADPP3 and
TADPP2-TT are different from those obtained based on ab-
sorption and electrochemical data.
vacuum. However, the hole mobility starts to decrease after
further increasing the annealing temperature (see Table 2).
The hole mobility of the as-prepared thin film of TADPP2-
TT was estimated to be 1.24ꢀ10À4 cm2 VÀ1 sÀ1. Similarly, the
hole mobility increases after thermal annealing at 1008C. It
is clear that hole mobilities of thin films of TADPP3 are
higher than those of TADPP2-TT (see Table 2). Moreover,
the hole mobilities of neither TADPP3 nor TADPP2-TT
are significantly affected by thermal annealing. As discussed
below, relatively low hole mobilities can be attributed to the
low crystallinity and poor morphology of thin films of
TADPP3 and TADPP2-TT.
Out-of-plane XRD patterns of thin films of TADPP3 and
TADPP2-TT were measured after annealing at different
temperatures. As depicted in Figure S2 in the Supporting In-
formation, only broad and weak diffraction peaks at around
228 were detected for a thin
Thin-Film Organic Field-Effect Transistors (OFETs)
To ensure effective charge-carrier transport to the electrodes
and reduce photocurrent loss, high mobility is a basic re-
quirement for photovoltaic materials. To estimate the
charge mobilities of thin films of TADPP3 and TADPP2-
TT, the respective bottom-gate/bottom-contact organic field-
effect transistors (OFETs) were fabricated with convention-
al techniques (see the Experimental Section). Based on the
transfer and output characteristics shown in Figure 4, thin
film of TADPP3 after anneal-
ing at 100 and 1208C. Similarly,
no sharp diffraction peaks were
also observed for a thin film of
TADPP2-TT. Thus, thin films
of
both
TADPP3
and
TADPP2-TT show poor crystal-
linity, even after thermal an-
nealing. Figures S3 and S4 in
the Supporting Information
show AFM images of thin films
of TADPP3 and TADPP2-TT
on
octadecyltrichlorosilane
(OTS)-modified SiO2 substrates
after annealing at different tem-
peratures. For a thin film of
TADPP3, the heights and sizes
of molecular domains increase
after thermal annealing, but
most of these molecular do-
mains are not interconnected.
Figure 4. The transfer and output characteristics of OFETs with thin films of TADPP3 (A and B) and
TADPP2-TT (C and D) after annealing at 1008C. The transistor channel width and channel length are 1400
and 50 mm, respectively.
Morphological
modifications
are rather minor after thermal
annealing for the thin film of
films of both TADPP3 and TADPP2-TT display p-type
semiconducting properties. The as-prepared OFET with
a thin film of TADPP3 shows a hole mobility of 2.18ꢀ
10À3 cm2 VÀ1 sÀ1, which slightly increases to 2.36ꢀ
10À3 cm2 VÀ1 sÀ1 after thermal annealing at 1008C for 1.0 h in
TADPP2-TT (see Figure S4 in the Supporting Information).
These results agree well with the fact that thin films of
TADPP3 and TADPP2-TT display low carrier mobilities.
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Deqing Zhang, Zitong Liu et al.
Photovoltaic Device Performance
By considering their HOMO/LUMO energies, TADPP3
and TADPP2-TT may function as p- and n-type organic
photovoltaic materials. First, TADPP3 and TADPP2-TT
were examined as p-type organic photovoltaic materials.
OSCs were fabricated with the structure indium tin oxide
(ITO)/poly(3,4-ethylenedioxythiophene)–poly(styrenesulfo-
nate) (PEDOT:PSS)/donor:PC71PM (PC61BM)/LiF/Al, in
which PC71PM and PC61BM were utilized as electron ac-
ceptors. The blend thin films of TADPP3 or TADPP2-TT
with either PC71PM or PC61BM show strong absorptions in
the visible region. For instance, the thin film of TADPP3
with PC71BM absorbs from l=330 to 680 nm with an ab-
sorption maximum at lꢀ554 nm, whereas that of TADPP2-
TT with PC71BM exhibits absorptions in the region of l=
340–660 nm with absorption maxima at lꢀ380 and 555 nm,
as shown in Figure S5 in the Supporting Information.
OSCs with blending films of TADPP3 and PC61BM in dif-
ferent weight ratios were examined. As listed in Table 3,
Figure 5. A) J–V curves and B) IPCE spectra of OSCs with the respective
blended thin films of TADPP3/TADPP2-TT and PC71BM; the weight
ratios are indicated.
Table 3. Photovoltaic performances with TADPP3/TADPP2-TT after
blending with either PC61PM/PC71BM or P3HT.
Donor/acceptor
Ratio (w/w) VOC
JSC
[mAcmÀ2
FF
[%]
PCE
[%]
[V]
G
]
a PCE of 1.11% and a VOC of 0.97 V (Table 3). Notably,
thermal annealing or the introduction of additives cannot
clearly improve the performances of solar cells with
TADPP3 or TADPP2-TT after blending with either
PC61BM or PC71BM.
1:1[a]
1:2[a]
1:3[a]
1:2[a]
0.92 2.54
0.87 3.17
0.78 3.22
0.99 6.24
0.97 3.88
1.03 1.21
1.10 1.97
1.05 0.98
1.11 2.72
29.1 0.68
34.1 0.94
34.1 0.86
40.0 2.47
30.0 1.11
37.3 0.47
40.3 0.87
36.8 0.38
35.7 1.08
TADPP3/PC61BM
TADPP3/PC71BM
TADPP2-TT/PC71BM 1:2[a]
1:1[a]
1:1[b]
P3HT/TADPP3
1:2[b]
Compound TADPP3 was also investigated as an n-type
photovoltaic material to construct solar cells after blending
with P3HT in different weight ratios. For example, Figure 6
shows the respective J–V curve and IPCE spectrum for the
solar cell with TADPP3 after blending with P3HT in
a weight ratio of 1:2 after thermal annealing. As listed in
Table 3, the VOC of these OSCs with TADPP3 and P3HT is
remarkably high, up to 1.11 V. This can be attributed to the
large energy difference (1.33 eV) between the HOMO of
P3HT and the LUMO of TADPP3. The solar cell with
TADPP3 and P3HT in a weight ratio of 1:1 shows a PCE of
0.47%, which increases to 0.87% after thermal annealing of
the active layer at 1508C for 10 min. This is mainly owing to
the increase in JSC from 1.21 to 1.97 mAcmÀ2. The PCE is
further enhanced to 1.08% by varying the weight ratio of
P3HT and TADPP3 to 2:1 after thermal annealing of the
active layer at 1508C. Similarly, optimization of the weight
ratios and thin-film thicknesses was performed, but the en-
hancement of PCE was not observed. Also, post-treatments,
including thermal annealing and the introduction of addi-
tives, did not lead to an increase in PCE.
2:1[b]
[a] As a casted thin film. [b] As a thin film after annealing at 1508C.
these OSCs exhibit relatively high VOC up to 0.92 V, but the
values of JSC and fill factor (FF) are low, and thus, PCE
values are lower than 1.0% for thin films of TADPP3 with
PC61BM in different weight ratios. The thin film of
TADPP3 and PC61BM in a ratio of 1:2 (w/w) yields a PCE
of 0.94%. Apart from PC61BM, PC71BM was also employed
as an electron acceptor to be blended with either TADPP3
or TADPP2-TT for OSCs. Figure 5 shows the respective J–
V curves and incident photon-to-current conversion efficien-
cy (IPCE) spectra for solar cells with TADPP3 or
TADPP2-TT after blending with PC71BM. The results
reveal that the blending of thin films of TADPP3 or
TADPP2-TT with PC71BM in a ratio of 1:2 (w/w) lead to
better photovoltaic performances. For instance, the PCE in-
creases to 2.47% for the solar cell with TADPP3 as the
active layer; the VOC is up to 0.99 V after optimization of
both ratios and thin-film thicknesses (Table 3 and Table S3
in the Supporting Information). The reduction of thin-film
thickness is beneficial for transporting charges to electrodes,
and thus, the PCE is enhanced. The combination of
TADPP2-TT with PC71BM in a ratio of 1:2 (w/w) yields
As depicted in Figures 5 and 6, the blending of thin films
of TADPP3/PC71BM, TADPP2-TT/PC71BM, and P3HT/
TADPP3 displayed broad IPCE spectra from lꢀ300 to
700 nm. These IPCE spectra correspond well to the respec-
tive UV/Vis absorption profiles (see Figures S5 and S6 in
the Supporting Information); this indicates that absorbed
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Deqing Zhang, Zitong Liu et al.
the surface morphology. The domain sizes increased to
about 150ꢀ200 nm2, and the surface roughness reached
5.09 nm. This probably resulted from the self-organization
of P3HT chains after thermal annealing. A strong peak at
4.58 and a weak signal at 23.68 were detected in the out-of-
plane and in-plane XRD patterns (see Figure S10 in the
Supporting Information) for a thin film of TADPP3 with
P3HT after annealing at 1508C. This may be due to the or-
dered packing of P3HT chains, which is beneficial for or-
dered structure formation and charge transport within the
thin film.
Conclusion
Two conjugated molecules, TADPP3 and TADPP2-TT,
were synthesized and investigated. Three and two D-A-D
moieties that contained dithienyldiketopyrrolopyrrole were
substituted at the meta positions of benzene in TADPP3
and TADPP2-TT, respectively; TADPP2-TT also included
one 2,2’:5’,2’’-terthiophene moiety. The respective cyclic vol-
tammetric and absorption spectra data revealed that
TADPP3 and TADPP2-TT possessed similar HOMO and
LUMO energies of approximately À5.2 and À3.4 eV, respec-
tively. This was consistent with DFT calculations, which indi-
cated that the 2,2’:5’,2’’-terthiophene moiety in TADPP2-TT
made no contribution to either the HOMO or LUMO.
Based on the transfer and output characteristics of OFETs,
thin films of TADPP3 and TADPP2-TT exhibited p-type
semiconducting behavior with hole mobilities of 2.36ꢀ10À3
and 3.76ꢀ10À4 cm2 VÀ1 sÀ1 after thermal annealing. Com-
pound TADPP3 could be utilized as both p- and n-type pho-
tovoltaic materials. A solar cell with a thin film of TADPP3
with PC71BM in a weight ratio of 1:2 exhibited a high VOC
of 0.99 V and a PCE of 2.47%. The blended thin film of
TADPP3 with P3HT in a weight ratio of 1:2 yielded a high
VOC of 1.11 V and a PCE of 1.08% after thermal annealing.
OSCs could be also fabricated with blended thin films of
TADPP2-TT and PC71BM, and VOC and PCE values
reached 0.97 V and 1.11%, respectively, when the weight
ratio of TADPP2-TT and PC71BM was 1:2. However, these
OSCs with TADPP3 and TADPP2-TT exhibited low JSC
and FF values; this could be attributed to low crystallinity
and poor morphologies for blended thin films based on
AFM and XRD studies. Further studies include optimiza-
tion of molecular structures with such 3D-conjugated frame-
works, which may enable to improve the thin-film morphol-
ogy and crystallinity.
Figure 6. A) J–V curves and B) IPCE spectra of OSCs with the respective
blended thin films of P3HT/TADPP3 after annealing at 1508C; the
weight ratios are indicated.
photons contribute to the photovoltaic conversion. The max-
imum monochromatic IPCEs appear at lꢀ500 nm for OSCs
with TADPP3/PC71BM, TADPP2-TT/PC71BM, and P3HT/
TADPP3.
The blended thin films of TADPP3 or TADPP2-TT with
either PC71BM or P3HT were characterized with AFM and
XRD techniques. Figure S7 in the Supporting Information
shows AFM height and phase images of the blended films
of TADPP3 with PC71BM (1:2 in weight ratio) and
TADPP2-TT with PC71BM (1:2 in weight ratio) without any
post-treatments. The surface morphology of the thin film of
TADPP3 with PC71BM is more uniform than that with
TADPP2-TT and PC71BM; thin films of TADPP3 and
TADPP2-TT with PC71BM exhibit root-mean-square (RMS)
roughnesses of 0.97 and 1.49 nm, respectively. Moreover, no
clear phase contrast was observed from the phase images
for either thin film. Out-of-plane and in-plane XRD data
reveal no strong peaks (see Figure S8 in the Supporting In-
formation), and the weak, broad signal at around 198 may
be due to the short intermolecular contacts of PC71BM.
These results reveal that thin films of TADPP3 and
TADPP2-TT with PC71BM are amorphous and show low
crystallinity. This is in agreement with the observation that
OSCs with these thin films as active layers possess low JSC
and FF values (see Table 3).
Figure S9 in the Supporting Information shows AFM
height and phase images of as-prepared and annealed
TADPP3/P3HT films. Interpenetrating networks were ob-
served in the AFM phase images. The height image of the
as-prepared thin film comprises small domains of around
90ꢀ110 nm2 with a surface roughness of 3.06 nm. After ther-
mal annealing at 1508C, there were significant changes in
Experimental Section
Materials and Characterization Techniques
The reagents and starting materials employed were commercially avail-
able and used without any further purification, unless specified otherwise.
Compound 5 was synthesized according to published procedures.[16c] 1H
and 13C NMR spectra were recorded on a Bruker AVANCE III 400 MHz
spectrometer. Elemental analysis was performed on
a Carlo Erba
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Deqing Zhang, Zitong Liu et al.
model 1160 elemental analyzer. MALDI-TOF MS were recorded with
a BEFLEX III spectrometer. TGA measurements was carried out on
a Shimadzu DTG-60 instrument under a flow of dry nitrogen, heated
1351.7 [M+H]+; elemental analysis calcd (%) for C81H102N4O4S4Si: C
71.96, H 7.06, N 4.41, S 9.48; found: C 71.88, H 7.11, N 4.52, S 9.39.
Synthesis of TADPP2-TT
from room temperature to 5508C, with a heating rate of 108CminÀ1
.
DSC (PerkinElmer 7 series thermal analyzers) measurements were per-
formed under a nitrogen atmosphere at a heating rate of 108CminÀ1. Ab-
sorption spectra were measured with a Jasco V-570 UV/Vis spectropho-
tometer. Cyclic voltammetric measurements were carried out in a conven-
tional three-electrode cell by using Pt wires of 2.0 mm in diameter as
working and counter electrodes and an Ag/AgCl reference electrode on
a computer-controlled CHI660C instrument at room temperature. The
GIXRD data were measured at 1W1A, Beijing Synchrotron Radiation
Facility. AFM images of the thin films were obtained on a Nanoscope
IIIa (Digital instruments) microscope operating in tapping mode.
K2CO3 (21.8 mg, 0.16 mmol) was added to a stirred solution of compound
7 (216 mg, 0.16 mmol) in THF (40 mL) at 258C, and the reaction mixture
was stirred for 30 min. Then, 5-bromo-2,2’:5’,2’’-terthiophene (103 mg,
0.32 mmol), [PdACHTNUTRGNEUNG(PPh3)4] (9 mg, 0.008 mmol), CuI (0.4 mg, 0.002 mmol),
and (iPr)2NH (5.0 mL) were added to the solution. The mixture was
heated to reflux and stirred overnight. After cooling to room tempera-
ture, water (25 mL) was added and the mixture was extracted with ethyl
acetate (3ꢀ25 mL). The organic phase was dried over MgSO4 and fil-
tered, and the filtrate was concentrated under reduced pressure. The
crude product was purified by flash chromatography with CH2Cl2/n-
hexane (v/v, 1:2) as the eluent to afford TADPP2-TT (152 mg, 67%).
M.p. 96.5–98.98C; 1H NMR (400 MHz, CD2Cl2): d=8.83 (d, J=4 Hz,
2H), 8.78 (d, J=4 Hz, 2H), 7.62–7.59 (m, 5H), 7.38–7.37 (m, 2H), 7.22–
7.17 (m, 4H), 7.14–7.13 (m, 1H), 7.07–7.02 (m, 3H), 6.97–6.95 (m, 1H),
3.95–3.92 (m, 8H), 1.80–1.77 (m, 4H), 1.30–1.16 (m, 32H), 0.85–0.76 ppm
(m, 24H); 13C NMR (100 MHz, CD2Cl2): d=161.5, 161.4, 140.7, 138.9,
135.3, 134.9, 133.2, 131.5, 131.3, 131.0, 129.9, 128.3, 127.5, 122.8, 108.9,
108.1, 96.7, 84.6, 45.8, 45.7, 39.2, 39.1, 30.1, 30.0, 29.7, 28.35, 28.32, 23.5,
23.0, 13.8, 10.2 ppm; MS (MALDI-TOF): m/z: 1441.9 [M+H]+; elemen-
tal analysis calcd (%) for C84H88N4O4S7: C 69.96, H 6.15, N 3.89, S 15.56;
found: C 69.74, H 5.89, N 3.65, S 15.37.
Synthesis of TADPP3
Compound 5 (600 mg, 1.0 mmol), [PdACHTNUGRTNEUNG(PPh3)4] (23.1 mg, 0.02 mmol), and
CuI (1.1 mg, 0.006 mmol) were loaded into a flame-dried, one-necked
flask mounted with a condenser. Degassed/anhydrous toluene (20 mL)
and (iPr)2NH (5 mL) were added, followed by the addition of 1,3,5-trie-
thynylbenzene (30 mg, 0.2 mmol). The reaction mixture was then heated
to 908C and stirred overnight. After cooling to room temperature, water
(25 mL) was added and the mixture was extracted with ethyl acetate (3ꢀ
25 mL). The organic phase was dried over MgSO4 and filtered. Then, the
filtrate was concentrated under reduced pressure. The crude product was
purified by flash chromatography with CH2Cl2/n-hexane (v/v, 1:2) as the
eluent to afford TADPP3 (248 mg, 72%). M.p. 210.5–211.78C; 1H NMR
(400 MHz, CDCl3): d=8.95 (d, J=4 Hz, 3H), 8.89 (d, J=4 Hz, 3H), 7.70
(s, 3H), 7.66–7.65 (m, 3H), 7.42–7.41 (m, 3H), 7.29–7.26 (m, 3H), 4.07–
4.02 (m, 12H), 1.89–1.88 (m, 6H), 1.42–1.26 (m, 48H), 0.94–0.89 ppm (m,
36H); 13C NMR (100 MHz, CDCl3): d=161.8, 161.7, 141.1, 139.1, 135.9,
135.3, 134.2, 133.7, 131.5, 131.1, 129.9, 128.7, 127.3, 123.7, 109.1, 108.2,
95.3, 84.2, 46.2, 46.1, 39.3, 39.2, 30.4, 30.3, 29.8, 29.5, 28.4, 23.7, 23.2, 14.2,
10.6 ppm; MS (MALDI-TOF): m/z: 1717.7 [M+H]+; elemental analysis
calcd (%) for C102H120N6O6S6: C 71.29, H 7.04, N 4.89, S 11.19; found: C
71.38, H 7.01, N 4.93, S 11.05.
Acknowledgements
This research was financially supported by the Chinese Academy of Sci-
ences, NSFC (no.51373181) and the State Key Basic Research Program.
We thank Prof. Yongfang Li for helpful discussions and permission to use
the facility for fabrication of OSCs. We also gratefully acknowledge the
assistance of scientists of the Diffuse X-ray Scattering Station at Beijing
Synchrotron Radiation Facility for measuring the GIXRD data.
Synthesis of Compound 6
nBuLi (1.6m in hexane, 1.13 mL, 1.8 mmol) was added to a stirred solu-
tion of 1,3,5-triethynylbenzene (300 mg, 2 mmol) in THF (20 mL) at
À788C, and the resulting mixture was stirred at À788C for 30 min.
TIPSCl (3.4 mL, 23.9 mmol) was then added and the reaction mixture
was stirred for another 30 min at À788C. The reaction was quenched by
the addition of a saturated aqueous solution of NH4Cl and extracted with
Et2O. The organic layer was dried over Na2SO4 and concentrated to
afford 6 (470 mg, 85%). The product was used in the next step without
further purification.
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Synthesis of Compound 7
Compound 5 (600 mg, 1.0 mmol), [PdACHTNUGRTNEUNG(PPh3)4] (23.1 mg, 0.02 mmol), and
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Received: January 19, 2014
Published online: && &&, 0000
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Chem. Asian J. 2014, 00, 0 – 0
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
These are not the final page numbers! ÞÞ

FULL PAPER
Solar Cells
Chenmin Yu, Chang He, Yang Yang,
Zhengxu Cai, Hewei Luo,
Wenqiang Li, Qian Peng,
Guanxin Zhang, Zitong Liu,*
Deqing Zhang*
&&&& — &&&&
New Conjugated Molecules with Two
and Three Dithienyldiketopyrrolopyr-
role (DPP) Moieties Substituted at
meta Positions of Benzene toward p-
and n-Type Organic Photovoltaic
Materials
Star-shaped small molecules: Two con-
jugated molecules, TADPP3 and
TADPP2-TT, have been developed
and applied in solution-processed
organic solar cells. Molecule TADPP3
can be applied as both a donor and an
acceptor. The resulting solar cells have
a high open-circuit voltage (>0.9 V)
and the power conversion efficiency
reaches 2.47%.
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Chem. Asian J. 2014, 00, 0 – 0
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!