Diketopyrrolopyrrole-based oligomers accessed via sequential C[Formula presented]H activated coupling for fullerene-free organic photovoltaics

Article abstract of DOI:10.1016/j.dyepig.2016.07.007

Exploring sustainable chemistry for renewable energy plays a key role in meeting the ever increasing energy demand without sacrificing the environment. In this study, two novel diketopyrrolopyrrol(DPP)-based π-conjugated oligomers (named as TPE-DPP₄ and BP-DPP₄) have been readily synthesized via a ligand-free Pd-catalyzed sequential activation of C[Formula presented]H bond in two steps with good yields starting from simple building blocks. Poly(3-hexylthiophene) is employed as an electron donor to blend with the new DPP-derived electron acceptors for the fullerene-free bulk heterojunction organic photovoltaics. The power conversion efficiency of 2.49% has been achieved, corresponding with an open-circuit voltage of 1.16?V, which is among the highest open-circuit voltages for the single-junction organic photovoltaics. The facilely accessible electron acceptors blended with cost-effective poly(3-hexylthiophene) donor for fullerene-free organic photovoltaics opens a new pathway to access renewable solar energy via sustainable chemistry.

Full text of DOI:10.1016/j.dyepig.2016.07.007

Dyes and Pigments 134 (2016) 139e147
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Diketopyrrolopyrrole-based oligomers accessed via sequential CeH
activated coupling for fullerene-free organic photovoltaics
, b,
Shi-Yong Liu a *, Wen-Qing Liu b, Cui-Xia Yuan b, Ai-Guo Zhong a, Deman Han a,
,
, ***
Bo Wang b, Muhammad Naeem Shah b, Min-Min Shi b **, Hongzheng Chen b
a Department of Pharmacy & Chemistry, Taizhou University, Taizhou, 317000, PR China
b Department of Polymer Science & Engineering, Zhejiang University, Hangzhou, 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Exploring sustainable chemistry for renewable energy plays a key role in meeting the ever increasing
Received 1 June 2016
Received in revised form
4 July 2016
Accepted 5 July 2016
Available online 7 July 2016
energy demand without sacrificing the environment. In this study, two novel diketopyrrolopyrrol(DPP)-
based
p-conjugated oligomers (named as TPE-DPP4 and BP-DPP4) have been readily synthesized via a
ligand-free Pd-catalyzed sequential activation of CeH bond in two steps with good yields starting from
simple building blocks. Poly(3-hexylthiophene) is employed as an electron donor to blend with the new
DPP-derived electron acceptors for the fullerene-free bulk heterojunction organic photovoltaics. The
power conversion efficiency of 2.49% has been achieved, corresponding with an open-circuit voltage of
1.16 V, which is among the highest open-circuit voltages for the single-junction organic photovoltaics.
The facilely accessible electron acceptors blended with cost-effective poly(3-hexylthiophene) donor for
fullerene-free organic photovoltaics opens a new pathway to access renewable solar energy via sus-
tainable chemistry.
Keywords:
Diketopyrrolopyrrol (DPP)
Direct arylation
Atom economy
Non-fullerene acceptor
P3HT
© 2016 Elsevier Ltd. All rights reserved.
Organic solar cell
1. Introduction
as electron acceptors in place of fullerenes for BHJ OPVs [7e14].
These fullerene-free OPVs have raised extensive attention and led
Bulk heterojunction organic photovoltaics (BHJ OPV) has
emerged as a promising renewable energy technology due to its
merits of low-cost, light-weight, solution-processability and good
flexibility [1e4]. Typically, the active layers of OPVs consist of ful-
lerenes as electron acceptors (A) and conjugated (macro) molecules
as electron donors (D) [5]. In spite of their widespread usage,
fullerene-based acceptors do suffer from drawbacks including
weak absorption of visible light, poor stability of morphology,
limited chemical and electronic tunability, and high production
cost, which may suppress the sustainability of OPV technology. In
recent years, the conjugated semiconducting polymers and oligo-
mers [6], which possess broad tunability in all the aspects of light
absorption, frontier orbital energy levels (FOEL), electron affinity,
molecular geometry and synthetic complexity, have been explored
to a rapid progress very recently [15e22], e.g., the indacenodithieno
[3,2-b]thiophene-2-(3oxo-2,3-dihydroinden-1-ylidene)malononi-
trile (ITIC) acceptor developed by the Zhan group [15]. Neverthe-
less, to ensure the cost-effectiveness and sustainability of fullerene-
free OPVs, the accessibility of both acceptor and donor materials
need to be carefully evaluated [23e25]. Green and sustainable
synthetic strategies [26e39] to the
p-conjugated materials using
readily available building blocks are favorable to reach the above
goal [23]. Moreover, the use of commercially available and well-
developed semiconducting materials with relatively low price is
recommendable [23e25].
In the current work, two novel DPP-based oligomer acceptors
have been designed and synthesized via direct arylation of CeH
bond (DACH) to pair with polymeric donor P3HT for fullerene-free
BHJ OPV applications. Concerning the sustainability, this P3HT-
DPPs donor-acceptor combination has some distinct advantages.
DPP is not only a commercialized dye chromophore [40], but also a
sustainable and readily available building block for the organic
semiconducting materials [41e44]. Previous investigation shows
that DPP unit is among the most synthetically accessible electron-
poor monomers due to its low synthetic complexity [23]. Also, as
* Corresponding author. Department of Pharmacy & Chemistry, Taizhou Univer-
sity, Taizhou, 317000, PR China.
** Corresponding author.
*** Corresponding author.
E-mail addresses: chelsy@zju.edu.cn (S.-Y. Liu), minminshi@zju.edu.cn
(M.-M. Shi), hzchen@zju.edu.cn (H. Chen).
http://dx.doi.org/10.1016/j.dyepig.2016.07.007
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

140
S.-Y. Liu et al. / Dyes and Pigments 134 (2016) 139e147
demonstrated by our previous study, the accessibility of DPP de-
rivatives can be further increased by employing the atom-efficient
CeH activation reaction [45]. As for the donor applied herein, P3HT
is one of the most frequently studied 2nd generation semi-
conducting polymers, which has a relatively lower price compared
with other high performance 3rd generation D-A semiconducting
polymers [25,46]. It is known that P3HT donor has a wide bandgap,
and high-lying LUMO (lowest unoccupied molecular orbital) and
HOMO (highest occupied molecular orbital) levels. Meanwhile, the
conjugated polymers or oligomers containing a DPP block typically
have narrow or medium bandgaps with higher LUMO levels
compared to the commonly-used acceptors such as PC61BM or
perylene diimide (PDI) which has four electron-withdrawing
carbonyl groups [47,48]. Therefore, the P3HT-DPP BHJ OPVs are
expected to achieve high open-circuit voltages (VOC) and a broad
range of light harvesting, due to the enlarged gap between LUMO of
DPP and HOMO of P3HT, and the complementary light absorptions
of the wide-bandgap P3HT and narrow-bandgap DPPs [49e53].
Taking all of the abovementioned aspects into account, two
novel non-planar oligomers both containing four DPP blocks have
been design and synthesized in good yields by direct arylation of
mono-phenyl capped DPP respectively using tetrakis(4-
bromophenyl)ethene (TBPE) and tetrabromo-biphenyl (TBBP) as
arylating reagents. From the viewpoint of molecular design, the
twisted non-planar [20e22,54,55], quasi-3D [56] and 3D
[52,53,57e59] molecular acceptors suggest that the optimization of
the morphology of BHJ layer and thus fill factors (FF) can be realized
by molecular geometry engineering. Tetraphenylethene (TPE) has
been widely used as a building block in the field of aggregation-
induced emission (AIE) [60,61] and very recently as a core for
PDI-based 3D non-fullerene acceptor [54]. In this study, for the first
time, the TPE and biphenyl (BP)-cored tetra-DPP (named as TPE-
DPP4 and BP-DPP4, Scheme 1) have been easily obtained via
sequential CeH activation starting from simple building blocks. To
the best of our knowledge, there are no previous reports using
strategy of sequential CeH activation for accessing structurally
complicated DPP-based oligomers. The non-planar conjugated
oligomers obtained herein were further evaluated as non-fullerene
acceptors in BHJ OPVs by using cost-effective P3HT as donor,
affording a highest PCE of 2.49% with an open-circuit voltage (Voc)
of 1.16 V, which is among the highest Voc for the single-junction
OPVs reported. The integration of easily accessible acceptors with
an economical donor for fullerene-free OPVs provided by us rep-
resents a step toward accessing renewable solar energy by sus-
tainable chemistry.
DMA. The precipitate was extracted with CH2Cl2 (3 ꢀ 40 mL). The
combined organic layer was washed with distilled water. Removal
of the solvent by rotary evaporator afforded the crude product,
which was then purified by column chromatography on silica gel
using the mixtures of CH2Cl2 and hexane as eluent (1.5:1, v/v) and
gave a deep red solid (790 mg, 70%). The thin layer chromatography
(TLC) analysis for the reaction is shown in Fig. S1 (see Supple-
mentary Information, SI). Ph-DPP is a known compound [45].
1H NMR (500 MHz, CDCl3)
d
8.97 (d, J ¼ 4.0 Hz, 1H), 8.89 (d,
J ¼ 3.5 Hz, 1H), 7.68 (d, J ¼ 7.5 Hz, 2H), 7.62 (d, J ¼ 5.0 Hz, 1H), 7.42
(ddd, J ¼ 5.0, 6.5, 4.0 Hz, 4H), 7.30e7.27 (m, 1H), 4.16e3.89 (m, 4H),
1.93 (d, J ¼ 5.5 Hz, 2H), 1.37e1.25 (m, 16H), 0.88 (ddd, J ¼ 5.5, 6.0,
3.5 Hz, 12H).
2.2. Synthesis of TPE-DPP4
Ph-DPP (300 mg, 0.5 mmol), 1.1,2,2-tetrakis(4-bromophenyl)
ethene (TBPE) (65 mg, 0.1 mmol), PivOH (15 mg, 0.15 mmol), and
anhydrous K2CO3 (83 mg, 0.6 mmol) were added into a Schlenk
tube. The mixed solid in the tube was purged by repetitions of
vacuum and nitrogen filling ( ꢀ 3). Then 10 mL anhydrous DMA
solution of Pd(OAc)2 (5 ꢀ 10ꢁ5 M) was added into the tube via
syringe. The reaction solution was put through freeze-vacuum-
thaw cycles three times to remove dissolved gases, and then
rigorously stirred at 110 ꢂC for 10 h under nitrogen atmosphere. The
post-treatment of the reaction are similar to that of Ph-DPP. The
crude product was purified by column chromatography on silica gel
using the mixtures of CHCl3 and ethyl acetate (EA) as eluent (100:1,
v/v) and gave a dark blue solid (235 mg, yield 86%, calculated from
TBPE).
1H NMR (600 MHz, CDCl3)
d
8.93 (dd, J ¼ 11.4, 4.2 Hz, 8H), 7.64
(d, J ¼ 7.5 Hz, 8H), 7.49 (d, J ¼ 8.1 Hz, 8H), 7.37 (dd, J ¼ 38.7, 7.3 Hz,
20H), 7.13 (d, J ¼ 7.7 Hz, 8H), 4.04 (s, 16H), 1.91 (s, 8H), 1.30 (d,
J ¼ 68.0 Hz, 65H), 0.86 (dd, J ¼ 34.4, 15.3 Hz, 48H).
13C NMR (151 MHz, CDCl3)
d 164.26, 152.32, 152.56, 146.39,
142.54, 142.23, 139.52, 135.77, 134.98, 134.45, 131.77, 131.51, 128.72,
128.20, 127.05, 111.01, 110.78, 48.61, 41.96, 32.99, 31.25, 26.33, 25.86,
16.71, 13.26.
MALDI-TOF MS (m/z): [M]þ calcd for C170H188N8O8S8, 2727.90;
found, 2728.42. Elemental Anal.: calcd: C, 74.85; H, 6.95; N, 4.11.
Found: C, 74.86; H, 6.95; N, 4.12.
2.3. Synthesis of BP-DPP4
Ph-DPP (300 mg, 0.5 mmol), 3.30,5.50-tetrabromo-1.10-biphenyl
(TBBP) (47 mg, 0.1 mmol), PivOH (15 mg, 0.15 mmol), and anhy-
drous K2CO3 (83 mg, 0.6 mmol) were added into a Schlenk tube.
The mixed solid in tube was purged by repetitions of vacuum and
nitrogen filling (ꢀ3). Then an anhydrous DMA (10 mL) solution of
Pd(OAc)2 (5 ꢀ 10ꢁ5 M) was added into the tube via syringe. The
reaction solution was put through freeze-vacuum-thaw cycles
three times to remove dissolved gases, and then rigorously stirred
at 110 ꢂC for 10 h under nitrogen atmosphere. The post-treatment
of the reaction are similar to that of Ph-DPP. The crude product
was purified by column chromatography on silica gel using the
mixtures of CHCl3 and ethyl acetate (EA) as eluent (100:1, v/v) and
gave a dark blue solid (216 mg, yield 85%, calculated from TBBP).
2. Experimental section
The general synthetic routes toward Ph-DPP, TPE-DPP4 and BP-
DPP4 are outlined in Scheme 1. The detailed synthetic procedures
are as follows.
2.1. Synthesis of Ph-DPP
DPP (1000 mg, 524.78, 1.90 mmol), PivOH (60 mg, 0.3 equiv.),
and anhydrous K2CO3 (316 mg, 1.2 equiv.) were added into a
Schlenk tube. The mixed solid in the tube was purged by repetitions
of vacuum and nitrogen filling ( ꢀ 3). Then a mixture of bromo-
benzene (300 mg, 1.90 mmol) and an anhydrous DMA (25 mL)
solution of Pd(OAc)2 (8 ꢀ 10ꢁ5 M) was added into the tube. The
reaction solution was put through freeze-vacuum-thaw cycles
three times to remove dissolved gases, and then rigorously stirred
at 110 ꢂC for 10 h under nitrogen atmosphere. After cooling to room
temperature, the mixture was poured into an aqueous solution of
NaCl (saturated, 250 mL) to remove the high boiling point solvent
1H NMR (600 MHz, CDCl3)
7.56 (m, 12H), 7.33 (m, 16H), 3.98 (s, 16H), 1.90 (s, 8H), 1.31 (d,
d
9.00 (s, 8H), 7.82 (d, J ¼ 26.8 Hz, 6H),
J ¼ 78.8 Hz, 64H), 0.80 (d, J ¼ 77.5 Hz, 48H).
13C NMR (151 MHz, CDCl3)
d 168.32, 163.96, 163.74, 152.46,
144.92, 140.06, 137.47, 135.68, 133.54, 132.53, 131.84, 131.28, 128.48,
126.99, 111.23, 110.32, 110.09, 48.62, 42.01, 32.46, 32.02, 31.28,
29.93, 26.36, 25.77, 16.78.
MALDI-TOF MS (m/z): [M]þ calcd for C156H178N8O8S8, 2549.67;

S.-Y. Liu et al. / Dyes and Pigments 134 (2016) 139e147
141
Scheme 1. Sequential Activation of CeH Bonds for Accessing TPE-DPP4 and BP-DPP4.

142
S.-Y. Liu et al. / Dyes and Pigments 134 (2016) 139e147
found, 2550.33. Elemental Anal.: calcd: C, 73.49; H, 7.04; N, 4.39.
Found: C, 73.47; H, 7.05; N, 4.40.
sequential CeH activation of thiophene-flanked DPP in two steps
with a global yield of more than 60% (70 % ꢀ 86 %). As shown in the
reactions of [2] and [3] (Scheme 1), four DPP blocks were smoothly
installed onto TPE or BP cores in one step simply starting from Ph-
DPP, and TBPE or TBBP. Throughout the synthetic route, CeH and
CeBr bonds were exclusively used as reactive sites to build new
CeC bonds. Neither organometallic reactants (e.g., organoborons,
organotins and organozincs) nor strong bases (e.g., LDA and BuLi)
were employed. Besides, non-halogenated and non-aromatic sol-
vent DMA was applied as reaction medium.
2.4. Device fabrication and characterization
OPV devices were fabricated on glass substrates commercially
pre-coated with a layer of ITO. Prior to fabrication, the substrates
were cleaned using detergent, de-ionized water, acetone, and iso-
propanol consecutively for every 15 min, and then treated in an
ultraviolet ozone generator for 15 min before being spin-coated at
Concerning the sustainable chemistry, an efficient catalyst with
high turnover number (TON, moles of substrate converted/moles of
catalyst) will be highly desirable [62]. In our study, the DACH re-
actions were accomplished in a short time (10 h) in the presence of
0.1 mol% Pd(OAc)2 without using phosphine ligands (Scheme 1).
This catalysis was simple but highly efficient. To precisely quantify
the ultralow catalyst loading for the reactions, stock DMA solution
of Pd(OAc)2, rather than solid Pd(OAc)2, was added into reaction
mixtures by syringes (see Experimental Section). These ligand-free
Pd(OAc)2-catalyszed DACH reactions with ultralow loading of
catalyst (0.1 mol%) have a higher TON by 1e2 orders of magnitude
than that of typical CeM/CeX bonds cross-couplings which usually
need 2.5e10 mol % ligand-stabilized palladium catalyst, e. g.,
Pd(PPh3)4.
Supposing the typical cross-couplings are employed instead, the
synthesis of TPE-DPP4 and BP-DPP4 will be tedious and less atom-
efficient than the sequential CeH activation. As shown in Scheme
S1 (see SI), from the same building blocks, six steps in total
would be required if Suzuki or Stille couplings were used. During
these processes, n-BuLi strong base and organotin or organoboron
reactants are needed [63] (Scheme S1, SI). This comparison shows
that the synthetic strategy we developed is more straightforward
with total two steps and single byproduct of HBr (Scheme 1). It
should be noted that the current synthetic route also has an
improvement in atom efficiency as compared with all our previous
ones using Suzuki coupling of mono-Br-DPP with phenylboronic
acid to prepare Ph-DPP (Scheme S2, SI) [45,64e66].
3000 rpm with
a
layer of 30 nm thick poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Af-
ter baking the PEDOT:PSS layer in air at 150 ꢂC for 15 min, the
substrates were transferred to a glovebox. The active layer was
spin-cast at 3000 rpm from a solution of P3HT and the tetra-DPPs in
chloroform with different blend weight ratios at a total solid con-
centration of 15 mg mlꢁ1. The samples might be annealed at 110 ꢂC
for 10 min. Then a 5 nm thick poly [(9,9-bis(30-(N,N-dimethyla-
mino) propyl)-2,7-fluorene)-alt-2.7-(9,9-dioctylfluorene)] (PFN)
film was deposited as the cathode buffer layer by the spin-coating
of a solution of 0.4 mg mlꢁ1 PFN in methanol because PFN could
reduce greatly the work function of the cathode, in favor of electron
collection in the OPVs. Subsequently, the samples were loaded into
a
vacuum
deposition
chamber
(background
pressure z 5 ꢀ 10ꢁ4 Pa) to deposit 100 nm thick aluminum cathode
with a shadow mask (the device area was 5.2 mm2). The J-V curves
were measured with a Keithley 2400 measurement source units at
room temperature in air. The photocurrent was measured under a
calibrated solar simulator (Abet 300W) at 100 mW cmꢁ2 and the
light intensity was calibrated with a standard photovoltaic (PV)
reference cell. IPCE spectra were measured with a Stanford lock-in
amplifier 8300 unit. The charge carrier mobilities of the P3HT:tetra-
DPPs films were measured using the SCLC method. Hole-only de-
vices were fabricated in a structure of ITO/PEDOT:PSS/P3HT:tetra-
DPPs(2:1)/MoO3/Al, electron-only devices were fabricated in a
structure of ITO/PFN/P3HT:tetra-DPPs(2:1)/PFN/Al. The device
characteristics were extracted by modeling the dark current under
forward bias using the SCLC expression described by the Mott-
Gurney law:
On the whole, the strategy of sequential CeH activation (Scheme
1) provided herein for accessing DPP-based p-functional materials
have a number of advantages in terms of sustainability [62],
including (1) straightforward synthesis from simple building blocks
on gram scale in few steps within short reaction time, (2) simple
and ligand-free catalysis with ultralow catalyst loading and high
TON, (3) bypassing the use of organometallic reactant and
dangerous strong base, (4) absence of active CeM bonds in the
reactants, and thus high compatibility with functional groups, (5)
non-halogenated and non-aromatic solvent used for the reactions,
(6) simple byproduct of HBr only, and, last but not least, (7) the
current strategy can be extended as a general and effective method
for accessing complex DPP-based functional materials.
9
8
V2
L3
J ¼ εrε0m
Here, εr z 3 is the average dielectric constant of the blend film, ε0 is
the permittivity of the free space, is the carrier mobility,
L z 100 nm is the thickness of the film, and V is the applied voltage.
m
3. Results and discussion
3.1. Synthesis and sustainability aspects
Scheme 1 depicts the synthetic routes to two DPP-based oligo-
mers TPE-DPP4 and BP-DPP4, and the detailed procedures are
described in the Experimental Section. Firstly, a mono-phenyl
capped DPP, Ph-DPP, was obtained by the direct arylation of one
3.2. Characterization and theoretical calculation
The target oligomers, TPE-DPP4 and BP-DPP4, were fully char-
acterized by 1H and 13C NMR, matrix-assisted laser desorption
ionization time of flight (MALDI-TOF) mass spectroscopy (MS) and
elemental analysis (spectra shown in SI). Both DPP-based oligomers
are readily soluble in common organic solvents such as CHCl3,
CH2Cl2, tetrahydrofuran and dichlorobenzene due to the presence
of solubilizing N-substituted 2-ethyl-hexyl chains. The thermal
properties of TPE-DPP4 and BP-DPP4 were checked by thermog-
ravimetric analysis (TGA, Fig. S2), which shows that both TPE-DPP4
and BP-DPP4 exhibit good thermal stability with 5% weight-loss
temperatures (Td) at 390 and 401 ꢂC under N2 atmosphere,
respectively. Differential scanning calorimetry (DSC) was used to
a
-CeH bond on thiophene-flanked DPP with one equiv of bromo-
benzene. The reaction was run under simple and ligand-free con-
ditions: 0.1 mol % Pd(OAc)2, 30 mol % pivalic acid (PivOH), 1.2 equiv
anhydrous K2CO3, and solvent N,N-dimethylacetamide (DMA) at
110 ꢂC for 10 h. It is noteworthy that this reaction can be carried out
on a gram scale to afford Ph-DPP in good yield (reaction [1] in
Scheme 1). After that, further arylation of the remaining
a-CeH
bond on Ph-DPP by multibromo-arenes, TBPE and TBBP, led to
formation of TPE and BP cored tetra-DPPs, i.e. TPE-DPP4 and BP-
DPP4 respectively. Overall, both oligomers were accessed by

S.-Y. Liu et al. / Dyes and Pigments 134 (2016) 139e147
143
study the crystallinity of TPE-DPP4 and BP-DPP4 (Fig. 1). The DSC
scanning of TPE-DPP4 exhibits both weak crystalline melting and
recrystallization peaks at 241.5 and 192 ꢂC respectively. Under the
similar DSC scans, neither melting nor recrystallization peaks could
be observed for BP-DPP4. These results suggest that TPE-DPP4 and
BP-DPP4 are low-crystalline and non-crystalline, respectively.
The geometries of the oligomers TPE-DPP4 and BP-DPP4 have
been calculated by density functional theory (DFT) method. As
shown in Fig. 2a and b, the optimized geometries indicate that both
TPE-DPP4 and BP-DPP4 are twisted and non-planar, which comes
from the non-planar nature of TPE and BP cores, the steric effect
among the DPP arms, and also the phenyl-thiophene linkages be-
tween cores and DPP blocks. The non-planar architecture of TPE-
DPP4 and BP-DPP4 will suppress the intermolecular packing [58],
which is responsible for their less crystalline solid state when
compared to their planar counter parts. This is likely true as the
linear/planar DPPBzFu has highly crystalline [67,68]. Although the
DPP arms of these non-planar oligomers have less molecular orbital
(MO) overlap, they are close to each other in space. The charges on
TPE-DPP4 and BP-DPP4 can delocalize throughout the DPP arms,
and can hop among each other. Taking advantage of the non-planar
geometries, the distribution and transport of charges in multi-
directions will lead to statistically better D-A charge separation
and collection [66].
with tetra-DPPs. The light absorption of P3HT donor and tetra-DPP
acceptors complements each other quite well.
Besides light absorption, suitable FOEL gaps between donor and
acceptor play an important role in driving photon-generated charge
dissociation and determining the parameters of BHJ OPV devices.
The FOELs of DPP-acceptors were electrochemically checked by
cyclic voltammetry (CV) in CH2Cl2 solution (Fig. S4). The LUMO and
HOMO are estimated from the E1/2 values in solution, using the
value of ꢁ5.1 eV for Fc/Fcþ. Fig. 2e shows the FOELs of the P3HT
donors and DPP-based acceptors. The FOELs of fullerene-based
acceptor PC61BM were illustrated for comparison purpose. Both
TPE-DPP4 and BP-DPP4 exhibit very similar HOMOs and LUMOs. As
illustrated in Fig. 2e, the gap between the LUMO of DPPs and the
HOMO of P3HT (1.12 eV) is much larger than that between PC61BM
and P3HT (0.74 eV), benefiting high Vocs for the BHJ OPVs based on
P3HT and DPPs. Notably, both of LUMO-LUMO (ꢁ2.74 eV vs
ꢁ3.64 eV) and HOMO-HOMO (ꢁ4.76 eV vs ꢁ5.08 eV) offsets be-
tween P3HT donor and tetra-DPP acceptors are larger than 0.3 eV,
which is favorable for either electron or hole transfer between D-A
interfaces [69]. That is, due to the suitable FOELs, the electrons on
P3HT will transfer onto tetra-DPPs while the holes on tetra-DPPs
will transfer onto P3HT, thus potentially increasing the photo-
generated current (Fig. S5, SI).
Table 1 summarizes the significant optical and electrochemical
properties of P3HT donor and tetra-DPP acceptors.
3.3. Optical and electrochemical properties
3.4. P3HT:tetra-DPPs fullerene-free BHJ OPV performances
To evaluate potential applications for OPV, the optical and
electronchemical properties of TPE-DPP4 and BP-DPP4 were
investigated. Fig. 2d shows the normalized individual UVevis ab-
sorption spectra of the DPP-acceptors, and the donor polymer P3HT
from spin-coated films. TPE-DPP4 and BP-DPP4 films exhibit very
similar absorption spectra, with peaks (lfmax) at 588 and 638 nm for
TPE-DPP4 and 587 and 639 nm for BP-DPP4. These peaks are red-
Motivated by the complementary absorption and well-matched
FOELs between P3HT donor and tetra-DPP acceptors, the devices
with a configuration of ITO/PEDOT:PSS/P3HT:tetra-DPPs/PFN/Al
were fabricated to investigate the performances of solution-
processed BHJ OPVs under the simulated AM 1.5 G irradiation
with intensity of 100 mWcmꢁ2. The devices based on P3HT:tetra-
DPPs BHJ processed by chloroform are firstly optimized with
three sets of D(P3HT)/A(tetra-DPPs) ratios, i.e. 3:1, 2:1 and 1:1. It
was found that D/A ratio of 2:1 can afford best performance after
thermal annealing at 110 ꢂC for 10 min. Without annealing, a low
PCE of 0.20% with a VOC of 1.23 V, a short current (JSC of
0.69 mA cm2, and an FF of 0.23 is obtained when the blend weight
ratio of donor (P3HT) and acceptor (TPE-DPP4) is 2:1 (Table S1, SI).
After thermal annealing at 110 ꢂC for 10 min, the device with the
same D:A blend ratio can give a maximum PCE of 2.49% with a Voc
of 1.16 V, a JSC of 4.55 mA cm2, and a FF of 0.47. The same trend was
also found in the BHJ of P3HT:BP-DPP4. By annealing treatment, the
PCE of P3HT:BP-DPP4 BHJ can dramatically improved from 0.4% to
1.18% (Table S1). Fig. 3a shows the current density-voltage (J-V)
characteristic curves of the optimal cells. The corresponding OPV
parameters are summarized in Table 2. For P3HT-based fullerene-
free OPVs, the distinct enhancement of PCE by annealing treatment
was also observed by our group previously [52]. The thermal
shifted compared to their solution absorption spectra (lms ax
,
Fig. S3), which can be ascribed to the enhanced intermolecular
interaction in the solid state of oligomers. The absorption band-
edge of the cast films (lfedge) for TPE-DPP4 and BP-DPP4 are 777
and 774 nm respectively, corresponding to the optical bandgaps
(Eogpt) of 1.59 and 1.60 eV. To maximize the light harvesting of BHJs,
integrating electron donors and acceptors with complementary
light absorption is highly desirable. As shown in Fig. 2d, P3HT film
has an absorption peak at 523 nm, which is blue-shifted compared
annealing may promote intermolecular p-p stacking in P3HT:tetra-
DPP BHJ films, leading to better charge transport and suitable phase
separation, which are desirable for JSC, FF and thus the PCE.
Impressively, the VOC of 1.16 V achieved herein represents one of
the highest values for the single-junction OPVs [49e53,70e72]. The
large offset between the LUMOs of tetra-DPPs and HOMO of P3HT
(Fig. 2e) contributed to the high VOCs.
3.5. IPCE measurements
To understand the photophysical properties, the IPCE (incident
photon-to-current conversion efficiency) of the P3HT:tetra-DPPs
BHJs were studied. Fig. 3b shows the IPCE spectra of the optimal
Fig. 1. DSC curves of TPE-DPP4 and BP-DPP4.
OPV device. The calculated JSCs from IPCE are 4.51 and

144
S.-Y. Liu et al. / Dyes and Pigments 134 (2016) 139e147
Fig. 2. DFT optimized geometries (ethyl-hexyl chains replaced by methyl groups) of (a) TPE-DPP4 and (b) BP-DPP4. (c) Structure of P3HT. (d) Uvevis absorption of neat films of TPE-
DPP4, BP-DPP4 and P3HT. (e) FOEL diagram of P3HT, TPE-DPP4, BP-DPP4 and PC61BM.
Table 1
Optical and electrochemical properties of P3HT donor and tetra-DPP acceptors.
D or A
lsmax (nm)
lfmax (nm)
lfedge (nm)
Eogpt (eV)
HOMO (eV)
LUMO (eV)
P3HT
TPE-DPP4
BP-DPP4
464
574/614
569/608
523
588/638
589/639
650
777
774
1.91
1.59
1.60
ꢁ4.76
ꢁ5.08
ꢁ5.06
ꢁ2.74
ꢁ3.64
ꢁ3.61
Fig. 3. (a) JeV curves of P3HT:tetra-DPPs BHJs and their OPV device configuration. (b) IPCE spectra of P3HT:tetra-DPPs BHJs.
2.94 mA cmꢁ2
,
respectively, which are consistent with the
from donor and acceptor domain can be dissociated effectively at
the D-A junction. The sufficient LUMO-LUMO and HOMO-HOMO
offsets (>0.3 eV) between the donor and acceptor provided
enough driving force for the photo-generated electron from P3HT
transfer onto tetra-DPPs, while photo-generated hole from tetra-
DPPs transfer onto P3HT (Fig. 2e and Fig. S5).
measured JSCs. Impressively, the IPCEs of P3HT:tetra-DPPs BHJs
extend from 320 nm to 700 nm, matching well with their corre-
sponding UVevis absorption spectra (Fig. S3, SI). It indicates that
light absorbed by both P3HT and tetra-DPPs were efficiently con-
verted into current in solar cells, suggesting excitons generated

S.-Y. Liu et al. / Dyes and Pigments 134 (2016) 139e147
145
Table 2
Photovoltaic parameters of the OPV devices based on P3HT:tetra-DPPs.
D:Aa
VOC [V]
JSC [mA cmꢁ2
]
FF
PCE [%]d
mh
me
e
[10ꢁ4 cm2 Vꢁ1sꢁ1
]
b
c
P3HT:TPE-DPP4
P3HT:TPE-DPP4
1.23
1.16
1.15
1.11
0.69
4.45
1.32
2.93
0.23
0.47
0.26
0.36
0.20 (0.16)
2.49 (2.43)
0.40 (0.37)
1.18 (1.14)
1.24
1.41
0.57
0.87
0.23
0.32
0.15
0.17
b
P3HT:BP-DPP4
c
P3HT:BP-DPP4
a
The D:A ratio is 2:1 (w/w).
As-cast.
b
c
Annealed at 110 ꢂC for 10 min.
d
e
The best and average (in brackets, from 10 devices) PCEs.
Hole and electron mobilities by SCLC method.
3.6. BHJ morphology and charge carrier mobility
4. Conclusion
Atomic force microscopy (AFM) and space-charge-limited-
current (SCLC) measurements were employed to investigate sur-
face morphology and charge mobility of the BHJ layers. AFM scans
were carried out for films spin-coated on ITO substrates. Fig. 4
shows the AFM images of all the as-cast and annealing-treated
P3HT:tetra-DPPs BHJs. In the as-cast films, P3HT is highly
miscible with tetra-DPPs (Fig. 4a, c, e and g). The RMS roughness of
the as-cast P3HT:TPE-DPP4 and P3HT:BP-DPP4 BHJs are 0.465 and
0.571 nm respectively. After thermal annealing, the RMS roughness
of the BHJ films increases to 0.629 and 0.723 nm accordingly. It can
be speculated that the thermal annealing helps to improve the
phase separation, crystallinity, and domain purity [52]. For SCLC
tests, all devices were fabricated by using the identical procedure
for solar cell preparation. The J-V curves for hole-only and electron-
only devices are shown in Fig. S7. The rightmost two columns in
Table 2 list the corresponding values. As Table 2 shows, both hole
mobility (mh) and electron mobility (me) of P3HT:TPE-DPP4 and
P3HT:BP-DPP4 BHJs are improved by thermal annealing treatment
that leads to the enhanced crystallinity. In addition, the mh and me of
P3HT:TPE-DPP4 BHJ are both higher than those of P3HT:BP-DPP4
BHJ. Compared with BP-DPP4, TPE-DPP4 has a larger conjugation
system and slightly higher crystallinity (Fig. S2), which may make
contributions to the improved charge mobility.
Diketopyrrolopyrro (DPP), as a commercialized dye chromo-
phore [40], is one of the most widely studied building blocks for
organic semiconducting materials [41e44]. CeH is the most
extensive chemical bond in organic compounds. The direct func-
tionalization of CeH bonds for accessing target molecules has
clearly attractive features for sustainable chemistry. In our work,
starting from three simple building blocks, two large conjugated
oligomers containing four DPP units have been readily constructed
merely in two steps via Pd-catalyzed sequential arylation of CeH
bonds. Neither organometallic reactants nor strong bases were
employed throughout the processes of synthesis. The obtained
DPP-based non-planar oligomers were tested as non-fullerene ac-
ceptors in OPVs by using commercially available P3HT as the donor,
affording a best PCE of 2.49% with a high Voc of 1.16 V. The inte-
gration of easily accessible acceptors with economical donor for
fullerene-free BHJ OPVs in our study represents a step toward
accessing renewable solar energy via sustainable chemistry.
Importantly, the synthetic route developed herein can be
extended as a general and effective method for accessing a wide
number of DPP derivatives. Besides, the sequential CeH activation
strategy for atom-efficient construction of
should be applicable for other building blocks beyond DPP moiety.
We believe, in the future, more and more -semiconducting ma-
p-conjugated oligomers
p
terials will be accessed via sequential CeH activation for organic
electronic applications.
Fig. 4. AFM height (a, b, c, d) and phase (e, f, g, h) images of the P3HT:tetra-DPPs (2:1 by weight) BHJ films with or without annealing treatment.

146
S.-Y. Liu et al. / Dyes and Pigments 134 (2016) 139e147
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Acknowledgements
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solar cells. Macromolecules 2015;48:453e61.
The National Natural Science Foundation of China (21244008,
21374075, 21575097, 21375092), China Postdoctoral Science
Foundation (Nos. 2013M540484 and 2014T70572), and Zhejiang
Provincial Natural Science Foundation of China(No. LY15B030001)
are appreciated for financial supports. SY Liu and WQ Liu contrib-
uted equally to this work)
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