Efficient diketopyrrolopyrrole-based small-molecule bulk-heterojunction solar cells with different electron-donating end-groups

Article abstract of DOI:10.1002/asia.201402223

A series of small molecules that contained identical π-spacers (ethyne), a central diketopyrrolopyrrole (DPP) unit, and different aromatic electron-donating end-groups were synthesized and used in organic solar cells (OSCs) to study the effect of electron-donating groups on the device performance. The three compounds, DPP-A-Ph, DPP-A-Na, and DPP-A-An, possessed intense absorption bands that covered a wide range, from 350 to 750 nm, and relatively low HOMO energy levels, from -5.50 to -5.55 eV. DPP-A-An, which contained anthracene end-groups, demonstrated a stronger absorbance and a higher hole mobility than DPP-A-Ph, which contained phenyl groups, and DPP-A-Na, which contained naphthalene units. The power-conversion efficiencies (PCEs) of OSCs based on organic:PC₇₁BM blends (1:1, w/w) with a processed DIO additive were 3.93% for DPP-A-An, 3.02 % for DPP-Na, and 2.26 % for DPP-A-Ph. These findings suggest that a DPP core that is functionalized with electron-donating capping groups constitutes a promising new class of solution-processable small molecules for OSC applications.

Full text of DOI:10.1002/asia.201402223

FULL PAPER
DOI: 10.1002/asia.201402223
Efficient Diketopyrrolopyrrole-Based Small-Molecule Bulk-Heterojunction
Solar Cells with Different Electron-Donating End-Groups
Yu Jin Kim,[a] Jang Yeol Back,[b] Seul-Ong Kim,[b] Chan-Woo Jeon,[c]
Chan Eon Park,*[a] and Yun-Hi Kim*[c]
Abstract: A series of small molecules
that contained identical p-spacers
(ethyne), a central diketopyrrolopyr-
role (DPP) unit, and different aromatic
electron-donating end-groups were syn-
thesized and used in organic solar cells
(OSCs) to study the effect of electron-
donating groups on the device perfor-
mance. The three compounds, DPP-A-
Ph, DPP-A-Na, and DPP-A-An, pos-
sessed intense absorption bands that
covered a wide range, from 350 to
750 nm, and relatively low HOMO
energy levels, from ꢀ5.50 to ꢀ5.55 eV.
DPP-A-An, which contained anthra-
cene end-groups, demonstrated a stron-
ger absorbance and a higher hole mo-
bility than DPP-A-Ph, which contained
phenyl groups, and DPP-A-Na, which
contained naphthalene units. The
power-conversion efficiencies (PCEs)
of OSCs based on organic:PC71BM
blends (1:1, w/w) with a processed DIO
additive were 3.93% for DPP-A-An,
3.02% for DPP-Na, and 2.26% for
DPP-A-Ph. These findings suggest that
a DPP core that is functionalized with
electron-donating capping groups con-
stitutes a promising new class of solu-
tion-processable small molecules for
OSC applications.
Keywords: absorption · alkynes ·
conjugation · cross-coupling · solar
cells
Introduction
tion and chain-end contamination, which can impede overall
performance and device consistency.[5,6]
Organic photovoltaic (OPV) devices are a promising alter-
native technology for the cost-effective and scalable produc-
tion of renewable energy.[1–3] Current research into OPV ma-
terials primarily focuses on the design and synthesis of semi-
conducting polymers that are capable of both light absorp-
tion and charge transport. Great advances in the develop-
ment of these polymers have brought about significant
improvements in their power-conversion efficiencies
(PCEs), which has recently exceeded 10%.[4] However, they
suffer from several drawbacks, such as batch-to-batch varia-
Electroactive small molecules have recently received at-
tention as alternatives to polymers for the development of
OPV materials that exhibit favorable electronic properties,
as well as batch-to-batch consistency. Small molecules not
only exhibit the inherent polydispersity of polymeric materi-
als, they also exhibit other remarkable benefits, such as easy
purification and easy mass production.[7–9] Moreover, they
can be synthesized and purified as single-molecular entities,
thus eliminating the sort of device variability that can arise
from material inhomogeneity. Several research groups have
reported such small molecules with optimized electronic and
optical properties.[10–12] To date, state-of-the-art solution-pro-
cessed small-molecule OPVs have demonstrated PCEs>
8%,[13] but their overall performance is still significantly
lower than that of their polymer counterparts.[14] One ap-
proach to close this gap is to design better active-layer mate-
rials.
To address this issue, we have designed new small mole-
cules that had a simple and symmetric D-A-D alternating
structure (D=donor, A=acceptor). We postulated that the
introduction of electron-rich aromatic moieties onto the
ends of such small molecules would facilitate favorable end-
to-end p–p interactions through their planar structure, as
well as larger electron-donating characteristics, leading to
enhanced charge transport. To investigate this approach,
a series of small molecules were synthesized. Each molecule
had different electron-rich end-groups: 2,5-bis(2-hexyldec-
[a] Y. J. Kim,+ Prof. C. E. Park
POSTECH Organic Electronics Laboratory
Department of Chemical Engineering
Pohang University of Science and Technology
Pohang, 790-784 (Republic of Korea)
E-mail: cep@postech.ac.kr
[b] J. Y. Back,+ S.-O. Kim
School of Materials Science&Engineering and
Research Institute for Green Energy Convergence
Technology (REGET), Gyeongsang National University
Jin-ju, 660-701 (Republic of Korea)
[c] C.-W. Jeon, Prof. Y.-H. Kim
Department of Chemistry and Research Institute of
Natural Science, Gyeongsang National University
Jin-ju, 660-701 (Republic of Korea)
E-mail: ykim@gnu.ac.kr
[+] These authors contributed equally to this work.
yl)-3,6-bis(5-(phenylethynyl)thiophen-2-yl)pyrrolo
role-1,4(2H,5H)-dione (DPP-A-Ph), 2,5-bis(2-hexyldecyl)-
3,6-bis(5-(naphthalen-2-ylethynyl)thiophen-2-yl)pyrrolo[3,4-
ACHTUNGTRENUN[NG 3,4-c]pyr-
AHCTUNGTRENNUNG
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402223.
ACHTUNGTRENNUNG
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Chan Eon Park, Yun-Hi Kim et al.
c]pyrrole-1,4
thracen-2-ylethynyl)thiophen-2-yl)-2,5-bis(2-
hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-
N
tion. The obtained compounds were characterized by
H NMR and IR spectroscopy and by mass spectrometry.
These three molecules had good solubility in common or-
ganic solvents, such as o-dichlorobenzene, chlorobenezene,
CHCl3, toluene, and THF, which is significant for preparing
solution-processable bulk-heterojunction small-molecule
solar cells. In the fabrication of solution-processed solar
cells, the thermal stability of the donor material is of crucial
importance. Therefore, the thermal properties of DPP-A-
Ph, DPP-A-Na, and DPP-A-An were investigated by using
thermogravimetric analysis (TGA) and DSC (Figure 1). All
three small molecules exhibited good thermal stability, as
observed from the TGA analysis. The decomposition tem-
A-An). Diketopyrrolopyrrole (DPP) was chosen as the elec-
tron-deficient core as a platform for investigating the effect
of these end-groups, because the incorporation of this
moiety allowed for control of the solution processability of
the small molecule and solid-state molecular ordering
through modulation of the N-alkyl substituents.[15,16] The
DPP core itself has also demonstrated promising optical
properties, charge-charier mobility, and photovoltaic perfor-
mance in numerous materials that incorporate small mole-
cules.[17,18] In addition, we decided to insert an ethyne link-
age between the DPP moiety and the electron-donating
end-group to afford a lower HOMO–LUMO band-gap
through extended conjugation and also to increase the ioni-
zation potential of the triad molecule based on the relatively
larger electron-withdrawing characteristics of sp hybridiza-
tion over sp2 hybridization.[19,20]
Results and Discussion
Synthesis and Thermal Properties
Our synthetic route to DPP-A-Ph, DPP-A-Na, and DPP-
A-An is shown in Scheme 1. These compounds were synthe-
sized by using Sonogashira coupling reactions and desilyla-
Figure 1. TGA of DPP-A-Ph, DPP-A-Na, and DPP-A-An.
Scheme 1. Synthesis of DPP-A-Ph, DPP-A-Na, and DPP-A-An.
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Chan Eon Park, Yun-Hi Kim et al.
peratures (Td) of DPP-A-Ph,
DPP-A-Na, and DPP-A-An, as
determined at 5% weight loss,
were 362, 379, and 3768C, re-
spectively. Interestingly, DSC
analysis (see the Supporting In-
formation) showed that DPP-
A-Ph exhibited a sharp melting
transition at about 1508C. For
DPP-A-Na, the incorporation
of naphthalene units onto both
ends of the molecule resulted in
Figure 2. Normalized UV/Vis absorption spectra of DPP-A-Ph, DPP-A-Na, and DPP-A-An a) as solutions in
CHCl3 and b) as thin films.
a
slight increase in melting
point to about 1648C. The melt-
ing transition of DPP-A-An
Table 1. Thermal, optical, and electrochemical properties of DPP-A-Ph,
was quite higher than that observed for DPP-A-Na. The sig-
nificantly higher melting temperature of DPP-A-An (about
2388C) compared to DPP-A-Ph and DPP-A-Na was likely
the result of a more-planar structure, which could lead to
stronger intermolecular p–p stacking interactions in the
solid state.[21]
DPP-A-Na, and DPP-A-An.
Small Td Td lmax
molecule [8C][a] [8C][b] [nm]
solution film
opt
lmax lonset
Eg
EHOMO ELUMO
[nm] [nm] [eV][c] [eV][d]
[eV][d]
film
687
DPP-A-
Ph
362
379
376
150
164
238
567,
609
576,
616
583,
622
540
596
612
1.81
1.76
1.68
ꢀ5.55
ꢀ5.53
ꢀ5.50
ꢀ3.92
ꢀ3.96
ꢀ3.97
DPP-A-
Na
DPP-A-
An
706
736
Optical Properties
The UV/Vis absorption spectra of all of these DPP-based
small molecules both as solutions in CHCl3 and as thin films
are shown in Figure 2; the corresponding data are listed in
Table 1. In solution (Figure 2a), all of the materials exhibit-
ed absorption bands in the UV- (350–400 nm) and visible-
light regions of the electromagnetic spectrum (400-650 nm).
The two absorption peaks are attributed to intramolecular
charge transfer (ICT) associated with the terminal donors
(phenyl, naphthalene, and anthracene; D) and the DPP core
(A).[22] The absorption maximum exhibited a considerable
red-shift on moving from DPP-A-Ph (lmax =567, 609 nm) to
DPP-A-Na (lmax =576, 616 nm) and DPP-A-An (lmax =583,
622 nm). This ICT absorption band was gradually red-shifted
as the donor end-group was changed from phenyl to naph-
thalene and anthracene, thereby agreeing well with the elec-
tron-rich nature of these three aromatic rings, as well as
with extension of the p-conjugated system.[23]
[a] Decomposition temperature that corresponded to 5% weight loss in N2,
as determined by TGA. [b] Melting temperature, as determined by DSC.
[c] Estimated from the absorption edge in the film (Egopt =1240/lonset).
[d] HOMO and LUMO energy levels of the small molecules, as obtained by
cyclic voltammetry.
of anthracene end-groups, thereby leading to the smallest
band-gap, which is useful for increasing the photocurrent in
organic solar cells.[26]
Electrochemical Properties
Cyclic voltammetry (CV) was employed to investigate the
oxidation and reduction behaviors of these small molecules
by using a Pt counter electrode in MeCN solution that con-
ꢀ1
tained 0.1 molLꢀ1 Bu4NPF6 at a scan rate of 50 mVs
Figure 2b shows the absorption spectra of spin-coated
films from solutions of DPP-A-Ph, DPP-A-Na, and DPP-
A-An in CHCl3. Compared to their corresponding spectra
in solution, all three compounds exhibited broader and
bathochromic absorption peaks for the ICT band, owing to
the donor–acceptor (D–A) interactions and to extended p–p
interactions caused by the polarity of the amide groups in
the DPP units.[24,25] The optical band-gaps were obtained by
extrapolating the absorption edges of the films for DPP-A-
Ph, DPP-A-Na, and DPP-A-An (Table 1). Owing to the
electron-donating terminal group, the optical band-gap nar-
rowed in the sequence: DPP-A-Ph (1.81 eV)>DPP-A-Na
(1.76 eV)>DPP-A-An (1.68 eV). As mentioned above, this
result can be ascribed to the increased conjugation of the
electron-donating end-group. Thus, DPP-A-An contained
the most-extended p-conjugation, owing to the introduction
(Figure 3). The oxidation-onset potentials of DPP-A-Ph,
DPP-A-Na, and DPP-A-An were located at 1.15, 1.13, and
1.10 V, respectively. The corresponding HOMO energy
levels of DPP-A-Ph, DPP-A-Na, and DPP-A-An were esti-
mated to be ꢀ5.55, ꢀ5.53, and ꢀ5.50 eV, respectively
(Table 1), by using the ferrocene reference value of ꢀ4.4 eV
below the vacuum level.[27] This trend in increasing HOMO
level are consistent with the electron-donating natures of
phenyl, naphthalene, and anthracene moieties. The HOMO
levels were all below ꢀ5.50 eV, which indicates that these
materials may be valuable for making stable OPV devices
that are resistant to oxidation by oxygen.[28] Moreover, these
low-lying HOMO levels can allow a high open-circuit volt-
age (Voc) for photovoltaic cells, because Voc is proportional
to the energy difference between the HOMO level of the
donor and the LUMO level of the acceptor.[29] DPP-A-An
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Chan Eon Park, Yun-Hi Kim et al.
Photovoltaic Performance
We fabricated bulk-heterojunc-
tion solar cells by using these
small molecules as the donor ma-
terial and a fullerene derivative,
PC61BM or PC71BM, as the ac-
ceptor material, with the tradi-
tional architecture of ITO/PE-
DOT:PSS/active-layer
(small-
molecule:PC61BM or PC71BM)/
LiF/Al (Figure 4a). The active
layers were spin-coated by using
CHCl3 solutions of the blends of
the small molecules and PCBM
(PC61BM or PC71BM) acceptor
in a 1:1 ratio. After trying vari-
ous blend ratios, this combina-
tion was found to be optimal.
Figure 4b,c shows the photocur-
rent-density–voltage curves (J–
V) under AM 1.5G illumination
at 100 mWcmꢀ2; the correspond-
ing performance parameters are
summarized in Table 2. All of
the devices achieved relatively
Figure 3. CVs of DPP-A-Ph (a), DPP-A-Na (b), and DPP-A-An (c) and d) energy-level diagram for the
OPV device structure.
shows the highest HOMO value (ꢀ5.50 eV) of these com-
pounds, owing to a decrease in the oxidation-onset potential
in the extended conjugated molecules. Anthracene groups
enhanced the conjugation of DPP-A-An. The LUMO levels
were determined to be ꢀ3.92, ꢀ3.96, and ꢀ3.97 eV, respec-
tively, from the reduction-onset potentials. The LUMO ener-
gies were approximately 0.33 eV above the LUMOs
(ꢀ4.3 eV) of a PC71BM n-type acceptor, which leads to
a driving force for energetically favorable electron transfer
(Figure 3b).[30]
To better understand the oxidative and reductive proper-
ties, theoretical calculations were also performed to deter-
mine the HOMO and LUMO energy levels by using density
function theory (DFT). Methyl groups were used in place of
long alkyl substituents to limit the computation time. The
optimized geometry and electron-density distribution of the
HOMOs and LUMOs of three DPP-based small molecules
are shown in the Supporting Information, Figure S2. In all
of the materials, the HOMOs and LUMOs were localized
along the conjugated backbone. The DFT-computed
HOMO/LUMO values showed similar trends to those ob-
served from the electrochemical and optical analyses; the
theoretically predicted HOMO and LUMO energy levels
were approximately 0.6–0.8 eV higher than the experimen-
tally calculated values. This result is related to various ef-
fects, such as conformational order in the bulk state and ef-
fects of the solvent and the electrolyte, which were not
taken into account.[31]
high VOC values (about 0.8 V), consistent with a low HOMO
level (<ꢀ5.50 eV). The blends of DPP-A-Ph, DPP-A-Na,
and DPP-A-An with PC61BM showed PCEs of 1.30, 2.13,
and 3.01%, respectively. However, the device performance
dramatically changed when using PC71BM as the acceptor
material. We obtained PCEs of 1.64, 2.47, and 3.63% for
DPP-A-Ph:PC71BM, DPP-A-Na:PC71BM, and DPP-A-
An:PC71BM, respectively (the small-molecule/PC71BM
weight ratio was fixed at 1:1). This change in PCE value was
mainly due to the improved Jsc (from 5.5 to 6.8 mWcmꢀ2 for
DPP-A-Ph:PC71BM, from 8.0 to 8.2 mWcmꢀ2 for DPP-A-
Na:PC71BM, and from 9.6 to 12.4 mWcmꢀ2 for DPP-A-
An:PC71BM). Because PC71BM absorbed more strongly in
the visible-light region than PC61BM, it was expected to con-
tribute more to the device current than PC61BM and, thus,
enhance the JSC value.[32,33] The JSC values of the three small-
molecule:PC71BM increased in the sequence: DPP-A-
Ph:PC71BM<DPP-A-Na:PC71BM<DPP-A-An:PC71BM.
The JSC value strongly depends on the number of excitons
that are generated in the photoactive layers and the photo-
response (the conversion of input photons into photocur-
rent), as well as on properties such as film morphology and
nanostructural order. (The film morphology and nanostruc-
tural order are discussed below.) The DPP-A-An films had
a more intense light absorption than than the DPP-A-Ph
and DPP-A-Na films, which likely contributed to larger ex-
citon and charge generation in the DPP-A-An:PC71BM de-
vices.[34] Moreover, the photoresponse of DPP-A-
An:PC71BM solar cells was higher than 52% at 593 nm,
whereas those of the devices that were fabricated from
DPP-A-Ph:PC71BM and DPP-A-Na:PC71BM were over
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Chan Eon Park, Yun-Hi Kim et al.
ment in the FF values (by 0.05–
0.7) of the devices. Thus, the
PCEs of DPP-A-Ph:PC71BM,
DPP-A-Na:PC71BM, and DPP-
A-An:PC71BM increased to
2.26, 3.02, and 3.93%, respec-
tively. The highest efficiency of
3.93%, with VOC =0.78 V, JSC
=
13.3 mWcmꢀ2, and FF=37.6%
(see the Supporting Informa-
tion, Table S1), was achieved by
the DPP-A-An:PC71BM device,
thus indicating that DPP-A-An,
which contained anthracene
end-groups, was also most ef-
fective small molecule in the
devices that were processed
with the DIO additive. The
EQE curves of the correspond-
ing devices that were processed
with DIO exhibited broad spec-
tra with enhanced efficiencies
(by about 10%) compared to
the devices without DIO, con-
sistent with the higher photo-
currents.[38]
Figure 4. a) Schematic representation of the photovoltaic cells. b,c) J–V curves for solar-cell devices based on
small-molecule:PC61BM and small-molecule:PC71BM blends (1:1, w/w), respectively. d) EQE spectra of the
corresponding small-molecule:PC71BM solar cells.
Table 2. Summary of the device-performance parameters for BHJ solar
Morphology
cells based on DPP-A-Ph, DPP-A-Na, and DPP-A-An.
Small molecule Acceptor Voc [V] Jsc [mAcmꢀ2
FF [%] PCE [%]
To investigate the effect of supramolecular structure on the
bulk-heterojunction-type active layer, we examined the top-
ography of the film surfaces by using AFM in tapping mode.
AFM height images of DPP-A-Ph:PC71BM, DPP-A-
Na:PC71BM, and DPP-A-An:PC71BM blends are shown in
Figure 6. The DPP-A-Ph:PC71BM film exhibited a relatively
formless and flat surface, with a root-mean-squared (RMS)
roughness of 0.30 nm. On the other hand, the DPP-A-
Na:PC71BM blend film clearly showed a phase-separated,
highly aggregated, and continuously connected pathway,
with a RMS roughness of 2.66 nm. Interestingly, the DPP-
A-An:PC71BM film displayed a distinct crystalline network,
]
DPP-A-Ph
DPP-A-Na
DPP-A-An
PC61BM 0.78
PC71BM 0.80
PC61BM 0.78
PC71BM 0.78
PC61BM 0.76
PC71BM 0.78
5.5
6.8
8.0
8.2
9.6
30.2
29.9
33.3
38.3
41.5
37.2
1.30
1.64
2.13
2.47
3.01
3.63
12.4
43% at 569 nm and about 34% at 543 nm, respectively (Fig-
ure 4d). The highest EQE spectrum for the DPP-A-
An:PC71BM device can be attributed to the higher conver-
sion of the input photons into
photocurrent at all absorption
wavelengths, which is consistent
with the highest observed cir-
cuit current.[35]
To further improve the pho-
tovoltaic performance of DPP-
based organic solar cells, we in-
troduced
a
1,8-diiodooctane
(DIO) additive.[36,37] Figure 5
shows the J–V and EQE curves
of the corresponding devices.
The incorporation of 1% DIO
(v/v) led to a substantial in-
crease in the JSC values (by 1.0–
Figure 5. a) Current-density–voltage characteristics of small-molecule:PC71BM solar cells that were processed
with 1,8-diiodooctane (DIO) under AM 1.5G illumination (100 mWcmꢀ2). b) EQEs of the corresponding devi-
2.0 mWcmꢀ2) and an improve- ces that were processed with DIO.
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Chan Eon Park, Yun-Hi Kim et al.
Figure 7. XRD patterns of the three small-molecule:PC71BM blend films.
An:PC71BM film contained a high fraction of DPP-A-An
crystallites,[41] in good agreement with the results of film
morphology. The XRD peak positions of DPP-A-Ph, DPP-
A-Na, and DPP-A-An were at 2qꢁ4.72, 5.84, and 6.778,
which corresponded to a d-spacing between the molecular
layers of 13.00, 10.51, and 9.06 ꢁ, respectively. The decrease
in d-spacing of the DPP-based small molecules is attributed
to the aromatic end-groups, which make the small molecules
more planar and, hence, facilitates close packing of the com-
pounds.[42]
Figure 6. AFM height images and TEM images of DPP-A-Ph:PC71BM
(a,d), DPP-A-Na:PC71BM (b,e), and DPP-A-An:PC71BM blend films
from CHCl3 (c,f).
with a large RMS roughness of 3.61 nm. This result most
likely originated from enhanced intermolecular interactions
between the anthracene-incorporated DPP-A-An molecules,
as also evidenced by the DSC analysis. The higher surface
roughness and the nanoscale texture of the crystal led to in-
creased internal light scattering and enhanced light absorp-
tion.[39,40] These characteristics contributed to the higher Jsc
value and efficiency of the DPP-A-An:PC71BM device rela-
tive to the DPP-A-Ph:PC71BM and DPP-A-Na:PC71BM de-
vices.
TEM was also used to study the bulk-phase morphology
of three DPP-based small-molecule:PC71BM films. As
shown in Figure 6, the bright regions can be attributed to
the small-molecule domains, whereas the dark regions may
be attributed to the PC71BM domains, owing to its high elec-
tron-scattering density. As expected, a clearer packing struc-
ture, a nanoscale pathway, and a distinct crystal network are
observed in the sequence: DPP-A-Ph:PC71BM<DPP-A-
Na:PC71BM<DPP-A-An:PC71BM. These results indicate
that the introduction of anthracene end-groups into the mol-
ecule structure helps the self-organization of the active layer
and produces a nanocrystalline network, which lead to effi-
cient charge separation and transport.
Hole Mobility
To quantitatively investigate the effect of inter-chain pack-
ing on their charge-transport properties, the hole mobilities
of the three DPP-based small molecules were measured by
using the space charge limit current (SCLC) method (for
the experimental details, see the Experimental Section). The
hole mobilities for DPP-A-Ph, DPP-A-Na, and DPP-A-An
were estimated to be 3.72ꢂ10ꢀ7, 1.90ꢂ10ꢀ6, and 3.72ꢂ
10ꢀ5 cm2 Vꢀ1 Sꢀ1, respectively, as determined from the plots
of logJD versus voltage (Figure 8). The enhancement in hole
mobility of DPP-A-An may arise from the denser packing
of molecules with anthracene groups, as evidenced by the
AFM and XRD analyses. These hole mobilities are suffi-
ciently high for application in small-molecule solar cells.
X-Ray Diffraction
To obtain insightful information on molecular packing in the
films of the PC71BM blends, XRD patterns (out-of-plane) of
the DPP-A-Ph:PC71BM, DPP-A-Na:PC71BM, and DPP-A-
An:PC71BM films were measured (see Figure 7). As shown
in Figure 7, all three films exhibited a 2q diffraction peak at
about 58, which indicated that the crystalline structure of
the small molecule existed in the blend films.
The DPP-A-Na:PC71BM film exhibited stronger XRD
diffraction peaks than the DPP-A-Ph:PC71BM blend film,
owing to better layer-by-layer packing of the molecules. The
DPP-A-An:PC71BM film exhibited even stronger crystalline
diffraction peaks, which suggested that the DPP-A-
Figure 8. logJ–V characteristics of hole-only devices by using SCLC mod-
eling.
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Chan Eon Park, Yun-Hi Kim et al.
Synthesis of 2-ethynylanthracene
Conclusions
A solution of (anthracen-2-ylethynyl)trimethylsilane (3.00 g 0.0109 mol)
in THF (60 mL) was added dropwise to degassed methanol (60 mL) and
K2CO3 (7.55 g, 0.0547 mol) and the mixture was stirred for 8 h. The reac-
tion was quenched with water and the organic phase was extracted with
ethyl ether. After drying over MgSO4, the liquid was concentrated under
vacuum, and the residue was purified by column chromatography on
a short silica gel column (n-hexane) to yield 2-ethynylanthracene as
To study the effect of different electron-donating groups on
small molecules on the performance of their corresponding
OPVs, three new DPP-based compounds were designed and
synthesized. Different electron-donating groups in the or-
ganic materials afforded different photophysical and photo-
voltaic properties, as well as different molecular packing
structures. DPP-A-An, which contained anthracene end-
groups, had a lower band-gap, a more-planar structure, and
a higher hole mobility compared to DPP-A-Ph, which con-
tained phenyl groups, and DPP-A-Na, which contained
naphthalene units. A higher PCE value was recorded for the
DPP-A-An blend with PC61BM compared to those with
DPP-A-Ph and DPP-A-Na. The highest PCE of 3.93% was
achieved for a device that was fabricated from DPP-A-
An:PC71BM and processed with DIO, with VOC =0.78 V,
a
yellow solid in 82% yield. M.p. 181–1828C; 1H NMR (300 MHz,
CDCl3): d=8.41 (s, 2H), 8.22 (s, 1H), 8.03–7.95 (m, 3H), 7.53–7.47 (m,
3H), 3.22 ppm (s, 1H); 13C NMR (500 MHz, CDCl3): d=136.35, 134.18,
133.58, 133.31, 133.25, 130.82, 130.20, 129.48, 129.20, 128.18, 126.44,
126.21, 125.88, 120.52, 85.55, 78.55 ppm; MS (EI): m/z: 202 ([M]+).
Synthesis of 2,5-bis(2-hexyldecyl)-3,6-bis(5-(phenylethynyl)thiophen-2-
yl)pyrroloCAHUTGTNREN[NUG 3,4-c]pyrrole-1,4ACHTUNTGREN(NUNG 2H,5H)-dione (DPP-A-Ph)
3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(2-hexyldecyl)pyrroloACTHNUGRTNE[NUG 3,4-c]pyrrole-
1,4(2H,5H)-dione (0.600 g, 0.7 mmol), ethynylbenzene (0.149 g,
ACHTUNGTRENNUNG
1.5 mmol), copper iodide (0.009 g, 0.05 mmol), diisopropylamine
(6.0 mL), and toluene (25.0 mL) were mixed in a three-necked flask and
degassed with nitrogen. Then, [PdACHTNUGTRNEUNG(PPh3)4] (0.015 g, 0.001 mmol) was
J
SC =13.3 mAcmꢀ2, and a fill factor (FF)=37.6% under
100 mWcmꢀ2 simulated AM 1.5 G solar irradiation. These
results suggest that a DPP core that is functionalized with
electron-donating capping groups is a promising new class
of solution-processable small molecules for OSC applica-
tions.
added and the mixture was heated at 608C for 8 h. The reaction was
quenched with water and the organic phase was extracted with CHCl3.
After drying over MgSO4, the liquid was concentrated under vacuum and
the residue was purified by column chromatography on a short silica gel
column (n-hexane/CHCl3, 10:1) to yield 2,5-bis(2-hexyldecyl)-3,6-bis(5-
(phenylethynyl)thiophen-2-yl)pyrrolo
a purple powder in 72% yield. M.p. 1508C; H NMR (300 MHz, CDCl3):
d=8.92–8.91 (s, 2H), 7.57–7.56 (m, 4H), 7.41–7.40 (m, 8H), 4.05–4.03(d,
ACHUTGTNREN[UNG 3,4-c]pyrrole-1,4ACHTUNGTREN(NNGU 2H,5H)-dione as
1
ACHTUNGTRENNUNG
4H), 1.97 (s, 2H), 1.35–1.25 (d, 48H), 0.89–0.84 ppm (m, 12H); 13C NMR
(500 MHz, CDCl3): d=161.18, 131.19, 130.99, 130.71, 129.50, 128.22,
127.15, 126.33, 125.89, 124.53, 124.39, 124.18, 123.35 116.61, 108.90, 97.46,
88.19, 46.59, 37.89, 31.95, 31.83, 31.35, 30.09, 29.76, 29.59, 29.34, 26.32,
26.28, 22.67, 14.08 ppm; HRMS (FAB): m/z calcd: 948.5661 [M+H]+;
Experimental Section
Materials
All of the reactions were performed under a nitrogen atmosphere by
using standard Schlenk techniques. All of the chemical reagents were
purchased from Aldrich and used as received. Other chemicals were used
unless otherwise specified. Ethynylbenzene,[43] 2-ethynylnaphthalene,[44]
and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-hexyldecyl)-2,5-dihydropyr-
ꢀ
found: 949.1352; IR (KBr): n˜ =3066 (aromatic C H), 2846, 2924 (aliphat-
ic C H), 2187 (C C), 1662 cmꢀ1 (C=O).
ꢀ
ꢂ
Synthesis of 2,5-bis(2-hexyldecyl)-3,6-bis(5-(naphthalen-2-
ylethynyl)thiophen-2-yl)pyrroloACTHNUGTRENNUG[3,4-c]pyrrole-1,4ACHTUNGTRNE(NUGN 2H,5H)-dione (DPP-A-
Na)
roloACHTUNGTRENNUNG
[3,4-c]pyrrole-1,4-dione[45] were synthesized according to literature
procedures.
3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(2-hexyldecyl)pyrrolo
1,4AHCTUNGTR(ENNNUG 2H,5H)-dione (0.500 g, 0.6 mmol), 2-ethynylnaphthalene (0.185 g,
ACHTUNGTREN[NGNU 3,4-c]pyrrole-
Synthesis
1.2 mmol), copper iodide (0.008 g, 0.04 mmol), diisopropylamine
(5.0 mL), and toluene (20.0 mL) were mixed in a three-necked flask and
degassed with nitrogen. Then, [PdACHTNUGTRNEUNG(PPh3)4] (0.013 g, 0.001 mmol) was
added and the mixture was heated at 608C for 8 h. The reaction was
quenched with water and the organic phase was extracted with CHCl3.
After drying over MgSO4, the liquid was concentrated under vacuum and
the residue was purified by column chromatography on a short silica gel
column (n-hexane/CHCl3, 7:1) to yield 2,5-bis(2-hexyldecyl)-3,6-bis(5-
Synthesis of (anthracen-2-ylethynyl)trimethylsilane
Under a nitrogen atmosphere, 2-bromoanthracene (10.00 g 0.0389 mol),
bis(triphenylphosphine)palladium(II)dichloride (0.846 g, 0.0012 mol), and
copper iodide (0.393 g, 0.0021 mol) were dissolved THF (200 mL). Trie-
thylamine (200 mL) and trimethylsilylacetylene (5.69 g 0.0580 mol) were
successively added dropwise. The mixture was stirred overnight at RT.
The reaction was quenched with a saturated aqueous solution of NH4Cl
and the organic layer was extracted with n-hexane and dried over
MgSO4. After concentration under vacuum, the residue was purified by
column chromatography on silica gel (n-hexane) to give (anthracen-2-yle-
(naphthalen-2-ylethynyl)thiophen-2-yl)pyrroloACTHNUGTRENNUG[3,4-c]pyrrole-1,4ACHTUNGTRENNUNG(2H,5H)-
1
dione as a purple powder in 67% yield. M.p. 1648C; H NMR (300 MHz,
CDCl3): d=8.96–8.94 (d, 2H), 8.11 (s, 2H), 7.87–7.85 (d, 6H), 7.62–7.53
(m, 6H), 7.46–7.44 (d, 2H), 4.07–4.05 (d, 4H), 2.00 (s, 2H), 1.37–1.26 (d,
48H), 0.89–0.85 ppm (m, 12H); 13C NMR (500 MHz, CDCl3): d=161.08,
131.20, 130.99, 130.71, 130.75, 130.55, 129.55, 129.12, 128.55, 128.20,
127.15, 126.33, 125.89, 124.53, 124.39, 124.18, 123.35 116.61, 108.91, 97.45,
88.19, 46.59, 37.90, 31.95, 31.80, 31.31, 30.09, 29.76, 29.50, 29.34, 26.31,
26.28, 22.67, 14.18 ppm; HRMS (FAB): m/z calcd: 1048.5974 [M+H]+;
thynyl)trimethylsilane as
(300 MHz, CDCl3): d=8.38
a
yellow solid in 68% yield. 1H NMR
(s, 2H), 8.19 (s, 1H), 8.03–7.99 (m, 2H), 7.95–
ꢀ
U
found: 1048.6512. IR (KBr): n˜ =3057 (aromatic C H), 2850, 2922 (ali-
ꢀ
ꢂ
7.92 (d, 1H), 7.52–7.45 (m, 3H), 0.33 ppm (s, 9H); MS (EI): m/z: 274
phatic C H), 2189 (C C), 1665 cmꢀ1 (C=O).
([M]+).
Synthesis of 3,6-bis(5-(anthracen-2-ylethynyl)thiophen-2-yl)-2,5-bis(2-
hexyldecyl)pyrrolo
ACHUTGTNRENN[GU 3,4-c]pyrrole-1,4ACTHUNGTRENNNUG
3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(2-hexyldecyl)pyrroloAHCTNUGTRENNNUG
1,4(2H,5H)-dione (0.500 g, 0.6 mmol), 2-ethynylanthracene (0.245 g,
ACHTUNGTRENNUNG
1.2 mmol), copper iodide (0.008 g, 0.04 mmol), diisopropylamine
(5.0 mL), and toluene (20.0 mL) were mixed in a three-necked flask and
7
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Chan Eon Park, Yun-Hi Kim et al.
degassed with nitrogen. Then, [Pd
(PPh3)4] (0.013 g, 0.001 mmol) was
calibrated at the National Renewable Energy Laboratory. External quan-
tum efficiency (EQE) spectra were recorded by using a photomodulation
spectroscopic set-up (model Merlin, Oriel), a calibrated Si UV detector,
and a SR570 low-noise current amplifier.
added and the mixture was heated at 608C for 8 h. The reaction was
quenched with water and the organic phase was extracted with CHCl3.
After drying over MgSO4, the liquid was concentrated under vacuum and
the residue was purified by column chromatography on a short silica gel
column (n-hexane/CHCl3, 3:1) to yield 3,6-bis(5-(anthracen-2-ylethy-
Hole-Mobility Measurements
nyl)thiophen-2-yl)-2,5-bis(2-hexyldecyl)pyrroloACTHUNTGRENNUG[3,4-c]pyrrole-1,4ACHTUNGTRNE(NUGN 2H,5H)-
The hole-only devices were fabricated with the configuration ITO/PE-
DOT:PSS/small-molecule:PC71BM/Au in an identical manner, except
that a 100 nm gold layer was deposited onto the polymer:PC71BM thin
film, instead of a LiF/Al layer. The Au layer was deposited at a low
speed (1 s) to avoid the penetration of Au atoms into the active layer.
Mobilities were extracted by fitting the current–voltage curves by using
the Mott–Curney relationship (space-charge-limited current), Equa-
tion (1), where J is the current density, L is the film thickness of the
active layer, mh is the hole mobility, er is the relative dielectric constant of
the transport medium, e0 is the permittivity of free space, V is the internal
voltage in the device, and V=VapplꢀVrꢀVbi, where Vappl is the voltage ap-
plied to the device, Vr is the voltage drop owing to contact resistance and
series resistance across the electrodes, and Vbi is the built-in voltage
owing to the relative difference in workfunctions of the two electrodes.
Vbi can be determined from the transition between the ohmic region and
the SCLC region.
dione as a dark purple powder in 75% yield. M.p. 2388C; 1H NMR
(300 MHz, CDCl3): d=8.97–8.96 (d, 2H), 8.42 (s, 4H), 8.27 (s, 2H), 8.04–
8.00 (m, 6H), 7.56–7.46 (m, 8H), 4.08–4.05 (d, 4H), 2.01 (s, 2H), 1.38–
1.27 (m, 48H), 0.88–0.84 ppm (t, 12H); 13C NMR (500 MHz, CDCl3): d=
161.18, 131.21, 131.00, 130.75, 130.73, 130.55, 130.33, 129.83, 129.55,
129.12, 129.10, 128.55, 128.20, 127.15, 126.33, 125.89, 124.53, 124.39,
124.18, 123.35 116.61, 108.91, 97.45, 88.19, 46.59, 37.90, 31.90, 31.85,
31.31, 30.09, 29.76, 29.51, 29.34, 26.31, 26.28, 22.77, 14.08 ppm; HRMS
(FAB): m/z calcd: 1148.6287 [M+H]+; found: 1148.7530; IR (KBr): n˜ =
ꢀ
ꢀ
ꢂ
3050 (aromatic C H), 2852, 2921 (aliphatic C H), 2186 (C C),
1665 cmꢀ1 (C=O).
General Characterization
1H NMR spectra were recorded on a Bruker AM-200 spectrometer;
13C NMR spectra were recorded on a Bruker Advance-300 spectrometer.
HRMS (EI) was performed on a LabRAM HR800 UV high-resolution
GCMS. MS (MALDI-TOF/TOF) was performed on a Voyager DE-STR
high-resolution 4800 TOF/TOF-MS. FTIR spectra were recorded on
a Bomen Michelson series FTIR spectrometer. TGA was performed on
a TA TGA 2100 thermogravimetric analyzer under a purified nitrogen at-
mosphere at a heating rate of 108Cminꢀ1. Differential scanning calorime-
try (DSC) was performed on a TA Instruments 2100 DSC under a nitro-
gen atmosphere. The sample was heated from 50 to 3008C at a rate of
108Cminꢀ1. UV/Vis absorption spectra were recorded on a Carry 5000
UV/Vis/NIR double-beam spectrophotometer. CV was performed on
a PowerLab/AD instrument model system in a 0.1m solution of tetrabu-
tylammonium hexafluorophospate (Bu4NPF6) in anhydrous MeCN as
a supporting electrolyte at a scan rate of 50 mVsꢀ1. A glassy carbon disk
(thicknessꢁ0.05 cm2) that was coated with a thin film of the small mole-
cule, a Ag/AgCl electrode, and Pt wire were used as the working elec-
trode, reference electrode, and counter electrode, respectively. Atomic
force microscopy (AFM) was performed in tapping mode on a Multimo-
de IIIa (Digital Instruments) to obtain surface images (surface area: 5ꢂ
5 mm2) of the small molecule:PC71BM blend films under ambient condi-
tions. TEM images were obtained on a HITACHI-7600 at 100 kV. The
small-moelcule:PC71BM layers floated from the water-soluble PE-
DOT:PSS substrate onto the surface of dematerialized water and were
“picked up” with 200-mesh copper TEM grids. X-ray spectroscopy was
9
8
V2
L3
ð1Þ
J ¼ e0ermh
Acknowledgements
This work was supported by a New and Renewable Energy grant from
the Korea Institute of Energy Technology Evaluation and Planning
(KETEP), funded by the Korea Government Ministry of Knowledge
Economy (No. 20123010010140).
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Received: March 12, 2014
Revised: April 14, 2014
Published online: && &&, 0000
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Chem. Asian J. 2014, 00, 0 – 0
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These are not the final page numbers! ÞÞ

FULL PAPER
Solar Cells
End of the road: Three new small mol-
ecules with identical p-spacers (acety-
lene), a central diketopyrrolopyrrole
(DPP) unit, and different aromatic
electron-donating end-groups (DPP-
A-Ph, DPP-A-Na, and DPP-A-An)
showed broad absorptions, low
HOMO levels, and high hole mobili-
ties. Solution-processed organic solar
cells based on DPP-A-An showed
power-conversion efficiencies as high
as 3.93%.
Yu Jin Kim, Jang Yeol Back,
Seul-Ong Kim, Chan-Woo Jeon,
Chan Eon Park,*
Yun-Hi Kim*
&&&& — &&&&
Efficient Diketopyrrolopyrrole-Based
Small-Molecule Bulk-Heterojunction
Solar Cells with Different Electron-
Donating End-Groups
&
&
10
Chem. Asian J. 2014, 00, 0 – 0
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!