Thieno[3,4-c]pyrrole-4,6-dione-based small molecules for highly efficient solution-processed organic solar cells

Article abstract of DOI:10.1002/asia.201301357

Two small molecules named BT-TPD and TBDT-TTPD with a thieno[3,4-c]pyrrole- 4,6-dione (TPD) unit were designed and synthesized for solution-processed bulk-heterojunction solar cells. Their thermal, electrochemical, optical, charge-transport, and photovoltaic characteristics were investigated. These compounds exhibit strong absorption at 460-560nm and low highest occupied molecular orbital levels (-5.36eV). Field-effect hole mobilities of these compounds are 1.7-7.7×10^-3cm²V^-1s ^-1. Small-molecule organic solar cells based on blends of these donor molecules and a acceptor display power conversion efficiencies as high as 4.62 under the illumination of AM 1.5G, 100mWcm^-2. Powering up: Two small molecules named BT-TPD and TBDT-TTPD with a thieno[3,4-c]pyrrole-4,6-dione (TPD) unit are designed and synthesized for solution-processed bulk-heterojunction solar cells. Small-molecule organic solar cells based on blends of these donor molecules and a phenyl-C₆₁-butyric acid methyl ester acceptor display power conversion efficiencies (η) as high as 4.62.

Full text of DOI:10.1002/asia.201301357

FULL PAPER
DOI: 10.1002/asia.201301357
ThienoACHTUNGTRENNUNG
Solution-Processed Organic Solar Cells
Jong-jin Ha,[b] Yu Jin Kim,[c] Jong-gwang Park,[b] Tae Kyu An,[c] Soon-Ki Kwon,*[b]
Chan Eon Park,*[c] and Yun-Hi Kim*[a]
Abstract: Two small molecules named
BT-TPD and TBDT-TTPD with
gated. These compounds exhibit strong
absorption at 460–560 nm and low
highest occupied molecular orbital
levels (ꢀ5.36 eV). Field-effect hole mo-
bilities of these compounds are 1.7–
a thieno
N
7.7ꢁ10ꢀ3 cm2 Vꢀ1 sꢀ1.
Small-molecule
unit were designed and synthesized for
solution-processed bulk-heterojunction
solar cells. Their thermal, electrochemi-
cal, optical, charge-transport, and pho-
tovoltaic characteristics were investi-
organic solar cells based on blends of
these donor molecules and a acceptor
display power conversion efficiencies
as high as 4.62% under the illumina-
tion of AM 1.5G, 100 mWcmꢀ2.
Keywords: bulk heterojunction
charge transfer · energy conversion ·
solar cells · small molecules
·
Introduction
parts. As such, the investigation into the design and devel-
opment of SMOSCs has not received as much interest as the
investigation into polymer solar cells. Therefore, various
techniques for polymer-based solar cells can be applied to
SMOSCs to increase their efficiency. Furthermore, research
on active-layer materials, especially donor materials, is es-
sential because these are a key factor for high PCEs of
small-molecule photovoltaic devices. To address this issue,
we designed new small molecules with simple and symmet-
ric D–A alternating structures (D=donor, A=acceptor)
Organic solar cells (OSCs) with bulk heterojunction (BHJ)
architectures have attracted significant interest as sources of
renewable energy, largely as a result of their advantages,
which include low cost, low weight, and high mechanical
flexibility.[1–4] Until recently, 8–9% power conversion effi-
ciencies (PCEs) for polymer-based solar cells have been
achieved by enormous efforts in active layer, device struc-
ture, and fabricating techniques.[5–7] Small-molecule organic
solar cells (SMOSCs) are emerging because of their easier
synthesis, less batch-to-batch variation, and more reproduci-
ble solar cell performances.[8–10] Consequently, peak photo-
voltaic efficiencies above 8% have been obtained by using
solution-processed small molecules.[11] However, their over-
all efficiency is low relative to that of their polymer counter-
based on thienoACTHUNTGRNEUNG[3,4-c]pyrrole-4,6-dione (TPD). We postulat-
ed that the planar character of TPD would be beneficial for
p-electron delocalization once incorporated into various
conjugated molecule units.[12,13] Its relatively strong electron-
withdrawing nature facilitates low-lying highest occupied
molecular orbital (HOMO) and lowest unoccupied molecu-
lar orbital (LUMO) energy levels, which are desired to in-
crease the open circuit voltage (Voc) in OSCs.[14] Finally,
alkyl side chains were introduced on the pyrrole ring to im-
prove the solubility and processability of the molecules.[15]
Ma et al. reported a PCE of 6.1% from an alternating TPD
copolymer by using benzodithiophene as the donor and thie-
nopyrrole-4,6-dione as the acceptor.[16] Tao and co-workers
achieved a PCE of 7.3% from an alternating copolymer by
using 4,4-bis(2-ethylhexyl)-dithieno[3,2-b:2ꢀ,3ꢀ-d]silole and
[a] Prof. Y.-H. Kim
Department of Chemistry & Research Institute of Natural Science
Gyeongsang National University
Jin-ju, 660-701 (Republic of Korea)
E-mail: ykim@gnu.ac.kr
[b] J.-j. Ha,+ J.-g. Park, Prof. S.-K. Kwon
School of Materials Science & Engineering and
Research Institute for Green Energy Convergence Technology
(REGET)
N-octylthienoACHTUNTRGNEUNG[3,4-c]pyrrole-4,6-dione (PDTSTPD) com-
Gyeongsang National University
Jin-ju, 660-701 (Republic of Korea)
E-mail: skwon@gnu.ac.kr
posed of dithienosilole and thienopyrrole-4,6-dione.[17] De-
spite their remarkable performances in polymeric solar cells,
TPD-containing small molecules are rarely employed for ap-
plications in organic photovoltaic devices. Herein, we report
the syntheses and BHJ solar-cell performances of TPD-con-
taining solution-processed small molecules. We designed two
compounds with distinct molecular structures. The first was
the (1,3-bis(5’-hexyl-2,2’-bithiophen-5-yl)-5-octyl-4H-thieno-
[c] Y. J. Kim,+ T. K. An, 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
[+] These authors contributed equally to this work.
AHCTUNGTREG[NNUN 3,4-c]pyrrole-4,6(5H)-dione (BT-TPD) small molecule con-
taining a D-A-D structural motif with a thiophene-capped
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301357.
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Yun-Hi Kim et al.
TPD as the core acceptor unit. The other was a 4-[5-(2-
ethyl-hexyl)-thiophen-2-yl]-8-[5-(2-ethyl-hexyl)-thiophen-2-
yl]-2,6-bis-11-[5-octyl-1,3-di-thiophen-2-yl-thieno[3,4-c]pyr-
AHCTUNGTRENNUNG
role-4,6-dionyl]-1,5-dithia-s-indacene (TBDT-TTPD) small
molecule bearing an A-D-A architecture, and it was com-
posed of a benzodithiophene (BDT) core donor unit and
thiophene-bridged TPD acceptor arm units. We hypothe-
sized that incorporation of the BDT donor unit as the core
in TBDT-TTPD would result in enhanced p conjugation, in-
creased absorption, and improved charge-transfer ability of
the small molecules as a consequence of its rigid charac-
ter.[18] These small molecule donors exhibited thermal stabil-
ity, excellent solubility in organic solvents, and strong optical
absorption properties in the near-infrared region.
Figure 1. TGA thermograms of BT-TPD and TBDT-TTPD.
Results and Discussion
The general synthetic routes to the monomers and small
molecules are outlined in Scheme 1. BT-TPD and TBDT-
TTPD were obtained as red and dark red solids, respective-
Scheme 1. Synthesis route to BT-TPD and TBDT-TTPD.
Figure 2. UV/Vis absorption spectra of BT-TPD and TBDT-TTPD: a) in
chlorobenzene solution with a concentration of 1ꢁ10ꢀ4 m and b) as spin-
coated films.
ly, by Stille coupling of 1,3-dibromo-5-octyl-4H-thienoACTHNUTRGNEU[GN 3,4-
c]pyrrole-4,6-(5H)-dione with 5-trimethylstannyl-5’-hexyl-
2,2’-bithiophene and of 2,6-bis(trimethyltin)-4,8-bis[5-(2-eth-
ylhexyl)thiophene-2-yl]benzo[1,2-b:4,5]dithiophene with 1-
(5-bromothiophen-2-yl)-5-octyl-3-(thiophen-2-yl)-4H-thieno-
firmed that both compounds are highly stable and suitable
for OSC fabrication and evaluation.
The optical properties of the chlorobenzene solutions and
thin films of small molecules were investigated by UV/Vis
absorption spectroscopy, as shown in Figure 2 and summar-
ized in Table 1. In chlorobenzene (1ꢁ10ꢀ4 m, Figure 2a), the
ACHTUNGTRENNUNG[3,4c]pyrrole-4,6-(5H)-dione, respectively. The structures
and purities of BT-TPD and TBDT-TTPD were confirmed
1
by H NMR spectroscopy and mass spectrometry. The small
BT-TPD solution showed a main absorption band at
molecules show good solubility at room temperature in or-
ganic solvents such as o-dichlorobenzene, chloroform, chlor-
obenzene, and THF. The thermal properties of BT-TPD and
TBDT-TTPD were investigated by thermogravimetric analy-
sis (TGA) (Figure 1). TGA revealed onset decomposition
temperatures (Td) of BT-TPD and TBDT-TTPD with 5%
weight loss of 442 and 4578C, respectively. This result con-
463 nm, whereas the main absorption band for TBDT-
TTPD was redshifted to 488 nm and contained a shoulder at
517 nm as a result of the higher degree of intramolecular
charge-transfer transitions (ICTs) resulting from the core
BDT units.[19] Moreover, the molar extinction coefficient
(emax) of TBDT-TTPD (24120 Lmꢀ1 cmꢀ1) appears to be
larger than that of BT-TPD (15780 Lmꢀ1 cmꢀ1), which indi-
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Yun-Hi Kim et al.
Table 1. Photophysical properties of small-molecule compounds.
level for both BT-TPD and
TBDT-TTPD was estimated to
be ꢀ5.36 eV (Table 1) by using
the ferrocene reference value
of ꢀ4.4 eV below the vacuum
level.[24] The LUMO levels
were determined to be ꢀ3.17
Small molecule
Td
emax/103
[mꢀ1 cmꢀ1
lmax
lmax
Eogpt
HOMO
[eV]
LUMO[eV]
[b]
[8C][a]
G
]
[nm] solution
[nm] film
[eV][c]
BT-TPD
TBDT-TTPD
442
457
15.780
24.120
463
488
448
2.19
1.97
ꢀ5.36
ꢀ5.36
ꢀ3.17
ꢀ3.39
524, 570
cates that TBDT-TTPD can absorb more photons.[20] In
comparison to their absorption spectra in dilute solution,
the high-energy band (lmax =448 nm) observed for BT-TPD
in a thin solid film is slightly blueshifted and broadened to
566 nm. This is likely the result of various aggregation pat-
terns of the BT-TPD molecules by combination of H- and J-
types.[21,22] The absorption spectrum of TBDT-TTPD as
a thin film is bathochromically shifted and exhibits absorp-
tion bands at 524 and 570 nm, which thus indicates effective
p–p stacking between the backbones of the molecules.[23]
The optical band gaps of BT-TPD and TBDT-TTPD were
estimated from the onsets to be 2.19 and 1.97 eV, respective-
ly. This indicates that TBDT-TTPD with long push–pull in-
teraction has a stronger ICT effect than BT-TPD, and this
leads to a smaller band gap, which is useful for increasing
the short circuit current in SMOSCs. Cyclic voltammetry
(CV) was employed to investigate the oxidation behaviors
of BT-TPD and TBDT-TTPD by using a platinum counter
electrode in an acetonitrile solution containing 0.1 molLꢀ1
Bu4NPF6 with a scan rate of 50 mVsꢀ1 (Figure 3a). The
onset oxidation potential for both small molecules was
0.96 V versus Ag/Ag+. The corresponding HOMO energy
and ꢀ3.39 eV, respectively, by using the calculated HOMO
value and the optical band gaps. All synthesized small mole-
cules not only have suitable HOMO energy levels to ensure
a high value of Voc (>0.8 V) in small-molecule solar cell ap-
plications, but their LUMO energies are approximately
0.9 eV above the energy levels of the LUMOs (ꢀ4.3 eV) of
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) n-type
acceptors, which generates a driving force for energetically
favorable electron transfer (see Figure 3b).[25,26]
High hole mobility (mh) is a basic requirement for effec-
tive photovoltaic active donors to ensure effective charge-
carrier transport to the electrodes and to reduce photocur-
rent loss in OSCs. To measure the hole mobility of the small
molecules, organic field effect transistor (OFET) devices
with a top-contact configuration were fabricated onto octyl-
trichlorosilane (OTS-8)-modified Si/SiO2 substrates. As
shown in Figure 4 and Table 2, OFET devices composed of
BT-TPD and TBDT-TTPD exhibited p-channel transfer
characteristics.
Figure 4. Typical transfer curves for BT-TPD and TBDT-TTPD devices.
Table 2. Device performance of solution-processed OFETs on BT-TPD
and TBDT-TTPD.
Material
Mobility
I
on/Ioff
Vth
[V]
SS
[cm2 Vꢀ1 sꢀ1
]
[Vdecadeꢀ1
]
BT-TPD
TBDT-TTPD
1.72ꢁ10ꢀ3
7.71ꢁ10ꢀ3
3.08ꢁ105
2.11ꢁ107
ꢀ12.7
ꢀ5.28
0.558
0.536
The field-effect hole mobility of BT-TPD was measured
to be 1.72ꢁ10ꢀ3 cm2 Vꢀ1 sꢀ1, and a slightly higher hole mobi-
lity of 7.71ꢁ10ꢀ3 cm2 Vꢀ1 sꢀ1 was observed in the TBDT-
TTPD-based device. This is a considerably high hole mobili-
ty for solution-processed small-molecule donors.[27,28] The
higher hole mobility of TBDT-TTPD-based devices relative
to that of BT-TPD-based devices may be due to the shorter
Figure 3. a) Cyclic voltammograms of BT-TPD and TBDT-TTPD as thin
films and b) the HOMO and LUMO energy levels of the small molecules
and PCBMs.
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p–p stacking distances and the better quality of the films on
OTS-treated Si/SiO2 substrates (see Figure S11, Supporting
Information), which leads to enhanced charge-transporting
properties.[29] To demonstrate the reliability of those values,
we also tested the hole mobility of the small molecule in
blended films by using the space charge limited current
(SCLC)ACHTUNGTRENNUNG( model (see the Supporting Information). The hole
mobility trends were similarly shown for field-effect hole
mobility and hole mobility in blended films. The hole mobi-
lity (mh) of BT-TPD was measured to be 9.28ꢁ
10ꢀ9 cm2 Vꢀ1 sꢀ1 for blends of BT-TPD/PC71BM (PC71BM=
[6,6]-phenyl-C71-butyric acid methyl ester)(, and a higher
hole mobility of 8.87ꢁ10ꢀ8 cm2 Vꢀ1 sꢀ1 was exhibited in the
TBDT-TTPD/PC61BM film. The difference in the hole mobi-
lity values is one order of magnitude, which also indicates
that the hole-transporting abilities of TBDT-TTPD enable
high hole mobility in blended films.[30] The photovoltaic
characteristics of small molecules were investigated by using
the conventional structure of ITO/PEDOT/PSS/BT-TPD
and TBDT-TTPD/PCBM/LiF/Al . To optimize the photovol-
taic conditions, devices with different small molecule/accept-
or weight ratios, fullerene derivative (PC61BM and
PC71BM), solvent, and thermal annealing (from 100 to
1608C) were examined (detailed photovoltaic data are pre-
sented in the Supporting Information). First, the blend ratio
was an important factor that influenced the device perfor-
mance; therefore, we carefully tuned the blend ratio from
1:0.4 to 1:2 (w/w), A weight ratio of 1:2 for the active layers
displayed the best performance for BT-TPD/PC61BM,
whereas the device having a weight ratio of 1:1 TBDT-
TTPD/PC61BM displayed optimal performance properties.
Devices using a different solvent (i.e., chloroform) and full-
erene (i.e., PC71BM) were also fabricated and compared at
the optimized blend weight ratios of BT-TPD/PC61BM and
TBDT-TTPD/PC61BM. The results demonstrated that the
best processing conditions involved the use of chloroben-
zene as the solvent for both BT-TPD- and TBDT-TTPD-
based devices. However, after changing the electron accept-
or from PC61BM to PC71BM, a higher PCE was obtained for
BT-TPD, whereas a lower PCE was obtained for TBDT-
TTPD. Finally, post-treatment with thermal annealing was
investigated to further enhance device performance. Ther-
mal annealing has been proven to be an efficient and easy
treatment to improve the morphology of active films.
Herein, we introduced an optimal annealing temperature,
and enhanced device performance was observed. Figure 5a
shows the key photocurrent density voltage curves under il-
lumination of AM 1.5G at 100 mWcmꢀ2, and the corre-
sponding performance parameters are summarized in
Table 3.
Figure 5. a) Current–voltage characteristics of BHJ devices based on BT-
TPD/PC71BM (1:2 w/w) (c) and TBDT-TTPD /PC61BM (1:1 w/w)
(c) under illumination of AM 1.5 G, 100 mWcmꢀ2. b) EQE spectra of
the corresponding devices.
Table 3. Device performance parameters for BHJ solar cells based on
BT-TPD and TBDT-TTPD.
Small molecule / PCBM (w/ Conditions Voc Jsc
FF PCE
w)
[V]
ACTHNUTRGNEUGN ] [%] [%]
BT-TPD/PC71BM (1:2)
BT-TPD/PC71BM (1:2)
as-cast
annealed
160
0.94 5.3
0.84 6.2
23.1 1.16
27.5 1.43
TBDT-TTPD/PC61BM (1:1) as-cast
TBDT-TTPD/PC61BM (1:1) annealed
120
0.87 7.9
0.97 9.1
56.6 3.90
52.0 4.62
4.62% for BT-TPD and TBDT-TTPD at 160 and 1208C, re-
spectively. The change in the PCE was mainly due to the im-
proved short circuit current density (Jsc)(from 5.3 to
6.2 mWcmꢀ2 for BT-TPD and from 7.9 to 9.1 mWcmꢀ2 for
TBDT-TTPD) and change properties such as film morphol-
ogy and nanostructure order, which will be discussed below.
Relative to that observed in the TBDT-TTPD-based devi-
ces, the smaller value of Jsc resulting from the slightly larger
band gap, smaller hole mobility, and smaller fill factor (FF)
translated into a lower PCE in the BT-TPD-based solar cell
devices. The external quantum efficiency (EQE) spectra of
these devices are shown in Figure 5b. The shapes of the
EQE curves for the devices based on small-molecule/PCBM
are similar to those of their absorption spectra, and this indi-
cates that most of the absorbed photons of the small mole-
All devices achieved relatively high values of Voc above
0.8 V, which is consistent with a low HOMO energy level
(ꢀ5.36 eV).[31] The BT-TPD and TBDT-TTPD small mole-
cules cast from chlorobenzene without thermal annealing
treatment showed a PCE of 1.16 and 3.90%, respectively.
However, device performance dramatically changed after
post-thermal annealing. We obtained a PCE of 1.43 and
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cules contribute to the generation of the photocurrent. The
higher and broader coverage of the EQE for both the an-
nealed BT-TPD/PC71BM (1:2 w/w) and TBDT-TTPD/
PC61BM (1:1 w/w) devices led to considerable increases in
the Jsc values.[32] The BT-TPD/PC71BM device exhibited
a maximum EQE of approximately 40% at 413 nm, whereas
the maximum EQE for the TBDT-TTPD/PC61BM-based
device was approximately 50% at 430 nm. These results sup-
port that the photocurrent in the TBDT-TTPD-based cell is
1.5 times higher than that in the BT-TPD/PC71BM-based
cell. As the performances of the solar cell devices were
closely associated with the nanomorphology of the active
layer, we investigated the film morphology of the as-cast
and annealed blended films by tapping mode atomic force
microscopy (AFM), as shown in Figure 6. For the BT-TPD/
roughness (from 0.52 to 0.85 nm) were observed relative to
that observed in the as-cast TBDT-TTPD/PC61BM film. The
higher aggregation and the larger roughness of the annealed
TBDT-TTPD/PC61BM film are indicative of a more highly
ordered structure, which indicates more efficient charge sep-
aration in the OSCs. (The FF value in the TBDT-TTPD/
PC61BM annealed film decreases as a result of an increase
in the rms roughness.) A film surface with higher roughness
and aggregation is also beneficial to internal light scattering
and enhances light absorption.[36] All of this results in
a higher Jsc value and increased efficiency of the annealed
TBDT-TTPD device relative to the as-cast TBDT-TTPD
device. The increase in the PCE after annealing in conjuga-
tion with the AFM results suggests that thermal annealing
causes a change in the distribution and/or a reorganization
of the components in the blend. To probe the distribution of
small-molecule/PCBM blended films in the bulk before and
after thermal annealing, 2D grazing incidence X-ray diffrac-
tion (GIXD) analysis measurements were performed. The
results of these experiments are shown in Figure 7.
Figure 6. AFM height images of a) as-cast BT-TPD/PC71BM, b) BT-TPD/
PC71BM annealed at 1608C with a blend ratio of 1:2 w/w, c) as-cast
TBDT-TTPD/PC61BM, d) TBDT-TTPD/PC61BM annealed at 1208C with
a blend ratio of 1:1 w/w.
Figure 7. GIXD patterns for blended BT-TPD/PC71BM films a) as-cast
and b) annealed at 1608C; c) TBDT-TTPD/PC61BM as-cast film;
d) TBDT-TTPD/PC61BM film annealed at 1208C for 10 min.
PC71BM film with a blend ratio of 1:2 and annealed at
1608C, a root-mean-square (rms) roughness (1.64 nm) small-
er than that found for the as-cast BT-TPD/PC71BM film was
observed, as was a clearly ordered network, which could ex-
plain the higher FF and photocurrent values. The relatively
smooth active layer surface could reduce the charge recom-
bination by evenly distributed morphological features as
well as defect-free contact with the metal cathode, which
would lead to increased FF values.[33,34]
A network of the small-molecule crystals is formed with
the ordered structure, which acts as an effective charge
transporter. The formation of a crystal network therefore
enhances charge transport, which thereby increases the cur-
rent density.[35] In the TBDT-TTPD/PC61BM (1:1 w/w) an-
nealed film at 1208C, increased phase aggregation and
To obtain GIXD patterns of blended films, small-mole-
cule/PCBM films were prepared on a Si/PEDOT/PSS sub-
strate by the spin-coating method. As shown in Figure 7, for
the BT-TPD/PC71BM as-cast film, no distinct diffraction
peaks can be assigned to BT-TPD, which indicates that the
majority of the small-molecule fraction has entered an
amorphous state.[37] Upon heating a BT-TPD/PC71BM film
to 1608C, however, at which point BT-TPD starts to self-as-
semble, the intensity of the reflection dramatically increased
with random orientation. In the as-cast TBDT-TTPD/
PC61BM film, a distinct reflection was detected in the out-of
plane direction at qz =0.375 ꢃꢀ1 (d=1.67 nm), whereas
a strong second-order peak was observed along the nominal
direction in the film annealed at 1208C. This implies the im-
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proved order in the crystallites.[38] This result suggests that
Experimental Section
the improvement in the PCE of the device observed upon
thermally annealing may be ultimately attributed to substan-
tially ordered donor domains. Comparison of the ordering
structures of BT-TPD and TBDT-TTPD was also performed
by using out-of-plane and in-plane X-ray diffraction (XRD,
Figure S18). In both the BT-TPD/PC71BM and TBDT-
TTPD/PC61BM blend films, the diffraction patterns for out-
of-plane and in-plane directions were consistent with the
2D-GIXD results. All two-blend films were shown to have
stronger crystalline diffraction peaks and a longer range or-
dering in the annealed films. For the BT-TPD/PC71BM
blend film, the lamellar (2q=2.998) and weak p–p stacking
(2q=13.378) peaks appear in out-of-plane and in-plane pat-
terns, respectively. The corresponding interlayer d spacing
and p–p intermolecular stacking distance were 20.53 and
4.60 ꢃ, respectively. In the TBDT-TTPD/PC61BM film, crys-
talline diffraction peaks (2q=3.488, d spacing=17.63 ꢃ)
with second-order diffraction and a strong p–p in-plane
peak (2q=17.458, p–p stacking distance=3.53 ꢃ) were ob-
served. These findings indicate that the TBDT-TTPD film is
composed of higher-order TBDT-TTPD crystallites and that
is has a stronger packing structure with a shorter intermolec-
ular distance.[39] The dominant long-range ordering and
shorter p–p stacking of TBDT-TTPD in the blend film
allows efficient charge transport, which would explain why
devices based on TBDT-TTPD showed higher hole mobility
and an almost twofold higher PCE than those based on the
less-ordered BT-TPD structure.
General methods
All chemicals were purchased from Aldrich and Alpha: DMF, THF,
NBS, nBuLi, toluene Pd(PPh3)2Cl2, and trimethyltin chloride were used
ACHTUNGTRENNUNG
without further purification. 1H NMR spectra were recorded by using
a BrukerAM-200 spectrometer. 13C NMR spectra were recorded with
a Bruker Advance-300 spectrometer. HRMS (EI) spectra were per-
formed with a high-resolution GC mass spectrometer with LabRAM
HR800 UV. MS (MALDI-TOF/TOF) spectra were performed with
a high-resolution MALDI-TOF/TOF mass spectrometer with a Voyager
DE-STR. Thermal analyses were performed with a TA TGA 2100 ther-
mogravimetric analyzer under
a nitrogen atmosphere at a rate of
108Cminꢀ1
.
UV/Vis absorption spectra were determined by using
a Carry 5000 UV/Vis–near-IR double-beam spectrophotometer. Cyclic
voltammetry (CV) was performed by using a PowerLab/AD instrument
model system in 0.1m solution of tetrabutylammonium hexafluorophos-
phate (Bu4NPF6) in anhydrous acetonitrile as supporting electrolyte at
a scan rate of 50 mVsꢀ1. A glassy carbon disk (ꢁ0.05 cm2) coated with
a thin small molecule film, an Ag/AgNO3 electrode, and a platinum wire
were used as the working electrode, reference electrode, and counter
electrode, respectively. An atomic force microscope (Multimode IIIa,
Digital Instruments) was used to obtain surface images of the active
layer blended small molecules with PCBM. 2D Grazing incident X-ray
diffraction (GIXD) was performed for structural analysis at the 9 A
beamline at the Pohang Accelerator Laboratory (PAL). The measure-
ments were performed with a sample-to-detector distance of 231.761 mm.
Data were typically collected for 10 s by using an X-ray radiation source
of l=1.1189 nm with a 2D charge-coupled detector (CCD) (Roper Sci-
entific, Trenton, NJ, USA). The samples were mounted on a home-built z
axis goniometer equipped with a vacuum chamber. The incidence angle
ai for the X-ray beam was set to 0.178, which was intermediate between
the critical angles of the films and the substrate (ac,f and ac,s). X-ray dif-
fraction (XRD) was also performed by using the 5 A beamline (wave-
length=1.071 ꢃ) at the PAL.
OFET device fabrication and characterization
Conclusions
Top-contact organic field-effect transistors (OFETs) composed of BT-
TPD and TBDT-TTPD were fabricated on a common gate of highly n-
doped silicon with a 300 nm thick thermally grown SiO2 dielectric layer.
Before substrates were modified with octyltrichlorosilane (OTS-8), a pira-
nha solution was used to clean the organic residues of silicon substrates;
the surfaces were ozone treated for 20 min. Cleaned substrates were
modified with hydrophobic OTS-8 from a toluene solution for 60 min at
room temperature, and films of semiconducting materials were spin
coated at 6000 rpm from 0.7 wt% toluene solutions with a thickness of
50 nm. Gold source and drain electrodes of the OFETs based on BT-
TPD and TBDT-TTPD were evaporated on top of the semiconductor
layers (80 nm). For all measurements, we used channel lengths of 50 mm
and channel widths of 2000 mm. The electrical characteristics of the
OFETs were measured in air by using both Keithley 2400 and 236
source/measure units.
In conclusion, two novel small molecules, that is, BT-TPD
and TBDT-TTPD, bearing TPD units as acceptor building
blocks were designed and synthesized. The two small mole-
cules show excellent thermal stability, strong absorption, rel-
atively low HOMO levels, and high hole mobility. SMOSCs
based on the blend of small-molecule/PCBM (1:1 or 1:2 w/
w) exhibit PCEs of 1.16–3.90% without any post-treatment.
Increased PCEs were achieved for both BT-TPD- and
TBDT-TTPD-based devices after thermal annealing, and
the highest PCE of 4.6% with a notable Voc of 0.97 V, Jsc of
9.1 mAcmꢀ2, and FF of 52.0% was obtained from the
TBDT-TTPD-based solar cell. The PCE of >4.6% is among
the highest reported for fully solution-processed photovolta-
ic devices based on the TPD motif. This work demonstrates
that these small molecules based on the TPD moiety offer
a good strategy for the development of novel photovoltaic
small-molecule-based donor materials with high values of Jsc
and Voc for high-efficiency OSCs.
Fabrication and characterization of the photovoltaic cells
The organic photovoltaic devices were fabricated on prepatterned ITO
(indium tin oxide) glass substrates with the structure of glass/ITO/
PEDOT/PSS/small molecule/PCBM/LiF/Al (a 0.09 cm2 active layer). The
ITO-coated glass substrates were first cleaned with detergent, deionized
water, acetone, and isopropyl alcohol by ultrasonication. UV/ozone treat-
ment of the cleaned ITO substrates was then performed at room temper-
ature for 30 min. PEDOT/PSS (Bay P VP AI 4083, Bayer AG) was spin
coated at 4000 rpm for 60 s onto the ITO glass with a thickness of 30–
40 nm after filtration by using a 0.45 mm PVDF filter. A blend of BT-
TPD or TBDT-TTPD and PC71BM or PC61BM with different weight
ratios (from 1:0.4 to 1:2 w/w) was solubilized overnight in chlorobenzene
or chloroform at a total concentration of 40 mgmLꢀ1. The blended solu-
tions were spin coated at 1200 rpm for 60 s (thicknessꢁ70–90 nm) on the
top of the PEDOT/PSS layer and annealed at various temperatures for
Chem. Asian J. 2014, 9, 1045 – 1053
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Yun-Hi Kim et al.
15 min on a hot plate in the glove box. The LiF and Al cathodes were
thermally deposited to thickness of 0.8 and 100 nm, respectively, onto the
surface of the active layer. The current versus voltage (J–V) characteris-
tics of each device without encapsulation were measured in the dark and
under an ambient condition AM 1.5G illumination of 100 mVcmꢀ2 pro-
duced by an Oriel 1 KW solar simulator with respect to a reference cell
PVM 132 calibrated at the National Renewable Energy Laboratory. The
external quantum efficiency (EQE) spectra were obtained by using a pho-
tomodulation spectroscopic setup (model Merlin, Oriel), a calibrated Si
UV detector, and a SR570 low-noise current amplifier.
of the solvents. Distillation at 1408C under high vacuum (10–2 torr) gave
2b (2.87 g, 6.96 mmol, 58%) as a colorless liquid. 1H NMR (300 MHz,
CDCl3): d=7.28 (d, 1H), 7.14 (d, 1H), 7.01 (d, 1H), 6.71 (d, 1H), 2.83 (t,
2H), 1.70 (m, 2H), 1.37 (m, 6H), 0.95 (t, 3H), 0.43 ppm (t, 9H).
5-Octyl-1,3-di(thiophen-2-yl)-4H-thienoACHTNUGTRNEUNG[3,4-c]pyrrole-4,6(5H)-dione (3)
Compound 1b (7.50 g, 17.73 mmol) and trimethyl(thiophen-2-yl)stannane
(16.53 g, 44.32 mmol) were dissolved in toluene (300 mL) in a pressure
tube. The solution was degassed by a nitrogen flow for 30 min. Then, Pd-
AHCTUNGTRENN(GNU PPh3)2Cl2 (0.37 g, 0.53 mmol) was added into the solution. The tube was
Hole and electron mobility measurements
capped and heated to 1108C overnight. The mixture was extracted with
dichloromethane. The combined organic layer was dried with anhydrous
MgSO4. After removing the solvent, the residue was purified by column
chromatography (silica gel, hexane/dichloromethane=4:1). Recrystalliza-
tion (hexanes) afforded 3 (4.88 g, 11.34 mmol, 64%) as a yellow solid.
1H NMR (300 MHz, CDCl3): d=8.00 (s, 2H), 7.45 (s, 2H), 7.15 (t, 2H),
Hole- or electron-only devices were fabricated by using the ITO/
PEDOT/PSS/small molecule/PCBM/Au and Al/small molecule/PCBM/Al
architectures, respectively. The mobility was extracted by fitting the cur-
rent–voltage curves by using the Mott–Curney relationship [space charge
limited current, Eq. (1)]:
3.68ACTHNUTRGENU(GN t, 2H), 1.72 (m, 2H), 1.32 (m, 10H), 0.88 ppm (t, 3H).
9
V2
L3
ð1Þ
J ¼ e0ermh
8
1-(5-Bromothiophen-2-yl)-5-octyl-3-(thiophen-2-yl)-4H-thienoACTHNUTRGNE[NUG 3,4-
c]pyrrole-4,6(5H)-dione (4)
in which J is the current density, L is the film thickness of
the active layer, mh is the hole mobility, er is the relative die-
lectric constant of the transport medium, e0 is the permittivi-
ty of free space, V is the internal voltage in the device, and
V=VapplꢀVrꢀVbi, for which Vappl is the applied voltage to
the device, Vr is the voltage drop resulting from contact re-
sistance and series resistance across the electrodes, and Vbi
is the built-in voltage arising from the relative work function
difference of the two electrodes. The value of Vbi can be de-
termined from the transition between the ohmic region and
the SCLC region.
N-Bromosuccinimide(2.27 g, 12.79 mmol) was added dropwise over
a period of 60 min to a solution of 3 (5.50 g, 12.79 mmol) in DMF
(100 mL) and CHCl3 (100 mL). After 2 h, the reaction mixture was
poured into water and extracted three times with chloroform. The organ-
ic phase was combined and dried with anhydrous magnesium sulfate.
After removal of the solvent, the crude product was purified by column
chromatography (dichloromethane/hexane=1:4). Recrystallization (hex-
anes) afforded 4 (3.44 g, 6.78 mmol, 53%) as a yellow solid. 1H NMR
(300 MHz, DMSO): d=7.99 (s, 1H), 7.83 (s, 1H), 7.70 (t, 1H), 7.35 (d,
1H), 7.24
3H).
ACHUTGTNREN(NUG d, 1H), 3.51ACHUTNGTRENN(NGU t, 2H), 1.50 (s, 2H), 1.27 (m, 10H), 0.84 ppm (m,
Synthesis of the monomers and small molecules
4,8-Bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b;4,5-b’]dithiophene (5b)
5-Hexyl-2,2’-bithiophene
(2a),
4,8-dehydrobenzo[1,2-b:4,5-b’]dithio-
A dry, three-necked, 250 mL, nitrogen-purged flask was charged with
a mixture of ethylhexylthiophene (9.34 g, 47.67 mmol) in THF (100 mL).
At 08C, nBuLi (2.5m in hexane, 19.06 mL, 47.67 mmol) was added drop-
wise over 30 min. The mixture was then warmed to 508C and stirred for
2 h. Compound 5a (5.00 g, 22.70 mmol) was added to the reaction mix-
ture, which was then stirred for 1.5 h at 508C. After cooling the reaction
mixture to ambient temperature, SnCl2·2H2O (7.69 g, 34.05 mmol) in
HCl (10%, 10 mL) was added, and the mixture was stirred for an addi-
tional 2 h. It was subsequently poured into ice water and extracted with
diethyl ether. The combined extract was dried with anhydrous MgSO4
and then concentrated. The crude product was purified by column chro-
matography (silica gel, petroleum ether) to give pure 5b (9.07 g,
15.56 mmol, 69%) as a pale-yellow solid.1H NMR (300 MHz, CDCl3):
d=7.70 (d, 2H), 7.49 (d, 2H), 7.38 (d, 2H), 6.92 (d, 2H), 2.94 (d, 4H),
1.79 (m, 2H), 1.57–1.34 (br, 16H), 0.99–0.92 ppm (m, 12H).
phene-4,8-dione (5a), and 5-octyl-4H-thienoACTHNUTRGENN[UG 3,4-c]pyrrole-4,6HCATUNGTRENN(GUN 5H)-dione
(1a) were synthesized by following literature procedures.[40–42] A detailed
synthesis scheme is presented in the Supporting information.
1,3-Dibromo-5-octyl-4H-thienoACTHNUTRGNE[UNG 3,4-c]pyrrole-4,6(5H)-dione (1b)
Compound 1a (4.90 g, 18.49 mmol) was dissolved in concentrated sulfuric
acid (27.6 mL) and trifluoroacetic acid (92.4 mL). N-Bromosuccinimide
(9.87 g, 55.47 mmol) was added in one portion, and the reaction mixture
was stirred at room temperature overnight. The brown solution was then
diluted with water (500 mL) and extracted with dichloromethane. The or-
ganic phase was dried with anhydrous magnesium sulfate and concentrat-
ed to afford the crude product as orange crystals. Purification by column
chromatography (silica gel, dichloromethane/hexane=1:1) followed by
recrystallization (aqueous ethanol) gave 1b (6.41 g, 15.16 mmol, 82%) as
white crystals. M.p. 104–1058C (760 torr, 1 torr=0.13 kPa). 1H NMR
(300 MHz, CDCl3): d=3.60 (t, 2H), 1.62 (q, 2H), 1.31 (m, 10H),
0.89 ppm (t, 3H).
2,6-Bis(trimethyltin)-4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-
b’]dithiophene (5c)
In
a dry, two-necked, 50 mL, nitrogen-purged flask, 5b (1.00 g,
1.72 mmol) was dissolved in anhydrous THF (15 mL). The solution was
cooled to 08C, and a solution of nBuLi (2.5m in hexane, 1.38 mL,
3.45 mmol) was added dropwise with stirring. The reaction mixture was
then stirred for 2 h at room temperature. Next, the reaction mixture was
cooled to 08C and chlorotrimethylstannane (0.68 g, 3.45 mmol) was
added in one portion. The reaction mixture was stirred at 08C for 30 min
and then warmed to room temperature over 2 h. Subsequently, the reac-
tion mixture was quenched by the addition of distilled water (10 mL),
and then the mixture was extracted with diethyl ether. Finally, the com-
bined organic phase was dried with anhydrous MgSO4 and concentrated
to obtain a yellow viscous crude product. Further purification by recrys-
tallization (ethanol) afforded pure 5c (1.12 g, 72%) as a yellow solid.
5-Trimethylstannyl-5’-hexyl-2, 2’-bithiophene (2b)
A solution of nBuLi (2.5m in hexanes, 5.56 mL, 13.92 mmol) was added
dropwise over a period of 30 min to a solution of 2a (3.00 g, 12.00 mmol)
in THF (60 mL). The reaction mixture was stirred at 508C for 2 h and
then cooled down to ꢀ788C. The solution was stirred for 12 h. All vola-
tile components were removed under high vacuum, and the residue was
taken up in toluene (15 mL). A solution of trimethyltin chloride (2.98 g,
15.00 mmol) in THF (400 ml) was then added slowly. The mixture was
first stirred at ꢀ788C for 3 h and then for an additional 12 h at ambient
temperature. Quenching with aqueous NH4Cl solution and extraction of
the organic layer with ether gave a black oily material upon evaporation
Chem. Asian J. 2014, 9, 1045 – 1053
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Yun-Hi Kim et al.
1H NMR (300 MHz, CDCl3): d=7.70 (s, 2H), 7.32 (d, 2H), 6.91 (d, 2H),
2.90 (d, 4H), 1.56–1.37 (br, 18H), 0.97 (m, 12H), 0.50 ppm (t, 18H).
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4,6(5H)-dione (BT-TPD)
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Compounds 2 (1.97 g, 4.77 mmol) and 7 (1.01 g, 2.38 mmol) were dis-
solved in dry THF (40 mL), and the solution was degassed with N2 for
10 min. Then, PdACHTUNGTRENNUNG(PPh3)2L2 (0.05 g, 0.07 mmol) was added. The mixture
was stirred at 1008C overnight under a nitrogen atmosphere. The reac-
tion mixture was poured into water and extracted three times with
chloroform. The organic phase was combined and dried with anhydrous
magnesium sulfate. After removal of the solvent, the crude product was
purified by column chromatography (chloroform/hexane=1:1). Recrys-
tallization (hexanes) afforded BT-TPD (0.96 g, 1.26 mmol, 53%) as a red
1
solid. H NMR (300 MHz, CDCl3): d=7.92 (d, 2H), 7.11 (d, 4H), 6.73 (d,
2H), 3.68 (t, 2H), 2.82 (t, 4H), 1.68 (m, 6H), 1.43–1.25 (br, 22H),
0.89 ppm (t, 9H). 13C NMR (500 MHz, CDCl3): d=163.0, 147.4, 141.8,
136.3, 134.1, 131.1, 130.7, 128.5, 125.5, 125.0, 124.3, 39.0, 38.2, 32.1, 31.9,
31.8, 30.6, 29.6, 29.5, 29.1, 28.9, 27.3, 23.0, 22.9, 14.4 ppm. HRMS (EI): m/
z: calcd for C42H51NO2S5: 761.252 [M+]; found: 765.25.
4-[5-(2-Ethylhexyl)thiophen-2-yl]-8-[5-(2-ethylhexyl)thiophen-2-yl]-2,6-
bis-11-[5-octyl-1,3-dithiophen-2-yl-thieno
dithia-s-indacene (TBDT-TTPD)
ACHTUNGTRNE[NUNG 3,4-c]pyrrole-4,6-dionyl]-1,5-
Compounds 5 (0.89 g, 0.98 mmol) and 9 (1.00 g, 1.96 mmol) were dis-
solved in dry toluene (30 mL), and the solution was degassed with nitro-
gen for 10 min. Then, PdACTHNUTRGNEUNG(PPh3)2Cl2 (0.02 g, 0.03 mmol) was added. The
mixture was stirred at 1008C overnight under a nitrogen atmosphere.
The reaction mixture was poured into water and extracted three times
with chloroform. The organic phase was combined and dried with anhy-
drous magnesium sulfate. The residue was purified by column chroma-
tography (silica gel, hexane/dichloromethane=1:1), which was followed
by recrystallization (hexanes) to give TBDT-TTPD (0.49 g, 0.34 mmol,
35%) as a dark red solid. 1H NMR (300 MHz, CDCl3): d=8.01 (d, 2H),
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2H), 6.97 (d, 2H), 3.66 (t, 4H), 2.95 (t, 4H), 1.70 (br, 8H), 1.57–1.29 (br,
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162.6, 146.2, 140.6, 139.6, 137.4, 136.9, 135.9, 135.6, 132.9, 132.7, 132.2,
130.9, 130.2, 128.8, 128.7, 128.6, 128.4, 126.1, 125.9, 125.6, 123.6, 120.3,
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1432.364; found: 1432.36
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Acknowledgements
This work was supported by a New & Renewable Energy of the Korea
Institute of Energy Technology Evaluation and Planning (KETEP) grant
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