Electron-rich anthracene semiconductors containing triarylamine for solution-processed small-molecule organic solar cells

Article abstract of DOI:10.1002/cssc.201200242

New electron-rich anthracene derivatives containing triarylamine hole stabilizers, 2,6-bis[5,5'-bis(N,N'-diphenylaniline)-2,2'-bithiophen-5-yl]-9,10- bis-[(triisopropylsilyl)ethynyl]anthracene (TIPSAntBT-TPA) and 2,6-bis(5,5'-bis{4-[bis(9,9-dimethyl-9H-fluoren-2-yl)amino]phenyl}-2, 2'-bithiophen-5-yl)-9,10-bis-[(triisopropylsilyl)ethynyl]anthracene (TIPSAntBT-bisDMFA), linked with π-conjugated bithiophene bridges, were synthesized and their photovoltaic characteristics were investigated in solution-processed small-molecule organic solar cells (SMOSCs). These new materials exhibited superior intramolecular charge transfer from triarylamine to anthracene, leading to a more electron-rich anthracene core that facilitated electron transfer into phenyl-C₆₁-butyric acid methyl ester. Compared with TIPSAntBT and triarylamine, these materials show a threefold improvement in hole-transporting properties and better photovoltaic performance in solution-processed SMOSCs, with the best power conversion efficiency being 2.96 % at a high open-circuit voltage of 0.85 V. Digging deeper: New electron-rich anthracene derivatives containing a triarylamine hole stabilizer, TIPSAntBT-TPA and TIPSAntBT-bisDMFA, exhibit superior intramolecular charge-transfer performance, resulting in a boost in hole-transporting properties (see picture).

Full text of DOI:10.1002/cssc.201200242

DOI: 10.1002/cssc.201200242
Electron-Rich Anthracene Semiconductors Containing
Triarylamine for Solution-Processed Small-Molecule
Organic Solar Cells
Hyeju Choi,[a] Haye Min Ko,[a] Nara Cho,[a] Kihyung Song,[c] Jae Kwan Lee,*[b] and
Jaejung Ko*[a]
New electron-rich anthracene derivatives containing triaryl-
amine hole stabilizers, 2,6-bis[5,5’-bis(N,N’-diphenylaniline)-2,2’-
bithiophen-5-yl]-9,10-bis-[(triisopropylsilyl)ethynyl]anthracene
(TIPSAntBT–TPA) and 2,6-bis(5,5’-bis{4-[bis(9,9-dimethyl-9H-fluo-
ren-2-yl)amino]phenyl}-2,2’-bithiophen-5-yl)-9,10-bis-[(triisopro-
pylsilyl)ethynyl]anthracene (TIPSAntBT–bisDMFA), linked with
p-conjugated bithiophene bridges, were synthesized and their
photovoltaic characteristics were investigated in solution-pro-
cessed small-molecule organic solar cells (SMOSCs). These new
materials exhibited superior intramolecular charge transfer
from triarylamine to anthracene, leading to a more electron-
rich anthracene core that facilitated electron transfer into
phenyl-C61-butyric acid methyl ester. Compared with TIPSAntBT
and triarylamine, these materials show a threefold improve-
ment in hole-transporting properties and better photovoltaic
performance in solution-processed SMOSCs, with the best
power conversion efficiency being 2.96% at a high open-circuit
voltage of 0.85 V.
Introduction
Solution-processed organic solar cells (OSCs) fabricated by
means of versatile printing techniques, such as doctor blade,
inkjet, and roll-to-roll, offer a range of benefits that include low
cost and light weights. In recent years, efforts have focused on
improving device performance with the aim of achieving
a power conversion efficiency (PCE) of 10%. New photoactive
materials, such as p-conjugated (semiconducting) polymers
and fullerene derivatives, have been developed in combination
with functional layers that provide buffering, charge transport,
and optical spacing.[1] Recently, OSCs that promise PCEs above
8% have become strong candidates for the next generation of
solar cells, threatening to replace inorganic thin-film and dye-
sensitized solar cells (DSSCs).[2] The highest efficiencies have
been reported for OSCs fabricated with bulk-heterojunction
(BHJ) materials comprised of low band gap semiconducting
polymers and [6,6]-phenyl-C61(or 71)-butyric acid methyl ester
(PC61(or 71)BM).[3] However, these polymers suffer from the dis-
advantages of low reproducibility of certain characteristics (for
example, weight-average molecular weight and polydispersity
index) and difficulties in purification. Thus, small-molecule or-
ganic semiconductors appear to be more attractive from the
viewpoint of mass production for commercial applications, and
considerable research effort has been focused on developing
efficient small-molecule materials for improved device perfor-
mance, with the short-term goal being a PCE comparable to
that of polymer-based solar cells.[4] Indeed, recent break-
throughs in realizing PCEs of above 6% have placed solution-
processed small-molecule organic solar cells (SMOSCs) squarely
in competition with polymer-based cells.[5]
role, inspired by low band gap semiconducting polymers or
push–pull molecular structures in nonlinear optics and DSSCs
because of their superior optoelectronic properties. Recently,
we also reported various symmetric or asymmetric push–pull
chromophores for solution-processed SMOSCs.[6] These push–
pull structures in SMOSCs enable efficient intramolecular
charge transfer (ICT), giving the chromophore better molar ab-
sorptivity. In addition, an electron-donating group, such as tri-
phenylamine (TPA), can play an important role in stabilizing
holes separate from excitons and improving the transporting
property of the hole carrier.[4,6]
However, these small molecules have often exhibited poor
molecular packing and intermolecular interactions in spin-cast
BHJ composite films with PC71BM, resulting in low charge-carri-
er mobility. Therefore, we became interested in solution proc-
essable and highly crystallizable small molecules, such as pen-
tacene and anthracene derivatives, as an efficient core motif
[a] H. Choi, H. M. Ko, N. Cho, Prof. J. Ko
Department of New Materials Chemistry
Korea University, Chungnam, 330-700 (South Korea)
Fax: (+82)44-860-1331
E-mail: jko@korea.ac.kr
[b] Prof. J. K. Lee
Department of Chemistry Education
Chosun University, Gwangju, 501-759 (South Korea)
Fax: (+82)62-230-7319
E-mail: chemedujk@chosun.ac.kr
[c] Prof. K. Song
Department of Chemical Education
Korea National University of Education
Chungbuk, 333-791 (South Korea)
Efficient organic semiconductors used in SMOSCs often have
structural core motifs,[4] such as oligothiophene, bridged di-
thiophene, benzothiadiazole, squaraine, or diketopyrrolopyr-
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200242.
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J. K. Lee, J. Ko et al.
for organic semiconductors in SMOSCs.[7] These small-molecule
semiconductors show excellent electron- and hole-transporting
properties, with field-effect mobilities over 1 cm2 Vꢀ1 s.[8] In this
study, we tried to develop new small-molecule organic semi-
conductors based on the soluble and crystallizable anthracene
derivative 6,13-bis(triisopropylsilylethylnyl)anthracene (TIPSAnt)
for solution-processed SMOSCs. Recently, Chung et al. reported
an SMOSC with a PCE of 1.4%, with bithiophene substituted
for the TIPSAnt derivative and a PC71BM BHJ-active layer.[9]
Although the single crystalline structure showed a high mobili-
ty of 0.2 cm2 Vꢀ1 s in a field-effect transistor device,[8] the low
PCE in the SMOSC was mainly the result of a low photocurrent.
This could be due to it having insufficient structural strength
to stabilize the excitons conducted by absorbed light in the
BHJ-active layer with PC71BM, as well as its low molar
absorptivity.
Results and Discussion
Synthesis and characterization
The synthetic methods used for the new semiconductors are
outlined in Scheme 2. All reactions were carried out in a ni-
trogen atmosphere and 2,6-dibromo-9,10-bis[(triisopropylsilyl)-
ethynyl]anthracene (1), 5-bis(N,N’-diphenylanilinyl)-2,2’-bithio-
phene, and 5-bis{4-[bis(9,9-dimethyl-9H-fluoren-2-yl)amino]-
phenyl}-2,2’-bithiophene were readily synthesized, according
to a procedure reported previously.[10]
The anthracene semiconductor TIPSAntBT–TPA was syn-
thesized by using a stepwise synthetic protocol. Consecutive
lithiation of 5-bis(N,N’-diphenylanilinyl)-2,2’-bithiophene with n-
butyllithium followed by the addition of 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane successfully produced 5-
bis(N,N’-diphenylanilinyl)-5’-(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)-2,2’-bithiophene (2). Then, a Suzuki coupling reaction
of 1 and 2 (1:2 molar ratio) successfully proceeded with mod-
erate yield, leading to the final product, TIPSAntBT–TPA.
TIPSAntBT–bisDMFA was synthesized by using the same syn-
thetic procedure with compound 3 instead of 2. The chemical
structures of the synthesized products, TIPSAntBT–TPA and
TIPSAntBT–bisDMFA, were verified by 1H and 13C NMR spectros-
copy and matrix-assisted laser desorption/ionization time of
flight (MALDI-TOF) mass spectrometry. These materials have
good solubility in common organic solvents, such as dichloro-
methane, chloroform, chlorobenzene, and toluene. These de-
tails are described further in the Supporting Information.
Herein, we report the synthesis and photovoltaic characteris-
tics of new electron-rich anthracene derivatives containing tri-
arylamine hole stabilizers, 2,6-bis[5,5’-bis(N,N’-diphenylaniline)-
2,2’-bithiophen-5-yl]-9,10-bis-[(triisopropylsilyl)ethynyl]anthra-
cene (TIPSAntBT–TPA) and 2,6-bis(5,5’-bis{4-[bis(9,9-dimethyl-
9H-fluoren-2-yl)amino]phenyl}-2,2’-bithiophen-5-yl)-9,10-bis-
[(triisopropylsilyl)ethynyl]anthracene (TIPSAntBT–bisDMFA), for
use as efficient solution-processed SMOSCs. These semiconduc-
tors were compared with the 2,6-di(2’-bithiophen-5-yl)-9,10-
bis-[(triisopropylsilyl)ethynyl]anthracene (TIPSAntBT) devices re-
ported previously and had a PCE of 2.96% with a high open-
circuit voltage of 0.85 V (Scheme 1).
In addition, two triarylamines of TPA and bisDMFA were
compared to investigate their role as electron donors and hole
stabilizers. Both of these newly synthesized materials,
TIPSAntBT–TPA and TIPSAntBT–bisDMFA, presented superior
ICT to anthracene through a p-conjugated bithiophene bridge,
producing a more electron-rich anthracene core that could fa-
cilitate electron transfer into PC71BM.
Optical properties and time-dependent DFT calculations
Figure 1 shows the UV/Vis absorption spectra of TIPSAntBT,
TIPSAntBT–TPA, and TIPSAntBT–bisDMFA in chlorobenzene and
as thin films. The corresponding optical properties are sum-
marized in Table 1.
Scheme 1. Molecular structures of the anthracene derivatives containing triarylamine and the device architecture of a solution-processed SMOSC.
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Electron-Rich Anthracene Semiconductors
Scheme 2. Synthesis of the new organic semiconductors TIPSAntBT–TPA and TIPSAntBT–bisDMFA.
al theory (TD-DFT) using the B3LYP functional/6-31G* basis set
might provide the information on these transition bands.
Figure 2 shows the calculated energy levels of these materials
in vacuo. The details are summarized in Table S1 in the Sup-
porting Information. The absorption bands observed at
a longer wavelength should result from ICT originating from
HOMO!LUMO monoexcitations, as shown in Figure 2.
Although TIPSAntBT was already reported by Chung et al.,[9]
there was little information about the four optical absorption
transitions observed in the 300–700 nm region. These absorp-
tion peaks could be assigned to the p–p* transition, which
predominantly originate from HOMO!LUMO+1, HOMOꢀ2!
LUMO, and HOMOꢀ1!LUMO transitions. The fourth peak, at
a longer wavelength, should result from ICT between the bi-
thiophene donor and the anthracene core.
Figure 1. UV/Vis absorption spectra of TIPSAntBT (black), TIPSAntBT–TPA
(blue), and TIPSAntBT–bisDMFA (red) in chlorobenzene (solid lines) and as
thin films (dashed lines).
As shown in Figure 1, the transition absorption bands of
TIPSAntBT (black), TIPSAntBT–TPA (blue), and TIPSAntBT–
bisDMFA (red) in cholorobenzene (solid line) were observed in
the wavelength region of 300–700 nm of the absorption spec-
tra. Molecular calculations by time-dependent density function-
On the other hand, TIPSAntBT–TPA and TIPSAntBT–bisDMFA
modified with triarylamine exhibited redshifted absorption
bands of 14 and 26 nm and molar absorptivities enhanced 3.2
and 3.6 times, respectively, relative to their unmodified coun-
terpart, TIPSAntBT. Their ICT bands exhibited high molar
absorption efficiencies
e
of
45626molꢀ1 cmꢀ1 at 528 nm and
51964molꢀ1 cmꢀ1 at 530 nm, re-
spectively. However, modifica-
tion with triarylamine alone
Table 1. Optical and electrochemical properties of the TIPSAntBT–TPA and TIPSAntBT–bisDMFA.
Compounds
lmax[a] [nm]
(e [molꢀ1 cmꢀ1])
lmax[b] [nm] Eonset, ox. [V]
/HOMO [eV][c] LUMO [eV][c]
Eonset, red. [V]/
DEonset [eV][d] Eonset [eV][e]
cannot
significantly
extend
TIPSAntBT[h]
TIPSAntBT–TPA
504 (14370)
528 (45626)
525
532
532
0.543/ꢀ5.343 1.590/ꢀ3.210 2.133
0.222/ꢀ5.022 1.348/ꢀ3.452 1.570
0.411/ꢀ5.211 1.381/ꢀ3.419 1.792
2.322
2.202
2.189
the p-conjugation length of
TIPSAntBT. A redshift of the ab-
sorption spectrum and a high
molar absorptivity are often ob-
served in molecular structures
with longer p conjugation. The
broadened transition band of
TIPSAntBT–TPA observed at 300–
500 nm might overlap with the
TIPSAntBT- bisDMFA 530 (51964)
[a] Absorption spectra were measured in chlorobenzene. [b] Absorption spectra were measured as solid-state
thin films. [c] Energy levels were calculated from oxidation/reduction onsets of cyclic voltammogram (CV) spec-
tra measured with a scan rate of 100 mVsꢀ1 in CH2Cl2 with 0.1m (n-C4H9)4NPF6 using a ferrocenium/ferrocene
(Fc) redox couple as an external reference. The HOMO and LUMO levels can be deduced from the oxidation
and reduction onsets, under the assumption that the energy level of Fc versus a normal hydrogen electrode
(NHE) is 4.8 eV below vacuum level. [d] DEonset was calculated by LUMOꢀHOMO. [e] Eonset was determined from
the onset of the absorption spectra. [g] These values were consistent with those reported in Ref. [9].
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when excited by light energy,
while TPA and bisDMFA can play
a key role in stabilizing holes
separate from excitons.
In
a
solid-state thin film,
the absorption spectrum of
TIPSAntBT
(Figure 1,
black
dashed line) showed
a
p–p*
transition band redshifted by
about 30 nm and broader
a
spectrum than that in solution,
indicating that its flattened
structure facilitated intermolecu-
lar packing interactions. Howev-
er, an interesting finding was
that films of TIPSAntBT–TPA
(Figure 1, blue dashed line) and
TIPSAntBT–bisDMFA
(Figure 1,
red dashed line) exhibited only
broader spectra with slightly
redshifted absorption bands.
Amorphous nonplanar TPA and
bisDMFA of TIPSAntBT–TPA and
TIPSAntBT–bisDMFA might inter-
rupt the intermolecular packing
interactions in solid-state films.
Nevertheless, TIPSAntBT–TPA (or
bisDMFA) exhibited about three-
Figure 2. Orbital energies of a) TIPSAntBT, b) TIPSAntBT–TPA, and c) TIPSAntBT–bisDMFA calculated by TD-DFT
using the B3LYP functional/6-31G* basis set.
transition bands of HOMO!LUMO+1 and HOMOꢀ2!LUMO
monoexcitations, whereas the transition at 450–600 nm seems
to originate from HOMO!LUMO excitation (f=1.044) with
minor HOMOꢀ1!LUMO monoexcitations (f=0.002), showing
very intense oscillator strengths (f) for the transitions. The ab-
sorption band of TIPSAntBT–bisDMFA exhibited similar transi-
tion characteristics (see the Table S1 in the Supporting Infor-
mation). From these results, we note that the presence of
more effective ICT from triarylamine to the anthracene core in
TIPSAntBT–TPA and TIPSAntBT–bisDMFA leads to redshifted
absorption and increased molar absorptivity, and bisDMFA
shows a slightly better electron-donating capability than that
of TPA.
To clarify the interpretation of these results, a molecular cal-
Figure 3. Isodensity surface plots of TIPSAntBT, TIPSAntBT–TPA, and TIP-
SAntBT–bisDMFA calculated by TD-DFT using the B3LYP functional/6-31G*
basis set
culation analysis of TIPSAntBT, TIPSAntBT–TPA, and TIPSAntBT–
bisDMFA was performed. Figure 3 shows the optimized struc-
tures of these materials calculated by means of TD-DFT using
the B3LYP functional/6-31G* basis set. The orbital densities of
the HOMO and LUMO of TIPSAntBT were observed for the p
conjugation of anthracene–dithiophene and the anthracene
core, respectively. The orbital density of the HOMO of
TIPSAntBT–TPA (or bisDMFA) was evenly distributed on anthra-
cene–dithiophene–TPA (or bisDMFA), but the orbital density of
the LUMO of TIPSAntBT–TPA (or bisDMFA) was predominantly
located on anthracene–dithiophene. This calculation reveals
that the ICT from triarylamine to the anthracene core can
occur effectively in TIPSAntBT–TPA and TIPSAntBT–bisDMFA
fold higher hole mobility than that of TIPSAntBT. These results
are discussed in detail below.
Electrochemical properties
Figure 4 shows cyclic voltammograms of TIPSAntBT,
TIPSAntBT–TPA, and TIPSAntBT–bisDMFA in dichloromethane.
The corresponding electrochemical properties are also sum-
marized in Table 1. The energy levels of the HOMOs and
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Figure 4. Electrochemical characterization of TIPSAntBT, TIPSAntBT–TPA, and
TIPSAntBT–bisDMFA in dichloromethane/tetrabutylammonium hexafluoro-
phosphate (TBAFP6; 0.1m) with a scan speed of 100 mVsꢀ1; potentials are
given versus the external reference Fc/Fc+.
LUMOs of these materials were determined from these CV
spectra. A platinum rod electrode, a platinum wire, and an Ag/
AgNO3 (0.10m) electrode were used as the working electrode,
counter electrode, and reference electrode, respectively. The
analyses were performed in an electrolyte consisting of a 0.1m
solution of TBAPF6 in dichloromethane at room temperature
under nitrogen, with a scan rate of 100 mVsꢀ1, and an Fc/Fc+
redox couple was used as an external reference. The HOMO
and LUMO levels can be deduced from the oxidation and re-
duction onsets, under the assumption that the energy level of
Fc versus NHE[11] is 4.8 eV below vacuum level. Cyclic voltam-
mograms of these materials in solid-state thin films could not
be recorded due to stripping of the film on the electrode.
Therefore, we determined the optical band gap by calculating
absorption thresholds from the absorption spectra of these
materials in chlorobenzene.
Figure 5. a) UV/Vis absorption spectra and b) space–charge limitation of J--V
characteristics of the TIPSAntBT–TPA/PC71BM (1:4; blue) and TIPSAntBT–
bisDMFA/PC71BM (1:4; red) BHJ films, the hole-only devices of which (indium
tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/TIPSAntBT–TPA (or bisDMFA):PC71BM/Au) were compared with
those of TIPSAntBT/PC71BM (1:1; black).
To investigate the space–charge effects, we extracted the
hole mobilities of TIPSAntBT–TPA and TIPSAntBT–bisDMFA
from the space–charge limitation of current (SCLC) J–V (current
density–voltage) characteristics obtained in the dark for hole-
only devices. The average hole mobilities were determined by
measuring 20 cells, each of which was measured three times
to compensate for errors in measurement. Compared with
TIPSAntBT, TIPSAntBT–TPA and TIPSAntBT–bisDMFA showed
improved efficiency at an 1:4 ratio with PC71BM, giving results
similar to those reported by Chung et al.[9] Figure 5b shows
the dark-current characteristics of ITO/PEDOT:PSS/Do-
nor:PC71BM/Au devices as a function of the bias, corrected by
the built-in voltage determined from the difference in work
function between Au and the HOMO level of TIPSAntBT–TPA
(or bisDMFA). Ohm’s law can be observed at low voltages as
an effect of thermal-free carriers. For the presence of carrier
traps in the active layer, there is a trap-filled-limit (TFL) region
between the ohmic and the trap-free SCLC regions. The SCLC
behavior in the trap-free region can be characterized by using
the Mott–Gurney square law [Eq. (1)].[13]
The HOMO levels of TIPSAntBT, TIPSAntBT–TPA, and
TIPSAntBT–bisDMFA, determined in solution by CV, were calcu-
lated to be 5.34, 5.02, and 5.21 eV, respectively. From these re-
sults, we can estimate that the Voc of devices fabricated by
using these materials and with PC71BM BHJ-active layers could
be in the range of 0.7–0.9 eV.[12] Indeed, the Voc values found in
experiments were consistent with these expected values.
Photophysical properties
Figure 5a shows the UV-visible absorption spectra of the
TIPSAntBT–TPA/PC71BM (1:4; blue) and TIPSAntBT–bisDMFA/
PC71BM (1:4; red) films, which exhibited the best performances
in a BHJ solar cell device. As shown in Figure 5a, the peaks in
the absorption bands of TIPSAntBT–TPA (or bisDMFA) in the
TIPSAntBT–TPA (or bisDMFA)/PC71BM BHJ composites deposit-
ed from chlorobenzene were redshifted by about 20 nm rela-
tive to those of their pristine films, indicating interference in in-
termolecular p–p packing interactions by PC71BM in the BHJ
film.
J ¼ ð9=8Þe mðV2=L3Þ
ð1Þ
in which e is the static dielectric constant of the medium
and m is the carrier mobility.
The hole mobilities of TIPSAntBT, TIPSAntBT–TPA, and
TIPSAntBT–bisDMFA, as evaluated by using Equation (1) (e=
3e0), were 3.32ꢁ10ꢀ6, 1.02ꢁ10ꢀ5, and 1.12ꢁ10ꢀ5 cm2 Vꢀ1 sꢀ1, re-
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J. K. Lee, J. Ko et al.
spectively. Although strong intermolecular packing interactions
of anthracene, for high hole mobility, was significantly inter-
rupted by amorphous nonplanar triarylamine, as shown in
Figure 5, the hole mobilities of TIPSAntBT–TPA and TIPSAntBT–
bisDMFA were about three times higher than that of flattened
TIPSAntBT without triarylamine. These results indicate that the
triarylamine group of TIPSAntBT–TPA (or bisDMFA) can effec-
tively stabilize the hole separated from the exciton in the BHJ
system with PC71BM to reduce charge recombination between
the donor and PC71BM, leading to an increase in the transport-
ing properties of the hole carrier.
Photovoltaic performances
The performances of TIPSAntBT–TPA and TIPSAntBT–bisDMFA
were investigated through the application of a photovoltaic
device fabricated with these materials on PC71BM BHJ films.
A study of the characteristics of over 300 solar cells revealed
that the most efficient photovoltaic cells were those obtained
from a BHJ system of TIPSAntBT–TPA (or TIPSAntBT–bisDMFA)
with PC71BM, which were optimized at a ratio of approximately
1:4. These BHJ films were cast on top of a PEDOT:PSS (Heraeus,
AI 4083) layer. The optimum thicknesses of the TIPSAntBT–
TPA/PC71BM and TIPSAntBT–bisDMFA/PC71BM BHJ films ob-
tained under these conditions were around 64 and 86 nm,
respectively.
Figure 6 shows the J–V curves under AM 1.5 illumination
conditions (100 mWcmꢀ2) and the incident-photon-to-current
efficiency (IPCE) spectra of TIPSAntBT–TPA (or bisDMFA)/
PC71BM BHJ solar cells fabricated under optimized processing
conditions, with 1-chloronaphtalene (CN) as the processing ad-
ditive[6a] (or TiOx, which can effectively act as an optical spacer
and buffer layer[14]). The corresponding values are summarized
in Table 2. The IPCE spectra of these devices, as shown in Fig-
ure 6b, exhibit curves that are well matched with their optical
absorptions, resulting in close correlation with their photocur-
rents in the J–V curves.
Figure 6. J–V curves (a) under AM 1.5 conditions (100 mWcmꢀ2) and IPCE
spectra (b) of TIPSAntBT–TPA (blue) or TIPSAntBT–bisDMFA (red)/PC71BM BHJ
solar cells fabricated under optimized processing conditions with 1% CN
processing additive with (c)/without (b) insertion of a TiOx layer or
without additive and TiOx layer (g).
As shown in Figure 6 and Table 2, the devices conventionally
fabricated with TIPSAntBT–bisDMFA/PC71BM and TIPSAntBT–
TPA/PC71BM exhibited a PCE of 0.96% (ꢁ0.12) with a short-cir-
cuit current density (Jsc) of 4.92 mAcmꢀ2, a fill factor (FF) of
0.33, and an open-circuit voltage (Voc) of 0.59 V, and a PCE of
0.81% (ꢁ0.11) with Jsc =4.57 mAcmꢀ2, FF=0.33, and Voc =
0.53 V, respectively. These values indicate an efficiency reduced
by 30–40% compared with that of TIPSAntBT without triaryl-
amine. This could be due to the inferior BHJ morphologies of
TIPSAntBT–bisDMFA/PC71BM and TIPSAntBT–TPA/PC71BM, as
shown in Figure 7. The processing additive approach with CN
clearly improved the performance of the devices because the
processing additive delivered better morphologies of
TIPSAntBT–bisDMFA/PC71BM and TIPSAntBT–TPA/PC71BM BHJ
film.[6a,14]
Table 2. Photovoltaic performances of the TIPSAntBT–TPA (or bisDMFA)/
PC71BM BHJ solar cells fabricated with various conditions.[a]
Donor
Treatment
Jsc
Voc
[V]
FF
Efficiency
[mAcmꢀ2
]
[%] [%]
TIPSAntBT[b]
TIPSAntBT–TPA
TIPSAntBT–TPA
TIPSAntBT–TPA
TIPSAntBT–bisDMFA none
TIPSAntBT–bisDMFA 1% CN
TIPSAntBT–bisDMFA 1% CN, TiOx 8.58
none
none
1% CN
1% CN, TiOx 6.07
4.25
4.57
5.19
0.85 38
0.53 33
0.56 32
0.72 40
0.59 33
0.77 36
0.85 41
1.38
0.81
0.95
1.97
0.96
1.90
2.96
4.92
6.80
[a] The optimum devices were fabricated with TIPSAntBT/PC71BM (1:1),
TIPSAntBT–TPA/PC71BM (1:4), and TIPSAntBT–bisDMFA/PC71BM (1:4).
Performances were determined under simulated 100 mWcmꢀ2 AM 1.5G il-
lumination. The light intensity was set by using calibrated standard sili-
con solar cells with a proactive window made from KG5 filter glass from
the National Renewable Energy Laboratory (NREL). The masked active
area of the device was 4 mm2. [b] These values were similar to those re-
ported in Ref. [9].
In addition, the insertion of a TiOx layer often allowed the
device to increase light absorption and improved contact be-
tween the active layer and the metal electrode.[13] Therefore,
we obtained the best PCE of 2.96% (ꢁ0.2) in devices fabricat-
ed with a TIPSAntBT–bisDMFA/PC71BM film treated with a CN
processing additive and a TiOx layer: Jsc =8.58 mAcmꢀ2, FF=
&6
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Electron-Rich Anthracene Semiconductors
(or bisDMFA) to anthracene
through a p-conjugated bithio-
phene bridge, leading to a more
electron-rich anthracene core.
This could facilitate electron
transfer into PC71BM, resulting in
improved
hole-transporting
properties. The results obtained
from the molecular engineering
approaches shown in this study
could provide a useful guide for
developing new materials for
use in solution-processed small-
molecule bulk hetero junction
solar cells.
Experimental Section
The BHJ films were prepared
under optimized conditions, ac-
cording to the following previously
reported procedure.[15] The ITO-
coated glass substrate was first
cleaned with detergent, ultrasoni-
cated in acetone and isopropyl al-
cohol, and subsequently dried
overnight in an oven. PEDOT:PSS
(Heraeus, Clevios P VP. AI 4083) in
aqueous solution was spin-cast to
Figure 7. Tapping-mode AFM surface topography of films cast from TIPSAntBT–TPA/PC71BM (1:4) BHJ composite
without (a) or with (b) treatment with 1% CN, and TIPSAntBT–bisDMFA/PC71BM (1:4) without (c) or with (d) treat-
ment with 1% CN.
form a film with a thickness of approximately 40 nm. The substrate
was dried for 10 min at 1408C in air and then transferred into
a glove box for spin-casting of the photoactive layer. The donor/
PC71BM BHJ blends, TIPSAntBT–TPA/PC71BM or TIPSAntBT–bis-
DMFA/PC71BM (weight ratio, 1:4), solutions were prepared in chlor-
obenzene at a concentration of 30 mgmLꢀ1, then heated to 608C
overnight, followed by spin-casting on top of the PEDOT layer. The
substrate was dried for 10 min at 808C in air. Then, the device was
evacuated to less than 1.33ꢁ10ꢀ5 Pa and an Al electrode about
100 nm thick was deposited on top. The solar cell efficiencies were
characterized under simulated 100 mWcmꢀ2 AM 1.5G irradiation
from a 1000 W Xe arc lamp (Oriel 91193). The light intensity was
adjusted by a Si solar cell double-checked with an NREL-calibrated
Si solar cell (PV Measurement Inc.). The applied potential and cell
current were measured by using a Keithley model 2400 digital
source meter. The J–V characteristics of the cell under these condi-
tions were determined by biasing the cell externally and measuring
the generated photocurrent. This process was fully automated by
using Wavemetrics software. The IPCE spectra for the cells were
measured on an IPCE measuring system (PV measurements).
0.41, and Voc =0.85 V. Compared with films without a TiOx layer
and without either CN treatment or TiOx insertion, the efficien-
cy was about 1.5 and 3 times greater, respectively.
Devices fabricated with TIPSAntBT–TPA/PC71BM film treated
with a CN processing additive and a TiOx layer also exhibited
a PCE of 1.97% (ꢁ0.15), with Jsc =6.07 mAcmꢀ2, FF=0.40, and
Voc =0.72 V, namely, about 33% less than that of TIPSAntBT–
bisDMFA/PC71BM. These results might be due to the
TIPSAntBT–bisDMFA/PC71BM BHJ films having a better surface
morphology than that of TIPSAntBT–TPA/PC71BM, resulting in
a 13% higher root-mean-square (rms) value, as shown in
Figure 7. These rougher nanoscale surfaces might be responsi-
ble for the more phase-segregated morphology of the
TIPSAntBT–bisDMFA/PC71BM BHJ films.
Conclusions
We have demonstrated the synthesis and photovoltaic charac-
teristics of two electron-rich anthracene derivatives, 2,6-
bis[5,5’-bis(N,N’-diphenylaniline)-2,2’-bithiophen-5-yl]-9,10-bis-
[(triisopropylsilyl)ethynyl]anthracene (TIPSAntBT–TPA) and 2,6-
bis(5,5’-bis{4-[bis(9,9-dimethyl-9H-fluoren-2-yl)amino]phenyl}-
2,2’-bithiophen-5-yl)-9,10-bis-[(triisopropylsilyl)ethynyl]anthra-
cene (TIPSAntBT–bisDMFA), containing triarylamine hole stabil-
izer, which exhibited a maximum power conversion efficiency
of 2.96% in solution-processed anthracene derivative-based
small-molecule organic solar cells (SMOSCs). These new materi-
als exhibited superior intramolecular charge transfer from TPA
The TiOx material was prepared according to the following previ-
ously reported procedure.[14] Titanium(IV) isopropoxide (10 mL) was
slowly added dropwise to a mixture of 2-methoxyethanol (50 mL)
and ethanolamine (5 mL) under argon. Then, the mixed solution
was heated to 808C for 2 h, followed by heating to 1208C for 1 h.
The two-step heating (80 and 1208C) was then repeated. The
dense TiOx solution was diluted with isopropanol to prepare the
typical TiOx precursor solution. This precursor solution was spin-
cast in air on top of the semiconducting photoactive layer, with
thicknesses in the range of 10–20 nm. Subsequently, the films were
heated at 808C for 10 min in air.
ChemSusChem 0000, 00, 1 – 9
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. K. Lee, J. Ko et al.
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Acknowledgements
This research was supported by the World Class University pro-
gram funded by the Ministry of Education, Science and Technolo-
gy through the National Research Foundation of Korea (R31-
2011-000-10035-0) and MEST for the Center for Next Generation
Dye-Sensitized Solar Cells (no. 2011-0001055).
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Received: April 5, 2012
Published online on && &&, 0000
&8
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ChemSusChem 0000, 00, 1 – 9
ÝÝ
These are not the final page numbers!

FULL PAPERS
Digging deeper: New electron-rich an-
thracene derivatives containing a triaryl-
amine hole stabilizer, TIPSAntBT–TPA
and TIPSAntBT–bisDMFA, exhibit superi-
or intramolecular charge-transfer perfor-
mance, resulting in a boost in hole-
transporting properties (see picture).
H. Choi, H. M. Ko, N. Cho, K. Song,
J. K. Lee,* J. Ko*
&& – &&
Electron-Rich Anthracene
Semiconductors Containing
Triarylamine for Solution-Processed
Small-Molecule Organic Solar Cells
ChemSusChem 0000, 00, 1 – 9
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
&
9
&
ÞÞ
These are not the final page numbers!