Post-deposition treatment of an arylated-carbazole conjugated polymer for solar cell fabrication

Article abstract of DOI:10.1002/adma.201201567

An arylated-carbazole conjugated polymer with a deep HOMO level has been developed. Solar cells based on blends of PCX3 and PC₇₁BM show efficiency of 3.9% with a V_oc of 0.96 V. The device performance can be improved to 5.1% by using polar solvent treatment, most likely as a result of modified interfacial properties.

Full text of DOI:10.1002/adma.201201567

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Post-Deposition Treatment of an Arylated-Carbazole
Conjugated Polymer for Solar Cell Fabrication
Xiaofeng Liu, Wen Wen, and Guillermo C. Bazan*
Solution processable bulk heterojunction (BHJ) solar cells are
emerging as potential low-cost alternatives for renewable energy
generation.[1–3] Considerable efforts have been taken in recent
years to pursue higher power conversion efficiencies (PCEs)
through the design of narrow bandgap donor-conjugated poly-
mers[4–6] and device optimization.[7–9] Donor material develop-
ment continues to dominate research, although efforts to find
fullerene alternatives are notable.[10–12] An important challenge
is designing conjugated polymers with relatively deep highest
occupied molecular orbital (HOMO) energy levels (to optimize
the open circuit voltage, Voc)[13–15] and appropriate absorption
profiles (to maximize the short circuit current, Jsc).[16–19] The
2,7-carbazole unit has proven as an excellent electron-rich frag-
ment to design appropriate backbone structures.[13,20–23] One
of the most well studied such carbazole copolymer, namely
PCDTBT, has gained particular attention recently.[13,21] The ini-
tial PCE reached 3.6% in a typical BHJ device with Voc of 0.88 V,
and has been subsequently improved to 6% by device structure
optimization.[22]
Despite the advantages of the BHJ structure, the PCEs can
be limited by undesirable morphology within the photoactive
layer.[24,25] Although thermal annealing, solvent vapour treat-
ment and high boiling-point additives are useful processing
options to tailor the organization of the BHJ blend,[26–28] these
methods need delicate control of the experimental variables.
Another strategy to improve PCE has emerged by incorporating
a conjugated polyelectrolyte interlayer. This study revealed that
polar solvent, such as methanol, has an independent positive
effect on the device performance.[29] Polar solvents are also
thought to modify the crystallinity within P3HT-containing
BHJ solar cells.[30,31]
structurally related to PCDTBT, as shown in Scheme 1A. The
deep HOMO level of the polymer leads to a high Voc of 0.96 V
and a PCE of up to 3.9% when blended with [6,6]-phenyl-C71-
butyric acid methyl ester (PC71BM). The PCE can be further
improved to 5.1% by simple post solvent treatment.
The synthetic route to the arylated carbazole monomer 6
and PCX3 is outlined in Scheme 1B. Initially, 3,5-dimethoxy-
1-iodobenzene (1) was coupled with 2,7-dibromocarbazole (2)
in the presence of copper(I) iodide leading to arylated carba-
zole precursor 3 in good yield. Demethylation of the methoxy
groups was achieved by treating 3 with BBr3. Octyloxy side
chains were subsequently introduced by reaction of 1-bromooc-
tane with dihydroxy carbazole 4 under mild conditions to yield
octyloxy substituted carbazole 5. It is noted that octyloxy side
chain substituted donor and acceptor units were designed to
balance solubility, intermolecular packing, weight percentage
of conjugated backbone and a coplanar configuration. The
carbazole monomer 6 was synthesized by double lithiation of
octyloxy carbazole 5 with n-butyllithium, followed by quenching
with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The
acceptor fragment 4,7-bis(5-bromothiophen-2-yl)-2,1,3-benzo-
thiadiazole (7) was obtained following a high yield procedure
adapted from the literature.[32] Finally, both monomers were
co-polymerized via a Suzuki-Miyaura coupling manner. As
described previously,[21] end-capping reaction was carried out
by using bromobenzene and phenylboronic acid to improve
polymer stability.[33] After a series of purification procedures,[34]
PCX3 was obtained in ∼90% yield. Characterization by NMR
spectroscopy is provided in the Supporting Information (Figure
S1). As collected from chloroform fractions, PCX3 can easily
form free-standing films (Figure S2), and was determined to
have a number average molecular weight (Mn) of 65 KDa and
a polydispersity index (PDI) of 2.5 by using high-temperature
(150 °C) gel permeation chromatography in 1,2,4-trichloroben-
zene (TCB) (Figure S3). PCX3 is soluble at room temperature
in common organic solvents, such as chloroform, chloroben-
zene (CB), orthodichlorobenzene (ODCB) and TCB.
In this contribution, we focus our attention on the effect
of polar solvent on BHJ device performance. To carry out our
studies, we focused on a new donor polymer, namely poly[N-
9′-(3,5-bis(octyloxy)phenyl)-2,7-carbazole-alt-5,5-(4′,7′-dithienyl-
5′,6′-bis(octyloxy)-2′,1′,3′-benzothiadiazole)] (PCX3), which is
The absorption spectrum of PCX3 in ODCB at room tem-
perature, shown in Figure 1a, exhibits two absorption peaks
centered at 392 nm and 530 nm, which upon heating at 100
˚C blue-shift to 388 nm and 518 nm, respectively. A film from
ODCB solution (1% w/w) shows broadened absorption bands
at 400 nm and 556 nm. The film absorption onset of 650 nm
corresponds to an optical bandgap of 1.91 eV, which is compa-
rable to that of PCDTBT.[13] Cyclic voltammetry (CV, Figure S4)
shows that PCX3 exhibits reversible oxidation and reduc-
tion processes at 0.60 V and -1.65 V, respectively, relative to
the ferrocene/ferrocenium couple. The HOMO (−5.40 eV) and
lowest unoccupied molecular orbital (LUMO, −3.15 eV) energy
levels can thus be calculated from these electrochemical values.
Dr. X. Liu,[+] W. Wen,[+] Prof. G. C. Bazan
Center for Polymers and Organic Solids
Department of Chemistry and Biochemistry
Department of Materials
University of California at Santa Barbara
Santa Barbara, California 93106, USA
E-mail: bazan@chem.ucsb.edu
W. Wen
State Key Laboratory of Electronic Thin Films and Integrated Devices
Department of Optoelectronic Information
University of Electronic Science and Technology of China (UESTC)
Chengdu 610054, P. R. China
[+] X.L. and W.W. contributed equally to this work.
DOI: 10.1002/adma.201201567
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Scheme 1. (A) Chemical structures of PCDTBT and PCX3. (B) Synthetic route to carbazole monomer and PCX3. a) K3PO4, CuI, ( )-trans-1,2-diamino-
cyclohexane, dioxane, 100 °C, overnight. b) BBr3, CH2Cl2, -20 °C to r.t., 48 h. c) 1-Bromooctane, K2CO3, DMF, 80 °C, 5 h. d) n-BuLi, THF, -78 °C, then
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, r.t., overnight. e) Pd2(dba)3, P(o-tol)3, Et4NOH (20% aq.), toluene, 97 °C, 16 h. f) bromobenzene,
1 h, then phenylboronic acid, 1 h.
Solar cell devices with a structure of ITO/PEDOT:PSS/
PCX3:PC71BM/Al were prepared and characterized under an
inert atmosphere. We first investigated the device perform-
ance with different PCX3/PC71BM weight ratios under AM
Compared to the HOMO level (−5.25 eV) of PCDTBT obtained
under identical experimental conditions, that of PCX3 is 0.15 eV
lower, which offers the possibility of Voc enhancement in devices
comprising fullerene acceptors.[35]
Figure 1. (a) UV-vis absorption spectra of PCX3 in ODCB and solid state. (b) J−V characteristics of 1:4 PCX3/PC71BM devices before and after ethanol
treatment. (c, d) Wetting time-dependent photovoltaic characteristics of the devices upon ethanol treatment.
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T a b le 1 . Summary of characteristics for PCX3/PC71BM BHJ solar cells.
PCX3/PC71BM
(w/w ratio)
J sc
Voc
[V]
FF
[%]
PCE
[%] avg
PCE
[%] best
[mA/cm2]
1:1
1:2
1:3
1:4
1.69
2.71
3.67
3.90
1.85
2.90
3.79
3.96
5.6 0.2
7.6 0.2
8.5 0.2
8.8 0.2
0.86 0.02
0.92 0.01
0.96 0.01
0.96 0.01
34.8
1
1
1
1
38.6
45.2
46.5
Figure 2. AFM height images (2 μm × 2 μm) of PCX3/PC71BM blend
films before (a) and after (b) ethanol treatment.
1.5G at 100 mW/cm2 conditions (Figure S5); Table 1 provides
a summary of the results. The optimized weight ratio of PCX3
to PC71BM was found to be 1:4, yielding a device with Voc
=
device is suppressed in the region between -1.0 and 1.0 V. The
shunt resistance (Rsh) was calculated as 5.3 × 105 Ω·cm2 and
1.3 × 106 Ω·cm2 for the pristine and ethanol treated devices,
respectively. These data are in agreement with the decreased
leakage current and slightly increased Voc in the devices. More-
over, the series resistance (Rs) of the ethanol-treated device is
17 Ω·cm2, which is lower than for a pristine device (62 Ω·cm2).
These results indicate that better diode characteristics were
obtained after simple ethanol treatment.
0.96 V and PCE = 3.9%. The film thickness of this device was
approximately 100 nm. Although the performance of solar cells
with weight ratios of 1:1 and 1:2 are considerably lower, the
behaviour of devices with ratios of 1:3 and 1:4 are comparable.
The higher Voc as compared with PCDTBT (0.88 V)[22] agrees
with a lower-lying HOMO level, as determined by electrochem-
istry. The EQE curve from the best device reveals broad pho-
toresponse from 300 nm to 700 nm, with a maximum value of
74% around 400 nm. The Jsc calculated from EQE curve under
standard AM 1.5G conditions yields 8.8 mA/cm2, which is in
accordance with that obtained from J−V measurements.
We found that the fill factor (FF) of the 1:4 PCX3/PC71BM
devices can be improved by treatment with ethanol as follows:
(i) ethanol was added atop the active layer, (ii) solvent was
removed by spin coating, (iii) the resulting film was placed
under vacuum overnight, and (iv) the cathode was evaporated.
With this ethanol treatment the FF increases from 46% to 53%,
leading to a PCE of 5.1%. The J−V characteristics of solar cells
with and without ethanol treatment are illustrated in Figure 1b.
An increase of Voc can also be observed, from 0.96 to 1.01 V,
as well as Jsc from 8.8 to 9.2 mA/cm2. The EQE spectra are in
agreement with the increase of Jsc (Figure S6). It is also worth
pointing out that other polar solvents, including acetonitrile
and methanol, have positive effects on device efficiencies with
only slightly lower PCEs compared to ethanol (Figure S7 and
Table S1). There is therefore a range of solvent options available
to execute this post treatment improvement.
The wetting time of ethanol (i.e. the time that ethanol is
present on top of active layer before the substrate is spun)
is relevant for the observed improvements. Figure 1c and 1d
present the wetting time dependence of Jsc, Voc, FF, and overall
PCE of 1:4 PCX3/PC71BM devices upon ethanol treatment, see
also Table S2. A wetting time of 2 min was found to work best.
Although there are slight decreases of PCE with wetting time
longer than 2 min, the Voc and FF remain around 1.0 V and
53%, respectively. In a parallel experiment, the devices were
exposed to ethanol vapour by simply introducing the device into
a beaker with ethanol (no direct contact between ethanol and
the device surface). One observes that these devices also show
comparable increase in FF to 53% (Figure 1b). These observa-
tions suggest that the spin-casted ethanol treatment does not
wash away any surface-accumulated components, i.e. it does
not simply serve the role of “cleaning” the surface.
Surface topographic features as function of ethanol treat-
ment were examined by atomic force microscopy (AFM). The
pristine 1:4 PCX3/PC71BM film exhibits a homogeneous, rela-
tively flat surface (Figure 2a) with a root-mean-squared (RMS)
surface roughness of 0.3 nm. Ethanol treatment causes slight
roughening; RMS = 1.2 nm (Figure 2b). However, there is no
observable change in the film thickness, as determined by pro-
filometry. Similar roughness increases were observed in films
treated with acetonitrile or methanol, with RMS values of 0.7
and 1.3 nm, respectively (Figure S9). It is also worth noting that
no obvious changes were observed in the absorption spectra of
1:4 PCX3/PC71BM films after ethanol treatment (Figure S10),
unlike the case of P3HT/PCBM blends.[30] This observation
suggests the absence of obvious reconstruction of the internal
donor structure. In agreement, we find nearly identical features
in the X-ray diffraction (XRD) measurements (Figure S11).
Further additional experiments were carried out to probe
the origin of the ethanol effect. In particular, PEDOT:PSS is
known to be sensitive to polar solvents.[36,37] A simple control
experiment was done by spin-coating 1:4 PCX3/PC71BM atop
PEDOT:PSS that had been previously treated with ethanol.
Sharp decreases in Voc and FF were observed (Figure S12). A
parallel experiment was performed by using thermally evapo-
rated MoOx as the ITO/PCX3/PC71BM interlayer. As shown in
Figure 3, ethanol treatment does not provide the improvements
observed with the PEDOT:PSS-containing device. This collected
set of observations indicates that part of the improvement
arises from modification of the PEDOT:PSS/BHJ interface, but
whatever modifications are occurring need this interface to be
created ahead of time.
As a final set of experiments, we used ultraviolet photo-
electron spectroscopy (UPS) to probe how ethanol treatment
modifies an organic/metal interface. Because of experimental
limitations, a testing structure different from the device configu-
ration had to be used. Specifically, PCX3/PC71BM blend solution
was spin-coated atop gold substrates. We recognize that the
J−V characteristics were also carried out under dark condi-
tions (Figure S8). The current density of the ethanol-treated
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is the binding energy relative to the work function (EF) of Au,
which is defined by the energy of the electron before excitation
relative to the vacuum level (Evac).[38] The Evac of the films were
determined by linear extrapolation of the secondary electron cut-
offs on the high-binding-energy side of the spectra (14–18 eV).
The shift in Evac indicates the magnitude of the interfacial
dipole (Δ).[39] For the pristine film, an interfacial dipole of
0.07 eV was obtained. Upon ethanol treatment, the value
increases to 0.33 eV. The energy diagrams of the organic/metal
interface are illustrated in Figure 4b. We take these features to
indicate that ethanol can penetrate the PCX3/PC71BM (even in
the absence of obvious solubility) to considerably influence the
contact properties of a “buried” interface.
In conclusion, a novel arylated-carbazole comonomer has
been developed and successfully incorporated in the conjugated
polymer PCX3. This aryl substituent offers the potential to tailor
the optical and electronic properties of the polymers, and is
particularly suited for modifying the HOMO energy level. The
high molecular weight polymer exhibits excellent solubility and
film-formation property. In combination with PC71BM one can
obtain solar cells with Voc of 0.96 V and a PCE of 3.9%. It was
also determined that post treatment of the preformed BHJ thin
film with ethanol, which is nontoxic, offers a simple method for
improving the overall device efficiency and may offer opportu-
nities to be incorporated in high throughput fabrication. How
this process works remains poorly under-
Figure 3. J−V characteristics of PCX3/PC71BM (1:4) devices before and
after solvent treatment by using MoOx as an anode buffer layer.
BHJ/Au interface is not identical to those in the solar cell device.
Nonetheless it provides a useful platform to examine ethanol
diffusion through the film. Figure 4a shows the UPS spectra
taken for the pristine and ethanol treated films. The abscissa
stood. There are obvious minor changes to
the surface topography, which suggest pos-
sible modification of the top metal contact.
However, we find the unexpected observa-
tion that (a) swelling by ethanol is sufficient
for attaining the improvements, and (b) evi-
dence that the swelling changes the proper-
ties of the buried PEDOT:PSS/BHJ interface.
Further understanding and optimization of
this interfacial tuning has the potential to
improve materials performance in a wide
range of BHJ structures.
Experimental Section
Characterization: 1H and 13C NMR were carried
out using
a Bruker Avance 500 spectrometer
in appropriate deuterated solvents at room
temperature (298 K) unless otherwise noted.
Chemical shifts were recorded as δ value (ppm)
relative to trace solvent signals.[40] Mass spectra
were taken from a Shimadzu GC-17A MS-QP5000
and a VG70 Magnetic Sector instrument. UV-visible
absorption spectra were collected on a Perkin
Elmer Lambda 750 spectrophotometer at different
temperature. Number-average molecular weight
(Mn) and weight-average molecular weight (Mw)
were determined by
a Polymer Laboratories
PL220 instrument at high temperature. Cyclic
voltammograms (CVs) were performed on a CHI
730B instrument in a standard three-electrode, one
compartment configuration equipped with Ag/AgCl
reference electrode, platinum counter electrode and
glassy carbon working electrode (dia. 3 mm). Glassy
carbon electrodes were polished with alumina before
Figure 4. (a) UPS spectra of Au, pristine film and ethanol treated film. (b) Energy diagrams near
the organic/metal interface. EA is electron affinity. IP is ionization potential.
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use. The CV curves were recorded in anhydrous acetonitrile (CH3CN)
with 0.1 M tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) as
the supporting electrolyte at scan rate 100 mV·s−1 . All solutions were
purged with dry argon for 15 minutes to deoxygenate the system. Films
were prepared by drop-casting a mixture of PCX3 in dry chloroform
(∼2 mg·mL−1 ) onto glassy carbon electrode under room temperature.
Fc/Fc+ was taken as an internal standard.
Synthetic procedures: All reagents and chemicals were purchased from
Alfa and Sigma-Aldrich and used without further purification. All solvents
used for reaction here were redistilled following standard procedures.
Column chromatography was carried out on flash silica gel (32-63μ,
Dynamic Adsorbents Inc.). The 1-iodo-3,5-dimethoxybenzene (1),[41]
2,7-dibromocarbazole (2)[42] and 4,7-bis(5-bromothiophen-2-yl)-5,6-
bis(octyloxy)-2,1,3-benzothiadiazole (7)[32] have already been prepared
in the literature. All other compounds were synthesized following
procedures described below.
0.90-0.87 (t, J = 7.0 Hz, 6H). 13C NMR (125 MHz, CDCl3, δ): 161.6, 142.0,
138.0, 123.7, 121.7, 121.6, 120.1, 113.5, 105.9, 101.5, 68.6, 31.9, 29.5,
29.4, 29.3, 26.2, 22.8, 14.2. FI-TOF-MS: Calculated for C34H43Br2NO2:
657.17, Found: 657.16.
2,7-Bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-N-9′′-(3,5-
bis(octyloxy)phenyl)carbazole (6) : To a stirred solution of 5 (1.2 g, 1.8
mmol, 1.0 eq) in dry THF (60 mL) at -78 °C under an inert atmosphere
was added dropwise a solution of n-BuLi (2.9 mL, 1.6 M in hexane, 4.6
mmol, 2.5 eq). The resulting mixture was stirred at -78 °C for 2 h to form
a thick suspension, after which 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (1.1 mL, 5.5 mmol, 3.0 eq) was added all at once. The
reaction mixture was stirred for an additional hour before slowly brought
to room temperature and stirred overnight. The mixture was then poured
into water, extracted with diethyl ether three times. The organic layer was
washed with brine, water and dried over magnesium sulfate. After the
solvent was removed, the residue was purified on a silica gel column
using 3% ethyl acetate in hexane as eluent. Monomer 6 (0.95 g, 69%)
was obtained as a white solid after recrystallization in ethanol.
1H NMR (500 MHz, CDCl3, δ): 8.17-8.15 (d, J = 7.5 Hz, 2H), 7.87
(s, 2H), 7.75-7.73 (d, J = 8.0 Hz, 2H), 6.68-6.67 (m, 2H), 6.61-6.60 (m,
1H), 4.00-3.98 (t, J = 6.5 Hz, 4H), 1.83-1.80 (m, 4H), 1.49-1.45 (m, 4H),
1.36-1.25 (m, 40H), 0.90-0.87 (t, J = 6.5 Hz, 6H). 13C NMR (125 MHz,
CDCl3, δ): 161.3, 141.3, 139.2, 126.0, 125.6, 120.0, 116.7, 106.6, 83.9,
68.6, 31.9, 29.5, 29.4, 26.2, 25.0, 22.8, 14.2. FI-TOF MS: Calculated for
C46H67B2NO6: 751.52, Found: 751.49.
N-9′-(3,5-Dimethoxyphenyl)-2,7-dibromocarbazole (3) : To
a
100
mL round bottom flask was charged 2,7-dibromocarbazole (4.0 g,
12.3 mmol, 1.0 eq), 1-iodo-3,5-dimethoxybenzene (3.9 g, 14.8 mmol,
1.2 eq), potassium phosphate (7.82 g, 36.9 mmol, 3.0 eq), copper iodide
(0.586 g, 3.1 mmol, 0.25 eq). The solid mixture was purged with argon
before adding anhydrous dioxane (30 mL). The mixture was stirred at
room temperature for 15 min, then ( )-trans-1,2-diaminocyclohexane
(0.72 mL, 0.5 eq) was added rapidly and the reaction was heated
at reflux overnight. The mixture was then filtered and washed with
dichloromethane. After removing the solvent, the crude product was first
purified by column chromatography (silica gel, 15% DCM in hexane as
eluent), then dissolved as-obtained solid in a small amount of boiling
chloroform. The title compound (4.2 g, yield: 73%) was obtained as an
off-white solid by precipitation chloroform solution in methanol (200 mL).
It should be noted that less than 4% of the compound was mono-iodized
and could not be separated.
Poly[N-9′-(3,5-Bis(octyloxy)phenyl)-2,7-carbazole-alt-5,5-(4′,7′-
dithienyl-5′,6′-bis(octyloxy)-2′,1′,3′-benzothiadiazole)]
(PCX3) :
To
a 20 mL flame dried reaction vial was added monomer 6 (375.8
mg, 0.5 mmol, 1.0 eq) and 4,7-bis(5-bromothiophen-2-yl)-5,6-
bis(octyloxy)-2,1,3-benzothiadiazole (7) (357.3 mg, 0.5 mmol, 1.0 eq),
tris(dibenzylideneacetone)dipalladium(0) (15 mg, 0.016 mmol, 0.033
eq) and tri(o-tolyl)phosphine (25 mg, 0.082 mmol, 0.164 eq) equipped
with a magnetic stirring bar under a nitrogen atmosphere. Dry toluene
(5 mL) was added and the reaction vial was sealed. A pre-degassed
aqueous tetraethylammonium hydroxide (20%, 2.0 mL) was added. The
rection mixture was then stirred at 97 °C for 16 h on a conventional oil
bath. Bromobenzene (60 μL) was added to the mixture and after 1 h, a
solution of phenylboronic acid (60 mg) in toluene (0.7 mL) was added.
Polymer end-capping was done in 2 h. The polymer was purified by
precipitation in methanol (300 mL), filtered, and subjected to soxhlet
extraction with methanol (12 h), acetone (5 h), hexane (12 h) and
chloroform (3 h). After the chloroform fraction was concentrated to
c.a. 100 mL, N,N-diethylphenylazothioformamide (35.4 mg, 0.16 mmol,
0.32 eq) was added as a palladium scavenger. The mixture was stirred
at 60 °C for 2 h under argon, followed by precipitation in methanol
(300 mL), filtered, and wash with methanol. The polymer was then
dissolved in chloroform (50 mL) and precipitated in methanol a second
time. The pure polymer (484 mg, 92%) was collected as a black purple
solid after filtration and dried in a vacuum oven.
1H NMR (500 MHz, CDCl3, δ): 7.95-7.93 (d, J = 10.0 Hz, 2H), 7.56
(s, 2H), 7.41-7.39 (d, J = 10.0 Hz, 2H), 6.64-6.62 (m, 3H), 3.87 (s, 6H).
13C NMR (125 MHz, CDCl3, δ): 162.1, 141.9, 138.2, 129.4, 123.8, 121.7,
120.3, 113.4, 105.6, 100.8, 55.8. EI-MS: Calculated for C20H15Br2NO2:
460.94, Found: 461.00.
N-9′-(3,5-Dihydroxyphenyl)-2,7-dibromocarbazole (4) : To a 100 mL flask
was added compound 3 (4.0 g, 8.68 mmol, 1.0 eq) and DCM (50 mL).
The clear solution was purged with argon for 30 min before cooled to
−78 °C using an acetone/dry ice bath. BBr3 (6.52 g, 26.0 mmol, 3.0 eq)
was dissolved in methylene chloride (20 mL) under argon atmosphere
and was added dropwise to the mixture in 10 min. The reaction was
allowed to warm to room temperature and stirred for 2 days. The
reaction mixture was then quenched with water carefully, extracted with
methylene chloride three times. The organic layer was washed with water
and dried over magnesium sulfate. The solvent was removed under
reduced pressure. The residue was purified by column chromatography
(silica gel, 5% ethyl acetate in methylene chloride as eluent). Compound
4 (2.56 g, 68%) was obtained as a white solid.
1H NMR (500 MHz, CDCl3, δ): 8.50 (br, 2H), 8.14-8.13 (d, J = 5.0
Hz, 2H), 7.78 (br, 2H), 7.69-7.68 (d, J = 5.0 Hz, 2H), 7.48 (br, 2H), 6.79
(br, 2H), 6.66 (br, 2H), 4.19 (br, 4H), 4.04 (br, 4H), 1.99-1.97 (m, 4H),
1.88-1.83 (m, 4H), 1.54-1.46 (m, 8H), 1.35-1.26 (m, 32H), 0.91-0.88 (m,
12H). Mn = 65 KDa, PDI = 2.5.
1H NMR (500 MHz, CDCl3, δ): 7.89-7.87 (d, J = 10.0 Hz, 2H), 7.57
(s, 2H), 7.35-7.33 (d, J = 10.0 Hz, 2H), 6.56-6.53 (m, 3H). 13C NMR (125
MHz, CDCl3, δ): 159.2, 141.6, 137.7, 123.2, 121.8, 121.4, 113.0, 105.6,
103.1. EI-MS: Calculated for C18H11Br2NO2: 430.92, Found: 430.35.
N-9′-(3,5-Bis(octyloxy)phenyl)-2,7-dibromocarbazole (5) : A suspension
of compound 4 (1.5 g, 3.5 mmol, 1.0 eq), 1-bromooctane (3.6 mL,
21.0 mmol, 6.0 eq), and potassium carbonate (5.8 g, 42 mmol, 12.0
eq) in dry N,N-dimethylformamide (15 mL) was degassed for three
times. The mixture was stirred at 70 °C for 5 h monitored by thin
layer chromatography (TLC). The suspension was then cooled to
room temperature and filtered off. The filtrate was directly loaded on
silica gel column, first using pure hexane as eluent to remove N,N-
dimethylformamide, then using 10% methylene chloride in hexane as
eluent. The title compound (2.1 g, 92%) was collected as a white powder
after removal of solvents.
Photovoltaic device Fabrication and Characterization: Polymer solar
cell devices with a sandwiched structure of glass/ITO/PEDOT:PSS)/
PCX3:PCBM/Aluminium were prepared according to the following
procedure. ITO-coated glass substrates were first ultrasonicated with
detergent, deionized water, acetone, and isopropyl alcohol for 30 min,
respectively. The substrates were subsequently treated with UV/ozone
for 20 min. PEDOT:PSS (Bayer Baytron 4083) layer was then spin-coated
on top of the ITO substrate with a thickness of ∼40 nm by using a
0.45 μm filter, and subsequently dried at 140 °C for 10 min in air. The
active layer, with a thickness around 100 nm was then spin-coated atop
PEDOT:PSS layer by using a CB:ODCB (1:3 v/v) solution with different
blend ratios of PCX3 and PC71BM (1:1, 1:2, 1:3 and 1:4 w/w, 10 mg·mL−1
of PCX3) under an inert atmosphere. The film thickness was controlled
by adjusting the spin speed between 800 and 2500 rpm. The films
1H NMR (500 MHz, CDCl3, δ): 7.93-7.91 (d, J = 10.0 Hz, 2H), 7.54 (s,
2H), 7.39-7.37 (d, J = 10.0 Hz, 2H), 6.59-6.57 (m, 3H), 3.99-3.97 (t, J =
5.0 Hz, 4H), 1.82-1.79 (m, 4H), 1.48-1.44 (m, 4H), 1.38-1.26 (m, 16H),
©
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Adv. Mater. 2012,
DOI: 10.1002/adma.201201567
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
5

www.advmat.de
www.MaterialsViews.com
[8] J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante,
A. J. Heeger, Science 2007, 317, 222.
[9] L. Yang, H. Zhou, S. C. Price, W. You, J. Am. Chem. Soc. 2012, 134, 5432.
[10] F. G. Brunetti, X. Gong, M. Tong, A. J. Heeger, F. Wudl, Angew.
Chem., Int. Ed. 2010, 49, 532.
[11] C. L. Chochos, S. P. Economopoulos, V. Deimede, V. G. Gregoriou,
M. T. Lloyd, G. G. Malliaras, J. K. Kallitsis, J. Phys. Chem. C 2007,
111, 10732.
[12] P. Sonar, G.-M. Ng, T. T. Lin, A. Dodabalapur, Z.-K. Chen, J. Mater.
Chem. 2010, 20, 3626.
[13] N. Blouin, A. Michaud, M. Leclerc, Adv. Mater. 2007, 19, 2295.
[14] Q. Zheng, B. J. Jung, J. Sun, H. E. Katz, J. Am. Chem. Soc. 2010, 132, 5394.
[15] L. Huo, J. Hou, S. Zhang, H.-Y. Chen, Y. Yang, Angew. Chem., Int. Ed.
2010, 49, 1500.
[16] K.-H. Ong, S.-L. Lim, H.-S. Tan, H.-K. Wong, J. Li, Z. Ma,
L. C. H. Moh, S.-H. Lim, J. C. de Mello, Z.-K. Chen, Adv. Mater.
2011, 23,1409.
[17] E. Wang, M. Wang, L. Wang, C. Duan, J. Zhang, W. Cai, C. He,
H. Wu, Y. Cao, Macromolecules 2009, 42, 4410.
[18] T.-Y. Chu, J. Lu, S. Beaupré, Y. Zhang, J.-R. Pouliot, S. Wakim,
J. Zhou, M. Leclerc, Z. Li, J. Ding, Y. Tao, J. Am. Chem. Soc. 2011,
133, 4250.
were dried at 70 °C for 10 min before directly subjecting to thermal
evaporation or post solvent treatment. Polar solvent was spin-coated at
2000 rpm on top of the active layer for 60 s. The solvents used here
were purchased from Sigma-Aldrich without further purification. For
the parallel experiments, thermally evaporated MoOx anode buffer layer
on ITO with thickness around 10 nm was used instead of PEDOT:PSS.
The thickness of PEDOT:PSS layer and active layer was determined by
a surface profilometer (Alfa step-500, Tencor). Finally, a 100 nm thick
Al layer was thermally evaporated through a shadow mask under a
pressure of 10−6 Torr. There were two devices on one single substrate
and each with an active area of 0.13 cm2. Photovoltaic characterization
was performed on a Keithley 2602 source measure unit with a 300 W Xe
arc lamp and an AM 1.5 global filter. The solar simulator illumination
intensity was measured using a KG1 filter from the National Renewable
Energy Laboratory (NREL) with silicon photovoltaics. External quantum
efficiency (EQE) spectra were measured using a 75 W Xe lamp, Newport
monochrometer, Newport optical chopper, and a Stanford Research
Systems lock-in amplifier. Power-density calibration was done by National
Institute of Standards and Technology traceable silicon photodiode.
UPS Measurement: A gold film of 75 nm was deposited on a pre-
cleaned Si substrate with a thin native oxide. Solutions containing
PCX3:PC71BM (1:4) in CB:DCB (1:3) mixed solvent with a concentration
of 2 mg mL−1 were then spin-cast on top of the Au film. For the film that
underwent ethanol treatment, ethanol was present on top of the active
layer for 2 min. Film fabrication was carried out in a N2-atmosphere
glovebox. To minimize the possibility of exposure to air the films
were then transferred from the N2-atmosphere dry box to the analysis
chamber inside an air-free sample holder. Subsequently, the samples
were kept inside a high-vacuum chamber overnight to remove solvent.
The UPS analysis chamber was equipped with a hemispherical electron-
energy analyzer (Kratos Ultra spectrometer), and was maintained at
1.33 × 10−7 Pa. The UPS was measured using He I (hv = 21.2 eV) source,
and the electron energy analyzer was operated at a constant pass energy
of 10 eV. During the measurements, a sample bias of -9 V was used in
order to separate the sample and the secondary edge for the analyzer. In
order to confirm the reproducibility of UPS spectra, we repeated these
measurements twice on two sets of samples.
[19] S. C. Price, A. C. Stuart, L. Yang, H. Zhou, W. You, J. Am. Chem. Soc.
2011,133, 4625.
[20] J. Li, F. Dierschke, J. Wu, A. C. Grimsdale, K. Müllen, J. Mater. Chem.
2006, 16, 96.
[21] N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair,
R. Neagu-Plesu, M. Belletête, G. Durocher, Y. Tao, M. Leclerc, J. Am.
Chem. Soc. 2007, 130, 732.
[22] S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon,
D. Moses, M. Leclerc, K. Lee, A. J. Heeger, Nat. Photon. 2009, 3, 297.
[23] J. Li, A. C. Grimsdale, Chem. Soc. Rev. 2010, 39, 2399.
[24] A. J. Moulé, K. Meerholz, Adv. Funct. Mater. 2009, 19, 3028.
[25] T. M. Clarke, J. R. Durrant, Chem. Rev. 2010, 110, 6736.
[26] J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger,
G. C. Bazan, Nat. Mater. 2007, 6, 497.
[27] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang,
Nat. Mater. 2005, 4, 864.
Supporting Information
[28] S. Miller, G. Fanchini, Y.-Y. Lin, C. Li, C.-W. Chen, W.-F. Su,
M. Chhowalla, J. Mater. Chem. 2008, 18, 306.
[29] J. H. Seo, A. Gutacker, Y. Sun, H. Wu, F. Huang, Y. Cao, U. Scherf,
A. J. Heeger, G. C. Bazan, J. Am. Chem. Soc. 2011, 133, 8416.
[30] H. Li, H. Tang, L. Li, W. Xu, X. Zhao, X. Yang, J. Mater. Chem. 2011,
21, 6563.
Supporting Information is available from the Wiley Online Library or
from the author.
[31] S. Nam, J. Jang, H. Cha, J. Hwang, T. K. An, S. Park, C. E. Park, J.
Mater. Chem. 2012, 22, 5543.
[32] R. Qin, W. Li, C. Li, C. Du, C. Veit, H.-F. Schleiermacher,
M. Andersson, Z. Bo, Z. Liu, O. Inganäs, U. Wuerfel, F. Zhang, J.
Am. Chem. Soc. 2009, 131, 14612.
Acknowledgements
The authors are grateful to the Department of Energy BES and the Air
Force Office of Scientific Research for support of this work.
Received: April 19, 2012
Published online:
[33] J. K. Park, J. Jo, J. H. Seo, J. S. Moon, Y. D. Park, K. Lee, A. J. Heeger,
G. C. Bazan, Adv. Mater. 2011, 23, 2430.
[34] K. T. Nielsen, K. Bechgaard, F. C. Krebs, Macromolecules 2005, 38, 658.
[35] M. C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldauf,
A. J. Heeger, C. J. Brabec, Adv. Mater. 2006, 18, 789.
[36] Y. Xia, J. Ouyang, J. Mater. Chem. 2011, 21, 4927.
[37] B. Peng, X. Guo, C. Cui, Y. Zou, C. Pan, Y. Li, Appl. Phys. Lett. 2011,
98, 243308.
[1] G. Yu, J. Gao, J. C. Hunnelen, F. Wudl, A. J. Heeger, Science 1995,
270, 1789.
[2] B. C. Thompson, J. M. J. Fréchet, Angew. Chem., Int. Ed. 2008, 47,
58.
[38] J. H. Seo, T.-Q. Nguyen, J. Am. Chem. Soc. 2008, 130, 10042.
[39] J. H. Seo, R. Yang, J. Z. Brzezinski, B. Walker, G. C. Bazan,
T.-Q. Nguyen, Adv. Mater. 2009, 21, 1006.
[40] G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb,
A. Nudelman, B. M. Stoltz, J. E. Bercaw, K. I. Goldberg, Organome-
tallics 2010, 29, 2176.
[3] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, Prog. Photovolt. Res.
Appl. 2010, 18, 144.
[4] Y.-J. Cheng, S.-H. Yang, C.-S. Hsu, Chem. Rev. 2009, 109, 5868.
[5] B. Carsten, F. He, H. J. Son, T. Xu, L. Yu, Chem. Rev. 2011, 111,
1493.
[6] J. Chen, Y. Cao, Acc. Chem. Res. 2009, 42, 1709.
[7] G. Li, C. W. Chu, V. Shrotriya, J. Huang, Y. Yang, Appl. Phys. Lett.
2006, 88, 253503.
[41] J. Zhao, R. C. Larock, J. Org. Chem. 2006, 71, 5340.
[42] A. W. Freeman, M. Urvoy, M. E. Criswell, J. Org. Chem. 2005, 70, 5014.
©
6
wileyonlinelibrary.com
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2012,
DOI: 10.1002/adma.201201567