Polythiophenoazomethines - Alternate photoactive materials for organic photovoltaics

Article abstract of DOI:10.1039/c4ta03202k

Solution-processable polyazomethines containing thiophenes were synthesized and used as the donor material in bulk heterojunction solar cells. The blue polymers exhibited similar electrochemical properties to the benchmark P3HT with the advantage of absor

Full text of DOI:10.1039/c4ta03202k

Journal of
Materials Chemistry A
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PAPER
Polythiophenoazomethines – alternate
photoactive materials for organic photovoltaics†
Cite this: J. Mater. Chem. A, 2014, 2,
5620
1
a
a
b
b
Andreanne Bolduc,‡ Satyananda Barik,§ Martin R. Lenze, Klaus Meerholz*
´
a
and W. G. Skene*
Solution-processable polyazomethines containing thiophenes were synthesized and used as the donor
material in bulk heterojunction solar cells. The blue polymers exhibited similar electrochemical
properties to the benchmark P3HT with the advantage of absorbing more of the visible spectrum. The
resulting photovoltaic devices using polyazomethines in the photoactive layer with PC60BM as the
Received 23rd June 2014
Accepted 24th July 2014
ꢀ2
acceptor showed power conversion efficiencies up to 0.22% under simulated 100 mW cm AM 1.5G
irradiation. The low efficiencies are ascribed to poor charge generation because of too coarse bulk
heterojunction morphology formation.
DOI: 10.1039/c4ta03202k
www.rsc.org/MaterialsA
9
10
Alternatively, Gilch and Horner–Emmons strategies have also
1
Introduction
been used for preparing vinylene conjugated polymers.
Conjugated polymers have received much attention as they offer Although these polymerization methods are successful, the
many new possibilities for devices combining unique optical, preparation of conjugated polymers using these protocols
1
electrical, and mechanical properties. Of particular interest are requires rigorous reaction conditions such as catalysts, inert
2d,11
their light harvesting and charge transfer properties, especially atmospheres and anhydrous solvents.
These coupling
when combined with electron acceptors such as the fullerene methods further produce by-products, requiring rigorous
derivative phenyl-C -butyric acid methyl ester (PC BM). These product purication for obtaining pristine materials and for
6
1
60
electron donor–acceptor systems are well suited for uses in ensuring optimal device performance. Interestingly, the PCEs
organic electronics, especially organic photovoltaic devices measured for devices prepared from a vast majority of these
2
12
(OPVDs).
synthetically demanding materials are below 0.5%. Given the
Much effort has focused on the design and preparation of ecological and economic rewards of OPVDs, alternate straight-
new polymers for achieving enhanced OPVDs with high power forward coupling methods not requiring stringent protocols
3
conversion efficiencies (PCE). PCEs up to 9.2% have been and little to no product purication are, therefore, of interest for
achieved by combining different small molecules and donor– preparing new photoactive conjugated polymers for use in
acceptor copolymer systems and optimizing the device fabri- OPVDs.
cation including thermal annealing and the use of different
Conjugated polymers derived from azomethines (–N]C–)
4
cathode materials. Even though research endeavors have are highly interesting alternatives to conventional coupling
concentrated on synthetic optimization, the polymerization of protocols in part due to their simple synthesis that does not
photoactive OPVD materials has exclusively used aryl–aryl require the use of stringent reaction conditions or metal cata-
5
6
coupling reactions including Suzuki, Yamamoto, Grignard lysts. Polythiophenoazomethine derivatives such as those in
7
8
metathesis, and C–H activation polymerization protocols.
Scheme 1 are especially interesting because they have optical
and electrochemical properties that are well-suited for use as
13
a
the light harvesting component in photovoltaic devices. Poly-
de Montr ´e al, CP 6128, succ. azomethines are also generally interesting because water is the
e
Laboratoire de caract ´e risation photophysique des mat ´e riaux conjugu ´e s, D ´e partement
de Chimie, Pavillon JA Bombardier, Universit
´
Centre-ville, Montreal, Quebec, Canada H3T 2B1. E-mail: w.skene@umontreal.ca
only by-product of the reaction (Scheme 1). As a result,
b
Department of Chemistry, University of Cologne, Luxemburger Str. 116, 50939,
Cologne, Germany. E-mail: klaus.meerholz@uni-koeln.de
†
Electronic supplementary information (ESI) available: NMR, absorbance and
FT-IR spectra, GPC elugram, AFM micrographs, and thermograms. See DOI:
1
0.1039/c4ta03202k
‡
Molecular Science and Technology, Department of Chemical Engineering and
Chemistry, Eindhoven University of Technology, Den Dolech 2, 5612 AZ, P.O.
Box 513 5600 MB, Eindhoven, The Netherlands.
§
Institute of Chemical and Engineering Sciences, Pesek Road, Jurong Island,
27833, Singapore.
6
Scheme 1 Acid catalyzed condensation of polyazomethines 1 and 2.
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postpolymerization purication of polyazomethines is not distilled TMEDA (9.8 g, 84 mmol) in anhydrous hexanes (50 mL)
required and they can be used as is directly from the polymer- under nitrogen, was added a solution of 2.0 M n-BuLi in hexane
ization mixture. They can therefore be thought of as green (42 mL, 84 mmol) drop-wise. Aer reuxing for 1.5 h, THF
ꢁ
materials with environmental benets.
(40 mL) was added and the solution was cooled at ꢀ50 C.
Despite the synthetic advantages of conjugated azomethines Anhydrous DMF (14 mL, 190 mmol) was added drop-wise. Aer
over currently used polymers in OPVDs, there are only few 2.5 h at room temperature, the reaction mixture was hydrolyzed
14
examples of azomethines used in such devices. These have with water (60 mL) and the mixture was extracted with ether.
focused predominately either on small molecules or polymers The organic layers were dried over MgSO and concentrated.
4
having limited degrees of conjugation. The latter have narrow The crude product was puried using column chromatography
absorbance in the visible and limited solubility in solvents used with hexanes/ethyl acetate (90/10 v/v) to give the product as a
1
for device preparation. Given the ecological advantages of pol- colorless oil (7.0 g, 72%). H-NMR (CDCl ) d ppm: 10.13 (s, 1H),
3
yazomethines, we were therefore incited to demonstrate that 9.98 (s, 1H), 7.63 (s, 1H), 2.92 (d, 2H), 1.61 (septet, 1H), 1.3
1
3
these easily prepared conjugated polymers consisting uniquely (m, 8H), 0.90 (dt, 6H). C-NMR (CDCl
3
): d ¼ 183.8, 183.4, 151.7,
of thiophenes could be used as the photoactive layer in OPVDs, 148.0, 144.3, 138.1, 41.9, 33.1, 32.8, 29.1, 26.0, 23.3, 14.4, 11.1.
while having key properties for use in such devices, including HR-MS( ) calculated for [C14
+
+
H
20
O
2
S + H] : 253.12840; found:
solution processability and broad absorption in the visible. 253.12842.
Herein, we present the polythiophenoazomethines 1 and 2
Scheme 1) for use in OPVDs as a proof-of-concept to demon- methine (2). The copolymerization was done by mixing 3-(2-eth-
strate that these polyazomethines can be used as materials for ylhexyl)-thiophene-2,5-dicarbaldehyde (200 mg, 0.79 mmol) and
photoactive layers in OPVDs. diethyl 2,5-diaminothiophene-3,4-dicarboxylate (204 mg, 0.79
mmol) in CHCl (5.0 mL) in a pressure tube. A catalytic amount of
Poly(3-(3-octane2-yl)thiophene)-co-3,4-diethoxy-thiophenoazo-
(
3
diluted TFA (30 mL) was then added. The pressure tube was sealed
and heated to 90 C for 72 h. The reaction mixture was cooled to
room temperature and the polymer was precipitated from a
2
Experimental
ꢁ
2.1 Synthesis
ꢀ1
ꢀ1
The synthesis of 1 (M
1
to known protocols.
n
¼ 10 kg mol , M
w
¼ 15 kg mol , PDI ¼ methanol–water mixture. The resulting blue solid (85%, 380 mg)
.5, GPC relative to polystyrene standards) was done according was ltered and washed with methanol, water and acetone and
13b,c,15,24
FT-IR: 2925, 2850, 1725, 1670, 1560, then dried under vacuum overnight. In another sample, the
1
425, 1220, 1195, 1155, 1095, 1025, 860, 845, 775, 745, 715, 635 solvent was removed under vacuo and the polymer was used as is
ꢀ
1
1
cm . Anal. Calcd for C36
3
1
H
56
N
2
O
5
S
2
∙11.65 H
7.65; H 6.58; N 2.30; S 10.82. Found: C 35.86; H 4.77; N 4.01; S ppm: 10.13 (m, CHO), 8.52 (m, –N]CH–), 8.07 (s, –Th–H), 4.42
0.71. The synthesis of 2 is given in the following. For NMR (m, 4H). 2.82 (m, –CH ), 1.44 (m, –CH–), 1.28 (m, –CH–CH –), 0.88
(m, –CH ). FT-IR: 2955, 2925, 2855, 1720, 1705, 1570, 1535, 1488,
-(2-Ethylhexyl) thiophene. In anhydrous THF (50 mL) was 1460, 1420, 1380, 1285, 1245, 1204, 1145, 1095, 1020, 965, 930,
2
O∙0.7 Sc(OTf)
3
3
: C for subsequent characterization. H NMR (400 MHz, CDCl ) d
2
2
spectra as well as TGA, DSC and GPC measurements see ESI.†
3
3
ꢀ
1
dissolved magnesium (2.65 g, 109 mmol) and 2-ethylhexyl 845, 780, 510 cm . Anal. calcd for C24H N O S : C 58.61; H 6.55;
32 2 5 2
bromide (15 g, 77 mmol) was added. The reaction mixture was N 5.69; S 13.02. Found: C 60.59; H 6.46; N 5.93; S 13.66. GPC
ꢁ
stirred for 30 min at 0 C. The magnesium was allowed to react relative to polystyrene molecular weight standards: M
¼ 10.8 kg
n
ꢀ
1
ꢀ1
as much as possible by reuxing the reaction mixture for 2 h to mol ; M
obtain the desired 2-ethylhexyl magnesium bromide. In another
w
¼ 15.7 kg mol ; PDI ¼ 1.45.

ask, 3-bromothiophene (7.0 g, 43 mmol) was dissolved in
anhydrous THF (100 mL) to which was then added [1,3-bis
diphenylphosphino) propane] nickel(II) chloride (200 mg, 0.4
2
.2 Spectroscopy and electrochemistry
The spectroscopic properties in solution (dichloromethane) and
thin lm were measured using a Cary 500 spectrophotometer. The
electrochemical properties of the polyazomethines were charac-
terized using a Bio-Logic VSP 300 potentiostat. The compounds
were dissolved in deaerated dichloromethane at 10 M with
NBu PF (0.5 M). A platinum electrode was used as the working
(
mmol). Three cycles of freeze–pump–thaw were performed to
ensure complete removal of oxygen. The prepared 2-ethylhexyl
magnesium bromide (Grignard reagent) was added to the red
coloured solution (second ask) by cannula and the brown
solution was then reuxed for 18 h. The solution was washed
with aqueous HCl (10% w/w) aer cooling to room temperature.
The organic phase was extracted with ethyl acetate, dried with
ꢀ4
4
6
electrode with a platinum wire as the auxiliary electrode. The
reference electrode was a saturated Ag/AgCl electrode. Ferrocene
was added to the solution as an internal reference (E ¼ 0.35 V vs.
pa
MgSO
4
ltered, and the solvent was evaporated. The crude
16
Ag/AgCl). The HOMO level of 1 was additionally measured in
thin lms via photoelectron spectroscopy in air with a Riken Keiki
AC-2 spectrometer (CSIRO-Clayton, Australia).
product was chromatographed on silica with 100% hexanes to
afford a colourless oil (4.9 g, 58%). H-NMR (CDCl
1
3
): d ¼ 7.26 (d,
1
0
4
H), 6.94 (dd, 2H), 2.63 (d, 2H), 1.58 (septet, 1H), 1.32 (m, 8H),
.91 (dt, 6H). C-NMR (CDCl
0.8, 34.7, 32.9, 29.3, 26.0, 23.5, 14.8, 11.2. HR-MS( ) calculated
1
3
3
): d ¼ 142.3, 129.2, 125.2, 121.0,
+
2.3 Device fabrication
+
for [C H S + H] : 197.12857; found: 197.12852.
All devices were fabricated on commercial indium–tin oxide
1
2
20
ꢀ
1
3-(2-Ethylhexyl) thiophene-2,5-dicarbaldehyde. To a solution (ITO)-coated glass (Thin Film Devices (TFD), 15 U sq ). The
of 3-(2-ethylhexyl) thiophene (7.5 g, 38 mmol) and freshly ITO was etched with acid and subsequently cleaned using
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chloroform, acetone, Mucasol detergent, and deionized water in values of both 1 and 2 were measured in solution by cyclic
an ultrasonic bath. The ITO substrates were next cleaned in an voltammetry (inset Fig. 1). The advantage of the photoelectron
ozone chamber for 10 min. PEDOT : PSS (Clevios P Al 4083, oxidation method is that the HOMO level can directly be
from Heraeus) was then spin coated on the ITO substrates at measured without relying on the electrochemical oxidation
18
3
000 rpm for 30 seconds, resulting in 35 nm of the hole-injec- onset. The measured HOMO by the photoelectron oxidation
tion layer. Residual water was removed by heat treatment for 2 method is consistent with the HOMO derived from the oxida-
ꢁ
min at 110 C. Concentrated stock solutions of equal weight tion onset according to the commonly accepted approximation:
onset
amounts of the corresponding polyazomethine and PC60BM HOMO ¼ ꢀe(Epa (1 or 2) + 4.72), where the potentials are
+
19
0
(
from Nano-C, Westwood, MA, USA) were individually prepared measured against Ag/Ag , by taking the E
o
of ferrocene that
ꢀ1
20
in chlorobenzene (ca. 25 mg mL ) under inert atmosphere was used as the internal reference to be 0.35 V vs. Ag/AgCl. The
and stirred at room temperature until homogeneous. The reduction potential measured by cyclic voltammetry can equally
solutions were then combined to obtain the desired be used to calculate the LUMO level according to: LUMO ¼
onset
polyazomethine : PC60BM ratios and then further stirred for ꢀe(Epc (1 or 2) + 4.72).
four hours. The active layer was deposited by spin coating from
g
The energy-gap (E ) can further be calculated from either the
solution onto the ITO/PEDOT substrates at different speeds to solution electrochemical data or thin lm absorbance onset.
obtain active layer lm thickness ranging between 25 and 80 Upon comparing the measured data summarized in Table 1 for
nm. The bulk heterojunction (BHJ) device was completed by the azomethines 1 and 2 to P3HT, it is apparent that the HOMO
evaporating 4 nm of Ca followed by 110 nm of Ag (99.9%, Alfa levels of the azomethines are slightly lower than that of P3HT.
Aesar) through a mask, leading to seven solar cells on each However, the LUMO levels of the polyazomethines are higher
2
substrate, each with an active area of 0.08 cm .
than the LUMO level of PC BM. Although the energy offset
60
between PC60BM and the azomethines is small, charge injection
into the electron accepting material from the excited azome-
thines is nonetheless expected.
2
.4 Device characterization
Device characterization was done by measuring the J–V char-
acteristics of the solar cells with a Keithley 2425 source-
measurement unit. The simulated AM 1.5 light was provided by
a ltered Xe lamp. The intensity of 100 mW cm of the AM1.5
light was determined using a calibrated inorganic solar cell
from the Fraunhofer Institute for Solar Energy (ISE) in Freiburg
The absorbance spectra of 1 relative to P3HT is displayed in
Fig. 2. From this gure, it can be seen that the polyazomethine
absorbs at 625 nm, ca. 200 nm bathochromically shied relative
to P3HT. The observed spectroscopic redshi is a result of the
high degree of conjugation concomitant with intramolecular
charge transfer effects of the imine/esters of the poly-
azomethines. Similar absorbance was also seen with 2 (Fig. 2).
The absorbance maximum at 625 nm as well as the absorbance
onset were found not to shi with concentration (see Fig. S9 in
the ESI†). This conrms that the absorbance is insensitive to
intermolecular effects. Also, the polyazomethines intrinsically
absorb more of the visible spectrum than P3HT. Meanwhile, the
similar spectral properties of both 1 and 2 in solution and thin
ꢀ2
(Germany) and a reference P3HT : PC60BM cell measured by the
same institution. A surface proler (Dektak, Veeco) was used to
determine the active layer thicknesses. UV-vis thin lm spectra
were taken with a Varian Cary 50 spectrometer.
3
Results and discussion
Despite the synthetic and purication advantages of poly-
azomethines in addition to their isoelectronic character to C]C
homologous polymers, they must have comparable opto-elec-
tronic properties to current photoactive OPVD materials in
order to be considered as viable alternatives for these materials.
For this reason, the opto-electronic properties of 1 and 2 were
investigated and compared to the benchmark photoactive
17
material, P3HT. The two polyazomethines were investigated
because their different alkylations were expected to lead to
different morphologies and hence various device performances,
while being soluble in common processing solvents. In partic-
ular, desired properties of 1 and 2 are: (i) broad absorption
across the visible spectrum for maximum solar spectrum
absorption; (ii) hole transport capability; (iii) HOMO and LUMO
levels less than ꢀ6.1 and ꢀ4.1 eV, respectively, for efficient
charge transfer between the azomethine and the commonly
used electron acceptor PC60BM.
Fig. 1 Photoelectron oxidation of 1 cast as a thin film on an ITO
Both the photoelectron spectroscopy and the redox proper-
ties of 1 were measured for determining the HOMO energy
levels. While the photoelectron oxidation was measured from a
coated glass slide. Inset: cyclic voltammograms of 1 ( ) and 2 ( ) in
ꢀ1
dichloromethane measured at 100 mV s
6
with TBAPF against
ferrocene/ferrocenium in anhydrous dichloromethane with ferrocene

lm of 1 deposited on ITO coated glass in air (Fig. 1), the redox added as an internal reference.
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Table 1 Comparative HOMO, LUMO and band gap energy values of
donors and PC60BM
b
Compound
HOMO
LUMO
E
g
(eV)
a
1
2
ꢀ5.5 (ꢀ5.6)
ꢀ5.5
ꢀ3.9
ꢀ4.0
ꢀ2.7
ꢀ4.1
1.6 (1.6)
1.5 (1.6)
2.0 (1.9)
2.0
c
P3HT
PC60BM
ꢀ5.1 (ꢀ4.8)
ꢀ6.1
d
a
b
Value in parentheses is from photoelectron spectroscopy. Value in
parentheses is the spectroscopically derived energy-gap from the
absorbance onset. From literature measured in solution. Value in
c
21
d
parentheses is measured in the solid state for P3HT.
From
22
literature.
Fig. 3 AFM phase images of 1 showing the different morphologies
formed on silica substrates at different concentrations; (a) 0.5 mg
ꢀ1
ꢀ1
ꢀ1
mL , (b) 1.0 mg mL and (c) 2.0 mg mL in chloroform and their
corresponding topography images (d)–(f).
Bulk-heterojunction OPVDs were fabricated with the poly-
azomethines using an optimized polymer/acceptor ratio of 1 : 4
and an active layer lm thickness of 80 nm (see Fig. S15 and S16
in the ESI†). The devices were fabricated using the conventional
ITO/PEDOT : PSS (35 nm)/polyazomethine : PC60BM (80 nm)/
Ca (4 nm)/Ag (120 nm) device architecture under anaerobic
conditions. Illumination of the devices with simulated AM1.5
ꢀ
2
Fig. 2 Absorbance spectra of 1 ( ) and 2 ( ) in solution, 1 in thin films
measured on glass slides ( ), P3HT in solution ( ) and in thin films ( ),
as well as PC60BM ( ).
(100 mW cm ) gave the J–V response reported in Fig. 4. The
averaged device characteristics of the investigated solar cells are
summarized in Table 2.
There was no change in device performance when using

lms provide sound evidence that the length of the pendant either puried or as-prepared 1. A J–V response could not be
solubilizing alkyl groups does not affect the optical properties. measured with a device fabricated without 1 or 2, i.e. pure
No spectral shis were seen with either 1 or 2 upon annealing PC60BM as active layer, conrming that the polyazomethine
aer spin coating onto glass slides. This is in contrast to P3HT indeed harvests photons and undergoes photoinduced electron
that shis to longer wavelengths upon annealing because of transfer with PC60BM.
increased ordering of the polymer chains and formation of
aggregated domains.
The theoretical maximum Voc of an OPVD can be derived
from the energy difference between the HOMO energy levels of
23
Discrete repeating patterns associated with highly ordered
crystalline materials were previously found to be absent in the
24
powder X-ray diffraction (XRD) of 1. Instead, the powder XRD
was consistent with limited intermolecular p-stacking with a
˚
separation distance of 23 A between the polymer backbones.
This large separation is in part a result of interdigitation of the
C10-alkyl chains. To further examine the intermolecular
packing and self-assembly behavior of 1, the polymer was spin
coated onto silica substrates from solutions of different
concentrations and the resulting morphologies were measured
by atomic force microscopy (AFM). As seen in Fig. 3, an unor-
dered (amorphous) morphology is adopted at low concentra-
ꢀ
1
tions (0.5 mg mL ). In contrast, an organized herringbone-like
ꢀ
1
morphology is received at high concentrations (2 mg mL ),
while small domains occur at intermediate concentrations. The
self-organization at higher concentrations is a result of inter-
digitation of the C10 alkyl chains, which is consistent with the
powder XRD data.
Fig. 4 Representative J–V response of photovoltaic devices prepared
of 1 : PC60BM (1 : 4) ( ) and 2 : PC60BM (1 : 4) ( ) under simulated
AM1.5 illumination of 100 mW cm intensity.
ꢀ2
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Table 2 Representative solar cell characteristics of the investigated
ꢀ2
OPVDs under simulated AM1.5 illumination of 100 mW cm
ꢀ2
Compound
Voc/V
JSC/mA cm
FF
%PCE
1
2
: PC60BM
: PC60BM
0.65
0.56
0.67
0.94
0.37
8.40
0.32
0.34
0.53
0.22
0.07
2.83
P3HT : PC60BM
Fig. 5 (a) AFM phase image of 5 ꢂ 5 mm area for a 1 : 4 PVD of
the polyazomethines and the LUMO of PC60BM and is equal to 1 : PC60BM device and (b) histogram analysis of the phase signal with
2
5
Gaussian fits: polyazomethine-rich phase (blue) and PCBM-rich phase
red). For AFM topography see Fig. S17 in the ESI.†
ca. 1.4 V. The discrepancy between the experimental and
theoretical Voc is comparable to most reported photoactive
materials, including our reference device prepared from
(
26
P3HT. This can be attributed to intramolecular charge transfer
of the donor and acceptors.
In contrast to the Voc, the ll factor (FF) is lower than in
devices prepared with conventional polymers. The low ll factor
most likely results from poor hole mobility in the photoactive
layer. This is conrmed by eld effect transistor measurements
of 1 that showed hole mobilities on the order of 10 cm V
. This is in contrast to the oligomeric analogue of 1 whose
mobilities are on the order of 10 cm V
27
alkyl substitution as well as optimization of the OPVD pro-
cessing conditions are required for improved device efficien-
cies. Nonetheless, the collective opto-electronic device
measurements conrm that the polyazomethines have inherent
opto-electronic properties that are suitable for use as a photo-
ꢀ1 active material in photovoltaics. Increased device efficiencies
are expected by modifying the polymer to promote the forma-
tion of a more favorable BHJ morphology.
ꢀ
8
2
ꢀ
1
24
s
ꢀ5
2
ꢀ1 ꢀ1 24
s . The increased
small molecular ordering in part results in a higher short-circuit
ꢀ
2
current density (JSC ¼ 3.7 mA cm ), and in turn, a larger PCE z
4
Conclusions
14a
1
.1%. The measured low mobilities of 1 are therefore not
In summary, a photovoltaic device using an all-thiophene
conjugated polyazomethine as the photoactive layer was
demonstrated. It was shown that the Voc of the device was
comparable to photoactive materials currently used (z0.7 V). A
small ll factor (<0.35) was measured, which could be attributed
to the low hole mobility that results from poor p-staking of the
thiophene repeating units.
Although the device PCE is not in the top performance
bracket, it nonetheless serves to illustrate that polyazomethines
can be used as photoactive and electron donor materials. In
fact, the measured performance is comparable to many conju-
gated polymers examined in photovoltaic devices and whose
synthesis is not straightforward, requiring multiple steps and
extensive purication. Taking into account the similar proper-
ties compared to many of its carbon counterparts together with
the ease of synthesis and no purication required for their
synthesis, polyazomethines are attractive alternatives for pho-
toactive materials. With additional material optimization
including shorter alkyl chains and different donor–acceptor
repeating units, improved device efficiencies are expected for
polyazomethine based OPVDs.
representative of poor intrinsic mobility properties of the
polymer, but rather result from limited p-stacking of the thio-
phene units, which is required for efficient interchain hopping.
The measured short-circuit current density (JSC) for the BHJ
devices is considerably smaller than that of P3HT : PC BM. In
general, low JSC values can be attributed to two main reasons: (i)
no photoinduced charge is produced due to coarse phase
separation of the donor and acceptor domains in the BHJ, i.e.
excitons do not reach the D/A interface within their lifetime (ii)
generated charges do not reach the electrode interface due to
the absence of continuous percolation pathways and/or high
degree of recombination.
6
0
Interestingly, AFM phase measurements (Fig. 5a) of the
device did not show a uniform morphology expected for a BHJ.
Instead, tightly packed grains of PC60BM measuring ca. 300 nm
in diameter were found. Histogram analysis of the phase image
indicates an almost complete separation into the pristine donor
and acceptor phases (Fig. 5b). The data could be tted with two
distributions, assigned to polyazomethine-rich (PC BM-poor)
6
0
and PC BM-rich (azomethine-poor) phases, yielding an area
6
0
ratio of 1 : 2.5. This morphology does not promote efficient
exciton dissociation into free charge carriers. Therefore, the low
photocurrent of 1 in blends with PC60BM is most likely the
result of poor charge generation. Favorable coarse 20 nm
Acknowledgements
domain size for efficient charge generation while maintaining The authors acknowledge nancial support from the Natural
continuous percolation pathways to the electrodes is expected Sciences and Engineering Research Council Canada for both a
by varying the length of the alkyl chains. It was originally DG and SRG in addition to the Centre for Self-Assembled
expected that 2 would exhibit better device performance Chemical Structures and additional equipment funding from
because of its mono alkylation that would favor smaller domain the Canada Foundation for Innovation. Appreciation is also
sizes. However, since no increase in the device performance was extended to NanoQu ´e bec. WGS also thanks the Alexander von
observed for 2 (see Table 2), additional modications besides Humboldt Foundation for fostering this collaborative research
15624 | J. Mater. Chem. A, 2014, 2, 15620–15626
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Journal of Materials Chemistry A
and the RSC for the JWT Jones Travelling Fellowship and CSIRO
for photoelectron spectrometer access. AB thanks NSERC,
Fonds de recherche Nature et technologies Qu ´e bec (FQRNT),
and the Universit ´e de Montr ´e al for graduate scholarships. AB
also acknowledges FQRNT for a mobility grant. We thank Dr
J ¨o rg-Bernd Bonekamp for analysis of the AFM images. Ms.
Zahra Awan is thanked for assistance with FT-IR measurements.
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