Anthradithiophene-benzothiadiazole-based small molecule donors for organic solar cells

Article abstract of DOI:10.1039/c3nj00824j

A new A-D-A small molecule involving anthradithiophene as a donor and benzothiadiazole as an acceptor unit has been synthesized by Stille coupling reaction. Its thermal, optical and electronic properties, hole mobility and photovoltaic properties have been fully characterized. The resulting material shows a broad absorption range (300-750 nm), a low band gap (1.59 eV) and a moderate hole mobility (8.81 × 10^-4 cm² V ^-1 s^-1). We used the new small molecule blended with PC₇₁BM as the active layer to fabricate solution-processed organic solar cells (OSCs), and employed a variety of post-treatment methods to optimize the device performance. With the help of polar solvent exposure, the highest power conversion efficiency (PCE) of 0.55% was obtained. These results would supply useful information to understand the relationship between molecular structure and photovoltaic properties of anthradithiophene/benzothiadiazole- based OSCs.

Full text of DOI:10.1039/c3nj00824j

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PAPER
Anthradithiophene-benzothiadiazole-based small
molecule donors for organic solar cells
Qida Liu,ab Ying Liu,a Yang Wang,a Ling Ai,a Xinhua Ouyang,a Lei Han*b and
Ziyi Ge*a
Cite this: NewJ.Chem., 2013,
37, 3627
A new A–D–A small molecule involving anthradithiophene as a donor and benzothiadiazole as
an acceptor unit has been synthesized by Stille coupling reaction. Its thermal, optical and electronic
properties, hole mobility and photovoltaic properties have been fully characterized. The resulting
material shows a broad absorption range (300–750 nm), a low band gap (1.59 eV) and a moderate hole
mobility (8.81 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1). We used the new small molecule blended with PC71BM as the
active layer to fabricate solution-processed organic solar cells (OSCs), and employed a variety of post-
treatment methods to optimize the device performance. With the help of polar solvent exposure, the
highest power conversion efficiency (PCE) of 0.55% was obtained. These results would supply useful
information to understand the relationship between molecular structure and photovoltaic properties of
anthradithiophene/benzothiadiazole-based OSCs.
Received (in Montpellier, France)
25th July 2013,
Accepted 11th August 2013
DOI: 10.1039/c3nj00824j
www.rsc.org/njc
Introduction
Organic solar cells have received intensive scientific research
because of their low-cost, light-weight, mechanical flexibility,
roll-to-roll production and the tunable chemical properties of
the photovoltaic materials.1,2 In the past decades, numbers of
scientific researchers have been trying to find an active layer
molecule incorporating good solution processability, fine
charge carrier mobility, wide optical absorption profile, large
absorption coefficient, and appropriate energy band.3,4 So far, much
of the research effort has been devoted to the design and synthesis
of multifused aromatic molecules, such as benzodithiophene
(BDT)5 and its derivatives, which are excellent building blocks to
synthesize organic photovoltaic materials. As shown in Scheme 1,
the extended p-conjugation system of naphthodithiophene (NDT)6–9
is expected to afford strong intermolecular orbital overlap and
their electron-donating properties have been recently reported
for highly efficient solution-processed organic solar cells.10–13
Anthradithiophene (ADT), in which two thiophenes are fused to
the ends of the anthracene core, have been widely utilized as
key building blocks in the development of superior organic
semiconductors applicable to organic electronic devices such
as organic field effect transistors (OFETs)14 and displayed high
Scheme 1 Chemical structures of acenedithiophene derivatives (BDT, NDT,
and ADT).
hole mobility, broad absorption band and large conjugate
plane,15 implying that ADT could be a promising donor building
block applied in OSCs. Surprisingly, so far the OSC devices using the
ADT-based polymers have only shown power conversion efficiencies
(PCE) of less than 1%.16 Considering the above, we tried to develop
small molecules based on ADT to expand its applications in OSCs.
We report here on the synthesis, characterization, and photovoltaic
properties of a novel small molecule material based on the
5,11-bis(5-alkylthiophen-2-yl)anthra[2,3-b:6,7-b0]dithiophene moiety.
In order to extend its conjugation and guarantee its good solubility,
we introduced 2-(2-ethylhexyl)thiophene in the molecular structure
(shown in Scheme 2). The extended p-conjugation system of ADT
with alkylthiophene is expected to afford strong intermolecular
orbital overlap and electron-donating properties by utilizing a
favorable ‘‘push–pull’’ architecture. This could thereby enhance
charge separation, transport, and energy level tunability. The
thermal, optical and electronic properties were fully characterized.
It has been indicated that this small molecule possesses good
a Ningbo Institute of Material Technology and Engineering (NIMTE),
Chinese Academy of Sciences (CAS), Zhuangshi Road 519, Ningbo,
Zhejiang 315201, PR China. E-mail: geziyi@nimte.ac.cn; Fax: +86-574-86685043;
Tel: +86-574-86690273
b The School of Materials Science and Chemical Engineering, University of Ningbo,
Fenghua Road 818, Ningbo, Zhejiang 315201, PR China
c
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Scheme 2 Synthesis of ADT(DPBT)2. Reagents and conditions: (a) 1,4-cyclohexanedione–ethanol–15% aq KOH; (b) 2-(2-ethylhexyl)thiophene–n-BuLi–THF, ꢁ78 1C;
(c) n-BuLi–trimethylstannylchloride, ꢁ78 1C; (d) bis(4-tert-butylphenyl)amine–4,7-dibromobenzo[c][1,2,5]thiadiazole–palladiumacetate–tri-t-butylphosphine–sodium-
t-butoxide; (e) anhydrous toluene–DMF–Pd(PPh3)4.
solubility, high thermal stability, broad absorption band and and SnCl2ꢂ2H2O (6.3 g, 28 mmol) in HCl (10%, 11.2 mL) was
moderate hole mobility. Moreover, by adjusting the ratio of added, and the mixture was stirred for another 3 h. The mixture
small molecules and PC71BM, changing the annealing temperature was quenched with 15 mL ice water and extracted by ethyl acetate
and exposing the active layer to different polar solvents, the highest three times. The organic phases were dried with anhydrous MgSO4
PCE of 0.55% was achieved. But the result did not show its good and evaporated. The crude product was purified by column
properties, so we compared the reported molecules and discussed chromatography on silica gel (petroleum ether) to give compound
the possible reasons of the unexpected results.
3 as a red solid (1.61 g, 78%). 1H NMR (CDCl3, 400 MHz, d/ppm):
8.45 (s, 2H), 8.40 (s, 2H), 7.42 (d, 2H), 7.28 (d, 2H), 7.10 (s, 2H),
7.04 (s, 2H), 2.96 (m, 4H), 1.76 (m, 2H), 1.56–1.33 (br, 16H), 1.04–
0.92 (m, 12H). 13C NMR (CDCl3, 100 MHz, d/ppm), d = 154.73,
153.64, 151.56, 145.10, 144.27, 134.90, 127.72, 126.82, 125.20,
Experimental
General
1H and 13C NMR spectra were recorded with a Bruker DMX-400 124.05, 123.24, 117.52, 117.17, 34.34, 32.40, 29.74, 28.86, 25.63,
spectrometer. Differential scanning calorimeter (DSC) was measured 23.01, 14.15, 10.82.
using a Metler Toledo DSC822 instrument, and thermogravimetric
Synthesis of compound 3. Under an argon atmosphere, a
analysis (TGA) was determined by a PerkinElmer Pyris. UV-visible solution of compound 2 (1.5 g, 2.24 mmol) in 60 mL of dry THF
absorption spectra were collected on a PerkinElmer Lambda 950. cooled to ꢁ78 1C, n-BuLi (1.03 mL, 2.46 mmol, 2.4 M in hexane)
Cyclic voltammetry (CV) of the polymer film was studied on was added dropwise. After stirring for 1 h, the mixture was
a CHI650B potentiostat. Surface images were obtained on a brought to room temperature. The stirring was continued for
Veeco Dimension 3100V atomic force microscope. Solvents for another hour, and then the reaction mixture was cooled to
reactions and photophysical measurements were all distilled ꢁ78 1C again. A solution of trimethylstannyl chloride (0.49 g,
after dehydration according to conventional methods.
2.46 mmol) was added in one portion. The mixture was slowly
warmed up to room temperature and stirred overnight. The
reaction mixture was quenched with 30 mL water and extracted
Materials
Compound 1 was synthesized according to the literature.17 by ethyl acetate three times. The organic phases were dried
Other compounds were synthesized as following:
with anhydrous MgSO4 and evaporated to afford a red crude
Synthesis of compound 2. To a solution of 2-(2-ethylhexyl)- solid. The crude product was purified by recrystallization with
thiophene (2.42 g, 10.20 mmol) in 80 mL of dry THF cooled ethanol to yield a red solid (1.54 g, 55%). 1H NMR (CDCl3,
to ꢁ78 1C under an argon atmosphere, was added dropwise 400 MHz, d/ppm): 8.45 (s, 2H), 8.39 (s, 2H), 7.19–7.10 (d, 2H),
4.25 mL of n-BuLi (2.4 M in hexane) and the mixture was stirred 7.05–7.04 (d, 2H), 3.00–2.95 (t, 4H), 1.79–1.75 (m, 2H), 1.54–1.40
for 30 min. Then compound 1 (1.0 g, 3.09 mmol) was added in (m, 16H), 1.05–0.96 (m, 12H), 0.51–0.37 (m, 18H). 13C NMR
one portion. After another 30 min, the mixture was moved to (CDCl3, 100 MHz, d/ppm), d = 146.23, 145.20, 141.73, 140.15,
room temperature and stirred until the complete dissolution of 137.28, 131.80, 129.51, 129.35, 129.16, 128.83, 125.18, 119.40,
compound 1. Finally, the mixture was protected from the light 118.25, 41.57, 34.49, 32.59, 29.02, 26.01, 23.09, 14.26, 11.04.
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Synthesis of compound 4. Bis(4-tert-butylphenyl)amine Results and discussion
Paper
(1.5 g, 5.34 mmol), 4,7-dibromobenzo[c][1,2,5]thiadiazole (1.57 g,
5.34 mmol), palladium acetate (0.03 g, 0.13 mmol), and tri-t-
butylphosphine (0.55 g, 2.67 mmol), sodium-t-butoxide (0.65 g,
6.68 mmol) were dissolved in a flask with anhydrous toluene
under a nitrogen atmosphere. The reaction mixture was stirred for
12 h at 100 1C. The mixture were extracted with dichloromethane
and washed with distilled water three times. The organic layer
was dried over anhydrous MgSO4 and evaporated to give the
crude product, and the crude product was purified by column
chromatography to yield a light-red powdery product (1.43 g,
Synthesis
The synthetic routes of ADT(DPBT)2 are outlined in Scheme 2.
n-BuLi was added to a solution of 2-(2-ethylhexyl)thiophene in
THF, then compound 1 and SnCl2ꢂ2H2O in HCl(aq) was added,
finally the target product (compound 2) can be gained with
extraction and purification by column chromatography in 78%
yield. n-BuLi was added to a solution of compound 2 in THF,
then trimethylstannyl chloride was added in one portion, and
compound 3 can be got with extraction and purification by
column chromatography in 55% yield. Compound 4 was
synthesized by Buchwald–Hartwig cross-coupling reaction with
a 54% yield, and ADT(DPBT)2 was synthesized by Stille coupling
reaction between compound 3 and compound 4 in 46.8% yield.
All the compounds were verified by 1H NMR and 13C NMR.
ADT(DPBT)2 is readily soluble in common organic solvents
such as chloroform, chlorobenzene and dichlorobenzene.
1
54%). H NMR (acetone, 400 MHz, d/ppm): 7.86–7.84 (d, 1H),
7.35–7.33 (d, 4H), 7.03–7.01 (d, 1H), 6.99–6.97 (d, 4H), 1.3
(s, 18H). 13C NMR (CDCl3, 100 MHz, d/ppm): d = 154.59,
150.56, 146.52, 144.86, 140.28, 132.58, 126.09, 123.74, 122.54,
106.53, 53.43, 34.35, 31.41.
Synthesis of the ADT(DPBT)2. Compound
3 (1.00 g,
0.80 mmol) and compound 4 (0.82 g, 1.65 mmol), Pd(PPh3)4
(0.10 mg, 8.0 ꢀ 10ꢁ5 mmol) were dissolved in a flask with
anhydrous toluene (20 mL) and DMF (5 mL). The flask was
Thermal properties
evacuated and refilled with argon three times. Then the mixture The thermal properties of ADT(DPBT)2 were analyzed by
was heated to 120 1C and stirred for 48 h under an argon thermogravimetric analysis (TGA) and differential scanning
atmosphere. Then, the reactant was cooled to room tempera- calorimetry (DSC) under a nitrogen atmosphere at a heating
ture and poured slowly into methanol (150 mL). The precipitate rate of 10 1C minꢁ1. The TGA analysis reveals that the 5%
was filtered to obtain a purple solid and further purified by weight-loss temperature (Td) of ADT(DPBT)2 is 414 1C (Fig. 1a).
column chromatography and recrystallized with methanol to This indicates that ADT(DPBT)2 has good thermal stability,
1
yield a purple-dark powdery product (0.56 g, 46.8%). H NMR which is very important in the device fabrication process and
(CDCl3, 400 MHz, d/ppm): 8.56 (s, 2H), 8.43–8.42 (d, 2H), 8.37 other kinds of applications. DSC analysis (Fig. 1b) reveals that
(s, 2H), 7.75–7.73 (s, 2H), 7.31–7.29 (d, 8H), 7.15–7.14 (t, 2H), ADT(DPBT)2 possesses a prominent melting temperature (Tm)
7.12 (s, 2H), 7.08–7.06 (m, 2H), 7.03–7.00 (d, 8H), 3.03–2.99 at 247.4 1C and a distinct glass transition temperature (Tg) at
(m, 4H), 1.83–1.79 (m, 2H), 1.35 (d, 36H), 1.57–1.43 (m, 16), 172.0 1C, though no obvious crystallization point was observed.
1.07–0.98 (m, 12H). 13C NMR (CDCl3, 100 MHz, d/ppm): d =
153.84, 151.00, 146.70, 146.37, 144.89, 141.62, 141.48, 140.61,
136.99, 129.90, 129.76, 129.46, 129.10, 126.08, 125.21, 124.10,
123.64, 121.41, 121.01, 118.41, 41.56, 34.56, 34.38, 32.62, 31.42,
29.69, 29.01, 26.07, 23.08, 14.25, 11.04. Elemental analysis (%)
calcd for C94H100N6S6: C, 74.94; H, 6.69; N, 5.58. Found: C,
75.18; H, 6.55; N, 5.37.
Optical properties
The UV-Vis absorption spectra of ADT(DPBT)2 in diluted chloro-
form solution and in thin film are shown in Fig. 2. It exhibits
relatively broad and strong absorption between 300 and 650 nm
for the solution. The absorption maximum at 527 nm should
result from an intramolecular charge transfer (ICT) between
the electron-rich unit (ADT) and the electron-deficient unit
(BT). The absorption in the thin film state showed a similar
absorption trend with a red shift of about 3 nm, and exhibited
broader absorptions than its corresponding absorption in
solution. The optical band gap calculated from the film absorption
edge is 1.69 eV.
Device fabrication
OSCs were fabricated on the basis of ITO/PEDOT:PSS/
ADT(DPBT)2:PC71BM/Ca/Al configuration and their performances
were measured in an argon glovebox (200B, MBraun) under an
AM 1.5 Solar Constant 575 PV simulator (Steuernagel Lichttecknik,
equipped with a metal halogenide lamp). The ITO electrodes (10 O
per square, Kintec Company) were cleaned in ultrasonic baths and
then modified by a 40 nm thick spin-cast layer of PEDOT/PSS
‘‘Clevios P VP AI 4083’’ (HC Stark), which was dried at 130 1C for
20 min. The Clevios suspension was stirred and filtered through a
0.45 mm membrane (Minisart RC 15, Sartorius) just prior to casting.
Active layer ADT(DPBT)2/PC71BM was spin-coated on top of the
PEDOT/PSS layer at 1000 rpm. A 20 nm thick layer of Ca and
100 nm Al were thermally evaporated through a shadow mask, at a
pressure of about 10ꢁ6 mbar. The mask geometry defined a device
area of 0.1257 cm2.
Fig. 1 Thermogravimetric analysis (TGA) of ADT(DPBT)2 performed at a heating
rate of 10 1C minꢁ1 under a nitrogen atmosphere.
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Fig. 4 Energy diagram of the HOMO–LUMO energy levels for ADT(DPBT)2 and
Fig. 2 UV-Vis absorption spectra of ADT(DPBT)2 in diluted chloroform solution
PC71BM.
and in thin film.
Hole mobility
Electrochemical properties
As an important parameter of small molecules for photovoltaic
applications, the hole mobility of ADT(DPBT)2 was investigated
by the space-charge-limited current (SCLC) model with a device
structure of ITO/PEDOT:PSS/ADT(DPBT)2:PC71BM (90 nm)/
Au.20 For the hole-only devices, SCLC is described by modified
Mott–Gurney’s law:
Cyclic voltammetry (CV) was employed to examine the electro-
chemical properties and evaluate HOMO and LUMO energy
levels of the polymers. Fig. 3 shows the CV curves of
ADT(DPBT)2 with a platinum plate as working electrode and
Ag/Ag+ as reference electrode in acetonitrile with a 0.1 M
solution of Bu4NPF6 at room temperature with a scanning rate
of 50 mV sꢁ1. For calibration, the redox potential of ferrocene/
ferrocenium (Fc/Fc+) was measured under the same conditions,
and it is located at 0.40 eV to the Ag/AgCl electrode. It is
assumed that the redox potential of Fc/Fc+ has an absolute
energy level of ꢁ4.80 eV under vacuum.18 The HOMO and
LUMO energy levels were calculated according to the following
equations:19
pffiffiffiffiffiffiffiffiffi
J ¼ ð9=8Þe0ermðV2=d3Þ exp½0:89b V=dꢃ
We can get the following formula:
ln(Jd3/V2) D 0.89b(V/d)1/2 + ln(9e0erm/8)
The results are plotted as ln(Jd3/V2) versus (V/d)1/2, as shown in
Fig. 5. J stands for current density, d is the thickness of the active
layer, V is the applied potential, e0 is the relative dielectric
constant of the blend (assuming that 3.5), er is the permittivity
ox
EHOMO = ꢁ(E + 4.4) eV
(1)
(2)
(3)
on
of free space (8.85 ꢀ 10ꢁ12 C Vꢁ1
s
ꢁ1), b is the field activation
factor and m is the hole mobility. The hole mobility of
red
ELUMO = ꢁ(E + 4.4) eV
on
ADT(DPBT)2 is evaluated to be 8.81 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1
.
Eegc = ꢁ(EHOMO ꢁ ELUMO) eV
Obviously, the hole mobility of ADT(DPBT)2 is moderate as the
photovoltaic donor material in small-molecule BHJ solar cells.21
The oxidation and reduction onset potentials of ADT(DPBT)2
are 0.81 and ꢁ0.78 eV, respectively. The HOMO and LUMO
energy levels can be calculated as ꢁ5.21 eV and ꢁ3.62 eV,
respectively. The relatively deep HOMO level of ADT(DPBT)2
indicates its relatively high stability against oxidation and is
beneficial for obtaining high VOC when using PCBM as the
acceptor. The corresponding electrochemical bandgap is 1.59 eV,
which is similar to the optical band gap. The HOMO and LUMO
energy levels of the ADT(DPBT)2 are illustrated in Fig. 4.
Theoretical calculations
To gain more insight into the geometrical and electronic
properties of the ADT(DPBT)2, density functional calculations
were conducted using the B3LYP/6-31G* basis and Gaussian 03
program package. To simplify the calculations, the two 2-ethyl-
hexyl substituents on the thiophene unit and the four 1-tert-butyl
substituent on the benzene unit were replaced by methyl groups.
The frontier molecular orbitals of the ADT(DPBT)2 are shown in
Fig. 5 ln(Jd3/V2) versus (V/d)1/2 plots of ADT(DPBT)2 for the measurement of
hole mobility by the SCLC method for the device ITO/PEDOT:PSS/ADT(DPBT)2:
PC71BM/Au.
Fig. 3 CV of ADT(DPBT)2 films cast on platinum wire in 0.1 M Bu4NPF6/CH3CN at
a scan rate of 100 mV sꢁ1
.
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Fig. 6 Frontier molecular orbitals of the ADT(DPBT)2. LUMO (top) and HOMO
(bottom).
Fig. 6. The dihedral twist angle between the central ADT core and
the benzothiadiazole bridge unit obtained from the optimized
geometry of the small molecule is 131, illustrating that the A–D–A
p-conjugated system is a quasi-planar conjugated system. The
molecular orbitals show that the HOMO is essentially localized
on the ADT core, in contrast, as the LUMO is distributed over the
entire A–D–A p-conjugated system. The calculated energy levels
(HOMO = ꢁ4.40 eV, LUMO = ꢁ2.48 eV) are considerably higher
than experimental data, but it should be underlined that solvent
effects are neglected in calculation.22
Photovoltaic properties
The photovoltaic properties were investigated with the conventional
device structure as follows: ITO/PEDOT:PSS/ADT(DPBT)2:PC71BM/
Ca/Al. Herein we selected PC71BM instead of PC61BM as the
electron acceptor to get better absorption in the visible range,
and fabricated a large number of devices under various conditions,
such as different weight ratios of ADT(DPBT)2/PC71BM, changing
the annealing temperature and employing a variety of polar solvent,
etc. The J–V curves and J–V parameters of the OSCs based on
ADT(DPBT)2/PC71BM under the illumination of AM 1.5G,
100 mW cmꢁ2 are shown in Fig. 7 and Table 1. As shown in
Table 1, a 1 : 1 ratio of ADT(DPBT)2–PC71BM gave a performance
of the OSCs with a JSC of 1.55 mA cmꢁ2, a VOC of 0.69 V and a
PCE of 0.26% and after 10 min of the thermal treatment at
150 1C, approximately 1.5 times higher PCE (0.39%) was
Fig. 7 J–V characteristics of the OSCs based on ADT(DPBT)2/PC71BM under the
illumination of AM 1.5G, 100 mW cmꢁ2. (a) Prepared with different weight ratios
of ADT(DPBT)2/PC71BM; (b) treated with different annealing temperature;
(c) exposed with different polar solvents. J–V characteristics of the OSCs in the dark.
Table 1 J–V parameters for the ADT(DPBT)2:PC71BM OSCs with different weight
ratio or treatments
Active layer [ADT(DPBT)2:PC71BM]
Weight
ratio (wt%) Treatment
JSC
VOC (V) (mA cmꢁ2
)
FF (%) PCE (%)
2 : 1
1 : 2
None
None
None
0.67
0.75
0.69
0.93
0.94
1.55
1.54
2.26
1.69
2.38
2.58
2.38
25.22 0.18
24.93 0.19
24.38 0.26
25.04 0.25
26.23 0.39
26.94 0.28
25.52 0.49
26.43 0.55
26.53 0.46
obtained. In order to further improve the performance of the 1 : 1
OSCs, three different polar solvents were used to optimize the
1 : 1
120 1C annealed 0.64
150 1C annealed 0.66
180 1C annealed 0.61
MeOH exposed 0.80
EtOH exposed
i-PrOH exposed 0.73
morphology of the active layer.23 The procedure was as follows:
after the active layer was spin-coated, 0.2 mL polar solvent 1 : 1
was added dropwise on the active layer and removed by spin
coating. With the help of polar solvent treatment, a large
increase of JSC and VOC were obtained and we got the highest
0.80
PCE of 0.55% after the ethanol treatment.
Zhong et al.25 had proved that a mixture of syn and anti isomers
This relatively low PCE did not reflect its superior properties, in the active layer led to a lower PCE than pure syn isomers; they
which were out of our expectation. The related research of held the opinion that anti isomers behave essentially as inactive
Jiang et al.24 also presented a similar phenomenon in which impurities, forming poorly-functioning crystalline domains that
polymers with a high hole mobility did not show high efficiency. hinder charge transport. We believe that the traditional synthesis of
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benzothiadiazole as acceptor unit by Stille coupling reaction,
and its thermal, optical and electronic properties, hole mobility
and photovoltaic properties have been fully characterized. We
used ADT(DPBT)2 blending with PC71BM as the active layer to
fabricate solution-processed OSCs and employed thermal
annealing and polar solvent treatment to optimize the device
performance. After exposure to polar solvents, the highest PCE
of 0.55% was obtained due to the optimized morphology of the
active layer, and the relatively low efficiency may be caused by
the existence of the two isomers. Though the device parameters
were not ideal, these results would supply useful information to
design, optimize and understand the relationship between
molecular structure and photovoltaic properties of anthra-
dithiophene/benzothiadiazole-based OSCs.
Acknowledgements
The present research was financially supported by the National
Natural Science Foundation (no. 21074144, no. 51273209, no.
21102156, no. 91122012), the NIMTE Foundation (Y20841QF07),
Qianjiang Talent Project, and the External Cooperation Program
of the Chinese Academy of Sciences (no. GJHZ1219).
Fig. 8 Tapping-mode AFM topography images (left) and 3D surface plots (right)
of the blend film of ADT(DPBT)2–PC71BM (1 : 1, w/w): (a) without any treatment;
(b) annealed at 150 1C; (c) exposed to ethanol (all images: 5.0 mm ꢀ 5.0 mm).
Notes and references
the ADT inevitably afforded an inseparable mixture of syn and anti
isomers resulting in undefined molecular conformation,26 and the
existence of the two isomers led to the irregular state of aggregation
and invalid phase separation so as to affect the electron and hole
transfer which may be the main reason for the low power conver-
sion efficiency.
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Morphology
As a very important factor to affect the performance of OSCs, the
morphological features of the photoactive layer were studied by
tapping-mode atomic force microscopy (AFM). The AFM topography
and 3d surface plots are shown in Fig. 8. It could be observed that
the blend films exhibited uneven surfaces with a lot of peak-like
spikes with a root-mean-square (RMS) roughness of 0.332 nm, which
could have a side effect on the exciton separation, thus leading to a
lower JSC and PCE. When the blend films were annealed at 150 1C or
exposed with ethanol, the root-mean-square (RMS) roughness
decreases to 0.276 nm for ADT(DPBT)2 :PC71BM (1:1, w/w, annealed
at 150 1C) and 0.251 nm for ADT(DPBT)2 :PC71BM (1 : 1, w/w,
exposed with ethanol), and led to a 1.5–3 times increase in JSC and
PCE compared with the devices without any treatment. As shown in
the images, morphologies of the blend films were moderately
homogenous and there was no large phase separation, indicating
that both of the small molecules possessed good miscibility with
PC71BM,27 which is not favorable for electron and hole transfer and
may be another reason for the low power conversion efficiency.
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3632 New J. Chem., 2013, 37, 3627--3633
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New J. Chem., 2013, 37, 3627--3633 3633