A high voltage solar cell using a donor-acceptor conjugated polymer based on pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-dione

Article abstract of DOI:10.1039/c4ta01248h

We report an improved synthesis of the electron-accepting unit pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-dione (BTI) and the synthesis and characterisation of two donor-acceptor copolymers containing it: poly[6-dodecyl-4,8-bis(2-thienyl)pyrrolo[3,4-f]-2,1

Full text of DOI:10.1039/c4ta01248h

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Materials Chemistry A
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A high voltage solar cell using a donor–acceptor
conjugated polymer based on pyrrolo[3,4-f]-2,1,3-
benzothiadiazole-5,7-dione†
Cite this: J. Mater. Chem. A, 2014, 2,
17925
Hairong Li,‡ab Shuangyong Sun,‡a Subodh Mhaisalkar,ab Melvin T. Zin,§c
ab
ab
*
*
Yeng Ming Lam
and Andrew C. Grimsdale
We report an improved synthesis of the electron-accepting unit pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-
dione (BTI) and the synthesis and characterisation of two donor–acceptor copolymers containing it: poly
[6-dodecyl-4,8-bis(2-thienyl)pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-dione-alt-9,9-dioctyl-2,7-bis(thien-
20-yl)-9H-fluorene] (P(BTI-F)), and poly[6-dodecyl-4,8-bis(thien-20-yl)pyrrolo[3,4-f]-2,1,3-benzothiadiazole-
5,7-dione-alt-4,8-bis(octyloxy)benzo[1,2-b:4,5-b0]dithiophene] (P(BTI-B)). By the incorporation of an imide
group onto a benzothiadiazole moiety, the HOMO level is lowered. To explore the effectiveness of
conjugation along the polymer backbone, two electron donors: fluorene and benzobithiophene, were
introduced. These represent a weak and a strong electron-donor, respectively. Both polymers have low
lying HOMOs and hence are thermally stable and also result in high open-circuit voltages (Voc) in
photovoltaic applications. Compared to P(BTI-B), P(BTI-F) has a larger bandgap due to the incorporation
of a weaker donor unit and thus a deeper lying HOMO level. Devices based on P(BTI-F) and [6,6]-phenyl-
C61-butyric acid methyl ester (PC61BM) can obtain a remarkably high Voc of 1.11 V, but a PCE of only 1.61%.
By contrast, polymer P(BTI-B) has stronger absorption in the longer wavelength range and can achieve a
well-dispersed nanomorphology with PC61BM for efficient exciton dissociation, achieving PCE of 3.42%,
with a Jsc of 9.71 mA cmꢀ2, Voc of 0.75, and FF of 0.47.
Received 13th March 2014
Accepted 5th September 2014
DOI: 10.1039/c4ta01248h
www.rsc.org/MaterialsA
been extensively explored. An important structural feature of
many BT-type materials is the attachment of the two anking
Introduction
Benzo[c][2,1,3]thiadiazole (BT)1–34 and its derivatives such as
benzo[1,2-c:4,5-c0]bis[1,2,5]thiadiazole (BBT),35–46 [1,2,5]-thia-
diazolo[3,4-g]quinoxaline (TQ)2,47–62 represent one of the most
widely studied class of electron-accepting units for making
donor–acceptor (D–A) polymers or small molecules. Attributed
to the tunable electronic structures (HOMO, LUMO, bandgap),
versatile chemical functionalization and robust properties,
their electronic device applications such as organic eld-effect
transistors (OFET), organic light-emitting diodes (OLED),
organic photovoltaics (OPV) and electrochromic devices have
thienyl units at the 4,7-positions of BT, which results in large
electron delocalization along the thiophene–benezene–thio-
phene direction due to strong quinoid character of the BT unit
and reduced steric hindrance, thereby reducing the band gap
and facilitating the intermolecular p–p interactions and charge
transport.44,61,63 However, one of the main disadvantages of BT
based monomers is their poor solubility and they must be
coupled with donor monomers that contain sufficiently long
alkyl chain to make the polymer soluble. In earlier work we
increased the solubility of such polymers by attaching alkoxy
chains on the 5,6-positions of BT but this resulted in strong
electron localization effect within BT and hence the bandgap
remained at ꢁ2 eV even when very electron-donating units such
as benzo[1,2-b:4,5-b0]dithiophene (BDT), dithieno[3,2-b:20,30-d]
silole and thieno[3,2-b]thiophene were coupled to these BT
polymers.6 To address this issue we herein report the synthesis
of halogenated pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-dione
(BTI) units enabling their more easy incorporation into poly-
mers or oligomers. The introduction of the imide group onto BT
not only facilitates the alkylation but also lowers the HOMO
level to possibly obtain a high solar cell open circuit voltage Voc
while maintaining a reduced band gap. Fluorene and benzobi-
thiophene representing a weak and a strong electron-donor,
aSchool of Materials Science and Engineering, Nanyang Technological University, Block
N4.1, 50 Nanyang Avenue, 639798, Singapore. E-mail: ymlam@ntu.edu.sg;
acgrimsdale@ntu.edu.sg
bEnergy Research Institute @ NTU (ERI@N), Nanyang Technological University, 50
Nanyang Drive, 637533, Singapore
c3M Research Centre, 100 Woodlands Avenue 7, 738205, Singapore
† Electronic supplementary information (ESI) available: Single crystal XRD data
le of the compound 5, named “BTI_XRD.cif”. CCDC 965762. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ta01248h
‡ These authors contributed equally to this work.
§ Current address: 3M Electronics Markets Materials Division, 222 Tianlin Road,
Shanghai 200233.
This journal is © The Royal Society of Chemistry 2014
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sꢀ1. The experiments were carried out at room temperature
with a conventional three electrode conguration consisting
of a platinum working electrode coated with polymer lm, a
gold counter electrode, and aSCE (saturated calomel elec-
trode) reference electrode (ꢀ4.40 eV relative to vacuum cali-
brated by Fc/Fc+).64 The HOMO, LUMO energy levels and
electrochemical band gaps were calculated by the following
equations:
HOMO ¼ ꢀ(4.40 + oxidation onset) eV
LUMO ¼ ꢀ(4.40 + reduction onset) eV
Eg ¼ LUMO ꢀ HOMO
(1)
(2)
(3)
Fig. 1 Chemical structures of the two BTI based polymers.
Single X-ray diffraction data were collected on a Bruker AXS
SMART CCD 3-circle diffractometer with a Mo-Ka radiation
respectively, were introduced to explore the effectiveness of
conjugation along the polymer backbonein two D–A conjugated
polymers, P(BTI-F) and P(BTI-B), shown in Fig. 1. The promising
device performance using blends of these copolymers with
fullerene acceptors in bulk heterojunction (BHJ) solar cells are
also reported, in comparison to the performance reported for
other polymers containing BTI units.
˚
(0.71073 A). The soware used was SMART22 for collecting
frames of data, indexing reections, and determining lattice
parameters; SAINT22 for integration of the intensity of
reections and scaling; SADABS23 for absorption correction;
and SHELXTL24 for space group determination, structure
solution, and least-squares renements on F2. Structures were
solved by direct methods and non-hydrogen atoms were
rened anisotropically. Hydrogen atoms were introduced at
xed distances from carbon atoms and assigned xed thermal
parameters. All calculations were performed on a Silicon
Graphics workstation, using programs provided by Siemens
Pvt. Ltd.
Experimental section
Materials and equipments
Chemicals and reagents were purchased from either Aldrich
or Alfa-Aesar and were used without further purication
unless otherwise stated. Tetrahydrofuran (THF) was freshly
distilled over sodium wire before use. N-Bromosuccinimide
Solar cells fabrication and characterization
(NBS) was recrystallised from distilled water before use. The active lms for the solar cells are made up of polymer:[6,6]-
Column chromatography was carried out with Merck silica- phenyl-C61-butyric-acid methyl ester (P61CBM) blended at
gel (230–400 mesh) while thin layer chromatography (TLC) different weight ratios (1 : 1, 1 : 2 and 1 : 3). Indium tin oxide
were performed on Merck silica-gel 60 PEG-backed plates (20 (ITO)/glass substrates were purchased from Kintec Company (7
cm ꢂ 20 cm). The NMR spectra were collected on a Bruker U sqꢀ1). The ITO/glass substrates were ultrasonic-cleaned by
DPX 400 spectrometer using chloroform-d as solvent and detergent, deionizer water, acetone and isopropanol for 15 min
tetramethylsilane (TMS) as internal standard. Matrix assisted each in sequence. Following that, the ITO substrates were
laser desorption/ionization time-of-ight (MALDI-TOF) mass exposed for 2 min to air plasma. A layer of 30 nm of poly(3,4-
spectra were obtained on a Shimadzu AXIMA Performance. ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS)
Elemental analysis was obtained via a Thermo Scientic Flash lm was spin-coated on ITO and then the substrates were baked
2000 Series CHNS Analyzer. Thermogravimetric analysis at 140 ꢃC for 10 min in a glove box. The polymer: P61CBM blend
(TGA) was carried out using a TA Instrument Q500 Ther- solutions were then spin-coated onto the PEDOT:PSS layer. The
mogravimetric Analyzer at a heating rate of 10 ꢃC minꢀ1 up to lm thickness of the active layers is around 70 nm. For some
900 ꢃC and at a nitrogen ow rate of 75 cm3 minꢀ1. Differ- devices, TiOx ethanol solution was spin-coated on the photo-
ential scanning calorimetry (DSC) was run on a TA Instrument active layer to form electron buffer layer and hydrolysed in the
Q10. Molecular weight of polymers was determined by gel ambient for 40 min. Finally, aluminium was deposited on the
permeation chromatography (GPC) method in chloroform, active layer under vacuum (10ꢀ6 torr) to form the cathode (ꢁ100
which was performed on Agilent GPC 1100 series model nm). The active area for each device was 0.07 cm2. Current
G1311A. In this method, column of PL gel 5 mm, MIXED-C was density–voltage (J–V) characteristics of the devices were
used and polystyrene was used as a standard, THF was used as measured using Agilent 4155C Semiconductor Parameter
the eluent at 20 ꢃC with ow rate of 1 mL minꢀ1. UV-Vis Analyzer under simulated AM 1.5G illumination (100 mW
absorption spectra were recorded on a Shimadzu UV-Vis 2501 cmꢀ2). The external quantum efficiency (EQE) measurement
spectrometer. Cyclic Voltammograms (CV) of the polymers was performed using the Merlin radiometer (Newport) with
were recorded on a CHI 660 electrochemical workstation in an calibrated Si photodiode (Hamamatsu) as the reference. Atomic
acetonitrile
solution
of
tetrabutylammoniun
hexa- force microscopy (AFM) images were obtained on the Digital
uorophosphate (Bu4NPF6) (0.1 M) at scanning rate of 100 mV Instruments Nanoscope IIIausing the tapping mode.
17926 | J. Mater. Chem. A, 2014, 2, 17925–17933
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Journal of Materials Chemistry A
5,6-Diamino-2-dodecylisoindoline-1,3-dione (3). Compound
2 (5 g, 13.3 mmol) and SnCl2 (5.1 g, 26.6 mmol) were dissolved
in the solvent mixture of EtOH (100 mL) and 37% HCl (40 mL),
the reaction mixture was brought to reux for 6 h. Cooled down
and precipitated from water, the white solid was washed thor-
oughly with water, MeOH and dried. The product was used
immediately without further purication due to its instability in
solution, though it was stable in the solid state as the 2HCl salt.
6-Dodecyl-4,8-dibromopyrrolo[3,4-f]-2,1,3-benzothiadiazole-
5,7-dione (4). At 0 ꢃC, SOCl2 (2.2 g, 18.6 mmol) was added to the
100 mL CHCl3 solution of compound 3 (1.6 g, 4.64 mmol) fol-
lowed by addition of Br2 (2.9 g, 18.6 mmol), the solution was
stirred for 0.5 h in ice-water bath, then warmed up to r.t. and
followed by addition of NEt3 (4.7 g, 46.4 mmol), aer overnight
the solution was poured into water, extracted with DCM. The
extract was washed with water, dried over anhydrous MgSO4
and ltered. The ltrate was evaporated and the residue was
puried by chromatography on silica eluting with DCM to yield
(4) as white crystals (1.48 g, 60%). 1H NMR (CDCl3) d: 3.77 (t, J ¼
7.2 Hz, 2H, NCH2), 1.73 (quintet, J ¼ 7.6 Hz, 2H, NCH2CH2),
1.24–1.35 (m, 18H, CH2), 0.86 (t, J ¼ 6.8 Hz, 3H, CH3). 13C NMR
(CDCl3) d: 164.5, 156.0, 130.6, 113.6, 40.0, 32.5, 30.29, 32.23,
30.1, 30.0, 29.8, 28.8, 27.5, 23.3, 14.7. MS (MALDI-TOF) m/z:
530.92 (M+). Calcd m/z (100%): 531.00. Anal. calcd for C20H25
-
Br2N3O2S: C, 45.21; H, 4.74; N, 7.91; S, 6.04%. Found: C, 45.37;
H, 4.70; N, 8.13; S, 5.91%.
6-Dodecyl-4,8-di(thien-20-yl)pyrrolo[3,4-f]-2,1,3-benzothiadi-
azole-5,7-dione (5). 1.2 g of compound 4 (2.26 mmol), 2.1 g of 2-
tributylstannylthiophene (5.65 mmol) and 95 mg of Pd
[PPh3]2Cl2 (6 mol%) were added to a 100 mL round bottom ask
purged with N2 gas, followed by addition of 50 mL of eshly
distilled THF. The reaction was stirred under reux for 1 day.
The cooled mixture was poured into water and extracted with
DCM. The organic layer was collected, dried over anhydrous
MgSO4 and concentrated. The ltrate was evaporated and the
residue was puried by chromatography on silica eluting with
Scheme 1 Synthetic routes to the two polymers. Reaction conditions:
(i) KOH, C12H25Br, EtOH, reflux. (ii) SnCl2, HCl (37%), EtOH, reflux. (iii)
Br2, SOCl2, CHCl3, NEt3, 0 ꢃC to r.t. (iv) 2-ThSnBu3, PdCl2(PPh3)2, THF,
reflux. (v) DMF, NBS, r.t. (vi) PdCl2(PPh3)2, THF, reflux. (vii) SOCl2, CHCl3,
pyridine, 0 ꢃC to reflux.
Synthesis
The synthetic route is summarized in Scheme 1. 5-Amino-6-
nitroisoindoline-1,3-dione (1)65 and 2,7-bis(5-(tributylstannyl)
thiophen-2-yl)-9,90-bisoctyl-9H-uorene (7)66 and 4,8-bis(octy-
loxy)-2,6-bis(tributylstannyl)benzo[1,2-b;3,4-b0]dithiophene (8)28
were synthesized according to literature procedures.
1
DCM to yield (5) as yellow crystals (1.1 g, 90% yield). H NMR
(CDCl3, d ppm): 7.90 (dd, J ¼ 4 Hz, 2H, ThH), 7.71 (dd, J ¼ 4.8
Hz, 2H, ThH), 7.28 (m, 2H, ThH), 3.72 (t, J ¼ 7.6 Hz, 2H, NCH2),
1.69 (quintet, J ¼ 7.2 Hz, 2H, NCH2CH2), 1.23–1.32 (m, 18H,
CH2), 0.87 (t, J ¼ 6.8 Hz, 3H, CH3). 13C NMR (CDCl3) d: 166.43,
157.2, 133.7, 132.2, 130.8, 127.7, 127.5, 127.3, 39.6, 32.5, 30.28,
30.22, 30.1, 30.0, 29.8, 28.9, 27.6, 23.3, 14.7. MS (MALDI-TOF) m/
z: 537.13 (M+). Calcd m/z (100%): 537.16. Anal. calcd for
5-Amino-2-dodecyl-6-nitroisoindoline-1,3-dione (2). Compo-
und 1 (10 g, 48.3 mmol), KOH (2.71, 17.0 g, 48.3 mmol) and
C
12H25Br (12.0 g, 48.3 mmol) was dissolved in EtOH (70 mL).
The solution was heated up to reux for 1 day, cooled down and
poured into water, and extracted with dichloromethane (DCM).
The extract was washed with water, dried over anhydrous
MgSO4 and ltered. The ltrate was evaporated and the residue
was puried by chromatography on silica eluting with
dichloromethane (DCM) to yield (2) as a yelow solid (14.5 g,
C
28H31N3O2S3: C, 62.54; H, 5.81; N, 7.81; S, 17.89%. Found:
62.66; H, 5.74; N, 7.95; S, 17.76%.
6-Dodecyl-4,8-bis(50-bromothiophen-20-yl)pyrrolo[3,4-f]-2,1,-
3-benzothiadiazole-5,7-dione (6). Compound 5 (1 g, 1.86 mmol)
and NBS (0.73 g, 4.10 mmol) were dissolved in 40 mL DMF and
stirred at r.t. overnight, the solution was poured into water and
ltered, the solid was thoroughly washed with water and MeOH,
then dried to yield (6) as an orange red solid in quantitative
yield. 1H NMR (CDCl3, d ppm): 7.78 (d, J ¼ 4 Hz, 2H, ThH), 7.22
(d, J ¼ 4 Hz, 2H, ThH), 3.73 (t, J ¼ 7.6 Hz, 2H, NCH2), 1.69
(quintet, J ¼ 7.2 Hz, 2H, NCH2CH2), 1.23–1.33 (m, 18H, CH2),
0.87 (t, J ¼ 6.8 Hz, 3H, CH3). 13C NMR (CDCl3) d: 166.3, 156.6,
1
85%). H NMR (CDCl3) d: 8.62 (s, 1H, PhH), 7.28 (s, 1H, PhH),
6.76 (br, 2H, NH2), 3.66 (t, J ¼ 7.2 Hz, 2H, NCH2), 1.65 (quintet, J
¼ 7.6 Hz, 2H, NCH2CH2), 1.24–1.31 (m, 18H, CH2), 0.87 (t, J ¼
6.8 Hz, 3H, CH3). 13C NMR (CDCl3) d: 167.18, 167.11, 149.5,
138.3, 134.5, 123.7, 119.5, 114.6, 39.3, 32.6, 30.3, 30.2, 30.1,
30.0, 29.8, 29.1, 27.5, 23.3, 14.8. MS (MALDI-TOF) m/z: 375.20
(M+). Calcd m/z (100%): 375.22. Anal. calcd for C20H29N3O4: C,
63.98; H, 7.79; N, 11.19%. Found: C, 64.20; H, 7.67; N, 11.37%.
This journal is © The Royal Society of Chemistry 2014
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134.7, 133.7, 130.5, 127.1, 126.6, 119.3, 39.7, 32.5, 30.29, 30.23, the synthetic details.67 The synthesis which we now report here
30.1, 30.0, 29.8, 28.9, 27.6, 23.3, 14.7. MS (MALDI-TOF) m/z: has considerable advantages over previous reports of BTI
694.91 (M+). Calcd m/z (100%): 694.98. Anal. calcd for C28H29
-
derivatives. The brominated dithienyl compound 6 has previ-
Br2N3O2S3: C, 48.35; H, 4.20; N, 6.04; S, 13.83%. Found: 48.56; ously been made, and used to make some reduced bandgap
H, 4.15; N, 6.21; S, 13.90%. polymers, including an analogue of P(BTI-B) bearing
a
6-Dodecyl-pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-dione (9). branched alkyl chain on the BTI unit, by a totally different and
At 0 ꢃC, SOCl2 (2.2 g, 18.6 mmol) was added to the 100 mL CHCl3 less efficient synthetic route involving synthesis of the BTI unit
solution of compound 3 (1.6 g, 4.64 mmol) followed by slow from a different heterocycle.68 Our route is shorter, more effi-
addition of anhydrous pyridine (3.7 g, 46.4 mmol) over 20 min, cient and more exible as it provides access to a wider range of
the solution was then brought to reux stirred for 4 h. The polymers and oligomers. Compound N-dodecyl-5,6-dicarboxylic
solution was cooled down, poured into water and extracted with imide-benzo[c][2,1,3]-thiadiazole (9), has also previously been
DCM. The extract was washed with water, dried over anhydrous made by Chi et al. by a different method,69 however all attempts
MgSO4 and ltered. The ltrate was evaporated and the residue to brominate the 4,7-positions of compound 9 including using
was puried by chromatography on silica eluting with DCM to bromine in strong acid at high temperature, were unsuccessful
yield (9) as white crystals (1.48 g, 78%). 1H NMR (CDCl3) d: 8.49 because of the deactivating imide and BT segments. Adding
(s, 2H, PhH), 3.77 (t, J ¼ 7.2 Hz, 2H, NCH2), 1.73 (quintet, J ¼ 7.6 bromine to the reaction mixture of compound 3 before adding
Hz, 2H, NCH2CH2), 1.24–1.34 (m, 18H, CH2), 0.87 (t, J ¼ 6.8 Hz, SOCl2, also failed to produce 4 as the brominated diamine is too
3H, CH3). 13C NMR (CDCl3) d: 167.2, 157.4, 132.3, 118.8, 39.6, unstable to survive the annulation to form BT. Therefore, we
32.6, 30.3, 30.2, 30.1, 30.0, 29.8, 29.0, 27.6, 23.3, 14.8. MS controlled the reaction conditions so that Br2 was added aer
(MALDI-TOF) m/z: 373.09 (M+). Calcd m/z (100%): 373.18. Anal. formation of a ring structure but before the nal dehydration to
calcd for C20H27N3O2S: C, 64.31; H, 7.29; N, 11.25; S, 8.58%. form BT stimulated by base. Our TLC results showed that as
Found: C, 64.57; H, 7.22; N, 11.34; S, 8.31%.
long as the time and temperature were properly controlled, the
P(BTI-F). Compound 6 (0.45 g, 0.753 mmol), 7 (0.85 g, 0.753 major product was compound 4 with a little mono-brominated
mmol) and Pd[PPh3]2Cl2 (30 mg, 6 mol%) were added to a 50 mL side product and a negligible amount of compound 9, both
round bottom ask purged with N2 gas, followed by addition of which could be removed by chromatography on silica.
20 mL of eshly distilled THF. The reaction was stirred under
The TGA and DSC results were shown in Fig. 2 and
reux for 2 days, then cooled down and poured into methanol summarized in Table 1. Both polymers showed very high
and ltered. The resulting solid was further puried by Soxhlet thermal stability. Decomposition (Td in Table 1) of P(BTI-F)
extraction using methanol, hexane and acetone to remove low starts at 400 ꢃC, while for P(BTI-B) it commences at 310 ꢃC. It is
MW polymers and impurities. Finally the polymers was extrac- obvious that the thermal stability is largely determined by the
ted out using chloroform, which was evaporated under reduced donor part wherein the bisthienyluorene (F) is much more
pressure to afford the polymer as a black solid (0.48 g, 62%). 1H
NMR (CDCl3) d: 8.05 (br, ArH), 7.88 (br, ArH), 7.68 (br, ArH), 7.59
(br, ArH), 7.38 (br, ArH), 3.74 (br, NCH2), 2.05 (br, 9-CH2 on
uorene), 1.72 (br, NCH2CH2), 1.10–1.35 (m, br, CH2), 0.87 (br,
CH3), 0.80 (br, CH3). Anal. calcd for (C65H73N3O2S5)n: C, 71.71;
H, 6.76; N, 3.86; S, 14.73%. Found: C, 71.19; H, 7.02; N, 4.08; S,
14.27%. GPC: Mn ¼ 11 263 g molꢀ1, Mw ¼ 21 560 g molꢀ1, PDI ¼
1.91.
P(BTI-B). P(BTI-B) was synthesized by copolymerization of
compound 6 and 8 by the same method as for polymer P(BTI-F),
as a black solid in 45% yield. 1H NMR (CDCl3) d: 8.05 (br, ArH),
7.76 (br, ArH), 7.64 (br, ArH), 7.38 (br, ArH), 4.31 (br, OCH2), 3.71
(br, NCH2), 1.94 (br, OCH2CH2), 1.62 (br, NCH2CH2), 1.25–1.34
(m, br, CH2), 0.90 (br, CH3). Anal. calcd for (C54H65N3O4S5)n: C,
66.15; H, 6.68; N, 4.29; S, 16.35%. Found: C, 66.44; H, 6.89; N,
3.80; S, 15.85%. GPC: Mn ¼ 11 454 g molꢀ1, Mw ¼ 17 934 g
molꢀ1, PDI ¼ 1.57.
Results and discussion
Synthesis and characterization
All polymers were fully characterized and are readily soluble in
chloroform, THF, chlorobenzene and 1,2-dichlorobenzene. The
most critical synthetic intermediate is N-dodecyl-5,6-dicarbox-
ylic imide-4,7-dibromo-benzo[c][2,1,3]thiadiazole (4). This
compound was previously reported by us but without describing normalized absorption spectra of the two polymers in chloroform.
Fig. 2 Top: TGA and DSC (inset) of the two polymers; bottom:
17928 | J. Mater. Chem. A, 2014, 2, 17925–17933
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Journal of Materials Chemistry A
Table
polymers
1 Thermal, optical and electrochemical properties of the
A single crystal X-ray diffraction study of compound 5
revealed that the central BTI core was almost planar while the
thiophene groups at 4,7-positions were twisted from the central
core (Fig. 4). The sulfur atoms on both thiophene groups
pointed towards the same direction relative to the plane of the
central core and oriented closer to the thiadiazole side. It was
interesting to observe that this molecule packed as a pair with
both molecules oriented in the same direction, i.e. alkyl chain to
alkyl chain, core to core. The measurement of torsion angle
between two thiophenes and the connected central benzene
also conrmed that there were four different values: 40.84ꢃ,
44.10ꢃ for one and 39.87ꢃ, 41.83ꢃ for the other, which also
indicated that both thiophene groups at 4,7-positions in this
molecule were not completely mirror symmetrical.
Polymers
P(BTI-F)
P(BTI-B)
HOMO (eV)
LUMO (eV)
Eg (eV)
ꢀ5.60
ꢀ3.74
1.86
ꢀ5.36
ꢀ3.68
1.68
lmax (nm)
551, 403
400
610, 444, 392
310
Td (ꢃC)
Tg (ꢃC)
93
124
stable than benzodithiophene (B). On the other hand, P(BTI-F)
has a lowerglass transition temperature (Tg in Table 1) than
P(BTI-B). Considering the same acceptor unit is in both poly-
mers, the sp3 conguration of the uorene C9 bearing the alkyl
chains in P(BTI-F) simply creates greater separation between the
polymer chains during packing as compared to the planar
benzodithiophene in P(BTI-B), resulting in higher exibility at
elevated temperatures.
Solar cell performance
The photovoltaic properties of both polymers were investigated
using the conventional device conguration of ITO/PEDOT:PSS/
polymer blend/Al with [6,6]-phenyl C61-butyric acid methyl ester
(PC61BM) as acceptor in the polymer blend. Fig. 5 shows the
lm absorption of P(BTI-F) and P(BTI-B) thin lms blended with
PC61BM at their optimized weight ratios for device operation.
The optimized blend ratios for the two polymers are different as
the molecular structure of the two polymers, their miscibility
with PC61BM and the resulting lm morphology are different.
The best weight ratio for P(BTI-F) and PC61BM solar cells is 1 : 2;
while for P(BTI-B) and PC61BM devices, the value is 1 : 1. These
blended polymer lms were prepared from 1,2-dichlorobenzene
solution without thermal annealing and the absorption proles
are normalized with respect to the absorption maximum peak
of PC61BM at 330 nm. It should be noted that even the proles
are normalized with lower PCBM content for P(BTI-B), the
absorption intensity from P(BTI-B) in P(BTI-B):P61CBM
composite is still stronger in the longer wavelength range than
that of P(BTI-F):PC61BM blend lm. This is very likely to
generate larger electrical current under short-circuit conditions
The absorption spectra of the polymers were recorded in
chloroform solution as shown in Fig. 2. The peak maxima (lmax
)
and electrochemical band gaps (Eg) are summarized in Table 1.
Comparing P(BTI-F) with P(BTI-B), the peak maximum attrib-
uted to the p–p* absorption of the conjugated polymer back-
bone for the former (560 nm) is 50 nm shorter than the latter
(610 nm), which implies the benzodithiophene in P(BTI-B)
increases the intramolecular charge transfer (ICT) between
donor and acceptor, causing more electronic delocalization
along the polymer backbones.70 The electron rich BDT unit has
been widely incorporated into D–A polymer structures, leading
to a range of the bandgaps even down to 1 eV when coupling
with a variety of acceptors,28,29,71–75 it is not surprising to observe
that the stronger donor–acceptor interaction in P(BTI-B)
signicantly lower the band gap as compared to the weaker
donor in P(BTI-F) (Eg in Table 1). Cyclic voltammetry measure-
ments (Fig. 3) revealed that both polymers had close lowest
unoccupied molecular orbital (LUMO) energy levels, which can
be attributed to the dominating effect of the electron-with-
drawing BTI unit. When comparing P(BTI-F) with P(BTI-B), the
stronger donor in the latter raises the level of the highest
occupied molecular orbital (HOMO) signicantly (Table 1).
Fig. 4 Molecular geometry of compound 5 and its crystal packing
viewed from axis b.†
Fig. 3 CV spectra of the polymers.
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Table 2 Photovoltaic performance of the polymers
Polymers
Voc (V)
Jsc (mA cmꢀ2
)
FF
PCE (%)
P(BTI-F)
P(BTI-B)
1.11
0.75
4.0
9.8
0.33
0.47
1.61
3.42
for a single-junction cell using a polymer:fullerene blend76–78
and this is mainly attributed to the lower lying HOMO energy
level (5.59 eV) of P(BTI-F).79 The series resistance (Rs) of the
devices were also extracted from the slope of their J–V curves
under light near the open circuit voltage. It was found that Rs for
the P(BTI-F):PC61BM device was 86 U cm2 and for the P(BTI-
B):PC61BM device it was 32 U cm2. Two possible reasons may
Fig. 5 Normalized UV-Vis absorbance of P(BTI-F):PC61BM and P(BTI-
B):PC61BM films.
for the devices based on P(BTI-B):PC61BM if transport in these contribute to a high series resistance – poor charge transport as
lms is not an issue.
a result of morphology or inherent mobility, and a rough
Fig. 6(a) shows current density–voltage (J–V) characteristics interface between the polymer and electrodes. From the AFM
of P(BTI-F):PC61BM and P(BTI-B):PC61BM solar cells under AM measurements (Fig. 7), it can be seen that P(BTI-F):PC61BM lm
1.5G illumination (100 mW cmꢀ2). The results are summarized is smoother and has larger nano-domains. The former obser-
in Table 2. P(BTI-F):PC61BM device exhibits a PCE of 1.61%, vation suggests that the contact between the lm and Al cathode
with Jsc of 4.36 mA cmꢀ2, Voc of 1.11 V, and FF of 0.33. By should be better and the latter observation suggests that the
contrast P(BTI-B):PC61BM device shows a PCE of 3.42%, with Jsc transport of the charges should be better. Based on the above
of 9.71 mA cmꢀ2, Voc of 0.75 and FF of 0.47. Hence, our P(BTI-
B):PC61BM device is more than twice as efficient as the device
based on P(BTI-F) in converting photons to electrons. The
device based on P(BTI-F):PC61BM has a very high Voc (1.11 eV) in
comparison with the device based on P(BTI-B):PC61BM. To best
of our knowledge this is one of the highest Voc value reporteds
Fig. 6 (a) Current density–voltage (J–V) characteristics of P(BTI-
F):PC61BM and P(BTI-B):PC61BM solar cells and (b) their corresponding Fig. 7 AFM images of (a) P(BTI-F):PC61BM and (b) P(BTI-B):PC61BM
external quantum efficiency (EQE) profiles.
blend films.
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Journal of Materials Chemistry A
discussion, series resistance of P(BTI-F):PC61BM device ought to calculated from EQE of P(BTI-F):PC61BM device and P(BTI-
be lower but this is not the case and therefore, the mobility of B):PC61BM device are 4.0 mA cmꢀ2 and 9.8 mA cmꢀ2 respec-
charge carriers in P(BTI-F):PC61BM lm must be poorer tively, which agrees well with the measured Jsc.
compared to P(BTI-B):PC61BM lm. The higher charge mobility
AFM was used to study the nano-morphology of the active
together with the higher absorption of polymer P(BTI-B) result layers of the polymer devices, which have a strong impact on
in the higher Jsc seen in P(BTI-B):PC61BM device. The FF of both their corresponding photovoltaic properties. From the topo-
devices are only around 40%, which may indicate that the graphical AFM image shown in Fig. 6, it can be found that both
charge transport of electrons and holes is not balanced in these lms have well-distributed nano-sized domains with small lm
organic solar cells.80 It is worth mentioning that compared to roughness. The root-mean-square (RMS) roughness (Rq) of
our previously reported polymers based on dicarboxylic imide- P(BTI-F):PC61BM lm and P(BTI-B):PC61BM are 0.57 nm and
substituted isothianaphthene units, the performance of solar 0.84 nm, respectively. This shows that both polymers are
cells based on the structurally similar dicarboxylic imide- reasonably miscible with PC61BM. While P(BTI-F):PC61BM lm
substituted benzothiadizole unit in this work has been signi- has larger domains; the lms based on P(BTI-B):PC61BM has
cantly improved due to the stronger acceptor nature of the latter ner domains but the roughness is higher. The larger phase
unit.66
separation in the blend lm of P(BTI-F):PC61BM can be linked
Recently Wang et al. have also reported two low bandgap to the bulky uorene unit in P(BTI-F), which further highlights
polymers with the same BDT-T-BTI backbone as in our polymer the role of side chains in determining the intermolecular
P(BTI-B), and that solar cells based on these two polymers also interaction and the blend morphology thereof. These ner
showed efficient performance.68 This agrees with our nding domains in P(BTI-B):PC61BM are benecial for the exciton
that the combination of dicarboxylic imide-substituted benzo- separation and the rougher lm does not seem to impact the
thiadiazole (BTI) unit and benzodithiophene (BDT) units can be series resistance negatively. As a result, the devices based on
an effective D–A system to construct low bandgap polymers for P(BTI-B):PC61BM showed higher short-circuit current (Jsc) and
efficient polymer solar cells. It can be seen from comparing ll factor (FF) and hence showed higher measured efficiency
their results with ours, that the photophysical properties of compared to P(BTI-F):PC61BM devices.
polymers based on BTI–BDT is very sensitive to the side chains
From the model developed by Scharber et al.79 one can
attached to the main chain, so that the device performance can predict maximum PCEs for these polymers with PC61BM of
vary considerably depending on the number, type and size of above 7% based on their bandgaps and LUMO levels. Our device
side chains employed. For instance, when linear alky chains are results were well below this for various reasons. Firstly, we have
adopted both for the BTI and BDT units in P(BTI-B), the polymer yet to determine what aliphatic chains will give the best chain
has a HOMO level of 5.44 eV; but a polymer with branched alkyl packing so as to optimise the photophysical and morphological
chains on both BTI and BDT units reported by Wang, has a properties, though given the superior performance of our P(BTI-
HOMO level of 5.36 eV.68 This shows linear alkyl chains can B) to a similar polymer with branched chains we can already
induce a deeper lying HOMO level (ca. 80 meV shi) than conclude that straight chains are better than branched ones;
branched alkyl chains, which may result from stronger inter- secondly, we used a standard device fabrication technique
molecular interactions between the main chains. One inter- without further optimization steps such as adopting blocking/
esting observation is that solar cells based on the conjugated injection layers, additives, or using PC71BM, etc. We anticipate
polymer with linear alkyl chains showed a Jsc which is more that use of any or all of these would lead to marked improve-
than 1.5 times higher than that of the polymer with branched ments in device efficiencies, especially for P(BTI-F).
alkyl chains (9.8 mA cmꢀ2 vs. 5.5 mA cmꢀ2) with similar Voc and
FF values, even though an inferior electron acceptor was used
(PC61BM vs. PC71BM). This implies that the polymer with linear
Conclusions
alkyl chains also possesses higher hole mobility, which can be We report an improved synthesis of pyrrolo[3,4-f]-2,1,3-benzo-
attributed to better p–p stacking of the polymer chains. On the thiadiazole-5,7-dione (BTI) and the synthesis and characteriza-
other hand, Wang et al. have shown that a BTI–BDT polymer tion of two new D–A polymers using BTI as the electron
with conjugated thienyl side groups on the BDT unit has a acceptor. These polymers showed high thermal stability, excel-
deeper lying HOMO energy level and much higher carrier lent solubility and reduced band gaps. Our preliminary photo-
mobility than that with branched alkyl chains.68 All these voltaic measurements indicated these polymers have
comparisons imply that for this BTI–BDT system, more considerable potential: P(BTI-F) had a deep lying HOMO level of
planarity and better p–p stacking are desired for better charge ꢀ5.6 eV which directly led to an exceptionally large Voc of 1.1 V,
transportation.
though the overall device performance (PCE 1.61%) was medi-
The external quantum efficiency (EQE) proles of the devices ocre due to poor charge carrier mobility. Replacing the weak
are shown in Fig. 6(b). It can be seen that photovoltaic cells donor uorene with dialkoxybenzodithiophene signicantly
based on P(BTI-B):PC61BM have much better EQEs than devices increased the Jsc, FF and PCE (3.42%) but markedly reduced the
based on P(BTI-F):PC61BM, especially between 625 nm and 750 Voc to 0.77 V. The efficiency of the latter materials was superior
nm. This is because that polymer P(BTI-B) absorbed more to that of a similar copolymer reported elsewhere with branched
strongly in this wavelength region. As expected, the Jsc of devices instead of linear alkyl substituents suggesting that linear alkyl
based on P(BTI-B):PC61BM is higher. The integrated Jsc chains promote higher device efficiencies, probably through
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better chain packing. While there remains considerable scope 15 W. Cai, M. Wang, J. Zhang, E. Wang, T. Yang, C. He,
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