2D π-conjugated benzo[1,2-b:4,5-b′]dithiophene- and quinoxaline-based copolymers for photovoltaic applications

Article abstract of DOI:10.1039/c3ra44238a

Two medium gap semiconducting polymers, P(1)-Q-BDT-4TR and P(2)-FQ-BDT-4TR, based on alternate units of alkyl-dithiophene substituted benzodithiophene (BDT) and quinoxaline units (without or with fluorine substitution), are synthesized and fully characterized. The polymers exhibit optical and electrical properties favorable for being employed as donors in BHJ OPV devices, such as: absorption spectra extending up to around 720 nm for a high solar spectrum coverage, deep lying HOMO energy levels for a high device open circuit voltage and LUMO energy levels higher than those of PC₆₁BM and PC ₇₁BM for an efficient exciton dissociation. In particular, the presence of alkyl-dithiophene side chains allows us to obtain a high 2D π-conjugation which promotes red shifted absorption profiles, low HOMO energy levels (<-5.6 eV) and enhanced environmental and thermal stability. Moreover, the introduction of the fluorine atom in the polymer backbone allows us to obtain efficient OPV devices, based on as-cast P(2)-FQ-BDT-4TR:PC ₆₁BM blend, showing a J_SC of -10.2 mA cm^-2, V_OC of 0.90 V, FF of 58% and PCE of 5.3%, without the need for any additional thermal treatment.

Full text of DOI:10.1039/c3ra44238a

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PAPER
2D p-conjugated benzo[1,2-b:4,5-b0]dithiophene- and
quinoxaline-based copolymers for photovoltaic
applications†
Cite this: RSC Adv., 2013, 3, 24543
a
b
Margherita Bolognesi, Desta Gedefaw, Dongfeng Dang,b Patrik Henriksson,b
*
*
Wenliu Zhuang,b Marta Tessarolo,c Ergang Wang,b Michele Muccini,c Mirko Serid
b
*
and Mats R. Andersson
Two medium gap semiconducting polymers, P(1)-Q-BDT-4TR and P(2)-FQ-BDT-4TR, based on alternate
units of alkyl-dithiophene substituted benzodithiophene (BDT) and quinoxaline units (without or with
fluorine substitution), are synthesized and fully characterized. The polymers exhibit optical and electrical
properties favorable for being employed as donors in BHJ OPV devices, such as: absorption spectra
extending up to around 720 nm for a high solar spectrum coverage, deep lying HOMO energy levels for
a high device open circuit voltage and LUMO energy levels higher than those of PC61BM and PC71BM
for an efficient exciton dissociation. In particular, the presence of alkyl-dithiophene side chains allows us
to obtain a high 2D p-conjugation which promotes red shifted absorption profiles, low HOMO energy
levels (<ꢀ5.6 eV) and enhanced environmental and thermal stability. Moreover, the introduction of the
fluorine atom in the polymer backbone allows us to obtain efficient OPV devices, based on as-cast P(2)-
FQ-BDT-4TR:PC61BM blend, showing a JSC of ꢀ10.2 mA cmꢀ2, VOC of 0.90 V, FF of 58% and PCE of 5.3%,
without the need for any additional thermal treatment.
Received 7th August 2013
Accepted 9th October 2013
DOI: 10.1039/c3ra44238a
www.rsc.org/advances
architectures where the photoactive layer is composed of an
interpenetrating network of bicontinuous electron-donor poly-
Introduction
Solution-processed organic solar cells (OPVs) represent the
newest generation of technologies in solar power generation,
offering benets in terms of low manufacturing costs (i.e. high-
throughput roll-to-roll processing), large area coverage,
compatibility with exible and light-weight substrates, earth-
abundant constituents and architectural tunability over
multiple length scales.1 During the last decade, the power
conversion efficiency (PCE) of these devices has increased
dramatically due to the development of new materials,2 engi-
neered interfaces,3 enhanced understanding of polymeric lm
microstructure and photophysics4 and optimization of the
devices architecture.5 The highest OPV performance to date has
been obtained for so-called bulk-heterojunction (BHJ) cell
mer (D) and electron-acceptor fullerene (A) domains, placed
between a semitransparent anode (ITO, indium tin oxide) and a
metal cathode.6 Despite the impressive achievements in power
conversion efficiency (PCE), there is still much room for the
improvement of the performance of BHJ solar cells.7 In partic-
ular, the properties of photoactive materials are one of the most
determining factors on the overall performance of a polymer
solar cell.8 An ideal donor polymer should fulll some funda-
mental requirements such as: broad absorption spectrum with
high absorption coefficient, high charge carrier mobility and
optimal energetic alignment of the HOMO and LUMO energy
levels with those of the acceptor material (PC61BM or PC71BM)
to ideally provide large open-circuit voltages (VOC),9 increased
environmental stability10 and efficient exciton dissociation at
the D–A interface.11 Through a rational materials development,
a wide variety of p-conjugated donor polymers, with desirable
chemical and physical properties, were prepared and employed
as efficient donor materials for BHJ solar cells.12 Among these,
the push–pull polymers, based on the combination of alternated
electron-rich (push) and electron-decient (pull) moieties, have
attracted much attention due to their intrinsic advantages and
potentials in optoelectronic devices.1a,13 This particular struc-
ture allows to effectively tune the energy of the HOMO–LUMO
levels, the band gap and thus the light absorption properties of
aLaboratory MIST E-R, Via P. Gobetti, 101, 40129, Bologna, Italy. E-mail: m.
bolognesi@bo.ismn.cnr.it; Tel: +39 051 6399173
bPolymer Chemistry, Department of Chemical and Biological Engineering, Chalmers
¨
University of Technology, SE-412 96, Goteborg, Sweden. E-mail: antenehe@
chalmers.se; mats.andersson@chalmers.se; Tel: +46 031 7723410; +46 031 7723401
cConsiglio Nazionale delle Ricerche (CNR), Istituto per lo Studio dei Materiali
Nanostrutturati (ISMN), Via P. Gobetti, 101, 40129, Bologna, Italy
dConsiglio Nazionale delle Ricerche (CNR), Istituto per la Sintesi Organica e la
`
Fotoreattivita (ISOF), Via P. Gobetti, 101, 40129, Bologna, Italy
† Electronic supplementary information (ESI) available: Square wave
voltammetry, NMR spectra, hole mobility measurements. See DOI:
10.1039/c3ra44238a
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alternating polymers.14 Moreover, recent studies demonstrate
that such push–pull structures can promote charge carrier
mobility due to the reduced interchain p–p stacking distance.15
The wide possibility of combinations of comonomers in
push–pull structures results in a wide variety of polymers with
very different physical–chemical and optoelectronic proper-
ties.16 Among these, it has been recently demonstrated the use
of push–pull alternating copolymers based on quinoxaline and
benzo[1,2-b:4,5-b0]dithiophene (BDT) moieties as highly effi-
cient donor materials for BHJ solar cells.17 The quinoxaline unit,
usually sandwiched between adjacent thiophene spacers to
limit the inter-monomers steric hindrance, is a strong electron
acceptor unit widely used as building block for optoelectronic
applications, since it can be easily modied and side-
Fig. 1 Structure of P(1)- Q-BDT-4TR and P(2)-FQ-BDT-4TR.
substituted to nely tune the optoelectronic properties of the dithiophene side groups to form a 2D-conjugated unit, is
resulting polymer.18 On the other hand, BDT has been widely combined with an unsubstituted or uorinated quinoxaline
used as electron-rich co-monomer thanks to its desirable moiety (Q and FQ, respectively) sandwiched between two thio-
peculiarities such as structural rigidity and planarity leading to phene rings. The functionalization of the BDT core with elec-
extended p-conjugation length and favorable inter-chain p–p tron-rich thiophene based side chains (4TR) is applied in order
stacking, other than the presence of additional substitution to obtain a broader absorption, a higher hole mobility and a
sites for the incorporation of side chains.
lower HOMO energy level of the resulting polymer. The synergic
The side substitution of the p-conjugated co-monomers is effect of the p-conjugated side chains and uorine atom
indeed an essential and widely spread technique used to substitution effectively allows to obtain a polymer with a strongly
modulate the chemical–physical properties of semiconducting red shied absorption maxima (Dlmax > 30 nm) and a lowered
polymers.19 For instance, the introduction of strong electron HOMO level (DHOMO > 0.1 eV), with respect to the analogous
withdrawing halogen atoms (e.g. uorine)20 on the polymer polymer (PBDT–TFQ)17b containing alkoxy side chains on the
backbone allows to tune its electronic properties such as HOMO BDT unit. As a result, an OPV device with a JSC ¼ ꢀ10.5 mA cmꢀ2
and LUMO energy levels and bandgap. In addition, it is known VOC ¼ 0.90 V and PCE ¼ 5.5% has been fabricated.
that the introduction of uorine atoms increases the polymer
,
charge carrier mobility by promoting stronger inter-chain
Experimental
General
interactions.21 On the other hand, the use of alkyl substituted
aromatic side groups on the copolymer backbone has been
demonstrated to both enhance the solubility of the copolymer 5,8-Dibromo-6,7-diuoro-2,3-bis(3-(octyloxy)phenyl)quinoxaline
and contribute to extend the p-conjugation from the backbone and 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(3-(octyloxy)phenyl)-
to the lateral substituents, leading to two-dimensional or 2D p- quinoxaline were prepared according to previously reported
conjugated systems.22 Thiophene-based p-conjugated side procedures.18b,23 Tetrahydrofuran (THF) was dried over Na/
chains, with the high electronic density and strong tendency to benzophenone and freshly distilled prior to use. Other reagents
aggregation of this group, are particularly suitable for the and solvents were commercial grade and used as received
described purposes. For example, Li et al.22a reported the without further purication.
synthesis and study of a 2D-conjugated polymer with alkyl-
1H NMR and 13C NMR spectra were acquired from a Varian
thiophene p-conjugated side chains, which showed red-shied Inova 400 MHz NMR spectrometer. Tetramethylsilane was used
absorption spectrum, improved thermal stability, lower HOMO as an internal reference with deuterated chloroform as solvent.
energy, signicantly higher hole mobility and overall enhanced Size exclusion chromatography (SEC) was performed on Waters
photovoltaic properties (PCEs up to 6%), in comparison with Alliance GPCV2000 with refractive index detector columns:
the corresponding alkoxy-substituted copolymer. Moreover, by Waters Styragelꢀ HT 6E ꢁ1, Waters Styragelꢀ HMW 6E ꢁ2.
further extending the p-conjugation of the side chain from a The eluent was 1,2,4-trichlorobenzene. The working tempera-
single thiophene unit to two or more, the described 2D-effects ture was 135ꢂC and the resolution time was 2 h. The samples,
could be further enhanced.
with concentration of 0.5 mg mLꢀ1, were ltered (lter: 0.45
Based on these considerations, we study here the effect of mm) prior to the analysis. The molecular weights were calculated
introducing conjugated side-chains (alkyl-dithiophenes) on through calibration with polystyrene standards.
quinoxaline – BDT based co-polymers by evaluating their optical,
Square-wave voltammetry (SWV) measurements were carried
electronic, morphological and photovoltaic properties and out on a CH-Instruments 650A Electrochemical Workstation. A
comparing them to analogous polymers with similar backbone three-electrode setup was used with platinum wires both as
structures (same push–pull units) and unconjugated alkyl side working electrode and counter electrode, and Ag/Ag+ used as
chains. In particular we report the synthesis and characteriza- reference electrode calibrated with Fc/Fc+. A 0.1 M solution of
tion of two p-type copolymers P(1)-Q-BDT-4TR and P(2)-FQ-BDT- tetrabutylammonium hexauorophosphate (Bu4NPF6) in anhy-
4TR (Fig. 1), where the BDT monomer, substituted with alkyl- drous acetonitrile was used as supporting electrolyte. The
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polymers were deposited onto the working electrode from room temperature and SnCl2$2H2O (3 g, 13.3 mmol) dissolved in
chloroform solution. In order to remove oxygen from the elec- 10% HCl (7 mL) was added gradually and stirred at room
trolyte, the system was bubbled with nitrogen prior to each temperature overnight. The reaction mixture was poured on
experiment. The nitrogen inlet was then moved to above the water and extracted with diethyl ether. The ether extract was
liquid surface and le there during the scans. HOMO and dried with anhydrous sodium sulfate and the solvent was
LUMO levels were estimated from the peak potentials of the removed to give a crude product. It was puried with silica gel
third scan by setting the oxidative peak potential of Fc/Fc+ vs. column chromatography using hexane–chloroform (15 : 1) as
the normal hydrogen electrode (NHE) to 0.63 V (ref. 24) and the eluent which yielded compound 5 (1.29 g, 77.3%).
NHE vs. the vacuum level to 4.5 V.25
1H NMR (400 MHz, CDCl3, d): 7.69 (1H, d, J ¼ 4 Hz), 7.50 (1H,
d, J ¼ 8 Hz), 7.38 (1H, d, J ¼ 4 Hz), 7.24 (1H, d, J ¼ 4 Hz), 7.09
(1H, d, J ¼ 4 Hz), 6.70 (1H, d, J ¼ 4 Hz), 2.77 (2H, d, J ¼ 8 Hz),
1.68 (1H, m), 1.35 (16H, m), 0.97 (6H, m).
Synthesis of monomers and polymers
Synthesis of 6,7-diuoro-2,3-bis(3-(octyloxy)phenyl)-5,8-
13C NMR (100 MHz, CDCl3, d): 144.45, 139.06, 139.05, 137.61,
di(thiophen-2-yl)quinoxaline (2). 5,8-Dibromo-6,7-diuoro-2,3- 136.55, 134.56, 128.83, 127.82, 125.97, 123.62, 123.57, 123.27,
bis(3-(octyloxy)phenyl)quinoxaline (1) (1.3 g, 1.775 mmol), 123.11, 39.99, 34.59, 33.18, 32.86, 31.90, 29.64, 28.86, 26.59,
Pd2(dba)3 (38 mg, 2.3 mol%), P(o-tolyl)3 (45 mg, 6.3 mol%) were 23.02, 22.69, 14.16, 14.13.
dissolved in THF (35 mL) and heated to reux. 2-(Tributyl
Synthesis of (4,8-bis(50-(2-butyloctyl)-[2,20-bithiophen]-5-yl)-
stannyl) thiophene (1.66 g, 1.41 mL, 4.44 mmol) was added drop benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl)bis(trimethylstannane) (6).
by drop to the reuxing reaction mixture and heating continued Compound 5 (1.15 g, 1.34 mmol) was dissolved in THF (45 mL)
for two days. The reaction mixture was cooled and THF was and stirred in an ice bath for 1 hour. n-BuLi, 2.5 M (1.29 mL,
removed by rotary evaporator. To the remaining crude greenish 3.216 mmol) was added drop-by-drop with a syringe over 10
material, hexane was added and a solid crushed out from the minutes. The reaction mixture was stirred in the bath for 1 hour
solution. The solid formed was separated by suction ltration and 20 minutes. The bath was removed and stirred for 1 hour
and recrystallized from isopropanol and yielded 2 (0.8 g, 62%). and 30 minutes. Trimethyl tin chloride (4.02 mL, 4.02 mmol)
1H NMR (400 MHz, CDCl3, d): 8.05 (1H, d, J ¼ 4 Hz), 7.62 (1H, was added in one portion and stirred overnight. The reaction
d, J ¼ 4 Hz), 7.36 (1H, s), 7.26 (4H, m), 6.95 (1H, m), 3.92 (2H, t), mixture was poured on water and extracted with diethyl ether.
1.76 (2H, m), 1.43 (10H, m), 0.90 (3H, t).
The ether extract was washed with distilled water. The ether
13C NMR (100 MHz, CDCl3, d): 159.01, 139.23, 130.80, 130.74, solution was dried with anhydrous sodium sulfate and solvent
129.90, 129.19, 126.60, 126.58, 122.73, 116.59, 115.61, 109.99, removed to yield a crude product which was puried by recrys-
68.12, 31.83, 29.33, 29.99, 29.13, 26.05, 22.68.
tallization from isopropanol two times. The solid was dried to
Synthesis of 5,8-bis(5-bromothiophen-2-yl)-6,7-diuoro-2,3- give compound 6 (1.13 g, 71.1%).
bis(3-(octyloxy)phenyl)quinoxaline (3). Compound 2 (0.8 g, 1.08
1H NMR (400 MHz, CDCl3, d): 7.70 (1H, s), 7.39 (1H, d, J ¼ 4
mmol) was dissolved in THF (40 mL) and NBS (0.38 g, 2.16 Hz), 7.25 (1H, d, J ¼ 4 Hz), 7.10 (1H, d, J ¼ 4 Hz), 6.70 (1H, d, J ¼
mmol) was added and stirred at room temperature for 3 hours 4 Hz), 2.77 (2H, d, J ¼ 8 Hz), 1.66 (1H, m), 1.32 (16H, m), 0.91
aer protecting the reaction from light by covering with (6H, m), 0.41 (9H, s).
aluminum foil. The mixture was poured on water and the
13C NMR (100 MHz, CDCl3, d): 144.26, 143.35, 142.91, 138.74,
orange solid formed was collected by ltration. The solid was 138.44, 137.38, 134.77, 130.89, 128.71, 125.93, 123.53, 123.13,
recrystallized twice from isopropanol and the uffy orange solid 121.88, 39.99, 34.59, 33.18, 32.87, 31.90, 29.65, 28.86, 26.60,
obtained was the desired product (0.5 g, 52%).
23.03, 22.69, 14.17, 14.13, ꢀ8.27.
1H NMR (400 MHz, CDCl3, d): 7.77 (1H, d, J ¼ 4 Hz), 7.51
Synthesis of P(1)-Q-BDT-4TR. Compound 6 (0.137 g, 0.116
(1H, s), 7.23 (1H, t, J ¼ 8 Hz), 7.16 (1H, d, J ¼ 4 Hz), 7.08 (1H, d, J mmol) and 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(3-(octyloxy)-
¼ 8 Hz), 7.00 (1H, dd, J ¼ 4 Hz, 8 Hz), 4.05 (2H, t), 1.82 (2H, m), phenyl)quinoxaline (7) (0.1 g, 0.116 mmol) were dissolved in
1.51 (10H, m), 0.91 (3H, t).
toluene (10 mL) and deaerated for 10 minutes. Pd2(dba)3 (2.55
13C NMR (100 MHz, CDCl3, d): 159.38, 151.62, 138.71, 132.37, mg, 2.79 ꢁ 10ꢀ3 mmol) and P(o-Toly)3 (4.24 mg, 0.014 mmol)
130.86, 129.34, 129.08, 122.90, 118.88, 117.29, 115.09, 68.30, were added and the mixture was purged with nitrogen gas for 25
ꢂ
31.85, 29.42, 29.32, 29.30, 26.17, 22.69, 14.13.
minutes. The reaction mixture was heated at 90 C for 7 hours
Synthesis of 4,8-bis(50-(2-butyloctyl)-[2,20-bithiophen]-5-yl)- under nitrogen atmosphere. The polymer was end capped by
benzo[1,2-b:4,5-b0]dithiophene (5). 5-(2-Butyloctyl)-2,20-bithio- adding tributyl(thiophen-2-yl)stannane (0.1 mL, 0.348 mmol)
phene (4) (2.27 g, 6.796 mmol) was dissolved in THF (35 mL) and and 2-bromothiophene (0.09 mL, 0.92 mmol) and heating for
cooled with an ice bath and stirred under nitrogen atmosphere. more than 5 hours aer the addition of each of the end capping
n-BuLi, 2.5 M (2.72 mL, 6.796 mmol) was added drop-by-drop to reagents. The polymer solution was then added to methanol
the reaction mixture over 18 minutes. The mixture was stirred and the solid formed was collected by ltration. The polymer
for 1 hour and 45 minutes in the ice bath and then at ambient was re-dissolved in chloroform and 10% aqueous solution of
temperature for 30 minutes. The reaction mixture was heated at sodium diethyldithiocarbamate trihydrate (100 mL) was added.
50 C for 1 hour and 15 minutes aerwards. Benzo[1,2-b:4,5-b0] The mixture was heated at 60 ꢂC for 1 hour and 30 minutes
ꢂ
dithiophene-4,8-dione (0.43 g, 1.95 mmol) was added and followed by stirring at room temperature overnight. The chlo-
ꢂ
heated at 50 C for 1 hour. The reaction mixture was cooled to roform soluble portion was separated and washed with distilled
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water three times. The chloroform solution was reduced to deposited sequentially without breaking vacuum (ꢃ3 ꢁ 10ꢀ6
small volume and then added to methanol. The solid formed Torr) using a thermal evaporator directly connected to the glove
was collected by ltration and then puried by Soxhlet extrac- box. The current–voltage (I–V) characteristics of complete OPV
tion using methanol, hexane and diethyl ether. Finally, the devices were recorded by a Keithley 236 source-measure unit
polymer remaining in the thimble was extracted with chloro- under simulated AM1.5 G illumination of 100 mW cmꢀ2 (Abet
form. Aer reducing to small volume, the solution was precip- Technologies Sun 2000 Solar Simulator). The light intensity was
itated by adding on methanol. The solid was collected by determined by a calibrated silicon solar cell tted with a KG5
ltration, dried in vacuum oven at 40 ꢂC overnight and yielded color glass lter to bring spectral mismatch to unity. The active
P(1)-Q-BDT-4TR as a brown solid (140 mg, 78%).
area of the solar cell was exactly 6 mm2. During testing, each cell
1H NMR (400 MHz, CDCl3, d): 7.5–6.5 (aromatic protons), was carefully masked, by calibrated mask, to prevent an excess
4.0–3. 5 (–OCH2–), 3.0–2.5 (–CH2 adjacent to thiophene), 2.0–0.5 photocurrent generated from the parasitic device regions outside
(other aliphatic protons).
Synthesis of P(2)-FQ-BDT-4TR. Compound 6 (0.11 g, 0.093 the glove box with oxygen and moisture free environment.
mmol) and compound 3 (0.0835 g, 0.093 mmol) were dissolved External Quantum Efficiency (EQE) was measured with a
the overlapped electrode area. All solar cells were tested inside
in toluene (5 mL) and deaerated with N2 gas for 10 minutes. home built system on encapsulated devices: monochromatic
Pd2(dba)3 (2.55 mg, 2.79 ꢁ 10ꢀ3 mmol) and P(o-Tolyl)3 (4.24 mg, light was obtained with a Xenon arc lamp from Lot-Oriel (300
0.014 mmol) were added and purged with nitrogen gas for 25 Watt power) coupled with a Spectra-Pro monochromator. The
ꢂ
minutes. The reaction mixture was heated at 90 C for 1 hour photocurrent produced by the device passed through a cali-
and 20 minutes under nitrogen atmosphere. The polymer solu- brated resistance (50 U) and the voltage drop signal was
tion was then precipitated by adding on methanol. The solid collected through the resistance with a Lock-In Digital Ampli-
formed was collected by ltration. The polymer was re-dissolved er-SR830. Signal was pulsed by means of an optical chopper
in chloroform by heating at 60 ꢂC for 1 hour and a 10% aqueous (ꢃ500 Hz frequency). A calibrated Silicon UV-enhanced photo-
solution of sodium diethyldithiocarbamate trihydrate (100 mL) diode was used as reference.
was added and stirred overnight at room temperature. The
chloroform soluble portion was separated and washed with
distilled water three times. The chloroform solution was reduced
Thin lms characterization
All thin-lm characterizations were performed in air. Solution
and thin lm optical absorption spectra were recorded with a
PerkinElmer Lambda 900 UV/Vis/NIR spectrophotometer.
Atomic Force Microscopy (AFM) images were taken with a Solver
Pro (NT-934 MDT) scanning probe microscope in tapping mode.
The absorption spectra of blend lms were recorded with a
JASCO V-550 spectrophotometer. AFM images and blend lms
absorption spectra were recorded directly on the tested devices,
in the device area out of the LiF/Al electrode. The hole mobility
of the polymers in the active layers was measured through the
space charge limited current (SCLC) method using devices with
the structure ITO/PEDOT:PSS/polymer:PC61BM/Au.26 The pro-
cessing conditions used for the active layers were the optimized
ones. Charge mobility was extracted by tting the current
density–voltage curves, recorded in dark conditions, to the
Mott–Gurney equation: J ¼ (9/8)(m3r30V2)dꢀ3, where J is the
measured current density, m is the hole mobility, 3r is the rela-
tive permittivity of the active layer, 30 is the permittivity of
vacuum, d is the thickness of the active layer and V is the
applied voltage.
to small volume and then added to methanol. The solid was
collected by ltration and then puried by Soxhlet extraction
using methanol, hexane, diethylether and dichloromethane.
Finally, chloroform was used to wash out what was le in the
thimble. The volume of the chloroform solution was reduced
and the polymer was precipitated by adding on methanol. The
solid was ltered and dried in vacuum oven at 40 ꢂC to yield P(2)-
FQ-BDT-4TR as a brown solid (96 mg, 65%).
1H NMR (400 MHz, CDCl3, d): 7.5–6.5 (aromatic protons),
4.0–3.5 (–OCH2–), 3.0–2.5 (–CH2 adjacent to thiophene), 2.0–0.5
(other aliphatic protons).
OPV devices fabrication and characterization
Patterned ITO-coated glasses (Rs ꢃ 10 U sqꢀ1) were cleaned in
sequential sonicating baths (for 15 min) in deionized water,
acetone and isopropanol. Aer the nal sonication step,
substrates were dried with a stream of N2 gas and then placed in
an oxygen plasma chamber for 10 min. Next, a thin layer
(ꢃ30 nm) of PEDOT:PSS (Clevios P VP Al 4083) was spun-cast on
the ITO surface and subsequently annealed at 150 ꢂC for 15 min.
The samples were then transferred inside the glove box (<0.1
ppm of O2 and H2O) for active layer and top contact deposition.
In the meantime, the active layer blend solutions were formu-
Results and discussion
Synthesis of monomers and polymers
lated inside the glove box. For optimized devices, a total polymer- Fig. 1 shows the structures of P(1)-Q-BDT-4TR and P(2)-FQ-BDT-
fullerene concentration of 30 mg mLꢀ1 in o-dichlorobenzene was 4TR (abbreviated as P(1) and P(2), respectively, in the text).
used for both polymers. The mixture solutions were stirred
The routes towards the synthesis of the quinoxaline and BDT
overnight at 80 ꢂC and then spun-cast on top of the ITO/ based monomers are illustrated in Scheme 1, while details are
PEDOT:PSS surface. Before cathode deposition, the substrates reported in the experimental section. Stille coupling reaction
were then either thermally annealed or le as-cast. To complete between 5,8-dibromo-6,7-diuoro-2,3-bis(3-(octyloxy)phenyl)qui-
the device fabrication, LiF/Al cathode (0.6 nm/100 nm) were next noxaline (1), synthesized as previously reported,23 and
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bithiophen]-5-yl)benzo[1,2-b:4,5-b0]dithiophene (5). Finally,
double lithiation of compound 5 followed by quenching with
trimethyl tin chloride yielded the desired monomeric unit (6).
The presence of four thiophene units as side chain made the
BDT monomer to have a rigid and planar structure with strong
tendency to crystallization. Hence the monomer was collected in
its solid state, despite the presence of long and branched side
chain, and it was possible to purify it by recrystallization from
isopropanol. Polymers P(1) and P(2) were prepared via Stille
coupling reaction of the corresponding quinoxaline based
monomers with distannylated BDT monomer in toluene in the
ꢂ
presence of Pd2(dba)3/P(o-Toly)3 as catalyst by heating at 90 C
under nitrogen atmosphere. The polymers were collected and
puried following standard procedures (details are given in the
Experimental section). The weight-average molecular weight
(Mw) and number-average molecular weight (Mn) of the copoly-
mers were determined by gel permeation chromatography (GPC)
using polystyrene standards and TCB as an eluent. The Mw and
Mn of P(1) resulted to be 47.4 and 16.3 kg molꢀ1 and those of P(2)
46.1 and 18.5 kg molꢀ1 respectively (Table 1). Since the two
polymers present very similar molecular weight and poly-
dispersity index, their properties and photovoltaic performances
are discussed on the basis of their different chemical structure.
Electrochemical and optical properties
The HOMO and LUMO energy levels of the two polymers,
reported in Table 1, were calculated by square wave voltam-
metry (SWV) (Fig. S1 in ESI†), extrapolating them from the
potentials of the rst oxidation and reduction peaks registered.
The HOMO and LUMO energy levels of P(1) were estimated
to lie at ꢀ5.68 eV and ꢀ3.14 eV respectively. In comparison, P(2)
exhibits a deeper HOMO level at ꢀ5.96 eV and a similar LUMO
level lying at ꢀ3.20 eV. The lowering of the HOMO of P(2)
comparing to the HOMO of P(1) by 0.28 eV conrms the strong
Scheme 1 Synthetic route for the preparation of monomers and of polymers
P(1) and P(2).
tributyl(thiophen-2-yl)stannane, gave compound 2 in excellent effect played by the electron withdrawing uorine atoms.21a,27 As
yield. Bromination of compound 2 with NBS in THF yielded the expected, the inuence of uorine on the frontier orbital ener-
dibromo functionalized monomeric unit (3) which was used for gies is more pronounced for the HOMO than for the LUMO, the
the preparation of P(2)-FQ-BDT-4TR (or P(2)).17b The other latter differing only by 0.06 eV between the two polymers.
quinoxaline based monomer (7) used for the synthesis of P(1)-Q- Interestingly, the polymers P(1) and P(2) exhibit lower HOMO
BDT-4TR (P(1)) was synthesized following a sequence of reac- energy levels (ꢀ5.68 and ꢀ5.96 eV, respectively) with respect to
tions previously reported.18b The BDT containing monomeric the analogous PBDT–TFQ17b polymer based on alkoxy side
unit (6) was also prepared by two successive steps in high yield. 5- chains, having HOMO and LUMO levels at ꢀ5.52 and ꢀ3.30 eV
(2-Butyloctyl)-2,20-bithiophene (4) was lithiated with n-BuLi and respectively. Even though a direct comparison between these
subsequently made to react with benzo[1,2-b:4,5-b0]dithiophene- values is not highly accurate for the different techniques used
4,8-dione. The material formed was reduced with SnCl2$2H2O for their measurement (square wave and cyclic voltammetry
and 10% HCl in situ to give 4,8-bis(50-(2-butyloctyl)-[2,20- respectively), P(1)- and P(2)-based devices could reasonably give
Table 1 Summary of molecular weights, optical and electrochemical properties of P(1) and P(2)
Solutionb
Polymer MN [kDa] MWa [kDa] PDI lmax [nm] lonset [nm]
Thin-lmc
a
optd
gap
E
[eV] lmax [nm]
lonset [nm] Eogapptd [eV] EHOMO [eV] ELUMO [eV]
P(1)
P(2)
16.3
18.5
47.4
46.1
2.9 388, 597
2.5 390, 601
700
700
2.07
2.05
388, 601, 636 720
394, 606, 640 720
1.73
1.73
ꢀ5.68
ꢀ5.96
ꢀ3.14
ꢀ3.20
Determined by GPC. b In o-dichlorobenzene. c Spin-coated from o-dichlorobenzene solutions on glass substrates. d
E
¼ 1240/lonset
.
a
opt
gap
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devices with a higher VOC, in agreement with the difference: appearing at 600 nm, while the more red-shied shoulder at
28
HOMOdonor ꢀ LUMOacceptor
.
Moreover, the measured HOMO 650 nm could be mainly due to inter-chain interactions.31 The
energies are in an ideal range to ensure good polymer air latter band then indicates the probable presence of molecular
stability (being the oxidation threshold from air around ꢀ5.2 aggregates also in solution, conrming the good aggregation
eV).29 In addition, considering that the LUMO level of the typical tendency of the polymers, promoted by their 2D highly
electron acceptors PC61BM and PC71BM are located at ꢀ4.03 p-conjugated structure. Note that the relative intensity of the CT
and ꢀ4.13 eV respectively,30 the offset between the excited state band versus the p–p* one is higher for P(2) than for P(1), this
of the donor polymer and of the molecular acceptor should indicating the stronger CT character of the former polymer
provide the driving force for an efficient exciton dissociation, comparing to the latter, due to the superior electron-with-
ensuring energetically favorable electron transfer.
drawing strength of the uorine-substituted moiety comparing
From the electrochemical data (EC), relatively high band to the unsubstituted quinoxaline one.
gaps are extrapolated for the two polymers in solution: 2.54 eV
Passing from solution to thin lm absorption spectra, a red
and 2.76 eV respectively for P(1) and P(2) (Table 1). These values, shi of the CT bands absorption maxima and onsets is regis-
despite a slight discrepancy likely dependent on the different tered for both P(1) and P(2), accompanied by an intensity
methods employed, are consistent with those estimated from increase of the absorption shoulder at 650 nm relative to the
the absorption spectra in solution (Eogappt: 2.07 and 2.05 eV for shoulder around 600 nm. This indicates a high aggregation
opt
gap
P(1) and P(2), respectively). However, the E estimated from tendency of the two polymers in the solid state comparing to
the absorption spectra in solid state demonstrate how the solution, in particular for P(2) comparing to P(1) probably due
polymer aggregation could determine an important decrease of to its discussed stronger CT character.
the energy gap due to the effect of polymeric chains interac-
It is interesting to note that both P(1) and P(2), comparing to
tions. Indeed, the absorption onset value (lonset) for P(1) and their analogous counterparts with alkoxy side chains P(BDT–
opt
P(2) lms is ꢃ720 nm, from which the resulting E are esti- DTQx)17a and PBDT–TFQ17b respectively, present a strong red
gap
mated to be ꢃ1.73 eV for both polymers.
shi (of 67 and 29 nm respectively) of the lower energy
The UV-visible absorption spectra of P(1) and P(2) in solution absorption maximum in thin lm, conrming the role of the 2D
and in the solid state deposited as thin lms on glass, with p-conjugated side chains in promoting a high intermolecular
spectra normalized on the higher energy absorption band, are p–p stack in the polymeric lm.
shown in Fig. 2. The two polymers show similar behavior both
A summary of the described optical properties of the two
in solution and in thin lm, with broad absorption spectra polymers, together with the electrochemical data, is shown in
spanning from the UV region to above 700 nm. In diluted Table 1.
solution a rst absorption band centered around 400 nm is
observed for both polymers, attributable p–p* transitions
localized on the 2D p-conjugated units, as conrmed by the
Photovoltaic performance
good overlap of this absorption band with the spectrum of the The photovoltaic performance of P(1) and P(2) polymers was
BDT monomeric precursor (6) in solution, also shown in Fig. 2. investigated by fabricating conventional BHJ solar cells having
A lower energy absorption band, characteristic of the charge the following structure: ITO/PEDOT:PSS/active layer/LiF/Al. The
transfer (CT) state arising between the BDT and quinoxaline active layers were prepared by spin-casting the polymer:PC61BM
units, is then observed between 500 and 700 nm, with two (or PC71BM) blends, with different (wt/wt) ratios, from o-dichlo-
relative maxima around 600 and 650 nm. In particular, an robenzene (o-DCB) solutions. The photovoltaic parameters of the
intramolecular CT state could be responsible for the peak corresponding devices are shown in Table 2, while the J–V curves
of optimized devices based on P(1) and P(2) with PC61BM or
PC71BM are reported in Fig. 3.
Within the devices prepared with blends of P(1) and PC61BM
in 1 : 2, 2 : 1 and 1 : 1 ratios, the device with an equal loading of
polymer and fullerene shows a higher PCE, with the increase in
PCE mainly originating from enhanced short-circuit current
density (JSC). The best P(1):PC61BM-based device shows a JSC of
ꢀ7.3 mA cmꢀ2, a VOC of 0.74 V and a FF of 58%, resulting in an
overall PCE of 3.2% (Fig. 3a and Table 2). For all P(1) : PC61BM
donor : acceptor ratios, the effect of thermal annealing is to
enhance the device JSC and FF while only slightly decreasing the
VOC (by ꢃ30–40 mV), indicating a probable reordering of the
polymer chains into a morphology which facilitates charge
separation, transport and improves the overall device effi-
ciency.32 Even a higher performance is obtained for the
P(1):PC71BM-based device with annealed active layer in 1 : 1
polymer:fullerene blend ratio, which shows enhanced JSC (ꢀ9.5
Fig. 2 Optical absorption of monomer 6 in CHCl3 solution and of polymers P(1)
and P(2) in solid state and in o-dichlorobenzene solution.
mA cmꢀ2) and consequently higher PCE (3.7%) comparing to
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Table 2 Summary of photovoltaic parameters of the device based on P(1) or P(2) and PC61BM or PC71BM blends, in different polymer : fullerene ratios and annealing
conditions. Values were averaged over 6 devices each. For optimized devices, the maximum values are reported in brackets
Polymer : PCxxBM ratio (wt/wt)
TANN. [ꢂC]
VOC [V]
JSC [mA cmꢀ2
]
FF [%]
PCE [%]
P(1) : PC61BM (1 : 2)
P(1) : PC61BM (1 : 2)
P(1) : PC61BM (1 : 1)
P(1) : PC61BM (1 : 1)
P(1) : PC61BM (2 : 1)
P(1) : PC61BM (2 : 1)
P(1) : PC71BM (1 : 1)
P(1) : PC71BM (1 : 1)
P(2) : PC61BM (1 : 2)
P(2) : PC61BM (1 : 2)
P(2) : PC61BM (1 : 1)
P(2) : PC61BM (1 : 1)
P(2) : PC61BM (2 : 1)
P(2) : PC61BM (2 : 1)
P(2) : PC71BM (1 : 1)
P(2) : PC71BM (1 : 1)
—
0.70
0.73
0.78
0.74
0.80
0.75
0.77
0.74
0.87
0.86
0.90
0.86
0.91
0.89
0.90
0.88
ꢀ5.8
ꢀ6.3
ꢀ6.5
ꢀ7.3
ꢀ3.6
ꢀ6.0
ꢀ7.1
ꢀ9.5
ꢀ7.9
ꢀ6.1
ꢀ10.2
ꢀ9.4
ꢀ6.9
ꢀ8.7
ꢀ9.3
ꢀ8.7
42
61
34
58
28
35
48
53
56
65
58
65
33
41
57
60
1.7
2.8
1.7
3.2
0.8
1.6
2.6
3.7
3.8
3.4
5.3 (5.5)
5.2 (5.3)
2.1
3.1
4.7
4.5
110/50
—
110/50
—
110/50
—
110/50
—
110/50
—
110/50
—
110/50
—
110/50
leads to: (i) red-shied absorption spectrum with enhanced
interchain p–p interactions, that correlates well with the
improved JSC; (ii) lower polymer HOMO energy, in agreement
with the larger device VOC
.
Devices based on the uorinated polymer P(2) in general
show an improved photovoltaic performance with respect to P(1)
(Fig. 3b and Table 2). By comparing the devices prepared with
the two different polymers in analogous conditions, an increase
in VOC by 0.13 V on average is observed passing from P(1) to P(2),
due to the discussed effect of the F atoms in lowering the poly-
mer HOMO level (Table 1).32,34 The best performance is obtained
for the devices based on as-cast P(2):PC61BM blend with a
polymer : fullerene ratio of 1 : 1 (wt/wt), showing on average a JSC
of ꢀ10.2 mA cmꢀ2, a VOC of 0.90 V, a FF of 58% and a PCE of
5.3% (5.5% for the best device). It has to be emphasized that the
synergic effect of the uorine atom and of the 2D p-conjugated
side chains in P(2) allows to obtain a higher VOC (0.90 V in the
best performing device) with respect to the analogous polymer
with alkoxy-side chains (PBDT–TFQ),17b which affords a VOC of
only 0.76 V (best performing device). Nevertheless, an overall
lower performance of P(2):PC61BM-based device comparing to
the PBDT–TFQ:PC61BM one is observed (5.3% and 6.9%
respectively). This could be explained on the basis of some
Fig. 3 J–V curves of the most representative devices based on: (a) P(1) and (b) P(2). differences, other than the substitution on the BDT core, such
as: the different length of the alkoxy side chains on the qui-
noxaline unit (octyl and hexyl respectively) and the different
cathode buffer layers employed in the devices (LiF and Ca,
the P(1):PC61BM-based device (Fig. 3a and Table 2), thanks to respectively).
the contribution of PC71BM to light absorption in the visible
By comparing the photovoltaic performance of P(2):PC61BM-
region and hence to charge carrier generation.33 Note that the based devices with that of the analogous optimized
optimized P(1):PC71BM-based device, compared to the one P(1):PC61BM-based ones, the higher efficiency in the former case
based on the analogous polymer with alkoxy side chains instead can be ascribed, other than to the higher VOC, to enhanced JSC
of 2D p-conjugated ones17a (P(BDT-DTQx):PC71BM-based and FF. This could be due to the effect of the F atoms in favoring
device), presents a doubled JSC (ꢀ9.5 versus ꢀ4.9 mA cmꢀ2) and the p–p polymer chains interactions, already promoted by the
a higher VOC (0.74 versus 0.65 V).
high 2D p-conjugation, which enhances the polymer light
This conrms the positive effect of the 2D p-conjugated side- absorption and most importantly its charge carrier mobility.21a
chains (comparing to alkoxy-side chains), which effectively Indeed, the hole mobility of the P(2):PC61BM optimized blend
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resulted to be three times higher with respect to the mobility of
the optimized P(1):PC61BM blend (2.4 ꢁ 10ꢀ4 cm2 Vꢀ1$sꢀ1 and
0.8 ꢁ 10ꢀ4 cm2 Vꢀ1$sꢀ1 respectively, Fig. S19 and S18 in ESI†).
Moreover, the strong p–p intermolecular interactions of P(2)
allow the achievement of a P(2):PC61BM blend lm with a
morphology giving optimized photovoltaic performances
without the need for additional thermal treatment.
Morphological analysis
Morphological analysis were done on annealed and as-cast
polymer:PC61BM lms in 1 : 1 ratio (optimized blend ratio) to
have a deeper insight on the different effect of annealing on the
morphology of the two polymers-based blends and on the cor-
responding device performances. By comparing P(1):PC61BM
as-cast and annealed lms, similar lm roughness of ꢃ1 nm
(calculated as Root Mean Square deviation, or RMS) are regis-
tered, even though the P(1):PC61BM annealed lm (Fig. 5b)
exhibits a supercial morphology with slightly reduced domain
size and ner nanostructures comparing to as-cast lm
(Fig. 5a). On the opposite, an evident increase in roughness is
found when passing from P(2):PC61BM as cast lm to the
annealed one, with RMS going from ꢃ1 nm to ꢃ3 nm. Note that
for P(2):PC61BM blends the annealed lm (Fig. 5d) exhibits a
less structured surface with relatively larger domains
comparing to the analogous as-cast lm (Fig. 5c), being this an
opposite trend in the lm nanostructuring with heat comparing
to P(1)-based blends.
As a result, thermal annealing leads to an increase in JSC and
in FF for the P(1):PC61BM-based devices, while for P(2)-based
devices annealing causes a decrease in JSC and a modest
increase in FF (Table 2). This is most probably due to a better
packaging/ordering of the P(1) polymer chains induced by
annealing, in agreement with the ner reorganization of the
active layer surface despite the identical surface roughness,
which improves the polymer electrical and optical properties.
On the other hand, the stronger interchain interactions of P(2)
comparing to P(1), due to the presence of the F atoms in the
polymer backbone and to its discussed stronger CT character,
leads to a more dened and ner polymer self-organization in
as-cast blend lms (Fig. 5c). As a consequence, since the effect
of annealing on the P(2):PC61BM blend is probably to induce
further aggregation, an increase of the polymer domains sizes
A
lower performance is obtained for the optimized
P(2):PC71BM-based device (Fig. 3b and Table 2), probably due to
a lower miscibility of the polymer with PC71BM leading to a less
favorable morphology of the P(2):PC71BM lm comparing to the
analogous PC61BM-based lm. However, the optimized
P(2):PC71BM based devices showed an efficiency of 4.7%, with a
JSC of ꢀ9.3 mA cmꢀ2, a VOC of 0.90 V and a FF of 57%.
The EQE spectra of the best polymer:PC61BM based devices,
as shown in Fig. 4a, have similar spectral features of the cor-
responding blends absorption spectra (Fig. 4b), with higher
values for the P(2)-based blend comparing to the P(1)-based one
(0.52 and 0.43 on the maximum respectively, corresponding to
the relative polymers absorption maxima). Convolution of these
EQE spectra with the 1.5AM solar spectrum give calculated
short circuit current densities in good agreement, within a
ꢃ10% experimental error, with those obtained from J–V
measurements.
Fig. 5 AFM images (size: 5 ꢁ 5 mm) of blend films on ITO/PEDOT:PSS of: (a)
P(1):PC61BM (1 : 1) as cast, RMS ¼ 0.8 nm; (b) P(1):PC61BM (1 : 1) annealed, RMS
¼ 0.9 nm; (c) P(2):PC61BM (1 : 1) as cast, RMS ¼ 1.3 nm; (d) P(2):PC61BM (1 : 1)
annealed, RMS ¼ 3.4 nm.
Fig. 4 Normalized absorption spectra (a) and EQE spectra (b) of optimized
devices based on annealed (1 : 1) P(1):PC61BM (black line) and as-cast (1 : 1)
P(2):PC61BM films (red line).
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and a higher separation of the fullerene and polymer phases
with a lower D–A interfacial area probably occurs, decreasing
the corresponding annealed device JSC and efficiency.
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In conclusion, we synthesized and characterized two donor-
acceptor alternating polymers based on quinoxaline and BDT co-
monomers. Both polymers have a highly 2D p-conjugated
structure obtained through the insertion of thiophene-conju-
gated side chains on the BDT unit, leading to red-shied
absorption spectra, improved thermal stability and lower HOMO
energies comparing to the analogous alkyl-substituted copoly-
mers. Moreover, the substitution of uorine atoms on the qui-
noxaline moiety in one of the two polymers was demonstrated to
improve the resulting thin lm optical, electrical and morpho-
logical properties by inducing a further lowering of its HOMO
energy level and by promoting intermolecular p–p interactions.
Electrochemical measurements, UV-visible absorption spectra
and morphological studies were carried out to get insights on
the differences between the two synthesized materials. BHJ solar
cells prepared in different conditions of polymer:fullerene blend
ratio, thermal treatment and acceptor nature allowed the
gradual optimization of the photovoltaic performances, other
than allowing to evidence the two polymers optical, electrical
and physical properties. As a result of the cooperative effect of
the uorine atoms and alkyl-dithiophene side substitution, the
best performing solar cells, using PC61BM as acceptor, showed a
PCE of 5.3% (5.5% for the best device) with JSC, FF and VOC of
ꢀ10.2 mA cmꢀ2, 58% and 0.90 V, respectively, without the need
for additional solvent additive or thermal treatments. This work
emphasizes the effectiveness of using highly 2D p-conjugated
units and highly electron withdrawing side substituents for the
design of efficient active donor materials having broader
absorption spectra, high photocurrents and VOC when employed
in BHJ organic solar cells.
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Acknowledgements
This work was nancially supported by the European
Commission FP7 collaborative project SUNFLOWER (FP7-ICT-
2011-7, Grant no. 287594), by Programma Operativo FESR 2007–
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2013 of Regione Emilia-Romagna – Attivita I.1.1 and by Progetto
Premiale CNR 2012 – Produzione di energia da fonti rinnovabili
(Iniziativa CNR per il Mezzogiorno, L. 191/2009 art. 2 comma
44). We also acknowledge Chalmers’ Area of Advance Materials
Science for funding. EW acknowledges the Swedish Research
Council for nancial support.
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24552 | RSC Adv., 2013, 3, 24543–24552
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