Fluorine substitution enhanced photovoltaic performance of a D-A 1-D-A2 copolymer

Article abstract of DOI:10.1039/c3cc44931a

A new alternating donor-acceptor (D-A₁-D-A₂) copolymer containing two electron-deficient moieties, isoindigo and quinoxaline, was synthesized. The photovoltaic performance of this polymer could be improved by incorporating fluorine a

Full text of DOI:10.1039/c3cc44931a

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Fluorine substitution enhanced photovoltaic
performance of a D–A1–D–A2 copolymer†
Dongfeng Dang,ab Weichao Chen,c Renqiang Yang,*c Weiguo Zhu,*a
Wendimagegn Mammod and Ergang Wang*b
Cite this: Chem. Commun., 2013,
49, 9335
Received 1st July 2013,
Accepted 10th August 2013
DOI: 10.1039/c3cc44931a
www.rsc.org/chemcomm
A new alternating donor–acceptor (D–A1–D–A2) copolymer containing
two electron-deficient moieties, isoindigo and quinoxaline, was
synthesized. The photovoltaic performance of this polymer could
be improved by incorporating fluorine atoms into the quinoxaline
units, resulting in an efficiency of 6.32%. This result highlights the
attractive promise of D–A1–D–A2 copolymers for high-performance
bulk heterojunction solar cells.
Scheme 1 Synthetic routes of the polymers PTQTI and PTQTI-F. Reagents and
conditions: (i) Pd2(dba)3, P(o-tol)3, toluene.
Over the last decade, polymer solar cells (PSCs) have been intensively absorption spectra and electronic properties of the copolymer
developed with power conversion efficiencies (PCEs) improving can be tuned by the second acceptor unit. The performance of
from 2% to 10%.1 This great progress could be registered not the copolymer is higher compared to its constituent parts D–A1
only from a better understanding and control of charge genera- and D–A2 polymers. Moreover, we have shown that compared to
tion and transportation, but also due to the development of a the random copolymer PTQTI-R with two acceptor units, the
wide array of novel donor–acceptor (D–A) conjugated polymers.1 alternating copolymer PTQTI possessed regioregular structure,
The combination of electron-rich units as donors and electron- which is beneficial for p-stacking.
deficient units as acceptors, forming internal donor–acceptor
Recently, conjugated polymers with fluorine substitution were
structures, was proved to be an effective method to develop low shown to have higher performance compared to their non-fluorinated
band gap polymers.1a,2 Numerous D–A polymers were designed analogues.5 The incorporation of fluorine atoms into D–A polymers
and synthesized, which exhibited quite high performances when can simultaneously lower the highest occupied molecular orbital
used as donors in PSCs.1–3 The photophysical and electronic (HOMO) and lowest unoccupied molecular orbital (LUMO) energy
properties of the D–A polymers are determined by the donor levels, maintaining almost the same optical band gap compared to
and acceptor units. However, alternating copolymers containing their corresponding non-fluorinated analogues.5 Benefiting from the
two acceptor units, forming D–A1–D–A2 structures, were rarely lower HOMO levels, solar cells comprising fluorinated polymers
investigated for their application in PSCs. Very recently, we have exhibited higher open-circuit voltage (Voc).5,6 In some cases, fluori-
shown the advantages of the D–A1–D–A2 strategy by demonstrating nated polymers also showed improved short-circuit current (Jsc) and
the improved performance of PTQTI (cf. Scheme 1) over the fill factor (FF), which were attributed to high mobilities and/or better
conventional D–A polymer TQ1 and PTI-1.4 In such a case, the morphology.5a,6a It was also reported that fluorine substitution on
quinoxaline units can improve the performance of the resulting
a Key Lab of Environment-Friendly Chemistry and Application of the Ministry of
Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China.
E-mail: zhuwg18@126.com
polymers.6b,7 Encouraged by these results from the literature, we
prepared a D–A1–D–A2 copolymer PTQTI-F by attaching two
fluorine atoms to the quinoxaline unit of PTQTI. PTQTI-F
exhibited a higher photovoltaic performance with a PCE of 6.32%
compared to its non-fluorinated analog PTQTI with a PCE of
4.82%. This is the highest efficiency registered for a D–A1–D–A2
type of polymer, which highlights the great potential of such
polymers for highly efficient solar cells.
b Department of Chemical and Biological Engineering/Polymer Technology,
¨
Chalmers University of Technology, SE-412 96 Goteborg, Sweden.
E-mail: ergang@chalmers.se
c Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Qingdao 266101, China. E-mail: yangrq@qibebt.ac.cn
d Department of Chemistry, Addis Ababa University, P.O. Box 1176, Addis Ababa,
Ethiopia
The synthesis of the polymers PTQTI and PTQTI-F is shown in
Scheme 1. The preparation of monomers 1 and 3 was described in
our previous publication.4 Compound 2 was synthesized starting
† Electronic supplementary information (ESI) available: Detailed synthetic pro-
cedures, electrochemical data, DFT calculations, AFM images and details of solar
cell fabrication. See DOI: 10.1039/c3cc44931a
c
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from the commercially available compound 1,4-dibromo-2,3-
difluorobenzene. Thus, 1,4-dibromo-2,3-difluorobenzene was
nitrated with a mixture of fuming nitric acid and trifluoro-
methanesulfonic acid8 to afford 1,4-dibromo-2,3-difluoro-5,6-
dinitrobenzene in high yield (75.2%). The commonly used
nitrating mixture comprising of fuming nitric acid and fuming
sulfuric acid was avoided because it yielded a mixture of the mono-
and dinitro-compounds which were difficult to separate.5b,8
1,4-Dibromo-2,3-difluoro-5,6-dinitrobenzene was reduced with
iron and condensed with 1,2-bis(3-octyloxyphenyl)ethane-1,2-
dione to afford 5,8-dibromo-6,7-difluoro-2,3-bis(3-octyloxyphenyl)-
quinoxaline. The details of the synthesis of monomer 2 are
described in the ESI.† Both PTQTI and PTQTI-F were synthesized
by the Suzuki coupling reaction as depicted in Scheme 1. Both of
them are readily soluble in organic solvents such as chloroform,
toluene and o-dichlorobenzene (oDCB). The molecular weights of
the two polymers were determined by size exclusion chromato-
Fig. 2 Cyclic voltammograms of PTQTI and PTQTI-F.
To investigate the influence of fluorine substitutions on the
graphy (SEC). The fluorinated polymer PTQTI-F exhibited a lower frontier energy levels of the copolymer, cyclic voltammetry (CV)
number-average molecular weight (Mn) of 58 000 with a poly- measurements were performed. The details of the CV measure-
dispersity index (PDI) of 3.0 compared to PTQTI with a Mn of ments and the calculation method of HOMO and LUMO levels
76 000 and a PDI of 3.7. Differential scanning calorimetry (DSC) from CV results are described in the ESI.† As shown in Fig. 2, the
revealed that there was no obvious thermal transition in the oxidation onset of PTQTI-F is 0.1 V higher than that of PTQTI
range of 20–320 1C for the two polymers.
measured under the same conditions. As a result, PTQTI-F exhibits a
The absorption spectra of the two polymers, both in chloroform HOMO level of À5.90 eV and a LUMO level of À3.97 eV as deduced
solution and in the solid state, are shown in Fig. 1. Both polymers from CV measurements, which are B0.1 eV lower than those of
show two absorption bands. The strong low-energy band can be PTQTI (HOMO of À5.80 eV and LUMO of À3.90 eV). This trend is
attributed to strong intramolecular charge transfer interaction also consistent with DFT calculation (HOMO of À5.01 eV and LUMO
between the electron-rich and electron-deficient units. Although of À2.96 eV for PTQTI-F, and HOMO of À4.90 eV and LUMO of
absorption onsets of D–A1–D–A2 copolymers were mainly determined À2.90 eV for PTQTI). Previous studies4,9 revealed that the LUMO
by the stronger electron-deficient units (isoindigo is stronger than levels of copolymers were mainly determined by the stronger
quinoxaline),9 the fluorinated quinoxaline-containing polymer acceptor units. However, the results described above clearly indicate
PTQTI-F showed a clear blue shift compared to PTQTI, which means the influence of fluorine substitution of weaker acceptor units on the
fluorine substitution on weaker acceptor units may still have visible energy levels of the D–A1–D–A2 copolymer.
influence on the photophysical properties of the resulting D–A1–D–A2
For comparison of the photovoltaic performances of the two
copolymers. Similar blue shifts have been observed in some other donor polymers, conventional bulk heterojunction solar cell
fluorinated D–A polymer systems.1b,5b,10 Density functional theory devices were fabricated with [6,6]-phenyl C61 butyric acid
(DFT) calculation showed that the introduction of fluorine atoms methyl ester (PC61BM) as an acceptor. The devices had a
onto quinoxaline does not increase the torsion angle of the polymer configuration of glass/ITO/PEDOT:PSS/polymer:PC61BM/Ca/Al
backbone (cf. ESI,† Fig. S2 side views of the simulated molecular and were tested under the illumination of AM 1.5G simulated
geometries of PTQTI and PTQTI-F). A plausible explanation for the solar light (100 mW cmÀ2). Based on our previous studies,4,12
planarity of fluorinated polymers is the strong interaction between the optimized D : A ratio of 1 : 1, processing solvent of 2.5% (wt)
fluorine atoms and C-4 hydrogens on the thiophene rings.11
of 1,8-diiodooctane (DIO) in oDCB and an active layer thickness
of B100 nm were used in this work. Moreover, PC61BM was chosen
as the acceptor instead of PC71BM, as the morphology of poly-
mer:PC61BM was better in this case.4,13 The typical current density–
voltage (J–V) curves for solar cells based on the two polymers are
shown in Fig. 3 and the corresponding external quantum efficiency
(EQE) profiles are shown in Fig. 4. PTQTI presents a PCE of 4.82%
with a short-circuit current density (Jsc) of 11.40 mA cmÀ2, a fill
factor (FF) of 0.52 and an open-circuit voltage (Voc) of 0.82 V. This
result is close to our previous result with a PCE of 5.03% regardless
of the difference in cathode material (calcium in this work and
LiF in previous work).4 PTQTI-F exhibited a higher Voc of 0.93 V,
an enhanced Jsc of 12.58 mA cmÀ2 and a FF of 0.54, resulting in
an increased PCE of 6.32%. The increased Voc can be attributed
to its deeper HOMO level, which is also observed in other
fluorinated D–A polymer systems.1b,5,6 It is striking to note that
Fig. 1 Absorption spectra of the polymers.
c
9336 Chem. Commun., 2013, 49, 9335--9337
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PTQTI-F-based film, which is favorable for charge transportation.
Therefore, the enhanced photocurrent of PTQTI-F can be partially
attributed to the improved morphology of the PTQTI-F:PC61BM
blend.5a,6a
In conclusion, a new fluorinated D–A1–D–A2 copolymer
PTQTI-F was synthesized. Bulk heterojunction solar cells fabricated
with this polymer as a donor and PC61BM as an acceptor realized a
PCE of 6.32%, which is the highest efficiency recorded for a
polymer containing two acceptor units. Moreover, enhancement
of the Voc by 0.1 V was achieved by PTQTI-F compared to non-
fluorine substituted analogues due to its deeper HOMO level. Thus,
fluorine substitution on the weaker acceptor unit is a useful
strategy to fine-tune energy levels of D–A1–D–A2 copolymers.
We thank the Swedish Research Council and National Natural
Science Foundation of China (51173199) for financial support.
Fig. 3 The J–V curves of the PSCs based on the two polymers.
Notes and references
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This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 9335--9337 9337