Regioregular pyridyl[2,1,3]thiadiazole-co-indacenodithiophene conjugated polymers

Article abstract of DOI:10.1039/c3cc43229g

Regioregular conjugated polymers containing alternating pyridyl[2,1,3]thiadiazole (PT) and indacenodithiophene (IDT) structural units were synthesized. In these copolymers, the pyridyl nitrogen atoms on PT are precisely arranged along the backbone so that

Full text of DOI:10.1039/c3cc43229g

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Regioregular pyridyl[2,1,3]thiadiazole-co-
indacenodithiophene conjugated polymers†
Cite this: DOI: 10.1039/c3cc43229g
Wen Wen,z Lei Ying,z Ben B. Y. Hsu, Yuan Zhang, Thuc-Quyen Nguyen and
Received 2nd May 2013,
Accepted 24th May 2013
Guillermo C. Bazan*
DOI: 10.1039/c3cc43229g
www.rsc.org/chemcomm
Regioregular conjugated polymers containing alternating pyridyl[2,1,3]-
Indacenodithiophene (IDT), which contains two thiophene rings
thiadiazole (PT) and indacenodithiophene (IDT) structural units held in a rigid arrangement via a central phenyl ring, has been recently
were synthesized. In these copolymers, the pyridyl nitrogen atoms shown to be a suitable electron rich moiety for the design of narrow
on PT are precisely arranged along the backbone so that each one band gap conjugated polymers for incorporation into organic solar
has an adjacent proximal and an adjacent distal counterpart across cells.6 For the donor–acceptor (D–A) copolymer containing IDT and PT,
the two IDT flanking units. We find that despite the absence of it was found that in combination with [6,6]-phenyl-C71-butyric acid
obvious differences in orbital energy levels and optical bandgap, methyl ester (PC71BM) one could obtain PCEs on the order of 3.4%.7
the regioregular materials exhibit larger charge carrier mobilities, Based on the recent success in achieving high charge carrier mobilities
as determined by using field effect transistor devices, and can yield discussed above, it seemed to be appropriate to consider how relevant
higher solar cell power conversion efficiencies when mixed with basic physical properties and device function would be influenced by
fullerenes in bulk heterojunction active layers.
controlling the regiochemistry in PT–IDT copolymers; a structural
consideration that has not received attention in the literature. In this
Bulk heterojunction (BHJ) solar cells based on conjugated polymers contribution, we show that, indeed, regioregular IDT–PT copolymers
and fullerene acceptors have attracted much attention due to the show improved performances relative to regiorandom counterparts.
possibility of solution fabrication.1 This approach has led to power
A two step procedure is required for the preparation of the
conversion efficiencies (PCE) of 8–10% based on the optimization of regioregular polymer, see Scheme 1. First, the cross-coupling reaction
processing technologies and the development of new narrow band of two equivalents of 4,7-dibromo-[1,2,5]thiadiazolo[3,4-c]pyridine
gap materials, primarily via the development of copolymers contain- (PTBr2) with (4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-sec-indaceno-
ing electron rich and electron poor fragments.2 For conjugated [1,2-b:5,6-b0]dithiophene-2,7-diyl)bis(trimethylstannane) (M1) provided
polymers containing unsymmetric structural units, it is well-known 4,40-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-sec-indaceno[1,2-b:5,6-b0]-
that molecular structure, especially in terms of regioregularity, dithiophene-2,7-diyl)bis(7-bromo-[1,2,5]thiadiazolo[3,4-c]pyridine) (M2).
influences relevant optoelectronic properties. Classic examples This reaction takes advantage of the higher reactivity of C–Br
include poly(3-alkylthiophene), poly(3-alkylthienylenevinylenes) and
poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene).3 We recently
showed that by controlling the orientation of the pyridyl[2,1,3]-
thiadiazole (PT) unit along the backbone vector one can obtain up
to two orders of magnitude improvement in the carrier mobility,
relative to counterparts that are not synthesized with specific control.4
Moreover, these regioregular PT copolymers have been recently shown
to be useful in the fabrication of field effect transistors (FETs) with
hole mobilities of up to 6.7 cm2 Vꢀ1 sꢀ1 5
.
Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry
and Materials, University of California, Santa Barbara, California 93106-7150,
USA. E-mail: bazan@chem.ucsb.edu
† Electronic supplementary information (ESI) available: Detailed synthesis of
monomers and polymers, GPC, CV and DSC characteristics, detailed device
fabrication and characterization including J–V curves and AFM images. See
DOI: 10.1039/c3cc43229g
Scheme 1 Synthesis of PIPT-RG.
‡ Both authors contributed equally to this work.
c
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adjacent to the pyridyl nitrogen on PT. In a second step, microwave- measurements and the assumptions in energy calculations.7 It was
assisted Stille polymerization of M1 with M2 generates the target also determined that nearly no difference in the HOMO values could
copolymer (PIPT-RG) with the regiochemistry as shown in Scheme 1. be determined by using ultraviolet photoelectron spectroscopy (UPS)
Specifically, for each PT unit one finds an adjacent distal PT (Fig. 1b). The onsets of reduction (Ered) of the two copolymers were
(N atoms pointing away) and an adjacent proximal PT (N atoms indistinguishable and located at approximately ꢀ1.20 V, corres-
pointing toward) fragments. Also for comparison with PIPT-RG, we ponding to a lowest unoccupied molecular orbital energy (ELUMO
)
prepared the IDT-co-PT copolymer via the established route reported of approximately ꢀ3.60 eV. Thus, the increase in regioregularity led
in the literature in which one equivalent of PTBr2 and one equivalent to no detectable differences in energy levels as could be determined
of the distannylated compound M1 are subjected to Stille-coupling by conventional characterization methods.
conditions.7 The resulting product is expected to be less structurally
precise and will be referred to as PIPT-RA.
To investigate charge transport properties of the polymers, bottom
gate, bottom contact field effect transistors (FETs) with the architec-
Both PIPT-RG and PIPT-RA were end-capped after polymerization ture Si/SiO2/DTS/polymer/Au were fabricated by spin-casting from
to remove unreacted bromide or trimethyltin end-groups,8 and were 0.4 wt/v% of solution in 1,2-dichlorobenzene. Decyltrichlorosilane
purified by Soxhlet extraction using methanol, acetone, hexane and (DTS) was used as a dielectric passivation layer. Fig. 2 shows
dichloromethane successively. The final polymers corresponded to characteristic output and transfer curves after the films were thermally
the remaining chloroform soluble fraction. Gel permeation chroma- annealed at 150 1C for 10 minutes. From these data one obtains
tography showed that the two polymers have very same molecular calculated hole mobilities of 0.2 cm2 Vꢀ1 sꢀ1 for PIPT-RG and
weight characteristics. The number average molecular weights are 0.04 cm2 Vꢀ1 sꢀ1 for PIPT-RA. This result indicates that better charge
60 kDa and 68 kDa for PIPT-RA and PIPT-RG, with polydispersities carrier transport can be realized by controlling the regioregularity of
of 2.5 and 2.4, respectively (Fig. S1, ESI†). No noticeable phase PIPT, which is consistent with previously demonstrated better perfor-
transitions were observed for either material using differential mance with other regioregular PT-containing polymers.4
scanning calorimetry up to 300 1C (Fig. S2, ESI†).
Solar cells with an architecture described by ITO/molybdenum
Fig. 1a shows the absorption profiles in the solid state. PIPT-RA oxide (MoO3)/polymer:PC71BM/Al were fabricated as testing platforms,
exhibits a band centred at 710 nm, which corresponds to inter- where the MoO3 anode buffer layer was processed by spin-coating
molecular charge transfer interactions between the donor and from aqueous MoO3 solution.9 We first screened device performance
acceptor moieties. A higher energy transition is also observed at with different polymer/PC71BM weight ratios under AM 1.5 G solar
417 nm, which has been previously assigned to a delocalized p–p* irradiation at 100 mW cmꢀ2 (Fig. S4 and Table S1, ESI†). The
transition. These observations are consistent with properties optimized weight ratio of the polymer to PC71BM was found to be
described in the literature.7 Fig. 1a also shows that PIPT-RG 1 : 4 and PCEs of 3.4% for PIPT-RA (denoted as A1 in Table 1) and
exhibits a nearly indistinguishable profile. From the onsets of 5.1% for PIPT-RG (denoted as A2 in Table 1) were obtained by thermal
absorption one can estimate the optical band gaps of the two annealing at 100 1C for 10 min,10 respectively. Current density–voltage
copolymers to be nearly equal (1.60 eV). However, it is worth (J–V) characteristics are shown in Fig. 3, and the corresponding
noting that the film absorption coefficient (a) of PTPT-RA (0.65 ꢁ parameters are summarized in Table 1. Device A2 exhibited higher
104 cmꢀ1) is smaller than that of PIPT-RG (0.77 ꢁ 104 cmꢀ1). This shunt resistance (Rsh = 6.1 MO cmꢀ2) and lower series resistances
slightly stronger intensity is not understood at this point, but may (Rs = 0.66 KO cmꢀ2) than device A1 (Rsh = 4.1 MO cmꢀ2 and Rs =
be related to differences in the orientation of the polymer chains 1.23 KO cmꢀ2), respectively (Fig. S6, ESI†), indicating that the leakage
relative to the substrate surface or subtle differences in interchain current in A2 is lower, and higher FF could be achieved.
packing, partly influenced by the regioregularity or the ordered
vector of the PT unit along the polymer main chain.
We note here that topographic analysis using atomic force
microscopy (AFM) shows that the PIPT-RG:PC71BM film is smoother
Electrochemical measurements using cyclic voltammetry (CV) (root mean square, rms, roughness of 0.5 nm) than PIPT-RA:PC71BM
were carried out to estimate the orbital energy levels (Fig. S3, (rms = 3.5 nm). Furthermore, AFM also shows that PIPT-RA:PC71BM
ESI†). The onsets of oxidation (Eox) are determined to be 0.50 V films exhibit round elevated features that suggest more pronounced
and 0.45 V, corresponding to the highest occupied molecular BHJ phase separation. Even though the AFM images mainly reflect
orbital energy level (EHOMO) of ꢀ5.30 eV and ꢀ5.25 eV for PIPT-RA the surface of film morphologies, the relative suppression of domain
and PIPT-RG, respectively. These values are similar to those
reported in the literature and are equivalent given the errors in
Fig. 2 Output curves (a) at a gate voltage (VG) of ꢀ60 V and transfer curves at a
Fig. 1 UV-Vis absorption spectra (a) and UPS spectra (b) of PIPT-RA and PIPT-RG
source–drain voltage (VSD) of ꢀ60 V (b) of films thermally annealed at 150 1C
in the solid state.
for 10 min.
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Table 1 Summary of device properties
regioregular PIPT-RG and the polymer obtained through
conventional synthesis (PIPT-RA) are essentially identical
within the certainties of the measurements. However, the FET
hole mobility of PIPT-RG is higher than that of PIPT-RA. Of
particular note is that the PCE values of BHJ solar cells with
PIPT-RG are significantly improved and that in these devices
one finds from single carrier diode measurements that the hole
Device Polymer:PC71BM Voc (V) Jsc (mA cmꢀ2
)
FF (%) PCE (%)
A1
A2
B1
B2
PIPT-RA:PC71BM 0.82
PIPT-RG:PC71BM 0.88
PIPT-RA:PC71BM 0.86
PIPT-RG:PC71BM 0.88
10.5
12.1
12.3
13.9
40
48
43
55
3.4
5.1
4.5
6.7
Devices A1 and A2 have conventional structure: ITO/MoO3/poly-
mer:PC71BM (1 : 4 wt : wt)/Al, while B1 and B2 comprise the inverted mobilities are also higher. With inverted device structures one can
structure: ITO/ZnO/polymer : PC71BM (1 : 4 wt : wt)/MoO3/Ag.
obtain with PIPT-RG:PC71BM a PCE of 6.7%. From a practical
perspective, these findings highlight the benefits of controlling
the regiochemistry of PT-containing narrow bandgap conjugated
polymers.
We acknowledge the financial support from Mitsubishi
Chemical Center for Advanced Materials (MC-CAM). We thank
Prof. Alan J. Heeger, Dr Yanming Sun, Dr Soo-Hyung Choi and
Dr Chan Luo for technical assistance and helpful discussions.
Notes and references
Fig. 3 J–V curves (a) and external quantum efficiency spectra (b) of devices
1 (a) Z. B. Henson, K. Mullen and G. C. Bazan, Nat. Chem., 2012, 4,
based on polymer:PC71BM.
699–704; (b) J. W. Chen and Y. Cao, Acc. Chem. Res., 2009, 42,
1709–1718; (c) Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev.,
2009, 109, 5868–5923; (d) C. Li, M. Y. Liu, N. G. Pschirer,
M. Baumgarten and K. Mu¨llen, Chem. Rev., 2010, 110, 6817–6855.
2 (a) J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty,
K. Emery, C. C. Chen, J. Gao, G. Li and Y. Yang, Nat. Commun., 2013,
4, 1–10; (b) Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat.
Photonics, 2012, 6, 593–597; (c) C. Cabanetos, A. El Labban,
size in PIPT-RG:PC71BM could be responsible for the improve-
ments in device function.
Hole- and electron-only devices were specifically fabricated
to estimate the charge mobilities of the blend films by a
space charge limited current (SCLC) model as shown in
Fig. S6 (ESI†). The electron mobility of two devices are compar-
´
J. A. Bartelt, J. D. Douglas, W. R. Mateker, J. M. J. Frechet,
M. D. McGehee and P. M. Beaujuge, J. Am. Chem. Soc., 2013, 135,
4656–4659; (d) Y. Huang, X. Guo, F. Liu, L. J. Huo, Y. N. Chen,
T. P. Russell, C. C. Han, Y. F. Li and J. H. Hou, Adv. Mater., 2012, 24,
3383–3389; (e) T. B. Yang, M. Wang, C. H. Duan, X. W. Hu, L. Huang,
J. B. Peng, F. Huang and X. Gong, Energy Environ. Sci., 2012, 5, 8208–8214.
3 (a) I. Osaka and R. D. McCullough, Acc. Chem. Res., 2008, 41, 1202–1214;
(b) R. S. Loewe and R. D. McCullough, Chem. Mater., 2000, 12, 3214–3221;
(c) I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. MacDonald,
M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. Zhang,
M. L. Chabinyc, R. J. Kline, M. D. McGehee and M. F. Toney, Nat. Mater.,
2006, 5, 328–333.
4 L. Ying, B. B. Y. Hsu, H. M. Zhan, G. C. Welch, P. Zalar, L. A. Perez,
E. J. Kramer, T. Q. Nguyen, A. J. Heeger, W. Y. Wong and
G. C. Bazan, J. Am. Chem. Soc., 2011, 133, 18538–18541.
5 H. R. Tseng, L. Ying, B. B. Y. Hsu, L. A. Perez, C. J. Takacs,
G. C. Bazan and A. J. Heeger, Nano Lett., 2012, 12, 6353–6357.
6 (a) I. McCulloch, R. S. Ashraf, L. Biniek, H. Bronstein, C. Combe,
J. E. Donaghey, D. I. James, C. B. Nielsen, B. C. Schroeder and
W. Zhang, Acc. Chem. Res., 2012, 45, 714–722; (b) Y. Zhang, J. Zou,
H. L. Yip, K. S. Chen, J. A. Davies, Y. Sun and A. K. Y. Jen, Macromolecules,
2011, 44, 4752–4758; (c) W. Zhang, J. Smith, S. E. Watkins, R. Gysel,
M. McGehee, A. Salleo, J. Kirkpatrick, S. Ashraf, T. Anthopoulos,
M. Heeney and I. McCulloch, J. Am. Chem. Soc., 2010, 132,
11437–11439; (d) K. S. Chen, Y. Zhang, H. L. Yip, Y. Sun, J. A. Davies,
C. Ting, C. P. Chen and A. K. Y. Jen, Org. Electron., 2011, 12, 794–801;
(e) Y. C. Chen, C. Y. Yu, Y. L. Fan, L. I. Hung, C. P. Chen and C. Ting,
Chem. Commun., 2010, 46, 6503–6505; ( f ) C. P. Chen, S. H. Chan,
T. C. Chao, C. Ting and B. T. Ko, J. Am. Chem. Soc., 2008, 130,
12828–12833.
able (2 ꢁ 10ꢀ4 cm2 Vꢀ1
s
ꢀ1), most likely implying that the
properties of the PC71BM pathway are not modified signifi-
cantly. However, the PIPT-RG:PC71BM device exhibited higher
hole mobility (1.3 ꢁ 10ꢀ4 cm2 Vꢀ1
s
ꢀ1) relative to that of the
PIPT-RA:PC71BM device (3.1 ꢁ 10ꢀ5 cm2 Vꢀ1
s
ꢀ1). Since the
collected current is dependent on the charge transporting
properties of the BHJ layer, the higher hole mobility may be
responsible for the larger Jsc observed for the PIPT-RG based
device A2. Moreover, more balanced charge carrier mobility of
PIPT-RG:PC71BM blends will lead to an improvement in FF.
Further optimization of solar cell performance was achieved
through the use of inverted structures of the type ITO/ZnO/
polymer : PC71BM (1 : 4 wt : wt)/MoO3/Ag. These devices are
denoted in Table 1 as B1 and B2, respectively. Here the active
layers were also annealed at 100 1C for 10 min before cathode
deposition. The best inverted device structure (B1) with PIPT-RA
provided a PCE of 4.5%, (Jsc = 12.13 mA cmꢀ2, Voc = 0.86, and
FF = 43%). The best performing device with PIPT-RG exhibited a
much larger PCE of 6.7% (Jsc = 13.9 mA cmꢀ2, Voc = 0.88 V and
FF = 55%). The J–V characteristics and EQE spectra are shown in
Fig. 3 and the device performances are summarized in Table 1.
Note that the integrations of EQE spectra (Fig. 3b) are consistent
with the Jsc results obtained from J–V measurements.
7 Y. Sun, S. C. Chien, H. L. Yip, Y. Zhang, K. S. Chen, D. F. Zeigler,
F. C. Chen, B. P. Lin and A. K. Y. Jen, J. Mater. Chem., 2011, 21,
13247–13255.
8 J. K. Park, J. Jo, J. H. Seo, J. S. Moon, Y. D. Park, K. Lee, A. J. Heeger
and G. C. Bazan, Adv. Mater., 2011, 23, 2430–2435.
9 (a) F. M. Liu, S. Y. Shao, X. Y. Guo, Y. Zhao and Z. Y. Xie, Sol. Energy
Mater. Sol. Cells, 2010, 94, 842–845; (b) S. Murase and Y. Yang, Adv.
Mater., 2012, 24, 2459–2462.
10 G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery and
Y. Yang, Nat. Mater., 2005, 4, 864–868.
In conclusion, we have observed that the PCEs of indaceno-
dithiophene-co-pyridyl[2,1,3]thiadiazole (IDT–PT) based copolymers
can be improved by precisely controlling the orientation of the
N-atom in the PT unit with respect to the polymer backbone.
One finds that the optical and electrochemical properties of the
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.