Tuning the absorption, charge transport properties, and solar cell efficiency with the number of thienyl rings in platinum-containing poly(aryleneethynylene)s

Article abstract of DOI:10.1021/ja074959z

The synthesis, characterization, and photophysics of a series of solution-processable and strongly visible-light absorbing platinum(II) polyynes containing bithiazole-oligo(thienyl) rings were presented. Tuning the polymer solar cell efficiency, as well a

Full text of DOI:10.1021/ja074959z

Published on Web 10/30/2007
Tuning the Absorption, Charge Transport Properties, and
Solar Cell Efficiency with the Number of Thienyl Rings in
Platinum-Containing Poly(aryleneethynylene)s
Wai-Yeung Wong,*,† Xing-Zhu Wang,†,‡ Ze He,† Kwok-Kwan Chan,†
Aleksandra B. Djurisˇic´,*,§ Kai-Yin Cheung,§ Cho-Tung Yip,§ Alan Man-Ching Ng,§
Yan Yan Xi,§ Chris S. K. Mak,| and Wai-Kin Chan|
Contribution from the Department of Chemistry and Centre for AdVanced Luminescence
Materials, Hong Kong Baptist UniVersity, Waterloo Road, Kowloon Tong,
Hong Kong, P. R. China, Key Laboratory of EnVironmentally Friendly Chemistry and
Applications of Ministry of Education, College of Chemistry, Xiangtan UniVersity, Xiangtan
411105, Hunan ProVince, P. R. China, and Departments of Chemistry and Physics, The
UniVersity of Hong Kong, Pokfulam Road, Hong Kong, P. R. China
Received July 5, 2007; E-mail: rwywong@hkbu.edu.hk; dalek@hkusua.hku.hk
Abstract: The synthesis, characterization, and photophysics of a series of solution-processable and strongly
visible-light absorbing platinum(II) polyynes containing bithiazole-oligo(thienyl) rings were presented. Tuning
the polymer solar cell efficiency, as well as optical and charge transport properties, in soluble, low-band
gap PtII-based conjugated poly(heteroaryleneethynylene)s using the number of oligothienyl rings is described.
These materials are highly soluble in polar organic solvents due to the presence of solubilizing bithiazole
moieties and show strong absorptions in the solar spectra, rendering them excellent candidates for bulk
heterojunction polymer solar cells. Their photovoltaic responses and power conversion efficiencies (PCEs)
depend to a large extent on the number of thienyl rings along the main chain, and some of them can be
used to fabricate highly efficient solar cells with PCEs of up to 2.7% and a peak external quantum efficiency
to 83% under AM1.5 simulated solar illumination, which is comparable to that of poly(3-hexylthiophene)-
based devices fabricated without additional processing (annealing or TiOx layer). The influence of the number
of thienyl rings and the metal group on the performance parameters and optimization of solar cell efficiency
was evaluated and discussed in detail. At the same blend ratio of 1:4, the light-harvesting ability and PCE
increase sharply as the thienyl chain length increases. The present work provides an attractive approach
to developing conjugated metallopolymers offering broad solar absorptions and tunable solar cell efficiency
and demonstrates the potential of metalated conjugated polymers for efficient power generation.
energy problem.1 Solar cells based on solution-processable
organic semiconducting polymers have attracted considerable
Introduction
Harvesting energy directly from sunlight using photovoltaic
technology is increasingly recognized as a solution to the world’s
interest as a low-cost alternative to inorganic semiconductors
for large-area and lightweight applications.2 While excellent
performance (5-6% efficiency) has been reported for solar cells
based on bulk heterojunctions consisting of poly(3-hexylth-
iophene) (P3HT) as a donor material and [6,6]-phenyl-C61-
butyric acid methyl ester (PCBM) as an acceptor material,3 it
has been recognized that novel materials need to be developed
in order to obtain solar cells with an efficiency exceeding 10%
(threshold for commercial applications).4 So, efforts have been
made for designing better materials to improve coverage of the
solar spectrum and hence enhance the efficiency of polymer
photovoltaic cells by extending the absorption to longer
wavelengths, increasing the absorption coefficients (ꢀmax), and
optimizing the charge transport properties for forming efficient
blends with fullerenes in bulk heterojunction cells.3a,h,4
† Hong Kong Baptist University.
‡ Xiangtan University.
§ Department of Physics, The University of Hong Kong.
| Department of Chemistry, The University of Hong Kong.
(1) (a) Shaheen, S. E.; Ginley, D. S.; Jabbour, G. E. MRS Bull. 2005, 30, 10.
(b) Barnham, K. W. J.; Mazzer, M.; Clive, B. Nature 2006, 5, 161.
(2) (a) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater.
2001, 11, 15. (b) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004,
16, 4533. (c) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007,
107, 1324. (d) Winder, C.; Saricftci, N. S. J. Mater. Chem. 2004, 14, 1077.
(3) (a) Kim, J. Y.; Kim, S. H.; Lee, H. H.; Lee, K.; Ma, W.; Gong, X.; Heeger,
A. J. AdV. Mater. 2006, 18, 572. (b) Reyes-Reyes, M.; Kim, K.; Carroll,
D. J. Appl. Phys. Lett. 2005, 87, 083506. (c) Kim, Y.; Cook, S.; Tuladhar,
S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles,
M.; McCulloch, I.; Ha, C. S.; Ree, M. Nat. Mater. 2006, 5, 197. (d) Li, G.;
Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat.
Mater. 2005, 4, 864. (e) Li, G.; Shortriya, V.; Yao, Y.; Yang, Y. J. Appl.
Phys. 2005, 98, 043704. (f) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley,
D. D. C.; Cook, S.; Durrant, J. R. Appl. Phys. Lett. 2006, 86, 063502. (g)
De Bettignies, R.; Leroy, J.; Firon, M.; Sentein, C. Synth. Met. 2006, 156,
510. (h) Mihailetchi, V. D.; Xie, H.; de Boer, B.; Popescu, L. M.;
Hummelen, J. C.; Blom, P. W. M.; Koster, L. J. A. Appl. Phys. Lett. 2006,
89, 012107. (i) von Hauff, E.; Parisi, J.; Dyakonov, V. J. Appl. Phys. 2006,
100, 043702-1. (j) Kim, K.; Liu, J.; Namboothiry, M. A. G.; Carroll, D.
L. Appl. Phys. Lett. 2007, 90, 163511.
(4) (a) Koster, L. J. A.; Mihailetchi, V. D.; Bloom, P. W. Appl. Phys. Lett.
2006, 88, 093511. (b) Scharber, M. C.; Mu¨hlbacher, D.; Koppe, M.; Denk,
P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. AdV. Mater. 2006, 18, 789.
(c) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A.
J.; Bazan, G. C. Nat. Mater. 2007, 6, 497.
9
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10.1021/ja074959z CCC: $37.00 © 2007 American Chemical Society

Pt-Containing Poly(aryleneethynylene)s with Thienyl Rings
A R T I C L E S
However, although several organic polymers with lower band
gaps than P3HT have been reported, the achieved power
conversion efficiency (PCE) was typically lower compared to
the best achieved results for P3HT based cells.5 Typically, the
efficiency of low band gap polymer based solar cells is in the
range ∼2.2-3.2%,5a-e while efficiencies exceeding 4-5% have
been only recently reported.5f,g It should be noted that most of
the efforts in developing new materials are focused on novel
donor materials5 and establishing what properties a desirable
donor material should have in order to result in efficient solar
cells based on a bulk heterojunction with PCBM.4b An alterna-
tive method for improving device performance could be placed
on changing the acceptor material, since it has been proposed
that lowering of the LUMO level of the acceptor would be more
beneficial to the cell performance than lowering the band gap
of the polymer.4a However, although efficient cells with other
acceptor molecules have been demonstrated,6 the majority of
polymer solar cells are based on a PCBM acceptor. Nevertheless,
a recent report that a polyfluorene based polymer yields similar
efficiency in bulk heterojunctions with P3HT and PCBM6a
highlights the need for expanding the flexibility of solar cell
designs to include a greater variety of functional materials.
However, due to the large number of possible material
combinations, and with each new material requiring optimization
of cell fabrication conditions to achieve good results, this would
be a very difficult task and inevitably most of the novel donor
polymer design efforts are focused on donors which would be
combined with PCBM. One possible solution to this problem
would be to develop a new class of polymers with easily tunable
properties which can be solution-processed under similar
conditions.
ment of a triplet state for energy conversion.8f,h We have recently
demonstrated efficient solar cells based on platinum metal-
lopolyyne/PCBM bulk heterojunctions.5f While further improve-
ments in the achieved solar cell efficiency are expected with
optimization of the processing conditions and device architec-
ture, a design limit will eventually be reached, as determined
by the charge mobilities and energy levels of the material. For
greater flexibility in the solar cell design, a class of polymers
with tunable functional properties is desirable. Here we report
some strongly visible-absorbing and band gap tunable metalated
conjugated polymers with the unexplored bis(oligothienyl)-2,2′-
bithiazole donor-acceptor (D-A) hybrid spacers, and solar cells
derived from them showed high PCEs of up to 2.7%. The work
allows an effective tuning of PCE by simple tailoring of the
number of thienyl rings, which changes the absorption features
and charge carrier mobilities of the resulting metallopolymers.
Results and Discussion
Synthetic Strategies and Chemical Characterization. The
chemical structures of new platinum(II) polyyne polymers P0-
P3 and their well-defined model compounds M0-M3 are shown
in Scheme 1. The dibromo precursors (L1-2Br, L2-2Br, and
L3-2Br) can be obtained from L0-2Br by successive coupling
of the 2-thienylmagnesium bromide with the corresponding
dibromide of the lower generation. Conversion of the dibromide
derivatives to their corresponding diethynyl congeners can be
readily achieved following the typical organic synthetic proto-
cols for alkynylation of aromatic halides.8b,d,u The Pt compounds
were prepared by the Sonogashira-type dehydrohalogenation
reaction between each of L0-L3 and the platinum chloride
precursors.8j The use of long nonyl chains on bithiazole is crucial
in increasing the solubility and improving the solution process-
ability/tractability of these metallopolymers. The polymers can
be purified by silica column chromatography and repeated
precipitation and isolated in good yield and high purity. All
these Pt compounds are thermally and air stable and soluble in
common chlorinated hydrocarbons and toluene. P0-P3 can cast
tough, free-standing thin films from their solutions readily, but
their solubility tends to decrease gradually as m increases. Gel-
Metal-containing conjugated organic polymers represent an
intriguing and promising class of materials,7 and platinum
alkynyls have been a popular candidate for inclusion into such
a polymeric backbone.8 While organometallic donor materials
are widely used in small-molecule solar cells,9 soluble π-con-
jugated organometallic polyynes were much less explored and,
in most cases, the efficiency was low due to low coverage in
the solar spectral region,8a,e,f,h although there can be an involve-
(5) (a) Zhou, Q.; Hou, Q.; Zheng, L.; Deng, X.; Yu, G.; Cao, Y. Appl. Phys.
Lett. 2004, 84, 1653. (b) Svensson, M.; Zhang, F.; Veenstra, S. C.; Verhees,
W. J. H.; Hummelen, J. C.; Kroon, J. M.; Ingana¨s, O.; Anderson, M. R.
AdV. Mater. 2003, 15, 988. (c) Zhang, F.; Mammo, W.; Andersson, L. M.;
Admassie, S.; Andersson, M. R.; Ingana¨s, O. AdV. Mater. 2006, 18, 2169.
(d) Hou, J.; Tan, Z.; Yan, Y.; He, Y.; Yang, C.; Li, Y. J. Am. Chem. Soc.
2006, 128, 4911. (e) Mu¨hlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.;
Waller, D.; Gaudiana, R.; Brabec, C. AdV. Mater. 2006, 18, 2884. (f) Wong,
W.-Y.; Wang, X.-Z.; He, Z.; Djurisˇic´, A. B.; Yip, C.-T.; Cheung, K.-Y.;
Wang, H.; Mak, C. S.-K.; Chan, W.-K. Nat. Mater. 2007, 6, 521. (g) Peet,
J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan,
G. C. Nat. Mater. 2007, 6, 497.
(6) (a) McNeill, C. R.; Abrusci, A.; Zaumsell, J.; Wilson, R.; McKiernan, M.
J.; Burroughes, J. H.; Halls, J. J. M.; Greenham, N. C.; Friend, R. H. Appl.
Phys. Lett. 2007, 90, 193506. (b) Yao, Y.; Shi, C.; Li, G.; Shotriya, V.;
Pei, Q.; Yang, Y. Appl. Phys. Lett. 2006, 89, 153507. (c) Alam, M. M.;
Jenekhe, S. A. Chem. Mater. 2004, 16, 4647.
(7) (a) Manners, I. Science 2001, 294, 1664. (b) Nguyen, P.; Go´mez-Elipe, P.;
Manners, I. Chem. ReV. 1999, 99, 1515. (c) Kingsborough, R. P.; Swager,
T. M. Prog. Inorg. Chem. 1999, 48, 123. (d) Abd-El-Aziz, A. S. Macromol.
Rapid Commun. 2002, 23, 995. (e) Holliday, B. J.; Swager, T. M. Chem.
Commun. 2005, 23. (f) Fustin, C.-A.; Guillet, P.; Schubert, U. S.; Gohy,
J.-F. AdV. Mater. 2007, 19, 1665. (g) Abd-El-Aziz, A. S.; Todd, E. K.
Coord. Chem. ReV. 2003, 246, 3. (h) Abd-El-Aziz, A. S.; Carraher, C. E.,
Jr.; Pittman, C. U., Jr.; Zeldin, M. Macromolecules Containing Metal and
Metal-Like Elements; Wiley-Interscience: New Jersey, 2005; Vol. 5. (i)
Wolf, M. O. J. Inorg. Organomet. Polym. Mater. 2006, 16, 189. (j) Abd-
El-Aziz, A. S., Manners, I., Eds. Frontiers in Transition Metal-Containing
Polymers; Wiley-Interscience: New Jersey, 2007. (k) Fyfe, H. B.; Mlekuz,
M.; Zargarian, D.; Taylor, N. J.; Marder, T. B. J. Chem. Soc., Chem.
Commun. 1991, 188.
(8) (a) Younus, M.; Ko¨hler, A.; Cron, S.; Chawdhury, N.; Al-Mandhary, M.
R. A.; Khan, M. S.; Lewis, J.; Long, N. J.; Friend, R. H.; Raithby, P. R.
Angew. Chem., Int. Ed. 1998, 37, 3036. (b) Long, N. J.; Williams, C. K.
Angew. Chem., Int. Ed. 2003, 42, 2586. (c) Manners, I. Synthetic Metal-
Containing Polymers; Wiley-VCH: Weinheim, 2004. (d) Wong, W.-Y.;
Ho, C.-L. Coord. Chem. ReV. 2006, 250, 2627. (e) Chawdhury, N.; Ko¨hler,
A.; Friend, R. H.; Wong, W.-Y.; Lewis, J.; Younus, M.; Raithby, P. R.;
Corcoran, T. C.; Al-Mandhary, M. R. A.; Khan, M. S. J. Chem. Phys.
1999, 110, 4963. (f) Guo, F.; Kim, Y. G.; Reynolds, J. R.; Schanze, K. S.
Chem. Commun. 2006, 1887. (g) Silverman, E. E.; Cardolaccia, T.; Zhao,
X.; Kim, K. Y.; Haskins-Glusac, K.; Schanze, K. S. Coord. Chem. ReV.
2005, 249, 1491. (h) Ko¨hler, A.; Wittman, H. F.; Friend, R. H.; Khan, M.
S.; Lewis, J. Synth. Met. 1996, 77, 147. (i) Wong, W.-Y. Comment Inorg.
Chem. 2005, 26, 39. (j) Wong, W.-Y. J. Inorg. Organomet. Polym. Mater.
2005, 15, 197. (k) Yam, V. W.-W. Acc. Chem. Res. 2002, 35, 555. (l)
Szafert, S.; Gladysz, J. A. Chem. ReV. 2003, 103, 4175. (m) Powell, C. E.;
Humphrey, M. G. Coord. Chem. ReV. 2004, 248, 725. (n) Bruce, M. I.;
Low, P. J. AdV. Organomet. Chem. 2004, 50, 179. (o) Uwe, U. H. F. Chem.
ReV. 2000, 100, 1605. (p) Davies, S. J.; Johnson, B. F. G.; Khan, M. S.;
Lewis, J. J. Chem. Soc., Chem. Commun. 1991, 187. (q) Lewis, J.; Khan,
M. S.; Johnson, B. F. G.; Marder, T. B.; Fyfe, H. B.; Wittmann, F.; Friend,
R. H.; Dray, A. E. J. Organomet. Chem. 1992, 425, 165. (r) Fujikura, Y.;
Sonogashira, K.; Hagihara, N. Chem. Lett. 1975, 1067. (s) Sonogashira,
K.; Takahashi, S.; Hagihara, N. Macromolecules 1977, 10, 879. (t)
Sonogashira, K.; Kataoka, S.; Takahashi, S.; Hagihara, N. J. Organomet.
Chem. 1978, 160, 319. (u) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.;
Hagihara, N. Synthesis 1980, 627.
(9) (a) Xue, J.; Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004,
84, 3013. (b) Robertson, N. Angew. Chem., Int. Ed. 2006, 45, 2338. (c)
Reagan, D. O.; Gra¨tzel, M. Nature 1991, 353, 737. (d) Gra¨tzel, M. Inorg.
Chem. 2005, 44, 6841.
9
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A R T I C L E S
Wong et al.
Scheme 1. Synthetic Pathways to Bithiazole-Oligo(thiophene) Based Ligand Precursors and Their Platinum Compoundsa
a Conditions and reagents: (i) Br2, H2SO4/H2O; (ii) dithiooxamide, EtOH; (iii) NBS, DMF/AcOH; (iv) 2-thienylmagnesium bromide, Pd(dppf)Cl2, THF;
(v) NBS, CHCl3/AcOH; (vi) Me3SiCtCH, Pd(OAc)2, PPh3, CuI, Et3N; (vii) K2CO3, MeOH/CH2Cl2; (viii) trans-[PtPh(Cl)(PEt3)2], CuI, Et3N; (ix) trans-
[Pt(n-Bu3P)2Cl2], CuI, Et3N.
Table 1. Photophysical Data of P0-P3
permeation chromatography on P0-P3 suggests a high-molec-
λ
abs in
λeme in
CH2Cl2
[nm]
ular-weight material with the degree of polymerization ∼10-
34 (∼56-80 heterocyclic rings in total) (Table 1). The structures
were unequivocally characterized using elemental analyses, mass
spectrometry, and IR and NMR spectroscopies (see Supporting
Information). To our knowledge, P3 represents a soluble metal
polyyne consisting of up to eight heterocycles in the repeat unit.
Photophysical and Electrochemical Characterization. The
absorption and emission spectra of the polymers and model
complexes were measured in CH2Cl2 solutions at 293 K (Figure
1). The absorption of P0-P3 is each dominated by an intense
band peaking at 452-481 nm, and these polymers can emit
strong fluorescence from the singlet excited states at 497-603
d
Tdec
C]
CH2Cl2
[nm]
a
b
polymer
Mw
Mn
DPc
[
°
P0
P1
P2
P3
67 930 35 570
29 850 17 350
36 060 17 590
28 740 15 700
34
14
13
10
355 452, 474
352 346, 468
497 (6.4, 1.93)
557 (5.7, 1.16)
359 378,f 476 592 (5.1, 1.34)
363 397,f 481 603 (3.2, 0.85)
a Weight-average molecular weight. b Number-average molecular weight.
c Degree of polymerization. d Onset decomposition temperature. e Quantum
yields and lifetimes in ns (Φ, τ) are shown in parentheses. f Shoulder peak.
nm. The absorption and emission properties of M0-M3 are
very similar to those of P0-P3, with the absorption maxima in
9
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Pt-Containing Poly(aryleneethynylene)s with Thienyl Rings
A R T I C L E S
Figure 2. Relationship of Eg and 1/a (a ) m + 1) for P0-P3.
value for Eg is estimated to be ca. 1.96 eV for m ) ∞ (Figure
2).13 The measured photoluminescence lifetimes for P0-P3 and
M0-M3 are all very short (ca. 0.85-2.22 ns), characteristic
of the spin-allowed singlet emission. These results and the small
Stokes shift preclude the emitting state as a triplet but a singlet
excited state instead.8a,d We observed no emission from a triplet
excited state over the measured spectral window between 1.2
and 3.1 eV for P0-P3. This is in accordance with the energy
gap law for Pt-containing conjugated polyynes and their model
diynes,14 and a tradeoff problem between the band gap and the
phosphorescence decay rate exists in such a metallayne system.
The nonradiative decay rate (knr)P from the triplet state increases
exponentially with the decreasing triplet-singlet energy gap
Figure 1. Absorption and PL spectra of P0-P3 in CH2Cl2 at 293 K (concn
according to the expression (knr)P
exp(-C∆ES-T), where
ca. 10-5 M).
∆ES-T is the energy gap for the T1-S0 transition and C is a
term controlled by the molecular parameters and vibrational
modes.14 Hence, a high band gap polymer will likely be
phosphorescent, whereas a low band gap value will be detri-
mental for the triplet emission. In addition, the highly extended
heteroaryl rings in the ligand reduces largely the influence of
heavy metal which is mainly responsible for the intersystem
crossing and thus phosphorescence. Therefore, it is not the triplet
state but mostly a charge-transfer (CT) excited state that
contributes to the efficient photoinduced charge separation in
the energy conversion for P0-P3,5f,8a which is different from
the Pt-monothiophene polyyne-based blends where charge
separation occurs via the triplet state of the polymer.8f From
these results, we attribute the localized states centered at 2.06-
2.50 eV to a strong CT-type interaction for the polymers. The
CT nature of the transition was also manifested by the solvent
dependent emission spectra of these Pt polyynes. For instance,
P3 exhibited a positive solvatochromism with a red shift of the
peak maximum of ∼52 nm from toluene (λem ) 566 nm) to
CH2Cl2 (λem ) 618 nm), which suggests a more polar excited
state. Likewise, the PL spectra of P2 are solvatochromic (λem
) 539 in toluene and 563 nm in CH2Cl2). We think that the
D-A interaction along the bithiazole-oligothienyl unit is
remarkably enhanced by the extensive π-delocalization in the
polymer backbone containing the electron-rich Pt ion. This gives
the range 437-473 nm. Due to the presence of a more extended
π-electron delocalized system through the rigid chain and the
creation of an alternate D-A chromophore10 based on a highly
electron-accepting bithiazole ring (n-doping)11 in combination
with the electron-donating thiophene rings (p-doping),12 the band
gaps of P0-P3 vary from 2.40 to 2.06 eV with the value for
red P3 being significantly reduced by ca. 0.34 eV relative to
the yellow P0. If the D and A moieties are separated by neutral
parts and extended through the Pt and alkynyl units, we can
get a smaller band gap, typical of the inorganic n-i-p-i
superlattice quantum well structures.10d However, the extent of
red shifts induced by increasing m is less pronounced, and there
would be little benefit in extending the π-conjugation by
increasing m above 7 or 8 (see Supporting Information). A linear
oligothienyl chain length dependence of Eg can be rationalized
from the plot of Eg against reciprocal chain length, and a limiting
(10) (a) Roncali, J. Chem. ReV. 1997, 97, 173. (b) Wong, W.-Y.; Choi, K.-H.;
Lu, G.-L.; Shi, J.-X. Macromol. Rapid Commun. 2001, 22, 461. (c) Ko¨hler,
A.; Younus, M.; Al-Mandhary, M. R. A.; Raithby, P. R.; Khan, M. S.;
Friend, R. H. Synth. Met. 1999, 101, 246. (d) Havinga, E. E.; Haeve, W.;
Wynberg, H. Synth. Met. 1993, 55-57, 299.
(11) (a) Yamamoto, T.; Suganuma, H.; Maruyama, T.; Inoue, T.; Muramatsu,
Y.; Arai, M.; Komarudin, D.; Ooba, N.; Tomaru, S.; Sasaki, S.; Kubota,
K. Chem. Mater. 1997, 9, 1217. (b) Cao, J.; Kampf, J. W.; Curtis, M. D.
Chem. Mater. 2003, 15, 404. (c) MacLean, B. J.; Pickup, P. G. J. Mater.
Chem. 2001, 11, 1357. (d) Politis, J. K.; Curtis, M. D.; Gonzalez, L.; Martin,
D. C.; He, Y.; Kanicki, J. Chem. Mater. 1998, 10, 1713. (e) Masui, K.;
Mori, A.; Okano, K.; Takamura, K.; Kinoshita, M.; Ikeda, T. Org. Lett.
2004, 6, 2011.
(12) (a) Perepichka, I. F.; Perepichka, D. F.; Meng, H.; Wudl, F. AdV. Mater.
2005, 17, 2281. (b) McCullough, R. D. AdV. Mater. 2005, 17, 2281. (c)
Fichou, D., Ed. Handbook of Oligo- and Polythiophenes; Wiley-VCH: New
York, 1999. (d) Roncali, J.; Blanchard, P.; Fre`re, P. J. Mater. Chem. 2005,
15, 1589. (e) Stott, T. L.; Wolf, M. O. Coord. Chem. ReV. 2003, 246, 89.
(13) (a) Zade, S. S.; Bendikov, M. Org. Lett. 2006, 8, 5243. (b) Ba¨uerle, P.
AdV. Mater. 1992, 4, 102. (c) Lahti, P. M.; Obrzut, J.; Karasz, F. E.
Macromolecules 1987, 20, 2023.
(14) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus, M.; Khan,
M. S.; Raithby, P. R.; Ko¨hler, A.; Friend, R. H. J. Am. Chem. Soc. 2001,
123, 9412.
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A R T I C L E S
Wong et al.
Table 2. Physical Data and Photovoltaic Performance of P0-P3; the Photovoltaic Performance Parameters Represent an Average from 5
to 20 Devices
2
polymer
Ega [eV]
E
ox [V]
E
HOMO [eV]
ELUMO b[eV]
V
oc [V]
I
sc [mA cm-
]
FF
PCE [%]
P0
P1
P2
P3
2.40 (2.46)
2.18 (2.28)
2.10 (2.22)
2.06 (2.19)
0.84
0.75
0.70
0.31
-5.91
-5.82
-5.79
-5.71
-3.51
-3.64
-3.69
-3.65
0.73
0.83
0.91 (0.79)d
2.33 (1.63)d
0.32
0.39
0.21
0.76
0.81 (0.90)c
0.88 (0.88)c
6.93 (6.39)c (5.88)d
6.50 (6.01)c (4.70)d
0.38 (0.46)c
0.44 (0.43)c
2.14 (2.66)c
2.50 (2.30)c
a Optical bandgaps determined from onset of absorption in solid state. The corresponding values obtained from the solution phases are shown in parentheses.
d
b Calculated from EHOMO + Eg. c At P2 or P3/PCBM (1:5, w/w) ratio. Isc values estimated from the integration of EQE spectrum under theoretical AM1.5
global irradiation.
rise to a CT-type transition that causes a substantial lowering
of the optical gap.
Cyclic voltammetry of the films for P0-P3 was performed
(Table 2). P0-P3 show an irreversible thienyl oxidation at 0.84,
0.75, 0.70, and 0.31 V, respectively, which agrees with the
phenomenon that formation of the heteroaromatic cation radicals
is favored by increased conjugation length.15 The electrooxi-
dation of oligothiophenes is often irreversible because the
electrogenerated cations readily undergo rapid coupling reactions
leading to higher oligomers or polymers. The stability of these
radical cations increases when m becomes longer.15 Hence, the
HOMO levels tend to be elevated with increasing m from P0
to P3.
Polymer Photovoltaic Behavior. Polymer solar cells were
fabricated by using each of P0-P3 as an electron donor and
PCBM as an electron acceptor (Table 2). The hole collection
electrode consisted of indium tin oxide (ITO) with a spin-coated
poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate) (PE-
DOT:PSS), while Al served as the electron collecting electrode.
It can be clearly observed that enhancing the absorption
coefficient of the band by increasing the conjugation length of
the chain with oligothienyl rings is a good strategy. The facile
functionalization of thiophene groups also offers relatively
efficient synthetic solutions to solubility, polarity, and band gap
tuning. A considerable increase in the short circuit current
density (Isc) and PCE can be observed in P2 and P3 relative to
P0 and P1 (Figure 3a). The results are shown for the 1:4 blend
ratio (for cell performance dependence on blend ratio for P2
and P3, see Figure 4). The increase in the number of rings is
also expected to result in an increase of the intrachain mobility
because of more extended π-conjugation. Since a clear trend
Figure 3. (a) J-V curves of solar cells with P0-P3:PCBM (1:4) active
in the increase in the mobility is observed for the blend films
(according to the results obtained by the space-charge-limited
current (SCLC) modeling shown in Figure 5), it is likely that
intrachain mobility significantly contributes to the bulk proper-
ties. Similar trends have been previously observed in monosi-
lanylene-oligothienylene copolymers with different numbers
of thienyl rings.15d Both P2 and P3 blended with PCBM exhibit
carrier mobilities of the order of 10-4 cm2 V-1 s-1 and have
comparable photovoltaic efficiencies, although P3 has a slightly
higher absorption at longer wavelengths. This is likely due to
the differences in film roughness and phase separation in P2:
PCBM and P3:PCBM blends (for AFM images of 1:4 and 1:5
blends for both polymers, see Figure 6). From Figure 5, we
note that the P3:PCBM blend can exhibit a reasonably balanced
layers under simulated AM1.5 solar irradiation. (b) EQE wavelength
dependencies of solar cells with P0-P3:PCBM (1:4) active layers.
charge transport, as required for efficient solar cell performance.3i
The mobilities of the blends with P0 and P1 are somewhat
lower, and we can observe that, for both electrons and holes,
the mobilities increase with an increasing number of thienyl
rings. Thus, not only optical but also charge transport properties
can be tuned by changing the length of oligothienyl chains. As
for the fill factor (FF), the relatively low values are at least
partly due to the fact that all processing (except PEDOT:PSS
annealing and electrode deposition) and measurements have
been done in an ambient atmosphere which likely results in the
presence of traps. We expect FF to improve for fabrication and
characterization to be performed in an inert gas environment.
A comprehensive study of charge transport and the influence
of traps is necessary to further improve the fill factor and overall
device performance. A high PCE of up to 2.50-2.66% can be
obtained for P2 and P3 under illumination of an AM1.5 solar
simulator, which can compete with that of P3HT-based devices
(15) (a) Garcia, P.; Pernaut, J. M.; Hapiot, P.; Wintgens, V.; Valat, P.; Garnier,
F.; Delabouglise, D. J. Phys. Chem. 1993, 97, 513. (b) Jestin, I.; Fre`re, P.;
Mercier, N.; Levillain, E.; Stievenard, D.; Roncali, J. J. Am. Chem. Soc.
1998, 120, 8150. (c) Diaz, A. F.; Crowley, J.; Bargon, J.; Gardini, G. P.;
Torrance, J. B. J. Electroanal. Chem. 1981, 121, 355. (d) Harima, Y.; Kim,
D.-H.; Tsutitori, Y.; Jiang, X.; Patil, R.; Ooyama, Y.; Ohshita, J.; Kunai,
A. Chem. Phys. Lett. 2006, 420, 387.
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Pt-Containing Poly(aryleneethynylene)s with Thienyl Rings
A R T I C L E S
Figure 5. Comparison between (a) hole (µh) and (b) electron (µe) mobilities
in P0-P3:PCBM blends obtained by the SCLC modeling.
Figure 6. AFM topography (left) and phase contrast (right) images of blend
films (a) P2:PCBM (1:4), (b) P2:PCBM (1:5), (c) P3:PCBM 1:4, and (d)
P3:PCBM (1:5). Scan size is 1 µm × 1 µm.
ability of P3 can be increased by ∼12 times relative to P0
simply by adding more thienyl rings along the polymeric
backbone. Yet, higher PCEs with excellent solution-process-
ability are notoriously difficult to achieve with the pure organic
polymers. Hence, the need to improve PCE to the practical level
requires the implementation of new materials, and a new
approach to capture sunlight for power generation involves
putting metals into the conjugated organic polymers. The
generally good solubility of platinum polyyne polymers over
their purely organic poly(heteroaryleneethynylene)s favors the
utilization of the metallopolyynes in the advance of polymer
photovoltaic devices. The novel approach reported here can
provide a useful avenue to PCE enhancement by extending the
Figure 4. Effect of blend composition on solar cell performance. The curves
shown are representative (close to average) curves from batches of 5-20
devices. (a) P2:PCBM, (b) P3:PCBM, and (c) P3HT:PCBM.
fabricated without additional processing (annealing or TiOx
layer) both reported in the literature3a and obtained by us (see
Supporting Information). The broad EQE curves for P2 and P3
cover almost the entire visible spectrum from 350 to 700 nm
with a maximum of 81.3 and 59.3%, respectively (Figure 3b).
Both the Voc and FF values of P2- and P3-based solar cells
(1:4) are quite close to each other. At the same blend ratio of
1:4, the PCE increases sharply from P0 to P3 (i.e., P0 < P1 <
P2 < P3), and it is remarkable to see that the light-harvesting
9
J. AM. CHEM. SOC. VOL. 129, NO. 46, 2007 14377

A R T I C L E S
Wong et al.
efficient and balanced charge transport are achieved. The
obtained results clearly demonstrate the potential of metalated
conjugated polymers in the future design of stable photovoltaic
devices for commercialization purpose. It is expected that
optimization of device architecture will result in further
improvements in the efficiency and that tunability of the energy
levels of the polymers is expected to be an asset in the
applicability of this class of materials to solar cells with different
acceptors.
Conclusion
In conclusion, we have demonstrated a versatile avenue for
the tuning of polymer photophysical and charge-transporting
properties and PCE in solar cells via the variation of oligothienyl
chain length in strongly absorbing polyplatinaynes with solu-
bilizing bithiazole units, and we have prepared highly efficient
solar cells with PCEs of up to 2.7% and peak EQE to 83%.
Given the excellent solution-processability as well as perfor-
mance advantage, this work has great potential for enhancing
the light-to-electricity conversion efficiencies of polymer solar
cells to a level of practical applications without the need for
exploiting the triplet excited states in promoting an efficient
photoinduced charge separation. While it may be a promising
research endeavor to use platinum polyynes with accessible
triplet states for polymer solar cells, these polymers usually
absorb only in the blue-violet spectral region (i.e., with wide
bandgaps), and consequently the efficiency was low due to low
coverage of the solar spectrum (e.g., only 0.27% in PCE for
the Pt polyyne with 2,5-diethynylthiophene spacer). In other
words, the design of metallopolyynes should be focused on the
development of not only phosphorescent polymeric materials
but also the low band gap counterparts for high-efficiency
photovoltaics. Our present work clearly sets out on an unex-
plored strategy toward an effective tuning of the solar cell
efficiency and charge transport properties in metalated conju-
gated polymers for the future development of polymer-based
solar power generation.
Figure 7. Real (n) and imaginary (k) parts of the refractive index of P0-
P3:PCBM blend films. Ellipsometry measurements also confirm that P3
has stronger absorption at longer wavelengths compared to polymers with
a fewer number of thienyl rings.
absorption to longer wavelengths and metal incorporation
without the involvement of the triplet excitons in the energy
conversion which is recently believed to be a good way of
making efficient polymer solar cells.8f,h
Generally, the amount of absorbed light depends not only
on the cutoff absorption wavelength but also on the intensity
of the absorption. A higher ꢀmax value of the polymer with a
similar degree of polymerization would be anticipated as we
increase m, and this can be exemplified by a gradual increase
in ꢀmax from M0 to M3 (see Supporting Information), resulting
in higher PCEs. To investigate in more detail the reasons for
the excellent performance of P2:PCBM and P3:PCBM cells,
we performed spectroscopic ellipsometry measurements of P0-
P3:PCBM blend films. The obtained real and imaginary parts
of the index of refraction are shown in Figure 7. It should be
noted that the relationship between the real and imaginary parts
of the index of refraction of blend films and the number of
thienyl rings is not simple, so that further study of phase
separation in the blends for different blend compositions
combined with more complex sample modeling is needed to
fully understand the optical properties of the blends. In addition,
the effects of surface roughness have not been taken into
account. However, ellipsometry measurements clearly confirm
that P3 has a stronger absorption at longer wavelengths
compared to polymers with fewer numbers of thienyl rings,
leading to improved absorption of the solar radiation (Figure
7). However, P3:PCBM blend film also exhibits a slightly higher
refractive index, so that as compared to the P2:PCBM blend
more light is reflected at the PEDOT:PSS/P3:PCBM interface,
resulting in a smaller increase in the efficiency of P3:PCBM
compared to P2:PCBM than that expected from the difference
in absorption and charge transport properties. Differences in
surface roughness and phase separation (domain sizes) in
different blends also likely play a role and require further study.
Therefore, the design of the low band gap polymers for
photovoltaic applications should consider not only lowering the
gap but also increasing the absorption coefficient of the polymer,
as well as optimizing the morphology of the blend films.
Intensely colored metalloorganic polymers thus represent a
promising class of materials for opening up new opportunities
in photovoltaic applications, provided that low band gap and
Experimental Section
General. Solvents were carefully dried and distilled from appropriate
drying agents prior to use. Commercially available reagents were used
without further purification unless otherwise stated. All reagents for
the chemical syntheses were purchased from Aldrich or Acros Organics.
PCBM and regioregular P3HT (Mw 20 000-50 000) were purchased
from American Dyes. PEDOT:PSS (Baytron VPAI 4083) was pur-
chased from H. C. Starck. Reactions and manipulations were carried
out under an atmosphere of prepurified nitrogen using Schlenk
techniques. All reactions were monitored by thin-layer chromatography
(TLC) with Merck precoated glass plates. Flash column chromatography
and preparative TLC were carried out using silica gel from Merck
(230-400 mesh). Fast atom bombardment (FAB) mass spectra were
recorded on a Finnigan MAT SSQ710 system. NMR spectra were
measured in CDCl3 on a Varian Inova 400 MHz FT-NMR spectrometer,
and chemical shifts are quoted relative to tetramethylsilane for 1H and
13C nuclei and H3PO4 for 31P nucleus.
Physical Measurements. UV/vis spectra were obtained on an HP-
8453 diode array spectrophotometer. The solution emission spectra and
lifetimes of the compounds were measured on a Photon Technology
International (PTI) Fluorescence QuantaMaster Series QM1 spectro-
photometer. The phosphorescence quantum yields were determined in
degassed CH2Cl2 solutions at 293 K against quinine sulfate in 0.1 N
9
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Pt-Containing Poly(aryleneethynylene)s with Thienyl Rings
A R T I C L E S
H2SO4 (ΦP ) 0.54).16 The decay curves were analyzed using a
Marquardt-based nonlinear least-squares fitting routine and were shown
to follow a single-exponential function in each case according to I )
I0 + A exp(-t/τ). Electrochemical measurements were made using a
Princeton Applied Research model 273A potentiostat. The cyclic
voltammetry experiment of the polymer film was performed by casting
the polymer on the glassy-carbon working electrode with a Pt wire as
the reference electrode, at a scan rate of 100 mV s-1. The solvent in
all measurements was deoxygenated MeCN, and the supporting
electrolyte was 0.1 M [nBu4N][BF4]. Ferrocene was added as an internal
reference after each set of measurements, and all potentials are quoted
31.90, 30.48, 29.55, 29.46, 29.42, 29.40, 29.31, 22.68, 14.12 (nonyl),
15.11, 8.03 ppm (Et); 31P NMR (CDCl3): δ ) 11.11 ppm (1JP-Pt
)
2626 Hz); FAB-MS: m/z: 1812 (M+). Anal. Calcd (%) for C80H116N2P4-
Pt2S6: C, 53.02; H, 6.45; N, 1.55. Found: C, 52.88; H, 6.35; N, 1.48.
M3: Dark red solid. Yield: 43% (Eluent: CH2Cl2/hexane (1:1, v/v)).
IR (KBr): ν(CtC) 2079 cm-1; 1H NMR (CDCl3): δ ) 7.31-7.29 (d,
4H, J ) 7.6 Hz, Ar), 7.11-7.07 (m, 6H, Ar), 7.01-6.95 (m, 8H, Ar),
6.82-6.79 (t, 2H, J ) 7.0 Hz, Ar), 6.76-6.75 (d, 2H, J ) 5.6 Hz,
Ar), 2.98-2.94 (t, 4H, J ) 7.4 Hz, alkyl), 1.79-1.71 (m, 28H, alkyl),
1.42-1.26 (m, 24H, alkyl), 1.13-1.05 (m, 36H, alkyl), 0.88-0.85 ppm
(t, 6H, J ) 6.6 Hz, alkyl); 13C NMR (CDCl3): δ ) 157.48, 155.76,
154.63, 139.02, 138.35, 137.70, 134.27, 132.84, 131.48, 129.95, 128.11,
127.71, 127.55, 127.38, 124.68, 123.77, 123.60, 123.31, 121.39, 102.18
(Ar and CtC), 31.91, 30.50, 29.56, 29.47, 29.43, 29.33, 22.70, 14.14
(nonyl), 15.11, 8.03 ppm (Et); 31P NMR (CDCl3): δ ) 11.10 ppm
(1JP-Pt ) 2625 Hz); FAB-MS: m/z: 1976 (M+); Anal. Calcd (%) for
C88H120N2P4Pt2S8: C, 53.48; H, 6.12; N, 1.42. Found: C, 53.50; H,
6.02; N, 1.47.
relative to that of the ferrocene-ferrocenium couple (taken as E1/2
)
+0.27 V relative to the reference electrode). The oxidation (Eox)
potentials and optical bandgaps (Eg) were used to determine the HOMO
and LUMO energy levels using the equations EHOMO ) -(Eox + 4.8)
eV and ELUMO ) (EHOMO + Eg) eV (versus the internal standard
ferrocene value of -4.8 eV with respect to the vacuum level).17 Thermal
analyses were performed with the Perkin-Elmer TGA6 thermal analyzer.
Synthesis of Platinum Model Complexes M0-M3. All of them
were synthesized following the dehydrohalogenating coupling between
trans-[PtCl(Ph)(PEt3)2]18 and the corresponding diterminal alkynes. A
typical procedure was given for M0 starting from L0.
Synthesis of Platinum Metallopolyynes P0-P3. The polymers were
prepared by the dehydrohalogenative polycondensation between trans-
[Pt(n-Bu3P)2Cl2]19 and each of L0-L3. A typical procedure was given
for P0 starting from L0.
Polymerization was carried out by mixing L0 (46.9 mg, 0.10 mmol),
trans-[Pt(n-Bu3P)2Cl2] (67.1 mg, 0.10 mmol), and CuI (3.00 mg) in
Et3N/CH2Cl2 (30 mL, 1:2, v/v). After stirring at room temperature
overnight under nitrogen, the solution mixture was evaporated to
dryness. The residue was redissolved in CH2Cl2 and filtered through a
short silica column using the same eluent to remove ionic impurities
and catalyst residues. After removal of the solvent, the crude product
was purified by precipitation in CH2Cl2 from MeOH two times.
Subsequent washing with hexane and drying in Vacuo gave a yellow
solid of P0 (92.7 mg, 87%). IR (KBr): ν(CtC) 2087 cm-1; 1H NMR
(CDCl3): δ ) 2.78 (br, s, 4H, alkyl), 2.08 (br, s, 16H, alkyl), 1.55-
1.25 (m, 48H, alkyl), 0.91-0.85 ppm (m, 24H, alkyl); 31P NMR
(CDCl3): δ ) 5.04 ppm (1JP-Pt ) 2324 Hz). Anal. Calcd (%) for
(C52H92N2P2PtS2)n: C, 58.56; H, 8.70; N, 2.63. Found: C, 58.46; H,
8.78; N, 2.47.
To a stirred mixture of L0 (46.9 mg, 0.10 mmol) and trans-[PtPh-
(Cl)(PEt3)2] (109 mg, 0.20 mmol) in Et3N (30 mL) and CH2Cl2 (30
mL) was added CuI (5.00 mg). The solution was stirred at room
temperature under nitrogen over a period of 12 h, after which all volatile
components were removed under vacuum. The crude product was taken
up in dichloromethane and purified on preparative silica TLC plates
with CH2Cl2/hexane (1:1, v/v) as eluent. The product M0 was obtained
1
as a yellow solid (107 mg, 72%). IR (KBr): ν(CtC) 2083 cm-1; H
NMR (CDCl3): δ ) 7.30-7.29 (m, 4H, Ar), 6.98-6.94 (t, 4H, J )
7.6 Hz, Ar), 6.82-6.78 (t, 2H, J ) 7.6 Hz, Ar), 2.81-2.77 (t, 4H, J )
8.0 Hz, alkyl), 1.74-1.70 (m, 28H, alkyl), 1.34-1.25 (m, 24H, alkyl),
1.12-1.04 (m, 36H, alkyl), 0.88-0.85 ppm (t, 6H, J ) 6.8 Hz, alkyl);
13C NMR (CDCl3): δ ) 157.30, 155.58, 155.01, 139.00, 127.98,
127.38, 121.39, 120.95, 99.11 (Ar + CtC), 31.91, 30.68, 29.85, 29.81,
29.70, 29.67, 29.35, 22.68, 14.11 (nonyl), 15.11, 8.00 ppm (Et); 31P
NMR (CDCl3): δ ) 11.38 ppm (1JP-Pt ) 2627 Hz); FAB-MS: m/z:
1483 (M+). Anal. Calcd (%) for C64H108N2P4Pt2S2: C, 51.81; H, 7.34;
N, 1.89. Found: C, 51.90; H, 7.23; N, 1.77.
1
P1: Yellow solid. Yield: 79%. IR (KBr): ν(CtC) 2085 cm-1; H
NMR (CDCl3): δ ) 6.97-6.95 (m, 2H, Ar), 6.79-6.78 (d, 2H, J )
4.4 Hz, Ar), 2.93-2.90 (t, 4H, J ) 7.2 Hz, alkyl), 2.13-2.09 (m, 12H,
alkyl), 1.76-1.72 (m, 4H, alkyl), 1.61-1.56 (m, 12H, alkyl), 1.52-
1.45 (m, 12H, alkyl), 1.39-1.26 (m, 24H, alkyl), 1.00-0.92 (m, 18H,
alkyl), 0.88-0.85 ppm (t, 6H, J ) 6.8 Hz, alkyl); 31P NMR (CDCl3):
δ ) 4.49 ppm (1JP-Pt ) 2320 Hz). Anal. Calcd (%) for (C60H96N2P2-
PtS4)n: C, 58.56; H, 7.86; N, 2.28. Found: C, 58.62; H, 7.66; N, 2.10.
M1: Red-brown solid. Yield: 73%. (Eluent: CH2Cl2/hexane (1:1,
1
v/v)). IR (KBr): ν(CtC) 2082 cm-1; H NMR (CDCl3): δ ) 7.31-
7.30 (d, 4H, J ) 6.8 Hz, Ar), 6.98-6.95 (m, 6H, Ar), 6.82-6.79 (d,
4H, J ) 12.4 Hz, Ar), 2.94-2.90 (t, 4H, J ) 7.8 Hz, alkyl), 1.76-
1.71 (m, 28H, alkyl), 1.41-1.26 (m, 24H, alkyl), 1.14-1.06 (m, 36H,
alkyl), 0.88-0.85 (t, 6H, J ) 7.0 Hz, alkyl); 13C NMR (CDCl3): δ )
156.82, 155.68, 153.67, 138.99, 131.53, 129.05, 128.31, 127.37, 126.86,
124.06, 121.40, 101.87 (Ar + CtC), 31.91, 30.55, 29.60, 29.58, 29.50,
29.47, 29.31, 22.67, 14.12 (nonyl), 15.11, 8.02 ppm (Et); 31P NMR
(CDCl3): δ ) 11.07 ppm (1JP-Pt ) 2623 Hz); FAB-MS: m/z: 1648
(M+). Anal. Calcd (%) for C72H112N2P4Pt2S4: C, 52.48; H, 6.85; N,
1.70. Found: C, 52.30; H, 6.67; N, 1.62.
P2: Red-brown solid. Yield: 59%. IR (KBr): ν(CtC) 2083 cm-1
;
1H NMR (CDCl3): δ ) 7.05 (br, s, 4H, Ar), 6.98-6.97 (d, 2H, J )
2.8 Hz, Ar), 6.75-6.74 (d, 2H, J ) 2.8 Hz, Ar), 2.94 (br, s, 4H, alkyl),
2.10 (br, s, 12H, alkyl), 1.78 (br, s, 4H, alkyl), 1.60-1.25 (m, 48H,
alkyl), 0.97-0.84 ppm (m, 24H, alkyl); 31P NMR (CDCl3): δ ) 4.53
ppm (1JP-Pt ) 2322 Hz); Anal. Calcd (%) for (C68H100N2P2PtS6)n: C,
58.55; H, 7.23; N, 2.01. Found: C, 58.43; H, 7.12; N, 1.94.
P3: Dark brown solid. Yield: 41%. IR (KBr): ν(CtC) 2083 cm-1
;
M2: Red solid. Yield: 63%. (Eluent: CH2Cl2/hexane (1:1, v/v)).
IR (KBr): ν(CtC) 2080 cm-1; 1H NMR (CDCl3): δ ) 7.32-7.30 (d,
4H, J ) 6.8 Hz, Ar), 7.06-7.04 (t, 4H, J ) 4.2 Hz, Ar), 6.98-6.95
(m, 6H, Ar), 6.82-6.79 (d, 2H, J ) 14.4 Hz, Ar), 6.76-6.75 (d, 2H,
J ) 4.0 Hz, Ar), 2.97-2.93 (t, 4H, J ) 7.8 Hz, alkyl), 1.81-1.70 (m,
28H, alkyl), 1.42-1.34 (m, 24H, alkyl), 1.14-1.06 (m, 36H, alkyl),
0.88-0.85 ppm (t, 6H, J ) 7.0 Hz, alkyl); 13C NMR (CDCl3): δ )
157.30, 155.72, 154.36, 139.16, 139.00, 132.54, 130.69, 130.16, 127.98,
127.72, 127.37, 123.76, 123.58, 123.09, 121.39, 102.16 (Ar + CtC),
1H NMR (CDCl3): δ ) 7.13-7.09 (m, 4H, Ar), 7.02-6.96 (m, 6H,
Ar), 6.82-6.79 (t, 2H, J ) 6.0 Hz, Ar), 2.95 (br, s, 4H, alkyl), 2.11
(br, s, 12H, alkyl), 1.79 (br, s, 4H, alkyl), 1.57-1.30 (m, 48H, alkyl),
0.98-0.84 ppm (m, 24H, alkyl); 31P NMR (CDCl3): δ ) 4.50 ppm
(1JP-Pt ) 2319 Hz). Anal. Calcd (%) for (C76H104N2P2PtS8)n: C, 58.55;
H, 6.72; N, 1.80. Found: C, 58.34; H, 6.79; N, 1.67.
Solar Cell Fabrication and Characterization. The device structure
was ITO/poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate)
(PEDOT:PSS)/polymer:PCBM blend/Al. ITO glass substrates (10 Ω
per square) were cleaned by sonication in toluene, acetone, ethanol,
and deionized water, dried in an oven, and then cleaned with UV ozone
(16) Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251.
(17) (a) Thelakkat, M.; Schmidt, H.-W. AdV. Mater. 1998, 10, 219. (b) Ashraf,
R. S.; Shahid, M.; Klemm, E.; Al-Ibrahim, M.; Sensfuss, S. Macromol.
Rapid Commun. 2006, 27, 1454.
(18) Chatt, J.; Shaw, B. L. J. Chem. Soc. 1960, 4020.
(19) Chatt, J.; Hayter, R. G. J. Chem. Soc., Dalton Trans. 1961, 896.
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A R T I C L E S
Wong et al.
for 300 s. As-received PEDOT:PSS solution was passed through the
0.45 µm filter and spin-coated on patterned ITO substrates at 5000
rpm for 3 min, followed by baking in N2 at 150 °C for 15 min. The
metallopolyyne:PCBM (1:4 by weight, unless specified otherwise)
active layer was prepared by spin-coating the toluene solution (4 mg
mL-1 of metallopolyyne, 16 mg mL-1 of PCBM) at 1000 rpm for 2
min. The substrates were dried at room temperature under low vacuum
medium approximation is no longer applicable and the effect of the
roughness cannot be taken into account. Thus, we have not used the
surface roughness correction for all films to obtain a better comparison
of the properties of the blends containing polymers with different
numbers of thienyl rings. It should be noted that the roughness of the
P2:PCBM 1:4 blend film in addition to the increased uncertainty in
determined optical functions will also result in the increased absorption
due to longer effective path length, in agreement with the measured
higher absorbance for these films (see Supporting information). The
mobility measurements for the P0:PCBM blend were also determined
by the time-of-flight (TOF) technique for comparison. The samples
for TOF measurements were prepared by drop-casting and then dried
under vacuum, followed by the evaporation of a metal electrode in
high vacuum (10-6 Torr). A nitrogen laser was used to generate pulsed
excitation (λ ) 337.1 nm). The transient photocurrent was monitored
by an oscilloscope Tektronix TDS 3052B. The mobilities determined
from TOF and SCLC were in good agreement, although the obtained
values were slightly lower from the TOF measurements.
(vacuum oven) for 1 h and then stored under high vacuum (10-5
-
10-6 Torr) overnight. An Al electrode (100 nm) was evaporated through
a shadow mask to define the active area of the devices (2 mm circle).
For comparison, solar cells with P3HT:PCBM active layers with
different blend ratios were also prepared. The procedure for cell
fabrication was the same, except that the P3HT:PCBM solution was
prepared in chlorobenzene and stirred overnight in N2 at 40 °C. All
the fabrication procedures (except drying, PEDOT:PSS annealing, and
Al deposition) and cell characterization were performed in air. Power
conversion efficiency was determined from J-V curve measurement
(using a Keithley 2400 sourcemeter) under white light illumination (at
100 mW cm-2). For white light efficiency measurements, an Oriel
66002 solar light simulator with an AM1.5 filter was used. The light
intensity was measured by a Molectron Power Max 500D laser power
meter. For the measurement of the external quantum efficiency, different
wavelengths were selected with an Oriel Cornerstone 74000 mono-
chromator, while the photocurrent was measured with a Keithley 2400
sourcemeter. The light intensity was measured with a Newport 1830-C
optical power meter equipped with a 818-UV detector probe. AFM
images have been obtained using Digital Instruments Nanoscope III
AFM. The scan rate was 1.2 µm s-1, and the tip force constant and
resonant frequency were 0.7-0.9 N m-2 and 50-130 Hz, respectively.
The charge carrier mobility was determined from the SCLC modeling.20
The device structure was ITO/PEDOT:PSS/blend film/Au for the hole
mobility determination, while for electron mobility determination the
device configuration was ITO/Mg/blend film/LiF/Al. The mobilities
were determined by fitting the measured J-V curves to the SCLC
model.20 The index of refraction of the material was estimated from
the modified Lorentz model fitting21 of the ellipsometry data (obtained
using JA Woollam V-VASE ellipsometer) for blend films spin-coated
on Si substrates. Surface roughness correction was not used in the
fitting. It should be noted that the surface roughness of P2:PCBM 1:4
blend films is significant, which may lead to underestimation of the
extinction coefficient. Due to the lateral size of the surface features
which is of the same order of magnitude as the wavelength, effective
Acknowledgment. This work was supported by a CERG
Grant from the Hong Kong Research Grants Council (HKBU
202607P) and a Faculty Research Grant from the Hong Kong
Baptist University (FRG/06-07/II-63). K.-K. Chan acknowledges
the receipt of a Prof. Sunney Chan Research Scholarship in
2007. Financial support from the Strategic Research Theme,
University Development Fund, and Seed Funding Grant and
Outstanding Young Researcher Award (administrated by The
University of Hong Kong) is also acknowledged. We also thank
S.-F. Cheung and Prof. K.-Y. Chan for AFM measurements.
The authors also wish to thank Prof. Yong Cao of South China
University of Technology, Guangzhou, China for his assistance
in repeating photovoltaic measurements.
Supporting Information Available: Synthesis and charac-
terization data of ligands, X-ray crystal structure of L2-2Br and
crystallographic data for the reported structure (CIF format),
absorption and emission spectra of L0-L3 and M0-M3,
solvent dependence of PL data for P2 and P3, the J-V curves
of the solar cells in the dark as well as the charge carrier
mobilities of P0 as determined by both TOF and SCLC methods.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(20) Mihailetchi, V. D.; Xie, H.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M.
AdV. Funct. Mater. 2006, 16, 699.
(21) Liu, Z. T.; Kwok, H. S.; Djurisˇic´, A. B. J. Phys. D: Appl. Phys. 2004, 37,
678.
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14380 J. AM. CHEM. SOC. VOL. 129, NO. 46, 2007