Platinum-acetylide polymers with higher dimensionality for organic solar cells

Article abstract of DOI:10.1002/asia.201100111

A new series of platinum(II)-acetylide polymers P1-P3 containing thiophene-triarylamine chromophores of different dimensions were synthesized and their electronic band structures, field-effect charge transport, and application in bulk heterojunction solar

Full text of DOI:10.1002/asia.201100111

FULL PAPERS
DOI: 10.1002/asia.201100111
Platinum–Acetylide Polymers with Higher Dimensionality for Organic Solar
Cells
Qiwei Wang,[a] Zhicai He,[b] Andreas Wild,[c] Hongbin Wu,*[b] Yong Cao,[b]
Ulrich S. Schubert,[c] Chung-Hin Chui,*[d] and Wai-Yeung Wong*[a]
On the occasion of the 150th anniversary of the Department of Chemistry, The University of Tokyo
1766
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Chem. Asian J. 2011, 6, 1766 – 1777

Abstract: A new series of platinum(II)-
acetylide polymers P1–P3 containing
thiophene-triarylamine chromophores
of different dimensions were synthe-
sized and their electronic band struc-
tures, field-effect charge transport, and
application in bulk heterojunction solar
cells were evaluated. These materials
are soluble in polar organic solvents
and show strong absorptions in the
solar spectra (with the highest absorp-
tion coefficient of 1.59ꢁ105 cmÀ1 from
thin films), thus rendering them excel-
lent candidates for bulk heterojunction
polymer solar cells. The spin-coated
polymer thin films showed p-channel
field-effect charge transport with hole
mobilities of 1.90ꢁ10À5 to 7.86ꢁ
10À5 cm2 VÀ1 sÀ1 for P1–P3 and an im-
proved charge carrier transport was
found for P2 with higher molecular di-
mensionality than P1. The dependence
of their photovoltaic properties and di-
mensionality was also investigated.
Even if the polymers possess relatively
high bandgaps and narrow absorption
bandwidths, the highest power conver-
sion efficiency of 2.24% can be ob-
tained based on blends of P3 with
[6,6]phenyl-C61-butyric acid methyl
ester (PCBM) (1:5, w/w) under AM1.5
simulated solar illumination. The pres-
ent work indicates that multidimen-
sional polymers exhibit a better photo-
voltaic performance over the linear
polymers under the same measurement
conditions and can provide an attrac-
tive approach to developing highly effi-
cient conjugated metallopolymers for
efficient power generation.
Keywords: dimensionality
· fluo-
rine · photovoltaics · platinum ·
polymers
Introduction
ization of devices with good performance based on one-di-
mensional (1D) p-conjugated systems requires a specific
control of molecular interactions and orientation, which
would complicate the device fabrication.[7] Such a low di-
mensionality is shown to cause an anisotropy of the optical
and charge-transport properties and such a constraint evi-
dently represents a problem when device fabrication by so-
lution-based processes, such as printing techniques, is used.
This question led us to develop a new type of polymer with
higher dimensionality accompanied by isotropic charge-
transport and optical properties, which might be expected to
reach a better performance. The design of two-dimensional
(2D) or even higher dimensional conjugated systems repre-
sents a possible chemical solution to this impasse for linear
conjugated systems.[8] A new approach toward extended 2D
conjugated systems involves the introduction of a side chain
substituent to the main conjugated polymer backbone. For
instance, Li and co-workers have synthesized various co-
polymers of poly(thienylenevinylene) and poly(thiophene)
with thienylenevinylene side groups which result in a broad-
ening of the absorption spectrum and thus improved light-
harvesting properties.[9] When fabricated in bulk heterojunc-
tion (BHJ) solar cells using [6,6]-phenyl-C61-butyric acid
methyl ester (PCBM) as the electron acceptor, these organic
polymers yielded PCE better than 3%.
Global environmental concern and the finite nature of fossil
fuels lead to a great demand for clean and renewable
energy, and the quest for this new energy resource becomes
one of the most important challenges in the 21st century.[1]
Solar energy is one of the best candidates as it is abundant,
renewable, and environmentally friendly. Such strong
demand for solar electricity stimulates much scientific re-
search for efficient, low-cost photovoltaic devices.[2] The so-
lution-processable organic semiconducting polymer based
solar cells have attracted considerable interest as a low-cost
alternative to inorganic semiconductors for large-area and
lightweight applications.[3] Polymer solar cells (PSCs) offer
important advantages over their inorganic counterparts
owing to their easy and fast processing capability and the
low cost of fabrication. Recently, rigid-rod conjugated or-
ganic polymers containing metal centers in the main chain
have evolved as a new class of active donor materials to cap-
ture energy from the sun.[4] Until now, the low-bandgap or-
ganic polymers recorded high power conversion efficiencies
(PCEs) of ~5–7.4%.[5] The morphology of the photoactive
layer is crucial and directly related to the PCEs of the solar
cells, which needs to be further optimized in order to reach
higher PCEs of the solar cells.[6] On the other hand, the real-
Fax : (+86)20-35902080
E-mail: hbwu@scut.edu.cn
[a] Dr. Q. Wang, Prof. W.-Y. Wong
Institute of Molecular Functional Materials
Areas of Excellence Scheme, University Grants Committee (Hong
Kong)
[c] A. Wild, Prof. U. S. Schubert
Laboratory of Organic and Macromolecular Chemistry (IOMC)
Friedrich-Schiller-University Jena
Humboldtstr. 10,d-07743 Jena (Germany)
and Department of Chemistry and Centre for
Advanced Luminescence Materials
Hong Kong Baptist University
Waterloo Road, Kowloon Tong, Hong Kong (P.R. China)
Fax : (+852)3411-7348
[d] Dr. C.-H. Chui
Institute of Textiles and Clothing
The Hong Kong Polytechnic University
Hung Hom, Hong Kong (P. R. China)
Fax : (+852)2773-1432
E-mail: rwywong@hkbu.edu.hk
[b] Z. He, Prof. H. Wu, Prof. Y. Cao
Institute of Polymer Optoelectronic Materials and Devices
State Key Laboratory of Functional Optoelectronics Materials and
Devices
E-mail: chchui@graduate.hku.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100111.
South China University of Technology
Guangzhou 510640 (P.R. China)
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The electron-rich organic triarylamine group, such as tri-
phenylamine (TPA) or trifluorenylamine (TFA), has been
widely used in organic solar cell research for their excellent
hole-transporting, pseudo-three-dimensional (3D), good
light-harvesting, and favorable steric properties.[10,11] Each of
them also shows good ease of oxidation for the nitrogen
center and the ability to transport charge carriers by means
of the radical cation species with high stability.[12] Because of
the steric interactions among the phenyl groups, TPA or
TFA adopts a non-planar propeller shape at the central ni-
trogen node which represents a promising multidimensional
system.[13] The use of TPA as a building block, in combina-
tion with linear conjugated systems for the design of photo-
voltaic active materials, serves as a good illustration of the
recent trend toward 3D organic semiconductors in which
these materials would allow realization of all kinds of elec-
tronic or photonic devices without any constraint in terms of
molecular orientation.[14] The fact that field-effect hole mo-
bility of around 10À2 cm2 VÀ1 sÀ1 and solar cells with efficien-
cy close to 2% can be fabricated with such organic TPA ma-
terials is likely to stimulate further research along this direc-
tion.[8] Thus, the development of new classes of 3D materials
represents an interesting and valuable target.
for photovoltaic devices.[15] Preliminary data also reveal a
better photovoltaic performance of metalated poly(aryle-
neethynylene)s over the metal-free one because of the more
favorable absorption and charge-transporting features.[4] In
view of this, we report here the synthesis, characterization,
and preliminary photovoltaic behavior of some TPA-based
metallopolymers with different molecular dimensions, while
the relationship between their photovoltaic properties and
dimensionality was studied. We also develop parent systems
containing longer conjugated branches in M3 and P3 specifi-
cally tailored for shifting the absorption toward longer
wavelengths relative to M2 and P2. It was shown that the
multidimensional polymer shows a better performance than
that for the linear polymer using the same device fabrication
process and measurement parameters. Purely in the proof-
of-concept demonstrations here, solar cells derived from
them exhibit PCEs of up to 2.24% even at short visible
wavelengths and narrow absorption bandwidth of the sun-
light irradiation spectrum.
Results and Discussion
Synthetic Methodologies and Chemical Characterization
Along another line, recent years have witnessed the rapid
emergence of various platinum-containing polymers for or-
ganometallic photovoltaic studies.[15] Solution-processable
organometallic polymer semiconductors, possessing the
donor–acceptor (D–A) architecture and platinum in the
backbone, were recently shown to exhibit broad absorption
bands owing to the intramolecular charge transfer (ICT) be-
tween the D and A units as well as small bandgaps (even
down to the near-infrared region) which make them suitable
The chemical structures and syntheses of the ligands L1–L3
are shown in Scheme 1, while those of new platinum(II)
polyyne polymers P1–P3 and their well-defined model com-
pounds M1–M3 are presented in Scheme 2. The methyl
group in the 1D-type compound is needed to prevent bromi-
nation by N-bromosuccinimide (NBS) and further reaction,
leading to the linear skeleton of P1. The bromide deriva-
tives (L1-Br to L3-Br) were prepared in good yields by bro-
minating the corresponding TPA derivatives with NBS in
chloroform/acetic acid at room temperature. In this step,
NBS was added carefully under the ice-bath condition to
prevent it from further bromination (see the Supporting In-
formation). Conversion of the bromide derivatives to their
corresponding ethynyl congeners can be readily achieved
following the typical organic synthetic protocols for alkyny-
lation of aromatic halides.[16] The platinum-acetylide com-
pounds were prepared by the Sonogashira-type dehydroha-
logenation between each of the ethynyl precursors (L1 to
L3) and suitable platinum chloro precursors.[17] The feed
mole ratio of the platinum precursors and the ethynyl li-
gands were 1:1 and 2:1 for polymer P1 and dimer M1 and
1.5:1 and 3:1 for polymers P2–P3 and trimers M2–M3, re-
spectively, and each product was carefully purified to
remove ionic impurities and catalyst residues. The dinuclear
and trinuclear platinum complexes serve as good discrete
molecular model complexes for the corresponding polymers
as far as their spectroscopic and photophysical properties
are concerned. The polymers can be purified by column
chromatography over alumina and repeated precipitation
and isolated in good yields of 45–58% and high purity. All
of these platinum compounds are thermally and air-stable
solids and they are soluble in common chlorinated hydrocar-
bons and toluene. Size exclusion chromatography (SEC) in-
Abstract in Chinese:
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Platinum–Acetylide Polymers for Organic Solar Cells
(Mn based on photodiode array
(PDA) detector) of P1–P3 cali-
brated
against
polystyrene
standards are 11190, 38300,
and 39700, respectively, and P1,
P2, and P3 can possess up to
approximately 11, 23, and 19
triarylamine units in total along
the polymer chain. However,
the data should be viewed with
caution in view of the common
hurdles associated with utilizing
SEC for rigid-rod polymers
which show appreciable differ-
ences in the hydrodynamic be-
Scheme 1. Synthetic pathways of L1–L3.
havior from those for flexible
vestigations of P1–P3 support their oligomeric/polymeric
nature (Table 1). The number-average molecular weights
polystyrene polymers.[18] The polymer samples were also
studied by SEC using N,N-dimethylacetamide (DMA) con-
Scheme 2. Synthetic pathways of PtII compounds M1–M3 and P1–P3.
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H. Wu, C.-H. Chui, W.-Y. Wong et al.
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Table 1. Molecular weight characteristics of P1–P3 estimated from
complexes exhibit a single resonance with a pair of platinum
SEC.[a]
satellites supporting the trans-arrangement of the phosphine
Mn (RI detector)[b]
Mn (PDA detector)[b]
DP[c]
1
ligands around the metal. The JP–Pt values in the platinum
P1
P2
P3
12300 (1.36)
38200 (3.14)
37800 (4.36)
11190 (1.74)
38300 (3.12)
39700 (4.13)
11
23
19
compounds (ca. 2611–2617 Hz for model complexes; 2316–
2320 Hz for polymers) are typical of those found for related
trans-PtP2 systems.[23] The formulas of M1–M3 were success-
fully confirmed from the intense molecular ion peaks in the
mass spectra. Crystals of L3-Br suitable for X-ray diffraction
studies were grown by slow evaporation of its solutions in a
CH2Cl2/n-hexane mixture at room temperature. A perspec-
tive drawing of L3-Br is shown in Figure 1. The fluorene
and thiophene units in these compounds were confirmed to
[a] Using DMA+0.08 mol% NH4PF6 as an eluent and polystyrene as a
standard. [b] Polydispersity index (PDI) in parentheses. [c] Degree of
polymerization calculated from Mn (using PDA detector).
taining 0.08 mol% NH4PF6 as an eluent.[19] The response
was detected by using both a refractive index (RI) as well as
a PDA detector. The chromatograms show the typical shape
of the system as confirmed by the p–p* absorption band at
around 400–450 nm.
The thermal properties of polymers were examined by
thermal gravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) under nitrogen. Analysis of the
TGA trace (heating rate of 208CminÀ1) for the polymers re-
veals an excellent thermal stability with onset decomposi-
tion temperatures (Tdec) above 3408C (Table 2), which are
À À
have a non-coplanar relationship. The three C N C angles
are close to 1208. Because of steric interactions among the
phenyl groups, TPA adopts a non-planar propeller shape
and thus represents an intermediate situation between the
planar 2D systems and true 3D conjugated systems. By pre-
venting tight p-stacking interactions, this pseudo-3D struc-
ture probably plays a major role in the resistance to crystal-
lization of these compounds.
higher than those for trans-
[-Pt
N
Photophysical and Redox Characterization
(Ar=C6H4: 3028C,[20] anthrylene: 3158C,[21] oligothienylene:
278–2908C,[22] and so forth). Their degradation patterns are
quite similar to each other which are ascribed to the elimi-
nation of PBu3 and butyl groups from the polymers. In DSC
studies, no phase transition or crystallization signals are de-
tected during repeated heating/cooling DSC cycles for both
polymers. This observation probably results from the stiff-
ness of the polymer chains.
The absorption and emission spectra of the ligands, poly-
mers (P1–P3), and model complexes (M1–M3) were mea-
sured in CH2Cl2 solutions at 293 K (see Figure 2). P1 and P2
show similarly structured broad absorption bands spanning
around 300–460 nm whereas P3 shows the spectral band be-
tween 315 and 480 nm. All of these absorption features are
dominated by intense ligand-centered p–p* transition in the
thiophene-triarylamine organic system. As thin films, P3
shows the highest absorption coefficient of 1.59ꢁ105 cmÀ1
(at 430 nm), while those of P1 and P2 are found to be 0.45ꢁ
105 cmÀ1 and 1.05ꢁ105 cmÀ1 (both at 413 nm) under the
same testing conditions, respectively. These absorption coef-
ficients are typical of many other low-bandgap organic poly-
mers reported recently with good PCEs in solar cells.[3b,24]
The coefficient of the multidimensional polymer P2 is
almost double that of the linear polymer P1. The absorption
properties of M1–M3 are simi-
The structures of new molecules were unequivocally char-
acterized using mass spectrometry, IR, and NMR spectros-
copies (see the Experimental section). The solution IR spec-
tra of these new metal complexes display a single sharp n-
A
thereby revealing a trans-configuration of the ethynylene li-
gands around the platinum center. The NMR spectral data
confirmed that these compounds have well-defined and sym-
metrical structures. 31PNMR spectra of the platinum(II)
lar to those of their polymeric
congeners and no further de-
Table 2. Photophysical and thermal data of M1–M3 and P1–P3.
Absorption (293 K)
CH2Cl2
labs [nm]
Emission (293 K)
CH2Cl2
Emission (77 K)
CH2Cl2
Tdec [8C]
[a]
scription is needed for these
materials. Even in the pres-
ence of an extended p-electron
delocalized system through the
electron-rich platinum ion and
Bandgap Eg [eV][b] lem [nm] F [%] tP [ns]
lem [nm]
tP [ns]
M1 402 (4.4)
2.74
2.73
2.68
451
474*
461
67.2
63.1
33.3
1.53
1.54
1.68
448
1.52
–
–
–
470*
460
M2 405 (10.2)
M3 430 (15.2)
1.06
487*
464
triarylamine-thiophene
seg-
467
18.49
ment, the electron-rich poly-
mers P1–P3 only achieve
rather high bandgaps of 2.59–
2.72 eV because of the lack of
D–A structure in the polymers.
Owing to the longer p-conju-
gation length of fluorene, P3
shows a better absorption fea-
ture than P2, and this would
482*
493*
P1 420 (4.5)[c]
P2 420 (10.5)[c]
P3 432 (15.9)[c]
2.72
2.68
2.59
470
471
468
494*
1.4
1.5
3.2
2.8
472 485* 495* 0.57
476 485* 495* 0.72
470
481*
493*
341
389
459
2.0
[d]
[d]
[a] Molar extinction coefficients (104 dm3 molÀ1 cmÀ1 for M1–M3, and 104 cmÀ1 for P1–P3) are shown in paren-
theses. [b] Optical bandgaps determined from the onset of absorption. [c] Data from thin film. [d] Lifetime is
too short to be determined. Asterisks indicate weak or shoulder bands.
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Platinum–Acetylide Polymers for Organic Solar Cells
a Ag/AgCl wire as the refer-
ence electrode, at a scan rate
of 50 mVsÀ1. The solvent in all
measurements was deoxygen-
ated MeCN, and the support-
ing electrolyte was 0.1m
[nBu4N]BF4. The relevant data
are collected in Table 3.
Photovoltaic Effect and Solar
Cell Behavior
Polymer solar cells were fabri-
cated by using each of M2 and
P1–P3 as an electron donor
and PCBM as an electron ac-
ceptor (Table 3). The hole col-
lection electrode consisted of
ITO with a spin-coated PE-
DOT:PSS, while Al served as
the electron collecting elec-
trode. Absorption spectra of
P1–P3 and M2 blended with
PCBM (1:5 by weight) are de-
picted in Figure 3. Each of the
spectra shows one major band
between 400 and 500 nm, con-
sistent with the solution ab-
sorption spectra. It can be
Figure 1. a) Molecular structure and b) space filling model: top view (top) and side-view (bottom) of L3-Br.
be one of the reasons leading to the better photovoltaic per-
formance of P3 (vide infra). P1–P3 and M1–M3 emit strong
spin-allowed fluorescence from the singlet excited states at
both 293 and 77 K. The emission is assigned on the basis of
the small Stokes shift (<50 nm) between the absorption and
emission and the short lifetimes (ca. 1.53–2.80 ns at 293 K)
in the ns time scale (Table 2). Even at 77 K, only singlet
emissions were observed in these compounds without the in-
volvement of triplet excited states.
The highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) levels of
stable M2 and P1–P3 were calculated based on the redox
potentials determined from electrochemical measurements
using cyclic voltammetry. The experiments were performed
by casting the polymer films on the ITO glass electrode with
clearly observed that enhancing the absorption coefficient
of the band by increasing the dimensionality of the polymer
is a good strategy for efficiency improvement. A considera-
ble increase in the short-circuit current density (Jsc) and
PCE can be observed in P2 and P3 relative to P1, and the
results are shown for 1:5 blend ratio (Figure 4). The 3D
polymers exhibit a 2–4 times larger PCE than the linear
one. This improvement has been attributed to the random
orientation of the conjugated segments in films of the 3D
compounds, which allows a better absorption of the incident
light compared to the vertically oriented chains of the linear
molecule. This is likely owing to the differences in film
roughness and phase separation in P2:PCBM and P3:PCBM
blends (vide infra). The open-circuit voltage (Voc) of bulk
heterojunction device was shown to correlate with the
energy of the HOMO of the polymer (ED) and the LUMO
of the acceptor (EA), according to Voc =1/eCAHTUNGTRENNNUG
So, an effective method to improve Voc is to manipulate the
ED (or ionization potential) by modifying the polymer struc-
ture. The Voc of P3 is higher compared with P1 or P2/PCBM
cells (0.74 V) owing to the lower ED of P3 (À5.67 eV versus
À5.50 to À5.49 eV for P1 and P2).[15a] Likewise, similar
mean Voc values were obtained for P1 and P2 which show
comparable ED. Bulk heterojunctions based on these sys-
tems are, however, characterized by a pronounced voltage-
dependent photocurrent, therefore resulting in low fill fac-
tors (FF), typically 27–56%. This common characteristic is
likely to arise from imbalanced charge transport across the
Table 3. Electrochemical data and frontier orbital energy levels for M2,
P1, P2, and P3.
Polymer Oxidation potential [V] Energy levels [eV]
Bandgap [eV]
Eg
[a]
[b]
[c]
opt[d]
Eonset,ox
EHOMO
ELUMO
M2
P1
P2
P3
+0.75
+0.78
+0.77
+0.95
À5.47
À5.50
À5.49
À5.67
À2.74
À2.78
À2.81
À3.08
2.73
2.72
2.68
2.59
[a] Eonset,ox are the onset potentials of oxidation. [b] EHOMO
=
À(Eonset,ox+4.72) eV. [c] Calculated from the optical bandgap and the
energy level of HOMO and ELUMO =(EHOMO+Egopt) eV. [d] Egopt =Optical
bandgap.
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H. Wu, C.-H. Chui, W.-Y. Wong et al.
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ble for the observed low FF.[28]
Also, all processing (except PE-
DOT:PSS annealing and elec-
trode deposition) and measure-
ments have been done under an
ambient atmosphere which
likely results in the presence of
traps. We expect FF to improve
when fabrication and character-
ization is performed in an inert
gas environment. Comprehen-
sive study of charge transport
and the influence of traps are
necessary to further improve
the FF and overall device per-
formance. Even if P1–P3 pos-
sess relatively high bandgaps
and narrow absorption bands in
the solar spectrum, we can
obtain the highest PCE of
2.24% based on blends of
P3:PCBM (1:5, w/w) in these
proof-of-concept experiments
(Table 4). Therefore, we would
anticipate a further improve-
ment of device performance by
designing other lower-bandgap
3D polymers in the future.
The light absorption in the
visible wavelength region is rel-
atively poor and the PCE ap-
pears to be low as PCBM does
not contribute much to the ab-
sorption in this region and the
blends consist of four or five
times as much PCBM as the
polymer. Therefore, external
quantum efficiency (EQE)
curves of the solar cells based
Figure 2. Normalized absorption (left) and emission (right) spectra of M1–M3 and P1–P3 in CH2Cl2 solution
at 293 K.
on these metallopolymers and PCBM were also studied for
P1 and P2 (Figure 5). In each case, the shape of the EQE
curve follows the shape of the absorption of the blend, thus
illustrating that all the light energy absorbed by the poly-
mer/PCBM blend film is to some extent converted into elec-
tricity. The excitons produced by absorption in the polymer
Table 4. Solar cell performance of best devices with M2 and P1–P3. The
numbers in parentheses denote average from 6 devices.
PSCs
Voc [V]
Jsc [mAcmÀ2
]
FF
Max. PCE [%]
P1:PCBM (1:5) 0.74 (0.74) 3.10 (2.99)
P2:PCBM (1:5) 0.77 (0.76) 5.10 (4.79)
P3:PCBM (1:5) 0.82 (0.82) 5.88 (4.09)
M2:PCBM (1:5) 0.65 (0.64) 2.35 (2.16)
P1:PCBM (1:4) 0.74 (0.75) 3.00 (2.83)
P2:PCBM (1:4) 0.76 (0.72) 4.94ACTHNUTRGNE(UNG 5.92)
P3:PCBM (1:4) 0.82 (0.84) 3.98 (4.14)
M2:PCBM (1:4) 0.61 (0.55) 2.56 (2.55)
0.38 (0.38) 0.88 (0.83)
0.43 (0.44) 1.67 (1.60)
0.56 (0.53) 2.24 (1.78)
0.27 (0.28) 0.41 (0.38)
0.37 (0.36) 0.83 (0.77)
0.44 (0.35) 1.64 (1.47)
0.54 (0.50) 1.75 (1.72)
0.30 (0.30) 0.46 (0.41)
Figure 3. Absorption spectra of M2 and P1–P3:PCBM blends (1:5, w/w).
device limited by the low hole mobility.[26] As a result, occur-
rence of the buildup of space charge[27] and subsequent bi-
molecular recombination will take place, which is responsi-
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Platinum–Acetylide Polymers for Organic Solar Cells
complicated fabrication procedures. These results open up a
feasible strategy to develop low-cost, high PCE materials in
photovoltaic applications. Therefore, besides optimization of
the bandgap of the conjugated branches, future work should
be focused on polymers of higher dimensionality and 3D
donors of this type should also be taken into account to ach-
ieve appropriate phase separation in the composite material.
The charge transport in platinum(II) acetylides has been
demonstrated.[31] When the platinum metal is conjugated
with an alkyne unit, the dp-orbitals (dxy and dxz) of the plati-
num overlap with the p-orbitals (py* and pz*) of the alkyne,
hence facilitating p-electron delocalization. This offers new
opportunities for investigating the solution-processable sem-
iconducting polymers in organic thin-film transistor (OTFT)
implementation. Charge transport in P1–P3:PCBM layers
was studied using thin film organic field-effect transistor
(OFET) configuration. OFET devices fabricated from P1–
P3 showed typical p-channel output characteristics when op-
erated in the accumulation mode. The saturation region
field-effect mobility was calculated from the transfer charac-
Figure 4. J–V curves of solar cells with M2 and P1–P3:PCBM (1:5, w/w)
active layers under simulated AM1.5 solar irradiation.
1/2
teristics of the OFETs by plotting ID against VG.[32] The re-
sulting average hole mobilities from measurements on 5–6
different devices of P1–P3 are 1.90ꢁ10À5, 7.86ꢁ10À5, and
2.77ꢁ10À5 cm2 VÀ1 sÀ1, respectively, which are quite consis-
tent with the lower Jsc of the devices. Their OFET output
and transfer characteristics are exemplified by the results
given in Figure 7 for P2-based top-contact, bottom-gate p-
channel OFETs (with highly n-doped Si as the gate elec-
trode, octadecyltrichlorosilane (OTS)-treated SiO2 as the
gate dielectric, and vacuum deposited gold as source and
drain electrodes) processed from chlorobenzene. Although
P2 has a slightly higher hole mobility than P3, the higher
PCE for P3 is plausibly attributed to the much improved
morphological features of the P3-based solar cells. However,
more efforts are still required to enhance the charge-trans-
port properties of these materials.
Figure 5. EQE wavelength dependencies of solar cells with P1:PCBM
and P2:PCBM (1:5) active layers.
are dissociated into charge carriers at the contact between
polymer and PCBM in the active layer, and are subsequent-
ly collected at the electrodes. The absorption is enhanced in
the high-energy absorption range 350–415 nm after blending
with 83% PCBM. The highest EQE values of P1 and P2 are
50.2% at 418 nm and 80.9% at 412 nm, respectively. The
broad EQE curves for P1 and P2 cover the spectrum up to
800 nm.
Conclusions
The control of the morphology formation (in particular,
for the morphology of the photoactive layer) directly deter-
mines the performance of PSC devices.[6,29] As the exciton
diffusion length is typically limited to about 10 nm in BHJ
cells, it is critically important that the donor and acceptor
materials in the photoactive layer are well intermixed by
forming phases at the nm range in the BHJ structure to
ensure a higher efficiency.[30] Atomic force microscopy
(AFM) was used to characterize the blends of P1–
P3:PCBM with blend ratios of 1:5, as shown in Figure 6. It
can be observed that the film of P1 is smooth and does not
show significant phase separation or formation of larger
PCBM domains, while films of P2 and P3 are rougher. Area
difference of 0.009%, 0.012%, and 0.102% for P1–P3 are
given by the roughness analysis. These data indicate that a
multidimensional polymer can afford a better surface mor-
phology than a linear polymer without the need for using
In summary, we have reported a new type of pseudo-3D
platinum-containing polymer and the photovoltaic results
obtained with the BHJs based on such conjugated organo-
metallic donors appear very promising with efficiencies in
the range of 1.64–2.24%, despite the limited overlap of the
polymer absorption with the solar radiation. Both the mor-
phology and PCE of these multidimensional platinum-con-
taining polymers are improved relative to their linear ana-
logue under identical testing conditions. While the polymer
structures are still not yet optimized for photovoltaic work,
because of their narrow absorption bandwidths in the high-
energy visible wavelengths, the moderate peak PCE of
2.24% obtained from solar cells made by P3 under AM1.5
solar illumination is encouraging. As a proof-of-principle,
the moderate efficiency is attributed to the pseudo-3D
charge-transport properties of the polymers. In other words,
it is envisioned that polymers of higher dimensionality can
Chem. Asian J. 2011, 6, 1766 – 1777
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1773

H. Wu, C.-H. Chui, W.-Y. Wong et al.
FULL PAPERS
Figure 6. AFM images of P1–P3:PCBM blend films (1:5, v/v). a) Height image, area is 2 mmꢁ2 mm; b) roughness analysis, area is 10 mmꢁ10 mm; c) phase
image, area is 10 mmꢁ10 mm and d) height image, area is 2 mmꢁ2 mm.
be exploited to develop new low-cost and high PCE materi-
als in solar cell application. We anticipate that the perfor-
mance of these devices can be further improved by device
optimization, as well as further chemical modifications of
these metallized systems with better absorption and charge-
transport properties. This demonstration will stimulate fur-
ther work on the development of new multidimensional or-
ganic or organometallic polymers. Besides geometrical and
space-filling parameters that control material organization,
the relevant electronic properties of 3D organometallic sem-
iconductors depend to a large extent on the chemical consti-
tution of the conjugated branches. For practical photovoltaic
applications, the bandgap of the materials reported here is
too large for efficient light-harvesting, and these systems can
be considered as good models for testing the feasibility of
the concept. Future work should include the synthesis of 3D
architectures with more p-conjugated branches in order to
reduce the HOMO–LUMO gap according to known syn-
thetic principles.
Experimental Section
General Information
Solvents were carefully dried and distilled from appropriate drying
agents prior to use. Commercially available reagents were used without
1774
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 1766 – 1777

Platinum–Acetylide Polymers for Organic Solar Cells
working electrode, a platinum counter electrode, and a Ag/AgCl (in 3m
KCl) reference electrode. The solvent in all measurements was deoxygen-
ated MeCN, and the supporting electrolyte was 0.1m [nBu4N]BF4. Thin
polymer films were deposited on the working electrode by dip-coating in
chlorobenzene solution (6 mgmLÀ1). The onset oxidation and optical
bandgap (Egopt) were used to determine the HOMO and LUMO energy
levels using the equations
E
HOMO =À(Eonset,ox+4.72) eV and ELUMO
=
(EHOMO+Egopt) eV (where the unit of potential is V versus Ag/AgCl).[9b,34]
Thermal analyses were performed with a Perkin–Elmer TGA6 thermal
analyzer.
Syntheses
The syntheses of all the ligand precursors are given in the Supporting In-
formation.
Platinum(II) model complexes
A typical procedure is given for M1. CuI (2 mg) was added to a stirred
mixture of L1 (11.8 mg, 0.025 mmol) and trans-[PtACTHNUTRGNEUNG
(PEt3)2PhCl][35]
(28.5 mg, 0.052 mmol) in freshly distilled triethylamine (6 mL) and
CH2Cl2 (6 mL). The solution was stirred at room temperature under ni-
trogen over a period of 24 h. After removal of the solvents, the crude
product was purified by column chromatography on silica gel eluting
with hexane/CH2Cl2 (1:1, v/v) to provide M1 as a yellow solid (16 mg,
0.011 mmol, 43%). IR (KBr): n˜
G
.
1HNMR (400 MHz,
CDCl3): d=7.41 (m, 4H, Ar), 7.32 (m, 4H, Ar), 7.09 (m, 2H, Ar), 7.00
(m, 12H, Ar), 6.81 (m, 4H, Ar), 2.33 (s, 3H, CH3), 1.75 (m, 24H,
PC2H5), 1.10 ppm (m, 36H, PC2H5); 13CNMR (100 MHz, CDCl3): d=
146.46, 144.69, 140.26, 139.09, 133.09, 129.99, 129.05, 129.00, 127.98,
127.33, 126.14, 125.01, 123.64, 121.76 (Ar), 121.31, 102.45 (CꢀC), 20.87
(CH3), 15.11, 8.05 ppm (PC2H5); 31PNMR (161 MHz, CDCl3): d=
9.94 ppm (1JPt-P =2613 Hz); FAB-MS: m/z 1487.9 [M+1]+. Anal. calcd
for C67H89NP4S2Pt2: C 54.13, H 6.03, N 0.94; found: C 54.34, H 6.12,
N 1.14.
Figure 7. a) Output and b) transfer characteristics of OFETs fabricated
from P2.
further purification unless otherwise stated. All reagents for the chemical
syntheses were purchased from Aldrich or Acros Organics. PCBM was
purchased from American Dyes. PEDOT:PSS (Baytron VPAI 4083) was
purchased from H. C. Starck. Reactions and manipulations were carried
out under an atmosphere of prepurified nitrogen using Schlenk tech-
niques. All reactions were monitored by thin-layer chromatography
(TLC) with Merck pre-coated glass plates. Flash column chromatography
and preparative TLC were carried out using silica gel from Merck (230–
400 mesh). Infrared spectra were recorded as KBr pellets using a Perkin–
Elmer Paragon 1000 PC or Nicolet Magna 550 Series II FTIR spectrome-
ter. Fast atom bombardment (FAB) mass spectra were recorded on a Fin-
nigan MAT SSQ710 system. NMR spectra were measured in CDCl3 on a
Bruker AV 400 MHz FT-NMR spectrometer and chemical shifts are
M2: Yellow solid. Yield: 67%. IR (KBr): n˜
G
;
1HNMR
(400 MHz, CDCl3): d=7.44 (m, 6H, Ar), 7.37 (m, 6H, Ar), 7.08 (d, J=
8.8 Hz, 6H, Ar), 7.03 (d, J=4.0 Hz, 3H, Ar), 6.98 (t, J=7.4 Hz, 6H, Ar),
6.81 (m, 6H, Ar), 1.74 (m, 36H, PC2H5), 1.10 ppm (m, 54H, PC2H5);
13CNMR (100 MHz, CDCl3): d=156.03, 145.94, 140.09, 139.07, 135.21,
129.54, 129.19, 128.02, 127.33, 126.20, 124.25, 121.93 (Ar), 121.30, 102.44
(CꢀC), 15.08, 8.05 ppm (PC2H5); 31PNMR (161 MHz, CDCl3): d=
9.95 ppm (1JPt-P =2611 Hz); MALDI-TOF: m/z calcd for C90H123NP6Pt3S3:
2085.6189; found: 2087.1284 [M+1]+. Anal. calcd for C90H123NP6S3Pt3:
C 51.82, H 5.94, N 0.67; found: C 51.68, H 6.10, N 0.88.
M3: Yellow solid. Yield: 50%. IR (KBr): n˜
G
;
1HNMR
(400 MHz, CDCl3): d=7.56 (m, 9H, Ar), 7.44 (m, 3H, Ar), 7.33 (m, 6H,
Ar), 7.16 (m, 6H, Ar), 7.09 (m, 3H, Ar), 6.98 (m, 6H, Ar), 6.83 (m, 6H,
Ar), 1.90 (m, 12H, C2H5), 1.76 (m, 36H, PC2H5), 1.11 (m, 54H, PC2H5),
0.40 ppm (t, J=7.4 Hz, 18H, C2H5); 13CNMR (100 MHz, CDCl3): d=
156.05, 151.33, 150.29, 147.32, 141.08, 140.34, 139.06, 136.09, 135.20,
132.87, 129.35, 128.02, 127.33, 127.12, 124.99, 124.39, 123.03, 122.24,
121.30, 120.16, 119.49 (Ar), 118.56, 102.52 (CꢀC), 56.15 (quat. C), 32.84,
15.07, 8.69, 8.06 ppm (PC2H5+C2H5); 31PNMR (161 MHz, CDCl3): d=
1
quoted relative to tetramethylsilane for H and 13C nuclei and H3PO4 for
31P nucleus. Size exclusion chromatography (SEC) values were obtained
using a Waters LC system equipped with a isocratic pump HPLC 1515,
an autosampler 717 plus (71P), a refractive index detector 2414, and a
photodiode array detector 2996. The following column setup was used
for the separation: A Waters Styragel pre-column, a Phenomenex Pheno-
gel 103 A (10 mm, 300ꢁ7.8 mm), and a Phenomenex Phenogel 103 A
(10 mm, 300ꢁ7.8 mm). Conditions for measurements were a 1 mLminÀ1
flow rate at 508C using DMA+0.08% NH4PF6 as an eluent. Calibration
was done against polystyrene (with narrow distributed standards).
9.99 ppm
(1JPtÀP =2617 Hz);
MALDI-TOF:
m/z
calcd
for
C123H159NP6Pt3S3: 2517.9012; found: 2519.6712 [M+1]+. Anal. calcd for
C123H159NP6S3Pt3: C 58.65, H 6.36, N 0.56; found: C 58.89, H 6.30, N 0.81.
Physical Measurements
Platinum(II) polymers
UV/Vis spectra were obtained on an HP-8453 diode array spectropho-
tometer. The solution emission spectra and lifetimes of the compounds
were measured on a Photon Technology International (PTI) Fluores-
cence QuantaMaster Series QM1 spectrophotometer. The emission quan-
tum yields were determined in degassed CH2Cl2 solutions at 293 K
against quinine sulfate in 0.1 n H2SO4 (FF =0.54).[33] 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
A typical procedure is provided for P1. The polymerization was carried
out by mixing L1 (33.1 mg, 0.70 mmol), trans-[PtACTHNUTRGNEUNG
(PBu3)2Cl2][36] (47 mg
0.70 mmol), and CuI (4.2 mg) in freshly distilled triethylamine (25 mL)
and CH2Cl2 (25 mL). The solution was stirred at room temperature for
24 h under a nitrogen atmosphere. The solvents were removed on a
rotary evaporator in vacuo. The residue was redissolved in CH2Cl2 and
filtered through a short alumina column using the same eluent to remove
ionic impurities and catalyst residue. After the removal of all volatile
components, the crude product was washed with hexane (15 mLꢁ3) fol-
lowed by methanol (15 mLꢁ3), and the crude solid was precipitated
from CH2Cl2/hexane and dried in vacuo to obtain P1 (36 mg, 48%) as a
case according to I=I0+Aexp
N
out at a scan rate of 50 mVsÀ1 using a eDAQ EA161 potentiostat electro-
chemical interface equipped with a thin film coated ITO covered glass
Chem. Asian J. 2011, 6, 1766 – 1777
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1775

H. Wu, C.-H. Chui, W.-Y. Wong et al.
FULL PAPERS
golden yellow solid. IR (KBr): n˜
G
;
1HNMR (400 MHz,
Fabrication and Characterization of Organic Field-Effect Transistors
CDCl3): d=7.40 (m, 4H, Ar), 7.09 (m, 2H, Ar), 7.02 (m, 8H, Ar), 6.79
(m, 2H, Ar), 2.32 (s, 3H, CH3), 2.13 (m, 12H, PC4H9), 1.58 (m, 12H,
shielded by H2O, PC4H9), 1.48 (m, 12H, PC4H9), 0.94 ppm (t, J=7.2 Hz,
Heavily p-doped Si wafers with 300 nm thermally grown SiO2 were used
both as the substrate and the gate electrode. The substrates were cleaned
by subsequent soaking in acetone, isopropanol, and de-ionized water in
an ultrasonic bath for 10 min, and dried with nitrogen. Then, the sub-
strates were treated with oxygen plasma for 5 min to remove any residual
organic materials and to create a high density of silanol groups at the sur-
face. Films of polymers P1–P3 were deposited on pre-treated SiO2/Si sub-
strates by spin-coating a 10 mgmLÀ1 solution of the polymer in chloro-
benzene, followed by annealing under nitrogen to remove the residual
solvent. The source and drain electrodes were defined by thermally evap-
orating gold (400 nm) through a shadow mask on top of the organic thin
film forming top contact geometry transistors. Channel length and width
of the obtained OFET were 0.1 and 10 mm, respectively. The OFET was
characterized under ambient conditions by an Agilent semiconductor pa-
rameter analyzer (model: 4155B).
18H, PC4H9); 31PNMR (161 MHz, CDCl3): d=3.19 ppm (1JPt–P
=
2318 Hz).
P2: L2/Pt ratio is approximately 1:1.8 from NMR analysis. Golden
yellow solid. Yield: 58%. IR (KBr): n˜
1HNMR
(400 MHz, CDCl3): d=7.42 (m, 6H, Ar), 7.05 (m, 9H, Ar), 6.78 (m, 3H,
Ar), 2.12 (m, 22H, PC4H9), 1.55 (m, 22H, PC4H9), 1.46 (m, 22H, PC4H9),
0.95 ppm (m, 34H, PC4H9); 31PNMR (161 MHz, CDCl3): d=3.24 ppm
(1JPt–P =2316 Hz).
;
P3: L3/Pt ratio is approximately 1:1.8 from NMR analysis. Golden
yellow solid. Yield: 45%. IR (KBr): n˜
(400 MHz, CDCl3): d=7.52 (m, 12H, Ar), 7.14 (m, 9H, Ar), 6.82 (m,
3H, Ar), 2.15 (m, 21H, PC4H9), 1.96 (m, 12H, C2H5), 1.55 (m, shielded
by the H2O proton, 42H, PC4H9), 0.97 (m, 33H, PC4H9), 0.40 ppm (m,
18H, C2H5); 31PNMR (161 MHz, CDCl3): d=3.33 ppm (1JPt–P =2320 Hz).
;
1HNMR
Acknowledgements
X-ray Crystallography
This work was supported by a grant from the University Grants Commit-
tee of HKSAR, China (Project No. [AoE/P-03/08]), Hong Kong Re-
search Grants Council (HKBU202410), and Hong Kong Baptist Universi-
ty (FRG2/09-10/091) and W.-Y.W. also thanks the Croucher Foundation
for a Croucher Senior Research Fellowship. H.-B.W. thanks the National
Natural Science Foundation of China (No. 50990065 and 60906032) for
the financial support. A. Wild and U. S. Schubert also acknowledge the
Dutch Polymer Institute (DPI) for financial support. We also thank Drs.
A. B. Djurisic and K.-Y. Cheung for technical assistance in solar cell fab-
rication and testing.
X-Ray diffraction data were collected at 293 K using graphite-monochro-
mated MoKa radiation (l=0.71073 ꢀ) on a Bruker Axs SMART 1000
CCD diffractometer. The collected frames were processed with the soft-
ware SAINT+[37] and an absorption correction (SADABS)[38] was ap-
plied to the collected reflections. The structure was solved by direct
methods (SHELXTL)[39] in conjunction with standard difference Fourier
techniques and subsequently refined by full-matrix least-squares analyses
on F2. Hydrogen atoms were generated in their idealized positions and
all non-hydrogen atoms were refined anisotropically. CCDC 822376,
CCDC 822377, CCDC 822378 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre at www.ccdc.cam.ac.uk/data_re-
quest/cif. Selected crystal data for L3-Br: C63H54NBr3S3, Mw =1160.98,
monoclinic, space group P21/n, a=11.982(2), b=18.270(2), c=
24.485(3) ꢀ, b=92.937(2)8, V=5353(1) ꢀ3, Z=4, 1calcd =1.441MgmÀ3, m-
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The device structure was ITO/poly(3,4-ethylene-dioxythiophene):poly(s-
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oxide (ITO) glass substrates (10 W per square) were cleaned by sonica-
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temperature under low vacuum in a vacuum oven for 1 h, and then
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Chem. Asian J. 2011, 6, 1766 – 1777

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Received: February 5, 2011
Published online: June 7, 2011
Chem. Asian J. 2011, 6, 1766 – 1777
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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