Solution-processable crystalline platinum-acetylide oligomers with broadband absorption for photovoltaic cells

Article abstract of DOI:10.1021/cm903329a

A series of solution-processable and crystalline platinum-acetylide oligomers containing a thienyl-benzothiadiazole-thienyl core and oligothiophene alkynyl ligands are synthesized and characterized. X-ray crystallography analysis indicates a two-dimensional arrangement of oligomers through CH- interactions in single crystals. These oligomers show two intense and broad absorption bands in the visible spectral region, with the short-wavelength absorption band being strongly dependent on the oligothiophene length. In neat films, all the oligomers form large crystalline domains of several hundred nanometers in size upon thermal treatment and exhibit space-charge limited current (SCLC) mobilities on the order of 10^-5-10^-4 cm²-V^-1-s^-1. The photovoltaic properties of these oligomers were evaluated by fabricating bulk heterojunction devices with fullerene derivatives (PC₆₁BM and PC₇₁BM) and some of these devices showed high-power conversion efficiencies (PCEs) of up to 3% and a peak external quantum efficiency (EQE) to 50% under AM 1.5 simulated solar illumination. The present work suggests that well-defined platinum oligomers with desirable light-absorbing and self-assembly properties have potential for solution-processed organic photovoltaics.

Full text of DOI:10.1021/cm903329a

Chem. Mater. 2010, 22, 2325–2332 2325
DOI:10.1021/cm903329a
Solution-Processable Crystalline Platinum-Acetylide Oligomers with
Broadband Absorption for Photovoltaic Cells
Xiaoyong Zhao, Claudia Piliego, BongSoo Kim, Daniel A. Poulsen, Biwu Ma,
ꢀ
David A. Unruh, and Jean M. J. Frechet*
Materials Sciences Division, Lawrence Berkeley National Laboratory and College of Chemistry,
University of California, Berkeley, California 94720-1460
Received October 30, 2009. Revised Manuscript Received January 29, 2010
A series of solution-processable and crystalline platinum-acetylide oligomers containing a thienyl-
benzothiadiazole-thienyl core and oligothiophene alkynyl ligands are synthesized and characterized.
X-ray crystallography analysis indicates a two-dimensional arrangement of oligomers through CH-π
interactions in single crystals. These oligomers show two intense and broad absorption bands in the
visible spectral region, with the short-wavelength absorption band being strongly dependent on the
oligothiophene length. Inneat films, all the oligomers formlarge crystalline domains of several hundred
nanometers in size upon thermal treatment and exhibit space-charge limited current (SCLC) mobilities
on the order of 10-5-10-4 cm2 V-1 s-1. The photovoltaic properties of these oligomers were evaluated
by fabricating bulk heterojunction devices with fullerene derivatives (PC61BM and PC71BM) and
some of these devices showed high-power conversion efficiencies (PCEs) of up to 3% and a peak
external quantum efficiency (EQE) to 50% under AM 1.5 simulated solar illumination. The present
work suggests that well-defined platinum oligomers with desirable light-absorbing and self-assembly
properties have potential for solution-processed organic photovoltaics.
Introduction
electron donor and fullerene derivatives as the electron
acceptor.10-13 In contrast to polymeric systems that in-
trinsically display large structural variations in molecular
weight, polydispersity, and regioregularity, conjugated
small molecules and oligomers possess well-defined struc-
tures that are relatively easy to modify and purify.14,15 In
addition, small molecules and oligomers show a strong
tendency to self-assemble into ordered crystalline domains
with superior charge transport properties14 and favorable
nanoscale phase separation may be achieved via self-
assembly of donor and acceptor molecules during solution
processing.16
Progress in organic photovoltaics (OPVs) requires sig-
nificant advances in the areas of material design and synth-
esis, film processing, and morphology control, as well as
device architecture.1-6 One of the most important break-
throughs in OPVs is the introduction of the bulk hetero-
junction (BHJ) device structure, in which the photoactive
thin film consists of a blend of electron donors and ac-
ceptors.7-9 To date, solution-processed BHJ photovoltaic
cells have achieved a power conversion efficiency (PCE) of
4%-6%, using various semiconducting polymers as the
BHJ OPVs based on solution-processable molecular
donors, consisting of small molecules, oligomers, or den-
drimers with fullerene acceptors, have been described,17-23
*Author to whom correspondence should be addressed. E-mail: frechet@
berkeley.edu.
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2010 American Chemical Society
Published on Web 02/16/2010
pubs.acs.org/cm

2326 Chem. Mater., Vol. 22, No. 7, 2010
Zhao et al.
affording PCEs that have evolved from <1%15 to a recent
value of 4.4%.24 Generally, device efficiencies have been
limited by issues related to the photoactive donors, includ-
ing poor overlap with the solar spectrum, low molar
absorptivity, and low charge mobility. Therefore, the
development of new molecular donors with desirable
absorption and charge transport properties is important
to further understand the variables that control the device
operation and device performance.
Platinum(II)-acetylide oligomers have been investi-
gated extensively for applications in optical limiting,
two-photon imaging, and phosphorescence-based sen-
sors.25-28 The direct incorporation of platinum(II)
into conjugated systems leads to a high yield of triplet
states and also enhances the intrachain charge transport
in these oligomers.29,30 Some donor-acceptor dyads
bridged with Pt(II) atoms have shown photoinduced
charge separation states with lifetimes on the order of
microseconds.31,32 Recently, platinum polyyne macro-
molecules have been explored as a new class of donor
materials in BHJ OPVs achieving PCEs of ∼3%.33-37
However, these polymers typically show a double-band
absorption with a significant gap (∼100 nm) between the
two bands, leading to a considerable loss of absorption
coverage of the solar spectrum. Moreover, these polymers
prepared by the Hagahara coupling reaction38 are intrin-
sically of large molecular weight distribution (polydi-
spersity of ∼2) and undefined end-groups. The amor-
phous nature of these polymers has limited their charge
transport properties, as suggested by the reported low
SCLC mobility (μhole ≈ 10-7 cm2 V-1 s-1).36
We now report the synthesis and characterization of a
series of crystalline platinum-acetylide oligomers with
well-defined structure and demonstrate their use as solu-
tion-processable molecular donors for efficient photo-
voltaic cells. The oligomers have been designed to (i) build
multichromophoric systems with broad absorption using
Pt2þ atoms as exciton confinement centers; (ii) extend the
absorption into the long wavelength range (beyond
600 nm) by incorporating intramolecular charge tran-
sfer (ICT) chromophores in the core; (iii) take advant-
age of oligothiophene-based alkynyl ligands to ensure
absorption in the range of 350-500 nm and promote
the solid-state packing of these molecules; (iv) prevent
strong aggregation using tetrahedral trialkylphosphine
ligands on the platinum center, thus providing their
solution processability. The optoelectronic properties
of these new compounds have been studied by UV-
vis absorption spectroscopy, density function theory
(DFT) calculations, and cyclic voltammetry (CV). The
self-assembly behavior of these oligomers in thin films has
been analyzed using atomic force microscopy (AFM) and
discussed in relation to their charge mobility. Finally, the
photovoltaic properties of these oligomers have been
evaluated by fabricating solution-processed BHJ devices
using these oligomers and fullerene derivatives.
Experimental Section
Synthesis. Compounds 1, 2, 3, 4, and 6a-6c were synthesized
following literature procedures.36,39-41 CuI and cis-[Pt(PEt3)2-
Cl2] were purchased from Aldrich Co. and used as received.
Pd(PPh3)2Cl2 was obtained from Strem Chemical Company,
and trimethylsilylacetylene was obtained from GFS Chemicals.
General Procedure for Sonogashira Coupling. A solution of
1.0 equiv of compounds 6a-6c, 1.2 equiv of trimethylsilylace-
tylene, Pd(PPh3)2Cl2 (3 mol %), and CuI (6 mol %) in a mixture
of THF/Et3N (v/v = 4/1) was degassed for 15 min and then
heated at 65 °C for 12 h. The solvent was then removed,
affording a brownish residue. The crude product was purified
by flash chromatography (silica gel, hexane). The typical yield is
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Schanze, K. S. Inorg. Chem. 2007, 46, 6483–6494.
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W.; Cheah, K. W. Inorg. Chem. 2009, 48, 2855–2864.
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2477–2488.
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Schanze, K. S. J. Am. Chem. Soc. 2002, 124, 12412–12413.
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Moore, M. H.; Pollack, S. K.; Shashidhar, R. J. Am. Chem. Soc.
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Commun. 2006, 1887–1889.
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Cheung, K. Y.; Yip, C. T.; Ng, A. M. C.; Xi, Y. Y.; Mak, C. S. K.;
Chan, W. K. J. Am. Chem. Soc. 2007, 129, 14372–14380.
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A. K. Y. Chem. Mater. 2008, 20, 5734–5736.
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1
86%-95%. Compound 7a: H NMR (400 MHz, CDCl3, δ):
7.22 (dd, J = 5.11, 1.02 Hz, 1H), 7.16 (dd, J = 3.61, 1.00 Hz,
1H), 7.12 (d, J = 3.80 Hz, 1H), 7.03-6.98 (m, 2H), 0.24 (s, 9H).
13C NMR (100 MHz, CDCl3, δ): 138.848, 136.656, 133.423,
127.906, 125.005, 124.236, 123.240, 121.751, 99.900, 97.368,
1
-0.153. Compound 7b: H NMR (400 MHz, CDCl3, δ): 7.31
(dd, J = 5.10, 0.97 Hz, 1H), 7.25 (dd, J = 3.60, 0.98 Hz, 1H),
7.17 (d, J = 3.83 Hz, 1H), 7.16-7.12 (m, 2H), 7.10-7.06 (m,
2H), 0.29 (s, 9H). 13C NMR (100 MHz, CD2Cl2, δ): 138.478,
136.977, 136.726, 135.162, 133.579, 128.007, 125.007, 124.895,
124.409, 124.009, 123.321, 121.824, 100.317, 97.071, -0.24.
Compound 7c: 1H NMR (400 MHz, CDCl3, δ): 7.22 (dd, J =
5.10, 1.07 Hz, 1H), 7.17 (dd, J = 3.60, 1.04 Hz, 1H), 7.12 (d, J =
3.83 Hz, 1H), 7.09-7.04 (m, 4H), 7.02 (dd, J = 5.10, 3.63 Hz,
1H), 6.99 (d, J = 3.83 Hz, 1H), 0.25 (s, 9H).
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Bongini, A. J. Org. Chem. 2004, 69, 4821.
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cycles 1987, 26, 1793–1796.

Article
General Procedure for Deprotection of Trimethylsilyl (TMS)
Chem. Mater., Vol. 22, No. 7, 2010 2327
2H), 6.87 (d, J = 3.82 Hz, 2H), 6.70 (d, J = 3.69 Hz, 2H), 2.11 (m,
24H), 1.22-1.11 (m, 36H); 31P NMR (202 MHz, CD2Cl2, δ): 10.65,
JPt-P = 2336 Hz; mp 261 °C; FAB MS (m/z) [MþH]þ Calcd
for C62H76N2P4Pt2S7, 1916.3; Found 1917.3; Anal. Calcd for
C78H80N2P4Pt2S11: C 48.89, H 4.42, N 1.46, S 18.41; Found: C
48.77, H 4.23, N 1.40, S 18.55.
Group. A solution of 1.0 equiv of compounds 7a-7c in THF/
MeOH (v/v = 3/1) was added 1.1 equiv of K2CO3. The mixture
was stirred at room temperature for 1 h. After removal of the
solvent, the crude product was loaded on a short plug of silica
gel (∼5 cm) and flushed with a large amount of hexane. The
pure products were obtained as yellow-orange solids upon the
evaporation of hexane. Note that these compounds quickly
changed to dark brownish solids upon storage for even a short
period of time. Thus, they were obtained a few hours prior to
X-ray Crystallography. Single crystals of BTD-Pt-T2 were
grown via the slow diffusion of hexane into dichloromethane
solutions. All measurements were made on a Bruker Model
SMART 1000 CCD area detector with graphite monochro-
1
ꢀ
˚
use, and the typical yield is >90%. Compound 8a: H NMR
mated Mo K (λ = 0.71073 A) radiation. The structure was
solved by direct methods (SHELXS-97). The crystal structure of
BTD-Pt-T2 consists of two molecules of the compound packed
in a triclinic unit cell with no symmetry relating them. The unit-
cell parameters are given as follows: triclinic, space group P1, a =
(400 MHz, CD2Cl2, δ): 7.26 (d, J = 4.99 Hz, 1H), 7.29-7.19 (m,
2H), 7.15-7.04 (m, 2H), 3.45 (s, 1H). Compound 8b: 1H NMR
(400 MHz, CD2Cl2, δ): 7.28 (d, J = 5.03 Hz, 1H), 7.21 (dd, J =
5.81, 3.92 Hz, 2H), 7.12 (dd, J = 3.75, 3.58 Hz, 2H), 7.06 (dd,
J = 4.94, 4.90 Hz, 2H), 3.49 (s, 1H). Compound 8c: 1H NMR
(400 MHz, CDCl3, δ): 7.28 (d, J = 5.17 Hz, 1H), 7.23 (d, J =
3.45 Hz, 2H), 7.17-7.10 (m, 4H), 7.07 (dd, J = 6.49, 4.34 Hz,
2H), 3.46 (s, 1H).
˚
˚
˚
14.106 (1) A, b = 15.157 (1) A, c = 15.541 (2) A, R = 97.079(1)°,
3
˚
β = 96.718(1)°, γ = 90.344(1)°, V = 3274.2(5) A , Z = 2.
Device Fabrication. All devices were fabricated on indium tin
oxide (ITO)-coated glass substrates (prepatterned, resistivity of
R = 20 Ω-1, Thin Film Devices, Inc.). A thin layer (30-40 nm)
of PEDOT:PSS (Clevios PH) was spin-coated onto the ITO glass
at 4000 rpm for 40 s and then baked at 140 °C for 15 min in air.
The photoactive layer containing the oligomer and fullerene
in different ratios was spin-cast from chloroform (10 mg/mL)
after passing through a 0.45 μm polytetrafluoroethylene filter.
The thickness for all devices, measured with a Dektak profil-
ometer, was ∼70 nm. An Al cathode (100 nm) was then
thermally evaporated under vacuum (∼10-7 torr) through a
shadow mask defining an active device area of ∼0.03 cm2. The
current-voltage (J-V) curves were measured using a Keithley
Model 236 source-measure unit under AM 1.5 G solar illumination
at 100 mW cm-2 (1 sun), using a Thermal-Oriel 300 W solar
simulator. A set of 16 devices were tested for each experimental
condition (concentration, blend ratio, annealing temperature, and
time), and experiments were repeated multiple times, to check the
reproducibility of the data. External quantum efficiency (EQE)
values were obtained with a monochromator and calibrated with a
silicon photodiode. The hole mobility was measured using a SCLC
model,42 and the device structure was ITO/PEDOT:PSS/Pt-
oligomer/Au. The zero-field charge mobility was obtained by
fitting the experimental data using the method reported in ref 42
and the values reported are the average of 5-8 devices.
Compound 5. 4,7-Bis(5-ethynylthiophen-2-yl)benzo[c][1,2,5]-
thiadiazole (4) (0.4 g, 1.15 mmol) in a mixture of THF/Et2NH
(50 mL/50 mL) was degassed for 30 min, then cis-[Pt(PEt3)2Cl2]
(1.27 g, 2.53 mmol) was added in one portion. The reaction flask
was wrapped in aluminum foil and stirred at room temperature
(rt) for 40 h. After evaporation of the solvent, the purple residue
was loaded on a silica column and washed first with CH2Cl2/
hexane (1/2), then CH2Cl2/hexane (1/1), and finally CH2Cl2.
The purple band was collected affording the title compound as a
1
purple solid (1.1 g, 75%). H NMR (500 MHz, CD2Cl2, δ):
7.93-7.86 (d, J = 3.82, 2H), 7.69 (s, 2H), 6.89-6.81 (d,
J = 3.81, 2H), 2.02-1.97 (m, 24H), 1.18-1.11 (m, 36H); 13C
NMR (125 MHz, CD2Cl2, δ): 152.52, 136.25, 131.11, 128.65,
127.39, 125.20, 124.86, 94.31, 93.07, 14.56, 7.77; 31P NMR (202
MHz, CD2Cl2, δ): 14.17, JPt-P = 2359 Hz.
BTD-Pt-T2, BTD-Pt-T3, and BTD-Pt-T4. A mixture of com-
pound 5 and the corresponding oligothiophene alkynyl ligands
in THF/Et2NH (2/1, v/v) was degassed for 30 min. A catalytic
amount of CuI (10 mol %) was then added and the resulting
mixture was stirred at rt for 36 h. After removal of the solvent,
the crude product was purified by flash chromatography (silica
gel, CH2Cl2/hexane 1:1, 2:1, then CH2Cl2). Further recrystalli-
zation of the products by vapor diffusion of hexane into CH2Cl2
solutions yielded crystalline products in good yields (50%-
81%). BTD-Pt-T2: 1H NMR (500 MHz, CD2Cl2, δ): 7.92
(d, J= 3.85 Hz, 2H), 7.70 (s, 2H), 7.12 (dd, J = 5.10, 0.86 Hz,
2H), 7.05 (d, J = 3.55 Hz, 2H), 6.93 (dd, J = 5.06, 3.63 Hz, 2H),
6.90 (d, J= 3.69 Hz, 2H), 6.87 (d, J= 3.83 Hz, 2H), 6.69 (d, J=
3.68 Hz, 2H), 2.15-2.06 (m, 24H), 1.21-1.13 (m, 36H); 31P
NMR (202 MHz, CD2Cl2, δ): 10.62, JPt-P = 2329 Hz; melting
point (mp) 234 °C; FAB MS (m/z) [M]þ Calcd for C62H76N2-
P4Pt2S7, 1587.8; Found 1588.4; Anal. Calcd for C62H76N2-
P4Pt2S7: C 46.90, H 4.82, N 1.75, S 14.14; Found: C 47.02, H
4.82, N 1.75, S 14.03. BTD-Pt-T3:1H NMR (400 MHz, CD2Cl2, δ):
7.97 (d, J = 3.75 Hz, 2H), 7.75 (s, 2H), 7.23 (d, J = 4.68 Hz, 2H),
7.17 (d, J = 2.72 Hz, 2H), 7.06 (d, J = 3.69 Hz, 2H), 7.01 (t, J =
4.22 Hz, 4H), 6.96 (d, J = 3.64 Hz, 2H), 6.92 (d, J = 3.73 Hz, 2H),
6.75 (d, J= 3.58 Hz, 2H), 2.82-1.72 (m, 24H), 1.34-1.09 (m, 36H);
31P NMR (162 MHz, CD2Cl2, δ): 11.94, JPt-P = 2334 Hz; mp
262 °C; FAB MS (m/z) [M]þ Calcd for C62H76N2P4Pt2S7, 1752.0;
Found 1752.5; Anal. Calcd for C70H80N2P4Pt2S9: C 47.99, H
4.60, N 1.60, S 16.47; found: C 48.31, H 4.54, N 1.56, S 16.74.
BTD-Pt-T4: 1H NMR(500 MHz, CD2Cl2, δ): 7.92 (d, J= 3.74 Hz,
2H), 7.71 (s, 2H), 7.19 (d, J= 5.14 Hz, 2H), 7.14 (d, J= 3.44 Hz,
2H), 7.08-7.00 (m, 6H), 6.97 (t, J= 4.82, 4H), 6.92 (d, J= 3.68 Hz,
Instrumentation. 1H, 13C, and 31P nuclear magnetic reso-
nance (NMR) spectra were recorded in deuterated solvents
such as CDCl3 or CD2Cl2, using Bruker Model AVB 400 and
Bruker Model DRX 500 spectrometers. UV-vis absorption
spectra were recorded at room temperature using a Carey
Model 50 Conc UV-visible spectrophotometer. Fluorescence
spectra were recorded on a SPEX Model Fluorolog II spectro-
meter. CV experiments were collected using a Solartron
Model 1285 potentiostat under the control of CorrWare II
software. A conventional three-electrode cell with gold working
electrodes 2 mm in diameter, a silver wire pseudo-reference
electrode (calibrated vs Fc/Fcþ), and a platinum wire coun-
terelectrode was purged with nitrogen and maintained under
a nitrogen atmosphere during all measurements. CH2Cl2 was
distilled over CaH2 prior to use and tetrabutyl ammonium
tetrafluoroborate (TBABF4) (0.1 M) was used as the support-
ing electrolyte. Tapping-mode atomic force microscopy (AFM)
was performed on a Veeco Nanoscope V scanning probe
microscope.
(42) Mihailetchi, V. D.; Xie, H. X.; de Boer, B.; Koster, L. J. A.; Blom,
P. W. M. Adv. Funct. Mater. 2006, 16, 699–708.

2328 Chem. Mater., Vol. 22, No. 7, 2010
Scheme 1. Synthesis of the Platinum-Acetylide Oligomers
Zhao et al.
Results and Discussion
structure of oligomer BTD-Pt-T2. The oligomer crystal-
lizes in a triclinic crystal system, and there are two
molecules within the unit cell with no symmetry between
them. Figure 1a shows the perspective drawing of BTD-
Pt-T2. The two Pt atoms have an approximately square
planar geometry, as typically observed for platinum-
acetylide oligomers.44 The CꢀC bond lengths are in the
Synthesis and X-ray Crystallography. These symmetri-
cal dinuclear platinum(II) oligomers were synthesized in a
convergent way, as outlined in Scheme 1. The core unit (3)
based on thienyl-benzothiadiazole-thienyl (BTD) was
synthesized following literature procedures.36 Because
of the electron-donating property of thiophene and the
strong electron-accepting character of benzothiadiazole,
this tricyclic push-pull conjugated system shows a large
charge transfer from the thiophene rings to the central
unit.43 After cleavage of the trimethylsilyl-protecting
groups using K2CO3, compound 4 was then reacted with
cis-[Pt(PEt3)2Cl2] in a mixture of THF/Et2NH at room
temperature for 12 h to afford the chloroplatinum(II)
complex (5) in 75% yield. The desired oligomers, BTD-Pt-
T(n), were then obtained in good yields by coupling 5
with different oligothiophene-based alkynyl ligands (8a, 8b,
and 8c) in the presence of catalytic amount of CuI. These
compounds were characterized by 1H, 31P NMR spectros-
copy, fast-atom bombardment-mass spectroscopy (FAB-
MS), and elemental analysis. The 31P NMR spectra of
compounds 5 and BTD-Pt-T(n) all show a single signal,
which confirms the highly symmetrical structure of these
˚
range of 1.151(12)-1.243(10) A, and the CꢀC bond
˚
between the central unit and the Pt (1.205(10)-1.243(10) A)
is slightly longer than that between the thiophene arms
˚
and the Pt center (1.151(12)-1.179(10) A), suggesting
a more pronounced electronic interaction between the
Pt(II) and the central unit. The close S N contacts
3 3 3
˚
˚
(S(1)-N(1)=3.054 A, S(3)-N(2)=2.963 A), which are
significantly shorter than the van der Waals radii of
the two interacting atoms, force the central unit to be
almost planar, leading to enhanced absorption. The tor-
sion angles between the central benzothiadiazole unit and
the two neighboring thiophene rings are -19.3(11)°
(S(1)-C(6)-C(7)-C(12)) and -9.0(11)° (S(3)-C(13)-
C(10)-C(11)). Although the steric requirement of the
trialkyl ligand on the metal center makes it difficult to
achieve a close packing of the platinum-acetylide oligo-
mers, examination of the crystal packing of oligomer BTD-
Pt-T2 reveals edge-to-face intermolecular interactions, as
shown in Figure 1b. The distance is in the range of
oligomers. The observed platinum satellites with JPt-P
2300 Hz in the 31P NMR spectra of these compounds are
characteristic of a trans-P-Pt-P configuration.
To further characterize the structure of these plati-
num-acetylide oligomers, we obtained the single-crystal
≈
45
˚
2.890-3.287 A, which is typical of CH-π interactions.
In analogy to many oligothiophenes for which edge-to-face
(44) Battocchio, C.; D’Acapito, F.; Fratoddi, I.; Groia, A.; Polzonetti,
G.; Roviello, G.; Russo, M. V. Chem. Phys. 2006, 328, 269–274.
(45) Nishio, M. CrystEngComm 2004, 6, 130–158.
(43) Karikomi, M.; Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Am.
Chem. Soc. 1995, 117, 6791–6792.

Article
Chem. Mater., Vol. 22, No. 7, 2010 2329
Figure 1. (a) OakRidge Thermal Ellipsoidal plot(ORTEP) diagramof BTD-Pt-T2, with thermal ellipsoids drawn at50% probability level. One ofthe two
molecules in the unit cell is shown, and H atoms have been omitted for clarity. (b) Intermolecular interactions in the crystals. (c) View down the long axis of
the linear molecule.
interactions are favored,46 oligomer BTD-Pt-T2 packs in a
herringbone geometry, as shown in the packing diagram in
Figure 1c. This particular two-dimensional arrangement of
BTD-Pt-T2, likely similar for the other two oligomers, could
be potentially beneficial for photovoltaic applications com-
pared to strong π-π interactions, which usually favor one-
dimensional in-plane charge transport.47
Optical and Electrochemical Properties. Figure 2 shows
the absorption spectra for the thin films of BTD-Pt-T(n)
spin-coated on quartz substrates from chloroform solu-
tions (10 mg/mL) and then annealed at 70 °C for 30 min.
All oligomers exhibit a broad and structureless absorp-
Figure 2. Absorption spectra of BTD-Pt-T(n) thin films after annealing
at 70 °C for 30 min.
tion band centered at 570 nm with a molar absorbance of
ca.104 cm-1, characteristic of the intramolecular charge-
transfer (ICT) transition. The absorption band at short
wavelength corresponding to a πfπ* transition shows
the expected dependence on the length of the oligothio-
phene “arms”. Increasing the oligothiophene length
(2T, 3T to 4T) leads to a significant red-shift of the
short-wavelength absorption from 385 nm for BTD-Pt-
T2, to 415 nm for BTD-Pt-T3, and 430 nm for BTD-Pt-
T4, while leaving the long-wavelength absorption peak
unchanged. A similar trend was observed in the absorp-
tion spectra for the series of oligomers in solution and in
preannealed thin films. Comparing the solution absorption
spectra (see Figure S1 in the Supporting Information), there
is a pronounced red-shift of ∼30 nm for the long-wave-
length absorption on thin films of all the oligomers,
indicating strong intermolecular interactions. A gentle
thermal treatment led to a slight enhancement of long-
wavelength absorbance and the evolution of a weak
shoulder in the absorption band. To understand the effect
of oligothiophene length on the optical properties, we
performed time-dependent density function theory (TD-
DFT) calculations for BTD-Pt-T2. Based on the calcula-
tion results, the origins of the two distinct absorption
bands at long wavelengths and short wavelengths were
assigned to the HOMO fLUMO and HOMO fLUMO þ 1
transitions, respectively. As illustrated from the contour
surface of molecular orbitals (see Figure S2 in the Sup-
porting Information), the HOMO of oligomer BTD-Pt-
T2 is mainly delocalized over the central core with
some contribution from the d orbitals of Pt and the
LUMO is highly localized on the benzothiadiazole unit.
In contrast, the LUMO þ 1 orbital is delocalized over the
ꢀ
(46) Murphy, A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107, 1066–1096.
(47) Sirringhaus, H. Adv. Mater. 2005, 17, 2411–2425.

2330 Chem. Mater., Vol. 22, No. 7, 2010
Zhao et al.
Table 1. Summary of Physical Properties of BTD-Pt-T(n)
ox
onset
red
onset
f
Eopt (eV)a
E
(V)b
E
(V)b
HOMO (eV)c
LUMO (eV)d
Eg (eV)e
μ )
hole (cm2 V-1 s-1
BTD-Pt-T2
BTD-Pt-T2
1.9
1.9
0.19
0.66
-1.75
-1.75
-5.29
-5.29
-3.35
-3.35
1.94
1.94
2.7 ꢁ 10-4
1.3 ꢁ 10-4 g
BTD-Pt-T3
BTD-Pt-T3
1.9
1.9
0.18
0.51
-1.72
-1.72
-5.28
-5.28
-3.38
-3.38
1.9
1.9
2.5 ꢁ 10-4
1.5 ꢁ 10-4 g
BTD-Pt-T4
BTD-Pt-T4
1.9
1.9
0.19
0.48
-1.72
-1.72
-5.29
-5.29
-3.38
-3.38
1.91
1.91
9 ꢁ 10-5
4.3 ꢁ 10-5 g
a Estimated from the absorption edge of BTD-Pt-T(n) thin films. b Determined by cyclic voltammetry and calculated with reference to ferrocene
ox
onset
red
onset
(5.1 eV vs vacuum). c EHOMO = -(E
þ 5.1) eV. d EHOMO = -(E
þ 5.1) eV. e Eg = -(EHOMO - ELUMO). f Zero-field charge mobility calculated
according to the SCLC model.42 g Annealed at 70 °C for 30 min.
oligothiophene alkynyl ligands with only minor contribu-
tions from the central unit, which explains the depen-
dence of the short-wavelength absorption band on the
oligothiophene length. In addition to the red-shift of the
high-energy absorption band, increasing the oligothio-
phene length also leads to a continuous increment of
absorbance for the longer-wavelength absorption, which
is likely due to the addition of red-edge absorption of the
short wavelength transition to the ICT transition. Both
absorption spectroscopy and TD-DFT calculation have
suggested that, in the current system, increasing the
length of oligothiophene does not shift the long-wave-
length ICT transition, but instead leads to considerable
broadening of the absorption by reducing the energy gap
for the HOMO f LUMOþ1 transition. Because the opti-
cal bandgap (Eopt) of conjugated systems is usually
determined from the onset of the long-wavelength absor-
ption, all oligomers exhibit a band gap of 1.9 eV, based on
their film absorption.
To further study the effect of “arm” length on the
electrochemical properties of these oligomers, cyclic vol-
tammetry (CV) was performed in anhydrous CH2Cl2
solution with 0.1 M TBABF4 as the supporting electro-
lyte. The potentials are reported relative to an internal
Fcþ/Fc standard and summarized in Table 1. The cyclic
voltammograms of BTD-Pt-T(n) are shown in Figure S3
in the Supporting Information. All oligomers exhibited a
reversible cathodic wave with an onset at approximately
-1.70 V, corresponding to one electron reduction of the
core unit, which is consistent with the data reported for
oligomeric and polymeric systems containing a similar
core unit.48 In contrast, all oligomers showed a similar
irreversible anodic wave with an onset at ∼0.20 V,
corresponding to the oxidation of the core unit and a
second anodic wave with the onset decreasing with the
oligothiophene length. Subsequent scans induced a gra-
dual increase in current response for the second oxidation
(see Figure S3 (insets) in the Supporting Information) and
the formation of a dark film on the gold electrode for all
oligomers resulting from electropolymerization of the
platinum-acetylide oligomers at higher voltage. These
results further confirm that varying the length of oligo-
thiophene “arms” does not affect the HOMO and LUMO
levels, which are determined from the first reduction and
Figure 3. Tapping-mode AFM images of thin films before annealing ((a)
BTD-Pt-T2, rms = 0.4 nm; (c) BTD-Pt-T3; rms = 0.5 nm; (e) BTD-Pt-
T4, rms = 1.4 nm) and after annealing((b) BTD-Pt-T2, rms = 7.6nm; (d)
BTD-Pt-T3, rms = 5.6 nm; (f) BTD-Pt-T4, rms = 14.3 nm). Image size is
2 μm ꢁ 2 μm.
oxidation to be -5.3 eV and -3.4 eV, respectively, for all
oligomers (see Table 1).
Self-Assembly and Charge Mobility. One important
feature of these platinum-acetylide oligomers is the
incorporation of alkynyl ligands based on oligothio-
phenes, which have been well-known for their self-assem-
bling properties.49 Because the self-assembly of electro-
active molecules has a strong effect on their optical and
electronic properties,49 we have examined the morphol-
ogy of thin films cast from all oligomers using atomic
force microscopy (AFM). Figure 3 shows the AFM
topographic images of thin films of BTD-Pt-T2 with
and without thermal treatment. The surface of the pris-
tine films of BTD-Pt-T2 is quite smooth, showing an
(48) Kitamura, C.; Tanaka, S.; Yamashita, Y. Chem. Mater. 1996, 8,
570–578.
(49) Mishra, A.; Ma, C. Q.; Bauerle, P. Chem. Rev. 2009, 109, 1141–
1276.

Article
Chem. Mater., Vol. 22, No. 7, 2010 2331
Table 2. Summary of Device Performance for BTD-Pt-T(n)
active layer (wt:wt = 1:4)
Voc (V)
short-circuit current density, Jsc (mA cm-2
)
fill factor, FF
photoconversion efficiency, PCE (%)
BTD-Pt-T2:PC61BM
BTD-Pt-T2:PC71BM
BTD-Pt-T3:PC61BM
BTD-Pt-T3:PC71BM
BTD-Pt-T4:PC61BM
BTD-Pt-T4:PC71BM
0.73
0.71
0.78
0.82
0.72
0.73
-4.3
-7.91
-5.4
-8.45
-5.3
-7.66
0.39
0.42
0.39
0.43
0.39
0.40
1.4
2.3
1.7
3.0
1.5
2.2
average root-mean-square (rms) roughness of 0.4 nm,
0.5 nm for BTD-Pt-T3, and 1.4 nm for BTD-Pt-T4. Large
crystalline domains of BTD-Pt-T2, on the order of
∼200 nm in size, were formed upon annealing at 70 °C,
presumably through CH-π interactions, as observed in
the single-crystal structure. Accordingly, the rms roughness
of the films increased significantly from 0.4 nm to 7.6 nm.
Similarly, particulate-shaped crystalline domains formed
for other complexes upon annealing, though with smaller
size, ∼70 nm for BTD-Pt-T3 and ∼130 nm for BTD-Pt-
T4. Compared with thin films without thermal treatment,
a considerable increase of the rms roughness after anneal-
ing was also observed for both BTD-Pt-T3 and BTD-Pt-
T4. The crystalline order in the neat films was also probed
by two-dimensional grazing-incidence X-ray scattering
(2D-GIXS) measurements (see Figure S4 in the Support-
ing Information). In agreement with AFM studies, the
pristine films of all the oligomers showed no observable
feature in the GIXS patterns, hence supporting the amor-
phous nature of these films. Upon thermal treatment at
70 °C for 30 min, concentric Debye rings with crescent peaks
in the z-direction appeared in the GIXS patterns, suggesting
that thermal annealing induced the formation of highly
ordered crystalline domains in the films. In addition, the
slightly stronger intensity of the peaks in the z-direction
indicates that there is some degree of long-range ordering,
with respect to the substrate in the annealed films.
Because charge mobility is strongly dependent on the
molecular packing and film morphology, we have mea-
sured the hole mobilities of these platinum-acetylide
oligomers using the hole-only devices of the following
structure: ITO/PEDOT:PSS/BTD-Pt-T(n)/Au. The mo-
bilities calculated using the SCLC model42 are summar-
ized in Table 1. These values are not affected significantly
by the oligothiophene length, likely because the holes are
preferably localized on the core unit, as suggested by CV
experiments. Hole mobility values on the order of
10-5-10-4 cm2 V-1 s-1 were obtained for preannealed
films of BTD-Pt-T(n), which are higher than those of
corresponding platinum-containing polymeric systems.36
The general trend of decreased hole mobility upon an-
nealing observed for all oligomers suggests that grain
boundaries are detrimental to hole transport, regardless
of molecular packing within the crystalline domains.
Photovoltaic Devices. The broad light absorption, self-
assembly into ordered domains, and good hole mobility
of these platinum-acetylide oligomers support their use
as electron donors in OPVs. Bulk heterojunction (BHJ)
devices were fabricated with PC61BM or PC71BM as the
acceptor in a simple device structure of ITO/PEDOT:PSS
(30 nm)/blend (∼70 nm)/Al. Since the blend ratio is key to
device performance, we have carefully examined the
effect of blend ratio of BTD-Pt-T3 with PC61BM/
PC71BM in a very broad range from 2:1 (wt:wt) to 1:9
(wt:wt). The best performance was obtained for devices
with a 1:4 weight ratio of BTD-Pt-T3:PCBM. We also
investigated different thermal annealing conditions10 to
optimize the device performance, and the optimal results
were obtained by performing the thermal treatment at 70 °C
for 30 min after electrode evaporation. All oligomers
were evaluated under standard AM 1.5G solar illumina-
tion in devices with a 1:4 weight ratio, using the optimized
annealing conditions. Table 2 summarizes the various
device characteristics (short-circuit current density (Jsc),
open circuit voltage Voc, fill factor (FF), and photocon-
version efficiency (PCE)). These photovoltaic (PV) cells
containing different platinum-acetylide oligomers gave
a similar Voc of 0.71-0.82 V, because of the fact that
the HOMO remains on the core unit, regardless of the
oligothiophene length. Among these new compounds,
BTD-Pt-T3 showed the best device performance. Figure
4a shows the current density versus voltage (J-V) char-
acteristics of devices of BTD-Pt-T3/PC61BM and BTD-
Pt-T3/PC71BM. Short-circuit current densities of 5.4 mA
cm-2 and 8.45 mA cm-2 have been achieved for BTD-Pt-
T3/PC61BM and BTD-Pt-T3/PC71BM, respectively.
Combined with high fill factors (FF = 0.39 for PC61BM
device and 0.43 for PC71BM device), these devices gave
power conversion efficiencies of 1.7% with PC61BM and
3.0% with PC71BM. Figure 4b shows the external quan-
tum efficiencies (EQEs) for these devices across the absor-
ption range of the photoactive materials. High EQEs of
40% and 50% at 440 nm have been achieved for the
PC61BM and PC71BM devices, respectively. Importantly,
for the device with PC61BM, which has a very low molar
absorbance and, therefore, does not contribute to the
light absorption in the range of 350-700 nm, the EQE
spectrum correlates very well with the absorption spec-
trum of BTD-Pt-T3 film. This indicates that the oligo-
thiophene “arms” also contribute largely to the photo-
current, despite the fact that the HOMO and LUMO
orbitals are localized on the core unit, as shown by TD-
DFT calculations and CV experiments. Compared to the
PC61BM device, the improved performance for PC71BM
device is mainly due to the large increment of the short-
circuit current, which could be attributed to the better
absorption properties of PC71BM in the visible range, as
evident from the EQE spectrum.50 Although BTD-Pt-T4
(50) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.;
Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem.,
Int. Ed. 2003, 42, 3371–3375.

2332 Chem. Mater., Vol. 22, No. 7, 2010
Zhao et al.
thienyl-benzothiadiazole-thienyl core and oligothio-
phene alkynyl ligands. The absorption spectra of these
oligomers are significantly broadened by tuning the
length of the oligothiophene alkynyl ligands, thus enhan-
cing the overlap of the absorption of these materials with
the solar spectrum. In addition to the light-absorbing
properties, incorporation of the oligothiophene “arms”
enables the solid state intermolecular packing of these
platinum-acetylide oligomers via edge-to-face interac-
tions. The use of these molecules as electron donor was
demonstrated in bulk heterojunction (BHJ) devices with
either PC61BM or PC71BM as electron acceptors in a
simple ITO/PEDOT:PSS/blend/Al device structure.
Solution-processed photovoltaic cells show bicontinuous
morphology, which is optimal for charge separation and
high power conversion efficiencies (PCEs). Some of these
devices show high PCEs of up to 3% and a peak external
quantum efficiency to 50% under AM 1.5 simulated solar
illumination. Our work suggests that solution-processa-
ble platinum-acetylide oligomers with broad light-
absorbing and self-assembly properties can lead to highly
efficient organic photovoltaic (OPV) devices. It is
expected that further improvement in device efficiency
can be achieved by fine-tuning the optical absorption,
redox, and solid-state packing properties of this class of
materials.
Figure 4. (a) Current density-voltage (J-V) of PV devices of BTD-Pt-
T3 under AM 1.5 G, 100 mW cm-2. (b) EQE spectra.
exhibits the broadest absorption and highest molar ab-
sorbance, its poor solubility in chloroform has limited the
photovoltaic performance. We have also investigated the
surface morphology of devices processed from 1:4 blend
of different oligomers with PC61BM. As shown in the
AFM topographical images (see Figures S5a-S5c in the
Supporting Information), despite the tendency of these
oligomers to form crystalline domains, the surface of the
blends remains smooth, with an average rms roughness of
∼0.3 nm. In the phase images (see Figures S5d-S5f in the
Supporting Information), two distinct domains (∼20 nm)
with fiberlike structures are observed, likely correspond-
ing to oligomers and PC61BM. It is believed that these
bicontinuous domains provide percolated pathways for
holes and electrons to be efficiently transported through
the blend and subsequently collected at the opposite
electrodes, thus ensuring the large photocurrent and high
overall PCE efficiency.
Acknowledgment. This work was supported by the Direc-
tor, Office of Science, Office of Basic Energy Sciences,
Materials Sciences and Engineering Division, of the U.S.
Department of Energy (under Contract No. DE-AC02-
05CH11231). GIXS measurements were performed at the
Stanford Synchrotron Radiation Laboratory, a national
user facility operated by Stanford University on behalf of
the U.S. Department of Energy, Office of Basic Energy
Sciences. We also acknowledge the use of the Molecular
Graphics and Computing Facility at UC, Berkeley which
was supported by a National Science Foundation Grant
(No. CHE-0233882).
Supporting Information Available: X-ray crystallography file
(CIF), and figures showing absorption spectra of all oligomers
in solution and thin films, time-dependent density function
theory (TD-DFT) calculation of BTD-Pt-T2, cyclic voltamme-
try (CV) of all the oligomers, two-dimensional grazing-inci-
dence X-ray scattering (2D-GIXS) patterns of neat films, and
AFM images of platinum-oligomers/PC61BM blends (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
Conclusions
In summary, we have designed and synthesized a series
of crystalline platinum-acetylide oligomers containing a