Low band gap dithieno[3,2-b:2′,3′-d]silole-containing polymers, synthesis, characterization and photovoltaic application

Article abstract of DOI:10.1039/b910443g

A series of low band gap silole-containing polymers were synthesized with different alkyl side chains and a power conversion efficiency (PCE) of 3.43% was obtained.

Full text of DOI:10.1039/b910443g

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Low band gap dithieno[3,2-b:20,30-d]silole-containing polymers, synthesis,
characterization and photovoltaic applicationw
Lijun Huo, Hsiang-Yu Chen, Jianhui Hou,* Teresa L. Chen and Yang Yang*
Received (in Berkeley, CA, USA) 28th May 2009, Accepted 27th July 2009
First published as an Advance Article on the web 18th August 2009
DOI: 10.1039/b910443g
A series of low band gap silole-containing polymers were
synthesized with different alkyl side chains and a power
conversion efficiency (PCE) of 3.43% was obtained.
level) at the cost of a lower open-circuit voltage (Voc),
however, silole-containing polymers with higher LUMO levels
(low-lying level) have been proven to reduce the bandgap
mostly by lowering the LUMO level.6 If a sufficient energy
level offset (0.3 B 0.5 V) between the LUMO level of donor
and the LUMO of PCBM is ensured for silole-containing
polymers, a low bandgap and low-lying HOMO energy level
and higher Voc will be realized simultaneously. In addition,
silole-containing polymers applied in photovoltaic devices
have also proved to be effective in improving hole mobility
and rendering higher efficiencies compared to its analog
without the silole unit.1f,7 Hence, silole-containing conjugated
polymers have been used in many photovoltaic devices
recently.7,8 For examples, by replacing the carbon atoms on
the 9-position of the fluorene units of PFDTBT with silicon
atoms, a higher efficiency of 5.4% was obtained from
poly[(2,7-dioctylsilafluorene)-2,7-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-
benzothiadiazole)-5,50-diyl] (PSiFDTBT).7 Also, when
PCPDTBT was modified by replacing the carbon atoms on
the 7-position of the cyclopenta-[2,1-b;3,4-b0]dithiophene with
silicon atoms to obtain poly[(4,40-bis(2-ethylhexyl)dithieno-
[3,2-b:20,30-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl]
(PSBTBT), the efficiency also improved from 3.2% to 5.1%.1f
Although the absorption spectra of the former is similar to
that of the latter, the substitution of the silicon atom in
PSBTBT improves the hole mobility to three times the value
of that of PCPDTBT. All these research results inspire further
development to explore new silole-containing derivatives.
To meet the requirements of good polymer photovoltaic
materials, silole-containing conjugated polymers with suitable
low-lying LUMO levels, low bandgaps, and high carrier
mobilities will be developed. A series of different alkyl
substituted dithienosilole-containing polymers, PSiDTBT6,
PSiDTBTEH and PSiDTBT12 (see Scheme 1), were synthesized
by Stille coupling reaction.
In the past two years, tremendous developments in polymer
solar cells (PSCs) have intrigued researchers to study new
conjugated polymer materials, to optimize polymer morphology,
and to investigate various physical device structures to
promote higher power conversion efficiencies (PCE).1 Among
the various types of organic photovoltaic devices investigated
so far, bulk-heterojunction solar cells, which are composed of
an interpenetrating network of donors and acceptors, have
played a leading role in realizing higher efficiencies. Until
now, efficiencies up to 4–5% have been achieved from
poly(3-hexylthiophene) (P3HT) as the donor and a soluble
fullerene derivative (C61-PCBM) as the acceptor.2 However,
the narrow absorption spectrum of P3HT ranging from
300–650 nm is one of the main reasons for the limited
efficiency of P3HT-based devices.
To overcome this problem, smaller bandgap polymers have
been used to achieve better sunlight harvesting. Although
many low bandgap photovoltaic materials have already been
designed and synthesized for this purpose, only a select
few have contributed to higher efficiencies. For example,
poly[(9,9-dialkylfluorene)-2,7-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-
benzothiadiazole)-5,50-diyl] (PFDTBT) and its derivatives
are
a well-known class of low band-gap photovoltaic
materials that give efficiencies of 2–3%.3 Another typical
low bandgap material, poly[(4,4-bis(2-ethylhexyl)-cyclopenta-
[2,1-b;3,4-b0]dithiophene)-2,6-diyl-alt-2,1,3-benzothiadiazole-
4,7-diyl] (PCPDTBT), exhibits a more red shifted absorption
edge at ca. 900 nm. The device performance based on
PCPDTBT shows a higher efficiency of 3.2%;1a One of its
derivatives, poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b]-
dithiophene-2,6-diyl-alt-4,7-bis(2-thienyl)-2,1,3-benzothiadiazole-
5,5-diyl] (PCPDTTBTT), also exhibits an efficiency of 2.1%.4
To achieve high efficiencies, an ideal donor material should
have not only a low bandgap (o1.6 eV), but also a high hole
mobility and well-matched HOMO/LUMO levels with the
acceptor such as PCBM.5
Different alkyl side chains were employed to increase the
solubility of the polymers. However, it was found that
PSiDTBT6 with two hexyl side chains was not able to be
dissolved in common organic solvents, including chloroform,
chlorobenzene, and dichlorobenzene. By replacing the hexyl
side chain with ethylhexyl and dodecyl, PSiDTBTEH and
PSiDTBT12 were synthesized, and both materials were readily
dissolved in common solvents. The corresponding weight
average molecular weight (Mw) of PSiDTBTEH is 8.0 K, with
a narrow polydispersity index (PDI) of 1.2 and that of
PSiDTBT12 is 13.3 K with a PDI of 1.5.
For many low-bandgap polymers synthesized by a D–A
strategy, most of them exhibit a lower HOMO level (high-lying
Department of Materials Science and Engineering & California
Nanosystems Institute, University of California Los Angeles,
Los Angeles, California 90095, USA. E-mail: jhhou@ucla.edu,
yangy@ucla.edu; Fax: +1-310-206-7353; Tel: +01-310-794-6491
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b910443g
Fig. 1 shows the absorption spectra of PSiDTBTEH and
PSiDTBT12 in dilute chloroform solutions (s) and solid thin
films (f). Both of the spectra from PSiDTBTEH and
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Scheme
1 Synthesis route of polymers: (i) butyllithium, THF,
ꢁ78 1C, 30 min. then DiR-SiCl2, ambient temp. 2 h; (ii) NBS, THF,
ambient temp. 2 h; (iii) butyllithium, ꢁ78 1C, 30 min; then trimethyltin
chloride, 1 h; (v) Pd(PPh3)4, toluene, reflux, 20 h.
Fig. 2 Cyclic voltammograms of the PSiDTBTEH and PSiDTBT12
films on a platinum electrode in acetonitrile solution containing
0.1 mol Lꢁ1 Bu4NPF6 at a scan rate of 20 mV sꢁ1
.
Electrochemical cyclic voltammetry has been widely
employed to investigate the redox behavior and to estimate
the HOMO and LUMO energy levels of a polymer.9 Fig. 2
shows the CV curves for PSiDTBTEH and PSiDTBT12.
Clearly, both polymers have reversible p-doping/n-doping
processes in the positive and negative scanning range. The
onset oxidation potential (Eox) is 0.19 V vs Fc/Fc+ for
PSiDTBTEH and 0.22 V for PSiDTBT12. The onset reduction
potential (Eox) is ꢁ1.63 V vs Fc/Fc+ for PSiDTBTEH and
ꢁ1.61 V for PSiDTBT12, respectively. The HOMO/LUMO
energy levels can also be obtained when ferrocene–ferrocenium
(Fc/Fc+) is used as the reference electrode (4.8 eV below the
vacuum level). The corresponding HOMO and LUMO levels
of PSiDTBTEH are ꢁ4.99 eV and ꢁ3.17 eV and that of
PSiDTBT12 are ꢁ5.02 eV and ꢁ3.19 eV, respectively. Clearly,
the EHOMO and ELUMO values of PSiDTBTEH and
PSiDTBT12 are quite similar, indicating that different
alkyl side chain lengths do not significantly affect the
HOMO/LUMO energy levels. The electrochemical band gap
(Eegc), taken as the difference between the onsets of oxidation
and reduction potentials, is equal to 1.82 eV for PSiDTBTEH
and 1.83 eV for PSiDTBT12. The electrochemical bandgap is
slightly larger than the corresponding optical bandgap due to
the interfacial barrier for charge injection.10 The hole mobility
of PSiDTBTEH was measured by the space-charge-limited
current (SCLC) method.11 The hole mobility of PSiDTBTEH
and PSiDTBT12 are 3 ꢂ 10ꢁ6 cm2 Vꢁ1 Sꢁ1 and 3.6 ꢂ 10ꢁ6 cm2,
respectively.
Fig. 1 Normalized UV-vis absorption spectra of PSiDTBTEH and
PSiDTBT12 in dilute CHCl3 solutions (s) and in solid films (f).
PSiDTBT12 show two obvious absorption peaks. The
short-wavelength absorption at ca. 400–500 nm and the long-
wavelength absorption at ca. 600–800 nm. Both of the
absorption peaks obtained from solid thin films are red-shifted
compared to those obtained in solution due to the more
aggregated configurations in solid state. For the absorbtion
peak of PSiDTBTEH in solution located at ca.630 nm, there is
a red shift of 9 nm compared to that of PSiDTBT12 in
solution. In solid thin films, similar absorption peaks of
ca. 652 nm and 644 nm were obtained for PSiDTBTEH and
PSiDTBT12, respectively, giving rise to a peak position
difference of 8 nm. At the same time, the corresponding
absoprtion edges of PSiDTBTEH and PSiDTBT12 in films
are located at ca. 810 nm and 821 nm, respectively. There is a
11 nm red shift of the onset absorption of PSiDTBT12
compared to that of PSiDTBTEH. This can be ascribed to
the different side chains present in PSiDTBT12, with two
symmetrical dedecyl side chains, and PSiDTBTEH, with two
ethylhexyl side chains. The former can form p–p* stacking
more easily and has a stronger crystalline tendency due to less
steric hindrance in the dodecyl side chains as in ethylhexyl side
chains. The optical bandgaps (Eogpt ) calculated from the
absorption edges of the PSiDTBTEH and PSiDTBT12 films
are 1.53 eV and 1.51 eV, respectively.
Polymer solar cells (PSCs) were fabricated with a structure
of ITO/PEDOT:PSS/Polymers:PCBM /Ca/Al. A polymer/
PC70BM (1 : 1, wt/wt) solution in chlorobenzene was used.
Fig. 3a shows J–V curves of the devices under illumination of
AM 1.5 G (100 mW cmꢁ2). The photovoltaic performance of
PSiDTBTEH exhibited a Voc of 0.60 V, Jsc of 9.76 mA, and
FF of 50.3%, giving rise to a power convency efficiency (PCE)
of 2.95%. Under the same conditions, PSiDTBT12 exhibited a
slightly higher Voc of 0.62 V, a higher Jsc of 10.67 mA, and an
equivalent FF of 51.8%, resulting in an overall efficiency of
3.43%. Compared to the PSiDTBTEH-based device, the
PSiDTBT12-based device exhibited better performance and
an improved efficiency by 16%. Although the molecular
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5570–5572 | 5571

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This work is supported by the Office of Naval Research
(grant No. N00014-04-1-0434, Program Manager Dr Paul
Armistead), Air Force Office of Scientific Research (grant #
FA9550-07-1-0264, Program Manager Dr Charles Lee), and
Solarmer Energy Inc.
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In conclusion, a series of low bandgap silole-containing
polymers were synthesized with different alkyl side chains.
The PSiDTBT12-based device exhibited a higher power
conversion efficiency (PCE) of 3.43% than that of the
PSiDTBTEH-based device and the overall performance of
silole-containing polymers was superior to its counterpart of
PCPDTTBTT without the silole unit.4 Considering the
sensitivity of alkyl chain length on device performance in this
series, the development of dithienosilole with different side
chain lengths could become a promising method for realizing
high efficiency polymer photovoltaic materials.
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This journal is The Royal Society of Chemistry 2009
5572 | Chem. Commun., 2009, 5570–5572