Thiophene-fused benzothiadiazole: A strong electron-acceptor unit to build D-A copolymer for highly efficient polymer solar cells

Article abstract of DOI:10.1021/cm501052a

A novel strong electron-acceptor, thieno[2,3-f]-2,1,3-benzothiadiazole-6- carboxylate (BTT), was first designed and synthesized. By introducing two thienyl groups into BTT and then copolymerizing with thienyl group substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) unit, a low band gap D-A copolymer (PBTT-TBDTT) was obtained. Compared with its polymer analogue (PBT-TBDTT) with benzothiadiazole (BT) as an acceptor, PBTT-TBDTT exhibits stronger intramolecular charge transfer. Thus, it shows much broader absorption covering almost the whole visible light region (in the range of 300-850 nm) and narrower optical band gap around 1.45 eV with a large IP (ionization potential) at 5.35 eV. The maximum efficiency of PBTT-TBDTT based device reaches 6.07% which is much higher than that of PBT-TBDTT (3.24%), indicating that BTT unit is a promising electron-acceptor moiety to construct low band gap D-A copolymers for PSCs with high photovoltaic performances.

Full text of DOI:10.1021/cm501052a

Article
pubs.acs.org/cm
Thiophene-Fused Benzothiadiazole: A Strong Electron-Acceptor Unit
to Build D−A Copolymer for Highly Efficient Polymer Solar Cells
Pengcheng Zhou,† Zhi-Guo Zhang,*,‡ Yongfang Li,‡ Xingguo Chen,*,† and Jingui Qin†
†Key Laboratory on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, China
‡Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China
S
* Supporting Information
ABSTRACT: A novel strong electron-acceptor, thieno[2,3-f]-
2,1,3-benzothiadiazole-6-carboxylate (BTT), was first designed
and synthesized. By introducing two thienyl groups into BTT
and then copolymerizing with thienyl group substituted
benzo[1,2-b:4,5-b′]dithiophene (BDTT) unit, a low band gap
D−A copolymer (PBTT-TBDTT) was obtained. Compared
with its polymer analogue (PBT-TBDTT) with benzothiadia-
zole (BT) as an acceptor, PBTT-TBDTT exhibits stronger
intramolecular charge transfer. Thus, it shows much broader
absorption covering almost the whole visible light region (in the
range of 300−850 nm) and narrower optical band gap around
1.45 eV with a large IP (ionization potential) at 5.35 eV. The
maximum efficiency of PBTT-TBDTT based device reaches
6.07% which is much higher than that of PBT-TBDTT (3.24%),
indicating that BTT unit is a promising electron-acceptor moiety to construct low band gap D−A copolymers for PSCs with high
photovoltaic performances.
absorption coefficient the D−A copolymers containing BT unit
commonly show a band gap from 1.7 to 1.9 eV, which were not
optimal for efficient sunlight harvesting due to its relatively
weak electron-withdrawing capability. In addition, the rigid
structure of BT unit without alkyl chain generally leads to low
molecular weight and poor solubility of the polymers based on
it. This can affect the device fabrication and of course the
performance of PSC devices. Thus, further structural
modification is necessary for the BT unit to reach better
device performance. Until now, much work has been done on
this and great achievements have been obtained. Thiadiazole,
quinoxaline, and triazole, for example, had been fused on the 5
and 6 positions of BT unit, respectively, affording the
corresponding acceptors of benzobisthiadiazole,13 [1,2,5]-
thiadiazolo[3,4-g] quinoxaline,14 and [1,2,5]thiadiazolo[3,4-
f ]benzotriazole.15 These acceptors exhibited much stronger
electron withdrawing ability than BT, which could potentially
reduce the band gap of resultant polymers. However, the strong
electron affinity of the acceptor units affected the charge
separation efficiency between the resultant polymer and the
fullerene derivative (such as PC61BM, (6,6)-phenyl-C61 butyric
acid methyl ester, or PC71BM, (6,6)-phenyl-C71 butyric acid
methyl ester, etc.) in the PSCs, thus leading to poor device
INTRODUCTION
■
Sunlight is a renewable and unlimited energy. The utilization of
photovoltaic devices to harvest solar energy and generate
electricity is a promising way to solve the problem of energy
shortage in the future. In recent years, polymer solar cells
(PSCs) have drawn much attention for their lightweight, low-
cost, solution-processability to form large-area and flexible
devices.1 And a number of excellent conjugated polymers with
PCEs > 8% have been reported.2−6 In order to harvest the
maximum photon flux for polymers, a narrow band gap that
absorbs a wide portion of solar spectrum, in particular photon
absorption in the near-infrared region is needed. It has been
proven that combination of an electron-rich donor (D) unit
and an electron-deficient acceptor (A) unit is a powerful
strategy in designing narrow band gap conjugated polymers.7
Thus, seeking novel efficient donor and acceptor units to
develop well-performed p-type conjugated polymers is of great
importance in bulk-heterojunction (BHJ) solar cells. So far, a
variety of good donor moieties have been designed such as
oligothiophene,8 carbazole,9 benzo[1,2-b:4,5-b′]dithiophene
(BDT),10 and dithienosilole (DTS).11 However, excellent
electron-accepting moieties such as diketopyrrolopyrrole
(DPP) and theinopyrazine enabling polymers to absorb
wavelengths over 800 nm remain relatively less.12
Due to its planar and rigid geometry, 2,1,3-benzothiadiazole
(BT) is regarded to be one of the most classical electron-
deficient acceptor units used in PSCs. However, despite its high
Received: March 19, 2014
Revised: May 11, 2014
Published: May 21, 2014
© 2014 American Chemical Society
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performances despite of their dramatically lowered band gap.
Therefore, to develop new acceptors, it is critical to find a
balance between the electron affinity and the band gap to
match the electron-donating material for PSCs application.
Here, we adopted an approach to increase the quinoid
population by fusing a thiophene ring on the BT unit (Figure
1). The BT unit is an aromatic heterocyclic compound
Scheme 1. Synthesis of Monomer M1, M3, and polymers
PBTT-TBDTT and PBT-TBDTT
a
Figure 1. Aromatic and quinoid resonance forms of BTT and BT for
comparison.
constituted of a six-member ring of benzene unit fused an
adjacent five-member ring of thiadiazole unit on its 2 and 3
positions. The six-member ring connected in the backbone has
even higher aromaticity than the five-member ring, so BT unit
is even more likely to adopt an aromatic form, and
consequently, the polymers with BT unit have relatively larger
band gap.7a Generally, the stabilized electronic quinoid state
can lead to a lower band gap that had been proven by Wudl and
his co-workers.16 Thus, reducing the aromaticity of benzene
group in the BT unit via the structural modification will allow it
a greater tendency to adopt the quinoid form through π-
electron delocalization. Recently, Yu and co-workers had
developed thieno[3,4-b]-thiophene (TT) unit by fusing a
thiophene ring with the thiophene ring connected in the
backbone, which successfully promoted quinoid population of
the thiophene unit, thereby reduced the bandgap of polymers.17
This inspired us to design a new acceptor based on BT unit. It
can be expected that fusing a thiophene ring on the 5 and 6
positions of BT unit will stabilize its quinoid structure and
further reduce its band gap.
a
Reagents and conditions: (a) NBS, Ac2O, 120 °C, 99.7%; (b) NBS,
BPO, 80 °C, 48.2%; (c) urotropin, HAc, H2O, HCl(aq), 130 °C,
29.8%; (d) ethyl 2-mercaptoacetate, K2CO3, CuO, 80 °C, 98%; (e)
LiOH·H2O, HCl(aq), 96%; (f) 2-butyloctan-1-ol, DCC, DMAP, 46%;
(g) SnCl2·2H2O, 75 °C; (h) Br2, SOCl2, Et3N, 60 °C; (i) 2-
tributylstannanylthiophene, Pd(PPh3)4, 110 °C, 87%; (j) NBS, 74%;
(k) NBS, 86%; (l) Pd(PPh3)4, toluene, 110 °C; (m) Pd(PPh3)4,
toluene, 110 °C.
EXPERIMENTAL SECTION
■
The synthetic procedures of BTT, PBT-TBDTT, and PBTT-
TBDTT are described in the Supporting Information (SI).
Scheme 1 shows the synthetic routes and approaches used for
them.
In this work, we presented the design and synthesis of a new
strong electron-acceptor, namely, thieno[2,3-f ]-2,1,3-benzo-
thiadiazole-6-carboxylate (BTT, see Scheme 1). In this
acceptor, the additional carboxyl group moiety was introduced
to further lower the HOMO energy level of BTT unit and
stabilize the quinoid structure. Meanwhile, introducing a long
alkyl chain in the ester could provide unique opportunity to
incorporate a solubilizing alkyl side-chain without disturbing
the planarity of polymer backbone. BTT is expected to be a
stronger acceptor and a more π-extended aromatic ring as
compared to BT. Thus, the incorporation of BTT into the
polymer main chain could lead to a deeper HOMO energy level
and smaller band gap (Eg). Additionally, it can also enhance
intermolecular interactions and thereby promote a strong π−π
stacking of polymeric backbones benefiting from the more rigid
structure.
As can be seen, compound 1 was synthesized by bromination
of 5-methyl-2-nitroaniline after protecting of the amino group.
Then the benzylic bromination of 1 was implemented to yield
compound 2. The key intermediate 3 was obtained via
Sommelet reaction in the presence of urotropin. Compound
4 was easily synthesized by using ethyl mercaptoacetate with
compound 3. The hydrolysis of compound 4 afforded a
carboxylic acid derivative 5. Then the alkylation of 5 yielded 6.
Compound 6 was reduced by SnCl2 and then reacted with Br2
and SOCl2 to give a benzothiadiazole derivative 8. BTT-T (9)
was obtained by Stille coupling of compound 8 with 2-
tributylstannanylthiophene. Bromination of 9 with NBS gave
4,8-di(5-bromothiophen-2-yl) thieno[2,3-f ]-2,1,3-benzothiadia-
zole-6-carboxylate (M1) as the monomer.
The PBTT-TBDTT copolymer was obtained via Stille
coupling reaction of M1 with M2 (2,6-bis(trimethyltin)-4,8-
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di(5-(2-butyloctyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′] dithio-
phene). The numerable average molecular weight (Mn) reached
58 kDa with polydispersity index (PDI) of 2.8. In addition, the
polymer, PBT-TBDTT as a compared analogue was also
obtained with Mn of 14 kDa and PDI of 6.2. Both polymers are
soluble in common organic solvents such as chloroform,
toluene, chlorobenzene, and THF. Moreover, owing to the
introduction of a branched long-chain alkyl group into BTT
unit, PBTT-TBDTT showed much larger molecular weight and
better solubility than PBT-TBDTT. This was beneficial for the
performance of the devices.18 The thermal gravimetrical
analysis (TGA) (see Supporting Information Figure S1)
indicated that the polymers are much stable up to 380 °C.
In addition, the much red-shifted absorption is observed in
the film state for both monomer units. Especially, in the film
state, the ICT absorption of BTT-T appeared a strong shoulder
peak at 543 nm, indicating a better molecular ordering. Eg
values of BTT-T and BT-T determined from the absorption
onset are 1.97 and 2.23 eV, respectively. The BTT-T unit
possessed a much narrower optical band gap. This is fairly
consistent with the results obtained from the cyclic
voltammetry (see Supporting Information Figure S4). These
results imply that BTT can be acted as a better electron-
acceptor to build low band gap conjugated copolymers for
PSCs.
UV−vis absorption spectra and energy level diagram of the
polymers, PBTT-TBDTT and PBT-TBDT, were displayed in
Figure 3. The comparisons of their optical and electrical
RESULTS AND DISSCUSSION
■
In order to have a better knowledge of BTT, the
physicolchemical properties of the monomer units, BT-T and
BTT-T (see Scheme 1), were discussed. Figure 2 shows their
Figure 2. UV−vis absorption spectra of BT-T and BTT-T in
chloroform solution (the line with symbols) and thin film spun from
chloroform (the full line and the dush line). The concentration of both
monomers in chloroform was 1 × 10−5 mol−1 L−1.
UV−vis absorption spectra. Obviously, BTT-T displays much
higher absorption coefficient of 7.1 × 104 M−1 cm−1 in the
high-energy band around 310 nm, which is over 2-fold than
that of BT-T. In the low-energy band from 350 to 650 nm, they
give the almost equivalent absorption coefficients of about 1.6
× 104 M−1cm−1. Moreover, BTT-T shows much more red-
shifted absorption than BT-T. The maximum absorption of
BTT-T at 504 nm in chloroform is 62 nm longer than that of
BT-T, which can be attributed to the extension of conjugated
backbone and stability of quinoid structure through fusing a
thiophene ring with an electron accepting ester group. This
indicates that BTT unit possesses much stronger electron
withdrawing ability than BT. The absorption and emission
spectra for BTT-T in different solvents were also measured
(See Supporting Information Table S2 and Figures S2 and S3).
As can be seen, the polarity of the solvents exerts little influence
on the UV−vis absorption. However, the bathochromism for
the photoluminescence is observed as the polarity of the
solvent increases. Although the solvachromism is not much
significant, the large Stokes shift (over 100 nm) is observed for
BTT-T at different solvents. This implies that the long
wavelength absorption is mainly attributed to the strong ICT.
This phenomenon is much similar to some BT derivatives, such
as BT-T.19,20
Figure 3. (a) UV−vis absorption spectra of PBT-TBDTT and PBTT-
TBDTT in chloroform solution (the line with symbols) and thin films
spun from chloroform (the full line and the dush line). (b) Energy
level diagram of the active layer components.
characteristics are provided in Table 1. As shown, the solution
absorption spectra of PBT-TBDTT and PBTT-TBDTT display
the maximum intramolecular charge transfer (ICT) peak at 615
and 690 nm, respectively, with a similar absorption coefficient
of 4.4 × 104 M−1 cm−1. In the film state, the maximum
absorption of PBT-TBDTT changes barely while PBTT-
TBDTT shows a small red-shift of 22 nm. For PBT-TBDT,
it shows a pronounced shoulder peak at around 662 nm in the
longer wavelength region, indicating the presence of the tight
intermolecular packing.
When compared to PBT-TBDTT, PBTT-TBDTT exhibits a
broader absorption spectra and a great red-shift over 70 nm in
the film state. This can be attributed to the extended π-
conjugated and strongly quinoid structure of BTT unit that can
be expected to improve the overall PCE of the device. The
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Table 1. Summary of the Optical and Electrical Characteristics for the Polymers
a
a
b
c
soln
film
polymers
Mn (kDa)
PDI
λmax (nm)
λmax (nm)
IP(CV) (eV)
EA(CV) (eV)
Eg (eV)
Egopt(eV)
PBT-TBDTT
14
58
6.2
2.8
427, 615, 662
439, 690
429, 613, 662
447, 712
5.18
5.35
3.06
3.39
2.12
1.96
1.70
1.45
PBTT-TBDTT
a
Ionization potential (IP) and electron affinity (EA) were determined from the CV using the onset of oxidation (Eoxonset) and the onset of
reduction (Eredonset) of thin films spun from chloroform on platinum electrode in acetonitrile solution with 0.1 M n-Bu4NPF6 and caculated by the
b
c
formulas IP (eV) = e (Eoxonset + 4.8) and EA (eV) = e (Eredonset + 4.8). Eg were obtained by Eg = IP − EA. The optical band gaps Egopt were
estimated from the absorption edges of thin film spectra.
optical band gaps estimated from the absorption edges of thin
film spectra are 1.70 and 1.45 eV for PBT-TBDTT and PBTT-
TBDTT, respectively. Interestingly, the optical band gap
obtained for PBTT-TBDTT is actually very close to ideal
donor polymer (1.5 eV) for PSCs as proposed by Frec
́
het,21
indicative of the great potential of the BTT unit as strong
acceptor to construct narrow band gap polymers. Cyclic
voltammetry (CV) was used to investigate the electrochemical
properties of the polymers and estimate their ionization
potentials (IP) and electron affinities (EA) (see Supporting
Information Figure S5). The optical and electrical properties of
the polymers are summarized in Table 1. PBTT-TBDTT shows
a larger IP at 5.35 eV compared to 5.18 eV for PBT-BDTT, this
may be benefitial for the open circuit voltage (VOC) of the
device.22 In addition, the EA value of the former is also
increased by 0.33 eV when compared to the latter. The
observable increment of both EA and IP makes BTT a stronger
acceptor used in PSCs. The band gaps (Eg) determined from
the CV were 2.12 and 1.96 eV for PBT-TBDTT and PBTT-
TBDTT, respectively. The corresponding values were larger
than the ones estimated from the absorption edges of thin film
spectra. This is caused by the reason that the electron and hole
remain electrostatically bound to one another in the excited
state (contrary to the ionized state).23
Polymer solar cells were fabricated from PBT-TBDTT or
PBTT-TBDTT as the donating material and (6,6)-phenyl-C71-
butyric acid methyl ester (PC71BM) as the accepting material
with a general device structure of ITO/PEDOT−PSS/
polymer:PC71BM/Ca (20 nm)/Al (90 nm),24 where the active
layer was spin-coated from o-dichlorobenzene solution. Besides,
PBTT-TBDTT with higher molecular-weight (Mn = 73 kDa,
PDI = 1.8) and lower molecular-weight (Mn = 49 kDa, PDI =
2.1) were also synthesized and discussed in BHJ solar cell
devices to study the influence of molecular weight on the device
performance. To balance the absorbance and the charge
transporting network of the photoactive layer, the weight ratios
of polymer and PC71BM were varied from 1:1 to 1:3. Clearly,
the 1:2 weight ratio seems to be the optimal ratio (see
Supporting Information Figure S6).
The current density−voltage (J−V) curves of the PSC
devices based on PBTT-TBDTT/PC71BM (1:2, w/w) under
the illumination of AM1.5G, 100 mW cm−2 are displayed in
Figure 4 and the corresponding results are summarized in
Table 2. For the higher molecular-weight PBTT-TBDTT (Mn
= 73 kDa), the PCE reached 5.10 0.35% (PCEmax = 5.45%)
with an open circuit voltage (VOC) of 0.75 0.01 V, a short-
circuit current density (JSC) of 12.63 0.40 mA cm−2 and a fill
factor (FF) of 53.0 3.5%. With the decreasing of Mn from 73
kDa, 58 kDa to 49 kDa, the corresponding PCE of the devices
dropped from 5.10 0.35%, 4.50 0.21% to 2.50 0.23%.
Lower Mn leads to poor device performance, this is in
accordance with the result that has been reported.18 Despite
the highest PCE value for the highest Mn sample, its poor
Figure 4. J−V curves of the polymer solar cells based on PBTT-
TBDTT with different Mn under illumination of AM1.5G, 100 mW
cm−2.
solubility makes the processing of the PBTT-TBDTT/PC71BM
film more challenging. A long-time (12 h) and high-
temperature (110 °C) stirring process of the PBTT-TBDTT/
PC71BM solution (8 mg/mL for PBTT-TBDTT) are needed
before use. This makes the deposition of the active layer
tedious. Specially, the poor solubility of this high Mn sample
(Mn = 73 kDa) makes its device performance sensitive to its
concentration. At a higher polymer donor concentration of 10
mg/mL, the device showed inferior device performance (see
Supporting Information Figure S6). Thus, in the following
study, only the 58 kDa Mn sample was used to optimize the
morphology for a better device performance. Also for a better
understanding of the thiophene fusing effect on the photo-
voltaic properties, the optimized device performance of its
polymer analogue PBT-TBDTT was also provided.
Figure 5a shows the J−V curve of the champion PSC based
on PBTT-TBDTT/PC71BM when 1,8-diiodooctane (DIO, 5%
v/v) was added as a solvent additive together with the PBT-
TBDTT/PC71BM champion device. The corresponding IPCE
(incident-photon-to-electron conversion efficiency) spectra are
displayed in Figure 5b. The optimal photovoltaic parameters of
the devices are summarized in Table 3. The primary optimized
solar cell device based on PBT-TBDTT/PC71BM exhibits a
V
OC of 0.68 0.01 V, a JSC of 8.25 0.20 mA cm−2, a fill factor
(FF) of 54.0 2.5% and an overall PCE of 3.10 0.14%
(PCEmax = 3.24%), while the corresponding values of PBTT-
TBDTT/PC71BM based device are 0.70 0.001 V, 13.15
0.25 mA cm−2, 63.0 1.5% and 5.90 0.17 (PCEmax = 6.07%)
under the same device fabrication conditions except for a DIO
additive (5% v/v). The better device performance of PBTT-
TBDTT over that of PBT-TBDTT clearly suggested that the
as-developed BTT acceptor unit is better than the widely used
BT acceptor unit to construct high efficiency conjugated
polymers.
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a
Table 2. Summary of the Photovoltaic Characteristics for PBTT-TBDTT with Different Molecular Weight
Mn (kDa)
Voc (V)
Jsc (mA cm−2
)
FF (%)
PCE (PCEmax) (%)
49
58
73
0.75 0.01
0.75 0.01
0.75 0.01
6.35 0.35
9.60 0.30
12.63 0.40
53.0 2.5
63.0 1.9
53.0 3.5
2.50 0.23 (2.73)
4.50 0.21 (4.71)
5.10 0.35 (5.45)
a
Device average values and standard deviation are based on more than 10 devices. The best efficiencies are given in parentheses.
Figure 6. Tapping mode AFM topography images (5 × 5 μm2) of the
composite films of (a) PBTT-TBDTT without DIO (b) PBTT-
TBDTT with DIO. Roughness measurement of the surface (RMS)
values are given to describe the smooth level of the morphology.
value of 1.40 nm is clearly visible for the film with 5% DIO
additive. The bicontinuous network with nanoscale phase
separation improved the exciton dissociation and carrier
collection efficiency, thus leading to an increase in the short-
circuit current density as well as the device efficiency.
Notably, the VOC of PBTT-TBDTT without DIO is about
0.75
0.01 V, that is ca. 0.07 V higher than that of PBT-
TBDTT (0.68 0.01 V). This is well in agreement with the
result from CV study, in which PBTT-TBDTT has lower IP
than PBT-TBDTT. However, the adding DIO additive seems
to be unfavorable for the VOC of the PSCs based on PBTT-
TBDTT. A drastic decrease from 0.75 0.01 V (without DIO)
to 0.70 0.01 V (with IDO) is observed. As reported in some
references,25 the blend morphology has quite a large effect on
V
OC. Accompanied with the change of morphology, interfacial
energy, exciton stabilization and dissociation along with carrier
recombination dynamics can also influence the VOC. Here,
these factors are possible reasons related to the decreased VOC
value for PBTT-TBDTT with DIO as additive.
Figure 5. (a) J−V curves of the champion polymer solar cells based on
PBT-TBDTT and PBTT-TBDTT under illumination of AM1.5G, 100
mW cm−2. (b) IPCE curves of the corresponding polymer solar cells.
Hole mobility of two polymers (PBTT-TBDTT and PBT-
TBDTT) was measured by space charge limit current (SCLC)
method. PBTT-TBDTT demonstrated an relatively high hole
mobility of 2.00 × 10−4 cm2 V−1 s−1 (see Supporting
Information Figure S7), which is two times higher than that
of PBT-TBDTT (8.89 × 10−5 cm2 V−1 s−1). The higher
mobility of PBTT-TBDTT is benefited from its more planar
conjugated structure as compared to PBT-TBDTT. This
facilitates improved carrier collection efficiency and can partly
account for the higher FF and JSC values for PBTT-TBDTT. In
the IPCE spectra, PBTT-TBDTT exhibits a much broader
photon response range from 300 to 900 nm with relatively high
values above 50%. This leads to an enhanced JSC for PBTT-
TBDTT. For PBT-TBDTT, the IPCE spectra only showed a
response range from 300 to 700 nm with values below 40%.
Besides the higher hole-mobility of PBTT-TBDTT, its planar
DIO is commonly used as a solvent additive in PSCs to
improve the device performances owing to its high boil point
and good solubility for fullerene aggregates. In this work, we
used DIO to optimize the morphology of PBTT-TBDTT based
device. This additive was also used in PBT-TBDTT based
device, unfortunately, no better device performance was
achieved. For PBTT-TBDTT based device, the influence of
additive concentration on device fabrication is described in the
Supporting Information (Table S1). The effect of DIO additive
is clearly visualized from atomic force microscopy (AFM)
images in Figure 6. Without DIO, the composite film exhibits
microscale phase separation with a root-mean-square (RMS)
value of 1.70 nm, suggesting poor dispersion of PC71BM in the
polymer matrix. In contrast, nanomorphology with a RMS
a
Table 3. Summary of the Photovoltaic Characteristics for the Polymers of PBT-TBDTT and PBTT-TBDTT (Mn = 58 kDa)
polymer
D/A
DIO (%)
Voc (V)
Jsc (mA cm−2
)
FF (%)
PCE (PCEmax) (%)
PBT-TBDTT
1:2
1:2
0
5
0.68 0.01
0.70 0.01
8.25 0.20
54.0 2.5
63.0 1.5
3.1 0.14 (3.24)
5.9 0.17 (6.07)
PBTT-TBDTT
13.15 0.25
a
Device average values and standard deviation are based on more than 10 devices. The best efficiencies are given in parentheses.
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structure resulted lower reorganization energies can also partly
account for its relatively higher IPCE value.26 Thus, benefiting
from broader absorption, lager IP and more planar structure,
PBTT-TBDTT exhibited enhanced JSC and VOC values and
spontaneously resulted in an overall improvement of the PCE
of the device.
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In organic solar cells, the energy loss (ΔE) under one sun
equivalent light intensity can be quantified by the difference
between eVOC and the lowest-component optical bandgap
27
(Eopt): ΔE = Eopt − eVOC
.
With the fact of the optical band
gap of fullerene acceptor is 1.70 eV, here Eopt means the optical
band gap of PBTT-TBDTT (1.45 eV). Thus, the calculated
energy loss of 0.75 eV is approaching 0.70 eV considered as the
minimal for charge generation in PSCs. Compared to the
optical band gap, the lower ΔE obtained implies that VOC was
optimized with PC71BM as acceptor. Recently, Janssen’s result
suggested a slightly increase in photon energy loss can lead to a
higher quantum efficiency for charge generation, indicative of a
large room for further enhancing the efficiency by slightly
turning the donor−acceptor coupling.28
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CONCLUSIONS
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In conclusion, thieno[2,3-f]-2,1,3-benzothiadiazole-6-carboxy-
late (BTT) was first designed and synthesized as a strong
electron acceptor and introduced to polymer solar cells to form
a novel D−A copolymer with efficient device performance and
compared with the traditional benzothiadiazole (BT) based
analogue. By the incorporation of the strong electron-
withdrawing BTT unit, a polymer with large IP (ionization
potential) and good solubility was obtained. The polymer
presented a narrow optical band gap around 1.45 eV and broad
absorption spectra in the visible and near-infrared region. The
maximum PSC device based on PBTT-TBDTT reached more
enhanced PCE of 6.07% than that of PBT-TBDTT, implying
that BTT will be acted as a novel promising building block
developed for organic semiconducting materials. Much work
has been done to tailor and optimize the molecular structure by
simply adjusting the thiadiazole and thienyl group and further
studies are under way.
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ASSOCIATED CONTENT
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Experimental details of the synthesis of the polymers, details of
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AUTHOR INFORMATION
■
C.; Subbiah, J.; Choudhury, K.; Mavrinskiy, A.; Pisula, W.; Bred
́
as, J.-
Corresponding Authors
L.; So, F.; Mullen, K.; Reynolds, J. R. J. Am. Chem. Soc. 2012, 134,
̈
8944−8957.
*Email: xgchen@whu.edu.cn.
*Email: zgzhangwhu@iccas.ac.cn.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Nos. 51173138 and 20972122) and the Specialized
Research Fund for the Doctoral Program of Higher Education
of China (No. 20100141110010).
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