Low-bandgap conjugated polymer for high efficient photovoltaic applications

Article abstract of DOI:10.1039/c0cc01087a

We investigate the morphological and performance of organic photovoltaics based on blended films of alternating poly(thiophene-phenylene-thiophene) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). The resulting fine-scale phase separation leads to enhanced performance and the highest power efficiency (6.4% under AM 1.5G (100 mW cm^-2)) when we use solvent annealing process.

Full text of DOI:10.1039/c0cc01087a

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Low-bandgap conjugated polymer for high efficient photovoltaic
applicationsw
Yi-Chun Chen, Chao-Ying Yu, Yu-Ling Fan, Ling-I Hung, Chih-Ping Chen* and Ching Ting*
Received 23rd April 2010, Accepted 9th July 2010
DOI: 10.1039/c0cc01087a
We investigate the morphological and performance of organic
photovoltaics based on blended films of alternating poly-
(thiophene–phenylene–thiophene) and [6,6]-phenyl-C71-butyric
acid methyl ester (PC71BM). The resulting fine-scale phase
separation leads to enhanced performance and the highest power
efficiency (6.4% under AM 1.5G (100 mW cmÀ2)) when we use
solvent annealing process.
(100 mW cmÀ2) condition. An alternating polymer (a-PTPTBT)
that consists of TPT as the electron-donating unit and
2,1,3-benzothiadiazole (BT) as the electron-accepting moiety
was synthesized by Pd(0)-catalyzed Stille coupling polymeri-
zation in chlorobenzene under microwave heating conditions
(Scheme 1).
The newly synthesized di-tin TPT precursor enables the
synthesis of various TPT-based polymers with well-defined
alternating structures. The structures of earlier synthesized
random TPT polymers are not controllable and are likely to
feature a random distribution of TPT and acceptor-rich
moieties. Details of the synthesis and characterization are
included in the ESI.w As a result of GPC, the a-PTPTBT
shows a Mw of 60.9 kg molÀ1 with a PDI of 2.51. The
a-PTPTBT possesses good thermal stability [5% weight-loss
temperature (Td) > 450 1C] as indicated by thermogravi-
metric analysis (TGA). When investigating the thermal beha-
vior of the polymer using differential scanning calorimetry
(DSC), we observed no clear thermal transitions in the
temperature range from 40 to 300 1C. Fig. 1 displays the
UV-vis absorption features of the polymer film in the solid
state. The polymer bandgap is 1.75 eV with an absorption
coefficient of 1.60 Â 105 cmÀ1 for the polymer film at lmax
(= ca. 570 nm). Two characteristic bands in the absorption
spectrum were found in which the shorter wavelength absor-
bance originated from the p–p* transition of the TPT units,
while the lower energy band was attributed to the intra-
molecular charge transfer (ICT) between the electron-rich
and the BT segments. The HOMO and LUMO energy levels
of a-PTPTBT are determined from cyclic voltammograms
(CVs) to be À5.36 and À3.52 eV, respectively. According to
a previous report, the LUMO and HOMO of PCBM are
À3.94 and À5.93 eV, respectively.7 The LUMO offset of the
polymer and PCBM is large enough for energetically efficient
charge transfer and therefore allows devices to exhibit higher
current output. Furthermore, the Voc can be improved by the
relatively deeper HOMO energy level of the polymer.
The research field of organic photovoltaics (OPVs) is
flourishing due to its promising application advantages such
as flexibility, light weight, potential high efficiency and
solution-processability.1–5 For OPVs with low band gap
(LBG) materials,6–10 processing into large area devices11–13
with long-term stability14 has been widely investigated. With
the innovation of materials engineering, the power conversion
efficiency (PCE) of OPVs has been improved up to 7.9% using
new active layer materials.15,16 This encouraging progress
advances such technology towards commercial application.17
To push the PCE of OPVs towards predicted theoretical
limits,18 simultaneously achieving a high short circuit current
(Jsc) and a high open circuit voltage (Voc) is critical. Theoreti-
cally, the extended absorption of sunlight at longer wavelengths
directly reflects the value of Jsc. Nevertheless, film charac-
teristic and optoelectronic properties like absorption coefficient
will change this value.7 The Voc of OPVs is dominated by the
difference between the LUMO of the acceptor (fullerene) and
the HOMO of the donor (polymer). Furthermore, the LUMO
difference between conjugated polymer and fullerene should
be greater than 0.3 eV to ensure efficient exciton dissociation.1–3
Because these parameters are often inter-correlated, it is a
great challenge to design and synthesize a polymer to meet all
the challenging points required for high efficient OPV
application.
Previous studies on thiophene/phenylene/thiophene (TPT)-
based derivatives have shown efficient OPV characteristics.6,7
These polymers exhibit better harvesting of solar flux with
deeper HOMOs that give impressively larger Voc (>0.8 V)
and possess OPV performance up to 4.3%. Those previous
polymers were synthesized from di-bromine TPT monomer
and the structures were limited to a random feature. Through
a rational molecular engineering approach, we describe an
alternating TPT based conjugated polymer for high efficiency
OPV, which exhibits a PCE of 6.4% under the AM 1.5G
Table 1 and Fig. 2(a) display the output characteristics of
the devices prepared under various fabrication conditions.
The a-PTPTBT/PC71BM cell exhibited averaged AM 1.5G
(100 mW cmÀ2) PCEs of ca. 5.4%. In comparison with the
Materials and Chemical Laboratories, Industrial Technology Research
Institute, 195, Sec. 4, Chung Hsing Road, Chutung, Hsinchu, 310,
Taiwan. E-mail: chihping_chen@itri.org.tw, cting@itri.org.tw;
Fax: 886-35-827694; Tel: 886-35-93588
w Electronic supplementary information (ESI) available: A description
of experimental procedures including structural characterization. See
DOI: 10.1039/c0cc01087a
Scheme 1 The synthetic route to a-PTPTBT.
Chem. Commun., 2010, 46, 6503–6505 6503
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Fig. 1 UV-vis absorption spectra of the a-PTPTBT and the blend
films.
devices based on similar random PTPTBT,6,7 the PCE increases
by ca. 30%, while Jsc and Voc show a slight improvement. This
is primarily due to the fill factor (FF). It is known that the hole
mobility of polymer should be higher for effective carrier
transport, which directly affects the FF of device. To evaluate
the contribution of polymer mobility, we employed hole-only
devices to characterize the hole mobility of blend films.19,20 We
calculated the field-independent mobility of the a-PTPTBT
and random PTPTBT derived devices to be 2.2 Â 10À9 m2/Vs
and 1.9 Â 10À10 m2/Vs, respectively. As expected, the mobility
of the alternating TPT derivate is one order of magnitude
higher than that for random derivates, which enables better
carrier transport within active layer and produces greater FF.
In an attempt to further improve device performance, we
utilized strategies based on thermal annealing, slow drying,
and solvent vapor annealing (SA).21–23 We found that the PCE
of devices can be significantly increased via the SA process.
The devices were placed inside a Petri dish for annealing
with saturated DCB vapor for 10–30 min. The most efficient
SA-fabricated device exhibited a PCE of 6.4% under AM
1.5G irradiation (100 mW cmÀ2), with Jsc, Voc, and FF values
of 11.2 mA cmÀ2, 0.85 V, and 0.67, respectively. In comparison
with the normal device, the FF of SA-fabricated device under-
went a significant increase of ca. 12%, whereas the Jsc
increased only slightly. The PCEs of these SA-fabricated
devices were reproducible, with a variation of only 10%
exhibited among five individual PSC devices. The performance
of this OPV device is among the highest ever reported.
To confirm the accuracy of the measurements, the external
quantum efficiency (EQE) of the devices was measured.
Fig. 2(b) displays the EQE spectra of the devices fabricated
with a-PTPTBT:PC71BM blend films. The SA-fabricated
device reveals a significant contribution of EQE in the wave-
length between 350 nm and 750 nm and is consistent with
UV-vis spectrum of blend film (Fig. 1) with the maximum
EQE being approximately 63% at 470 nm. Convolution of the
spectral response with the photon flux AM 1.5G spectrum
Fig. 2 (a) Current density-potential characteristics of a-PTPTBT/
PC71BM devices under illumination with AM 1.5G solar simulated
light (100 mW cmÀ2). (b) The EQE spectra of devices fabricated with
a-PTPTBT /PC71BM.
EQE curves is different after SA process. The relative enhance-
ment of EQE from 600 nm to 700 nm is likely due to different
spatial distribution of a-PTPTBT and PCBM.24 We also
determined the SCLC hole and electron mobilities of the
respective devices. The result shows that hole mobility of
SA-fabricated device is 2.4 Â 10À9 m2/Vs and reveals no
significant change between the normal and SA devices. The
determined electron mobilities of SA and normal devices are
2.6 Â 10À8 and 1.6Â 10À8 m2/Vs, respectively. A small increase
of the electron mobility from 1.6 Â 10À8 to 2.6 Â 10À8 m2/Vs is
observed after the SA process. The preliminary result indicates
that the carrier mobilities in both devices are balanced and
within the reasonable range.
Besides the mobility, we suspect the differing morphology
also contributes to variations in device performance.
To confirm this correlation, we recorded TEM images of
respective blend films (Fig. 3). Because the electron scattering
density of PCBM is higher than that of the conjugated
polymer, the a-PTPTBT domains appear as bright regions
whereas the dark regions can be attributed to PCBM domains.
Table 1 OPV parameters of devices based on PTPTBT polymers
PCE (%)
Jsc/mA cmÀ2
Voc/V
FF (%)
Best
Ave
gives an estimate for the Jsc under irradiation to be 10.6 mA cmÀ2
.
a-PTPTBTa
a-PTPTBTb
PTPTBTb
8.1
0.87
0.86
0.80
0.85
64.9
60.4
53.0
67.2
4.56
5.56
4.30
6.41
4.3
5.4
4.0
6.1
10.7
10.1
11.2
Due to the mismatch between EQE and photon flux
AM 1.5G, there is an approximately 5% mismatch between
convolution and solar simulator data. The EQE of normal
device is close to SA device and is consistent with the Jsc values
obtained from these two methods. Note that the shape of the
a-PTPTBT-SAb
a
b
Using PC61BM (Purchased from Nano-C) as acceptors. Using
PC71BM (Purchased from Solenne) as acceptors.
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the material’s energy bandgap. A further improvement in
PCE can be expected if the Jsc of the device can be further
enhanced.
We thank the Ministry of Economic Affairs, Taiwan, for
financial support.
Notes and references
1 B. C. Thompson and J. M. J. Frechet, Angew. Chem., Int. Ed.,
2008, 47, 58.
2 P. Heremans, D. Cheyns and B. Rand, Acc. Chem. Res., 2009, 42,
1740.
3 M. Helgesen, R. Søndergaard and F. C. Krebs, J. Mater. Chem.,
2010, 20, 36.
Fig. 3 TEM images of a-PTPTBT : PC71BM films before (a) and
after solvent annealing (b). (The scale bar is 200 nm).
4 G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Mater., 2009,
21, 1323.
5 J. Lu, F. Liang, N. Drolet, J. Ding, Y. Tao and R. Movileanu,
Chem. Commun., 2008, 5315.
6 C.-P. Chen, S.-H. Chan, T.-C. Chao, C. Ting and B.-T. Ko, J. Am.
Chem. Soc., 2008, 130, 12828.
7 C.-Y. Yu, C.-P. Chen, S.-H. Chan, G.-W. Hwang and C. Ting,
Chem. Mater., 2009, 21, 3262.
8 S. H. Park, A. Roy1, S. Beaupre, S. Cho, N. Coates, J. S. Moon,
D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics,
2009, 3, 297.
9 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger
and G. C. Bazan, Nat. Mater., 2007, 6, 497.
10 M. Wienk, M. Turbiez, J. Gilot and R. A. J. Janssen, Adv. Mater.,
2008, 20, 2556.
The normal blend film [Fig. 3(a)] exhibits morphology
with separated a-PTPTBT domains and PCBM domains;
the fibrillar features of the polymer are clearly seen. In
contrast, the SA film [Fig. 3(b)] reveals that it consists of
well-defined blends of PCBM domains (having grain size
ca. 20–30 nm) surrounded by a-PTPTBT domains. The SA
process optimizes the miscibility of the polymer with PCBM,
leading to better phase separation between polymer chains and
PCBM molecules formation of interpenetrating networks
(IPNs). Furthermore, the morphology of the SA blend film
is much more uniform and there is no large phase separation.
As a result, the SA blend system has preferred IPN morpho-
logy to ensure the efficient charge separation and transport
required in higher cell performance. For the widely investi-
gated P3HT system, in which the fibril-like structure typically,
correlates to the high crystallinity as well as the performance.25
The solvent vapor treatment for P3HT system resulted in fine-
tuned of nanoscale morphology and the enhancement of the
carrier mobility.26–28 Unlike P3HT system, our observation
for a-PTPTBT system is different after SA. To our knowledge,
the correlation between morphology, performance and carrier
mobility is not clear for new designed polymers. For instance,
the use of additive (alkane-dithiols) has resulted in a significant
improvement of performance for a device based on poly[2,6-(4,4-
bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]dithiophene)-alt-4,7-
(2,1,3-benzothiadiazole)] (PCPDTBT).9,29 The well-organized
morphology of the PCPDTBT device was observed in the
presence of an additive. Nevertheless, the hole mobility of the
device showed no increase either from space charge limited
current (SCLC) model29 or FETs9 result. We speculate that
the effect stems from the spatial distribution of donor and
acceptor domains rather than intrinsic differences in carrier
mobilities.9,29 Further study is underway to verify the difference
between these polymers.
11 F. C. Krebs, S. A. Gevorgyan and J. Alstrup, J. Mater. Chem.,
2009, 19, 5442.
12 F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 465.
13 L. Blankenburg, K. Schultheis, H. Schache, S. Sensfuss and
M. Schrodner, Sol. Energy Mater. Sol. Cells, 2009, 93, 476.
14 M. Jørgensen, K. Norrman and F. C. Krebs, Sol. Energy Mater.
Sol. Cells, 2008, 92, 686.
15 H. Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu,
Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649.
16 Y. Liang, Y. Wu, D. Feng, S.-T. Tsai, H.-J. Son, G. Li and L. Yu,
J. Am. Chem. Soc., 2009, 131, 56.
17 F. C. Krebs, T. D. Nielsen, J. Fyenbo, M. Wadstrøm and
M. S. Pedersen, Energy Environ. Sci., 2010, 3, 512.
18 M. C. Scharber, D. Muhlbacher, M. Koppe, P. Denk, C. Waldauf,
A. J. Heeger and C. J. Brabec, Adv. Mater., 2006, 18, 789.
19 O. G. Reid, K. Munechika and D. S. Ginger, Nano Lett., 2008, 8,
1602.
20 C. Melzer, E. J. Koop, V. D. Mihailetchi and P. W. M. Blom, Adv.
Funct. Mater., 2004, 14, 865.
21 G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and
Y. Yang, Nat. Mater., 2005, 4, 864.
22 W. L. Ma, C. Y. Yang, X. Gong, K. Lee and A. J. Heeger, Adv.
Funct. Mater., 2005, 15, 1617.
23 J. K. Lee, W. L. Ma, C. J. Brabec, J. Yuen, J. S. Moon, J. Y. Kim,
K. Lee, G. C. Bazan and A. J. Heeger, J. Am. Chem. Soc., 2008,
130, 3619.
24 H. Y. Chen, H. Yang, G. Yang, S. Sista, R. Zadoyan, G. Li and
Y. Yang, J. Phys. Chem. C, 2009, 113, 7946.
25 C. W. Chu, H. Yang, W. J. Hou, J. Huang, G. Li and Y. Yang,
Appl. Phys. Lett., 2008, 92, 103306.
26 Y. Yao, J. Hou, Z. Xu, G. Li and Y. Yang, Adv. Funct. Mater.,
2008, 18, 1783.
27 G. Li, Y. Yao, V. Shrotriya and Y. Yang, Adv. Funct. Mater.,
2007, 17, 1636.
28 L. M. Chen, Z. R. Hong, G. Li and Y. Yang, Adv. Mater., 2009,
21, 1434.
To conclude, we have demonstrated high performance OPV
devices by using alternating TPT copolymer. The alternating
TPT polymer shows higher SCLC hole mobility than its
random derivative. We used SA treatment to achieve optimized
bulk hetero-junction morphology. The devices with PCEs up
to 6.4% and high fill factor of over 67% were obtained.
Further investigations are currently underway to fine-tune
29 M. Dante, A. Garcia and T.-Q. Nguyen, J. Phys. Chem. C, 2009,
113, 1596.
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Chem. Commun., 2010, 46, 6503–6505 6505