A High-Mobility Low-Bandgap Copolymer for
Efficient Solar Cells
a
Ping Ding, Cheng-Che Chu, Bo Liu, Bo Peng, Yingping Zou,* Yuehui He,
Kechao Zhou, Chain-Shu Hsu*
Introduction
Polymer solar cells (PSCs) have attracted significant research interest from industry and the academic commu-nity for tomorrow’s photovoltaics, because of their advantages of easy fabrication, low cost, lightweighted-ness, and flexibility.[1]In the past few years, such solar cells
have witnessed tremendous progress. A power conversion
efficiency (PCE) of 5–8% has been reached.[2] Although present research indicates that PSCs have a bright future, the efficiency and stability need to be improved towards commercialization. To achieve high performance PSCs, the greatest challenge is to develop low bandgap polymers with broad absorption and high hole mobility.[3]The use of an electron rich donor and electron deficient acceptor in the backbone was proved to be the most effective strategy for obtaining low bandgap polymers.[4]Coplanar geometries
and rigid structures usually lead to high charge mobility.[5]
Last year, Bo et al.[6] reported a planar alternating copolymer, HXS-1, obtained from 2,7-carbazole and 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzothiadiazole, which shows a promising PCE of 5.4% using 1,8-diiodooctane as an additive. However, the disadvantage of this polymer is the solar spectral mismatch between its absorption spectrum and solar irradiation with an absorption edge of 650 nm, which limited the ultimate efficiency of this polymer. Inspired by the purpose of harvesting more sunlight, there is a necessity to further lower the bandgap of this kind of polymer.
In the past two years, copolymers based on electron donating benzodithiophenes (BDTs) have demonstrated great potentials in polymer photovoltaics. A class of BDT-P. Ding, B. Peng, Y. Zou
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China
E-mail: [email protected] (Y. Zou) C. Chu, C. Hsu
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan
E-mail: [email protected] (C. Hsu) B. Liu, Y. He, K. Zhou, Y. Zou
State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China
a:
Supporting information for this article is available at the bottom of the article’s abstract page, which can be accessed from the journal’s homepage at http://www.mcp-journal.de, or from the author.
A new high-mobility low-bandgap polymer, PBDT-DODTBT, based on benzodithiophene and
5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzothiadiazole has been synthesized through a
stan-dard Stille coupling reaction. The polymer is soluble in common organic solvents, such as
chloroform, tetrahydrofuran, and chlorobenzene and has
excellent film forming properties. Preliminary studies of
the copolymer showed the charge mobility as high as
7.15 10
3cm
2V
1s
1from
SCLC
measurement.
Initial photovoltaic cells based on the composite
struc-ture of ITO/PEDOT:PSS/PBDT-DODTBT:
aPC
71BM (1:2)/Ca/
Al showed an open-circuit voltage of 0.76 V, a power
conversion efficiency of 4.02%, and a short-circuit current
of 8.96 mA cm
2.
based copolymers with optimized electronic and optical properties led to promising PCEs as high as 7.4%.[7]By fusing
a benzene with two flanking electron donating thiophene units, the BDT unit seems to be a more electron-rich building block than those units such as fluorene or carbazole, moreover, BDT has an entirely planar and symmetrical structure, which is beneficial to obtaining high charge mobility. Taking all this into account, copolymerizing BDT with 5,6-bis(octyloxy)-4,7-di(thio-phen-2-yl)benzothiadiazole should lead to some interesting features such as broad absorption, high mobility, etc.
In this work, a new copolymer, PBDT-DODTBT, obtained from BDT units and 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl) benzothiadiazole (DODTBT), as shown in Scheme 1, was designed, synthesized, and used in solar cells. This
low-bandgap polymer shows good solubility in common organic solvents, broad absorption in the wavelength range from 360 to 758 nm, and a high hole mobility up to 7.15 103cm2 V1 s1. The polymer solar cells based on PBDT-DODTBT as the donor and PC71BM as the acceptor
exhibit a high PCE of 4.02% under the AM 1.5G one sun condition.
Experimental Part
The general synthetic route of the comonomers and copolymer is sketched in Scheme 1 and the detailed synthesis is given in the Supporting Information. Device fabrication and characterization are also shown in the Supporting Information.
C8H17Br, EtOH K2CO3, 75oC,40h C2H5OH,SnCl2, con.HCl 80o C,8h 65%HNO3, 100%HNO3 CH2Cl2, HOAc, RT, 40h SOCl2, N(C2H5)3, CH2Cl2 reflux,6h Br2, HBr ref lux, 6h N S N C8H17O OC8H17 Br Br NBS , RT CHCl3, HOAc S Sn(C4H9)3, Pd(PPh3)4 , 100oC ,2days
2
1
3
4
5
toluene N S N S S OC8H17 C8H17O N S N S S Br Br OC8H17 C8H17O OC8H17 OC8H17 OH OH NO2 NO2 C8H17O C8H17O NH3Cl NH3Cl C8H17O C8H17O N S N C8H17O C8H17O6
7
S S O O Sn Sn Pd(PPh3)4, toluene 100oC, 2days+
S S O O N S N OC8H17 C8H17O S n S N S N S Br OC8H17 C8H17O S Br7
8
PBDT-DODTBT
Results and Discussion
Synthesis and Characterization of PBDT-DODTBT The synthetic routes of the comonomers and the correspond-ing polymer are outlined in Scheme 1. Compounds 5[8]and 8[9] were prepared according to the known literature procedures. Compound 5 underwent a Stille coupling reaction with 2-(tributylstannyl) thiophene to obtain compound 6, this step revealed a quite satisfactory yield. A dibromination step of compound 6 using N-bromosucci-nimide (NBS) as brominating agent gave comonomer 7 in 78% yield. Compound 7 copolymerized with compound 8 through a Stille coupling reaction to afford the target polymer, PBDT-DODTBT. The polymer was purified by sequential Soxhlet extraction with methanol, hexane, and CHCl3. The CHCl3 fraction was then reduced in volume,
precipitated into methanol, and collected by filtration, to yield a purple-dark solid with a 91% yield. The chemical structure of the polymer was verified by1H NMR spectro-scopy as shown in Figure 1. The copolymer displayed excellent solubility in common solvents such as tetrahy-drofuran (THF), CHCl3, dichlorobenzene, and so forth. The
molecular weight of the polymer was determined by gel permeation chromatography (GPC) in THF solution relative to polystyrene standards. The weight-average molecular weight (Mw) was 25.7 kg mol1, with a PDI of 2.7.
Thermal Stability
The thermal stability of the polymer was investigated by thermogravimetric analysis (TGA) as shown in Figure 2. The
TGA analysis reveals that the 5% weight loss temperature (Td) of PBDT-DODTBT is 323 8C in air, mainly because of the
removal of the alkoxy groups. The thermal stability of the polymer is favorable for its application in PSCs and other optoelectronic devices.
Optical Properties
The photophysical characteristics of PBDT-DODTBT were investigated by UV-vis absorption spectra in dilute chloro-form solution and in a solid film spin-coated on a quartz substrate. Figure 3a shows the UV-vis absorption spectra of the polymer solution and film. The absorption peak of the polymer in solution was 548 nm, whereas in the film state the absorption peak shifted to 600 nm with the onset wavelength at 758 nm. The absorption showed a red shift from the solution to the film probably resulting from the stronger interchain interaction of the polymer in the solid state.[10]The optical bandgap (Eg) was calculated from
the onset of the absorption to be 1.64 eV, which is lower than that of HXS-1 (ca. 1.95 eV). These results indicate that BDT is a more efficient electron rich low band-gap unit compared to that of carbazole.
Electrochemical Properties
In order to investigate the electrochemi-cal properties of PBDT-DODTBT, and estimate its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, cyclic voltammetry (CV) was carried out in a 0.1M solution of
tetrabutylammo-nium hexafluorophosphate (Bu4NPF6) in
Figure 1.1H NMR spectrum of PBDT-DODTBT in CDCl3.
100 200 300 400 500 600 50 60 70 80 90 100
Weight (%)
Temperature (
oC)
Figure 2. TGA curve of PBDT-DODTBT with a heating rate of 20 8C min1.
acetonitrile (CH3CN) at room temperature under argon with
a scanning rate of 50 mV s1. All the potentials are reported vs. Ag/Agþwith the ferrocene/ferrocenium couple as an internal standard. The CV curve of the polymer is shown in Figure 3b. The copolymer exhibited a quasi-reversible oxidation and irreversible reduction. From the value of the onset oxidation potential and the onset reduction potential of the polymer, the HOMO and the LUMO levels were estimated to be 5.17 and 3.61 eV, respectively.[11]The HOMO level is lower than that of P3HT (4.76 eV),[12]which is expected to be more stable than that of P3HT. We calculated the HOMO and LUMO energy levels of the polymer according to the equations: [13] HOMO ¼
–e(Eox
onþ 4.71 V), LUMO ¼ –e(Eredon þ 4.71 V). The
correspond-ing electrochemical bandgap is 1.56 eV, which is similar to the optical bandgap within the experimental error.
X-Ray Analysis
To evaluate the crystallinity of the polymer, X-ray diffraction measurements were taken for the spin-coated films on SiO2substrate. Figure 4 shows the XRD spectrum of
the thin film of PBDT-DODTBT at room temperature. It is clear that there is no apparent peak at the small angle region, but the peak at 24.78 corresponds to a short p-p distance of 3.6 A˚ between the polymer main chains, which is even shorter than that of HXS-1, indicating that PBDT-DODTBT possesses strong long range p-p stacking crystal-linity.[14]
Hole Mobility
Hole mobility is an important parameter of the conjugated polymers for photovoltaic applications. We investigated the hole mobility of PBDT-DODTBT with the space-charge-limited current (SCLC) model with a device structure of indium/tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) (PEDOT:PSS)/PBDT-DODTBT (210 nm)/ Au (100 nm). Details concerning the SCLC method are shown in the Supporting Information. The results are plotted as ln(Jd3/V2) vs. (V/d)0.5, as shown in Figure 2c. Herein, J stands for current density, d is the thickness of the device, and V ¼ Vappl Vbi, where Vappl is the applied
potential and Vbiis the built-in potential.
JSCLC¼ 9 8"o"rmo ðVVbiÞ2 d3 exp 0:89g ffiffiffiffiffiffiffiffiffiffiffiffiffiffi VVbi d r " # (1)
According to Equation (1)[15] and Figure 5, the hole
mobility of the polymer is evaluated to be 7.15 103cm2 V1 s1. The high hole mobility is mostly because of the rigidity, short p-p stacking distance, and planarity of the polymer backbone. Obviously, the hole mobility of PBDT-DODTBT is relatively high as the photovoltaic donor material in PSCs. The high hole mobility Figure 3. a) UV-Vis absorption spectra of PBDT-DODTBT in chloroform solution and in film state. b) Cyclic voltammogram of a PBDT-DODTBT film on a platinum electrode in 0.1 mol L1Bu4NPF6, CH3CN solution at a scanning rate of 50 mV s1.
60 50 40 30 20 10 0 200 400 600 800 In tens ity 2 theta (o) PBDT-DODTBT
Figure 4. X-Ray diffraction pattern of a powdery PBDT-DODTBT sample.
of the polymer is expected to facilitate charge transport and reduce recombination loss in the PSCs, therefore, it can lead to an improved device performance.
Photovoltaic Properties
All these characteristics (low bandgap, high hole mobility, preferred energy levels, etc) indicate that the copolymer PBDT-DODTBT should be an excellent candidate for photovoltaic applications. Therefore, bulk heterojunction PSCs with the use of PBDT-DODTBT as the donor and fullerene derivative PC71BM as the acceptor were
fabricated with the structure of ITO/PEDOT:PSS/PBDT-DODTBT:PC71BM (1:2)/Ca/Al. Details concerning device
fabrication and characterization are provided in the Supporting Information. Figure 6 shows a typical current
density/voltage (J/V) curve and the external quantum efficiency (EQE) of the polymer solar cells. The device demonstrated a high short-circuit current density (Jsc) of
8.96 mA cm2, an open-circuit voltage (Voc) of 0.76 V, and a
good fill factor (FF) of 0.59. A high FF indicates a balanced transport in the PSCs. The resulting PCE reached up to 4.02% without annealing and additives. The complementary absorbance of PC71BM and PBDT-DOTBT gives a relatively
high photoconversion efficiency over the wavelength range of 390–630 nm, with monochromatic EQE values above 40%. It is worth noting that the Jsccalculated by integrating
the EQE curve with an AM 1.5G reference spectrum agrees well with the Jsc value obtained from the J/V
measure-ments.[16]Initial results are promising and we are currently
aiming at trying this polymer with other electron acceptors and different device conditions.
Morphology
The morphology of the photoactive layer is very important for the performance of PSCs, and in some cases the performance of the device strongly depends on its morphological features.[17]
Therefore, we used atomic force microscopy (AFM) to investigate the surface morphology of the active layer. AFM topographic and phase images prepared of PBDT-DODTBT:PC71BM (1: 2) in dichlorobenzene are shown in
Figure 7. The film is smooth [RMS (root mean square) roughness ¼1.3 nm) and connective between the neighbor-ing domains from the topographic image, which can in part explain the high hole mobility of this copolymer. In addition, from AFM, the polymer and fullerene domains are homogeneously distributed throughout the surface of the film, in other words, the nanometer-scale interpene-trating network can be formed in this blend, which can benefit not only the charge separation but also the charge transport, therefore, leading to a high FF. The very fine
Figure 6. a) Current density/voltage (J/V) curves of a PBDT-DODTBT:PC71BM (1:2, w/w) device under 100 mW cm2, AM 1.5 solar irradiation
and in the dark. b) EQE of the PBDT-DODTBT:PC71BM BHJ solar cell.
0 200 400 600 -36 -35 -34 -33 -32 -31 -30
ln
(Jd
3 /V
2
)(mAcm/V
2
) (V/d
)0.5(V/cm)0.5
PEDOT:PSS/PBDT-DODTBT(210nm)/Au(100nm)Figure 5. Current/voltage data from the device ITO/PEDOT:PSS/ polymer/Au, plotted in the format of ln(Jd3/V2) vs. (V/d)0.5, where
domains and no large phases contributed partially to the high photovoltaic performance of the polymer.
Conclusion
We have designed and synthesized a new high mobility low bandgap polymer, PBDT-DODTBT, based on benzodithio-phene and 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)ben-zothiadiazole through a Stille coupling reaction in a good yield. Preliminary studies of the copolymer showed the hole mobility as high as 7.15 103cm2 V1 s1from the SCLC model. Broad and good absorption, and high hole mobility combined with a nanometer-scale morphology leads to a polymer with promising photovoltaic properties. Initial bulk heterojunction solar cells based on the blends of PBDT-DODTBT with PC71BM had a high PCE up to 4% under
100 mW cm2, AM 1.5 solar illumination. A high PCE from the PSCs based on PBDT-DODTBT is ascribed to high EQE plateau values between 390–630 nm over 40%, high mobility, and good morphology. Optimization of PSCs based on PBDT-DODTBT is underway, a PCE of higher than 5% can be anticipated through careful device engineering. We firmly believe this polymer has the promising potential for future photovoltaics.
Acknowledgements: The authors acknowledge Miao Yang for the synthesis of the intermediates and Prof. Yongfang Li and Prof. Chunyue Pan for great help. This work was supported by National Natural Science Foundation for Distinguished Young Scholar (NO.50825102), National Natural Science Foundation of China (NOs. 50803074, 50633050, 20976199), Lieying Project, the Fundamental Research Funds for the Central Universities, the Opening Fund of State Key Laboratory of Powder Metallurgy.
Received: August 6, 2010; Revised: August 30, 2010; Published online: October 21, 2010; DOI: 10.1002/macp.201000473
Keywords: benzodithiophene; conjugated polymers; low band-gap; high mobility; polymer solar cells
[1] [1a] A. J. Heeger, Chem. Soc. Rev. 2010, 39, 2354; [1b] P. L. T. Boudreault, S. Beaupre´, M. Leclerc, Polym. Chem. 2010, 1, 127. [2] [2a] E. G. Wang, L. Wang, L. F. Lan, C. Luo, W. L. Zhuang, J. B. Peng, Y. Cao, Appl. Phys. Lett. 2008, 92, 03330; [2b] Y. P. Zou, A. Najari, P. Berrouard, S. Beaupre´, R. B. Aich, Y. Tao, M. Leclerc, J. Am. Chem. Soc. 2010, 132, 5330; [2c] S. H. Park, A. Roy, S. Beaupre´, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger, Nat. Photon. 2009, 3, 297; [2d] J. H. Hou, H. Y. Chen, S. Zhang, G. Li, Y. Yang, J. Am. Chem. Soc. 2008, 130, 16144; [2e] X. W. Zhan, D. B. Zhu, Polym. Chem. 2010, 1, 409. [3] [3a] J. W. Chen, Y. Cao, Acc. Chem. Res. 2009, 11, 1709; [3b] Y. F. Li, Y. P. Zou, Adv. Mater. 2008, 20, 2952; [3c] Y. J. Cheng, S. H. Yang, C. S. Hsu, Chem. Rev. 2009, 11, 5868; [3d] B. Liu, W. P. Wu, B. Peng, Y. Q. Liu, Y. H. He, C. Y. Pan, Y. P. Zou, Polym. Chem. 2010, 1, 678.
[4] [4a] Z. H. Zhang, B. Peng, B. Liu, C. Y. Pan, Y. F. Li, Y. H. He, K. C. Zhou, Y. P. Zou, Polym. Chem. 2010, DOI: 10.1039/ c0py00136h; [4b] W. Yue, Y. Zhao, S. Y. Shao, H. K. Tian, Z. Y. Xie, Y. H. Geng, F. S. Wang, J. Mater. Chem. 2009, 19, 2199; [4c] Y. P. Zou, D. Gendron, R. B. Aı¨ch, A. Najari, Y. Tao, M. Leclerc, Macromolecules 2009, 42, 2891; [4d] B. Liu, A. Najari, C. Y. Pan, M. Leclerc, D. Q. Xiao, Y. P. Zou, Macromol. Rapid Commun. 2010, 31, 391.
[5] E. Ahmed, F. S. Kim, H. Xin, S. A. Jenekhe, Macromolecules. 2009, 42, 8615.
[6] R. P. Qin, W. W. Li, C. H. Li, C. Du, C. Veit, H. F. Schleiermacher, M. Andersson, Z. S. Bo, Z. P. Liu, O. Ingana¨s, U. Wuerfel, F. Zhang, J. Am. Chem. Soc. 2009, 131, 14612.
[7] [7a] Y. Liang, D. Feng, Y. Wu, S. T. Tsai, G. Li, C. Ray, L. Yu, J. Am. Chem. Soc. 2009, 131, 7792; [7b] H. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, G. Li, Nat. Photon. 2009, 3, 649.
[8] M. Helgesen, S. A. Gevorgyan, F. C. Krebs, R. A. J. Janssen, Chem. Mater. 2009, 21, 4669.
[9] J. H. Hou, M. H. Park, S. Zhang, Y. Yao, L. M. Chen, J. H. Li, Y. Yang, Macromolecules 2008, 41, 6012.
[10] Y. P. Zou, D. Gendron, R. Neagu-Plesu, M. Leclerc, Macromol-ecules 2009, 42, 6361.
[11] Q. J. Sun, H. Q. Wang, C. H. Yang, Y. F. Li, J. Mater. Chem. 2003, 13, 800.
Figure 7. AFM images of PBDT-DODTBT/PC71BM (1:2) prepared from 1,2-dichlorobenzene solution: topographic image (left) and phase image
[12] J. H. Hou, Z. A. Tan, Y. Yan, Y. J. He, C. H. Yang, Y. F. Li, J. Am. Chem. Soc. 2006, 128, 4911.
[13] [13a] Q. Sun, H. Wang, C. Yang, Y. F. Li, J. Mater. Chem. 2003, 13, 800; [13b] Y. F. Li, Y. Cao, J. Gao, D. L. Wang, G. Yu, A. J. Heeger, Synth. Met. 1999, 99, 243.
[14] I. Osaka, G. Sauve, R. Zhang, T. Kowalewski, R. D. McCullough, Adv. Mater. 2007, 19, 4160.
[15] [15a] G. G. Malliaras, J. R. Salem, P. J. Brock, C. Scott, Phys. Rev. B 1998, 58, 13411; [15b] H. C. F. Martens, H. B. Brom, P. W. M. Blom, Phys. Rev. B 1999, 60, 8489.
[16] S. Gu¨nes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007, 107, 1324.
[17] C. V. Hoven, X. D. Dang, R. C. Coffin, J. Peet, T. Q. Nguyen, G. C. Bazan, Adv. Mater. 2010, 22, E63.
