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D-A-D structured organic molecules with diketopyrrolopyrrole acceptor unit for solution-processed organic solar cells Jing Zhang, Chang He, Zhi-Guo Zhang, Dan Deng, Maojie Zhang and Yongfang Li Phil. Trans. R. Soc. A 2014 372, 20130009, published 10 March 2014
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D-A-D structured organic molecules with diketopyrrolopyrrole acceptor unit for solution-processed organic solar cells Jing Zhang, Chang He, Zhi-Guo Zhang, Dan Deng,
Research
Maojie Zhang and Yongfang Li
Cite this article: Zhang J, He C, Zhang Z-G, Deng D, Zhang M, Li Y. 2014 D-A-D structured organic molecules with diketopyrrolopyrrole acceptor unit for solution-processed organic solar cells. Phil. Trans. R. Soc. A 372: 20130009. http://dx.doi.org/10.1098/rsta.2013.0009
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
One contribution of 8 to a Theo Murphy Meeting Issue ‘Theo Murphy International Scientific Meeting between the UK and China on the chemistry and physics of functional materials’.
Subject Areas: materials science Keywords: organic solar cells, solution-processable organic molecules, diketopyrrolopyrrole-containing molecules Authors for correspondence: Chang He e-mail: [email protected] Yongfang Li e-mail: [email protected]
Four solution-processable D-A-D structured organic molecules with diketopyrrolopyrrole (DPP) as acceptor unit and triphenylamine (TPA) or (4-hexyl) thieno [3,2-b]thiophene (HTT) as donor unit, DPP8TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT, were designed and synthesized for the application as donor materials in solution-processed organic solar cells (OSCs). The molecules show broad absorption and relatively lower highest occupied molecular orbital energy levels. Photovoltaic properties of the molecules were investigated by fabricating the bulkheterojunction OSCs with the molecules as donor and PC71 BM as acceptor. Power conversion efficiency of the OSC based on DPP8-HTT reached 1.5% under the illumination of AM1.5, 100 mW cm−2 .
1. Introduction Solution-processable organic small molecules have attracted much interest recently for applications in organic solar cells (OSCs) [1–10] owing to their advantages of high purity and definite molecular weight in comparison with conjugated polymers, and the low-cost solution processability in comparison with vaccum-deposited small molecule photovoltaic materials. In the molecular design of photovoltaic donor materials in solution-processed OSCs, good solubility, narrower band gap with broad absorption, relatively lower HOMO (the highest occupied molecular orbital) energy level and higher hole mobility are 2014 The Author(s) Published by the Royal Society. All rights reserved.
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(a) Synthesis and thermal properties The four linear molecules were synthesized by the palladium-catalysed Heck and Suzuki reaction, as shown in Scheme 1. Monomers 1–5 were synthesized according to the literature [24,27,30]. The four compounds are soluble in common organic solvents, such as CHCl3 , tetrahydrofuran (THF), chlorobenzene and toluene. The thermal stability of the compounds was investigated by thermogravimetric analysis (TGA). The temperatures with 5% weight loss are at 368◦ C, 363◦ C, 366◦ C and 337◦ C for DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT, respectively, as shown in figure 1. The stability of the materials is good enough for application in optoelectronic devices.
(b) Absorption spectra Figure 2 shows the UV-vis absorption spectra of the four compounds in chloroform solutions and in solid films. The optical absorption peak wavelength (λ) of the compounds is summarized in table 1. Benefitting from their D-A molecular structure, the absorption spectra of the four compound solutions in chloroform exhibit an absorption peak at around 350 nm and a strong absorption band in the wavelength range from 400 to 700–750 nm, as shown in figure 2a. The strong absorption band in the visible region could be assigned to the intramolecular charge transfer transition between the donor moiety (TPA, TT) and DPP acceptor unit. Owing to the stronger electron donating effects of the TPA unit, DPP8-TPA and DPP8-TPAOR solutions exhibit more red-shifted absorption (by ca 30 nm) than that of DPP6-HTT and DPP8-HTT solutions.
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2. Results and discussion
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key issues for high photovoltaic performance [11]. Among the requisites of the properties, good solubility of the organic donor materials is required for solution-processing. Broad absorption, relatively lower HOMO level and higher hole mobility of the donor materials are needed for greater short circuit current (Jsc ), higher open-circuit voltage (Voc ) and larger fill factor (FF) to get higher power conversion efficiency (PCE) of the OSCs. Roncali and colleagues were the first to use triphenylamine (TPA)-containing molecules in the solution-processed OSCs [12–14]. Benefitting from the three-dimensional propeller structure and good hole-transporting property of TPA units, TPA-containing molecules show good solubility and attractive photovoltaic performance [12–23]. The highest PCE of the solution-processed OSCs based on a star-shaped TPA-containing molecule as donor and PC71 BM as acceptor reached 4.3% [20]. Nguyen and colleagues introduced diketopyrrolopyrrole (DPP) unit into the solutionprocessable organic molecule photovoltaic materials [24–26]. A linear D-A-D structured molecule DPP(TBFu)2 with DPP as acceptor (A) unit, benzofuran as donor (D) unit and thiophene as pi-bridges, possesses a broad absorption and suitable electronic energy levels. The solutionprocessed OSCs based on DPP(TBFu)2 as donor and PC71 BM as acceptor demonstrated a PCE of 4.4% [25]. Here, we designed and synthesized a series of new linear and D-A-D structured molecule donor materials, DPP8-TPA, DPP8-TPA-OR, DPP6-(4-hexyl)thieno[3,2-b]thiophene (HTT) and DPP8-HTT, with DPP as core and acceptor unit, TPA or thieno[3,2-b]thiophene (TT) as donor unit with different substituents for modulating the solubility, absorption spectra, HOMO energy levels and hole mobility of the molecules. The TPA unit used in the molecules is for good solubility and higher hole mobility as mentioned above [12–23]. The consideration of using the TT unit in DPP8-HTT and DPP6-HTT is from the planar structure and higher hole mobility of the molecules containing the fused thiophene unit [27–29]. The four molecule films of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT show good thermal stability and broad absorption band in the range 350–750 nm. The solution-processed OSCs based on the blend of DPP8-HTT and PC71 BM (1 : 3, w/w) reached 1.50% under the illumination of AM1.5G, 100 mW cm−2 .
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O
N
2
+ S
N C8H17
I
OC8H17
DPP8-TPA
Br
C8H17O S
N 3
O
OC8H17 C8H17O
Br
N
S O
N C8H17
S
Br
N
S
S
+ C6H13 N
S
O
O
S
N
II S
5
O
OC8H17
C8H17 N O
S
C6H13
Br
N
S
DPP8-TPA-OR
O S
OC8H17
C8H17 N O
N
1
N
S
S S C6H13
4
S
S S
DPP8-HTT
B O O
Br
N C8H17
C6H13
C6H13 N O
S O
S N C6H13
C6H13 S S
DPP6-HTT
Scheme 1. Synthetic routes of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT. I Pd(OAc)2 , NaOAc, n-Bu4 NBr, DMF, 100◦ C for 24 h, under N2 ; II Pd2 (dba)3 , K3 PO4 (2 mol l−1 ), toluene, 120◦ C reflux for 24 h.
100
weight (%)
80 60 DPP8-TPA DPP8-TPA-OR DPP6-HTT DPP8-HTT
40 20
0
100
300 400 200 temperature (°C)
500
Figure 1. TGA plots of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT. For all four molecules, the absorption peaks of their films (figure 2b) red shifted in comparison with their dilute solutions, and the red shift of the DPP6-HTT and DPP8-HTT films is much greater than that of DPP8-TPA and DPP8-TPA-OR films, indicating that there is stronger intermolecular interaction in the TT-containing compound films than that of the TPA-containing compound films. Obviously, better planarity of the TT-containing molecules benefits the stronger intermolecular interaction. Moreover, the absorption spectrum of DPP8-TPA and DPP6-HTT films
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N O
S
O
3
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Br
S
N
N
C8H17 N O
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(a) 1.0
absorbance
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0.8
4
DPP8-TPA solution DPP8-TPA-OR solution DPP6-HTT solution DPP8-HTT solution
0.6 0.4 0.2 0 400
normalized absorbance
(b)
600
1.0
800 DPP8-TPA film DPP8-TPA-OR film DPP6-HTT film DPP8-HTT film
0.5
0 400
800 1000 600 wavelength (nm)
1200
Figure 2. (a,b) UV-vis absorption spectra of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT in chloroform solution and film state. (Online version in colour.)
Table 1. Absorption spectra and electrochemical properties of compounds DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT. compounds
λsolution (nm)
λfilm (nm)
red Eonset (V)/LUMO (eV)
ox Eonset (V)/HOMO (eV)
DPP8-TPA
370/626/664
379/640/693
−1.19/−3.52
0.43/−5.14
DPP8-TPA-OR
371/639/676
376/641/703
−1.18/−3.53
0.32/−5.03
DPP6-HTT
364/595/636
392/641/716
−1.13/−3.58
0.55/−5.26
DPP8-HTT
364/598/634
379/609/685
−1.18/−3.53
0.47/−5.18
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is more red shifted than that of DPP8-TPA-OR and DPP8-HTT films, respectively, indicating that fewer or shorter side chains in the molecules result in stronger intermolecular interactions.
(c) Electrochemical properties Electrochemical cyclic voltammetry was carried out for measuring the HOMO and LUMO energy levels of the compounds. Figure 3 shows the cyclic voltammograms of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT films and the values of the onset reduction potentials (Ered onset ); onset ) of the compounds are listed in table 1. It can be seen from figure 3 oxidation potentials (Eox onset
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5
DPP8-TPA
current (mA)
DPP8-HTT
–2
–1 0 potential (V versus Ag/Ag+)
1
Figure 3. Cyclic voltammograms of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT films on Pt electrode in an acetonitrile solution of 0.1 mol l−1 Bu4 NPF6 (Bu = butyl) with a scan rate of 100 mV s−1 . that the oxidation processes of the two HTT-based molecules DPP6-HTT and DPP8-HTT are reversible, but the reduction processes of the four compounds are irreversible. The HOMO, LUMO energy levels and the electrochemical band gaps of the four materials were estimated, respectively, from the onset oxidation and reduction potentials according to red the following equations [31,32]: EHOMO = −e(Eox onset + 4.71) (eV), ELUMO = −e(Eonset t + 4.71) (eV) and Eg = ELUMO − EHOMO . The LUMO energy levels of the four compounds are around −3.5 eV, which change little for different molecules. But the HOMO energy levels of the molecules change greatly from −5.03 eV for DPP8-TPA-OR to −5.26 eV for DPP6-HTT (table 1). The molecules of DPP8-HTT and DPP6-HTT containing the TT unit show lower HOMO energy levels, which is beneficial to higher Voc of the OSCs with the compounds as donor. The electrochemical band gaps of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT are 1.61, 1.50, 1.68 and 1.65 eV, respectively. Compared with the HOMO (−5.87 eV) and LUMO (−3.91 eV) energy levels for PC71 BM, [33] the compounds are suitable to be used as photovoltaic donors with PC71 BM as acceptor on the energy level point of view.
(d) Morphology of the blend films The morphology of the photoactive layers is very important for the photovoltaic performance of OSCs [34]. We used atomic force microscopy (AFM) to investigate the morphology of the four organic molecules: PC71 BM (1 : 3, w/w) blend films. All of the AFM images of the blend films exhibit typical amorphous morphology without any crystalline domains, as shown in figure 4. The blend films of DPP8-TPA/PC71 BM and DPP6-HTT/PC71 BM exhibit bigger aggregates, compared with that of DPP-8-TPA-OR/PC71 BM and DPP8-HTT/PC71 BM.
(e) Photovoltaic properties Bulk-heterojunction OSCs were fabricated by using the four compounds as donor, PC71 BM [35] as acceptor and Al as the negative electrode. The device structure is ITO/PEDOT: PSS/photoactive layer/Al. The active layer is composed of a blend film of the organic molecule donor and PC71 BM acceptor with a weight ratio of 1 : 3. Figure 5 shows the current density–voltage curves of the OSCs in the dark and under the illumination of AM 1.5, 100 mW cm−2 . The photovoltaic performance data of the OSCs, including Voc , Jsc , FF and PCE values, are summarized in table 2 for a clear comparison. The best PCE of the device based on DPP8-HTT reached 1.50% with a higher Voc of 0.73 V and a Jsc of 6.62 mA cm−2 . In comparison to the photovoltaic performance of DPP8-TPA and DPP8-HTT, it can be seen that DPP8-HTT showed higher Voc and higher Jsc values than
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DPP6-HTT
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DPP8-TPA-OR
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(a)
5
(b)
6
10.0 nm
5.0 nm
0 nm 50
0
5
0
Figure 4. AFM images of the blend films of organic molecules: PC71 BM (1 : 3, w/w), the organic molecules are (a) DPP8-TPA, (b) DPP8-TPA-OR, (c) DPP6-HTT and (d) DPP8-HTT. The scan size of the images is 5 × 5 μm. (Online version in colour.) Table 2. Photovoltaic performance of ITO/PEDOT : PSS/organic molecules: PC71 BM (1 : 3, w/w)/Al negative electrode under the illumination of AM 1.5, 100 mW cm−2 . organic molecule in the active layer
Voc (V)
Jsc (mA cm−2 )
FF (%)
PCE (%)
DPP8-TPA
0.58
5.00
30.0
0.90
DPP8-TPA-OR
0.63
5.66
34.0
1.20
DPP6-HTT
0.73
4.36
28.3
0.90
DPP8-HTT
0.73
6.62
30.5
1.50
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that of DPP8-TPA, which indicates that the planar structure of the TT unit is beneficial to the improved photovoltaic performance of the compounds. The poorer photovoltaic performance of the OSC based on DPP6-HTT could be owing to the lower solubility of DPP6-HTT with too short side chains. In conclusion, a series of linear D-A-D molecules, DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT with DPP as core and acceptor unit and TPA or TT as donor unit, was designed and synthesized for application as donor materials in solution-processed OSCs. The four compound films show broad absorption band and lower band gap. The donor unit of TPA or TT and the side chains of the molecules influence the photovoltaic performance of the compounds significantly. The OSC devices based on a blend of DPP8-HTT and PC71 BM (1 : 3, w/w) exhibited a PCE of 1.50% with a Voc of 0.73 V and a Jsc of 6.62 mA cm−2 , under the illumination of AM1.5, 100 mW cm−2 . The results indicate that the planar donor unit of TT is better than the TPA unit in the D-A-D structured DPP-containing molecules photovoltaic materials.
3. Experimental section (a) Chemicals TPA, n-butyllithium (2.50 mol l−1 in hexane), tetrabutylammonium bromide, sodium acetate, palladium acetate, Pd2 (dba)3 , K3 PO4 , toluene and DMF were obtained from Acros Organics. THF was dried over Na/benzophenoneketyl and freshly distilled prior to use. Monomers 1–5
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0 5
(d)
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(c)
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(b) 100
10
1
1 0.1
0.1
0.01 0.01 10–3 10–4
(c)
(d) DPP6-HTT dark DPP6-HTT illuminated
100
10
1
1
0.1
0.1
0.01 10–3 –1.5
DPP8-HTT dark DPP8-HTT illuminated
100
10
0.01 –1.0
–0.5
0 0.5 voltage (v)
1.0
1.5
10–3 –1.5
–1.0
–0.5
0 0.5 voltage (v)
1.0
1.5
Figure 5. (a–d) Current density–voltage characteristics of the OSC devices based on the blends of linear molecules/PC71 BM (1 : 3, w/w) with Al as the negative electrode in the dark and under the illumination of AM1.5, 100 mW cm−2 .
were synthesized according the literature [24,27,30]. Other chemicals were common commercial chemicals and were used as received.
(b) Measurements Nuclear magnetic resonance (NMR) spectra were taken on a Bruker DMX-400 spectrometer. MALDI-TOF spectra were recorded on a Bruker BIFLEXIII. Absorption spectra were taken on a Hitachi U-3010 UV-vis spectrophotometer. The film on quartz used for UV measurements was prepared by spin-coating with 1% chloroform solution. The TGA measurement was performed on a Perkin-Elmer TGA-7 apparatus. The electrochemical cyclic voltammetry was carried out with a Zahner IM6e electrochemical workstation in a 0.1 mol l−1 tetrabutylammonium hexafluorophosphate (Bu4 NPF6 ) acetonitrile solution. A glassy carbon electrode coated with the sample film was used as the working electrode, a Pt wire and Ag/Ag+ (0.01 M AgNO3 in acetonitrile) were used as the counter and reference electrodes, respectively.
(c) Fabrication and characterization of organic solar cells OSCs were fabricated in the configuration of the traditional sandwich structure with an ITO positive electrode and a metal negative electrode. Patterned ITO glass with a sheet resistance of 30 Ω sq−1 was purchased from CSG Holding Co., Ltd (China). The ITO glass was cleaned in an ultrasonic bath of acetone and isopropanol and treated by UVO (ultraviolet ozone cleaner, Jelight Company, USA). Then a thin layer (30 nm) of PEDOT: PSS (poly(3,4-ethylenedioxythiophene)poly(styrene sulfonate)) (Baytron PVP A1 4083, Germany) was spin-coated on the ITO glass. Subsequently, the photosensitive layer was prepared by spin-coating the blend solution of linear materials and PC71 BM (1 : 3 w/w) on the top of the PEDOT:PSS layer (ca 30 nm thickness) and baked at 80◦ C for 30 min. The concentration of the solution was 10 mg ml−1 in chlorobenzene. The
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DPP-TPA OR dark DPP-TPA OR illuminated
100
10
10–3
current density (mA cm–2)
7
1000
DPP8-TPA dark DPP8-TPA illuminated
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20130009
current density (mA cm–2)
(a)
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DPP8-TPA: monomer 1 (0.682 g, 1 mmol), N,N-diphenyl-N-(4-vinylphenyl)amine (0.596 g, 2.2 mmol), palladium (II) acetate (5 mg), tetra-n-butylammonium bromide (103 mg, 0.32 mmol) and sodium acetate anhydrous (1.64 g, 20 mmol) were dissolved and kept in degassed N,Ndimethyl formamide (30 ml), under argon at 100◦ C for 24 h. The mixture was poured into water (30 ml). The precipitate was filtered, washed with water and dissolved in dichloromethane, and dried over anhydrous sodium sulfate. After evaporation of the solvent, the residue was purified by column chromatography (petroleum/dichloromethane, 1 : 1). Yield: 656 mg (62%). 1 H NMR (CDCl3 , 400 MHz, δ/ppm): 7.7 (d, 2H), 7.3 (d, 2H), 7.05(d, 4H), 7.01(t, 8H), 6.99 (m, 8H), 6.46 (m, 8H), 6.41(d, 4H), 4.05 (d, 4H), 2.10 (m, 2H), 1.39 (m, 20H), 0.93 (t, 6H). MALDI-TOF MS: 1063.66, calcd for C70 H70 N4 O2 S2 1063.56. Anal. Calcd for C70 H70 N4 O2 S2 : C, 79.09; H, 6.59; N, 5.27. Found: C, 77.99; H, 6.79; N, 5.05. DPP8-TPA-OR: monomer 1 (0.682 g, 1 mmol), N,N-di(n-octyloxyphenyl)-4-(vinylphenyl) amine (1.16 g, 2.2 mmol), palladium (II) acetate (5 mg), tetra-n-butylammonium bromide (103 mg, 0.32 mmol) and sodium acetate anhydrous (1.64 g, 20 mmol) were dissolved and kept in degassed N,N-dimethyl formamide (30 ml), under argon at 100◦ C for 24 h. The mixture was poured into water (30 ml). The precipitate was filtered, washed with water and dissolved in dichloromethane, dried over anhydrous sodium sulfate. After the evaporation of the solvent, the residue was purified by column chromatography (petroleum/dichloromethane, 2 : 1). Yield: 320 mg (20%). 1 H NMR (CDCl3 , 400 MHz, δ/ppm): 8.91 (d, 2H), 7.3 (d, 4H), 7.15(d, 2H), 7.09 (m, 10H), 7.00 (m, 2H), 5.88 (m, 12H), 4.10 (t, 4H), 3.05 (d, 4H), 2.10 (m, 2H), 1.74 (m, 4H), 1.33 (m, 42H), 0.90 (t, 12H). MALDI-TOF MS: 1575.9, calcd for C102 H134 N4 O6 S2 1576.31. Anal. Calcd for C102 H134 N4 O6 S2 : C, 77.76; H, 8.51; N, 3.55. Found: C, 77.68; H, 8.64; N, 3.52. DPP6-HTT: In a three-necked, oven-dried 100 ml round-bottom flask, monomer 4 (0.626 g, 1 mmol) was mixed with 15 ml of anhydrous toluene and 10 ml of 2.0 M potassium phosphate and the resulting mixture was degassed for 10 min. Monomer 5 (0.77 g, 2.20 mmol), tris(dibenzylideneacetone)dipalladium(0) (14 mg, 0.0153 mmol) and tri-tert-butylphosphonium tetrafluoroborate (18 mg, 0.0620 mmol) were then added to the mixture and then degassed again for 5 min. The reaction mixture was stirred and heated to 120◦ C under argon overnight. The reaction mixture was allowed to cool down to room temperature, then was poured into 300 ml of methanol and stirred for 30 min. The precipitated solid was then collected by vacuum filtration and washed with several portions of distilled water, methanol, isopropanol and petroleum ether. The crude product was purified by flash chromatography using chloroform as eluent, and the solvent was removed in vacuum to obtain a pure product. Yield: 520 mg (57%). 1 H NMR (CDCl3 , 400 MHz, δ/ppm): 8.97 (d, 2H), 7.48 (s, 2H), 7.34 (d, 2H), 6.99 (s, 2H), 4.12(t, 4H), 2.74 (t, 4H), 1.77(m, 8H), 1.47 (m, 4H), 1.36 (m, 20H), 0.91 (t, 12H). MALDI-TOF MS: 912.3, calcd for C50 H60 N2 O2 S6 913.4. Anal. Calcd for C50 H60 N2 O2 S6 : C, 65.78; H, 6.57; N, 3.07. Found: C, 65.35; H, 6.64; N, 3.13. DPP8-HTT: In a three-necked, oven-dried 100 ml round-bottom flask, monomer 1 (0.683 g, 1.00 mmol) was mixed with 15 ml of anhydrous toluene and 10 ml of 2.0 M potassium phosphate and the resulting mixture was degassed for 10 min. Monomer 5 (0.77 g, 2.20 mmol), tris(dibenzylideneacetone)dipalladium(0) (14 mg, 0.0153 mmol), and tri-tert-butylphosphonium tetrafluoroborate (18 mg, 0.0620 mmol) were then added to the mixture and then degassed again for 5 min. The reaction mixture was stirred and heated to 120◦ C under argon overnight. The reaction mixture was allowed to cool down to room temperature, and then it was poured
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thickness of the photoactive layer was measured using an Ambios Technology XP-2 profilometer. Finally, an Al electrode (ca 120 nm) was vacuum-evaporated on the photoactive layer under a shadow mask in the vacuum of ca 10−4 Pa. The current–voltage (J–V) measurement of the devices was conducted on a computer-controlled Keithley 236 Source Measure Unit. A xenon lamp coupled with AM1.5 solar spectrum filters was used as light source, and the optical power at the sample was 100 mW cm−2 .
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(no. 2014CB643501), NSFC (no. 51373181) and the China Postdoctoral Science Foundation (no. 2012M510554).
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Funding statement. This work was supported by The Ministry of Science and Technology of China
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into 300 ml of methanol and stirred for 30 min. The precipitated solid was then collected by vacuum filtration and washed with several portions of distilled water, methanol, isopropanol and petroleum ether. The crude product was purified by flash chromatography using chloroform as eluent, and the solvent was removed in vacuum to obtain a pure product. Yield: 490 mg (51%). 1 H NMR (CDCl , 400 MHz, δ/ppm): 8.96 (d, 2H), 7.46 (s, 2H), 7.33 (d, 2H), 7.02 (s, 2H), 4.02 (t, 4H), 3 2.74 (t, 4H), 1.94 (t, 2H), 1.77 (m, 4H), 1.38 (m, 34H), 0.94 (m, 12H). MALDI-TOF MS: 968.4, calcd for C54 H68 N2 O2 S6 969.52. Anal. Calcd for C54 H68 N2 O2 S6 : C, 66.94; H, 7.02; N, 2.89. Found: C, 65.97; H, 7.13; N, 2.84.
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18. Zhang J, Yang Y, He C, Li YF. 2011 Red-emission organic light-emitting diodes based on solution-processable molecules with triphenylamine core and benzothiadiazole-thiophene arms. Sci. China Chem. 54, 695–698. (doi:10.1007/s11426-010-4189-3) 19. Gao L, Zhang J, He C, Shen SL, Zhang Y, Liu HT, Sun QJ, Li YF. 2013 Synthesis and photovoltaic properties of a star-shaped molecule based on triphenylamine core and branched terthiophene end groups’. Sci. China Chem. 56, 997–1003. (doi:10.1007/s11426-013-4878-9) 20. Shang HX, Fan HJ, Liu Y, Hu WP, Li YF, Zhan XW. 2011 A solution-processable star-shaped molecule for high-performance organic solar cells. Adv. Mater. 23, 1554–1557. (doi:10.1002/ adma.201004445) 21. Xue LL, He JT, Gu X, Yang ZF, Xu B, Tian WJ. 2009 Efficient bulk-heterojunction solar cells based on a symmetrical D-π -A-π -D organic dye molecule. J. Phys. Chem. C 113, 12 911–12 917. (doi:10.1021/jp8080996) 22. Li ZF et al. 2011 Design and synthesis of solution processable small molecules towards high photovoltaic performance. J. Mater. Chem. 21, 2159–2168. (doi:10.1039/c0jm02510k) 23. Wen SP, Pei JN, Zhou YH, Li PF, Xue LL, Li YW, Xu B, Tian WJ. 2009 Synthesis of 4,7-diphenyl2,1,3-benzothiadiazole-based copolymers and their photovoltaic applications. Macromolecules 42, 4977–4984. (doi:10.1021/ma900598c) 24. Tamayo AB, Walker B, Nguyen T-Q. 2008 A low band gap, solution processable oligothiophene with a diketopyrrolopyrrole core for use in organic solar cells. J. Phys. Chem. C 112, 11 545–11 551. (doi:10.1021/jp8031572) 25. Walker B, Tamayo AB, Dang XD, Zalar P, Seo JH, Garcia A, Tantiwiwat M, Nguyen T-Q. 2009 Nanoscale phase separation and high photovoltaic efficiency in solution-processed, small-molecule bulk heterojunction solar cells. Adv. Funct. Mater. 19, 3063–3069. (doi:10.1002/ adfm.200900832) 26. Ripaud E, Demeter D, Rousseau T, Boucard-Cetol E, Allain M, Po R, Leriche P, Roncali J. 2012 Structure–properties relationships in conjugated molecules based on diketopyrrolopyrrole for organic photovoltaics. Dyes Pigments 95 126–133. (doi:10.1016/j.dyepig.2012.03.021) 27. He YJ, Wu WP, Zhao GJ, Liu YQ, Li YF. 2008 Poly(3,6-dihexyl-thieno [3,2-b] thiophene vinylene): synthesis, field-effect transistors and photovoltaic properties. Macromolecules 41, 9760–9766. (doi:10.1021/ma801923c) 28. Baek NS, Hau SK, Yip HL, Acton O, Chen KS, Jen AKY. 2008 High performance amorphous mettallated pi-conjugated polymers for field-effect transistors and polymer solar cells. Chem. Mater. 20, 5734–5736. (doi:10.1021/cm8016424) 29. Tang WH, Singh SP, Ong KH, Chen ZK. 2010 Synthesis of thieno[3,2-b]thiophene derived conjugated oligomers for field-effect transistors applications. J. Mater. Chem. 20, 1497–1450. (doi:10.1039/b920112b) 30. Deng D, Yang Y, Zhang J, He C, Zhang MJ, Zhang ZG, Zhang ZJ, Li YF. 2011 Triphenylamine-containing linear D-A-D molecules with benzothiadiazole as acceptor unit for bulk-heterojunction organic solar cells. Org. Electron. 12, 614–622. (doi:10.1016/j.orgel.2011.01.013) 31. Sun QJ, Wang HQ, Yang CH, Li YF. 2003 Synthesis and electroluminescence of novel block copolymers containing crown ether spacers. J. Mater. Chem. 13, 800–806. (doi:10.1039/ b209469j) 32. Hou JH, Tan ZA, Yan Y, He YJ, Yang CH, Li YF. 2006 Synthesis and photovoltaic properties of two-dimensional conjugated polythiophenes with bi(thienylenevinylene) side chains. J. Am. Chem. Soc. 128, 4911–4916. (doi:10.1021/ja060141m) 33. He YJ, Zhao GJ, Peng B, Li YF. 2010 High yield synthesis, electrochemical and photovoltaic properties of indene-C70 bisadduct. Adv. Funct. Mater. 20, 3383–3389. (doi:10.1002/adfm. 201001122) 34. Li G, Shrotriya V, Huang JS, Yao Y, Moriarty T, Emery K, Yang Y. 2005 High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nat. Mater. 4, 864–868. (doi:10.1038/nmat1500) 35. Wienk MM, Kroon JM, Verhees WJH, Knol J, Hummelen JC, van Hal PA, Janssen RAJ. 2003 Efficient methano[70]fullerene/MDMO-PPV bulk heterojunction photovoltaic cells. Angew. Chem. Int. Ed. 42, 3371–3375. (doi:10.1002/anie.200351647)
D-A-D structured organic molecules with diketopyrrolopyrrole acceptor unit for solution-processed organic solar cells Jing Zhang, Chang He, Zhi-Guo Zhang, Dan Deng, Maojie Zhang and Yongfang Li Phil. Trans. R. Soc. A 2014 372, 20130009, published 10 March 2014
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D-A-D structured organic molecules with diketopyrrolopyrrole acceptor unit for solution-processed organic solar cells Jing Zhang, Chang He, Zhi-Guo Zhang, Dan Deng,
Research
Maojie Zhang and Yongfang Li
Cite this article: Zhang J, He C, Zhang Z-G, Deng D, Zhang M, Li Y. 2014 D-A-D structured organic molecules with diketopyrrolopyrrole acceptor unit for solution-processed organic solar cells. Phil. Trans. R. Soc. A 372: 20130009. http://dx.doi.org/10.1098/rsta.2013.0009
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
One contribution of 8 to a Theo Murphy Meeting Issue ‘Theo Murphy International Scientific Meeting between the UK and China on the chemistry and physics of functional materials’.
Subject Areas: materials science Keywords: organic solar cells, solution-processable organic molecules, diketopyrrolopyrrole-containing molecules Authors for correspondence: Chang He e-mail: [email protected] Yongfang Li e-mail: [email protected]
Four solution-processable D-A-D structured organic molecules with diketopyrrolopyrrole (DPP) as acceptor unit and triphenylamine (TPA) or (4-hexyl) thieno [3,2-b]thiophene (HTT) as donor unit, DPP8TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT, were designed and synthesized for the application as donor materials in solution-processed organic solar cells (OSCs). The molecules show broad absorption and relatively lower highest occupied molecular orbital energy levels. Photovoltaic properties of the molecules were investigated by fabricating the bulkheterojunction OSCs with the molecules as donor and PC71 BM as acceptor. Power conversion efficiency of the OSC based on DPP8-HTT reached 1.5% under the illumination of AM1.5, 100 mW cm−2 .
1. Introduction Solution-processable organic small molecules have attracted much interest recently for applications in organic solar cells (OSCs) [1–10] owing to their advantages of high purity and definite molecular weight in comparison with conjugated polymers, and the low-cost solution processability in comparison with vaccum-deposited small molecule photovoltaic materials. In the molecular design of photovoltaic donor materials in solution-processed OSCs, good solubility, narrower band gap with broad absorption, relatively lower HOMO (the highest occupied molecular orbital) energy level and higher hole mobility are 2014 The Author(s) Published by the Royal Society. All rights reserved.
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(a) Synthesis and thermal properties The four linear molecules were synthesized by the palladium-catalysed Heck and Suzuki reaction, as shown in Scheme 1. Monomers 1–5 were synthesized according to the literature [24,27,30]. The four compounds are soluble in common organic solvents, such as CHCl3 , tetrahydrofuran (THF), chlorobenzene and toluene. The thermal stability of the compounds was investigated by thermogravimetric analysis (TGA). The temperatures with 5% weight loss are at 368◦ C, 363◦ C, 366◦ C and 337◦ C for DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT, respectively, as shown in figure 1. The stability of the materials is good enough for application in optoelectronic devices.
(b) Absorption spectra Figure 2 shows the UV-vis absorption spectra of the four compounds in chloroform solutions and in solid films. The optical absorption peak wavelength (λ) of the compounds is summarized in table 1. Benefitting from their D-A molecular structure, the absorption spectra of the four compound solutions in chloroform exhibit an absorption peak at around 350 nm and a strong absorption band in the wavelength range from 400 to 700–750 nm, as shown in figure 2a. The strong absorption band in the visible region could be assigned to the intramolecular charge transfer transition between the donor moiety (TPA, TT) and DPP acceptor unit. Owing to the stronger electron donating effects of the TPA unit, DPP8-TPA and DPP8-TPAOR solutions exhibit more red-shifted absorption (by ca 30 nm) than that of DPP6-HTT and DPP8-HTT solutions.
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2. Results and discussion
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key issues for high photovoltaic performance [11]. Among the requisites of the properties, good solubility of the organic donor materials is required for solution-processing. Broad absorption, relatively lower HOMO level and higher hole mobility of the donor materials are needed for greater short circuit current (Jsc ), higher open-circuit voltage (Voc ) and larger fill factor (FF) to get higher power conversion efficiency (PCE) of the OSCs. Roncali and colleagues were the first to use triphenylamine (TPA)-containing molecules in the solution-processed OSCs [12–14]. Benefitting from the three-dimensional propeller structure and good hole-transporting property of TPA units, TPA-containing molecules show good solubility and attractive photovoltaic performance [12–23]. The highest PCE of the solution-processed OSCs based on a star-shaped TPA-containing molecule as donor and PC71 BM as acceptor reached 4.3% [20]. Nguyen and colleagues introduced diketopyrrolopyrrole (DPP) unit into the solutionprocessable organic molecule photovoltaic materials [24–26]. A linear D-A-D structured molecule DPP(TBFu)2 with DPP as acceptor (A) unit, benzofuran as donor (D) unit and thiophene as pi-bridges, possesses a broad absorption and suitable electronic energy levels. The solutionprocessed OSCs based on DPP(TBFu)2 as donor and PC71 BM as acceptor demonstrated a PCE of 4.4% [25]. Here, we designed and synthesized a series of new linear and D-A-D structured molecule donor materials, DPP8-TPA, DPP8-TPA-OR, DPP6-(4-hexyl)thieno[3,2-b]thiophene (HTT) and DPP8-HTT, with DPP as core and acceptor unit, TPA or thieno[3,2-b]thiophene (TT) as donor unit with different substituents for modulating the solubility, absorption spectra, HOMO energy levels and hole mobility of the molecules. The TPA unit used in the molecules is for good solubility and higher hole mobility as mentioned above [12–23]. The consideration of using the TT unit in DPP8-HTT and DPP6-HTT is from the planar structure and higher hole mobility of the molecules containing the fused thiophene unit [27–29]. The four molecule films of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT show good thermal stability and broad absorption band in the range 350–750 nm. The solution-processed OSCs based on the blend of DPP8-HTT and PC71 BM (1 : 3, w/w) reached 1.50% under the illumination of AM1.5G, 100 mW cm−2 .
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O
N
2
+ S
N C8H17
I
OC8H17
DPP8-TPA
Br
C8H17O S
N 3
O
OC8H17 C8H17O
Br
N
S O
N C8H17
S
Br
N
S
S
+ C6H13 N
S
O
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II S
5
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C8H17 N O
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O S
OC8H17
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1
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S S C6H13
4
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B O O
Br
N C8H17
C6H13
C6H13 N O
S O
S N C6H13
C6H13 S S
DPP6-HTT
Scheme 1. Synthetic routes of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT. I Pd(OAc)2 , NaOAc, n-Bu4 NBr, DMF, 100◦ C for 24 h, under N2 ; II Pd2 (dba)3 , K3 PO4 (2 mol l−1 ), toluene, 120◦ C reflux for 24 h.
100
weight (%)
80 60 DPP8-TPA DPP8-TPA-OR DPP6-HTT DPP8-HTT
40 20
0
100
300 400 200 temperature (°C)
500
Figure 1. TGA plots of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT. For all four molecules, the absorption peaks of their films (figure 2b) red shifted in comparison with their dilute solutions, and the red shift of the DPP6-HTT and DPP8-HTT films is much greater than that of DPP8-TPA and DPP8-TPA-OR films, indicating that there is stronger intermolecular interaction in the TT-containing compound films than that of the TPA-containing compound films. Obviously, better planarity of the TT-containing molecules benefits the stronger intermolecular interaction. Moreover, the absorption spectrum of DPP8-TPA and DPP6-HTT films
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O
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(a) 1.0
absorbance
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0.8
4
DPP8-TPA solution DPP8-TPA-OR solution DPP6-HTT solution DPP8-HTT solution
0.6 0.4 0.2 0 400
normalized absorbance
(b)
600
1.0
800 DPP8-TPA film DPP8-TPA-OR film DPP6-HTT film DPP8-HTT film
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0 400
800 1000 600 wavelength (nm)
1200
Figure 2. (a,b) UV-vis absorption spectra of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT in chloroform solution and film state. (Online version in colour.)
Table 1. Absorption spectra and electrochemical properties of compounds DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT. compounds
λsolution (nm)
λfilm (nm)
red Eonset (V)/LUMO (eV)
ox Eonset (V)/HOMO (eV)
DPP8-TPA
370/626/664
379/640/693
−1.19/−3.52
0.43/−5.14
DPP8-TPA-OR
371/639/676
376/641/703
−1.18/−3.53
0.32/−5.03
DPP6-HTT
364/595/636
392/641/716
−1.13/−3.58
0.55/−5.26
DPP8-HTT
364/598/634
379/609/685
−1.18/−3.53
0.47/−5.18
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is more red shifted than that of DPP8-TPA-OR and DPP8-HTT films, respectively, indicating that fewer or shorter side chains in the molecules result in stronger intermolecular interactions.
(c) Electrochemical properties Electrochemical cyclic voltammetry was carried out for measuring the HOMO and LUMO energy levels of the compounds. Figure 3 shows the cyclic voltammograms of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT films and the values of the onset reduction potentials (Ered onset ); onset ) of the compounds are listed in table 1. It can be seen from figure 3 oxidation potentials (Eox onset
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5
DPP8-TPA
current (mA)
DPP8-HTT
–2
–1 0 potential (V versus Ag/Ag+)
1
Figure 3. Cyclic voltammograms of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT films on Pt electrode in an acetonitrile solution of 0.1 mol l−1 Bu4 NPF6 (Bu = butyl) with a scan rate of 100 mV s−1 . that the oxidation processes of the two HTT-based molecules DPP6-HTT and DPP8-HTT are reversible, but the reduction processes of the four compounds are irreversible. The HOMO, LUMO energy levels and the electrochemical band gaps of the four materials were estimated, respectively, from the onset oxidation and reduction potentials according to red the following equations [31,32]: EHOMO = −e(Eox onset + 4.71) (eV), ELUMO = −e(Eonset t + 4.71) (eV) and Eg = ELUMO − EHOMO . The LUMO energy levels of the four compounds are around −3.5 eV, which change little for different molecules. But the HOMO energy levels of the molecules change greatly from −5.03 eV for DPP8-TPA-OR to −5.26 eV for DPP6-HTT (table 1). The molecules of DPP8-HTT and DPP6-HTT containing the TT unit show lower HOMO energy levels, which is beneficial to higher Voc of the OSCs with the compounds as donor. The electrochemical band gaps of DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT are 1.61, 1.50, 1.68 and 1.65 eV, respectively. Compared with the HOMO (−5.87 eV) and LUMO (−3.91 eV) energy levels for PC71 BM, [33] the compounds are suitable to be used as photovoltaic donors with PC71 BM as acceptor on the energy level point of view.
(d) Morphology of the blend films The morphology of the photoactive layers is very important for the photovoltaic performance of OSCs [34]. We used atomic force microscopy (AFM) to investigate the morphology of the four organic molecules: PC71 BM (1 : 3, w/w) blend films. All of the AFM images of the blend films exhibit typical amorphous morphology without any crystalline domains, as shown in figure 4. The blend films of DPP8-TPA/PC71 BM and DPP6-HTT/PC71 BM exhibit bigger aggregates, compared with that of DPP-8-TPA-OR/PC71 BM and DPP8-HTT/PC71 BM.
(e) Photovoltaic properties Bulk-heterojunction OSCs were fabricated by using the four compounds as donor, PC71 BM [35] as acceptor and Al as the negative electrode. The device structure is ITO/PEDOT: PSS/photoactive layer/Al. The active layer is composed of a blend film of the organic molecule donor and PC71 BM acceptor with a weight ratio of 1 : 3. Figure 5 shows the current density–voltage curves of the OSCs in the dark and under the illumination of AM 1.5, 100 mW cm−2 . The photovoltaic performance data of the OSCs, including Voc , Jsc , FF and PCE values, are summarized in table 2 for a clear comparison. The best PCE of the device based on DPP8-HTT reached 1.50% with a higher Voc of 0.73 V and a Jsc of 6.62 mA cm−2 . In comparison to the photovoltaic performance of DPP8-TPA and DPP8-HTT, it can be seen that DPP8-HTT showed higher Voc and higher Jsc values than
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(a)
5
(b)
6
10.0 nm
5.0 nm
0 nm 50
0
5
0
Figure 4. AFM images of the blend films of organic molecules: PC71 BM (1 : 3, w/w), the organic molecules are (a) DPP8-TPA, (b) DPP8-TPA-OR, (c) DPP6-HTT and (d) DPP8-HTT. The scan size of the images is 5 × 5 μm. (Online version in colour.) Table 2. Photovoltaic performance of ITO/PEDOT : PSS/organic molecules: PC71 BM (1 : 3, w/w)/Al negative electrode under the illumination of AM 1.5, 100 mW cm−2 . organic molecule in the active layer
Voc (V)
Jsc (mA cm−2 )
FF (%)
PCE (%)
DPP8-TPA
0.58
5.00
30.0
0.90
DPP8-TPA-OR
0.63
5.66
34.0
1.20
DPP6-HTT
0.73
4.36
28.3
0.90
DPP8-HTT
0.73
6.62
30.5
1.50
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that of DPP8-TPA, which indicates that the planar structure of the TT unit is beneficial to the improved photovoltaic performance of the compounds. The poorer photovoltaic performance of the OSC based on DPP6-HTT could be owing to the lower solubility of DPP6-HTT with too short side chains. In conclusion, a series of linear D-A-D molecules, DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT with DPP as core and acceptor unit and TPA or TT as donor unit, was designed and synthesized for application as donor materials in solution-processed OSCs. The four compound films show broad absorption band and lower band gap. The donor unit of TPA or TT and the side chains of the molecules influence the photovoltaic performance of the compounds significantly. The OSC devices based on a blend of DPP8-HTT and PC71 BM (1 : 3, w/w) exhibited a PCE of 1.50% with a Voc of 0.73 V and a Jsc of 6.62 mA cm−2 , under the illumination of AM1.5, 100 mW cm−2 . The results indicate that the planar donor unit of TT is better than the TPA unit in the D-A-D structured DPP-containing molecules photovoltaic materials.
3. Experimental section (a) Chemicals TPA, n-butyllithium (2.50 mol l−1 in hexane), tetrabutylammonium bromide, sodium acetate, palladium acetate, Pd2 (dba)3 , K3 PO4 , toluene and DMF were obtained from Acros Organics. THF was dried over Na/benzophenoneketyl and freshly distilled prior to use. Monomers 1–5
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(b) 100
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Figure 5. (a–d) Current density–voltage characteristics of the OSC devices based on the blends of linear molecules/PC71 BM (1 : 3, w/w) with Al as the negative electrode in the dark and under the illumination of AM1.5, 100 mW cm−2 .
were synthesized according the literature [24,27,30]. Other chemicals were common commercial chemicals and were used as received.
(b) Measurements Nuclear magnetic resonance (NMR) spectra were taken on a Bruker DMX-400 spectrometer. MALDI-TOF spectra were recorded on a Bruker BIFLEXIII. Absorption spectra were taken on a Hitachi U-3010 UV-vis spectrophotometer. The film on quartz used for UV measurements was prepared by spin-coating with 1% chloroform solution. The TGA measurement was performed on a Perkin-Elmer TGA-7 apparatus. The electrochemical cyclic voltammetry was carried out with a Zahner IM6e electrochemical workstation in a 0.1 mol l−1 tetrabutylammonium hexafluorophosphate (Bu4 NPF6 ) acetonitrile solution. A glassy carbon electrode coated with the sample film was used as the working electrode, a Pt wire and Ag/Ag+ (0.01 M AgNO3 in acetonitrile) were used as the counter and reference electrodes, respectively.
(c) Fabrication and characterization of organic solar cells OSCs were fabricated in the configuration of the traditional sandwich structure with an ITO positive electrode and a metal negative electrode. Patterned ITO glass with a sheet resistance of 30 Ω sq−1 was purchased from CSG Holding Co., Ltd (China). The ITO glass was cleaned in an ultrasonic bath of acetone and isopropanol and treated by UVO (ultraviolet ozone cleaner, Jelight Company, USA). Then a thin layer (30 nm) of PEDOT: PSS (poly(3,4-ethylenedioxythiophene)poly(styrene sulfonate)) (Baytron PVP A1 4083, Germany) was spin-coated on the ITO glass. Subsequently, the photosensitive layer was prepared by spin-coating the blend solution of linear materials and PC71 BM (1 : 3 w/w) on the top of the PEDOT:PSS layer (ca 30 nm thickness) and baked at 80◦ C for 30 min. The concentration of the solution was 10 mg ml−1 in chlorobenzene. The
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current density (mA cm–2)
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DPP8-TPA: monomer 1 (0.682 g, 1 mmol), N,N-diphenyl-N-(4-vinylphenyl)amine (0.596 g, 2.2 mmol), palladium (II) acetate (5 mg), tetra-n-butylammonium bromide (103 mg, 0.32 mmol) and sodium acetate anhydrous (1.64 g, 20 mmol) were dissolved and kept in degassed N,Ndimethyl formamide (30 ml), under argon at 100◦ C for 24 h. The mixture was poured into water (30 ml). The precipitate was filtered, washed with water and dissolved in dichloromethane, and dried over anhydrous sodium sulfate. After evaporation of the solvent, the residue was purified by column chromatography (petroleum/dichloromethane, 1 : 1). Yield: 656 mg (62%). 1 H NMR (CDCl3 , 400 MHz, δ/ppm): 7.7 (d, 2H), 7.3 (d, 2H), 7.05(d, 4H), 7.01(t, 8H), 6.99 (m, 8H), 6.46 (m, 8H), 6.41(d, 4H), 4.05 (d, 4H), 2.10 (m, 2H), 1.39 (m, 20H), 0.93 (t, 6H). MALDI-TOF MS: 1063.66, calcd for C70 H70 N4 O2 S2 1063.56. Anal. Calcd for C70 H70 N4 O2 S2 : C, 79.09; H, 6.59; N, 5.27. Found: C, 77.99; H, 6.79; N, 5.05. DPP8-TPA-OR: monomer 1 (0.682 g, 1 mmol), N,N-di(n-octyloxyphenyl)-4-(vinylphenyl) amine (1.16 g, 2.2 mmol), palladium (II) acetate (5 mg), tetra-n-butylammonium bromide (103 mg, 0.32 mmol) and sodium acetate anhydrous (1.64 g, 20 mmol) were dissolved and kept in degassed N,N-dimethyl formamide (30 ml), under argon at 100◦ C for 24 h. The mixture was poured into water (30 ml). The precipitate was filtered, washed with water and dissolved in dichloromethane, dried over anhydrous sodium sulfate. After the evaporation of the solvent, the residue was purified by column chromatography (petroleum/dichloromethane, 2 : 1). Yield: 320 mg (20%). 1 H NMR (CDCl3 , 400 MHz, δ/ppm): 8.91 (d, 2H), 7.3 (d, 4H), 7.15(d, 2H), 7.09 (m, 10H), 7.00 (m, 2H), 5.88 (m, 12H), 4.10 (t, 4H), 3.05 (d, 4H), 2.10 (m, 2H), 1.74 (m, 4H), 1.33 (m, 42H), 0.90 (t, 12H). MALDI-TOF MS: 1575.9, calcd for C102 H134 N4 O6 S2 1576.31. Anal. Calcd for C102 H134 N4 O6 S2 : C, 77.76; H, 8.51; N, 3.55. Found: C, 77.68; H, 8.64; N, 3.52. DPP6-HTT: In a three-necked, oven-dried 100 ml round-bottom flask, monomer 4 (0.626 g, 1 mmol) was mixed with 15 ml of anhydrous toluene and 10 ml of 2.0 M potassium phosphate and the resulting mixture was degassed for 10 min. Monomer 5 (0.77 g, 2.20 mmol), tris(dibenzylideneacetone)dipalladium(0) (14 mg, 0.0153 mmol) and tri-tert-butylphosphonium tetrafluoroborate (18 mg, 0.0620 mmol) were then added to the mixture and then degassed again for 5 min. The reaction mixture was stirred and heated to 120◦ C under argon overnight. The reaction mixture was allowed to cool down to room temperature, then was poured into 300 ml of methanol and stirred for 30 min. The precipitated solid was then collected by vacuum filtration and washed with several portions of distilled water, methanol, isopropanol and petroleum ether. The crude product was purified by flash chromatography using chloroform as eluent, and the solvent was removed in vacuum to obtain a pure product. Yield: 520 mg (57%). 1 H NMR (CDCl3 , 400 MHz, δ/ppm): 8.97 (d, 2H), 7.48 (s, 2H), 7.34 (d, 2H), 6.99 (s, 2H), 4.12(t, 4H), 2.74 (t, 4H), 1.77(m, 8H), 1.47 (m, 4H), 1.36 (m, 20H), 0.91 (t, 12H). MALDI-TOF MS: 912.3, calcd for C50 H60 N2 O2 S6 913.4. Anal. Calcd for C50 H60 N2 O2 S6 : C, 65.78; H, 6.57; N, 3.07. Found: C, 65.35; H, 6.64; N, 3.13. DPP8-HTT: In a three-necked, oven-dried 100 ml round-bottom flask, monomer 1 (0.683 g, 1.00 mmol) was mixed with 15 ml of anhydrous toluene and 10 ml of 2.0 M potassium phosphate and the resulting mixture was degassed for 10 min. Monomer 5 (0.77 g, 2.20 mmol), tris(dibenzylideneacetone)dipalladium(0) (14 mg, 0.0153 mmol), and tri-tert-butylphosphonium tetrafluoroborate (18 mg, 0.0620 mmol) were then added to the mixture and then degassed again for 5 min. The reaction mixture was stirred and heated to 120◦ C under argon overnight. The reaction mixture was allowed to cool down to room temperature, and then it was poured
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(d) Synthesis
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thickness of the photoactive layer was measured using an Ambios Technology XP-2 profilometer. Finally, an Al electrode (ca 120 nm) was vacuum-evaporated on the photoactive layer under a shadow mask in the vacuum of ca 10−4 Pa. The current–voltage (J–V) measurement of the devices was conducted on a computer-controlled Keithley 236 Source Measure Unit. A xenon lamp coupled with AM1.5 solar spectrum filters was used as light source, and the optical power at the sample was 100 mW cm−2 .
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(no. 2014CB643501), NSFC (no. 51373181) and the China Postdoctoral Science Foundation (no. 2012M510554).
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Funding statement. This work was supported by The Ministry of Science and Technology of China
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into 300 ml of methanol and stirred for 30 min. The precipitated solid was then collected by vacuum filtration and washed with several portions of distilled water, methanol, isopropanol and petroleum ether. The crude product was purified by flash chromatography using chloroform as eluent, and the solvent was removed in vacuum to obtain a pure product. Yield: 490 mg (51%). 1 H NMR (CDCl , 400 MHz, δ/ppm): 8.96 (d, 2H), 7.46 (s, 2H), 7.33 (d, 2H), 7.02 (s, 2H), 4.02 (t, 4H), 3 2.74 (t, 4H), 1.94 (t, 2H), 1.77 (m, 4H), 1.38 (m, 34H), 0.94 (m, 12H). MALDI-TOF MS: 968.4, calcd for C54 H68 N2 O2 S6 969.52. Anal. Calcd for C54 H68 N2 O2 S6 : C, 66.94; H, 7.02; N, 2.89. Found: C, 65.97; H, 7.13; N, 2.84.
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