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Solution-Processable Organic Molecule for High Performance Organic Solar Cells with Low Acceptor Content Kun Wang, Bing Guo, Zhuo Xu, Xia Guo, Maojie Zhang, and Yongfang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07085 • Publication Date (Web): 22 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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ACS Applied Materials & Interfaces

Solution-Processable Organic Molecule for High Performance Organic Solar Cells with Low Acceptor Content Kun Wang†+, Bing Guo†+, Zhuo Xu†, Xia Guo†,* Maojie Zhang†,* Yongfang Li†,‡* †

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical

Engineering and Materials Science, Soochow University, Suzhou 215123, China. E-mail: [email protected], [email protected] ‡

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of

Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected] [+]

These authors contributed equally to this work.

Abstract: A new planar D2-A-D1-A-D2 structured organic molecule with bithienyl benzodithiophene (BDT) as central donor unit D1 and fluorine-substituted benzothiadiazole (BTF) as acceptor unit and alkyl-dithiophene as end group and donor unit D2, BDT-BTF, was designed and synthesized for the application as donor material in organic solar cells (OSCs). BDT-BTF shows a broad absorption in visible region, suitable HOMO energy level of –5.20 eV and high hole mobility of 1.07×10-2 cm2/(V s), benefitted from its high coplanarity and strong crystallinity. The OSCs based on BDT-BTF as donor (D) and PC71BM as acceptor (A) at a D:A weight ratio of 3:1 without any extra treatment exhibit high photovoltaic performance with Voc of 0.85 V, Jsc of 10.48 mA/cm2, FF of 0.66 and PCE of 5.88%. The morphological study by TEM reveals that the blend of BDT-BTF and PC71BM (3:1, w/w) possesses an 1

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appropriate interpenetrating D/A network for the exciton separation and charge carrier transport which agrees well with the good device performance. The optimized D: A weight ratio of 3:1 is the lowest acceptor content in the active layer reported so far for the high performance OSCs, and the organic molecules with the molecular structure like BDT-BTF could be promising high performance donor materials in solution-processable OSCs. Keywords: organic solar cells, D2-A-D1-A-D2 organic molecules, coplanarity, crystallization, active layer with low fullerene content

1. INTRODUCTION The organic solar cells (OSCs) based on solution-processed organic small molecules have attracted great attention in recent years,1–12 because the small molecule photovoltaic materials have several advantages, such as versatile chemical structures, easier tuning of electronic energy levels and electron mobility, and better reproducibility of photovoltaic performance. In general, molecular design plays key roles in developing high performance solution-processable organic molecule photovoltaic materials.6-10, well-known,

the

commonly

used

fullerene

derivative

acceptor,

13-32

It is

such

as

phenyl-C61-butyric acid methyl ester (PCBM), show weaker extinction coefficient in visible region than that of the donor photovoltaic materials. Therefore, higher molecule content and lower fullerene acceptor content in the active layer blend films would be desirable from the sunlight harvesting point of view. Thus it is an important 2


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challenge for molecular design of the donor materials to get high efficiency OSCs with low fullerene content in the active layer.33-41 Sun et al. found that the crystalline donor materials may induce the fullerene acceptor to form percolated pathways in the active layer, and hence can form appropriate donor: acceptor interpenetrating network with a lower fullerene content.33, 34

Recently, the acceptor-donor-acceptor (A-D-A)-type organic molecules have

become the research focus for the solution-processable donor materials in OSCs, due to their broad absorption, deeper the highest occupied molecular orbital (HOMO) energy level and higher hole mobility.1, 2, 8-10, 35 These properties are beneficial for getting larger short circuit current density (Jsc), higher open circuit voltage (Voc) and higher fill factor (FF) to achieve a higher PCE. Therefore, in this work we tried to design the molecular structure of crystalline organic donor molecule with A-D-A structure to realize efficient OSCs with lower fullerene content. Among various types of photovoltaic materials of solution-processable organic molecules

and

conjugated

polymers,

the

materials

containing

benzo[1,2-b:4,5-b’]dithiophene (BDT) unit exhibit excellent photovoltaic properties due to the symmetric and planar conjugated structure of BDT unit.42 Especially, photovoltaic materials based on two-dimension-conjugated BDT with thiophene conjugated side chains (2D-BDT) exhibited superior photovoltaic performance.43-46 However, only limited number of organic molecules containing 2D-BDT unit were reported.6-8,

35-38

On the other hand, fluorine-substituted benzothiadiazole (BTF)

possess strong electron-withdrawing ability and excellent planarity, and it have been 3



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proven to be a promising building block in organic molecule donor materials reported by Bazan’s group.1,47-49 Based on this, we designed and synthesized a new solution-processable

organic

molecule

donor

material:

7,7'-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b'] dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thi adiazole) (BDT-BTF) (as shown in Scheme 1) through Stille coupling reaction, with a 2D-BDT core and two bithiophene-BTF arms. The OSCs based on BDT-BTF: PC71BM blend film as active layer give a PCE of 5.88% with a low fullerene content of 25% (3:1, w/w).

Scheme 1. Synthetic route and molecular structure of BDT-BTF.

2. RESULTS AND DISCUSSION BDT-BTF exhibits excellent solubility in the organic solvents of chloroform, toluene, chlorobenzene (CB), o-dichlorobenzene, etc. Thermogravimetric analysis (TGA) indicates good thermal stability of the compound BDT-BTF with a 5% weight-loss temperature at 425 °C under a N2 atmosphere (see Figure S1 in 4


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Supporting Information (SI)). Figure 1 shows the DSC (differential scanning calorimetry) thermogram of BDT-BTF. There are an endothermic peak at 213 oC during increasing temperature and an exothermic peak at 196 oC during decreasing temperature, indicating stronger crystallization of BDT-BTF. The crystallization characteristics of BDT-BTF may be ascribed to its larger π-conjugated structure and higher coplanarity which favors ordered intermolecular π-π stacking.

Figure 1. DSC thermogram of BDT-BTF with a scan rate of 10 °C per minute under nitrogen atmosphere.

2.1 Absorption Spectra. Figure 2 shows the UV−vis absorption spectra of BDT-BTF in dilute CB solution and in solid film. The absorption spectrum of BDT-BTF solution displays a maximum absorption peak at 558 nm with a shoulder peak at 498 nm. In thin film, the absorption spectrum of BDT-BTF red-shifted significantly, with a concomitant more intense peak at 648 nm. These changes are characteristic for J-type (slipped) stacking50 which may be attributed to the ordered packing of BDT-BTF. Enhanced absorption peak of BDT-BTF film will be beneficial for the improvement of Jsc of the OSCs with the 5



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compound as donor. Furthermore, the planarity of the organic molecule is desirable to extend intramolecular π-delocalization and enhance intermolecular π-π interaction, which are beneficial for the charge carrier transport in the photovoltaic devices. The absorption edge of BDT-BTF film is at 696 nm which corresponds to an optical bandgap of 1.78 eV.

Figure 2. UV–vis absorption spectra of BDT-BTF in chlorobenzene solution and in solid film.

2.2 Theoretical Calculations Theoretical calculation was carried out by density functional theory (DFT) at the B3LYP/6-311+G(d, p) level to understand the molecular coplanarity and the HOMO and LUMO (the lowest unoccupied molecular orbital) energy levels of BDT-BTF. In order to avoid excessive computation demand in the calculation, the 2-ethylhexyl substituent on thiophene conjugated side chains of the molecule was replaced by hexyl group. The optimized geometry of BDT-BTF and calculated dihedral angles are shown in Figure 3a, where θ1 is dihedral angle between the thiophene unit in side 6


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chain and the BDT unit, θ2 is dihedral angle between BT and BDT units, θ3 is dihedral angle between BT and thiophene units, θ4 is dihedral angle between the two thiophene bridges. The dihedral angle between the thiophene unit in side chain and the BDT unit is 59.2o which is coincident with the reported result in literatures.51, 52 The dihedral angels of θ2, θ3 and θ4 in the backbone are 0.7o, 1.7o and 16.3o which are all less than 17o, indicating the planar conformation of this small molecule. Furthermore, the molecular geometry shows a linear backbone conformation from the side view. The calculated orbital distribution of HOMO and LUMO of BDT-BTF are shown in Figure 3b. The calculated HOMO and LUMO energy levels of BDT-BTF are −5.08 eV and −3.08 eV, respectively. The molecular orbital distributions in Figure 3b indicate that the HOMO of BDT-BTF is delocalized along the whole π-conjugated backbone of the molecule while its LUMO is mainly localized on the BTF-based acceptor segment of BDT-BTF.

Figure 3. (a) Top view and side view of the optimized geometry and (b) the molecular orbital distributions of HOMO and LUMO of BDT-BTF obtained from the DFT calculations. 7



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2.3 HOMO and LUMO Energy Levels The electrochemical cyclic voltammogram of BDT-BTF film (Figure 4) was measured for determining its electronic energy levels. The HOMO and LUMO energy levels are estimated from the onset oxidation and reduction potentials in the cyclic voltammogram. The estimated HOMO and LUMO energy levels of BDT-BTF are −5.20 eV and −3.24 eV, respectively, corresponding to an electrochemical bandgap of 1.96 eV, in reasonable agreement with the optical bandgap calculated from film absorption onset wavelength. The HOMO and LUMO energy levels of BDT-BTF are appropriate for the application as donor material with PCBM acceptor to generate high exciton dissociation efficiency and higher Voc in the OSCs devices. In addition, the multi-oxidation peaks in the cyclic voltammogram indicate that the doping/dedoping processes to form polarons and bipolarons in the conjugated molecule are reversible.

Figure 4. Cyclic voltammogram of BDT-BTF film on a glassy carbon electrode measured in a 0.1 mol/L Bu4NPF6 acetonitrile solution at a scan rate of 50 mV/s. 8


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2.4 Photovoltaic Properties For studying the photovoltaic performance of BDT-BTF, OSCs were fabricated with BDT-BTF as donor and PC71BM as acceptor. The device structure is indium tin oxide

(ITO)/poly

(3,4-ethylenedioxythiophene):poly

(styrene

sulfonate)

(PEDOT:PSS)/BDT-BTF:PC71BM/Ca (20 nm)/Al (80 nm). Figure 5a shows the current density-voltage (J−V) curves of the OSCs with different donor/acceptor (D/A) weight ratios from 1:1 to 4:1 in their active layers, and Table 1 lists the photovoltaic parameters of the OSCs for a clear comparison. The photovoltaic performance of the OSCs is tightly related to the D/A ratios in the blend active layer of the devices. The devices based on BDT-BTF: PC71BM with a weight ratio of 1:1 (the content of PC71BM acceptor is 50%) showed a PCE of 2.65%, with a Voc of 0.83 V, Jsc of 5.80 mA/cm2, and a FF of 55%. Interestingly, the PCE of the OSCs increased with the decrease of the acceptor content until the content of PC71BM acceptor is decreased to 25% (D/A weight ratio of 3:1). At the D/A weight ratio of 3:1, the OSC demonstrated the highest PCE of 5.88%, with a Voc of 0.85 V, Jsc of 10.48 mA/cm2, and a FF of 66%. Further decreasing the content of PC71BM acceptor to 20% (D.A weight ratio of 4:1), the PCE of the OSC dramatically declined to 2.17%. The optimized D/A weight ratio of 3:1 for our new molecule BDT-BTF is the lowest optimized content (25%) of fullerene acceptor in OSCs reported in literatures so far. Furthermore, the PCE of 5.88% is achieved without any extra treatment such as thermal annealing, solvent additives and solvent annealing etc., which facilitate the large scale roll-to-roll 9



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production in future applications.

Figure 5. (a) J–V characteristics and (b) EQE curves of the OSCs based on BDT-BTF: PC71BM blend film with different D/A weight ratios.

Table 1. Photovoltaic Parameters of the OSCs Based on BDT-BTF:PC71BM with Different D/A Weight Ratios under the Illumination of AM 1.5G, 100 mW/cm2.

a

D/A ratio

Voc

Jsc

FF

PCE

Thickness

(w/w)

(V)

(mA/cm2)

(%)

(%)

(nm)

1:1

0.83±0.01

5.80±0.28

55±2

2.65 (2.54±0.10) a

123

1.5:1

0.82±0.01

8.19±0.29

63±1

4.23 (4.13±0.15)

120

2:1

0.83±0.01

9.54±0.02

63±2

4.99 (4.85±0.23)

100

2.5:1

0.84±0.01

10.79±0.18

62±1

5.62 (5.58±0.16)

95

3:1

0.85±0.01

10.48±0.11

66±1

5.88 (5.76±0.12)

100

3.5:1

0.85±0.01

7.62±0.21

61±1

3.95 (3.89±0.15)

100

4:1

0.85±0.01

4.90±0.19

52±2

2.17 (2.01±0.15)

98

The values in the parentheses are the device average PCEs and their standard

deviation based on more than 20 devices.

Normally, the photovoltaic performance of OSCs is sensitive to thermal annealing for many small molecule donor materials1, 6, 7, 26, 53. So, we investigated the 10


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effect of the thermal annealing on the photovoltaic performance of the OSCs based on the blend of BDT-BTF: PC71BM (w/w=3:1). The thermal annealing was performed at different temperatures for 10 minutes. The corresponding J-V curves and the photovoltaic parameters of the OSCs are shown in Figure S2 and Table S1 in SI. It can be seen that the thermal annealing results in slight decrease of the photovoltaic performance of the OSCs. It is well known that, the D/A weight ratio in the active layers of the OSCs is of great importance for the formation of appropriate donor-acceptor interpenetrating network for high efficiency charge separation and transportation. The light absorption of the active layer mainly relies on donor materials and the fullerene acceptor is mainly for electron separation and transportation. For enhancing the light absorption of the active layers, less fullerene acceptor content should be desirable so long as an appropriate donor-acceptor interpenetrating network can be formed. The optimized D/A weight ratios depend on the molecular structures and the crystalline characteristics of the organic molecule donors. For the star-shaped organic molecule donors with triphenylamine (TPA) unit as core and TPA or thiophene as end groups, the optimized D/A (PCBM as acceptor) weight ratio is 1:354-55 with PCE of ca. 1.3~2.4% for its corresponding OSCs. When the end group of the star-shaped molecules was changed to an electron-withdrawing group such as dicyanovinyl end group, then the optimized D/A (PCBM as acceptor) weight ratio became 1:256 with an improved PCE of ca. 3%. For the high performance planar A-D-A structured organic molecule donor materials reported in recent years, the content of fullerene acceptor is 11



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further decreased to an optimized D/A weight ratio of 1:0.88-9, 1.5:135 or 7:333-34 with the PCE improved to 6~9%. Obviously, decreasing the content of fullerene acceptor in the active layers favors the improvement of the photovoltaic performance of the OSCs. The low fullerene content of 25% in the optimized active layer of BDT-BTF:PC71BM indicates that the organic molecules with the molecular structure like BDT-BTF could be promising high performance donor materials in solution-processable OSCs. In addition, low fullerene content is beneficial to reduce the cost of OSCs because the price of PC71BM is quite expensive. The external quantum efficiency (EQE) curves of the OSCs are shown in Figure 5b. The EQE curves of the OSCs with different D/A ratio cover a wide wavelength range from 350-700 nm, which agrees with the absorption spectra of the active layer. The current densities calculated from the EQE curves under the standard solar spectrum (AM 1.5G) are consistent with the Jsc values obtained from the J–V measurement with deviation less than 10%. 2.5 Hole and Electron Mobilities In order to study the effect of D/A weight ratios on the charge transport properties of the blend active layers, the hole and electron mobilities were measured by using the space charge limited current (SCLC) method. Figure 6 shows the current-voltage plots for the measurements, and Table 2 lists the hole and electron mobilities of the active layers at different D/A ratios. The hole mobility of pure BDT-BTF is 1.07×10−2 cm2/(V s). After blending the PC71BM acceptor, the hole mobility decreased gradually with the increase of the acceptor content, but the hole 12


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mobility of the film with a D/A weight ratio of 2:1 is still in the order of 10-3 cm2/(V s). Importantly, for the active layer with the optimized D/A weight ratio of 3:1, the hole and the electron mobilities are well balanced with a hole mobility of 3.9×10−3 cm2/(V s) and an electron mobility of 2.4×10−3 cm2/(V s), which is consistent with the high photovoltaic performance of the OSCs with the D/A weight ratio of 3:1.

Figure 6. J−V characteristics of (a) hole-only and (c) electron-only diodes and corresponding curves according to the SCLC model for (b) hole-only and (d) electron-only diodes of BDT-BTF and BDT-BTF: PC71BM blend films with different D/A weight ratios.

Table 2. Electron and Hole Mobilities of the BDT-BTF:PC71BM Blend Films with Different D/A Ratios D/A ratio

Electron mobility 13


Hole mobility


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(cm2/(V s))

(cm2/(V s))

BDT-BTF

--

1.07±0.3×10-2

2:1

3.1±0.3×10-3

1.7±0.4×10-3

3:1

2.4±0.5×10-3

3.9±0.2×10-3

4:1

9.3±0.6×10-3

4.8±0.7×10-3

2.6 Morphology of Active Layers For further explaining the effect of the D/A weight ratio on the photovoltaic performance of the OSCs based on BDT-BTF: PC71BM, we measured morphology of the active by transmission electron microscopy (TEM). Figure 7 shows the TEM images of the active layers of BDT-BTF: PC71BM with different D/A weight ratios. The bright regions in Figure 7 correspond to BDT-BTF-rich domains and the dark regions correspond to PC71BM-rich domains57,

58

. Large-scale phase-separated

morphology can be clearly seen in the TEM image for the blend film with D:A weight ratio of 1:1 (50% PC71BM content) (Figure 7a), which may cause more geminate and bimolecular recombination and result in lower Jsc of the OSC. With decreasing the fullerene acceptor content, the size of the PC71BM aggregates reduced gradually (Figure 7b-d). For the active layer with D:A weight ratio of 3:1 (25% fullerene content), an appropriate D/A interpenetrating network is observed with size of ca. 10~20 nm for the BDT-BTF and PC71BM networks (Figure 7c). This morphology is beneficial for charge separation and transport, which agrees well with the optimized photovoltaic performance of the OSCs with the D:A weight ratio of 3:1.

14


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Figure 7. TEM images of BDT-BTF: PC71BM blend films with different PC71BM content (D/A weight ratio): (a) 50% (1:1), (b) 33% (2:1), (c) 25% (3:1), (d) 20% (4:1).

In order to further explore the miscibility of the compound BDT-BTF with PC71BM, and surface morphology of the photoactive layer, we measured atomic force microscopy (AFM) morphology of the blend film with D/A ratio of 2:1, 3:1 and 4:1, as shown in Figure S3 in SI. The blend films show similar root mean square (rms) roughness, which is 1.46, 1.47, and 1.51 nm for the blend films with D/A weight ratio of 2:1, 3:1 and 4:1 respectively. The AFM results indicate that BDT-BTF has a good miscibility with PC71BM in the active layers. An optimized D/A interpenetrating network was formed when the D/A weight ratio is 3:1 (Figure S3(b)), which correlates well with the images observed from TEM. The interpenetrating network of

15



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the blend films benefits to improve the charge carrier collection efficiency for a high Jsc33, 59. Furthermore, the effect of the D/A weight ratios on the crystalline structure of the BDT-BTF:PC71BM blend layers was studied by XRD (X-ray diffraction) measurement. The XRD pattern of BDT-BTF, as shown in Figure S4 in SI, exhibits 4 diffraction peaks at 5.5o, 6.5o, 8.3o, 13.0o, indicating that the highly ordered crystallinity of BDT-BTF existed. Figure 8 shows XRD patterns of the BDT-BTF/PC71BM blend films with different D/A weight ratios from 1:1 to 4:1, prepared with CB solvent on a Si substrate. The XRD pattern exhibits two diffraction peaks at 2θ = 6.2o (100) and 12.4o (200) with a d spacing value of 14.5 Å which corresponds to the interchain distance separated by the alkyl side chains of BDT-BTF. The value is smaller than other organic small molecule donor materials26, 28, 38, which indicates that the side chains of BDT-BTF in the blend film are more densely packed. Moreover, it can be seen clearly that with the decrease of the acceptor content as the D/A ratio changed from 1:1 to 3:1, the 100 peak is enhanced significantly, which is beneficial for the charge carrier transport. However, the diffraction peak was apparently decreased with further decreasing the acceptor content for the D/A ratio of 4:1. These results agree very well with the effect of the acceptor content on the TEM morphologies and the charge carrier mobilities of the blend film.

16


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Figure 8. X-ray diffraction patterns of BDT-BTF/PC71BM (w/w, from 1:1 to 4:1) blend films drop-cast from CB solution onto a Si substrate.

3. CONCLUSIONS In summary, a highly planar D2-A-D1-A-D2 structured organic molecule BDT-BTF with thiophene-substituted benzodithiophene (BDT) as central donor unit, fluorine-substituted benzothiadiazole (BTF) as acceptor unit and hexyl-bithiophene as end donor units, was designed and synthesized. The compound exhibits a crystalline structure, suitable LUMO and HOMO energy levels, broad and strong absorption in the visible region, and high hole mobility. The PCE reached 5.88% for the OSCs based on BDT-BTF: PC71BM with a low fullerene acceptor content of 25% and without any extra treatment (such as thermal annealing, solvent annealing and solvent additive). The organic molecules with the molecular structure like BDT-BTF could be promising high performance donor materials in solution-processable OSCs.

4. EXPERIMENTAL SECTION

17



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Organic solar cells were fabricated and the hole/electron mobilities were measured by space charge limited current method, as described in our previous publications.17, 18 Synthesis of BDT-BTF 4-Bromo-5-fluoro-7-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole unit

(BTF)

and

(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']-

dithiophene-2,6-diyl)bis(trimethylstannane) (BDT) were synthesized according to reported method.20a The synthetic route of BDT-BTF was shown in Scheme 1, and the detailed synthesis processes are described in the following: Compound BDT (0.4 g, 0.44 mmol) and compound BTF (0.64 g, 1.33 mmol) were dissolved in 30 ml dry toluene in a double-neck round-bottom flask. The reactant mixture was purged with argon for 20 min to remove O2, and then Pd(PPh3)4 (0.04 g, 0.035 mmol) was added into the reactant. After argon flushing for another 20 min, the reactant was heated to reflux for 24 h under the protection of argon. Then, the reactants mixture was poured into water (100 ml) and extracted with chloroform (50 ml x 3). The organic layer was washed with water for twice and dried over anhydrous MgSO4. After removal of solvent, the crude product was purified by silica gel with chloroform/petroleum (2:1) as eluant to obtain compound BDT-BTF (0.41 g, 67%) as a black-blue solid. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.49 (s, 2H), 7.62 (d, 2H), 7.39 (d, 2H), 7.28 (d, 2H), 6.99 (d, 2H), 6.83 (d, 2H), 6.73 (d, 2H), 6.50 (d, 2H), 3.00 (d, 4H), 2.61 (m, 4H), 1.85 (m, 2H), 1.65-1.55 (m, 8H), 1.53-1.42(m, 8H), 1.35-1.22 (m, 16H), 1.13-1.09 (t, 6H), 1.04-1.01 (t, 6H), 0.91-0.89(t, 6H). 13C NMR (400 MHz, 18


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CDCl3), δ (ppm): 158.87, 158.35, 153.25, 149.47, 146.29, 145.49, 144.27, 140.59, 139.04, 137.45, 136.39, 135.57, 134.51, 133.54, 128.79, 128.52, 126.35, 125.57, 124.86, 123.88, 123.42, 115.63, 110.6. 41.60, 34.36, 32.73, 31.57, 31.33, 30.11, 29.13, 28.87, 25.80, 23.20, 22.60, 14.33, 14.08, 11.12. MS (MALDI:TOF) m/z: calcd for C74H76F2N4S10[M], 1378.32; found, 1378.67. (see Figures S6~S7 in SI for 13C NMR spectra and MALDI-TOF plot of BDT-BTF) ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (91333204, 91433117, 51203168, 51422306), the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12030200), the Postdoctoral research start-up funding of Soochow University (32317366, 32317400), and the Jiangsu Postdoctoral Grant (7131707314). Supporting Information: TGA plot of BDT-BTF with a heating rate of 10 °C/min under nitrogen atmosphere (Figure S1), J–V characteristics of the devices based on BDT-BTF: PC71BM blend film (w/w = 3:1) with thermal annealing at different temperatures for 10 minutes (Figure S2), Photovoltaic Parameters of the OSCs based on BDT-BTF: PC71BM (w/w = 3:1) with thermal annealing at different temperatures for 10 minutes, under illumination of AM 1.5G, 100 mW/cm2 (Table S1), AFM (5 × 5 µm) topography of BDT-BTF: PC71BM blend films (Figure S3), X-ray diffraction patterns of BDT-BTF film drop-cast from CB solution onto a Si substrate (Figure S4), 1H NMR spectra of compound BDT-BTF in CDCl3 (Figure S5), 13C NMR spectra of compound BDT-BTF in CDCl3 (Figure S6) and MALDI-TOF plot of compound BDT-BTF 19



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Graphic Abstract

Solution-Processable Organic Molecule for High Performance Organic Solar Cells with Low Acceptor Content Kun Wang, Bing Guo, Xia Guo,* Maojie Zhang,* Yongfang Li*

A new planar D2-A-D1-A-D2 structured organic molecule, BDT-BTF, was designed and synthesized for the application as donor material in organic solar cells (OSCs). The OSCs based on BDT-BTF as donor (D) and PC71BM as acceptor (A) at a D:A weight ratio of 3:1 without any extra treatment exhibit higher photovoltaic performance with PCE of 5.88%. The optimized D:A weight ratio of 3:1 is the lowest acceptor content in the active layer reported so far for the high photovoltaic performance OSCs, and the organic molecules with the molecular structure like BDT-BTF

could

be

promising

high

performance

solution-processable OSCs.

30


donor

materials

in

Solution-Processable Organic Molecule for High ... - ACS Publications

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