Binary Nonchlorinated and Nonaromatic Solvent-Processed PTB7

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C: Energy Conversion and Storage; Energy and Charge Transport

Binary Non-Chlorinated and Non-Aromatic Solvent Processed PTB7:PC BM and PTB7-Th:PC BM Active Layers Showing Comparable Efficiency to Chlorobenzene in Organic Solar Cells 71

71

Chang Liu, Yongxiang Zhu, Yong Cao, and Junwu Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11318 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Binary Non-Chlorinated and Non-Aromatic Solvent Processed PTB7:PC71BM and PTB7-Th:PC71BM Active Layers Showing Comparable Efficiency to Chlorobenzene in Organic Solar Cells Chang Liu, Yongxiang Zhu, Yong Cao, and Junwu Chen* Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, P. R. China.

ABSTRACT: For the eventual commercialization of solution processing organic solar cells (OSCs), using a non-aromatic and non-chlorinated solvent of low toxicity is highly desirable. In this work, 1,4-dioxane (DIOX) as a new non-aromatic and non-chlorinated solvent and its mixing with terpinolene (TPO) to form a binary non-aromatic and nonchlorinated mixture were introduced to process PTB7:PC71BM and PTB7-Th:PC71BM BHJ active layers for conventional and inverted OSCs, with comparisons to chlorobenzene (CB). Despite poor film-forming with DIOX as the solvent, a DIOX:TPO (58:42) mixture as the solvent is optimal to afford high quality BHJ blend films. As a result, the DIOX:TPO (58:42) mixture processed PTB7:PC71BM and PTB7Th:PC71BM active layers showed power conversion efficiencies (PCEs) of 7.6% and 8.3% respectively in the conventional OSCs and the efficiency could be elevated to 8.30% and 9.39% in inverted OSCs, respectively. All the photovoltaic performances are comparable to those of the CB cases. Our results suggest the mixing of some nonchlorinated and non-aromatic solvents is an attractive strategy to expand valuable 1

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solvents towards environment-friendly fabrication of OSCs.

1. INTRODUCTION During the last two decades, a number of studies of bulk-heterojunction (BHJ) organic solar cells (OSCs) have been devoted to increasing the power conversion efficiencies (PCEs). As a result, the PCE of single BHJ OSC device is approaching 14%.1 Relative to vacuum deposition, solution processing is a convenient way to prepare the BHJ active layers, during which organic materials need to be dissolved in solvents. Chlorobenzene (CB),2 o-dichlorobenzene (DCB),3 and chloroform4 are the typical solvents utilized to dissolve conjugated donor materials and conjugated acceptor materials. Since these hazardous halogenated solvents impact human health and the environment negatively, a low toxicity solvent should be a more promising choice for the solution processing of OSCs. So far, major low toxicity solvents belong to nonchlorinated aromatic solvents, including toluene,5,6 xylene,7-9 trimethylbenzene,10,11 nbutylbenzene,12 benzaldehyde,13 anisole,14,15 2-methylanisole,16 acetonphenone,17 and tetrahydronaphthalene.18 There are relatively rare reports on non-chlorinated and nonaromatic solvents that are regarded more promising for the processing OSCs. So far, Nmethyl-2-pyrrolidone (NMP),19 terpinolene (TPO),20 tetrahydrofuran (THF),21 and 2methyl-tetrahydrofuran (2-MeTHF)2224 are the rare examples. Generally, the abovementioned low toxicity solvents showed somewhat lower PCEs in processing various BHJ active layers if compared with the typical chlorinated solvents. It is highly desirable but also challenging that a non-chlorinated and non-aromatic solvent can 2


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display the best efficiency in processing the currently promising materials. To overcome the restricted solubility of a single non-chlorinated solvents to both conjugated donor and acceptor materials, a binary solvent mixture would be a better choice. Theoretically, a mixed solvent can show largely tuned Hansen solubility parameters, which would be helpful for the quality control of a BHJ active layer. There have been some reports on binary solvents based on non-chlorinated aromatic solvent mixtures.13,17,18 However, reports on binary non-chlorinated and non-aromatic solvents are relatively rare, in which a PCE of 6.8% has been achieved for carbon disulphide and acetone as binary solvent in processing a low band gap polycarbazole PCDTBT based active layer.25 Developing new binary non-chlorinated and non-aromatic solvent would be a necessary and effective way to replace chlorinated solvents. PTB7

and

PTB7-Th

(Figure

1a)

with

the

thieno[3,4-

b]thiophene/benzodithiophene backbone unit are two well-known low band gap polymers donors because of some record efficiency performances reported in the past several years for single junction BHJ PSCs.26 Recently, PTB7-Th based active layers showing high short-circuit current densities (Jsc) in binary and ternary PSCs with outstanding PCE up to 14% have been demonstrated,27,28 from which tandem PSCs with record high PCE of 17.29% can be achieved with the PTB7-Th based ternary blend film.29 It should be noted that the high efficiency BHJ active layers of PTB7 and PTB7Th are processed with chlorobenzene (CB). Terpinolene (TPO in Figure 1b) is the first non-chlorinated and non-aromatic solvent to process the PTB7 based active layer, whose LD50 (Rat oral) is 4390 mg/kg. The PCE for the terpinolene processed 3



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PTB7:PC71BM active layer is 6.42%, still lower than that of 7.34% for CB.20

(a) C4H9 C2H5 S

O S

S S O

n

S

S S

S F

C2H5

PTB7

S F

S

O

O

n

O

O

C4H9

PTB7-Th

PC71BM

(b) O

O

terpinolene (TPO)

1,4-dioxane (DIOX)

Figure 1. (a) Chemical structures of PTB7, PTB7-Th, and PC71BM. (b) Chemical structures of terpinolene (TPO) and 1,4-dioxane (DIOX), two non-chlorinated and nonaromatic solvents. To the best of our knowledge, 1,4-dioxane (DIOX in Figure 1b) as a nonchlorinated and non-aromatic solvent has not previously been reported in the processing of a BHJ active layer. DIOX (LD50 Rat oral = 5170 mg/kg) is considered to possess lower toxicity than CB (LD50 Rat oral = 1110 mg/kg). However, in this work, DIOX as a single component solvent to process PBT7 and PTB7-Th based OSCs cannot afford ideal efficiencies. Delightedly, the binary solvent based on DIOX and TPO was quite effective and PCEs of 8.30% and 9.39% were achieved for PBT7 and PTB7-Th based inverted OSCs, respectively. The efficiencies are comparable to those achieved with CB. In addition, with a conventional device configuration, the DIOX:TPO binary solvent also displayed comparable PCEs to those of CB. The results suggest the mixing of two non-chlorinated and non-aromatic solvents is an attractive strategy to expand 4


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valuable solvents, paving a way to replace chlorinated aromatic solvents such as CB in processing of highly efficient BHJ active layers.

2. RESULTS AND DISCUSSION 2.1 Hansen solubility parameters (HSPs) and solubility of 1,4-dioxane and terpinolene Despite the high toxicity, CB possesses the advantage of good solubility to various organic semiconductors. Also, the boiling point (bp.) of 132 °C for CB usually contributes a suitable evaporation speed to achieve good BHJ morphology. Relatively, the bp. of TPO is 185 °C, obviously higher than that of CB. In the previous report, TPO showed enough high solubilities to PTB7 and PC71BM. However, TPO processed PTB7:PC71BM active layer showed a lower PCE of 6.42%, relative to 7.34% for CB.20 In the binary solvent of TPO and DIOX, the much lower bp. of 101 °C for DIOX would result in a faster evaporation, from which a TPO:DIOX mixture processed BHJ blend film may obtain a morphology similar to CB.

5



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Table 1. Hansen solubility parameters (HSPs), Distance in Hansen space (Ra), and Relative energy difference (RED) of different solvents to PC71BM. solute or solvent

bp (°C)

δd (MPa1/2)

δp (MPa1/2)

δhb (MPa1/2)

Ra (MPa1/2)

RED (MPa1/2)

ref

PC71BM

--

20.2

5.40

4.5

0

--

31

132

19.0

4.30

2.0

3.63

0.52

32

101.3

19.0

1.80

7.4

5.21

0.72

32

185

16.8

2.75

2.5

7.56

1.08

20

DIOX:TPO=50:50

17.9

2.3

5

5.57

0.80

This work

DIOX:TPO=55:45

18

2.2

5.2

5.49

0.78

This work

DIOX:TPO=58:42

18.1

2.2

5.3

5.34

0.76

This work

DIOX:TPO=60:40

18.1

2.2

5.4

5.36

0.77

This work

CB DIOX TPO

In order to obtain the best solubility to PC71BM, Hansen solubility parameters (HSPs) are used to predict the optimal ratio of DIOX and TPO in solvent blends. The HSPs of a solvent consists of three components; the energy of the dispersion forces between molecules (δd), the energy resulting from permanent dipole moments (δp), and the energy of hydrogen bonds (δhb).30 This has been previously used as a powerful technique to determine the solubility of a solvent. As listed in Table 1, the HSPs of PC71BM, CB, DIOX, and TPO were obtained from literature.31,32 The CB probably show the best solubility to PC71BM, based on the smallest Distance in Hansen Space (Ra) of 3.63 MPa1/2 and Relative Energy Difference (RED) of 0.52 MPa1/2 to PC71BM if compared with DIOX and TPO. The calculation of RED is according to a radius of solubility of 7.0 for PC71BM (based on 10 mg/mL threshold).33 The Ra and RED values PC71BM for DIOX are relatively lower than those of TPO. We calculated the HSPs of four DIOX:TPO mixtures, from which the Ra and RED of the DIOX:TPO mixtures to PC71BM could be obtained (Table 1). Among the four mixtures, the DIOX:TPO 6


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mixture of 58:42 by volume exhibits the best RED value of 0.76 MPa1/2 when dissolving PC71BM. Table 2. The measured solubilities (mg/mL) of PTB7, PTB7-Th, and PC71BM in CB, DIOX, TPO, and the DIOX:TPO mixture (58:42 by volume) at room temperature Solvent

CB

DIOX

TPO

DIOX:TPO

PTB7

34a

4

22a

18

PTB7-Th

31

3

20

16

PC71BM

70a

10

27a

22

Solute

a

From ref 20.

The solubilities of PTB7, PTB7-Th and PC71BM in CB, DIOX, TPO, and the DIOX:TPO mixture (58:42 by volume) were optically measured at room temperature (Table 2). The measurements were conducted according to a report by Nguyen et al, utilizing UV absorbance intensity which shows a linear relationship with concentration.31 With PTB7 as the solute, solubilities of 34, 4, 22, and 18 mg/mL were achieved for CB, DIOX, TPO, and the DIOX:TPO mixture, respectively. With PTB7Th as the solute, solubilities of 31, 3, 20, and 16 mg/mL were achieved for CB, DIOX, TPO, and the DIOX:TPO mixture, respectively. PC71BM showed solubilities of 70, 10, 27 and 22 mg/mL in CB, DIOX, TPO, and the DIOX:TPO mixture, respectively. Generally, the DIOX:TPO mixture can show solubilities of more than 15 mg/mL when dissolving PTB7, PTB7-Th, and PC71BM, suggesting its feasibility in processing of BHJ active layers. Unexpectedly, the real solubilities of DIOX to PTB7, PTB7-Th, and PC71BM are all not high and this may be an obstacle to achieve high performance OSCs. 2.2 Surface morphology of pristine and blend films 7



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To the best of our knowledge, there is no report that DIOX is utilized to process a BHJ blend film. Firstly, surface morphologies of neat films of PTB7, PTB7-Th, and PC71BM processed with DIOX were evaluated on atomic force microscopy (AFM) (Figure 2). The neat films of PTB7 and PTB7-Th show extremely rough morphologies, corresponding to root-mean-square (RMS) roughness of 131 and 153 nm, respectively. However, smoother surface can be found for the DIOX processed pristine PC71BM film, whose RMS value is 0.846 nm. Relative to the PC71BM film, the rougher surfaces for PTB7 and PTB7-Th should be related to the much lower solubilities of DIOX to the two polymers, reflecting the possible drawback of DIOX as a single solvent to process semiconducting materals. (a)

PTB7, RMS = 131 nm

(b) PTB7-Th, RMS = 153 nm

(c) PC BM, RMS = 0.846nm 71

Figure 2. AFM height images (5 μm × 5 μm) of DIOX processed pristine films for (a) PTB7, (b) PTB7-Th, and PC71BM. The surface morphologies of the PTB7:PC71BM and PTB7-Th:PC71BM blend films processed with DIOX, DIOX:TPO mixture (58:42 by volume), and CB are shown in Figure 3. The DIOX processed PTB7:PC71BM (Figure 3a) and PTB7-Th:PC71BM (Figure 3d) blend films exhibit large-sized domains and their RMS roughness values are of 83 and 88 nm, respectively. In combination with TPO, the DIOX:TPO mixture (58:42 by volume) can afford much better BHJ blend films, with largely decreased RMS of 1.52 and 1.36 nm for PTB7:PC71BM (Figure 3b) and PTB7-Th:PC71BM 8


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(Figure 3e), respectively. Coincidently, the RMS values of the DIOX:TPO mixture are almost identical to those of blend films processed with CB (Figures 3c and 3f). The very small difference for the DIOX:TPO mixture and CB processed blend films suggests they may show similar optical and electrical properties in devices, despite the different solubilites to the two polymers and PC71BM. (a) DIOX, RMS = 83 nm

(b) DIOX:TPO, RMS = 1.52 nm

(c)

CB, RMS = 1.53 nm

(d)

(e) DIOX:TPO, RMS = 1.36 nm

(f)

CB, RMS = 1.47 nm

DIOX, RMS = 88 nm

Figure 3. AFM height images (5 μm × 5 μm) of PTB7:PC71BM (a,b,c) and PTB7Th:PC71BM (d,e,f) blend films spin-casted from (a,d) DIOX, (b,e) DIOX:TPO, and (c,f) CB. 2.3 UV absorption and photoluminescence spectra of blend films We further compared the absorption spectra of the DIOX:TPO mixture (58:42 by volume) and CB processed blend films (Figures 4a and 4b). To eliminate the minor thickness variations of the different blend films, the UV spectra are given in normalized manners, based on the PTB7 absorption peak at 682 nm and the PTB7-Th absorption peak at 710 nm. As shown in Figure 4a, the two PTB7:PC71BM blend films almost show overlapped absorptions except the small difference around 390 nm. But for the 9



PTB7-Th:PC71BM blend films, there are absorption differences in wavelength range from 300 to 550 nm. Generally, the slightly changed absorptions may correspond to the changing of aggregations of polymer or PC71BM inside the BHJ blend films when casted from different solvent systems.

(b)

2.0

Normalized absorbance (a.u.)

Normalized absorbance (a.u.)

(a) PTB7:PC71BM, CB PTB7:PC71BM, DIOX:TPO

1.5

1.0

0.5

0.0 300

400

500

600

700

800

2.0

PTB7-Th:PC71BM, CB PTB7-Th:PC71BM, DIOX:TPO

1.5

1.0

0.5

0.0

900

300

400

Wavelength(nm)

(c)

(d)

4x10

PTB7:PC71BM, CB PTB7:PC71BM, DIOX:TPO

3x105 2x105 5

1x10

700

800

900

PTB7-Th, CB PTB7-Th:PC71BM, CB

5

6x10

PTB7-Th:PC71BM, DIOX:TPO

5x105 4x105 3x105 2x105 1x105

0 600

600

PTB7-Th, DIOX:TPO

5

7x10

PTB7, DIOX:TPO

Photoluminescence (a.u.)

5

500

Wavelength (nm)

PTB7, CB

5x105

Photoluminescence (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

0 650

700 750 800 Wavelength (nm)

850

900

750 800 850 Wavelength(nm)

900

Figure 4. UV absorption spectra of (a) PTB7:PC71BM and (b) PTB7-Th:PC71BM blend films casted with the CB and the DIOX:TPO mixture. Photoluminescence spectra of (c) the pristine PTB7 films and the PTB7:PC71BM blend films casted with the CB and the DIOX:TPO mixture and (d) the pristine PTB7-Th films and the PTB7-Th:PC71BM blend films casted with the CB and the DIOX:TPO mixture. Excitation wavelength: 500 nm for PTB7 based films and 700 nm for PTB7-Th based films. 10


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To evaluate exciton quenching effects of the BHJ blend films casted from the different solvent systems, we employed the photoluminescence (PL) spectroscopy. As shown in Figure 4c, under excitations, the neat films of PTB7 casted from CB and the DIOX:TPO mixture (58:42 by volume) show PL peaks at ~752 nm whereas for the PTB7:PC71BM blend films casted from CB and the DIOX:TPO mixture, very high PL quenching extents for the polymer emissions of 99.2% and 99.1%, respectively, can be found. Similarly, the PL emissions at 757 nm for the two neat films of PTB7-Th casted from CB and the DIOX:TPO mixture can be almost completely quenched by PC71BM in the blend films, based on quenching extents of 98.4% and 98.5%, respectively (Figure 4d). The results indicate the DIOX:TPO mixture processed PTB7 and PTB7Th based blend films should possess similar domain sizes to the corresponding CB processed blend films, and excitons formed under the excitations can well migrate to the polymer/fullerene interfaces, resulting in the PL quenching by PC71BM. The results also imply the DIOX:TPO mixture processed PTB7:PC71BM and PTB7-Th:PC71BM blend films would have big potential to achieve high photovoltaic performances. 2.4 Photovoltaic performances Because of the high efficiency, PTB7:PC71BM and PTB7-Th:PC71BM blend films have been widely utilized as the active layers in OSCs to evaluate new electrode interlayers.2,34 Conventional and inverted device configurations are usually compared.35 In order to give the thorough comparison of the CB and DIOX:TPO mixture processed PTB7:PC71BM and PTB7-Th:PC71BM blend films, the photovoltaic performances with the two device configurations were investigated in the meantime. The conventional 11



devices were fabricated with a configuration of ITO/PEDOT:PSS/polymer:PC71BM (1:1.5)/PFN/Al, where an alcohol-soluble polymer PFN film (5 nm)36 was the electron selection layer for the Al cathode. The inverted devices were fabricated as ITO/ZnO/PFN/polymer:PC71BM (1:1.5)/MoO3/Al. The bi-interlayer of ZnO/PFN on the ITO cathode has been reported before.37 The optimal ratios of 1:1.5 for both PTB7:PC71BM and PTB7-Th:PC71BM active layers have been reported previously.8 1,8-Diiodooctane (DIO) is a necessary solvent additive in the processing of PTB7:PC71BM and PTB7-Th:PC71BM active layers. In this study, an optimal content of 3% for CB is according to literature,2,8,26 while a 5% DIO is an optimized content for the DIOX:TPO mixture (Table S1). The measurements of the PSCs were carried out under illumination of AM1.5G simulated solar light at 100 mW/cm2. In Table 1, the DIOX:TPO mixture of 58:42 by volume shows an optimal RED among the four mixtures. With the four DIOX:TPO mixtures as the solvent, we compared the photovoltaic performances of the corresponding PTB7:PC71BM active layers in the conventional structure OSCs (Figure S2). As listed in Table S2, the OSC for the the DIOX:TPO mixture of 58:42 by volume displays the highest PCE of 7.6%, based on Voc of 0.75 V, Jsc of 15.4 mA/cm2 and FF of 65.9%. The other three mixtures afford inferior PCEs between 7.24% and 7.45%. It should be noted that all the efficiencies are higher than that of 6.42% for TPO in a previous report, initially demonstrating the value of a binary solvent based on DIOX and TPO. The highest PCE achieved by the the DIOX:TPO mixture of 58:42 by volume agrees with the ideal composition as suggested by the calculated HSPs and related RED values. Thereby, in 12


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the following investigations of photovoltaic performances of the PTB7:PC71BM and PTB7-Th:PC71BM active layers, the optimal DIOX:TPO mixture of 58:42 by volume was utilized to compare with CB and DIOX, as listed in Table 3.

Table 3. Photovoltaic performances of DIOX, DIOX:TPO (58:42) mixture, and CB processed BHJ blend films in two OSC structures Active layer

solvent

Voc

Jsc

FF

PCE c

(V)

(mA/cm2)

(%)

(%)

PTB7:PC71BMa

DIOX

0.02

1.88

19.12

0.01

PTB7:PC71BMb

DIOX

0.01

2.05

12.00

0.01

PTB7:PC71BMa

DIOX:TPO

0.75

15.36

65.94

7.60 (7.45)

PTB7:PC71BMb

DIOX:TPO

0.76

15.34

71.19

8.30 (8.11)

PTB7:PC71BMa

CB

0.73

15.86

65.61

7.59 (7.42)

PTB7:PC71BMb

CB

0.74

16.07

68.49

8.17 (8.02)

PTB7-Th:PC71BMa

DIOX

0.02

1.92

21.61

0.01

PTB7-Th:PC71BMb

DIOX

0.02

1.91

20.05

0.01

PTB7-Th:PC71BMa

DIOX:TPO

0.81

16.39

62.49

8.30 (8.15)

PTB7-Th:PC71BMb

DIOX:TPO

0.80

17.20

68.26

9.39 (9.18)

PTB7-Th:PC71BMa

CB

0.80

16.30

63.49

8.28 (8.10)

PTB7-Th:PC71BMb

CB

0.79

17.21

68.39

9.30 (9.14)

a

Conventional structure. b Inverted structure. devices is listed in the parenthesis.

c

The average PCE based on over 8

13



(a)

2

(b) 70 PTB7, DIOX PTB7, DIOX:TPO PTB7, CB PTB7-Th, DIOX PTB7-Th, DIOX:TPO PTB7-Th, CB

-2 -4 -6

60 EQE (%)

Current density (mA/cm2)

0

-8

50 40 30

-10 -12

20

-14

10

-16 -0.2

0.0

0.2

0.4

0.6

0 300

0.8

Voltage (V)

400 500 600 700 Wavelength (nm)

800

900

800

900

(d)

0

70

-2

PTB7, DIOX PTB7, DIOX:TPO PTB7, CB PTB7-Th, DIOX PTB7-Th, DIOX:TPO PTB7-Th, CB

-6 -8

60 EQE (%)

-4

-10

50 40 30

-12

PTB7, DIOX:TPO PTB7, CB PTB7-Th, DIOX:TPO PTB7-Th, CB

20

-14

10

-16 -18 -0.2

PTB7, DIOX:TPO PTB7, CB PTB7-Th, DIOX:TPO PTB7-Th, CB

80

2

(c) Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

0.0

0.2

0.4

0.6

0 300

0.8

Voltage (V)

400

500 600 700 Wavelength (nm)

Figure 5. (a) J-V characteristics and (b) EQE curves of conventional OSCs with PTB7 and PTB7-Th based active layers casted from different solvents. (c) J-V characteristics and (d) EQE curves of inverted OSCs with PTB7 and PTB7-Th based active layers casted from different solvents. The J-V characteristics of the conventional and inverted devices are shown in Figures 5a and 5c, respectively. With pure DIOX as the solvent, the resulting PTB7 and PTB7-Th based OSCs display abnormal J-V curves. The Voc values of the devices are close to zero, accompanying very low Jsc values of ~2 mA/cm2 and FF less than 22% (Table 3). The close to short-circuit state of the solar cells indicates that there are pinholes inside the DIOX processed blend films. The low solubilitis of materials in pure 14


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DIOX and poor surface mopology of the blend films would be responsible for the above poor device performances. With the addition of 42 vol.% TPO to DIOX, the resulting DIOX:TPO mixture can afford very efficient PTB7:PC71BM active layers. The conventional and inverted OSCs show good PCEs of 7.60% and 8.30%, respectively. Thus the binary solvent strategy can overcome the drawback of a single DIOX or TPO in casting active layers. The higher PCE achieved by the inverted OSCs is mainly due to the obviously improved FF of 71.1%. With CB as the solvent to process PTB7:PC71BM active layers, the conventional and inverted OSCs display PCEs of 7.59% and 8.17%, respectively. Therefore, in the conventional structure, the OSCs with the DIOX:TPO mixture processed PTB7 active layer show comparable efficiency to the CB case, while in the inverted structure, the DIOX:TPO mixture can give slight higher efficiency. With PTB7-Th to replace PTB7 in active layers, better photovoltaic performances can be achieved. For the conventional OSCs, the DIOX:TPO (58:42) mixture processed PTB7-Th:PC71BM active layer can show PCE of 8.30%. Similar to the PTB7 case, the PCE for PTB7-Th based inverted OSC can be elevated to 9.39%, due to the improved Jsc and FF. For the CB processed PTB7-Th:PC71BM active layers, the conventional and inverted OSCs display PCEs of 8.28% and 9.30%, respectively. Thereby the photovoltaic performances of the DIOX:TPO mixture processed PTB7-Th active layers are quite attractive, paving a way to replace chlorinated aromatic solvents in processing of highly efficient BHJ active layers. With the DIOX:TPO (58:42) mixture and CB as the solvents, the external quantum 15



efficiency (EQE) spectra for the conventional and inverted OSCs are shown in Figures 5b and 5d. For the conventional OSCs, there are some differences for the EQE responses between the DIOX:TPO (58:42) mixture and CB processed PTB7 based active layers as well as the corresponding PTB7-Th based active layers (Figure 5b). These differences for the EQE responses should be related to the observed differences of absorption spectra as shown in Figures 4a and 4b, when casted with different solvents. The integrated Jsc values for the DIOX:TPO mixture and CB processed PTB7 based active layers are 15.13 and 15.58 mA/cm2, respectively, quite close to the respective 15.36 and 15.86 mA/cm2 obtained in J-V measurements. For the PTB7-Th based active layers, the integrated Jsc values for the DIOX:TPO mixture and CB are 16.05 and 16.08 mA/cm2, respectively, comparable to their ~16.35 mA/cm2 obtained in J-V measurements. For the inverted OSCs (Figure 5b), the PTB7 based active layers casted from the DIOX:TPO mixture and CB show integrated Jsc values of 14.90 and 15.86 mA/cm2, respectively, while the corresponding values for PTB7-Th based active layers are 16.96 and 16.99 mA/cm2, respectively, all higher than those of corresponding conventional devices. In order to study the effects of processing solvents on the bulk charge carrier mobility of the blend films, the space charge limited current (SCLC) method was used to measure the hole (µh) and electron mobilities (µe) (Figure S2). The hole- and electron-only devices were fabricated with ITO/PEDOT:PSS/active layer/MoO3/Al and ITO/ZnO/active layer/PFN/Al structures, respectively. The extracted hole and electron mobilities are listed in Table S3. There are very small variations for the hole and 16


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electron mobilities of PTB7:PC71BM blend films when casted from the CB and DIOX:TPO (58:42) mixture. This is an important evidence that the two active layers can show comparable photovoltaic performances in either the conventional or the inverted OSCs. The CB and DIOX:TPO (58:42) mixture processed PTB7-Th:PC71BM blend films also demonstrate the similar behaviors for the hole and electron mobilities. More importantly, the µh and µe for all the blend films are very close and the calculated µh/µe values between 1.4 and 1.65 indicate fairly balanced transports, supporting their high FF achieved by the OSCs.

2.5 Bulk-heterojunction morphology of blend films We investigated the morphologies of the blend films using transmission electron microscopy (TEM) measurements to examine the phase separation of the blend films (Figure 6). The thicknesses for the blend films are ~100 nm. The PTB7 and PTB7-Th based blend films processed with the DIOX:TPO mixture exhibit similar degrees of dispersion homogeneity for the polymers and fullerene phases if compared with CB. The majorities of the polymers and fullerene phases are well-penetrated networks with small domain sizes less than 50 nm. Only few areas belong to large phase separations more than 100 nm, but can also show some mixing of the donor/acceptor phases. The morphologies may account for the good PCEs achieved for the DIOX:TPO mixture and CB, where PTB7 and PTB7-Th based active layers would show comparable charge separations and charge carrier transports.

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(a)

(b)

(c)

(d)

Figure 6. TEM images of (a,b) PTB7:PC71BM and (c,d) PTB7-Th:PC71BM blend films caste from (a,c) the DIOX:TPO (58:42) mixture and (b,d) CB.

3. CONCLUSIONS In summary, DIOX as a new non-aromatic and non-chlorinated solvent and DIOX:TPO as a binary non-aromatic and non-chlorinated mixture were introduced to process PTB7:PC71BM and PTB7-Th:PC71BM BHJ active layers for conventional and inverted OSCs, with comparisons to CB. DIOX processed active layers show disappointed photovoltaic performances due to poor film-forming ability arising from limited solubilities to polymers PTB7 and PTB7-Th. An optimal DIOX:TPO (58:42) mixture, predicted by the HSPs derived RED, can afford high quality BHJ blend films, as verified by AFM, TEM, PL quenching, and balanced carrier transports. As a result, all the DIOX:TPO (58:42) mixture processed PTB7:PC71BM and PTB7-Th:PC71BM active layers can display comparable PCEs to CB in either the conventional or the 18


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inverted OSCs. Particularly, PCEs of 8.30% and 9.39%, achieved by the DIOX:TPO mixture processed PTB7 and PTB7-Th based active layers in inverted OSCs respectively, are slightly higher than the corresponding 8.17% and 9.3% for the CB cases. Our results highlight the potential of mixing some low toxicity solvents to replace CB, towards environment-friendly fabrication of OSCs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jp-2018-113185. Calculations of Hansen space Ra and Relative Energy Difference (RED), experimental details for the device fabrication and characterization, and supplementary data (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (J. Chen) ORCID Junwu Chen: 0000-0003-0190-782X Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

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The authors thank the financial support of National Natural Science Foundation of China (U1401244, 21225418, 51521002, 91633301), National Basic Research Program of China (973 program 2013CB834705), Natural Science Foundation of Guangdong Province (2016A030312002), and GDUPS (2013).

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TOC Graphic

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Binary Nonchlorinated and Nonaromatic Solvent-Processed PTB7

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