Achieving over 9.8% Efficiency in Nonfullerene Polymer Solar Cells by

Subscriber access provided by LONDON METROPOLITAN UNIV

Article

Achieving over 9.8% Efficiency in Nonfullerene Polymer Solar Cells by Environmental-Friendly Solvent-Processing Yue Wu, Yan Zou, Hang Yang, Yaowen Li, Hongkun Li, Chaohua Cui, and Yongfang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11488 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces 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.

Page 1 of 19

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

ACS Applied Materials & Interfaces

Achieving over 9.8% Efficiency in Nonfullerene Polymer Solar Cells by Environmental-Friendly Solvent-Processing Yue Wu1, Yan Zou1, Hang Yang1, Yaowen Li1, Hongkun Li1, Chaohua Cui*1, and Yongfang Li1, 2 1. Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China. 2. CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: [email protected] (C. H. Cui)

ABSTRACT Nowadays, most of the solution-processed high-efficiency polymer solar cell (PSC) devices are fabricated by halogenated solvents (such as chlorobenzene, 1,2dichlorobenzene, and chloroform etc) which are harmful to people and the environment. Therefore, it is essential to develop high-efficiency PSC devices processed by environmental-friendly solvent-processing for their industrialization. In this regard, we report a new alkylthio chain-based conjugated polymer PBDB-TS as donor material for environmental-friendly solvent-processed PSCs. PBDB-TS possesses a low-lying HOMO energy level at -5.42 eV and a good solubility in toluene and o-xylene. By using o-xylene and 1% N-methylpyrrolidone as processing solvent, following by the thermal annealing treatment for PBDB-TS:ITIC blend films, well-developed morphological features and balanced charge transport properties are observed, leading to a high power conversion efficiency (PCE) of 9.85%, which is higher than that of the device cast from halogenated solvent (PCE = 9.65%). The results suggest that PBDB-TS is an attractive donor material for non-halogen solvents-processing PSCs.

Keywords: polymer solar cells; non-halogen solvent-processing; alkylthio chains; donor material; power conversion efficiency

1

ACS Paragon Plus Environment


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

INTRODUCTION Solution-processed bulk heterojunction (BHJ) polymer solar cells (PSCs) are considered as very attractive low-cost and environmental-friendly photovoltaic technologies.1-7 During the past decades, the power conversion efficiencies (PCEs) of the PSCs have been significantly improved to over 10% by a combination of photoactive materials (donor and acceptor materials) innovation, interface engineering, and device manipulation.8-20 Very recently, the development of nonfullerene acceptors, which exhibit strong and broad absorption in the visible region to well match the donor materials for improving the light harvesting capability, enables the efficiency to over 13%.21 Although the PCEs of PSCs have been driven to reach a milestone, most of the highefficiency devices are fabricated by halogenated solvents such as chlorobenzene (CB), 1,2-dichlorobenzene, chloroform etc. In addition, the frequently-used 1,8-diiodooctane (DIO) solvent additive for fine tuning the morphology of the blend films is also halogenated solvent. These halogenated solvents are high toxicity and costly. Therefore, it is essential to develop PSCs which can still realize high photovoltaic performance when using environmental-friendly solvents for devices fabrication. In recent years, many attempts have been dedicated to find environmental-friendly alternative solvents to address this issue.15, 22-31 For instance, Yan et al. used 1,2,4-trimethylbenzene (TMB) as host solvent and 1-phenylnaphthalene as additive solvent to achieve a remarkable PCE of 11.7% from the device based on PffBT4T-C9C13:PC71BM blend films;15 Hou et al employed o-xylene (o-XY) and N-methylpyrrolidone (NMP) to fabricate the PBDTTS1:PC71BM-based PSC devices, and a promising PCE of 9.47% was obtained;28 a high PCE of 8.1% was realized by Cao et al. from the PSC device based on PDTSTPD:PC71BM blend films cast from TMB and 1,5-dimethylnaphthalene.27 Very importantly, the non-halogen solvents TMB and o-XY have certain advantages over chlorobenzenes of low toxicity and production cost. Moreover, such methylbenzenes are biodegradable and can be decomposed to alleviate the harm of health and environment.22 Despite the above achievements, in comparison with the extensively reported highefficiency PSCs processed by halogenated solvents, high performance non-halogen solvents-processing PSCs are still less common in literatures. One major reason for the much poorer performance of PSCs processed by non-halogen solvents is that the reported 2


Page 2 of 19

Page 3 of 19

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


state-of-the-art donor materials (or acceptor materials) typically exhibit poor solubility in non-halogenated solvents, resulting in poor BHJ morphology with excessively large domains.15 In this regard, it is desirable to develop new photoactive materials (donor and acceptor materials) which are favorable to well-developed BHJ morphology from nonhalogen solvents processing for high-efficiency PSCs. In this work, we report promising environmental-friendly solvent-processed nonfullerene PSCs by using a new conjugated polymer (namely PBDB-TS, as shown in Scheme 1) as donor and ITIC32 as acceptor. The positive effect of alkylthio side-chains in down-shifting the HOMO (highest occupied molecular orbital) energy level of the donor materials for high open-circuit voltage (Voc) values of the PSC devices has been demonstrated in our previous studies.33-38 As expected, over 0.9 V of the Voc are obtained from the PBDB-TS:ITIC-based devices by using either chlorobenzene or o-XY as processing solvent. Very importantly, well-developed morphological features and balanced charge transport properties of the PBDB-TS:ITIC blend film were realized when using o-XY and 1% NMP as processing solvent, following by thermal annealing at 160 °C for 10 min, delivering a promising PCE of 9.85%, with Voc = 0.92 V, FF = 0.647, and Jsc = 16.63 mA cm-2. As a contrast, the optimal device cast from CB solvent exhibits a lower PCE of 9.65%, with Voc = 0.93 V, Jsc = 15.93 mA cm-2, and FF = 0.651. The outcome of this study indicates that PBDB-TS is an attractive donor material for nonhalogen solvents-processing PSCs.

RESULTS AND DISCUSSION Synthesis and Characterization of the Polymer The monomers BDTT-S33 and BDD39 were synthesized by the literature methods. The co-polymer PBDB-TS was synthesized by the Stille coupling reaction in toluene with Pd(PPh3)4 as catalyst (as shown in Scheme 1). More detailed characterization of the polymer is described in the Supporting Information. The polymer shows good solubility in chloroform, CB, toluene and o-XY, etc. The number-average molecular weight (Mn) of PBDB-TS is 20.9 kDa with polydispersity (PDI) of 1.8, which was measured by high-temperature gel-permeation chromatography (GPC) using 1,2,4-trichlorobenzene as the eluent under 160 °C. The decomposition 3



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 4 of 19

temperature (Td) at 5% weight-loss of PBDB-TS estimated by thermogravimetric analysis (TGA) is over 300 °C (Figure S1, see Supporting Information), indicating the good thermal stability of PBDB-TS for photovoltaic material application.

S

S

S S

S

S O

S Sn

Sn S

+

Br

O

S

Toluene Pd(PPh3)4 S

S

Br

o

110 C 11 h

O

O S

S

S S

S

n

S

S

BDD

S

S

BDTT-S

PBDB-TS

Scheme 1. Synthetic routes and chemical structure of PBDB-TS.

Photophysical Properties and Electronic Energy Levels Figure 1 shows the absorption spectra of PBDB-TS in dilute o-XY solution and a thin solid film on a quartz plate. The absorption spectrum of PBDB-TS in o-XY covers the wavelength range from 300 to 675 nm, with two absorption peaks at 586 nm and 622 nm, respectively. In comparison with the absorption spectrum in dilute o-XY solution, the absorption edge of PBDB-TS film red-shifted ~21 nm to 698 nm, corresponding to a bandgap of 1.78 eV. Two well-defined absorption peaks can be observed in the absorption spectrum of PBDB-TS film at 583 nm and 626 nm, attributing to π-π* transition and interchain π-π* transition resulting from the π-π stacking of the polymeric backbones, respectively.39-41 Identical spectra of PBDB-TS can be seen in both o-XY solution and thin film state, suggesting that the polymer molecules are strongly aggregated in solution state even if the polymer can be dissolved into o-XY solvent.

4


Page 5 of 19

Solution in o-XY Film

Normalized Absorbance (a.u.)

1.0 0.8 0.6 0.4 0.2 0.0 300

400

500 600 Wavelength (nm)

700

800

Figure 1. Normalized absorption spectra of PBDB-TS in o-XY solution and thin film.

Electrochemical cyclic voltammetry was performed to determine the LUMO (lowest unoccupied molecular orbital) and HOMO energy levels of PBDB-TS.42 As shown in Figure 2, the onset oxidation potential (φox) and the onset reduction potential (φred) of PBDB-TS are 0.71 and -1.10 V vs. Ag/Ag+, respectively. Thus, the LUMO and HOMO levels of PBDB-TS are calculated from φred and φox are -3.61, -5.42 eV, respectively, corresponding to the electrochemical bandgap of 1.81 eV. The relatively low-lying HOMO energy level of PBDB-TS can potentially guarantee a high Voc value of the PSC device, since the Voc is closely relate to the HOMO energy level of the donor material.43 2 1 Current (mA)

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


0 -1 -2 -3 -4 -1.5

-1.0 -0.5 0.0 0.5 + Potential (V vs. Ag/Ag )

1.0

1.5

Figure 2. Cyclic voltammogram of the PBDB-TS thin film in 0.1 mol/L Bu4NPF6 acetonitrile solution at a scan rate of 100 mV s-1.

Density functional theory (DFT) modeling provides the frontier molecular orbital surfaces patterns and optimized geometry of PBDB-TS (as shown in Figure 3). As shown 5



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

in Figure 3a, the electron density in the LUMO of PBDB-TS mainly distributes in the BDD unit, while the electron density in the HOMO is delocalized along the conjugated backbone. The dihedral angles of BDTT-S-thiophene and BDD-thiophene are 9.9° and 5.2° (see Figure 3b), respectively, indicating the relatively good planarity of conjugated backbone of PBDB-TS.

Figure 3. (a) The frontier molecular orbital surfaces and (b) molecular geometry of PBDB-TS calculated by DFT/B3LYP/6-31G(d, p).

Photovoltaic Performance BHJ PSC device with an inverted device structure of glass/indium tin oxide (ITO)/ZnO/PBDB-TS:ITIC/MoO3/Al were fabricated for investigating photovoltaic performance of PBDB-TS. For the initial photovoltaic characterizations, donor (D)/acceptor (A) weight ratios of 1:0.5, 1:0.8, 1:1, and 1:1.2 were scanned from CB solution, and the optimal weight ratio is 1:0.8 for the devices (Figure S2, Figure S3 and Table S1, see Supporting Information). By using o-XY as processing solvent, the optimized device fabrication conditions include with 1% NMP as solvent additive, and then the blend films were annealed at 160 °C for 10 min. While the optimal treatment for the devices cast from CB solvent is by using 0.5% NMP as solvent additive, following by thermal annealing at 160 °C for 10 min. Detailed optimal photovoltaic performance of the devices processed by different amount of DIO or NMP as solvent additive in CB and oXY is listed in Table S2 in Supporting Information. Plots of PCEs against different treatment conditions for the PSCs based on PBDB-TS:ITIC (1:0.8, w/w) are shown in 6


Page 6 of 19

Page 7 of 19

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


Figure 4 for clear comparison. The J-V curves of the optimal devices are shown in Figure 5 and the detailed photovoltaic device parameters are summarized in Table 1. It is well-known that Voc is also decided by the morphology of blend films besides just the HOMO energy level of the donor. Thus, varied Voc values (range from 0.92 to 0.96 V) were obtained from the devices when using different solvent additive or posttreatment to tune the morphology of the blend films. The device processed by CB solvent shows a moderate PCE of 8.08%, with a high Voc of 0.96 V, a FF of 0.594, and a Jsc of 14.2 mA cm-2. By using 0.5% DIO as additive solvent, following by thermal annealing at 160 °C for 10 min for the PBDB-TS:ITIC (1:0.8, w/w) blend films, a higher Jsc and FF can be realized, leading to a higher PCE of 9.54%. Replacing DIO by NMP as additive to process the active layer, a slightly higher PCE of 9.65% was achieved. The PSC devices prepared from o-XY shows a moderate PCE of 7.9%, while an improved PCE of 9.39% is obtained by using 1% DIO as additive solvent. Very importantly, the combination treatment of active layer with 1% NMP as additive and then thermal annealing at 160 °C for 10 min, a very promising PCE of 9.85% was realized, with Voc = 0.92 V, FF = 0.647, and Jsc = 16.63 mA cm-2. It is worth to point out that, unlike DIO, the additive NMP is non-halogenated solvent of low toxicity. In addition, the PSCs processed with environmental-friendly non-halogen solvent showed a higher PCE than that of the halogen processed device.

Table 1. Photovoltaic parameters of the PSCs based on PBDB-TS:ITIC (1:0.8, w/w). Solvent

Solvent additive

Voc [V]

Jsc [mA cm-2]

FF [%]

PCE [%]

PCEave [%][a]

CB

w/o

0.96

14.20

59.4

8.08

7.72 ± 0.16

CB

0.5% DIO

0.96

14.48

60.9

8.43

8.28 ± 0.15

CB

0.5% DIO[b]

0.94

15.19

67.0

9.54

9.25 ± 0.14

CB

1% NMP

0.94

13.96

61.5

8.04

7.82 ± 0.16

CB

1% NMP[b]

0.93

15.93

65.1

9.65

9.05 ± 0.32

7



o-XY

w/o

0.94

14.40

58.4

7.90

7.77 ± 0.08

o-XY

1% DIO

0.95

15.48

63.9

9.39

9.28 ± 0.10

o-XY

1% DIO[b]

0.92

15.40

63.8

9.06

8.72 ± 0.20

o-XY

1% NMP

0.92

15.44

60.4

8.38

8.22 ± 0.12

o-XY

1% NMP[b]

0.92

16.63

64.7

9.85

9.64 ± 0.11

[a]

PCEave stands for the average PCE obtained from 10 devices.

[b]

The active layer was

annealed at 160 °C for 10 min. 10.5 Cast from CB solvent Cast from o-XY solvent

10.0 9.5

PCE (%)

9.0 8.5 8.0 7.5 7.0

a

a

N M P 1%

D IO

D IO

N M P 1%

a

1%

DI O

0. 5%

1%

0. 5%

D IO

6.5 w /o

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 8 of 19

Figure 4. Plots of PCEs against different treatment conditions for the PSCs based on PBDB-TS:ITIC (1:0.8, w/w). The PCE values were calculated from 10 devices for each treatment. a The active layer was annealed at 160 °C for 10 min.

8


Page 9 of 19

(a) CB

(b) o-XY

Current density (mA cm )

0 -2

-2

Current density (mA cm )

0

w/o 0.5% DIO

-5

o

0.5% DIO + 160 C annealing 1% NMP o

1% NMP + 160 C annealing

-10

-15

0.0

0.2

0.4 0.6 Voltage (V)

0.8

w/o 1% DIO

-5

o

1% DIO + 160 C annealing 1% NMP o


-10

-15

0.0

1.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

Figure 5. J-V curves of the PSCs based on PBDB-TS:ITIC (1:0.8, w/w) processed by (a) CB with different treatment; (b) o-XY with different treatment.

External quantum efficiency (EQE) spectra were measured to examine the accuracy of Jsc from the J-V measurements. As shown in Figure 6, all the devices exhibit broad EQE spectra from 300 to 800 nm with the maximum over 60%. The integrated Jsc values of the EQE curves are rather consistent (within 5% mismatch) with the Jsc values obtained by J-V measurements. The maximum EQE value of the PSC device cast from oXY + 1% NMP + 160 °C annealing reached 76% at 576 nm, with integrated Jsc value of 16.21 mA cm-2. 75

(a) CB

75

60

(b) o-XY

60

45

EQE (%)

EQE (%)

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


w/o

30

0.5% DIO

45 w/o

30

1% DIO

o

o

0.5% DIO + 160 C annealing

1% DIO + 160 C annealing

1% NMP

15 0 300

400

500

600

700

1% NMP

15

o


o


0 300

800

400

Wavelength (nm)

500 600 700 Wavelength (nm)

800

Figure 6. EQE curves of the PSCs based on PBDB-TS:ITIC (1:0.8, w/w) processed by (a) CB with different treatment; (b) o-XY with different treatment.

Mobility Measurements 9



The high and balanced charge-carrier property is essential for high performance photovoltaic materials in PSCs, since a balanced hole and electron transport of blend films is beneficial to suppress the pace-charge build-up for achieving high photovoltaic device performance.44 To study the charge-carrier mobilities of the PBDB-TS-based blend films cast from CB and o-XY, the hole and electron mobilities were measurement by space-charge-limited-current (SCLC) method. As shown in Figure 7 and Table 2, a high SCLC hole mobility of 2.69 × 10-4 cm2 V-1 s-1 was observed from the pure PBDBTS film cast from CB solvent exhibited. In comparison with the hole and electron mobilities of CB processed PBDB-TS:ITIC (1:0.8, w/w) blend films (µh = 1.77 × 10-4 cm2 V-1 s-1, µe = 1.29 × 10-5cm2 V-1 s-1), a higher hole and electron mobilities (µh = 7.09 × 10-4 cm2 V-1 s-1, µe = 6.68 × 10-5cm2 V-1 s-1) were observed from the o-XY processed blend films (as shown in Figure 7b-e). Moreover, the PBDB-TS:ITIC (1:0.8, w/w) blend films cast from o-XY exhibit a more balanced µh/µe ratio (µh/µe = 10.6) than that of the blend films cast from CB (µh/µe = 13.7). Generally, the ratio of µh/µe is related to the charge transport, in which the ratio close to 1 indicates that hole and electron can equally arrive at electrode so as to reduce charge recombination. This is beneficial for enhancing Jsc and FF values. In this case, the more balanced µh/µe ratio (10.6 vs. 13.7) led to a higher Jsc (16.63 mA cm-2 vs. 15.93 mA cm-2) and similar FF (0.647 vs. 0.651) values, respectively, and thus a higher PCE was achieved from the PSC device processed by oXY + 1% NMP + 160 °C annealing. The µh/µe ratio of the device cast from o-XY + 0.5% DIO + 160 °C annealing is 2.64, which is agree with the highest FF of 0.67 of the corresponding device. 140 (a)

Hole only device Fitting curve

120

J0.5/A0.5m-1

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 19

100

80

µh = 2.69 × 10-4 cm2 v-1 s-1 60 2.0

2.5

3.0

3.5

4.0

Vappl-Vbi-Vbr/V 10


4.5

Page 11 of 19

25

100

J0.5/A0.5m-1

J0.5/A0.5m-1

(c)


(b)

80

Electron only device Fittting curve

20

15

60

µh = 1.77 × 10-4 cm2 v-1 s-1 40 1.5

2.0

2.5

3.0

3.5

4.0

µe = 1.29 × 10-5 cm2 v-1 s-1 10 1.5

4.5

2.0

120

(d)

J0.5/A0.5m-1

J0.5/A0.5m-1

100

80

2.5

3.0

3.5

4.0

4.5

0.0

5.0

0.5

(g)

J0.5/A0.5m-1

µh = 3.01 × 10-4 cm2 v-1 s-1 2.0

2.5

3.0

3.5

1.5

2.0

2.5

3.0

60

µe = 1.14 × 10-4 cm2 v-1 s-1

40 1.5

1.0

Electron only device Fittting curve

80

120

80

4.5

Vappl-Vbi-Vbr/V


160

4.0

µe = 6.68 × 10-5 cm2 v-1 s-1

Vappl-Vbi-Vbr/V (f)

3.5

14

7

µh = 7.09 × 10-4 cm2 v-1 s-1

2.0

3.0

Electron only device Fitting curve

(e)

21


60

2.5

Vappl-Vbi-Vbr/V

Vappl-Vbi-Vbr/V

J0.5/A0.5m-1

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


4.0

4.5

1.0

1.5

Vappl-Vbi-Vbr/V

2.0

2.5

3.0

3.5

Vappl-Vbi-Vbr/V

Figure 7. J0.5 vs. (Vapp-Vbi-Vbr) plots of (a) pristine PBDB-TS film cast from CB; (b) hole-only devices and (c) electron only devices of PBDB-TS:ITIC (1:0.8, w/w) blend film cast from CB + 1% NMP + 160 °C annealing; (d) hole-only devices and (e) electron only devices of PBDB-TS:ITIC (1:0.8, w/w) blend film cast from o-XY + 1% NMP + 160 °C annealing; (f) hole-only devices and (g) electron only devices of PBDB-TS:ITIC (1:0.8, w/w) blend film cast from o-XY + 0.5% DIO + 160 °C annealing. 11



Table 2. Hole and electron mobilities of the blend films of PBDB-TS:ITIC (1:0.8, w/w) cast from different processing conditions. Processing treatment

µh (cm2 v-1 s-1)

µe (cm2 v-1 s-1)

µh /µe

CB + 1% NMP + 160 °C annealing

1.77 × 10-4

1.29 × 10-5

13.7

o-XY + 1% NMP + 160 °C annealing

7.09 × 10-4

6.68 × 10-5

10.6

CB + 0.5% DIO + 160 °C annealing

3.01 × 10-4

1.14 × 10-4

2.64

XRD and Morphology Analysis X-ray diffraction (XRD) analysis was carried out to study the crystalline structures of BDB-TS film and PBDB-TS:ITIC (1:0.8, w/w) blend film (as shown in Figure 8). The BDB-TS film shows a strong (100) diffraction peak at 2θ = 4.4°, corresponding to a d100spacing of 20.0 Å. Clear diffraction (100) peak also can be observed in the PBDBTS:ITIC (1:0.8, w/w) blend films at 2θ = 4.5° (d100-spacing = 19.6 Å), indicating the lamellar structure of PBDB-TS:ITIC blend film. The ordered and well-defined structure in the blend films should be beneficial for the charge transportation and photovoltaic performance of the PBDB-TS-based devices. 14000 PBDB-TS PBDB-TS:ITIC (1:0.8 w/w)

12000

Intensity (CPS)

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 12 of 19

10000 8000 100

6000 4000 2000

100

0 5

10

15

20

25

30

2θ (degree)

Figure 8. XRD patterns of PBDB-TS film and PBDB-TS:ITIC (1:0.8, w/w) blend films.

12


Page 13 of 19

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


To provide an in-depth understanding on the morphological properties of the PBDBTS:ITIC films, the surface and bulk morphologies of the blend films processed with CB + 0.5% DIO + 160 °C annealing, CB + 1% NMP + 160 °C annealing, o-XY + 1% DIO, and o-XY + 1% NMP + 160 °C were characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM), respectively. As shown in Figure 9, the four blend films cast from different treatments show rather flat and uniform surfaces. The root-mean-square roughness (RMS) of the PBDB-TS:ITIC (1:0.8, w/w) blend films processed with CB + 0.5% DIO + 160 °C annealing and CB + 1% NMP + 160 °C annealing are 2.76 and 3.92 nm, respectively, while the RMS of the blend films cast from o-XY + 1% DIO, and o-XY + 1% NMP + 160 °C annealing are 2.22 and 3.22 nm, respectively. It is very evident from the AFM images that the blend films cast from either CB or o-XY with DIO additive showed lower RMS values than that processed with NMP additive. In comparison with the blend films cast from CB solvent, lower RMS values were obtained from the o-XY solvent processed blend films with corresponding additive treatment, respectively. The TEM images of the four blend films all show homogeneous morphology with nanosize segregated crystallites (as shown in Figure 10). Due to the established formation of continuous charge transport channels, such morphology is more favorable for effective exciton diffusion, charge separation, and transport, which could be beneficial to achieve high efficiency of the device. It is clear from these micrographs that when replacing CB by o-XY as the processing solvent, a well-defined phase separation is realized in PBDB-TS:ITIC blend films, which led to a more balanced charge transport of the device.

13



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

CB 0.5 % DIO + 160 oC annealing

Page 14 of 19

o-XY

1%NMP + 160 oC annealing

1%NMP + 160 oC annealing

1% DIO

20 nm

a

RMS =2.76 nm

b

RMS =3.92 nm

500 nm 0

2 µm

2 µm

0

2 µm

0

2 µm 0 nm 50 mV

h

500 nm

500 nm

j

i

2 µm

g

500 nm

RMS =3.22 nm

500 nm 0

f

2 µm

d

RMS =2.22 nm

500 nm 0

e

0

c

500 nm

2 µm

0

0

2 µm

0 mV

l

k

Figure 9. AFM (a)-(d) topography, (e)-(h) phase, and (i)-(l) 3D topography images of the blend films cast from different processing treatment. The size of the images is 5 µm × 5 µm.

CB

o-XY

a

b

c

d

14


Page 15 of 19

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


Figure 10. TEM images of the blend films cast from (a) CB + 0.5% DIO + 160 °C annealing, (b) o-XY + 1% DIO, (c) CB + 1% NMP + 160 °C annealing, and (d) o-XY + 1% NMP + 160 °C annealing, respectively.

CONCLUSION In summary, a new alkylthio chain-based conjugated polymer PBDB-TS was developed as donor material for PSCs. PBDB-TS showed low-lying HOMO energy level and good solubility in non-halogen solvents such as toluene, o-XY, etc. Very importantly, a well-defined phase separation is realized in PBDB-TS:ITIC blend films which processed by o-XY solvent with 1% NMP as a non-halogenated additive, and then annealing at 160 °C for 10 min. Moreover, the o-XY processed blend films exhibited higher charge-carrier mobilities and relatively more balanced µh/µe ratio than that of the CB processed blend films. The combination of the well-developed morphological features and balanced charge transport properties, the non-halogen solvent processed PSCs deliver a promising PCE of 9.85%, which is higher than that of the device processed by CB solvent (PCE = 9.65%). The outcome of this study indicates that

PBDB-TS is an attractive donor material for non-halogen solvents-processing PSCs.

ASSOCIATE CONTENT Supporting Information The following Supporting Information is available free of charge on the ACS Publication website. Experimental details for synthesis of PBDB-TS, material characterization, device fabrication and characterization, and mobility measurement. TGA plots of PBDB-TS, summary of devices performance obtained from different D/A weight ratios or processed treatment.

AUTHOR INFORMATION Corresponding Author [email protected] (C. H. Cui)

15



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

ACKNOWLEDGEMENTS We thank National Natural Science Foundation of China (51603136, 91633301, 91333204), Jiangsu Provincial Natural Science Foundation (BK20150327), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China Postdoctoral Science Foundation (2015M581855 and 2017T100395), and Natural Science Foundation of the Jiangsu Higher Education Institutions of China (15KJB430028) for financial support.

REFERENCES (1) Zhou, H.; Yang, L.; You, W. Rational Design of High Performance Conjugated Polymers for Organic Solar Cells. Macromolecules 2012, 45, 607-632. (2) Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153-161. (3) Kang, H.; Kim, G.; Kim, J.; Kwon, S.; Kim, H.; Lee, K. Bulk-Heterojunction Organic Solar Cells: Five Core Technologies for Their Commercialization. Adv. Mater. 2016, 28, 7821-7861. (4) Yao, H.; Ye, L.; Zhang, H.; Li, S.; Zhang, S.; Hou, J. Molecular Design of BenzodithiopheneBased Organic Photovoltaic Materials. Chem. Rev. 2016, 116, 7397-7457. (5) Lin, Y.; Li, Y.; Zhan, X. Small Molecule Semiconductors for High-efficiency Organic Photovoltaics. Chem. Soc. Rev. 2012, 41, 4245-4272. (6) Cao, W.; Xue, J. Recent Progress in Organic Photovoltaics: Device Architecture and Optical Design. Energy Environ. Sci. 2014, 7, 2123-2144. (7) Chen, Y.; Wan, X.; Long, G. High Performance Photovoltaic Applications Using SolutionProcessed Small Molecules. Acc. Chem. Res. 2013, 46, 2645-2655. (8) Zhang, S.; Ye, L.; Zhao, W.; Yang, B.; Wang, Q.; Hou, J. Realizing over 10% Efficiency in Polymer Solar Cell by Device Optimization. Sci. China: Chem. 2015, 58, 248-256. (9) Qin, Y.; Chen, Y.; Cui, Y.; Zhang, S.; Yao, H.; Huang, J.; Li, W.; Zheng Z.; Hou, J. Achieving 12.8% Efficiency by Simultaneously Improving Open-Circuit Voltage and Short-Circuit Current Density in Tandem Organic Solar Cells. Adv. Mater. 2017, 29, 1606340. (10) Zhao, W.; Li, S.; Zhang, S.; Liu X; Hou, J. Ternary Polymer Solar Cells based on Two Acceptors and One Donor for Achieving 12.2% Efficiency. Adv. Mater. 2017, 29, 1604059. (11) Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Mukherjee, S.; Ade H.; Hou J. Energy-Level Modulation of Small-Molecule Electron Acceptors to Achieve over 12% Efficiency in Polymer Solar Cells. Adv. Mater. 2016, 28, 9423-9429. (12) Bin, H.; Gao, L.; Zhang, Z.-G.; Yang, Y.; Zhang, Y.; Zhang, C.; Chen, S.; Xue, L.; Yang, C.; Xiao, M.; Li, Y. 11.4% Efficiency Non-fullerene Polymer Solar Cells with Trialkylsilyl Substituted 2DConjugated Polymer as Donor. Nat. Commun. 2016, 7, 13651. (13) Zhang, S.; Ye L.; Hou, J. Breaking the 10% Efficiency Barrier in Organic Photovoltaics: Morphology and Device Optimization of Well-Known PBDTTT Polymers. Adv. Energy Mater. 2016, 6, 1502529. (14) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. 16


Page 16 of 19

Page 17 of 19

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


(15) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027 (16) Zhao, F.; Dai, S.; Wu, Y.; Zhang, Q.; Wang, J.; Jiang, L.; Ling, Q.; Wei, Z.; Ma, W.; You, W.; Wang, C.; Zhan, X. Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency. Adv. Mater. 2017, 29, 1700144. (17) Zhang, M.; Wang, J.; Zhang, F.; Mi, Y.; An, Q.; Wang, W.; Ma, X.; Zhang, J.; Liu, X., Ternary Small Molecule Solar Cells Exhibiting Power Conversion Efficiency of 10.3%. Nano Energy 2017, 39, 571-581. (18) Zhang, M.; Zhang, F.; An, Q.; Sun, Q.; Wang, W.; Zhang, J.; Tang, W., Highly Efficient Ternary Polymer Solar Cells by Optimizing Photon Harvesting and Charge Carrier Transport. Nano Energy 2016, 22, 241-254. (19) Zhang, M.; Zhang, F.; An, Q.; Sun, Q.; Wang, W.; Ma, X.; Zhang, J.; Tang, W., Nematic Liquid Crystal Materials as a Morphology Regulator for Ternary Small Molecule Solar Cells with Power Conversion Efficiency Exceeding 10%. Journal of Materials Chemistry A 2017, 5, 3589-3598. (20) Sun, Q.; Zhang, F.; An, Q.; Zhang, M.; Ma, X.; Zhang, J., Simultaneously Enhanced Efficiency and Stability of Polymer Solar Cells by Employing Solvent Additive and Upside-Down Drying Method. ACS Appl. Mater. Interfaces 2017, 9, 8863-8871. (21) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc., 2017, 139, 7148-7151. (22) Chueh, C.-C.; Yao, K.; Yip, H.-L.; Chang, C.-Y.; Xu, Y.-X.; Chen, K.-S.; Li, C.-Z.; Liu, P.; Huang, F.; Chen, Y.; Chen, W.-C.; Jen, A. K. Y. Non-halogenated Solvents for Environmentally Friendly Processing of High-performance Bulk-heterojunction Polymer Solar Cells. Energy Environ. Sci. 2013, 6, 3241-3248. (23) Deng, Y.; Li, W.; Liu, L.; Tian, H.; Xie, Z.; Geng, Y.; Wang, F. Low Bandgap Conjugated Polymers Based on Mono-fluorinated Isoindigo for Efficient Bulk Heterojunction Polymer Solar Cells Processed with Non-chlorinated Solvents. Energy Environ. Sci. 2015, 8, 585-591. (24) Sprau, C.; Buss, F.; Wagner, M.; Landerer, D.; Koppitz, M.; Schulz, A.; Bahro, D.; Schabel, W.; Scharfer, P.; Colsmann, A. Highly Efficient Polymer Solar Cells Cast from Non-halogenated Xylene/anisaldehyde Solution. Energy Environ. Sci. 2015, 8, 2744-2752. (25) Zhang, H.; Yao, H.; Zhao, W.; Ye, L.; Hou, J. High-Efficiency Polymer Solar Cells Enabled by Environment-Friendly Single-Solvent Processing. Adv. Energy Mater. 2016, 6, 1502177. (26) Burgués-Ceballos, I.; Machui, F.; Min, J.; Ameri, T.; Voigt, M. M.; Luponosov, Y. N.; Ponomarenko, S. A.; Lacharmoise, P. D.; Campoy-Quiles, M.; Brabec, C. J. Solubility Based Identification of Green Solvents for Small Molecule Organic Solar Cells. Adv. Funct. Mater. 2014, 24, 1449-1457. (27) Cai, W.; Liu, P.; Jin, Y.; Xue, Q.; Liu, F.; Russell, T. P.; Huang, F.; Yip, H.-L.; Cao, Y. Morphology Evolution in High-Performance Polymer Solar Cells Processed from Nonhalogenated Solvent. Adv. Sci. 2015, 2, 1500095. (28) Zhao, W.; Ye, L.; Zhang, S.; Sun, M.; Hou, J. A Universal Halogen-free Solvent System for Highly Efficient Polymer Solar Cells. J. Mater. Chem. A 2015, 3, 12723-12729. (29) Farahat, M. E.; Perumal, P.; Budiawan, W.; Chen, Y.-F.; Lee, C.-H.; Chu, C.-W. Efficient Molecular Solar Cells Processed from GreenSolvent Mixtures. J. Mater. Chem. A 2017, 5, 571582. (30) Farahat, M. E.; Tsao, C.-S.; Huang, Y.-C.; Chang, S. H.; Budiawan, W.; Wu, C.-G.; Chu, C.-W. Toward Environmentally Compatible Molecular Solar Cells Processed from Halogen-free Solvents. J. Mater. Chem. A 2016, 4, 7341-7351.

17



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

(31) Park, G. E.; Choi, S.; Park, S. Y.; Lee, D. H.; Cho M. J.; Choi, D. H. Eco-Friendly SolventProcessed Fullerene-Free Polymer Solar Cells with over 9.7% Efficiency and Long-Term Performance Stability. Adv. Energy Mater. 2017, DOI: 10.1002/aenm. (32) Lin, Y.; Wang, J.; Zhang, Z. G.; Bai, H.; Li, Y.; Zhu, D.; Zhan, X. An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv Mater 2015, 27, 1170-1174. (33) Cui, C.; Wong, W.-Y.; Li, Y. Improvement of Open-circuit Voltage and Photovoltaic Properties of 2D-conjugated Polymers by Alkylthio Substitution. Energy Environ. Sci. 2014, 7, 2276-2284. (34) Cui, C.; He, Z.; Wu, Y.; Cheng, X.; Wu, H.; Li, Y.; Cao, Y.; Wong, W.-Y. High-performance Polymer Solar Cells based on a 2D-conjugated Polymer with an Alkylthio Side-chain. Energy Environ. Sci. 2016, 9, 885-891. (35) Cui, C.; Wong, W.-Y. Effects of Alkylthio and Alkoxy Side Chains in Polymer Donor Materials for Organic Solar Cells. Macromol. Rapid Commun. 2016, 37, 287-302. (36) Cheng, X.; Wan, Q.; Wu, Y.; Guo, B.; Guo, X.; Li, Y.; Zhang, M.; Cui, C.; Li, Y. Toward High Open-circuit Voltage by Smart Chain Engineering in 2D-conjugated Polymer for Polymer Solar Cells. Sol. Energy Mater. Sol. Cells 2016, 149, 162-169. (37) Wu, Y.; Cheng, X.; Xu, G.; Li, Y.; Cui, C.; Li, Y. Manipulating the Photovoltaic Properties of Small-Molecule Donor Materials by Tailoring End-capped Alkylthio Substitution. RSC Adv. 2016, 6, 108908-108916. (38) Cui, C.; Guo, X.; Min, J.; Guo, B.; Cheng, X.; Zhang, M.; Brabec, C. J.; Li, Y. High-Performance Organic Solar Cells Based on a Small Molecule with Alkylthio-Thienyl-Conjugated Side Chains without Extra Treatments. Adv. Mater. 2015, 27, 7469-7475. (39) Qian, D.; Ye, L.; Zhang, M.; Liang, Y.; Li, L.; Huang, Y.; Guo, X.; Zhang, S.; Tan, Z. a.; Hou, J. Design, Application, and Morphology Study of a New Photovoltaic Polymer with Strong Aggregation in Solution State. Macromolecules 2012, 45, 9611-9617.

18


Page 18 of 19

Page 19 of 19

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


Table of Contents

19


Achieving over 9.8% Efficiency in Nonfullerene Polymer Solar Cells by

Recommend Documents