Diblock Copolymer PF-b

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Diblock Copolymer PF-b-PDMAEMA as Effective Cathode Interfacial Material in Polymer Solar Cells Ligang Yuan, Jie Li, Zhao-Wei Wang, Peng Huang, Kai-cheng Zhang, Yanfeng Liu, Kai Zhu, Zhendong Li, Tiantian Cao, Bin Dong, Yi Zhou, Mi Zhou, Bo Song, and Yongfang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11648 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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ACS Applied Materials & Interfaces

Diblock Copolymer PF-b-PDMAEMA as Effective Cathode Interfacial Material in Polymer Solar Cells Ligang Yuan,1 Jie Li,2 Zhao-wei Wang,1 Peng Huang,1 Kai-cheng Zhang,1 Yanfeng Liu,1 Kai Zhu,1 Zhendong Li,1 Tiantian Cao,1 Bin Dong,1 Yi Zhou,1* Mi Zhou,2* Bo Song,1* Yongfang Li1,3 1

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical

Engineering and Materials Science, Soochow University, Suzhou, 215123, China 2

College of Materials Science and Engineering, Zhejiang University of Technology, Zhejiang

310014, China 3

CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key

Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China *Corresponding authors. E-mail: [email protected]; [email protected]; [email protected]

Abstract: An

alcohol-soluble

diblock

poly[2,7-(9,9-dihexylfluorene)]15-block-poly[2-(dimethylamino)ethyl

copolymer methacrylate]75

(denoted as PF15-b-PDMAEMA75) was employed as cathode interfacial layer (CIL) in p-i-n polymer solar cells (PSCs). PF15-b-PDMAEMA75 contains a conjugated rigid block and a non-conjugated flexible block grafted with polar amino groups, and can effectively lower the work function of Al cathode, and decrease series resistance of the devices. When being

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applied as CILs in PSCs based on PTB7-Th:PC71BM, the champion power conversion efficiency of 8.80% was achieved, which is slightly higher than that of the PSCs using the well-known PFN as CILs under our experimental conditions, and much better than that of PSCs using Ca as CILs. The improvement of the performance is mainly attributed to the enhanced open-circuit voltage and fill factor. According to the best of our knowledge, this is the first time using diblock copolymer as CILs in PSCs, and this study may provide a novel avenue for design and synthesis of interfacial materials for high-performance PSCs. Key words: alcohol soluble, diblock copolymer, p-i-n polymer solar cells, cathode interfacial layer, PF15-b-PDMAEMA75, PTB7-Th

1. INTRODUCTION Bulk heterojunction polymer solar cells (PSCs) have attracted considerable attention in the past decades because of their superior advantages such as light weight, mechanical flexibility and the potential for large-scale roll-to-roll fabrication through solution processing.1-3 Recently, PSCs have shown the bright commercial prospect with the increasing power conversion efficiency (PCE) over 13%.4-10 Lots of efforts have been made on the rational design of donor/acceptor materials and the morphologies optimization of active layer.10-14 Nevertheless, attention should also be paid on the anode / cathode interfacial layer (AIL / CIL), which play an equally essential role in enhancing the performance of PSCs,15-20 for the AILs or CILs can reduce the interfacial energy barriers and promote the charge extraction.21, 22 For p-i-n PSCs, Ca is often used as CIL owning to its low work function, however, Ca is


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rather sensitive to moisture and oxygen, and hence affect the stability of the resulting devices.23 On this purpose, inorganic candidates, such as metal fluorides (LiF,24 CsF25) and metal oxides (ZnO,26,27 TiOx28) were explored. These kinds of materials are natively sensitive to UV-light,29 and show a relatively low electron extraction ability because of their poor interfacial contact with active layers.24-28,

30, 31

Alternatively, alcohol-soluble conjugated

polymers (ASCPs), due to the orthogonal solubility with active layer and excellent optoelectronic properties, have attracted increasing attention.9, 32-39 In general, ASCPs are structurally composed of π-conjugated backbones and surfactant-like side-groups, such as amino, sulfonic or zwitterionic groups. These two components function differently in PSCs. The conjugated backbones contribute to the opto- and electro-properties in form of absorption, emission, energy levels and charge transport abilities. The polar side-groups promote the solubility in alcohol, among which the amino-terminated side-groups were most popular owing to their excellent interface modification ability.21, 40 The representative compound was poly[(9,9-bis(3´-(N,N-dimethylamino)

propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)],

known as PFN and reported by Cao et al. 1, 41,42 In

the

present

study,

diblock

copolymer

poly[2,7-(9,9-dihexylfluorene)]15-b-poly[2-(dimethylamino)ethyl methacrylate]75 (denoted as PF15-b-PDMAEMA75) was employed as a new type of CIL in PSCs based on PTB7-Th: PC71BM. It is hoped that the conjugated poly-fluorene backbone could contribute to a high conductivity, and the flexible poly-methacrylate with polar amino group could endow good solubility in alcohol and reduce work function of the cathode. The results indicate that the PSCs using PF15-b-PDMAEMA75 as CIL exhibited a champion power conversion efficiency



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(PCE) of 8.80% with open circuit voltage (Voc) of 0.78 V, short-circuit current (Jsc) of 16.72 mA cm-2 and fill factor (FF) of 66.3%. Notably, the PCE was slightly higher than that of the PSCs based on PFN (8.53%) and much higher than that of the PSCs based on Ca (7.53%). These results indicate that PF15-b-PDMAEMA75 can be a good candidate of CILs, and using block copolymer as CIL can be a novel strategy for exploring interfacial materials in PSCs.

2. RESULTS AND DISCUSSION The molecular structure of PF15-b-PDMAEMA75 is provided in Figure 1a. The synthesis and characterization was described somewhere else.43 The molecular weight and molar mass dispersity were 11.1 kDa and 1.12 characterized by GPC, which means a well-defined rod-coil

diblock

copolymer

has

been

obtained.

Bearing

the

relatively

longer

poly-methacrylate block, PF15-b-PDMAEMA75 shows good solubility in methanol (MeOH), orthogonal to chlorobenzene employed in processing active layers of the PSCs. The UV-vis absorption spectra of PF15-b-PDMAEMA75 block copolymer were shown in Figure S1 in supporting information (SI). In film state, PF15-b-PDMAEMA75 showed the absorption edge at 435 nm, and the absorption is similar to that of the polymer in MeOH solution. According to the absorption onset of the polymer film, the optical band gaps (Egopt) was estimated to be 2.85 eV. The highest occupied molecular orbital (HOMO) level of PF15-b-PDMAEMA75 was measured by electrochemical cyclic voltammetry (CV). As shown in Figure 1b, the onset oxidation potential (Vox) of PF15-b-PDMAEMA75 is 0.74 V versus Ag/Ag+. Its HOMO (EHOMO) energy level was determined to be -5.45 eV according to the empirical equation, EHOMO = -e Vox - 4.71 eV.44, 45 From the Egopt and EHOMO, its LUMO energy level (ELUMO) was calculated to be -2.60 eV. To make a good comparison, all the


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energy levels were presented in Figure 1c. It is clear that the deeper HOMO levels of PF15-b-PDMAEMA75 implied that it could block holes from PTB7-Th donor. 46, 47

(a)

(c) PF15-b-PDMAEMA75

-2.6 -2.6

0.2 0.0

-0.2 -0.4 -0.2

0.0

0.2

0.4

+

0.6

Potential (V vs. Ag/Ag )

0.0 -0.4

-3.6 -3.9 -4.0 -4.8 -5.0

-5.1 ITO PEDOT:PSS

0.0

0.4

0.8

1.2

-6.0

+

Potential (V vs. Ag/Ag )

PFN

0.1

Fc/Fc

Energy Level (eV)

0.2

Current (mA)

+

PF15-b-PDMAEMA75

-3.0

0.4

PC71BM

0.3

PTB7-Th

(b) 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


-4.3 Al

-5.2 -5.5 -5.9

-5.6

Figure 1. (a) Molecular structure of PF15-b-PDMAEMA75. (b) CV curve of PF15-b-PDMAEMA75 film on a glassy carbon electrode measured in a 0.1 mol L-1 of acetonitrile solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6) at a scan rate of 50 mV s-1. (c) The energy level diagram of the materials involved in the PSCs.

To study the effect of PF15-b-PDMAEMA75 as the CIL, the PSCs with configuration of ITO/PEDOT:PSS/PTB7-Th:PC71BM/CIL/Al were fabricated. It was reported that the post-treatment of the active layer with MeOH can passivate the surface traps and enhance the build-in voltage, leading to improved Voc and FF of the resulting devices.48, 49 To exclude the influence of solvent treatment, herein the devices with the active layer post-treated with MeOH were also fabricated under parallel conditions, and used as control to compare with the PSCs with different CILs. The active area of the devices was defined by a shadow mask



with an aperture of 7.57 mm2. The thickness of PF15-b-PDMAEMA75 was modulated by varying the concentration from 0.25 to 1.00 mg mL-1 and the corresponding thickness of PF15-b-PDMAEMA75 films were shown in Figure S2 in SI. Figure S3 in SI shows the current density-voltage (J-V) curves and external quantum efficiency (EQE) curves of the PSCs with different CIL thickness, and the effect of the CIL thickness on the devices performance was displayed in Table S1 in SI. The optimized thickness of the PF15-b-PDMAEMA75 CIL is 4 nm. As diblock copolymer, PF15-b-PDMAEMA75 is ready to form self-assemblies in alcohols. To exclude the influence of the assembly on the device performance, herein three alcohols, such as MeOH, ethanol and isopropanol, were applied as solvents in preparation of the CILs. As shown by the photovoltaic results (J-V curves and corresponding parameters are presented in supporting information as Figure S4 and Table S2), the PCEs and the detailed parameters did not show significant differences, suggesting that the type of alcohols has little effect on the performance of the resulting PSCs. Figure 2 shows the J-V curves and external quantum efficiency (EQE) curves of the optimized PSCs with different CILs and the detailed photovoltaic parameters of the devices were listed in Table 1. The PSCs without CIL attained a maximum PCE of 7.67%, while the PSCs post-treated with MeOH reached a PCE of 8.29% with Voc of 0.77 V, Jsc of 16.71 mA cm-2 and FF of 64.3%. These results agree well with that reported in the previous literatures.50, 51 After inserting a thin layer of PF15-b-PDMAEMA75 as CIL, the PSCs showed a maximum PCE of 8.80% (8.68 ± 0.08% in average) with Voc of 0.78 V, Jsc of 16.72 mA cm-2 and FF of 66.3%. In comparison, the PSCs using Ca and PFN as CILs achieved maximum PCE of 7.53% and 8.53%, both lower than that of the devices using PF15-b-PDMAEMA75 as CIL. These results indicate that the diblock copolymer


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PF15-b-PDMAEMA75 is a promising candidate of CIL. The EQE curves were here employed to confirm the Jscs and the integrated current density (Jint), and the detailed values are attached in Table 1. It is clear that the Jints were comparable with their corresponding Jsc, indicating the photovoltaic performance data obtained from J-V curves should be reliable. We also noticed that the enhancement of the device performance is mainly attributed to the increased Voc and FF. 0 -4 -2

-8

(b) W/O MeOH Ca PFN PF15-b-PDMAEMA75

80

60

EQE (%)

(a) J (mA cm )

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


-12

40

W/O MeOH Ca PFN PF15-b-PDMAEMA75

20 -16 0.0

0.2

0.4

0.6

0.8

0

300

400

V (V)

500

600

700

800

900

Wavelength (nm)

Figure 2. (a) J-V and (b) EQE curves of the devices based on PTB7-Th:PC71BM without and with different CILs. Table 1. Photovoltaic parameter data of the devices on PTB7-Th:PC71BM without or with different CILs. CILs W/O MeOH Ca PFN PF15-b-

Voc (V) 0.73

Jsc -2

(mA cm ) 16.95

FF (%)

PCE (%)

61.8

7.67

(0.72 ± 0.01) (16.83 ± 0.25) (60.1 ± 0.9) 0.77

16.45

65.6

(0.77 ± 0.00) (16.71 ± 0.10) (64.3 ± 0.1) 0.75

14.98

67.7

(0.75 ± 0.00) (14.96 ± 0.02) (67.2 ± 0.5) 0.78

16.47

67.4

(0.78 ± 0.00) (16.49 ± 0.03) (66.7 ± 0.9) 0.79

16.80

66.7

PDMAEMA75 (0.78 ± 0.01) (16.72 ± 0.09) (66.3 ± 0.5)

Jint

Rs -2

Rsh 2

(mA cm )

(Ω cm )

(Ω cm2)

16.90

7.1

630

16.72

5.1

834

14.72

4.8

917

16.43

4.1

869

16.50

3.6

833

(7.27 ± 0.24) 8.29 (8.25 ± 0.06) 7.60 (7.53 ± 0.10) 8.63 (8.53 ± 0.14) 8.80 (8.68 ± 0.08)

To make an insight investigation on the reasons of the improvement of the performance of the PSCs when using PF15-b-PDMAEMA75 as CIL. The surface morphologies of the 7 ACS Paragon Plus Environment


PF15-b-PDMAEMA75 on PTB7-Th:PC71BM were measured by atom force microscopy (AFM), and the work function (WF) of Al electrodes before and after modification of PF15-b-PDMAEMA75 were determined by peak force kelvin probe force microscopy (KPFM), and the corresponding results were presented in Figure S5 and Table S3 in SI. Either the MeOH-treatment or modification of PF15-b-PDMAEMA75 did show significant influence on the surface morphology, as well as the roughness reflected by root-mean-square (RMS) value. Nevertheless, the modification of PF15-b-PDMAEMA75 on Al led to a decrease of WF from 3.92 to 3.49 eV, which is in accordance with the relatively high Voc of the devices when using PF15-b-PDMAEMA75 as CIL. The reduced WF of the Al electrode would increase build-in voltage, thus leading to a higher Voc of the PSCs.40, 52 The reduced WF of Al electrode can be ascribed to the formation of interfacial dipoles due to the amino groups of PF15-b-PDMAEMA75, which led to an improved FF by boosting the charge extraction and transportation.53-55 To investigate whether introducing PF15-b-PDMAEMA75 CIL can improve the carrier transportation, the J-V characteristics of devices were performed in the dark. As shown in Figure 3, the reverse current of the PSCs with PF15-b-PDMAEMA75 CIL in the region from -1 to 0 V was greatly suppressed compared with that of the PSCs without CIL (either post-treated with MeOH or not), suggesting that the diblock copolymer CIL can indeed improve the electron-transporting and hole-blocking properties. These results also imply that the charge recombination at the interface of Al electrode and active layer is decreased, which makes a reasonable explanation to the relatively higher FF of the PSCs with PF15-b-PDMAEMA75 as CIL.19, 56 At the forward current in the region from 0 to 1.5 V, a


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slightly higher injection current was observed for the PSC with PF15-b-PDMAEMA75 as CIL, indicating more electron were injected from the cathode side. This result was in accordance with the series resistance (Rs) and shunt resistance (Rsh) calculated from the slope of J-V curves at Voc and Jsc, respectively. It is known that Rs and Rsh can greatly affect the FF,55 and decrease of Rs and/or increase of Rsh is directing to high FFs.57 As shown in Table 1, all the CILs can result in decrease of Rs and increase of Rsh comparing with the PSCs without CIL. In particular, the PSC with PF15-b-PDMAEMA75 as CIL showed the lowest Rs with value of 3.6 Ω cm2, indicating that an ideal ohmic contact was formed at the interfaces between the active layer and Al cathode, which is beneficial to the charge extraction and transportation. In addition, the decreased Rs and the relatively high Voc are also because of the reduced potential drop across the devices.58 W/O MeOH PF15-b-PDMAEMA75

2

10

-2

J (mA cm )

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

10

-2

10

-4

10

-6

10

-0.8

-0.4

0.0

0.4

0.8

1.2

V (V)

Figure 3. J-V characteristics of the PSCs using PF15-b-PDMAEMA75 as CIL measured in the dark, and the PSCs without CIL and the active layer post-treated with MeOH were applied as control. The data were plotted on a semilog scale.

In the following, the charge transportation properties of PF15-b-PDMAEMA75 were studied using the space charge limited current (SCLC) method.20, 51, 59 The electron- and hole-only

devices

with

configuration

of

ITO/PEI/PTB7-Th:PC71BM/CIL/Al


and


ITO/PEDOT:PSS/PTB7-Th:PC71BM/CIL/MoO3/Al were fabricated, respectively. The energy level diagrams of electron- and hole-only devices were shown in Figure S6 in SI. The carrier mobilities were calculated from the J-V curves measured in the dark, and the curves were fitted with the Mott-Gurney law,60 i.e. J=9εrε0µV2/8L3. Here, εr is the dielectric constant of the active layer, ε0 the permittivity of free space (ε0 = 8.85 × 10-12 F m-1), µ the mobility, L the thickness of the photoactive layer (~ 100 nm), and V the effective voltage. The J1/2–V curves were shown in Figure 4 and the corresponding carrier mobilities of devices were summarized in Table 2. The devices without CIL and not post-treated with MeOH exhibited the lowest µe and µh with values of 1.38 × 10-4 and 2.35 × 10-4 cm2 V-1 s-1, respectively. The µe and µh of the PSCs post-treated with MeOH raised to 2.67 × 10-4 and 4.45 × 10-4 cm2 V-1 s-1, respectively. Upon addition of PF15-b-PDMAEMA75 interlayer, µe and µh increased to 6.16 × 10-4 and 7.05 × 10-4 cm2 V-1 s-1, respectively. The charge mobility increase can also be verified by electrochemical impendence spectroscopy (EIS) results, which indicate the decreased interfacial resistance upon addition of the diblock copolymer CIL. The enhanced µe and µh of the PSCs with PF15-b-PDMAEMA75 interlayer might be attributed to the amine groups contained in PF15-b-PDMAEMA75.7 In addition, the inter-mixing of PF15-b-PDMAEMA75 with PC71BM at the interface may also contribute the charge mobility.54,61-63 The µe and µh became higher and balanced, which makes another good explanation for the relatively high FF of the corresponding PSCs.


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(a) 40

(b)

W/O MeOH PF15-b-PDMAEMA75


60

J (A /m)

30

40

1/2

1/2

J (A /m)

20

1/2

1/2

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


10

1

2

20

3

0

1

Vappl-Vbi (V)

Vappl-Vbi (V)

2

3

Figure 4. J1/2-V characteristics of the (a) electron- and (b) hole-only devices. Table 2. Electron and hole mobility of the devices without (including that active layer post-treated with MeOH) and with PF15-b-PDMAEMA75 as CIL. µe (10-4 cm2 V-1 s-1)

µh (10-4 cm2 V-1 s-1)

W/O

1.38 ± 0.16

2.35 ± 0.21

MeOH

2.67 ± 0.18

4.45 ± 0.32

PF15-b-PDMAEMA75

6.16 ± 0.44

7.05 ± 0.52

EIS was employed to assess the contribution of CILs on the charge transportation in PSCs. The results are presented in Figure 5 (the original data with extended scanning range is shown in Figure S7 in SI), where the inset is the equivalent circuit used to fit the curves. The equivalent circuit consists of a contact resistance (Rc) in series, and two parallel resistance-constant phase elements (R1 // C1 and R2 // C2). Rc can be assigned to the contribution of the series resistance of the electrodes and the external circuit. R1 // C1 and R2 // C2 can be regarded as the contribution of the active layer and interfacial layers, respectively.20, 64 As shown in Table 3, the post-treatment of active layer with MeOH resulted in a decrease of R1 from 39.8 to 26.9 Ω, which can be attributed to the vertical distribution of active layer caused by MeOH.49 Noteworthily, the addition of PF15-b-PDMAEMA75 did not 11 ACS Paragon Plus Environment


make notable difference on R1, since MeOH was used as solvent during the film preparation. Clearly, upon insertion of PF15-b-PDMAEMA75 layer, R2 decreased from 37.2 to 19.3 Ω. The significant decrease of R2 indicates that a facilitated charge transportation have been achieved, leading to the enhanced FF in PSCs. W/O W/O (fitted) MeOH MeOH (fitted) PF15-b-PDMAEMA75

80 60

-Z'' (Ω )

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

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Rc

R1

R2

C1

C2

PF15-b-PDMAEMA75 (fitted)

40 20 0

20

40

60

80

100

120

Z' (Ω)

Figure 5. Nyquist plots of devices without (including that active layer post-treated with MeOH) and with PF15-b-PDMAEMA75 as CIL measured in the dark. (Inset: the equivalent circuit used in this work) Table 3. Parameters fitted from the Nyquist for PSCs with/without PF15-b-PDMAEMA75 CILs. Rc (Ω)

R1 (Ω)

C1 (nF)

R2 (Ω)

C2 (nF)

W/O

16.6

39.8

1.7

37.2

1.8

MeOH

15.9

26.9

1.9

36.4

4.6

PF15-b-PDMAEMA75

16.6

26.1

5.8

19.3

2.4

At last, the contribution of PF15-b-PDMAEMA75 as CIL to the stability of the PSCs was investigated. In doing so, the devices without encapsulation were stored in a glovebox filled with N2. As shown in Figure 6a, the average PCE of the devices based on PF15-b-PDMAEMA75 as CIL remained more than 90% of the initial average PCE in 50 days, much stable compared with the two control devices whose PCE dropped to less than 80%. And we have measured the actual lifetime results under one sun conditions. All the PCEs decay rapidly (Figure 6b), much faster than those of the storage stability. We speculate that


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photoactive layer should be decomposed either by the strong light illumination or the heat generated in the cells. The bright side of this experiment is that the PCEs with the diblock copolymer CIL are still better than those without all through the detection. The stability improvement could be attributed the inserted PF15-b-PDMAEMA75 layer, which should in certain extent shield the photoactive layers from oxygen and moisture, both fatally toxic to PSCs.65 These results suggest that the PF15-b-PDMAEMA75 can not only improve the device efficiency, but also enhance the stability.

(b) 1.0

1.0

Normalized PCE

(a) Normalized PCE

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


0.9

0.8 W/O MeOH PF15-b-PDMAEMA75

0.7

0.6

0

10

20

30

40

50

60


0.8 0.6 0.4 0.2 0.0

0

Time (day)

20

40

60

80

Time (hour)

Figure 6. The normalized PCEs of PSCs without (including that active layer post-treated with MeOH) and with PF15-b-PDMAEMA75 as CIL (a) versus stored time and (b) versus stored time under one sun illumination. All the devices were not capsuled and stored in glovebox filled with N2.

3. CONCLUSIONS In summary, an alcohol-soluble diblock copolymer PF15-b-PDMAEMA75 was employed as CIL in PSCs for the first time. As CIL, PF15-b-PDMAEMA75 showed better photovoltaic performance in comparison with the well-known PFN and commonly-used Ca. Upon addition of PF15-b-PDMAEMA75 as CIL, the PSCs based on PTB7-Th:PC71BM reached a higher PCE of 8.80%, which is mainly attributed to the significant enhancement of Voc and FF. The 13 ACS Paragon Plus Environment


KPFM, J-V curves in the dark, SCLC and EIS results indicate that the lowered WF of Al electrode and decreased interfacial resistance should be responsible for the enhancement of the device performance. In addition, the diblock copolymer PF15-b-PDMAEMA75 also helps to improve the stability of the resulting devices. 4. EXPERIMENTAL SECTION Materials: The ITO glass (≤ 15 Ω/square) was purchased from CSG Holding Co., Ltd. PEDOT:PSS (Clevios PVP AI 4083) was acquired from Nichem Co. PTB7-Th and PC71BM were purchased from 1-Material Co., Ltd. and Solarmer Materials Inc., respectively. 1, 8-diiodoctane (DIO, ≥ 95.0%) was purchased from TCI Chemicals. MeOH (anhydrous, 99.9%) and chlorobenzene (CB, anhydrous, 99.8%) were purchased from J&K Technology Co., Ltd. Fabrication of PSCs: The ITO/glass substrates were cleaned sequentially by sonication with detergent, deionized water, acetone, ethanol and isopropanol, twice for each solvent and 15 min for each time. Then the substrates were dried in N2 flow, followed by ultraviolet-zone treatment for 20 min. PEDOT: PSS was spin-coated on the ITO substrates, and dried at 150 °C for 15 min in air to afford a ∼ 35 nm thickness layer. Then the substrates were transferred into a glovebox filled with N2. A blend solution was prepared by dissolving PTB7-Th and PC71BM by a 1:1.5 weight ratio in CB with DIO as the additive (CB: DIO = 97:3, v/v). An approximate 100 nm of PTB7-Th: PC71BM blend film was spin-coated on top of the PEDOT: PSS layer. Then, the solutions of PF15-b-PDMAEMA75 in MeOH with the varied concentration (0.25 - 1.00 mg mL-1) were spin-coated onto the active layer at the speed of 3000 rpm for 60 s. For comparison, PFN and Ca were also used as CILs of the PSCs. The


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MeOH solution of PFN (0.02 mg mL-1) containing 2 vol% of acetic acid was spin-coated on the active layer to obtain a 5 nm film. For Ca CIL, ∼ 20 nm Ca was evaporated in high-vacuum chamber (pressure < 2.5 × 10-4 Pa). Finally, Al cathodes (∼ 80 nm) were thermally deposited in vacuum with a shadow mask to define the active area. Measurement and characterization: The CV measurement was performed on a Zahner Ennium IM6 Electrochemical Workstation (Zahner Zennium, Germany) with glassy carbon disk, platinum wire, and Ag/Ag+ electrode as working electrode, counter electrode, and reference electrode, respectively. The CV tests were conducted in a 0.1 M acetonitrile solution of Bu4NPF6. During the measurement, ferrocene/ferrocenium (Fc/Fc+) was used as an internal standard, which was assigned an absolute energy of 4.71 eV vs. vacuum level. The J-V curves of all devices were tested under a 1 sun, AM1.5 G, 100 mW cm-2, using a SS-F5-3A (Enli Technology CO., Ltd.) solar simulator (AAA grade) with an aperture of area of 0.0757 cm2. Silicon standard cell (SRC-00019) was used to calibrate the light intensity. The J-V characteristics of the PSCs were recorded with a Keithley 2450 source meter. The EQE spectra were measured by using a Solar Cell Spectral Response Measurement System QE-R3011 (EnliTechnology CO., Ltd.). The film thickness was determined by using a Profilometer (KLA Tencor D-100). AFM (Bruker, Santa Barbara, CA) images were gained with peak force quantitative nano-mechanical mode on Nanoscope VIII. The WFs of thin films were measured in air by peak force kelvin probe force microscopy (KPFM). EIS was performed on an IM6 electrochemical workstation (Zahner Zennium, Germany) in a frequency range from 0.1 Hz to 4 ×106 Hz with perturbation amplitude of 10 mV in the dark, the applied voltages were the respective Voc.



ASSOCIATED CONTENT Supporting information UV-vis spectra, concentration dependent thickness of PF15-b-PDMAEMA75; J-V, EQE and dark J-V curves and corresponding device parameters of the PSCs with various thicknesses of CILs; effect of alcohols on the device performance; AFM images of PTB7-Th:PC71BM layer and PF15-b-PDMAEMA75@PTB7-Th:PC71BM; comparison of WFs of Al modified with PF15-b-PDMAEMA75 and PFN; Energy level diagrams of electron- and hole-only devices; the origin data of EIS. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51673139, 91333204), A Priority Academic Program Development of Jiangsu Higher Education Institutions, State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials. REFERENCES (1) He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y., Single-junction Polymer Solar Cells with High Efficiency and Photovoltage. Nat. Photon. 2015, 9, 174-179. (2) Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J., A Roll-to-Roll Process to Flexible Polymer Solar Cells: Model Studies, Manufacture and Operational Stability Studies. J. Mater. Chem. 2009, 19, 5442. (3) Krebs, F. C., Fabrication and Processing of Polymer Solar Cells: A Review of Printing and Coating Techniques. Sol. Energy Mater. Sol. Cells 2009, 93, 394-412. (4) Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganas, O.; Gao, F.; Hou, J., Fullerene-free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734-4739. 16 ACS Paragon Plus Environment

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Table of contents C6H13

C6H13

O CH3 CH2OC C CH3

CH3

H2 C

C O

Br 75

O H2C CH2 N H3C CH3

0 -4 -2

15

J (mA cm )

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 PFN PF15-b-PDMAEMA75

-8 -12 -16 0.0

0.2

0.4

V (V)


0.6

0.8

Diblock Copolymer PF-b

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