Novel Nonconjugated Polymer as Cathode Buffer Layer for Efficient

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A Novel Nonconjugated Polymer as Cathode Buffer Layer for Efficient Organic Solar Cells Yunhao Cai, Li Chang, Longzhen You, Bingbing Fan, Hongliang Liu, and Yanming Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07691 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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ACS Applied Materials & Interfaces

A Novel Nonconjugated Polymer as Cathode Buffer Layer for Efficient Organic Solar Cells Yunhao Cai,† Li Chang,§ Longzhen You,† BingBing Fan,† Hongliang Liu,*,‡ and Yanming Sun*,†

†

School of Chemistry, Beihang University, Beijing 100191, P. R. China.

‡

Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and

Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China §

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous

Metal Chemistry and Resources Utilization of Gansu Province and Department of Chemistry, Lanzhou University, Lanzhou 730000, P. R. China. KEYWORDS: Organic solar cells, cathode buffer layer, nonconjugated polymer, efficiency, stability.

ABSTRACT: A novel nonconjugated polymer named PAMPS-Na was designed and synthesized. The PAMPS-Na has good solubility in polar solvents, such as water, methanol, and ethanol, which can be used as the cathode buffer layer in organic solar cells through solution processing without damaging the underlying active layer. Moreover, it was found that PAMPS-Na can significantly decrease the Al work function when it was modified with Al. To reveal its universal application in organic photovoltaic devices, a variety of photovoltaic donor materials, including two medium-bandgap polymers, a wide-bandgap polymer, and a small molecule donor were employed to fabricate organic solar cells (OSCs). Compared with OSCs with Ca/Al electrode, the devices based on PAMPS-Na/Al exhibited higher photovoltaic 1

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performance, mainly due to the increased short-circuit current. Additionally, OSCs with PAMPS-Na/Al displayed better ambient stability than devices with Ca/Al. It is also interesting to find that the performance of the devices can tolerate a wide change of PAMPS-Na’s thickness, enabling the potential for large-scale fabrication of OSCs. The results suggest that PAMPS-Na is a promising candidate as the cathode buffer layer to improve the efficiency and stability of organic solar cells.

INTRODUCTION

The key advantages of bulk-heterojunction (BHJ) organic solar cells (OSCs) including low-cost, light weight, flexibility and ease of fabrication have distinguished this technology as promising alternative to silicon-based solar cells.1-4 Impressive advances in power conversion efficiencies (PCEs) owing to combined improvements in design of novel photovoltaic materials, use of innovative device structure, engineering of interfacial layer, and optimization of BHJ morphology have been recently achieved.5-13 In a standard BHJ solar cell, the active layer is typically sandwiched between a transparent tin-dopedindium oxide (ITO) and a metal electrode.14 In pursuit of high-performance OSCs, one of the main challenges is to control the interface between the active layer and the electrodes.15-18 The contact barrier at the interfaces must be efficiently reduced to form a good ohmic contact.19 The broad selection of interface materials can not only establish a tunable energy level alignment between the contact interface to favor the charge extraction and transportation but also increase interfacial stability of OSCs eventually.16, 20-21 2


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In

conventional

devices,

poly

(3,4-ethylenedioxythiophene):

poly(styrenesulfonate) (PEDOT:PSS) is commonly used as the anode buffer layer to increase the work function of anode (usually ITO).22-23Low work function (WF) active metals such as Ba and Ca have been used as the cathode at the expense of sensitivity toward environmental moisture and oxygen. Accordingly, high WF metals including Ag, Al, or Au are adopted to improve the stability.24 On the other hand, insertion of solution-processable and stable cathode buffer layer has been identified to be another effective strategy to realize high performance and stable OSCs.17, 25-28 A number of n-type metal oxide semiconductors, such as zinc oxide (ZnO),29-30 titanium dioxide (TiO2),31-32metal salts including Lithium fluoride(LiF),33-35 Caesium fluoride (CsF),36 Cesium carbonate (Cs2CO3)37-38, etc. have been used as electron transport layer. Unfortunately, these inorganic cathode buffer layers are not well compatible with the organic active layer, resulting in inferior electron extraction ability. Furthermore, most of them usually susceptible to degradation by surface adsorption of oxygen and UV-irradiation.16 In

recent

years,

various

conjugated

polymers

such

as

poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluore ne)] (PFN)39-40, ethoxylated polyethylenimine (PEIE). etc.41 have emerged as promising interfacial layer materials in organic solar cells.

16,42,43-45

However, the

acquisitions of some of these conjugated polymers require complicated synthesis routes along with time-consuming purification procedure.46-47 Therefore, it is highly desirable to develop new buffer layer with low processing cost and simple synthesis 3



route, enabling the potential commercialization of organic solar cells. In this contribution, we reported high-performance OSCs via employing poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt) (PAMPS-Na) as the cathode buffer layer. This nonconjugated polymer has good solubility in polar solvents such as water, methanol and alcohol. More importantly, PAMPS-Na can be easily synthesized with a high yield via simple radical polymerization. In order to certify the universality of PAMPS-Na on the photovoltaic performances of OSCs, bulk-heterojunction devices adopting two medium-bandgap materials PTB7, PTB7-Th, a wide-bandgap polymer PBDT-ST1, a small molecule p-DTS(FBTTh2)2 as electron donors and PC71BM as electron acceptor were fabricated. The sodium salt solution was simply prepared by spin-coating its methanol solution onto the active layer at room temperature, without any thermal annealing or any other post-treatment. It was interesting to find that the new cathode buffer layer can effectively improve the Jsc and further the PCE of corresponding devices. The PCE of PTB7:PC71BM-based device with a PAMPS-Na buffer layer reached 8.24%, which is 13% increased in comparison with traditional Ca/Al electrode. For OSC using PTB7-Th as electron donor, an improved PCE of 9.16% was achieved after insertion of the new cathode buffer layer, while the control device delivers a PCE of 8.32%. When adopting wide-bandgap D-A type polymer PBDT-ST1 and small molecule p-DTS(FBTTh2)2 as donor materials, an optimum PCE of 9.09% and 7.94% was obtained respectively by PAMPA-Na based devices, which are greatly improved (13% and 8% enhancement) in comparison with those (8.05% and 7.30%) of the control devices. It should be 4


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noted that, the performance of the devices using PAMPS-Na is relatively insensitive to the thickness of the cathode buffer layer. In addition, the device with PAMPS-Na as cathode buffer layer was proved to possess better stability in ambient in relative to device without it. Our work has successfully manifested that PAMPS-Na is a high-efficient and cost-effective cathode buffer layer for use in organic solar cells.

EXPERIMENTAL SECTION Device fabrication ： OSCs were fabricated with a conventional architecture of ITO/PEDOT:PSS/active layer/PAMPS-Na or Ca/Al. ITO-coated glasses were sequential ultrasonic cleaned for 15 min in soapy water, deionized water, acetone and isopropyl alcohol. ITO glasses were dried overnight in a vacuum oven and treated with UV ozone for 30 min before use. A 40 nm thick PEDOT:PSS (Heraeus Clevios P VP A 4083) layer was spin-cast on top of the ITO substrates, followed by annealed at 150 °C for 10 min in air. The active layers were deposited by spin coating the dichlorobenzene solution of PTB7:PC71BM (1:1.5 w/w, PTB7 is 10 mg mL−1), the chlorobenzene solution of PTB7-Th:PC71BM (1:1.5 w/w, PTB7-Th is 10 mg mL−1), the chloroform solution of PBDT-ST1:PC71BM (1:1 w/w, PDBT-T1 is 7 mg mL−1) and

the chlorobenzene

solution

of

p-DTS(FBTTh2)2:PC71BM (1.4:1

w/w,

p-DTS(FBTTh2)2 is 20 mg mL−1) on the top of PEDOT:PSS for 40 s, respectively. Subsequently, a thin layer of PAMPS-Na (1.5 mg mL−1 in methanol) was spin coated for 40 s on the top of the active layer and functioned as cathode buffer layer, without any additional thermal annealing or post-treatment. The PAMPS-Na methanol

5



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solution was simply made by adding the PAMPS-Na powder into the methanol solvent and then stirred for 2 hours. Finally, the substrates were transferred to a vacuum chamber and the metal cathode (Al or Ca/Al) was thermally deposited at a vacuum level of 5 × 10-5 Pa. The photoactive area of the devices is 4.8 mm2, calibrating by a shadow mask. Characterization and measurements：In UV-vis absorption measurement, a Hitachi (model U-3010) UV-vis spectrophotometer was used. Cyclic voltammetric (CV) measurement was performed in a conventional three-electrode cell using a Pt plate as the working electrode, Pt wire as the counter electrode, and Ag/Ag+ electrode as the reference electrode on a Zahner IM6e Electrochemical Workstation in a tetrabutylammonium hexafl uorophosphate (Bu4NPF6)

(0.1 M ) acetonitrile

solution at a scan rate of 20 mV s−1. Tapping mode AFM images of the films were acquired from Digital Instruments Dimenion V SPM System with NanoScope V controller. The current density-voltage (J-V) characteristics of the devices were measured with a Keithley 2400 source measurement unit under the illumination of AM 1.5 G, 100 mW cm-2 with a Enlitech solar simulator (Taiwan, China). The light intensity was calibrated with a standard silicon solar cell. The external quantum efficiencies (EQE) of solar cells were analyzed using a solar cell quantum efficiency measurement system PV measurement QEX10. The layer thickness was measured using an Ambios Technology XP-2 surface profilometer.

RESULTS AND DISCUSSIONS

6


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PAMPS-Na is synthesized by free radical polymerization (Scheme S1). The polymerization was performed in distilled water with AMPS as the monomer and APS as the initiator with monomer and initiator weight ratio of 100:1. The AMPS was first converted into the sodium form as obtained by previous titration of AMPS (5 g, 24.2 mmol) in distilled water by dropwise addition of NaOH solution until adjusting the pH at 10. The reaction mixture was purged with the nitrogen gas for 30 min and polymerization was carried out under stirring at 60 oC for 12 h. The PAMPS-Na powder was obtained by subsequent vacuum freeze-drying process. As

displayed

in

Figure

S1,

the

PTB7-Th:PC71BM

and

PTB7-Th:PC71BM/PAMPS-Na films show nearly identical UV–vis absorption spectra, which demonstrates that the newly developed cathode buffer layer has no impact on the absorption of the active layer. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of PAMPS-Na were determined by cyclic voltammetry (CV), the HOMO and LUMO of PAMPS-Na were calculated to be -6.51 eV and -3.24 eV, respectively (Figure S2). Ultraviolet photoelectron spectroscopy (UPS) was performed to clarify the work function change of PAMPS-Na modified Al electrode. As shown in Figure 1c, the bare Al exhibited a typical work function of -4.30 eV, in accordance with the value reported in the literature.48 Evidently, upon a thin layer of PAMPS-Na deposited on the Al substrate, the work function of Al was shifted to -3.51 eV, which is thought to be favorable of electron collection.

7



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Figure 1. (a) Chemical structures of the donor materials and PAMPS-Na. (b) Energy level diagram of each component material used in the fabrication of OSCs. (c) UPS structure of Al and Al/PAMPS-Na. The effects of PAMPS-Na on the photovoltaic performance of OSCs were investigated

with

a

conventional

structure

of

ITO/PEDOT:PSS/Active

layer/PAMPS-Na/Al, where PTB7, PTB7-Th, PBDT-ST1 and p-DTS(FBTTh2)2 were used as the donors and PC71BM was used as the acceptor. Figure 2 presents current density-voltage (J-V) curves of the devices with different cathode under the illumination of AM 1.5 G, 100 mW/cm2. The resulted Jsc, Voc, FF, and PCE values, as determined from the J-V curves are summarized in Table 1. OSCs based on PTB7 with traditional Ca/Al cathode showed a PCE of 7.29%, with a Jsc of 14.37 mW/cm2, a Voc of 0.79 V, a FF of 65%, the introduction of the PAMPS-Na layer led to obvious increasement in Jsc to 15.80 mW/cm2, and PCE to 8.24% with a Voc of 0.79 V, a FF of 66%. In the case of PTB7-Th-based device, a PCE of 8.32% with a Jsc of 14.83 8


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mW/cm2, a Voc of 0.80 V and a FF of 70% was obtained using Ca/Al cathode. Device with a thin PAMPS-Na cathode interlayer showed a higher PCE of 9.16%, with an enhanced Jsc of 15.99 mW/cm2 and a Voc of 0.80 V and a FF of 71%. Similarly, PAMPS-Na displayed remarkable performance in the OSCs based on wide-bandgap D-A copolymer PBDT-ST1. The control device showed a PCE of 8.05%, a Voc of 0.89 V and a FF of 70%. With PAMPS-Na as the electron transport layer, same Voc of 0.89 V was obtained, while both the Jsc and FF were improved to 13.80 mW/cm2 and 74%. Finally, a high PCE of 9.09 was achieved. Furthermore, when using a small molecule p-DTS(FBTTh2)2 as the electron donor and PC71BM as the acceptor, the device without PAMPS-Na cathode buffer layer exhibited a PCE of 7.30%, with a Jsc of 13.62 mW/cm2, a Voc of 0.80 V, a FF of 67%. In relative to the control device, an improved Jsc of 14.60 mW/cm2, a FF of 68% and a Voc of 0.80 V was produced, leading to a PCE of 7.94%. Apparently, in comparing with the PAMPS-Na-based devices with the Ca/Al based OSCs, the higher PCEs were mainly ascribed to the increased Jsc and FF, which partially resulted from the high and more balanced charge carrier mobility in PAMPS-Na/Al-based devices. The external quantum efficiencies (EQEs) of these OSCs with Ca/Al or PAMPS-Na/Al cathode are presented in Figure 2e-h. Similar curves cover a wide spectral range but higher EQEs of devices with a thin PAMPS-Na were observed. OSCs based on PTB7:PC71BM displayed highest EQE of 64% at 560 nm using traditional Ca/Al cathode, while the device with PAMPS-Na showed improved light response in the range of 550-800 nm, showing a maximum EQE of 69%. For the 9



PTB7-Th: PC71BM-based and PBDT-ST1:PC71BM-based devices, the shape of the EQE curves of the device with PAMPS-Na were almost same with those of the control devices across the entire wavelength range. However, higher light responses between 340-720 nm, and 360-650 nm along with highest EQE values of 71% and 77% were achieved for PTB7-Th and PBDT-ST1-based devices with a thin PAMPS-Na layer. Besides, in comparison with the devices with Ca/Al cathode, the EQE spectra of OSCs based on p-DTS(FBTTh2)2:PC71BM with a PAMPS-Na exhibits a notable enhancement in wavelength range from 345-430 nm, and 490-600 nm. These results were consistent well with the current values from the J−V measurements and the

-12 -16

0

-8 -12 -16

PAMPS-Na/Al Ca/Al

-8

0.4 Voltage (V)

0.6

PAMPS-Na/Al Ca/Al

0.0

0.8

PTB7

(f)

70

0.2

0.4 0.6 Voltage (V)

-12

PAMPS-Na/Al Ca/Al

0.0

0.8

PTB7-Th

(g)

50

IPCE (%)

IPCE(%)

30

40 30

0

p-DTS(FBTTh2)2:PC71BM PAMPS-Na/Al Ca/Al

-4

-8

-12

20

10

10

0 300

PBDT-ST1

0.0

(h)

80 70

400

500 600 700 Wavelength (nm)

800

900

0 300

0.2

0.4 Voltage (V)

Ca/Al PAMPS-Na/Al

0.6

0.8

p-DTS(FBTTh2)2

60

30

10 800

1.0

40

20

500 600 700 Wavelength (nm)

PAMPS-Na/Al Ca/Al

0.8

50

20

400

0.4 0.6 Voltage (V)

60

50

40

80 70

60

0.2

IPCE (%)

0.2

60

0 300

(d)

PBDT-ST1:PC71BM

-4

2

Ca/Al PAMPS-Na/Al

-4

-16

0.0

70

(c)

PTB7-Th:PC71BM

-2

-8

(e)

0

2

Ca/Al PAMPS-Na/Al

-4

Current density(mA/cm )

(b)

PTB7:PC71BM

Current density (mA cm )

0

-2

Current density (mA cm )

(a)

Current density(mA/cm )

higher Jsc achieved in devices inserting a PAMPS-Na layer than the control devices.

IPCE(%)

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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50 40 30 20 10

400

500 600 Wavelength (nm)

700

800

0 300

400

500

600

700

800

Wavelength (nm)

Figure 2. (a-d) J-V curves of PTB7/PTB7-Th/PBDT-ST1/ p-DTS(FBTTh2)2:PC71BM solar cells with and without PAMPS-Na layer and (e-h) the corresponding EQE curves of these organic solar cells.

10


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Table 1. Device parameters of the OSCs with and without PAMPS-Na. Active layer

Electrode

Voc (V)

Jsc (mA cm-2)

FF

PCE (%)a)

PTB7:PC71BM

Ca/Al

0.79±0.003

14.25±0.12

0.63±0.02

7.29 (7.09±0.13)

PAMPS-Na/Al

0.79±0.007

15.64±0.16

0.64±0.02

8.24 (7.93±0.16)

Ca/Al

0.80±0.004

14.57±0.26

0.69±0.01

8.32 (8.04±0.18)

PAMPS-Na/Al

0.80±0.006

15.76±0.23

0.70±0.01

9.16 (8.82±0.10)

PBDT-ST1:

Ca/Al

0.89±0.006

12.72±0.20

0.68±0.02

8.05 (7.70±0.18)

PC71BM

PAMPS-Na/Al

0.89±0.005

13.62±0.18

0.73±0.01

9.09 (8.85±0.11)

p-DTS(FBTTh2)2:

Ca/Al

0.80±0.003

13.41±0.21

0.65±0.02

7.30 (6.99±0.16)

PAMPS-Na/Al

0.80±0.004

14.45±0.15

0.67±0.01

7.94 (7.75±0.12)

PTB7-Th:PC71BM

PC71BM

a) The average PCE values were both obtained from 10 devices

On account of investigating the charge transport properties, we fabricated hole-only and electron-only devices with configuration of ITO/MoOx/donor:PC71BM (with or without PAMPS-Na)/MoOx/Al and ITO/Al/donor:PC71BM (with or without PAMPS-Na)/Ca/Al respectively. As plotted in Figure S3, S4, devices with traditional Ca/Al cathode exhibited relatively low hole mobilities of 8.24×10-4, 1.09×10-3, 6.51×10-4, 7.59×10-4 cm2 V−1 s−1 and electron mobilities of 1.63×10-3, 1.46×10-3, 2.39×10-3 and 1.90×10-3 cm2 V−1 s−1 for PTB7:PC71BM, PTB7-Th:PC71BM, PBDT-ST1:PC71BM and p-DTS(FBTTh2)2:PC71BM blends, respectively. After incorporation of PAMPS-Na as the cathode buffer layer, all the electron mobilities were remarkably enhanced. The electron mobilities of these devices were increased to 2.19×10-3, 2.50×10-3, 3.06×10-3, 2.49×10-3 cm2 V−1 s−1, respectively. Besides, improved hole mobilities of 1.23×10-3, 1.58×10-3, 1.63×10-3 and 1.16×10-3 cm2 V−1 s−1 were also obtained in devices with a PAMPS-Na thin layer. The higher carrier mobilities and more balanced carrier transport in the active layers help explain the 11



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better Jsc and PCEs achieved in OSCs using PAMPS-Na as a cathode buffer layer. Detail data were concluded in Table 2.

Table 2. Hole mobilities and electron mobilities of the devices with Ca/Al and PAMPS-Na/Al electrode. Active layer

Electrode

Hole mobility (cm2 V−1 s−1)

Electron mobility (cm2 V−1 s−1)

PTB7:PC71BM

Ca/Al PAMPS-Na/Al Ca/Al PAMPS-Na/Al Ca/Al PAMPS-Na/Al Ca/Al PAMPS-Na/Al

8.24×10-4 1.23×10-3 1.09×10-3 1.58×10-3 6.51×10-4 1.63×10-3 7.59×10-4 1.16×10-3

8.36×10-4 2.19×10-3 7.51×10-4 2.50×10-3 9.40×10-4 3.06×10-3 9.70×10-4 1.98×10-3

PTB7-Th:PC71BM PBDT-ST1:PC71BM p-DTS(FBTTh2)2:PC71B M

In order to further reveal the origin of the improvement in Jsc and FF, the J–V characteristics of all the devices were also measured under dark conditions. (Figure 3) Obviously, in the region -1.5 to 0 V, the reverse currents of the devices using PAMPS-Na as cathode interlayer were significantly suppressed in comparison with the bare Ca/Al control device. This result manifests that PAMPS-Na possesses favorable hole-blocking properties and prevents hole current leakage through the cathode interface at reversed bias, giving rise to corresponding devices’ performance.49 While in the region over 1 V, higher injection current were observed from PAMPS-Na based devices, which is indicative of improved electron extraction from Al electrode.

12


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(b) 3

1

10

-1

10

-3

10

-5

10

-2

-1

0 1 Voltage (V)

10

0

10

-2

10

-4

10

-6

10

-2

2

(c)

PTB7-Th:PC71BM Ca/Al PAMPS-Na/Al

2

2

Current density (mA/cm )

PTB7:PC71BM Ca/Al PAMPS-Na/Al

10

2


(a)

-1

0 1 Voltage (V)

2

(d) 3

2

2

10

PBDT-ST1:PC71BM Ca/Al PAMPS-Na/Al


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


0

10

-2

10

-4

10

-6

10

-2

-1

0 1 Voltage (V)

10

p-DTS(FBTTh2)2:PC71BM Ca/Al PAMPS-Na/Al

1

10

-1

10

-3

10

-5

10

-2

2

-1

0 1 Voltage (V)

2

Figure 3. Dark currents of the (a) PTB7:PC71BM, (b) PTB7-Th:PC71BM, (c) PBDT-ST1:PC71BM and (d) p-DTS(FBTTh2)2:PC71BM solar cells with or without PAMPS-Na layer.

To gain a deeper insight into the improved Jsc, atomic force microscopy (AFM) with tapping mode was subsequently carried out to study the surface morphologies of the films with or without PAMPS-Na modifier. As shown in Figure 4, the pristine PTB7:PC71BM,

PTB7-Th:PC71BM,

PBDT-ST1:PC71BM

and

p-DTS(FBTTh2)2:PC71BM films displayed roughness of 0.99, 3.33, 1.53 and 2.48 nm, respectively. After deposition of PAMPS-Na upon the active layer, the surfaces became smoother and more homogeneous with a decreased roughness of 0.88, 1.02, 1.07 and 2.20 nm, respectively, indicating that PAMPS-Na has good film forming 13



properties on these blends. In consequence, a better charge transport is assumed from good interfacial adhesion between the Al cathode and the photoactive layers, which certainly accounts for the observed superior device performance50,51.

Figure 4. AFM images of the solar cells (a-d) without PAMPS-Na and (e-h) with PAMPS-Na layer. We also investigated the effect of cathode buffer layer thickness on the photovoltaic performance of OSCs based on PTB7-Th:PC71BM with PAMPS-Na thickness ranging from 9 to 24 nm. Figure S4 displays the J-V characters and corresponding photovoltaic parameters of the devices are listed in Supplementary Table S1. It was found that the PCE remains 8.63% when the thickness of PAMPS-Na is 19 nm. Further increase the thickness of the new cathode buffer layer, the performance of the device decreases gradually. However, when the thickness of PAMPS-Na is 24 nm, a PCE of 7.51% was still achieved. More importantly, it is generally accepted that interfacial buffer layers play a key role in both the photovoltaic performance and stability of OSCs.52 The stability of devices based on PTB7-Th:PC71BM in the ambient were measured. OSCs were stored and periodically measured in air for 30 days. It was found that OSCs based on 14


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PAMPS-Na showed better stability than devices without it. The device without PAMPS-Na experienced about 64% degradation in PCE after being exposed to ambient in 30 days, while OSCs based on PAMPS-Na can still maintained 77% of its initial efficiency after 30 days. This result testifies the great promise of PAMPS-Na’s application in OSCs. (a)

(b) Ca/Al PAMPS-Na/Al

0.8 0.6 0.4 0.2 0.0

Ca/Al PAMPS-Na/Al

1.0 Normalized Jsc (%)

Normalized Voc (V)

1.0

0

5

10

15 20 Time (d)

25

0.8 0.6 0.4 0.2 0.0

30

0

(c)

5

10

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25

30

(d) Ca/Al PAMPS-Na/Al

0.8 0.6 0.4 0.2 0.0

0

5

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15 20 Time (d)

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Ca/Al PAMPS-Na/Al

1.0 Normalized PCE (%)

1.0 Normalized FF (%)

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


0.8 0.6 0.4 0.2 0.0

30

0

5

10

15

20

25

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Time (d)

Figure 5. Degradation of normalized (a) Voc, (b) Jsc, (c) FF, and (d) PCE values of PTB7-Th:PC71BM based devices with and without PAMPS-Na cathode buffer layer.

CONCLUSION

In conclusion, a nonconjugated polymer PAMPS-Na was synthesized and developed as cathode buffer layer for boosting the electron transport and collection character of OSCs. In comparison with OSCs using traditional Ca/Al electrode, improved photovoltaic performance was obtained in the devices employing 15



PAMPS-Na as electron transport layer. The better performance is mainly ascribed to the improved short-circuit current, benefiting from the enhanced carrier transport. It was also observed that the thickness of PAMPS-Na has no significant influence on the device performance, making it possible for its application in large-area device fabrication. Moreover, PAMPA-Na affords an even more appealing feature in its ability in performing as a good moisture/oxygen scavenger to protect device from degradation in air during a long period. As a consequence, it is expected that PAMPS-Na can serve as a stable cathode buffer layer for high performance OSCs.

ASSOCIATED CONTENT

Supporting information Synthetic route of PAMPS-Na, the absorption spectra of PTB7-Th:PC71BM (with or without PAMPS-Na films), cyclic voltammograms of PAMPS-Na, J-V curves of hole-only devices/electron-only devices with and without PAMPS-Na layer, J−V characteristics of different PAMPS-Na thickness under AM 1.5G irradiation (100 mW cm−2), photovoltaic parameters of PTB7-Th:PC71BM solar cells without and with PAMPS-Na layer, photovoltaic parameters of PTB7-Th:PC71BM solar cells with different PAMPS-Na thickness. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Dr. Hongliang Liu: Email ([email protected]); *Dr. Sun Yanming: Email ([email protected].) 16


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Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of China (NSFC) (No. 51473009, 21674007, 21734001), the International Science & Technology Cooperation Program of China (2014DFA52820), the Academic Excellence Foundation of BUAA for PhD Students, and the Youth Innovation Promotion Association, CAS (2016026).

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Novel Nonconjugated Polymer as Cathode Buffer Layer for Efficient

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