Comparison of Three n-Type Copolymers Based on

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Comparison of three n-type copolymers based on benzodithiophene and naphthalene diimide /perylene diimide/ fused perylene diimides for all-polymer solar cells application Jing Yang, Yuli Yin, Fan Chen, Yong Zhang, Bo Xiao, Liancheng Zhao, and Erjun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06306 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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ACS Applied Materials & Interfaces

Comparison

of

benzodithiophene

three

n-type

copolymers

and

naphthalene

diimide

based

on

/perylene

diimide/fused perylene diimides for all-polymer solar cells application Jing Yang,a,c# Yuli Yin,b# Fan Chen,a,c Yong Zhang,b* Bo Xiao,a,c Liancheng Zhao,b Erjun Zhoua* a

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in

Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China b

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001,

China c

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

E-mail: [email protected];

[email protected]

#Both authors contribute equally.

Keywords: all-polymer solar cells; non-fullerene acceptor; benzodithiophene; fused perylene diimide; naphthalene diimide

Abstract: All-polymer solar cells (all-PSCs) have gained large attention in recent years, due to their tunable energy levels and absorption spectra for both polymeric donor and acceptor. Comparing to the numerous polymeric donors, the development of polymeric acceptors was relatively slow. Rylene diimide based polymers are regarded as the most promising n-type polymers, which were widely investigated in the past decade and some novel rylene diimide structures are constantly designed. In this work, three n-type polymers with donor-acceptor (D-A) alternative backbone structure, named as PNDI-BDT, PPDI-BDT and PFPDI-BDT, were synthesized. In these 1

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polymers, naphthalene diimide (NDI), perylene diimide (PDI) and recently developed fused perylene diimide (FPDI) were utilized as electron-withdrawing segment respectively, and benzodithiophene (BDT) with thiophenes as conjugated side chains was utilized as electron-rich unit. The optical properties, electron energy levels, charge transport properties, photovoltaic performance, charge recombination loss and surface morphology were systematically investigated. After optimizing the device fabrication conditions, PNDI-BDT, PPDI-BDT and PFPDI-BDT based photovoltaic cells realized the power conversion efficiencies (PCEs) of 0.88%, 3.74% and 5.65%, respectively. Our results indicate that FPDI is a better electron-deficient segment in comparison with NDI and PDI, for the design of n-type photovoltaic polymers.

1. Introduction In the past two decades, all-polymer solar cells (all-PSCs) have been strongly investigated due to their advantages such as the enhanced absorption spectra in long-wavelength region, adjustable molecular energy levels, superior thermal and morphological stability, and suitable viscosity for solution processing procedure1-9. The power conversion efficiencies (PCEs) of all-PSCs were significantly increased from initial 1.9% in 1999 10 to 10.1% in 2017 11, because of the development of novel n-type polymeric acceptors and the selection of suitable p-type polymeric donors. However, the photovoltaic parameters, including the short-circuit current (Jsc), the open-circuit voltage (Voc), and the filled factor (FF) of all-PSCs still largely lagged behind those of the current state-of-the-art photovoltaic cells based on small molecular non-fullerene acceptors12-19. Further improvement of the PCE of all-PSCs is dependent on the exploitation of novel photovoltaic polymers, especially for n-type materials and the engineering of the photovoltaic device fabrication. Among the various n-type semiconductor polymers, rylene diimide-based polymers are considered as the most promising electron acceptors because of the advantages of the high electron mobility, good thermal and chemical stability and ease 2


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of structure modification20. The earliest used rylene dimide-containing photovoltaic polymer was the copolymer of dithienothiophene (DTT) and perylene diimide (PDI), which was first reported by Zhan et al. in 200721. After that, much effort was put into the structure modification of PDI-based polymers. For example, PDI can be copolymerized with many electron-donating segments such as vinylene (V)22-23, thiophene (T)22,

24

, tellurophene (Te)25, fluorene (F)26, carbazole (C)22,

benzodithiophene (BDT) 27 etc. to modulate the optoelectronic properties. On the other hand, PDI can also be fused with a thiophene ring on one side to construct an asymmetric thianaphthene fused perylenediimide (TDI). Photovoltaic devices using the copolymer of TDI and thiophene (PTDI-T) showed obviously higher performance than its symmetric analogue28. Besides, the aromatic ring of PDI can be further extended by fusing two PDI units through the bay region to obtain a novel building block of fused perylene diimide (FPDI). FPDI unit have been copolymerized with vinylene (V)29, bithiophene (2T)30, difluorobithiophene (2FT)30 and 4,7-dithienyl-2,1,3-benzothiadiazole (DTBT)31, respectively. Due to the large π-electron delocalization and high electron mobilities, FPDI-based polymers realized promising photovoltaic performance with PCEs of 5.35-8.59%. Another widely used rylene diimide was naphthalene diimide (NDI), and the most representative NDI-containing n-type semiconductor was P(NDI2OD-T2) (N2200), which was first reported in 2009 by Facchetti32 as a kind of high-mobility n-channel polymer for organic thin-film transistor. After that, N2200 was applied to the field of all-PSCs, although the PCEs were relatively low by using poly(3-hexylthiophene) as electron donor33-34. NDI was also copolymerized with electron-rich building blocks like fluorene (F)35, carbazole (C)36, selenophene (Se)37-38, thiophene (T)39, thienylene-benzothiophene (T-BTh)40 etc. It is worth noting that incorporating electron-withdrawing unit such as fluorine41, cyanovinylene42 etc. into the backbone could also improve the photovoltaic performance. As so far, the highest PCE of all-PSCs has broken 10%11, which is based on NDI-based polymer. In comparison with NDI and PDI, FPDI-based polymers are relatively rare and the 3



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comprehensive comparison among these three rylene diimide based polymers has not been

reported.

Thus,

we

should

investigate

the

relationship

of

structure-properties-performance in detail, which could provide guideline information to design novel and promising n-type photovoltaic polymers. As we known, benzodithiophene (BDT) with two-dimensional (2D) conjugated side chain has been utilized to construct p-type polymers because of their advantages of high hole mobility, enhanced light absorption and excellent photovoltaic performance43. Herein, we selected the BDT with conjugated thiophene side chains as the donor segment to copolymerize with NDI, PDI and FPDI electron-deficient units respectively. The optoelectronic properties of the three n-type polymers of PNDI-BDT, PPDI-BDT and PFPDI-BDT (their chemical structures were shown in Figure 1) were investigated. All-PSCs were fabricated and the photovoltaic properties, charge recombination, carrier mobilities, photoluminescence spectra and blend films morphology were systematically studied, where PNDI-BDT, PPDI-BDT and PFPDI-BDT were used as electron acceptors the classic polymer of PTB7-Th was chose as the electron donor. Our results demonstrated that FPDI was a promising acceptor unit to construct n-type photovoltaic polymers. 2. Results and Discussion 2.1 Synthesis of the materials The synthetic routes of PNDI-BDT, PPDI-BDT and PFPDI-BDT were shown in Scheme S1. NDI-2Br, PDI-2Br and FPDI-2Br were synthesized via the reported methods30, 44-45. The polymer acceptors of PNDI-BDT, PPDI-BDT and PFPDI-BDT were synthesized via Stille reaction. The detailed synthesis procedures were described in the Supporting Information. Figure S1 gave the 1H NMR spectra of three polymers. All these polymers show good solubility in the commen solvents, including chlorobenzene, o-dichlorobenzene and chloroform. The number-average molecular 4


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weight (Mn) are 24.1, 30.8 and 16.2 kDa for PNDI-BDT, PPDI-BDT and PFPDI-BDT, respectively, with the polydispersity index of 2.02, 2.30 and 2.26 (Figure S2), which are measured through gel permeation chromatography (GPC). Thermogravimetric analysis (TGA) of PNDI-BDT, PPDI-BDT and PFPDI-BDT was measured and the decomposition temperature (5% weight loss) of three polymers arrived over 398 °C in a nitrogen atmosphere (see Figure S3). 2.2 Electrochemical and optical behaviors Cyclic voltammetry (CV) was adopted to investigate the electrochemical behaviors of three rylene dimide-based polymers and PTB7-Th, where ferrocene/ ferrocenium (Fc/Fc+) couple was used to celebrate the results46. The CV curves of PTB7-Th and three n-type polymers are provided in Figure S4. According to the onsets of the reduction and oxidation potential, the values of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels could be calculated to be -3.55 and -5.65 eV for PNDI-BDT, -3.83 and -5.73 eV for PPDI-BDT, -3.97 and -5.57 eV for PFPDI-BDT and -3.47 and -5.18 eV for PTB7-Th (as shown in Figure 1). All the three polymer acceptors have suitable energy levels difference with PTB7-Th. Figure 2 shows the absorption spectra of three polymers in CHCl3 solutions and in films. In terms of the absorption in CHCl3 solution, PFPDI-BDT showed the highest ε (the molar extinction coefficient) of 8.85×104 L mol-1 cm-1 at 400 nm in comparison with PNDI-BDT and PPDI-BDT. The results indicated that ε is highly dependent on the size of rylene dimides building blocks maybe because of the improved 5



intermolecular charge transfer, which can provide important guidelines for the understanding of photovoltaic materials with high ε values. The absorption edges of PNDI-BDT, PPDI-BDT and PFPDI-BDT in film located at 760, 760 and 750 nm, with the optical bandgaps (Eg) of 1.63, 1.63 and 1.65 eV, respectively.

Figure 1. The chemical structures of PNDI-BDT, PPDI-BDT, PFPDI-BDT and the p-type polymer of PTB7-Th, together with their energy level diagrams.

5

1.0x10

(a)

(b)

PNDI-BDT in CF PPDI-BDT in CF PFPDI-BDT in CF

4

8.0x10

4

6.0x10

4

4.0x10

4

2.0x10

0.0 300

400

500

600

700

PNDI-BDT in film PPDI-BDT in film PFPDI-BDT in film

1.0

Normalized absorbance

Molar extinction coefficient (L mol-1 cm-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

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0.8

0.6

0.4

0.2

0.0 300

800

400

500

600

700

800

Wavelength (nm)

Wavelength (nm)

Figure 2. UV-vis absorption spectra of PNDI-BDT, PPDI-BDT and PFPDI-BDT (a) in chloroform solution and (b) in pristine thin film on quartz plates. 6


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2.3 Photovoltaic properties of all-PSCs PNDI-BDT, PPDI-BDT and PFPDI-BDT were paired with PTB7-Th to investigate fthe photovoltaic properties with the conventional photovoltaic device architecture of ITO / PEDOT:PSS /active layer /Ca /Al. Photovoltaic performance of the devices was optimized by using some typical methods, such as tuning the different donor/ acceptor (D/A) weight ratios, introducing different additives and utilizing different annealing temperatures. The results were provided in Table S1-S3. Figure 3a depicted the J-V (current density-voltage) curves of the optimized devices and the corresponding photovoltaic parameters (VOC, JSC, FF and PCE with standard deviations from ten devices) were listed in Table 1. In the optimized condition, PTB7-Th: PFPDI-BDT based photovoltaic devices showed the PCE of 5.65%, which is higher than that of the other two polymeric acceptors. The high PCE of PFPDI-BDT based devices comes −2

from the high FF of 0.57 and JSC of 12.87 mA cm and. Photovoltaic devices of PPDI-BDT and PNDI-BDT showed relatively low PCE of 3.74% and 0.88%, respectively. The values of VOC decreased from 0.84V to 0.80V and then to 0.77V for PNDI-BDT, PPDI-BDT and PFPDI-BDT based devices, respectively, which come from the gradually downshifted of LUMO in the order of PNDI-BDT, PPDI-BDT and PFPDI-BDT. In contrast to the trend of VOC, the JSC of PNDI-BDT, PPDI-BDT and PFPDI-BDT based devices significantly increased, which might be related, at least in part, the enhancement of the LUMO offset. Furthermore, the absorption spectra of the mixed films in the optimized conditions were also measured. As shown in Figure S5, blend film of PTB7-Th: PFPDI-BDT exhibited obviously enhanced absorption intensity in 365-575nm compared to the other two blend films, which was beneficial to capture more sunlight to generate photo current. External quantum efficiency (EQE) plots of three photovoltaic devices were showed in Figure 3b. All the devices realized the wide response from 350 nm to 750 nm, due to the absorption of both kinds of polymeric materials. The EQE of PTB7-Th: PFPDI-BDT based device was higher than the other two acceptors-based devices in 7



the whole spectra range with the maximum over 60%, which explained the high JSC of PFPDI-BDT based devices. The calculated JSC according to the EQE curves matched well with the JSC measured under the simulated solar light with the error less than 5%. In addition, the charge recombination in the photovoltaic devices could be investigated by the light intensity dependence of Jsc, which were shown in Figure 3c. In general, a power-law relationship of JSC∝Iα should be followed and α=1 indicates that the bimolecular recombination can be ignored47. In this work, we can fit the double-logarithmic curves of JSC vs I, and calculate the values of α, which are 0.912, 0.947 and 0.954 for PNDI-BDT, PPDI-BDT and PFPDI-BDT, respectively. The results revealed that charge carrier losses due to bimolecular recombination were existed to some extent. The increased FF may come from this decreased charge recombination loss for PNDI-BDT, PPDI-BDT and PFPDI-BDT. Furthermore, photoluminescence (PL) quenching experiments were also performed to investigate the process of exciton dissociation. The photoluminescence of all the three blend films were almost completely quenched compared with the photoluminescence intensity of the donor polymer, when exciting at the maximum absorption wavelength of the donor polymer (701nm), as shown in Figure 3d. The PL quenching results proved that efficient exciton dissociation occurred in all the devices. 5

(a)

70

PTB7-Th:PNDI-BDT PTB7-Th:PPDI-BDT PTB7-Th:PFPDI-BDT

(b)


60

0

50

EQE (%)

Current density(mA/cm2)

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

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-5

40 30 20

-10

10 0 300

-15 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

400

Voltage(V)

500

600

Wavelength (nm)

8


700

800

Page 9 of 16

10

(c)


α=0.912 α=0.947 α=0.954

6

1.0x10

(d)

PTB7-Th@701nm PTB7-Th:PNDI-BDT@701nm PTB7-Th:PPDI-BDT@701nm PTB7-Th:PFPDI-BDT@701nm

5

PL Intensity (CPS)

8.0x10

JSC(mA cm-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


1

5

6.0x10

5

4.0x10

5

2.0x10

0.1 0.0

1

10

Light intensity (mW cm-2)

740

100

760

780

800

820

840

860

880

Wavelength (nm)

Figure 3. (a) the J-V curves and (b) the EQE curves of all-PSCs based on PTB7-Th: PNDI-BDT (PPDI-BDT or PFPDI-BDT); (c) Light intensity dependence of JSC; (d) The photoluminescence spectra of PTB7-Th: PNDI-BDT (PPDI-BDT or PFPDI-BDT) blend films, together with pristine donor film excited at 701nm.

Table 1. Photovoltaic results of all-PSCs based on PTB7-Th and there rylene diimide polymers. J

a)

SC

Acceptors

V

OC

(V)

(mA

J

b)

SC

−2

(mA −2

cm )

cm )

FF

PCE (%)

2

µh

−1 −1

(cm V s )

PNDI-BDT

0.84

3.38

3.50

0.31

0.88 (0.84±0.03)c)

2.50×10

PPDI-BDT

0.80

9.95

9.48

0.47

3.74 (3.64±0.08)c)

2.37×10

PFPDI-BDT

0.77

12.87

13.00

0.57

5.65 (5.48±0.12)c)

2.92×10

a)

measured from J–V characteristics; calculated from 10 cells.

b)

Calculated from EQE results;

c)

-6

-5

-4

2

-7

7.46×10

-6

2.30×10

-6

4.45×10

The average PCE are

It was proved that the morphology of donor and acceptor blend films significantly influence the photovoltaic results. The formation of continuous interpenetrating pathways for electron and hole transport is vital to achieve promising photovoltaic results. In this work, atomic force microscopy (AFM) was used to check the surface morphology of the blend films. From Figure 4, it could be found that all the active layers exhibited relatively smooth surface and uniform microstructures with roughness of 1.15-1.28 nm, which suggested that the different photovoltaic results of PNDI-BDT, PPDI-BDT and PFPDI-BDT based devices did not come from the film


−1 −1

(cm V s )

2.4 Morphology, crystallinity and charge carrier mobilities

9

µe


quality. Furthermore, the grazing incidence wide angle X-ray scattering (GIWAXS) measurements were carried out to investigate the crystallinity in the neat and mixed films. From Figure S6,

three neat polymer acceptors thin films exhibited weak

crystalline, especially for PPDI-BDT and PFPDI-BDT, which may result from the relatively poor planarity of the polymers. In addition, the branching position for the alkyl chains in these three kinds of rylene diimide might also affect the crystallinity. On the other hand, a pronounced (010) diffraction peak related with π-π stacking could be found at qz = 1.57 A-1 for PTB7-Th, which indicated that the polymer backbone of PTB7-Th mainly adopted a face-on orientation. When blending with the three polymer acceptors, the orientation of PTB7-Th was retained to some extent, thus diffraction peaks at around qz=1.52 A-1 could be found for three mixed films. In addition, the charge mobilities of the active layer could be measured by the space-charge-limited-current (SCLC) method, which could gain insight of to explain the different photovoltaic results. Figure 5 showed the J-V curves of the hole-only and electron-only devices and their hole and electron mobilities calculated from Mott−Gurney equation in the SCLC region (slope = 2)48 were summarized in Table 1. PTB7-Th: PFPDI-BDT combination exhibited hole (µh) and electron mobility (µe) of 2.92×10-4 and 4.45×10-6 cm2 V−1 s−1, respectively, both of which are higher than that of the other two devices. The device based on PTB7-Th: PPDI-BDT illustrated moderate µh and µe of 2.37×10-5 and 2.30×10-6 cm2 V−1 s−1, respectively. The d PTB7-Th: PNDI-BDT combination displayed the lowest carrier mobilities (µh= 2.50× 10-6 cm2 V−1 s−1, µe= 7.46×10-7 cm2 V−1 s−1). The enhancement of electron mobility from PNDI-BDT to PPDI-BDT and to PFPDI-BDT should come from the increased size of rylene diimide rings, which increased the possibility of intermolecular charge transport, which should be also related with the increased FF. These charge transport results agreed well with the photovoltaic results of the three polymers.

10


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Figure 4. AFM height images of (a) PTB7-Th: PNDI-BDT, (b) PTB7-Th: PPDI-BDT and (c) PTB7-Th: PFPDI-BDT films.

(b) Electron-only devices

(a) Hole-only devices 10 2

2

100


10 1

Slope = 2 SCLC 0.1 0.01

Current Density (mA/cm )

1000

Current Density (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



1

Slope = 2 SCLC

0.1

Slope = 1 Ohmic conduction

Slope = 1 Ohmic conduction

0.01 1E-3 0.1

0.1

1

1

Voltage (V)

Voltage (V)

Figure 5. J-V curves of the (a) hole-only and (b) electron-only devices

3. Conclusion In this paper, we designed and synthesized three rylene dimide-based polymers of PNDI-BDT, PPDI-BDT and PFPDI-BDT, which composed of NDI, PDI and FPDI as electron-deficient unit and BDT containing conjugated thiophene side chain as electron-rich segment. The three polymers displayed relatively deep LUMO levels of -3.55 to -3.97 eV, which enabled them to be used as acceptors in all-PSCs. Considering the complementary spectra and matched energy levels, PTB7-Th was utilized as the p-type polymer to fabricate photovoltaic devices with the three polymer acceptors. 11



PTB7-Th: PFPDI-BDT combination realized a PCE of 5.65% in the optimized condition, which was higher than that of PPDI-BDT (3.74%) and PNDI-BDT based device (0.88%). The high PCE of PFPDI-BDT based devices comes from the high photo current response from 350-750nm, less charge recombination in the blend film, uniform microstructure and high hole and electron mobility. Our result indicated that comparing with classic NDI and PDI unit, FPDI was also an effective electron-deficient segment to design n-type photovoltaic polymers.

Acknowledgements The authors thank the support from National Key Research and Development Program of China (2017YFA0206600), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH033), the National Natural Science Foundation of China (NSFC, Nos. 51673048, 21602040, 51473040, 21644006, 51403044), the National Natural Science Foundation of Beijing (No. 2162045), Natural Science Foundation of Heilongjiang Province of China (E2018036) and the Fundamental Research Funds for the Central Universities (Harbin Institute of Technology).

Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI:

Synthesis, 1H NMR and molecular weights of polymers; TGA; photovoltaic

results.

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Comparison of Three n-Type Copolymers Based on

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