Z-Shaped Fused-Chrysene Electron Acceptor for Organic Photovoltaics

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Z-Shaped Fused-Chrysene Electron Acceptor for Organic Photovoltaics Bing Lu, Yiqun Xiao, Tengfei Li, Kuan Liu, Xinhui Lu, Jiarong Lian, Pengju Zeng, Junle Qu, and Xiaowei Zhan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10834 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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Z-Shaped Fused-Chrysene Electron Acceptor for Organic Photovoltaics Bing Lu,†,‡ Yiqun Xiao, ⊥ Tengfei Li,‡ Kuan Liu,‡ Xinhui Lu, ⊥ Jiarong Lian,*,† Pengju Zeng,† Junle Qu,† and Xiaowei Zhan*,‡ †

Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education

and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. ‡

Department of Materials Science and Engineering, College of Engineering, Key

Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China.



Department of Physics, The Chinese University of Hong Kong, New Territories

999077, Hong Kong, China.

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ABSTRACT: A new fused-chrysene electron-donating core is synthesized, where chrysene is condensed with two thiophenes via two dihydrobenzene rings. Based on this

building

block

coupled

with

2

electron-accepting

end

groups

1,1-dicyanomethylene-3-indanone, a new Z-shaped fused-ring electron acceptor, FCIC, is designed and synthesized. FCIC shows intense absorption in 500−850 nm region, with a maximum molar absorptivity of 1.5 × 105 M–1 cm–1, a bandgap of 1.50 eV, and a charge mobility of 2.5 × 10–4 cm2 V–1 s–1. The ternary organic photovoltaic cells based on PTB7-Th/F8IC/FCIC yield an efficiency of 12.6%, higher than that of the binary cells of PTB7-Th/F8IC (10.7%) and PTB7-Th/FCIC (7.21%). Relative to the PTB7-Th/F8IC binary blend, the addition of FCIC leads to improvement in the open-circuit voltage, short-circuit current and fill factor.

KEYWORDS: organic photovoltaic, chrysene, fused-ring electron acceptor, Z-shaped, nonfullerene acceptor.

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Introduction In the last two decades, organic solar cells (OSCs) have achieved rapid development and present unique advantages, including facile solution processing, low cost, low toxicity, light weight, light transparency, and flexibility.1-5 In this field, fullerenes and their derivatives have dominated electron acceptor materials for more than 20 years.6 However, the deficiencies of fullerene acceptors, including finite tunability of energy levels, weak visible light absorption and poor morphological stability, restrict continued advance of this field. Organic nonfullerene acceptors have been developed as potential alternatives to overcome these drawbacks of fullerene acceptors.7-10 In 2015, our group discovered a brand-new series of nonfullerene acceptors, called "fused-ring electron acceptor (FREA)", such as the milestone molecule ITIC.11 These FREAs generally consist of a linear-shaped A-D-A structure, where a central electron-rich unit (e.g., indacenodithieno[3,2-b]thiophene) is coupled with 2 electron-deficient groups (e.g., 1,1-dicyanomethylene-3-indanone (IC)). FREAs exhibit several special merits, such as intense and wide visible and even near infrared (NIR) absorption, and variable electronic energy levels.12,13 For the sake of boosting the power conversion efficiency (PCE), continuous efforts have been devoted to modifying the central cores,14-20 side chains21-26 and electron-withdrawing end groups.27-32 Now FREA-based single-junction devices have shown PCEs of 14-16%.33-36 These FREAs with different molecular structure generally have linear molecular geometry and the construction strategy of the central fused cores is fusion 3 ACS Paragon Plus Environment

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of the electron-rich aromatic rings with cyclopentadienyl rings. Here, we create a new fused-chrysene electron-donating core, where chrysene is condensed with two thiophenes via two dihydrobenzene rings; a new Z-shaped FREA, FCIC, is synthesized based on this core (Figure 1a). Chrysene is a common polycyclic aromatic hydrocarbon with high structural stability,37,38 rigid conjugated system and wide bandgap,39 and is widely used in organic field-effect transistors37,39-40 and organic light-emitting diode.41,42 As far as we know, there are no chrysene-based photovoltaic materials reported up to date. In contrast to general design of linear-shaped

FREAs,

this

twisted

Z-shaped

structure

can

prevent

over

self-aggregation of acceptor and facilitates miscibility of donor and acceptor. FCIC exhibits intense absorption from 500 to 850 nm, a bandgap of 1.50 eV, and a charge mobility of 2.5 × 10–4 cm2 V–1 s–1. The devices (Figure 1a) consisting of ternary blends of donor PTB7-Th,43 our previously reported acceptor F8IC,44 and FCIC yield PCEs up to 12.6%. Experimental Section Details for materials synthesis and characterization as well as device fabrication and characterization see Supporting Information. Results and Discussion The synthesis procedure for FCIC is depicted in Scheme S1. FCIC was well characterized by NMR spectra, mass spectra and elemental analysis (details see Supporting Information). FCIC can dissolve in often-used solvents, e.g., dichloromethane, chlorobenzene and chloroform. FCIC shows high thermal 4 ACS Paragon Plus Environment

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decomposition temperature of 377 oC, measured through thermogravimetric analysis (TGA). (Figure S1).

b)

c) -3.59

0.4 0.2

-4.00

F8IC

-3.86

0.6

0.0 300

LUMO

FCIC

0.8

PTB7-Th F8IC FCIC

PTB7-Th

1.0

Energy (eV)

Normalized absorbance (a.u.)

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

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-5.49

-5.43

-5.20 400

500

600

700

800

900 1000

HOMO

Wavelength (nm)

Figure 1. a) Molecular structures of donor and acceptors, and the OSC device structure; b) thin-film absorption spectra and c) energy levels of donor and acceptors. In CHCl3 solution, FCIC presents intense absorption in 500-735 nm region with maximum peak at 679 nm and a maximum molar absorptivity of 1.5 × 105 M–1 cm–1 (Figure S2a). FCIC thin film shows bathochromic absorption compared to its solution, and an intense absorption band locates at 500−850 nm. The optical bandgap of FCIC, calculated from the absorption edge, is 1.50 eV. Cyclic voltammetry was used to estimate the oxidation and reduction potentials of FCIC (Figure S2b), from which the 5 ACS Paragon Plus Environment

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highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies are estimated to be −5.49 and −3.86 eV, respectively. The electron mobility (μe) of FCIC was measured to be 2.5 × 10–4 cm2 V–1 s–1 using a method of space charge limited current (SCLC) (Figure S3).45 We fabricated solar cells using inverted sandwich structure of indium tin oxide (ITO) glass/ZnO/PTB7-Th:FCIC/MoOx/Ag. We chose PTB7-Th as donor due to the matched energy levels between PTB7-Th and FCIC (Figure 1c). The PTB7-Th:FCIC (1:1.5, w/w) devices employing diphenyl ether as solvent additive (0.5%, v/v) exhibit the best efficiency of 7.21% with an open-circuit voltage (VOC) of 0.850 V, short-circuit current density (JSC) of 14.5 mA cm–2 and fill factor (FF) of 58.2% (Table S1). To obtain wide absorption and high JSC, our previously reported NIR-absorbing FREA, F8IC, was used as the third component since F8IC shows intense absorption in 700-1000 nm region and complements the absorption spectra of the PTB7-Th:FCIC blend (Figure 1b). The ternary OSCs (ITO/ZnO/PTB7-Th:F8IC:FCIC/MoOx/Ag) were fabricated. The D/A weight ratio is fixed at 1/1.5 and the FCIC/F8IC weight ratio varies from 0/1, 0.1/0.9, 0.2/0.8, 0.3/0.7, 0.5/0.5 to 1/0 (Table S2). When increasing the content of FCIC, VOC exhibits a monotonic increase due to the higher LUMO of FCIC (Figure S4a), whilst the JSC and FF go up first and then down (Figures S4b and S4c). The ternary cell optimized by incorporating 10 wt% FCIC in acceptors using diphenyl ether as solvent additive (0.5% v/v) affords the best PCE of 12.6% with VOC of 0.668 V, JSC of 25.9 mA cm–2, and FF of 72.6% (Table 1 and 6 ACS Paragon Plus Environment

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Figure 2a). Compared with the binary blends of PTB7-Th/FCIC and PTB7-Th/F8IC, the JSC and FF of the ternary device are synergistically increased. Table 1. Performance of optimized binary and ternary OSCs a acceptor

VOC (V)

FCIC

F8IC

F8IC/FCIC a

JSC (mA cm–2)

FF (%)

PCE (%)

58.2

7.21

0.850

14.5

(0.847±0.005)

(14.1±0.6)

0.643

23.5

70.8

10.7

(0.642±0.003)

(23.3±0.9)

(70.0±0.7)

(10.4±0.4)

0.668

25.9

72.6

12.6

(0.663±0.006)

(25.4±0.8)

(71.4±1.6)

(12.4±0.2)

calc JSC (mA cm–2) 14.3

(57.9±1.4) (7.15±0.06)

23.0

24.5

Average values listed in parenthesis are calculated from 15 devices. The external quantum efficiency (EQE) spectra of the binary blends of

PTB7-Th/FCIC and PTB7-Th/F8IC and ternary blend of PTB7-Th/F8IC/FCIC are shown in Figure 2b. The corresponding values of ternary blend in the 500-800 nm region are improved compared to the binary blend of PTB7-Th/F8IC since stronger absorption of FCIC at 500–800 nm complements that of F8IC (Figure S5). b) 5

80

0

70

PTB7-Th/F8IC/FCIC PTB7-Th/F8IC PTB7-Th/FCIC

-5

60

EQE (%)

-2)

a) Current density (mA cm

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-10 -15

50 40 30 20

-20

10

-25 0.00

0.25

0.50

0 300

0.75

PTB7-Th/F8IC/FCIC PTB7-Th/F8IC PTB7-Th/FCIC 400

500

600

700

800

900 1000

Wavelegth (nm)

Voltage (V)

Figure 2. a) J–V curves and b) EQE spectra of the best binary and ternary devices. The thermal stability tests of the devices were implemented at 80 oC in glovebox 7 ACS Paragon Plus Environment

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(Figure S6). The binary device based on PTB7-Th/F8IC maintains 58% of its initial PCE, while the PTB7-Th/FCIC and PTB7-Th/F8IC/FCIC devices retain 81% and 78%, respectively, after heating for 180 min, indicating FCIC enhances the device stability. To study the charge dynamics processes in devices, the dependence of the photocurrent density (Jph) on the effective voltage (Veff) of OSCs is investigated (Figure 3a). The ratio of JSC to saturation photocurrent density (Jsat) reflects the charge extraction properties under short-circuit condition. Under short-circuit condition, the JSC/Jsat values of FCIC, F8IC and F8IC/FCIC-based OSCs are 94.6%, 95.4% and 97.4%, respectively, implying that ternary devices exhibit a little more efficient charge extraction. To figure out the charge recombination behavior, we measured JSC with varied incident light intensity (Plight). The JSC depends on Plight following a power-law relationship of JSC ∝ Plightα. If α approaches 1, the bimolecular charge recombination of the device is negligible. As shown in Figure 3b, the α is 0.912, 0.974, and 0.981 for FCIC, F8IC and F8IC/FCIC-based devices, respectively, indicating bimolecular charge recombination is decreased in the ternary OSCs. b)

a)

10

-2

PTB7-Th/F8IC/FCIC PTB7-Th/F8IC PTB7-Th/FCIC

1 0.1

10

Jsc ( mW cm )

-2

JSC( 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

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1

1

Veff (V)

PTB7-Th/F8IC/FCIC PTB7-Th/F8IC PTB7-Th/FCIC 10

-2

Light intensity ( mW cm )

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100

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Figure 3. a) Jph-Veff and b) JSC-Plight curves of the best cells. The charge mobilities of the binary and ternary blends were measured (Figure S7). In comparison with the binary blends, PTB7-Th/F8IC/FCIC ternary blend exhibits the highest hole mobility (μh = 1.1 × 10–3 cm2 V–1 s–1) and μe (8.8 × 10–4 cm2 V–1 s–1) with more balanced charge mobilities (μh/μe = 1.2) (Table S3). The higher and more balanced charge mobilities contribute to reduced carrier recombination, leading to higher FF of the ternary OSCs. We used atomic force microscopy (AFM) to image the surface film morphology of the blends (Figure S8). The binary and ternary blends show smooth and uniform surface morphology, with root-mean-square roughnesses of 2.03 nm, 1.48 nm and 1.14 nm for FCIC, F8IC and F8IC/FCIC-based blend films, respectively. In order to study the molecular packing in the active layer, grazing-incidence wide-angle X-ray scattering (GIWAXS) were performed.46 Figure S9 presents the two-dimensional (2D) GIWAXS patterns and the intensity profiles of pristine donor and acceptor films. Figure 4 shows the GIWAXS patterns and the intensity profiles of PTB7-Th/FCIC, PTB7-Th/F8IC, PTB7-Th/F8IC/FCIC blend films. PTB7-Th film shows a preferential face-on packing, consistent with previous report.47 FCIC film presents obvious edge-on oriented lamellar stacking with the lamellar peak appeared at qz = 0.34 Å−1, while the F8IC film shows bimodal molecular packing with the face-on lamellar and π-π peak at qr = 0.42 Å−1 and qz = 1.77 Å−1 and the relatively weaker edge-on lamellar and π-π peak at qz = 0.43 Å−1 and qr = 1.39 Å−1. The PTB7-Th/FCIC blend shows the lamellar peak at qr = 0.29 Å−1 (d-spacing = 20.3 Å, 9 ACS Paragon Plus Environment

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crystallite coherence length (CCL) = 46.2 Å) and the π-π peak at qz = 1.61 Å−1 (d = 3.90 Å, CCL = 7.38 Å), originated from the face-on oriented PTB7-Th crystalline domains. The intense lamellar (100) peak at qz= 0.35 Å−1 (d = 18.0 Å, CCL = 84.8 Å) is assigned to FCIC, indicating a slightly tighter packing of FCIC in the blend film than in the pure FCIC film. The PTB7-Th/F8IC exhibits strong face-on ordering for both PTB7-Th donor and F8IC acceptor domains. The two (100) peaks at qr = 0. 27 Å−1 (d = 23.3 Å, CCL = 63.5 Å) and qr = 0. 44 Å−1 (d = 14.3 Å, CCL = 110 Å) are assigned to PTB7-Th and F8IC domains, respectively. In addition, the π-π stacking peak of PTB7-Th appears at qz = 1.64 Å−1 (d = 3.83 Å, CCL = 18.3 Å). There exist two π-π peaks of F8IC located at qz = 1.85 Å−1 (d = 3.40 Å, CCL = 35.9 Å) and qz = 1.43 Å−1 (d = 4.40 Å, CCL = 37.1 Å). These two peaks are also observed in the pure F8IC film, indicating the coexistence of core group and end group π-π stacking in the F8IC crystalline domains. The enhanced crystallinity and closer π-π stacking are in accordance with the high carrier mobility compared to the PTB7-Th/FCIC film. For the ternary PTB7-Th/FCIC/F8IC film, the crystalline packing of three phases are well maintained. The face-on lamellar peaks which appear at qr = 0. 28 Å−1 (d = 22.4 Å, CCL = 81.5 Å) and qr = 0. 44 Å−1 (d = 14.3 Å, CCL = 102 Å) are assigned to the crystalline domains of PTB7-Th and F8IC, respectively, while the edge-on lamellar peak at qz = 0.38 Å−1 is attributed to FCIC. A wider and stronger π-π scattering can be fitted by three peaks at qz =1.69 (PTB7-Th), 1.41 and 1.84 (F8IC) Å−1. The corresponding CCLs are estimated to be 32.3, 21.5 and 43.8 Å. The enhanced crystallinity and increased coherence lengths compared to PTB7-Th/F8IC film could 10 ACS Paragon Plus Environment

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contribute to enhanced and balanced electron and hole mobilities and thereby improved device performance. To study the phase separation of blended films, grazing-incidence small-angle X-ray scattering (GISAXS) was used, which are presented in Figures 4e and S10. The correlation length of the intermixing phases and acceptor domains can be extracted by fitting the intensity profiles by the Debye–Anderson–Brumberger (DAB) and

fractal-like

network

models.

PTB7-Th/FCIC,

PTB7-Th/F8IC,

and

PTB7-Th/F8IC/FCIC show correlation length of the intermixing phase of 38.4, 55.6 and 56.7 nm, respectively, while their fitted acceptor phase size (2Rg) is 32.8, 53.4 and 40.4 nm correspondingly. The decreased acceptor phase size of ternary blend is beneficial to exciton splitting, leading to an improved JSC of the devices.

Figure 4. a−c): 2D GIWAXS patterns of the blend films; d) the intensity profiles along qr (dashed lines) and qz (solid lines) directions; e) GISAXS intensity profiles 11 ACS Paragon Plus Environment

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along qr with the best fittings. Conclusions In summary, a new Z-shaped fused-chrysene electron acceptor FCIC was synthesized. FCIC shows high thermal stability and wide absorption from 500 to 850 nm with high molar absorptivity. The PCE of ternary devices consisting of PTB7-Th/F8IC/FCIC blend is 12.6%, which is higher than that of FCIC and F8IC-based binary-blend OSCs. As the third component, the new electron acceptor FCIC simultaneously improves VOC, JSC and FF compared with the binary OSCs based on PTB7-Th/F8IC. The upshifted LUMO of FCIC contributes to the increased VOC; the improved JSC is related to the complementation of F8IC/FCIC light absorption; the enhanced crystallinity and increased coherence length contribute to enhanced and balanced mobility and thereby improved FF. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Detailed experimental procedures; syntheses and characterization of materials; TGA, SCLC, AFM, GIWAXS and GISAXS; OSC fabrication and characterization. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (X.Z.). *E-mail: [email protected] (J.L.).

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT X. Z. thanks the NSFC (Nos. 51761165023 and 21734001). Y. X and X. L. thank the financial

support

from

NSFC/RGC

Joint

Research

Scheme

(Grant

No.

N_CUHK418/17). REFERENCES 1.

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