Organic solar cells based on non-fullerene acceptors of nine fused-ring by modifying end groups

Journal Pre-proof Organic solar cells based on non-fullerene acceptors of nine fused-ring by modifying end groups Jingbo Xiao, Tingting Yan, Tao Lei, Yanbo Li, Yufang Han, Liang cao, Wei Song, Songting Tan, Ziyi Ge PII:

S1566-1199(20)30048-3

DOI:

https://doi.org/10.1016/j.orgel.2020.105662

Reference:

ORGELE 105662

To appear in:

Organic Electronics

Received Date: 6 January 2020 Revised Date:

17 February 2020

Accepted Date: 18 February 2020

Please cite this article as: J. Xiao, T. Yan, T. Lei, Y. Li, Y. Han, L. cao, W. Song, S. Tan, Z. Ge, Organic solar cells based on non-fullerene acceptors of nine fused-ring by modifying end groups, Organic Electronics (2020), doi: https://doi.org/10.1016/j.orgel.2020.105662. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Graphical abstract

1

Organic solar cells based on non-fullerene acceptors of nine fused-ring by modifying end groups Jingbo Xiaoa,b, Tingting Yanb, Tao Leib, Yanbo Lib, Yufang Hanb, Liang caob, Wei Songb, Songting Tana,*, Ziyi Geb,* a

Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of

Education, Xiangtan University, College of Chemistry, Xiangtan University, Xiangtan 411105, P. R. China b

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

Ningbo, 315201, P.R. China *Corresponding author. E-mail address: [email protected] (S. Tan), [email protected] (Z. Ge).

2

ABSTRACT A

series

of

small

molecule

acceptors

(SMAs)

based

on

a

benzodithiophene-pyrrolobenzothiadiazole-based nine fused-ring core and different end groups of 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2FIC),

3-(dicyanomethylene)indian-1-one

(IC)

and

2-(3-ethyl-4-oxo-thiazolidin-2-ylidene) malononitrile (RCN) have been designed and synthesized. The SMAs X94FIC, X9IC, X9Rd and X9T4FIC were chosen as electron acceptors with blending the donor polymer PBDB-T to prepare the organic solar cells (OSCs) and investigate photoelectric performance. Surprisingly, X94FIC showed an excellent photo-response up to 1000 nm, while X9IC and X9T4FIC also possess a photo-response to 900 nm. A power conversion efficiency (PCE) of 7.08% was obtained for the active layers PBDB-T:X94FIC. This result demonstrates that small molecule acceptor containing the nine fused-ring core and the end group with di-fluorine atoms is promising candidate for the development of high performance non-fullerene OSCs. Keywords: Organic solar cell; Non-fullerene acceptor; Small molecule; Fused-ring; Synthesis

1. Introduction In the past few years, organic solar cells have developed rapidly, and bulk heterojunction (BHJ) [1] organic solar cells (OSCs) are one of the most commonly used device structures based on p-type organic semiconductor electron donors blended with n-type organic semiconductor electron acceptors as active layer and have been achieved great progress [2-7]. It is attribute to the development of high performance donors [8-12], acceptors [13-18] and electrode buffer layer materials [19, 20]. For 3

acceptor materials, they are classified into fullerene acceptors such as PCBM [21-24] and non-fullerene acceptors. Non-fullerene acceptors (NFAs) are mainly small molecule acceptor (SMAs) [25-27] and polymer acceptors. Compared with polymer acceptors, the SMAs have the advantages of strong absorption, adjustable energy level, high carrier mobility, excellent phase separation morphology, and easy modification. Therefore, the development of polymer acceptor materials relative to SMAs are jog along. At the same time, n-type donors also play an important role in organic solar cells as hole transport carriers for photovoltaic devices, while P3HT is the most representative donor photovoltaic material [28, 29]. To produce a high performance OPVs, the most important are the match of the HOMO level of donors and the LUMO level of acceptors, high and balanced charge-carrier mobility, and complementary absorption bands in the Vis-NIR range of donors and acceptors. In addition, both the material composition ratio of the blend films and the morphology of the active layer affect the power conversion efficiencies (PCEs) of the OPVs. In the last ten years, the A-D-A type fused-ring SMAs have received widespread attention from researchers due to its advantages such as adjustable energy levels and strong absorption, its rapid development has greatly promoted the development of OSCs and achieved high efficiency. For example, zhan’ group reported a series of A-D-A type SMAs [30-33], such as IDIC, ITIC etc. For most reported SMAs, the absorption wavelength of the electron acceptors were lower than 800 nm, while the PCEs of the OSCs after matching the appropriate polymer donor materials could exceed 13%. Recently, the non-fullerene small molecule acceptor Y6 that synthesized by Zou and coworkers has an absorption peak exceeding 800 nm. [34] At the same time, Y6 has a HOMO level of -5.65 eV and a LUMO level of -4.10 eV, after matching with a polymer donor PM6, a high PCE of 15.7% was obtained, which is one of the most efficient acceptor materials so far. In this paper, we plan to synthesize SMAs with near-infrared absorption based on benzodithiophene (BDT) and benzothiadiazole (BT) to improve the efficiency of OSCs. BDT has good symmetry, the molecular conformation presents a planar 4

conjugated structure and small steric hindrance, which is a proven excellent donor unit and it could improve the electron mobility of SMAs [35-41]. Based on the above considerations, we designed and synthesized four SMAs based on a new benzodithiophene-pyrrolobenzothiadiazole central nine fused-ring unit. Firstly, 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b'] dithiophene (BDT-OEH) contains four alkoxy side chains, which can effectively improve the solubility of SMAs. Then, three different end

groups

malononitrile

of 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)

(2FIC),

3-(dicyanomethylene)indian-1-one

(IC)

and

2-(3-ethyl-4-oxo-thiazolidin-2-ylidene) malononitrile (RCN) were used to adjust the light absorption and energy levels of SMAs. In addition, we also added a π-bridge between the central core and the end group to adjust the performance. Finally, we obtained four SMAs, X94FIC, X9IC, X9Rd and X9T4FIC (Scheme 1). For X94FIC, X9IC and X9T4FIC, they were found to have narrow band gaps of 1.41 eV, 1.47 eV, and 1.47 eV, respectively, and X9Rd showed a wide band gap of 1.71 eV. In particular, the light absorption band of X94FIC reached an amazing 1000 nm, while X9IC and X9T4FIC also have a photo-response to 900 nm, which is beneficial for improving the PCE of OCSs.

2. Experimental 2.1. Materials and methods Toluene, tetrahydrofuran, dichloromethane and DMF were used after water removal strictly. All experimental drugs were reagent-grade chemicals purchased from formal commercial sources, and used directly unless specified. The compounds from each synthesis steps were purified by extraction, silica gel column chromatography and recrystallization. The chemical structures of the experimental products were determined by nuclear magnetic resonance spectra (NMR), which were measured on an Avance DPX-400 with deuterated chloroform or dichloromethane as the solvent and tetramethylsilane (TMS) as internal reference. In addition, ESI-TOF-MS was used to determine the molecular mass of SMAs, and elemental analysis was used to 5

characterize the proportion of atoms in SMAs. The theoretical energy levels of the acceptors were calculated by density functional theory (DFT). The electrochemical properties of the acceptors were measured by cyclic voltammetry on a CHI660E electrochemical workstation and the energy level of the acceptors were estimated from the obtained redox potential. The UV-visible absorption spectra of the solutions and films were measured by a PerkinElmer Lambda 950 spectrophotometer. The surface morphologies and roughness of the films were tested by atomic force microscope (AFM, Veeco Dimension 3100 V). 2.2. Synthesis All synthetic routes, and the solvents and reagents used in the synthesis are shown in Scheme 1. 2.2.1.

Synthesis

of

(4,

8-bis(octyloxy)benzo[1,2-b:4,5-b’]

dithiophen-2-yl)

trimethylsilane (2) Compound 1 (3.00 g, 6.72 mmol) was dissolved in anhydrous THF (30 mL) at -78 °C under nitrogen for twenty minutes, and n-BuLi (5.04 mL, 8.06 mmol, 1.6 M in hexane) was gradually added dropwise to the above solution. After stirring at -78 °C for 2 hours, trimethylchlorosilane (949.08 mg, 8.74 mmol) was added to the three-necked flask. After half an hour, the reaction flask was transferred to room temperature and stirred overnight. Water was added to quench the reaction. The reaction mixture was extracted with twice, the organic layers dried over anhydrous MgSO4. After vacuum drying to remove solvent, the crude material was purified on a silica gel column eluting with dichloromethane/petroleum ether (1/5, v/v) as the eluent, the result is a flaxen liquid. Compound 2 (2.96 g, 85% yield). 1H NMR (400 MHz, CDCl3) δ: 7.57 (s, 1H), 7.46 (d, J = 4.8 Hz, 1H), 7.35 (d, J = 4.9 Hz, 1H), 4.19 (s, 4H), 1.62 – 0.92 (m, 30H), 0.40 (s, 9H).

6

Scheme 1. Synthesis routes of X94FIC, X9IC, X9Rd and X9T4FIC.

7

2.2.2.

Synthesis

of

(4,

8-bis(octyloxy)-6-(trimethylstannyl)benzo[1,2-b:4,5-b’]

dithiophen-2-yl) trimethylsilane (3) Compound 2 (2.96 g, 5.71 mmol) was dissolved in anhydrous THF (25 mL) under a nitrogen atmosphere at -78 °C. After twenty minutes, n-BuLi (4.28 mL, 6.85 mmol, 1.6 M in hexane) was slowly added dropwise to the above solution. Stirring was continued at -78 °C for 2 hours, and then tributyltin chloride (7.42 mL, 7.43 mmol, 1 M) was added to the three-necked flask. After half an hour, the reaction flask was transferred to room temperature and stirred overnight. The reaction was quenched by the addition of water after the next day. The reaction mixture was extracted twice with dichloromethane. After removing the solvent by vacuum spinning, the crude product was recrystallized twice to obtain a compound 3 of flaxen solid (3.31 g, 85% yield). 1H NMR (400 MHz, CDCl3) δ 7.56 (s, 1H), 7.55 – 7.47 (m, 1H), 4.19 (dd, J = 5.3, 3.3 Hz, 4H), 1.66 – 0.86 (m, 30H), 0.49 – 0.21 (m, 18H). 2.2.3. Synthesis of Compound 4 Compound 3 (3.31 g, 4.86 mmol), 4, 7-dibromo-5, 6-dinitrobenzo[c] [1,2,5]thiadiazole (776.70 mg, 2.03 mmol) and Pd(PPh3)4 (0.23 g, 0.20 mmol) was dissolved in 30 mL of anhydrous toluene, and stirred at 110 °C overnight under nitrogen with light-free conditions. The reaction flask and solvent were cooled to room temperature, and then concentrated under reduced pressure. The crude product was purified with on a silica gel column eluting with dichloromethane/petroleum ether (1/3, v/v) as the eluent, the crude product was recrystallized twice to obtain a compound 4 of yellow solid (2.05 g, 80% yield). 1H NMR (400 MHz, CDCl3) δ 7.92 (s, 2H), 7.59 (s, 2H), 4.23 (s, 8H), 1.63 – 0.90 (m, 60H), 0.43 (s, 18H). 2.2.4. Synthesis of Compound 5 Compound 4 (2.05 g, 1.62 mmol) and triethyl phosphate (2.69 g, 16.24 mmol) were dissolved in o-dichlorobenzene (o-DCB, 50 mL) under nitrogen. The temperature was raised to 180 °C and stirred for 48 hours. The mixture was then cooled to room temperature, extracted with chloroform and the organic layer dried 8

over Na2SO4. The solvent was removed under reduced pressure to obtain a crude compound 5 without further purification. 1H NMR (400 MHz, CDCl3) δ 9.57 (s, 2H), 7.54 (s, 2H), 4.34 (d, J = 79.2 Hz, 8H), 1.20 (ddd, J = 90.3, 76.6, 24.5 Hz, 60H), 0.46 (s, 18H). 2.2.5. Synthesis of Compound 6 Compound 5 (2.05 g, 1.62 mmol), 1-bromo-2-ethylhexane (3.13 g, 16.24 mmol), potassium iodide (66 mg, 0.4 mmol) and potassium carbonate (2.24 g, 16.24 mmol) were dissolved in the N, N-dimethylmethanamide (DMF, 30 mL) under nitrogen. After being heated at 90 °C and stirred for 24 hours, the solution was removed under vacuum and extracted with chloroform. The organic layers were combined and dried over MgSO4, filtered and purified with column chromatography on silica gel using dichloromethane/petroleum ether (1/4, v/v) as the eluent to give a red solid. Compound 6 (1.35 g, 65% yield). 1H NMR (400 MHz, CDCl3) δ 7.56 (d, J = 5.4 Hz, 2H), 7.46 (d, J = 5.4 Hz, 2H), 6.14 – 3.76 (m, 12H), 1.59 – 0.71 (m, 90H). 2.2.6. Synthesis of Compound 7 Compound 6 (1.35 g, 1.06 mmol) and DMF (386.9 mg, 5.30 mmol) were dissolved in anhydrous 1, 2-dichloroethane (ClCH2CH2Cl, 30 mL), and phosphorus oxychloride (POCl3, 812.0 mg, 5.30 mmol) was added dropwise to the above solution at 0 °C. The mixture was stirred at 0 °C for 2 h. The reaction was then heated to 85 °C and allowed to react overnight, and the mixture was slowly added dropwise to iced water (300 mL) with stirring and neutralized with Na2CO3 (aq). The reaction mixture was extracted with dichloromethane. The organic layer was dried over anhydrous Na2SO4. After removal of the solvent, the crude material was purified eluting with silica gel eluting with petroleum ether/dichloromethane (1:1, v/v). The title compound was obtained as an orange solid. Compound 7 (998 mg, 70%). 1H NMR (400 MHz, CDCl3) δ 10.13 (s, 2H), 8.23 (s, 2H), 5.25 (dd, J = 12.9, 4.7 Hz, 2H), 4.57 (d, J = 13.4 Hz, 2H), 4.42 – 4.24 (m, 6H), 4.11 – 3.98 (m, 2H), 1.94 – 0.56 (m, 90H). 2.2.7. Synthesis of Compound 8 9

Compound 6 (467.0 mg, 0.37 mmol) and N-bromosuccinimide (NBS, 163.0 mg, 0.92 mmol) were dissolved in chloroform (25 mL) and degassed with N2 for 10 min. The reaction system was sealed quickly. Then, the mixture was stirred at room temperature for 10 hours under N2 atmosphere. The mixture was extracted with CH2Cl2 and H2O. Then, the organic layer was dried over anhydrous MgSO4, filtrated and evaporated using a rotary evaporator. The solid was purified by silica gel column chromatography (petroleum ether/dichloromethane = 1: 2). The product was obtained as purple-red solid with yield of 72%. Compound 8 (378 mg). 1H NMR (400 MHz, CDCl3) δ 7.54 (s, 2H), 5.23 (s, 2H), 4.67 – 4.53 (m, 2H), 4.37 – 4.21 (m, 6H), 4.10 – 3.95 (m, 2H), 1.77 – 0.77 (m, 90H). 2.2.8. Synthesis of Compound 9 Compound 8 (190.0 mg, 132.0 µmol), 4-octyl-5-(4,4,5,5-tetramethyl-1,3,2dioxaborolan -2-yl)thiophene-2-carbaldehyde (138.6 mg, 396 µmol), K2CO3 (365.0 mg, 2.64 mmol, 1.0 M in H2O), and Pd(PPh3)4 (7.6 mg, 6.6 µmol) in toluene (20 mL) was vigorously stirred at 100 °C under nitrogen. The mixture was extracted with CH3Cl and H2O. Then, the organic layer was dried over anhydrous MgSO4, filtrated and evaporated using a rotary evaporator. The solid was purified by silica gel column chromatography (petroleum ether/dichloromethane = 1: 3). The product was obtained as purple-red solid. Compound 9 (148 mg, yield of 65%). 1H NMR (400 MHz, CDCl3) δ 9.92 (d, J = 4.9 Hz, 2H), 7.71 (d, J = 8.0 Hz, 2H), 7.66 (s, 2H), 5.28 (s, 2H), 4.64 (s, 2H), 4.35 (dd, J = 12.3, 6.5 Hz, 6H), 4.11 (s, 2H), 2.92 (s, 4H), 2.22 – 0.48 (m, 120H). 2.2.9. Synthesis of Compound X9IC Compound 7 (0.20 g, 0.15 mmol), 1, 1-dicyanomethylene-3-indanone (IC) (145.5 mg, 0.75 mmol), pyridine (1 mL) and chloroform (30 mL) were dissolved in a round bottom flask under nitrogen. The mixture was stirred at 70 °C overnight. After cooling to room temperature, the crude mixture was extracted with ethyl acetate and purified with column chromatography on silica gel using dichloromethane/petroleum 10

ether (2/1, v/v) as the eluent to give a black solid (176 mg, 70% yield). 1H NMR (400 MHz, CDCl3) δ 9.04 (s, 2H), 8.78 (d, J = 7.0 Hz, 2H), 8.44 (s, 2H), 8.04 (dd, J = 10.8, 4.0 Hz, 2H), 7.91 – 7.79 (m, 4H), 5.35 (s, 2H), 4.64 (d, J = 9.8 Hz, 2H), 4.46 (d, J = 23.3 Hz, 6H), 4.24 (d, J = 24.4 Hz, 2H), 2.28 – 2.13 (m, 2H), 2.04 – 1.57 (m, 22H), 1.54 – 1.37 (m, 14H), 1.35 – 0.76 (m, 40H), 0.54 (s, 4H), 0.40 – 0.10 (m, 8H). Elemental analysis for [C100H114N8O6S5], calculated: C, 71.31%; H, 6.82%; N, 6.65%; O, 5.70%; S, 9.52%; found: C, 71.18%; H, 6.91%; N, 6.78%; O, 5.62%; S, 9.51%. ESI-TOF-MS: m/z [M+] calcd for [C100H114N8O6S5], 1683.7498; found: 1683.7549. 2.2.10. Synthesis of Compound X94FIC By employing the similar procedure as used for X9IC, compound 7 (0.20 g, 0.15 mmol) and 2FIC (172.5 mg, 0.75 mmol) were used for the synthesis of X94FIC. The title compound was obtained as a black solid (184 mg, 70%). 1H NMR (400 MHz, CDCl3) δ 8.99 (s, 2H), 8.58 (dd, J = 9.7, 6.5 Hz, 2H), 8.41 (s, 2H), 7.82 – 7.71 (m, 2H), 5.31 (s, 2H), 4.60 (d, J = 9.3 Hz, 2H), 4.42 (d, J = 23.6 Hz, 6H), 4.19 (d, J = 23.6 Hz, 2H), 2.16 (d, J = 19.0 Hz, 2H), 2.01 – 1.48 (m, 30H), 1.31 – 0.91 (m, 38H), 0.80 (dd, J = 18.1, 6.7 Hz, 8H), 0.49 (s, 4H), 0.17 (dd, J = 43.0, 37.8 Hz, 8H). Elemental analysis for [C100H110F4N8O6S5], calculated: C, 68.39%; H, 6.31%; F, 4.33%; N, 6.38%; O, 5.47%; S, 9.13%; found: C, 68.22%; H, 6.43%; F, 4.41%; N, 6.19%; O, 5.39%; S, 9.36%. ESI-TOF-MS: m/z [M+] calcd for [C100H110F4N8O6S5], 1755.7122; found: 1755.8424. 2.2.11. Synthesis of Compound X9Rd By employing the similar procedure as used for X9IC, compound 7 (0.20 g, 0.15 mmol) and 2-(3-ethyl-4-oxo-thiazolidin-2-ylidene)-malononitrile (144.9 mg, 0.75 mmol) were used for the synthesis of X9Rd. The title compound was obtained as a dark red solid (182 mg, 75%).1H NMR (400 MHz, CDCl3) δ 8.29 (s, 2H), 7.97 (s, 2H), 5.32 (s, 2H), 4.63 (s, 2H), 4.39 (dd, J = 14.0, 6.9 Hz, 10H), 4.13 (s, 2H), 2.14 (s, 2H), 2.03 – 1.57 (m, 20H), 1.47 (t, J = 6.9 Hz, 22H), 1.29 – 0.74 (m, 40H), 0.52 (s, 4H), 0.19 (dd, J = 43.1, 37.0 Hz, 8H). Elemental analysis for [C92H116N10O6S7], calculated: 11

C, 65.68%; H, 6.95%; N, 8.33%; O, 5.71%; S, 13.34%; found: C, 65.77%; H, 7.08%; N, 8.21%; O, 5.75%; S, 13.19%. ESI-TOF-MS: m/z [M+H+] calcd for [C92H116N10O6S7], 1681.7208; found: 1681.8329. 2.2.12. Synthesis of Compound X9T4FIC By employing the similar procedure as used for X9IC, compound 9 (150.0 mg, 0.09 mmol) and 2F-IC (100.0 mg, 0.44 mmol) were used for the synthesis of X9T4FIC. The title compound was obtained as a dark red solid (149 mg, 80%).1H NMR (400 MHz, CDCl3) δ 8.86 (s, 2H), 8.59 (dd, J = 9.9, 6.5 Hz, 2H), 7.90 (s, 2H), 7.80 – 7.70 (m, 4H), 5.31 (s, 2H), 4.65 (s, 2H), 4.39 (d, J = 15.2 Hz, 6H), 4.15 (s, 2H), 2.97 (s, 4H), 2.19 (d, J = 6.1 Hz, 2H), 2.01 – 1.63 (m, 20H), 1.51 – 1.04 (m, 56H), 1.03 – 0.82 (m, 30H), 0.55 (s, 4H), 0.17 (dd, J = 12.4, 5.8 Hz, 8H). Elemental analysis for [C124H146F4N8O6S7], calculated: C, 69.43%; H, 6.86%; F, 3.54%; N, 5.22%; O, 4.48%; S, 10.46%; found: C, 69.32%; H, 6.94%; F, 3.41%; N, 5.38%; O, 4.39%; S, 10.56%. ESI-TOF-MS: m/z [M+] calcd for [C124H146F4N8O6S7], 2143.9380; found: 2143.9366. 2.3. Fabrication of organic solar cells The ITO glass with the sheet resistance of ~15 Ω·sq-1 was divided into squares with a side length of 15 mm. The ITO substrate was sonicated for 20 min in detergent, water, acetone, and isopropanol, respectively. Nitrogen blowing was used to remove dust on the dried ITO substrate. Then, ITO substrate was treated with ultraviolet ozone for 20 min. The PEDOT: PSS (Clevios P VP 4083) was spin-coated on ITO to form a thin film as hole transport layer under 3000 rpm, and then annealed at 130 °C for 20 min. The active layer materials were dissolved in chloroform (14 mg/mL), and PDINO was dissolved in methanol (1.5 mg/mL) stirred overnight. After the material was completely dissolved in chloroform, the solution was spin-coated on the PEDOT: PSS film with the thickness of the active layer about 100 nm. Next, a PDINO solution (1.5 mg/mL) was spin-coated on the active layer at 3000 rpm. Finally, an Al layer with a thickness of 100 nm was evaporated on the active layer under a vacuum 12

pressure of 5×10-5 Pa, and the effective area of the device was 4 mm2. In the glove box filled with N2, the J-V characteristics, EQE and other photoelectric properties of the devices were investigated.

3. Results and discussion 3.1. Synthesis and thermogravimetric All synthetic routes are shown in Scheme 1. In order to obtain the target acceptor molecules, a series of routine reactions were applied, such as Stille coupling, Suzuki reaction, and Knoevenagel condensation reaction. First, one side of the BDT was protected with trimethylsilane and then a trimethylstannane on the other side. The compound 3 was coupled with 4,7-dibromo-5,6-dinitrobenzo[c] [1,2,5]thiadiazole, and then subjected to an N-hybrid ring reaction. The obtained unpurified compound 5 was directly connected to an isooctyl chain at the N atom. Subsequently, the compound 6 was reacted with POCl3 and DMF to obtain the compound 7 with aldehyde groups. Finally, the SMAs were obtained by Knoevenagel condensation reaction with different end groups. The crude products synthesized at each step were purified by silica gel columns, recrystallization or sedimentation, then the pure compounds were dried in a vacuum oven. These compounds were finally identified by nuclear magnetic resonance (NMR) and time-of-flight mass spectrometry (TOF-MS). All 1H NMR and TOF-MS were listed in the Supporting Information (Fig. S1- Fig. S16). The thermal properties of the four SMAs were characterized by thermogravimetric analysis (TGA) were shown in Fig. S17. All SMAs showed high thermal stability up to 300 °C. A 5% weight loss of X94FIC/X9IC/X9Rd/X9T4FIC were found at 330 °C, 328 °C, 331 °C and 318 °C, respectively, indicated that they can work normally and stably exist at work temperature. 3.2. Electrochemical properties

The density functional theory (DFT) theoretical calculation was performed to 13

understand the electron cloud distribution and molecular energy levels of SMAs. To simplify the calculation process, all long alkyl chains in SMAs were replaced by methyl groups. It can be seen from Fig. 1 that the fused-ring main chains of SMAs exhibited a planar conformation, which was conducive to molecular accumulation. The theoretical molecular energy levels HOMO/LUMO of X94FIC, X9IC, X9Rd, and X9T4FIC were -5.39 eV/-3.55 eV, -5.28 eV/-3.39 eV, -5.31 eV/-3.23 eV, and -5.18 eV/-3.42 eV, respectively. Then, the molecular energy levels of SMAs were determined by cyclic voltammetry (CV) (Fig. 2b), which used ferrocene/ferrocenium (Fc/Fc+) of the redox couple (4.8 eV below the vacuum level) as the internal calibration. The molecular energy levels HOMO/LUMO of X94FIC, X9IC, X9Rd and X9T4FIC determined by cyclic voltammetry were -5.58 eV/-4.17 eV, -5.53 eV/-4.06 eV, -5.51 eV/-3.80 eV, and -5.51 eV/-4.10 eV, respectively. Although the molecular energy level values obtained by the two methods are different, the same trend of change was showed. The energy level of X9IC increased relative to X94FIC, and X9Rd possesses the widest band gap of 1.71 eV, X94FIC and X9T4FIC possess the narrowest band gap of 1.41 eV among SMAs (Table 1).

Fig. 1. DFT-calculated frontier molecular orbitals of the methyl substituted SMAs of X94FIC, X9IC, X9Rd and X9T4FIC.

14

3.3. Optical properties The UV-visible absorption spectra of X94FIC, X9IC, X9Rd, and X9T4FIC in films were shown in Fig. 2a. Obviously, SMAs in the films showed two absorption bands at 400-1000 nm. The maximum absorption peaks of X94FIC, X9IC, X9Rd, and X9T4FIC were 783, 761, 620 and 743 nm, respectively. End-group 2FIC had stronger electron withdrawing ability than end-group IC and RCN, so X94FIC showed red shift in the films absorption. End group RCN has the weakest electron withdrawing ability, and the light absorption of films showed a significant blue shift. Relative to in solutions, the absorption of all SMAs films showed a red shift (Fig. S18). In addition, the light absorption of the active layer films that after optimizing the donor/acceptor ratio and the active layer solution concentration was provided (Fig. S19). Although X9T4FIC and X9IC had similar absorption spectral ranges, it can be seen from the light absorption of the blend films that the light absorption intensity of X9T4FIC was significantly weaker than X9IC. The absorption intensity of SMAs is significantly smaller than that of the donor component PBDB-T, which was a major reason for the lower short-circuit current densities (Jsc) of the devices. Table 1. Energy levels and optical band gaps of SMAs.

λmax/sol.

λmax/film Egopt(eV) HOMO(eV) LUMO(eV) Egec(eV)

SMAs (nm)

(nm)

X94FIC

749

783

1.25

-5.58

-4.17

1.41

X9IC

723

761

1.29

-5.53

-4.06

1.47

X9Rd

608

620

1.45

-5.51

-3.80

1.71

X9T4FIC

689

743

1.29

-5.51

-4.10

1.41

15

Fig. 2. (a) UV-Vis absorption spectrum of SMAs in films, (b) cyclic voltammetry of SMAs in acetonitrile with 0.1 M Bu4NPF6 at a potential scan rate of 100 mV s −1, (c) chemical structures of PBDB-T and PDINO, (d) energy levels of donor and acceptors.

3.4. Photovoltaic properties Combining the absorption spectra and molecular energy levels of SMAs, PBDB-T was selected as the donor component of the active layer to fabricate OSCs (Fig. 2c and 2d). The device structure that chose to test photovoltaic performance was ITO/PEDOT: PSS (Clevios P VP 4083)/donor: SMAs/PDINO (perylene diimide functionalized with amino N-oxide)/Al type sandwich structure. First, PBDB-T and X94FIC as the active layer materials was used to fabricate OSCs with different D/A ratios and different active layer solution concentrations, and tried different spin coating speeds and annealing temperatures (Fig. S20 and S21, Table S1 and S2). Finally, an optimal condition was D/A ratio of 1:1, solution concentration of 14 mg/mL, spin coating speed of 3000 rpm, and annealing temperature of 100 °C were found. The amount of additive chloronaphthalene (CN) was 0.5% of the volume of active layer solution. The highest OSCs PCE under the optimal conditions was 7.08%. 16

Then, other photovoltaic devices of SMAs were tested under the same conditions. The PCE of devices with the active layer D/A of PBDB-T:X9IC, PBDB-T:X9Rd and PBDB-T:X9T4FIC under optimal conditions were 6.29%, 2.48%, and 3.22%, respectively. The specific parameters was shown in Table 2. The J-V curves are shown in Fig. 3a. The Jsc of the devices by using X94FIC/X9IC/X9Rd/X9T4FIC as acceptor were 14.67 mA/cm2, 11.40 mA/cm2, 6.70 mA/cm2 and 7.01 mA/cm2, and the open-circuit voltages (Voc) were 0.73 V, 0.86 V, 1.08 V and 0.85 V, respectively (Table 2). The voltage was proportional to the difference between the HOMO value of PBDB-T and the LUMO value of SMAs. At the same time, the external quantum efficiency (EQE) of the four devices were tested as shown in Fig. 3b. The integral value of the EQE curve matched well with Jsc values from J-V curves. Compared to the other three SMAs, PBDB-T: X94FIC-based OSCs exhibited a broader photo-response coverage of 300-1000 nm. In addition, PBDB-T: X9IC/X9T4FIC-based OSCs also exhibited an excellent photo-response of 300-900 nm. But its photo-response intensity of acceptor component was significantly weaker than the donor component. This phenomenon was consistent with the measured light absorption phenomenon of the blend films, which indicated that although the SMAs had a wide photo-response, their intensity was weaker, resulting in lower Jsc and PCE of OSCs.

Fig. 3. (a) J-V curves of PBDB-T: acceptors-based devices, (b) EQE curves of the PBDB-T: acceptors-based devices.

17

Table 2. Photovoltaic properties and the charge carrier mobility of PBDB-T: SMAs-based devices.

Voc

Jsc

FF

µe

PCE

µh µe/µh

Active layer -2

2

-1 -1

2

-1 -1

(V)

(mA cm )

(%)

(%)

(cm V s ) (cm V s )

PBDB-T:X94FIC

0.73

14.67

66.1

7.08

6.10×10-4

1.49×10-4

4.09

PBDB-T:X9IC

0.86

11.40

63.9

6.29

6.18×10-4

1.41×10-4

4.38

PBDB-T:X9Rd

1.08

6.70

34.1

2.48

6.58×10-4

0.64×10-4

10.28

PBDB-T:X9T4FIC

0.85

7.01

54.1

3.22

5.16×10-4

1.04×10-4

4.96

Moreover, the dependence of Jsc on illumination light intensity (Plight) was characterized to explain the relationship between charge transport and bimolecular recombination in the devices, this is generally expressed by

[42, 43]. As

show in Fig. 4a, the slope (α) of PBDB-T: X94FIC/X9IC/X9Rd/X9T4FIC-based devices were 0.992, 0.995, 0.930 and 0.994, respectively. When the value of α is equal to 1, the recombination in the blend films are negligible basically, so the bimolecular recombination in the PBDB-T: X94FIC/X9IC/X9T4FIC-based blend films were very small, while the PBDB-T: X9Rd-based blend film had a large reorganization. The photocurrent density-effective voltage (Jph-Veff) characteristic curves as shown in Fig. 4b. Under the short-circuit conditions, the Jph/Jsat

Fig. 4. (a) Light intensity dependence of Jsc of the optimized devices, (b) Jph versus Veff plot. 18

value of the PBDB-T:X94FIC/X9IC/X9Rd/X9T4FIC-based devices were 95%, 93%, 77% and 86%, respectively, indicated that the X94FIC-based device had the highest exciton dissociation and charge collection efficiency, while X9Rd-based device had the lowest exciton dissociation and charge collection efficiency. As a result, it can be seen from the above characterization that X94FIC-based devices had higher FF and Jsc. 3.5. Charge transport mobility The hole (µh) and electron mobility (µe) of blend films of PBDB-T: X94FIC/X9IC/X9Rd/X9T4FIC-based were investigated via SCLC method and the J-V curves were shown in Fig. 5. The data extracted from the J-V curve, and then

(V − V ) 2 9 calculated by the equation [44, 45] J = ε 0ε r µ appl 3 bi to obtain the carrier 8 L mobility

were

shown

in

Table

2.

The

hole

mobility

of

PBDB-T:

X94FIC/X9IC/X9Rd/X9T4FIC-based devices were 1.49 × 10-4, 1.41×10-4, 0.64×10-4 and 1.04×10-4 cm2 V-1 s-1, respectively. The electron mobility of PBDB-T: X94FIC/X9IC/X9Rd/X9T4FIC-based devices were 6.10×10-4, 6.18×10-4, 6.58×10-4 and 5.16×10-4 cm2 V-1 s-1, respectively. The corresponding ratio (µe/µh) are 4.09, 4.38, 10.28 and 4.96 respectively. The closer the ratio is to 1, the better it is for Jsc and FF. These results indicated that the Jsc and FF of PBDB-T:X94FIC-based devices were more balanced than other SMAs, and therefore had the best Jsc and FF.

Fig. 5. J-V curves of (a) electron-only and (b) hole-only devices based on PBDB-T: SMAs.

19

3.6. Film morphology Based on the above tests and analysis, in order to further analyzing the factors affecting device performance, the atomic force microscope (AFM) was used to study the morphology of the active layer. The root mean square (RMS) in the AFM height map (Fig. 6) was characterized the roughness of the surface of the blend films. The roughness

of

the

PBDB-T:X94FIC/X9IC/X9Rd/X9T4FIC-based

blend

films

respectively were 1.36 nm, 1.10 nm, 1.98 nm and 1.39 nm in the case of annealing. The roughness were 1.33 nm, 1.07 nm, 1.86 nm, and 1.32 nm in the case of non-annealing, respectively. It was obvious that although the roughness under annealing conditions were bigger than that under the as-cast conditions, the overall difference was not significant. It could be seen from the phase diagram (Fig. S22) that the active layer had an obviously fibrous network structure after TA treatment. Fiber shape micro-phase separation of active layer is beneficial for effective exciton charge separation and photo-generated charge transfer. Therefore, the X9Rd-based films exhibited the highest roughness morphology, while X94FIC-based films had better micro-phase separation than X9IC due to the influence of F atom, which was beneficial to PCE of OSCs.

Fig. 6. Atomic force microscope (AFM) height images of PBDB-T: SMAs blend films, TA treatment films of PBDB-T: X94FIC/X9IC/X9Rd/X9T4FIC-based (a, b, c, d), the as-cast films of PBDB-T: X94FIC/X9IC/X9Rd/X9T4FIC-based (e, f, g, h), (2×2 µm2).

20

Conclusions In summary, we designed and synthesized four small molecule acceptors X94FIC, X9IC, X9Rd, and X9T4FIC with different end groups under the same fused-ring core. A series of characterization and testing methods were used to analyze the thermogravimetric analysis, light absorption, photoelectric properties and film morphology of SMAs. Compared with the other three SMAs, X94FIC achieved the highest PCE of 7.08% after matching the donor PBDB-T. X94FIC obtained the highest Jsc due to its red-shifted absorption compared to other SMAs. Although X9Rd obtains the largest Voc, its absorption spectrum overlaps with PBDB-T, so Jsc was the lowest. Meanwhile, PBDB-T/X94FIC-based devices have excellent interpenetrating network structures, which was beneficial to improve PCE of device. Although the photo-response of SMAs could reach up to 1000 nm by analyzing the EQE curve and UV-visible light absorption spectrum, their weak light absorption intensity may be the main reason for the lower PCEs.

Acknowledgement This work was supported by the National Science Foundation of China (Grants Nos. 21875204, 21805236, 21574144, 51773212, 61705240 and 21674123), the National Science Fund for Distinguished Young Scholars (21925506), the National Key R&D Program of China (2017YFE0106000), Ningbo Municipal Science and Technology Innovative Research Team (2015B11002 and 2016B10005), CAS Interdisciplinary Innovation

Team,

CAS

Key

Project

of

Frontier

Science

Research

(QYZDBSSW-SYS030), Ningbo S&T Innovation 2025 Major Special Programme (2018B10055).

21

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27

Highlights A detailed

method

to

synthesize

nine

fused-ring

SMAs

based

on

benzodithiophene and benzothiadiazole. Systematic investigation of SMAs by CV, TGA, UV–vis spectra and so on. Performance comparison of SMAs based on the same fused-ring modified with different end-groups. SMAs based on nine fused-ring showed an excellent photo-response up to 1000 nm.

Conflicts of interest There are no conflicts to declare.