Enhancing the Performance of Organic Solar Cells by Hierarchically

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Enhancing the Performance of Organic Solar Cells by Hierarchically Supramolecular Self-Assembly of Fused-Ring Electron Acceptors Yahui Liu, Cai'e Zhang, Dan Hao, Zhe Zhang, Liangliang Wu, Miao Li, Shiyu Feng, Xinjun Xu, Feng Liu, Xuebo Chen, and Zhishan Bo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01319 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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Chemistry of Materials

Enhancing the Performance of Organic Solar Cells by Hierarchically Supramolecular Self-Assembly of Fused-Ring Electron Acceptors Yahui Liu,† Cai’e Zhang,† Dan Hao,† Zhe Zhang,† Liangliang Wu,† Miao Li,† Shiyu Feng,† Xinjun Xu,*,† Feng Liu,*,‖ Xuebo Chen,† Zhishan Bo*,† † Key Laboratory of Energy Conversion and Storage Materials, Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, P.R. China ‖Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai 200240, China ABSTRACT: Three novel nonfullerene small molecular acceptors ITOIC, ITOIC-F and ITOIC-2F were designed and synthesized with easy chemistry. The concept of supramolecular chemistry was successfully used in the molecular design, which includes noncovalently conformational locking (via intra-supramolecular interaction) to enhance the planarity of backbone and electrostatic interaction (inter-supramolecular interaction) to enhance the π-π stacking of terminal groups. Fluorination can further strengthen the inter-supramolecular electrostatic interaction of terminal groups. As expected, the designed acceptors exhibited an excellent device performance when blending with polymer donor PBDB-T. In comparison with the parent acceptor molecule DC-IDT2T reported in literature with a power conversion efficiency (PCE) of 3.93%, ITOIC with a planar structure exhibited a PCE of 8.87% and ITOIC-2F with a planar structure and enhanced electrostatic interaction showed a quite impressive PCE of 12.17%. Our result demonstrates the importance of comprehensive design in the development of high performance nonfullerene small molecular acceptors.

Organic solar cells (OSCs) are considered as one of the promising renewable energy conversion devices due to their low cost, light-weight and convenient roll-to-roll printing.1,2 Electron acceptor material is the key component in OSCs to achieve efficient photoelectric conversion. In the past decades, fullerene derivatives were dominantly used as the acceptors due to their good electron-transporting abilities. However, drawbacks of fullerene acceptors, such as low absorption coefficient and mismatching of energy levels with polymer donors, limit the improvement in power conversion efficiency (PCE) of OSCs. Recently, fused-ring electron acceptors (FREAs) have received extensive attention due to their tunable energy levels, broad and strong absorption, and low-cost synthesis.3-7 PCE of OSCs using FREAs was rapidly improved to larger than 10%, showing a promising commercialization prospect.8-17 Currently, in the aspect of molecular design, most of the reported FREAs focused on tuning the energy levels, light absorption properties, and charge transport behaviors through covalent modifications.18-21 Besides, there is a trendancy to use concepts of intramolecular or intermolecular interactions to affect supramolecular assembly. For example, Bo’s group and Huang’s group untilized intramolecular interactions such as S⋅⋅⋅O, O⋅⋅⋅H and S⋅⋅⋅N to design highefficiency FREAs;22,23 through theoritical calculations Yi et al. demonstrated that intermolecular interaction (via terminal π-π stacking) plays a dominant role in determining the chargetransport behavior of the acceptor-π-acceptor type FREA.24 In addition, inspired by the work of Zhang et al. who have used electrostatic interactions to fabricate supramolecular polymers,25 here we utilize suparmolecular interactions (including both intramolecular and intermolecular ineractions) to design FREAs for high-efficiency organic solar cells. The

chemical structures of the designed FREAs are shown in Scheme 1. Such FREAs are composed of a central ladder type planar aromatic core, two 3,4-bis(hexyloxy)-substituted 2,5thiophenylene spacers (bridging units), and two cyanoindanone type end-capping units. The central ladder type aromatic core bears four 4-hexylphenyl side chains, which can prevent the planar aromatic ladder structure to form too close stacking. The two 3,4-bis(hexyloxy)-substituted 2,5-thienylene units can form intramolecular noncovalent bonding with the thiophene units at the central core and the two terminal cyanoindanone units, locking the flat conformation of the aromatic backbone. Due to the electron-donating effect of alkoxyl side chain, bis(hexyloxy)-substituted thiophene can provide more stronger donor-acceptor (D-A) electrostatic interaction in comparison with the mono hexyloxy substituted thiophene unit. Moreover, the introduction of fluoro substitution on the two terminal cyanoindanone units can broaden the absorption of acceptor molecule and enhance the π-π stacking of flanking groups via intermolecular electrostatic interaction to facilitate the charge transportation. And the formation of flat conformation by conformational locking can reduce non-radiative energy loss to achieve higher open circuit voltage in devices.22 Three FREA molecules (ITOIC, ITOIC-F and ITOIC-2F) with various fluoro substituents were designed based on a parent compound DC-IDT2T (shown in Supporting Information).26 It is noteworthy that DC-IDT2T based photovoltaic devices only exhibited a moderate performance. However, ITOIC with 3,4-bis(hexyloxy)thiophene as the spacer instead of thiophene (DC-IDT2T) can achieve a PCE of 8.87% in ITOIC:PBDB-T based inverted devices. Since fluoro atom is strong electron withdrawing, the introduction of fluoro sub-

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Chemistry of Materials stituents at the terminal cyanoindanone unit can enhance the electrostatic interactions. As expected, the intermolecular stacking distance can be reduced, and the electron mobility of the blend film was seen to increase from 2.43×10-4 for ITOIC to 6.02×10-4 cm2 V-1 s-1 for ITOIC-2F. A quite impressive PCE of 12.17% with a short circuit current up to 21.04 mA/cm-2 was achieved for ITOIC-2F based solar cells. The synthetic routes of ITOIC, ITOIC-F and ITOIC-2F are shown in Scheme 1. The starting material 1 was purchased

and used as received. Compounds ThO6, 2, 3, and 4 were synthesized according to reported procedures.12,27 IDT-CHO was synthesized by Stille coupling of ThO6 and 1 with Pd(PPh3)4 as the catalyst precursor in a yield of 89%. Knoevenagel condensation of IDT-CHO with different terminal groups (2, 3 and 4) afforded ITOIC, ITOIC-F and ITOIC-2F in yields of about 60%. The detailed syntheses are described in supporting information.

Scheme 1. Reagents and conditions: i) Pd(PPh3)4, toluene, reflux; ii) pyridine, CHCl3, room temperature. (a) 1.0

Normalized absorption

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(b)

ITOIC (s) ITOIC (f) ITOIC-F (s) ITOIC-F (f) ITOIC-2F (s) ITOIC-2F (f)

0.8 0.6

ITOIC (s) ITOIC-F (s) ITOIC-2F (s)

0F

to

2F

ITOIC (f) ITOIC-F (f) ITOIC-2F (f)

32nm(0.07eV) 23nm(0.05eV)

12nm(0.03eV)

0.4 0.2 0.0 400

500

600

700

800

900

1000

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Wavelength (nm)

735

770

805

Wavelength (nm)

Figure 1. (a) UV-vis absorption of ITOIC, ITOIC-F and ITOIC-2F in chloroform solutions and as thin films. (b) The enlarged version of absorption peaks in (a).

UV-vis absorption spectra of ITOIC, ITOIC-F and ITOIC-2F in CHCl3 solutions and thin films are shown in Figure 1a. In solutions, ITOIC, ITOIC-F and ITOIC-2F exhibited a broad absorption in the region of 600 to 900 nm with the absorption maximum located at 722, 732 and 737 nm, respectively. Besides, as shown in Figure S1, the maximum molar extinction coefficients of ITOIC, ITOIC-F and ITOIC-2F in chloroform solutions were 1.46×105, 1.58×105 and 1.69×105 M-1cm-1, respectively. In going from solution to films, as shown in Figure 1b, the absorption became broader and the absorption maximum of ITOIC, ITOIC-F and ITOIC-2F was red-shifted to 734, 755 and 769 nm, respectively. Time-dependent density functional theory (TDDFT) calculations can provide a possible explanation of this phe-

nomenon. The wavelengths of the monomer and dimer model, excitation energies (∆E, eV), oscillator strengths (f) and transition contributions calculated with TDDFT at the CAMB3LYP/6-31G* level were listed in Table S1. The absorption wavelength of dimer model exhibits an obviously red-shift compared with that of monomer. Therefore, the red-shift of absorption in going from solution to film can be ascribed to the co-facial stacking in the solid state. From solution to film, the redshift is about 12 nm for ITOIC, 23 nm for ITOIC-F, and 32 nm for ITOIC-2F, corresponding to energy shifts of 0.03 eV, 0.05 eV and 0.07eV. The value of redshift is associated with π-π interaction of the terminal groups. Thus, fluorination at the end group can enhance the co-facial stacking and the electron communication. According to the absorption as

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thin films, the onset wavelength (λonset) of ITOIC, ITOIC-F and ITOIC-2F are 800, 827 and 855 nm, respectively. The optical band gap of ITOIC, ITOIC-F and ITOIC-2F was calculated to be 1.55, 1.50 and 1.45 eV, respectively, according to the equation: Egopt = 1240/λonset. Their electrochemical properties were investigated by cyclic voltammetry (CV). According to the equation: EHOMO/LUMO = -e(Eonset, ox/red + 4.71 eV),28 the highest occupied molecular orbital (HOMO) and LUMO energy levels were calculated and listed in Table S2. The CV curves and energy band diagram are shown in Figure S1. The proposed hierarchically supramolecular self-assembly of these acceptor molecules is also investigated by density functional theory (DFT) calculations. As shown in Figure S2, ITOIC, ITOIC-F and ITOIC-2F exhibit small dihedral angles about 3o between the terminal groups and 2,5-thienylene bridge units and 4o between 2,5-thienylene bridge units and central IDT unit. The introduction of 3,4-bis(hexyloxy)substituted 2,5-thienylene units (DOT) allows the formation of noncovalently conformational locking via S-O and O-H interaction.22,29,30 The HOMO and the LUMO of these molecules were also estimated by DFT calculation, which decreased when F atom was added, agreed well with CV meas-

urements. Dimer packing model was constructed similar to reported ITIC terminal stacking model.24,31 DFT calculations at the B3LYP-D3/6-31G(d) level were performed to optimize the structure model to investigate intermolecular π-π interactions. Taking ITOIC-2F as an example, it can be divided into three segments: IDT core, 3,4-bis(hexyloxy)-substituted 2,5thienylene units (DOT) unit and terminal group (2F-IC). The aromatic side chain in IDT core would prevent a full co-facial stacking, and π-π interaction could only happen through terminal groups. The initial guess was set by overlapping 2F-IC groups (state 1), and then dimer slid in parallel to find the energy minimum. Under DFT optimization, state 4 exhibited the lowest energy in which 2F-IC overlapped with DOT. State 2 and State 3 display intermediate energy with the slide of dimers. This result is quite reasonable since the electron donating DOT units was favorably interaction with electron accepting 2F-IC segments through quadrapole interactions.32-35 It should be noted that the introduction of F atoms in the acceptor unit can enhance intermolecular interaction due to its strong electron-withdrawing ability. And in DFT calculation, the co-facial stacking distance reduces when F atom is presented, which was 3.44 Å for ITOIC, 3.24 Å for ITOIC-F and 3.15 Å for ITOIC-2F.

Figure 2. Stacking modes of two neighboring molecules of ITOIC-2F via DFT calculation.

(a)

(b) 75

0

ITOIC ITOIC-F ITOIC-2F

-5

60 EQE (%)

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-10 -15

45 30

ITOIC ITOIC-F ITOIC-2F

15 -20 0.0

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0 300

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500 600 700 800 Wavelength (nm)

Figure 3. J-V and EQE curves of ITOIC, ITOIC-F and ITOIC-2F based devices. Table 1. Photovoltaic Parameter of ITOIC, ITOIC-F and ITOIC-2F Based Devices. Device ITOIC

Voc (V)

a

ITOIC-Fb ITOIC-2F

b

Jsc (mA/cm2)

FF (%)

PCE (%)

1.024±0.001

15.73±0.41 (15.71)

c

55.1±0.5

8.87±0.19 (8.79)d

0.946±0.002

18.60±0.18 (18.00)c

60.5±0.7

10.65±0.11 (10.49)d

0.897±0.007

c

64.5±1.1

12.17±0.17 (11.88)d

21.04±0.30 (20.32)

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Annealed at 140 °C for 5 min; b Annealed at 160 °C for 5 min; c Calculated by EQE measurement; d Average PCE of 10 devices.

Figure 4. GIWAXS pattern of as cast thin films of ITOIC family acceptors and BHJ thin films with thermal annealing (a); line-cut profile of neat (b) and BHJ (c) thin films (solid-line: out-of-plane; dotted-line: in-plane). .

Bulk-heterojunction solar cells were fabricated with an inverted structure of ITO/ZnO (30 nm)/active layer (100 nm)/MoO3 (8 nm)/Ag (100 nm). The ZnO layer was prepared as pervious reported36 and the active layer is composed of PBDB-T (shown in SI) and FREA (ITOIC, ITOIC-F or ITOIC-2F). The devices were optimized in various parameter spaces with details (Table S3-S8). The optimized device parameters are summarized in Table 1. The current densityvoltage (J-V) and external quantum efficiency (EQE) curves are shown in Figure 3. ITOIC based devices gave a PCE of 8.87% with an open circuit voltage (Voc) of 1.02 V, a current density (Jsc) of 15.73 mA/cm-2, and a fill factor (FF) of 55.1%. In comparison, ITOIC-F based devices gave a PCE of 10.65% with a decreased of Voc 0.95 V, a higher Jsc of 18.60 mA/cm-2, and a higher FF of 60.5%. With the increase of the number of F atoms, ITOIC-2F based devices show a high PCE of 12.17% with much improved Jsc and FF (21.04 mA/cm-2 and 64.5%). F atoms can enhance the intermolecular donor-acceptor interaction and lower the optical bandgap of small molecules, benefitting for good charge transport and broad photon to current response (Figure 3b). The calculated Jsc from EQE curve is 18.00 mA/cm-2 and 20.32 mA/cm-2, which were within 5% error relative to those obtained from JV curve. Solid state packing of neat acceptor film and BHJ blends were studied by grazing incidence x-ray wide-angle scattering (GIWAXS). Acceptor neat films without thermal annealing showed quite strong crystallinity as seen from diffraction peaks. They all take face-on orientations as seen from the π-π stacking and (100) lamellae stacking directions. The (100) peaks for all three molecules are all located around 0.31 Å-1, corresponding to a distance of 20.2 Å. However, the crystal

coherence length (CCL) increases with F atom addition, which is 145.7 Å for ITOIC, 330.5 Å for ITOIC-F and 369.8 Å for ITOIC-2F. The π-π stackings for ITOIC, ITOIC-F and ITOIC-2F are at 1.730, 1.737 and 1.764 Å-1, corresponding to a distance of 3.63, 3.61 and 3.56 Å, and the CCLs are 27.3, 32.2 and 33.9 Å, respectively. Such results well demonstrated the success of our molecular design, of using conformation locker and fluorine atom substitution to enhance electrostatic interactions and thus π-π stacking. It should be noted that thermal annealing could further enhance structure order, and the tiny peak at 0.445 Å that represents another primitive unit cell vector becomes clear and sharp in in-plane direction with a CCL of ~320 Å for ITOIC-2F. ITOIC family acceptors also possess good structure order in BHJ blends, as seen from Figure 4c. However, the lamellae stacking and π-π stacking details cannot be fully extracted since polymer diffraction features are similar and fully merged. Yet from diffraction peak shape, we can clearly see an order improvement from ITOIC to ITOIC-2F based blends. Blended thin film morphology was investigated by transmission electron microscope (TEM) and atomic force microscope (AFM). As shown in Figure S6 and S7, these three FREAs show similar film morphology with fibrils of the polymer donor (PBDB-T) surrounded by the acceptor-rich phase. The diameter of the semi-crystalline polymer fibers were about 10-20 nm of the blend films which is among the suitable region (4.5-30 nm) beneficial for the generation and splitting of excitons.37,38 Such a fibril network structure with good transport properties help to boost device performances. The surface of BHJ thin films was also dominated by a fibrillar texture, and thus polymer network is thoroughly distributed inside BHJ thin films.

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Chemistry of Materials To further understand effect of fluoro substitution at the end group on the photovoltaic performance, hole and electron mobilities were measured by using space charge limited current (SCLC) method with device structure of ITO/PEDOT:PSS (30 nm)/active layer(100 nm)/Au (100 nm) and FTO/active layer (100 nm)/Al (100 nm), respectively. The hole and electron mobility of ITOIC based devices were measured to be 3.69×10-4 and 2.43×10-4 cm2V-1s-1 (see Figure S8 and Table S9). ITOIC-F based devices exhibited a higher hole and electron mobility of 5.32×10-4 and 4.92×10-4 cm2V-1s-1. In contrast, the charge mobility based on ITOIC-2F further promoted to 6.03×10-4 and 6.02×10-4 cm2V-1s-1 for hole and electron, respectively. The largely increased electron mobilities in fluorinated acceptors can be attributed to the more efficient intermolecular connectivity. The µh/µe of ITOIC, ITOIC-F and ITOIC-2F was 1.52, 1.08 and 1.00, respectively. These more and more balanced hole and electron mobility is beneficial for higher FF, which is the same tendency as the measurement of devices. Moreover, as shown in Figure S8, the electron mobilities of ITOIC, ITOIC-F and ITOIC-2F neat films were also measured to be 2.49×10-5, 6.53×10-5 and 1.27×10-4 cm2V-1s-1 with the above-mentioned device structure. Although the electron mobilities of neat films were slightly lower than those of the corresponding blend films, the trend is consistent In summary, three FREAs with the A-D1-D2-D1-A type structure were carefully designed by the aid of the concept of noncovalently conformational locking (to extend the molecular planarity) and intermolecular D-A interaction (to enhance the connectivity among molecules). Accordingly, an efficiently intermolecular connectivity through the local π-π stacking of the terminal D1-A units can be realized, which leads to an increased electron mobility. Meanwhile, the π-π stacking distance both by calculated and GIWAXS measurements was seen to decrease with increasing the electron-withdrawing ability of the A unit, arising from the enhanced intermolecular electrostatic interactions. As a result, the electron mobility of the polymer donor:FREA blend film was largely promoted, which resulted in an excellent photovoltaic performance. Even more intriguing, ITOIC-2F exhibits a PCE of 12.17% with a Jsc of 21.04 mA/cm-2. Our results indicate that choosing appropriate units to obtain D-A enhanced interaction for increasing the intermolecular connectivity is an effective way to improve the photovoltaic performance of nonfullerene organic solar cells.

ASSOCIATED CONTENT Supporting Information. Experimental details, DFT calculations, CV, TGA, AFM and TEM image, SCLC measurements and OPV fabrication and measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

[email protected] [email protected] * [email protected] *

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

Financial support from the National Natural Science Foundation of China (21574013 and 51673028) and Program for Changjiang Scholars and Innovative Research Team in University is gratefully acknowledged.

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Chain Substitution on the Photovoltaic Performance. J. Mater. Chem. A 2017, 5, 7776-7783. (18) Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Ade, H.; Hou, J. Significant Infulence of the Methoxyl Substitution Position on Optoelectronic Properties and Molecular Packing of Small-Molecule Electron Acceptors for Photovoltaic Cells. Adv. Energy Mater. 2017, 7, 1700183. (19) Wang, W.; Yan, C.; Lau, T. K.; Wang, J.; Liu, K.; Fan, Y.; Lu, X.; Zhan, X. Fused Hexacyclic Nonfullerene Acceptor with Strong Near-Infrared Absorption for Semitransparent Organic Solar Cells with 9.77% Efficiency. Adv. Mater. 2017, 29, 1701308. (20) Yao, H.; Cui, Y.; Yu, R.; Gao, B.; Zhang, H.; Hou, J. Design, Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra-Narrow Band Gap. Angew. Chem. Int. Edit. 2017, 129, 3091-3905. (21) Yang, F.; Li, C.; Lai, W.; Zhang, A.; Huang, H.; Li, W. Halogenated Conjugated Molecules for Ambipolar Field-Effect Transistors and Non-Fullerene Organic Solar Cells. Mater. Chem. Front. 2017, 1, 1389-1395. (22) Liu, Y.; Zhang, Z.; Feng, S.; Li, M.; Wu, L.; Hou, R.; Xu, X.; Chen, X.; Bo, Z. Exploiting Noncovalently Conformational Locking as a Design Strategy for High Performance Fused-Ring Electron Acceptor Used in Polymer Solar Cells. J. Am. Chem. Soc. 2017, 139, 3356-3359. (23) Ye, P; Chen, Y; Wu, J; Wu, X; Yu, S; Xing, W; Liu, Q; Jia, X; Peng, A; Huang, H. Wide Bandgap Small Molecular Acceptors for Low Energy Loss Organic Solar Cells. J. Mater. Chem. C 2017, 5, 12591-12596. (24) Han, G.; Guo, Y.; Song, X.; Wang, Y.; Yi, Y. Terminal π-π Stacking Determines Three-Dimensional Molecular Packing and Isotropic Charge Transport in an A-π-A Electron Acceptor for NonFullerene Organic Solar Cells. J. Mate. Chem. C 2017, 5, 4852-4857. (25) Liu, Y.; Yu, Y.; Gao, J.; Wang, Z.; Zhang, X. Water-Soluble Supramolecular Polymerization Driven by Multiple Host-Stabilized Charge-Transfer Interactions. Angew. Chem. Int. Edit. 2010, 49, 6576-7679. (26)Bai, H.; Wang, Y., Cheng, P.; Wang, J.; Wu, Y.; Hou, J.; Zhan, X. An Electron Acceptor Based on Indacenodithiophene and 1,1Dicyanomethylene-3-indanone for Fullerene-Free Organic Solar Cells. J. Mater. Chem. A 2015, 3, 1910-1914. (27) Cecconi, B.; Manfredi, N.; Ruffo, R.; Montini, T.; RomeroOcaña, I.; Fornasiero, P.; Abbotto, A. Tuning Thiophene-Based Phenothiazines for Stable Photocatalytic Hydrogen Production. ChemSusChem 2015, 8, 4216-4228.

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