Article Cite This: Chem. Mater. 2018, 30, 4307−4312
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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*,† †
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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 S Supporting Information *
ABSTRACT: Three novel non-fullerene 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 intrasupramolecular interaction) to enhance the planarity of backbone and electrostatic interaction (intersupramolecular interaction) to enhance the π−π stacking of terminal groups. Fluorination can further strengthen the intersupramolecular electrostatic interaction of terminal groups. As expected, the designed acceptors exhibited excellent device performance when blended with polymer donor PBDB-T. In comparison with the parent acceptor molecule DC-IDT2T reported in the 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 non-fullerene small molecular acceptors.
O
demonstrated that intermolecular interaction (via terminal π−π stacking) plays a dominant role in determining the charge transport behavior of the acceptor−π-acceptor type FREA.24 In addition, inspired by the work of Zhang et al., who used electrostatic interactions to fabricate supramolecular polymers,25 here we utilize supramolecular 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 2,5-thiophenylene spacers (bridging units), and two cyanoindanone type endcapping units. The central ladder type aromatic core bears four 4-hexylphenyl side chains, which can prevent the planar aromatic ladder structure from forming overly close stacking. The two 3,4-bis(hexyloxy)-substituted 2,5-thienylene units can form intramolecular noncovalent bonds with the thiophene units at the central core and the two terminal cyanoindanone units, locking the flat conformation of the aromatic backbone. Because of the electron-donating effect of the alkoxyl side chain, bis(hexyloxy)-substituted thiophene can provide more
rganic solar cells (OSCs) are considered as one of the promising renewable energy conversion devices because of their low cost, light weight, and convenient roll-to-roll printing.1,2 The electron acceptor material is the key component in OSCs for achieving efficient photoelectric conversion. In the past several decades, fullerene derivatives were dominantly used as the acceptors because of their good electron-transporting abilities. However, drawbacks of fullerene acceptors, such as a low absorption coefficient and mismatching of energy levels with polymer donors, limit the improvement in the power conversion efficiency (PCE) of OSCs. Recently, fused-ring electron acceptors (FREAs) have received extensive attention because of their tunable energy levels, broad and strong absorption, and low-cost synthesis.3−7 The PCE of OSCs using FREAs was rapidly improved to >10%, showing promising prospects for commercialization.8−17 Currently, with respect to 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 interactions to design high-efficiency FREAs; 22,23 through theoretical calculations, Yi et al. © 2018 American Chemical Society
Received: March 29, 2018 Revised: June 11, 2018 Published: June 11, 2018 4307
DOI: 10.1021/acs.chemmater.8b01319 Chem. Mater. 2018, 30, 4307−4312
Article
Chemistry of Materials Scheme 1a
a
Reagents and conditions: (i) Pd(PPh3)4, toluene, reflux; (ii) pyridine, CHCl3, room temperature.
Figure 1. (a) Ultraviolet−visible absorption of ITOIC, ITOIC-F, and ITOIC-2F in chloroform solutions and as thin films. (b) Enlarged version of absorption peaks in panel a.
V−1 s−1 for ITOIC-2F. A quite impressive PCE of 12.17% with a short circuit current of 21.04 mA/cm2 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 and 2−4 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 (2−4) afforded ITOIC, ITOIC-F, and ITOIC-2F in yields of ∼60%. The detailed syntheses are described in the Supporting Information. Ultraviolet−visible (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− 900 nm, with the absorption maxima 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−1 cm−1, respectively. In going from solution to films, as shown in Figure 1b, the absorption became broader and the absorption maxima of ITOIC, ITOIC-F, and
stronger donor−acceptor (D−A) electrostatic interaction in comparison with the interaction of the mono hexyloxysubstituted thiophene unit. Moreover, the introduction of a fluoro substituent on the two terminal cyanoindanone units can broaden the absorption of the acceptor molecule and enhance the π−π stacking of flanking groups via intermolecular electrostatic interaction to facilitate the charge transportation. The formation of a flat conformation by conformational locking can reduce nonradiative energy loss to achieve higher open circuit voltages in devices.22 Three FREA molecules (ITOIC, ITOIC-F, and ITOIC-2F) with various fluoro substituents were designed on the basis of a parent compound DC-IDT2T (shown in the Supporting Information).26 It is noteworthy that DC-IDT2T-based photovoltaic devices exhibited an only 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. Because the fluoro atom is a strong electron-withdrawing group, the introduction of fluoro substituents 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 cm2 V−1 s−1 for ITOIC to 6.02 × 10−4 cm2 4308
DOI: 10.1021/acs.chemmater.8b01319 Chem. Mater. 2018, 30, 4307−4312
Article
Chemistry of Materials
Figure 2. Stacking modes of two neighboring molecules of ITOIC-2F determined via DFT calculation.
Table 1. Photovoltaic Parameters of ITOIC-, ITOIC-F-, and ITOIC-2F-Based Devices device a
ITOIC ITOIC-Fb ITOIC-2Fb
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
1.024 ± 0.001 0.946 ± 0.002 0.897 ± 0.007
15.73 ± 0.41 (15.71)c 18.60 ± 0.18 (18.00)c 21.04 ± 0.30 (20.32)c
55.1 ± 0.5 60.5 ± 0.7 64.5 ± 1.1
8.87 ± 0.19 (8.79)d 10.65 ± 0.11 (10.49)d 12.17 ± 0.17 (11.88)d
Annealed at 140 °C for 5 min. bAnnealed at 160 °C for 5 min. cCalculated by EQE measurement. dAverage PCE of 10 devices.
a
Figure 3. J−V and EQE curves of ITOIC-, ITOIC-F-, and ITOIC-2F-based devices.
eV),28 the highest occupied molecular orbital (HOMO) and LUMO energy levels were calculated and are 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 of ∼3° between the terminal groups and 2,5-thienylene bridge units and ∼4° between 2,5-thienylene bridge units and the 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 interactions.22,29,30 The HOMO and the LUMO of these molecules were also estimated by DFT calculation, which decreased when the F atom was added, and agreed well with CV measurements. A dimer packing model similar to the reported ITIC terminal stacking model24,31 was constructed (Figure 2). 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, we can divide it into three segments: IDT core, 3,4bis(hexyloxy)-substituted 2,5-thienylene (DOT) unit, and terminal group (2F-IC). The aromatic side chain in the IDT core could prevent a full cofacial stacking, and π−π interaction
ITOIC-2F were red-shifted to 734, 755, and 769 nm, respectively. Time-dependent density functional theory (TDDFT) calculations can provide a possible explanation of this phenomenon. The wavelengths of the monomer and dimer model, excitation energies (ΔE, electronvolts), oscillator strengths (f), and transition contributions calculated with TDDFT at the CAM-B3LYP/6-31G* level are listed in Table S1. The absorption wavelength of the dimer model exhibits an obvious red-shift compared with that of the monomer. Therefore, the red-shift of absorption in going from solution to film can be ascribed to the cofacial stacking in the solid state. From solution to film, the red-shift is approximately 12 nm for ITOIC, 23 nm for ITOIC-F, and 32 nm for ITOIC-2F, corresponding to energy shifts of 0.03, 0.05, and 0.07 eV, respectively. The value of the red-shift is associated with π−π interaction of the terminal groups. Thus, fluorination at the end group can enhance cofacial stacking and electron communication. According to the absorption as thin films, the onset wavelengths (λonset) of ITOIC, ITOIC-F, and ITOIC-2F are 800, 827, and 855 nm, respectively. The optical band gaps of ITOIC, ITOIC-F, and ITOIC-2F were 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 4309
DOI: 10.1021/acs.chemmater.8b01319 Chem. Mater. 2018, 30, 4307−4312
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Chemistry of Materials
Figure 4. (a) GIWAXS patterns of as-cast thin films of ITOIC family acceptors and BHJ thin films with thermal annealing. Line-cut profiles of (b) neat and (c) BHJ thin films (solid lines, out of plane; dotted lines, in plane).
small molecules, benefiting good charge transport and broad photon to current response (Figure 3b). The calculated Jsc values from the EQE curve are 18.00 and 20.32 mA/cm2, which were within 5% error relative to those obtained from the J−V curve. Solid state packing of neat acceptor film and BHJ blends was 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) lamellar 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 upon addition of F atoms, 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, respectively, corresponding to distances of 3.63, 3.61, and 3.56 Å, respectively, and the CCLs are 27.3, 32.2, and 33.9 Å, respectively. Such results successfully 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 the in-plane direction with a CCL of ∼320 Å for ITOIC-2F. ITOIC family acceptors also possess good structural order in BHJ blends, as seen from Figure 4c. However, the lamellar stacking and π−π stacking details cannot be fully extracted because polymer diffraction features are similar and fully merged. However, from the diffraction peak shape, we can clearly see an order improvement from ITOIC- to ITOIC-2F-based blends. The blended thin film morphology was investigated by transmission electron microscopy (TEM) and atomic force
could happen through only terminal groups. The initial guess was set by overlapping 2F-IC groups (state 1), and then the dimer slid in parallel to find the energy minimum. Under DFT optimization, state 4 exhibited the lowest energy in which 2FIC overlapped with DOT. State 2 and state 3 display intermediate energies with the slide of dimers. This result is quite reasonable because the electron-donating DOT unit favorably interacted with electron-accepting 2F-IC segments through quadrupole interactions.32−35 It should be noted that the introduction of F atoms into the acceptor unit can enhance intermolecular interaction because of its strong electronwithdrawing ability. In the DFT calculation, the cofacial stacking distance decreases when the F atom is presented, which was 3.44 Å for ITOIC, 3.24 Å for ITOIC-F, and 3.15 Å for ITOIC-2F. Bulk-heterojunction solar cells were fabricated with an inverted ITO/ZnO (30 nm)/active layer (100 nm)/MoO3 (8 nm)/Ag (100 nm) structure. The ZnO layer was prepared as perviously reported,36 and the active layer is composed of PBDB-T (shown in the Supporting Information) and FREA (ITOIC, ITOIC-F, or ITOIC-2F). The devices were optimized in various parameter spaces with details (Tables S3−S8). The optimized device parameters are summarized in Table 1. The current density−voltage (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/cm2, and a fill factor (FF) of 55.1%. In comparison, ITOIC-F-based devices gave a PCE of 10.65% with a decreased Voc of 0.95 V, a higher Jsc of 18.60 mA/cm2, and a higher FF of 60.5%. With the increase in the number of F atoms, ITOIC-2F-based devices show a high PCE of 12.17% with much improved Jsc and FF values (21.04 mA/cm2 and 64.5%, respectively). F atoms can enhance the intermolecular donor−acceptor interaction and lower the optical band gap of 4310
DOI: 10.1021/acs.chemmater.8b01319 Chem. Mater. 2018, 30, 4307−4312
Article
Chemistry of Materials microscopy (AFM). As shown in Figures 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 diameters of the semicrystalline polymer fibers were approximately 10−20 nm of the blend films, which is a range within the suitable region (4.5−30 nm) that is beneficial for the generation and splitting of excitons.37,38 Such a fibril network structure with good transport properties helps to improve device performances. The surface of BHJ thin films was also dominated by a fibrillar texture, and thus, the polymer network is thoroughly distributed inside BHJ thin films. To further understand effect of fluoro substitution at the end group on the photovoltaic performance, hole and electron mobilities were measured by using the space charge limited current (SCLC) method with an ITO/PEDOT:PSS (30 nm)/ active layer (100 nm)/Au (100 nm) and FTO/active layer (100 nm)/Al (100 nm) device structures, respectively. The hole and electron mobilities of ITOIC-based devices were measured to be 3.69 × 10−4 and 2.43 × 10−4 cm2 V−1 s−1, respectively (see Figure S8 and Table S9). ITOIC-F-based devices exhibited higher hole and electron mobilities of 5.32 × 10−4 and 4.92 × 10−4 cm2 V−1 s−1, respectively. In contrast, the charge mobility based on ITOIC-2F further increased to 6.03 × 10−4 and 6.02 × 10−4 cm2 V−1 s−1 for the hole and electron, respectively. The largely increased electron mobilities in fluorinated acceptors can be attributed to the more efficient intermolecular connectivity. The μh/μe values of ITOIC, ITOIC-F, and ITOIC-2F were 1.52, 1.08. and 1.00, respectively. These more and more balanced hole and electron mobilities are beneficial for a 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 cm2 V−1 s−1, respectively, with the device structure mentioned above. 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 with 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 from both calculated and GIWAXS measurements was seen to decrease with an increase in 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 intriguingly, ITOIC-2F exhibits a PCE of 12.17% with a Jsc of 21.04 mA/cm2. 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 non-fullerene organic solar cells.
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Experimental details, DFT calculations, CV, TGA, AFM, and TEM images, SCLC measurements, and OPV fabrication and measurements (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Xinjun Xu: 0000-0002-0750-352X Xuebo Chen: 0000-0002-9814-9908 Zhishan Bo: 0000-0003-0126-7957 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21574013 and 51673028) and the Program for Changjiang Scholars and Innovative Research Team in University is gratefully acknowledged.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01319. 4311
DOI: 10.1021/acs.chemmater.8b01319 Chem. Mater. 2018, 30, 4307−4312
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DOI: 10.1021/acs.chemmater.8b01319 Chem. Mater. 2018, 30, 4307−4312
