Bis(naphthothiophene diimide)indacenodithiophenes as Acceptors

The ordering natures of IDT-NTI -2EH and -1HH in the neat thin films and in the bulk-heterojunction layer were investigated by two-dimensional grazing...
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Communication Cite This: Chem. Mater. 2017, 29, 9618-9622

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Bis(naphthothiophene diimide)indacenodithiophenes as Acceptors for Organic Photovoltaics Johan Hamonnet,† Masahiro Nakano,*,† Kyohei Nakano,‡ Hiroyoshi Sugino,† Kazuo Takimiya,*,†,§ and Keisuke Tajima‡ †

Emergent Molecular Function Research Group, RIKEN Center for Emergent Matter Science (CEMS), 2-1, Hirosawa, Wako, Saitama 351-0198, Japan ‡ Emergent Functional Polymers Research Team, RIKEN Center for Emergent Matter Science (CEMS), 2-1, Hirosawa, Wako, Saitama 351-0198, Japan § Department of Chemistry, Graduate School of Science, Tohoku University, 6-3, Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan S Supporting Information *

O

RyDI-based electron-deficient unit, we have recently reported the monothiophene-fused NDI, naphtho[2,3-b]thiophene diimide (NTI, Figure 1b), for organic semiconducting materials.24,25 The NTI unit can be integrated via its vacant thiophene α-position into π-extended systems with planar πstructures, which gives red-shifted absorption and π−π stacking in the solid state. As a model compound for NTI-based acceptors, we combined the NTI unit with indaceno[1,2-b:5,6b′]dithiophene (IDT), a frequently used core-unit in superior NFAs.8,9,25−31 In this work, we synthesized and characterized NTI-IDT-NTI-triads, namely IDT-NTI-2EH and IDT-NTI1HH,32 and utilized them as NFAs (Figure 1c). Compared to a NFA with a similar triad structure, PDI-IDT-PDI triad,33 IDTNTIs have a planar π-conjugated skeleton (Figure S1ab). Owing to the effective π-conjugation in the planar structures, IDT-NTIs exhibited red-shifted absorption of up to 780 nm. By combining with PBDB-T10,34,35 (Figure 1d) as a donor material with the complementary absorption, the resulting OPVs yielded promising PCEs as high as 9%, comparable to the best PCEs so far reported for RyDI-based OPVs.19 Scheme 1 shows the synthesis of IDT-NTIs. The NTI-units, 3a and 3b, were prepared from N,N′-unsubstituted NTI (1) (the synthesis is detailed in Supporting Information). The Nsubstituents, 2-ethylhexyl (2EH) or 1-hexylheptyl (1HH), were introduced on the unsubstituted imide N-positions of 1 by Mitsunobu reaction using readily available corresponding alcohols to afford 2a and 2b. The following bromination at the α-position of the fused-thiophene gave 3a and 3b, respectively. The target compounds, IDT-NTI-2EH and IDTNTI-1HH, were synthesized by Stille coupling reaction of 3a or 3b with 2,7-bis(trimethylstannyl)-4,4,9,9-tetrakis(4-hexylphenyl)-s-indaceno[1,2-b:5,6-b′]dithiophene.36 IDT-NTI-2EH and IDT-NTI-1HH were fully characterized by 1H and 13C NMR, IR, and high-resolution mass spectra (see Supporting Information). Note that the present 3-step synthesis of IDTNTIs featuring the alkylation on the NTI core enables facile molecular modifications of NTI-based acceptors. IDT-NTIs were soluble in common organic solvents, such as chloroform,

rganic photovoltaics (OPVs) have recently received considerable attention because of their unique advantages, such as lightweight, mechanical flexibility, and solutionprocessability.1−3 Fullerene-based acceptors have played an important role in the improvement of power conversion efficiency (PCE) of OPVs; in particular, OPVs based on [6,6]phenyl-C71-butyric acid methyl ester (PC71BM) and appropriate π-conjugated donor materials have shown PCEs higher than 11% in single junction solar cells.4,5 However, fullerene derivatives have several drawbacks, such as weak absorption in the visible region, poor chemical stability, and limited tunability of their chemical structures and electronic properties. As a consequence, nonfullerene acceptors (NFAs) is now emerging as an alternative. The performances of OPVs based on NFAs have been rapidly improved in recent years,6−9 and several small-molecular NFAs have been reported to afford OPVs with PCEs comparable to or higher than those of the best OPVs with fullerene derivatives.10−16 In the small-molecular NFAs, rylene diimide (RyDI, Figure 1a),17 e.g., perylene diimide (PDI) and naphthalene diimide (NDI), have been employed as a building unit due to their strong electron affinity and good thermal stability.16−23 In particular, the PCEs of the OPVs with PDI-based smallmolecular acceptors have already exceeded 9%.18−20 As a novel

Received: September 4, 2017 Revised: November 5, 2017 Published: November 7, 2017

Figure 1. Chemical structures of RyDIs (a), NTI (b), IDT-NTIs (c), and PBDB-T (d). © 2017 American Chemical Society

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DOI: 10.1021/acs.chemmater.7b03733 Chem. Mater. 2017, 29, 9618−9622

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1HH, its absorption edge was slightly blue-shifted compared with that of IDT-NTI-2EH (Δλedge ∼ 30 nm). The absorption band at 600−780 nm has the largest absorption coefficient (ε) (IDT-NTI-2EH: 6.6 × 104 cm−1, IDT-NTI-1HH: 6.2 × 104 cm−1) in the spectrum. Additionally, IDT-NTIs have another absorption band at 400−500 nm (IDT-NTI-2EH: ε = 4.0 × 104 cm−1, IDT-NTI-1HH: ε = 3.4 × 104 cm−1). The simulated absorption spectrum of the model compound of IDT-NTI reproduced the experimental results with the absorption at 732 nm (oscillator strength: f = 1.61) and at 449 nm (f = 1.12) (see Supporting Information, Figure S4). With these two absorption bands in the spectrum, IDT-NTIs can cover wide range of the solar spectrum. PBDB-T, which exhibits complementary absorption to IDTNTIs (Figure 2b) and favorable EHOMO/ELUMO, was chosen as a suitable donor material for fabrication of OPVs (Figure 3a).

Scheme 1. Synthetic Route to IDT-NTIs

chlorobenzene, and o-dichlorobenzene at room temperature. Their good thermal stability was confirmed by thermogravimetric (TG) measurements; the decomposition temperatures (5% weight loss) of IDT-NTI -2EH and -1HH were 466 and 411 °C, respectively (Figure S2). The HOMO and LUMO energy levels (EHOMO and ELUMO, respectively) of IDT-NTIs were estimated from the onset potentials of oxidation and reduction waves in the cyclic voltammograms (Figure 2a) recorded with their thin films

Figure 3. Schematic energy-levels diagrams of MoO3, IDT-NTIs, PBDB-T, and ZnO (a). J-V characteristics (b) and EQE spectra (c) of IDT-NTI-2EH:PBDB-T and IDT-NTI-1HH:PBDB-T based OPVs.

Their photovoltaic parameters with optimized solvent and donor/acceptor (D/A) weight ratio are summarized in Table 1 (the details of the optimization are shown in Table S1). Figure 3b,c shows the current density−voltage (J-V) curves and the EQE spectra, respectively. From Figure 3b, typical photovoltaic responses with relatively high open-circuit voltage (VOC), shortcircuit current density (JSC), and fill factor (FF) are confirmed. The spectra in Figure 3c cover a wide range of wavelength ca. 350−800 nm, in agreement with the absorption spectra of the active materials, which demonstrates the contribution of both donor and acceptor to the current generation. The maximum PCEs of the OPVs based on as-spun films of IDT-NTI-1HH:PBDB-T and IDT-NTI-2EH:PBDB-T were 5.05% and 6.26%, respectively. The OPVs with IDT-NTI1HH showed higher VOC (1.02 V) than those with IDT-NTI2EH (0.91 V) as expected from the higher ELUMO. On the other hand, the IDT-NTI-2EH-based devices showed a higher JSC, reflecting the red-shifted absorption of the acceptor material and the higher EQE (Figure 3c) than that of IDT-NTI-1HHbased devices (vide infra). These results indicate that the Nsubstituents in IDT-NTIs can influence the OPV properties. Another influence caused by the alkyl substituent was observed upon thermal annealing of the active layers (Table S2): The annealed OPVs with IDT-NTI-2EH (210 °C, 60 min.) showed enhanced PCEs of up to 9.07% with a JSC of 14.38 mA cm−2

Figure 2. Cyclic voltammograms (a) and absorption spectra in the thin-film state (b) of IDT-NTIs.

deposited on the working electrode. The estimated ELUMO of IDT-NTI -2EH and -1HH were −3.9 and −3.8 eV, respectively. The slightly higher ELUMO of IDT-NTI-1HH can be explained by the enhanced electron donating nature of the 1HH group, where the first carbon atom connecting to the imide nitrogen atom has two alkyl branches. A similar effect of the branching position of N-alkyl groups on ELUMO was observed in related molecules.37 This result indicates that the ELUMO of NTI-based acceptors can be controlled by modification of substituents on the imide nitrogen atoms. In contrast to the ELUMO, EHOMO of both IDT-NTIs were almost the same (EHOMO = −5.4 eV), which is consistent with the relatively small contribution of the NTI moieties to the HOMO (Figure S1c). Figure 2b shows the absorption spectra of IDT-NTIs in the thin-film state. The effective π-conjugation gave the red-shifted absorption of up to 780 nm compared with the absorption of NTI (∼510 nm). Reflecting the higher ELUMO of IDT-NTI9619

DOI: 10.1021/acs.chemmater.7b03733 Chem. Mater. 2017, 29, 9618−9622

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Chemistry of Materials Table 1. Photovoltaic Parameters of OPVs Based on IDT-NTI-2EH:PBDB-T and IDT-NTI-1HH:PBDB-T Acceptor IDT-NTI-2EH

a

IDT-NTI-1HHb a

Anneal

VOC (V)

JSC (mA cm−2)

FF

PCE (%)

w/o 210 °C, 60 min. w/o

0.91 ± 0.00 0.92 ± 0.00 1.01 ± 0.01

11.71 ± 0.06 14.43 ± 0.05 8.07 ± 0.22

0.58 ± 0.01 0.69 ± 0.00 0.59 ± 0.01

6.26c (6.21 ± 0.05) 9.07c (9.01 ± 0.06) 5.05c (4.83 ± 0.22)

Optimized D/A weight ratio was 1.3:1.0. bOptimized D/A weight ratio was 1.0:1.0. cMaximum value.

peaks assignable to the face-on stacking (0.31 Å−1 at ∼qz = 1.74 Å−1 and 0.066 Å−1 at qxy = 0.31 Å−1) were smaller than those of the nonannealed film (0.42 Å−1 at ∼qz = 1.71 Å−1 and 0.127 Å−1 at qxy = 0.30 Å−1). The better crystallinity could explain the higher JSC and FF of the annealed devices; as reported by Zusan et al. and Bernardo et al., dissociation of charge-transfer states is improved by the carrier delocalization, which can be correlated with an increased carrier mobility in well-ordered active materials.38,39 To confirm this hypothesis in the present devices, the mobilities of the annealed and the nonannealed films of IDT-NTI-2EH:PBDB-T were evaluated by the steadystate space-charge-limited current (SCLC) technique (Figure S7, Table S3). The electron mobility of the annealed film was 1.4 × 10−3 cm2 V−1 s−1, which is higher than that of the nonannealed one (2.4 × 10−4 cm2 V−1 s−1). Moreover, the exciton dissociation probability (Pdiss) in the OPV devices were estimated from the plot of photogenerated current density (Jph) versus effective voltage (Veff) (Figure S9).40 The annealed devices gave an improved Pdiss of 84% (nonannealed device: 73%). As a result, it can be confirmed that effective exciton dissociation in the annealed device gives the improved JSC and FF. Concerning the IDT-NTI-1HH:PBDB-T films, annealing caused a drastic change in crystallinity: from the amorphouslike nature (Figure 4d) to the well-ordered crystalline film, as confirmed by the many peaks both on the qz and qxy axes (Figure 4f). In opposition to the IDT-NTI-2EH:PBDB-T films, better ordering decreased the OPV performances. In order to understand this counterintuitive phenomena, the morphological properties of the blend films of both IDT-NTI-2EH:PBDBT and IDT-NTI-1HH:PBDB-T were investigated by transmission electron microscopy (TEM, Figure S10). The TEM images of the annealed film of IDT-NTI-2EH:PBDB-T (210 °C, 60 min.) showed more homogeneous morphology with a uniform nanostructure than that of as-cast film. In contrast, aggregates were observed in the TEM image of the annealed film of IDT-NTI-1HH:PBDB-T whereas such aggregates could not be observed in the image of the as-cast film. This morphological change of IDT-NTI-1HH:PBDB-T films could be directly correlated to the change of ordering observed in the 2D-GIXD. It can be associated with a 3D-like crystallite formation with a large-scale phase separation, which can disturb the effective charge carrier generation and transport in the OPVs. This explains the diminished photovoltaic properties of the annealed devices of IDT-NTI-1HH. It is worth mentioning that the N-substituents on the NTI unit also affect the ordering nature in the thin-film state, thereby the OPV properties. In summary, IDT-NTIs were synthesized and evaluated as acceptors in OPVs. IDT-NTIs have two absorption bands; one is at 600−780 nm, which is red-shifted compared with the absorption of NTI (∼510 nm) thanks to the planar πconjugated structure, and the other is at 400−500 nm. Owing to these two bands, IDT-NTIs can absorb a wide range of the solar spectrum. By combining with a donor polymer, PBDB-T, with complementary absorption the resulting OPVs yielded

and a FF of 0.69. In contrast, thermal annealing did not improve the photovoltaic properties of IDT-NTI-1HH-based OPVs (90, 120, 180 °C), and, after thermal annealing at 210 °C, they were drastically diminished (PCE < 1%). The ordering natures of IDT-NTI -2EH and -1HH in the neat thin films and in the bulk-heterojunction layer were investigated by two-dimensional grazing incidence X-ray diffraction (2D-GIXD, Figures 4 and S5). The 2D diffraction

Figure 4. 2D-GIXD diffraction images of the neat films of IDT-NTI2EH (a) and IDT-NTI-1HH (b), the nonannealed blend films of IDTNTI-2EH:PBDB-T (c) and IDT-NTI-1HH:PBDB-T (d), and the annealed blend films of IDT-NTI-2EH:PBDB-T and IDT-NTI1HH:PBDB-T (210 °C, 60 min.).

image of IDT-NTI-2EH shows peaks assignable to the face-on stacking (Figure 4a, ∼qz = 1.75 Å−1 and qxy = 0.33 Å−1); this ordering is beneficial for the carrier transport in OPVs with a vertical device structure. In contrast, IDT-NTI-1HH did not show any detectable peaks (Figure 4b). The lower ordering nature of IDT-NTI-1HH could be caused by the branching position in the 1HH groups close to the π-conjugated skeleton. This likely disturbs π−π stacking of IDT-NTI-1HH in the thinfilm state. As observed in Figure 4c,d, these ordering natures were maintained in the bulk-heterojunction layers, which could explain the higher JSC of the nonannealed OPVs based on IDTNTI-2EH:PBDB-T. After the thermal annealing (210 °C, 60 min.), the crystallinity in IDT-NTI-2EH:PBDB-T films was improved (Figure 4e): the full widths at half-maximum (FWHM) of the 9620

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morphology leads to over 11% efficiency for inverted small-molecule organic solar cells. Nat. Commun. 2016, 7, 13740. (6) Anthony, J. E. Small-Molecule, Nonfullerene Acceptors for Polymer Bulk Heterojunction Organic Photovoltaics. Chem. Mater. 2011, 23, 583−590. (7) Lin, Y.; Zhan, X. Non-fullerene acceptors for organic photovoltaics: an emerging horizon. Mater. Horiz. 2014, 1, 470−488. (8) Nielsen, C. B.; Holliday, S.; Chen, H.-Y.; Cryer, S. J.; McCulloch, I. Non-fullerene electron acceptors for use in organic solar cells. Acc. Chem. Res. 2015, 48, 2803−2812. (9) Liang, N.; Jiang, W.; Hou, J.; Wang, Z. New developments in non-fullerene small molecule acceptors for polymer solar cells. Mater. Chem. Front. 2017, 1, 1291−1303. (10) Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganäs, O.; Gao, F.; Hou, J. Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734−4749. (11) Yang, Y.; Zhang, Z.-G.; Bin, H.; Chen, S.; Gao, L.; Xue, L.; Yang, C.; Li, Y. Side-Chain Isomerization on an n-type Organic Semiconductor ITIC Acceptor Makes 11.77% High Efficiency Polymer Solar Cells. J. Am. Chem. Soc. 2016, 138, 15011−15018. (12) Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Mukherjee, S.; Ade, H.; Hou, J. Energy-Level Modulation of Small-Molecule Electron Acceptors to Achieve over 12% Efficiency in Polymer Solar Cells. Adv. Mater. 2016, 28, 9423−9429. (13) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148−7151. (14) Bin, H.; Gao, L.; Zhang, Z.-G.; Yang, Y.; Zhang, Y.; Zhang, C.; Chen, A.; Xue, L.; Yang, C.; Xiao, M.; Li, Y. 11.4% Efficiency nonfullerene polymer solar cells with trialkylsilyl substituted 2Dconjugated polymer as donor. Nat. Commun. 2016, 7, 13651. (15) Chen, S.; Liu, Y.; Zhang, L.; Chow, P. C. Y.; Wang, Z.; Zhang, G.; Ma, W.; Yan, H. A Wide-Bandgap Donor Polymer for Highly Efficient Non-fullerene Organic Solar Cells with a Small Voltage Loss. J. Am. Chem. Soc. 2017, 139, 6298−6301. (16) 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, 3356−3339. (17) Zhan, X.; Facchetti, A.; Barlow, S.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R.; Marder, S. R. Rylene and Related Diimides for Organic Electronics. Adv. Mater. 2011, 23, 268−284. (18) Fernández-Lázaro, F.; Zink-Lorre, N.; Sastre-Santos, A. Perylenediimides as non-fullerene acceptors in bulk-heterojunction solar cells (BHJSCs). J. Mater. Chem. A 2016, 4, 9336−9346. (19) Liu, J.; Chen, S.; Qian, D.; Gautam, B.; Yang, G.; Zhao, J.; Bergqvist, J.; Zhang, F.; Ma, W.; Ade, H.; Inganäs, O.; Gundogdu, K.; Gao, F.; Yan, H. Fast charge separation in a non-fullerene organic solar cell with a small driving force. Nat. Energy 2016, 1, 16089. (20) Meng, D.; Fu, H.; Xiao, C.; Meng, X.; Winands, T.; Ma, W.; Wei, W.; Fan, B.; Huo, L.; Doltsinis, N. L.; Li, Y.; Sun, Y.; Wang, Z. Three-Bladed Rylene Propellers with Three-Dimensional Network Assembly for Organic Electronics. J. Am. Chem. Soc. 2016, 138, 10184−10190. (21) Duan, Y.; Xu, X.; Yan, H.; Wu, W.; Li, Z.; Peng, Q. Pronounced Effects of a Triazine Core on Photovoltaic Performance-Efficient Organic Solar Cells Enabled by a PDI Trimer-Based Small Molecular Acceptor. Adv. Mater. 2017, 29, 1605115. (22) Liu, Y.; Zhang, L.; Lee, H.; Wang, H.-W.; Santala, A.; Liu, F.; Diao, Y.; Briseno, A.-L.; Russell, T. P. NDI-Based Small Molecule as Promising Nonfullerene Acceptor for Solution-Processed Organic Photovoltaics. Adv. Energy Mater. 2015, 5, 1500195. (23) Srivani, D.; Gupta, A.; Bhosale, S. V.; Puyad, A. L.; Xiang, W.; Li, J.; Evans, R. A.; Bhosale, S. V. Non-fullerene acceptors based on central naphthalene diimide flanked by rhodamine or 1,3-indanedione. Chem. Commun. 2017, 53, 7080−7083. (24) Chen, W.; Nakano, M.; Kim, J.-H.; Takimiya, K.; Zhang, Q. Naphtho[2,3-b]thiophene diimide (NTI): a mono-functionalisable

PCEs as high as 9%. These performances are comparable to those of the best OPVs with PDI-based acceptors, suggesting that NTI is a promising unit for superior NFAs. In addition, the substituents in the imide groups of the NTI unit, 1HH and 2EH, affected the physicochemical properties and the ordering natures in the thin-film state, which gave distinct photovoltaic properties. N-substituted NTIs can be synthesized easily from the N,N′-unsubstituted NTI, which allows us to modify structures and properties of NTI-based materials for further development of efficient NFAs. Molecular design, synthesis, and evaluation of new NTI-based acceptors are now underway in our group.

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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.7b03733. Experimental details, additional data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*M. Nakano, E-mail: [email protected]. *K. Takimiya, E-mail: [email protected]. ORCID

Masahiro Nakano: 0000-0002-9231-4124 Kyohei Nakano: 0000-0003-2493-2817 Kazuo Takimiya: 0000-0002-6001-1129 Keisuke Tajima: 0000-0003-1590-2640 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by JSPS KAKENHI Grant Numbers 15H02196, 16K05900, Iketani Science and Technology Foundation, and the Strategic Promotion of Innovative Research and Development from the Japan Science and Technology Agency (JST). HRMSs were measured at the Molecular Structure Characterization Unit, RIKEN Center for Sustainable Resource Science (CSRS). The DFT calculations using Gaussian 09 were carried out by using the RIKEN Integrated Cluster of Clusters (RICC). Elemental analysis and TEM measurement were carried out at RIKEN materials characterization team. 2D-GIXD experiments were performed at BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). We thank Dr. T. Koganezawa for supporting the GIXD measurements.

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b03733 Chem. Mater. 2017, 29, 9618−9622

Communication

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DOI: 10.1021/acs.chemmater.7b03733 Chem. Mater. 2017, 29, 9618−9622