Bis-Silicon-Bridged Stilbene: A Core for Small-Molecule Electron

By utilizing a wide bandgap polymer donor, poly[(2,6-(4,8-cbis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-d...
0 downloads 0 Views 1MB Size
Download PDF
Communication Cite This: Chem. Mater. 2018, 30, 587−591

pubs.acs.org/cm

Bis-Silicon-Bridged Stilbene: A Core for Small-Molecule Electron Acceptor for High-Performance Organic Solar Cells Zhongbo Zhang†,‡ and Xiaozhang Zhu*,†,‡ †

Chem. Mater. 2018.30:587-591. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 10/08/18. For personal use only.

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

R

and co-workers reported a small molecular DTS(PTTh2)2 based on DTS exhibiting a high saturation hole mobility of 0.12 cm2 V−1 s−1 and Ion/Ioff ∼ 107,28 and they fabricated solution-processable small-molecule solar cells with a designed small-molecule donor p-DTS(FBTTh2)2 and achieved an encouraging PCE over 8% as well.32 However, the photovoltaic performance of small-molecule NFAs based on silole derivatives was far from satisfactory, a DBS-based smallmolecule NFA only delivered a PCE of 2.76%.33 Yamaguchi and co-workers reported a series of π-conjugated polymers based on BSS, which exhibit an intense blue to greenish-blue emission.34 The easy tuning of absorption and emission scope and extended coplanar skeleton of BSS make it a valuable building block for the design of new organic π-functional materials. By incorporating strongly electron-deficient terminal groups onto BSS, new electron-accepting materials are likely to be promising for OPV applications. In our previous work, we introduced thieno[3,4-b]thiophene (TbT) as a bridge unit between strong donor−acceptor pairs utilizing its quninodal resonance35 to enhancing the π− conjugation along the molecular backbone.5,36−38 This effective strategy, “enhancing the quinoidal resonance of D-A system”, has been successfully extended to the design of donor and acceptor materials, both of which achieved excellent OPV performance. Herein, a new small-molecule acceptor NSTI with a BSS core, 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1ylidene)malononitrile (INCN-2F) terminals, and 2-alkylsubstituted TbT linkers was designed and synthesized (Scheme 1). Our molecular design principles are as follows: (i) BSS possesses rigid and coplanar structure which is beneficial for charge transport and the four alkyl chains affiliated to silicon atoms are out of the BSS plane that can restrict excessive aggregation in blend films; (ii) INCN-2F has strong electronaccepting ability that can down shifts LUMO (the lowest unoccupied molecular orbital) energy levels; (iii) introduction of TbT narrows the optical bandgap by quinoid-resonance effect, which is beneficial for light harvesting and thus achieving high short-circuit current (Jsc). We found that NSTI exhibited a narrow optical bandgap of 1.58 eV, inducing a strong and broad absorption that covers the spectrum range of 550−800 nm, and appropriate energy levels as an electron acceptor. By utilizing a

ecently, the research on nonfullerene electron acceptors has provided a new opportunity to promote the rapid development of high-performance organic photovoltaics (OPVs).1,2 Compared with fullerene acceptors, nonfullerene acceptors (NFAs) possess significant advantages of tunable electronic structure and the potential of lowering production cost. Over the past decade, considerable efforts have been dedicated to developing NFAs. An important kind of smallmolecule NFAs based on an electron-rich π-extended fused rings as presented by indacenodithiophene 3−9 and indacenodithieno[3,2-b]thiophene10−16 with an acceptor− donor−acceptor (A-D-A) framework show very promising photovoltaic performance and have received widespread attentions. Such A-D-A architecture ensures good conjugated planarity and facilitates π-electron delocalization, which benefits the decrease of energy bandgap and the extension of light absorption wavelength. Inspiringly, fullerene-free organic photovoltaics using ITIC and its analogues as electron acceptor and conjugated polymers as electron donor exhibited impressive power conversion efficiencies (PCEs) of above 12%,10,15−19 which are even higher than those using PC71BM acceptor. Current molecular design for ideal small-molecule NFAs with an A-D-A structure mainly focused on the adjustment of bulky π-conjugated heteroaromatic cores,4,20,21 π-bridges, 3,5,6,9 and terminal electron-deficient moieties.10,14,15,22 Among these, developing new polycyclic arene and heteroarenes as the central building blocks is considered to be an effective idea to pursue attractive small-molecule NFAs. Very recently, new rigid ladder-type fused moieties based on benzo[1,2-b:4,5-b′] dithiophene and two cyclopenta[2,1-b;3,4b′]dithiophene have been reported and PCEs over 10%.23−25 Silole is a five-membered silacyclic that possesses σ*−π* conjugation arising from the interaction between the σ* orbital of two exocyclic σ-bonds on the silicon atom and the π* orbital of the butadiene moiety, enduing silole with fascinating optoelectronic properties such as strong fluorescence in the solid state and high electron-accepting properties.26,27 Fused silole derivatives such as dibenzo[b,d]silole (DBS), dithieno[3,2-b:2′,3′-d]silole (DTS), and bis-silicon-bridged stilbene (BSS) are attractive building blocks for the design of polymer and small-molecule electronic and optoelectronic materials that are widely applied in thin-film transistors (OTFTs),28 lightemitting diodes (OLEDs),29 and OPVs,28,30,31 attributing to their electron-rich nature, planar molecular structure, and electronic tunability through structural modifications. Heeger © 2018 American Chemical Society

Received: November 24, 2017 Revised: January 6, 2018 Published: January 10, 2018 587

DOI: 10.1021/acs.chemmater.7b04930 Chem. Mater. 2018, 30, 587−591

Communication

Chemistry of Materials Scheme 1. Synthesis and Chemical Structure of NSTIa

with marked red shift (ca. 50 nm) and displayed a strong shoulder peak at 656 nm, indicating that NSTI forms ordered packing structure during the film forming process. The absorption onset of the NSTI thin film was located at 786 nm, which corresponds to a narrow optical bandgap of 1.58 eV that is 0.01 eV smaller than that of ITIC (Table S5 in Supporting Information). The absorption spectrum of PBDB-T in thin film is also plotted in Figure 1a for comparison. Apparently, the complementary absorption of PBDB-T and NSTI is desirable for enhancing light harvest so as to increase Jsc of the solar cells. The electrochemical property of NSTI was examined by cyclic voltammetry measurement and shown in Figure S2 (Supporting Information). The potentials were internally calibrated using the ferrocene/ferrocenium (Fc/Fc+) redox couple (4.8 eV below the vacuum level) and the HOMO (highest occupied molecular orbital) and LUMO energy levels were estimated based on the onsets of the oxidation and reduction curves. The HOMO and LUMO energy levels of NSTI are −5.54 and −3.87 eV, respectively, which are slightly deeper than those of ITIC (HOMO/LUMO: −5.48/−3.83) (Table S5 in Supporting Information). The energy diagram relative to the vacuum level is shown in Figure 1b. The LUMO energy offset between PBDB-T and NSTI is 0.71 eV, which may lead to efficient electron transfer from PBDB-T to NSTI. On the other hand, since the acceptor absorption also contributes to the overall photocurrent, the hole transfer from the acceptor to the donor need to be considered. The efficient hole transfer from NSTI to PBDB-T is confirmed by the high EQE values of the device in the spectral range of 700− 800 nm, where the light absorption of the PBDB-T is negligible.43 To evaluate the photovoltaic performance of NSTI, we fabricated the OPVs based on PBDB-T donor with a conventional device architecture of ITO/PEDOT:PSS/PBDBT:NSTI/PDINO/Al were fabricated, where PDINO is an efficient cathode buffer layer.44 After screening the D:A ratio/ concentration and processing solvent, the active layers were prepared by spin-coating the PBDB-T:NSTI (1:1, w/w) solution with a total weight concentration of 18 mg mL−1 in chlorobenzene and the optimized thickness of the active layer was ∼100 nm (Table S1−S3 in Supporting Information). OPV devices based on the as-cast PBDB-T:NSTI film gave a high PCE of 7.92% with a Voc of 0.807 V, a high Jsc of 14.17 mA cm−2, and a FF of 69.2%. Processing additives played an important role in organic photovoltaic systems to optimize the active layer film morphology and further improving the device performance. Upon adding 0.5% v/v 1-chloronaphthalene (1CN) to the solvent, the Jsc of the resulting OPVs was significantly increased to 16.47 mA cm−2. At the same time, the Voc was enhanced to 0.832 V and the FF increased to 75.2%. The combined improvements lead to a PCE of 10.33%, which is the highest PCE for OPVs using silole-based small-molecule acceptors. The current density−voltage (J−V) curves of the ascast and with 1-CN as additive devices are shown in Figure 2a, and their corresponding photovoltaic parameters are summarized in Table S1. The external quantum efficiency (EQE) curves of the optimal NSTI-based devices without and with 1CN covered a wide wavelength range from 300 to 800 nm (Figure 2b), which originated from complementary absorptions of the donor and acceptor materials. The EQE values for a device with 1-CN were much higher than those for the as-cast device, reaching a maximum value of 75% at 660 nm indicating

a

Reagents and conditions: (a) tributyl(2-(2-ethylhexyl)thieno[3,4b]thiophen-6-yl)stannane, Pd(PPh3)4, toluene/DMF, 100 °C; (b) POCl3, DMF, CH2Cl2, rt; (c) INCN-2F, pyridine, chloroform, reflux.

wide bandgap polymer donor, poly[(2,6-(4,8-cbis(5-(2ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T) (characteristics of PBDB-T can be found in Table S5 Supporting Information) and NSTI acceptor, OPVs achieved the highest PCE value of 10.33% with an open-circuit voltage (Voc) of 0.832 V, a standout fill factor (FF) of 75.2%, and Jsc of 16.47 mA cm−2 after device optimization, which is so far the best reported for OPVs using silole-based organic photovoltaic materials.33,39−42 The synthetic route of compound NSTI is shown in Scheme 1. The central unit bis-silicon-bridged stilbene 1 was synthesized according to the literature.34 After the Stille coupling reaction of compound 1 and tributyl(2-(2-ethylhexyl)thieno[3,4-b]thiophen-6-yl)stannane38 followed by Vilsmeier− Haack reaction, dialdehyde 2 was synthesized as a yellow solid in 61% yield. Subsequently, by Knoevenagel condensation of compound 2 with INCN-2F, we synthesized the target compound NSTI in 82% yield as a dark brown solid, which exhibits good solubility in common organic solvents. Thermogravimetric analysis indicates good thermal stability of NSTI with a 5% weight loss at 334 °C under a N2 atmosphere (see Figure S1 in Supporting Information). The UV−vis absorption spectrum of NSTI was examined in both dilute chloroform solution and thin film (Figure 1a). The chloroform solution of NSTI exhibited broad absorption in the 350−750 nm region. The absorption peak of NSTI solution was located at around 668 nm with a maximum molar extinction coefficient of 1.89 × 105 M−1 cm−1. The NSTI film exhibited a broadened absorption spectrum (550−800 nm)

Figure 1. (a) Normalized UV−vis absorption spectra of NSTI in chloroform (blue line), solid state (red line), PBDB-T in the solid state (black line). (b) Energy diagram relative to vacuum level. 588

DOI: 10.1021/acs.chemmater.7b04930 Chem. Mater. 2018, 30, 587−591

Communication

Chemistry of Materials

Jph is saturated (Jsat) at Veff higher than 2 V, suggesting that charge recombination is minimized at higher voltage due to the high internal electric field in the devices. The charge dissociation and charge collection probability (P(E,T)) in the devices could be estimated from the value of Jph/Jsat.45 Under their short-circuit and maximal power output conditions, the P(E,T) values are 95%, 80% for the as-cast device, and 97%, 83% for the device with 1-CN, respectively. The increased P(E,T) values indicate that the device processed with additives exhibits higher exciton dissociation rate and a more efficient charge collection efficiency compared to those of the as-cast device.45,46 To understand the charge recombination behavior of the OPVs, the effect of light intensity (P) and short-circuit current density was then studied (Figure 3b). Generally, the relationship between Jsc and light intensity can be described by the formula of Jsc ∝ Pα. If all free carriers are swept out and collected at the electrodes prior to recombination, α should be equal to 1, whereas α < 1 indicates some extent of bimolecular recombination.45,47 The α value for the device processed with 1-CN is 0.95, which is higher than 0.92 for the as cast device. It is indicated that there is more efficient transportation of carriers and less bimolecular recombination in the device processed with additives. This reduced bimolecular recombination thus resulted in high Jsc and better device performance. Typically, charge recombination is directly related with the FF of the devices. The lower bimolecular recombination in the additive treated devices agrees well with higher FF value of the device processed with 1-CN (75.2%) relative to that as cast device (69.2%). To understand the relationship between OPV performance and the molecular packing of BHJ films, we investigated the pure and blend films by the grazing incidence X-ray diffraction (GIXD). Figure 4 shows the 2D GIXD patterns and the

Figure 2. (a) J−V curves of the optimized OPVs based on PBDBT:NSTI (1:1, w/w) without (blue line) and with 1-CN (red line) additive under the illumination of AM 1.5G, 100 mW cm−2 (inset shows the histogram of the PCE counts for 50 devices with 1-CN). (b) EQE spectra of the corresponding devices.

rather efficient photoelectricity conversion in the range of 375− 750 nm with EQE values over 50%. Comparing to the current densities obtained from the J−V measurement, the Jsc values calculated from the EQE curves under the standard solar spectrum (AM 1.5 G) were 13.56 and 16.06 mA cm−2, respectively, which represented a small deviation of less than 5%. Balanced hole/electron mobility played an important role in obtaining high performance in OPVs. By using the space charge limited current (SCLC) method with the hole/electron-only devices in the optimized blend ratio, the hole and electron mobilities were investigated (see Figure S4 in Supporting Information). By fitting the J−V curves from both hole- and electron-only devices, the carrier mobilities of PBDB-T:NSTI blends were determined. For the as-cast blend film, the hole and electron mobilities were estimated as 1.59 × 10−4 and 3.00 × 10−5 cm2 V−1 s−1, respectively. After mixing of 0.5% v/v 1CN to the solvent, the hole and electron mobilities are improved to 1.97 × 10−4 and 1.06 × 10−4 cm2 V−1 s−1, respectively. In comparison, the mobilities of the blend film processing of 1-CN are relatively higher, and more balance for hole and electron transportation (Table S1 in Supporting Information). The higher and balanced charge carrier mobilities of the film processed with additives are beneficial to the efficient exciton dissociation and charge transportation, which may contribute to the higher Jsc and FF of the corresponding as-cast OPVs. By plotting the photocurrent density (Jph, defined as JL−JD, where JL and JD are the current densities under illumination and in the dark, respectively) as a function of effective voltage (Veff, defined as V0−Vbias, where V0 is the voltage at which Jph is zero and Vbias is the applied external voltage bias), we investigated the exciton dissociation and charge collection properties in the devices without and with 1-CN. As can be seen from Figure 3a,

Figure 4. (a) 2D GIXD patterns and (b) line cut profiles for pure films of NSTI, PBDB-T and blend films of PBDB-T:NSTI with/without 1CN (red line, out-of-plane; black line, in-plane).

corresponding line cut profiles in the in-plane (IP) and out-ofplane (OOP) direction of the pure films and blend films with and without 1-CN treatment. For the NSTI pure film, it showed a strong degree of crystallinity, the π−π stacking peak was located at 1.74 Å−1 (d-spacing of 3.61 Å) in the IP direction. The PBDB-T pure film shows obvious lamellar (100) peak at 0.29 Å−1 (lamellar d-spacing of 21.8 Å) in the IP direction and strong π−π stacking (010) peak at 1.68 Å−1 (dspacing of 3.75 Å) in the OOP direction. The as-cast blend film without additive showed an edge-on orientation, and the π−π stacking peak was located at 1.74 Å−1 (d-spacing of 3.61 Å) in the IP direction. For the blend film treated with 1-CN, it showed lamellar (100) peak at 0.29 Å−1 (lamellar d-spacing of

Figure 3. (a) Photocurrent density versus effective applied voltage (Jph−Veff) characteristics. (b) Light intensity dependence of the short circuit current of the devices. 589

DOI: 10.1021/acs.chemmater.7b04930 Chem. Mater. 2018, 30, 587−591

Communication

Chemistry of Materials

(3) Lin, Y.; Zhang, Z.; Bai, H.; Wang, J.; Yao, Y.; Li, Y.; Zhu, D.; Zhan, X. High-Performance Fullerene-Free Polymer Solar Cells with 6.31% Efficiency. Energy Environ. Sci. 2015, 8, 610−616. (4) Lin, Y.; He, Q.; Zhao, F.; Huo, L.; Mai, J.; Lu, X.; Su, C. J.; Li, T.; Wang, J.; Zhu, J.; Sun, Y.; Wang, C.; Zhan, X. A Facile Planar FusedRing Electron Acceptor for As-Cast Polymer Solar Cells with 8.71% Efficiency. J. Am. Chem. Soc. 2016, 138, 2973−2976. (5) Liu, F.; Zhou, Z.; Zhang, C.; Vergote, T.; Fan, H.; Liu, F.; Zhu, X. A Thieno[3,4-b]thiophene-Based Non-fullerene Electron Acceptor for High-Performance Bulk-Heterojunction Organic Solar Cells. J. Am. Chem. Soc. 2016, 138, 15523−15526. (6) Yao, H.; Chen, Y.; Qin, Y.; Yu, R.; Cui, Y.; Yang, B.; Li, S.; Zhang, K.; Hou, J. Design and Synthesis of a Low Bandgap Small Molecule Acceptor for Efficient Polymer Solar Cells. Adv. Mater. 2016, 28, 8283−8287. (7) Lin, Y.; Zhao, F.; Wu, Y.; Chen, K.; Xia, Y.; Li, G.; Prasad, S. K.; Zhu, J.; Huo, L.; Bin, H.; Zhang, Z. G.; Guo, X.; Zhang, M.; Sun, Y.; Gao, F.; Wei, Z.; Ma, W.; Wang, C.; Hodgkiss, J.; Bo, Z.; Inganas, O.; Li, Y.; Zhan, X. Mapping Polymer Donors toward High-Efficiency Fullerene Free Organic Solar Cells. Adv. Mater. 2017, 29, 1604155. (8) Liu, F.; Zhou, Z.; Zhang, C.; Zhang, J.; Hu, Q.; Vergote, T.; Liu, F.; Russell, T. P.; Zhu, X. Efficient Semitransparent Solar Cells with High NIR Responsiveness Enabled by a Small-Bandgap Electron Acceptor. Adv. Mater. 2017, 29, 1606574. (9) 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. (10) 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, 9423−9249. (11) Lin, Y.; Zhao, F.; He, Q.; Huo, L.; Wu, Y.; Parker, T. C.; Ma, W.; Sun, Y.; Wang, C.; Zhu, D.; Heeger, A. J.; Marder, S. R.; Zhan, X. High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics. J. Am. Chem. Soc. 2016, 138, 4955−4961. (12) Yang, Y.; Zhang, Z.; 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. (13) Xie, D.; Liu, T.; Gao, W.; Zhong, C.; Huo, L.; Luo, Z.; Wu, K.; Xiong, W.; Liu, F.; Sun, Y.; Yang, C. A Novel Thiophene-Fused Ending Group Enabling an Excellent Small Molecule Acceptor for High-Performance Fullerene-Free Polymer Solar Cells with 11.8% Efficiency. Solar RRL 2017, 1, 1700044. (14) Yao, H.; Ye, L.; Hou, J.; Jang, B.; Han, G.; Cui, Y.; Su, G. M.; Wang, C.; Gao, B.; Yu, R.; Zhang, H.; Yi, Y.; Woo, H. Y.; Ade, H.; Hou, J. Achieving Highly Efficient Nonfullerene Organic Solar Cells with Improved Intermolecular Interaction and Open-Circuit Voltage. Adv. Mater. 2017, 29, 1700254. (15) Zhao, F.; Dai, S.; Wu, Y.; Zhang, Q.; Wang, J.; Jiang, L.; Ling, Q.; Wei, Z.; Ma, W.; You, W.; Wang, C.; Zhan, X. Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency. Adv. Mater. 2017, 29, 1700144. (16) 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. (17) Zhao, W.; Zhang, S.; Zhang, Y.; Li, S.; Liu, X.; He, C.; Zheng, Z.; Hou, J. Environmentally Friendly Solvent-Processed Organic Solar Cells That Are Highly Efficient and Adaptable for the Blade-Coating Method. Adv. Mater. 2017, 1704837. (18) Xu, X.; Yu, T.; Bi, Z.; Ma, W.; Li, Y.; Peng, Q. Realizing over 13% Efficiency in Green-Solvent-Processed Nonfullerene Organic Solar Cells Enabled by 1,3,4-Thiadiazole-Based Wide-Bandgap Copolymers. Adv. Mater. 2017, 1703973. (19) Bin, H.; Yang, Y.; Peng, Z.; Ye, L.; Yao, J.; Zhong, L.; Sun, C.; Gao, L.; Huang, H.; Li, X.; Qiu, B.; Xue, L.; Zhang, Z.; Ade, H.; Li, Y. Effect of Alkylsilyl Side-Chain Structure on Photovoltaic Properties of Conjugated Polymer Donors. Adv. Energy Mater. 2017, 1702324.

21.8 Å) in the IP direction and the molecule packing was changed to a face-on orientation, which is advantageous for the charge transport. The π−π stacking (010) diffraction peak in the OOP direction was located at 1.71 Å−1 (d-spacing of 3.67 Å). The coherence length deduced from the full width at halfmaximum of OOP (010) peaks were calculated via Scherrer equation, the coherence lengths of the PBDB-T and NSTI are increased from 2.25/2.50 nm for the as cast blend film to 2.90/ 5.33 nm for 1-CN treated blend film, which means that higher ordering of NSTI packing is formed after 1-CN treatment. In summary, a new small-molecule nonfullerene electron acceptor NSTI was designed and synthesized by using bissilicon-bridged stilbene as the core unit, INCN-2F as ending moieties, and TbT serving as the bridge to enhance the quinoidal resonance. NSTI exhibits a broadened absorption spectrum (550−800 nm) that is complementary with the absorption of PBDB-T for achieving high Jsc. Through device optimization, we obtained the optimal efficiency of 10.33% with a Jsc of 16.47 mA cm−2, and a FF of 75.2%, which is so far the best reported for OPV devices utilizing silole-based organic photovoltaic materials. Our work indicates that the bis-siliconbridged stilbene should be a very promising moiety for the development of high-performance fullerene-free OPVs.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04930. General experimental methods, thermal gravimetric analysis, cyclic voltammetry measurement, charge transport property, OPV device data, the AFM, TEM, and NMR spectra of all new compounds (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*X. Zhu. E-mail: [email protected]. ORCID

Zhongbo Zhang: 0000-0001-9711-1097 Xiaozhang Zhu: 0000-0002-6812-0856 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank National Key R&D Program of China (2017YFA0204700), National Basic Research Program of China (973 Program, 2014CB643502), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12010200), and the National Natural Science Foundation of China (21661132006, 21572234) for the financial support. We appreciate Prof. Changduk Yang at Department of Energy Engineering, School of Energy and Chemical Engineering Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798 (South Korea) for GIXD measurements.

■

REFERENCES

(1) 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. (2) Lin, Y.; Zhan, X. Non-Fullerene Acceptors for Organic Photovoltaics: An Emerging Horizon. Mater. Horiz. 2014, 1, 470−488. 590

DOI: 10.1021/acs.chemmater.7b04930 Chem. Mater. 2018, 30, 587−591

Communication

Chemistry of Materials (20) Lin, Y.; Wang, J.; Zhang, Z.; Bai, H.; Li, Y.; Zhu, D.; Zhan, X. An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170−1174. (21) Dai, S.; Zhao, F.; Zhang, Q.; Lau, T. K.; Li, T.; Liu, K.; Ling, Q.; Wang, C.; Lu, X.; You, W.; Zhan, X. Fused Nonacyclic Electron Acceptors for Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2017, 139, 1336−1343. (22) Baran, D.; Kirchartz, T.; Wheeler, S.; Dimitrov, S.; Abdelsamie, M.; Gorman, J.; Ashraf, R. S.; Holliday, S.; Wadsworth, A.; Gasparini, N.; Kaienburg, P.; Yan, H.; Amassian, A.; Brabec, C. J.; Durrant, J. R.; McCulloch, I. Reduced Voltage Losses Yield 10% Efficient Fullerene Free Organic Solar Cells with > 1 V Open Circuit Voltages. Energy Environ. Sci. 2016, 9, 3783−3793. (23) Kan, B.; Feng, H.; Wan, X.; Liu, F.; Ke, X.; Wang, Y.; Wang, Y.; Zhang, H.; Li, C.; Hou, J.; Chen, Y. Small-Molecule Acceptor Based on the Heptacyclic Benzodi(cyclopentadithiophene) Unit for Highly Efficient Nonfullerene Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 4929−4934. (24) Li, Y.; Zhong, L.; Gautam, B.; Bin, H. J.; Lin, J. D.; Wu, F. P.; Zhang, Z. J.; Jiang, Z. Q.; Zhang, Z. G.; Gundogdu, K.; Li, Y. F.; Liao, L. S. A Near-Infrared Non-Fullerene Electron Acceptor for High Performance Polymer Solar Cells. Energy Environ. Sci. 2017, 10, 1610− 1620. (25) Wang, J.; Wang, W.; Wang, X.; Wu, Y.; Zhang, Q.; Yan, C.; Ma, W.; You, W.; Zhan, X. Enhancing Performance of Nonfullerene Acceptors via Side-Chain Conjugation Strategy. Adv. Mater. 2017, 29, 1702125. (26) Yamaguchi, S.; Tamao, K. Silole-Containing σ- and πConjugated Compounds. J. Chem. Soc., Dalton Trans. 1998, 3693− 3702. (27) Yamaguchi, S.; Tamao, K. A Key Role of Orbital Interaction in the Main Group Element-containing π-Electron Systems. Chem. Lett. 2005, 34, 2−7. (28) Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency. Nat. Mater. 2012, 11, 44−48. (29) Chan, K. L.; McKiernan, M. J.; Towns, C. R.; Holmes, A. B. Poly(2,7-dibenzosilole): A Blue Light Emitting Polymer. J. Am. Chem. Soc. 2005, 127, 7662−7663. (30) Hou, J.; Chen, H. Y.; Zhang, S.; Li, G.; Yang, Y. Synthesis, Characterization, and Photovoltaic Properties of a Low Band Gap Polymer Based on Silole-Containing Polythiophenes and 2,1,3Benzothiadiazole. J. Am. Chem. Soc. 2008, 130, 16144−16145. (31) Chu, T. Y.; Lu, J.; Beaupre, S.; Zhang, Y.; Pouliot, J. R.; Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; Tao, Y. Bulk Heterojunction Solar Cells Using Thieno[3,4-c]pyrrole-4,6-dione and Dithieno[3,2b:2′,3′-d]silole Copolymer with a Power Conversion Efficiency of 7.3%. J. Am. Chem. Soc. 2011, 133, 4250−4253. (32) Wang, D. H.; Kyaw, A. K. K.; Gupta, V.; Bazan, G. C.; Heeger, A. J. Enhanced Efficiency Parameters of Solution-Processable SmallMolecule Solar Cells Depending on ITO Sheet Resistance. Adv. Energy Mater. 2013, 3, 1161−1165. (33) Patil, H.; Gupta, A.; Alford, B.; Ma, D.; Privér, S. H.; Bilic, A.; Sonar, P.; Bhosale, S. V. Conjoint use of Dibenzosilole and Indan-1,3dione Functionalities to Prepare an Efficient Non-Fullerene Acceptor for Solution-Processable Bulk-Heterojunction Solar Cells. Asian J. Org. Chem. 2015, 4, 1096−1102. (34) Xu, C.; Yamada, H.; Wakamiya, A.; Yamaguchi, S.; Tamao, K. Ladder Bis-Silicon-Bridged Stilbenes as a New Building Unit for Fluorescent π-Conjugated Polymers. Macromolecules 2004, 37, 8978− 8983. (35) Zhang, C.; Zhu, X. Thieno[3,4-b]thiophene-Based Novel SmallMolecule Optoelectronic Materials. Acc. Chem. Res. 2017, 50, 1342− 1350. (36) Zhang, C.; Li, H.; Wang, J.; Zhang, Y.; Qiao, Y.; Huang, D.; Di, C.; Zhan, X.; Zhu, X.; Zhu, D. Low-Bandgap Thieno[3,4-c]pyrrole-4,6dione-Polymers for High-Performance Solar Cells with Significantly Enhanced Photocurrents. J. Mater. Chem. A 2015, 3, 11194−11198.

(37) Xu, S.; Zhou, Z.; Fan, H.; Ren, L.; Liu, F.; Zhu, X.; Russell, T. P. An Electron-Rich 2-alkylthieno[3,4-b]thiophene Building Block with Excellent Electronic and Morphological Tunability for High-Performance Small-Molecule Solar Cells. J. Mater. Chem. A 2016, 4, 17354− 17362. (38) Zhang, Z.; Zhou, Z.; Hu, Q.; Liu, F.; Russell, T. P.; Zhu, X. 1,3Bis(thieno[3,4-b]thiophen-6-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)dione-Based Small-Molecule Donor for Efficient Solution-Processed Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 6213−6219. (39) Lin, Y.; Li, Y.; Zhan, X. A Solution-Processable Electron Acceptor Based on Dibenzosilole and Diketopyrrolopyrrole for Organic Solar Cells. Adv. Energy Mater. 2013, 3, 724−728. (40) Wu, J.; Ma, Y.; Wu, N.; Lin, Y.; Lin, J.; Wang, L.; Ma, C. 2,2Dicyanovinyl-End-Capped Oligothiophenes as Electron Acceptor in Solution Processed Bulk-Heterojunction Organic Solar Cells. Org. Electron. 2015, 23, 28−38. (41) Gupta, A.; Hangarge, R. V.; Wang, X.; Alford, B.; Chellapan, V.; Jones, L. A.; Rananaware, A.; Bilic, A.; Sonar, P.; Bhosale, S. V. Crowning of Dibenzosilole with a Naphthalenediimide Functional Group to Prepare an Electron Acceptor for Organic Solar Cells. Dyes Pigm. 2015, 120, 314−321. (42) Liu, X.; Xie, Y.; Cai, X.; Li, Y.; Wu, H.; Su, S.; Cao, Y. Synthesis and Photovoltaic Properties of A−D−A Type Non-Fullerene Acceptors Containing Isoindigo Terminal Units. RSC Adv. 2015, 5, 107566− 107574. (43) Lin, H.; Chen, S.; Li, Z.; Lai, J. Y.; Yang, G.; McAfee, T.; Jiang, K.; Li, Y.; Liu, Y.; Hu, H.; Zhao, J.; Ma, W.; Ade, H.; Yan, H. HighPerformance Non-Fullerene Polymer Solar Cells Based on a Pair of Donor-Acceptor Materials with Complementary Absorption Properties. Adv. Mater. 2015, 27, 7299−7304. (44) Zhang, Z.; Qi, B. Y.; Jin, Z. W.; Chi, D.; Qi, Z.; Li, Y. F.; Wang, J. Perylene Diimides: A Thickness-Insensitive Cathode Interlayer for High Performance Polymer Solar Cells. Energy Environ. Sci. 2014, 7, 1966−1973. (45) Proctor, C. M.; Kuik, M.; Nguyen, T.-Q. Charge Carrier Recombination in Organic Solar Cells. Prog. Polym. Sci. 2013, 38, 1941−1960. (46) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Device Physics of Polymer: Fullerene Bulk Heterojunction Solar Cells. Adv. Mater. 2007, 19, 1551−1566. (47) Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in PolymerFullerene Bulk Heterojunction Solar Cells. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 245207.

591

DOI: 10.1021/acs.chemmater.7b04930 Chem. Mater. 2018, 30, 587−591