Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

Abstract

•NTI is synthesized for a new family of polymer donors•The strategy of introducing thiophene can significantly enhance the PCE of NTI-polymer•PNTB-2T-based binary and ternary devices exhibit the efficiencies of 16.72% and 17.35% With the emergence of highly efficient non-fullerene electron acceptors, there was a demand to develop new efficient polymer donors to match these narrow band-gap molecules. Designing novel electron-deficient monomers that could effectively lower the polymer’s HOMO level and fine-tune the polymer’s physical properties is vital for polymer donor evolution. Herein, we developed a new monomer naphthalenothiophene imide (NTI) and NTI-based polymer donors—PNTB and PNTB-2T. Polymer chain modification strategy, which works by inserting extra thiophenes, enhanced the efficiency of 3.81% for PNTB:Y6 to 16.72% for the PNTB-2T:Y6 binary device, and eventually enhanced the efficiency to 17.35% for the PNTB-2T:Y6:PC71BM ternary device. PNTB-2T-based devices also exhibit excellent batch-to-batch reproducibility in the photovoltaic performance that distinguishes them from other high-performance polymer donors. This work provides new insights into the electron-deficient monomer and high-performance polymer donor renovation of organic solar cells. Developing novel electron-deficient monomers is crucially important for the future development of non-fullerene organic solar cells. Here, we report a new electron-deficient monomer naphthalenothiophene imide (NTI) and NTI-based copolymers PNTB and PNTB-2T. The strong electron-withdrawing ability of NTI could effectively lower the HOMO level of the polymer that facilitates the tuning of the polymer’s physical properties while simultaneously maintaining the low-lying HOMO value. Polymer chains modification by introducing extra thiophenes results in the closed π−π stacking and ordered polymer chain packing of PNTB-2T and is proven to be an effective strategy to increase the device performance. Thus, solar cell devices for PNTB-2T:Y6 exhibit an efficiency of 16.72%, which is further enhanced to 17.35% by the third component PC71BM, whereas that for PNTB:Y6 is only 3.81%. Importantly, PNTB-2T-based devices exhibit excellent batch-to-batch reproducibility. These features make this family of polymer donors very promising and the design strategies will enlighten future development of high-performance polymer donors for organic solar cells (OSC). Developing novel electron-deficient monomers is crucially important for the future development of non-fullerene organic solar cells. Here, we report a new electron-deficient monomer naphthalenothiophene imide (NTI) and NTI-based copolymers PNTB and PNTB-2T. The strong electron-withdrawing ability of NTI could effectively lower the HOMO level of the polymer that facilitates the tuning of the polymer’s physical properties while simultaneously maintaining the low-lying HOMO value. Polymer chains modification by introducing extra thiophenes results in the closed π−π stacking and ordered polymer chain packing of PNTB-2T and is proven to be an effective strategy to increase the device performance. Thus, solar cell devices for PNTB-2T:Y6 exhibit an efficiency of 16.72%, which is further enhanced to 17.35% by the third component PC71BM, whereas that for PNTB:Y6 is only 3.81%. Importantly, PNTB-2T-based devices exhibit excellent batch-to-batch reproducibility. These features make this family of polymer donors very promising and the design strategies will enlighten future development of high-performance polymer donors for organic solar cells (OSC). 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The strong electron-withdrawing ability of NTI facilitates polymer chain modification while maintaining a low-lying HOMO value. Although PNTB-2T has two more electron-donating thiophene units in each repeating unit than PNTB, both polymers exhibited low-lying HOMO energy levels (−5.60 eV for PNTB; −5.52 for PNTB-2T). Polymer chains modification impacts polymers’ physical and film-forming properties, resulting in PNTB-2T exhibiting closed π−π stacking, ordered polymer chain packing, and efficient hole transport. After blending with electron acceptor Y6, PNTB-2T-based solar cell devices exhibited PCE of 16.72% with Voc of 0.872 V, further enhanced to 17.35% by third component PC71BM, while the PCE for PNTB is only 3.81%. Importantly, four batches of PNTB-2T with varying molecular weight (Mn: 47.16–114.61 KDa) and polydispersity index (PDI: 2.69–2.87) exhibited the best efficiency higher than 16.4% in the parallel experiments. The excellent batch-to-batch reproducibility and high efficiency indicate that NTI is a promising construction unit for next-generation non-fullerene polymer donors especially for the upcoming large-scale industrial production of OSCs. NTI was synthesized by simply using the Suzuki coupling reaction between compounds 1 and 3,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene (Scheme 1). In 2016, Marcin et al., reported the synthesis of NMI-pyrrole hybrid that bears some kind of similarity in the chemical structure with NTI.43Zhylitskaya H. Cybińska J. Chmielewski P. Lis T. Stępień M. Bandgap engineering in pi-extended pyrroles. A modular approach to electron-deficient chromophores with multi-redox activity.J. Am. Chem. Soc. 2016; 138: 11390-11398Crossref PubMed Scopus (37) Google Scholar The NMI-pyrrole hybrid was synthesized by 8 step reactions from the starting material of acenaphthene and was used to construct π-extended porphyrins, which exhibit promising optical properties. It should be noted that we did not find any similar units to NTI that have been used to construct D-A type polymer donors. The monomer 3 was synthesized by Stille reaction between NTI-2Br and 2-(tributylstannyl)-4-ethylhexylthiophene, followed by bromination. The polymer PNTB and PNTB-2T (Figures 1A and 1B ) were prepared by Stille coupling reaction of monomer 3 with 4 and 5, respectively, using Pd2(dba)3 as a catalyst. In PNTB-2T, longer alkyl chains (butyloctyl) in benzo[1,2-b:4,5-b']dithiophene (BDT) units are used to increase the solubility because the polymer with ethylhexyl groups is insoluble even in refluxing chlorobenzene, which makes solution processing impracticable. These two polymers show good solubility in a commonly used solvent such as chloroform, and chlorobenzene. The gel permeation chromatography (GPC) measurement was performed at 150°C using trichlorobenzene as the eluent. The molecular weight and polydispersity index (PDI) for three batches of PNTB and five batches of PNTB-2T are presented in Figures S1–S8. Thermogravimetric analysis (TGA) indicates that both polymers have good thermal stability with decomposition temperature (5% weight loss) at 438°C for PNTB and 389°C for PNTB-2T (Figure S9), which meet requirements of device fabrication. The differential scanning calorimetry (DSC) measurement shows no clear endothermic and exothermic peak of PNTB and PNTB-2T (Figure S10) at a scan range of 40°C–300°C.Figure 1The structures, energy levels, and optical properties of polymersShow full caption(A and B) The polymer structure of PNTB (A) and PNTB-2T (B).(C and D) The top and side view of calculated geometries of two repeating units of PNTB (C) and PNTB-2T (D).(E) Schematic energy levels of PNTB, PNTB-2T, and Y6.(F) Absorption spectra of PNTB and PNTB-2T in chloroform solution, as-cast film.View Large Image Figure ViewerDownload Hi-res image Download (PPT) (A and B) The polymer structure of PNTB (A) and PNTB-2T (B). (C and D) The top and side view of calculated geometries of two repeating units of PNTB (C) and PNTB-2T (D). (E) Schematic energy levels of PNTB, PNTB-2T, and Y6. (F) Absorption spectra of PNTB and PNTB-2T in chloroform solution, as-cast film. To investigate the polymer-conjugated backbone planarity of these two polymers, molecular geometry of two repeating units for polymers were calculated based on the density functional theory (DFT) using Gaussian package B3LYP/6–31G∗. The long alkyl chains were replaced by a methyl group, which facilitates the calculation. The calculated molecular geometries are shown in Figures 1C and 1D. The NTI unit exhibits good planarity with all atoms of the core in the same plane. The steric hindrance exists at bay area, resulting in a dihedral angle of 35.6° and 32.4° between NTI and thiophene for the units of PNTB-2T and PNTB, respectively. The alkyl group in thiophene also affects conjugated polymer chain planarity and dihedral angle of 22.1° and 26.8° are observed between 3-methylthiophene and thiophene/BDT units for PNTB-2T and PNTB. From the side view of calculated geometries of two repeating units of PNTB, it can be seen that steric hindrance leads to large long-range torsion of the conjugated polymer chain of PNTB, which is consistent with large π−π stacking distance of 4.04 Å observed in grazing-incidence wide-angle X-ray scattering (GIWAXS) measurement. Compared with PNTB, the side view of PNTB-2T exhibits relatively better long-range planarity of the conjugated core. The better planarity of the conjugated core of PNTB-2T favors closed π−π stacking and ordered polymer chain packing, which is verified by GIWAXS data. The LUMO and HOMO orbitals of two repeating units of the polymers PNTB and PNTB-2T were calculated by using DFT at the B3LYP/6–31G∗ level and are summarized in Figure S11. The HOMO orbital is delocalized over the whole conjugated chain of the polymer PNTB and PNTB-2T. The LUMO orbital is mainly localized on the NTI unit with extension onto the conjugated polymer chain. The frontier orbital energy level of PNTB and PNTB-2T was determined by cyclic voltammetry (CV) with ferrocene (−4.80 eV) as a standard reference (Figure S12). The ELUMO for PNTB and PNTB-2T were calculated to be −3.53 and −3.43 eV from onset reduction potentials, while EHOMO were calculated to be −5.60 and 5.52 eV from onset oxidation (Table 1). Compared with HOMO values between −5.45 and −5.52 eV for other highly efficient polymer donors, which have a fluoride 4,8-bis(5-(2ethyl hexyl)thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene (BDTT) unit, NTI-based polymer donors have obviously lower HOMO energy level that enables the high Voc values of organic photovoltaics (OPV) devices.14Liu Q. Jiang Y. Jin K. Qin J. Xu J. Li W. Xiong J. Liu J. Xiao Z. Sun K. et al.18% efficiency organic solar cells.Science Bulletin. 2020; 65: 272-275Crossref Scopus (1531) Google Scholar,27Zhang M.J. Guo X. Ma W. Ade H. Hou J.H. A large-bandgap conjugated polymer for versatile photovoltaic applications with high performance.Adv. Mater. 2015; 27: 4655-4660Crossref PubMed Scopus (555) Google Scholar,32Chao P. Chen H. Zhu Y. Lai H. Mo D. Zheng N. Chang X. Meng H. He F. A benzo[1,2-b:4,5-c′]dithiophene-4,8-dione-based polymer donor achieving an efficiency over 16%.Adv. Mater. 2020; 32: 1907059Crossref Scopus (49) Google Scholar, 33Xu X. Feng K. Bi Z. Ma W. Zhang G. Peng Q. Single-junction polymer solar cells with 16.35% efficiency enabled by a platinum(II) complexation strategy.Adv. Mater. 2019; 31: e1901872Crossref PubMed Scopus (436) Google Scholar, 34Fan B.B. Zhang D.F. Li M.J. Zhong W.K. Zeng Z.M.Y. Ying L. Huang F. Cao Y. Achieving over 16% efficiency for single-junction organic solar cells.Sci. China Chem. 2019; 62: 746-752Crossref Scopus (718) Google Scholar The slightly elevated LUMO and HOMO energy level of PNTB-2T than that for PNTB is due to more electron-donating thiophene units. The ΔEHOMO offset between these two polymer donors and Y6 acceptor is 0.05 and 0.13 eV, which is enough to drive exciton dissociation in blend films according to some non-fullerene systems.44Yao H. Cui Y. Qian D. Ponseca Jr., C.S. Honarfar A. Xu Y. Xin J. Chen Z. Hong L. Gao B. et al.14.7% efficiency organic photovoltaic cells enabled by active materials with a large electrostatic potential difference.J. Am. Chem. Soc. 2019; 141: 7743-7750Crossref PubMed Scopus (227) Google Scholar The UV-vis absorption of these two polymers in chloroform solution at a concentration of 1.5 × 10−5 g ml−1 was measured and recorded, as shown in Figure 1F. The maximum absorption of PNTB in chloroform solution was at 511 nm, while that for PNTB-2T was red shifted by 48 to 559 nm. The films cast from the chloroform solution exhibited maximum absorption peaks at 541 and 566 nm for PNTB and PNTB-2T, respectively. The maximum absorption peak of the film was red shifted to 30 nm for PNTB, and 7 nm for PNTB-2T. The film absorption coefficient at absorption maximum was 6.9 × 104 and 9.8 × 104 cm−1 for PNTB and PNTB-2T, respectively, which is shown in Figure S13 and summarized in Table 1. The film absorption range of these two polymers (450–650 nm) complements 600–900 nm for Y6 that favors solar energy harvesting. The conventional photovoltaic devices with the configuration of ITO/PEDOT:PSS/PNTB or PNTB-2T:Y6/PNDIT-F3N/Ag were fabricated by employing Y6 as the electron acceptor. The measurement of OPV devices was performed under a simulated solar illumination of 100 mW cm−2 AM 1.5G in a nitrogen atmosphere. The J-V curves, EQE spectra, and parameters of photovoltaic devices are presented in the Figures 2A and 2B and Table 2. The active layer film of PNTB:Y6 and PNTB-2T:Y6 with a thickness around 100 nm were spin-casted from chloroform solution. Solar cell devices optimization by varying donor/acceptor weight ratio and solution concentration were tested. A little amount of additive and thermal annealing at 110°C proved to be effective to enhance the photovoltaic performance of these devices. With 1:1.5 of D:A mass ratio, 13 mg ml−1 concentration, and 0.5% DIO additive (volume ratio), solar cell devices of PNTB:Y6 annealed at 110°C gave the highest PCE of 3.81%; Voc of 0.899 V; Jsc of 8.68 mA cm−2; FF of