Materials Today Energy
Alkoxyphenyl or alkylphenyl side-chained Thieno[2,3-f]benzofuran polymer for efficient non-fullerene solar cells
Graphical abstract
Introduction
In the past two decades, polymer solar cells (PSCs) have drawn extensive attentions due to the diversified structure, low cost, light weight, and the potential solution processing large-area printing technology [[1], [2], [3], [4], [5]]. Traditionally, PSC's active layer is fabricated with a bulk-heterojunction (BHJ) architecture with p-type polymers as donors and n-type semiconductors as acceptors [6,7]. In order to improve the efficiency of PSCs, great efforts have been made over the past decades, including designing new donor and acceptor materials, controlling active layer morphology, optimizing device structures, and etc. [5,[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]] In 2015, Zhan et al. proposed a novel high-efficiency n-type fused-ring small molecule acceptor-ITIC [19]. Compared with phenyl-C71-butyric-acid-methyl ester (PC71BM), ITIC exhibits the advantages of broadening the absorption spectrum, low highest molecular occupied orbit (HOMO) energy level and lowest molecular unoccupied orbit (LUMO) energy level, good electron transport ability and easy miscibility with polymer donors [[19], [20], [21]]. Until now, PSC's PCE have reached up to 14% with ITIC derivatives as the small molecule acceptor [22]. Therefore, it is necessary to develop more novel polymer donor materials that match ITIC to further improve the PCE of PSCs.
Benzodithiophene (BDT) has been widely applied in BHJ PSCs owing to its planar conjugated structure, adjustable molecular energy levels, easily modifiable structure and high mobility [[23], [24], [25], [26], [27], [28]]. Asymmetric thieno[2,3-f]benzofuran (TBF) has a similar molecular structure with BDT by replacing one of the thiophene rings of BDT with a furan ring. However, TBF unit attracted less attention compared to the well-studied BDT unit. In fact, as reported in the previous literatures, the oxygen atom in the furan ring with the higher electronegativity can downshift HOMO energy levels and induce π-π stacking [[29], [30], [31]]. Meanwhile, TBF unit does not produce excessive molecular aggregation due to its asymmetric structure, which may be beneficial for improve photovoltaic properties. For example, Zou and coworkers designed and synthesized a series of conjugated polymers based on TBF unit and PCE exceed 6% [[32], [33], [34], [35], [36], [37]]. Three conjugated polymers based on TBF unit have been reported by our group [38]. The PTBFEH-BDD:ITIC based devices achieved a higher PCE of 11.13%, which is comparable with its BDT counterparts. In view of the above consideration, TBF unit probably will be a promising electron donor building block.
As is well-known, optimizing the side chains is another effective way to improve device performance [[39], [40], [41], [42], [43], [44], [45], [46]]. The side chains not only impact the solubility, molecular packing and polymer morphology, but also great affect the electronic state of the polymer by atoms modifying, which would affect the band gap and photovoltaic properties of the conjugated polymer [47]. Generally, two-dimensional (2D) conjugated side chains are widely applied and explored due to broad and strong visible absorption, large conjugated planes and crystallization properties [33]. Yuan et al. have synthesized two 2D conjugated polymers PBO-m-FPO and PBO-p-FPO by introducing fluorinated alkoxyphenyl conjugated side chains on BDT unit, which have exhibited desirable photovoltaic performance [48]. Liu et al. have reported a series of 2D-BDT conjugated polymers with alkoxyphenyl, alkylthio phenyl and alkylphenyl as the conjugated side chains. alkylphenyl-substituted polymer PBT1-C delivered a high PCE of 12.7% [8]. In BDT monomers, the fluorinated alkoxyphenyl and alkylphenyl conjugated side chains have proven to be helpful in improving photovoltaic performance, how these concepts function in TBF-based polymers are still unknown.
In this work, two polymers (PTBFPF-BDD and PTBFP-BDD), based on asymmetrical building block-TBF modified with the fluorinated alkoxyphenyl and alkylphenyl side chains and BDD acceptor, were designed and synthesized. The chemical structures of these two polymers are shown in Scheme 1. These two polymers were characterized by thermogravimetric analysis (TGA), UV–vis absorption spectroscopy (UV) and cyclic voltammetry (CV). As a result, both polymers presented similar absorption characteristics in the film (strong and broad absorption at 350–690 nm) and showed a medium band gap of 1.79 eV, which displayed complementary absorption spectrum with ITIC. PSCs based on PTBFP-BDD:ITIC showed a higher PCE of 8.50% with VOC of 0.88 V, JSC of 16.01 mA/cm2 and FF of 59.80%. In addition, the introduction of fluorine atom lower the HOMO energy levels, and thus PTBFPF-BDD:ITIC-based devices exhibit a higher VOC. The PCE of the devices under the optimized device fabrication conditions were 8.22% (a VOC of 0.92 V, JSC of 15.54 mA/cm2 and FF of 57.33%) for PTBFPF-BDD.
Section snippets
Synthesis of materials
All reagents and solvents used in the experiment were purchased from commercial source. Toluene and tetrahydrofuran (THF) were dried using sodium under an argon atmosphere and the other solvents used without any further purification. Thieno [2,3-f]benzofuran-4,8-dione was synthesized according to the literature [36]. The synthetic routes for the new monomers and polymers are shown in Scheme 1.
Synthesis of 4-bromo-1-((2-ethylhexyl)oxy)-2-fluoro-benzene (2). 4-Bromo-2-fluorophenol (5 g,
Synthesis and characterization
The chemical structures and synthetic routes of monomers and polymers are shown in Scheme 1. The chemical structures of monomers and intermediates were determined by 1H NMR and 13C NMR spectra (Figs. S9–16). Polymers PTBFPF-BDD and PTBFP-BDD were prepared by Stille coupling reaction based on M1 or M2 and BDD-Br monomer with Pd2(dba)3 and P(o-tol)3 as catalysts in the toluene. Polymers can be dissolved well in chlorobenzene and o-dichlorobenzene. The detailed synthesis steps are given in the
Conclusions
In summary, two novel donor-acceptor conjugated polymers PTBFPF-BDD and PTBFP-BDD, based on fluorinated alkoxyphenyl or alkylphenyl groups as the substituents of electron-donating thieno [2,3-f] benzofuran (TBF) unit and an electron-deficient BDD unit, were designed and synthesized. The optimal PCE of PTBFPF-BDD and PTBFP-BDD based devices both over 8% after thermal annealing. PTBFP-BDD based devices achieved a higher PCE of 8.50% with a VOC of 0.88 V, JSC of 16.01 mA/cm2 and FF of 59.80%, due
Author contribution
Dongxue Wang, does the materials synthesis and most of the materials characterization and prepare the paper.
Wen Cui, does most of the device work.
Feng Li, help the materials characterization and prepare the paper.
Mingqun Yang, help the monomer synthesis and prepare the paper.
Xin Jing, does most of the DFT calculation and some devices work.
Liangmin Yu, design the whole work and prepare the paper.
Mingliang Sun, design/supervise the whole work and prepare the paper.
Conflict of interest
There are no conflicts to declare.
Acknowledgements
The authors are deeply grateful to Fundamental Research Funds for the Central Universities (201822002) and Natural Science Foundation of Shandong Province (ZR2018MEM023), and National Natural Science Foundation of China (U1806223).
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