Abstract
AbstractControlling the energetics and backbone order of semiconducting polymers is essential for the performance improvement of polymer‐based solar cells. The use of fluorine as the substituent for the backbone is known to effectively deepen the molecular orbital energy levels and coplanarize the backbone by noncovalent interactions with sulfur of the thiophene ring. In this work, novel semiconducting polymers are designed and synthesized based on difluoronaphthobisthiadiazole (FNTz) as a new family of naphthobisthiadiazole (NTz)–quaterthiophene copolymer systems, which are one of the highest performing polymers in solar cells. The effect of the fluorination position on the energetics and backbone order is systematically studied. It is found that the dependence of the solar cell fill factor on the active layer thickness is very sensitive to the fluorination position. It is thus further investigated and discussed how the structural features of the polymers influence the photovoltaic parameters as well as the diode characteristics and bimolecular recombination. Further, the polymer with fluorine on both the naphthobisthiadiazole and quaterthiophene moieties exhibits a quite high power conversion efficiency of 10.8% in solar cells in combination with a fullerene. It is believed that the results would offer new insights into the development of semiconducting polymers.
Highlights
This suggests that the F2–F0 backbone is more rigid than the F0–F0 backbone, most likely due to the noncovalent F⋅⋅⋅S interactions between the FNTz moiety and the neighboring alkylthiophene moiety, the coplanarity is similar at room temperature, as mentioned above
The results indicate that the F2–F0 and F2–F2 films were composed of both fractions of the edge-on and face-on orientations, whereas the F0–F0 and F0–F2 films were mostly dominated by the edge-on orientation
The low fill factor (FF) in F2–F0based cell compared to the F0–F0 cell would rather be ascribed to the increased bimolecular recombination as proven by the larger ζ, the origin is yet unclear
Summary
Semiconducting polymers are an important class of functional materials that can be solution-processed to form thin films on plastic substrates and can be applied to various flexible optoelectronic devices.[1,2,3] One of the devices that are most strongly reliant on the properties of semiconducting polymers is organic photovoltaics (OPVs), in which the semiconducting polymers are typically used as the p-type (electron donor) material in combination with fullerene derivatives or nonfullerene small molecules as the n-type (electron acceptor) material.[4,5,6,7,8,9,10,11,12] Research of OPVs has seen great advances in the last decade owing to the development of a wide variety of semiconducting polymers with donor–acceptor motifs wherein electron-rich and electron-deficient π-conjugated building units (donor and acceptor) are alternately incorporated in the backbone.[13,14,15,16,17,18,19,20,21,22] This design strategy has enabled us to tune polymer. The desired structural features for the semiconducting polymers are high crystallinity and backbone orientation with the “face-on” motif.[40,41,42,43] Such favorable structural features would bring about high charge carrier mobility, which is crucial for a high fill factor (FF) It would enable the use of thick active layers, which are beneficial for increasing light absorption and JSC.[44,45]. We reported that an NTz-based polymer, PNTz4T (Figure 1a), showed crystalline structures with the face-on orientation in the polymer/fullerene blend film, which led to PCEs of ≈10% in the PC71BM-based cell.[9] More recently, we developed fluorinated NTz-based polymers, PNTz4TF2 and PNTz4TF4 (Figure 1b), in which two and four fluorine atoms were introduced into the β-positions of the bithiophene moiety, respectively (Figure 1b).[50] Both fluorinated polymers had deeper HOMO energy levels than PNTz4T, whereas they had wider optical bandgaps than PNTz4T. PNTz4T, PNTz4TF2, PNTz4TF4, PFN4T, and PFN4TF2 will be hereinafter called F0–F0, F0–F2, F0–F4, F2–F0, and F2–F2, respectively (Figure 1a–c)
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