English

• All-polymer solar cells (all-PSCs), comprising a binary blend of conjugated polymer electron donor and polymer electron acceptor as the photoactive layer, have attracted great attention due to their merits of excellent morphology stability and superior mechanical properties [1-3]. While a large family of electron-rich building blocks have been reported, only a few electron-deficient building blocks have been developed, such as naphthalene diimide, perylenediimide, and 1,1-dicyanomethylene-3-indanone [4-10]. As shown in Scheme 1, most of these electron-deficient building blocks are based on either amide unit or cyan units. Enhancing the power conversion efficiency (PCE) of all-polymer solar cells is highly dependent on the advancement of new polymer acceptors. An optimal polymer acceptor should possess appropriate lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels (E_LUMO/HOMO), a broad absorption spectrum, high electron mobilities, and effective phase separation morphology when combined with polymer donors [11,12]. To make further advancement to all-PSCs, it is essential to enlarge the library of polymer acceptors via developing diversified building blocks, particularly new electron-deficient units.

  Scheme 1

  

  Scheme 1.  Examples of electron-deficient building blocks based on amide, cyan, and B←N units (this work).

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  Boron (B) is considered a common electron-withdrawing center due to its unoccupied orbitals. Therefore, incorporating B electron-withdrawing center into backbones could be a viable approach for creating electron-deficient units [13,14]. For instance, conjugated backbones containing B←N coordination bond are promising electron-deficient units that have been widely studied [15-21]. Since the groundbreaking research in 2015, Liu’s group have been dedicated to studying polymer acceptors containing B←N, which have demonstrated a power conversion efficiency (PCE) of 10% in all-polymer solar cells (all-PSCs) and a PCE of 27% in all-polymer indoor photovoltaics [21,22]. A typical example of a polymer acceptor containing B←N was PBN-12, an alternating copolymer composed of a double B←N bridged bipyridine (BNBP) unit and a 4,7-dithienyl-2,1,3-benzothiadizole unit. Huang et al. successfully designed and synthesized a novel electron-deficient unit containing B←N bond, namely BNIDT [18,23]. By copolymerizing BNIDT with thiophene and 3,4-difluorothiophene, two new conjugated polymers, BN-T and BN-2fT were developed. Using PBDB-T as the donor and BN-2fT as the acceptor in all-PSCs, an impressive efficiency of 8.78% was achieved. For B←N type polymer acceptors, despite achieving high efficiencies ranging from 5% to 10%, the main bottleneck hindering further improvement lies in the poor short-circuit current (J_SC) values, typically lower than 15 mA/cm². This can also be partially attributed to the limited absorption coverage which is generally less than 800 nm. The workgroup of Chujo reported on the azobenzene complex (BAz) with a fused structure through boron coordination and investigated its optical properties [24]. BAz acts as a strong electron acceptor due to the inherent electron deficiency of the N=N double bond and the boron–nitrogen (B←N) coordination bond, which significantly reduced the energy of LUMO. The N=N photoisomerization [25-28] was limited by the formation of the B←N coordination bond, ultimately benefiting the charge transport within the conjugated polymer. However, the photovoltaic performance based on N=N double bond has not yet been reported.

  Herein, we introduce the fluorine atom to design and synthesize fluorinated fused azobenzene boron (FBAz) as a new class of electron-deficient building block for polymer electron acceptors and demonstrated its all-PSCs device performance. Fluorination has been proved to be an effective strategy to improve the PCE of PSCs, due to the adjustment of material properties such as the energy level of molecules, absorption spectrum, molecular arrangement, and the morphology of active layer [29-33]. By copolymerizing FBAz with IDT, three novel conjugated polymers named P2f, P3f and P5f are developed. It is shown that these polymers possess wide absorption spectra covering 500–1000 nm, and low-lying energy levels. Generally, polymer acceptors with extended λ_edge values to the near-infrared region, especially approaching 900 nm have been scarcely reported [34,35]. Using PTB7-Th as the donor and P3f as the acceptor, all-PSCs afford a moderate efficiency of 2.70%. Inspiringly, adding PC₆₁BM as the third component can boost device efficiency to 5.36%. Considering that the structure of FBAz is totally different from these classical units, this work opens up a new class of electron-deficient unit for constructing efficient polymer acceptors.

  The molecular structures and synthetic routes of FBAz, P2f, P3f and P5f were shown in Scheme 2. The FBAz units was prepared in a three-step synthesis starting from commercially available fluorinated 4-bromoaniline. Firstly, we tried Mills reaction of compound 1 to afford azo compound 2 and subsequent nucleophilic aromatic substitution reaction (SNAr) with OH^‒ as the nucleophile to afford ligand 3. The FBAz complexes were synthesized in high yields by the condensation reaction at 100 ℃ in toluene between the corresponding azobenzene tridentate ligands and boron trifluoride etherate (BF₃·Et₂O), respectively. FBAz complexes are highly stable in air, aqueous solution and could be purified by column chromatography on silica gel. The chemical structure of FBAz units were confirmed by ¹H NMR and ¹³C NMR spectroscopy (Supporting information). Next, using the obtained FBAz complexes as monomers coupled with the electron-rich IDT tin reagent, three FBAz-IDT copolymers P2f, P3f and P5f were synthesized. The incorporation of an electron-rich moiety was expected to enhance the conjugation and extend the absorption spectrum, thereby reducing the band gap. All the three polymers exhibited good solubility in normal chlorinated aromatic solvents, such as chloroform (CF), chlorobenzene (CB) and o-dichlorobenzene (o-DCB). According to gel permeation chromatography (GPC) with THF as eluent, the number-average molecular weight (M_n) and polydispersity index (PDI) were estimated to be 60 kDa/2.75 for P2f, 55 kDa/4.23 for P3f and 75 kDa/4.14 for P5f. According to thermogravimetric analysis (TGA) under nitrogen atmosphere P2f, P3f and P5f all exhibited excellent thermal stability with the thermal decomposition temperatures (T_d) at 5% weight loss higher than 390 ℃ (Supporting information).

  Scheme 2

  

  Scheme 2.  The synthetic route of monomers and copolymers.

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  To investigate the position-dependent substituent effect of fluorine atom in the ground state, the UV–vis absorption was measured in chloroform solution and neat film, and the corresponding optical data were shown in Table 1. As shown in Fig. 1, all of the polymers had strong absorption in the range of 500–1000 nm due to the efficient intramolecular charge transfer from the electron-rich group to the electron-deficient FBAz unit. In the chloroform solution, the P2f and P5f exhibited similar absorption spectra with the maximum absorption peaks at 810 and 812 nm, respectively. While the absorption bands of P3f in solution was broad and red-shifted with maxima at 864 nm. Compared to the solution, the films of P2f, P3f and P5f exhibited a slightly red-shifted maximum absorption peak at 817, 875 and 824 nm with a shoulder peak at 732, 740 and 638 nm, respectively, originating from the weak intermolecular interaction in the polymer chains [36-38]. Two vibronic bands observed for them are probably assigned to 0–0 and 0–1 transitions [39,40]. From the onset absorption wavelength in films, the optical band gaps (E_g) of P2f, P3f and P5f were estimated to be 1.44, 1.33 and 1.36 eV, respectively.

  Table 1

  

  Table 1.  Key material properties of the acceptor polymers.

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  Polymers	λabsmax (nm)a	λfilmmax (nm)b	λabsonset (nm)b	Egopt (eV)c	EHOMO (eV)d	ELUMO (eV)d	Egcv (eV)e
  P2f	810	817	876	1.44	−5.60	−3.99	1.61
  P3f	864	875	946	1.33	−5.66	−3.97	1.69
  P5f	812	824	922	1.36	−5.61	−4.19	1.42
  a In CHCl3 solution (0.08 mg/mL). b Spin-coated from CHCl3 solution (10 mg/mL) onto the quartz. c Calculated from the absorption edge of polymer film Egopt = 1240/λabsonset. d EHOMO = -e (Eoxonset + 4.8) (eV); ELUMO = -e (Eredonset + 4.8) (eV). e Egcv = e (Eoxonset - Eredonset).

  Figure 1

  

  Figure 1.  UV–vis absorption spectra of P2f, P3f and P5f in solution (a) and in films (b). (c) Schematic representation of the LUMO/HOMO energy level alignments of the polymer donor and the acceptors. (d) Chemical structure of PTB7-Th.

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  Cyclic voltammetry (CV) measurements were carried out on films of the three polymers to estimate the LUMO/HOMO energy levels of the three polymers (Fig. S21 in Supporting information). The E_LUMO/HOMO values of the three polymers were estimated from the onsets of the reduction and oxidation potentials in the cyclic voltammograms. As shown in Fig. 1c, the E_LUMO/E_HOMO values of P2f, P3f and P5f were calculated to be −3.99/−5.60 eV, −3.97/−5.66 eV and −4.19/−5.61 eV, respectively. The E_LUMO/_HOMO values of P2f, P3f and P5f matched well with that of polymer donor poly[(ethylhexylthiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7-Th). In view of the energy level alignments, P2f, P3f and P5f can be used as polymer electron acceptors for all-PSCs.

  To elucidate the polymer backbone configuration and energy levels. The dimers were fully optimized at B3LYP-D3/def2-SVP level with Gaussian 09 package in combination with frequency validation to confirm local minima [41]. The dimers were constructed to simulate the corresponding polymeric chains, and the terminals were truncated with hydrogen atoms (Figs. S22 and S23 in Supporting information). Interestingly, an azo nitrogen atom can link to the boron atom to form poly-fused heterocycles with a very strong B←N coordination bond. In P2f, P3f and P5f dimers, the B←N bond length was 1.605, 1.604 and 1.606 Å, respectively. In P3f and P5f dimers, there was a distinct halogen bond between the fluorine atom and the neighboring thiophene sulfur atom. Our theoretical results show that, the crinkling of the polymer plane arises mainly from the tetrahedral boron atom, and the nonplanarity can be described by the angle θ corresponding to the center of the B←N bond and the centroids of the fused six-membered and five-membered heterocycles. The θ values of P2f, P3f and P5f dimers were 151.1°, 151.2° and 151.3°, implying that the location of the substituted F atoms and the halogen bonds have limited influence on the planarity of the chains. The same results can also be concluded from their HOMOs and LUMOs. The HOMOs correspond to the conjugated π bonding orbitals, and the LUMOs were relative to the π* antibonding orbitals. However, the halogen bonds did not apparently enhance and extend the popularity of the conjugated π electrons, but lower the orbital energies and the HOMO/LUMO energy gaps. Compared with an energy gap of 1.87 eV in the P2f dimer, the gaps of P3f and P5f dimers were reduced to 1.69 and 1.75 eV, respectively.

  Furthermore, the electron mobilities (μ_e) of P2f, P3f and P5f were estimated using the space-charge-limited current (SCLC) method with the current density−voltage (J−V) curves of the electron-only devices (Fig. S26 in Supporting information). The electron mobility of P3f (2.6 × 10^–6 cm² V^-1 s^-1) is higher than that of P2f (1.6 × 10^–6 cm² V^-1 s^-1) and P5f (8.9 × 10^–7 cm² V^-1 s^-1). The high electron mobility of the P3f is attributed to the excellent charge transport properties.

  To investigate and compare the photovoltaic properties of the three polymers, we fabricated and tested the bulk heterojunction all-PSCs with a structure of ITO/ZnO/PTB7-Th:polymer/MoO₃/Al, where the synthesized polymers were used as the acceptor materials and PTB7-Th as donor material, and the ZnO and MoO₃ were used as the electron and hole transporting layers, respectively. The active layer was spin coated from a blend of polymer donor and polymer acceptor in o-DCB solution. The LUMO and HOMO gap between PTB7-Th (E_LUMO = −3.59 eV, E_HOMO = −5.20 eV) and P2f, P3f and P5f are large enough to allow efficient excitation dissociation. Fig. 2c showed the J−V curves under AM 1.5G illumination (100 mW/cm²) and the external quantum efficiency (EQE) spectra of optmized devices. Table S2 (Supporting information) listed the corresponding open-circuit voltage (V_OC), J_SC, fill factor (FF), and power conversion efficiency (PCE) under simulated AM 1.5G sunlight illumination. The PTB7-Th:P3f device achieved a PCE of 2.7% with V_OC of 0.81 V, J_SC of 7.47 mA/cm², and FF of 0.45, which was higher than that of the PTB7-Th:P2f device (PCE = 2.3%). The PTB7-Th:P5f showed a low PCE of only 0.83%. P3f with moderate all-PSC device performance represented a novel strategy to develop promising polymer acceptors based on the FBAz unit. The three all-PSC devices all showed broad EQE responses from 300 nm to 950 nm. The higher J_SC of the P3f device compared to that of the P2f device aligns well with their respective external quantum efficiency (EQE) values (EQE_max = 0.20 for P3f and EQE_max = 0.15 for P2f). The calculated J_SC values from the integrations of the EQE spectra value and the AM 1.5G spectrum agree well with the J_SC obtained from the J-V curves within an error of 5%. Accordingly, we anticipate that these polymers will be useful as the third component for the ternary solar cells to enhance the light absorption and act as the energy bridge.

  Figure 2

  

  Figure 2.  (a) Device structure, (b) energy level alignments, (c) J–V curves, and (d) EQE spectra of the all-PSC devices based on the PTB7-Th:P2f, PTB7-Th:P3f and PTB7-Th:P5f blends. (e) J–V curves for PTB7-Th:PC₆₁BM, PTB7-Th:P3f:PC₆₁BM.

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  The much higher J_SC values and FF values of the devices based on P3f compared to those of P2f and P5f were attributed mainly to the enhanced electron mobility of the polymer acceptor. The measurements of hole and electron mobilities in blended films were conducted using the space charge limited current (SCLC) method (Supporting information). Hole-only and electron-only diodes were fabricated using the following architectures: ITO/PEDOT:PSS/PTB7-Th:Acceptor/MoO₃/Al for holes, and ITO/ZnO/PTB7-Th:Acceptor/PDINN/Al for electrons. The hole mobilities of PTB7-Th:P2f, PTB7-Th:P3f and PTB7-Th:P5f were 2.19 × 10^–4, 3.25 × 10^–4 and 8.81 × 10^–4 cm² V^-1 s^-1 respectively. The electron mobility of PTB7-Th:P2f (1.53 × 10^–6 cm² V^-1 s^-1) was comparable to that of PTB7-Th:P3f (1.98 × 10^–6 cm² V^-1 s^-1). The more balanced electron and hole transport in PTB7-Th:P2f and PTB7-Th:P3f partially contributes to its higher efficiency compared to PTB7-Th:P5f.

  The morphology of the blend films was studied by atomic force microscopy (AFM). The AFM height and phase images were presented in Fig. 3. The PTB7-Th:P3f film had a root mean square roughness (RMS) value of 1.63 nm, which was smaller than that of PTB7-Th:P2f film (RMS = 2.28 nm). The smaller RMS of PTB7-Th:P3f film implied that the film was more homogeneous compared with that of the PTB7-Th:P2f film. As shown in the AFM phase images (Figs. 3d-f), PTB7-Th:P2f and PTB7-Th:P3f formed nano fiberous structures, which were beneficial for the charge dissociation and transport, leading to higher J_SC and FF, and thus achieving better PCE. However, when the acceptor was changed to P5f, the film became rougher, with a RMS roughness of 13.8 nm. More importantly, the presence of many big black dots suggests the morphology is far from ideal for the photo-to-electric conversion, thus resulting in a rather poor device performance.

  Figure 3

  

  Figure 3.  AFM height and phase images of PTB7-Th:P2f (a, d), PTB7-Th:P3f (b, e), and PTB7-Th:P5f (c, f).

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  Based on the result above, the low device efficiency is primarily attributed to poor electron mobility of acceptor materials. As is known to all, PC₆₁BM, due to its high electron mobility, has been widely used as electron transporting material in organic solar cells [42,43]. Therefore, adding PC₆₁BM is expected to boost electron mobility, enhance J_SC, and ultimately increase device efficiency. As shown in Fig. 2e and Table S4 (Supporting information), with device optimization, the efficiency of PTB7-Th:P3f:PC₆₁BM has reached a maximum of 5.36%. However, the device efficiency based on PTB7-Th:PC₆₁BM was only 1.56%. The EQE spectra of PTB7-Th:P3f:PC₆₁BM ternary solar cells demonstrate higher and broader absorption in comparison with PTB7-Th:P3f and PTB7-Th:PC₆₁BM binary solar cells, leading to improved efficiency (Fig. S27 in Supporting information). The PTB7-Th:P3f:PC₆₁BM blend shows a uniform morphology with well-defined phase separation, in contrast to the PTB7-Th:P3f blend (Fig. S28 in Supporting information). The AFM height image of PTB7-Th:P3f:PC₆₁BM blend shows a smooth surface with a root-mean-square (RMS) roughness of 0.86 nm, suggesting that the active materials are well mixed in the film. Therefore, the enhancement in device efficiency can be attributed to three primary factors. First, PC₆₁BM contributes to improved electron mobility, thereby facilitating more efficient charge transport. Second, the complementary absorption characteristics of PC₆₁BM and P3f further optimize the device performance. P3f, with its absorption range extending up to 1000 nm, significantly broadens the spectrum of sunlight that can be effectively utilized, thus enhancing the overall power conversion efficiency of the device. Third, PC₆₁BM doping optimizes the morphology of the active layer, resulting a better phase separation structure.

  Finally, the photostability was evaluated as the change of the UV–vis absorption spectra with irradiation by transilluminator (365 nm, 6500 µW/cm²) in the diluted solution (1.0 × 10^–5 mol/L in chloroform), as shown in Fig. S29 (Supporting information). The spectra of P2f, P3f and P5f were preserved even after irradiation with UV for 60 min, demonstrating that these three polymers exhibit excellent photostability.

  In summary, we have developed FBAz as a new class of electron-deficient building block for polymer electron acceptors and demonstrated its efficient all-PSC device performance. The B←N bridging unit endows FBAz with fixed configuration and low LUMO/HOMO energy level. Three novel D-A alternating FBAz-based polymers P2f, P3f and P5f with different fluorine substitution positions were synthesized. Through fluorinated BAz compounds, it was revealed that the influence of fluorination was systematically understandable and the fluorination was effective in controlling energy gaps of π-conjugated polymers. The study disclosed high potentiality of FBAz compounds as a strong electron acceptor. As films, all these polymers exhibited a broad absorption ranging from 500 nm to 1000 nm. P5f with the fluorine substitution on the C-5 position showed deeper LUMO energy level than P2f and P3f. The preliminary all-PSCs device based on PTB7-Th:P3f blend exhibits a PCE value of 2.70%. Inspiringly, adding PC₆₁BM as the third component can boost device efficiency to 5.36%. The better PCE of P3f-based devices is attributed to bi-continuous interdigitated network morphology and balanced hole and electron mobilities of the blended films. These results indicate that FBAz-based polymers hold significant potential for applications in all-polymer solar cells. Given that the structure of FBAz is fundamentally distinct from these classical units, this study introduces a novel class of electron-deficient unit for the development of efficient polymer acceptors.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Jiabin Zhang: Writing – original draft, Investigation, Formal analysis, Data curation. Xiaoke Zhang: Formal analysis, Data curation. Lilei Wang: Data curation. Lingpeng Yan: Writing – review & editing, Conceptualization. Xueli Cheng: Writing – review & editing, Data curation. Tao Li: Writing – review & editing, Supervision, Funding acquisition, Conceptualization.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 22375123), the Shuguang Program of Shanghai Education Development Foundation, the Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering (No. 2022SX-TD012). We are also grateful to Prof. Jun Liu and Dr. Yingze Zhang (Changchun Institute of Applied Chemistry) for their valuable support in device optimization.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111064.

  
  
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• 

  Scheme 1  Examples of electron-deficient building blocks based on amide, cyan, and B←N units (this work).

  下载: 全尺寸图片 幻灯片
  

  Scheme 2  The synthetic route of monomers and copolymers.

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  Figure 1  UV–vis absorption spectra of P2f, P3f and P5f in solution (a) and in films (b). (c) Schematic representation of the LUMO/HOMO energy level alignments of the polymer donor and the acceptors. (d) Chemical structure of PTB7-Th.

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  Figure 2  (a) Device structure, (b) energy level alignments, (c) J–V curves, and (d) EQE spectra of the all-PSC devices based on the PTB7-Th:P2f, PTB7-Th:P3f and PTB7-Th:P5f blends. (e) J–V curves for PTB7-Th:PC₆₁BM, PTB7-Th:P3f:PC₆₁BM.

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  Figure 3  AFM height and phase images of PTB7-Th:P2f (a, d), PTB7-Th:P3f (b, e), and PTB7-Th:P5f (c, f).

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  Table 1.  Key material properties of the acceptor polymers.

  Polymers	λabsmax (nm)a	λfilmmax (nm)b	λabsonset (nm)b	Egopt (eV)c	EHOMO (eV)d	ELUMO (eV)d	Egcv (eV)e
  P2f	810	817	876	1.44	−5.60	−3.99	1.61
  P3f	864	875	946	1.33	−5.66	−3.97	1.69
  P5f	812	824	922	1.36	−5.61	−4.19	1.42
  a In CHCl3 solution (0.08 mg/mL). b Spin-coated from CHCl3 solution (10 mg/mL) onto the quartz. c Calculated from the absorption edge of polymer film Egopt = 1240/λabsonset. d EHOMO = -e (Eoxonset + 4.8) (eV); ELUMO = -e (Eredonset + 4.8) (eV). e Egcv = e (Eoxonset - Eredonset).

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