A double B←N bridged bipyridine (BNBP)-based polymer electron acceptor: all-polymer solar cells with a high donor : acceptor blend ratio

Abstract

A new polymer electron acceptor (P-BNBP-CDT) composed of an alternating double B←N bridged bipyridine (BNBP) unit and a cyclopenta-[2,1-b:3,4-b′]-dithiophene (CDT) unit has been developed. P-BNBP-CDT exhibits strong light absorption in the visible range of 500–650 nm and suitable LUMO/HOMO energy levels (E_LUMO/HOMO) of −3.45 eV/−5.64 eV, which are very complementary to that (E_LUMO/HOMO = −3.2 eV/−5.2 eV) of the widely-used polymer donor, poly(3-hexylthiophene) (P3HT). All-polymer solar cells (all-PSCs) with P3HT as an electron donor and P-BNBP-CDT as an electron acceptor exhibit power conversion efficiencies (PCEs) exceeding 1.0% with high donor : acceptor blend ratios (w : w, from 0.5 : 1 to 9 : 1). The highest PCE of these devices is 1.76% with a high donor : acceptor blend ratio of 5 : 1. These results not only indicate that BNBP-based polymers are promising for P3HT : polymer acceptor devices, but also suggest the potential for low cost and facile device processing of all-PSCs.

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Introduction

Polymer solar cells (PSCs) as an important kind of clean energy technique have attracted considerable attention due to their great advantages of low cost, light weight and mechanical flexibility.¹ PSCs usually use conjugated polymers as electron donors and fullerene derivatives as acceptors in the active layer.^2,3 Poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PC₆₁BM) are the most representative donor and acceptor materials, respectively. The PSC device based on the P3HT:PC₆₁BM blend has shown great performance improvement through device optimization. However, the narrow absorption between 200 and 400 nm of PC₆₁BM and the large LUMO/HOMO energy level (E_LUMO/HOMO) offsets (0.68 eV/0.9 eV) between P3HT and PC₆₁BM limit the further development of the P3HT:PC₆₁BM device.⁴ Recently, conjugated polymers as electron acceptors have attracted much attention because they show the advantages of tunable absorption properties and energy levels of polymer acceptors, and improved morphology stability of the blends.^5,6 For example, Kim et al. developed all-polymer solar cells (all-PSCs) with high strength and flexibility due to the stable polymer : polymer blend morphologies.^5e,f The all-PSCs based on the P3HT : polymer acceptor blend have been intensively studied. Neher et al., Ma et al., Loi et al. and Sirringhaus et al. have demonstrated detailed morphology optimizations of all-PSCs using poly((N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl)-alt-5,5′-(2,2-bithiophene)) (N2200 or P(NDI2ODT2)) as electron acceptors.⁷ Li et al., Miyake et al. and Kim et al. optimized processing conditions of the polymer donor : acceptor blends using the conventional 2,1,3-benzothiadiazole-based conjugated polymers as electron acceptors.⁸ Though the P3HT : polymer acceptor blend-based all-PSCs have shown considerable progress, it is still crucial to develop new polymer acceptors, which should show broad absorption and suitable E_LUMO/HOMO that are well matched with that of P3HT.

Following our strategy to develop polymer acceptors using the B←N unit (boron–nitrogen coordination bond),⁹ we have reported a series of polymer electron acceptors based on a novel electron-deficient building block, double B←N bridged bipyridine (BNBP).¹⁰ Conjugated polymers containing the BNBP unit exhibit tunable E_LUMO/HOMO, strong absorbance in the visible range and high electron mobility, which are very desirable for polymer acceptors. As a result, all-PSCs with these BNBP-based polymer acceptors show remarkably high PCEs and high open-circuit voltage (V_oc). These results strongly indicate that BNBP-based polymer acceptors are very promising for all-PSCs.

In this manuscript, we report a novel BNBP-based polymer acceptor, P-BNBP-CDT, possessing broad absorption and suitable energy levels. As shown in Scheme 1, P-BNBP-CDT is the alternating copolymer of the BNBP unit and the cyclopenta-[2,1-b:3,4-b′]-dithiophene (CDT) unit. P-BNBP-CDT exhibits strong light absorption in the visible range of 500–650 nm, which is very complementary to the absorption between 400 and 600 nm of P3HT. The relatively high-lying E_LUMO/HOMO of −3.45 eV/−5.64 eV of P-BNBP-CDT matches well with that of P3HT. All-PSCs with P-BNBP-CDT as the acceptor and P3HT as the donor exhibit PCE values exceeding 1.0% with high donor : acceptor blend ratios (w : w, from 0.5 : 1 to 9 : 1), and a good PCE of 1.76% with a high donor : acceptor blend ratio of 5 : 1 was obtained. This is very rarely observed in the reported PSCs, and only several specific polymer/fullerene PSCs exhibit this phenomenon.¹¹ For example, Hou et al. reported polymer/fullerene PSCs exhibiting PCEs of over 5% with a blend ratio of 4 : 1 and only 2.03% with a high blend ratio of 9 : 1.^11a Ma et al. demonstrated polymer/fullerene PSCs with PCEs of over 4% and only 0.32% with the blend ratios of 3.3 : 1 and 10 : 1, respectively.^11b Moreover, these all-PSC devices produce a V_oc of ca. 1.0 V, which is higher than that of the P3HT:PC₆₁BM device by 0.4 V. These results not only indicate that BNBP-based polymers are promising for the P3HT : polymer acceptor devices, but also suggest the potential for low cost and facile device processing of all-PSCs.

Scheme 1 Synthetic route of P-BNBP-CDT. Reagents and conditions: (a) n-BuLi, THF, −78 °C, then C₁₂H₂₅Br, from −78 °C to 65 °C; (b) BF₃·Et₂O, Et₃N, CH₂Cl₂, 50 °C; (c) Pd₂(dba)₃, P(o-tolyl)₃, toluene, 120 °C.

Results and discussion

Synthesis and characterization

Scheme 1 shows the synthetic route of P-BNBP-CDT. The two monomers were prepared following procedures previously published in the literature.^9c,10aP-BNBP-CDT was synthesized via Stille-polymerization of the two monomers with Pd₂(dba)₃/P(o-tolyl)₃ as the catalyst system. According to gel permeation chromatography (GPC) with 1,2,4-trichlorobenzene as the eluent at 150 °C, the number-average molecular weight (M_n) is 15.4 kDa and the polydispersity (PDI) is 1.88. According to thermogravimetric analysis (TGA), P-BNBP-CDT shows good thermal stability with a thermal decomposition temperature (T_d) at 5% weight loss of over 380 °C (ESI†). In addition, the polymer exhibits good solubility in common organic solvents, including chloroform (CHCl₃), chlorobenzene (CB), and o-dichlorobenzene (o-DCB).

Absorption properties

The absorption spectrum of P-BNBP-CDT in CHCl₃ solution shows a strong absorption band from 500 to 650 nm with the main absorption peak at λ = 628 nm and a high molar absorption coefficient of 1.08 × 10⁵ M⁻¹ cm⁻¹ (Fig. 1). In the thin film, the absorption spectrum is red-shifted by 20 nm due to intermolecular interactions in the aggregation state. According to the onset absorption wavelength in the film, the optical bandgap of P-BNBP-CDT is estimated to be 1.85 eV. The strong absorption in the visible range of P-BNBP-CDT is expected to improve the solar light absorption of the active layer of all-PSCs (Table 1).

Fig. 1 UV/Vis absorption spectra of P-BNBP-CDT in CHCl₃ solution and in film.

Table 1 Photophysical properties, electrochemical properties and energy levels of P-BNBP-CDT

λ abs (nm)	ε max (M^−1 cm^−1)	λ em (nm)	λ abs (nm)	λ onset (nm)	E ^optg ^b (eV)	E ^oxonset (V)	E ^redonset (V)	E HOMO (eV)	E LUMO (eV)	E ^elecg ^b (eV)
a Measured in dilute CHCl3 solution. b Measured in thin film. c Onset potential vs. Fc/Fc^+. d Calculated using the equations (EHOMO = −(4.80 + E^oxonset) eV, ELUMO = −(4.80 + E^redonset) eV).
628	108 000	655	648	670	1.85	+0.84	−1.35	−5.64	−3.45	2.19

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Fluorescence properties

Fig. 2 shows the fluorescence spectra of P-BNBP-CDT in organic solvents with different polarities, including toluene, CH₂Cl₂, THF and CHCl₃. The fluorescence peaks of P-BNBP-CDT in various organic solvents are all around 655 nm, which are almost insensitive to the polarities of solvents. It is well known that D–A type conjugated molecules or polymers (alternating electron-rich unit and electron-deficient unit) usually exhibit red-shifted fluorescence peaks in polar solvents compared to in non-polar solvents. Therefore, the absence of solvatochromic effects of P-BNBP-CDT confirms that P-BNBP-CDT is not a typical D–A type conjugated polymer. This is fully consistent with the theoretical calculation results of P-BNBP-CDT, which show both the delocalized LUMO and the delocalized HOMO over the polymer backbones (vide infra).

Fig. 2 Normalized fluorescence spectra of P-BNBP-CDT in different solvents.

Theoretical calculations

To elucidate the polymer backbone configuration and electronic structure of P-BNBP-CDT, density functional theory (DFT) calculations at the B3LYP/6-31G* level of theory were performed using the model compound containing four repeating units with the long alkyl chains replaced by methyl groups.¹² As shown in Fig. 3, the model compound of P-BNBP-CDT exhibits a “wavy” backbone configuration. A slight twisted conformation is observed with an optimized twist angle of 21° between the BNBP and CDT units. It is noteworthy that both the calculated LUMO and HOMO of the model compound are delocalized over the electron-deficient BNBP units and the electron-rich CDT units. These delocalized molecular orbitals are similarly observed in the reported BNBP-based polymers, suggesting that BNBP-containing conjugated polymers are not typical D–A type conjugated polymers, even if a highly electron-rich CDT unit is introduced in P-BNBP-CDT.

Fig. 3 Kohn–Sham molecular orbitals of the model compound of P-BNBP-CDT (B3LYP/6-31G*).

Electrochemical properties

To further investigate the electronic structure of P-BNBP-CDT, cyclic voltammetry (CV) measurements were carried out with the thin film of P-BNBP-CDT. The ferrocene/ferrocenium (Fc/Fc⁺) reference was used as an internal standard, which was assigned an absolute energy of −4.80 eV vs. the vacuum level. As shown in Fig. 4, the cyclic voltammogram exhibits irreversible reduction and oxidation waves with the onset potentials of E^red_onset = −1.35 V and E^ox_onset = +0.84 V. Accordingly, the LUMO/HOMO energy levels of P-BNBP-CDT are estimated to be −3.45 eV/−5.64 eV. Its E_HOMO/LUMO are higher than that of typical BNBP-based polymers, indicating that the electron-rich CDT unit can enhance the E_HOMO/LUMO of P-BNBP-CDT due to its delocalized HOMO/LUMO. Moreover, its E_HOMO/LUMO is higher than that (E_HOMO/LUMO = −6.10 eV/−3.88 eV) of PC₆₁BM by ca. 0.4 eV, suggesting that high V_oc could be obtained in P-BNBP-CDT-based all-PSCs.

Fig. 4 Cyclic voltammogram of P-BNBP-CDT in film. Scan rate: 100 mV s⁻¹; working electrode: glassy carbon; reference electrode: Ag/AgCl; supporting electrolyte: 0.1 M n-Bu₄NClO₄ in CH₃CN. Fc/Fc⁺ = ferrocene/ferrocenium.

Photovoltaic properties

The absorption properties and energy levels of P3HT and P-BNBP-CDT were compared. As shown in Fig. 5a, P3HT shows absorption between 400 and600 nm in the visible range, which is very complementary to that of P-BNBP-CDT. The E_LUMO/HOMO offsets between P3HT and P-BNBP-CDT are 0.25 eV/0.44 eV, which could enable the photo-induced electron transfer from P3HT to P-BNBP-CDT and the photo-induced hole transfer from P-BNBP-CDT to P3HT. Thus, we used P-BNBP-CDT as the electron acceptor and P3HT as the electron donor to fabricate all-PSC devices. All-PSCs were fabricated with the configuration of ITO/PEDOT:PSS/P3HT:P-BNBP-CDT/LiF/Al. The active layer was spin-coated from the blend of donor : acceptor polymers in CHCl₃ solution without any additives.

Fig. 5 (a) UV/Vis absorption spectra of P-BNBP-CDT and P3HT in thin films; (b) the energy level alignments of P-BNBP-CDT and P3HT; (c) schematic of the all-PSC device structure.

Fig. 6 shows the current density–voltage (J–V) curves and the external quantum efficiency (EQE) spectra of the P3HT:P-BNBP-CDT devices under AM 1.5G illumination (100 mW cm⁻²). The photovoltaic parameters are summarized in Fig. 7 and Table 2. The P3HT : P-BNBP-CDT (w : w, 0.5 : 1) device exhibits a photovoltaic performance with an open-circuit voltage (V_oc) of 0.99 V, a short-circuit current density (J_sc) of 2.96 mA cm⁻², and a fill factor (FF) of 0.43, corresponding to a PCE of 1.26%. The preliminary device performance suggests enough E_LUMO/HOMO offsets between P3HT and P-BNBP-CDT, and efficient charge separation in the blend.

Fig. 6 (a and b) J–V curves and (c and d) EQE spectra of the P3HT : P-BNBP-CDT devices with different donor : acceptor blend ratios.

Fig. 7 V _oc, J_sc, FF and PCE versus the donor : acceptor blend ratios of the P3HT : P-BNBP-CDT devices.

Table 2 The photovoltaic parameters for the P3HT:P-BNBP-CDT devices

Donor (w : w)	V oc (V)	J sc (mA cm^−2)	J ^(cal.)sc (mA cm^−2)	FF	PCEmax (%)	PCEave^a (%)	EQE
a The average PCE value is calculated from four devices.
P3HT (0.5 : 1)	0.99	2.96	2.83	0.43	1.26	1.15	0.18
P3HT (1 : 1)	0.98	3.56	3.48	0.41	1.44	1.32	0.25
P3HT (2 : 1)	0.99	3.73	3.66	0.40	1.47	1.38	0.26
P3HT (3 : 1)	0.98	4.16	4.02	0.36	1.48	1.36	0.29
P3HT (4 : 1)	0.97	4.60	4.49	0.35	1.57	1.44	0.27
P3HT (5 : 1)	1.01	4.98	4.86	0.35	1.76	1.67	0.31
P3HT (6 : 1)	1.01	4.56	4.42	0.37	1.69	1.60	0.32
P3HT (7 : 1)	1.00	4.59	4.45	0.34	1.58	1.49	0.31
P3HT (8 : 1)	0.95	4.14	4.02	0.32	1.26	1.15	0.26
P3HT (9 : 1)	0.94	3.88	3.77	0.30	1.10	1.01	0.24
P3HT (10 : 1)	0.92	3.12	3.01	0.27	0.79	0.68	0.21

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The all-PSC devices were carefully optimized and the effects of the blend ratios were investigated (Fig. 6). The thicknesses of the blend films are in the range of 80–100 nm. When increasing the blend ratios of the devices from 0.5 : 1 to 5 : 1, the J_sc and PCE values gradually enhance. Upon further increasing the blend ratios of the devices from 5 : 1 to 10 : 1, the J_sc and PCE values exhibit decreasing trends (Fig. 7). The V_oc and FF values show small changes in the variations of donor : acceptor ratios. Thus, the P3HT : P-BNBP-CDT (w : w, 5 : 1) device shows the highest PCE of 1.76% with a V_oc of 1.01 V, a J_sc of 4.98 mA cm⁻² and a FF of 0.35. It is very attractive that the P3HT : P-BNBP-CDT devices can give PCEs exceeding 1% with very high blend weight ratios from 0.5 : 1 to 9 : 1. Moreover, these all-PSC devices produce a V_oc of ca. 1.0 V, which is higher than that of the P3HT : PC₆₁BM device by 0.4 V. This is due to the higher E_LUMO of P-BNBP-CDT compared with that of PC₆₁BM. These results indicate that all-PSC devices can be fabricated using a large amount of P3HT with good device performance, suggesting the important potential for low cost and facile device processing of all-PSCs.¹¹

The EQE spectra of the P3HT:P-BNBP-CDT devices are shown in Fig. 6c and d. The all-PSC devices all show a broad EQE response from 300 nm to 700 nm, contributed from the absorption of both P3HT and P-BNBP-CDT. The P3HT : P-BNBP-CDT (w : w, 0.5 : 1) device shows the maximum EQE of 0.18 in 500–600 nm. When increasing the blend ratios of the devices from 0.5 : 1 to 5 : 1, the maximum EQE values gradually increase to 0.31. Upon further increasing the blend ratios from 5 : 1 to 10 : 1, the maximum EQE values decreased. The J_sc values calculated from the integration of EQE spectra with the AM 1.5G spectrum are consistent with the J_sc values obtained from the J–V scans within 10% error.

We also investigated the charge carrier mobilities of the blend films using the space-charge-limited current (SCLC) method with the electron-only and hole-only devices (ESI†). The data are shown in Fig. S10 and listed in Table S2 (ESI†). In the active layers, the hole/electron mobility (μ_h/μ_e) ratios are 6/1 for the P3HT : P-BNBP-CDT (w : w, 0.5 : 1) blend, 8/1 for the P3HT : P-BNBP-CDT (w : w, 5 : 1) blend, and 46/1 for the P3HT : P-BNBP-CDT (w : w, 9 : 1) blend, respectively. The relatively balanced charge carrier mobilities of the P3HT : P-BNBP-CDT (w : w, 0.5 : 1 and 5 : 1) devices are consistent with their good FF values.

Based on these experimental results, it is reasonable that the P3HT : P-BNBP-CDT (w : w, 5 : 1) device shows the best device performance. Increasing the P3HT : P-BNBP-CDT weight ratios from 0.5 : 1 to 5 : 1 can enhance light absorption in the range of 350–600 nm from P3HT and keep the balanced charge carrier mobilities, which probably results in increasing the J_sc values and maintaining the FF values. Further increasing the P3HT : P-BNBP-CDT weight ratios from 5 : 1 to 9 : 1 leads to the imbalance of charge carrier mobilities and poor exciton dissociation due to the small amount of P-BNBP-CDT, probably decreasing the J_sc and FF values. As a result, while the P3HT : P-BNBP-CDT devices produce PCEs exceeding 1% with high blend weight ratios, the P3HT : P-BNBP-CDT (w : w, 5 : 1) device shows the best device performance.

At last, the morphology of the optimal P3HT:P-BNBP-CDT device was characterized using transmission electron microscopy (TEM) and atomic force microscopy (AFM) (Fig. 8). The TEM image exhibits nanostructures with uniform fibrous aggregation and no large-size aggregation can be observed. The AFM height and phase images of the blend exhibit fine phase-separation morphology with domain sizes of around 20–40 nm and a smooth surface with a root-mean-square (RMS) roughness of 1.21 nm. Such a phase separation morphology is very beneficial for all-PSCs.

Fig. 8 (a) The TEM image and (b and c) the AFM height and phase images of the P3HT : P-BNBP-CDT (w : w, 5 : 1) blend, respectively.

Conclusions

In conclusion, we have developed a new conjugated polymer composed of alternating BNBP and CDT units. P-BNBP-CDT exhibits strong light absorption in the visible range and suitable LUMO/HOMO energy levels, which match well with those of P3HT. All-PSCs with P-BNBP-CDT as the acceptor and P3HT as the donor exhibit PCE values exceeding 1.0% with high donor : acceptor blend ratios from 0.5 : 1 to 9 : 1, with the highest PCE of 1.76% with a high donor : acceptor blend ratio of 5 : 1. These results not only indicate that BNBP-based polymers are promising for P3HT : polymer acceptor devices, but also show the potential for low cost and facile device processing of all-PSCs.

Experimental section

Materials

Commercially available solvents and reagents were used without further purification unless otherwise mentioned. Ether, toluene, THF and CH₂Cl₂ were dried using sodium or calcium hydroxide before use. n-BuLi (2.5 M in hexane) was purchased from J&K Chemical Industries and used as received.

Measurements and characterization

¹H, ¹³C and ¹¹B NMR spectra were measured using a Bruker AV-400 (400 MHz for ¹H, 100 MHz for ¹³C and 128 MHz for ¹¹B) spectrometer in CDCl₃ at 25 °C. Chemical shifts are reported in δ ppm using CHCl₃ (7.26 ppm) as the internal standard for ¹H NMR and using CDCl₃ (77.16 ppm) as the internal standard for ¹³C NMR. The external standard of BF₃·OEt₂ was used for the ¹¹B NMR spectrum. Elemental analysis was performed on a VarioEL elemental analyzer. The molecular weight of the polymer was determined using gel permeation chromatography (GPC) on a PL-GPC 220-type at a temperature of 150 °C. 1,2,4-Trichlorobenzene (TCB) was used as the eluent and monodisperse polystyrene was used as the standard. UV/Vis absorption spectra and fluorescence spectra were measured using a Shimadzu UV-3600 spectrometer and a Hitachi F-4500 spectrometer, respectively, in spectral grade solvents. Thermal analyses were performed on a Perkin-Elmer 7 instrument under a nitrogen flow at a heating rate of 10 °C min⁻¹. The transmission electron microscope (TEM) image was obtained using a JEM-1011 (JEOL Co., Japan) operated at an accelerating voltage of 100 kV. Cyclic voltammetry (CV) was performed on a CHI660a electrochemical workstation using Bu₄NClO₄ (0.1 M) in acetonitrile as the electrolyte solution and ferrocene as the internal reference, at a scan rate of 100 mV S⁻¹. The CV cell consisted of a glassy carbon electrode, a Pt wire counter electrode, and a standard calomel reference electrode. For CV measurements, the polymer was casted on the working electrode. The redox potentials were calibrated with ferrocene as the internal standard. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the materials were estimated using the following equations: E_HOMO/LUMO = −(4.80 + E^ox_onset/E^red_onset) eV. All reactions were performed under an argon atmosphere.

Synthesis

5,5′-Dibromo-N3,N3′-didodecyl-[2,2′-bipyridine]-3,3′-diamine 7 (2). Under argon, to a solution of 1 (402 mg, 1.18 mmol) in THF (35 mL) was added n-BuLi (2.5 M in hexane, 0.96 mL, 2.41 mmol) dropwise at −78 °C. After the mixture was stirred at −78 °C for 1 h, n-C₁₂H₂₅Br (1.75 g, 7.06 mmol) was added under argon. The mixture was stirred for 4 h at 65 °C. After removing the solvents under reduced pressure, the brown oil was purified using silica gel column chromatography (CH₂Cl₂/hexane = 1/1). 2 was obtained as a light yellow solid in 57% yield (450 mg, 0.67 mmol). ¹H NMR (400 MHz, CDCl₃, 25 °C): δ 9.62 (s, 1H), 7.83 (d, J = 2.0 Hz, 1H), 7.09 (d, J = 2.0 Hz, 1H), 3.14 (dd, J = 3.6, 11.6 Hz, 2H), 1.76–1.67 (m, 2H), 1.46–1.27 (m, 18H), 0.88 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃, 25 °C): δ 146.56, 137.79, 132.50, 119.94, 119.26, 43.01, 32.08, 29.82, 29.75, 29.52, 29.48, 28.96, 27.42, 22.85, 14.28. Anal. calcd for C₃₄H₅₆Br₂N₄: C, 60.00; H, 8.29; Br, 23.48; N, 8.23. Found: C, 59.20; H, 8.53; N, 8.10.

Dibromo-substituted BNBP-C12 (3). Under argon, BF₃·Et₂O solution (3.6 mL, 29.2 mmol) was added to a solution of 2 (900 mg, 1.46 mmol) and Et₃N (1.8 mL) in dry CH₂Cl₂ (60 mL). The mixture was stirred at 50 °C for 1 h with the color changing from yellow to orange. After removing the solvents, the orange solid was purified using silica gel column chromatography (CH₂Cl₂/hexane = 2/1). 3 was obtained as an orange solid in 75% yield (852 mg, 1.10 mmol). ¹H NMR (400 MHz, CDCl₃, 25 °C): δ 8.19 (s, 1H), 7.61 (s, 1H), 3.55 (t, J = 6.8 Hz, 2H), 1.69–1.62 (m, 2H), 1.46–1.24 (m, 18H), 0.88 (t, J = 5.2 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃, 25 °C): δ 144.07, 128.77, 125.30, 124.70, 122.40, 44.32, 32.07, 29.80, 29.78, 29.75, 29.72, 29.50, 29.46, 27.70, 27.24, 22.84, 14.27. Anal. calcd for C₃₄H₅₄B₂Br₂F₄N₄: C, 52.61; H, 7.01; B, 2.79; Br, 20.59; F, 9.79; N, 7.22. Found: C, 52.25; H, 7.47; N, 7.02.

Polymer P-BNBP-CDT. Starting materials of 3 (153 mg, 0.21 mmol), the CDT monomer (155 mg, 0.20 mmol), Pd₂(dba)₃ (3.7 mg, 0.004 mmol) and P(o-tolyl)₃ (9.7 mg, 0.032 mmol) were placed in a two-necked flask under argon, and then dried toluene (10 mL) was added. After the mixture was stirred at 120 °C for 18 h, an end-capping reaction was carried out by adding bromobenzene (200 mg, 1.28 mmol). After cooling, the resulting organic phase was extracted with toluene (150 mL) and washed with water. After the solvents were removed, the residue was dispersed in methanol and the precipitate was collected. The polymer was purified by washing with hot hexane, and the solution was collected. The obtained dark solid was dispersed in acetonitrile, which was collected and dried under vacuum overnight. Yield: 163 mg (80%). ¹H NMR (400 MHz, CDCl₃, 25 °C): δ 8.40 (br, 2H), 7.55 (br, 2H), 7.43 (br, 2H), 3.70 (br, 4H), 2.02 (br, 4H), 1.77 (br, 4H), 1.30–0.67 (br, 72H). ¹¹B NMR (128 MHz, CDCl₃, 25 °C): δ 0.97 (s). GPC (TCB, polystyrene standard, 150 °C): M_n = 15 400, PDI = 1.88. Anal. calcd for C₅₉H₉₀B₂F₄N₄S₂: C, 69.67; H, 8.92; B, 2.13; F, 7.47; N, 5.51; S, 6.30. Found: C, 69.39; H, 8.98; N, 5.30; S, 6.45.

Fabrication and characterization of polymer solar cells

Indium tin oxide (ITO) glass substrates were cleaned via sequential ultrasonication in detergent, deionized water, acetone, and isopropyl alcohol, followed by drying at 120 °C for 30 min and treated with UV-ozone for 25 min. Then PEDOT:PSS (Baytron P Al 4083) was spin-coated on the ITO glass substrates at 5000 rpm for 40 s to give a thickness of 40 nm, followed by baking at 125 °C for 30 min. The active layer was spin-coated with the corresponding solution in CHCl₃ (8 mg mL⁻¹) at 2500 rpm for 60 s. Finally, the device was transferred to a vacuum chamber, and LiF (1 nm)/Al (100 nm) was sequentially deposited via thermal evaporation at a pressure of about 4 × 10⁻⁴ Pa. The active area of each device was 8 mm². The current density (J–V) characteristics of the PSC devices were measured using a computer-controlled Keithley 236 source meter and an Oriel 150 W solar simulator with an AM 1.5G filter and a light intensity of 100 mW cm⁻².

Acknowledgements

This work was financially supported by the 973 Project (No. 2014CB643504), the Nature Science Foundation of China (No. 51373165, 21574129, 21404099), the “Thousand Talents Program” of China, the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12010200), the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the State Key Laboratory of Supramolecular Structure and Materials in Jilin University (No. sklssm201608).

Notes and references

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12. DFT calculations were performed using Gaussian 09 program: M. J. Frisch, et al., Gaussian 09, revision A.02, Gaussian, Inc., Wallingford, CT, 2009 For details, see ESI†.

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Footnote

† Electronic supplementary information (ESI) available: Characterization and thermal properties of the polymer, as well as all-PSC device performance and charge-transporting properties. See DOI: 10.1039/c6qm00245e

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