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A2-D-A1-D-A2-type small molecule acceptors incorporated with electron-deficient core for non-fullerene organic solar cells
Solar Energy 197 (2020) 511–518
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A2-D-A1-D-A2-type small molecule acceptors incorporated with electrondeficient core for non-fullerene organic solar cells ⁎
Min Zhang, Min Zeng, Huajie Chen, Lanyan Li, Bin Zhao , Songting Tan
T
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Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Small molecule acceptors Synthesis Noncovalent conformational locks Non-fullerene Organic solar cells Photovoltaic performance
Two A2-D-A1-D-A2-type small molecule acceptors (SMAs), DFB-dIDT and BT-dIDT with 2,5-difluorobenzene (DFB) or benzothiadiazole (BT) as electron-withdrawing core (A1) and a derivative of indanone as A2 units, were prepared for applications in organic solar cells (OSCs). The results indicate that BT unit is more beneficial to forming multiple noncovalent conformational locks of N⋯S and N⋯H between BT and IDT unit than DFB in the core, so BT-dIDT showed better molecular coplanarity, higher-lying HOMO energy level, more red-shifted spectrum, superior molar absorption coefficient (1.60 × 105 M−1 cm−1 at 696 nm), more complementary absorption spectrum with PBDB-T and better photovoltaic performance than DFB-dIDT. As a result, the BTdIDT-based OSCs blending with PBDB-T exhibited higher power conversion efficiency (PCE) value of 10.52% with higher Jsc of 18.59 mA cm−2 than that of the DFB-dIDT-based devices (PCE of 6.71% with Jsc of 15.58 mA cm−2). These results demonstrate that the A2-D-A1-D-A2-type SMAs incorporated with a suitable electron-deficient core are promising candidates for high performance OSCs.
1. Introduction In recent years, non-fullerene small molecule acceptors (SMAs) in OSCs have been the subject of hot research, due to the advantages of the extended light absorption and tunable energy levels (Jiang et al., 2019; Liu et al., 2019; Song et al., 2018; Yuan et al., 2019a; Zhao et al., 2019). By now, through unremitting efforts, the PCEs of single-junction SMAbased organic solar cells have been excitingly improved to over 16% (Cui et al., 2019; Fan et al., 2019; Sun et al., 2019; Yuan et al., 2019b). Large quantities of SMAs have been designed and synthetized (Duan et al., 2017; Gautam et al., 2015; Han et al., 2017; Kim et al., 2015; Li et al., 2019; Tang et al., 2018; Tang et al., 2019; Wang et al., 2016; Zhang et al., 2017). Among them, the linear fused-ring non-fullerene acceptors with A-D-A structure, in which A is for electron-withdrawing unit and D is for electron-donating unit, are regarded as the most promising strategy for non-fullerene SMAs for the reason that coplanar molecular geometry leads to low bandgap and red-shifted absorption. Since the first fused-ring SMA was reported in 2015 (Lin et al., 2015), researchers have been sparing no efforts to improve their photovoltaic properties through modification of the side chains, electron-withdrawing groups, and fused-ring core. The solution processability, miscibility with donors and intermolecular packing can be adjusted by modification of the side chains (Dey, 2019; Yan et al., 2018; Yan et al.,
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2017; Yao et al., 2016). And variation of end groups is effective to improve the electrochemical and optical properties (Feng et al., 2017; Jia et al., 2017; Li et al., 2016; Liu et al., 2016; Wu et al., 2015; Xiao et al., 2017; Xie et al., 2017; Yao et al., 2017; Yu et al., 2017). Notably, the fused aromatic heterocycles, including the representative IDT (Bin et al., 2016a; Kan et al., 2017; Yang et al., 2016; Zhao et al., 2016; Zuo et al., 2017) and IDTT (Holliday et al., 2016; Liu et al., 2017), can facilitate π-electron delocalization, extend light absorption, and enhance intermolecular π-π stacking. To further broaden absorption spectra and reduce bandgap, many efforts have been devoted to developing larger fused aromatic heterocycles with high planarity for SMAs. However, these SMAs with large fused aromatic heterocycles usually suffered from perishing solubility in solution and strong aggregation at the solid state, which would cause large phase separations and reduce the exciton diffusion/separation efficiencies in the devices (Bin et al., 2016b; Che et al., 2018; Li, H. et al., 2018; Li, S. et al., 2018; Tan et al., 2019). Moreover, these SMAs have to suffer from an increased cost for complex synthesis routes and tedious purification process. Since Jackson et al. (2013) point out that utilization of unconventional hydrogen-bonding interactions, such as nitrogen-hydrogen (CH⋯N) and xygen-hydrogen (CH⋯O) might strengthen the planarity of a polymer backbone in 2013, Bo (Liu et al., 2017) and Chen (Geng
Corresponding authors. E-mail addresses: [email protected] (B. Zhao), [email protected] (S. Tan).
https://doi.org/10.1016/j.solener.2020.01.032 Received 20 December 2019; Received in revised form 3 January 2020; Accepted 13 January 2020 0038-092X/ © 2020 Published by Elsevier Ltd on behalf of International Solar Energy Society.
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Fig. 1. Structures of DFB-dIDT and BT-dIDT and device structure of OSCs.
−5.16 eV, which is higher than DFB-dIDT (−5.29 eV). From the frontier molecular orbitals of the two SMAs, the LUMO electron density of DFB-dIDT distributes extreme non-uniform and almost none in the center unit. In contrast, BT-dIDT shows a much well-distribution in whole molecular skeleton. Consequently, the LUMO level of BT-dIDT is 0.06 eV lower than DFB-dIDT, which results in a smaller bandgap, so BT-dIDT would possess a more red-shifted absorption spectrum than that of DFB-dIDT.
et al., 2019; Li, S. et al., 2018) groups adopted noncovalently conformational locking as an effective tactic to obtain planar acceptors. In such strategy, the coplanarity could be extended through a locked conformation between adjacent aromatic rings by a single bond instead of a fused ring. Usually, most SMAs with noncovalently conformational locking were constructed by incorporating with electron-donating units, such as alkoxythiophene (Liu et al., 2018) and dialkoxybenzene (Chen et al., 2018), only a few SMAs with noncovalently conformational locking were constructed by introducing an electron-withdrawing difluorobenzene (Li, S. et al., 2018). In this work, we report two new A2-D-A1-D-A2 type SMAs, DFBdIDT and BT-dIDT (Fig. 1), with IDT as D unit, 2-(5,6-difluoro-3-oxo2,3-dihydro-1H-inden-1-ylidene) malononitrile as A2 units, and 2,5-difluorobenzene (DFT) and benzothiadiazole (BT) as A1 unit, respectively. On the one hand, the introduction of the intermediate A1 units achieve noncovalently conformational locking and tune absorption spectra, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the SMAs. On the other hand, two IDT units are chemically bonded with A1 to avoid excessive intermolecular aggregation of acceptor molecules and obtain suitable microphase separation in active layer. The effects of A1 structures in the core on the optical properties, energy levels, carrier mobilities, and photovoltaic performances are systematically studied. It was found the introduction of BT unit as the core is beneficial to better molecular coplanarity, which results in more red-shifted and complementary absorption spectrum with PBDB-T and better photovoltaic performance than DFB-dIDT with DFB as the core. When blending with PBDB-T, BTdIDT-based OSCs delivered a decent PCE of 10.52%. The results demonstrate that introducing the electron-withdrawing unit with noncovalently conformational locking to the SMAs is a potential method to adjust the absorptions and energy levels for efficient OSCs.
2.2. Optical and electrochemical properties UV–vis absorption spectra of DFB-dIDT and BT-dIDT in 1.0 × 10−5 mol L−1 CHCl3 solution and in neat solid film are depicted in Fig. 3(a) and (b), respectively. In dilute chloroform solution, DFB-dIDT shows a peak absorption coefficient of 1.45 × 105 M−1 cm−1 in the range of 530–730 nm. Compared with DFB-dIDT, the maximum absorption peak of BT-dIDT exhibits a distinct redshift of 36 nm and a higher absorption coefficient of 1.60 × 105 M−1 cm−1 owing to the stronger intramolecular charge transfer (ICT) transition, which is consistent with the theoretical calculation in previous discussion. In thin films, the maximum absorption peaks of DFB-dIDT and BT-dIDT display redshifts of 10 and 22 nm, respectively, compared with their absorptions in solution, owing to the enhanced intermolecular interaction (Chen et al., 2019; Zhu et al., 2017). DFB-dIDT exhibits a broad absorption band of 530–750 nm with the maximum absorption at 670 nm (Table 1), while BT-dIDT displays a markedly red-shifted absorption band in the range of 550–820 nm with the maximum absorption at 718 nm. The optical bandgap of BT-dIDT is calculated to be 1.54 eV, which indicates a decrease by 0.10 eV in comparison with that of DFB-dIDT (1.64 eV, 754 nm). More importantly, BT-dIDT, with the polymeric donor PBDBT, exhibits a more complementary absorption spectrum, which is helpful to harvest more photons for a higher Jsc. The HOMO and LUMO energy levels of DFB-dIDT and BT-dIDT were determined from cyclic voltammetry (Fig. 4a) and calculated to be −5.53/−3.87 and −5.34/−3.87 eV, respectively. The higher-lying HOMO energy level and lower bandgap of BT-dIDT are attributed to better molecular coplanarity for the stronger noncovalent conformational locks, which are also in accordance with the DFT calculated results. The LUMO energy levels of the SMAs are lower than that of PBDB-T (−3.18 eV) (Fig. 4b), and the HOMO energy levels of the SMAs are lower than that of PBDB-T (−5.23 eV), which imply an effective charge transfer could occur between PBDB-T and the SMAs. The photoluminescence (PL) spectra were measured to gain insight into the excitation energy transfer of the blend film. The outcomes were presented in Fig. 5. The pure film of PBDB-T excited at 580 nm and exhibit a PL emission band in the range of 625–825 nm. DFB-dIDT (excited at 660 nm) and BT-dIDT (excited at 700 nm) exhibit the PL emission in the domain of 700–850 nm and 725–850 nm, respectively. Compared with the pure films, the blends film of PBDB-T: DFB-dIDT
2. Results and discussion 2.1. Structural calculation Density functional theory (DFT) at the B3LYP/6-31G* level were utilized to analyze the geometry structures of DFB-dIDT and BT-dIDT with methyl substituents taking the place of hexyl ones for simplification. As demonstrated in Fig. 2, a large dihedral angle of 12.85° is observed between 2,5-difluorobenzene and IDT units in DFB-dIDT. However, the dihedral angle between the benzothiadiazole and IDT in BT-dIDT is only 0.89°, which indicates there is stronger noncovalent interaction of N⋯S and N⋯H between BT and IDT unit in BT-dIDT than F⋯H in DFB-dIDT (Huang et al., 2017; Jackson et al., 2013; Yang et al., 2018; Yum et al., 2014; Zhang et al., 2016). The stronger noncovalent conformational locks of BT-dIDT are beneficial to better molecular coplanarity, which would cause to a higher-lying HOMO energy level. Therefore, the theoretical calculated HOMO energy level of BT-dIDT is 512
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Fig. 2. Optimized molecular geometries (a) top view, (b) side view, (c) LUMO, and (d) HOMO molecule orbitals of DFB-dIDT (left) and BT-dIDT (right).
Fig. 3. UV–vis absorption spectra of DFB-dIDT and BT-dIDT. Table 1 Optical and electrochemical data of DFB-dIDT, BT-dIDT and PBDB-T. Materials
DFB-dIDT BT-dIDT PBDB-Tc a b c
λmax (nm) Solution
Film
660 696 /
670 718 626
λonset (nm)
Egopt (eV)
HOMOa (eV)
LUMOa (eV)
Egecb (eV)
754 804 686
1.64 1.54 1.80
−5.53 −5.34 −5.23
−3.87 −3.87 −3.18
1.66 1.47 2.05
EHOMO/LUMO = −(4.80 + Eox/red − EFc/Fc+) eV. The potential for ferrocene vs. Ag/AgCl is 0.39 V. Calculated according to Egec = ELUMO − EHOMO. The parameter is based on references (Qian et al., 2012).
coated from chlorobenzene solution. PBDB-T was selected as donor due to its appropriate energy level and complementary absorption with the SMAs. The corresponding photovoltaic parameters are summarized in Table 2. The optimized OSCs based on PBDB-T: DFB-dIDT provided a PCE of 6.71% with a Voc of 0.86 V, a Jsc of 15.58 mA cm−2, and FF of 49.87% (Fig. 6a). In comparison, the as-cast devices based on PBDB-T: BT-dIDT showed a superior PCE of 8.45% with a Voc of 0.87 V, a higher Jsc of 16.68 mA cm−2, and a FF of 57.91%. Through further optimization of adding 1,8-diiodooctane (DIO) (DIO:CB, v:v, 1.2%:1) and thermal annealing (10 min, 100 °C) a better PCE of 10.52% with a Voc of 0.87 V, a Jsc of 18.59 mA cm−2, a higher FF of 64.83% was obtained.
and PBDB-T: BT-dIDT show a noticeable PL quenching, which indicates an efficient photoinduced charge transfer between PBDB-T and the SMAs. Such a result is a precondition for striving for high photovoltaic performance. 2.3. Photovoltaic properties The photovoltaic performances of DFB-dIDT and BT-dIDT were evaluated with an inverted device structure of ITO/ZnO (30 nm)/active layer/MoO3 (5 nm)/Al (100 nm) with 1.5 G, 100 mW cm−2, solar spectrum filters as the light source, in which the active layers were spin513
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Fig. 4. (a) Cyclic voltammograms of DFB-dIDT and BT-dIDT in chloroform solution; (b) energy levels of DFB-dIDT and BT-dIDT relative to the vacuum level.
Fig. 5. Photoluminescence spectra of DFB-dIDT and BT-dIDT-based blend films. Table 2 Photovoltaic parameters of OSCs. Active layera
DIO (vol%)
Voc (V)
PBDB-T: PBDB-T: PBDB-T: PBDB-T:
0 1.0 0 1.2
0.90 0.86 0.87 0.87
a b c
DFB-dIDT DFB-dIDTb BT-dIDT BT-dIDTb
± ± ± ±
Jsc (mA cm−2) 0.01 0.01 0.01 0.00
(0.90) (0.86) (0.87) (0.87)
12.75 15.57 16.52 18.59
± ± ± ±
0.49 0.59 0.21 0.46
PCEc (%)
FF (%) (12.91) (15.58) (16.68) (18.59)
41.23 48.56 57.40 64.61
± ± ± ±
0.45 0.71 0.66 0.76
D/A (wt/wt) = 1:1. Annealing for 10 min at 100 °C. Average values from more than 10 devices and the optimal ones are in brackets.
Fig. 6. (a) J-V curves. (b) EQE spectra of DFB-dIDT and BT-dIDT-based devices.
514
(42.00) (49.87) (57.91) (64.83)
4.49 ± 0.18 (4.90) 6.37 ± 0.29 (6.71) 8.15 ± 0.32 (8.45) 10.12 ± 0.33 (10.52)
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Fig. 7. Height and phase images (3 μm × 3 μm) for DFB-dIDT (a, c) and BT-dIDT-based blend films (b, d).
Fig. 8. J1/2-V plot for DFB-dIDT and BT-dIDT-based blend films (a) electron and (b) hole.
(Eloss = Egopt − eVoc) of PBDB-T: BT-dIDT is 0.67 eV, which is 0.11 eV lower than that of PBDB-T: DFB-dIDT (0.78 eV), which may stem from better microphase separation and less charge recombination in PBDB-T: BT-dIDT blend film. Both the DFB-dIDT and BT-dIDT based OSCs exhibited good photoresponse in scope of 300–750 and 300–800 nm (Fig. 6b), respectively. Notably, the BT-dIDT based device displayed a broader external quantum efficiency (EQE) band than that of the counterpart based on DFB-dIDT, which can be put down to the more complementary absorptions of BT-dIDT and PBDB-T. In addition, the broader EQE spectrum of PBDB-T: BT-dIDT led to an improved integrated current density of 17.98 mA cm−2 compared with the 14.91 mA cm−2 of PBDB-T: DFB-dIDT.
Table 3 Summary of charge transport properties. Devicesa
μe (cm2 V−1 s−1)
μh (cm2 V−1 s−1)
μh/μe
PBDB-T: DFB-dIDT PBDB-T: BT-dIDT
1.14 × 10−5 1.15 × 10−4
2.32 × 10−4 2.47 × 10−4
20.35 2.15
a
D/A (wt/wt) = 1:1, 100 °C annealing for 10 min.
As mentioned before, BT-dIDT possesses more red-shifted and complementary absorption spectrum with PBDB-T and higher absorption coefficient which leads to larger Jsc value in its OSC. Moreover, the larger Jsc and FF values could hint a more favorable morphology in PBDB-T: BT-dIDT blend film and more efficient exciton dissociation and charge transport in its OSC. Significantly, the energy loss 515
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Fig. 9. (a) Jph versus Veff and (b) Jsc versus light intensity of DFB-dIDT and BT-dIDT-based devices.
on DFB-dIDT and BT-dIDT as acceptor are 0.851 and 0.888, respectively, indicating the charge recombination of bimolecular is more effectively restrained in BT-dIDT-based OSCs. Those are the reasons why the OSCs based on BT-dIDT show higher Jsc, FF, and PCE values.
2.4. Film morphologies and charge carrier mobilities To further figure out effects of the SMAs on photovoltaic performance, the surface morphology of the optimal blend films was investigated by atomic force microscopy (AFM). As demonstrated in Fig. 7, the optimized active layer based on DFB-dIDT showed distinct granular aggregation with large domain size and significantly high rootmean-square (RMS) value of 5.22 nm. For comparison, BT-dIDT-based blend film shows fibrillar nanowires, smaller domain size and smoother surface morphology with a smaller RMS of 3.97 nm, which is in favour of efficient exciton dissociation and charge transport in BT-dIDT blend film, so the OSCs based on BT-dIDT show higher Jsc and FF values than those of the OSCs based on DFB-dIDT. The space-charge-limited current (SCLC) method (Peng et al., 2019; Xie et al., 2019) was used to evaluate the electron and hole mobilities of the PBDB-T: SMA blend film with the configurations of ITO/ZnO/active layer/LiF/Al and ITO/PEDOT: PSS/active layer/MoO3/Al, respectively. As shown in Fig. 8 and Table 3, the electron and hole mobilities of PBDB-T: DFB-dIDT are 1.14 × 10–5 and 2.32 × 10−4cm2 V−1 s−1, and those of PBDB-T: BT-dIDT are 1.15 × 10–4 and −4 2 −1 −1 2.47 × 10 cm V s , respectively. Obviously, the electron mobility of PBDB-T: BT-dIDT blend film is approximately ten times of that of PBDB-T: DFB-dIDT because PBDB-T: BT-dIDT blend film possesses better microphase separation (Fig. 7). Besides, the ratio of hole to electron mobility (μh/μe) of PBDB-T: BT-dIDT is 2.15, which is much smaller than that of PBDB-T: DFB-dIDT (20.35). The higher and more balanced mobilities of PBDB-T: BT-dIDT is in keeping with the features of the film morphology, which can be contributed to achieving higher Jsc and FF. To discuss charge recombination phenomenon, the relationship between photocurrent density (Jph) and the effective voltage (Veff) was investigated (Zhao et al., 2018). Jph values would reach up to saturation (Jsat) at 2 V on the basis of the hypothesis that the saturated photocurrent density (Jsat) only has to do with the gross quantity of incident photons, in which case all photogenerated excitons would decomposed into free carriers collected by the electrodes subsequently. Therefore, the ratio of JSC/Jsat can be used to feature the charge extraction. As shown in Fig. 9 (a), the charge collection probability P(E, T) calculated from Jph/Jsat are 0.722 and 0.768 for DFB-dIDT and BT-dIDT based OSCs respectively. Since the high domain purity of mixed phase is expected to obtain a decreased charge recombination and high FF (Ye et al., 2018), the BT-dIDT-based blend film with fibrillar nanowires and much smaller domain size (Fig. 7) possesses more efficient exciton dissociation and charge extraction, which brings about inhibit charge recombination and improved Jsc, FF, and PCE (Table 2). The Jsc versus light density (P) was measured (Fig. 9(b)) and Jsc ∝ Pα can be used to profile the dependence of them, in which α value is close to 1, indicating the case where the bimolecular charge recombination is unconspicuous (Li et al., 2017). The value of α based
3. Conclusion In summary, two A2-D-A1-D-A2 SMAs constructed by electronwithdrawing benzothiadiazole and 2,5-difluorobenzene as core, respectively, were designed, synthesized and characterized. Compared to DFB-dIDT, BT-dIDT showed stronger molar absorption coefficient, more red-shifted absorption spectrum, more complementary spectrum and better microphase separation with PBDB-T, which leads to higher charge mobilities, more balanced charge mobilities, more efficient exciton dissociation, more efficient charge extraction and less charge recombination. Consequently, the BT-dIDT-based OSC exhibited a higher PCE of up to 10.52% for higher Jsc of 18.59 mA cm−2 and FF of 64.83% than those of the OSCs based on DFB-dIDT (PCE = 6.71%, Jsc = 15.58 mA cm−2, FF = 49.87%). The results demonstrated the SMAs with A2-D-A1-D-A2-type framework bearing an electron-deficient core with noncovalently conformational locking should be an effective strategy for designing non-fullerene acceptors and fabricating high-efficiency OSCs. 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. Acknowledgements This work was supported by the fund from National Natural Science Foundation of China (Grants Nos. 21875204, 51873177). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2020.01.032. References Bin, H., Gao, L., Zhang, Z.-G., Yang, Y., Zhang, Y., Zhang, C., Chen, S., Xue, L., Yang, C., Xiao, M., 2016a. 11.4% Efficiency non-fullerene polymer solar cells with trialkylsilyl substituted 2D-conjugated polymer as donor. Nat. Commun. 7, 13651. Bin, H., Zhang, Z.-G., Gao, L., Chen, S., Zhong, L., Xue, L., Yang, C., Li, Y., 2016b. Nonfullerene polymer solar cells based on alkylthio and fluorine substituted 2D-conjugated polymers reach 9.5% efficiency. J. Am. Chem. Soc. 138 (13), 4657–4664. Che, X., Li, Y., Qu, Y., Forrest, S.R., 2018. High fabrication yield organic tandem photovoltaics combining vacuum-and solution-processed subcells with 15% efficiency. Nat. Energy 3 (5), 422–427.
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Solar Energy journal homepage: www.elsevier.com/locate/solener
A2-D-A1-D-A2-type small molecule acceptors incorporated with electrondeficient core for non-fullerene organic solar cells ⁎
Min Zhang, Min Zeng, Huajie Chen, Lanyan Li, Bin Zhao , Songting Tan
T
⁎
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Small molecule acceptors Synthesis Noncovalent conformational locks Non-fullerene Organic solar cells Photovoltaic performance
Two A2-D-A1-D-A2-type small molecule acceptors (SMAs), DFB-dIDT and BT-dIDT with 2,5-difluorobenzene (DFB) or benzothiadiazole (BT) as electron-withdrawing core (A1) and a derivative of indanone as A2 units, were prepared for applications in organic solar cells (OSCs). The results indicate that BT unit is more beneficial to forming multiple noncovalent conformational locks of N⋯S and N⋯H between BT and IDT unit than DFB in the core, so BT-dIDT showed better molecular coplanarity, higher-lying HOMO energy level, more red-shifted spectrum, superior molar absorption coefficient (1.60 × 105 M−1 cm−1 at 696 nm), more complementary absorption spectrum with PBDB-T and better photovoltaic performance than DFB-dIDT. As a result, the BTdIDT-based OSCs blending with PBDB-T exhibited higher power conversion efficiency (PCE) value of 10.52% with higher Jsc of 18.59 mA cm−2 than that of the DFB-dIDT-based devices (PCE of 6.71% with Jsc of 15.58 mA cm−2). These results demonstrate that the A2-D-A1-D-A2-type SMAs incorporated with a suitable electron-deficient core are promising candidates for high performance OSCs.
1. Introduction In recent years, non-fullerene small molecule acceptors (SMAs) in OSCs have been the subject of hot research, due to the advantages of the extended light absorption and tunable energy levels (Jiang et al., 2019; Liu et al., 2019; Song et al., 2018; Yuan et al., 2019a; Zhao et al., 2019). By now, through unremitting efforts, the PCEs of single-junction SMAbased organic solar cells have been excitingly improved to over 16% (Cui et al., 2019; Fan et al., 2019; Sun et al., 2019; Yuan et al., 2019b). Large quantities of SMAs have been designed and synthetized (Duan et al., 2017; Gautam et al., 2015; Han et al., 2017; Kim et al., 2015; Li et al., 2019; Tang et al., 2018; Tang et al., 2019; Wang et al., 2016; Zhang et al., 2017). Among them, the linear fused-ring non-fullerene acceptors with A-D-A structure, in which A is for electron-withdrawing unit and D is for electron-donating unit, are regarded as the most promising strategy for non-fullerene SMAs for the reason that coplanar molecular geometry leads to low bandgap and red-shifted absorption. Since the first fused-ring SMA was reported in 2015 (Lin et al., 2015), researchers have been sparing no efforts to improve their photovoltaic properties through modification of the side chains, electron-withdrawing groups, and fused-ring core. The solution processability, miscibility with donors and intermolecular packing can be adjusted by modification of the side chains (Dey, 2019; Yan et al., 2018; Yan et al.,
⁎
2017; Yao et al., 2016). And variation of end groups is effective to improve the electrochemical and optical properties (Feng et al., 2017; Jia et al., 2017; Li et al., 2016; Liu et al., 2016; Wu et al., 2015; Xiao et al., 2017; Xie et al., 2017; Yao et al., 2017; Yu et al., 2017). Notably, the fused aromatic heterocycles, including the representative IDT (Bin et al., 2016a; Kan et al., 2017; Yang et al., 2016; Zhao et al., 2016; Zuo et al., 2017) and IDTT (Holliday et al., 2016; Liu et al., 2017), can facilitate π-electron delocalization, extend light absorption, and enhance intermolecular π-π stacking. To further broaden absorption spectra and reduce bandgap, many efforts have been devoted to developing larger fused aromatic heterocycles with high planarity for SMAs. However, these SMAs with large fused aromatic heterocycles usually suffered from perishing solubility in solution and strong aggregation at the solid state, which would cause large phase separations and reduce the exciton diffusion/separation efficiencies in the devices (Bin et al., 2016b; Che et al., 2018; Li, H. et al., 2018; Li, S. et al., 2018; Tan et al., 2019). Moreover, these SMAs have to suffer from an increased cost for complex synthesis routes and tedious purification process. Since Jackson et al. (2013) point out that utilization of unconventional hydrogen-bonding interactions, such as nitrogen-hydrogen (CH⋯N) and xygen-hydrogen (CH⋯O) might strengthen the planarity of a polymer backbone in 2013, Bo (Liu et al., 2017) and Chen (Geng
Corresponding authors. E-mail addresses: [email protected] (B. Zhao), [email protected] (S. Tan).
https://doi.org/10.1016/j.solener.2020.01.032 Received 20 December 2019; Received in revised form 3 January 2020; Accepted 13 January 2020 0038-092X/ © 2020 Published by Elsevier Ltd on behalf of International Solar Energy Society.
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Fig. 1. Structures of DFB-dIDT and BT-dIDT and device structure of OSCs.
−5.16 eV, which is higher than DFB-dIDT (−5.29 eV). From the frontier molecular orbitals of the two SMAs, the LUMO electron density of DFB-dIDT distributes extreme non-uniform and almost none in the center unit. In contrast, BT-dIDT shows a much well-distribution in whole molecular skeleton. Consequently, the LUMO level of BT-dIDT is 0.06 eV lower than DFB-dIDT, which results in a smaller bandgap, so BT-dIDT would possess a more red-shifted absorption spectrum than that of DFB-dIDT.
et al., 2019; Li, S. et al., 2018) groups adopted noncovalently conformational locking as an effective tactic to obtain planar acceptors. In such strategy, the coplanarity could be extended through a locked conformation between adjacent aromatic rings by a single bond instead of a fused ring. Usually, most SMAs with noncovalently conformational locking were constructed by incorporating with electron-donating units, such as alkoxythiophene (Liu et al., 2018) and dialkoxybenzene (Chen et al., 2018), only a few SMAs with noncovalently conformational locking were constructed by introducing an electron-withdrawing difluorobenzene (Li, S. et al., 2018). In this work, we report two new A2-D-A1-D-A2 type SMAs, DFBdIDT and BT-dIDT (Fig. 1), with IDT as D unit, 2-(5,6-difluoro-3-oxo2,3-dihydro-1H-inden-1-ylidene) malononitrile as A2 units, and 2,5-difluorobenzene (DFT) and benzothiadiazole (BT) as A1 unit, respectively. On the one hand, the introduction of the intermediate A1 units achieve noncovalently conformational locking and tune absorption spectra, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the SMAs. On the other hand, two IDT units are chemically bonded with A1 to avoid excessive intermolecular aggregation of acceptor molecules and obtain suitable microphase separation in active layer. The effects of A1 structures in the core on the optical properties, energy levels, carrier mobilities, and photovoltaic performances are systematically studied. It was found the introduction of BT unit as the core is beneficial to better molecular coplanarity, which results in more red-shifted and complementary absorption spectrum with PBDB-T and better photovoltaic performance than DFB-dIDT with DFB as the core. When blending with PBDB-T, BTdIDT-based OSCs delivered a decent PCE of 10.52%. The results demonstrate that introducing the electron-withdrawing unit with noncovalently conformational locking to the SMAs is a potential method to adjust the absorptions and energy levels for efficient OSCs.
2.2. Optical and electrochemical properties UV–vis absorption spectra of DFB-dIDT and BT-dIDT in 1.0 × 10−5 mol L−1 CHCl3 solution and in neat solid film are depicted in Fig. 3(a) and (b), respectively. In dilute chloroform solution, DFB-dIDT shows a peak absorption coefficient of 1.45 × 105 M−1 cm−1 in the range of 530–730 nm. Compared with DFB-dIDT, the maximum absorption peak of BT-dIDT exhibits a distinct redshift of 36 nm and a higher absorption coefficient of 1.60 × 105 M−1 cm−1 owing to the stronger intramolecular charge transfer (ICT) transition, which is consistent with the theoretical calculation in previous discussion. In thin films, the maximum absorption peaks of DFB-dIDT and BT-dIDT display redshifts of 10 and 22 nm, respectively, compared with their absorptions in solution, owing to the enhanced intermolecular interaction (Chen et al., 2019; Zhu et al., 2017). DFB-dIDT exhibits a broad absorption band of 530–750 nm with the maximum absorption at 670 nm (Table 1), while BT-dIDT displays a markedly red-shifted absorption band in the range of 550–820 nm with the maximum absorption at 718 nm. The optical bandgap of BT-dIDT is calculated to be 1.54 eV, which indicates a decrease by 0.10 eV in comparison with that of DFB-dIDT (1.64 eV, 754 nm). More importantly, BT-dIDT, with the polymeric donor PBDBT, exhibits a more complementary absorption spectrum, which is helpful to harvest more photons for a higher Jsc. The HOMO and LUMO energy levels of DFB-dIDT and BT-dIDT were determined from cyclic voltammetry (Fig. 4a) and calculated to be −5.53/−3.87 and −5.34/−3.87 eV, respectively. The higher-lying HOMO energy level and lower bandgap of BT-dIDT are attributed to better molecular coplanarity for the stronger noncovalent conformational locks, which are also in accordance with the DFT calculated results. The LUMO energy levels of the SMAs are lower than that of PBDB-T (−3.18 eV) (Fig. 4b), and the HOMO energy levels of the SMAs are lower than that of PBDB-T (−5.23 eV), which imply an effective charge transfer could occur between PBDB-T and the SMAs. The photoluminescence (PL) spectra were measured to gain insight into the excitation energy transfer of the blend film. The outcomes were presented in Fig. 5. The pure film of PBDB-T excited at 580 nm and exhibit a PL emission band in the range of 625–825 nm. DFB-dIDT (excited at 660 nm) and BT-dIDT (excited at 700 nm) exhibit the PL emission in the domain of 700–850 nm and 725–850 nm, respectively. Compared with the pure films, the blends film of PBDB-T: DFB-dIDT
2. Results and discussion 2.1. Structural calculation Density functional theory (DFT) at the B3LYP/6-31G* level were utilized to analyze the geometry structures of DFB-dIDT and BT-dIDT with methyl substituents taking the place of hexyl ones for simplification. As demonstrated in Fig. 2, a large dihedral angle of 12.85° is observed between 2,5-difluorobenzene and IDT units in DFB-dIDT. However, the dihedral angle between the benzothiadiazole and IDT in BT-dIDT is only 0.89°, which indicates there is stronger noncovalent interaction of N⋯S and N⋯H between BT and IDT unit in BT-dIDT than F⋯H in DFB-dIDT (Huang et al., 2017; Jackson et al., 2013; Yang et al., 2018; Yum et al., 2014; Zhang et al., 2016). The stronger noncovalent conformational locks of BT-dIDT are beneficial to better molecular coplanarity, which would cause to a higher-lying HOMO energy level. Therefore, the theoretical calculated HOMO energy level of BT-dIDT is 512
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Fig. 2. Optimized molecular geometries (a) top view, (b) side view, (c) LUMO, and (d) HOMO molecule orbitals of DFB-dIDT (left) and BT-dIDT (right).
Fig. 3. UV–vis absorption spectra of DFB-dIDT and BT-dIDT. Table 1 Optical and electrochemical data of DFB-dIDT, BT-dIDT and PBDB-T. Materials
DFB-dIDT BT-dIDT PBDB-Tc a b c
λmax (nm) Solution
Film
660 696 /
670 718 626
λonset (nm)
Egopt (eV)
HOMOa (eV)
LUMOa (eV)
Egecb (eV)
754 804 686
1.64 1.54 1.80
−5.53 −5.34 −5.23
−3.87 −3.87 −3.18
1.66 1.47 2.05
EHOMO/LUMO = −(4.80 + Eox/red − EFc/Fc+) eV. The potential for ferrocene vs. Ag/AgCl is 0.39 V. Calculated according to Egec = ELUMO − EHOMO. The parameter is based on references (Qian et al., 2012).
coated from chlorobenzene solution. PBDB-T was selected as donor due to its appropriate energy level and complementary absorption with the SMAs. The corresponding photovoltaic parameters are summarized in Table 2. The optimized OSCs based on PBDB-T: DFB-dIDT provided a PCE of 6.71% with a Voc of 0.86 V, a Jsc of 15.58 mA cm−2, and FF of 49.87% (Fig. 6a). In comparison, the as-cast devices based on PBDB-T: BT-dIDT showed a superior PCE of 8.45% with a Voc of 0.87 V, a higher Jsc of 16.68 mA cm−2, and a FF of 57.91%. Through further optimization of adding 1,8-diiodooctane (DIO) (DIO:CB, v:v, 1.2%:1) and thermal annealing (10 min, 100 °C) a better PCE of 10.52% with a Voc of 0.87 V, a Jsc of 18.59 mA cm−2, a higher FF of 64.83% was obtained.
and PBDB-T: BT-dIDT show a noticeable PL quenching, which indicates an efficient photoinduced charge transfer between PBDB-T and the SMAs. Such a result is a precondition for striving for high photovoltaic performance. 2.3. Photovoltaic properties The photovoltaic performances of DFB-dIDT and BT-dIDT were evaluated with an inverted device structure of ITO/ZnO (30 nm)/active layer/MoO3 (5 nm)/Al (100 nm) with 1.5 G, 100 mW cm−2, solar spectrum filters as the light source, in which the active layers were spin513
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Fig. 4. (a) Cyclic voltammograms of DFB-dIDT and BT-dIDT in chloroform solution; (b) energy levels of DFB-dIDT and BT-dIDT relative to the vacuum level.
Fig. 5. Photoluminescence spectra of DFB-dIDT and BT-dIDT-based blend films. Table 2 Photovoltaic parameters of OSCs. Active layera
DIO (vol%)
Voc (V)
PBDB-T: PBDB-T: PBDB-T: PBDB-T:
0 1.0 0 1.2
0.90 0.86 0.87 0.87
a b c
DFB-dIDT DFB-dIDTb BT-dIDT BT-dIDTb
± ± ± ±
Jsc (mA cm−2) 0.01 0.01 0.01 0.00
(0.90) (0.86) (0.87) (0.87)
12.75 15.57 16.52 18.59
± ± ± ±
0.49 0.59 0.21 0.46
PCEc (%)
FF (%) (12.91) (15.58) (16.68) (18.59)
41.23 48.56 57.40 64.61
± ± ± ±
0.45 0.71 0.66 0.76
D/A (wt/wt) = 1:1. Annealing for 10 min at 100 °C. Average values from more than 10 devices and the optimal ones are in brackets.
Fig. 6. (a) J-V curves. (b) EQE spectra of DFB-dIDT and BT-dIDT-based devices.
514
(42.00) (49.87) (57.91) (64.83)
4.49 ± 0.18 (4.90) 6.37 ± 0.29 (6.71) 8.15 ± 0.32 (8.45) 10.12 ± 0.33 (10.52)
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Fig. 7. Height and phase images (3 μm × 3 μm) for DFB-dIDT (a, c) and BT-dIDT-based blend films (b, d).
Fig. 8. J1/2-V plot for DFB-dIDT and BT-dIDT-based blend films (a) electron and (b) hole.
(Eloss = Egopt − eVoc) of PBDB-T: BT-dIDT is 0.67 eV, which is 0.11 eV lower than that of PBDB-T: DFB-dIDT (0.78 eV), which may stem from better microphase separation and less charge recombination in PBDB-T: BT-dIDT blend film. Both the DFB-dIDT and BT-dIDT based OSCs exhibited good photoresponse in scope of 300–750 and 300–800 nm (Fig. 6b), respectively. Notably, the BT-dIDT based device displayed a broader external quantum efficiency (EQE) band than that of the counterpart based on DFB-dIDT, which can be put down to the more complementary absorptions of BT-dIDT and PBDB-T. In addition, the broader EQE spectrum of PBDB-T: BT-dIDT led to an improved integrated current density of 17.98 mA cm−2 compared with the 14.91 mA cm−2 of PBDB-T: DFB-dIDT.
Table 3 Summary of charge transport properties. Devicesa
μe (cm2 V−1 s−1)
μh (cm2 V−1 s−1)
μh/μe
PBDB-T: DFB-dIDT PBDB-T: BT-dIDT
1.14 × 10−5 1.15 × 10−4
2.32 × 10−4 2.47 × 10−4
20.35 2.15
a
D/A (wt/wt) = 1:1, 100 °C annealing for 10 min.
As mentioned before, BT-dIDT possesses more red-shifted and complementary absorption spectrum with PBDB-T and higher absorption coefficient which leads to larger Jsc value in its OSC. Moreover, the larger Jsc and FF values could hint a more favorable morphology in PBDB-T: BT-dIDT blend film and more efficient exciton dissociation and charge transport in its OSC. Significantly, the energy loss 515
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Fig. 9. (a) Jph versus Veff and (b) Jsc versus light intensity of DFB-dIDT and BT-dIDT-based devices.
on DFB-dIDT and BT-dIDT as acceptor are 0.851 and 0.888, respectively, indicating the charge recombination of bimolecular is more effectively restrained in BT-dIDT-based OSCs. Those are the reasons why the OSCs based on BT-dIDT show higher Jsc, FF, and PCE values.
2.4. Film morphologies and charge carrier mobilities To further figure out effects of the SMAs on photovoltaic performance, the surface morphology of the optimal blend films was investigated by atomic force microscopy (AFM). As demonstrated in Fig. 7, the optimized active layer based on DFB-dIDT showed distinct granular aggregation with large domain size and significantly high rootmean-square (RMS) value of 5.22 nm. For comparison, BT-dIDT-based blend film shows fibrillar nanowires, smaller domain size and smoother surface morphology with a smaller RMS of 3.97 nm, which is in favour of efficient exciton dissociation and charge transport in BT-dIDT blend film, so the OSCs based on BT-dIDT show higher Jsc and FF values than those of the OSCs based on DFB-dIDT. The space-charge-limited current (SCLC) method (Peng et al., 2019; Xie et al., 2019) was used to evaluate the electron and hole mobilities of the PBDB-T: SMA blend film with the configurations of ITO/ZnO/active layer/LiF/Al and ITO/PEDOT: PSS/active layer/MoO3/Al, respectively. As shown in Fig. 8 and Table 3, the electron and hole mobilities of PBDB-T: DFB-dIDT are 1.14 × 10–5 and 2.32 × 10−4cm2 V−1 s−1, and those of PBDB-T: BT-dIDT are 1.15 × 10–4 and −4 2 −1 −1 2.47 × 10 cm V s , respectively. Obviously, the electron mobility of PBDB-T: BT-dIDT blend film is approximately ten times of that of PBDB-T: DFB-dIDT because PBDB-T: BT-dIDT blend film possesses better microphase separation (Fig. 7). Besides, the ratio of hole to electron mobility (μh/μe) of PBDB-T: BT-dIDT is 2.15, which is much smaller than that of PBDB-T: DFB-dIDT (20.35). The higher and more balanced mobilities of PBDB-T: BT-dIDT is in keeping with the features of the film morphology, which can be contributed to achieving higher Jsc and FF. To discuss charge recombination phenomenon, the relationship between photocurrent density (Jph) and the effective voltage (Veff) was investigated (Zhao et al., 2018). Jph values would reach up to saturation (Jsat) at 2 V on the basis of the hypothesis that the saturated photocurrent density (Jsat) only has to do with the gross quantity of incident photons, in which case all photogenerated excitons would decomposed into free carriers collected by the electrodes subsequently. Therefore, the ratio of JSC/Jsat can be used to feature the charge extraction. As shown in Fig. 9 (a), the charge collection probability P(E, T) calculated from Jph/Jsat are 0.722 and 0.768 for DFB-dIDT and BT-dIDT based OSCs respectively. Since the high domain purity of mixed phase is expected to obtain a decreased charge recombination and high FF (Ye et al., 2018), the BT-dIDT-based blend film with fibrillar nanowires and much smaller domain size (Fig. 7) possesses more efficient exciton dissociation and charge extraction, which brings about inhibit charge recombination and improved Jsc, FF, and PCE (Table 2). The Jsc versus light density (P) was measured (Fig. 9(b)) and Jsc ∝ Pα can be used to profile the dependence of them, in which α value is close to 1, indicating the case where the bimolecular charge recombination is unconspicuous (Li et al., 2017). The value of α based
3. Conclusion In summary, two A2-D-A1-D-A2 SMAs constructed by electronwithdrawing benzothiadiazole and 2,5-difluorobenzene as core, respectively, were designed, synthesized and characterized. Compared to DFB-dIDT, BT-dIDT showed stronger molar absorption coefficient, more red-shifted absorption spectrum, more complementary spectrum and better microphase separation with PBDB-T, which leads to higher charge mobilities, more balanced charge mobilities, more efficient exciton dissociation, more efficient charge extraction and less charge recombination. Consequently, the BT-dIDT-based OSC exhibited a higher PCE of up to 10.52% for higher Jsc of 18.59 mA cm−2 and FF of 64.83% than those of the OSCs based on DFB-dIDT (PCE = 6.71%, Jsc = 15.58 mA cm−2, FF = 49.87%). The results demonstrated the SMAs with A2-D-A1-D-A2-type framework bearing an electron-deficient core with noncovalently conformational locking should be an effective strategy for designing non-fullerene acceptors and fabricating high-efficiency OSCs. 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. Acknowledgements This work was supported by the fund from National Natural Science Foundation of China (Grants Nos. 21875204, 51873177). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2020.01.032. References Bin, H., Gao, L., Zhang, Z.-G., Yang, Y., Zhang, Y., Zhang, C., Chen, S., Xue, L., Yang, C., Xiao, M., 2016a. 11.4% Efficiency non-fullerene polymer solar cells with trialkylsilyl substituted 2D-conjugated polymer as donor. Nat. Commun. 7, 13651. Bin, H., Zhang, Z.-G., Gao, L., Chen, S., Zhong, L., Xue, L., Yang, C., Li, Y., 2016b. Nonfullerene polymer solar cells based on alkylthio and fluorine substituted 2D-conjugated polymers reach 9.5% efficiency. J. Am. Chem. Soc. 138 (13), 4657–4664. Che, X., Li, Y., Qu, Y., Forrest, S.R., 2018. High fabrication yield organic tandem photovoltaics combining vacuum-and solution-processed subcells with 15% efficiency. Nat. Energy 3 (5), 422–427.
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