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Small molecule acceptors with indacenodithiophene–benzodithiophene–indacenodithiophene as donating cores for solution-processed non-fullerene solar cells
Chemical Physics Letters 726 (2019) 7–12
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Small molecule acceptors with indacenodithiophene–benzodithiophene–indacenodithiophene as donating cores for solution-processed non-fullerene solar cells
T
Jianing Zhua,1, Xiangjun Zhenga,c,1, Hua Tana, , Hongyi Tana, Jia Yanga, Junting Yub, , ⁎ Weiguo Zhua,c, ⁎
⁎
a
College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, PR China Key Laboratory of Theoretical Organic Chemistry and Functional Molecule, Ministry of Education, College of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, PR China c School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou University, Changzhou 213164, PR China b
HIGHLIGHTS
narrow bandgap non-fullerene acceptors BDT(IDT-IC) and BDT(IDT-IC-2F) • Two PBDB-T: BDT(IDT-IC) solar cell devices yield a PCE of 4.98%. • The • The PBDB-T: BDT(IDT-IC-2F) solar cell devices yield a PCE of 6.21%. 2
2
were obtained.
2
2
ARTICLE INFO
ABSTRACT
Keywords: Low bandgap Non-fullerene acceptor Benzodithiophene Polymer solar cells
Two acceptor-donor1-donor2-donor1-acceptor (A-D1-D2-D1-A)-type narrow bandgap small molecules BDT(IDTIC)2 and BDT(IDT-IC-2F)2 with indacenodithiophene–benzodithiophene-indacenodithiophene (BDT-(IDT)2) as donating cores and 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2FIC) as end groups, have been synthesized and investigated as nonfullerene acceptors in solution-processed polymer solar cells (PSCs). The optical bandgaps of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 are 1.59 eV and 1.54 eV, respectively. Using PBDB-T as donor, the power conversion efficiencies (PCEs) of 4.98% and 6.21% are achieved by PSCs based on BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, respectively.
1. Introduction Fullerene-free polymer solar cells (PSCs) have gained great progresses in recent years with power conversion efficiencies (PCEs) over 13% [1–3]. Compared to traditional fullerene-based acceptors, nonfullerene acceptors (NFAs) exhibit great advantages, such as finely tunable energy levels, strong light absorption ability, wide light absorption range and large electron mobilities [4–12]. More importantly, NFAs PSCs generally exhibit lower voltage loss due to the weak driving force for exciton separation [13]. Currently, NFAs with the acceptor-donor-acceptor (A-D-A)
backbone architecture have drawn particular interests due to their easytuned energy levels and high device performances. NFAs based on indaceno[2,1-b:6,5-b′]dithiophene (IDT) and indacenodithieno[3,2-b] thiophene (IDTT) as the central building blocks have demonstrated excellent photovoltaic performance [14–20]. The incorporation of these materials as electron acceptors has resulted in increased PCEs of 14% in single-junction solar cells and 17% in tandem solar cells [3,21]. Although the high efficiency has been obtained, it is still necessary to further improve the efficiency by developing new photovoltaic materials especially narrow bandgap NFAs (optical bandgap < 1.6 eV) for future commercialization of this technology [22–24].
Corresponding authors at: College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, PR China (W. Zhu). E-mail addresses: [email protected] (H. Tan), [email protected] (J. Yu), [email protected] (W. Zhu). 1 The two authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.cplett.2019.04.024 Received 1 February 2019; Received in revised form 9 April 2019; Accepted 10 April 2019 Available online 11 April 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 726 (2019) 7–12
J. Zhu, et al.
One effective strategy for designing ADA structure NFAs materials with narrow bandgap is to increase the electrondonating ability of donor part by extending its effective conjugate length [25]. The most common method is to connect the neighboring aromatic rings via covalent bonds to restrain the rotation of the single bond to increase the rigidity of the backbones [26–29]. However, the strategy of introducing an additional D unit on central core to extend the conjugation length to develop narrow bandgap NFAs with acceptor-donor1-donor2-donor1acceptor (A-D1-D2-D1-A) structure is rarely reported [30,31]. In this work, two AD1-D2-D1A-type narrow bandgap NFAs, BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, with an IDT-benzodithiophene (BDT)-IDT core as donating units and 2-(3-oxo-2,3-dihydroinden-1ylidene)malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1Hinden-1-ylidene)malononitrile (2FIC) as terminal accepting units were successfully synthesized and applied in highperformance PSCs. BDT is a rather widely used building block in the PSCs community, partially due to its symmetric and planar conjugated structure [20,32].
Fig. 1. UV–vis absorption spectra of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 in solution and films.
summarized in Scheme 1. 1H NMR, 13C NMR, MALDI-TOF MS are applied to identify the chemical structure of BDT(IDT-IC)2 and BDT(IDTIC-2F)2. From the thermogravimetric analysis (TGA) measurement in Fig. S1, the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 show good stability with 5% weight loss at 335 and 324 °C, respectively.
2. Results and discussion 2.1. Synthesis and thermal properties The chemical structures and synthetic routes of the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 is documented in the supporting information and
Scheme 1. Synthetic route of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2. 8
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Table 1 The photophysical and electrochemical data of the compounds.
BDT(IDT-IC)2 BDT(IDT-IC-2F)2 a b c d
a (opt) /V
λmax/nm (film)
λedge/nm (film)
Eg
676 690
780 805
1.59 1.54
Eox/V
Ered/V
EHOMOb/eV
ELUMOc/eV
Eg
d (elec) /eV
1.27 1.25
−0.30 −0.29
−5.53 −5.51
−3.96 −3.97
1.57 1.54
Optical energy gap determined from the onset position of the absorption band. HOMO position determined from onset of oxidation. LUMO position determined from onset of reduction. Electrochemical energy gap.
2.2. Optical and electrochemical properties The absorption spectra of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 in chloroform solution or in solid films are shown in Fig. 1, and the related parameters are summarized in Table 1. In dilute chloroform solution, BDT(IDT-IC)2 exhibits a strong absorption in 400–770 nm with a maximum peaking at 666 nm. Relative to its solution, BDT(IDT-IC)2 film exhibits a redshift (12 nm) with an absorption onset of 780 nm, corresponding to an optical bandgap Egopt of 1.59 eV. It is worth noting that BDT(IDT-IC-2F)2 film exhibits a significant redshift (20 nm) and a broader absorption peak relative to BDT(IDT-IC)2 film and the BDT(IDT-IC-2F)2 film exhibits a narrower Egopt of 1.54 eV. It indicates the BDT(IDT-IC-2F)2 with fluorinated IC end-capped groups has a more intensive intramolecular charge transfer (ICT) effect than BDT(IDTIC)2, which could be beneficial to near-infrared light utilization, thus an increased JSC of BDT(IDT-IC-2F)2-based PSCs can be expected. The electrochemical properties of BDT(IDT-IC)2 and BDT(IDT-IC2F)2 was measured by cyclic voltammetry (CV) with the ferrocene/ ferrocenium (Fc/Fc+) redox couple (4.8 eV below the vacuum level) as an internal reference. The oxidation/reduction potentials (Eoxon and Eredon) of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 are 1.27/−0.30 V and 1.25/−0.29 V, respectively (Fig. S2). As the formal potential of Fc/Fc+ is 0.54 V, the highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 can be calculated according to the following equations, EHOMO = −(Eox + 4.8 − EFc/Fc+) eV, ELUMO = −(Ered + 4.8 − EFc/Fc+) eV, respectively [33,34]. The HOMO and LUMO energy levels are −5.53 and −3.96 eV for BDT(IDT-IC)2, respectively. And the EHOMO and ELUMO of BDT(IDT-IC-2F)2 were −5.51 and −3.97 eV, respectively. The energy diagram relative to the vacuum level is shown in Fig. 2. The relatively high ELUMO for BDT(IDT-IC)2 is beneficial to achieve high VOC in PSCs, given the strong correlation between the VOC and the difference between the EHOMO of donor and the ELUMO of acceptor in PSCs [35–40].
Fig. 3. The J-V curves of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2-based PSCs.
2.3. Photovoltaic properties To investigate photovoltaic properties of the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, we fabricated PSCs with conventional structure of ITO/PEDOT:PSS/active layer/PFN/Al. Upon the optimization of film preparation (Figs. S3–S10), the PBDB-T: BDT(IDT-IC)2 devices showed a PCE of 4.98% with a Voc of 0.89 V, a Jsc of 10.97 mA cm−2 and a FF of 51.02%. The PBDB-T:BDT(IDT-IC-2F)2 devices showed a PCE of 6.21% with a Voc of 0.83 V, a Jsc of 13.60 mA cm−2 and a FF of 55.00% (Fig. 3). The related data are summarized in Table 2. The BDT(IDT-IC2F)2-based cells gave a lower Voc than the BDT(IDT-IC)2-based cells because of the deeper LUMO of BDT(IDT-IC-2F)2. However, the BDT(IDT-IC-2F)2-based devices gave much higher Jsc and FF than BDT(IDT-IC)2-based devices. From BDT(IDT-IC-2F)2-based devices to BDT(IDT-IC)2-based devices, the external quantum efficiencies (EQE) spectra broaden and intensify (Fig. 4) [41,42]. The maximum EQE of 65% at 590 nm for the BDT(IDT-IC-2F)2-based device and 56% at 609 nm for the BDT(IDT-IC)2-based device are obtained. It may be explained the reason why the BDT(IDT-IC-2F)2-based devices possessed a higher Jsc value than that of the BDT(IDT-IC)2-based devices. The calculated Jsc values are 10.95 and 13.56 mA cm−2 integrated from the EQE values for the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2-based devices, Table 2 Parameters of PSCs based on BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 measured at 100 mW cm−2 AM 1.5 G illumination.
Fig. 2. Energy level diagram of the device structure with active materials.
Acceptor
Jsc/mA cm−2
Voc/V
FF/%
PCEmax/%
PCEave/%
BDT(IDT-IC)2 BDT(IDT-IC)2a BDT(IDT-IC-2F)2 BDT(IDT-IC-2F)2a
8.66 10.97 11.83 13.60
0.92 0.89 0.87 0.83
48.31 51.02 49.85 55.00
3.85 4.98 5.13 6.21
3.56 4.80 4.77 5.92
a
9
Through a simple thermal annealing (TA) treatment at 130 °C.
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Table 3. The pristine films showed a higher hole and electron mobilities with balanced charge transport (Fig. S11). After TA treatment, the charge transport properties were improved greatly. The PBDBfilms showed a hole mobility of T:BDT(IDT-IC-2F)2 1.72 × 10−4 cm2 V−1 s−1 and an electron mobility of 1.11 × 10−5 cm2V−1 s−1. The PBDB-T: BDT(IDT-IC)2 films showed a hole mobility of 9.82 × 10−5 cm2 V−1 s−1 and an electron mobility of 4.83 × 10−6 cm2 V−1 s−1. Obviously, the higher hole-electron mobility of BDT(IDT-IC-2F)2-based devices is responsible for the higher FF level in the devices. Atomic force microscope (AFM) and transmission electron microscope (TEM) were carried out to study the morphology of the PBDBT:BDT(IDT-IC)2 and PBDB-T:BDT(IDT-IC-2F)2 blend films (Figs. 5 and 6). From AFM and TEM images, both of the blend films show obvious network interpenetrating structure, which was available to promote the exciton dissociation and charge transportation of devices. Root-meansquare (RMS) roughnesses of 3.90 and 3.53 nm for the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 blend films are observed, which indicates that the BDT(IDT-IC-2F)2 blend films exhibit a smoother surface morphology, which would lead to efficient charge transport. The X-ray diffraction patterns of PBDB-T:BDT(IDT-IC)2 and PBDBT:BDT(IDT-IC-2F)2 blend films were carried out in order to investigate the molecular interactions in detail and are displayed in Fig. S12. For the PBDB-T:BDT(IDT-IC-2F)2 film, the broad peak at around 23.8° corresponds to a π–π stacking distance of 3.72 Å. However, the corresponding distance for PBDB-T:BDT(IDT-IC)2 are increased, to 20.6° and
Fig. 4. The EQE spectra of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2-based PSCs.
respectively. The difference between the measured JSC and the calculated Jsc values is within 1%. It indicates that our photovoltaic measurement is accurate and reliable. To understand the influence of charge carrier transport on photovoltaic performance, the hole-only and electron-only devices of pristine and blend films were investigated using space-charge-limited current (SCLC) method [43,44], and the related data are summarized in
Table 3 Hole mobility and electron mobility of the devices measured by SCLC method. Active layer
D/A ratio (wt%)
μh (cm2 V−1 s−1)
μe (cm2 V−1 s−1)
μh/μe
PBDB-T: BDT(IDTIC)2 PBDB-T: BDT(IDTIC-2F)2 BDT(IDT-IC)2
1:1a 1:1b 1:1.5a 1:1.5b –a –b –a –b
7.30 × 10−5 9.82 × 10−5 8.44 × 10−5 1.72 × 10−4 5.50 × 10−5 1.55 × 10−4 1.16 × 10−4 2.83 × 10−4
1.34 × 10−6 4.83 × 10−6 5.71 × 10−6 1.11 × 10−5 1.82 × 10−5 5.47 × 10−5 1.11 × 10−4 2.20 × 10−4
54.5 20.33 14.85 15.5 3.02 2.83 1.05 1.29
BDT(IDT-IC-2F)2 a b
Without annealing. Annealing at 130 °C for 5 min.
Fig. 5. TEM images of optimized PBDB-T: BDT(IDT-IC)2 blend film (left) and PBDB-T: BDT(IDT-IC-2F)2 blend film (right) (scale bar: 200 nm).
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Fig. 6. AFM images of optimized PBDB-T: BDT(IDT-IC-2F)2 blend film (left) and PBDB-T: BDT(IDT-IC)2 blend film (right).
4.26 Å, respectively. These results indicate that PBDB-T:BDT(IDT-IC2F)2 film has a better π-π stacking of the main chain, which can lead to higher FF values in the OSCs
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3. Conclusion In conclusion, two A-D-A-type small molecules BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 with BDT-(IDT)2 as donating cores and IC or 2FIC as end groups have been synthesized. The optical bandgaps of BDT(IDTIC)2 and BDT(IDT-IC-2F)2 are 1.59 eV and 1.54 eV, respectively. Using PBDB-T as donor, the PCEs of 4.98% and 6.21% are achieved by PSCs based on BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, respectively. Our results demonstrate that introducing an additional D unit on central core to extend the conjugation length is useful to develop narrow bandgap NFAs. Declaration of interest statement The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was supported by the National Natural Science Foundation of China (51403178, 51573154, 51673031), the project of hunan natural science foundation (2018JJ2391, 2015JJ3113), the Scientific Research Fund of Hunan Provincial Education Department (14C1099, YB2015B025, 13A102). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.04.024. References [1] Y. Zhang, H.F. Yao, S.Q. Zhang, Y.P. Qin, J.Q. Zhang, L.Y. Yang, W.N. Li, Z.X. Wei, F. Gao, J.H. Hou, Sci. China Chem. 61 (2018) 1328–1337. [2] H.H. Gao, Y.N. Sun, X.J. Wan, X. Ke, H.R. Feng, B. Kan, Y.B. Wang, Y.M. Zhang, C.X. Li, Y.S. Chen, Adv. Sci. 5 (2018) 1800307. [3] H. Zhang, H.F. Yao, J.X. Hou, J. Zhu, J.Q. Zhang, W.N. Li, R.N. Yu, B.W. Gao, S.Q. Zhang, J.H. Hou, 30 (2018) 1800613. [4] X.Z. Che, Y.X. Li, Y. Qu, S.R. Forrest, Nat. Energy 3 (2018) 422–427.
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Research paper
Small molecule acceptors with indacenodithiophene–benzodithiophene–indacenodithiophene as donating cores for solution-processed non-fullerene solar cells
T
Jianing Zhua,1, Xiangjun Zhenga,c,1, Hua Tana, , Hongyi Tana, Jia Yanga, Junting Yub, , ⁎ Weiguo Zhua,c, ⁎
⁎
a
College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, PR China Key Laboratory of Theoretical Organic Chemistry and Functional Molecule, Ministry of Education, College of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, PR China c School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou University, Changzhou 213164, PR China b
HIGHLIGHTS
narrow bandgap non-fullerene acceptors BDT(IDT-IC) and BDT(IDT-IC-2F) • Two PBDB-T: BDT(IDT-IC) solar cell devices yield a PCE of 4.98%. • The • The PBDB-T: BDT(IDT-IC-2F) solar cell devices yield a PCE of 6.21%. 2
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were obtained.
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ARTICLE INFO
ABSTRACT
Keywords: Low bandgap Non-fullerene acceptor Benzodithiophene Polymer solar cells
Two acceptor-donor1-donor2-donor1-acceptor (A-D1-D2-D1-A)-type narrow bandgap small molecules BDT(IDTIC)2 and BDT(IDT-IC-2F)2 with indacenodithiophene–benzodithiophene-indacenodithiophene (BDT-(IDT)2) as donating cores and 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2FIC) as end groups, have been synthesized and investigated as nonfullerene acceptors in solution-processed polymer solar cells (PSCs). The optical bandgaps of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 are 1.59 eV and 1.54 eV, respectively. Using PBDB-T as donor, the power conversion efficiencies (PCEs) of 4.98% and 6.21% are achieved by PSCs based on BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, respectively.
1. Introduction Fullerene-free polymer solar cells (PSCs) have gained great progresses in recent years with power conversion efficiencies (PCEs) over 13% [1–3]. Compared to traditional fullerene-based acceptors, nonfullerene acceptors (NFAs) exhibit great advantages, such as finely tunable energy levels, strong light absorption ability, wide light absorption range and large electron mobilities [4–12]. More importantly, NFAs PSCs generally exhibit lower voltage loss due to the weak driving force for exciton separation [13]. Currently, NFAs with the acceptor-donor-acceptor (A-D-A)
backbone architecture have drawn particular interests due to their easytuned energy levels and high device performances. NFAs based on indaceno[2,1-b:6,5-b′]dithiophene (IDT) and indacenodithieno[3,2-b] thiophene (IDTT) as the central building blocks have demonstrated excellent photovoltaic performance [14–20]. The incorporation of these materials as electron acceptors has resulted in increased PCEs of 14% in single-junction solar cells and 17% in tandem solar cells [3,21]. Although the high efficiency has been obtained, it is still necessary to further improve the efficiency by developing new photovoltaic materials especially narrow bandgap NFAs (optical bandgap < 1.6 eV) for future commercialization of this technology [22–24].
Corresponding authors at: College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, PR China (W. Zhu). E-mail addresses: [email protected] (H. Tan), [email protected] (J. Yu), [email protected] (W. Zhu). 1 The two authors contributed equally to this work. ⁎
https://doi.org/10.1016/j.cplett.2019.04.024 Received 1 February 2019; Received in revised form 9 April 2019; Accepted 10 April 2019 Available online 11 April 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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One effective strategy for designing ADA structure NFAs materials with narrow bandgap is to increase the electrondonating ability of donor part by extending its effective conjugate length [25]. The most common method is to connect the neighboring aromatic rings via covalent bonds to restrain the rotation of the single bond to increase the rigidity of the backbones [26–29]. However, the strategy of introducing an additional D unit on central core to extend the conjugation length to develop narrow bandgap NFAs with acceptor-donor1-donor2-donor1acceptor (A-D1-D2-D1-A) structure is rarely reported [30,31]. In this work, two AD1-D2-D1A-type narrow bandgap NFAs, BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, with an IDT-benzodithiophene (BDT)-IDT core as donating units and 2-(3-oxo-2,3-dihydroinden-1ylidene)malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1Hinden-1-ylidene)malononitrile (2FIC) as terminal accepting units were successfully synthesized and applied in highperformance PSCs. BDT is a rather widely used building block in the PSCs community, partially due to its symmetric and planar conjugated structure [20,32].
Fig. 1. UV–vis absorption spectra of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 in solution and films.
summarized in Scheme 1. 1H NMR, 13C NMR, MALDI-TOF MS are applied to identify the chemical structure of BDT(IDT-IC)2 and BDT(IDTIC-2F)2. From the thermogravimetric analysis (TGA) measurement in Fig. S1, the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 show good stability with 5% weight loss at 335 and 324 °C, respectively.
2. Results and discussion 2.1. Synthesis and thermal properties The chemical structures and synthetic routes of the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 is documented in the supporting information and
Scheme 1. Synthetic route of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2. 8
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Table 1 The photophysical and electrochemical data of the compounds.
BDT(IDT-IC)2 BDT(IDT-IC-2F)2 a b c d
a (opt) /V
λmax/nm (film)
λedge/nm (film)
Eg
676 690
780 805
1.59 1.54
Eox/V
Ered/V
EHOMOb/eV
ELUMOc/eV
Eg
d (elec) /eV
1.27 1.25
−0.30 −0.29
−5.53 −5.51
−3.96 −3.97
1.57 1.54
Optical energy gap determined from the onset position of the absorption band. HOMO position determined from onset of oxidation. LUMO position determined from onset of reduction. Electrochemical energy gap.
2.2. Optical and electrochemical properties The absorption spectra of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 in chloroform solution or in solid films are shown in Fig. 1, and the related parameters are summarized in Table 1. In dilute chloroform solution, BDT(IDT-IC)2 exhibits a strong absorption in 400–770 nm with a maximum peaking at 666 nm. Relative to its solution, BDT(IDT-IC)2 film exhibits a redshift (12 nm) with an absorption onset of 780 nm, corresponding to an optical bandgap Egopt of 1.59 eV. It is worth noting that BDT(IDT-IC-2F)2 film exhibits a significant redshift (20 nm) and a broader absorption peak relative to BDT(IDT-IC)2 film and the BDT(IDT-IC-2F)2 film exhibits a narrower Egopt of 1.54 eV. It indicates the BDT(IDT-IC-2F)2 with fluorinated IC end-capped groups has a more intensive intramolecular charge transfer (ICT) effect than BDT(IDTIC)2, which could be beneficial to near-infrared light utilization, thus an increased JSC of BDT(IDT-IC-2F)2-based PSCs can be expected. The electrochemical properties of BDT(IDT-IC)2 and BDT(IDT-IC2F)2 was measured by cyclic voltammetry (CV) with the ferrocene/ ferrocenium (Fc/Fc+) redox couple (4.8 eV below the vacuum level) as an internal reference. The oxidation/reduction potentials (Eoxon and Eredon) of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 are 1.27/−0.30 V and 1.25/−0.29 V, respectively (Fig. S2). As the formal potential of Fc/Fc+ is 0.54 V, the highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 can be calculated according to the following equations, EHOMO = −(Eox + 4.8 − EFc/Fc+) eV, ELUMO = −(Ered + 4.8 − EFc/Fc+) eV, respectively [33,34]. The HOMO and LUMO energy levels are −5.53 and −3.96 eV for BDT(IDT-IC)2, respectively. And the EHOMO and ELUMO of BDT(IDT-IC-2F)2 were −5.51 and −3.97 eV, respectively. The energy diagram relative to the vacuum level is shown in Fig. 2. The relatively high ELUMO for BDT(IDT-IC)2 is beneficial to achieve high VOC in PSCs, given the strong correlation between the VOC and the difference between the EHOMO of donor and the ELUMO of acceptor in PSCs [35–40].
Fig. 3. The J-V curves of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2-based PSCs.
2.3. Photovoltaic properties To investigate photovoltaic properties of the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, we fabricated PSCs with conventional structure of ITO/PEDOT:PSS/active layer/PFN/Al. Upon the optimization of film preparation (Figs. S3–S10), the PBDB-T: BDT(IDT-IC)2 devices showed a PCE of 4.98% with a Voc of 0.89 V, a Jsc of 10.97 mA cm−2 and a FF of 51.02%. The PBDB-T:BDT(IDT-IC-2F)2 devices showed a PCE of 6.21% with a Voc of 0.83 V, a Jsc of 13.60 mA cm−2 and a FF of 55.00% (Fig. 3). The related data are summarized in Table 2. The BDT(IDT-IC2F)2-based cells gave a lower Voc than the BDT(IDT-IC)2-based cells because of the deeper LUMO of BDT(IDT-IC-2F)2. However, the BDT(IDT-IC-2F)2-based devices gave much higher Jsc and FF than BDT(IDT-IC)2-based devices. From BDT(IDT-IC-2F)2-based devices to BDT(IDT-IC)2-based devices, the external quantum efficiencies (EQE) spectra broaden and intensify (Fig. 4) [41,42]. The maximum EQE of 65% at 590 nm for the BDT(IDT-IC-2F)2-based device and 56% at 609 nm for the BDT(IDT-IC)2-based device are obtained. It may be explained the reason why the BDT(IDT-IC-2F)2-based devices possessed a higher Jsc value than that of the BDT(IDT-IC)2-based devices. The calculated Jsc values are 10.95 and 13.56 mA cm−2 integrated from the EQE values for the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2-based devices, Table 2 Parameters of PSCs based on BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 measured at 100 mW cm−2 AM 1.5 G illumination.
Fig. 2. Energy level diagram of the device structure with active materials.
Acceptor
Jsc/mA cm−2
Voc/V
FF/%
PCEmax/%
PCEave/%
BDT(IDT-IC)2 BDT(IDT-IC)2a BDT(IDT-IC-2F)2 BDT(IDT-IC-2F)2a
8.66 10.97 11.83 13.60
0.92 0.89 0.87 0.83
48.31 51.02 49.85 55.00
3.85 4.98 5.13 6.21
3.56 4.80 4.77 5.92
a
9
Through a simple thermal annealing (TA) treatment at 130 °C.
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Table 3. The pristine films showed a higher hole and electron mobilities with balanced charge transport (Fig. S11). After TA treatment, the charge transport properties were improved greatly. The PBDBfilms showed a hole mobility of T:BDT(IDT-IC-2F)2 1.72 × 10−4 cm2 V−1 s−1 and an electron mobility of 1.11 × 10−5 cm2V−1 s−1. The PBDB-T: BDT(IDT-IC)2 films showed a hole mobility of 9.82 × 10−5 cm2 V−1 s−1 and an electron mobility of 4.83 × 10−6 cm2 V−1 s−1. Obviously, the higher hole-electron mobility of BDT(IDT-IC-2F)2-based devices is responsible for the higher FF level in the devices. Atomic force microscope (AFM) and transmission electron microscope (TEM) were carried out to study the morphology of the PBDBT:BDT(IDT-IC)2 and PBDB-T:BDT(IDT-IC-2F)2 blend films (Figs. 5 and 6). From AFM and TEM images, both of the blend films show obvious network interpenetrating structure, which was available to promote the exciton dissociation and charge transportation of devices. Root-meansquare (RMS) roughnesses of 3.90 and 3.53 nm for the BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 blend films are observed, which indicates that the BDT(IDT-IC-2F)2 blend films exhibit a smoother surface morphology, which would lead to efficient charge transport. The X-ray diffraction patterns of PBDB-T:BDT(IDT-IC)2 and PBDBT:BDT(IDT-IC-2F)2 blend films were carried out in order to investigate the molecular interactions in detail and are displayed in Fig. S12. For the PBDB-T:BDT(IDT-IC-2F)2 film, the broad peak at around 23.8° corresponds to a π–π stacking distance of 3.72 Å. However, the corresponding distance for PBDB-T:BDT(IDT-IC)2 are increased, to 20.6° and
Fig. 4. The EQE spectra of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2-based PSCs.
respectively. The difference between the measured JSC and the calculated Jsc values is within 1%. It indicates that our photovoltaic measurement is accurate and reliable. To understand the influence of charge carrier transport on photovoltaic performance, the hole-only and electron-only devices of pristine and blend films were investigated using space-charge-limited current (SCLC) method [43,44], and the related data are summarized in
Table 3 Hole mobility and electron mobility of the devices measured by SCLC method. Active layer
D/A ratio (wt%)
μh (cm2 V−1 s−1)
μe (cm2 V−1 s−1)
μh/μe
PBDB-T: BDT(IDTIC)2 PBDB-T: BDT(IDTIC-2F)2 BDT(IDT-IC)2
1:1a 1:1b 1:1.5a 1:1.5b –a –b –a –b
7.30 × 10−5 9.82 × 10−5 8.44 × 10−5 1.72 × 10−4 5.50 × 10−5 1.55 × 10−4 1.16 × 10−4 2.83 × 10−4
1.34 × 10−6 4.83 × 10−6 5.71 × 10−6 1.11 × 10−5 1.82 × 10−5 5.47 × 10−5 1.11 × 10−4 2.20 × 10−4
54.5 20.33 14.85 15.5 3.02 2.83 1.05 1.29
BDT(IDT-IC-2F)2 a b
Without annealing. Annealing at 130 °C for 5 min.
Fig. 5. TEM images of optimized PBDB-T: BDT(IDT-IC)2 blend film (left) and PBDB-T: BDT(IDT-IC-2F)2 blend film (right) (scale bar: 200 nm).
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Fig. 6. AFM images of optimized PBDB-T: BDT(IDT-IC-2F)2 blend film (left) and PBDB-T: BDT(IDT-IC)2 blend film (right).
4.26 Å, respectively. These results indicate that PBDB-T:BDT(IDT-IC2F)2 film has a better π-π stacking of the main chain, which can lead to higher FF values in the OSCs
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3. Conclusion In conclusion, two A-D-A-type small molecules BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 with BDT-(IDT)2 as donating cores and IC or 2FIC as end groups have been synthesized. The optical bandgaps of BDT(IDTIC)2 and BDT(IDT-IC-2F)2 are 1.59 eV and 1.54 eV, respectively. Using PBDB-T as donor, the PCEs of 4.98% and 6.21% are achieved by PSCs based on BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, respectively. Our results demonstrate that introducing an additional D unit on central core to extend the conjugation length is useful to develop narrow bandgap NFAs. Declaration of interest statement The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was supported by the National Natural Science Foundation of China (51403178, 51573154, 51673031), the project of hunan natural science foundation (2018JJ2391, 2015JJ3113), the Scientific Research Fund of Hunan Provincial Education Department (14C1099, YB2015B025, 13A102). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.04.024. References [1] Y. Zhang, H.F. Yao, S.Q. Zhang, Y.P. Qin, J.Q. Zhang, L.Y. Yang, W.N. Li, Z.X. Wei, F. Gao, J.H. Hou, Sci. China Chem. 61 (2018) 1328–1337. [2] H.H. Gao, Y.N. Sun, X.J. Wan, X. Ke, H.R. Feng, B. Kan, Y.B. Wang, Y.M. Zhang, C.X. Li, Y.S. Chen, Adv. Sci. 5 (2018) 1800307. [3] H. Zhang, H.F. Yao, J.X. Hou, J. Zhu, J.Q. Zhang, W.N. Li, R.N. Yu, B.W. Gao, S.Q. Zhang, J.H. Hou, 30 (2018) 1800613. [4] X.Z. Che, Y.X. Li, Y. Qu, S.R. Forrest, Nat. Energy 3 (2018) 422–427.
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