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Cite This: ACS Appl. Energy Mater. 2018, 1, 1888−1892
An Asymmetrical Polymer Based on Thieno[2,3‑f ]benzofuran for Efficient Fullerene-Free Polymer Solar Cells Yueyue Gao,†,§ Ruoxi Zhu,† Zhen Wang,‡ Fengyun Guo,† Zhixiang Wei,‡ Yulin Yang,§ Liancheng Zhao,† and Yong Zhang*,† †
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China § School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China
ACS Appl. Energy Mater. 2018.1:1888-1892. Downloaded from pubs.acs.org by AUCKLAND UNIV OF TECHNOLOGY on 01/29/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: A novel conjugated D−A polymer, PTBFBz, with asymmetrical thieno[2,3-f ]benzofuran (TBF) and fluorobenzotriazole (Bz) units was designed and synthesized for the first time. PTBFBz showed the strong absorption with a wide bandgap of 1.89 eV. Power conversion efficiency of nonfullerene polymer solar cells based on PTBFBz:ITIC with thermal and sequential solvent annealing reached up to 8.33%, which is the highest reported value among the asymmetrical TBF-based polymer solar cells. The results demonstrate that the asymmetrical TBF unit can be one of the most promising building blocks in constructing the wide bandgap polymers for highly efficient polymer solar cells.
KEYWORDS: thieno[2,3-f ]benzofuran, fluorobenzotriazole, nonfullerene acceptor, polymer solar cells, solvent annealing
B
ment, in pursuing highly efficient NF-PSCs, the device normally requires a wide bandgap conjugated polymer donor since the most efficient NF acceptors possess the narrow bandgap of ∼1.50 eV with the absorption range 650−900 nm. For example, Li and Zhang et al. reported a series of benzodithophene (BDT)-based wide bandgap polymers with ITIC or m-ITIC as the nonfullerene electron acceptors and achieved the PCE of up to 11%.9,10 Hou et al. also developed wide bandgap polymers or narrow bandgap nonfullerene acceptors to obtain the state-of-the-art PSCs.11 In general, a symmetrical configuration with C2h symmetry is believed to be best in designing and constructing the building blocks of conjugated polymers for PSCs because of the facile synthesis, planar structures, and regular and ordered chain stacking behaviors, etc. The symmetrical units, such as BDT and indacenodithiophene (IDT), etc., have been considered as the most promising building blocks of the highly efficient conjugated polymers for PSCs with the current state-of-the-art PCEs of up to 14%.12,13 However, recent studies have demonstrated that the conjugated polymers with asymmetrical building blocks can also achieve the promising and even better photovoltaic performances compared to those of the symmetrical counterparts.14−16 For examples, Yang et al. found that BDT polymers with asymmetrical alkoxyl and aromatic
ulk heterojunction (BHJ) polymer solar cells (PSCs), which are incorporated with the blend layer of a p-type conjugated polymer and an n-type organic semiconductor between anode and cathode, are regarded as one of the alternative technologies to the traditional silicon-based solar cells with the most potential, which is mainly ascribed to their intrinsic properties, i.e, cost-effective, lightweight, flexible, and large area solar panels via the roll-to-roll process.1,2 Within the BHJ layer, the conjugated polymer is the crucial component of the active layer and plays an important role in achieving high performance PSCs since it mainly determines the sunlight absorption, exciton generation, hole transport, and film morphology, etc.3,4 Currently, the most efficient design for the polymer is based on the alternative donor−acceptor (D−A) strategy, in which the optical, electrochemical, and photovoltaic properties could be easily tuned through the control of the intramolecular charge transfer (ICT) between the donor and acceptor units. To date, the power conversion efficiencies (PCEs) of PSCs have approached up to 14% along with the synergic developments of materials and device engineering.5−8 Recently, PSCs based on nonfullerene (NF) acceptors (such as ITIC, IEIC) have received much attention due to their low cost, excellent optical absorption, and tunable energy levels compared to those of the fullerene derivatives. In NF-PSCs, it is known that the complementary absorption between polymer donor and nonfullerene acceptor is necessary and important to avoid the competition of absorbing sunlight so as to reach the maximized utilization of solar photons. To fulfill this require© 2018 American Chemical Society
Received: April 11, 2018 Accepted: May 7, 2018 Published: May 7, 2018 1888
DOI: 10.1021/acsaem.8b00574 ACS Appl. Energy Mater. 2018, 1, 1888−1892
Letter
ACS Applied Energy Materials
Figure 1. Chemical structures of polymer donor PBDTBz, PTBFBz, and acceptor ITIC.
Figure 2. (a) Absorption of PTBFBz in solution and PTBFBz and ITIC in the film state. (b) The schematic energy level diagram of materials applied in the PSCs.
annealing at 130 °C for 2 min and THF annealing for 1 min. These values are much higher than that of its BDT counterpart polymer PBDTBz (PCE = 5.51%).22 The synthetic route of monomer and polymer is shown in Scheme S1. Monomers 2,6-bis(trimethyltin)-4,8-bis(2ethylhexylthiophene)thieno[2,3-f ]benzofuran (TBF-T) and 4,7-bis(5-bromothiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)4,5-dihydro-2H-benzo[d][1,2,3] triazole (BzBr) were synthesized according to the previously reported literature methods.23,24 PTBFBz was synthesized via the Stille polymerization with Pd2(dba)3 and P(o-tol)3 as catalysts. It is noted that PTBFBz is an irregioregular polymer due to the asymmetric TBF unit. The number-average molecular weight (Mn) and polydispersity index (PDI) of PTBFBz were 21.4 kDa and 1.92, respectively, which were measured via size exclusion chromatography (SEC) by using 1,2,4-trichlorobenzene as eluent and monodisperse polystyrene as standard. The thermogravimetric analysis (TGA) curve (Figure S1) shows that PTBFBz possesses a decomposition temperature (Td, 5% weight loss) at 343 °C, and differential scanning calorimetry (DSC) (Figure S1) shows that there is no significant transition for PTBFBz observed. Due to the asymmetric properties, PTBFBz shows good solubility in chloroform, chlorobenzene, and o-dichlorobenzene, which is more favorable in the process of PSC fabrication.
substituents could achieve PCE exceeding 9% by combining both advantages of the substituents.17,18 As one of the symmetric BDT’s analogies, the asymmetrical thieno[2,3f ]benzofuran (TBF) unit has attracted wide research attention as a promising donor unit in the donor−acceptor type polymer. Various medium and narrow bandgap TBF-based polymers have been developed with acceptor units such as benzodithiophene-4,8-dione (BDD) and show the PCEs of up to 6.80% in fullerene-based PSCs and up to 7.13% in NF-PSCs.19−21 However, these polymers showed the large absorption overlaps with most of the efficient NF acceptors, which is not beneficial for harvesting the solar photons. Therefore, developing efficient wide bandgap donor polymers has become one of the important strategies in pursuing the remarkable NF-PSCs. Until now, to the best our knowledge, the wide bandgap TBFbased polymers are seldom reported. In this work, we designed and synthesized a novel polymer PTBFBz (Figure 1) based on asymmetrical TBF and fluorobenzotriazole (Bz). PTBFBz presented a strong absorption in the range 300−650 nm and showed a wide bandgap of 1.89 eV, which displayed excellent complementary absorption with ITIC. In addition, PTBFBz displayed a lower highest occupied molecular orbital (HOMO) energy level of −5.45 eV and strong interchain π−π interaction. NF-PSCs based on PTBFBz:ITIC achieved a PCE of 8.33% with Voc of 0.79 V, Jsc of 15.28 mA/cm2, and a high FF of 69% with both thermal 1889
DOI: 10.1021/acsaem.8b00574 ACS Appl. Energy Mater. 2018, 1, 1888−1892
Letter
ACS Applied Energy Materials Table 1. Photovoltaic Performances of PSCs Based on PTBFBz:ITIC under Different Treatments D/A
thermal annealinga
solvent annealingb
Voc (V)
Jsc (mA/cm2)
FF (%)
PCEmax (%)
PCEavg (%)
ref
PTBFBz:ITIC
no 90 °C 110 °C 130 °C 150 °C 130 °C no 100 °Cc
no no no no no yes no no
0.80 0.79 0.79 0.80 0.79 0.79 0.74 0.73
14.60 14.50 14.75 14.77 14.70 15.28 11.73 13.11
56 59 60 60 59 69 59 58
6.54 6.76 6.99 7.09 6.85 8.33 5.18 5.51
6.42 6.67 6.83 6.96 6.74 8.12
this work
PBDTBz:ITIC a
22
2 min. bTHF annealing for 1 min. c10 min.
Figure 3. (a) J−V curves of the devices based on PTBFBz/ITIC with different thermal annealing temperatures and THF annealing. (b) EQEs of the devices based on PTBFBz/ITIC with 130 °C for 2 min, 130 °C for 2 min, and THF annealing for 1 min.
the fullerene-based PSCs. However, in NF-PSCs, it has been demonstrated that the smaller HOMO energy offsets are also able to provide enough driving force to facilitate the exciton dissociation.27 As shown in the photoluminescence (PL) spectra of the PTBFBz:ITIC blend film (Figure S3), one can find that the emissions of PTBFBz and ITIC were significantly quenched when the film was excited both at 540 and 700 nm, which demonstrates the efficient exciton dissociation in the PTBFBz/ITIC blend film. In order to investigate the geometry and electronic structure of PTBFBz, theoretical calculation was performed by using the density functional theory (DFT) with the B3LYP/6-31G* basis (Figure S4). It is found that PTBFBz adopts a nearly planar backbone configuration, which could be ascribed to the strong noncovalent interactions of F···S, F···O, and F···H.28 The HOMO and LUMO energy levels of PTBFBz were calculated to be −4.67 and −2.56 eV, respectively, which are smaller than those from CV measurements since the simplified model is used in the DFT calculation. Photovoltaic properties of PTBFBz were investigated by the fullerene-free PSCs with a structure of ITO/PEDOT:PSS/ PTBFBz:ITIC/Ca/Al. The detailed fabrication procedures are shown in Supporting Information. First, we optimized the weight ratio of the donor and acceptor to maximize the photovoltaic performance. As depicted in Figure S5 and Table 1, the device with the weight ratio of 1:1.5 gave the best PCE of 6.54% with Voc of 0.80 V, Jsc of 14.60 mA/cm2, and FF of 56%. To improve the photovoltaic performance, thermal annealings at various temperature and time were applied. The optimized temperature was at 130 °C, and the optimized PCE of 7.09% with a Jsc of 14.77 mA/cm2 and FF of 60% was obtained after thermal annealing at 130 °C for 2 min (Figure 3a and Table 1). A longer or shorter thermal annealing time led to a lower PCE
The UV−vis absorption spectra of PTBFBz in chlorobenzene and in the film state are displayed in Figure 2a, and the corresponding data are summarized in Table S1. In solution and in the film state, PTBFBz shows two distinct absorption bands, in which the high energy band from 300 to 400 nm corresponds to the delocalized π−π* transition of the polymer backbone, and the low energy band in the range 400−650 nm is ascribed to the ICT peak between TBF and Bz units.25 The absorption of PTBFBz in solution presented well-defined vibronic splitting peaks at 546 and 587 nm, while the two vibronic splitting peaks were red-shifted to 565 and 614 nm in the film state, which is due to the better planarity of the TBF unit in the film state. The optical bandgap of PTBFBz was estimated to be 1.89 eV from the onset of PTBFBz absorption in the film state. The absorption spectrum of ITIC is also depicted in Figure 2a, and one can find that PTBFBz shows better complementary with that of ITIC with the broad cover of the absorption of 400−800 nm, which is beneficial for harvesting more sunlight to obtain a better Jsc in PSCs. Cyclic voltammetry (CV) was used to investigate the HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels of PTBFBz. The cyclic voltammogram of PTBFBz in film is displayed in Figure S2, and the schematic energy level diagram of materials applied in the PSCs is shown in Figure 2b. The HOMO and LUMO energy levels were estimated to be −5.45 and −3.46 eV (Table S1), respectively, in which the relatively lower HOMO energy level is beneficial for achieving a higher Voc for PSCs. Meanwhile, the LUMO energy offset between PTBFBz and ITIC is beyond 0.3 eV, which could provide enough downhill driving force for exciton dissociation and electron transfer.26 It is interesting that the HOMO energy offset between PTBFBz and ITIC is found to be only ∼0.09 eV, which is believed to be not enough for exciton dissociation in 1890
DOI: 10.1021/acsaem.8b00574 ACS Appl. Energy Mater. 2018, 1, 1888−1892
Letter
ACS Applied Energy Materials
morphology of fiber-like network nanostructure, indicating the strong interchain π−π interaction of polymer chains, which was also observed in the X-ray diffraction (XRD) (Figure S7) with the dπ−π of 4.11 Å and dlamellar of 21.74 Å. Meanwhile, a large roughness of 8.95 nm and a few sharp distributions were observed in the AFM images of PTBFBz:ITIC film with thermal annealing only (Figure 4c). The large roughness could partly explain the observed lower FF in the device with thermal annealing only. On the other hand, as shown in Figure 4b, a clear and uniform fiber-like bicontinuous network with nanoscale phase separation with domain size of 10−20 nm was observed for PTBFBz:ITIC film with thermal/solvent annealing. The AFM image also showed a smoother surface with roughness of 4.61 nm and a uniform distribution after thermal/solvent annealing (Figure 4d). The results are in good agreement with previous reports that the molecules of donor and acceptor could rearrange to form well-organized nanostructures through solvent annealing. The improved morphology is consistent with the measured higher FF in the device after solvent annealing, therefore resulting in an enhanced PCE. In summary, PTBFBz with an asymmetrical TBF unit as donor and Bz as acceptor was designed and synthesized. PTBFBz possessed good thermal stability and strong absorption in the range 300−650 nm and lower HOMO energy level of −5.45 eV. Fullerene-free PSCs based on PTBFBz:ITIC achieved a remarkable PCE of 8.33% with Voc of 0.79 V, Jsc of 15.28 mA/cm2, and a high FF of 69% after the thermal and solvent annealing cotreatments. This value is much higher than those of its counterpart PBDTBz-based best device (PCE = 5.51%, Voc = 0.73 V, Jsc = 13.11 mA/cm2, and FF = 58%). To the best of our knowledge, the PCE of 8.33% is the highest value among TBF-based PSCs. The results also indicate that the TBF unit has great potential in the design of highly efficient asymmetrical wide bandgap polymers for PSCs.
as a result of a decreased Jsc (Table S2), which may be due to the unfavorable morphology formed. It is known that the thermal and solvent annealing cotreatment is also an efficient method to increase FF by improving the morphology of the active layer.29 To this end, the device was first thermally treated with 130 °C for 2 min and then solvent annealing was further applied under THF atmosphere for 1 min. It is interesting that the PCE was further improved to 8.33% due to the significantly increased FF of 69% (Table 1), which is largely associated with the more favorable morphology after both treatments as proved by the TEM and AFM (Figure 4b,d). It is noted that the PCE
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Figure 4. TEM images of PTBFBz:ITIC film under thermal annealing (a) and thermal/solvent annealing (b). The AFM height images of PTBFBz:ITIC film under thermal annealing (c) and thermal/solvent annealing (d) (the inset is the 3D height image).
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00574. Synthesis, characterizations and device fabrications; thermal, electrochemical, and PL curves; DFT calculations; XRD curves and the summary of device performances; and NMR images of monomers and polymers (PDF)
of PTBFBz is ca. 50% higher than the BDT counterpart, PBDTBz-based device (5.51%, Table 1). The significant improvement in the photovoltaic performance upon solvent annealing was mainly due to the molecular rearrangements of either PTBFBz or ITIC during the slowly drying process so as to form well-organized and more favorable nanostructures with careful control of the THF solvent evaporation time. External quantum efficiency (EQE) curves of PTBFBz:ITIC devices with thermal and thermal/solvent annealing are shown in Figure 3b. One can find that both devices showed similar profiles in the 300−800 nm regions. The broad and strong photocurrent response should be due to the contributions of both PTBFBz and ITIC. The maximum EQE value was 66.9% at 600 nm for the device with thermal annealing, and the maximum EQE value was increased to 70.4% at 600 nm for the device with thermal/solvent annealing, which agreed well with J−V measurement. The transmission electron microscopy (TEM) and atomic force microscopy (AFM) in a tapping-mode were applied to investigate the effect of thermal and solvent annealing on the morphology of PTBFBz:ITIC blend film. As shown in Figure 4a, PTBFBz:ITIC film with thermal annealing features a
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhixiang Wei: 0000-0001-6188-3634 Yulin Yang: 0000-0002-2108-662X Yong Zhang: 0000-0002-9587-4039 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledges the support from the National Natural Science Foundation of China (51403044, 21644006). Y.Z. acknowledges the support from the Fundamental Research Funds for the Central Universities (Harbin Institute of Technology). 1891
DOI: 10.1021/acsaem.8b00574 ACS Appl. Energy Mater. 2018, 1, 1888−1892
Letter
ACS Applied Energy Materials
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DOI: 10.1021/acsaem.8b00574 ACS Appl. Energy Mater. 2018, 1, 1888−1892
