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An Asymmetrical Polymer Based on Thieno[2,3f]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 ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00574 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Applied Energy Materials

An Asymmetrical Polymer Based on Thieno[2,3-f]benzofuran for Efficient Fullerene-Free Polymer Solar Cells Yueyue Gao1,3, Ruoxi Zhu1, Zhen Wang2, Fengyun Guo1, Zhixiang Wei2, Yulin Yang3, Liancheng Zhao1, Yong Zhang1,* 1

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin

150001, China 2

CAS Center for Excellence in Nanoscience, National Center for Nanoscience and

Technology, Beijing 100190, China 3

School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin

150001, China E-mail: [email protected]

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Abstract: A novel conjugated D-A polymer, PTBFBz, with asymmetrical thieno[2,3f]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 non-fullerene 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, non-fullerene acceptor, polymer solar cells, solvent annealing


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Bulk 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 most potential alternative technologies to the traditional silicon based solar cells, which is mainly ascribed to their intrinsic properties, i.e, cost-effective, light-weight, flexible and large area solar panels via roll-to-roll process.1,

2

Among the BHJ layer, the conjugated polymer as the crucial component of the active layer 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. Up 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 been paid much attention due to their low cost, excellent optical absorption and tunable energy levels compared to 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 requirement, in pursuing highly efficient NF-PSCs, it 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 of 650 to 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 non-fullerene electron acceptors and achieved the PCE of up to 11%.9, 10 Hou et al also developed wide band-gap polymers or the narrow band-gap non-fullerene acceptors to obtain the state-of-the-art PSCs.11 - 3 - Environment ACS Paragon Plus


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In general, a symmetrical configuration with C2h symmetry is believed to be best in designing and constructing the build blocks of conjugated polymers for PSCs since its facial synthesis, planar structures, and its 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 that of the symmetrical counterparts.14-16 For examples, Yang et al found that BDT polymers with asymmetrical alkoxyl- and aromatic substitutes could achieve PCE of exceeding 9% by combining both advantages of the substitutes.17, 18 As one of the symmetric BDT’s analogies, the asymmetrical thieno[2,3-f]benzofuran (TBF) unit has appealed wide research attention as a promising donor unit in donor-acceptor type polymer. Various medium and narrow bandgap TBF-based polymers have been developed with the accetpor units such as benzodithiophene4,8-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 presented 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. Up to now, to the best our knowledge, the wide bandgap TBF-based 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 of 300-650nm and showed a wide bandgap of 1.89 eV, which displayed excellent complementary absorption with ITIC. Besides, PTBFBz displayed 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 - 4 - Environment ACS Paragon Plus

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mA/cm2 and a high FF of 69% with both thermal annealing at 130oC for 2 mins 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 2,6bis(trimethyltin)-4,8-bis(2-ethylhexylthiophene)thieno[2,3-f]benzofuran (TBF-T) and 4,7bis(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 literatures.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 irregioregual polymer due to the asymmetric TBF unit. The numberaverage molecular weight (Mn) and polydispersity indexes (PDIs) of PTBFBz were 21.4 kDa and 1.92, respectively, which were measured via the size exclusion chromatography (SEC) by using 1,2,4-trichlorobenzene as eluent and monodisperse polystyrene as standard. Thermogravimetric analysis (TGA) curve (Figure S1) shows that PTBFBz possesses a decomposition temperature (Td, 5%weight loss) at 343oC 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. NF Acceptor

Donor C8H 17

EH

C6 H13

S

N

N

N

S ∗

S S

S S

∗

n

F

C6 H13

F

C6 H 13

S O

PBDTBz

HE

S

NC CN

N

N

S S

S

C6 H13

N

O

∗

n

F

F

S

HE

CN O

C 6 H13

S

NC S

C8 H17

EH

∗

S S

PTBFBz


ITIC

C6 H13


Figure 1. The chemical structures of polymer donor PBDTBz, PTBFBz and acceptor ITIC.

Figure 2. (a) the absorption of PTBFBz in solution, PTBFBz and ITIC in film state; (b) the schematic energy level diagram of materials applied in the PSCs

The UV-vis absorption spectra of PTBFBz in chlorobenzene and film state are displayed in Figure 2a, and the corresponding data are summarized in Table S1. In solution and film state, PTBFBz both shows two distinct absorption bands, in which the high energy band from 300 nm to 400 nm corresponds to the delocalized π-π* transition of polymer backbone, and the low energy band in the range of 400-650 nm is ascribed to the intramolecular charge transfer (ICT) peak between TBF and Bz units.25 The absorption of PTBFBz in solution presented well defined vibronic splitting peaks at 546nm and 587nm, while the two vibronic splitting peaks were red-shifted to 565 nm and 614 nm in film state, which is due to the better planarity of 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 film state. The absorption spectrum of ITIC is also depicted in Figure 2a, and it can find that PTBFBz shows better complementary with that of ITIC withthe broad cover of the absorption of 400 nm to 800 nm, which is beneficial for harvesting more sunlights 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 eV and -3.46 eV (Table S1), - 6 - Environment ACS Paragon Plus

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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 the fullerene-based PSCs. However, in NF-PSCs, it has 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 PTBFBz:ITIC blend film (Figure S3), it can find that the emission of PTBFBz and ITIC were significantly quenched when exciting the film both at 540 nm and 700 nm, which demonstrate 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/631G*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 eV and -2.56 eV, respectively, which is smaller than that from CV measurements since the simplified model in the DFT calculation.



Figure 3. (a) J-V curves of the devices based on PTBFBz/ITIC with different thermal annealing temperatures and THF annealing. (b) the EQEs of the devices based on PTBFBz/ITIC with 130oC for 2 mins; 130oC for 2 mins and THF annealing for 1 min.

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 oC, 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 oC for 2 mins (Figure 3a and Table 1). A longer or shorter thermal annealing time led to a lower PCE as a result of a decreased Jsc (Table S2), which may 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 thermal treated with 130oC for 2 mins and then further applied a solvent annealing 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 of PTBFBz is ca. 50% higher than it's BDT counterpart PBDTBz based device (5.51%, Table 1). The significant improvement on 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 a well-organized and more favorable nanostructures with the carefully control of the THF solvent evaporation time.


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External quantum efficiency (EQE) curves of PTBFBz:ITIC devices with thermal and thermal/solvent annealing are shown in Figure 3b. It can find both devices showed similar profiles in 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 were agreed well with J-V measurement. Table 1. Photovoltaic performances of PSCs based on PTBFBz:ITICunder different treatments.

D/A

PTBFBz:ITIC

PBDTBz:ITIC a

Thermal annealinga

Solvent Annealingb

Voc (V)

Jsc (mA/cm2)

FF (%)

PCEmax (%)

PCEavg (%)

no 90oC 110 oC 130 oC 150oC 130 oC no 100 oCc

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 -

2 mins;bTHF annealing for 1 min; c10 mins.


Reference

This work

[35]


Figure 4. The 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).

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 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 distribution 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 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 10-20nm 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 the uniform distribution after thermal/solvent annealing (Figure 4d). The results are in agreement well with the previous reports that the molecules of donor and acceptor could rearrange to form well-organized nanostructures through the solvent annealing. The improved morphology is consistent with the measured higher FF in the device after solvent annealing, and therefore, resulting in an enhanced PCE. In summary, PTBFBz with asymmetrical TBF unit as donor and Bz as acceptor was designed and synthesized. PTBFBz possessed good thermal stability, strong absorption in the range of 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 co-treatments. This value is much higher than those of its counterpart PBDTBz-based best device (PCE = 5.51%,

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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 TBF unit has the great potential in the design of highly efficient asymmetrical wide bandgap polymers for PSCs. ASSOCIATED CONTENT Supporting Information available: The synthesis, characterizations and device fabrications, thermal, electrochemical and PL curves, DFT calculations, XRD curves and the summary of device performances, NMR images of monomers and polymers.

Acknowledgement The authors thank the supports from the National Natural Science Foundation of China (51403044, 21644006). Y. Zhang thanks the support from the Fundamental Research Funds for the Central Universities (Harbin Institute of Technology).

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Table of Content An Asymmetrical Polymer Based on Thieno[2,3-f]benzofuran for Efficient FullereneFree Polymer Solar Cells Yueyue Gao, Ruoxi Zhu, Zhen Wang, Fengyun Guo, Zhixiang Wei, Yulin Yang, Liancheng Zhao, Yong Zhang*


An Asymmetrical Polymer Based on Thieno[2,3-f ... - ACS Publications

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