Absorptive Behaviors and Photovoltaic Performance Enhancements of

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Absorptive behaviors and photovoltaic performances enhancements of alkoxy-phenyl modified indacenodithieno[3,2b]thiophene-based non-fullerene acceptors Baofeng Zhao, Weiping Wang, Jingming Xin, Haimei Wu, Hongli Liu, Zhaoqi Guo, Zhiyuan Cong, Wei Ma, and Chao Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03606 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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ACS Sustainable Chemistry & Engineering

Absorptive behaviors and photovoltaic performances enhancements of alkoxy-phenyl modified indacenodithieno[3,2-b]thiophene-based nonfullerene acceptors Baofeng Zhao, a, † Weiping Wang,

a,†

Jingming Xin, b Haimei Wu, a Hongli Liu, a Zhaoqi Guo, a

Zhiyuan Cong, a Wei Ma, b, * Chao Gao a, * a

State Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an Modern Chemistry Research Institute, NO.168 of East Zhangba Road, Xi’an, Shaanxi, 710065, China. E-Mail: [email protected] (C. Gao).

b

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, NO. 28 of West Xianning Road, Xi’an 710049, China. E-Mail: msewma@ xjtu.edu.cn (W. Ma).

ABSTRACT: Non-fullerene (NF) small molecular acceptors are very attractive for further improving the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) to overcome the limited absorptive region and fixed energy levels drawbacks of fullerene-based electronic acceptors (PC61BM and PC71BM). The acceptor-donor-acceptor (A-D-A) type oligomers (ITIC) containing

an

electron-rich

core

(four

hexyl-phenyl-substituted

indacenodithieno[3,2-

b]thiophene) as donor motif sealed with 2-(3-oxo-indane-1-ylidene)-malononitrile as acceptor

1 Environment ACS Paragon Plus

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motif has been intensively investigated due to their excellent absorptive and photovoltaic properties. Side-chain modifications have been proved to be an effective approach to modulate the energy levels and absorptive behaviors of conjugated polymers as well as conjugated small molecules. Through introduction of various side-chain and end groups, a series promisingly modified ITIC-based small molecules have been synthesized and well-studied. Herein, we reported three novel alkoxy-phenyl modified ITIC-type NF acceptors (namely pO-ITIC, mOITIC and FpO-ITIC), in which 4-hexyloxy-phenyl, 3-hexyloxy-phenyl and 3-fluorine-4hexyloxy-phenyl side-chains were connected on the indacenodithieno[3,2-b]thiophene core as the electron-donating segments of the A-D-A molecules. Both three small molecules exhibit good solubility in common solvents, finely tunable energy levels and adjustable optical bandgaps. The 4-hexyloxy-phenyl and 3-hexyloxy-phenyl substituted materials possess relatively low bandgap (1.61 eV for pO-ITIC and 1.63 eV for mO-ITIC) and a 4.7% enhancement in maximum extinction coefficient compare to that of ITIC. As the result of the better absorption behaviors, inverted polymer solar cells based on pO-ITIC blended with PTB7-Th achieves an open-circuit voltage (Voc) of 0.80 V, a short-circuit current (Jsc) of 14.79 mA/cm2, and a fill factor (FF) of 59.1%, leading to a high power conversion efficiency (PCE) of 7.51% relative to the 7.31% PCE of ITIC-based device. By using a new thiazolothiazole-based wide-bandgap polymer (PTZ-DO, 1.98 eV) with deep HOMO energy level (-5.43 eV) to match the optical absorption and molecular energy levels with the three NF acceptors, excellent PCE of 9.28% for mO-ITIC and 9.03% for pO-ITIC are obtained, which show 15.3% and 12.2% increment relative to that of 8.05% for ITIC, respectively. This finding should offer useful guideline for the design of novel NF acceptors for highly efficient PSCs through alkoxy-phenyl side-chain modified on the electron-donating moiety of A-D-A organic small molecules.


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KEYWORDS: Non-fullerene acceptors, alkoxy-phenyl modification, inverted devices, polymer solar cells

INTRODUCTION Bulk heterojunction (BHJ) polymer solar cells (PSCs) by blending a conjugated polymer donor and a fullerene derivatives acceptor (PC61BM/PC71BM) in their active layer, have been intensively investigated in the past decade for the advantages of light weight, flexibility, low cost, and suitability for the roll-to-roll fabrication.[1-4] At present, the fullerene-based BHJ PSCs have already exhibited power conversion efficiencies (PCEs) of about 12%.[5-7] However, it is difficult to further improve the PCEs of fullerene-based BHJ PSCs because of the intrinsic drawbacks of PCBM, such as weak absorption in the visible spectral region, fixed energy levels, and inherent tendency of easy aggregation in the blend films.[8,9] To overcome the above mentioned drawbacks of fullerene-based acceptors, non-fullerene (NF) acceptors have been attracted much attention in few years due to their strong absorption behaviors in the visible and even near-infrared spectral region with tunable optical bandgaps and energy levels, which are conductive to obtain complementary absorption spectra with a variety of conjugated polymer donors. [10,11] Up till now, above 13% PCE have been boosted in NF small molecular acceptor-based PSCs,

[12,13]

in which the acceptor-donor-acceptor (A-D-A) small

molecular acceptors, especially the four 4-hexyl-phenyl substituted indacenodithieno[3,2b]thiophene (IDTT) as electron-rich core end capped

by 2-(3-oxo-indane-1-ylidene)-

malononitrile (ITIC, Scheme 1) and its derivatives, are the key driving forces. [14-31]



Page 4 of 29

As is well known, side-chain modifications both in D and A segments could finely adjust the energy levels, bandgaps and charge transport properties of conjugated polymers and following this approach, improved PCEs have been obtained for a variety of alternating polymers.[32-34] Fortunately, this strategy remains valid in conjugated polymers and A-D-A structural NF small molecules containing indacenodithiophene (IDT) or IDTT electronic-donating cores. By using 3-hexyl-phenyl instead of 4-hexyl-phenyl as side chain, a relatively deep highest occupied molecular orbital (HOMO) energy level and thus enhanced open circuit voltage (Voc) and PCE (7.5%) was accomplished for an indacenodithiophene-quinoxaline based polymer (PIDTTQ-m).

[35]

Improved PCE was

also observed when the same side-chain was introduced to the IDTT-quinoxaline based polymer (PIDTT-Q-m).[36] In our previous work, a series of alternating polymers possessing gradient tunable bandgaps and energy levels based on alkoxy-phenyl modified indacenodithiophene (IDT) and fluorinated quinoxaline derivatives (0F, 1F and 2F) were synthesized, in which the 3-fluorine-4-hexyloxy-phenyl substituted IDT and quinoxaline based polymer (FO-TQ) exhibited a high PCE of 5.97% for its high hole mobility and suitable energy level when blended with PC71BM.[37] Zhan el al. synthesized a thienyl side-chains substituted ITIC derivative (ITIC-Th),[38] which possessed a deep HOMO energy level (-5.66 eV) and could match with a wide-band-gap polymer donor (PDBT-T1) for complementary absorption. As a result, enhanced PCE of 9.6% was achieved for the PDBT-T1:ITIC-Th-based devices. Through replacing the 4-hexyl-phenyl side chain of ITIC by 3-hexyl-phenyl group, a novel NF acceptor (m-ITIC) was reported and a high PCE

of

11.77

%

was

obtained

when

blending

with

a

medium

bandgap

fluorobenzotriazole-based polymer (J61).[39] Through introducing methyl in the electron-


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withdrawing unit 2-(3-oxo-indane-1-ylidene)-malononitrile to finely modulate the energy levels of ITIC, two new methyl-modified ITIC derivatives (IT-M and IT-DM) with upshifted lowest unoccupied molecular orbital (LUMO) levels were synthesized. Consequently, higher open circuit voltage (Voc) of 0.94 V and enhanced PCE over 12% was achieved for the PBDB-T:IT-M based inverted device.[40] Very recently, through replacing the phenyl ring in the INIC by thienyl ring or fluorinated phenyl rings, several highly efficient non-fullerene acceptors were also reported. [41,42] Compare to alkyl groups, the alkoxy substituents in the donor segment possess relatively strong electron-donating ability, which are favourable to the absorptive properties and energy levels adjustment of the conjugated materials.[43-45] However, alkoxy side-chain modified ITIC-type NF acceptors and their photovoltaic performances have not been investigated. Taking account of the above considerations, we herein design and synthesize three alkyloxylphenyl substituted ITIC-based NF acceptors, named pO-ITIC, mO-ITIC and FpO-ITIC (Scheme 1), in which four 4-hexyloxy-phenyl, 3-hexyloxy-phenyl and 3-fluorine-4-hexyloxyphenyl groups are adopted as the out-of-plane side chains of IDTT unit. The effects of the alkoxy side-chains modifications and their position as well as the addition of fluorine atom on the absorptive performances, energy levels and photovoltaic properties of the resulting NF acceptors were systemically investigated. The three side-chain modified molecules exhibit strong absorptive behaviors with adjusted bandgaps and finely modulated energy levels. Especially the 4-hexyloxy-phenyl and 3-hexyloxy-phenyl modified molecules (pO-ITIC and mO-ITIC) possess enhanced maximum extinction coefficients in contrast to that of ITIC. When blended with a low-bandgap donor polymer (poly({4,8-bis[(5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5b′]dithiophene-2,6-diyl}-alt-{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl})



(PTB7-Th)

[46]

Page 6 of 29

in inverted structural devices, the ITIC-based device show a PCE of 7.31%.

However, increased PCE of 7.51% and 7.32% were achieved for the pO-ITIC and mO-ITIC due to their better absorptive properties and suitable energy levels. Moreover, by blending with a thiazolothiazole-based wide-bandgap polymer (PTZ-DO, 1.98 eV) to achieve more complementary absorption, promising PCE of 9.28% for mO-ITIC and 9.03% for pO-ITIC are observed, which are 15.3% and 12.2% increment relative to that of 8.05% for ITIC, respectively, implying the effectiveness of alkoxy-phenyl modified ITIC derivatives as NF-acceptors in PSCs. C6H13 NC

C6H13O

C6H13

CN S

O

S O

NC

S

NC C6H13

O

O

S

CN C6H13O

NC CN OC6H13

pO-ITIC

C6H13O

C6H13O CN S

OC6H13 NC

CN

S C6H13O

O

S NC OC6H13

CN

mO-ITIC

OC6H13

F S

O

S

S

O

S

ITIC

NC S

S

C6H13

OC6H13

CN

F

O

S

S F C6H13O

S

F NC CN OC6H13

FpO-ITIC

Scheme 1. Structures of ITIC, pO-ITIC, mO-ITIC and FpO-ITIC.

RESULTS AND DISCUSSION

Acceptors design and synthesis Chemical structures of the three acceptors and ITIC are given in Scheme 1. Density functional theory (DFT) at B3LYP/6-31G (g, d)

[30]

level were performed to investigate the molecular

geometries and frontier orbitals of ITIC and the three acceptors. The detailed computations are described in Table S1 and the calculated energy levels and bandgaps are displayed in Table 1. The HOMO/LUMO energy level of ITIC is -5.45/-3.34 eV. Obviously, when 4-hexyl-phenyl side chain in the IDTT core is replaced by 4-hexyloxy-phenyl or 3-hexyloxy-phenyl group, simultaneously elevated HOMO/LUMO energy levels are observed, -5.37/-3.29 eV for pO-ITIC and -5.37/-3.27 eV for mO-ITIC, respectively, implying the stronger electronic-donating


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property of the alkoxy group in comparison to alkyl group. Interestingly, pO-ITIC and mOITIC exhibit the same HOMO energy levels (-5.37 eV), meaning the alkoxy substituted position has no influence on the HOMO levels of the two small molecules. However, LUMO energy level of pO-ITIC (-3.29 eV) show 0.02 eV deeper than that of mO-ITIC (-3.27 eV), giving rise to the relatively small bandgap of pO-ITIC which is conducive to the Jsc of the PSCs. Relative to pOITIC and mO-ITIC, both HOMO and LUMO energy level of FpO-ITIC (-5.47/-5.36 eV) are simultaneously deepened due to the four fluorine atoms’ introduction, which are consistent with other fluorinated-donor-segment based D-A molecules.[47,48] The calculated bandgaps of pOITIC and mO-ITIC are smaller than that of ITIC, indicating their good light absorption performances. C6H13O OC6H13 OC6H13

C6H13O

a

S

+ HO

Br S

b

S OH

C6H13O

OHC

S

C6H13O

S

S

S

1

OC6H13

+

COOC2H5

Br

S C6H13O HO

a

b

S OH

pO-ITIC

C6H13O

M1

S

S OC6H13

S

c

OHC

S

S

S

C6H13O

F

OC6H13 a

+

OC6H13 F S HO

Br

S

b

S OH

F

S F

S

F C6H13O

F

F

OC6H13

S O CH O 6 13

M2

NC

CN

OC6H13

c

S

F

F OHC

mO-ITIC

OC6H13

C6H13O

S F

S C6H13O

S

C6H13O

OC6H13

C6H13O

O

S

S

OC6H13

3 C6H13O

CN NC

CHO

S

S

OC6H13

C6H13O

C6H13O

d

S

S

OC6H13

C6H13O

OC6H13

OC6H13 C6H13O

NC CN OC6H13

C6H13O

OC6H13

C2H5OOC S

S

OC6H13

2

O

S S

O

CHO

S

S OC6H13

C6H13O

S

S

S

OC6H13

CN

d

c

OC6H13

NC

C6H13O

S

S S

S

OC6H13

OC6H13

F S

OC6H13

4

C6H13 d

S SF

S F C6H13O

OC6H13

M3

NC

CN

F

CHO O

OC6H13

S

F

O

S

S F C6H13

S

F NC C6H13

CN

FpO-ITIC

Scheme 2. Synthetic routes of the acceptors. (a) THF, n-BuLi, -78 oC;(b) CH3COOH, H2SO4; (c) BuLi, DMF, -78 oC; (d) CHCl3, pyridine, INIC, 40 oC.

The synthetic routines of the three NF-acceptors are depicted in Scheme 2. The indacenodithieno[3,2-b]thiophene derivatives containing 4-hexyloxy-phenyl, 3-hexyloxy-phenyl and 3-fluorine-4-hexyloxy-phenyl side-chains were lithiated with n-butyllithium and quenched by dimethylformamide (DMF) to afford the corresponding dialdehydes monomers (M1, M2 and



M3),

followed

by

a

Knoevenagel

condensation

with

Page 8 of 29

2-(3-oxo-2,3-dihydroinden-1-

ylidene)malononitrile (INIC) to afford the final NF-acceptors. All the intermediates, as well as the pO-ITIC, mO-ITIC and FpO-ITIC, were fully characterized by 1H-NMR,

13

C-NMR

(Figure S1-S18, ESI) and elemental analysis. The three NF-acceptors can be easily dissolved in organic solvents such as chloroform, tetrahydrofuran, and ortho-dichlorobenzene (oDCB) at room temperature, which ensure the film fabrication through a solution process. Properties of the acceptors. All three NF acceptors have decomposition temperature (Td, defined as the 5% weight-loss temperature) over 300 oC. FpO-ITIC shows the highest Td (396 oC for FpO-ITIC, 384 oC for pO-ITIC and 338

o

C for mC6O-ITIC respectively, Figure S19) as determined by

thermogravimetric analysis (TGA) under nitrogen, indicating their good thermal stabilities. The optical absorptive properties of the NF-acceptors were measured by ultraviolet-visible (UV-vis) absorption spectroscopy both in solutions (see Figure 1a) and thin films (Figure 1b). The three acceptors in chloroform solution (10−6 M) exhibit strong absorption behaviors in the 500−800 nm region with a maximum absorption peak (λmax) at 679 nm for pO-ITIC, 674 nm for mO-ITIC and 662 nm for FpO-ITIC. While for the films, the λmaxs were red-shifted to 702 nm (pO-ITIC), 696 nm (mO-ITIC) and 680 nm (FpO-ITIC), respectively, indicating better planarity structure of the NF-acceptors in solid state. Figure 1a and 1b also gives the absorption spectra of ITIC. The molar extinction coefficient (εmax) of ITIC in CHCl3 solution in this study is 2.09×105 M−1 cm−1. Improved εmaxs of 2.19×105 M−1 cm−1 are observed for pO-ITIC and mOITIC in CHCl3 solution, which is 4.7% bigger than that of ITIC. On the contrary, a little decreased εmax of 1.96×105 M−1 cm−1 is found for FpO-ITIC solution. The absorption edges


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(λedges) of the three acceptors and ITIC films are 771 nm (pO-ITIC), 764 nm (ITIC), 760 nm (mO-ITIC) and 755nm (FpO-ITIC), respectively. Then the optical bandgaps (Egopt) can be calculated as 1.61 eV for pO-ITIC, 1.62 eV for ITIC, 1.63 eV for mO-ITIC and 1.64 eV for FpO-ITIC, separately. Obviously, the 4-hexyloxy-phenyl substituted compound pO-ITIC possesses the smallest bandgap, which is consistent with the DFT calculated results. The strong and broad absorption behavior of pO-ITIC and mO-ITIC, meaning their better light harvest ability, benefits the short-circuit current density (Jsc) enhancement in this NF-acceptor-based devices. The absorption data were also exhibited in Table 1. Cyclic voltammetry (CV) were employed to investigate the effects of the side-chains on the frontier energy levels (HOMO/LUMO) of the acceptors (Figure S21). The HOMO/LUMO energy levels (EHOMO/ELUMO) of the three NF-acceptors are calculated from the onset oxidation/reduction potentials (Eox/Ered) by the equations: EHOMO=-e(Eox + 4.71) (eV); ELUMO=e(Ered + 4.71) (eV), where the unit of potential is V vs Ag/Ag+.[49] The measured EHOMO/ELUMO of ITIC and PTB7-Th in this study is -5.50/-3.71 eV and -5.28/-3.09 eV (Figure S21 and S23), respectively. For the three alkoxy-modified acceptors, the obtained EHOMO/ELUMO is -5.49 eV/3.71 eV for pO-ITIC, -5.50 eV/-3.71 eV for mO-ITIC and -5.61 eV/-3.72 eV for FpO-ITIC, respectively. Relative to ITIC, the energy levels of 4-hexyloxy and 3-hexyloxy modified molecules (pO-ITIC and mO-ITIC) exhibit slightly differences, indicating the alkoxy side chain has little effect on the molecular energy levels of ITIC. Although similar ELUMO is obtained for FpO-ITIC, a 0.11 eV deeper EHOMO is observed, indicating the weak electrondonating property of this materials and is agree with other fluorinated-donor unit materials.[47,48] The variation trends in the molecular energy levels of the acceptors are consistent with the acquired results from theoretical calculations. The energy levels diagrams of the three acceptors



as well as ITIC were summarized in Figure 1c. Apparently, the LUMO level differences between PTB7-Th and the four small molecular acceptors are all above 0.65 eV, which ensure enough driving force for efficient charge separation. The photoluminescence (PL) spectra PTB7-Th/acceptors blends (w/w=1:1) as well as the pure PTB7-Th film were measured (excited at 706 nm) and the plots were displayed in Figure 1d. Pure PTB7-Th film exhibits a PL emission band in the range of 720~820 with PL peak at around 790 nm. The PTB7-Th/acceptors blends show significant decreased PL emission intensities (over 99% fluorescence quenching), implying good molecule miscibility and efficient charge

2.0

1.5

a

ITIC pO-ITIC mO-ITIC FpO-ITIC

1.0

0.5

0.0 350 400 450 500 550 600 650 700 750 800

Normalized Absorption (a.u)

Absorbance (105 M-1 cm-1)

transfer between PTB7-Th and the three ITIC derivatives. [10]

1.0 0.8 0.6 0.4 0.2

0.0 350 400 450 500 550 600 650 700 750 800 850

-3.09

ITIC

-3.72

FpO-ITIC

-5.5

-3.74

mO-ITIC

-5.0

-3.71

-3.71

pO-ITIC ITIC

-4.5

c ITIC pO-ITIC ITIC

Energy levels (eV)

-3.5 -4.0

Wavelength (nm)

-5.50

-5.61

d

0.8 0.6 0.4

PTB7-Th PTB7-Th:pO-ITIC PTB7-Th:mO-ITIC PTB7-Th:FpO-ITIC

0.2 0.0 720

-5.28 -5.50 -5.49

1.0

PL Intensity (a.u)

-3.0

b

pO-ITIC mO-ITIC FpO-ITIC ITIC PTB7-Th

Wavelength (nm)

PTB7-Th

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

740

760

780

800

820

840

860

Wavelength (nm)

Figure 1. (a) Absorption coefficients of ITIC, pO-ITIC, mO-ITIC and FpO-ITIC in chloroform solutions; (b) Absorption spectra of ITIC, pO-ITIC, mO-ITIC, FpO-ITIC and PTB7-Th films; (c) Schematic illustration of HOMO/LUMO energy levels of ITIC, pO-ITIC, mO-ITIC, FpO-ITIC and PTB7-Th. (d) PL spectra of PTB7-Th, PTB7-Th:pO-ITIC, PTB7-Th:mO-ITIC and PTB7-Th:FpO-ITIC blends.


Page 10 of 29

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Table 1. Properties of the three acceptors λedge

Egopt a)

HOMO b)

LUMO b)

Eg b)

Eox c)

Ered c)

HOMO c)

LUMO c)

[nm]

[eV]

[eV]

[eV]

[eV]

[V]

[V]

[eV]

[eV]

pO-ITIC

771

1.61

-5.37

-3.29

2.08

0.78

-1.00

-5.49

-3.71

mO-ITIC

760

1.63

-5.37

-3.27

2.10

0.79

-0.97

-5.50

-3.74

FpO-ITIC

755

1.64

-5.47

-3.36

2.11

0.90

-0.99

-5.61

-3.72

ITIC

764

1.62

-5.45

-3.34

2.11

0.79

-1.00

-5.50

-3.71

Acceptor

a)

Estimated from the absorption edge of the polymer films (Egopt =1240/λedge (nm)). b) DFT calculated results. c)

Cyclic voltammetry measured data.

The crystallinity and molecular organization of the NF acceptors were investigated by grazing incident wide-angle X-ray diffraction (GIWAXS) as shown in Figure 2. For ITIC, both the lamellar (100) reflection (at 0.49 Å-1) and π-π stacking (010) reflection (at 1.64 Å-1) are competitively observed along the out-of-plane direction, confirming that face-on and edge-on orientations coexisted. The relatively weak π-π stacking (010) for a neat ITIC film is properly related to it slightly poor self-organization behavior with the para-alkyl-phenyl substitution. Thus, the crystal coherence length (CCL) in this direction obtained by the Scherrer equation

[50]

is 21.31Å. As for the 4-hexyloxy-phenyl and 3-fluorine-4-hexyloxy-phenyl modified acceptors (pO-ITIC and FpO-ITIC), the lamellar stacking peaks get broad on two different directions with the alkoxy indicating weak side-chain packing affected by alkoxy modification. At the same time, no apparent π-π stacking peaks could be observed within and beyond detect plane according to the 1D line-cuts, which elucidates weak crystallinity of pO-ITIC and FpO-ITIC compared to pure ITIC. However, for the 3-hexyloxy-phenyl modified mO-ITIC, the crystallinity of ortho-substituted acceptor is slightly higher than two para-substituted acceptors due to a (010) scattering peak emerged on 1.77 Å-1, while no obvious (100) peak exists in the out-of-plane direction.



Figure 2. GIWAXS images of the ITIC film (a), pO-ITIC film (b), mO-ITIC film (c) and FpO-ITIC film (d); Line cuts of the GIWAXS images of ITIC film, pO-ITIC film, mO-ITIC film and FpO-ITIC film (e).

Characteristics of Photovoltaic Devices To study the photovoltaic performances of the NF acceptors, inverted PSCs based on PTB7Th blended with the three acceptors were fabricated with a device configuration of ITO/ZnO/poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3-N,N-dimethylamino)propyl)-2,7fluorene] (PFN)/PTB7-Th:acceptors/MoO3/Al. To improve the devices performance, the 10 nm thin PFN interface between the ZnO and active layer was used as the cathode interface.[51] For spin-coating blend films, ortho-dichlorobenzene (oDCB) was adopted as the solvent. To optimize the devices, a variety of PTB7-Th/acceptor ratio (wt/wt) of 1:1, 1:1.3 and 1:1.5, were used to fabricate the corresponding device (Figure S24 and Table S2). Obviously, optimal donor/acceptor ratios of the three NF small molecules are 1:1. The current density versus voltage (J-V) characteristics and external quantum efficiency (EQE) plots of the PTB7-Th/three NF acceptors-based devices under optimal weight ratios (1:1) are displayed in Figure 3 and the photovoltaic parameters are summarized in Table 2. For the sake of contrast, the photovoltaic performance of the ITIC-based device with optimal donor/acceptor ratio (1:1.3) is also provided


Page 12 of 29

Page 13 of 29

in this study.

[10]

It should be pointed out that the obtained PCE value of ITIC-based device is

7.31%, slightly higher than that in the literature (6.80%),[10] mainly due to the improved FF of inverted device in this study. Enhanced PCE of 7.51%, with a VOC of 0.80 V, short-circuit current (Jsc) of 14.79 mA cm−2, and fill factor (FF) of 59.1% was obtained for pO-ITIC relative to that of ITIC. The increment in PCE of pO-ITIC is ascribed to the slightly improved Jsc which is originated from the narrowed bandgap and large extinction coefficient as illustrated above. For mO-ITIC, although the VOC (0.80 V) and FF (60.1%) have slightly changes, a little decreased Jsc of 14.19 mA cm−2 is observed relative to pO-ITIC due to the tinily blue-shifted absorption spectrum. However, this Jsc value is still slightly bigger than that of ITIC for the higher extinction coefficient of mO-ITIC, leading to almost equal PCE value of mO-ITIC (7.33%) and ITIC (7.31%). As to FpO-ITIC-based device, relatively small VOC of 0.78 V, Jsc of 12.99 mA cm−2, FF of 56.7% and PCE of 6.17% are observed for its bad absorption performances. The EQE spectra of the PTB7-Th/NF acceptors blends are displayed in Figure 3b. Both devices show strong photoresponses from 500 to 800 nm, agreeing well with their absorption spectra. The pO-ITIC-based device shows enhanced EQE response relative to ITIC and other two-acceptors-based devices, with EQE response values >70 % from 556 to 700 nm, which is consistent with the high Jsc of this molecule. The mO-ITIC-based device show medium strong EQE response and the FpO-ITIC-based device exhibit the lowest EQE, which are ascribed to their bandgaps and extinction coefficients of the two acceptors. 2

-2 -4 -6

b

70

0 PTB7-Th:ITIC PTB7-Th:pO-ITIC PTB7-Th:mpO-ITIC PTB7-Th:FpO-ITIC

60

EQE (%)

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-8

50 40 30

-10

20

-12

a

-14

-16 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Voltage (V)

PTB7-Th:ITIC PTB7-Th:pO-ITIC PTB7-Th:mO-ITIC PTB7-Th:FpO-ITIC

10 0 300

400

500

600

700

800

Wavelength (nm)



Page 14 of 29

Figure. 3 (a) J-V plots of optimal PTB7-Th/NF acceptors-based devices under the illumination of AM 1.5G, 93.1 mW cm-2. (b) Corresponding EQE curves of the optimal PTB7-Th/NF acceptors-based devices. Table 2. Photovoltaic performances of optimal PTB7-Th/NF acceptors devices under the illumination of AM 1.5G, 93.1 mW cm-2 Voc Jsc FF [V] [mA/cm2] [%] 1:1 0.80 14.79 59.1 PTB7-Th:pO-ITIC 1:1 0.80 14.19 60.1 PTB7-Th:mO-ITIC 1:1 0.78 12.99 56.7 PTB7-Th:FpO-ITIC 1:1.3 0.80 13.82 61.6 PTB7-Th:ITIC a) Average PCE values are achieved from 10 independent devices. Active layer

D/A

PCEmax(PCEave) a) [%] 7.51 (7.47) 7.33 (7.24) 6.17 (6.10) 7.31 (7.25)

Morphology and Charge Transport Properties The morphology of the three acceptros:PTB7-Th blend films was investigated by atomic force microscopy (AFM). For the AFM height images (Figure 4a-c), both blends are very smooth with a root-mean-square (RMS) roughness of 1.47 nm for pO-ITIC, 1.09 nm for mO-ITIC and slightly bigger RMS of 2.19 nm for FpO-ITIC, respectively. The relatively smooth surfaces of the films indicate good miscibility of the donor and acceptor components within their blend films. In the AFM phase images (Figure S25), both of the three blend films show welldistributed nanofibrillar networks around tens of nanometers. It should be pointed out that the more preferred domain size of ∼15 nm are observed in the PTB7-Th:pO-ITIC and PTB7Th:mO-ITIC blends relative to that of PTB7-Th:FpO-ITIC blend, agreeing with their superior device performances. The aggregation morphologies as well as the degree of crystallinity of the blending thin films were studied by GIWAXS. GIWAXS 2D patterns and 1D line-cuts of the PTB7-Th:ITIC (1:1.3, w/w), PTB7-Th:pO-ITIC (1:1, w/w), PTB7-Th:mO-ITIC (1:1, w/w) and PTB7-Th:FpOITIC (1:1, w/w) film are given in Figure 5. For ITIC blend, it exhibits a relatively sharp (010) scattering peak with 15.2Å CCL in the out-of-plane direction, while in-plane (100) peak located


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at 0.29 Å-1 with 60.67 Å CCL. After blending with polymer, alkoxy modification acceptors show enhanced side-chain packing (increasing (100) CCL, 64.98Å for PTB7-Th:pO-ITIC, 65.68Å for PTB7-Th:FpO-ITIC and 74.53Å for PTB7-Th:mO-ITIC) which is different from pure acceptors’ films discussed above. The (100) scattering peak represents the lamellar spacing in the plane,

[52,53]

relating to the length of the side chain of this four ITIC-based acceptors. Improved

side-chain packing of the three alkoxy-modified ITIC derivatives is benefit to the crystallinity of the molecules, suggesting enhanced electron mobility as described below.

Figure 4. (a) AFM height image (4 µm×4 µm) for PTB7-Th:pO-ITIC (1:1) blend, Rms=1.47 nm , (b) AFM height image (4 µm×4 µm) for PTB7-Th:mO-ITIC (1:1) blend, Rms=1.09 nm; (c) AFM height image (4 µm×4 µm) for PTB7-Th:FpO-ITIC (1:1) blend, Rms=2.19 nm.

Figure 5. GIWAXS 2D patterns for (a) PTB7-Th:ITIC blend, (b) PTB7-Th:pO-ITIC blend, (c) PTB7Th:mO-ITIC blend, (d) PTB7-Th:FpO-ITIC blend and (e) 1D line-cuts.



Page 16 of 29

The bulk charge mobility properties of the optimized three NF acceptor blend films were investigated by the space-charge limited current (SCLC) method. The hole (µh) and electron (µe) mobilities

were

measured

with

device

ethylenedioxythiophene):poly(styrene-sulfonate)

configurations

(PEDOT:PSS)/active

of

ITO/poly(3,4-

layer/MoO3/Al

and

ITO/ZnO/PFN/active layer/PFN/Al, respectively. As shown in Figure S26 (ESI), the three blends exhibit µh values of 1.63×10-4 cm−2V−1s−1 for pO-ITIC, 1.40×10-4 cm−2V−1s−1 for mOITIC and 1.57 ×10-4 cm−2V−1s−1 for FpO-ITIC, respectively. As for the electron mobilities (Figure S27), highest µe of 4.26×10-5 cm−2V−1s−1 is observed for mO-ITIC. A slightly decreased µe of 3.48×10-5 cm−2V−1s−1 and the lowest value of 8.56×10-6 cm−2V−1s−1 are obtained for the pO-ITIC and FpO-ITIC. The ratios of µh to µe for the three acceptors are 4.68 (pO-ITIC), 3.29 (mO-ITIC) and 18.34 (FpO-ITIC). The more balanced electron and hole mobility and the higher extinction coefficient of pO-ITIC and mO-ITIC are contributed to their enhanced Jsc and FF values. Considering the absorption mismatch between PTB7-Th and the NF acceptors (Figure 1a) as well as their unsatisfied VOC and PCE, to further investigate the three acceptors’ performances in PSCs and obtain better PCE, a new wide-bandgap (1.98 eV) alternating polymer (PTZ-DO, Figure 6a) based on benzo[1,2-b:4,5-b']dithiophene (BDT) and thiazolothiazole (TZ) was synthesized,[54] in which the 2,3-dioctylthienyl side-chain modified BDT was adopted to deepen the HOMO energy level of the polymer.[33] The polymer PTZ-DO is synthesized through typical Stille-coupling reaction as shown in Scheme S1. PTZ-DO exhibits deep HOMO energy level of -5.43 eV and a high-lying LUMO energy level of -2.93 eV (Figure S28), which ensure sufficient drive forces for effective charge transportation between PTZ-DO and the acceptors. Besides, film absorption spectrum of PTZ-DO possesses excellent complementary absorption with the


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four NF acceptors (Figure 6b). Figure 6c gives the J-V plots of inverted PSCs based on PTZDO and the NF acceptors with the same donor/acceptor weight ratio of 1:1, the corresponding photovoltaic parameters are summarized in Table 3. The PTZ-DO/ITIC-based device shows a largely improved VOC of 0.92 V ascribed to the deeper HOMO energy level of the polymer, a slightly increased JSC of 14.30 mA cm-2 and a similar FF of 61.2 % relative to PTB7-Th/ITIC blend, leading to an improved PCE of 8.05%. The fluorinated acceptor FpO-ITIC-based device exhibits unsatisfactory PCE of 6.69% due to its bad VOC, JSC and FF as described ahead. In contrast to ITIC, although the same VOC of 0.92 V are found for pO-ITIC and mO-ITIC based devices, simultaneously enhanced JSC (14.56 mA cm-2 and 15.18 mA cm-2) and FF (67.4% and 66.4%) are observed as anticipated, resulting in high PCE of 9.03% and 9.28%, respectively. In comparison with that of PTB7-Th-based device, the four PTZ-DO-based devices (Figure 6d) show relatively higher photoresponse from 300 to 500 nm due to the strong absorption of PTZDO film in this region. The PCEs of pO-ITIC and mO-ITIC are 12.2% and 15.3% increments relative to that of ITIC (8.05%), indicating better performances as NF acceptors of alkoxyphenyl modified A-D-A small molecules in PSCs. As shown in AFM morphology images (Figure 7), the root-mean-square (RMS) roughness of PTZ-DO:ITIC, PTZ-DO:pO-ITIC, PTZ-DO:mO-ITIC and PTZ-DO:FpO-ITIC are 0.787, 0.786, 0.820 and 0.801 nm, respectively. The four films exhibit very similar morphology and smooth surface, reflecting the good mixing properties of the donor and acceptor materials in the blend films.


a

C8H17

C8H17

S

C6H13 N

S *

S

S

S

C6H13 S N

S

S C8H17

C8H17

* n

1.0 0.8

ITIC pO-ITIC mO-ITIC FpO-ITIC PTZ-DO

0.4 0.2 0.0 300

400

2

0 -2 -4 -6

500

600

700

70

c ITIC pO-ITIC mO-ITIC FpO-ITIC

-8 -10

d

60 50 40 30 ITIC pO-ITIC mO-ITIC FpO-ITIC

20

-12 10

-14 -16 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Voltage (V)

800

Wavelength (nm)

EQE (%)

2

b

0.6

PTZ-DO

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Normalized Absorption (a.u)


0 300

400

500

600

700

800

Wavelength (nm)

Figure 6. (a) Chemical structure of PTZ-DO; (b) absorption spectra of ITIC, pO-ITIC, mO-ITIC, FpOITIC and PTZ-DO films; (c) the J-V curves of PSCs based on the optimal PTZ-DO/NF acceptors (w/w=1:1) devices under the illumination of AM 1.5G, 100 mW cm-2; (d) the corresponding EQE curves of the PTZDO/NF acceptors devices. Table 3. Photovoltaic performances of PTZ-DO/NF acceptors (w/w=1:1) devices under the illumination of AM 1.5G, 100 mW cm-2 Voc Jsc FF PCEmax(PCEave) a) 2 [V] [%] [mA/cm ] [%] 0.92 14.56 67.4 9.03 (8.84) PTZ-DO: pO-ITIC 0.92 15.18 66.4 9.28 (8.96) PTZ-DO: mO-ITIC 0.88 12.55 60.6 6.69 (6.59) PTZ-DO: FpO-ITIC 0.92 14.30 61.2 8.05 (7.83) PTZ-DO: ITIC a) Average PCE values are achieved from 10 independent devices. Active layer


Page 18 of 29

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Figure 7. AFM height images (5 µm×5 µm) of a) PTZ-DO:ITIC (1:1, w/w), rms=0.787 nm, b) PTZ-DO:pOITIC(1:1, w/w), rms=0.786 nm, c) PTZ-DO:mO-ITIC(1:1, w/w), rms=0.820 nm and d) PTZ-DO:FpOITIC(1:1, w/w) blend films, Rms=0.801 nm.

CONCLUSIONS In summary, through alkoxy-phenyl side-chain modifications, we synthesized three novel ITICtype of NF acceptors. Both three small molecules exhibit good solubility in common solvents and finely tunable energy levels and optical bandgaps. The 4-hexyloxy-phenyl and 3-hexyloxyphenyl substituted materials possess relatively low bandgap (1.61 eV for pO-ITIC and 1.63 eV for mO-ITIC) and 4.7% enhancements in maximum extinction coefficient compare to that of ITIC. As the result of the better absorption behaviors, inverted polymer solar cells based on pOITIC blended with PTB7-Th achieves improved PCE of 7.51% relative to that of ITIC (7.31%). When using thiazolothiazole-based wide-bandgap polymer with deep HOMO energy level to optimize both the optical absorption and molecular energy level differences between donor and NF acceptors, excellent PCE of 9.28% for mO-ITIC and 9.03% for pO-ITIC are obtained, which is 15.3% and 12.2% increment relative to 8.05% PCE of ITIC. We believe the finding in this work should provide useful guideline for the design of novel NF acceptors for highly efficient PSCs through alkoxy-phenyl side-chain modified on the electron-donating moiety of AD-A organic small molecules. EXPERIMENTAL SECTION



Materials. All used chemicals and solvents in this work were purchased from TCI or Alfa & Aesar. Tetrahydrofuran (THF) is dried over sodium (Na)/benzophenone ketyl and then freshly distilled before use. The structures and synthesized routines of the intermediates and acceptors are shown in Scheme 2. Compound 1 was prepared according to the literatures. [10] PTZ-DO was synthesized according to the Scheme S1.

Synthesis of pO-ITIC. 4-Hexyloxy-1-bromobenzene (6.4 g) in 30 mL tetrahydrofuran (THF) at -78 ºC was added n-BuLi (12.5mL, 2.0 M in hexane) under nitrogen, then the mixture was kept stirring at -78 ºC for 1 h. After that a solution of compound 1 (1.93 g) in THF (20 mL) was added slowly and then stirred for 4 h. Water was added to quench the reaction and the mixture was extracted with ethyl acetate. After the removal of solvent, the crude product was charged into 100 mL flask and acetic acid (50 mL) and conc. H2SO4 (1 mL) were added. Then the mixture was refluxed for 2 h. After pouring into water, the mixture was extracted with ethyl acetate. The resulting crude compound 2 was purified by silica gel to give an yellow solid (2.95g, 61%). 1

H NMR (500 MHz, CDCl3), δ 7.46 (s, 2H), 7.27 (q, 4H), 7.19 (m 8H), 6.79 (m, 8H), 3.89 (t, J = 5 Hz, 8H),

1.72 (m, 8H), 1.41 (m, 8H), 1.31 (m, 16H), 0.88 (t, J = 5 Hz, 12H). 13C NMR (125 MHz, CDCl3), δ 158.26, 153.61, 146.36, 141.74, 136.01, 134.95, 133.66, 129.25, 126.36, 120.41, 116.71, 114.34, 67.91, 62.23, 31.60, 29.29, 25.77, 22.62, 14.05.

In a three-neck flask under the protection of argon, compound 2 (2.20 g) and trimethylethyldiamine (1.2 mL) was dissolved in anhydrous THF (30 mL) and anhydrous hexane (20 mL). After cooling down to -78 oC, a n-BuLi solution (2.0 M in hexane, 3.0 mL) was added dropwise within in 15 min under vigorously stirring. Then the mixture was allowed to stir for 1 h under ambient temperature and then 1.5 mL dimethylformamide (DMF) was added. After stirred at -40 oC for 2h and then warmed to room temperature. The reaction was poured into water and extracted by ethyl acetate. The obtained organic layer was dried over anhydrous magnesium sulfate (MgSO4), after recrystallized with mixture of ethanol and hexane, the compound M1 was obtained as a yellow crystals (1.8g, 75% yield). 1H NMR (500 MHz, CDCl3), δ 9.88 (s, 2H), δ 7.93 (s, 2H), 7.57 (s, 2H), 7.16 (d, J = 10 Hz, 8H), 6.82 (d, J = 10 Hz, 8H), 3.90 (t, J = 5 Hz, 8H), 1.74 (m, 8H), 1.42 (m, 8H), 1.30 (m, 16H), 0.88 (t, J = 5 Hz, 12H).13C NMR (125 MHz, CDCl3), δ 182.81, 158.59, 155.01, 149.26,


Page 20 of 29

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147.03, 144.44, 141.83, 140.05, 136.38, 133.74, 129.85, 129.03, 117.82, 114.61, 67.98, 62.39, 31.57, 29.23, 25.74, 22.60, 14.03.

In a 100 mL dry flask, compound M1 (569 mg) and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (485 mg) were dissolved in CHCl3 (40 mL), then pyridine (1.5 mL) were added under nitrogen. The mixture was stirred at 40 °C under nitrogen for 6 h, then poured into water and extracted with chloroform. Then the product of pO-ITIC (530 mg, yield 71%) was purified by silica gel column chromatography by using dichloromethane as eluent. 1H NMR (500 MHz, CDCl3), δ 8.85 (s, 2H), 8.68 (d, J = 10 Hz, 2H), 8.19 (s, 2H), 7.91(d, J = 10 Hz, 2H), 7.73 (m, 4H), 7.61 (s, 2H ), 7.24 (d, J = 10 Hz, 8H), 6.86 (d, J = 10 Hz, 8H), 4.00 (t, J = 5 Hz, 8H), 1.73 (m, 8H), 1.42 (m, 8H), 1.30 (m, 16H), 0.88 (t, J = 5 Hz, 12H). 13C NMR (125 MHz, CDCl3), δ 188.08, 160.33, 158.72, 156.04, 152.57, 148.06, 146.88, 143.68, 140.03, 139.60, 138.17, 136.87, 136.82, 135.17, 134.47, 133.47, 129.09, 125.31, 123.77, 122.79, 118.34, 114.76, 114.61, 114.55, 69.44, 68.00, 62.49, 31.56, 29.24, 25.74, 22.59, 14.01. Anal. Calcd for C94H82N4O6S4: N 3.76, C 75.67, H 5.54. Found: N 3.70, C 75.68, H 5.57. HRMS (MALDI) m/z: Calcd for C94H82N4O6S4:1490.5112, 1491.5144, 1492.5150; Found: 1490.5096, 1491.5176, 1492.5230.

Synthesis of mO-ITIC. 3-Hexyloxy-1-bromobenzene (9.3 g) in tetrahydrofuran (30 mL) at -78 ºC was added n-BuLi (18 mL, 2.0 M in hexane) under nitrogen, then the mixture was kept stirring at -78 ºC for 1 h. After that a solution of compound 1 (3.0 g) in THF (20 mL) was added slowly and then stirred for 1 h. Water was added to quench the reaction and the mixture was extracted with ethyl acetate. After the removal of solvent, the crude product was charged into 100 mL flask and acetic acid (50 mL) and conc. H2SO4 (1 mL) were added. Then the mixture was refluxed for 2 h. After pouring into water, the mixture was extracted with ethyl acetate and washed with brine. The resulting crude compound 3 was purified by silica gel to give an yellow solid (4.1g, 63%). 1H NMR (500 MHz, CDCl3), δ 7.51 (s, 2H), 7.25 (s, 2H), 7.16 (t, J = 10 Hz, 4H), 6.84 (m, 8H), 6.77 (m, 4H), 3.86 (t, J = 5 Hz, 8H), 1.69 (m, 8H), 1.37 (m, 8H), 1.27 (m, 16H), 0.86 (t, J = 5 Hz, 12H).

13

C NMR (125 MHz, CDCl3), δ 159.15, 152.68, 145.41, 144.42, 143.55, 141.75, 136.15, 133.68,

129.37, 126.44, 120.49, 120.34, 117.05, 115.09, 112.88, 67.97, 63.56, 31.60, 29.19, 25.71, 22.59, 14.04.



The compound 3 (2.20 g) and trimethylethyldiamine (1.2 mL) was dissolved in 30 mL of anhydrous THF and 20 mL of anhydrous hexane in a three-neck flask under the protection of argon. The solution was cooled to -78 oC, and a solution of n-BuLi (2.0 M in hexane, 3.0 mL) was added dropwise with stirring in 15 min. After this addition, the reaction mixture was allowed to ambient temperature and stirred for 1 h and then 1.5 mL dimethylformamide (DMF) was added. The reaction mixture was stirred at -40 oC for 2h and then warmed to room temperature. Subsequently, the reaction mixture was poured into water and extracted with ethyl acetate. Then, the organic layer was dried over anhydrous magnesium sulfate (MgSO4) and concentrated to afford the yellow crude product. The crude product was recrystallized twice from the mixture of ethanol and hexane, and finally afforded compound M2 as a yellow crystals (1.8g, 75% yield). 1H NMR (500 MHz, CDCl3), δ 9.88 (s, 2H), 7.93 (s, 2H), 7.62 (s, 2H), 7.20 (t, J = 10 Hz, 4H), 6.80 (m, 12H), 3.87 (t, J = 5 Hz, 8H), 1.70 (m, 8H), 1.37 (m, 8H), 1.28 (m, 16H), 0.86 (t, J = 5 Hz, 12H). 13C NMR (125 MHz, CDCl3), δ 182.79, 159.33, 154.08, 149.75, 146.06, 144.47, 143.32, 141.83, 140.09, 136.53, 129.77, 129.73, 120.35, 118.14, 115.00, 112.92, 68.06, 63.67, 31.57, 29.16, 25.68, 22.57, 14.03.

In a 100 mL dry flask, M2 (569 mg) and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (485 mg) were dissolved in CHCl3 (40 mL), then, pyridine (1.5 mL) were added under nitrogen. The mixture was stirred at 40 °C under nitrogen for 6 h, then poured into water and extracted with chloroform. Then the product of mO-ITIC (450 mg, yield 61%) was purified by silica gel column chromatography by using dichloromethane as eluent. 1H NMR (500 MHz, CDCl3), δ 8.88 (s, 2H), 8.68 (d, J = 5 Hz, 2H), 8.16 (s, 2H), 7.87(dd, J = 10 Hz, 2H), 7.73 (m, 4H), 7.65 (s, 2H ), 7.23 (t, J = 10 Hz, 4H), 6.89-6.82 (br, 12H), 4.01-3.91 (br, 8H), 1.73 (m, 8H), 1.43 (m, 8H), 1.26 (m, 16H), 0.83 (t, J = 5 Hz, 12H). 13C NMR (125 MHz, CDCl3), δ 188.02, 160.33, 159.49, 154.93, 153.05, 147.01, 146.92, 143.55, 143.04, 140.04, 139.59, 138.23, 136.88, 136.85, 135.17, 134.42, 129.83, 125.35, 123.63, 122.74, 120.04, 118.75, 114.76, 114.61, 114.54, 113.61, 69.47, 68.12, 63.81, 31.66, 29.27, 25.74, 22.58, 14.03. Anal. Calcd for C94H82N4O6S4: N 3.76, C 75.67, H 5.54. Found: N 3.73, C 75.54, H 5.45. HRMS (MALDI) m/z: Calcd for C94H82N4O6S4:1490.5112, 1491.5144, 1492.5150; Found: 1490.5037, 1491.5093, 1492.5135.


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Synthesis of FpO-ITIC. 2-fluoro-4-hexyloxy-1-bromobenzene (9.9 g) in THF (30 mL) at -78 ºC was added n-BuLi (18 mL, 2.0 M in hexane) under nitrogen, then the mixture was kept stirring at -78 ºC for 1 h. After that a solution of compound 1 (3 g) in THF (20 mL) was added slowly and then stirred for 4 h. Water was added to quench the reaction and the mixture was extracted with ethyl acetate. After the removal of solvent, the crude product was charged into 100 mL flask and acetic acid (50 mL) and conc. H2SO4 (1 mL) were added. Then the mixture was refluxed for 2 h. After pouring into water, the mixture was extracted with ethyl acetate and washed with brine. The resulting crude compound 4 was purified by silica gel to give an yellow solid (4.5g, 65%). 1H NMR (500 MHz, CDCl3), δ 7.42 (s, 2H), 7.30 (s, 4H), 7.02(dd, 4H), 6.94 (dd, 4H), 6.85(t, J = 5 Hz, 4H), 3.97(t, J = 5 Hz, 8H), 1.77 (m, 8H), 1.43 (m, 8H), 1.29 (m, 16H), 0.88 (t, J = 5 Hz, 12H). 13C NMR (125 MHz, CDCl3), δ 153.35, 152.91, 151.39, 146.52, 145.39, 143.13, 142.11, 136.07, 135.11, 133.30, 126.79, 123.69, 120.54, 116.73, 116.26, 114.53, 69.36, 61.86, 31.52, 29.17, 25.61, 22.57, 14.01.

The compound 4 (2.30 g) and trimethylethyldiamine (1.2 mL) was dissolved in 30 mL of anhydrous THF and 20 mL of anhydrous hexane in a three-neck flask under the protection of argon. The solution was cooled to -78 oC, and a solution of n-BuLi (2.0 M in hexane, 3.0 mL) was added dropwise with stirring in 15 min. After this addition, the reaction mixture was allowed to ambient temperature and stirred for 1 h and then 1.5 mL dimethylformamide (DMF) was added. The reaction mixture was stirred at -40 oC for 2h and then warmed to room temperature. Subsequently, the reaction mixture was poured into water and extracted with ethyl acetate. Then, the organic layer was dried over anhydrous magnesium sulfate (MgSO4) and concentrated to afford the yellow crude product. The crude product was recrystallized twice from the mixture of ethanol and hexane, and finally afforded compound M3 as a yellow crystals (1.4g, 58% yield). 1H NMR (500 MHz, CDCl3), δ 9.91 (s, 2H), 7.97 (s, 2H), 7.53 (s, 2H), 6.94(m, 8H), 6.90 (m, 4H), 4.21 (t, J = 5 Hz, 8H), 1.77 (m, 8H), 1.43 (m, 8H), 1.32 (m, 16H), 0.88 (t, J = 5 Hz, 12H). 13C NMR (125 MHz, CDCl3), δ 182.81, 154.28, 153.40, 151.44,149.23, 146.89, 146.81, 146.01, 144.79, 142.10, 139.54, 136.42, 133.89, 133.84, 129.85, 123.74, 117.81, 115.89, 115.73, 114.70, 69.83, 61.99, 31.50, 29.13, 25.59, 22.57, 14.00.

In a 100 mL dry flask, M3 (605 mg) and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (485 mg) were dissolved in CHCl3 (40 mL), then, pyridine (1.5 mL) were added under nitrogen. The mixture was stirred



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at 40 °C under nitrogen for 6 h, then poured into water and extracted with chloroform. Then the product of FpO-ITIC (510 mg, yield 65%) was purified by silica gel column chromatography by using dichloromethane as eluent. 1H NMR (500 MHz, CDCl3), δ 8.88 (s, 2H), 8.70 (d, J = 5Hz, 2H), 8.23 (s, 2H), 7.92 (d, J = 5 Hz, 2H), 7.77 (m, 4H), 7.56 (s, 2H), 7.02(m, 8H), 6.93 (t, J = 10 Hz, 4H), 4.00 (t, J = 5 Hz, 8H), 1.78 (m, 8H), 1.44 (m, 8H), 1.32 (m, 16H), 0.87 (t, J = 5 Hz, 12H).

13

C NMR (125 MHz, CDCl3) , δ 188.11, 160.25, 155.17,

153.47, 152.18, 151.50, 147.01, 146.93, 146.09, 143.70, 140.06, 139.73, 138.07, 136.88, 136.70, 135.30, 134,59, 133.65, 125.38, 123.82, 123.28, 118.26, 115.96, 115.80, 114.85, 114.45, 69.88, 69.40, 62.06, 31.50, 29.14, 25.60, 22.56, 14.00. Anal. Calcd for C94H78F4N4O6S4: N:3.58, C 72.19, H 5.03. Found: N 3.43, C 72.09, H 4.99. HRMS (MALDI) m/z: Calcd for C94H78F4N4O6S4: 1563.4813, 1564.4845, 1565.4852; Found: 1563.4762, 1564.4832, 1565.4863.

Synthesis of PTZ-DO. PTZ-DO was synthesized according to the synthetic routines of Scheme S1. 2,6Bis(trimethyltin)-4,8-di(2,3-dioctylthiophen-5-yl)-benzo[1,2-b:4,5-b’]dithiophene derivatives (6)

[54]

(5)

[33]

thiazolothiazole

were synthesized according to the literatures. Compound 5 (64 mg, 0.1 mmol) and

compound 6 (113 mg, 0.1 mmol) were dissolved in dry toluene (25 mL) in a 50 ml flask with two necks under protection of argon. The mixture were flushed with argon for 30 min, then Pd(PPh3)4 (7 mg) was added and reaction system was stirred at 100 oC for 6 h under argon. After cooling to the room temperature, the solution was poured into the methanol (200 mL), filtered through filter and soxhlet-extracted with methanol, hexane and chloroform successively. The chloroform solution was extracted to a small volume and poured into methanol again. Finally, the polymer was collected by filtration and dried under vacuum at 50 oC overnight and afforded PTZ-DO as a dark-red solid (78 mg, 62%). GPC (150 oC, 1,2,4-tricholorobenzene, polystyrene standard): Mn=24.3 kDa, Mw=56.3 kDa, PDI=2.31. Anal. calcd for C74H100N2S8 (%): C, 69.76; H, 7.91; found (%): C, 69.56; H, 7.89.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Measurements and characterization section, 1H NMR and

13

C NMR spectra, calculated HOMO and LUMO


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distribution, TGA curves, cyclic voltammograms (CV) and absorption spectra of the three acceptors, J–V curves, EQE spectra and photovoltaic performances of PTB7-Th: acceptors based devices. AFM phase image, hole and electron mobilities of the PTB7-Th: acceptors blend films, synthetic route and CV of PTZ-DO. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email [email protected] (W. Ma) *Email [email protected] (C. Gao). Author Contributions †

These authors contribute equally to this work.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Dr. W. Ma thanks for the support from Ministry of Science and Technology of the People's Republic of China (No. 2016YFA0200700), NSFC (No. 21504066). Dr. C. Gao thanks for the support from Key Scientific and Technological Innovation Team Project of Shaanxi Province (2016KCT-28) and Key Project in Industrial Field of Shaanxi Province (2017ZDXM-GY-046). X-ray data was acquired at beamlines 7.3.3 at the Advanced Light Source, supporting by the



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For Table of Contents Use Only

Alkoxy-phenyl side-chain modified small molecular electron acceptors toward improved photovoltaic performance relative to alkyl-phenyl substituted molecule.


Absorptive Behaviors and Photovoltaic Performance Enhancements of

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