Isomerically Pure Benzothiophene-Incorporated Acceptor: Achieving

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Isomerically Pure Benzothiophene-Incorporated Acceptor: Achieving Improved Voc and Jsc of Nonfullerene Organic Solar Cells via End Group Manipulation Shao-Ling Chang, Kai-En Hung, Fong-Yi Cao, Kuo-Hsiu Huang, Chain-Shu Hsu, Chuang-Yi Liao, Chia-Hao Lee, and Yen-Ju Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08462 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019

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ACS Applied Materials & Interfaces

Isomerically

Pure

Benzothiophene-Incorporated

Acceptor: Achieving Improved Voc and Jsc of Nonfullerene Organic Solar Cells via End Group Manipulation Shao-Ling Changa, Kai-En Hunga, Fong-Yi Caoa, Kuo-Hsiu Huanga, Chain-Shu Hsua,b, Chuang-Yi Liaoc, Chia-Hao Leec and Yen-Ju Chenga,b*

aDepartment

of Applied Chemistry, National Chiao Tung University, 1001 University

Road, Hsinchu Taiwan 30010. bCenter

for Emergent Functional Matter Science, National Chiao Tung University, 1001

University Rd., Hsinchu, Taiwan 30010. cRaynergy

Tek Incorporation, 2F, No. 60, Park Avenue 2, Hsinchu Science Park,

Hsinchu 30844, Taiwan

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 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

KEYWORDS:

Acceptor

strength,

Benzothiophene-fused

Page 2 of 34

acceptor,

Ladder-Type

Structure, Non-Fullerene Acceptor, Organic Photovoltaics

Abstract Benzene-based 1,1-dicyanomethylene-3-indanone (IC) derivatives have been widely utilized as the end-group to construct acceptor-donor-acceptor type non-fullerene acceptors (A-D-A type NFAs). Extension of the end-group conjugation of NFAs is a rational strategy to facilitate intermolecular stacking of the end-groups which are responsible for efficient electron transportation. A bicyclic benzothiophene-based endgroup

acceptor,

2-(3-oxo-2,3-dihydro-1H-benzo[b]cyclopenta[d]thiophen-1-ylidene)

malononitrile), denoted as -BC was designed and synthesized. Knoevenagel condensation of the

unsymmetrical

1,3-diketo-precursor

with one equivalent of

malononitrile selectively reacts with the keto group attached at the -position of thiophene unit, leading to the isomerically pure benzothiophene-fused -BC. The welldefined -BC with extended conjugation was condensed with three different ladder-type diformylated donors to form three new A–D–A NFAs named as BDCPDT-BC, DTCC-BC and ITBC, respectively. The corresponding IC-based BDCPDT-IC, DTCC-IC and ITIC


2

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model compounds were also synthesized for comparison. The incorporation of electronrich benzothiophene unit in the end-group upshifts the LUMO energy levels of the NFAs, which beneficially enlarges the Voc values. On the other hand, the benzothiophene unit in -BC not also imparts an optical transition in the shorter wavelengths around 340-400 nm for better light harvesting ability but also promotes antiparallel  stacking of the end-groups for efficient electron transport. The OPV devices using PBDB-T polymer and BC-based NFAs all showed the improved Voc and Jsc values. The BDCPDT-BC-, and DTCC-BC-based devices exhibited a PCE of 10.82% and 10.74%, respectively, which outperformed the corresponding BDCPDT-IC-, and DTCC-IC-based devices (9.33% and 9.25%). More importantly, the ITBC-based device delivered a highest PCE of 12.07% with a Jsc of 19.90 mA/cm2, a Voc of 0.94 V, and an FF of 64.51%, representing a 14% improvement relative to the traditional ITIC-based device (10.5%).

1. Introduction


3


Page 4 of 34

Organic photovoltaic cells (OPVs) have emerged as promising technologies because of their potentials in producing flexible, large-area and semitransparent electronics.1-2 During the past two decades, a great deal of effort has been focused on developing p-type polymers whereas n-type materials are mostly restricted to the fullerene derivatives.3-6 However, further advance in fullerene-based OPVs has proven challenging owing to several intrinsic shortcomings of the mono-adduct fullerenes including feeble absorption, poor photochemical stability, untunable energy level and unstable morphology against heating. In an effort to break through the bottleneck, there has been an increasing interest in developing non-fullerene acceptors (NFAs) in recently years.7-19 In 2015, Zhan et al. first developed an n-type material ITIC containing a ladder-type indacenodithieno[3,2-b]thiophene (IDTT) central core coupled with 1,1dicyanomethylene-3-indanones (usually denoted as IC) as the end groups (EG). This strategy has successfully achieved remarkable OPV performance.20-22 Since then, an electron rich ladder-type donor (LD) end-capped with two acceptors (A) have emerged as the mainstream NFA architecture for OPVs. By molecular engineering of central LD and terminal A moieties, the optical and electronic properties of the A–LD–A materials


4

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can be systematically tailored. To date, a large volume of studies have focused on the development of LD conjugated building blocks which usually consist of multifused benzene/thiophene rings with bulky solubilizing groups attaching at sp3-carbon-bridges in a symmetrical manner.23-25 A variety of strategies have been implemented to tailor the electronic and steric effects of the central ladder-type donors including extending ladder’s conjugation length,26-30 employing different heteroaromatic components (such as

selenophene

and

carbazole)31-33,

side-chain

modification,34-36

and

using

unsymmetrical central core.37-40 Unlike more diverse modification on central donor units, the research on end-group acceptors is mostly limited to the modification of the IC structure which comprises a phenyl ring fused with a 1-one-3-dicyanovinylidenyl cyclopentane unit. Introduction of electron-rich (methyl, methoxy) or electron-deficient substituents (halogens) on the phenyl ring of IC41-46 or replacement of the phenyl ring of IC with thiophene unit have been implemented to fine-tune the electronic properties.47-49


5


Multifused ring

Heteroaromatic rings

Substituent effects O

O

O

NC

X X NC

IC

NC X= F,Cl

CN

NC

S

X= F,Cl, Br,Me, OMe

-TC

CN NC

O Isomers

NC

-TC

CN

S S

S

CN

O

S

O

CN

X CN

Page 6 of 34

NC

CN

This work O

S

S O

-TTC Isomers -TTC

NC

CN

-BC single Isomer

Figure 1. Chemical structures of IC, substituted IC, TC, TTC isomers and -BC.

To avoid the steric hindrance exerted from the central core, A-LD-A NFAs tend to selfassemble through  stacking between the neighbouring end-groups, which in turn form a channel for intermolecular electron-hopping.49-51 It is envisaged that elongation of conjugation of the end-group acceptors will strengthen such intermolecular  interactions to facilitate electron transport.52 We recently developed a thieno[2,3-

d]thiophene-fused end-group TTC acceptor which was used to make an A-LD-A-type NFA named as BDCPDT-TTC.53 Introducing the electron-rich thieno[2,3-d]thiophene (TT) unit into the end-group TTC of BDCPDT-TTC not only upshifts LUMO/HOMO energy level to obtain a larger Voc but also provides extra absorption at the 350-450 nm to enhance Jsc, demonstrating that thiophene-incorporated multifused ring end-groups are an effective and promising design. Unfortunately, upon knoevenagel condensation of the


6

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unsymmetrical 1,3-diketo-precursor with one equivalent of malononitrile inevitably yields two inseparate TTC regioisomers (-TTC and -TTC) which are further condensed with the diformylated donor to form a mixture of the final NFA (Figure 1). The previously developed asymmetrical thiophene-based -TC and -TC shown in Figure 1 also suffer the isomer problem. The undefined structure of the NFA might adversely cause uncertain and unpredicted intermolecular interactions and packing which in turn significantly affect the macroscopic device performance. It is of desire to develop a new class of unsymmetrical multi-fused ring end-group without having regioisomer problem. Achieving highly regioselective condensation of the unsymmetrical cyclopenta-1,3-dione moiety with malononitrile is thus important to circumvent the isomeric problem. It is known that benzothiophene has a stronger electron density (nucleophilicity) at C3 than C2 (Figure 2). Based on this property, we designed a new end-group precursor 1H-benzo[b]cyclopenta[d]thiophene-1,3(2H)-dione (BTO) by fusing the C2-C3 junction of benzothiophene with the cyclopenta-1,3-dione moiety. The C4 carbonyl group in BTO tends to adopt more charge-separated resonance structure due to the sulfur electron-donating ability (Figure 2). In other words, the C6 carbonyl group is much more reactive than C4 for condensation. Indeed, condensation of BTO with one equivalent of malononitrile affords the C6-olefinated -BC


7


Page 8 of 34

2-(3-oxo-2,3-dihydro-1H-benzo[b]cyclopenta[d]thiophen-1-ylidene)

malononitrile)

(denoted as -BC) exclusively as the single product which was unambiguously verified by its X-ray single crystal structure. No C4-olefinated -BC was observed. The isomerically pure end-group -BC was condensed with three different diformylated heptacyclic

ladder-type

cores,

benzodi(cyclopentadithiophene)

(BDCPDT)54

and

dithieno(cyclopentacarbazole) (DTCC)55 and (indacenodithieno[3,2-b]thiophene) IDTT20 units, forming three brand new A–LD–A NFAs named as BDCPDT-BC, DTCC-BC and ITBC, respectively (Figure 3). For comparison, the three corresponding IC-based NFAs were also prepared for comparison. (a) S

S

S

Minor (unstable)

Major (stable)

Benzothiophene

(b) O 3 S 2 1 BTO

5 6 O

S

NC

O

O 4

4

NC 6 O

CN

CN 4

S

6

CN NC -BC (single isomer)

S

O

-BC (not observed)

Figure 2. (a) Resonance structures of benzothiophene; (b) resonance structures of BTO and the regioselective condensation of isomerically pure -BC.


8

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The NFAs incorporating the more electron-rich and coplanar -BC end-group showed the higher-lying LUMO energy levels, stronger high-energy absorption and strengthened end-group interactions. The devices using the BC-based NFAs have achieved improved Voc and Jsc simultaneously leading to the exceptional OPV efficiencies over 12%, which outperforms the devices using the corresponding IC-based NFAs.


9


Page 10 of 34

Figure 3. Molecular structures of the NFAs in this research.

2. Results and discussion

Molecular Design, Synthesis and Characterization. The synthetic routes of the -BC, BDCPDT-BC, DTCC-BC, and ITBC are shown in Scheme 1. The compound 1 was easily converted into acyl chloride compound 2 by thionyl chloride. In the presence of AlCl3 and malonyl dichloride, the compound 2 underwent intramolecular electrophilic acylation to furnish the intermediate BTO in 40 % yield. Subsequent knoevenagel condensation with one equivalent of malononitrile yielded the -BC as the single pure isomer in a high 72 % yield. The X-ray single crystal structure of -BC (Figure 6) confirms that the dicyanovinylidenyl group and sulfur atom of the thiophene are situated at the same side. Finally, the target products, BDCPDT-BC, DTCC-BC, and ITBC were synthesized by knoevenagel condensation of -BC with BDCPDT-CHO, DTCC-CHO, and IDTT-CHO, respectively. The three BC-based NFAs showed good solubility in organic solvents. From thermogravimetric analysis (TGA), BDCPDT-BC, DTCC-BC, and ITBC showed higher thermal decomposition temperature (Td) of 388 oC, 352 oC, and 433 oC, respectively, relative to BDCPDT-IC, DTCC-IC, and ITIC (371, 346, 345 oC),20,54,55 demonstrating that employing rigid aromatic -BC with the extended conjugation as the end group further enhances their thermal stability. As expected, the phenyl


10

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groups at the sp3-carbons reduce the tendency of crystallization. From differential scanning calorimetry (DSC) analysis, no thermal transition was observed for these three BC-based NFAs. (Figure S1).

O OH S

Cl

SOCl2, DMF CHCl3

O

S

O

O

Cl

O NC

AlCl3, CH2Cl2 40 %

S

2

1

R

O

Cl O

CN

CH3COONa, DMSO 72 %

S

R

R

CN

NC -BC

BTO

R CN

S OHC

S

S

CHO

S

NC R

BDCPDT-CHO

R R = C6H13

R' R'

R'

S

O

S

S

S

S

O

S

CN

CN R

R R = C6H13

BDCPDT-BC O

R'

S -BC NC OHC

S

S

N R'

CHO

Pyridine Chloroform

R' R'

R'

CN

O

S

O S

NC

R' = C8H17

DTCC-CHO

R'

S

S

N

CN

R'

R'

CN NC R' = C8H17

R'

DTCC-BC R

OHC

R

R

R NC

S

S S

S

R

IDTT-CHO

O

S CHO NC

R R = C6H13

S

S S

S

O

CN S

CN R

R

ITBC

R = C6H13

Scheme 1. Synthetic route of -BC, BDCPDT-BC, DTCC-BC and ITBC.

Optical and Electrochemical Properties. The absorption of BDCPDT-BC, ITBC, and DTCC-BC in dilute solution and thin-film are shown in Figure S2 and Table 1. Compared with the IC-based BDCPDT-IC, ITIC, and DTCC-IC counterparts, BDCPDT-


11


Page 12 of 34

BC, ITBC, and DTCC-BC in chloroform solution exhibit a blue-shifted maximum absorption peak at 680, 664, and 643 nm but with larger extinction coefficients of 1.59 x 105, 1.52 x 105, and 1.85 x 105 M-1cm-1 in CHCl3, respectively (Figure S3). The maximum absorption (λmax) of BDCPDT-BC, ITBC, and DTCC-BC (717, 691 and 671 nm) was further red-shifted by 37, 27, and 28 nm, respectively, in the thin film state (Figure S2). Although incorporating the more electron-donating -BC as end group results in decreased intermolecular charge transfer (ICT), the BC-based NFAs turn out to have broader absorption spectrum and keep the similar absorption onset with the corresponding IC-based NFAs in thin film (Figure 4), indicating that employing π-extended -BC might enhance π–π interactions in solid state. More importantly, the BC-based NFAs show additional stronger absorption at 350460 nm in solution (with high extinction coefficients: BDCPDT-BC: 6.20 x 104, ITBC: 6.20 x 104, and DTCC-BC: 7.74 x 104 M-1cm-1) and in thin films which are absent in the corresponding IC-based NFAs (Figure 4 and S3). The extra absorption in shorter wavelengths is mainly from the optical transition of -BC units. The energy levels and electrochemical properties of BC-based NFAs estimated from cyclic voltammetry (CV) oxidation and reduction potentials are depicted in Figure S4 and Table 1. The lowest


12

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unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels were determined to be −3.85/−5.40, −3.90/−5.49, and −3.81/−5.46 eV for BDCPDT-BC, ITBC, and DTCC-BC, respectively (Figure 4d). BCbased NFAs exhibited slightly higher LUMO energy levels relative to the corresponding IC-based NFAs (BDCPDT-IC= −3.87 eV, ITIC = −4.01 eV, and DTCC-IC= −3.83 eV) which are expected to obtain the higher Voc in OPV devices.


13


Page 14 of 34

Figure 4. Absorption spectra of the BC-based NFAs and the corresponding IC-based NFAs in thin film (a) BDCPDT-BC, BDCPDT-IC, (b) DTCC-BC, DTCC-IC, (c) ITBC and ITIC; (d) energy levels of BC-based NFAs and the corresponding IC-based NFAs; (e) OPV inverted device structure and molecular structures of polymer donor PBDB-T and BC-based NFAs.

Table 1. The optical and electrochemical properties of the BC-based NFAs.

λmax (nm) Compound

CHCl

Film

ε × 105

λonset

Egopt

EHOMO

ELUMO

Egele

(M− 1cm− 1)

(nm)a

(eV)b

(eV) c

(eV) c

(eV) d

3

BDCPDT-IC

696

727

1.44

808

1.53

−5.41

−3.87

1.54

BDCPDT-BC

680

717

1.59

808

1.53

−5.40

−3.85

1.55

ITIC

664

705

1.36

780

1.59

−5.61

−4.01

1.60

ITBC

664

691

1.52

780

1.59

−5.49

−3.90

1.59

DTCC-IC

655

681

1.71

745

1.66

−5.47

−3.83

1.64

DTCC-BC

643

671

1.85

743

1.67

−5.46

−3.81

1.65

aAbsorption

edge of the films.bEgopt = 1240/λonset, calculated from the thin-film. cEnergy levels estimated from CV. d

Egele = ELUMO − EHOMO.

Density Functional Theory Calculations and X-ray Crystallographic Analysis. Quantum chemical calculations were used to study the molecular conformation and electronic influences of the BC-based NFAs (Figure 5). The distances between the sulfur (S) and oxygen (O) shown in Figure 5 are only ca.2.75 Å. Conformation lock by such S … O interactions would


14

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provide a highly planar in NFAs structure, which is beneficial to π‐π intermolecular packing and electron transport.46 The calculated HOMO/LUMO levels are −5.50 eV/−3.54 eV for BDCPDTBC, −5.57 eV/−3.55 eV for ITBC, and −5.62 eV/ −3.44 eV for DTCC-BC, respectively, which is consistent with experimental results and suggests that more electron-rich -BC end-group upshifts LUMO energy levels relative to the IC-based NFAs (the calculated LUMO levels are −3.58, −3.56, and −3.51 eV for BDCPDT-IC, ITIC, and DTCC-IC, respectively.) The dipole moments of the half-molecule of BDCPDT-IC, BDCPDT-BC, and BDCPDT-TTC are estimated to be 7.03 D, 6.86 D, and 6.32 D, respectively, indicating that the ICT effect in BC-based NFAs are in the middle between that in IC and TTC-based NFAs.


15


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Figure 5. Top view and side view of the optimized geometry of (a) BDCPDT-BC, (b) ITBC and (c) DTCC-BC with highlighting the intramolecular S···O distance, and their calculated HOMO/LUMO frontier molecular orbitals.

The molecular packing of -BC can be elucidated by the X-ray crystallographic analysis (Figure 6a). A large dipole moment of 6.05 D between the electron-rich benzothiophene and electronwithdrawing dicyanovinylidene moieties promotes strong electrostatic dipole–dipole and  interactions. Consequently, the dipolar -BC molecule self-assembles into an antiparallel faceto-face packing with a closest  distance of ca. 3.42 Å. This result implies that the BC-based NFAs are very likely to adopt the antiparallel  stacking of the end-groups (Figure 6b) to create a channel for the electron transport.49


16

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Figure 6. (a) Crystal packing of -BC showing the antiparallel  stacking with a closest distance of 3.42 Å; (b) size view of the proposed molecular packing of BDCPDTBC using the end-groups for antiparallel  stacking.

Photovoltaic

Characteristics.

OPV

devices

were

fabricated

with

an

inverted

configuration using the three new BC-based NFAs as the n-type materials and PBDB-T as the p-type polymer donor. To systematically investigate the end-group effect, the devices using the three corresponding IC-based NFAs were also fabricated for comparison.20,54,55 The J-V curves and corresponding external quantum efficiency (EQE) results are shown in Figure 7 and device parameters are summarized in Table 2.

In comparison with the BDCPDT-IC-based control device54, the BDCPDT-BCbased device showed an improvement in both Voc and Jsc from 0.86 to 0.92 V and 16.56 to 18.55 mA/cm2, respectively, achieving a higher PCE of 10.82%. Meanwhile, the DTCC-BC-based device also achieved an enhanced performance with a Voc of 0.98 V, a Jsc of 17.22 mA/cm2, and an FF of 63.62%, leading to a higher PCE of 10.74% relative to DTCC-IC-based device (9.25%). Furthermore, the ITBC-based device delivered a


17


Page 18 of 34

highest PCE of 12.07% with an impressive Jsc of 19.90 mA/cm2, a Voc of 0.94 V, and an FF of 64.51%, which also outperformed the ITIC-based device (10.05%). The ITBCbased device exhibited the highest EQE response at 600 nm (85%). Moreover, it should be noted that all of BC-based devices showed significantly increased EQEs (350-450 nm) relative to the corresponding IC-based devices (Figure S6). The improved Voc values is attributed to the up-shifted LUMO and the markedly increased Jsc values are due to the enhanced absorption and intermolecular interactions.

Table 2. Parameters of ITO/ZnO/PBDB-T:NFA/MoO3/Ag devices. Device a

PBDB-T:BDCPDT-IC

PBDB-T:BDCPDT-BC

PBDB-T:DTCC-IC

PBDB-T: DTCC-BC b

Blend

Voc

Jsc

FF

PCE

ratio

(V)e

(mA/cm2) e

(%)e

(%)e

(wt%) 1:1

0.86

16.56

(0.85 ± 0.01)

(16.83 ± 0.44)

0.92

18.55

(0.92 ± 0.00)

(18.31 ± 0.47)

0.97

14.26

(0.97 ± 0.00)

(14.26 ± 0.04)

0.98

17.22

(0.99 ± 0.01)

(17.16 ± 0.82)

1:1

1:1.5

1:1.5

65.52 (63.76 ± 1.52) 63.41 (62.49 ± 1.08) 64.08 (64.08 ± 1.66) 63.62 (61.54± 1.85)

9.33 (9.14 ± 0.08)

10.82 (10.52 ± 0.21)

9.25 (9.25 ± 0.29)

10.74 (10.46 ± 0.28)


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PBDB-T:

ITICc

1:1

PBDB-T: ITBCd

awith

1:1.2

0.92

16.75

(0.93 ± 0.01)

(16.74 ± 0.04)

0.94

19.90

(0.94 ± 0.00)

(19.61 ± 0.53)

65.23 (64.16 ± 0.99) 64.51 (63.48 ± 1.80)

10.05 (10.01 ± 0.07)

12.07 (11.70 ± 0.37)

0.5 vol % DIO as the additive and CB as the solvent b o-xylene as the solvent. c with 1 vol % DIO as the additive. d with 0.5 vol

% CN as the additive. eThe average values with standard deviation of 5 devices.

Figure 7. (a) J−V curves and (b) EQE spectra of the devices.

Morphological Characterization. 2D grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed to investigate the molecules packing and orientation of BC-based NFAs blend films.56 The 2D diffraction images and the corresponding 1D patterns in the out-of-plane direction are shown in Figure 8. The PBDB-T:BDCPDT-BC, PBDBT:DTCC-BC and PBDB-T:ITBC blend films showed obvious (010) diffractions at qz of 1.80, 1.84, and 1.62 Å−1, respectively. The face-on π−π stacking spacing was calculated to be ca. 3.48


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Page 20 of 34

Å for PBDB-T:BDCPDT-BC, 3.41 Å for PBDB-T:DTCC-BC, and 3.87 Å for PBDB-T:ITBC blends, respectively, which are shorter than those of IC-based counterparts (3.6 Å for PBDBT:BDCPDT-IC, 3.86 Å PBDB-T:DTCC-IC and 3.93 Å for PBDB-T:ITIC49 blends) (Figure S7). It is envisaged that incorporation of -BC end-group with extended conjugation could enhance intermolecular interactions of NFAs, leading to the shorter π−π stacking distances and improvement of the device characteristics.

Figure 8. GIWAXS images of the blend films of (a) PBDB-T:BDCPDT-BC, (b) PBDBT:DTCC-BC, and (c) PBDB-T:ITBC; and their corresponding 1-D patterns (d) in the outof-plane direction.

SCLC Mobility and Photoluminescence Quenching. The charge carrier mobilities of the PBDB-T:NFAs blend films were measured using space-charge limit current (SCLC) model


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with the structure of ITO/PEDOT:PSS/active layer/Au for hole nobility and ITO/ZnO/active layer/Al for electron mobility (Figure S8 and Table 3). The hole and electron mobilities are 2.28×10−4/1.26×10−5cm2V−1s−1 for PBDB-T:BDCPDT-BC, 8.83×10−5/1.38×10−5 for PBDBT:DTCC-BC, and 1.09×10−4/5.82×10−5 for PBDB-T:ITBC, respectively. The PBDB-T:ITBC device exhibited relatively high electron mobility and more balanced μh/μe of 1.87. In addition, relative to the corresponding IC-based devices (Table 3), all the BC-based devices achieved simultaneously improved electron/hole mobility and more balanced μh/μe due to the stronger π−π stacking interactions as inferred from GIWAXS measurements (Figure 8).

Table 3. Hole and electron mobility of PBDB-T: NFAs measured by SCLC model. Device

μh (cm2V−1s−1)

μe (cm2V−1s−1)

μh/ μe

PBDB-T:BDCPDT-IC

3.22 ×10−5

1.59×10−6

20.27

PBDB-T:BDCPDT-BC

2.28×10−4

1.26×10−5

18.06

PBDB-T:DTCC-IC

5.58×10−5

1.12 ×10−5

5.00

PBDB-T: DTCC-BC b

8.83×10−5

1.38 ×10−5

6.41

PBDB-T: ITICc

1.50×10−5

3.05×10−6

4.91

PBDB-T: ITBCd

1.09×10−4

5.82×10−5

1.87


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Photoluminescence (PL) measurements were performed to investigate the behavior of exciton dissociation and charge transfer between the donor and acceptor. The PL spectra of pristine PBDB-T, neat BC-based NFAs, and PBDB-T:BC-based NFAs blend films are shown in Figure S9. When the PBDB-T:BDCPDT-BC, PBDB-T:DTCC-BC, and PBDB-T:ITBC blends were excited at 700 nm, 670 nm, and 700 nm, respectively, the emission of the BC-based NFAs were significantly quenched (by 78.9% for BDCPDT-BC, 95.1% for DTCC-BC, and 84.5% for ITBC). These results indicated that the exciton dissociation and charge transfer can occur efficiently with a small HOMO energy offset (ΔEHOMO) of 0.04 eV for the PBDB-T:BDCPDT-BC blend film.

3. Conclusion In summary, we have developed a new benzothiophene-fused acceptor -BC. Due to the different reactivity of the unsymmetrical 1,3-diketo precursor, condensation of BTO with malononitrile preferentially occurs at the keto group attached at the -position of thiophene unit, leading to the isomerically pure -BC. The -BC was coupled with three different diformylated ladder-type donors to afford three well-defined A–D–A NFAs,


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BDCPDT-BC, DTCC-BC and ITBC, respectively, without isomeric mixtures. The corresponding IC-based BDCPDT-IC, DTCC-IC and ITIC counterparts were also synthesized for comparison to investigate the end-group effect. The OPV devices using the BC-based NFAs as the acceptors and PBDB-T as the p-type donor exhibited the improved Voc and Jsc values compared to the devices with the corresponding IC-based NFAs. The incorporation of electron-rich benzothiophene unit at the end-group of NFAs brings several beneficial characteristics such as (1) imparts additional absorption in the shorter wavelengths around 340-400 nm for better light-harvesting ability; (2) promotes antiparallel  stacking of the end-groups for efficient electron transport; (3) upshifts the HOMO/LUMO energy levels of NFAs. As a result, Jsc and Voc values can be simultaneously improved. The PBDB-T:ITBC device delivered an impressive Jsc of 19.90 mA/cm2, a Voc of 0.94 V, an FF of 64.51% and a highest PCE of 12.07%, which represents a 14% improvement relative to the traditional PBDB-T: ITIC-based device (10.5%).


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4. Experimental Section

4.1 Experimental Procedures. Benzo[b]thiophene-2-carboxylic acid (1), benzo[b]thiophene-2-carbonyl chloride (2), BDCPDTCHO, DTCC-CHO, and IDTT-CHO were synthesized as reported20,54,55,57. The detailed synthetic procedures of BC-based NFAs were provided in the supporting information. 4.2 Fabrication of the Devices.

All the NFAs device were fabricated with an inverted architecture of ITO/ZnO/active layer/MoO3/Ag. The polymer PBDB-T and BC-based NFAs were dissolved in chlorobenzene, the concentration and spin coating speed of active layer is 12 mg mL−

1

and 4000 rpm, respectively. The detailed processing of inverted

configuration was described in previous report53.

ASSOCIATED CONTENT

The Supporting Information including synthetic procedures, computational details, DSC/TGA measurements, cyclic voltammogram, device fabrication and characterization, and 1H and 13C NMR spectra is available free of charge on the ACS Publications.


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AUTHOR INFORMATION

Corresponding Author E-mail: [email protected] ACKNOWLEDGMENT We gratefully thank the financial support from the Ministry of Science and Technology and the Ministry of Education, and Center for Interdisciplinary Science (CIS) of the National Chiao Tung University, Taiwan. We gratefully acknowledge National Synchrotron Radiation Research Center (NSRRC), Dr. U-Ser Jeng, and Dr. Chun-Jen Su for supports GIWAXS experiments. We also acknowledge National Center for Highperformance Computing in Taiwan for computer time and facilities.

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Graphical abstract


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Isomerically Pure Benzothiophene-Incorporated Acceptor: Achieving

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