Butterfly Effects Arising from Starting Materials in Fused-Ring Electron Acceptors

Tengfei Li, Yao Wu, Jiadong Zhou, Mengyang Li, Jingnan Wu, Qin Hu, Boyu Jia, Xiran Pan,

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Butterﬂy Eﬀects Arising from Starting Materials in Fused-Ring

Electron Acceptors

Maojie Zhang, Zheng Tang, Zengqi Xie, Thomas P. Russell, and Xiaowei Zhan*

Cite This: https://dx.doi.org/10.1021/jacs.0c09800

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sı Supporting Information

ABSTRACT: We designed and synthesized a series of fused-ring electron acceptors

FREAs) based on naphthalene-fused octacyclic cores end-capped by 3-(1,1-

(

dicyanomethylene)-5,6-diﬂuoro-1- indanone (NOICs) using a bottom-up approach.

The NOIC series shares the same end groups and side chains, as well as similar

fused-ring cores. The butterﬂy eﬀects, arising from diﬀerent methoxy positions in the

starting materials, impact the design of the ﬁnal FREAs, as well as their molecular

packing, optical and electronic properties, charge transport, ﬁlm morphology, and

performance of organic solar cells. The binary-blend devices based on this NOIC

series show power conversion eﬃciencies varying from 7.15% to 14.1%, due to the

diﬀerent intrinsic properties of the NOIC series, morphologies of blend ﬁlms, and

voltage losses of devices.

INTRODUCTION

physical properties, and photovoltaic performance of the

resultant FREAs. For instance, when replacing the carbon-

■

Organic solar cells (OSCs) have the advantages of being

lightweight, ﬂexible, semitransparent, and printable over large-

areas from solution and hold promise for building integrated

bridge in IDIC with carbon−oxygen bridge, two isomers are

formed due to the diﬀerent oxygen positions, which signiﬁcantly

photovoltaics, wearable devices, and indoor photovoltaics.

aﬀect the PCEs of the devices (1.97% vs 10.42%).

The photoactive layers of OSCs consist of electron donor and

acceptor materials. In the past 3 decades, there has been a

tremendous amount of research focusing on the development of

Here, we present a bottom-up approach for the rational design

of high-performance FREAs and investigate the eﬀects of the

starting materials on molecular structures and properties of

FREAs. We choose 2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxabor-

olan-2-yl)naphthalene and its methoxylated isomers as starting

materials to synthesize a series of FREAs, NOICs (Chart 1). The

NOIC series of molecules share the same end-capping groups

and side chains, as well as similar fused-ring cores, but show

diﬀerent single crystal structures, absorption spectra, energy

levels, and electron mobilities. The binary-blend OSCs based on

the NOIC series and PM6 (Figure S1) have PCEs ranging

from 7.15% to 14.1%. These results indicate that the butterﬂy

eﬀects resulting from small diﬀerences in the starting materials

signiﬁcantly aﬀect the properties of the ﬁnal FREAs, under-

scoring the need for bottom-up design for high-performance

FREAs.

high-performance photovoltaic materials. Since we intro-

duced the star molecule ITIC and pioneered the concept of

fused-ring electron acceptors (FREAs), FREAs have developed

signiﬁcantly and have dominated the area of acceptor

−14

materials.

FREAs have the desirable characteristics of easy

molecular tailoring, tunability of energy levels, and strong

absorption in the visible and near-infrared (NIR) regions, due to

their modular acceptor−donor−acceptor (A−D−A) structure.

The optical and electronic properties, crystallinity, ﬁlm

morphology, and photovoltaic performance of FREAs can be

13,15−27

tuned by engineering the fused-ring cores,

end-capping

8−38

6,39−44

groups,

and side chains.

According to Shockley−Queisser limitation, extending optical

absorption and reducing energy loss can improve eﬃciencies of

OSCs. To realize improvement of device performance, rational

design of molecular structures and precise modulation of

properties of organic photovoltaic materials are essential but

challenging. Nowadays, molecular design of FREAs generally

adopts a top-down strategy that emphasizes the ﬁnal molecular

structure of FREAs, overlooking the starting materials and

synthesis process. However, subtle changes on the starting

materials may signiﬁcantly perturb the chemical structures,

Received: September 13, 2020

XXXX American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Article

Chart 1. Chemical Structures of SM1−4 and NOIC Series

RESULTS AND DISCUSSION

Characterization. Molecules in the NOIC series have good

solubility in chloroform and o-dichlorobenzene at room

temperature and good thermal stability with decomposition

■

Synthesis. The syntheses of the NOIC series are illustrated

in Schemes 1 and S1 −S7 . First, we designed and synthesized

NOIC by the Suzuki coupling reaction between starting material

SM1 and ethyl 2-bromothieno[3,2-b]thiophene-3-carboxylate

temperatures at 5% weight loss (T ) over 330 °C in nitrogen, as

determined by thermogravimetric analysis (TGA) (Figure S2,

Table 1).

(

TT-AB), a nucleophilic addition, intramolecular Friedel−

The single crystal data and structural reﬁnement of the NOIC

series are given in Tables S1 and S2. All compounds have planar

and rigid A−D−A backbones, due to the S···O interactions

between the outermost thiophene in the “D” units and the

carbonyl group in the “A ” units. The intramolecular S···O

interactions are shown in Figure S3 with distances of ∼2.7 Å for

the D−A fragments of NOIC series. In addition, the S···O

interactions in the core with a distance of 3.09 Å are observed for

NOIC2. For the stacking patterns shown in Figure 1, NOIC has

a 3D conjugated framework, due to the intermolecular π−π

interactions of adjacent D−A moieties (including 2FIC and

partial core skeleton) analogous to other FREAs reported

elsewhere. Introduction of methoxy groups at diﬀerent positions

leads to diﬀerent conformations and, therefore, packing

arrangements. Methoxy groups at the (4,8) positions seem to

intensify the disorder of alkyl chains, causing four independent

molecular conformations in the unit cell. NOIC1 shows π−π

stacking in directions orthogonal to the molecular long axis,

leading to less overlap of the molecular backbone, a low packing

index of 45.7%, and a large void percentage of 41.70%. Methoxy

groups at the (3,7) positions introduce conformational locks by

multiple S···O interactions, which are conducive to a planarity

and rigidity of A−D−A skeleton. Although the stacking

framework of NOIC2 is similar to that of NOIC, the stacking

Crafts cyclization, Vilsmeier−Haack reaction, and Knoevenagel

condensation with 3-(1,1-dicyanomethylene)-5,6-diﬂuoro-1-

indanone (2FIC) in sequence (Scheme S2). To probe the

butterﬂy eﬀects in FREAs resulting from the diﬀerent methoxy

positions in starting materials, we substituted SM1 with three

isomers SM2−4 where the methoxy position changed (Chart 1,

Scheme 1). Theoretically, ﬁve FREAs (NOIC1−5) can be

obtained, among which NOIC1 and NOIC2 are isomers with

diﬀerent methoxy positions; NOIC2 and NOIC3 are homo-

logues from diﬀerent reaction routes with the same starting

material SM3; NOIC3 and NOIC4 are isomers with diﬀerent

oxygen positions; NOIC4 and NOIC5 are homologues from

diﬀerent reaction routes with the same starting material SM4.

The syntheses of NOIC1 and NOIC2 are similar to that of

NOIC. Unlike NOIC1 and NOIC2, NOIC5 was not

synthesized, since no target compound 13 was obtained after

the regular nucleophilic addition reaction (Scheme S7 ). On the

other hand, NOIC3 and NOIC4 are synthesized using SM3 and

SM4 as starting materials, respectively, after the following

reactions: Suzuki coupling reaction with TT-AB, demethylation

reaction, acid-promoted intramolecular transesteriﬁcation,

nucleophilic addition, intramolecular dehydration cyclization,

Vilsmeier −Haack reaction, and Knoevenagel condensation with

FIC (Schemes S5 and S6 ).

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Scheme 1. Synthetic Routes for NOIC Series

Table 1. Basic Properties of NOIC Series

solution

ﬁlm

ε (10 M cm⁻¹

−1

)

λ_m_a_x(nm)

E_g^o^p^t(eV)

HOMO (eV)

LUMO (eV)

μ_e(10⁻⁴cm V s⁻¹

−1

)

acceptor

T_d(°C)

λ_m_a_x(nm)

NOIC

344

336

340

352

687

762

702

644

687

2.5

2.4

2.1

2.0

2.4

722

802

748

668

730

1.55

1.38

1.49

1.62

1.55

−5.76

−5.41

−5.64

−5.83

−5.64

−4.03

−4.02

−3.99

−3.95

−3.90

6.2

7.1

9.0

0.66

1.2

NOIC1

NOIC2

NOIC3

NOIC4

of 2FIC fragments increases, which is beneﬁcial for charge

transport and exciton migration. As for NOIC3, the covalent

cyclization of oxygen disrupts the coplanarity of the fused ring

system and the steric crowding of the alkyl chains results in more

complicated stacking pattern. Though ﬁbrous crystals of NOIC4

were obtained, they are unstable, and crystal structure data could

not be obtained.

NOIC1 and NOIC2 are similar, indicating that the diﬀerent

methoxy positions have negligible eﬀect on the packing. This is

not consistent with single-crystal data where diﬀerences are

observed, which may result from diﬀerent conditions of crystal

growth. NOIC3 and NOIC4 are slightly crystalline, showing

−1

only weak (100) reﬂections at 0.30 Å (d = 20.9 Å) and 0.35

−

(d = 18.0 Å) in the in-plane direction, respectively. All the

The ordering of the neat NOIC ﬁlms was investigated by

grazing-incidence wide-angle X-ray scattering (GIWAXS). 2D

GIWAXS proﬁles and related 1D plots are shown in Figure S4.

NOIC is highly crystalline and shows two diﬀerent crystal forms

in the ﬁlm. Both crystal forms show multiple orders of reﬂections

NOIC series of acceptors have a face-on orientation in the ﬁlm.

−

The π−π stacking peaks of the NOIC series are 1.80 Å , 1.84

−

−1

Å , 1.84 Å , 1.61 Å , and 1.71 Å in the out-of-plane

direction. The corresponding packing distance is 3.49 Å, 3.41 Å,

3.41 Å, 3.90 Å, and 3.67 Å.

with primary reﬂections at 0.31 Å⁻(d = 20.3 Å) and 0.46 Å (d

−1

The optical absorption data of the NOIC series are

−

13.7 Å) in the in-plane direction. NOIC1 and NOIC2 are also

summarized in Table 1. In CHCl solution (∼10 M), NOIC

−

highly crystalline, with primary reﬂections at 0.45 Å (d = 14.0

Å) and 0.46 Å (d = 13.7 Å), respectively, along with two higher

order reﬂections in the in-plane direction. All the reﬂections of

series compounds show similar maximum molar extinction

coeﬃcients (ε) of (2−2.5) × 10 M cm but notably diﬀerent

peak absorption wavelengths (Figure S5a ). In comparison to

−

−1

Journal of the American Chemical Society

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Figure 1. Molecular stacking patterns of (a) NOIC, (b) NOIC1, (c) NOIC2, and (d) NOIC3 in the crystal structures.

Article

Figure 2. (a) Absorption spectra of NOIC series as thin ﬁlms. (b) Energy levels of NOIC series.

−1 −1

NOIC (λ_m_a_x= 687 nm), the absorption peaks of NOIC1 and

NOIC2 red-shift to 762 and 702 nm, respectively, due to the

electron-donating eﬀects of methoxy groups on the fused-ring

cores. The peak of NOIC3 blue-shifts to 644 nm, while that of

NOIC4 is the same as that of NOIC. The absorption peaks of

(6.2 × 10⁻cm V s ), NOIC1 and NOIC2 have slightly

−

−1 −1

higher electron mobilities (μ = (7.1−9.0) × 10 cm V s ),

while NOIC3 and NOIC4 have signiﬁcantly lower mobilities

−4

−1 −1

((0.66−1.2) × 10 cm V s ). The higher electron mobilities

of NOIC, NOIC1, and NOIC2 are due to the denser molecular

packing in the ﬁlms, as indicated by GIWAXS. Relative to the

methoxylated NOIC1 and NOIC2, NOIC3 and NOIC4 with

carbon−oxygen bridges show lower electron mobilities (Table

1).

Photovoltaic Performance. The widely used donor

material PM6 has strong absorption in the 400−650 nm region

(Figure S1b ), complementing that of the NOIC series of

acceptors, and has energy levels that align well with those of the

NOIC series of acceptors. Hence, photovoltaic devices with an

inverted structure of ITO/ZnO/active layer/MoO /Ag using

PM6:NOIC series blends as photoactive layers were fabricated.

Devices prepared from ﬁlms of PM6 and the diﬀerent NOICs

(1:1 weight ratio) dissolved in CHCl containing 0.2 vol % of

1,8-diiodooctane (DIO) as an additive spin-coated and annealed

at 110 °C for 10 min showed the best performance (Figure S8,

Table S3 ).

PM6:NOIC-based devices had an open-circuit voltage (V

of 0.885 V, short-circuit current density (J ) of 18.1 mA cm ,

CHCl -cast thin ﬁlms red-shift 24−46 nm relative to those in

opt

solutions (Figure 2a). The optical bandgaps (E values) of the

NOIC series ﬁlms are 1.38−1.62 eV, calculated from their

absorption edges. Relative to methoxylated NOIC1 and

NOIC2, NOIC3 and NOIC4 with carbon−oxygen bridges

show blue-shifted absorption and larger bandgaps.

Electrochemical properties of the NOIC series were measured

by cyclic voltammetry (CV) (Figure S6). In comparison to

NOIC (LUMO/HOMO: −4.03/−5.76 eV), NOIC1 and

NOIC2 exhibit similar LUMO levels (−4.02 eV and −3.99

eV) but higher HOMO levels (−5.41 eV and −5.64 eV), due to

the electron-donating eﬀects of the methoxy groups on the

fused-ring cores. NOIC3 shows a higher LUMO (−3.95 eV) and

a lower HOMO level (−5.83 eV), while NOIC4 shows higher

LUMO and HOMO levels of −3.90 eV and −5.64 eV,

respectively. Relative to the methoxylated NOIC1 and

NOIC2, NOIC3 and NOIC4, with carbon−oxygen bridges,

show higher LUMO energy levels.

)

−

Electron transport properties normal to the surface of ﬁlms of

ﬁll factor (FF) of 71.2%, and PCE of 11.4% (Figure 3 a, Table 2).

Due to the red-shifted and broadened absorption (Figure S5b),

PM6:NOIC1-based devices showed a notably enhanced J_S_Cof

the NOIC series were investigated by the space charge limited

current (SCLC) method (Figure S7 ). Compared to NOIC

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Figure 3. Photovoltaic performance of optimal OSCs based on PM6:NOIC series: (a) J−V characteristics, (b) EQE spectra, (c) V versus light

intensity, and (d) J versus light intensity.

Table 2. Device Data of Optimal OSCs Based on PM6:NOIC Series (1:1, w/w)

μ (10⁻cm V s⁻¹

−1

)

J_S_C(mA cm⁻²

calcd J_S_C(mA cm⁻²

acceptor

NOIC

V_O_C(V)

)

FF (%)

PCE (%)

)

μ_h

μ_e

μ_h/μ_e

0.880 ± 0.007 (0.885)

0.863 ± 0.004 (0.864)

0.919 ± 0.005 (0.927)

0.925 ± 0.006 (0.930)

0.931 ± 0.007 (0.944)

17.7 ± 0.3 (18.1)

21.1 ± 0.7 (21.9)

20.2 ± 0.3 (20.6)

12.6 ± 0.4 (12.9)

16.6 ± 0.4 (16.8)

70.3 ± 0.6 (71.2)

66.9 ± 1.6 (65.9)

73.4 ± 0.7 (73.8)

58.4 ± 1.2 (59.8)

11.0 ± 0.1 (11.4)

12.2 ± 0.1 (12.5)

13.6 ± 0.2 (14.1)

6.81 ± 0.17 (7.15)

9.63 ± 0.23 (10.1)

17.9

21.1

20.4

12.3

16.4

1.5

2.3

2.5

0.12

0.38

1.1

2.4

2.5

0.34

0.86

1.4

0.96

1.0

0.35

0.44

NOIC1

NOIC2

NOIC3

NOIC4

62.2 ± 1.3 (63.7)

.2% DIO with thermal annealing at 110 °C for 10 min. Average values with standard deviation were obtained from 20 devices, and values in

parentheses are from the best devices.

1.9 mA cm⁻with a V_O_Cof 0.864 V and FF of 65.9%, yielding a

The external quantum eﬃciency (EQE) spectra of the best-

performing devices based on PM6:NOIC series (Figure 3b)

resemble absorption spectra of the blend ﬁlms (Figure S5b). In

comparison with PM6:NOIC-based devices with a maximum

EQE value (EQE_m_a_x) of 78%, PM6:NOIC1 has a similar EQE_m_a_x

of 76%, PM6:NOIC2 shows a higher EQE_m_a_xof 82%, while

higher PCE of 12.5%. Due to the red-shifted absorption and a

higher μ of NOIC2, PM6:NOIC2-based devices had a higher

J_S_Cof 20.6 mA cm and FF of 73.8% and a V_O_Cof 0.927 V,

yielding the highest PCE of 14.1%. Due to the blue-shifted

absorption and a lower μ of NOIC3, PM6:NOIC3-based

devices showed a dramatically reduced J_S_Cof 12.9 mA cm and

FF of 59.8% with a V_O_Cof 0.930 V, yielding a PCE of only 7.15%.

−2

^P^M⁶^:^N^O^I^C³^aⁿ^d^P^M⁶^:^N^O^I^C⁴^s^h^o^w^m^u^c^h^l^o^w^e^r^E^Q^Emax

−

values of 60% and 69%. The J_S_Cvalues calculated by integrating

the EQE spectra with the AM 1.5G reference spectrum are close

to those obtained from J−V measurements (Table 2).

The charge generation/extraction properties were studied by

Devices based on PM6:NOIC4 had a PCE of 10.1% with a V

−

^o^f⁰^.⁹⁴⁴^V^,^JSC of 16.8 mA cm , and FF of 63.7%. Relative to

methoxylated NOIC1 and NOIC2, NOIC3 and NOIC4 with

carbon−oxygen bridges yielded much lower PCEs.

measuring the photocurrent density (J ) as a function of the

eﬀective voltage (V ) (Figure S9). Theoretically, at a high V

eff

Journal of the American Chemical Society

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Figure 4. (a) 2D GIWAXS patterns, (b) scattering proﬁles of out-of-plane and in-plane, and (c) RSoXS proﬁles in log scale for PM6:NOIC series blend

ﬁlms.

(

2.35 V), the saturation photocurrent density (J ) is only

remained at 89.4% and 95.2% of their initial values, respectively,

which are better than that of PM6:NOIC based devices (82.5%),

while PM6:NOIC3- and PM6:NOIC4-based devices showed

poorer stability with PCEs decaying to 49.9% and 55.2% of the

initial values, respectively. With heating at 100 °C for 180 min,

the PCEs of PM6:NOIC1- and PM6:NOIC2-based devices

remained at 97.1% and 93.8% of the initial values, respectively,

which are better than that of PM6:NOIC based devices (91.2%),

while PM6:NOIC3- and PM6:NOIC4-based devices showed

poorer stability, with PCEs decaying to 84.1% and 78.3% of the

initial values, respectively. Relative to methoxylated NOIC1 and

NOIC2, NOIC3 and NOIC4 with carbon−oxygen bridges

showed worse thermal stability and photostability of the devices.

Film Morphology. The surface morphology of PM6:NOIC

series blend ﬁ lms was investigated by atomic force microscopy

(AFM) (Figure S12). The blend ﬁlms have similar root-mean-

square roughnesses of 4.00−4.62 nm. The molecular packing

and orientation behavior of blend ﬁlms were explored by

GIWAXS (Figure 4). The donor PM6 shows a (100) peak at

sat

limited by the amount of photons absorbed in the active layer.

Therefore, the ratio between J_S_Cand J_s_a_treﬂects the charge

extraction eﬃciency under short-circuit conditions. The devices

based on PM6:NOIC3 show a J /J of 0.916, which is lower

SC sat

than those of the other devices (0.943−0.958).

The charge recombination properties were evaluated by

investigating V_O_Cand J_S_Cunder diﬀerent illumination intensities

(P_l_i_g_h_t). The relationship between V_O_Cand P_l_i_g_h_tcan be described

as V_O_C∝ ln(P_l_i_g_h_t). The predominant types of recombination in

the devices could be deduced from the slope values: k T/q for

bimolecular recombination, 2k T/q for monomolecular recom-

bination, and 0.5k T/q for surface recombination (k is the

Boltzmann constant, T is temperature, and q is the elementary

charge).

series of acceptors are in the range between 0.94 and 1.25 k T/q,

suggesting that bimolecular recombination is dominant in all

devices (Figure 3c). The relationship between J and P_l_i_g_h_tcan

be described as J_S_C∝ P

bimolecular recombination losses for the solar cell under a short

circuit condition. We ﬁnd that all of the devices show similar α

values (0.952−0.973), regardless of the acceptors used,

indicating weak bimolecular recombination losses (Figure 3d).

The charge transport properties of PM6:NOIC series blend

ﬁlms were measured by the SCLC method (Figure S10). The

9−51

The slopes for the solar cells based on the NOIC

, where α = 1 implies negligible

light

−

0.32 Å in both the in-plane and out-of-plane directions with a

preferred edge-on orientation (Figure S4). The (100) peak of

PM6 in the out-of-plane direction remains when blended with

the NOIC series of acceptors. It is diﬃcult to deﬁne a preferred

orientation of PM6 when blended with NOIC, NOIC3, and

−

NOIC4, since the (100) reﬂections at ∼0.30 Å of NOIC,

NOIC3, and NOIC4 merge together with the (100) peak of

PM6 in the in-plane direction. PM6 remains edge-on when

blended with NOIC1 and NOIC2. The strong (100) peaks of

−

PM6:NOIC blend ﬁlm shows hole mobility (μ ) of 1.5 × 10

cm V

−1 −1

−4

−1 −1

s and μ of 1.1 × 10 cm V s with a μ /μ ratio of

e h e

.4; PM6:NOIC1 and PM6:NOIC2 blend ﬁlms show higher

−

−1

and more balanced mobilities (μ = (2.3−2.5) × 10 cm V

the acceptors are located at 0.50 Å , 0.45 Å , and 0.44 Å in

PM6:NOIC, PM6:NOIC1, and PM6:NOIC2 blend ﬁlms,

respectively. The corresponding coherent lengths, determined

from a Scherrer analysis, are 8.7 nm, 19.1 nm, and 17.8 nm,

which are all larger than those in pure ﬁlms (8.0 nm, 17.2 nm,

and 15.2 nm), indicating PM6 can induce an ordering of these

three acceptors in the blend ﬁlms. The relatively larger coherent

lengths of NOIC1 and NOIC2 beneﬁt electron transport in the

acceptor phase. The π−π stacking peaks of PM6:NOIC series

−

−4

−1 −1

s ; μ = (2.4−2.5) × 10 cm V s ; μ /μ = 0.96−1.0),

which beneﬁt the J and FF; PM6:NOIC3 and PM6:NOIC4

blend ﬁlms show lower and less balanced mobilities (μ =

−

−1

−4

−1

(

0.12−0.38) × 10 cm V ; μ = (0.34−0.86) × 10 cm V ;

μ /μ = 0.35−0.44), leading to lower J and FF (Table 2).

Stability of the PM6:NOIC series-based devices was

investigated under illumination or heating conditions without

encapsulation in a glovebox with inert atmosphere (Figure S11).

Under AM 1.5G illumination at 100 mW cm⁻for 180 min,

PCEs of PM6:NOIC1- and PM6:NOIC2-based devices

−1

are located at 1.78 Å , 1.80 Å , 1.82 Å , 1.76 Å , and 1.76

−

(d = 3.53 Å, 3.49 Å, 3.45 Å, 3.57 Å, and 3.57 Å) in the out-of-

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Table 3. Detailed V_O_CLosses of Optimal OSCs Based on PM6:NOIC Series

acceptor

V_O_C(V)

E_g^P^V(eV)

E_C_T(eV)

EQE_E_L

ΔE_C_T(eV)

ΔV_n_o_n_r_a_d(V)

ΔV_r_a_d(V)

V_l_o_s_s(V)

1.18 × 10⁻

1.24 × 10⁻

1.61 × 10

2.20 × 10⁻

4.54 × 10⁻

0.24

0.10

0.13

0.25

0.13

0.28

0.22

0.28

0.27

0.25

0.21

0.28

0.23

0.25

0.30

0.73

0.60

0.64

0.77

0.68

NOIC

0.89

0.86

0.93

0.94

1.62

1.46

1.57

1.70

1.62

1.38

1.36

1.44

1.45

1.49

NOIC1

NOIC2

NOIC3

NOIC4

−

Figure 5. Reduced EL and EQE spectra of optimal OSCs based on PM6:NOIC series.

plane direction, respectively. The smaller π−π packing distances

facilitate charge transport in the PM6:NOIC1 and PM6:NOIC2

blends, resulting in better performance of the devices.

Information): the voltage loss due to charge generation

(ΔE_C_T/q), the nonradiative recombination voltage loss

(ΔV_n_o_n_r_a_d), and the voltage loss due to radiative recombination

Resonant soft X-ray scattering (RSoXS) was used to

investigate the morphology of PM6:NOIC series blend

(ΔV ). Detailed values are given in Table 3.

rad

ΔE is deﬁned as the diﬀerence between the energy of the

3,54

ﬁlms.

To enhance contrast, an energy at the carbon edge

bandgap of the blend ﬁlm (E ) and the energy of the charge

transfer (CT) state (E ). In this work, E is determined from

of 285.0 eV was used. The scattering peaks of PM6:NOIC,

−

PM6:NOIC1, and PM6:NOIC2 located at 0.0099 Å , 0.0097

the crossing point between the normalized photoluminescence

(PL) and absorption spectra of the neat acceptor ﬁlm, because

the acceptor has a lower bandgap compared to that of PM6

(Figure S13 ). Furthermore, E_C_Tis determined by ﬁtting to the

low energy part of sensitive EQE (sEQE) spectra, using methods

−

−1

Å , and 0.0103 Å correspond to average mesh sizes of 31.6

nm, 32.4 nm, and 30.5 nm, respectively (Figure 4c). This mesh

arises from a network-like arrangement of the ﬁbrillar crystals of

the acceptor in a mixed donor−acceptor matrix. Well-deﬁned

interferences are seen for PM6:NOIC, PM6:NOIC1, and

PM6:NOIC2, indicating the establishments of a well-deﬁned

crystalline network of the acceptors which is beneﬁcial for charge

transport and corresponds to the better performance. The

interference seen for PM6:NOIC2 is better characterized as a

shoulder on the monotonically decreasing scattering seen at

smaller scattering vectors for all systems, suggesting a better

mixing between PM6 and NOIC2, thus hindering crystalliza-

tion. This is the origin of the smaller coherent length for

57,59

described previously in the literature (Figure 5).

From these

measurements, we ﬁnd that, in comparison to NOIC and

NOIC3 (ΔE = 0.24−0.25 eV), NOIC1-, NOIC2-, and

NOIC4-based devices have notably smaller ΔE (0.10−0.13

eV), indicating smaller driving force for charge transfer in these

devices. However, devices based on NOIC2, NOIC3, and

NOIC4 have a higher E_C_T(1.44−1.49 eV), compared to the

devices based on NOIC and NOIC1, which is the main reason

for the higher V_O_C(0.93−0.94 V).

PM6:NOIC2 in the GIWAXS. No obvious scattering

interferences were evident for PM6:NOIC3 and PM6:NOIC4

blend ﬁlms, suggesting a suppression of the formation of a well-

deﬁned network structure due to the mixing between the donor

and acceptor. From the GIWAXS data, it is evident that well-

deﬁned crystal reﬂections are seen for the entire NOIC series.

Consequently, it is not the crystalline ordering that is

dominantly aﬀected by the donor−acceptor mixing but rather

the arrangements of the crystalline ﬁbrils in the mixed donor−

acceptor matrix. This crystalline network is important for charge

transport, and as such, a disruption of the network will lead to

recombination losses or traps that will detrimentally inﬂuence

performance.

The electroluminescence external quantum eﬃciencies

(EQE ) of the solar cells are also measured, as shown in

Figure S14 , and ΔV

values, calculated according to

equation S2, are listed in Table 3. The EQE_E_Lis 1.24 × 10

nonrad

−4

for NOIC1-based device, leading to the smallest ΔV of 0.22

nonrad

V, while the devices based on the other acceptors have relatively

larger Δ V

of 0.25−0.28 V. ΔV is calculated using

nonrad

rad

equation S2 , and we ﬁnd that NOIC-based devices have the

smallest ΔV of 0.21 V. NOIC2- and NOIC3-based devices

rad

have higher ΔV (0.23 and 0.25 V, respectively), while the

rad

highest values of 0.28 and 0.30 V are obtained for NOIC1- and

NOIC4-based devices. Finally, in comparison with NOIC-based

devices (V_l_o_s_s= 0.73 V), NOIC1- and NOIC2-based devices

exhibit notably smaller V_l_o_s_sof 0.60 and 0.64 V, respectively, due

Voltage Loss. To understand the diﬀerences in V_O_C(0.86−

.94 V), we examined V in the solar cells based on the

to the much smaller ΔE . The small but suﬃcient driving force

loss

6−58

PM6:NOICs.

The total voltage loss can be divided into

for charge transfer in the solar cell is beneﬁcial for device

three parts according to the equation S2 (Supporting

performance. Although NOIC3- and NOIC4-based devices

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

orcid.org/0000-0003-0036-2362

Article

exhibit relatively higher V_O_C, the high ΔE for NOIC3 and high

Authors

ΔV for NOIC4 result in too high V of 0.77 and 0.68 V,

Tengfei Li − Department of Materials Science and Engineering,

College of Engineering, Key Laboratory of Polymer Chemistry

and Physics of Ministry of Education, Peking University,

Beijing 100871, China; orcid.org/0000-0002-0501-1505

Yao Wu − Department of Polymer Science and Engineering,

University of Massachusetts Amherst, Amherst, Massachusetts

rad

loss

respectively, partially responsible for the low PCEs of devices.

The diﬀerent losses discussed here can also be related to the

formation or lack of formation of a well-deﬁned network of

crystals, as discussed earlier, arising from the extent of mixing of

the donor and acceptor.

01003, United States; orcid.org/0000-0002-6680-7347

Jiadong Zhou − Institute of Polymer Optoelectronic Materials

and Devices, State Key Laboratory of Luminescent Materials

and Devices, South China University of Technology,

Guangzhou 510640, China

Mengyang Li − Center for Advanced Low-Dimension Materials,

State Key Laboratory for Modiﬁcation of Chemical Fibers and

Polymer Materials, College of Materials Science and

Engineering, Donghua University, Shanghai 201620, China

Jingnan Wu − Laboratory of Advanced Optoelectronic

Materials, College of Chemistry, Chemical Engineering and

Materials Science, Soochow University, Suzhou 215123, China

Qin Hu − Department of Polymer Science and Engineering,

University of Massachusetts Amherst, Amherst, Massachusetts

CONCLUSIONS

■

We designed ﬁve NOIC series molecules and synthesized from

starting materials SM1 and its three methoxylated isomers

SM2−4. The NOIC series has the same end groups and side

chains as well as similar fused-ring cores but shows diﬀerent

molecular packing, light absorption, energy levels, and electron

mobilities. The binary-blend OSCs based on the NOIC series

show notably diﬀerent PCEs varying from 7.15% to 14.1%, due

to the diﬀerent properties of NOICs, morphology of blend ﬁlms,

and voltage losses of devices. NOIC2-based devices have the

highest PCE of 14.1%. This study reveals that the butterﬂy

eﬀects arising from the diﬀerent methoxy positions in the

starting materials signiﬁcantly aﬀect properties of FREAs. (i)

Methoxy substitution on the fused-ring core is beneﬁcial for red-

shifted absorption, stronger crystallinity, higher charge mobility,

lower voltage loss, higher device eﬃciency, and better stability,

while carbon−oxygen bridge show negative eﬀects. (ii) The

methoxylation position and oxygen heterocyclization position

have signiﬁcant impact on molecular packing, absorption,

energy levels, electron mobilities, ﬁlm morphology, and

photovoltaic performance of FREAs. These results indicate

that minor changes on the starting materials could signiﬁcantly

aﬀect the chemical structures, physical properties, and photo-

voltaic performance of the resultant acceptors. The often-used

top-down design emphasizes the ﬁnal molecular structure but

ignores the starting materials and synthesis process and may be

insuﬃcient for rational design. The bottom-up strategy is

important for designing high-performance nonfullerene accept-

ors and should be taken into account.

1003, United States; Materials Sciences Division, Lawrence

Berkeley National Laboratory, Berkeley, California 94720,

United States

Boyu Jia − Department of Materials Science and Engineering,

College of Engineering, Key Laboratory of Polymer Chemistry

and Physics of Ministry of Education, Peking University,

Beijing 100871, China

Xiran Pan − Department of Materials Science and Engineering,

College of Engineering, Key Laboratory of Polymer Chemistry

and Physics of Ministry of Education, Peking University,

Beijing 100871, China

Maojie Zhang − Laboratory of Advanced Optoelectronic

Materials, College of Chemistry, Chemical Engineering and

Materials Science, Soochow University, Suzhou 215123,

China; orcid.org/0000-0002-6102-5856

Zheng Tang − Center for Advanced Low-Dimension Materials,

State Key Laboratory for Modiﬁcation of Chemical Fibers and

Polymer Materials, College of Materials Science and

Engineering, Donghua University, Shanghai 201620, China;

ASSOCIATED CONTENT

■

sı Supporting Information

Zengqi Xie − Institute of Polymer Optoelectronic Materials and

Devices, State Key Laboratory of Luminescent Materials and

Devices, South China University of Technology, Guangzhou

510640, China; orcid.org/0000-0003-0826-840X

Thomas P. Russell − Department of Polymer Science and

Engineering, University of Massachusetts Amherst, Amherst,

Massachusetts 01003, United States; Materials Sciences

Division, Lawrence Berkeley National Laboratory, Berkeley,

California 94720, United States; orcid.org/0000-0001-

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/jacs.0c09800.

Detailed experimental procedures including synthesis,

characterization, device fabrication, and additional

characterization data, such as TGA, single crystals,

GIWAXS, absorption spectra, CV, SCLC, AFM, PL

spectra, EQE , and optimization and stability of the OSC

devices (PDF)

6384-5826

Single-crystal structures of NOIC (2031646), NOIC1

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.0c09800

2031647), NOIC2 (2031648), and NOIC3 (2031649)

CIF)

Notes

AUTHOR INFORMATION

The authors declare no competing ﬁnancial interest.

■

Corresponding Author

ACKNOWLEDGMENTS

Xiaowei Zhan − Department of Materials Science and

Engineering, College of Engineering, Key Laboratory of

Polymer Chemistry and Physics of Ministry of Education,

Peking University, Beijing 100871, China; orcid.org/0000-

■

X.Z. thanks the NSFC (Grants 21734001 and 51761165023).

T.P.R. and Y.W. are supported by the U.S. Oﬃce of Naval

Research under Contract N00014-17-1-2244. Portions of this

research were conducted at beamlines 7.3.3 and 11.0.1.2 of

002-1006-3342 ; Email: xwzhan@pku.edu.cn

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

(17) Li, T.; Dai, S.; Ke, Z.; Yang, L.; Wang, J.; Yan, C.; Ma, W.; Zhan,

Article

Advanced Light Source, Materials Science Division, The

Molecular Foundry, Lawrence Berkeley National Laboratory,

which was supported by the Oﬃce of Science, Oﬃce of Basic

Energy Sciences, of the U.S. Department of Energy under

Contract No. DE-AC02-05CH11231. Z.X. thanks the NSFC

X. Fused tris(thienothiophene)-based electron acceptor with strong

near-infrared absorption for high-performance as-cast solar cells. Adv.

Mater. 2018, 30 (10), 1705969.

(

18) Xiao, Z.; Jia, X.; Li, D.; Wang, S.; Geng, X.; Liu, F.; Chen, J.; Yang,

−2

S.; Russell, T. P.; Ding, L. 26 mA cm J _S _C from organic solar cells with a

(

Grants 21733005 and 51761135101).

low-bandgap nonfullerene acceptor. Sci. Bull. 2017, 62 (22), 1494−

1496.

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