Angular-Shaped 4,9-Dialkylnaphthodithiophene-Based Octacyclic

Jun 29, 2018 - Fong-Yi Cao , Wen-Chia Huang , Shao-Ling Chang , and Yen-Ju Cheng*. Department of Applied Chemistry, National Chiao Tung University ...
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Article Cite This: Chem. Mater. 2018, 30, 4968−4977

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Angular-Shaped 4,9-Dialkylnaphthodithiophene-Based Octacyclic Ladder-Type Non-Fullerene Acceptors for High Efficiency TernaryBlend Organic Photovoltaics Fong-Yi Cao, Wen-Chia Huang, Shao-Ling Chang, and Yen-Ju Cheng* Department of Applied Chemistry, National Chiao Tung University, 1001 University Road, Hsinchu, Taiwan 30010

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S Supporting Information *

ABSTRACT: An angular-shaped 4,9-didodecylnaphthodithiophene-based octacyclic ladder-type structure was developed. This new ladder-type donor (LD) was coupled with electronwithdrawing IC and FIC units to form two A-LD-A type nonfullerene acceptor materials, NCIC and NCFIC. NCIC and NCFIC as the n-type acceptors are blended with a p-type donor PBDB-T to form the complementary absorption and suitable energy level alignments. The binary-blend PBDBT:NCIC and PBDB-T:NCFIC devices achieved an efficiency of 7.3% and 7.5%, respectively. PC71BM was incorporated to form D1:A1:A2 ternary blends which further strengthen the absorption at shorter wavelengths. Introduction of PC71BM not only efficiently improves the absorption but also provides multiple channels for exciton dissociation and electron transport to dramatically improve Jsc. It is interesting to find that the Voc of the ternary-blend devices is reversely in proportional to the added amount of PC71BM. The device using the PBDBT:NCIC:PC71BM (1:1:1 in wt %) showed an improved PCE of 8.32%. Moreover, the optimized device using the PBDBT:NCFIC:PC71BM (1:1:1.5 in wt %) blend achieved the highest efficiency of 9.18%.

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INTRODUCTION Organic photovoltaic solar cells (OPVs) are promising for producing large-area modules and flexible electronics.1−13 For long periods of time, n-type acceptor materials in OPVs are exclusively dominated by fullerene derivatives such as [6,6]phenyl-C61(or C71)-butyric acid methyl ester (PC61BM or PC71BM) due to their high electron affinity, high electron mobility, and isotropic electron transport.14−25 Consequently, the p-type polymers used in fullerene-based solar cells need to be specifically designed in order to match the properties of ntype fullerene acceptors. However, fullerene derivatives also have several intrinsic drawbacks such as insufficient absorption, untunable energy levels, and unstable morphology. To circumvent the deficiencies, recent research efforts have been centered on the development of organic-based non-fullerene acceptors (NFAs).26−29 We have been focused on the development of electron-rich ladder-type donors (denoted as LD) to construct various donor−acceptor copolymers.30−39 The rigidification and planarization of the LDs restrict rotational motions, extend effective conjugated length, and promote intermolecular interactions, which is beneficial to broaden light absorption and improve hole mobility.40 In view of these advantageous features, there has been a renewal of interest in employing LD units to construct new NFAs. For example, indacenodithiophene (IDT)41−46 and indacenodithieno[3,2,b]thiophene (IDTT)47−52 have been utilized as the central cores to couple with two electron-deficient 1,1dicyanomethylene-3-indanone (IC) acceptor to form A-LD-A © 2018 American Chemical Society

NFAs which have successfully achieved superior efficiencies.53−57 Through the electron push−pull interactions, the organic-based A-LD-A-type NFA can possess strong absorption covering UV−vis and near-infrared regions to improve the short-circuit current density (Jsc). By varying the combination of LD and A units, the energy levels of NFAs can be adjusted to reduce the energy loss of the exciton dissociation and improve the open-circuit voltage (Voc).58−60 Based on this design principle, a number of high-performance A-LD-A-type NFAs using different LD units have been developed.43−60 We recently reported a non-fullerene acceptor BDCPDT-IC using a heptacyclic ladder-type benzodi(cyclopentadithiophene) (BDCPDT) where the central benzodithiophene (BDT) subunit is fused with two outer cyclopentathiophene moieties.56 It is of great interest to further design an extended octacyclic ladder-type structure by replacing the central tricyclic benzodithiophene unit in BDCPDT with a tetracyclic naphthodithiophene (NDT) unit. Angular-shaped 5,10dialkylnaphthodithiophene unit has been incorporated into various donor−acceptor conjugated materials showing good molecular packing, high crystallinity, charge transportation properties, and thus high efficiencies in solar cell applications.61−70 However, another isomeric angular-shaped 4,9dialkyl naphthodithiophene (4,9-NDT) derivative has not Received: March 14, 2018 Revised: June 28, 2018 Published: June 29, 2018 4968

DOI: 10.1021/acs.chemmater.8b01089 Chem. Mater. 2018, 30, 4968−4977

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Chemistry of Materials

Figure 1. Chemical structures of PBDB-T, PC71BM, NCIC, and NCFIC.

Scheme 1. Synthesis of NCIC and NCFIC

been well explored due to the lack of useful synthetic method.71,72 We recently successfully designed a useful synthetic strategy to regiospecifically prepare angular-shaped 4,9-dialkylated naphthodithiophene derivatives, which allows us to systematically investigate the side-chain substitution isomeric effects.31,34,39 Compared to the 5,10-dialkylnaphthodithiophene-based counterparts, the 4,9-dialkylnaphthodithiophene-incorporated polymers with aliphatic chains at inner 4,9-positions might reduce steric hindrance to maintain the backbone coplanarity. Additionally, the synthesis of 4,9-

dialkylnaphthodithiophene is much easier than that of 5,10dialkylnaphthodithiophene.73 The 4,9-NDT-based donor− acceptor copolymers have exhibited promising photovoltaic properties. In this research, we design and synthesize an octacyclic ladder-type 4,9-didodecylnaphthodithiophenyldi(cyclopentathiophene) structure (denoted as NC) embedding a central 4,9-didodecylnaphthodithiophene unit fused with two outer cyclopentathiophene moieties. The octacyclic NC was formylated to condense with IC or fluorinated IC (FIC, 1,1dicyanamethylene-5,6-difluoro-3-indanone74,75) units, forming 4969

DOI: 10.1021/acs.chemmater.8b01089 Chem. Mater. 2018, 30, 4968−4977

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Figure 2. (a) UV−vis absorption spectra of PBDB-T, NCIC, and NCFIC in solution and thin film. (b) Energy levels of PBDB-T, NCIC, NCFIC, and PC71BM estimated by cyclic voltammetry.

Table 1. Summary of the Intrinsic Properties of NCIC and NCFIC λmax (nm) NFA NCIC NCFIC

5

−1

extinction coefficient (×10 cm 2.09 2.24

−1

M )

o-DCBa

film

λonsetb (nm)

Egopt c (eV)

IPd (eV)

EAd (eV)

Egele d (eV)

675 690

700 755

785 821

1.58 1.51

−5.37 −5.43

−3.73 −3.94

1.64 1.49

a o-DCB = o-dichorobenzene. bCalculated in the solid state. cEgopt = 1240/λonset. dDetermined by cyclic voltammetry (IP = ionization potential, EA = electron affinity).

o-dichlorobenzene. The detailed synthetic procedure, mass spectrometry, 1H NMR, and 13C NMR of new compounds are shown in the experimental section of the Supporting Information. From the thermogravimetric analysis (TGA) measurement shown in Figure S1, the decomposition temperature (Td) of NCIC and NCFIC is 356 and 376 °C, respectively. From the differential scanning calorimetry (DSC) measurement as shown in Figure S2, NCIC displayed two melting points (Tm) at 89 and 173 °C during heating and two crystallization points at 38 and 165 °C during cooling, indicating that NCIC has high crystallinity. However, NCFIC did not show melting point and crystallization transition. This discrepancy might be associated with the electron-withdrawing fluoro atom in NCFIC. Optical and Electrochemical Properties. The normalized absorption spectra of PBDB-T, PC71BM, NCIC, and NCFIC in thin film and solution state are shown in Figure 2a. The optical and electrochemical properties are listed in Table 1. In solution state, NCIC displays strong absorption in 600− 700 nm region with an extinction coefficient of 2.09 × 105 M−1 cm−1 at the λmax of 675 nm, while NCFIC exhibits more bathochromic absorption with λmax at 690 nm (2.24 × 105 M−1 cm−1) as a result of the two additional electron-withdrawing fluorine atoms. The extinction coefficients of NCIC and NCFIC in solution are shown in Figure S3. Compared to the solution state, the λmax of NCIC and NCFIC in thin film is redshifted by 25 and 65 nm, respectively. The greater red-shifted λmax of NCFIC suggests that the fluorinated FIC in NCFIC might induce stronger intermolecular interactions. In addition, the optical bandgaps of NCIC and NCFIC are estimated to be 1.58 and 1.51 eV, respectively. The cyclic voltammetry (CV) of NCIC and NCFIC is shown in Figure S4. The ionization potential (IP) and electron affinity (EA) of NCIC and NCFIC were estimated to be −5.37/−3.73 eV and −5.43/−3.94 eV, respectively. The lower-lying IP/EA levels of NCFIC are attributed to the stronger electron-accepting ability of the FIC moiety. The electrochemical bandgaps of NCIC and NCFIC are 1.64 and 1.49 eV, respectively.

two materials called NCIC and NCFIC, respectively (Figure 1). PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene4,8-dione))]) was selected as the p-type polymer to blend with NCIC and NCFIC. NCFIC displays more red-shifted absorption and higher extinction coefficient. Moreover, the introduction of the fluorine atom can enhance inter/intramolecular interactions and facilitate charge transport.57 The OPV devices using the binary-blend PBDB-T:NCIC and PBDB-T:NCFIC blend have achieved an efficiency of 7.3% and 7.5%, respectively. PC71BM, used as the second acceptor, was added into the PBDB-T:NCIC and PBDB-T:NCFIC system to form ternary blends. The introduction of PC71BM efficiently strengthens the absorption around 350−500 nm and facilitates electron transport.76−83 The ternary-blend device with the PBDB-T:NCIC:PC71BM (1:1:1 in wt %) showed an improved PCE of 8.32%, with a Voc of 0.88 V, a higher Jsc of 16.78 mA cm−2, and an FF of 56.4%, while the optimized ternary-blend device with the PBDB-T:NCFIC:PC71BM (1:1:1.5 in wt %) yielded the highest PCE of 9.18% with a Voc of 0.84 V, a higher Jsc of 17.79 mA cm−2, and an FF of 61.41%.

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RESULTS AND DISCUSSION Molecular Design, Synthesis, and Characterization. The synthesis of NCIC and NCFIC is described in Scheme 1. The synthesis of compound 1 has been reported previously.30 Stille coupling of compound 1 with ethyl 2-bromothiophene-3carboxylate afforded compound 2 in 86% yield. Compound 2 was reacted with 4-hexylphenylmagnesium bromide to yield compound 3, which underwent acid-catalyzed cyclization to form the NC (4) in 50% yield. The formylation of compound 4 generated compound 5 in 92% yield. Reaction of compound 5 with 1,1-dicyanomethylene-3-indanone (IC) and 1,1dicyanamethylene-5,6-difluoro-3-indanone (FIC) afforded the final products of NCIC and NCFIC, respectively. Both NCIC and NCFIC can be dissolved in chloroform, chlorobenzene, or 4970

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Figure 3. J−V curves and EQE spectra of the devices with the binary and ternary blends of PBDB-T:NCIC:PC71BM (a, b) and PBDBT:NCFIC:PC71BM (c, d).

Table 2. Characteristics of the Devices with the Blending System of PBDB-T:NCIC:PC71BM and PBDB-T:NCFIC:PC71BM blending system PBDB-T:PC71BM PBDBT:NCIC:PC71BM

PBDBT:NCFIC:PC71BM

blending ratioa (wt %)

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

Elossb (eV)

μhole (cm2 V−1 s−1)

μele (cm2 V−1 s−1)

1:1 1:1:0

0.82 (0.81 ± 0.01) 1.00 (1 ± 0)

13.44 (13.56 ± 0.12) 12.69 (12.82 ± 0.24)

62.0 (60.9 ± 1.1) 57.6 (56.2 ± 0.9)

6.84 (6.69 ± 0.15) 7.31 (7.21 ± 0.1)

1 0.61

1.08 × 10−6

5.98 × 10−7

1:1:0.5 1:1:1 1:1:1.5 1:1:0

0.90 0.88 0.84 0.88

0.67

2.23 9.08 1.77 1.00

1:1:0.5 1:1:1 1:1:1.5

0.86 (0.85 ± 0.01) 0.84 (0.83 ± 0.01) 0.84 (0.84 ± 0.01)

(0.91 (0.87 (0.84 (0.88

± ± ± ±

0.01) 0.01) 0) 0)

15.55 16.78 17.03 15.19

(15.28 (17.25 (16.48 (15.09

± ± ± ±

0.2) 0.47) 0.56) 0.07)

16.97 (17.6 ± 0.63) 16.98 (16.79 ± 0.77) 17.79 (17.81 ± 0.2)

52.7 56.4 56.4 56.3

(50.8 (54.8 (58.5 (56.0

± ± ± ±

1.4) 1.6) 0.6) 0.7)

61.8 (60.2 ± 1.6) 61.9 (61.3 ± 2.0) 61.4 (59.8 ± 1.1)

7.38 8.32 8.29 7.52

(7.04 (8.22 (8.09 (7.44

± ± ± ±

0.24) 0.1) 0.19) 0.1)

9.01 (8.99 ± 0.02) 8.83 (8.57 ± 0.17) 9.18 (8.89 ± 0.29)

× × × ×

10−5 10−6 10−5 10−6

1.98 × 10−5 2.49 × 10−5 2.60 × 10−5

3.99 9.52 2.64 6.97

× × × ×

10−6 10−6 10−6 10−7

7.18 × 10−6 8.14 × 10−6 1.73 × 10−5

a With 0.5 vol % DIO as the additive. The average values with standard deviation over 10 cells are shown in parentheses. bEloss = Egopt − eVoc. The molecular weight of PBDB-T is around 30 kDa with PDI of 1.5.

Photovoltaic Characteristics. The inverted devices with ITO/ZnO/PBDB-T:NFAs/MoO3/Ag configuration were prepared. The J−V curves and the external quantum efficiency (EQE) spectra are shown in Figure 3 (Table 2). The PBDBT:NCIC (1:1 in wt %) device with 0.5 vol % 1,8-diiodooctane (DIO) exhibited a PCE of 7.31% with a high Voc of 1.00 V, a Jsc of 12.69 mA cm−2, and an FF of 57.6%. The high-lying electron affinity of NCIC to reduce the energy loss of the charge separation between donor and acceptor results in the high Voc. In addition, the PBDB-T:NCFIC (1:1 in wt %) device with 0.5 vol % DIO led to a PCE of 7.52% with a Voc of 0.88 V, a higher Jsc of 15.19 mA cm−2, and an FF of 56.2%. The Jsc values obtained from the J−V measurements are rather consistent with the values calculated from the EQE spectra.

The binary systems display relatively weak photocurrent response at 350−500 nm in the EQE spectra. In order to strengthen the absorption at the shorter wavelength region, we introduced PC71BM as the second acceptor to form ternary blending systems. As shown in Figures 3a and 3b, the ternary devices using the PBDB-T:NCIC:PC71BM blend showed significant improvement in Jsc and much higher PCE compared to these of the binary system. The EQE of the ternary devices reveal dramatic enhancement at 350−500 nm. The optimized PBDB-T:NCIC:PC71BM (1:1:1 in wt %) ternary device showed the PCE of 8.32% with a Voc of 0.88 V, a higher Jsc of 16.78 mAcm−2, and an FF of 56.4%. Similarly, the ternaryblend PBDB-T:NCFIC:PC71BM device with the optimized ratio of 1:1:1.5 in wt % delivered the highest PCE of 9.18% 4971

DOI: 10.1021/acs.chemmater.8b01089 Chem. Mater. 2018, 30, 4968−4977

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Figure 4. (a) Voc and (b) Jsc versus different ternary blending ratio of PBDB-T:NCIC:PC71BM; (c) Voc and (d) Jsc versus different ternary blending ratio of PBDB-T:NCFIC:PC71BM.

Table 3. Characteristics of the Devices with the Blending System of NCIC:PC71BM and NCFIC:PC71BMa blending system

blending ratio (wt %)

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

NCIC:PC71BM NCFIC:PC71BM

1:1 1:1

0.98 (1 ± 0.02) 0.76 (0.78 ± 0.02)

5.85 (5.56 ± 0.29) 0.17 (0.18 ± 0.01)

42.8 (42.2 ± 0.6) 36.8 (33.1 ± 2.7)

2.45 (2.35 ± 0.11) 0.05 (0.046 ± 0.005)

The average values with standard deviation over five cells are shown in parentheses.

a

with a Voc of 0.84 V, an FF of 61.41%, and the highest Jsc of 17.79 mA cm−2. We found that the Voc of the NCIC-based devices gradually decreases as the amount of the introduced PC71BM increases (Figure 4a,c). Considering the fact that Voc is correlated to the photon energy loss (Eloss) which is defined as the potential energy difference between the absorbed photon and the released electron, we calculated the Eloss of devices based on the equation of Eloss = Egopt − eVoc.84−86 The binary PBDBT:PC71BM reference device was also fabricated for comparison. The Eloss of the binary PBDB-T:NCIC device is 0.61 eV. The PBDB-T:NCFIC device shows a larger Eloss of 0.67 eV, which is associated with the larger ionization potential energy difference between the PBDB-T and NCFIC. Nevertheless, the PBDB-T:PC71BM-based binary device shows a much higher Eloss of 1 eV. Although the Jsc is effectively improved as the amount of PC71BM increases in the ternary blend, the proportion of higher Eloss (exciton dissociation between PBDB-T and PC71BM) also increases, thereby resulting in lower Voc. Furthermore, the composition-dependent Voc of the ternary devices also reveals the good miscibility among PBDBT, NC-based NFAs, and PC71BM.87,88 As shown in Figures 4b and 4d, the Jsc gradually enhances as the amount of PC 7 1 BM increases in the PBDBT:NCIC:PC71BM and PBDB-T:NCFIC:PC71BM ternary

blends. To gain deeper insight into exciton dissociation and charge transport between NC-based NFAs and PC71BM in the ternary blend, we intentionally fabricated the binary NCIC:PC71BM and NCFIC:PC71BM devices (Table 3). The EQE spectrum and the corresponding absorption spectrum are shown in Figure 5. The NCIC:PC71BM device presented a PCE of 2.45% with a Voc of 0.98 V, a Jsc of 5.85 mA cm−2, and an FF of 42.8%. Furthermore, the device displayed EQEs of 20%, 33%, and 38% at the wavelength of 400, 663, and 728 nm, respectively. The EQE profile is essentially consistent with the UV−vis absorption of NCIC:PC71BM blend (Figure 5a), suggesting that exciton dissociation can also efficiently take place from NCIC to PC71BM. As shown in Figure 5c (1, 2, and 3 electron transfer channels), due to the extra exciton dissociation pathway between NCIC and PC71BM (channel 3), the Jsc of the ternary PBDB-T:NCIC:PC71BM devices was enhanced by 23% (1:1:0.5 in wt %), 32% (1:1:1 in wt %), and 34% (1:1:1.5 in wt %), respectively, compared to the binary device (PBDBT:NCIC). In contrast, the NCFIC:PC71BM device displayed a much lower PCE of 0.05% with only a weak EQE of 3% at 390 nm (Figure 5b) which corresponds to the absorption of PC71BM, indicating that the slightly higher-lying electron affinity energy of PC71BM prohibits exciton dissociation from NCFIC to 4972

DOI: 10.1021/acs.chemmater.8b01089 Chem. Mater. 2018, 30, 4968−4977

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Figure 5. EQE spectra of the devices and the corresponding UV−vis absorption spectra with the blend of (a) NCIC:PC71BM and (b)NCFIC:PC71BM. The energy diagram and the charge transport pathways of the devices with the blend of (c) PBDB-T:NCIC:PC71BM and (d) PBDB-T:NCFIC:PC71BM.

s−1 and the most balanced hole/electron mobility ratio of 1.5. The increased hole/electron mobility results in the more efficient charge transport/collection and thus higher current density. We used two-dimensional grazing-incidence wide-angle Xray diffraction (GIWAXS) to investigate the molecular orientation. In Figure S6, the neat PBDB-T thin film exhibited a strong (010) diffraction in the out-of-plane direction at qz = 1.76 Å−1 corresponding to the periodic π−π stacking with a distance (dπ) of ca. 3.57 Å. The result suggests that the polymer crystallites adopt a face-on orientation. Furthermore, neat NCIC and NCFIC films displayed edge-on orientations with obvious diffraction in-plane direction at qxy = 1.75 and 1.87 Å−1 corresponding to the periodic dπ of ca. 3.59 and 3.36 Å, respectively. The shorter dπ of NCFIC indicates the fluorinated NCFIC might produce stronger intermolecular interaction than NCIC. The result is consistent with the larger red-shifted λmax absorption of NCFIC from solution to solid state. As shown in Figure 6, the diffractions of the PBDB-T:NCIC and PBDB-T:NCFIC blending films present π−π stacking signals in both in-plane and out-of-plane directions, indicating that the thin films possess both face-on and edge-on crystalline orientations. However, after the incorporation of PC71BM, the π−π stacking signal of the ternary blend in the out-of-plane direction becomes more pronounced. The predominant face-

PC71BM. As shown in Figure 5d, the exciton dissociation in the PBDB-T:NCFIC:PC71BM blend can only efficiently occur at the PBDB-T:NCFIC or PBDB-T:PC71BM (channel 1 and 2) interface. Therefore, compared to the PBDB-T:NCIC:PC71BM ternary system, the Jsc of the ternary PBDB-T:NCFIC:PC71BM devices shows smaller enhancement of 12% (1:1:0.5 in wt), 12% (1:1:1 in wt), and 17% (1:1:1.5 in wt) relative to the binary device (PBDB-T:NCFIC). Furthermore, the EQE enhancement in the short wavelength region of the PBDBT:NCFIC:PC71BM device should mainly come from the additional PBDB-T:PC71BM interface. We also fabricated the hole-only ITO/PEDOT:PSS/active layer/Au and electron-only Al/active layer/Al devices to evaluate the hole and electron mobility by space-charge limit current (SCLC) model (Figure S5). The hole and electron mobilities were estimated to be 1.08 × 10−6/5.98 × 10−7 and 1.00 × 10−6/6.97 × 10−7 cm2 V−1 s−1 for PBDB-T:NCIC and PBDB-T:NCFIC, respectively. The ternary-blend PBDBT:NCIC:PC71BM and PBDB-T:NCFIC:PC71BM devices show increased hole/electron mobility. The optimized ternary-blend of PBDB-T:NCIC:PC71BM (1:1:1 in wt %) displays the highest electron mobility of 9.52 × 10−6 cm2 V−1 s−1 and the most balanced hole/electron mobility ratio of 0.95. Moreover, the ternary blend of PBDB-T:NCFIC:PC71BM (1:1:1.5 in wt %) with the best device performance also showed the highest electron mobility of 1.73 × 10−5 cm2 V−1 4973

DOI: 10.1021/acs.chemmater.8b01089 Chem. Mater. 2018, 30, 4968−4977

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Figure 6. Two-dimensional GIWAXS images of the films with PBDB-T:NCIC:PC71BM (1:1:0, 1:1:0.5, 1:1:1, and 1:1:1.5 in wt %) and PBDBT:NCFIC:PC71BM (1:1:0, 1:1:0.5, 1:1:1, and 1:1:1.5 in wt %).

%) and PBDB-T:NCFIC:PC71BM (1:1:1.5 in wt %) showed the improved PCE of 8.32% and 9.18%, respectively.

on orientation is beneficial for the vertical charge transport. Furthermore, the one-dimensional in-plane and out-of-plane GIWAXS patterns of the films are shown in Figure S7. In order to investigate the surface morphology of the blend films, we measured the AFM images of the binary and ternary blend films with different blending ratios. As shown in Figure S8, we found the surface roughness of PBDB-T:NCFIC is rougher than that of PBDB-T:NCIC attributed to the high crystallinity of flourinated NCFIC. The ternary-blend films show similar surface morphology with the binary-blend films, indicating that the three components have good miscibility without altering the surface morphology.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01089.

CONCLUSIONS

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In summary, an angular-shaped 4,9-didodecylnaphthodithiophene-based octacyclic ladder-type structure was developed. This coplanar LD building block was further coupled with electron-withdrawing IC and FIC units to form two A-LD-A type acceptor materials NCIC and NCFIC. NCIC and NCFIC as the n-type acceptors are combined with a ptype donor PBDB-T to form the complementary absorption and suitable energy level alignments. The PBDB-T:NCIC device achieved an efficiency of 7.3%. The device with PBDBT:NCFIC blend showed a higher efficiency of 7.5% due to the better light-harvesting ability of NCFIC. To further broaden the absorption at shorter wavelengths, the second acceptor A2, PC71BM, was incorporated to form D1:A1:A2 ternary blends. Introduction of PC71BM not only efficiently improves the absorption but also provides multiple channels for exciton dissociation and electron transport in the ternary-blend bulk heterojunction, leading to the dramatic improvement of Jsc. The devices using the PBDB-T:NCIC:PC71BM (1:1:1 in wt

DSC/TGA measurements, cyclic voltammogram, SCLC measurements, 1-D and 2-D GIWAXS images, AFM images, device fabrication and characterization, synthetic procedures, 1H and 13C NMR spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.-J.C.). ORCID

Yen-Ju Cheng: 0000-0003-0780-4557 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Ministry of Science and Technology, Taiwan (Grant No. MOST107-3017-F009-003) and Ministry of Education, Taiwan (SPROUT Project-Center for Emergent Functional Matter Science of National Chiao Tung University). We thank the National Synchrotron Radiation Research Center (NSRRC) and Dr. U-Ser Jeng and Dr. Chun-Jen Su at the BL23A1 station. 4974

DOI: 10.1021/acs.chemmater.8b01089 Chem. Mater. 2018, 30, 4968−4977

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