Research Article www.acsami.org

1,8-Naphthalimide-Based Planar Small Molecular Acceptor for Organic Solar Cells Jicheng Zhang,† Xuejuan Zhang,† Hongmei Xiao,† Guangwu Li,† Yahui Liu,† Cuihong Li,*,† Hui Huang,‡ Xuebo Chen,*,† and Zhishan Bo*,† †

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ College of Materials Science and Optoelectronic Technology University of Chinese Academy of Sciences, No.19A Yuquan Road, Beijing, 100049, China S Supporting Information *

ABSTRACT: Four small molecular acceptors (SM1−4) comprising a central benzene core, two thiophene bridges and two 1,8-naphthalimide (NI) terminal groups were designed and synthesized by direct C−H activation. SM1 has a planar chemical structure and forms H-aggregation as films. By attachment of different substituents on the central benzene ring, the dihedral angles between the two NI end groups of SM1−4 gradually increased, leading to a gradual decrease of planarity. SM1−4 all possess a high-lying LUMO level, matching with wide band gap (WBG) polymer donors which usually have a high-lying LUMO level. When used in OSCs, devices based on SM1 and WBG donor PCDTBT-C12 gave higher electron mobility, superior film morphology and better photovoltaic performance. After optimization, a PCE of 2.78% with a Voc of 1.04 V was achieved for SM1 based devices, which is among the highest PCEs with a Voc higher than 1 V. Our results have demonstrated that NI based planar small molecules are potential acceptors for WBG polymer based OSCs. KEYWORDS: organic solar cells, nonfullerene acceptor, planar small molecules, H-aggregation, wide band gap polymer, high open circuit voltage, C−H activation

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molecular acceptor could also increase its electron mobility.10,11 Unfortunately, both routes will significantly decrease the LUMO energy level of the acceptor.5,12 When polymers with a high-lying LUMO energy level are used as the donor, large offset between LUMO energy levels of donor and acceptor will lead to a great energy loss, which will reduce the open circuit voltage (Voc) and PCE of devices.13 So far, for most high efficiency nonfullerene acceptors, the LUMO energy level is similar to PC71BM (around −4.04 eV), which mismatches with donor materials with a high lying LUMO level, such as wide band gap (WBG) polymers.5 To further increase the PCE of WBG OSCs, exploiting new acceptor materials possessing suitable LUMO energy level and high electron mobility is distinctly necessary. In comparison with the widely used perylene bisimide acceptor unit which is a strong electron withdrawing group and usually needs a twisted chemical structure to achieve good mixing properties in blend films,14 1,8-naphthalimide (NI) is a weaker electron withdrawing group. NI-based small molecular

INTRODUCTION Fullerene derivatives, such as (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM), have been widely used as acceptor materials in organic solar cells (OSCs) because of their advantages of high electron affinity and mobility.1,2 However, the weak absorption and the difficulty in chemical modification of fullerene derivatives limit their future practical applications.3,4 By structural modification, nonfullerene acceptors could not only enhance the photo absorption but also provide a tunable molecular energy level to match that of donor materials.5 Recently, nonfullerene acceptors have attracted more and more attentions and PCE over 8% has been acquired.6 In nonfullerene photovoltaic devices, the electron mobility (μe) of acceptor is usually lower than the hole mobility (μh) of donor.7,8 The unbalanced charge mobility would induce charge recombination and lead to lower short circuit current density (Jsc) and fill factor (FF).8 Therefore, exploiting nonfullerene acceptors with a high electron mobility is extraordinarily important for nonfullerene OSCs. High electron mobility acceptor molecules usually comprise strong electron-withdrawing units, such as perylene diimide.9 Besides, incorporating more electron withdrawing groups like cyano into the small © XXXX American Chemical Society

Received: October 25, 2015 Accepted: February 4, 2016

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DOI: 10.1021/acsami.5b10211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis Routes of Small Molecular Acceptors SM1-4a

a

Reagents and conditions: (i) THF, 1-octanol, potassium tert-butoxide, 60 °C; (ii) Pd(OAc)2, PCy3.HBF4, K2CO3, DMAc, 120 °C.

in a yield of 82% by Williamson reaction of 1,4-dibromo-2,5difluorobenzene and 1-octanol with potassium tert-butoxide as the base and tetrahydrofuran (THF) as the solvent. Intermediates (4a, 4b) and small molecular acceptors (SM1− 4) were synthesized by direct C−H activation. Although C−H activation was widely used in the synthesis of conjugated small molecules, optoelectronic materials prepared by C−H activation were rarely reported.16,17 The C−H activation of 3a and 3b with excessive thiophene using Pd(OAc)2 and PCy3· HBF4 as the catalyst precursors gave 4a and 4b in yields of 81% and 82%, respectively. SM1 was subsequently synthesized in a yield of 80% by C−H activation of excessive compound 4a and 1,4-dibromobenzene. Similarly, direct C−H activation of excessive compound 4b and 1,4-dibromo-2-fluorobenzene afforded SM2 in a yield of 82%. Cross-coupling of excessive 4a with 1,4-dibromo-2-fluoro-5-(octyloxy)benzene and 1,4dibromo-2,5-bis(octyloxy) benzene by C−H activation afforded SM3 and SM4 in yields of 75% and 77%, respectively. Although SM1 cannot be dissolved in chloroform (CF), it is fully soluble in chlorobenzene (CB) and 1,2-dichlorobenzene (DCB) at room temperature. Whereas SM2, SM3, and SM4 could be fully dissolved in CF, CB, and DCB at room temperature. Thermal properties of SM1−4 were investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) in a nitrogen atmosphere at a heating rate of 10 °C/min. As shown in Figure S1, TGA traces indicated that SM1, SM2, SM3, and SM4 are of good thermal stability with 5% weight loss at 457, 452, 417, and 409 °C, respectively. The incorporation of octyloxy substituent on the central benzene made the small molecules less stable. As shown in Figure S2, DSC studies revealed that SM1−4 all exhibited good crystallinity. The melting peaks of SM1, SM2, SM3, and SM4 are at 196, 169, 174, and 175 °C, respectively, and the crystallization peaks of SM1, SM2, SM3, and SM4 are at 169, 104, 143, and 143 °C, respectively. No obvious glass transition could be observed for PCDTBT-C12 in the temperature range of 50 to 250 °C, indicating that PCDTBT-C12 could be amorphous. As shown in Figure S3, XRD experiments of SM1− 4 also revealed that these small molecules are crystalline, and SM1 displayed the sharpest peaks, demonstrating its superior crystallinity. DFT Calculations. To shed light on the relationship between chemical structures of SM1−4 and their planarity,

acceptors usually possess a higher lowest unoccupied molecular orbital (LUMO) level, which can match the energy levels of many WBG polymers.12 However, NI-based small molecular acceptors generally exhibit a low electron mobility, which will lead to unbalanced electron and hole mobility when blending with polymer donors.7 Here, we demonstrate a strategy for designing NI based planar small molecular acceptors to facilitate the electron transport in the active layer. Four NI-based small molecular acceptors SM1−4 were designed, synthesized by direct C−H activation, and used as nonfullerene acceptors for OSCs. SM1− 4 all comprise one central benzene ring, two thiophene bridges, and two NI end groups. By attaching different substituents on the central benzene ring, the dihedral angle between two NI units can be tuned. Optical studies indicated that planar SM1 as film can form typical H-aggregation which might be beneficial for the charge carrier transportation.15 As expected, devices based on blend films of planar SM1 and PCDTBT-C12 gave higher electron mobility, as well as better photovoltaic performance; whereas the blend films of nonplanar SM4 and PCDTBT-C12 exhibited low electron mobility and worse photovoltaic performance. The morphology of active layer is also affected by the chemical structures of the acceptor, active layers based on planar SM1 and PCDTBT-C12 are prone to form bicontinuous interpenetrating networks after thermal annealing; whereas nonplanar SM2−4 are prone to form spherical aggregates in the blend films. These small molecules are of high-lying LUMO levels (−3.55 eV ∼ −3.70 eV) which would produce higher Voc when blended with WBG polymer donors. For planar SM1-based devices, a PCE of 2.78% with a Voc of 1.04 V was achieved after thermal annealing at 90 °C for 3 min, which is among the highest PCEs with a Voc higher than 1V. As a comparison, nonplanar SM4-based devices only afforded a pretty low PCE of 0.13%. Our results have demonstrated that NI-based planar small molecular acceptors are potential for WBG polymer-based OSCs.

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RESULTS AND DISCUSSION Materials Synthesis and Characterization. As shown in Scheme 1, four NI-based small molecular acceptors (SM1−4) were designed and synthesized to investigate the influence of molecular planarity on their photovoltaic performance. First of all, 1,4-dibromo-2-fluoro-5-(octyloxy)benzene was synthesized B

DOI: 10.1021/acsami.5b10211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Chemical structures (a1−d1) and corresponding top view (a2−d2) and side view (a3−d3) of optimized geometries of SM1−4 by DFT calculations (B3LYP/6-31G(d)).

Figure 2. UV−vis absorption spectra of SM1−4 and PCDTBT-C12 in DCB solution (a) and as films (b).

central benzene ring, the neighboring thiophene ring and the central benzene ring would twist in different dihedral angles, leading to different dihedral angels for the two NI end groups in optimized geometries of SM1−4. SM1 presented a dihedral angle of 34° between two NI planes. But with one fluorine group attached on the central benzene ring, a larger dihedral angle of 57° between the two NI units was obtained for SM2. With two substituents on the central benzene ring, the two NI units in SM3 and SM4 were almost perpendicular to each other with dihedral angles of 83° and 89°, respectively. These results indicated that attaching different substituents on the central benzene ring had a significant influence on the planarity of small molecular acceptors, and the planarity of small molecular

computational studies were carried out by using density functional theory (DFT) approaches (B3LYP/6-31G(d)). For simplicity, alkyl and alkoxyl chains were replaced by methyl and methoxy groups. Optimized geometries of SM1−4 are shown in Figure 1 and corresponding dihedral angles between different aromatic rings are summarized in Table S1. In consideration of the complexity of their chemical structures, the top view and side view of optimized geometries of SM1−4 were based on the top view of the left NI ring and the side view of two NI rings, respectively, to investigate the dihedral angles between two NI groups. The dihedral angles between the NI end group and the thiophene bridge of SM1−4 were all calculated to be around 46°, which was independent of the benzene core. With different substituents attaching on the C

DOI: 10.1021/acsami.5b10211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Table 1. Optical and Electrochemical Properties of SM1-4 and PCDTBT-C12

a

materials

λmax (nm) solution

λmax (nm) film

λonset (nm)film

Eg,opt (eV)a

HOMO (eV)

LUMO (opt,eV)

SM1 SM2 SM3 SM4 PCDTBT-C12b

428 424 437 458 397, 539

405 415 448 492 401, 579

510 525 545 577 630

2.43 2.36 2.27 2.15 1.97

−5.93 −5.98 −5.95 −5.80 −5.23

−3.50 −3.62 −3.68 −3.65 −3.26

Calculated from the absorption onset of the film, Eg,opt = 1240/λonset. bFrom ref 19.

Figure 3. (a) Cyclic voltammetry curves of SM1−4. (b) Cyclic voltammetry curve of Fc/Fc+. (c) Energy level diagrams of PCDTBT-C12, SM1−4, and PC71BM.

Figure 4. J−V curves for blend devices based on PCDTBT-C12:SM1−4 without (a) and with (b) thermal annealing at 90 °C for 3 min.

to form J-aggregation. By introducing different substituents on the central benzene ring, in going from solution to film a blue shift of 23 nm for SM1, a blue shift of 9 nm for SM2, a red shift of 11 nm for SM3, and a red shift of 34 nm for SM4 were observed, indicating that the aggregation of SM1−4 was changed from H-aggregation to J-aggregation. Film absorption onsets of SM1−4 are 510, 525, 545, and 577 nm, respectively. Optical band gaps (Eg,opt) were therefore calculated to be 2.43, 2.36, 2.27, and 2.15 eV, respectively, and the related data are summarized in Table 1. Electrochemical Properties. Electrochemical properties of SM1−4 and PCDTBT-C12 were studied by cyclic voltammetry in a 0.1 M Bu4NPF6 CH3CN solution on a glass carbon working electrode. As shown in Figure 3, the potential value of ferrocene/ferrocenium (Fc/Fc+) redox couple is 0.10 eV, and the onset oxidation potential (Eox) of SM1 is 1.23 eV. The HOMO energy level of SM1 was then determined to be −5.93 eV according to the equation EHOMO = −e[Eox + 4.80 − E(Fc/Fc+)]. Similarly, HOMO energy levels of SM2, SM3 and SM4 were determined to be −5.98, −5.95, and −5.80 eV, respectively. LUMO energy levels of SM1, SM2, SM3 and SM4

acceptors decreased step by step from SM1 to SM2, SM3, and SM4. Optical Properties. UV−vis absorption characteristics of SM1−4 and PCDTBT-C12 were measured in DCB solutions (1 × 10−5 M) and as films. As shown in Figure 2, SM1, SM2, SM3, and SM4 all exhibited an intense absorption band in the visible region peaked at 428, 424, 437, and 458 nm, respectively. The absorption spectrum of SM1 was obviously blue-shifted in going from solution to film, demonstrated that SM1 formed H-aggregation in the solid state.15,18 With a less planar structure, SM2 as films exhibited a slightly blue-shifted absorption, revealing that the H-aggregation of SM2 was weaker than that of SM1. H-aggregation of small molecules was propitious to the charge transport in devices, which might lead to higher Jsc and PCE.15 Attaching one F atom and one octyloxy chain on the central benzene ring, SM3 as films showed a red-shifted absorption. With two octyloxy chains attached on the central benzene ring, a further red shift of absorption was observed for SM4 films. The strongly twisted conformation and the steric hindrance of the octyloxy chain(s) on SM3 and SM4 would weak the parallel stacking and prone D

DOI: 10.1021/acsami.5b10211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Table 2. Summary of Photovoltaic Properties of PCDTBT-C12:SM1-4 under the Illumination of AM 1.5 G, 100 mW/cm2 devices PCDTBT-C12:SM1 PCDTBT-C12:SM2 PCDTBT-C12:SM3 PCDTBT-C12:SM4 a

thermal annealing no 90 °C no 90 °C no 90 °C no 90 °C

3 min 3 min 3 min 3 min

Voc (V)

Jsc (mA/cm2)

FF

PCE % (best/avea)

thickness (nm)

1.36 1.04 1.19 1.11 1.11 1.09 1.25 1.24

1.82 4.79 1.10 2.21 0.42 1.71 0.34 0.37

0.24 0.56 0.26 0.30 0.26 0.30 0.26 0.28

0.59/0.56 2.78/2.65 0.34/0.30 0.74/0.63 0.12/0.11 0.56/0.44 0.11/0.10 0.13/0.12

59 52 62 60 70 66 66 57

Average PCE was calculated from more than 10 devices.

Figure 5. EQE curves of PCDTBT:SM1−4 based devices before (a) and after (b) thermal annealing at 90 °C for 3 min.

based devices, a PCE of 2.78% with a Voc of 1.04 V, a Jsc of 4.79 mA/cm2 and an FF of 0.56 was achieved. Such an obvious increase is largely originated from the improvement of Jsc and FF. Thermal annealing treatment can enhance the aggregation of small molecules, which is beneficial for the improvement of μe in devices.18,22,23 The significant enhancement of μe after thermal annealing treatment (vide infra) endowed devices with more balanced electron and hole mobility, and led to the improvement of Jsc and FF.24 Whereas, thermal annealing treatment could also increase the reverse saturation current (J0) and lead to a reduced Voc in devices.25 It is worth noting that the relatively planar small molecules SM1 and SM2 gave higher μe, Jsc, and PCE; whereas the nonplanar small molecules SM3 and SM4 furnished lower μe, Jsc, and PCE. With the increasing of dihedral angle between the two NI units, the PCE of SM1-, SM2-, SM3-, and SM4-based PSCs decreased gradually to be 2.78%, 0.74%, 0.56%, and 0.13%, respectively. Our results demonstrated for the first time that NI based planar small molecular acceptors are better than nonplanar ones for wide band gap material based OSCs. To investigate the accuracy of Jsc from J−V measurement, external quantum efficiencies (EQEs) of devices based on PCDTBT-C12:SM1−4 without and with thermal annealing treatment were measured. As shown in Figure 5, all devices showed a broad photoresponse in the range of 350−700 nm. Without thermal annealing treatment, the maximum EQE values of devices based on SM1−4 were 14.8% at 441 nm for SM1, 9.4% at 422 nm for SM2, 3.7% at 439 nm for SM3, and 5.3% at 401 nm for SM4, demonstrating that PCDTBT-C12 and SM1−4 both contributed to the photocurrent generation. After thermal annealing at 90 °C for 3 min, the EQE responses of all devices were enhanced. Jsc values calculated from the integration of EQE curves agreed roughly with that obtained from J−V measurement. Transport Properties. To investigate the influence of aggregation of SM1−4 on the charge transportation, space-

were calculated by the equation ELUMO = EHOMO + Eg,opt to be −3.50, −3.62, −3.68, and −3.65 eV, respectively. It is wellknown that the introduction of fluoro atom has an obviously influence on the HOMO and LUMO levels of small molecule.20,21 For SM2 and SM3, the introduction of one F atom on the central benzene ring led to a down-shifted LUMO energy level. However, in comparison with SM1, SM4 with the introduction of two alkoxy chains on the central benzene ring also gave a lower LUMO energy level and a higher HOMO energy level. Obviously, LUMO energy levels of SM1−4 are significant higher than that of PC71BM (−4.04 eV), matching well with the wide band gap polymer PCDTBT-C12 (−3.26 eV). These data are also summarized in Table 1. Photovoltaic Properties. To investigate photovoltaic properties of SM1−4 as the acceptor, devices with a structure of ITO/PEDOT:PSS (30 nm)/PCDTBT-C12:SM1−4/LiF (0.6 nm)/Al (100 nm) were fabricated. For all OSCs, devices fabricated from DCB solutions with a donor−acceptor weight ratio of 1:2 gave the best performance. The current density− voltage (J−V) curves of PCDTBT-C12:SM1−4 based devices are shown in Figure 4 and detailed photovoltaic parameters are summarized in Table 2. After optimization, the plain devices fabricated with PCDTBT-C12:SM1 provided a PCE of 0.59% with a Voc of 1.36 V under AM 1.5 G illumination at 100 mW cm−2, a Voc of 1.36 V is the highest reported Voc as we known in single junction OSCs. Compared with SM1, as prepared devices based on SM2, SM3, and SM4 exhibited lower PCEs of 0.34%, 0.12%, and 0.11%, respectively. Additionally, these devices all exhibited low FF in the range from 0.24 to 0.26, which could be ascribed to the unbalanced hole and electron mobilities in plain devices (vide infra). After thermal annealing at 90 °C for 3 min, all devices exhibited improved photovoltaic performances. The Voc for all devices is above 1.04 V, which is higher than most PC71BM based devices, indicating that the energy loss for WBG based devices can be significantly reduced by using SM1−4 as the acceptor. For PCDTBT-C12:SM1 E

DOI: 10.1021/acsami.5b10211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces charge limited current (SCLC) method was used to evaluate μh and μe of PCDTBT-C12:SM1−4 based devices. μh was measured by hole-only devices with a structure of ITO/ PEDOT:PSS (30 nm)/PCDTBT-C12:SM1−4/Au (100 nm), and μe was investigated by electron-only devices with a configuration of FTO/PCDTBT-C12:SM1−4/Al (100 nm). After the measurement, dark J−V curves were fitted by the Mott−Gurney equation: J = 9εoεrμV2/8L3. Fitted curves for μh and μe of PCDTBT-C12:SM1−4 based devices are shown in Figure S4 and related data are summarized in Table 3. μh of

more balanced charge mobilities in the active layers, which would lead to higher Jsc and FF. Noticeably, as changed from a planar structure to a nonplanar structure, μe of SM1−4-based devices without or with thermal annealing were all gradually decreased, demonstrating that the planar small molecules are beneficial for the electron transport in NI-based devices. Film Morphology. Since the film morphology has an obvious influence on the photovoltaic performance of OSCs, atomic force microscopy (AFM) was used to investigate the surface morphology of PCDTBT-C12:SM1−4 blend films. As shown in Figure 6 and Figure S6, the as prepared PCDTBTC12:SM1, PCDTBT-C12:SM2, PCDTBT-C12:SM3, and PCDTBT-C12:SM4 blend films all showed a relatively smooth surface with root-mean-square (RMS) roughness values of 0.73, 0.95, 2.29, and 0.55 nm, respectively. After thermal annealing at 90 °C for 3 min, all films became rougher, indicating the aggregation of SM1−4 was enhanced after thermal treatment.26,27 The enhanced aggregation of SM1−4 would therefore be propitious to the increase of μe and lead to a more balanced charge transport in devices, which was also verified by the SCLC results (vide supra). Notably, planar SM1 based blend films exhibited the roughest film morphology after thermal treatment; whereas nonplanar SM4 based films displayed a relatively smooth film. Transmission electron microscopy (TEM) was used to further investigate the morphology of blend films. Because crystalline domains formed by SM1−4 possess higher electron scattering density than amorphous PCDTBT-C12 domains, the black domains observed in TEM images (Figure 7) could be ascribed to the aggregation of SM1−4.28 For PCDTBT-C12:SM1 blend films without any thermal treatment, a plenty of spherical domains with diameters of 10−15 nm could be observed (Figure S5, ESI). After thermal annealing at 90 °C for 3 min, the black domains formed by SM1 were obviously disappeared and a uniform distribution SM1 in the polymer matrix was observed where the size of the SM1 aggregates became much smaller, which is beneficial for the efficient extraction of the photogenerated charge carriers.29 For PCDTBT-C12:SM2− 4-based blend films, after thermal annealing spherical aggregates could still be clearly observed. These results manifested that the planarity of NI-based acceptors not only influence their aggregation, but also the morphology of blend films.

Table 3. Summary of μe and μh of the Blend of PCDTBTC12:SM1-4 without and with Thermal Annealing Treatment devices PCDTBT-C12:SM1 PCDTBT-C12:SM2 PCDTBT-C12:SM3 PCDTBT-C12:SM4

thermal annealing no 90 °C no 90 °C no 90 °C no 90 °C

3 min 3 min 3 min 3 min

μh (cm2 V−1 s−1) 6.7 3.0 4.3 1.1 8.9 2.7 7.0 1.0

× × × × × × × ×

10−6 10−5 10−6 10−5 10−6 10−5 10−6 10−5

μe (cm2 V−1 s−1) 3.1 1.1 5.7 9.7 3.3 4.7 1.2 1.1

× × × × × × × ×

10−8 10−5 10−10 10−7 10−10 10−7 10−10 10−9

plain devices based on PCDTBT-C12:SM1, PCDTBTC12:SM2, PCDTBT-C12:SM3, and PCDTBT-C12:SM4 were calculated to be 6.7 × 10−6, 4.3 × 10−6, 8.9 × 10−6, and 7.0 × 10−6 cm2 V−1s−1, respectively. Similarly, μe of plain devices based on PCDTBT-C12:SM1, PCDTBT-C12:SM2, PCDTBT-C12:SM3, and PCDTBT-C12:SM4 were calculated to be 3.1 × 10−8, 5.7 × 10−10, 3.3 × 10−10, and 1.2 × 10−10 cm2 V−1s−1, respectively. Such low μe values would induce inferior Jsc. Besides, the mismatch between μe and μh would also cause unbalanced charge transport in the active layers and lead to lower FF.24 After thermal annealing at 90 °C for 3 min, μh of PCDTBT-C12:SM1, PCDTBT-C12:SM2, PCDTBTC12:SM3, and PCDTBT-C12:SM4 based devices were slightly increased to 3.0 × 10−5, 1.1 × 10−5, 2.7 × 10−5, and 1.0 × 10−5 cm2 V−1s−1, respectively. Nevertheless, μe of PCDTBTC12:SM1, PCDTBT-C12:SM2, PCDTBT-C12:SM3, and PCDTBT-C12:SM4 based devices were markedly improved to 1.1 × 10−5, 9.7 × 10−7, 4.7 × 10−7, and 1.1 × 10−9 cm2 V−1s−1, respectively. Significantly enhanced μe can provide

Figure 6. AFM height images of PCDTBT-C12:SM1−4-based active layers without (a−d) and with (a1−d1) thermal annealing at 90 °C for 3 min. (a, a1) PCDTBT-C12:SM1, (b, b1) PCDTBT-C12:SM2, (c, c1) PCDTBT-C12:SM3, (d, d1) PCDTBT-C12:SM4. F

DOI: 10.1021/acsami.5b10211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

nitrogen protection overnight. After the reaction mixture was cooled down to room temperature; water (30 mL) was added; the resulted mixture was extracted with dichloromethane (DCM, 30 mL × 3); and the combined organic layers were washed with brine (50 mL × 2), dried over anhydrous MgSO4 and evaporated to dryness. The residue was further purified on a silica gel column with petroleum ether as an eluent to afford 2a as a colorless solid (3.13 g, 82%). 1H NMR (400 MHz, CDCl3): δ (ppm) 7.26 (s, 1H), 7.15 (s, 1H), 4.14−4.16 (t, J = 4.0 Hz, 2H), 1.00−1.78 (m, 15H).13C NMR (100 MHz, CDCl3): δ (ppm) 130.41, 130.32, 129.97, 129.59, 122.05, 122.01, 30.25, 29.57, 29.21, 28.15, 28.09, 27.63, 22.62, 19.51. General Procedure for the Preparation of 4a and 4b. A mixture of thiophene (1.68 g, 20.0 mmol), compound 3 (10.0 mmol), anhydrous K2CO3 (2.76 g, 20.0 mmol), and DMAc (20 mL, 0.5 mol/ L) in a Schlenk tube was carefully degassed before and after Pd(OAc)2 (67.3 mg, 0.30 mmol) and PCy3.HBF4 (221 mg, 0.60 mmol) were added, and the reaction mixture was stirred at 120 °C under nitrogen protection for 1 d. Then the mixture was allowed to cool down to room temperature; water (100 mL) was added; the resulted mixture was extracted with DCM (50 mL × 3); and the combined organic layers were washed with brine (100 mL × 2), dried over anhydrous MgSO4 and evaporated to dryness. The residue was further purified on a silica gel column. N-Octyl-4-(thien-2-yl)-1,8-naphthalimide (4a). Compound 3a was used to synthesize 4a according the procedure described above. The residue was purified on a silica gel column eluting with DCM/ petroleum ether (1:1, v/v) to afford 4a as a slightly yellow solid (3.17 g, 81%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.52−8.57 (m, 3H), 7.72−7.73 (d, J = 7.6 Hz, 1H), 7.66−7.68 (t, J = 8.0 Hz, 1H), 7.45− 7.46 (d, J = 5.0 Hz, 1H), 7.26−7.27 (d, J = 3.2 Hz, 1H), 7.15−7.17 (t, J = 5.0 Hz, 1H), 4.14−4.16 (t, J = 3.6 Hz, 2H), 1.20−1.68 (m, 15H).13C NMR (100 MHz, CDCl3): δ (ppm) 164.23, 163.89, 139.79, 139.00, 132.17, 131.15, 130.53, 130.02, 128.79, 128.59, 127.83, 127.51, 127.10, 123.02, 122.05, 122.01, 40.49, 31.75, 29.29, 29.16, 28.12, 27.11, 22.56, 13.99. N-Dodecyl-4-(thien-2-yl)-1,8-naphthalimide (4b). Compound 3b was used to synthesize 4b according the procedure described above. The residue was purified on a silica gel column eluting with DCM/petroleum ether (1:1, v/v) to afford 4b as a slightly yellow solid (3.67 g, 82%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.47−8.52 (m, 3H), 7.69−7.70 (d, J = 7.6 Hz, 1H), 7.64−7.65 (t, J = 8.0 Hz, 1H), 7.42−7.43 (d, J = 5.0 Hz, 1H), 7.20−7.21 (d, J = 3.2 Hz, 1H), 7.13− 7.15 (t, J = 5.0 Hz,1H), 4.10−4.11 (t, J = 3.6 Hz, 2H), 0.86−1.97 (m, 23H).13C NMR (100 MHz, CDCl3): δ (ppm) 164.19, 163.97, 140.11, 140.21, 139.57, 134.65, 129.89, 129.51, 128.81, 128.31, 128.20, 127.97, 127.82, 122.96, 122.02, 121.99, 40.49, 32.87, 31.16, 30.24, 30.10, 29.97, 29.59, 28.72, 27.59, 25.41, 22.31, 14.57. General Procedure for the Preparation of SM1−4. A mixture of compound 2 (1.0 mmol), compound 4 (3.0 mmol), anhydrous K2CO3 (1.38 g, 10.0 mmol) and DMAc (2 mL, 0.5 mol/L) in a Schlenk tube was degassed before and after Pd(OAc)2 (13 mg, 0.06 mmol) and PCy3·HBF4 (46 mg, 0.14 mmol) were added. The reaction mixture was stirred at 120 °C under nitrogen protection for 1 d. Then the reaction mixture was allowed to cool down to room temperature; water (30 mL) was added; the resulted mixture was extracted with DCM (30 mL × 3); and the combined organic layers were washed with brine (50 mL × 2), dried over anhydrous MgSO4 and evaporated to dryness. The residue was further purified on a silica gel column. SM1. 1,4-Dibromobenzene (2a) and compound 4a were used to synthesize SM1 according the procedure described above. The residue was purified on a silica gel column eluting with chlorobenzene/THF (10:1, v/v) to afford SM1 as a yellow solid (0.69 g, 80%). 1H NMR (400 MHz, oDCB-d4): δ (ppm) 8.89−8.80 (m, 3H), 8.00−8.01 (d, J = 7.6 Hz, 1H), 7.97 (s, 2H), 7.86−7.90 (t, J = 8.0 Hz, 1H), 7.72−7.73 (d, J = 3.6 Hz, 1H), 7.59−7.60 (d, J = 3.6 Hz, 1H), 4.54−4.58 (t, J = 8.0 Hz, 2H), 1.13−2.13 (m, 15H). 13C NMR (100 MHz, oDCB-d4): δ (ppm) 163.97, 163.79, 140.22, 138.59, 132.15, 132.10, 131.77, 131.52, 130.23, 130.16, 129.59, 128.64, 127.32, 127.07, 126.50, 123.12, 122.15, 121.97, 40.33, 33.65, 29.77, 29.53, 28.17, 27.05, 22.19, 18.72. Anal.

Figure 7. TEM images of PCDTBT-C12:SM1−4 based active layers after thermal annealing at 90 °C for 3 min: (a) PCDTBT-C12:SM1, (b) PCDTBT-C12:SM2, (c) PCDTBT-C12:SM3, and (d) PCDTBT-C12:SM4.

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CONCLUSIONS In summary, we have synthesized a series of novel NI-based small molecular acceptors (SM1−4) by C−H activation reaction. These small molecular acceptors are composed of a central benzene core, two thiophene bridges and two NI terminal groups. SM1−4 all exhibited good crystallinity and solubility. By introducing different substituents on the central benzene ring, the dihedral angles between the two NI rings in SM1, SM2, SM3, and SM4 increased. Along with this change, μe of devices was gradually decreased, demonstrating that planar small molecules are beneficial for the electron transport in the blend films. With higher electron mobility, enhanced Jsc and PCE were achieved. Besides, only planar small molecule SM1 and PCDTBT-C12 blend films exhibited uniform morphology where the size of SM1 aggregates became much smaller. A PCE of 0.59% with a Voc of 1.36 V was acquired for PCDTBT-C12:SM1 based plain devices. After thermal annealing, the PCE was further improved to 2.78%. Compared to most high efficiency OSCs, the relatively low Jsc of SM1−4 based devices impinged the further increase of PCE. Designing and synthesizing a NI based acceptor which possesses a more planar chemical structure to increase the μe and a slightly downshifted LUMO level to afford better charge separation could further increase the Jsc of PCDTBT-C12 based devices. Our results demonstrated that the planarity of small molecular acceptors has significant influences on the performance of nonfullerene OSCs and NI based planar acceptors are potential acceptors for WBG polymer based OSCs.

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EXPERIMENTAL SECTION

N-Octyl-4-bromo-1,8-naphthalimide,30 N-dodecyl-4-bromo-1,8-naphthalimide,31 and 1,4-dibromo-2,5-bis(octyloxy)benzene31 were synthesized according to the literature procedures. 1,4-Dibromo-2-fluoro-5-(octyloxy)benzene (2a). A mixture of 1,4-dibromo-2,5-difluorobenzene (2.71 g, 10.0 mmol), potassium tertbutoxide (1.17 g, 10.5 mmol) and THF in a Schlenk tube was carefully degassed before 1-bromooctane (2.03 g, 10.5 mmol) was added by a syringe, and the reaction mixture was then stirred at 60 °C under G

DOI: 10.1021/acsami.5b10211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Calcd for C44H44N2O4 (%): C, 75.67; H, 6.12; N, 3.27. Found (%): C, 75.38; H, 6.51; N, 3.46. SM2. 1,4-Dibromo-2-fluorobenzene (2b) and compound 4b were used to synthesize SM2 according the procedure described above. The residue was purified on a silica gel column eluting with chlorobenzene/ THF (10:1, v/v) to afford SM2 as a yellow solid (0.81 g, 82%). 1H NMR (400 MHz, oDCB-d4): δ (ppm) 8.59−8.6 (m, 6H), 7.77−7.81 (m, 3H), 7.72−7.73 (d, J = 3.6 Hz, 1H), 7.67−7.68 (m, 2H), 7.54 (s, 1H), 7.49−7.50 (d, J = 3.6 Hz, 2H), 7.43−7.44 (d, J = 3.6 Hz, 1H), 7.38−7.39 (d, J = 3.6 Hz, 1H), 4.34−4.37 (t, J = 7.6 Hz, 4H), 0.92− 1.95 (m, 21H). 13C NMR (100 MHz, oDCB-d4): δ (ppm) 164.17, 163.95, 139.51, 139.21, 131.95, 131.27, 131.77, 131.55, 131.49, 130.18, 130.09, 129.87, 129.43, 129.34, 128.57, 127.75, 127.29, 127.09, 126.95, 124.87, 122.85, 121.79, 41.27, 35.73, 30.12, 29.87, 29.49, 29.16, 28.58, 28.17, 27.94, 25.98, 23.29, 21.55. Anal. Calcd for C44H44N2O4 (%): C, 75.42; H, 6.84; N, 2.84. Found (%): C, 75.58; H, 6.91; N, 3.02. SM3. 1,4-Dibromo-2-fluoro-5-(octyloxy)benzene (2c) and compound 4a were used to synthesize SM3 according the procedure described above. The residue was purified on a silica gel column eluting with chlorobenzene/THF (10:1, v/v) to afford SM3 as a light red solid (0.75 g, 75%). 1H NMR (400 MHz, oDCB-d4): δ (ppm) 8.80−8.91 (m, 6H), 7.97−8.06 (m, 4H), 7.87−7.89 (m, 3H), 7.65− 7.69 (m, 2H), 7.57−7.59 (d, J = 6.4 Hz, 1H), 4.45−4.57 (m, 6H), 1.07−2.25 (m, 45H). 13C NMR (100 MHz, oDCB-d4): δ (ppm) 163.98, 163.27, 141.39, 140.99, 129.85, 129.77, 129.54, 128.72, 128.51, 127.48, 127.31, 127.24, 127.18, 127.11, 126.97, 126.79, 126.53, 126.49, 123.77, 122.98, 122.18, 40.89, 34.87, 31.52, 30.19, 29.91, 29.41, 28.52, 28.44, 28.19, 27.59, 25.87, 23.41, 19.87, 18.53. Anal. Calcd for C44H44N2O4 (%): C, 74.22; H, 6.73; N, 2.79. Found (%): C, 74.01; H, 7.04; N, 2.52. SM4. 1,4-Dibromo-2,5-bis(octyloxy)benzene (2d) and compound 4a were used to synthesize SM4 according the procedure described above. The residue was purified on a silica gel column eluting with chlorobenzene/THF (10:1, v/v) to afford SM4 as a light red solid (0.86 g, 77%). 1H NMR (400 MHz, oDCB-d4): δ (ppm) 8.97−8.98 (m, 1H), 8.80−8.86 (m, 3H), 8.13−8.14 (d, J = 4.0 Hz, 1H), 8.07− 8.09 (d, J = 7.6 Hz, 1H), 7.86−7.90 (m, 1H), 7.80 (s, 1H), 7.68−7.69 (d, J = 4.0 Hz, 1H), 4.50−4.57 (m, 4H), 1.09−2.29 (m, 30H). 13C NMR (100 MHz, oDCB-d4): δ (ppm) 164.27, 164.19, 140.35, 139.51, 131.05, 131.01, 130.89, 130.69, 130.12, 130.03, 129.85, 128.51, 126.92, 126.87, 126.79, 126.70, 123.92, 122.05, 120.87, 40.51, 33.77, 30.52, 29.97, 29.51, 28.87, 28.75, 27.19, 26.54, 25.33, 22.52, 19.81, 18.77. Anal. Calcd for C44H44N2O4 (%): C, 75.50; H, 7.60; N, 2.52. Found (%): C, 75.17; H, 7.84; N, 2.66.

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Foundation (2132042), Program for Changjiang Scholars and Innovative Research Team in University and the Fundamental Research Funds for the Central Universities are gratefully acknowledged.

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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/acsami.5b10211. 1 H NMR, TGA, DSC and XRD spectra of SM1−4; SCLC curves of hole and electron devices; TEM image of PCDTBT-C12:SM1-based active layer before thermal annealing treatment; and detailed parameters of DFT calculations of SM1−4 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express thanks for the financial support financial support from the NSF of China (91233205), Beijing Natural Science H

DOI: 10.1021/acsami.5b10211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b10211 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX