Tuning the central donor core via intramolecular noncovalent interactions based on D(A-Ar)2 type small molecules for high performance organic solar cells

Solar Energy 161 (2018) 138–147

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Tuning the central donor core via intramolecular noncovalent interactions based on D(A-Ar)2 type small molecules for high performance organic solar cells ⁎

⁎

Zhongbin Qiua, Xiaopeng Xub, Liang Yanga, Yong Peia, , Mengbin Zhuc, Qiang Pengb, , Yu Liua,c,

T

⁎

a

College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in the Ministry of Education, Xiangtan University, Xiangtan 411105, PR China Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, and State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, PR China c Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, School of Materials Science & Engineering, Changzhou University, Changzhou 213164, PR China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Small molecule Diketopyrrolopyrrole Non-covalent interaction Organic solar cells

Two narrow band-gap small molecules with D(A-Ar)2 framework, namely DMPh(DPP-Py)2 and DFPh(DPP-Py)2, were designed and synthesized for high-performance small molecule organic solar cells (SMOSC), in which the 1,4-dimethoxybenzene (DMPh) and 1,4-difluorobenzene (DFPh) were employed as rigid donor cores, respectively, and the pyrenere (Py) unit is selected as terminal-capping groups on an electron-deficient diketopyrrolopyrrole (DPP)-based linear backbone. The impacts of the fluorine-sulfur (F⋯S) atoms and oxygen-sulfur (O⋯S) atoms noncovalent interaction on their absorption spectra, molecular energy levels, morphological properties, hole mobilities and photovoltaic properties were investigated thoroughly. The fluorinated DFPh(DPP-Py)2 possess a relatively lower-lying HOMO energy level, better miscibility of the blend with PC71BM, as well as higher mobility in comparison with those of the methoxyled DMPh(DPP-Py)2. As a consequence, the OSCs devices based on DMPh(DPP-Py)2 and DFPh(DPP-Py)2 exhibited PCEs of 5.47% and 7.54%, respectively. Obviously, the device based on DFPh(DPP-Py)2 presented a better performance, which should be ascribed to the improved simultaneously Voc of 0.77 V, Jsc of 15.3 mA cm−2, and FF of 64%. The results indicated that the choice of the fluorination designation on the molecular backbone is an effective approach to develop D(A-Ar)2 type small molecule donors for highly efficient solar cell applications.

1. Introduction As a new kind of energy, organic photovoltaic (OPV) has been paid more and more attention because of its advantages such as light weight, flexibility and low cost, etc (Li, 2011; Collins et al., 2017; Tang et al., 2016; Polman et al., 2016) With the rapid development in recent years, the highest power conversion efficiency (PCE) of single layer organic polymer and small molecules (SMs) photovoltaic devices has reached 13.1% (Zhao et al., 2017) and 11.53% (Wan et al., 2017), respectively. Meanwhile, the PCE of the tandem solar cells and the ternary blend organic solar cells are respectively 12.7% (Kan et al., 2015) and 12.1% (Kumari et al., 2017), respectively. Although the PCE of organic small molecules solar cells (OSMSCs) compared to organic polymer solar cells (OPSCs) is still low, SMs have some advantages as compared to polymers, for example, definite structure, easy separation and purification

(Collins et al., 2017; Mazzio and Luscombe, 2015). However, even now organic photovoltaic materials have made a great progress, there are still some challenges in the application. Especially for most OSMOSCs, the structure-property relationship has still not been made clear for photovoltaic SMs materials, and it is difficult to maintain the balance the open circuit voltage (Voc), short circuit current density (Jsc) and fill factor (FF) in organic solar cells (OSCs) Mazzio and Luscombe, 2015. As is known to all, in OSMSCs, donor materials of original innovation play a crucial scientific factor to obtain high performance devices. Therefore, so as to improve efficiency of OSCs, recently, all kinds of noncovalent bonding had been introduced in photovoltaic materials. Due to the fact thatthe intramolecular noncovalent bonding plays an important role for optimizing molecular structures (Zhang et al., 2016a; Zhang et al., 2015; Guo et al., 2016; Jo et al., 2014; Yum et al., 2014; Guo et al., 2013; Kawashima et al., 2016; Liu et al., 2015; Uddin et al.,

⁎ Corresponding authors at: College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in the Ministry of Education, Xiangtan University, Xiangtan 411105, PR China (Y. Liu). E-mail addresses: [email protected] (Y. Pei), [email protected] (Q. Peng), [email protected] (Y. Liu).

https://doi.org/10.1016/j.solener.2017.12.042 Received 14 October 2017; Received in revised form 18 December 2017; Accepted 21 December 2017 0038-092X/ © 2017 Elsevier Ltd. All rights reserved.

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minimize the influence of the length of the alkyl chain on the molecular packing as comparing with the DFPh(DPP-Py)2, and then, the introduced of the methoxy can make the absorption spectrum red shift, thus leading to improve the Jsc of the device (Zhang and Li, 2014; Shin et al., 2015). At the same time, the introduction of fluorine atoms can reduce the highest occupied molecular orbital (HOMO) levels and increase the carrier mobility of its corresponding SMs, and expects to obtain an enhanced device performance (Zhang et al., 2016b; Deng et al., 2016; Wang et al., 2016a; Peng et al., 2011). On the other side, the fused ring C2-pyrene (Py) end-capping group can influences intermolecular interactions which may promote molecular packing, improve the charge carrier transport, enhance molecular packing and conducive active layer morphology favorable for high device PCE (Lee et al., 2011; Yu et al., 2016). Sometimes, to obtain a wide range of absorption and narrow band gaps, the DPP fragment is often used as the acceptor (A) unit, because of its strong electron withdrawing ability, high carrier mobility, strong light absorption and good planarity (Tang et al., 2015; Duan et al., 2015; Chen et al., 2016; Yu et al., 2016). Prompted by the above mentioned considerations, to further investigate the different influence of O⋯S noncovalent interaction and F⋯S noncovalent interaction based donor (D) covalent bridge cores on photovoltaic property of D(A-Ar)2 type SMs, two conjugated SMs of DMPh(DPP-Py)2 and DFPh(DPP-Py)2 were constructed using the DMPh and DFPh units as the donor cores, the electron-deficient DPP and polycyclic aromatic hydrocarbons Py units were respectively used as arm acceptor (A) and terminal aryl (Ar) units. Their synthetic routes are presented in Scheme 1. Their photophysical, electrochemical and

2016; Zhang et al., 2016b; Deng et al., 2016). Through the introduction of intramolecular noncovalent interaction onto the photovoltaic materials, we can adjust the molecular levels, the band gaps, broaden the absorption spectra, reduce energy loss, and the crystallization of the blend films with PC71BM micro phase separation scale (Zhang et al., 2016a; Zhang et al., 2015; Guo et al., 2016; Jo et al., 2014; Yum et al., 2014; Guo et al., 2013; Kawashima et al., 2016; Liu et al., 2015; Uddin et al., 2016; Zhang et al., 2016b; Deng et al., 2016). The reported works for intramolecular noncovalent interactions which mainly include fluorine and sulfur atoms (F⋯S) Jo et al., 2014; Kawashima et al., 2016; Wang et al., 2016a, oxygen-sulfur atoms (O⋯S) Zhang et al., 2015; Guo et al., 2016, 2013, nitrogen-sulfur atoms (N⋯S) non-covalent interactions (Yum et al., 2014) and hydrogen (H) bonding interactions (Zhang et al., 2016b), make an outstanding contribution to enhance the coplanarity and the crystallinity of the SMs, and expected to improve the performance of OSMSCs. In this paper, two D(A-Ar)2 linear molecules based OeS and FeS non covalent bonds with a methoxy functionalized 1,4-dimethoxybenzene (DMPh) and 1,4-difluorobenzene (DFPh) rigid central core respectively, an electron-deficient diketopyrrolopyrrole (DPP) as the acceptor arms, and grafting C2-pyrene (Py) end-capping groups, named as DMPh(DPP-Py)2 and DFPh(DPP-Py)2 (Chart 1), were designed and synthesized for efficient photovoltaic materials. Herein, phene (Ph) is selected as the central core duo to their intrinsic ridgidity, symmetry and planar backbone structure, and can induce better molecular packing with improved backbone aggregation (Wang et al., 2016a). Inserting methoxy on the Ph core as side chains is conducive to

Chart 1. Structure of SMs.

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Scheme 1. Synthetic routes of SMs. Reaction condition: (a) n-BuLi, THF, 2-isobutyl-4,4,5,5-tetramethyl1,3,2-dioxaborolane, −78 °C; (b) 1,4-dioxane, KOAc, Pd(dppf)2Cl2, bis(pinacolato)diboron, 80 °C; (c) dtbpy, [Ir(OMe)COD]2, bis(pinacolato)diboron, cyclohexane, 80 °C; (d) Pd(PPh3)4, ethanol, K2CO3 (2 M), toluene, 80 °C.

photovoltaic performances were preliminary investigated. The fluorination central core of DFPh(DPP-Py)2 induce a reasonably lower-lying HOMO energy level, higher mobility and more excellent of the blend film formed with PC71BM in comparison with those of DMPh(DPP-Py)2based on methoxy core, finally enhancing the photovoltaic properties mainly due to elevate the Voc, Jsc, and FF of the related solar cell devices. As a result, the DMPh(DPP-Py)2/PC71BM-based OSCs under optimal conditions present a PCE of 5.47% with Voc of 0.66 V, Jsc of 14.50 mA cm−2, and FF of 55.5%, while OSCs based on DFPh(DPPPy)2/PC71BM exhibit a higher 7.54% with enhanced simultaneously Voc of 0.77 V, Jsc of 15.30 mA/cm2 and FF of 64.0%, respectively, which triggered by its lower HOMO level, higher carrier mobility and more excellent blend surface morphology of donor/acceptor (Jo et al., 2014; Wang et al., 2016a). This value of PCE is one of the highest values reported among D(A-Ar)2 linear type SMs so far for the OSCs-based on DPP as arm A units (Shin et al., 2015; Qin et al., 2014; Li et al., 2013; Gao et al., 2015; Li et al., 2016; Virginia et al., 2016). The results indicate that the incorporating the F⋯S noncovalent interaction on rigidity skeleton cores can contribute remarkably PCE in compared with the O⋯S noncovalent interaction onto the D(A-Ar)2 linear type SMs.

at a scan rate of 50 mV/s. A Pt plate, a Pt wire and an Ag/AgCl electrode were used as working electrode, counter electrode, reference electrode, respectively. The D(A-Ar)2 type SMs were respectively coated on the Pt plate surface and all potentials were corrected against ferrocene/ferrocenium (Fc/Fc+). The theoretical study was performed on the 631G∗∗ basis set in Gaussian 09 using the density functional theory (DFT), as approximated by the B3LYP. The surface morphology of the SMs/PC71BM blend film was investigated by an atomic force microscopy (AFM) on a Veeco, DI multimode NS-3D apparatus in a tapping mode under normal air condition at RT with a 5 µm scanner. 2.2. Device fabrication and characterization The SMs solar cells were prepared with the device structure of ITO/ PEDOT:PSS/SMs:PC71BM/Ca/Al. Patterned ITO-coated glasses were ultrasonically cleaned sequentially with ITO detergent, deionized water, acetone and isopropanol for 20 min each time, and then treated with oxygen plasma for 6 min. A layer of PEDOT:PSS was spin coated (5000 rpm) onto the ITO glass. After baking at 140 °C for 15 min, the substrates were transferred into a glove box. Subsequently, the substrates were transferred to a glove box filled with N2. The chloroform (CF) solutions of SM with PC71BM (5 mg/mL, SM) with different ratio were stirred overnight at room temperature. The solutions were spincoated to prepare the active layer at 2500 rpm on PEDOT:PSS modified ITO coated glass. Finally, a 20 nm Ca layer and a 100 nm Al layer were successively deposited onto the active layer under high vacuum by a shadow mask to define the area of 0.09 cm−2. Thermal annealing was performed by placing the completed devices on a digitally controlled hotplate at different temperatures in a nitrogen-filled glove box. The thicknesses of the spun-cast films were recorded by a profilometer (Alpha-Step 200, Tencor Instruments). The external quantum efficiency (EQE) was measured with a Stanford Research Systems model SR830 DSP lock-in amplifier coupled with WDG3 monochromator and a 150 W xenon lamp. Reflectance spectra were measured with HITACHI U-4100 spectrophotometer. Hole mobility was measured by the space-chargelimited current (SCLC) method in the hole-only device with a device structure of ITO/PEDOT:PSS/activelayer/MoO3/Au.

2. Experimental section 2.1. Measurement and characterization 1 H NMR and 13C NMR spectra were recorded at 400 MHz and 100 MHz on a Bruker Avance-400 spectrometer using CDCl3 as solvent and tetramethylsilane (TMS) as internal standard, respectively. Mass spectra (MS) were measured on a Bruker Daltonics BIFLEX Ш MALDITOF analyzer. Elemental analyses were carried out with a Vario EL III elemental analyzer. UV–Vis absorption spectra were recorded on a Shimadzu UV-1800 spectrophotometer. Thermogravimetric analysis (TGA) was measured on a Perkin-Elmer Diamond TG/DTA thermal analyzer at a scan rate of 10 °C/min under nitrogen atmosphere. The differential scanning calorimetric (DSC) measurement was made on a TA DSCQ10 instrument at a heating rate of 10 °C/min under nitrogen atmosphere. The powder XRD was recorded by a RINT2000 vertical goniometer operated at 40 kV voltage and a 250 mA current with Cu Kα radiation. Cyclic voltammetry (CV) was conducted on a CHI620 voltammetric analyzer under argon atmosphere in an anhydrous acetonitrile solution of tetra(n-butyl) ammonium hexafluorophosphate (0.1 M)

2.3. Materials and synthesis All reactions were made under nitrogen atmosphere. All reagents 140

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2.3.4. Synthesis of DMPh(DPP-Py)2 To a dry three neck 50 mL flask filled with ethanol (1.0 mL) and toluene (10.0 mL) were added compound DPPPy-Br (166 mg, 0.162 mmol), 1 (30 mg, 0.077 mmol), 2.0 M K2CO3 aqueous solution (0.4 mL), tris-(dibenzylideneacetone)dipalladium (1.4 mg) and tri(otolyl)phosphine (1.8 mg). The reaction mixture was heated to 80 °C and refluxed for overnight under nitrogen atmosphere. After cooled to room temperature and the mixture was poured into petroleum ether to form precipitation. The residue was collected by filtration through 0.45 μm Teflon filter and purified by column chromatography on silica gel using chloroform as eluent to provide a dark powder (129 mg, yield 82.4%). 1 H NMR (400 MHz, CDCl3) δ (ppm): 9.17 (d, J = 4.1 Hz, 2H), 8.98 (d, J = 4.0 Hz, 2H), 8.13 (s, 4H), 7.98 (t, J = 11.5 Hz, 10H), 7.77 – 7.64 (m, 2H), 7.59 (s, 2H), 7.52 (s, 2H), 7.23 (s, 2H), 4.11 (d, J = 8.2 Hz, 4H), 4.07 (s, 6H), 3.93 (d, J = 7.1 Hz, 4H), 2.03 (d, J = 59.7 Hz, 4H), 1.50 – 1.12 (m, 84H), 0.94 – 0.74 (m, 24H). MALDI-MS (m/z) of C132H166N4O6S4 for [M+]: calcd. 2032.17; found, 2031.42. Elemental Analysis of C132H166N4O6S4: calcd. C, 77.98; H, 8.23; N, 2.76; S, 6.31; found, C, 77.82; H, 8.304; N, 2.488; S, 5.82.

and solvents were purchased from Energy and Aldrich corporations. Toluene was distilled according to common methods. They were used without further purification unless stated otherwise. Compounds 1 were synthesized according to the reported literature (Kerszulis et al., 2014; Peter et al., 2000). The DMPh(DPP-Py)2 and DFPh(DPP-Py)2 were prepared by Suzuki Cross-Coupling reaction. The synthetic route is shown in Scheme 1 and their structures are consistent with molecular formulas characterized by 1H NMR and MALDI-TOF in the Experimental Section (Fig. S1–S8, see ESI†). 2.3.1. Synthesis of 2,2′-(2,5-difluoro-1,4-phenylene)bis(4,4,5,5tetramethyl-1,3,2-dioxaborolane) (2) To a mixture of compound 1,4-dibromo-2,5-difluorobenzene (2.00 g, 7.40 mmol), bis(pinacolato)diboron (4.70 g, 18.5 mmol), potassium acetate (4.00 g, 44.4 mmol) and [1,10-bis-(diphenylphosphino) ferrocene]dichloropalladium (210 mg, 0.287 mmol) were dissolved in dry 1,4-dioxane (25 mL). The reaction mixture was heated to 80 °C and refluxed for 24 h under nitrogen atmosphere. After cooling to room temperature (RT), the mixture was extracted with dichloromethane (DCM) (3 × 20 mL) and then combined organic layer was dried over anhydrous magnesium sulfate followed by filtration. The solvent was removed off under reduced pressure by rotary evaporation and the residue was purified through a flash silica gel column with petroleum ether (PE)/DCM (V/V, 2:1) as the eluent to give a white solid (1.98 g, yield 74.1%). 1H NMR (400 MHz, CDCl3, TMS), δ (ppm):δ 7.35 (s, 2H), 1.36 (s, 24H).

2.3.5. Synthesis of DFPh(DPP-Py)2 Compound DFPh(DPP-Py)2 was prepared according to the synthetic process of the above compound DMPh(DPP-Py)2 and give a dark powder (120 mg, yield 72.9%). 1H NMR (400 MHz, CDCl3) δ (ppm): 9.07 (d, J = 4.0 Hz, 4H), 8.04 (s, 4H), 7.89 (t, J = 11.3 Hz, 8H), 7.77 – 7.62 (m, 4H), 7.51 (s, 4H), 7.38 – 7.28 (m, 4H), 4.08 (d, J = 8.9 Hz, 4H), 3.99 (d, J = 5.5 Hz, 4H), 1.96 (s, 4H), 1.50 – 1.16 (m, 84H), 0.83 (dd, J = 21.0, 8.2 Hz, 24H). MALDI-MS (m/z) of C132H166N4O6S4 for [M+]: calcd. 2008.94; found, 2008.337. Elemental Analysis of C130H160F2N4O4S4: calcd. C, 77.72; H, 8.03; N, 2.79; S, 6.38; found, C, 80.49; H, 8.431; N, 2.597; S, 6.398.

2.3.2. Synthesis of 4,4,5,5-tetramethyl-2-(pyren-2-yl)-1,3,2-dioxaborolane (3) To a mixture of compound pyrene (2.00 g, 9.89 mmol), bis(pinacolato)diboron (2.79 g, 11.0 mmol), 4,4-di-tert-butyl bipyridine (48 mg, 0.18 mmol) and [Ir(OMe)COD]2 (60.0 mg, 0.09 mmol) were dissolved in dry anhydrous cyclohexane (50 mL). The reaction mixture was heated to 80 °C and refluxed for 12 h under nitrogen atmosphere. After cooled to room temperature and the mixture was poured into brine. Then the aqueous phase was extracted with chloroform (3 × 25 mL). The resulting organic layer was washed with brine, dried over anhydrous MgSO4 followed by filtration. The solvent was removed off under reduced pressure by rotary evaporation and the residue was purified through a flash silica gel column with PE/DCM (3:1, V/V) as eluent to provide a white powder (1.38 g, yield 60%). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.63 (s, 2H), 8.17 (d, J = 7.6 Hz, 2H), 8.08 (dd, J = 19.7, 9.0 Hz, 5H), 1.46 (s, 12H).

3. Results and discussion 3.1. Synthesis and thermal properties The synthetic routes of DMPh(DPP-Py)2 and DFPh(DPP-Py)2 are depicted in Scheme 1. The intermediate DPPPy-Br was synthesized from compound 3 and 4 through the Suzuki coupling reaction. DMPh(DPPPy)2 and DFPh(DPP-Py)2 were finally synthesized from the corresponding borate esters 1 or 2 with DPPPy-Br via Suzuki coupling reaction. Both SMs exhibited excellent film forming properties and good solubility in general organic solvents, such as CF, chlorobenzene (CB), and 1, 2-dichlorobenzene (o-DCB). The thermal stability of DMPh(DPPPy)2 and DFPh(DPP-Py)2 were evaluated by thermogravimetric analysis under inert atmosphere (TGA) (Fig. S9, see ESI†). As shown in Fig. S9, the high decomposition temperatures (Td) of 381.0 and 408.0 °C were observed for DMPh(DPP-Py)2 and DFPh(DPP-Py)2 at 5% weight-loss, respectively (Table 1). Such good thermal properties for DMPh(DPP-Py)2 and DFPh(DPP-Py)2 are enough for avoiding the degradation of the active layer and device deformation in OSMSCs. Meanwhile, the recorded DSC curves are also shown in Fig. 1a, and their corresponding data are summarized in Table 1. As shown in Fig. 1a, DMPh(DPP-Py)2 exhibited a higher melting temperature (Tm) at 300.0 °C and a higher crystallization temperature (Tc) at 275.0 °C, in comparison with a Tm of 281.0 °C and a Tc at 255.0 °C for the fluoridation DFPh(DPP-Py)2. Furthermore, these

2.3.3. Synthesis of DPPPy-Br To a mixture of compound compound 2 (362 mg, 1.10 mmol), 3 (1.0 g, 1.10 mmol), 2.0 M K2CO3 aqueous solution (5.0 mL) and tetrakis (triphenylphosphine)- palladium (25.0 mg) were dissolved in dry anhydrous ethanol (10.0 mL) and toluene (50.0 mL). The reaction mixture was heated to 80 °C and refluxed for 24 h under nitrogen atmosphere. After cooled to room temperature and the mixture was poured into brine. Then the aqueous phase was extracted with chloroform (3 × 25 mL). The resulting organic layer was washed with brine, dried over anhydrous MgSO4 followed by filtration. The solvent was removed off under reduced pressure by rotary evaporation and the residue was purified through a flash silica gel column with PE/DCM (2:1, V/V) as eluent to provide a purple powder (545 mg, yield 48.2%). 1H NMR (400 MHz, CDCl3) δ (ppm): 9.03 (d, J = 4.1 Hz, 1H), 8.89 (d, J = 3.8 Hz, 1H), 8.44 (s, 2H), 8.20 (d, J = 7.6 Hz, 3H), 8.12 (s, 4H), 8.05 – 7.99 (m, 1H), 7.76 (d, J = 4.1 Hz, 1H), 7.63 (d, J = 5.0 Hz, 1H), 7.63 (d, J = 5.0 Hz, 1H), 7.63 (d, J = 5.0 Hz, 1H), 7.29 (d, J = 4.9 Hz, 1H), 4.13 (d, J = 7.6 Hz, 2H), 4.06 (d, J = 7.7 Hz, 2H), 2.05 (s, 1H), 1.94 (s, 1H), 1.30 (dd, J = 46.6, 15.1 Hz, 42H), 0.91–0.76 (m, 12H). MALDI-MS (m/z) of C62H79BrN2O2S2 for [M+]: calcd. 1028.34; found, 1028.44.

Table 1 TGA, DSC and XRD patterns of SMs.

*

SMs

Td (°C)

Tm (°C)

Tc (°C)

2-theta1 (deg)

D1 (ang)

2-theta2 (deg)

D2 (ang)

SM1 SM2

381 408

300 281

275 255

5.378 5.525

16.42 15.98

24.00 25.45

3.70 3.50

SM1 and SM2 replaced DMPh(DPP-Py)2 and DMPh(DPP-Py)2, respectively.

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Fig. 1. (a) Differential scanning calorimetry (DSC) curves of SMs and (b) XRD patterns of the small molecule films on silicon wafers.

Tms on the heating process and Tcs on the cooling process indicated that DMPh(DPP-Py)2 and DFPh(DPP-Py)2 had a tendency to crystallize. The obvious crystallization peaks might be beneficial for the molecular stacking in the solid state, and resulting in an higher carrier mobility of the both SMs (Wang et al., 2016a; Fei et al., 2015).

Table 2 Optical and electrochemical properties of SMs. SMs

SM1 SM2

3.2. X-ray diffraction (XRD) analysis

λmax (nm) Solution

Film

λonset Film (nm)

654 641

659,746 647,729

845 796

Eg opt (eV)a

EHOMO (eV)b

ELUMO (eV)b

Egele (eV)

1.47 1.56

−5.14 −5.24

−3.45 −3.48

1.69 1.76

Calculated from the absorption band edge of the films, Eg opt = 1240/λonset. Calculated from empirical equation: EHOMO = −(Eox + 4.73) eV, ELUMO = −(EHOMO + 4.73) eV; The formal potential of Fc/Fc+ was 0.069 V vs. Ag/AgCl measured in this work. a

We used the X-ray diffraction (XRD) patterns to further investigate the crystallinity of the DMPh(DPP-Py)2 and DFPh(DPP-Py)2. As shown in Fig. 1b, the first peaks of DMPh(DPP-Py)2 and DFPh(DPP-Py)2 were showed at 2θ = 5.38° and 5.53°, respectively, which should be attributed to the (1 0 0) diffraction (d100) from a lamellar packing structure. Furthermore, the d100 spacings (d1s) of DMPh(DPP-Py)2 and DFPh(DPPPy)2 were calculated approximately 16.42 Å and 15.98 Å, respectively. Apparently, the crystallinity of DMPh(DPP-Py)2 is higher than that of DFPh(DPP-Py)2, which is consistent with what is reported in the literature (Yum et al., 2014) and the DSC result mentioned above. Notably, both SMs have a small d1s value, which may be attributed to the different introduction of intermolecular noncovalent interactions, and promoted the lamellar stacking of the whole molecule. More weak and broad (0 1 0) peaks which are originating from the π-π stacking of DMPh(DPP-Py)2 at 2θ = 24.00° and DFPh(DPP-Py)2 at 25.45°, respectively. Accordingly, the corresponding π-π stacking distances (dπ) between the coplanar SM skeletons is also calculated to be 3.70 Å and 3.50 Å for DMPh(DPP-Py)2 and DFPh(DPP-Py)2, respectively. Compared with DMPh(DPP-Py)2, the smaller dπ value of DFPh(DPP-Py)2 show a stronger π-π stacking between molecules, which is more conducive to improve its values of Jsc and FF in the photovoltaic cells.

b

absorption data are also summarized in Table 2. As shown in Fig. 2a, both SMs showed similar absorption in the wavelength region of 300–450 nm, which should be ascribed to the π-π∗ transition of the conjugated backbone. The intramolecular charge transfer (ICT) peak of DFPh(DPP-Py)2 exhibited blue shift of 13 nm as comparison with that of DMPh(DPP-Py)2. The reason is that the methoxy group is an electron donating group, which will promote the π-delocalization of the molecular system, while the fluorine atom is a strong electron withdrawing unit, which will limit the π-delocalization in the molecular system (Guo et al., 2016; Jo et al., 2014; Kawashima et al., 2016; Hu et al., 2015). As shown in Fig. 2b, in the thin films, both SMs exhibit an apparent redshifted and broadened absorption profiles as comparison with their solution appearances. With the introduction of C2-pyrene onto the end units, the shoulder absorption of DMPh(DPP-Py)2 and DMPh(DPP-Py)2 were red shift to 746 nm and 729 nm, respectively. Mainly due to two aspects: the terminal group of C2-pyrene promotes the self-assembly ability of the molecules; (Lee et al., 2011) Methoxy and fluorine atoms can enhance the intermolecular interactions and the closed π-π stacking property via the supramolecular forces induced by O⋯H, O⋯S, F⋯H, and F⋯S noncovalent interactions. As showed in Table 2, the optical band gaps (Eg opt) of DMPh(DPP-Py)2 and DFPh(DPP-Py)2 were

3.3. Optical absorption and electrochemical property Solution and solid film absorption spectra of DMPh(DPP-Py)2 and DFPh(DPP-Py)2 are measured and provided in Fig. 2. The detailed

Fig. 2. (a) UV–vis absorption spectra of SMsin chloroform solutions and (b) in thin solid films.

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Fig. 3. Temperature-dependent absorption spectra at a temperature interval of 20 °C: (a) DMPh(DPP-Py)2 in dilute o-dichlorobenzene (2.95 × 10−5 mol L−1); (b) DFPh(DPP-Py)2 in dilute o-dichlorobenzene (1.59 × 10−5 mol L−1);

3.4. Theoretical calculations

determined to be 1.47 and 1.56 eV, respectively, which were calculated by the absorption edges of their thin films. Fig. 3a and b shows the temperature-dependent absorption spectra of the both SMs in o-dichlorobenzene (o-DCB) solution which were elevated from 20 to 80 °C with an interval of 20 °C, respectively. In oDCB solution, the absorption intensity of all shoulder peaks decreased progressively with the temperature increased from 20 °C to 80 °C, which indicates that these SMs can form the strong π–π interaction at low temperature. Meanwhile, both of them still show an obvious shoulder peak at 80 °C, which indicates that SMs can form a strong and stable intermolecular aggregation (Fan et al., 2017a, b, c). To further investigate the electrochemical properties of DMPh(DPPPy)2 and DFPh(DPP-Py)2, the cyclic voltammetry (CV) using Ag/AgCl electrode as a reference and redox potential of ferrocene/ferrocenium (Fc/Fc+) as calibrated standard was investigated. Fig. 4a shows the recorded CV curves of DPh(DPP-Py)2 and DFPh(DPP-Py)2, and their detailed parameters are summarized in Table 2. versus Ag/Ag+, the onset oxidation potentials and reduction potentials (Eox/Ered) of DPh(DPP-Py)2 and DFPh(DPP-Py)2 were observed to be 0.41/0.51 V and −1.28/ −1.25 V, respectively, and the corresponding HOMO and LUMO energy levels (EHOMO and ELUMO) of the SMs were calculated according to the empirical equation (Fan et al., 2017c): EHOMO = - (Eox + 4.73) eV and ELUMO = - (Ered + 4.73) eV. As results, the EHOMO and ELUMO values of DMPh(DPP-Py)2 and DFPh(DPP-Py)2 are −5.14/−3.45 eV and −5.24/ −3.48 eV, respectively. Obviously, introduction of methoxy groups onto donor core can increase the HOMO energy level of DMPh(DPP-Py)2, Zhang et al., 2015; Guo et al., 2016 whereas introduction of F atoms onto donor core lowers the HOMO energy level of DFPh(DPP-Py)2 Jo et al., 2014; Kawashima et al., 2016; Huang et al., 2016; Bin et al., 2016. The relatively deeper HOMO levels could be expected to obtain a higher Voc in OSCs applications, which is consistent with the literature reported (Zhang et al., 2015; Guo et al., 2016; Jo et al., 2014; Yum et al., 2014; Kawashima et al., 2016; Huang et al., 2016; Bin et al., 2016).

To further study the influence of F⋯S conformation lock and O⋯S conformation lock in their molecules on molecular planarity and energy levels, the HOMO and LUMO distributions of the both SMs were calculated by the density functional theory (DFT) (B3LYP; 6-31G∗) method. Fig. 5 presented the optimal geometries and wave functions (HOMO and LUMO) of DMPh(DPP-Py)2 and DFPh(DPP-Py)2, respectively. As shown in Fig. 5a, the dihedral angle θ1 of DMPh(DPP-Py)2 and DFPh(DPP-Py)2 were 10.98° and 4.40°, respectively, due to introducing conformational locks. While introducing the planarity Py as ends, the dihedral angle θ2 of SMs was 7.18°. The results demonstrated that introducing F⋯S conformational locks onto the molecules are more planarity than that of the O⋯S conformation lock. Furthermore, the Py group was introduced as terminals can enhance the planarity of the molecules (Lee et al., 2011; Yu et al., 2016). As shown in Fig. 5b, the electron densities of LUMO and HOMO are main delocalization along on the whole conjugated skeleton for these SMs. Additionally, compared to that of the central DMPh core, the electron density of the cenrtral DFPh core makes less of a contribution to the HOMO state. However, the electron densities of LUMO+1 and HOMO−1 are mainly concentrated in the electron-deficient DPP units. The whole DFT results indicated that the effective charge-transfer process would be possible between the DMPh or DFPh skeleton core and DPP arms. As results, the HOMO and LUMO levels of DMPh(DPP-Py)2 and DFPh(DPP-Py)2 were determined to be −4.84/−2.85 eV and −4.98/−3.01 eV, respectively, Obviously, the change tangency is agreement with those results obtained from CV properties.

3.5. Photovoltaic properties To investigate the effects of photovoltaic properties of the introducing different conformational locks onto the benzene cores, the OSCs Fig. 4. (a) Cyclic voltammograms (CV) of SMs at a scan rate of 50 mV s−1. (b) Schematic energy diagram of the materials used in the OSMSCs.

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Fig. 5. (a) Optimized molecular structures for, (b) molecular orbital surfaces of LUMO +1, LUMO, HOMO, and HOMO−1 for SMs obtained by Gaussian 09 at the B3LYP/631G(d) level.

Fig. 6. (a) J-V curves and (b) EQE spectra of the optimized SMs/PC71BM cells under AM.1.5G illumination at100 mW/cm−2.

Table 3 Photovoltaic and hole mobilities data of the optimized devices based on SMs. SMs

Voc (V)

Jsc (mA cm−2)

FF (%)

PCEmax/ave (%)

Rs (Ω·cm2)

Rsh (Ω·cm2)

μh (cm2 V−1 s−1)

SM1 SM2

0.68 0.77

14.5 15.3

55.5 64.0

5.47/5.23 7.54/7.21

15.4 10.4

543.0 750.1

1.3 × 10−4 4.5 × 10−4

Encouragingly, under the above all kinds of treatments, both DMPh (DPP-Py)2 and DFPh(DPP-Py)2 based devices showed significant increase in the PCEs, respectively. Obviously, these exciting results should also attributed to the introduction of conformation locks (Zhang et al., 2015; Guo et al., 2016; Jo et al., 2014; Yum et al., 2014; Fan et al., 2015, 2016; Liu et al., 2014; Zhou et al., 2011; Dang et al., 2014). As results, the active layers were finally spin-coated from the chloroform (CF) solutions of the SMs and PC71BM with an optimal weight ratio of 1:1 (w/w), speed of 2500 rpm from CF solutions, added 0.5% chloronaphthalene (CN) (v/v) into the solution as the processing additive, and then SVA lasts for 30 s to optimize the surface morphology of the active blends, respectively. The J-V characteristics of the best

were fabricated with a device architecture of ITO/PEDOT:PSS/ SMs:PC71BM/Ca/Al, and were measured under the illumination of one Sun (AM 1.5 G, 100 mW cm−2). The performances were optimized fabricated by various processing parameters, such as the D/A (SMs/ PC71BM, w/w) blend ratio, the usage of processing additives, the annealing temperature, time of solvent vapor annealing (SVA). The detailed device fabrication process is presented in the Experimental Section (Fig. S10–S14, see ESI†). Fig. S10–S14 show the J-V characteristics of SMs/PC71BM-based devices at different blend ratios, 1,8diiodooctane (DIO) or CN additive concentrations, annealing temperatures, the time of SVA, respectively. The corresponding photovoltaic data are summarized in Table S1–S5, respectively. 144

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Fig. 7. J-V characteristics of the hole-only devices fabricated from SMs:PC71BM blends at ambient temperature.

To better understand why the cells based on DFPh(DPP-Py)2 exhibits a higher Jsc than that of the DMPh(DPP-Py)2, the EQE curves of the devices with the SMs/PC71BM blend under the optimized condition were also investigated. As depicted in Fig. 6b, the EQE response of DMPh(DPP-Py)2- and DFPh(DPP-Py)2-based devices exhibited a broad profile ranging from 300 nm to 900 nm. For the DMPh(DPP-Py)2/ PC71BM blend, the observed EQE value exceeds 40% with the corresponding wavelength from 354 to 761 nm and the maximum EQE of 63.2% at 394 nm. While for the DFPh(DPP-Py)2/PC71BM blend, the observed EQE value is greater than 40% and the corresponding wavelength is from 354 nm to 745 nm, and the maximum EQE of 67.5% at 633 nm. Obviously, compared with DMPh(DPP-Py)2/PC71BM blend, the maximum EQE value of DFPh(DPP-Py)2PC71BM blend is larger and located at broader wavelength, which will be more favorable for the increase of Jsc in the DFPh(DPP-Py)2-based devices. Clearly, the EQE results agreed well with the measured Jsc values from J-V assessments (within 5% error), which could be explained why the devices based on DFPh(DPP-Py)2 can obtain higher Jsc (Wang et al., 2016a; Fan et al., 2016; Wang et al., 2016b). 3.6. Reflection spectra, charge mobilities and film morphologies

Fig. 8. AFM height images (a, c) and phase images (b, d) of the SMs based active blends. (a, b) DMPh(DPP-Py)2:PC71BM, RMS = 2.52 nm; (c,d) DFPh(DPP-Py)2: PC71BM, RMS = 2.04 nm.

To investigate the change of the EQE what reason be cause, the total light absorption was implemented using reflectance measurements for the incident light from the ITO side of real device structures (Ginting et al., 2017). As shown in Fig. 7a. The reflectance spectra consistent with EQE curve indicates that the increase of EQE is mainly affected by optical properties. Meanwhile, to further survey hole mobility (μh) of the blend films, the hole-device were then fabricated with a configuration of ITO/PEDOT:PSS/SMs:PC71BM/MoO3/Au, and the space charge limited current (SCLC) could be estimated using the MottGurney equation, (Zhang et al., 2016b; Deng et al., 2016; Wang et al., 2016a) and the calculated μh data are listed in Table 3. As shown in Fig. 7b, the μh of DMPh(DPP-Py)2:PC71BM and DFPh(DPP-Py)2:PC71BM devices were calculated to be 1.3 × 10−4 and 4.5 × 10−4 cm2 V−1 s−1, respectively. Clearly, the larger μh of DFPh(DPP-Py)2:PC71BM blend was also responsible for the Jsc improvements. Furthermore, the morphology of the active layers with fine nanophase separation is significant factor to improve device performance in OSMSCs. The surface morphologies of the DMPh(DPP-Py)2:PC71BM and DFPh(DPP-Py)2:PC71BM films were determined by atomic force microscopy (AFM) measurements in a surface area of 5 × 5 µm2. As shown in Fig. 8, the average root mean square (RMS) roughness values of the DMPh(DPP-Py)2:PC71BM and DFPh(DPP-Py)2:PC71BM were confirmed to be 2.52 and 2.04 nm, respectively. Consequently, the DFPh(DPPPy)2:PC71BM film presented a more homogeneous morphology with the relatively smaller domain size than that obtained from the DMPh(DPPPy)2:PC71BM film. The small RMS of DFPh(DPP-Py)2:PC71BM film demonstrate that the introduction of F⋯S noncovalent interaction would urge a better compatibility of this small molecular donor and PC71BM

DMPh(DPP-Py)2 and DFPh(DPP-Py)2 are shown in Fig. 6a, and the device parameters, such as Jsc, Voc, FF and PCE, were deduced from the J-V characteristics and are summarized in Table 3. As shown in Fig. 6a, the device based on DMPh(DPP-Py)2 as donor material exhibited a PCE of 5.47% with a Voc of 0.68 V, Jsc of 14.5 mA cm−2 and FF of 55% (the average efficiency of the 15 parallel devices is 5.23%). Encouragingly, whereas using DMPh(DPP-Py)2 as the donor material, the device presented a higher PCE of 7.54% with a Voc of 0.77 V, Jsc of 15.3 mA cm−2 and FF of 64% (the average efficiency of the 15 parallel devices is 7.21%). Distinctly, the better efficiency of the DFPh(DPP-Py)2 device should clearly be attributed to enhanced Voc, Jsc and FF simultaneously. The higher Voc of 0.77 V obtained should originate from the low-lying HOMO levels of DFPh(DPP-Py)2 as tested in the CV experiment (Wang et al., 2016a; Fan et al., 2016). The Jsc and FF values of the DFPh(DPPPy)2-based device were more higher than those of DMPh(DPP-Py)2based device, which could be attributed to the higher charge mobility and much better phase separation formed in the active blend layer (Wang et al., 2016a; Fan et al., 2016; Wang et al., 2016b). To the best of our knowledge, these were the highest PCE, Jsc and FF values recorded to date as compared with previously reported D(A-Ar)2 type SMs in BHJ-OSCs. Compared with the optimized device of the DMPh(DPPPy)2, the optimized device of the DFPh(DPP-Py)2 has a smaller the series resistance (Rs) and larger shunt resistance(Rsh), which indicated it has better exciton separation and less exciton recombination. This will help to improve the Jsc and carrier mobility. 145

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acceptor, which would facilitate the carrier transport and thus induce higher Jsc and FF values, and leading to higher PCE.

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4. Conclusions In summary, two photovoltaic SMs donors with D(A-Ar)2 frame structure, namely DMPh(DPP-Py)2 and DFPh(DPP-Py)2, were designed and synthesized through introducing intramolecular noncovalent bonds, As results, the fluorinated DFPh(DPP-Py)2 exhibits a lower-lying HOMO level, better blending with PC71BM, more excellent separation of the exciton between the donor and the PC71BM, and higher mobility in comparison with those of DMPh(DPP-Py)2. As expected, the DFPh (DPP-Py)2:PC71BM device exhibited significant elevations of Voc, Jsc and FF synchronously, which should be attributed to the influence of F⋯S noncovalent interaction. After optimized devices, the OSCs presented largely improved PCEs of 5.47% and 7.54% for DMPh(DPPPy)2:PC71BM and DFPh(DPP-Py)2:PC71BM devices, respectively. The results demonstrated that this type of SMs is promising donor materials for application in OSCs. Acknowledgements This work was supported by the National Science Foundation of China (51573154, 51573107, 91633301), the Foundation of State Key Laboratory of Polymer Materials Engineering (sklpme2014-3-05, 20152-01), and the Collaborative Innovation Center of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization (2011). Appendix A. Supplementary material The detailed data of the optimal BHJ solar cells and their corresponding J-V curves, the NMR spectra and MALDI-MS data are shown in Supporting Information, which is available from the xxxxxx or from the author. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.solener.2017.12.042. References Bin, H.J., Zhang, Z.G., Gao, L., Chen, S.S., Zhong, L.W., Xue, L., Yang, C., Li, Y.F., 2016. Non-fullerene polymer solar cells based on alkylthio and fluorine substituted 2Dconjugated polymers reach 9.5% efficiency. J. Am. Chem. Soc. 138 (13), 4657–4664. Chen, J.H., Duan, L.R., Xiao, M.J., Wang, Q., Liu, B., Xia, H., Yang, R.Q., Zhu, W.G., 2016. Tuning the central fused ring and terminal units to improve the photovoltaic performance of Ar(A–D)2type small molecules in solution-processed organic solar cells. J. Mater. Chem. A 4 (13), 4952–4961. Collins, S.D., Ran, N.A., Heiber, M.C., Nguyen, T.Q., 2017. Small is powerful: recent progress in solution-processed small molecule solar cells. Adv. Energy Mater. 1602242. Dang, D.F., Chen, W.C., Himmelberger, S., Tao, Q., Lundin, A., Yang, R.Q., Zhu, W.G., Salleo, A., Müller, C., Wang, E.G., 2014. Enhanced photovoltaic performance of indacenodithiophene-quinoxaline copolymers by side-chain modulation. Adv. Energy Mater. 4 (15), 1400680. Deng, D., Zhang, Y.J., Zhang, J.Q., Wang, Z.Y., Zhu, L.Y., Fang, J., Xia, B.Z., Wang, Z., Lu, K., Ma, W., Wei, Z.X., 2016. Fluorination-enabled optimal morphology leads to over 11% efficiency for inverted small-molecule organic solar cells. Nat. Commun. 7, 13740. Duan, X.W., Xiao, M.J., Chen, J.H., Wang, X.D., Peng, W.H., Duan, L.R., Tan, H., Lei, G.T., Yang, R.Q., Zhu, W.G., 2015. Improving photovoltaic performance of the linear A-ArA-type small molecules with diketopyrropyrrole arms by tuning the linkage position of the anthracene core. ACS Appl. Mater. Interfaces 7 (33), 18292–18299. Fan, Q.P., Liu, Y., Yang, P.G., Su, W.Y., Xiao, M.J., Chen, J.H., Li, M., Wang, X.D., Wang, Y.F., Tan, H., Yang, R.Q., Zhu, W.G., 2015. Benzodithiophene-based two-dimensional polymers with extended conjugated thienyltriphenylamine substituents for high-efficiency polymer solar cells. Org. Electron. 23, 124–132. Fan, Q.P., Liu, Y., Jiang, H.G., Su, W.Y., Duan, L.R., Tan, H., Li, Y.Y., Deng, J.Y., Yang, R.Q., Zhu, W.G., 2016. Fluorination as an effective tool to increase the photovoltaic performance of indacenodithiophene-alt-quinoxaline based wide-bandgap copolymers. Org. Electron. 33, 128–134. Fan, Q.P., Su, W.Y., Meng, X.Y., Guo, X., Li, G.D., Ma, W., Zhang, M.J., Li, Y.F., 2017a. High-performance non-Fullerene polymer solar cells based on fluorine substituted wide bandgap copolymers without extra treatments. Solar RRL 1 (5), 1700020. Fan, Q.P., Su, W.Y., Guo, X., Zhang, X., Xu, Z., Guo, B., Jiang, L., Zhang, M.J., Li, Y.F., 2017b. A 1,1′-vinylene-fused indacenodithiophene-based low bandgap polymer for

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