Achieving Balanced Charge Transport and Favorable Blend

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Achieving Balanced Charge Transport and Favorable Blend Morphology in Non-Fullerene Solar Cells via Acceptor End Group Modification Minghui Hao, Tao Liu, Yiqun Xiao, Lik-Kuen Ma, Guangye Zhang, Cheng Zhong, Zhanxiang Chen, Zhenghui Luo, Xinhui Lu, He Yan, Lei Wang, and Chuluo Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05327 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Chemistry of Materials

Achieving Balanced Charge Transport and Favorable Blend Morphology in Non-Fullerene Solar Cells via Acceptor End Group Modification Minghui Hao,†,‡,# Tao Liu,*,Δ,†,# Yiqun Xiao,|| Lik-Kuen Ma,Δ Guangye Zhang,Δ Cheng Zhong,§ Zhanxiang Chen,§ Zhenghui Luo,§ Xinhui Lu,|| He Yan,*,Δ Lei Wang,*,† and Chuluo Yang*,†,§ †Shenzhen

Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China. ‡Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China ΔDepartment of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration & Reconstruction, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ||Department of Physics, Chinese University of Hong Kong, New Territories, Hong Kong, China §Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, China ABSTRACT: Generally, the electron-withdrawing substitution on the end-capping group of the acceptor-donor-acceptor type small-molecule acceptor (SMA) narrows the optical bandgap and that the electron-donating group lifts the lowest unoccupied molecular orbital (LUMO) energy level of non-fullerene SMA, which increase the short-circuit current density (JSC) and open circuit voltage (VOC) of the organic solar cells (OSCs), respectively, however their synergistic effect on the properties of SMA has remained elusive. Here, we first report a new end-capping group (EG), namely, CFDCI, that concurrently possesses electron-withdrawing fluorine substitute and electron-donating methyl group. A prototype SMA (namely ITCF) based on CFDCI and its two control counterparts were prepared to fully understand the structure-property relationship that the new EG exerts on the resultant SMA. The ITCF demonstrated a moderately crystalline morphology in pristine film and more balanced charge transport properties as well as a reduced amount of bimolecular recombination in blend film in comparison with its counterparts. The ITCF-based devices demonstrated a high power conversion efficiency (PCE) of 13.25% with an outstanding fill factor (FF) of 78.8%, which significantly outperformed their counterparts. Our study provides an important strategy to judiciously tune the properties of the SMAs for improving the performance of the OSCs.

INTRODUCTION Organic solar cells (OSCs) have attracted much attention due to their virtues of low cost, light-weight, flexibility and facile solution processability.1-3 Over the past two decades, fullerene derivatives have been the dominant choice for electron acceptors,4 leading to OSCs with power conversion efficiencies (PCEs) of ca. 11%.5-7 However, the intrinsic drawbacks of fullerene materials, such as the high-cost production, limited absorption and poor chemical and electronic adjustability, have impeded the further development of OSCs.8 To overcome these problems, a range of non-fullerene small-molecule acceptors (SMAs) have been developed and achieved great success.9-16 In this respect, SMAs with a planar acceptordonor-acceptor (A-D-A) configuration have been the intriguing representative which exhibited PCEs of over

13% in OSCs with appropriate donor materials and systematic device optimization.17-29 Typically, A-D-A-type SMAs contain two parts: a laddertype donor core with outstretched side chains19-23, 30-44 and electron-withdrawing end-capping groups (EGs),45-52 wherein the EGs can facilitate the tuning of optical bandgaps, energy levels and crystallization behavior of the molecules. Thus, the modification of EGs to construct electron acceptors in view of further increasing the PCEs is of great importance. Recently, electron-withdrawing functionalities, such as fluorine atom and chlorine atom,18, 26, 31, 41, 53-56 have been introduced onto the classic EG, i.e., 1,1-dicyanomethylene-3-indanone (DCI, Scheme 1), to narrow the optical bandgaps of the electron acceptors via enhancing the intramolecular charge transfer (ICT) effect, which lead to the increase of the short-circuit current density (JSC). Moreover, the resulting electron acceptors

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showed improved charge transport due to the noncovalent bonding.31 However, such acceptors have a reduced optical bandgap generally due to the downshifted lowest unoccupied molecular orbital (LUMO) levels.31, 53-55 As a consequence, OSCs based on such acceptors suffered from a loss of the open circuit voltage (VOC), which in turn may be detrimental to the PCE improvement. Besides the introduction of electron-withdrawing groups, electrondonating groups were also introduced onto the DCI moiety.57 It has been demonstrated that the electrondonating groups attached on the EGs can elevate the LUMO levels of the acceptors, leading to the increase of VOCs in OSC devices.46, 57, 58 Moreover, such groups can induce higher phase purities and more ordered donor aggregates, and thus are beneficial for charge transport.46, 57 However, electron acceptors modified by EGs with electron-donating groups exhibited blue-shifted absorption spectra,57, 58 which is theoretically harmful to the JSC. Note that the previous modification of the DCI is through the introduction of electron-withdrawing or electron-donating groups individually. However, how the electronwithdrawing and the electron-donating groups concurrently attached onto the EGs affect the properties of the resultant electron acceptors has remained non-existent and unclear. One possible reason is the synthetic inconvenience of such EGs, which, along with other possibilities, needs to be further explored. Herein, we report the design and synthesis of a novel EG, namely, 5-methyl-6-fluoro-3-dicycanovinylindan-1-one (as well as its isomer 5-fluoro-6-methyl-3-dicycanovinylindan1-one, abbreviated as CFDCI, Scheme 1), that concurrently possesses the electron-withdrawing and the electrondonating groups (fluorine atom and methyl group, respectively) to study how the combination of electronwithdrawing and electron-donating groups affect the properties of the resulting molecules and the performances of the corresponding OSC devices. A prototype SMA ITCF (Scheme 2) based on CFDCI was synthesized with the electron donor unit indacenodithieno[3,2-b]thiophene (IDTT) as the central core. Additionally, we prepared its counterparts—IT-DM and IT-4F—as the control molecules, which were synthesized by Knoevenagel condensation reaction between IDTT and DCI modified with two methyl groups or two fluorine atoms (DCI-DM and EG-2F, respectively, Scheme 1) according to the literature, to explore the influences of the newly established EG on the resultant electron acceptor. ITCF shows an absorption edge and a LUMO level between those of IT-DM and IT-4F. Thus, the ITCF-based device shows a VOC of 0.91 V and a JSC of 18.48 mA cm−2, which are between those of the IT-DM and IT-4F devices. Importantly, due to the balanced charge transport as well as the favorable morphology of ITCF, which are induced by the EG moiety, the ITCF-based device demonstrated a fill factor (FF) as high as 78.8%, which is one of the highest FFs for OSCs.27, 29, 38, 39, 48, 59 More importantly, the ITCF-based OSC device shows the best PCE of 13.25%, which is clearly higher than the IT-DM (12.05%) and the IT-4F-based (11.66%) devices. Our study demonstrates the strategy that concurrently introducing the electron-withdrawing and the electrondonating groups to delicately tune the properties of the

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electron acceptors is an effective way to boost the PCE of the OSCs. CN O

CN

CN

CN

CN

CN O

O

CN

O

CN

R1 = methyl , R2 = F or

H 3C

DCI

CH3

DCI-DM

F

F

EG-2F

R1

R2

R1 = F , R2 = methyl

CFDCI

Scheme 1. Structures of the EGs RESULTS AND DISCUSSION Material Synthesis. The synthetic routes of CFDCI and the prototype electron acceptor ITCF are shown in Scheme 3. The nitration of 4-methylphthalic anhydride accompanied with a hydrolysis afforded acid 1, which then underwent an esterification to give compound 2 in 43% yield. Nitro compound 2 was reduced to the amine derivative 3 in 95% yield. The fluorination of compound 3 via Sandmeyer reaction produced compound 4 in 79% yield. The phthalic esters of compound 4 were converted to 1,3-indandiones by Claisen condensation reaction, affording compound 5 in 39% yield. The EG CFDCI was obtained by Knoevenagel condensation of compound 5 with malononitrile in 76% yield. The target acceptor ITCF was prepared with a yield of 92% by the Knoevenagel condensation of CFDCI and the dialdehyde compound IDTT-CHO. Note that due to the asymmetric structure and the similar reactivity of the two keto-carbonyls of the precursor, the corresponding product CFDCI is composed of two isomers, with molar ratio of one to another of 1.7 according to the 1H NMR spectra in the Supporting Information (Figure S26). Hence, the ITCF is a mixture comprised of three isomers, which cannot be separated due to their similar chemical structures. The control acceptors, IT-DM and IT-4F, were prepared according to the literature methods.18, 57 The chemical structure of ITCF was fully characterized using 1H, 19F and 13C NMR spectroscopy, and high-resolution mass spectrometry. At room temperature, ITCF is readily soluble in common organic solvents, such as chloroform, chlorobenzene, and ortho-dichlorobenzene. The thermal stability of ITCF was determined by thermogravimetric analysis (TGA). The decomposition temperature of ITCF is 368 oC with 5% weight loss (Figure S1), indicating its good thermal stability. DFT Calculation. Density functional theory (DFT) calculations at the B3LYP/def2-SVP level with Grimme’s D3 dispersion correction were performed to obtain the optimized geometries of the EGs and acceptors. Then the orbital levels were obtained at the -tuned LCPBE*/def2-SVP level60 using Gaussian09 program. Regarding the EGs (Figure S2), the calculated LUMO and highest occupied molecular orbital (HOMO) levels of CFDCI are between


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H 3C

CH3

S

O S

NC

NC CN

S

CN

O

S

IT-DM

C6H13

C6H13

F CH3

H 3C

CH3

F

S CN

S

O S

NC

NC

IT-4F

C6H13

C6H13

CN O

S

C6H13

F

CH3

ITCF

O

S

NC S

C6H13

CN

S

CN

S

O

C6H13

C6H13

C6H13

C6H13

NC F

transport.61 The HOMO of ITCF is mainly distributed along the conjugated central core, while the LUMO delocalizes over the entire molecule (Figure S4). This is in line with the results obtained for IT-DM and IT-4F. The calculated LUMO energy level of ITCF (−2.61 eV, −2.62 eV, and −2.63 eV for the three isomers, respectively) is between that of IT-DM (LUMO: −2.50 eV) and IT-4F (LUMO: −2.75 eV). This result could be attributed to the electron-donating nature of the methyl unit that lifts the LUMO level and the electron-withdrawing property of the fluorine atom that lowers the LUMO level when attached onto the EGs.31, 57 Indeed, methyl and the fluorine atom show different electron distributions according to the electrostatic potential

C6H13

C6H13

F F

Scheme 2. Molecular Structures of the Acceptor Molecules (ITCF is represented by one of its three isomers) those of DCI-DM and EG-2F. ITCF possesses a highly planar backbone (Figure S3), which would be beneficial to intermolecular π-π interactions, facilitating charge

O

O

O

O O

HO

HNO3, H2SO4

OH

O

SOCl2, MeOH

110 oC

O O

CH3

O 2N

two steps, 43%

1

O

HBF4, NaNO2

O NaH, ethyl acetate

O

CH3

4

OHC

S S

CHO

S

+

CFDCI

NC

CHCl3, 65 C

C6H13

F

R2

CH3

R1 = methyl , R2 = F or R1 = F , R2 = methyl

R2

CFDCI

C6H13

C6H13

S S

CN

R1

76%

NC CN

S O

S

92% C6H13

IDTT-CHO

CN

ethanol, 25 C

O NC

C6H13

O

o

pyridine o

CN AcOK

5

R1

CH3

CN

O

39%

C6H13

S

O

3

HCl/H2O 80 C

100 C

toluene, 100 C

C6H13

H 2N

95%

o

o

o

79%

CH3

2

O O

F

O O

O

HCl, Zn powder ethyl acetate, 25 oC

reflux O 2N

CH3

ice water

O

C6H13

R4

R3

R1 = R3 = methyl , R2 = R4 = F or R1 = R3 = F , R2 = R4 = methyl or R1 = R4 = methyl , R2 = R3 = F

ITCF

Scheme 3. Synthetic Route of CFDCI and ITCF map (Figure S5). In addition, to calculate the dimer binding energies, dimer structures with intermolecular π-π stacking formed between the terminal acceptor units were built based on studies of molecular dynamic and crystal structure.62, 63 The geometries were then optimized at BLYP/def2-SVP level with the resolution-of-identity (RI) approximation64 using ORCA 4.0 program65 (Figure S6). BSSE correction were considered by Grimme’s gCP method.66 The dispersion items were conducted by Grimme's D3 version with BJ damping function.67 The ITCF dimer has a binding energy of −37.36 kcal mol−1, which is close to that of the IT-DM dimer (−37.81 kcal mol−1), and larger than that of the IT-4F dimer (−35.34 kcal mol−1). The binding energy calculations indicated that ITCF and IT-DM have a stronger tendency for intermolecular packing than IT-4F. Optical and Electrochemical Properties. J71 (Scheme S2) was selected as the donor polymer to pair with the SMAs for assessing their photovoltaic performances. As shown in Figure 1b, the absorption spectra of ITCF, IT-DM and IT-4F are spectrally complementary to that of J71. The absorption of the blend films cover the wavelength range

of ~ 400−800 nm (Figure S7). Compared to the solution state absorption, the absorption spectra of the three SMAs in the solid state displayed a redshift due to aggregation. Interestingly, the absorption profiles of ITCF in both solution and film are essentially identical to those of ITDM and IT-4F yet with the maximum absorption peaks and absorption onsets between those of IT-DM and IT-4F. The calculated optical bandgaps are 1.63, 1.57, and 1.54 eV for IT-DM, ITCF, and IT-4F (Table 1), respectively. This experimental result implies that the methyl group blueshifts the absorption while fluorine atom redshifts the absorption, which is in agreement with literature results.31, 57 Notably, when one of the methyl groups of the EG in ITDM is replaced by a fluorine atom, the absorption of the resultant molecule (i.e., ITCF) exhibits a relatively large redshift (31 nm). When the second one was further replaced by another fluorine atom, the resulting molecule (IT-4F) demonstrates a further redshift by only 11 nm. The max molar extinction coefficient of ITCF (Figure 1a, Table1), which is 1.93 × 105 M−1 cm−1, is close to that of ITDM (1.82 × 105 M−1 cm−1) and IT-4F (2.05 × 105 M−1 cm−1).


Chemistry of Materials To elucidate the combined effects of the methyl and fluorine atom on the electrochemical properties of ITCF, its electronic energy levels were measured by electrochemical cyclic voltammetry (CV) and compared with those of IT-DM and IT-4F (Figure 1c, Table 1). The three acceptors exhibited irreversible oxidation and reduction waves in the CV measurements. Based on the onset reduction potentials in the CV measurements, the LUMO levels of IT-DM, ITCF, and IT-4F were estimated to be −3.82, −3.95, and −4.06 eV, respectively. The energy levels of 2.0x105

(b)

IT-DM ITCF IT-4F

1.5x105

Normalized Absorption

 (M-1 cm-1)

(a)

1.0x105 5.0x104 0.0

400

500

600

700

J71 IT-DM ITCF IT-4F

1.0 0.8 0.6 0.4 0.2 0.0

800

400

500

Wavelength (nm)

0.0

IT-DM ITCF IT-4F Fc/Fc+

-2.0x10-4

Energy Levels (eV)

(d) -3.0

1.0x10-4

-1.0x10-4

0.0

0.5

700

800

-3.24

-3.5

-3.82

-4.0 -4.5

IT-DM

-5.0

1.0

Potential (V vs. Ag/AgCl)

1.5

-6.0

-3.95

-4.06

J71

-5.5

-3.0x10-4

-1.5 -1.0 -0.5

600

Wavelength (nm)

(c)

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-5.40 0

2

-5.58 4

ITCF

IT-4F

-5.59

6 8 10 molecules

-5.63 12

14

16

Figure 1. UV-vis spectra of (a) acceptors in chloroform solution (10−5 M) and (b) acceptors and J71 in thin film. (c) cyclic voltammograms of IT-DM, ITCF, and IT-4F at a scan rate of 100 mV s−1. (d) energy diagram relative to the vacuum level. J71 (HOMO = −5.40 eV; LUMO = −3.24 eV) are compatible with those of the acceptors (Figure 1d). The LUMO level of ITCF showed a decline of ca. 0.13 eV compared to that of IT-DM, whereas a rise of ca. 0.11 eV compared to that of IT-4F. Methyl, the electron-donating unit, lifts the LUMO levels while fluorine, the electron-accepting unit, lowers the LUMO levels. The trends in the LUMO levels are in good agreement with the results acquired from the DFT calculations and the results previously reported.31, 41, 53, 57 These results imply that the LUMO level of ITCF is the consequence of synergistic effects of the methyl and fluorine atom. The HOMO level of ITCF was estimated to be −5.59 eV, which is comparable to that of IT-DM (−5.58 eV) and IT-4F (−5.63 eV), indicating that the EGs have little effect on the HOMO levels of the electron acceptor molecules relative to their roles in tuning the LUMO levels, which is consistent with literature results.31, 57 The deep HOMO levels would be a positive factor for obtaining a large driving force for exciton dissociation.68 Morphology and Charge Transport of Pure Films. The surface morphology of the pristine films based on the

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three acceptors were measured by atomic force microscopy (AFM) (Figure S8). From the height images, the pristine IT-DM film exhibited a relatively rough top surface with a root-mean-square (RMS) roughness of 1.43 nm. The ITCF film demonstrated a smooth surface, with an RMS roughness of 0.731 nm. The IT-4F film showed a smoother surface, with an RMS roughness of 0.409 nm. These results imply that the crystallization propensity of ITCF is between those of IT-DM and IT-4F. This trend is also reflected in the phase images: with the reducing number of methyl or the increasing number of fluorine atom in the EG of the acceptor, i.e., from IT-DM to ITCF then to IT-4F, the films evolved gradually from the granular texture to a fine-grained texture. In addition, the J71 film exhibited a similar top surface to that of ITCF with an RMS roughness of 0.615 nm. The packing and orientation of acceptors and J71 were studied by grazing incidence wide-angle X-ray scattering (GIWAXS) (Figure S9).69 The detailed structure parameters are summarized in Table S1. J71 shows distinct (100) diffraction peaks in both the in-plane (IP) direction and the out-of-plane (OOP) direction while the π-π stacking (010) shows at 1.62 Å−1 in the OOP direction, corresponding to a preferential face-on orientation. The ITCF neat film shows a predominant face-on orientation with the π-π stacking diffraction peak position and d-spacing close to those of IT-DM. It is known that IT-DM has a strong aggregation tendency due to its low solubility,57 therefore, the much smaller coherence length of ITCF neat film (5.59 Å−1) than that of IT-DM (20.15 Å−1) and the decreased π-π stacking diffraction peak intensity of ITCF relative to that of IT-DM indicated that the excessive aggregation of IT-DM was substantially alleviated for ITCF. ITCF achieves a slightly higher electron mobility (μe) of 7.44 × 10−4 cm2 V−1 s−1 (measured using the space-charge-limited current (SCLC) method)70 than IT-DM (6.97 × 10−4 cm2 V−1 s−1, Figure S10 and Table S2). This result suggests that the combination of methyl and fluorine atom in the EG endows ITCF with morphology which is more favorable for charge transport. The IT-4F neat film exhibits an amorphous feature according to its feature from the GIWAXS results (Figure S9), which is in good agreement with the literature result.24 Note that IT-4F shows the highest electron mobility among the three acceptors (8.07 × 10−4 cm2 V−1 s−1), which may be associate with the noncovalent F∙∙∙S and F∙∙∙H bonds that facilitate charge transport.7 Photovoltaic Properties and Recombination Mechanisms. Photoluminescence (PL) spectra of the pristine donor and acceptor films and the blend films were measured to investigate the exciton dissociation efficiency. As shown in Figure S11, compared to the pure films, all blend films show significant PL quenching, suggesting that the exciton dissociation in the blend films are efficient. To evaluate the photovoltaic properties of ITCF, we fabricated OSC devices based on ITCF as acceptor and J71 as donor with a conventional device architecture of ITO (indium tin

Table 1. Optical and Electrochemical Data of the IT-DM, ITCF, and IT-4F acceptor λmaxsol (nm) λmaxfilm (nm) λonsetfilm (nm) εmax (M−1 cm−1)a Egopt (eV)b EHOMOCV (eV)c IT-DM 669 694 759 1.82 × 105 1.63 −5.58


ELUMOCV (eV)d −3.82

Egcv (eV) 1.76

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ITCF 683 718 792 1.93 × 105 1.57 −5.59 −3.95 1.64 IT-4F 690 726 803 2.05 × 105 1.54 −5.63 −4.06 1.57 aMolar extinction coefficient at λ b opt film c = 1240/λonset . HOMO energy level estimated from max in solution. Calculated from Eg the onset oxidation potential. dLUMO energy level estimated from the onset reduction potential. using the formula Eloss = Eg − eVOC. The Eloss values of the OSCs are 0.61-0.66 eV, which are relatively small.71 A high JSC of 18.48 mA cm−2 was achieved for the ITCF-based device, which is between those of the IT-DM- and IT-4Fbased devices. This result agrees well with the incident photon-to-current conversion efficiency (IPCE) spectra shown in Figure 2b. The IPCE (Figure S12b) onset of ITCF is between those of IT-DM and IT-4F, which is in accordance with the absorptions of the blend (Figure S7). Additionally, the ITCF device outperforms the IT-DM and IT-4F device in terms of FF. This can be ascribed to its more balanced electron and hole mobility and the favorable morphology of the active blend as discussed below. As a consequence, the optimized ITCF-based device demonstrated the best PCE of 13.25%, which is clearly higher than those of the IT-DM- (12.05%) and IT-4F(11.66%) based devices. In addition, the average PCE obtained from 20 devices for the ITCF device (12.81±0.24%) is clearly higher than that of the IT-DM (11.91±0.15%) and IT-4F (11.42±0.13%) devices. The detailed photovoltaic parameters of the typical 20 devices for ITCF are shown in Figure S13.Notably, the high FF (78.8%) that the ITCF-based device achieved is one of the highest values reported in all OSC devices.

oxide)/PEDOT:PSS (poly(3,4ethylenedioxythiophene):poly(styrenesulfonate))/J71:ITCF/ZrAcac (zirconium acetylacetonate)/Al, where ZrAcac was chosen as the cathode interlayer for lowering the work function of Al. The weight ratio of J71:ITCF is 1:1. The active layers were prepared by spin-coating at 2000 r.p.m. from the J71:ITCF blend solution with a total blend concentration of 16 mg mL−1 in chloroform. For comparison, the IT-DM-/IT-4Fbased devices were fabricated under the same conditions. The devices were optimized by thermal annealing (TA) at 100 oC for 5 minutes. The annealed devices showed improved device performances with the increase of JSCs and FFs as well as a negligible decrease of VOCs when compared with the as-prepared devices (Figure S12a and Table S3). Figure 2a shows the optimized current densityvoltage (J-V) curves of the OSCs, and the corresponding photovoltaic parameters are listed in Table 2. The optimized ITCF-based devices demonstrated a VOC of 0.91 V, which is between those of the IT-DM (1.015 V) and IT-4F (0.803 V) devices. This result is fairly reasonable considering that the VOC is correlated with the differences between the HOMO of the donor and the LUMO of the acceptor. The photon energy loss (Eloss) is calculated

Table 2. Photovoltaic Parameters of OSCs under AM 1.5G Illumination, 100 mW cm−2 Blends VOC (V)a JSC (mA cm−2)a Jcalc. (mA cm−2)b FFa PCE (%)a IT-DM:J71 1.015 (1.014±0.004) 16.74 (16.62±0.14) 16.338 0.709 (0.706±0.004) 12.05 (11.91±0.15) ITCF:J71 0.910 (0.903±0.006) 18.48 (18.27±0.10) 18.101 0.788 (0.777±0.008) 13.25 (12.81±0.24) IT-4F:J71 0.803 (0.801±0.003) 19.58 (19.36±0.18) 19.012 0.742 (0.736±0.004) 11.66 (11.42±0.13) aThe values in parentheses are the average values with standard deviations obtained from 20 devices. bJ SC integrated from the IPCE for the optimized devices. (b)

80

(d)

(c)

0

-8

IPCE (%)

J71:IT-DM J71:ITCF J71:IT-4F

-4

-12

Jph (mA/cm2)

60


40

10


20

-16 -20 0.0

0.2

0.4

0.6

Voltage (V)

0.8

1.0

0 300

400

500

600

700

Wavelength (nm)

800

1

0.1

1

Veff (V)

Current Density (mA/cm2)

(a) 4 Current Density mA/cm2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



10

1

1

10 Light Intensity (mW cm-2)

100

Figure 2. (a) J-V characteristics of the optimized devices; (b) IPCE spectra of the optimized devices under illumination of an AM 1.5 G at 100 mW cm−2; (c) Jph versus Veff characteristics of the optimized devices; and (d) JSC versus light intensity of the optimized devices.



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Figure 3. AFM height and phase images of the optimized blend films based on J71:IT-DM (a, d), J71:ITCF (b, e), and J71:IT4F (c, f). In-plane (dashed lines) and out-of-plane (solid lines) GIWAXS intensity profiles of blend films (g). On the other hand, the higher FF of ITCF with respect to IT-DM and IT-4F, suggests that our methodology—fine tuning the EG—is also beneficial for reducing the exciton recombination of the blend device. This is further supported by the analysis of photocurrent density (Jph) versus effective voltage (Veff) (Figure 2c), where Jph=JL − JD (JL and JD are the current density under illumination and in the dark, respectively) and Veff = V0 − Va (V0 is the voltage at Jph = 0 and Va is the applied bias). The summary of results is presented in Table S4 (Supporting Information). The ratios of Jph/Jph,sat are used to assess the overall efficiency of exciton dissociation and charge collection. The exciton dissociation and charge collection probabilities of the optimized devices were enhanced compared with those of the as-cast devices (Table S2). Under optimized conditions, both the exciton dissociation and charge collection probabilities of ITCF (97.8% and 91.1%) are higher than those of IT-DM (95.0% and 80.5%) and IT-4F (96.3% and 88.1%) (Table S2). The effective exciton dissociation and charge collection efficiencies in ITCF-based devices partially contribute to its high FF. To gain deeper insight into the relationship between light absorption and photocurrent generation in the devices, the maximum exciton generation rate (Gmax) was calculated. The Gmax could be described follow the equation, Jsat = qLGmax, where q is the elementary charge, L is the thickness of the active layer, and Jsat stands for the saturated photocurrent density.72 The Gmaxs of the optimized devices were evaluated to be 1.10 × 1028 m−3 s−1 for IT-DM, 1.18 × 1028 m−3 s−1 for ITCF, and 1.27 × 1028 m−3 s−1 for IT-4F, respectively. The Gmax of ITCF is between that of IT-DM and IT-4F, which can be explained by that the absorption onset of the ITCF blend film is between that of the IT-DM and IT-4F blend films.73 In addition, we also measured the JSC of each optimized device under different light intensities (Plight) to study the charge recombination behavior (Figure 2d). The relationship between JSC and Plight can be described as JSC ∝ Pαlight,74, 75 where the power-law exponential factor α implies the extent of bimolecular recombination. The α of the ITCF-based device is 0.98, while the IT-DM- and IT-4F-

based devices show relatively lower α values of 0.94 and 0.95, respectively. This suggests that less bimolecular recombination occurred in the ITCF device, supporting its high FF.76 The charge transport behaviors of the active layers were investigated by the SCLC method (Figure S14). The holeand electron-only devices were fabricated with the structures of ITO/MoOX/active layer/MoOX/Al and ITO/ZnO/active layer/ZrAcac/Al, respectively. The detailed parameters are summarized in Table S2. The charge transport properties of the annealed blend films were improved compared with the as-prepared blend films, as evidenced by the increased hole and electron mobilities and the lowered μh/μes. This result is one of the reasons that the annealed devices demonstrated better performances than those of the as-prepared devices. As for the annealed devices, a hole mobility of 8.38 × 10−4 cm2 V−1 s−1 was achieved for the ITCF-based device, which is between those of the IT-DM- (8.07 × 10−4 cm2 V−1 s−1) and IT-4F- (8.81 × 10−4 cm2 V−1 s−1) based device, whereas the electron mobility (5.32 × 10−4 cm2 V−1 s−1) for the ITCFbased device is higher than those of the IT-DM- (4.13 × 10−4 cm2 V−1 s−1) and IT-4F- (4.89 × 10−4 cm2 V−1 s−1) based device. Notably, the ITCF-based device demonstrated a more balanced charge transport with a μh/μe of 1.58, which is lower than those of the IT-DM- (1.95) and IT-4F- (1.80) based device, accounting for its high FF. Morphology of Blend Films. Figure 3a-f and Figure S15 shows the surface morphologies of the blend films. The morphologies of the as-cast blend films (Figure S15) are similar to the morphologies of the thermally annealed blend films, with an increase in the RMS roughness for the latter. The slight increase in the RMS implies that the crystallinity of the film is enhanced, which is favorable for charge transport. This result is consistent with the enhanced JSC and FF in the corresponding OSC device. The ITCF blend film has a nice surface morphology with fine nanoscale phase separation, and the RMS of ITCF blend film is roughness between that of IT-DM and IT-4F blend film, whose surface morphology is rougher or smoother than that of the ITCF blend film.


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The molecular packing and orientation of the blend films were investigated by GIWAXS (Figure 3g and Figure S16).69 The lamellar packing peaks of all blend films are at qr ~ 0.30 Å−1, originated from the J71 component (Figure S9). The as-cast blend films showed weak (010) π-π diffraction peaks at qz ~ 1.70 Å−1, which were significantly strengthened after being annealed at 100 oC, indicative of the predominant face-on oriented π-π stacking in the optimized blends. These characteristics support that the annealed blend films achieved higher charge carrier mobilities than the as-cast blend films did. By Scherrer equation,77 the π-π stacking coherence length of annealed ITCF blend film was estimated to be 22.86 Å, larger than that of the annealed IT-DM (17.66 Å) and IT-4F (17.52 Å) blend films, in agreement with the observed higher charge carrier mobility and FF. CONCLUSION In summary, we developed a novel EG, CFDCI, which possesses both electron-withdrawing and electrondonating groups (fluorine atom and methyl, respectively) to study how the combination of electron-withdrawing and electron-donating groups affect the properties of the resulting molecules and the performances of the corresponding OSC devices. We find that the prototype molecule ITCF, based on CFDCI, was endowed with finetuned the optical bandgap, energy levels, and the crystallinity via fluorine atom and methyl substitution onto the end-capping group, compared with the control molecules possessing fluorine atom or methyl individually. ITCF:J71 active layer exhibited more balanced charge transport and favorable morphology, and lower bimolecular recombination than its counterparts, and thus, ITCF-based device achieved a best PCE of 13.25%, clearly higher than its counterparts, and an outstanding FF of 78.8% that is among the top values of all OSC devices. The case in our work indicates that concurrently introducing fluorine atom and methyl onto the end-capping group to tune the properties of SMAs could be an effective way to boost the PCE of OSCs. Furthermore, the flexibility of the strategy—tune the properties of the acceptors through the continuous adjustment of the electron-withdrawing or/and electron-donating groups—allows us to improve the performance of OSCs.

ASSOCIATED CONTENT Supporting Information Detailed experimental procedures including synthesis, characterization, and device fabrication, NMR spectra of compounds, DFT-based theoretical calculation results, UV-vis and PL spectra, SCLC mobilities, AFM images, GIWAXS linecuts and additional characterization data. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *C.Y.: [email protected]. *L.W.: [email protected]. *H.Y.: [email protected].

*T.L.: [email protected].

Author Contributions #M.H.

and T.L. contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This study was financially supported by the National Natural Science Foundation of China (No.21572171, No.51773118, and No.51873160) and the Research Grant Council of Hong Kong (General Research Fund No.14314216 and Theme-based Research Scheme No. T23-407/13-N).

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Achieving Balanced Charge Transport and Favorable Blend

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