Nonfullerene Acceptors with Enhanced Solubility and Ordered

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Non-fullerene Acceptors with Enhanced Solubility and Ordered Packing for High-Efficiency Polymer Solar Cells Yahui Liu, Miao Li, Xiaobo Zhou, Qingqing Jia, Shiyu Feng, Pengcheng Jiang, Xinjun Xu, Wei Ma, Haibei Li, and Zhishan Bo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00928 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Energy Letters

Non-fullerene Acceptors with Enhanced Solubility and Ordered Packing for High-Efficiency Polymer Solar Cells Yahui Liu,

a, †

Miao Li, a, † Xiaobo Zhou,

b

Qing-Qing Jia,

c

Shiyu Feng,

a

Pengcheng Jiang,a

Xinjun Xu, a,* Wei Ma,b,* Hai-Bei Li, c Zhishan Bo a,* a

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry,

Beijing Normal University, Beijing 100875, China. E-mail: [email protected]; [email protected] b

State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an

710049, China. E-mail: [email protected] c

School of Ocean, Shandong University, Weihai 264209, China. E-mail: [email protected]

ABSTRACT The performance of polymer solar cells (PSCs) is commonly improved using additives or annealing treatment. However, these processes are accompanied with disadvantages including poor reproducibility and stability. Herein, a molecular design strategy was proposed to obtain additive- and annealing-free PSCs. IDTOT2F containing two alkoxyl side chains at the central unit of the non-fullerene acceptor IDTT2F was developed. This molecular design results in excellent solubility in solutions, ordered molecular packing in films, slightly elevated energy levels, and higher film absorption coefficient. Compared with its counterpart IDTT2F, its

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improved solubility provides an active layer with better morphology, its ordered molecular packing enhances the charge mobility in blend films, and its slightly elevated energy level furnishes higher open-circuit voltage of devices. As a result, IDTOT2F-based devices display a maximum power conversion efficiency of 12.79%, which is one of the highest values reported for a PSC fabricated without any extra treatment.

TOC GRAPHICS

In recent years, polymer solar cells (PSCs) containing non-fullerene acceptors (NFAs), especially fused ring electron acceptors (FREAs), have been attracting increasing attention. NFAs possess many advantages such as adjustable energy levels, facile synthesis, tunable absorption, and high molar extinction coefficients.1-4 After several years of development, the power conversion efficiency (PCE) of single-junction PSCs based on NFAs has reached over 12%.5-8 Very recently, a PCE exceeding 13% was realized for single-junction devices.9-11 The efficiency of non-fullerene PSCs can be further improved by careful molecular design.12-18 The effects of the side chains, bridging units, end groups, and core units of NFAs on the photovoltaic


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performance of their devices has been widely investigated.19-25 Almost all high-performance PSCs reported to date have used either a solvent additive or annealing treatment.23, 26, 27 The additive or annealing process adjusted the nanomorphology of the active layer, substantially increasing the efficiency of these PSCs.28-30 However, there are also disadvantages associated with these processes. Because the additive usually possesses a high boiling point, the residual additive in the device can induce undesirable morphological changes, deteriorate the reproducibility of device performance, and accelerate photo-oxidative degradation of the active layer by acting as a radical initiator.31-33 In the case of annealing treatment, the high temperature or solvent vapor used may induce uncontrollable large-scale phase separation in the multicomponent system,34-36 increase the likelihood of oxidation or degradation of the active layer,37 and be incompatible with the flexible substrates used in roll-to-roll manufacturing. Therefore, the use of an additive or annealing in device fabrication is unfavorable for practical applications. It is highly desirable to produce additive- and annealing-free PSCs that display high performance. It is well known that in high-efficiency PSCs, the donor polymers need to form nanofibers before the precipitation of acceptors during the drying of the active layer.38-40 Such a process guarantees the domain size and phase purity of each component. If a small molecule acceptor exhibits insufficient solubility in a processing solvent, it can precipitate from the solution first or together with the polymer donor during solvent evaporation, leading to overlarge domain size and poor phase purity in the active layer.41-43 Usually, a solvent additive or annealing treatment is required to overcome this predicament. To avoid the shortcomings introduced by using an additive or annealing treatment, NFAs suitable to fabricate additive- and annealing-free highefficiency PSCs are required.


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Generally, the introduction of a long alkyl side chain can effectively improve the solubility of small molecule acceptors and thus postpone their precipitation during the drying of the active layer. However, the introduction of alkyl chains usually brings other problems such as impeding the ordered packing of molecules, which decreases the charge transport ability, lowers carrier mobility, and deteriorates device performance.15, 22, 44, 45 Here we propose a new strategy to cope with this dilemma by designing and synthesizing a highly soluble FREA. The attachment of two alkoxyl substituents at the 3,8-positions of the 4,9-dihydro-s-indaceno1,2-b:5,6-b'dithiophene unit markedly enhances the solubility of acceptor molecules without preventing the molecular backbone from forming a rigid planar ladder structure through intramolecular S–O interactions. As a result, the fabricated NFAs can be used to produce high-performance additive- and annealing-free PSCs. Two NFAs (denoted as IDTOT2F and IDTT2F) with different substituents on the central core unit were developed in this work. IDTOT2F bears two hexyloxy groups at its central core, whereas IDTT2F does not. Compared with IDTT2F, IDTOT2F exhibits much better solubility in the processing organic solvent and more ordered molecular packing in films. Accordingly, the power conversion efficiency (PCE) of devices containing these materials was improved from 8.85% for that with IDTT2F to 12.79% for the PSC with IDTOT2F. In addition, compared with the reported analogue IEICO-4F, which possesses an alkoxyl substituent at the thiophene bridge unit,46-48 PSCs with IDTOT2F exhibit much better device performance. The PCE of 12.79% obtained for IDTOT2F-based devices is one of the highest values reported for additive- and annealing-free PSCs.6, 10, 49 The synthetic route used to obtain IDTT2F and IDTOT2F is shown in Scheme 1. Intermediate aldehyde 2 was synthesized in a high yield of 91% by Stille coupling of compound


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1 and 5-bromothiophene-2-carbaldehyde. Compound 3 and 4 were prepared according to reported procedures.50,

51

Suzuki cross-coupling of 3 and 4 was carried out in a mixture of

aqueous NaHCO3 and tetrahydrofuran (THF) with Pd(PPh3)4 as the catalyst precursor to afford 5 with a yield of 73%. Freshly prepared (4-hexylphenyl)lithium was added to a solution of 5 in THF, and then the formed alcohol was converted to compound 6 with a total yield of 34% by treatment with BF3•Et2O. Bromination of 6 with N-bromosuccinimide (NBS) in chloroform at room temperature afforded compound 7 with a yield of 80%. Similarly, compound 8 was prepared by Suzuki coupling of 7 and (5-formylthiophen-2-yl)boronic acid in a mixture of THF and aqueous NaHCO3 with Pd(PPh3)4 as the catalyst. Finally, IDTT2F and IDTOT2F were synthesized in yields of about 60% by Knoevenagel condensation of the corresponding aldehyde and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile. Scheme 1. Synthesis of IDTT2F and IDTOT2F. C6H13

C6H13

C6H13

C6H13

C6H13

F F

Sn

S S

Sn

OHC

S

CHO

S

i

NC

S

S

S

C6H13

O NC

ii

S

S

CN

S

CN

O C6H13

C6H13

C6H13

C6H13

C6H13

F F

C6H13

2

1

IDTT2F

C6H13 COOEt Br

O B

COOEt S

C6H13O

S

O

Br

S EtOOC

iii

OC6H13

COOEt

4

3

C6H13 OC6H13

S OC6H13

S

iv

C6H13O

5

C6H13

C6H13

6 C6H13

C6H13

C6H13

C6H13

C6H13

F F

Br

v

OC6H13

S S

C6H13O C6H13

C6H13

7

Br

OHC

vi

OC6H13

S

S

S

S

C6H13O C6H13

C6H13

O

OC6H13

S CHO

vii

C6H13

NC

CN

NC

S

S

CN

S

C6H13O C6H13

O C6H13

F F

8 IDTOT2F


5

ACS Energy Letters

Conditions and reagents: i) 5-bromothiophene-2-carbaldehyde, Pd(PPh3)4, toluene, reflux; ii) pyridine, CHCl3, RT; iii) Pd(PPh3)4, NaHCO3, THF/H2O; iv) (1) n-BuLi, 1-bromo-4hexylbenzene, −78 °C, THF; (2) BF3•Et2O, CH2Cl2, RT; v) NBS, CHCl3; vi) (5-formylthiophen2-yl)boronic acid, Pd(PPh3)4, THF/H2O, reflux; vii) 2-(5,6-difluoro-3-oxo-2,3-dihydro-1Hinden-1-ylidene)malononitrile, pyridine, CHCl3, RT.

(a)

(b)

Normalized Absorbance

1.0

Normalized Absorbance

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

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IDTT2F IDTOT2F

0.8 0.6 0.4 0.2 0.0

1.0 IDTT2F IDTOT2F

0.8 0.6 0.4 0.2 0.0

400

500

600

700

800

900

400

Wavelength (nm)

500

600

700

800

900

1000

Wavelength (nm)

Figure 1. UV-vis absorption spectra of IDTT2F (black) and IDTOT2F (red) in (a) dilute chloroform and (b) thin films. The ultraviolet–visible (UV-vis) absorption properties of IDTT2F and IDTOT2F in dilute chloroform solutions and as thin films were investigated; the resulting spectra are shown in Figure 1. In the dilute chloroform solutions, IDTT2F and IDTOT2F exhibited broad absorption bands in the range of 350 to 800 nm with absorption maxima located at 716 and 723 nm, respectively, as presented in Figure 1a. The absorption spectra of IDTT2F and IDTOT2F in thin films were red-shifted and broadened compared with those of the solutions, with absorption maximum located at 752 and 766 nm, respectively. As shown in Figure S1, the corresponding film absorption coefficients of IDTT2F and IDTOT2F were 7.5 × 104 and 9.9 × 104 cm−1, respectively. And the corresponding optical band gaps (Eg) of IDTT2F and IDTOT2F were


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calculated to be 1.46 and 1.44 eV, respectively, according to the equation Eg=1240/λedge. The electronic and optical properties of the materials are summarized in Table 1. The introduction of alkoxyl side chains onto the central unit had little influence on the absorption spectrum of the materials. In comparison, the reported acceptor IEICO-4F with an alkoxyl substituent at the thiophene bridge unit displayed an extremely narrow optical band gap of 1.24 eV. The electrochemical properties of IDTT2F and IDTOT2F were investigated by cyclic voltammetry (CV) measurements. According to the equation EHOMO/LUMO = −e(Eonset,ox/red + 4.71) (eV),52 the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were calculated to be −5.57 and −4.03 eV, respectively, for IDTT2F and −5.54 and −3.94 eV, respectively, for IDTOT2F. The energy diagrams for the materials are shown in Figure S1 and Table S1. The introduction of alkoxyl side chains onto the central unit slightly elevated the energy levels of the acceptor because of their weak electron-donating ability. Table 1. Electronic and optical properties of IDTT2F, IDTOT2F, and previously reported IEICO-4F. λmax (nm)

λmax (nm)

in solution a

as film

IDTT2F

716

IDTOT2F IEICO-4Fd

Acceptors

Eg b (eV)

HOMO c (eV)

LUMO c (eV)

689, 749

1.46

-5.57

-4.03

723

699, 765

1.44

-5.54

-3.94

803

755, 869

1.24

-5.44

-4.19


7


a

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Measured in dilute chloroform solutions. b Calculated from the absorption band edge of films

using the equation: Eg=1240/λedge. c Measured by cyclic voltammetry. d From ref. 46. Density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d) level of theory to investigate the energy levels and chemical geometries of IDTT2F and IDTOT2F with simplified side chains. Meanwhile, the geometry of IEICO-4F with simplified side chains was also optimized to allow direct comparison. As shown in Figure S2 and S3, the backbones of simplified IDTT2F, IDTOT2F, and IEICO-4F are planar with small dihedral angles and the sp3 hybrid 4-alkylphenyl side chains are distributed on both sides of the backbone plane. The energy levels of the three molecules determined via DFT calculations were quite different. IDTT2F and IDTOT2F exhibited a similar HOMO energy level, which was much deeper than that of IEOIC4F. IDTOT2F displayed a slightly higher LUMO energy level than those of IDTT2F and IEICO-4F. This is beneficial to obtain a higher open circuit voltage (Voc) for IDTOT2F-based devices. The calculated band gap values of IDTT2F and IDTOT2F were both about 2.0 eV, which is much larger than that of IEICO-4F (1.73 eV) and consistent with the experimental optical Eg values. Bulk heterojunction PSCs with an inverted device structure of indium tin oxide (ITO)/zinc oxide (ZnO; 30 nm)/active layer (100 nm)/molybdenum oxide (MoO3; 80 Å)/silver (Ag; 100 nm) were then fabricated. The ZnO layer was prepared according to a reported procedure.53 Various factors such as polymer concentration, active layer composition, additives, and spin coating rate were systematically optimized. The optimized weight ratio of PBDB-T to IDTT2F or IDTOT2F in the active layer was 1:1 (w/w), the optimal spin-coating rate was 1500 rpm, and the optimal polymer concentration was 4 mg/mL. The optimization process is described in detail in the Supporting Information. The photovoltaic data for the optimized devices are listed in Table 2,


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and the corresponding current density–voltage (J-V) curves are shown in Figure 2. the IDTT2Fbased devices achieved a maximum PCE of 8.85% with a Voc of 0.81 V, short-circuit current density (Jsc) of 18.51 mA/cm2, and fill factor (FF) of 0.59. In comparison, the IDTOT2F-based devices displayed enhanced overall performance, with a Voc of 0.85 V, Jsc of 20.87 mA/cm2, and FF of 0.72, leading to a maximum PCE of 12.79%. The distribution of device performance (PCEs) based on IDTT2F and IDTOT2F was also shown in Figure S4. It is worth noting that the above results were achieved for devices with spin-coated active layers; no extra treatment such as using additives or annealing was needed. If additives or annealing are used, the device performance will be deteriorated (Table S4 and S5). The details about optimized device parameters were shown in Table S2−S6. Benefiting from the slightly higher LUMO energy level of IDTOT2F, the device with IDTOT2F showed a larger Voc than those of the devices with IDTT2F and IEOIC-4F. The reason for the higher Jsc and FF of the IDTOT2F-based devices compared with those of the IDTT2F-based ones is discussed below.

(a)

(b)

0 -5

80 60

IDTT2F IDTOT2F

-10

EQE (%)

2 Current Density(mA/cm )

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

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-15

40

IDTT2F IDTOT2F

20

-20 0

-25 0.0

0.2

0.4

0.6

Voltage(V)

0.8

300

400

500

600

700

800

Wavelength (nm)


900

9

ACS Energy Letters

(d) 100

(c) 20 IDTT2F IDTOT2F 10

S=

IDTT2F IDTOT2F

9 0.9 S

Jph(mA/cm2)

Jsc (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

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.94 =0

50

Light Intensity (mW/cm2)

100

10

1

0.01

0.1 Veff(V)

1

Figure 2. Solar Cell parameters for IDTT2F and IDTOT2F based devices: (a) current density-voltage curves, (b) external quantum efficiency (EQE) at different excitation wavelengths, (c) dependence of short-circuit current density (Jsc) on the excitation light intensity, and (d) dependence of photocurrent density (Jph) on the effective voltage (Veff). Table 2. Photovoltaic parameters of devices with IDTT2F, IDTOT2F, and previously reported IEICO-4F.

Active layer

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PBDB-T:IDTT2F=1:1

0.81

18.51 (17.66)a

59

8.85 (8.75)b

PBDB-T:IDTOT2F=1:1

0.85

20.87 (19.91)a

72

12.79 (12.57)b

PBDB-T:IEICO-4F=1:1

0.77

20.10

56

8.66c

a

Calculated from EQE curves; b Average PCE of over ten devices; c From ref. 46.

External quantum efficiency (EQE) curves of the PSCs with IDTT2F and IDTOT2F were measured under monochromatic light. As shown in Figure 2, a broad photo-to-current response


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from 300 to 800 nm was observed for both IDTOT2F- and IDTT2F-based devices, and the former displayed higher EQE values than the latter. The film morphology and charge mobility of the active layer may responsible for this effect (vide infra). The Jsc values of IDTT2F- and IDTOT2F- based devices determined from EQE curves were 17.66 and 19.91 mA/cm2, respectively, which were within 5% error of the values obtained from the J-V curves. Photocurrent analysis was performed to better understand the origin of the considerably improved FF of IDTOT2F compared with that of IDTT2F. Figure 2c displays the light intensity (Plight) dependence of Jsc in logarithmic coordinates. The relationship of Jsc ∝ PlightS was found, where S is the exponential factor. The fitted slopes for IDTT2F- and IDTOT2F-based devices were 0.94 and 0.99, respectively. When the slope approaches 1, the generated excitons can be effectively swept out and the bimolecular recombination in devices can be neglected,54-56 which gave the high FF of the IDTOT2F-based devices. To further investigate the charge recombination process in these devices, curves of photocurrent density (Jph) as a function of effective voltage (Veff) were measured. Jph was calculated from Jph = JL – JD, where JL and JD are the current densities under illumination and in the dark, respectively. Veff was estimated by Veff = V0 – V, where V0 is the voltage when JL = JD and V is the applied voltage.57, 58 As shown in Figure 2d, both IDTOT2F- and IDTT2F-based devices reached the saturation current density (Jsat) when Veff approached 1 V, but the device with IDTOT2F showed a slightly higher Jsat than that of the device with IDTT2F, indicating that less geminate and bimolecular recombinations occur in IDTOT2F-based devices than in the IDTT2F ones.59 In addition, the Jph/Jsat ratios under short-circuit conditions were calculated to be 92.2% and 95.3% for the IDTOT2F- and IDTT2F-based devices, respectively. These results indicate that the IDTOT2F-based devices


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display enhanced photovoltaic performance because of their higher exciton dissociation and charge collection efficiencies than those of the IDTT2F-based devices.60, 61 One reason for the high PCE of additive- and annealing-free PSCs containing IDTOT2F lies in the enhancement of solubility after the introduction of alkoxyl side chains onto the core unit. Although their exact solubility was difficult to quantify because of the intense dark color of these NFAs in common device-processing solvents (such as CHCl3, chlorobenzene, dichlorobenzene, and toluene), a qualitative solubility experiment was carried out to directly compare the solubility of the NFAs using a poor solubility solvent (n-hexane). As shown in Figure S5, for the same solid content (0.5 mg/mL) of the NFAs in n-hexane, IDTT2F was slightly soluble, IEICO4F was almost insoluble, and IDTOT2F was highly soluble. The enhanced solubility of IDTOT2F relative to that of IDTT2F can postpone the precipitation process during drying of the active layer; thus, a more uniform film morphology with polymer nanofibrils surrounded by the acceptor phase is expected. To explain the excellent photovoltaic performance of the IDTOT2F-based devices, the morphology of the blend films used as the active layer was investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements; the results are presented in Figure 3. Compared with IDTT2F, IDTOT2F exhibited smoother morphology with a root mean square roughness (RMS) of 1.91 nm and thinner polymer fibers. Because of the relatively low solubility of IDTT2F, it precipitated much faster than IDTOT2F during the drying of the spin-coated films, resulting in poorer morphology with a larger RMS value of 2.25 nm. Resonant soft X-ray scattering (R-SoXS) measurements were performed to reveal the domain size and average domain purity of the blend films (Figure S6).62,

63

The PBDB-

T:IDTOT2F and PBDB-T:IDTT2F blend films exhibited similar domain sizes of about 18 nm.


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Such small domains mean that abundant donor:acceptor interfaces are able to efficiently facilitate exciton separation, which contributes to the high Jsc of 18.51 and 20.87 mA/cm2 obtained for the IDTT2F- and IDTOT2F-based devices, respectively.

Figure 3. AFM images of (a) PBDB-T:IDTT2F and (b) PBDB-T:IDTOT2F films. TEM images of (c) PBDB-T:IDTT2F and (d) PBDB-T:IDTOT2F films.

Figure 4. (a) GIWAXS 2D patterns of IDTT2F, IDTOT2F, PBDB-T:IDTT2F, and PBDBT:IDTOT2F films. (b) Corresponding scattering profiles in the in-plane (black) and out-of-plane (red) directions. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements revealed the molecular packing and orientation texture of PBDB-T:IDTT2F and PBDB-T:IDTOT2F blend


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films (Figure 4).64 For the pure IDTOT2F film, the out-of-plane (OOP) profile showed a pronounced (010) π–π stacking diffraction peak at 1.72 Å−1 with a coherence length of about 2.5 nm. Furthermore, the in-plane (IP) profile of the pure IDTOT2F film exhibited strong (100) and (200) lamellar packing diffraction peaks. This indicates that the pure IDTOT2F film exhibits high crystallinity and a face-on dominant molecular orientation with respect to the substrate. The pure IDTT2F film exhibited weak (010) π–π stacking and (100) lamellar packing diffraction peaks, indicating its low crystallinity. Both blend films displayed a high degree of molecular order with strong IP (100) lamellar packing and OOP (010) π-π stacking diffraction peaks. The (010) peaks of PBDB-T, IDTT2F, and IDTOT2F films were all located at about q = 1.75 Å−1, so it was difficult to differentiate the (010) peaks of the donor and acceptor. But the PBDBT:IDTOT2F blend exhibited a pronounced IP (200) lamellar packing peak, which could originate from IDTOT2F, indicating that IDTOT2F in a blend film also exhibits higher crystallinity than that of IDTT2F in a blend film. This result is consistent with the much higher electron mobility of IDTOT2F-based devices than that of IDTT2F-based ones. To understand the considerable difference of the photovoltaic performance of the IDTT2Fand IDTOT2F-based devices, hole and electron mobilities of blend films were measured using the space charge limited current method. Device structures of ITO/PEDOT:PSS/active layer/Au and FTO/active layer/Al were used to investigate hole and electron mobilities, respectively. As shown in Figure S7, the determined hole and electron mobilities of the IDTT2F-based device were 1.03×10−5 and 3.10×10−6 cm2 V−1 s−1, respectively. In contrast, the IDTOT2F-based device possessed higher hole and electron mobilities of 4.80×10−5 and 4.00×10−5 cm2 V−1 s−1, respectively. The higher and more balanced hole and electron mobilities of the IDTOT2F-based device readily explain its higher Jsc and FF.


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ACS Energy Letters

In conclusion, the NFA IDTOT2F was prepared and used to fabricate high-performance additive- and annealing-free PSCs. The introduction of two alkoxyl side chains onto the acceptor core simultaneously achieved high solubility and ordered packing. The good solubility of this small molecule acceptor is helpful to form appropriate morphology in its blend films, and the ordered packing of the small molecule acceptor results in blend films with high charge mobility. Compared with its analogue IDTT2F, the slightly improved energy levels of IDTOT2F caused by the introduction of alkoxyl side chains afforded devices with higher Voc. In addition, the improved solubility of IDTOT2F relative to that of IDTT2F with no alkoxyl side chains on the central unit and IEICO-4F with alkoxyl side chains on the bridging unit led to blend films with better morphology. The alkoxyl side chains of IDTOT2F also promoted the formation of more ordered packing relative to that of IDTT2F, as verified by GIWAXS measurements. The dilemma of simultaneously obtaining good nanoscale morphology and ordered molecular packing in donor:acceptor blend films can be well resolved using our strategy. High-performance PSCs produced without any extra treatment were realized with a maximum PCE of 12.79% for IDTOT2F-based devices. In comparison, IDTT2F- and IEICO-4F-based control devices only gave PCEs of 8.85% and 8.66%, respectively. Our research has provided an effective strategy to optimize the chemical structure of NFAs to control the morphology and charge transport of blend films to achieve high-performance additive- and annealing-free PSCs. ASSOCIATED CONTENT Supporting Information. Synthesis, NMR, DFT calculations, AFM and TEM images, and mobility. AUTHOR INFORMATION


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E-Mail: [email protected] (X. Xu); [email protected] (W. Ma); [email protected] (Z. Bo) †

Y. Liu and M. Li contribute equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (21574013, 21421003 and 51673028) and Program for Changjiang Scholars and Innovative Research Team in University is gratefully acknowledged. X.X. acknowledges the Fundamental Research Funds for the Central Universities. W.M thanks for the support from Ministry of science and technology (No. 2016YFA0200700), NSFC (21504066, 21534003, and 21421003). X-ray data was acquired at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors thank Chenhui Zhu at beamline 7.3.3, and Cheng Wang at beamline 11.0.1.2 for assistance with data acquisition.

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Nonfullerene Acceptors with Enhanced Solubility and Ordered

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