Efficient Polymer Solar Cells Having High Open-Circuit Voltage and

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Efficient Polymer Solar Cells Having High Open-Circuit Voltage and Low Energy Loss Enabled by a Main-Chain Twisted Small Molecular Acceptor Zhiyuan Cong, Baofeng Zhao, Zhenyu Chen, Weiping Wang, Haimei Wu, Jianqun Liu, Jianrong Wang, Liuchang Wang, Wei Ma, and Chao Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03499 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Materials & Interfaces

Efficient Polymer Solar Cells Having High OpenCircuit Voltage and Low Energy Loss Enabled by a Main-Chain Twisted Small Molecular Acceptor Zhiyuan Cong, a,† Baofeng Zhao, a,† Zhenyu Chen,b Weiping Wang, a Haimei Wu, a Jianqun Liu, a Jianrong Wang, a Liuchang Wang, c Wei Ma, b, * Chao Gao a, * a

State Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an Modern Chemistry Research Institute, NO.168 of East Zhangba Road, Xi’an, 710065, China. E-Mail: [email protected] (C. Gao).

b

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, NO. 28 of West Xianning Road, Xi’an, 710049, China. E-Mail: msewma@ xjtu.edu.cn (W. Ma). c School

of Chemical Engineering, Xi'an University, No. 168 of South Taibai Road, Xi’an, 710065, China.

1 Environment ACS Paragon Plus

ACS Applied Materials & Interfaces 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

ABSTRACT: A new main-chain twisted small molecular acceptor with non-halogenated end groups (i-IEICO), is designed and synthesized. In contrast to its planar analogue IECIO, iIEICO possesses obviously twisted backbone, leading to significant hypsochromic shift in film absorption, slight enhancement in solution extinction coefficient and significantly elevated molecular energy level. Benefited from these features, i-IEICO is matched well with two wide bandgap polymer donor materials (J52 and PBDB-T) both in absorption spectra and molecular energy levels. Relative to the planar molecule IEICO-based devices, the open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF) of the i-IEICO-based devices are simultaneously improved, giving rising to a 10.48% power conversion efficiency (PCE) (with J52) and 8.79% (with PBDB-T), respectively. Moreover, J52:i-IEICO device exhibits a high VOC of 0.96 V accompanied by a small energy loss of 0.64 eV, which can be further improved to 1.01 V and 0.59 eV for the PBDB-T-based device. The obtained VOC of i-IEICO-based devices are among one of the highest values of either J52 or PBDB-T-based binary devices, suggesting the effectiveness of main-chain twisted strategy coupled with end group modification to achieve highly efficient non-fullerene acceptors with low energy loss and high VOC.

KEYWORDS: Polymer solar cells, Main-chain twisted small molecules, Non-fullerene acceptors, Energy loss, Power conversion efficiency.


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INTRODUCTION In the past few years, polymer solar cells (PSCs) have been systematically and deeply studied because of their superiorities of light weight, low cost and potential roll-to-roll processes.1 The appropriate combinations of newly designed donor and acceptor materials, coupled with morphology optimizations, give rising to an intensively boost in the power conversion efficiencies (PCEs). In contrast to the commonly used fullerene derivatives-based electron-acceptors, small molecular acceptors (SMAs) with acceptordonor-acceptor-type (A-D-A) structures are very attractive for their adjustable molecular energy levels as well as strong absorption characteristics both in visible and even in nearinfrared (NIR) region.2,3 Recently, as the extensively efforts to finely design and tailor the structures of SMAs, PSCs have achieved tremendous progresses. The attractive PCE with >13%

4-12

for single active layer devices and >17%

13

for tandem devices have been

reported by several research groups. Although these significant results have been successfully achieved, PCEs of PSCs are still far behind those of sc-Si and perovskite solar cells.

14

One of the main reasons is the

large energy loss (Eloss) caused low open-circuit voltage (VOC) in PSCs. The Eloss can be calculated by subtracting eVOC from Eg, in which Eg represents the optical bandgap of the low bandgap material in absorption layer. Commonly, the Eloss for fullerene derivativesbased PSCs with high PCE (>9%) are often above 0.7 eV.

15–17

However, the driving

forces (especially the photo-induced holes transfer driving forces) in non-fullerene acceptors-based PSCs can be decreased to a very small value, which is favourable to reduce the Eloss and achieve high VOC. 18-21



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In our previous studies, a highly efficient A-D-A-type main-chain twisted SMA iIEICO-4F,22,23 an isomer of IEICO-4F,24 was successful synthesized. Despite an excellent efficiency (13.2%) was realized in the optimized J52:i-IEICO-4F device,22 the relatively low VOC (0.85 V) resulted in the high Eloss (0.71 eV) in this donor/acceptor blend. Generally, the adoption of strongly electron-withdrawing end groups like fluorinated end group 2F-INCN

6,24

or chlorinated end group 2Cl-INCN

10,25

can

effectively promote the intramolecular charge transfer (ICT) to red-shift the absorption spectra as well as deepen the molecular energy levels of the acceptors. On the contrary, the uses of weakly electron-withdrawing end group like thienyl substitution

27

26

or with alkyl

can slightly suppressed the ICT, leading to finely elevated lowest

unoccupied molecular orbital (LUMO) energy levels of the SMAs. To further investigate the correlation between structure and performance of the mainchain twisted SMAs so as to obtain highly efficient and low Eloss donor/acceptors system, up-shifted the LUMO energy levels of the SMAs may be a prospective approach. For this reason, herein, we synthesized a new main-chain twisted SMA (i-IEICO, Figure 1), in which the non-halogenated end group INCN

28

was used to replace the 2F-INCN as the

terminal unit in i-IEICO-4F to decrease the ICT and elevate the LUMO energy level. For the better comparison, the planar analogue IEICO 29 was also synthesized in this work. Unlike IEICO, i-IEICO shows twisted main-chain configuration ascribed to the intramolecular steric hindrance (Figure 1b). As anticipated, i-IEICO possesses a rising LUMO energy level with improved extinction coefficient in contrast to IEICO. PSC based on J52

30

(Figure 1a) and i-IEICO shows promising PCE of 10.48% with a high

VOC of 0.96 V, corresponding to a low Eloss of 0.64 eV. Furthermore, another polymer 4 Environment ACS Paragon Plus

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(PBDB-T, Figure 1a) with wide bandgap and slightly deep highest occupied molecular orbital (HOMO) energy level was also used to blend with i-IEICO. The best PBDB-T:iIEICO device exhibits a moderate PCE (8.79%) with a high VOC up to 1.01 V, yielding a very small Eloss of 0.59 eV. Given the importance of end group modification and mainchain twisted strategy of SMAs on the photovoltaic performances and energy losses of PSCs, we believe it can provide an effective guideline to construct excellent main-chain twisted SMAs not only having high PCE but also with low Eloss.

RESULTS AND DISCUSSION

Structural properties, optical performances and energy levels of the acceptor. The new acceptor i-IEICO was synthesized through a similar routine towards i-IEICO4F

22

(Scheme S1). After the Knoevenagel reaction between the key intermediate

dialdehyde compound 1 and INCN, i-IEICO was obtained as a black solid (71.0%). Then NMR spectra and elemental analysis were used to characterize the structure of i-IEICO (Figure S1, S2). Thermogravimetric analysis (TGA) illustrates that the 5% weight-loss temperature (Td) of i-IEICO is up to 353 oC (Figure S3), indicating its good thermal stability.



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Figure 1. (a) Chemical structures of J52 and PBDB-T; (b) Molecular structures, side-view and topview of IEICO and i-IEICO.

Quantum chemistry calculation through density functional theory (DFT) method with B3LYP/6-31G(d) basis set 31 was taken to estimate the structure-properties relationship of the two SMAs. To simplify the calculations and avoid the effects of twining alkyl on calculations, –CH3 and –OCH3 groups were used to replace the alkyl and alkoxy side chains on the phenyl and thiophene units. Obviously, the backbone of IEICO maintains plane

configuration

with

the

corresponding

dihedral

angles

between

the

indacenodithiophene (IDT) unit and conjugated π-bridge segment (θ2 and θ3), the πbridge segment and INCN unit (θ1 and θ4) ranged from 0.1o to 0.9o. However, the backbone of i-IEICO presents obvious warp with enlarged dihedral angles (θ1=θ4=5.9o, θ2= θ3=18.6o, Figure 1b), mainly due to the increased intramolecular steric hindrance. The calculated LUMO energy level of i-IEICO (-3.17 eV) are up-shifted about 0.12 eV relative to that of IEICO (-3.29 eV, Figure S4, Table S1) and 0.16 eV in comparison with that of i-IEICO-4F (-3.33 eV),

22

implying bigger VOC is anticipated in i-IEICO-

based PSCs. Besides, the calculated bandgaps of the two acceptors are 1.76 eV (IEICO) and 1.80 eV (i-IEICO). The slightly large bandgap of i-IEICO is consistent with its


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main-chain twisted configuration. Moreover, small space distances (2.04 Å for IEICO and 2.01 Å for i-IEICO) between the ‘O’ atoms of carbonyl groups and the ‘H’ atoms of the neighboured thiophene rings are observed, implying the weak O-H interactions, which suggests "E" configurations of these two molecules.

(b)

0.4 0.0 300

400

500

600

700

800

Wavelength (nm)

900

1000

15

i-IEICO

-3.0

Eox onset= 0.60V

Energy Levels (eV)

20

Ered onset= -1.03V

10 5 0 -5 -10 -15

0.0 300

(d)-2.5

(c)35 30 25

0.4

IEICO

Eox onset= 0.60V

Ered onset= -0.80V

+

500

600

700

800

Wavelength (nm)

-2.90

-4.0

1000

-3.68 -3.91

-5.21

-5.26

-4.5 -5.0 -5.5

900

-3.16

-3.5

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Voltage (V vs.Ag/Ag )

400

i-IEICO

0.8

0.8

IEICO

5

1.2

J52 film PBDB-T film IEICO film i-IEICO film

PBDB-T

1.6

1.2

J52

Normalized Absorbance

IEICO in CB i-IEICO in CB

-1

-1

Absorbance (10 M cm )

(a)2.0

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


-5.31

-5.31

Figure 2. (a) Molar extinction coefficients of the two SMAs in chlorobenzene; (b) Normalized film absorption spectra of the four materials from chlorobenzene solutions; (c) CV plots of IEICO and i-IEICO; (d) Energy levels sketch of the four materials.

The optical absorption spectra of IEICO and i-IEICO in chlorobenzene (CB) solutions and solid films from CB solutions are displayed in Figure 2a and 2b. A strong absorption in 550-750 nm peaked at 693 nm is observed for i-IEICO solution, despite an obvious 73 nm blue-shift compared to the absorptive peak of IEICO (766 nm). Besides, the i-IEICO



solution shows more effective photon absorbability with a molar extinction coefficient (εmax) of 1.80×105 M-1cm-1, which is 13.2% bigger than that of IEICO (1.59×105 M-1cm1).

The IEICO thin film shows broad absorption region both in visible and NIR region

with an absorption edge (edge) reaching to 897 nm, suggesting a small optical bandgap (Eg=1240/edge) of 1.38 eV. Solid film of i-IEICO shows palpably narrowed absorptive spectrum with edge of 775 nm, indicating a strongly blue-shift (122 nm) in contrast to that of IEICO. Consequently, the calculated optical bandgap of i-IEICO is 1.60 eV, which is 0.22 eV bigger than that of IEICO. Unlike IEICO with obviously blue-shifted edge (64 nm) relative to its fluorinated analogue (IEICO-4F), the edge blue-shift between iIEICO and i-IEICO-4F is only 25 nm (Figure S5), which is mainly resulted from the suppressed ICT of the main-chain twisted molecules. Clearly, i-IEICO exhibits complementary absorption (Figure 2b) with J52,30 which is beneficial to promoting the short-circuit current density (JSC) in PSCs. Electrochemical cyclic voltammetry (CV) of the two materials were measured in Figure 2c, from the obtained onset potentials of oxidation/reduction of IEICO (0.60/−0.80 V) and i-IEICO (0.60/-1.03 V), the HOMO/LUMO

32

energy levels of the two SMAs were then deduced as

−5.31/−3.91 eV for IEICO and 5.31/-3.68 eV for i-IEICO, respectively. The same HOMO energy level but an obviously up-shifted LUMO energy level of -3.68 eV is observed for iIEICO, which is 0.23 eV higher than that of IEICO. The CV plots and absorption spectra clearly reveal that the introduction of twisted structure in i-IEICO can significantly increase the bandgap, which is mainly caused by the raised LUMO energy level. We also measured the CV plots of i-IEICO-4F and J52 films (Figure S6 and S7), the obtained LUMO/HOMO energy levels of i-IEICO-4F and J52 are -3.82/-5.44 eV and -2.90/-5.21 eV, respectively. The up-


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shifted LUMO energy level of i-IEICO is propitious to get better VOC in PSCs due to the enlarged energy gap 33 between the LUMO of i-IEICO and the HOMO of J52. Grazing incident wide-angle X-ray diffraction (GIWAXS) was used to investigate the crystallinity and molecular organization of the two acceptors pure films, 34,35 which were given in Figure 3. Overall, the twisted structure of i-IEICO prevents the molecular packing at both sidechain and backbone direction in comparison with the IEICO. For the lamellar packing, both (100) peaks are located at ~0.33 Å-1, while the (100) peak of i-IEICO is broader than that of the IEICO, which imply that IEICO has the stronger lamellar packing. It could be found that the out of plane peak at ~0.8 Å-1 is absent in i-IEICO film. Meanwhile, the (010) peak in IEICO is significantly sharper than that of i-IEICO. The coherence lengths of these two acceptors are calculated as 35 (IEICO) and 31 Å (i-IEICO), respectively, suggesting the stronger π-π stacking in IEICO than that in i-IEICO. Thus, the crystallinity of i-IEICO film is weaker than its planar counterpart, which is due to the twisted main-chain structure as we discussed above.

Figure 3. GIWAXS 2D patterns for (a) pure IEICO film, (b) pure i-IEICO film, (c) 1D line-cuts.



Characteristics of Photovoltaic Devices To investigate the photovoltaic properties, inverted PSCs with device structure of indium tin oxide (ITO)/ZnO/J52:acceptors/MoO3/Al were prepared in a glove box and measured in atmosphere. CB was employed as the solvent for spin-coating. The optimal J52:i-IEICO weight ratio (1:1, w/w) and thermal annealing (TA) condition (130 oC 10 min) without any additive were adopt to fabricate the devices (Figure S8, S9, Table S2, S3). For the sake of contrast, the J52:IEICO device (1:1, w/w, 130 oC 10 min) was also prepared. The current density (J)-voltage (V) curves of the best IEICO and i-IEICO devices were plotted in Figure 4a. The device of IEICO shows a JSC of 11.39 mA cm-2, a VOC of 0.872 V and a FF of 51.7%, leading to a moderate PCE of 5.13%. However, the twisted molecule i-IEICO-based device exhibits simultaneously enhanced photovoltaic parameters, with VOC of 0.960 V, JSC of 18.76 mA cm-2 and FF of 58.2%, yielding a high PCE of 10.48%. The improved VOC and JSC of the twisted molecule based device are ascribed to the broaden energy level differences and more complementary absorption spectra between J52 and i-IEICO. Despite the PCE of J52:i-IEICO is lower than the PCE of J52:i-IEICO-4F device (13.18%),22 as a result of the elevated LUMO energy level of i-IEICO, the Eloss of this SMA-based device is decreased to 0.64 eV, about 0.07 eV smaller than that of i-IEICO-4F-based device. Although a very small Eloss (0.51 eV) is found in the J52:IEICO blend, the low photocurrent and PCE are mainly caused by the inefficient charge generation of the device.36 External quantum efficiency (EQE) responses of the best devices of the two SMAs are shown in Figure 4b. The IEICO device shows broad but low photoresponse intensity, exhibiting the highest EQE value (39.3%) at 540 nm. On the contrary, the device of iIEICO exhibits strong photoresponse with 70.0% EQE value peaked at 555 nm, despite


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relatively narrow photoresponse region (300-780 nm). The EQE spectra of the two acceptors-based devices are in line with the observed JSC in Figure 4a. The photovoltaic parameters of the two SMAs are collected in Table 1. The effects of incident light intensity (Plight) on the JSC and VOC 37 were further studied to investigate the charge recombination behaviours in the two SMAs-based devices (Figure 4c, 4d). Both of them show good correlation of JSC Plightα with corresponding α value of 0.984 for IEICO and 0.988 for i-IEICO, separately. The obtained α value suggest that at short circuit condition, the bimolecular charge recombination in the two SMAs-based devices is weak. The relationship between Plight and VOC shows the n values of 1.276 for IEICO and 1.223 for i-IEICO, respectively. The slightly small n value of i-IEICO indicates the suppressed trap-assisted recombinations, which is in consistent with the better photovoltaic performance as shown in Table 1. 2 0 J52:IEICO -2 J52:i-IEICO -4 -6 -8 -10 -12 -14 -16 -18 -20 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

(b) 80 70 60

EQE (%)

2

Current density (mA/cm )

(a)

(c)

50 40 30 20

J52:IEICO J52:i-IEICO

10 0 300

Voltage (V)

400

(d) 1.00

J52:IEICO =0.984 J52:i-IEICO =0.988

600

700

800

900

J52:i-IEICO n=1.223 kBT/q

10

VOC (V)

-2

500

Wavelength (nm)

J52:IEICO n=1.276 kBT/q

0.95

JSC (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


0.90 0.85 0.80

1

10

-2

Light intensity (mW cm )

100

0.75

10

-2



100


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Figure 4. (a) J-V curves of IEICO and i-IEICO-based PSCs; (b) EQE responses of the best PSCs; (c) Jsc–Plight, and (d) Voc–Plight curves of J52:IEICO and J52:i-IEICO devices. Table 1. Photovoltaic results of the J52:i-IEICO and J52:IEICO-based devices under the illumination of AM1.5G (100 mW/cm2)

Device

VOC [V]

JSC [mA/cm2]

FF [%]

PCEmax(PCEavea) [%]

J52:IEICO

0.872

11.39

51.7

5.13(4.88)

J52:i-IEICO 0.960 from 15 devices.

18.76

58.2

10.48 (10.26)

a) Obtained

Morphology and Charge Transport Properties To get insight into how does the improved photovoltaic performances in the nonplanar moleculebased device come from, space charge limited current (SCLC) 38 were carried out to measure the carriers mobilities of these two SMAs-based blends, and the details were displayed in Figure S10. Despite slightly small μh and μe of J52:i-IEICO blend (2.6410-4 and 1.0710-4 cm2V-1s-1) are observed relative to that of J52:IEICO blend (3.7110-4 and 1.2810-4 cm2V-1s-1), the small μh/μe ratio (2.43) of i-IEICO vs. 2.89 of IEICO indicates more balanced charge transporting in the i-IEICO-based device and agrees well with its improved JSC and FF. Atomic force microscopy (AFM) was also used to study the morphologies of J52:IEICO and J52:i-IEICO blend films. After TA treatment, the two films exhibit smooth surfaces with similar morphological characteristics (Figure 5). A small root-mean-square roughness of 1.79 nm for IEICO film and 1.09 nm for i-IEICO film are investigated, which suggest both SMAs have good miscibility to form appropriate morphologies with J52 within their blend films. The slightly bigger roughness of IEICO blend film indicates the better crystallinity

39

and is in good

accordance with its higher μh and μe. Moreover, to investigate the phase-separated features in the 12 Environment ACS Paragon Plus

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active layers, the transmission electron microscope (TEM) images of these two blend films under optimized TA treatments were also carried out. Obviously, the well nanofiber features with appropriate domain size can be seen in i-IEICO blend film (Figure S11), which is to the advantage of higher JSC and FF. Conversely, very large domains are found in J52:IEICO blend, which is accounted for the lowed JSC and FF of this planar acceptor-based device.

Figure 5. Surface morphology of the blend films.(a) AFM height image of J52:IEICO (1:1) as cast film (roughness, 2.31 nm; (b) AFM height image of J52:IEICO (1:1) 130 oC TA film (roughness, 1.79 nm); (c) AFM height image of J52:i-IEICO (1:1) as cast film (roughness, 1.15 nm); (d) AFM height image of J52:iIEICO (1:1) 130 oC TA film (roughness, 1.09 nm).

Performances of i-IEICO with PBDB-T



To further study the performances of i-IEICO in PSCs and to get low Eloss, another promising PBDB-T

40

(Figure 1a) was also used to blend with the two acceptors because of its relatively

deep HOMO energy level (-5.26 eV, Figure 2d and S12) and wide bandgap. Obviously, PBDBT also exhibits totally complementary absorption with these two acceptors but owns a slightly small bandgap relative to J52 (Figure 2b). Figure 6a and 6b give the J-V and EQE plots of the best IEICO and i-IEICO devices with the collected photovoltaic parameters in Table 2. Like the results of J52-based devices, the PBDB-T:i-IEICO device exhibits a high PCE of 8.79% in contrast to that of 3.38% for the PBDB-T:IEICO-based device. More importantly, the VOC of the optimized PBDB-T:i-IEICO device reaches a notable value of 1.006 V. As a consequence, the Eloss of this device is decreased to 0.59 eV, about 0.05 eV smaller than that of J52:i-IEICO blend. The Plight on the JSC and VOC of the two acceptors-based devices were also studied (Figure 6c and 6d). The similar α values (0.959 for IEICO and 0.947 for i-IEICO) indicates nearly identical weak bimolecular charge recombination. Moreover, the small n value of 1.20 (i-IEICO) vs. 1.46 (IEICO) in Figure 6d suggests the trap-assisted recombinations in the twisted molecule is suppressed, which is in accord with its better JSC and FF. As far as we know, the VOC data of J52:i-IEICO-based device (0.96 V) and PBDB-T:i-IEICO-based device (1.01 V) are ranking one of the highest values of either J52 22,25,30,41 or PBDB-T18, 25,42-53-based binary devices. 2 0

(b)80 70

PBDB-T:IEICO PBDB-T:i-IEICO

-2 -6 -8 -10

50 40 30

-12

20

-14

10

-16

0.0

0.2

0.4

0.6

Voltage (V)

PBDB-T:IEICO PBDB-T:i-IEICO

60

-4

EQE (%)

-2

(a) 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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0.8

1.0

0 300

400

500

600

700

Wavelength (nm)


800

900

Page 15 of 22

(c)

(d) 1.05

PBDB-T:IEICO =0.959 PBDB-T:i-IEICO =0.947

10

PBDB-T:IEICO n=1.46 kBT/q PBDB-T:i-IEICO n=1.20 kBT/q

1.00

-2

JSC (mA cm )

0.95

VOC (V)

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


0.90 0.85 0.80

1 10

-2


100

0.75

10

-2


100

Figure 6. (a) J-V plots of the best devices based on PBDB-T and two SMAs; (b) EQE curves of the best devices; (c) Jsc–Plight, and (d) Voc–Plight curves of the two SMAs:PBDB-T-based devices. Table 2. Photovoltaic results of the PBDB-T:IEICO and PBDB-T:i-IEICO-based PSCs under the illumination of AM1.5G (100 mW/cm2)

Device

VOC [V]

JSC [mA/cm2]

FF [%]

PCEmax(PCEavea) [%]

PBDB-T:IEICO

0.894

7.16

52.8

3.38 (3.27)

1.006 PBDB-T:i-IEICO a) Obtained from 15 devices.

14.67

59.6

8.79 (8.43)

AFM were also taken to investigate the morphologies of the two SMAs:PBDB-T blend films under optimized conditions (Figure S13). The two films show similar morphological features with a small roughness of 1.70 nm for IEICO film and 1.68 nm for i-IEICO film, suggesting their good miscibility with PBDB-T. The degree of crystallinity and aggregation morphologies of the blend films were also studied by GIWAXS. The 2D patterns and 1D line-cuts of the pure PBDB-T, PBDB-T:IEICO, and PBDB-T:i-IEICO films are displayed in Figure 7. It could be found that the π-π stacking of PBDB-T is peaked at q=1.67 Å-1. As for the two acceptors, the out of plane (010) peaks of IEICO and i-IEICO are located at 1.77 and 1.76 Å-1, respectively. The d-spacing

54

is calculated from the fitting data, 3.6 Å for acceptors in both PBDB-T:IEICO and

PBDB-T:i-IEICO, indicating that i-IEICO molecule arrangement is as compact as IEICO in



blend. Coherence length of PBDB-T in these two blend are 20 Å (PBDB-T:IEICO) and 24 Å (PBDB-T:i-IEICO), respectively. This suggests that the i-IEICO blend has enhanced crystallinity of PBDB-T compared with that of IEICO blend, which is beneficial to improving the hole mobility. 55,56 Considering the absorptive mismatch of the J52 and PBDB-T with IEICO, a low bandgap polymer donor (PTB7-Th)

57

was also selected to evaluate the photovoltaic performances with

these two SMAs in PSCs. Despite more complementary absorption between PTB7-Th and IEICO was achieved (Figure S14), a poor PCE of 2.36% accompanied by a low VOC (0.848 V) was found in PTB7-Th:IEICO device (Figure S15 and Table S4). Although the absorption spectra of PTB7-Th and the nonplanar molecule were overlapped from 520 to 800 nm, their blend achieved a high PCE of 6.64% with a big VOC of 0.957 V, suggesting the universally good performances of this main-chain twisted acceptor in wide/narrow bandgap polymers-based PSCs. Obviously, all the above results demonstrate the good performances and low energy loss features of the main-chain twisted molecules with non-halogenated end groups, which may provide a valuable guideline to design and build highly efficient main-chain twisted A-D-A acceptors with appropriate end groups.


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Figure 7. GIWAXS 2D patterns for (a) pure PBDB-T film, (b) PBDB-T:IEICO blend, (c) PBDB-T:i-IEICO blend, and (d) 1D line-cuts.

CONCLUSIONS To sum up, we have successfully obtained a nonplanar SMA named i-IEICO, by attaching the non-halogenated end-group INCN to a main-chain twisted molecule. In contrast to its planar analogue IEICO, intensely hypsochromic shift in film absorption with enhanced molar extinction coefficient and up-shifted molecular energy level are achieved in i-IEICO, which ensure its matching with two wide bandgap polymers (J52 and PBDB-T). PSC based on J52 as electron donor and i-IEICO as electron acceptor exhibits promising PCE of 10.48% with a high VOC of 0.96 V, corresponding to an Eloss of 0.64 eV. Moreover, a high VOC of 1.01 V is realized for the PBDB-T:i-IEICO-based device, giving ring to a further decreased Eloss of 0.59 eV. Given



the importance of end-group modification and main-chain twisted strategy on the performances and energy losses of non-fullerene acceptors, we think it can offer an effective guideline to construct excellent SMAs not only owning high PCE but also with high VOC and low Eloss. ASSOCIATED CONTENT Supporting Information. Synthesis and measurement details, NMR spectra for the acceptor, CV of the materials, photovoltaic properties of the device optimizations, J1/2-V plots, and AFM and TEM imagines are available. AUTHOR INFORMATION Corresponding Author *Email [email protected] (W. Ma) *Email [email protected] (C. Gao). Author Contributions C. Gao and W. Ma conceived the idea of the work. C. Gao, Z. Cong and W. Wang designed the molecule structure of the electron acceptor, and Z. Cong and W. Wang synthesized the compounds. B. Zhao and J. Wang fabricated and characterized the devices. Z. Chen and W. Ma performed the GIWAXS measurements. H. Wu and J. Liu determined the optical gap, the energy levels of the materials. L. Wang performed the DFT calculations. C. Gao coordinated the project. Z. Cong, C. Gao and W. Ma wrote the manuscript. All authors contributed to the data analysis and discussed the results. †

Z. Cong and B. Zhao contribute equally.

Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT Dr. C. Gao thanks for the financial supports from Xi'an Science and Technology Project (No.201805035YD13CG19(1)), the Key Scientific and Technological Innovation Team Project of Shaanxi Province (2016KCT-28) and the Project in Industrial Field of Shaanxi Province (2017ZDXM-GY-046 and 2018GY-122). Dr. W. Ma Thanks for the financial supports from Ministry of Science and Technology (No. 2016YFA0200700), NSFC (21875182, 21534003) and 111 project 2.0 (BP2018008). 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 acknowledge Chenhui Zhu at beamline 7.3.3, and Cheng Wang at beamline 11.0.1.2 for assistance with data acquisition. REFERENCES 1

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Efficient Polymer Solar Cells Having High Open-Circuit Voltage and

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