Difluorobenzoxadiazole-based conjugated polymers for efficient non-fullerene polymer solar cells with low voltage loss

Journal Pre-proof Difluorobenzoxadiazole-based conjugated polymers for efficient non-fullerene polymer solar cells with low voltage loss Kunxiang Huang, Miaomiao Li, Mu He, Ziqi Liang, Yanhou Geng PII:

S1566-1199(19)30568-3

DOI:

https://doi.org/10.1016/j.orgel.2019.105541

Reference:

ORGELE 105541

To appear in:

Organic Electronics

Received Date: 26 August 2019 Revised Date:

22 October 2019

Accepted Date: 30 October 2019

Please cite this article as: K. Huang, M. Li, M. He, Z. Liang, Y. Geng, Difluorobenzoxadiazole-based conjugated polymers for efficient non-fullerene polymer solar cells with low voltage loss, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105541. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Graphical abstract

Difluorobenzoxadiazole-based conjugated polymers for efficient non-fullerene polymer solar cells with low voltage loss Kunxiang Huang,a Miaomiao Li,*, a, b Mu He,c Ziqi Lianga and Yanhou Genga, b a

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P.

R. China. b

Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University,

and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China. c

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China. * Corresponding author. E-mail: [email protected] (M. Li).

Abstract Two donor-acceptor (D-A) conjugated copolymers based on difluorobenzoxadiazole (ffBX) and oligothiophenes, i.e., PffBX-2T and PffBX-TT, were designed and synthesized for polymer solar cells (PSCs). Compared to the polymers based on difluorobenzothiadiazole (ffBT) units, the two ffBX-based polymers presented identical optical bandgaps (~1.62 eV), but lower highest occupied molecular orbital (HOMO) energy levels. Owing to the down-shifted HOMO levels, the PSCs based on PffBX-2T and PffBX-TT showed lower voltage loss, and the open-circuit voltage (Voc) was ~0.1 V higher than that of the devices with the ffBT-based polymer. As a result, higher photovoltaic performance was achieved for the devices based on the ffBX-based polymers. The power conversion efficiencies (PCEs) of the non-fullerene PSCs with PffBX-2T and PffBX-TT as the donor were 8.72% and 10.12%, respectively. The superior device performance of PffBX-TT resulted from the efficient exciton dissociation and charge transport as well as weak charge recombination, which could be ascribed to the favorable face-on packing of the conjugated backbones and the desired morphology in the blend film. Our study demonstrates that difluorobenzoxadiazole is a promising building block for constructing conjugated polymers for high-performance non-fullerene PSCs.

Keywords difluorobenzoxadiazole; alkylthiophene side chains; voltage loss; non-fullerene polymer solar cells

1. Introduction Solution-processed bulk heterojunction (BHJ) polymer solar cells (PSCs) have attracted widespread attention because of their low cost, light weight, flexibility and large-area fabrication through roll-to-roll processing [1-5]. Recently, the development of the acceptor-donor-acceptor (A-D-A)-type fused-ring small molecule acceptors opened up a new landscape for PSCs [6-16]. Extensive studies have been devoted to design of non-fullerene acceptor molecules for elevating the efficiencies of PSCs

[17-21]. In addition to the innovations of a large number of small molecule acceptors, high photovoltaic performance of non-fullerene PSCs also heavily depends on the development of high-performance donor polymers [22-27]. It is important to design polymer donor materials that are matched with non-fullerene acceptors, and systematically study structure-property relationship that can guide rational design of donor polymers. In principle, the ideal polymer donors should possess complementary absorption with the non-fullerene acceptors, and fine-tuned the highest occupied molecular orbital (HOMO) energy level to ensure sufficient exciton dissociation and low voltage loss (∆Voc), in order to simultaneously achieve high short circuit current density (Jsc) and open circuit voltage (Voc) in PSC devices. Moreover, the appropriate aggregation behavior of the donor polymers is a key factor for the formation of proper BHJ morphology with reasonable phase separation and highly crystalline domains, which plays an important role in realizing efficient charge generation and transport. The donor-acceptor (D-A) conjugated donor polymers with oligothiophenes and difluorobenzothiadiazole (ffBT) as building blocks have attracted much attention owing to their structural amenability and accessibility [28-37]. In general, this type of polymers with bulky or branched alkyl side-chains on β-positions of thiophene rings exhibit the typical temperature-dependent aggregation behaviour, and the processing of the polymer solution at elevated temperature could efficiently control polymer aggregation and crystallization during the film cooling and drying process. Thus, proper BHJ morphology with appropriate phase separation and ordered nanostructures were successfully prepared by optimizing processing temperature of photoactive layer. Recently, we introduced alkylthiophene as side chains to the polymers based on ffBT and quaterthiophene units [38], since 2D conjugated unit has been proved to be able to enhance the π-π interaction of the polymers [39-42]. The resulting polymers (PffBT-2T

and

PffBT-TT,

Chart

1)

were

also

characterized

with

temperature-dependent aggregation behaviour, and presented ordered face-on molecular packing and appropriate blend film morphology, leading to a

high-performance fullerene-based PSCs. These ffBT-based polymers with alkyl or alkylthiophene as side chains display good absorption in visible region with absorption onset at ~760 nm, which is complementary with the near-infrared absorbing non-fullerene acceptors. However, using this type of ffBT-based polymers as donor materials, the PSCs with fullerene or non-fullerene acceptors always exhibit relatively low Voc with voltage loss (∆Voc) of 0.7~0.9 V[28, 31, 38], which is far higher than what is currently considered to be required for efficient PSCs (0.5~0.6 V) [43-45]. This suggests that the HOMO levels of these ffBT-based polymers are not deep enough to maximize the Voc, especially when the polymers are matched with some non-fullerene acceptors with deep lowest unoccupied molecular orbital (LUMO) energy levels. In several previous reports, it was found that replacing the benzothiadiazole (BT) unit with its analogue benzoxadiazole (BX) could result in down-shifted HOMO energy level but almost unchanged optical bandgap (



)

[46-48]. For example, Yan et. al. replaced ffBT units in PffBT4T-2OD with difluorobenzoxadiazole (ffBX), and synthesized PffBX4T-2DT that also exhibited the typical temperature-dependent aggregation in solution [46]. The HOMO energy level of this polymer was 0.12 eV deeper than that of the analogue polymer PffBT4T-2OD, and the



(1.66 eV) was almost identical to PffBT4T-2OD (1.65 eV). The

fullerene-based device with PffBX4T-2DT as donor yielded enhanced Voc and reduced ∆Voc, in comparison to the device with PffBT4T-2OD.

Chart 1 Chemical structures of PffBT-2T, PffBT-TT, PffBX-2T and PffBX-TT.

Given that alkylthiophenes are one kind of promising side chains for tuning the aggregation of the polymer in solution and the intermolecular stacking of the conjugated backbones in film, in this work, we designed and synthesized two D-A conjugated polymers, i.e., PffBX-2T and PffBX-TT (Chart 1), based on ffBX and oligothiophene building blocks with alkylthiophene as side chains. The photophysical and electrochemical properties of the ffBX based polymer were studied in detail, and compared to the results of the ffBT based polymers (PffBT-2T and PffBT-TT). In comparison to the PSCs based on the ffBT polymer, the PSCs with the ffBX polymers as the donor material displayed lower ∆Voc and significantly enhanced Voc. As a result, superior PCEs of 8.72% and 10.12% were achieved for PffBX-2T and PffBX-TT based devices, respectively. The higher photovoltaic performance of PffBX-TT based device could be ascribed to the formation of the blend films containing interpenetrated networks with proper domain sizes and face-on orientation of PffBX-TT backbones, which enables more efficient exciton dissociation and charge transport. 2. Results and Discussion 2.1. Density Functional Theory Calculations

Fig.

1

DFT-calculated (B3LYP/6-31G) frontier molecular orbitals of the

methyl-substituted trimers of PffBX-2T, PffBX-TT, PffBT-2T and PffBT-TT.

Initially, density functional theory (DFT) calculations at the B3LYP/6-31G (d, p) level were applied to evaluate the energy levels of the trimmers of PffBX-2T and PffBX-TT as well as the ffBT-based polymers (PffBT-2T and PffBT-TT). As shown in Fig. 1, the HOMO orbitals is delocalized over the entire conjugated backbone of the polymer, while the LUMO orbitals mainly distribute along the ffBT unit with extension onto the whole conjugated backbone units. The calculated HOMO/LUMO energy levels for PffBX-2T and PffBX-TT are -4.93/-2.98 eV and -5.02/-3.01 eV, respectively, which are lower than those of PffBT-2T (-4.84/-2.89 eV) and PffBT-TT (-4.93/-2.91 eV),

respectively.

Owing

to

the

weaker

electron

donating

ability

of

thieno[3,2-b]thiophene (TT) relative to 2,2'-bithiophene (2T), the polymers (PffBX-TT and PffBT- TT) with TT units show slightly deeper HOMO and LUMO energy levels relative to their 2T analogues (PffBX-2T and PffBT-2T). Besides, the calculated bandgaps of PffBX-2T (1.95 eV) and PffBX-TT (2.04 eV) are nearly identical to those of PffBT-2T (1.95 eV) and PffBT-TT (2.02 eV), respectively. 2.2. Materials Synthesis and Characterization

Scheme 1 Synthetic route to PffBX-2T and PffBX-TT. Reagents and conditions: i) Pd(PPh3)4, toluene, microwave; ii) N-bromosuccinimide (NBS), THF, 0 oC to r.t.; iii) Pd2(dba)3/ P(o-tol)3, toluene, 110 oC.

Table 1 Number-average molecular weights (Mn), molar mass dispersities (Ð), and photophysical and electrochemical parameters of PffBX-2T, PffBX-TT, PffBT-2T and PffBT-TT. a

LUMOb

HUMOb

[eV]

[eV]

[eV]

449, 631, 711

1.61

-2.91

-5.40

418, 656, 704

418, 646, 703

1.62

-2.96

-5.45

423

454, 636, 700

456, 635, 699

1.63

-3.10

-5.27

417

434, 647, 688

436, 644, 698

1.62

-3.13

-5.30

λmax [nm]

Mn

Td,5%

[kDa]/Đ

[oC]

solution

film

PffBX-2T

72/1.99

398

452, 643, 715

PffBX-TT

67/2.05

393

PffBT-2Tc

45.7/2.38

PffBT-TTc

58.0/2.16

polymer

a

The optical bandgap

was calculated from the film absorption onset;

b

The

HOMO and LUMO energy levels were calculated according to = ) eV and = - (4.80+ ) eV, in which and (4.80+ represent oxidation and reduction onset potentials, respectively, versus the half potential of Fc/Fc+; c The relevant data for PffBT-2T and PffBT-TT listed in the table were according to literature [38].

The synthetic route to PffBX-2T and PffBX-TT is depicted in Scheme 1. Microwave-assist Stille coupling of compound 1 and 2 followed by bromination with N-bromosuccinimide (NBS) afforded monomer 4 in a two-step yield of ~72%. PffBX-2T and PffBX-TT

were synthesized by typical Stille

cross-coupling polycondensation in yields of 84% and 90%, respectively. The synthetic details are given in the Supplementary Information (SI). The chemical structures of the intermediates and the polymers were verified by NMR spectra (Fig. S1-S8) and elemental analyses. The molecular weights of the two polymers were measured by gel permeation chromatography (GPC) using polystyrene as standard and 1,2,4-trichlorobenzene as solvent at 150 oC. As shown in Table 1, the number-average molecular weights (Mn) were 72 and 67 kDa for PffBX-2T and PffBX-TT with corresponding molar mass dispersities (Ð) of 1.99 and 2.05, respectively. The two polymers were well-soluble in hot chlorinated aromatic solvents, such as chlorobenzene (CB) and o-dichlorobenzene (o-DCB). The

thermal properties of the two polymers were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), as shown in Fig. S9 and S10. Both polymers exhibited good thermal stability with 5% weight loss temperatures (Td) exceeding 390 oC, and no obvious thermal transitions were observed from room temperature to 300 oC.

Temperature(οC)

PffBX-2T

Absorbance

30οC

110οC

400

500 600 700 800 Wavelength (nm)

30 40 50 60 70 80 90 100 110

900

(b)

Temperature(οC)

PffBX-TT

30οC

Absorbance

(a)

ο

110 C

400

500 600 700 800 Waelength (nm)

(c)

30 40 50 60 70 80 90 100 110

900

Normalized Absorbance (a.u.)

2.3. Photophysical and Electrochemical Properties 1.0

PffBX-2T PffBX-TT

0.8 0.6 0.4 0.2 0.0

400

500 600 700 800 Wavelength (nm)

900

Fig. 2 UV-vis-NIR absorption spectra of PffBX-2T solution (a), PffBX-TT solution (b) and films of the two polymers (c). Solution absorption spectra were measured in CB (10−5 mol/L of the repeating unit) during a heating process from 30 to 110 oC.

The absorption spectra of PffBX-2T and PffBX-TT in CB at different temperatures and in thin film are illustrated in Fig. 2, and relevant data are summarized in Table 1. Similar to the properties of ffBT- or ffBX-based polymers with bulky or branched alkyl side-chains [31,38], both polymers exhibited temperature-dependent aggregation behavior in solution, as revealed by their solution absorption spectra measured at different temperatures. When the temperature was elevated from 30 to 110 oC, the maximum absorption peak gradually blue-shifted, the vibronic shoulder peak became weakened and

the maximum absorption coefficient decreased, suggesting

disaggregation of the polymers at high temperatures. The disaggregation temperature, defined as at which the vibronic shoulder peaks caused by aggregation disappear, was 110 oC for PffBX-2T, while for PffBX-TT, the vibronic shoulder peaks did not vanish completely even at 110 oC. Temperature-dependent solution absorption spectra were also measured in o-DCB (Fig. S11), and similar aggregation behavior was observed. The stronger aggregation ability of PffBX-TT could be attributed to the more rigid

nature of TT unit relative to 2T unit. Besides, the disaggregation temperatures of PffBX-2T and PffBX-TT were higher than those of PffBT-2T (80 oC) and PffBT-TT (110 oC), respectively. The ffBX unit has a larger dipole moment than ffBT, which will induce stronger intermolecular interaction and could be a reason for the stronger aggregation ability of the ffBX-based polymers [46]. Besides, compared to their alkyl analogues such as PffBX4T-2DT, PffBX-2T and PffBX-TT also exhibited stronger aggregation tendency, although the alkylthiophene side chains are even more bulky than the alkyl chains in PffBX4T-2OD. This could be because the alkylthiophene side chains in PffBX-2T and PffBX-TT enlarge aromatic framework and thus enhance ) calculated from the film

intermolecular interaction. The optical band gaps (

absorption onsets were 1.61 and 1.62 eV for PffBX-2T and PffBX-TT, respectively, which were identical to those of PffBT-2T (1.63 eV) and PffBT-TT (1.62 eV). Thin film cyclic voltammograms (CV, see Figure S12) were measured to study the electrochemical properties of the two polymers. As shown in Table 1, the HOMO and LUMO energy levels calculated from the redox onset potentials, were -5.40 and -2.91 eV for PffBX-2T, and -5.45 and -2.96 eV for PffBX-TT, respectively. Compared to PffBT-2T (HOMO = -5.27 eV) and PffBT-TT (HOMO = -5.30 eV) based on BT units, PffBX-2T and PffBX-2T showed ~0.13 eV downshifted HOMO energy levels, in agreement with the DFT calculation results.

F8IC

5

(c)

70 60

0

50 -5

EQE (%)

(b)

(a)

Current Density (mA cm-2)

2.4. Photovoltaic Properties

PffBX-2T : F8IC PffBX-TT : F8IC

-10 -15

40 30 20

PffBX-2T:F8IC PffBX-TT:F8IC

10 -20 0.0

0.2 0.4 Voltage (V)

0.6

0.8

0 300 400 500 600 700 800 900 1000 Wavelength (nm)

Fig. 3 Chemical structure of F8IC (a), and current density-voltage (J-V) characteristics (b) and the external quantum efficiency (EQE) curves (c) of PffBX-2T and PffBX-TT based PSCs processed with CB upon thermal annealing at 110 oC.

Table 2 Photovoltaic parameters of the PSC devices based on PffBX-2T, PffBX-TT and PffBT-2T. Jsca polymer

solvent

treatment

Voca

(V)

FFa (%)

PCEa (%)

∆Voc (V)

-2

(mA cm ) as cast

0.73 (0.73)

14.54 (14.52)

54.68 (54.26)

5.81 (5.76)

0.55

110 °C TA

0.74 (0.74)

16.80 (16.50)

56.74 (56.02)

7.07 (6.94)

0.54

as cast

0.75 (0.74)

18.86 (18.46)

51.29 (51.02)

7.23 (6.99)

0.55

110 °C TA

0.74 (0.74)

19.60 (19.40)

59.59 (59.40)

8.72 (8.53)

0.54

as cast

0.76 (0.76)

18.42 (18.30)

58.68 (58.31)

8.29 (8.11)

0.52

110 °C TA

0.76 (0.76)

20.02 (19.91)

60.32 (60.00)

9.25 (9.08)

0.52

as cast

0.77 (0.77)

19.11 (18.96)

61.07 (60.68)

9.05 (8.86)

0.51

110 °C TA

0.76 (0.76)

20.42 (19.98)

65.48 (65.25)

10.12 (9.91)

0.52

As cast

0.67 (0.67)

19.12 (18.89)

52.36 (51.65)

6.70 (6.53)

0.61

110 °C TA

0.66 (0.66)

20.38 (20.03)

60.50 (60.05)

8.13 (7.93)

0.62

o-DCB PffBX-2T CB

o-DCB PffBX-TT CB

PffBT-2T

a

CB

Optimal and statistical results are listed outside of parentheses and in parentheses,

respectively. The average values are obtained from over 20 devices.

The low-bandgap conjugated molecule F8IC [49] (Fig. 3a) that has appropriate HOMO/LUMO energy levels and complementary absorption with PffBX-2T and PffBX-TT, was used as the acceptor material. The PSCs based on the two donor polymers were fabricated with a device architecture of indium tin oxides (ITO)/poly(3,4-ethylenedioxythiopene):poly(styrenesulfonate)(PEDOT:PSS)/polymer :F8IC/PDINO/Al, where PDINO (see Fig. S13) is an electron-transporting material developed by Li et al [50]. Two solvents (o-DCB and CB) were used as the processing solvent to optimize the device performance. The detailed device data obtained from different fabrication conditions are outlined in Table S1-S4. The current density-voltage (J-V) curves of the optimized PSCs based on PffBX-2T and PffBX-TT are shown in Fig. 3b, and the photovoltaic performance parameters are summarized in

Table 2. When using o-DCB as processing solvent, PffBX-2T and PffBX-TT based devices without post-treatment showed PCEs of 5.81% and 8.29%, respectively. Upon thermal annealing (TA) at 110 oC, the devices based on both polymers displayed enhanced PCEs (7.07% for PffBX-2T and 9.25% for PffBX-TT) with higher Jscs and fill factors (FFs). Similarly, the devices processed with CB also delivered improved photovoltaic performance after TA treatment. Compared to the o-DCB processed devices, PffBX-2T and PffBX-TT based devices prepared from CB exhibited higher Jscs and FFs, thus yielding superior PCEs. The CB processed device based on PffBX-2T displayed a PCE of 8.72% with a Voc of 0.74 V, a Jsc of 19.60 mA cm−2 and a FF of 59.6%. A higher PCE of 10.12% was achieved for the CB processed device based on PffBX-TT, with a Voc of 0.76 V, a Jsc of 20.42 mA/cm2 and a FF of 65.5%. The better photovoltaic performance of the device with CB as processing solvent could be ascribed to the more favorable BHJ morphology, which will be discussed below. As a comparison, the PSC with the ffBT-based polymer (PffBT-2T) was also fabricated. Compared to the BX analogue (PffBX-2T), PffBT-2T based device under the optimized condition showed similar Jsc (20.38 mA cm−2) and FF (60.5%), but much lower Voc of 0.66 V, thus giving an inferior PCE of 8.13%. The significantly enhanced Voc for the devices based on PffBX-2T and PffBX-TT were benefited from the down-shifted HOMO energy levels of the donor polymers. In addition, PffBX-2T and PffBX-TT based devices showed ∆Voc (∆Voc = Eg/q - Voc, where the optical bandgap Eg of blend layer is calculated from the onset of the EQE spectrum) of 0.54 and 0.52 V, respectively, which were lower than that of PffBT-2T based device (0.62 V). The external quantum efficiency (EQE) curves of PffBX-2T and PffBX-TT based devices processed with CB are shown in Fig. 3c. Similar to PffBX-2T based device (Fig. S14), the devices based on PffBX-2T and PffBX-TT also exhibited broad photocurrent response extending to ~970 nm, which is attributed to the narrow band gap of F8IC. The calculated Jsc values estimated from the integration of the EQE curves were 19.23, 19.81 and 19.71 mA cm-2 for PffBX-2T, PffBX-TT and PffBT-2T based devices, respectively, which coincided with which extracted from J-V curves within an acceptable deviation of ~5%.

Fig. 4 The photocurrent density versus effective voltage (Jph - Veff) characteristics (a), light-intensity dependence of Jsc (b), transient photocurrent (c) and transient photovoltage (d) measurements for the PSCs under the optimized conditions.

To probe the reason for the difference of the photovoltaic performances between PffBX-2T and PffBX-TT, the charge transport, generation and recombination dynamics were investigated for the CB processed devices based on PffBX-2T:F8IC and PffBX-TT:F8IC blend films. The carrier mobility of the blend films were measured by space-charge-limited current (SCLC) method. As shown in Fig. S15 and Table S5, both PffBX-2T:F8IC and PffBX-TT:F8IC blend films showed increased hole (µh) and electron (µe) mobilities after TA treatment. The µh and µe of PffBX-TT:F8IC films with TA were 2.70 × 10-4 and 1.34 × 10-4 cm2 V-1 s-1, respectively, which were somewhat higher than those of PffBX-2T:F8IC films (µh = 2.24 × 10-4, µe = 0.97 × 10-4 cm2 V-1 s-1). From the photocurrent density (Jph) versus effective voltage (Veff) characteristics (Fig. 4a) for the optimized PSCs based on PffBX-2T:F8IC and PffBX-TT:F8IC, the Jph increased with increasing Veff, and reached a saturated value (saturation current density, Jph,sat) at high Veff. The ratio of Jph/Jsat can be used to estimate the overall charge dissociation probability [51]. At the short-circuit conditions, the ratios of Jph/Jsat were 93.1% and 96.4% for PffBX-2T and PffBX-TT based devices, respectively. At the maximal power output conditions, the ratio was 74.4% for PffBX-2T based device and

82.3% for PffBX-TT based device. The higher Jph/Jsat ratios for PffBX-TT based device indicate more efficient exciton dissociation and charge collection. To investigate charge recombination and transport in the PSC devices, the dependence of Jsc on the light intensity (Ilight) was measured. As shown in Fig. 4b, plots of log(Jsc) and log(Ilight) exhibit linear dependence with a slope of α which is related to recombination losses. α = 1 implies negligible bimolecular recombination, and α < 1 suggests the presence of bimolecular recombination [52, 53]. Compared to PffBX-2T based device with α value of 0.887, PffBX-TT based device showed a higher α of 0.959. This indicates that bimolecular recombination is effectively suppressed in PffBX-TT based device. Fig. 4c and 4d show transient photocurrent and photovoltage decay kinetics in the devices [54-56]. The charge sweep-out time at short-circuit condition calculated from the traces transient photocurrent measurement was 0.42 µs for PffBX-2T based device and 0.27 µs for PffBX-TT based device. The higher charge mobility of PffBX-TT:F8IC blend film could contribute to the shorter charge extraction time. The carrier lifetime obtained from transient photovoltage measurement was 45 µs for PffBX-2T based device, while PffBX-TT based devices exhibited longer carrier lifetime of 71 µs. Since the lifetime of charge carriers at open-circuit voltage is dominated by recombination, the much longer carrier lifetime in PffBX-TT based devices may be resulted from the less charge recombination. These results indicate that the higher Jsc and FF of the PSCs based on PffBX-TT are benefited from more efficient exciton dissociation and charge transport with less bimolecular recombination. 2.5. Film Microstructures and Morphology

(a)

In plane

(b)

Out of plane

PffBX-2T PffBX-2T PffBX-TT PffBX-TT

5

10

15 20 2θ θ (degree)

(c)

25

5

30

In plane

10

15 20 2θ θ (degree)

(d)

25

30

Out of plane

PffBX-2T:F8IC_as cast

PffBX-2T:F8IC_as cast PffBX-2T:F8IC_TA PffBX-2T:F8IC_TA PffBX-TT:F8IC_as cast

PffBX-TT:F8IC_as cast

PffBX-TT:F8IC_TA

5

10

15 20 2θ θ (degree)

PffBX-TT:F8IC_TA

25

30

5

10

15 20 2θ θ (degree)

25

30

Fig. 5 In-plane and out-of-plane XRD patterns of PffBX-2T (a, b) and PffBX-TT (c, d) neat (a, c) and blend (b, d) films prepared from CB.

The molecular packing properties of the two polymers in films were investigated by X-ray diffraction (XRD), and the XRD data are summarized in Table S6-S8. As shown in Fig. 5a and 5b, PffBX-2T neat film showed an intense (100) diffraction peak in the out-of-plane XRD pattern with crystalline coherent length of 113.9 Å, and a pronounced (010) diffraction in the in-plane XRD pattern with coherent length of 32.18 Å. High order diffraction peaks up to (400) were also observed along the out-of-plane direction for PffBX-2T. In the out-of-plane XRD patterns of PffBX-TT neat film, there was an obvious (010) diffraction peak with coherent length of 27.3 Å, and a weak (100) peak was also observed. In addition, PffBX-TT presented a strong (100) diffraction in the in-plane direction, and the corresponding coherent length was 35.6 Å. These results indicate that PffBX-2T backbones adopted highly-ordered edge-on alignment on the substrate, while PffBX-TT backbones exhibit more preferential face-on molecular packing. Besides, compared to PffBX-2T, PffBX-TT displayed a much lower coherent length of (100) diffraction, which could be attributed to its relatively shorter repeating unit and therefore smaller steric hindrance for the interdigitation of side chains. The in-plane and out-of-plane XRD patterns of polymer:F8IC blend films are shown in Fig. 5c and 5d, respectively. Compared to the pristine blend films, the blend

films with TA treatment showed improved crystallinity, as indicated by the stronger diffraction peaks. In both in-plane and out-of-plane directions, the TA-treated blend films displayed intense (100) diffraction peaks at 2θ = 6.2~6.6° which were assigned to F8IC lamellar packing according to the XRD patterns of F8IC neat film (Fig. S16), and obvious (200) diffractions at 2θ = ~12.7° existed in the in-plane XRD patterns, suggesting the highly ordered molecular packing of F8IC in the two TA-treated blend films. In addition, the (100) peaks arising from lamellar packing of the polymers were also observed at 2.9~3.2° in the in-plane and out-of-plane directions for PffBX-2T and PffBX-TT based blend films. PffBX-2T formed highly ordered (100) packing with much larger coherent length (in out-of-plane direction) than PffBX-TT, in accordance with the XRD data of the neat polymer films. The broad (010) peaks in the out-of-plane XRD patterns of both PffBX-2T:F8IC and PffBX-TT:F8IC films could be ascribed to the overlapped diffractions of the polymers and F8IC. Besides, PffBX-2T:F8IC blend film also showed a pronounced (010) diffraction peak in the in-plane direction, which should be derived from the π-π stacking of PffBX-2T. This indicates that both edge-on and face-on orientations of the conjugated that that both edge-on and face-on orientations of the conjugated backbones existed in PffBX-2T based blend film. For PffBX-TT:F8IC blend film, the (010) diffraction only appeared in the out-of-plane direction, signifying preferred face-on molecular packing for both PffBX-TT and F8IC. The ordered backbone packing with face-on orientation in PffBX-TT:F8IC film is favorable for the charge carrier transport in PSC devices, and a key factor for the superior photovoltaic performance of PffBX-TT.

Fig.6 AFM images of PffBX-2T:F8IC (a, c) and PffBX-TT:F8IC (b, d) blend films processed with CB (a, b) and o-DCB (c, d).

The morphology of the CB and o-DCB processed blend films with TA treatment was investigated by atomic force microscope (AFM). As shown in Fig. 6, PffBX-2T:F8IC and PffBX-TT:F8IC blend films prepared from CB presented uniform and smooth features with root-mean-square (RMS) roughness of 1.43 nm and 1.39 nm, respectively. The proper phase separation and well-percolated bicontinuous network were observed for both PffBX-2T:F8IC and PffBX-TT:F8IC blend films processed with CB. However, when using o-DCB as the processing solvent, PffBX-2T:F8IC and PffBX-TT:F8IC blend films exhibited excessive phase separation, which were unbeneficial to exiton dissociation. It could be because o-DCB with higher boiling point provides longer polymer self-organization time during the film-casting process, and hence induces the large aggregates of the polymers that possess strong aggregation ability. 3. Conclusions In conclusion, two polymers based on ffBX and oligothiophene units with alkylthiophene as side chains, i.e. PffBX-2T and PffBX-TT, were synthesized and characterized. Both polymers exhibited temperature-dependent aggregation behavior, and their aggregation ability was stronger than that of ffBT-based derivatives (PffBT-2T and PffBT-TT) and the ffBX based polymers with branched alkyl side chains (such as PffBX4T-2OD). The strong aggregation trend of PffBX-2T and PffBX-TT in

solution could be ascribed to the large dipole moment of ffBX units and the enlarged aromatic framework caused by the alkylthiophene side chains. Most importantly, the two ffBX based polymers displayed down-shifted HOMO energy levels in comparison to the ffBT based polymers. As a result, the PSCs with PffBX-2T and PffBX-TT as donor showed Voc of ~0.75 V which was about ~0.1 V higher that of PffBT-2T based device, and the ∆Voc of the devices was lower than that of PffBT-2T based device. The device based on PffBX-2T showed a PCE of 8.72%, while a higher PCE of 10.12% was achieved for PffBX-TT based device with more efficient exciton dissociation and charge collection as well as weaker charge recombination owing to the ordered face-on moleulcar packing. Our study demonstrates that ffBX is a versatile and promising building block for constructing conjugated polymers for high-performance non-fullerene PSCs.

Acknowledgements This work is supported by the National Natural Science Foundation of China (no. 51703158 and 21574128) and China Scholarship Council (no. 201806255059).

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Highlights Difluorobenzoxadiazole-based polymers with alkylthiophene side chains were synthesized. PffBX-2T and PffBX-TT based PSC devices showed low voltage loss of 0.54 and 0.52 V. A high PCE up to 10.12% was achieved for PSC devices based on PffBX-TT.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: