Star-like n-type conjugated polymers based on naphthalenediimide for all-polymer solar cells

Accepted Manuscript Star-like n-type conjugated polymers based on naphthalenediimide for all-polymer solar cells Baitian He, Zhenye Li, Tao Jia, Jingming Xin, Lei Ying, Wei Ma, Fei Huang, Yong Cao PII:

S0143-7208(18)30829-5

DOI:

10.1016/j.dyepig.2018.06.004

Reference:

DYPI 6809

To appear in:

Dyes and Pigments

Received Date: 13 April 2018 Revised Date:

20 May 2018

Accepted Date: 4 June 2018

Please cite this article as: He B, Li Z, Jia T, Xin J, Ying L, Ma W, Huang F, Cao Y, Star-like n-type conjugated polymers based on naphthalenediimide for all-polymer solar cells, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.06.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

ACCEPTED MANUSCRIPT

Star-like n-Type Conjugated Polymers based on Naphthalenediimide for All-Polymer Solar Cells

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Baitian He,† Zhenye Li,† Tao Jia,† Jingming Xin,‡ Lei Ying,* † Wei Ma,‡ Fei Huang,*†

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

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†

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and Yong Cao†

Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China ‡

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

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University, Xi’an 710049, P. R. China

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Corresponding Authors

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*E-mail: [email protected] (L. Y.) *E-mail: [email protected] (F. H.)

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ACCEPTED MANUSCRIPT ABSTRACT: We developed n-type conjugated polymers that contain a naphthalene diimide moiety with star-like molecular geometry that are synthesized by incorporating

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1,3,5-trisphenyl-benzene as the branched unit into the linear copolymer N2200. The developed copolymers, NDIPh1 and NDIPh2, exhibited frontier molecular orbital energy levels and optical properties similar to those of N2200, whereas the melting and

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crystallization temperatures were simultaneously decreased due to the incorporated

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branched central unit. Both copolymers showed good miscibility with the widely used electron-donating

copolymer

poly[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-alt-(4-(2-ethyl hexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]

(PTB7-Th).

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All-polymer solar cells (All-PSCs) based on PTB7-Th:NDIPh1 presented an impressive power conversion efficiency of 6.87%, which obviously exceeded the

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5.75% efficiency obtained from devices based on N2200. The enhanced photovoltaic

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performance, by virtue of the improved short-circuit current density, can be attributed to the slightly enhanced electron mobility, higher exciton dissociation rates, and more efficient charge collection of NDIPh1-based devices. These observations indicate that the developed star-like NDIPh copolymers can be promising n-type acceptors for the construction of high-performance all-PSCs.

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ACCEPTED MANUSCRIPT Keywords: all-polymer solar cells; star-like copolymer; n-type conjugated polymer; naphthalene diimide

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1. INTRODUCTION In the past two decades, bulk-heterojunction polymer solar cells (PSCs), which consist of a conjugated polymer donor and an n-type semiconductor acceptor, have attracted

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tremendous attention for their unique advantages, such as their light weight,

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mechanical flexibility, large area and flexibility for roll-to-roll solution processes.1-4 Although all-polymer solar cells (all-PSCs) composed of both polymer donors and polymer acceptors present specific advantages, such as easily tunable absorption spectra, frontier molecular orbitals, remarkable miscibility of two components, and

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good thermal stability of film morphology,5-9 the overall photovoltaic performances of all-PSCs have lagged behind those devices based on electron-accepting fullerene

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derivatives or the recently emerged non-fullerene materials, primarily owing to the

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formation of the favorable phase separation that can facilitate charge separation and transportation. 10-15 To improve the overall photovoltaic performances of all-PSCs, much effort has been devoted to developing highly efficient n-type conjugated polymers, which has led to the emergence of a range of new electron-accepting polymers with appropriate frontier molecular orbital energy levels and obviously

3

ACCEPTED MANUSCRIPT broadened absorption spectra, associated with excellent thermal and mechanical stabilities.16-24

extensively

used

is

naphthalene

diimide

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Among the currently available n-type conjugated polymer acceptors, the most (NDI)-based

copolymer,

poly[[N,N′-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,

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5′-(2,2′-bithiophene)] (with the commercial name N2200).25 This copolymer has high

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electron affinity, good electron mobility, and well-extended absorption from the visible to the near-infrared region, and it has been widely used to fabricate all-PSCs that have achieved impressive power conversion efficiencies (PCEs) via integration with a range of electron-donating conjugated copolymers.6,7,7,26 Recently, based on the blend film of

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N2200 and a wide-bandgap conjugated polymer consisting of imide functionalized benzotriazole (PTzBI), an all-PSC with a remarkable PCE greater than 9% was

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achieved by carefully optimizing the molecular weight of N2200 and the processing

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solvents, which indicates great potential for achievement of a high PCE based on all-PSCs.7 One critical reason for the relatively low PCE is that it is easy for the n-type copolymer N2200 aggregate and form large crystalline domains due to the rigid and planar structure of the NDI unit, which will lead to suboptimal morphology and thus low short-circuit current density.7,19,23, This issue can be circumvented with a variety of processing techniques, such as spin-casting the bulk-heterojunction film at raised

4

ACCEPTED MANUSCRIPT temperatures, using different solvents or solvent additives, and post-treating the bulk-heterojunction films before the deposition of cathode. 27 , 28 , 29 An alternative

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strategy is to develop N2200 derivatives by modifying the molecular architecture, which can be achieved by incorporating a certain amount of the third component with a starburst configuration into the original linear main chain.30-34

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To address these issues, in this study we modulate the molecular topology of

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N2200 derivatives (Scheme 1) by incorporating a small amount of starburst monomer of 1,3,5-tris(4-bromophenyl)benzene during the Stille coupling polymerization of 2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic-N,N'-bis(2-octyldodecyl)diimide and bistannylated bithiophene (Th2-SnMe3). The resulting copolymers exhibit a gradually

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decreased crystallization temperature (Tc) as the molar ratio of the branched core unit increases, which can be attributable to the reduced rigidity of the molecular backbone

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and intermolecular stacking.23 Devices based on the branched N2200 derivatives

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exhibit enhanced PCEs of up to 6.87% as a result of the formation of more favorable film morphology, enhanced electron mobility, and reduced bimolecular recombination.

5

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of NDIPh1 and NDIPh2.

2. RESULTS AND DISCUSSION

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Scheme 1. Molecular structures of PTB7-Th, N2200 and potential chemical structure

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A series of NDI derivatives with branched architectures were synthesized by Stille polymerization by using a bistannylated bithiophene (Th2-SnMe3), a dibromo-NDI

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(NDI-Br2), and a 1,3,5-(4-bromophenyl)benzene (Ph-Br3) as the starting materials.

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Based on the molar feed ratio of the incorporated Ph-Br3 of 1% and 2%, the target copolymers are denoted as NDTPh1 and NDIPh2, respectively. Because the target starburst polymers were prepared by polycondensation, the length of the arms in the branched polymers theoretically cannot be consolidated. It is also worth noting that due to the relatively small feed ratio of the tribromo-monomer Ph-Br3, the resulting copolymers would also contain a certain number of copolymers with linear structures. 35 - 38 Moreover, considering that these copolymers were synthesized by 6

ACCEPTED MANUSCRIPT step-growth polycondensation, thus the length of each single arms of these resulted branched polymers would not be the same.37,37, 39 Nevertheless, the resulting

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copolymers can be dissolved in common organic solvents, such as toluene, chloroform, and chlorobenzene and can present uniform film by spin-casting; however, both resulting

copolymers

cannot

be

easily

dissolved

tetrahydrofuran,

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2-methyltetrahydrofuran, etc.

in

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The average molecular weights (Mn) of these copolymers were determined by high-temperature gel permeation chromatography in 1,2,4-trichlorobenzene using polystyrene as a standard; the relevant results are summarized in Table 1. We note that the estimated relative molecular weight of the resulting copolymers may deviate the

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essential values, yet since the lack of reliable strategy to determine the exact molecular weight of conjugated polymers with such star-burst architecture, the PS calibration

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has been widely used in the community to estimate the relative molecular weight of 40

thus herein we also estimate

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copolymers with starburst or branched structures,30, 39,

the relative molecular weight of the resulting copolymers using PS calibration. The resulting starburst polymers show a higher Mn (104.6 kDa for NDIPh1, and 93.8 kDa for NDIPh2) than the linear counterpart (N2200, Mn = 85.7 kDa), which can be explained that the branched structure can allow for the propagation of molecular weight

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ACCEPTED MANUSCRIPT through different directions. It is also worth pointing out that the molecular weights of copolymers have pronounced effects on photovoltaic performance.23,41

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Considering the relatively small feed ratio (1 mol% or 2 mol%) of the tribromomonomer Ph-Br3, the common characterization, such as nuclear magnetic resonance and element analysis, could not provide exact evidence of the differences

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between the linear polymer and their branched counterparts.39 However, the differences

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between the resulting starburst copolymers and the linear counterpart (N2200) can be revealed by their thermal properties and their viscosity in solution. From Figure S4 (see the SI) one can clearly note that the viscosities of both NDIPh1 and NDIPh2 are much lower than the linear N2200. Moreover, the differential scanning calorimetric

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characteristics (Figure 1) clearly show that the melting temperatures (Tm) of the resulting copolymers decreased gradually from 328°C for N2200 to 321°C for the

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branched copolymer NDIPh1 and 316°C for NDIPh2. The crystallization temperatures

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(TC) of these copolymers also decreased gradually from 307°C for N2200 to 298°C for NDIPh1 and 286°C for NDIPh2. The simultaneous decreases in Tm and TC can be attributed to the reduced intermolecular stacking of polymers due to the bulk 1,3,5-tribphenylbenzene branched unit. It is also worth noting that the melting enthalpy (∆Hm, defined as the area of the melting peak) of N2200 is 13.43 J g-1, which decreased slightly to 9.42 and 6.75 J g-1 for NDIPh1 and NDIPh2, respectively. These variations

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ACCEPTED MANUSCRIPT are consistent with the trends of Tm and Tc, which indicate that the crystallinity of the polymer acceptors decreased gradually upon enhancement of the incorporated

N2200

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branched unit.42

cooling

Exo

NDIPh1

heating 200

250 300 o Temperature ( C)

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Heat flow

NDIPh2

Figure 1. (a) Differential scanning calorimetric characteristics of N2200, NDIPh1, and

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under nitrogen.

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NDIPh2. The measurement was performed at a heating/cooling rate of 10°C min-1

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Optical and Electrochemical Properties The normalized ultraviolet–visible light (UV–vis) absorption spectra of the branched NDIPh1 and NDIPh2, both in chloroform solution and as thin films, are demonstrated in Figure 2, and the detailed parameters are summarized in Table 1. In chloroform solution, each of these polymers showed two characteristic absorption bands of the donor-acceptor type of conjugated polymers, in which the short wavelength absorption around 390 nm originated from the π–π* transition of the main chain and the long 9

ACCEPTED MANUSCRIPT wavelength absorption peaked at about 700 nm,23, 33 which can be correlated to the intramolecular charge transfer effect. The resulting branched N2200 derivatives show

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UV–vis absorption spectra nearly identical to that of the linear counterpart N2200, whereas the intramolecular charge transfer characteristics between 700 and 820 nm were slightly blue-shifted in thin films. Additionally, we measured the UV-visible

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absorption profiles in solution as function of the temperature (Figure S6, SI). It is noted

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that with the increase of temperature, the absorbance at the long wavelength gradually blue-shifted, which can be attributed to the breakup of the aggregation of polymer

1.2

1

In solution

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0.8

0.6

0.2

400

EP

0.4

0 300

(b)

N2200 NDIPh1 NDIPh2

500

600

700

800

900

Wavelength (nm)

Normalized absorption (a.u.)

(a) Normalized absorption (a.u.)

chains.

1.2

As thin films

N2200 NDIPh1 NDIPh2

1

0.8

0.6

0.4

0.2

0 300

400

500

600

700

800

900

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Wavelength(nm)

Figure 2. Normalized UV−vis absorption spectra of copolymers in chloroform solutions (a) and as thin films (b). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of polymer acceptors were measured by cyclic voltammetry (CV) (Figure S7, SI). The resulting copolymers exhibited nearly identical

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ACCEPTED MANUSCRIPT energy levels of about –5.88 eV, which is about 0.1 eV deeper than that of the linear counterpart copolymer N2200. The resulting copolymers exhibited the LUMO energy levels of –3.90 ± 0.02 eV, which are very similar to that of the linear counterpart

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polymer N2200.32, 33

Table 1. Molecular weight, absorption, electrochemical properties, and thermal

Mna

PDIa

(kDa)

λmaxsol

λmaxfilm

HOMOb

(nm)

(nm)

(eV)

LUMOb

Egcv

Tm

TC

∆Hm

(eV)

(eV)

(°C)

(°C)

(J g-1)

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Polymer

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transition for polymers.

85.7

1.83

706

718

-5.78

-3.87

1.91

328

307

13.4

NDIPh1

104.6

1.91

702

703

-5.88

-3.91

1.97

321

298

9.4

NDIPh2

93.8

2.72

-5.85

-3.88

1.87

316

286

6.8

a

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N2200

700

701

Determined by gel permeation chromatography (1,2,4-trichlorobenzene) against PS

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standards. b Measured by cyclic voltammetry.

Photovoltaic Properties All-PSCs were fabricated to evaluate the photovoltaic properties of the resulting branched NDIPh-derivatives, which have the conventional configuration of ITO/PEDOT:PSS/PTB7-Th:acceptor/PFN-Br/Al.

A

thin

layer

(~5

nm)

of

water-/alcohol-soluble poly[(9,9-bis(3-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9 11

ACCEPTED MANUSCRIPT -dioctylfluorene)]dibromide (PFN-Br) was used as the cathode interlayer because it can facilitate electron collection via the formation of an interfacial dipole.3,43 The initial

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optimization of photovoltaic performances of these devices were carried out with PTB7-Th:NDIPh1. Detailed data of the devices optimization, including the donor-to-acceptor weight ratios, processing solvent additives, thermal annealing

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temperatures, and thickness of the active layer, are shown in the Supporting

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Information (Figure S8 and Table S1–S4, SI). The current density−voltage (J−V) curves and external quantum efficiency (EQE) spectra of the optimized devices are depicted in Figure 3, and the relevant photovoltaic parameters are summarized in Table 2.

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The optimized weight ratio of PTB7-Th:NDIPh1 was determined to be 1.3:1, and film processed with chlorobenzene (CB) with the co-additive of dibenzyl ether (DBE,

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0.3 vol%) and 1,8-diiodooctane (DIO, 0.2 vol%) presented the best performances with

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a thickness of approximately 100 nm.44,45 All devices exhibited the same open-circuit voltage (VOC) of 0.81 V, which is identical to that obtained from devices based on linear N2200. The control device based on PTB7-Th:N2200 showed a moderate PCE of 5.75% (JSC = 13.84 mA cm−2; FF = 51.36%), which is consistent with the previously reported values.19 The device based on the copolymer NDIPh1 showed the best performance, with a PCE of 6.87% (JSC = 15.83 mA cm−2; FF = 53.60%), which is

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ACCEPTED MANUSCRIPT achieved by virtue of the obviously improved JSC. However, we note that the device based on a copolymer NDIPh2 consisting of higher branched unit exhibited a slightly

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decreased PCE of 6.07% (JSC = 13.83 mA cm−2; FF = 54.18%). Moreover, we also fabricated all-PSC based on the wide-bandgap copolymer PTzBI as the electron-donating polymer. However, the resulting devices presented moderate

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photovoltaic performances with PCE of 7% Table S5 and Table S6, SI), which is

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slightly lower than those obtained from all-PSCs based on linear N2200 as the electron-accepting polymer. This observation can be understood since two n-type copolymers NDIPh1 and NDIPh2 exhibited poor solubility in the processing solvent of 2-methyltetrahydrofuran (MeTHF), which cannot form uniform film during

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spin-casting procedure.

The dependence of the photovoltaic performances on the molar ratio of the

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branched unit has also been observed in the branched N2200-derivatives with medium

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or low molecular weight; however, the overall performances of these devices based on a lower Mn are inferior to those observed from the counterpart copolymer with a high Mn (Figure S9, Table S7 and S8, SI). The enhanced efficiency of these devices based on branched copolymer acceptors with higher molecular weight can be attributed to the increases in JSC and FF, which can be correlated to the enhanced exciton dissociation and charge carrier mobility (which will be discussed in the following section).

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ACCEPTED MANUSCRIPT To investigate the charge transport properties, we characterized the electron mobility of acceptors via space charge limited current (SCLC) measurements in device

configurations

with

a

device

structure

of

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electron-only

ITO/ZnO/acceptor/PFN-Br/Al. The corresponding J−V characteristics are shown in Figure S10 (SI), and the electron mobility is summarized in Table 2. The observed

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electron mobility (µe) of the resulting copolymer NDIPh1 is 1.19 × 10-3 cm2 V-1 s-1,

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which is greater than that of 0.33 × 10-3 cm2 V-1 s-1 found for N2200. The greater electron mobility is favorable for reduction of the bimolecular recombination in the solar cells and thus enhanced the photocurrent.23 In contrast, NDIPh2 exhibited a slightly lower µe than that of N2200 (0.33 × 10-3 cm2 V-1 s-1), which is consistent with

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the moderate JSC of the solar cell device. Moreover, a photoluminescence quenching experiment was carried out to investigate exciton diffusion and dissociation in blend

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films (Figure S11, SI). The quenching of the photoluminescence intensity of the

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PTB7-Th:NDIPh1 or PTB7-Th:NDIPh2 blend films is more pronounced than that of the PTB7-Th:N2200 blend film, indicating more efficient exciton dissociation and charge generation in the blend film with PTB7-Th:NDIPh-derivatives. These findings are consistent with the enhanced PCE of the NDIPh-derivatives polymer acceptor-based devices.

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ACCEPTED MANUSCRIPT To confirm the accuracy of the obtained JSC, we measured the EQE spectra of the all-PSCs (Figure 2b). All blend films showed a broad photocurrent response from 300

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to 800 nm. Devices based on PTB7-Th:NDIPh1 showed a slightly greater response, with an EQE of 65% at 640 nm, which is consistent with the higher JSC. Moreover, the enhanced Jsc values of the devices based on a branched acceptor correlated well with

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0

80 70

PTB7-Th:N2200

PTB7-Th:N2200 PTB7-Th:NDIPh1 PTB7-Th:NDIPh2

16

40

8

30 20

-15

0

0.2

0.4

0.6

-2

SC

-10

12

50

J

EQE (%)

PTB7-Th:NDIPh2

(mA cm )

60

PTB7-Th:NDIPh1 -5

4

10

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(a)

Current density (mA/cm 2)

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the changes in their spectral responses in the EQE curve.

0 300

0.8

400

Voltage (V)

500

600

700

800

0 900

Wavelength (nm)

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Figure 3. (a) J–V characteristics and (b) EQE spectra of devices with photoactive layer

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of PTB7-Th:acceptor (1.3:1, wt:wt). Table 2. Photovoltaic parameters of the optimized PTB7-Th:acceptor and electron mobility of acceptor.

D:A

Voc

Jsc

Jsc, EQEc

FF

PCE b (PCEbest)

µe

(V)

(mA cm-2)

(mA cm-2)

(%)

(%)

(cm2 V-1 s-1)

0.81±0.01

13.50±0.34

13.09

50.67±0.71

5.59±0.16 (5.75)

0.33×10-3

a

PTB7-Th:N2200

15

ACCEPTED MANUSCRIPT PTB7-Th:NDIPh1

0.81±0.01

14.50±0.49

14.46

55.88±0.86

6.75±0.12 (6.87)

1.19×10-3

PTB7-Th:NDIPh2

0.81±0.01

13.62±0.37

13.09

53.34±0.65

5.94±0.13 (6.07)

0.21×10-3

D:A = 1.3:1; all blend films were processed by chlorobenzene with 0.3 vol% DBE and

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a

0.2 vol% DIO. b The PCE values are the average values of a series of 12 separated

c

Obtained from the integration of EQE spectra.

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devices; and the best values (PCEbest) are the highest PCE value of 12 separated devices.

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Charge Generation, Recombination, and Extraction Properties

To compare the exciton dissociation and charge collection processes of the all-PSCs consisting of PTB7-Th as the electron-donating polymer and N2200 or

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NDIPh-derivatives as the acceptor, we investigated the curves for the photocurrent density (Jph) as a function of the effective voltage (V ) of the devices, and the relevant eff

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characteristics are shown in Figure 3. The photocurrent density Jph is defined as Jph = JL

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− JD, where JL and JD are the light and dark current densities, respectively, and the effective voltage Veff is defined as Veff = V0 − Vbias, where V0 is the voltage when Jph is zero and Vbias is the applied bias voltage.7,46 For the high value of Veff (>2V), we suppose that all photo-generated excitons were dissociated into free charges and collected by electrodes and that the Jph reaches saturation (Jsat). Thus, the maximum exciton generation rates (Gmax = Jsat/qL), where Jsat is the saturated photocurrent density, q is the electronic charge, and L is the thickness of the active layer of devices, are 16

ACCEPTED MANUSCRIPT summarized in Table S9 (see SI). The devices based on NDIPh1 and NDIPh2 presented slightly higher Gmax values (1.12 × 1028) than that of the PTB7-Th:N2200 based devices

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(1.07 × 1028), which indicates that the exciton generation of the devices based on NDIPh1 is faster than N2200 counterpart and agrees with the corresponding JSC values. In addition, the charge dissociation probability P(E, T) can be determined by

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normalizing Jph with Jsat (Figure 4b). Under short-circuit conditions, the P(E, T) values

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are 0.836 and 0.871 for the devices based on N2200 and NDIPh1, respectively. These results indicate that the all-PSCs based on NDIPh1 have higher exciton dissociation rates and more efficient charge collection than the devices based on N2200 acceptor. To understand the charge recombination behavior of all-PSCs, the dependence of

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JSC at various light intensities was further investigated; the results are presented in Figure 4c. The correlation between JSC and the illumination intensity (P) can be

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expressed quantitatively as JSC ∝ (Plight)S, where Plight is the light intensity and S is the

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exponential factor.47 When the bimolecular recombination of the charge carriers is weak, JSC shows a linear dependence on Plight with the S value close to 1. From Figure 4a, one can note that the S value was estimated to be 0.989 and 0.927 for the devices based on NDIPh1 and NDIPh2, respectively, both of which are higher than the estimated value of 0.904 for the device based on N2200. This observation indicates that

17

ACCEPTED MANUSCRIPT the bimolecular recombination for PTB7-Th:NDIPh-derivative devices was slightly

(b)

(a) PTB7-Th:N2200

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decreased relative to that of PTB7-Th:N2200.

1

PTB7-Th:N2200

PTB7-Th:NDIPh1

PTB7-Th:NDIPh1 PTB7-Th:NDIPh2

PTB7-Th:NDIPh2

0.8

ph

J

0.4

1

0.001

0.01

0.1 V (V)

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0.2

SC

0.6

ph

J /J

sat

-2

(mA cm )

10

1

0 0.001

10

eff

0.01

0.1

V

eff

1

10

(V)

PTB7-Th:N2200

10

PTB7-Th:NDIPh1

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1

1

EP

-2

JSC (mA cm )

PTB7-Th:NDIPh2

10

-2

Normalized photocurrent (a.u.)

(d) 1.2

(c)

Light intensity (mW cm )

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0.8 0.6 0.4 0.2 0 -0.2

100

PTB7-Th:N2200 PTB7-Th:NDIPh1 PTB7-Th:NDIPh2

1

-0.1

0

0.1

0.2

0.3

0.4

Time (us)

Figure 4. (a) Jph–Veff and (b) Jph/Jsat–Veff curves, (c) JSC as a function of light intensity and (d) transient photocurrent of the optimized devices.

Transient photocurrent measurements were used to probe the charge extraction process of all-PSCs.48 The measurements were carried out at 0 bias, where the charge

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ACCEPTED MANUSCRIPT extraction time (t) was obtained by nonlinearly fitting the curves of JSC versus time, and the relevant characteristics are shown in Figure 4d and Table S10. The devices based on

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NDIPh-derivatives exhibited charge extraction times of 0.14 and 0.18 µs for NDIPh1 and NDIPh2, respectively, both of which are slightly shorter than that obtained from the device based on N2200 (0.23 µs), which indicates the more efficient charge extraction

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due to the shorter charge extraction time for the branched acceptors, which is consistent

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with the slightly higher JSC and PCE values observed.

Film Morphology

The investigation of the influence of the branched structure of copolymers on the film

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morphology was carried out with atomic force microscopy (AFM), transmission electron microscopy (TEM) and grazing incidence wide angle X-ray scattering

EP

(GIWAXS). All blend films exhibited a very flat film morphology, with a

AC C

root-mean-square roughness value of approximately 1 nm (Figure S12, SI). In contrast, from the TEM images one can find that blend film based on PTB7-Th:N2200 (Figure S12a, SI) showed fibrous structures across the entire film, while the blend films based on PTB7-Th:NDIPh1 (Figure S12b, SI) and PTB7-Th:NDIPh2 (Figure S12c, SI) are essentially featureless. This observation clearly indicated that, although PTB7-Th has good miscibility with both N2200 and the resulting starburst copolymers, the resulting NDIPh1 and NDIPh2 presented slightly improved miscibility of the BHJ layer based 19

ACCEPTED MANUSCRIPT on the starburst copolymers, which can correlated to the improved current density of the corresponding devices.

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GIWAXS was also used to characterize the molecular packing and crystallinity of the pure films and PTB7-Th:NDIPh-derivative blends (Figure 5 and Figure S13, SI). For the pure PTB7-Th film (Figure 5a), a broad π-π stacking peak can be observed from

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an out-of-plane one-dimensional line-cut at q = 1.60 Å-1, whereas the (010) peak of the

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pure N2200 film (Figure 5b) was also located at q = 1.60 Å-1, but with a sharp shape that indicates stronger crystallinity. The strong (010) peak in the out-of-plane direction clearly indicated the preferential face-on orientation of molecular chain of pure film of PTB7-Th and N2200 regarding to the substrate. After blending, the PTB7-Th:N2200

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system presented a π-π stacking peak at q = 1.61 Å-1, and the coherence length was 21.29 Å, as calculated from the Scherrer equation to describe crystallinity.49 For the

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PTB7-Th:NDIPh-derivative blends, the (010) peaks of PTB7-Th:NDIPh1 and

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PTB7-Th:NDIPh2 had slightly lower coherence lengths (19.08 and 19.82 Å, respectively) than PTB7-Th:N2200. These results also demonstrate that the crystallinity of the blend films based on the branched NDIPh1 and NDIPh2 was slightly lower than that of N2200, which indicates that the N2200 derivatives could form more favorable film morphology with PTB7-Th and achieve more efficient exciton dissociation, thus enhanced the photovoltaic performances. It is also worth noting that,

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ACCEPTED MANUSCRIPT since the GIWAXS patterns of all blend films exhibited a strong (010) peak in the out-of-plane direction, it is reasonable to surmise that the two components have good

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miscibility, and the polymer backbones retain face-on orientation in these blend films.

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Figure 5. Grazing-incidence wide angle X-ray scattering 2D patterns pure films: PTB7-Th (a), N2200 (b); and blend films: PTB7-Th:N2200 (c), PTB7-Th:NDIPh1 (d). In-plane and out-of-plane (e) line cuts for pure and blend films.

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ACCEPTED MANUSCRIPT 3. CONCLUSION We developed a series of n-type branched polymer acceptors by introducing different of

1,3,5-tris(4-bromophenyl)benzene

units

into

bithiophene

and

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numbers

2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic-N,N'-bis(2-octyldodecyl)diimide,

which are denoted NDIPh1 and NDIPh2. The resulting copolymers have good

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miscibility with a narrow bandgap conjugated copolymer PTB7-Th. All-PSCs based on

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PTB7-Th:NDIPh1 blends exhibited a high PCE of 6.87% when processed with chlorobenzene in the presence of DBE (0.3 vol%) and DIO (0.2 vol%) as the co-solvent additives, which is obviously greater than the PCE of 5.75% obtained from the control device based on PTB7-Th:N2200. The enhanced photovoltaic performances of the

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resulting devices benefit from the branched structures of the resulting polymer acceptors, which can facilitate exciton generation and dissociation. These results

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indicate that the construction of N2200 derivatives with a branched architecture is a

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promising approach that has great potential for the construction of all-PSCs.

Supporting Information Additional figures as mentioned in the text, including synthesis of monomer and polymer, TGA and DSC curves, 1HNMR spectra and device fabrication and characterizations.

22

ACCEPTED MANUSCRIPT Notes

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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work was financially supported by the Ministry of Science and Technology of

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China (No. 2014CB643501 and 2016YFA0200700) and the Natural Science

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Foundation of China (No. 51673069, 91633301, 21504066 and 21534003), and the Science and Technology Program of Guangzhou, China (No. 2017A050503002). X-ray data were acquired at beamlines 7.3.3 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the

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U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors acknowledge Dr Xiao-Fang Jiang and Qingwu Yin for the transient photocurrent study,

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acquisition.

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and also acknowledge Chenhui Zhu at beamline 7.3.3 for assistance with data

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NDIPh1

0 PTB7-Th:N2200

-10

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PTB7-Th:NDIPh1

-5

PCE = 5.75%

-15 0.2

0.4 0.6 Voltage (V)

0.8

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0

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PCE = 6.87%

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-2

Current density (mA cm )

N2200

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Table of Contents Entry

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ACCEPTED MANUSCRIPT Highlights

Two star-like n-type conjugated polymers containing naphthalene diimide unit were

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developed. The crystallization temperatures of the star-like copolymers decreased regarding to the linear counterpart copolymer.

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The photovoltaic performances of devices based on star-like n-type copolymers

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as the electron-acceptor outperform that of linear N2200.