Influence of Bridging Groups on the Photovoltaic Properties of Wide

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Influence of bridging groups on the photovoltaic properties of wide bandgap poly(BDTT-alt-BDD)s Tahir Rehman, Zhi-Xi Liu, Tsz-Ki Lau, Zhi-Peng Yu, Minmin Shi, Xinhui Lu, Chang-Zhi Li, and Hongzheng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16628 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Applied Materials & Interfaces

Influence of Bridging Groups on the Photovoltaic Properties of Wide Bandgap poly(BDTT-alt-BDD)s Tahir Rehman,†,a Zhi-Xi Liu, †, a Tsz-Ki Lau,b Zhipeng Yu, a Minmin Shi, a Xinhui Lu,b Chang-Zhi Li, a* and Hongzheng Chena* a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key

Laboratory of Silicon Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China. *E-mail address: [email protected]; [email protected]. b

Department of Physics, The Chinese University of Hong Kong, New Territories, Hong Kong,

China. KEYWORDS: Polymer solar cell, Wide bandgap polymers, non-fullerene acceptor, Backbone conformation, Bulk heterojunction

ABSTRACT: To further advance polymer solar cells requires the fast evolution of π-conjugated materials, as well as better understanding of their structure-property relationships. Herein, we present three co-polymers (PT1, PT2, PT3) made through tuning π-bridges (without any group, thiophene, and 3-hexylthieno[3,2-b]thiophene) between electron rich (D: BDTT) and deficient (A: BDD) units. The comparative studies reveal the unique correlation that the tune of π-bridge on the polymeric backbone governs the solid stacking and photovoltaic properties of resultant

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poly(BDTT-alt-BDD)s, which provide an effective way to deliver new and efficient polymer with feasible processability. That is, polymers with either twist zigzag backbone (PT1) or with linear co-planar backbone (PT2) result in both inferior photovoltaic performance upon simple solution casting. Among them, PT3 with extended zigzag backbone and planar segments exhibits suitable processability and retains good efficiency in non-fullerene solar cells through single solvent-cast without involving tedious treatments. This work illustrates the tuning of D-π-A polymer backbone facilitates efficient materials with feasible processability, promising for scaleup fabrication.

1. INTRODUCTION: Bulk heterojunction (BHJ) polymer solar cells (PSCs) have great potential to realize mass production via solution fabrication,1-4 whereas, the photovoltaic performance of PSCs is closely correlated to the blend morphology of BHJ layer. To ensure efficient photo-charge generation and transport in BHJ layer, the electron donor and acceptor blend needs forming interpenetrated bi-continuous nanoscale networks,5 which are usually achieved through complicated pre- and/or post-treatments of BHJ films,6-7 such as slow growth,8 solvent annealing9 and additives10 etc. Some of the tedious treatments for BHJ morphology optimization would be impractical to be extended into scale-up fabrication of PSCs. Therefore, it is desirable to achieve efficient PSCs without the expense of complicated processing treatments, for instance, only simple solution-cast with single solvent. In this regard, the correlation between structural feature and self-assembly behavior of conjugated polymer remains to be important, yet less understood.11 The comparative study of conjugated polymers with the tailored structural factors, including the combination of electron rich donor (D) and electron deficient acceptor (A) unit,12-18 modulation of alkyl chain,19-21 and bridging groups,22-24 can be explored to understand this


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structure-property relationship. Recently, wide bandgap (WBG) donor co-polymers have grabbed attentions of researchers, after the discovery of near-IR non-fullerene acceptors,25-29 which exhibit complementary absorption and deep energy levels to simultaneously improve short-circuit current (JSC) and open-circuit voltage (VOC) of non-fullerene based PSCs. Among them, poly(BDTT-alt-BDD) is one of the most successful WBG polymers (BDTT: 4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene as electron rich unit and BDD: 1,3bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c'] dithiophene-4,8-dione as electron deficient unit).3032

It is worthy to note that solution processing of poly(BDTT-alt-BDD) analogues with non-

fullerene acceptor blend usually requires small volume percent of 1,8-diiodooctane (DIO) additive for optimization of BHJ morphology. It catches our interest to deploy this highperformance polymer as model for understanding the effect of polymer backbone to its selfassembly behaviour, which may be beneficial to further develop new polymers retaining highperformance with simple solution processability, which can be promising to future scale-up fabrication of polymer solar cells. π-Bridge groups in polymers are usually small aromatic rings, incorporated between D and A units to develop D-π-A polymers. Appropriate selection of bridging group can tune the conjugated backbone,33-34 intramolecular charge transfer,34-37 as well as the self-assembly24, 38-39 of organic semiconductors. It may serve as a knob to engineer conjugated polymer with suitable optoelectronic properties and solution processability for accessing efficient solar cells. Herein, we develop three wideband gap co-polymers (PT1, PT2 and PT3) by employing BDTT and linear octyl chain substituted BDD units, with different π-bridge groups. There is no π-bridging unit for PT1 (PBDTT-BDD), whereas PT2 (PBDTT-T-BDD) and PT3 (PBDTT-HTT-BDD) possess thiophene (T) and 3-hexylthieno[3,2-b]thiophene (HTT) bridging groups, respectively.


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Their comparative studies reveals by the incorporation of different π-bridge significantly alters the backbone conformation and conjugation of resultant polymers, which further influences optoelectronic and self-assembly properties of polymers, eventually affecting the blend morphology and photovoltaic performance of derived PSCs. PT1 and PT3 adapt zigzag backbone conformation, whereas PT1 without bridge exhibits largely twisted conformation (BDTT-BDD) over that of PT3. PT2, on the other hand, has linear co-planar conformation and strong aggregation tendency. As results, PT1 and PT2 induce either loose or strong aggregation within solution, resulting in both inferior PCEs in PSCs upon singlesolvent casting. Among them, PT3 with the extended zigzag conformation and planner segments exhibits moderate aggregation tendency, which possesses feasible solution-processability to deliver efficient photovoltaic performance without special treatments. PT3 yields simple solution-cast BHJs with power conversion efficiency (PCE) of 9.2% in non-fullerene based PSCs. This work indicates the tuning of polymeric bridging group can enables new D-π-A polymers simultaneous with simple processability and efficient photovoltaic performance, which would be desirable approach to develop materials for scale-up fabrication of PSCs.

2. RESULTS AND DISCUSSION 2.1. Synthetic procedures The synthetic route for monomers is depicted in Scheme 1. and details are given in the supporting information (SI). The synthesis of 3,4-thiophene dicarboxylic acid (4) revisited via the deamination of amino substituted thiophene (3). Gewald reaction was employed to synthesize amino substituted thiophene from very cheap starting materials (1 and 2),40-41 and the deamination of (3) was done in THF by the t-BuONO and hydrolysis by the sodium hydroxide to


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yield corresponding dicarboxylic acid (4). This synthetic strategy exhibits relatively high yield and reduces difficult steps reported previously.42 To synthesize BDD (6) unit,43-44 Friedel-Crafts acylation of 2,5-dioctylthiophene was applied with 2,5-dibromo-3,4-thiophene-dicarbonyl dichloride. To incorporate different bridging groups, Pd-catalyzed Stille coupling between BDD (6) and corresponding tin substituted π-bridge groups (Th-Sn and HTT-Sn) were performed, then followed by the bromination, finally yielding electron deficient BDD units (M1 and M2). The HTT-Sn was synthesized by the modification of previously reported method (SI).45 Finally, three co-polymers were synthesized by classic Stille-coupling polymerization as depicted in the Scheme 2. Polymerization performed in toluene at 110 °C under N2 atmosphere for 36 h, crude polymers were precipitated out in methanol and collected by filtration, subjected to soxhlet extraction by the methanol, acetone, hexane and chloroform, same as described in previous works46-48. The molecular weights were determined by using high temperature gel permeation chromatography (GPC), using 1,2,4-Trichlorobenzene (TCB) as the eluent and monodispersed polystyrene as the internal standard (Table 1).


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Scheme 1. Synthetic route for M1 and M2. Reaction conditions: (i) S8, Et3N, DMF, 50 °C for overnight; (ii) t-BuONO, THF, reflux for 3 h; then, 2M NaOH, reflux for overnight; (iii) Br2, AcOH, RT, overnight; (iv) oxalyl chloride, DCM, RT, overnight; then, 2,5-dioctylthiophene, C2H4Cl2, AlCl3, 0 °C to RT, 3 h; (v) Pd(PPh3)4, toluene, reflux overnight (vi) NBS, THF, 0 °C, then RT 5 h.

Scheme 2. Synthetic route of conjugated polymers, PT1, PT2 and PT3 via Stille coupling polymerization. Reaction conditions: (Vii) Pd2(dba)3, P(o-tol)3, toluene 110 °C for 36 h.


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2.2. Electrochemical, thermal and optical properties of polymers Energy levels for all three polymers were determined from the onset-oxidation potentials of corresponding films by the cyclic voltammetry (Figure 1a). The HOMO energy calculated from onset-oxidation potential according to the equation EHOMO = -[(Eoxonset –E1/2(Ferrocene)) + 4.8] eV. Whereas, the LUMO energy levels estimated by using the equation ELUMO = -[Egopt + EHOMO], data are summarized in Table 1. A suitable π-bridge group can modulate the conjugation and intra-charge transfer between both BDTT and BDD units. PT1 without π-bridge group have the deepest HOMO energy level compared to other polymers. This is due to the steric hindrance between BDTT and BDD, which twists backbone to decrease the effective conjugation of polymer, thus to deepen energetics and broaden bandgap.49 The PT3 with HTT bridge group comparatively have lower ionization potential than PT2 with T bridge. The thermal properties of all polymers investigated under nitrogen atmosphere by the thermogravimetric analysis (TGA). All polymers possess good thermal stability, and the thermal decomposition temperature (Td) at a 5% weight loss observed for PT1, PT2 and PT3 are 394 °C, 440 °C, and 410 °C respectively (Figure 1b).

Figure 1. (a) Cyclic voltammograms for Polymers PT1, PT2 and PT3 films; (b) TGA plots of PT1, PT2 and PT3 under N2.


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Table 1. Optical properties, energy levels and molecular weight of polymers

λmaxSol

λmaxFilm

(nm)a

(nm)

ε Eoxonset -1 (10 M (eV) cm-1)b (V)

PT1

537

586

1.87

2.97

PT2

613

619

1.77

PT3

579

584

1.83

Polymers

a

Egopt

EHOMO

ELUMO

Mn

Mw

(eV)

(eV)

kDa

kDa

0.66

-5.46

-3.59

16.20

32.50

4.79

0.55

-5.35

-3.58

66.40

169.50 2.50

4.62

0.57

-5.37

-3.54

66.60

153.20 2.30

4

PDI

2.00

Measured from Chloroform solution, b Measured from 1,2-Dichlorobenzene solution. The UV-vis spectra for all three polymers in solution and thin film (spin-cast onto glass

substrate) are shown in Figure 2 and data are summarized in Table 1. The solution absorption spectra recorded in varied solvent, chloroform (CF), chlorobenzene (CB) and 1,2dichlorobenzene (DCB), to study the effect of solvent on polymer self-assembly and temperature-dependent aggregation property. Main absorption band of three polymers located in the range of 500-700 nm, consisting of backbone charge transfer peaks (A0-0) and shoulder peaks (A0-1). The A0-1 peaks originated from the polymer inter-chain π-π* transition, and the stronger A0-1 peak suggests more ordered microstructure of polymer in solution and film.50 It can be seen that PT2 formed strong J-aggregates in both solution and film, verified from the peak ratio between A0-0 and A0-1 in the UV-vis absorption spectra.51-52 PT1 displays no (or much weak) aggregation in solution. PT3 retains similar absorption profile in both solution and film with lower shoulder A0-1 peaks than its A0-0 peaks, indicating the aggregation tendency of PT3 in between of PT1 (weak) and PT2 (strong). When compared to PT2, hypsochromic and hypochromic shift for PT1 and PT3 solution was observed, attributed to the loose aggregation behaviors of zigzag polymeric backbone (Figure S1).


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We further studied the temperature dependent UV-vis absorption spectra (in DCB) of three polymers from room temperature to 100 °C at 20 °C intervals. The PT1 absorption spectra remained almost unchanged with increasing temperature, indicating no (or much weak) aggregation and self-assembly in solution. Upon increasing temperature, the A0-1 peak intensity for PT2 and PT3 decreased but no blue-shift in the main absorption band (A0-0), indicating the deaggreation of polymer assembly at high temperature, whereas the polymer backbone remains planar for maintaining good intrachain charge transfer.53 From these observations, we estimate that three polymers exhibit quite different aggregation tendency due to the varied backbone curvature, wherein PT1 shows no (or much weak) aggregation in solution, while PT2 is strongly aggregated. And the extended zigzag PT3 shows moderate aggregation tendency in between those of PT1 and PT2. This variation of self-assembly and packing behaviors for polymers upon tuning bridge groups between BDTT and BDD moieties, which would further influence the morphology and overall photovoltaic performance of polymer based blends.


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Figure 2. UV-visible absorption spectra of polymers, thin film (spin cast from CB solution) and dilute solution with different solvent of PT1 (a), PT2 (c), and PT3 (e); temperature dependent absorption spectra of the PT1 (b), PT2 (d) and PT3 (f) in DCB from RT to 100 °C.

2.3. Density functional theory (DFT) Calculations: To understand the effects of the π-bridging groups on the electronic properties and conformation of polymeric backbones, density functional theory (DFT) calculations at the B3LYP/6-31G(d) level are employ to simulate dimer models (Figure 3). As shown in Figure 3a, typical zigzag conformation (Z-type) with 60°angle observed for PT1 and PT3, whereas, the PT2 adapts linear conformation (L-type). The large dihedral angle between BDTT and BDD is observed for PT1, attributing to the steric hindrance upon direct coupling of two large aromatic groups together. With the incorporation of HTT and T bridges, this dihedral angle can be released to promote better planarity for BDTT--BDD than that of BDTT-BDD structure (Figure 3b). Such difference in backbone conformation originates from the different bridging group. The corresponding calculated molecular orbitals (HOMO/LUMO) of PT1, PT2 and PT3 are -2.50/4.94 eV, -2.48/-4.83 eV and -2.48/-4.81 eV as shown in Figure 3c and 3d, indicating that the energy levels and the backbone conformation can be tuned by the different conjugated segments. It can also explain that strong aggregation in PT2 is due to L-type and planar conformation. PT1 and PT3 adapt zigzag backbone conformation, whereas PT1 exhibits highly twist BDTT-BDD conformation without bridge unit, resulting in not only blue shift of absorption profile, but also much weak aggregation behaviors. Among them, PT3 with extended zigzag conformation remains good planarity of BDTT--BDD segments, which ensures polymer with not only moderate aggregation tendency in solution and film, but also good optoelectronic properties.


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Figure 3. The optimized geometries of PT1, PT2 and PT3; (a) top view, (b) side view; The frontier molecular orbitals of (c) LUMO and (d) HOMO; Color code: grey (C), red (O) and yellow (S). 2.4. Photovoltaic Properties With above information in hand, we fabricated PSCs with inverted device architecture of ITO/ZnO/active layer (Polymer: ITIC)/MoO3/Ag (Figure 4a&b and SI). The photovoltaics properties characterized under illumination of solar simulator, AM 1.5G illumination (100 mW cm-2). For fair comparison, we made PSCs of all three polymer:ITIC blends under same condition by simple solution cast from CB solvent without any special treatments. The Figure 4c and Table 2 show current density-voltage (J-V) characteristics. PT1 based PSCs exhibit quite low photovoltaic performance of 2.13% PCE. PT2 based PSCs achieve reasonable performance of


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9.0% PCE with averaged PCE of 8.71%, which is, however, relatively difficult to process due to the strong aggregation tendency of linear and planar polymer. Among them, champion PCE of 9.21% and averaged PCE of 9.11% were achieved from PT3 based PSC made with simple solution fabrication, and comparatively high Voc with little compromise in JSC. Mobility of each blend was measured from space charge limited current (SCLC) method (Table 2 and SI). PT1 based blend showed one order of lower magnitude hole mobility than blends based on PT2 and PT3, which can be attributing to the twist BDTT-BDD conformation of PT1 without bridge decreases optoelectronic and self-assembly properties of polymer. The moderate aggregation property of PT3 having extended zigzag conformation facilitates simple BHJ blend processability, and retaining good optoelectronic characteristics of blend film. In term of simple solution-cast with single solvent, either strong aggregation of linear PT2 or very weak aggregation of PT1 results in BHJ film with inferior performance, comparing to PT3 with moderate aggregation tendency. These observations indicate that the modulation of polymer backbone with π-bridge unit largely affects the intrinsic self-assembly and optoelectronic properties of each polymer, which eventually impact on the photovoltaic characteristics of BHJ blends.


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Figure 4. (a) Energy level diagram of each layer in (b) inverted device structure (c) Current density-voltage (J-V) Plot and (d) external quantum efficiency (EQE) spectra of the PSCs based on PT1: ITIC, PT2: ITIC and PT3: ITIC under the illumination of AM 1.5 G (100 mW cm -2). (e) The plot of photocurrent (JPh) versus effective voltage (Veff), (f) and VOC as a function of light intensity.


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Table 2. Photovoltaics parameters of polymers: ITIC based PSCs from simple solutionfabrication.

Sample

PT1

PT2

PT3 a

VOC

JSC

PCE

(V)

(mA cm )

0.95

5.77

0.39

2.13

0.96 ±0.01

5.59 ±0.22

0.37 ±0.02

1.98 ±0.16

0.84

16.33

0.64

9.00

0.85 ±0.01

15.89 ±0.29

0.65 ±0.01

8.71 ±0.25

0.88

15.52

0.68

9.21

0.88 ±0.00

15.63 ±0.13

0.67 ±0.01

9.11 ±0.11

FF

-2

a

(%)

2

-1 -1

Mobility (cm V s ) Hole

Electron

2.18x10

-6

3.48x10

-6

1.17x10

-5

3.43x10

-6

2.83x10

-5

4.35x10

-6

The average PCE obtained from 20 devices.

External quantum efficiency (EQE) of PSCs based different polymers were recorded (Figure 4d), and the calculated JSC values from EQEs matched well with experimental JSC values. PSCs based on all three polymers exhibit photo-response ranging from 350-800 nm. PT1 based PSC have quite low EQE, 21% at 590 nm. The photo-response for PT2 and PT3 based PSC exceeded over 75% but with varied shapes. The PT2 based PSC showed strong photoresponse at 450-650 nm (accounting to polymer absorption region) with highest EQE of 78%. While, PT3 showed strong photo-response on longer wavelength (550-750 nm), accounting mainly for acceptor (ITIC) absorption, with highest EQE of 76 %. There is a little red shift in PT3 EQE spectra compared to other polymers. To investigate the exciton dissociation and charge extraction process of the optimize PSCs, we estimated the exciton probabilities, P(E,T) of devices from the plot of photocurrent (JPh) versus effective voltage54 (Veff) (Figure 4e). The photocurrent density, JPh is defined as JL- JD, where JL


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and JD are current densities under dark and illumination. The Veff is defined as Vo - V, the difference between the voltage (Vo) at JPh = 0 and the applied voltage (V). The estimated values of P(E,T) under maximum power output condition yielded 66.9%, 97.4% and 98.4% of PT1, PT2 and PT3 based devices, respectively. This indicates PT3 based PSC possess the most efficient exciton dissociation and charge extraction. The charge recombination process in optimized PSCs characterize by dependence of VOC on light intensity,55-56 as shown in Figure 4f. The slope of VOC versus lnP (P is light intensity) defines trap induced or Shockley-Read-Hall (SRH) recombination of carriers. In principle, the slope equal to 2kBT/q suggests the dominance in trap assisted recombination (where, kB, T and q are the Boltzmann constant, kelvin temperature and elementary charge, respectively). The value of slope kBT/q decreases with extending πconjugation of polymer backbone, thus, suppressing trap assisted recombination. Furthermore, we measured the linear dependence of JSC on light intensity57-58 Figure S2, by using formula JSC ∝ Pα, where α should be equal to 1 when all the free carriers swept out and collected at the electrodes, hence, negligible biomolecular recombination. The estimated values of α are 0.98, 0.97 and 0.99 for PT1, PT2 and PT3 based PSCs, respectively. Their comparative studies suggest PT3 based PSCs have efficient exciton dissociation, lower trap assisted and biomolecular recombination, which leads to high performance.

2.5. Photoluminescence Photoluminescence (PL) measurements performed to understand the exciton dissociation in blends.59-60 The blend films of each polymer prepared by following same procedure to device fabrication. Both blends and neat films were excited at 584 nm and emission spectra recorded from 610-900 nm (Figure 5). Neat ITIC film shows strong emission peak at 800 nm. Comparing


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with zigzag form of PT1 and PT3, the neat film of linear PT2 has relatively weak PL. In the studied blends, PL spectra of PT2 and PT3 based blends quenched strongly, suggesting efficient charge and energy transfer between polymer donor and ITIC.61 Whereas, the PL quench of PT1: ITIC blend remains incomplete with residue ITIC emission, which may be ascribed to the nonideal bend morphology and donor-acceptor interfacial states.

Figure 5. PL spectra of polymers and corresponding BHJ layers, (a) PT1, ITIC and PT1: ITIC; (b) PT2, ITIC and PT2: ITIC; (b) PT3, ITIC and PT3: ITIC. All were excited at 584 (nm).

2.6. Morphology The surface morphologies of cast BHJs for all three polymers were investigated by the atomic force microscopy (AFM). As shown in Figure 6a-6f, all three BHJ films exhibit relatively smooth surface, and fine features with curved fibre texture are observed for PT3, indicating smaller aggregates. Grazing incidence wide-angle X-ray scattering (GIWAXS)62 measurements were further performed to understand molecular packing and crystallinity of the BHJ film. Bragg scattering profiles for PT1, PT2, PT3, ITIC neat films, and their corresponding as cast blend films are shown in Figure 6g and Figure S3. The intensity profiles of both pristine polymers and


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blends clearly show the presence of stronger (010) π-π stacking peaks at out-of-plane (OOP) direction, than those at in-plane (IP) direction. The stacking distances (Table S1) are extracted to reveal that blends generally exhibit closer π-π stacking than that of neat polymers, which help improving molecular packing and charge transport of blends. Among those polymer blends with good performance, PT2:ITIC possess slightly shorter π-π stacking distance (3.61 Å), than that of PT3:ITIC blend (3.77 Å). It is because that linear polymer tends to form strongly stacked selfassemblies (than polymers with Zigzag backbones), usually requiring careful processing tuning to optimize blend morphology. However, Zigzag polymer, PT3 allows simple processing to reach good performance.

Figure 6. Tapping-mode AFM topography and phase images for as-cast films, (a, b) PT1: ITIC, (c, d) PT2: ITIC and (e, f) PT3: ITIC; (g) GIWAXS intensity profiles at out-of-plane (solid line) and in-plane (dotted line) for as-cast films of PT1: ITIC, PT2: ITIC, PT3: ITIC, Neat PT1, PT2, PT3 and ITIC.


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3. CONCLUSIONS In summary, three co-polymers (PT1, PT2 and PT3) were systematically investigated on the modulation of bridge groups between BDTT-BDD structures. We reveal that the selection of different bridge group can significantly alter the backbone conformation of resultant polymers, which further impacts on the optoelectronic and self-assembly properties of polymers, eventually influencing their corresponding blend morphologies and photovoltaic characteristics. Among all three polymers, PT3 containing 3-hexylthieno[3,2-b]thiophene bridging group adapts the extended zigzag backbone conformation, which retains 9.21% PCE with ITIC acceptor through simple solution-cast with single solvent, better than those of than of PT2 and PT1 based PSC, respectively. This study provides new examples to access balance between performance and feasible processability of polymer solar cell, which may be beneficial future scale-up fabrication of PSCs.

ASSOCIATED CONTENT The supporting Information contains, materials and instruments, synthetic details of intermediates and PT1, PT2, PT3 polymers, device fabrication, 2D-GIWAXS pattern and NMR spectra of intermediates and polymers. AUTHOR INFORMATION Corresponding Author E-mail address: [email protected], [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. T. Rehman and Z.-X. Liu contributed equally. ACKNOWLEDGMENT This research was funded by National Natural Science Foundation of China (Nos. 21722404, 21674093, 21503203 and 51473142), 973 program (No. 2014CB643503), International Science and Technology Cooperation Program of China (ISTCP) (Grant No. 2016YFE0102900), National Key R&D program of China (2017YFA0403403) and Zhejiang Province Science and Technology Plan (No. 2018C01047). C.-Z. Li thanks the support by Zhejiang Natural Science Fund for Distinguished Young Scholars (LR17E030001), and the Fundamental Research Funds for the Central Universities. X.L. thanks Research Grant Council of Hong Kong (General Research Fund No. 14314216). ABBREVIATIONS PSC, Polymer solar cell; BHJ, bulk heterojunction; WBG, wide bandgap; JSC Short-circuit current; VOC, open circuit voltage; PCE, Power conversion efficiency; DIO, 1,8-diiodooctane; AFM, Atomic force microscopy; GIWAXS, Grazing incidence wide angle X-ray scattering . REFERENCES (1) Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45 (5), 723-733. (2) Nielsen, C. B.; Holliday, S.; Chen, H.-Y.; Cryer, S. J.; McCulloch, I. Non-Fullerene Electron Acceptors for Use in Organic Solar Cells. Acc. Chem. Res. 2015, 48 (11), 2803-2812. (3) Krebs, F. C. Fabrication and Processing of Polymer Solar Cells: A Review of Printing and Coating Techniques. Sol. Energy Mater. Sol. Cells 2009, 93 (4), 394-412. (4) Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chem. Rev. 2014, 114 (14), 7006-7043. (5) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies Via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270 (5243), 1789-1791.


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5

PT1 PT2 PT3

-2

Current Desity (mA cm )

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0 Poly(BDTT-π-BDD) PT 1-3

-5 NONE PT1

-10 PT2 60

-15 0.0

0.2

0.4

0.6

0.8

1.0

PT3

Voltage (V)


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Influence of Bridging Groups on the Photovoltaic Properties of Wide

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