Linear solubilizing side chain substituents enhance the photovoltaic properties of two-dimensional conjugated benzodithiophene-based polymers

Accepted Manuscript Linear solubilizing side chain substituents enhance the photovoltaic properties of twodimensional conjugated benzodithiophene-based polymers Jian-Ming Jiang, Putikam Raghunath, Yu-Che Lin, Hsi-Kuei Lin, Chen-Lin Ko, Yu-Wei Su, M.C. Lin, Kung-Hwa Wei PII:

S0032-3861(15)30319-0

DOI:

10.1016/j.polymer.2015.10.033

Reference:

JPOL 18200

To appear in:

Polymer

Received Date: 18 July 2015 Revised Date:

5 October 2015

Accepted Date: 16 October 2015

Please cite this article as: Jiang J-M, Raghunath P, Lin Y-C, Lin H-K, Ko C-L, Su Y-W, Lin MC, Wei KH, Linear solubilizing side chain substituents enhance the photovoltaic properties of two-dimensional conjugated benzodithiophene-based polymers, Polymer (2015), doi: 10.1016/j.polymer.2015.10.033. 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.

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Graphical Abstract Linear solubilizing side chain substituents enhance the photovoltaic

polymers

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properties of two-dimensional conjugated benzodithiophene-based

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Jian-Ming Jiang,a Putikam Raghunath,b Yu-Che Lin,a Hsi-Kuei Lin,a Chen-Lin Ko,a Yu-Wei Su,a M. C. Lin,b,c and Kung-Hwa Wei＊a

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Linear solubilizing side chain substituents enhance the photovoltaic properties of two-dimensional conjugated benzodithiophene-based polymers

Department of Materials Science and Engineering, National Chiao Tung University,

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a

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Jian-Ming Jiang,a Putikam Raghunath,b Yu-Che Lin,a Hsi-Kuei Lin,a Chen-Lin Ko,a ＊ Yu-Wei Su,a M. C. Lin,b,c and Kung-Hwa Wei a

300, Hsinchu, Taiwan

Center for Interdisciplinary Molecular Science Department of Applied Chemistry,

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b

National Chiao Tung University, 300, Hsinchu Taiwan Department of Chemistry, Emory University, Atlanta, GA, USA

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c

e-mail address: [email protected]

ACCEPTED MANUSCRIPT ABSTRACT: In this study we synthesized medium-bandgap, two-dimensional (2-D) conjugated polymers comprising electron-rich benzo[1,2-b:4,5-b´]dithiophene (BDT) units

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presenting conjugated thiophene (T) side chains and electron-deficient alkoxy-modified 2,1,3-benzooxadiazole (BO) moieties. We introduced various

solubilizing substituents—linear alkyl, alkoxy, and alkythio units—on the thiophene

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side chains to obtain a series of 2-D conjugated D–π–A polymers: PBDTT-C-BO,

PBDTT-O-BO, and PBDTT-S-BO. The solubilizing substituents of the BDT units

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altered the solubility, conformations, and electronic properties of the synthesized conjugated polymers, allowing tuning of the photovoltaic properties when blended with fullerenes. We investigated the effects of the different linear solubilizing substituents of the BDT units on the structural, optical, and electronic properties (e.g., band gap energies) of the resulting 2-D conjugated polymers, as determined from

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quantum-chemical calculations, UV–Vis absorption spectra, and grazing-incidence X-ray diffraction. Atomic force microscopy and transmission electron microscopy images revealed the morphologies of active layers comprisingthese 2-D conjugated

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polymers and the fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester

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(PC71BM). Through rational structural modifications of the solubilizing substituents in the 2-D conjugated polymers with alkoxy, alkythio or alkyl units, the resulting PCEs varied from 5.4 to 7.5%. A polymer solar cell based on a blend of PBDTT-C-BO and PC71BM exhibited the best photovoltaic performance among our three studied systems, with a high short-circuit current density (Jsc) of 15.7 mA cm–2 and a power conversion efficiency of 7.5%, without the need for any processing additives or post-treatment processes, highlighting the importance of careful selection of appropriate solubilizing substituent that attached to the donor segments when

ACCEPTED MANUSCRIPT designing efficient D–π–A polymers for use in solar cells. Keywords: photovoltaics, two-dimensional conjugated polymers, sidechain structure,

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solubilizing substituent

ACCEPTED MANUSCRIPT Introduction Thin-film polymer solar cells (PSCs) based on bulk heterojunction (BHJ) structures incorporating conjugated polymers possessing delocalized π electrons and

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fullerene derivatives are being studied extensively because they allow the fabrication of light-weight, large-area, flexible devices using low-cost solution processing

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methods.1–3 Tremendous efforts have been made toward improving the power

conversion efficiencies (PCEs) of polymer BHJ devices that incorporate conjugated

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polymers and fullerene derivatives as their electron-donating and -accepting components, respectively.4–8 In recent years the field of polymer photovoltaics has experienced tremendous advances, both in our understanding of the underlying

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photo-physical processes9 and in the morphology10 of the active layer of the devices. To obtain even higher PCEs, the search continues for optimized structures

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combining high absorption coefficients with broad solar absorption, thereby

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improving the harvesting of solar light and, hence, increasing the short-circuit current density (Jsc). Nevertheless, an improvement in PCE not only requires a greater value of Jsc but also suitable energy levels to ensure a high open-circuit voltage (Voc), and a well-defined morphology to ensure a reasonable fill factor (FF). The ability to increase light harvesting while maintaining a deep highest occupied molecular orbital (HOMO) and high solubility remain challenging when designing new conjugated polymers as materials for organic photovoltaic applications.11 In attempts to harvest

ACCEPTED MANUSCRIPT more photons and tune the energy levels, several polymers have been developed featuring conjugated electron donor/acceptor (D/A) units in their main chains12–35 or

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two-dimensional (2-D) conjugated configurations.36–49 At present, the efficiencies of PSCs are reaching beyond 10% as a result of our better understanding of the

photon-to-electron conversion mechanism and the development of novel materials

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and tandem device architectures.50–54

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Generally, the band gaps and energy levels of conjugated polymers are determined primarily by the molecular structure of the polymer backbone, rather than its alkyl solubilizing groups.55 Nevertheless, the solubilizing side chain substituents can not only influence the solubility of the polymers in the processing solvent but also

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the packing of the polymers in the thin film state, and thus play significant roles in modulating the values of Voc and Jsc of BHJ solar cells fabricated from polymers

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containing even identical conjugated backbones.46,56–58 The solubilizing side chains

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are particularly important in imparting sufficient solubility of the resulting polymers for use in solution-processable organic electronics. Thus, improvements in PCEs require careful selection of polymer solubilizing side chains. Electron-donating solubilizing side chains (e.g., linear alkyl, alkoxy, and alkylthio groups) can donate some electron density to a conjugated polymer backbone. Because the electron-donating ability of sulfur is weaker than oxygen, and because the sulfur atom

ACCEPTED MANUSCRIPT has some π-acceptor capability, due to pπ(C)–dπ(S) orbital overlap where divalent sulfur accepts π-electron density from the porbital of the carbon–carbon double bond

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into its empty 3d orbitals,46,59 polymers containing alkylthio side chains can exhibit unique optoelectronic properties.

One of the most effective approaches to optimizing the performance of PSCs is

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the incorporation of extended π-electron cores in the polymer backbone, thereby

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strengthening intermolecular interactions and enhancing π–π overlap between neighboring polymers. In addition, the electrostatic interactions of D–A-type semiconducting polymers can enhance their intermolecular interactions, thereby facilitating the formation of long-range-ordered structures that are important for

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efficient carrier transport.60,61 These strong intermolecular interactions can, however, decrease the solubility of the polymers significantly. For this reason, D–A polymers

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possessing highly π-extended cores require many long and/or bulky solubilizing

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groups to ensure sufficient solubility for fabrication of efficient devices. From the viewpoint of material design, another important concern is the choice of appropriate side chains. Installation of optimal side chains can enhance both intermolecular interactions and crystallinity, factors that lead to appropriate morphologies and effective carrier transport.62–64 In addition, the architecture (e.g., linear, branched), length, and number of side chains can all dramatically affect the molecular orientation

ACCEPTED MANUSCRIPT and morphology of the polymer backbone.65–67Because PSCs feature a vertically arranged electrode relative to their surfaces, the incorporated polymers should ideally adhere in a face-on orientation, known to facilitate carrier transport, to result in high

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values of Jsc and no loss of FF.68 Thus, optimizing the solubilizing side chain

substituents of the polymer and investigating the relationships between the polymer

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thin film’s structure and its photovoltaic properties are essential aspects when

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developing high-performance PSCs. Several studies have demonstrated that octyl-based polymers can deliver slightly lower values of Voc, but with significantly improved values of Jsc, relative to corresponding 2-ethylhexyl–based polymers.56,57 To investigate the effects of the solubilizing side chain substituents, in this study

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we synthesized three 2-D conjugated D–π–A copolymers having their solubilizing side chain groups positioned perpendicular to their main chains. Here, we used BDT

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moieties presenting conjugated thiophene side chains that were modified with linear

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alkyl, alkoxy, or alkythio solubilizing substituent side chains as the donor units, and thiophene as the π-bridge. The reason that we choose the alkoxy-benzooxadiazole unit (BO) as the acceptor unit is owing to the fact that BO can effectively lower the HOMO energy levels of the synthesized donor-acceptor conjugated polymers and thus can lead to a high device Voc when blended with fullerene derivatives. 32–35 We probed the effects of these structures on the absorption spectra, energy levels, hole mobility

ACCEPTED MANUSCRIPT (in films), and photovoltaic performance of the three 2-Dconjugated D–π–A copolymers. Morphological studies, quantumchemical calculations, and

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grazing-incidence wide-angle X-ray (GIWAX) diffraction patterns also provided insight into the different properties of our three tested systems. Experimental Section

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Materials and Synthesis

annane) (M1),46

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(4,8-Bis(5-octylthiophen-2-yl)benzo[1,2-b:4,5-b´]dithiophene-2,6-diyl)bis(trimethylst

(4,8-bis(5-(octyloxy)thiophen-2-yl)benzo[1,2-b:4,5-b´]dithiophene-2,6-diyl)bis(trimet hylstannane) (M2),46,69

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(4,8-bis(5-(octylthio)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimet hylstannane) (M3),46,58 and

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4,7-bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)benzo[c][1,2,5]oxadiazole (M4)70

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were prepared according to reported procedures. The general procedures for Stille polymerization of the three 2-D polymers are put in supporting information. [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) was purchased from Nano-C. All other reagents were used as received without further purification, unless stated otherwise. Measurements and Characterization

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H NMR spectra were recorded using a Varian UNITY 300MHz spectrometer.

Thermo gravimetric analysis (TGA) was performed using a TA Instruments Q500 apparatus; the thermal stabilities of the samples were determined under N2 atmosphere

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by measuring their weight losses while heating at a rate of 20 °C min–1. Size

exclusion chromatography (SEC) was performed using a Waters chromatography unit

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interfaced with a Waters 1515 differential refractometer; polystyrene was the standard;

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the temperature of the system was set at 45 °C; CHCl3 was the eluent. UV–Vis spectra of dilute solutions (1 × 10–5 M) of samples in dichlorobenzene (DCB) were recorded at room temperature (ca. 25 °C) using a Hitachi U-4100 spectrophotometer. Solid films for UV–Vis spectroscopic analysis were obtained by spin-coating the polymer

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solutions onto a quartz substrate. Cyclic voltammetry (CV) of the polymer films was performed using a BAS 100 electrochemical analyzer operated at a scan rate of 50

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mV s–1; the solvent was anhydrous MeCN, containing 0.1 M

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tetrabutylammoniumhexafluorophosphate (TBAPF6) as the supporting electrolyte. The potentials were measured against a Ag/Ag+ (0.01 M AgNO3) reference electrode; the ferrocene/ferrocenium ion (Fc/Fc+) pair was used as the internal standard (0.09 V). The onset potentials were determined from the intersection of two tangents drawn at the rising and background currents of the cyclic voltammograms. HOMO energy levels were estimated relative to the energy level of the ferrocene reference (4.8 eV

ACCEPTED MANUSCRIPT below vacuum level). Topographic and phase images of the polymer:PC71BM films (surface area: 1×1µm2) were obtained using a Digital Nanoscope III atomic force

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microscope operated in the tapping mode under ambient conditions. The thickness of the active layer of the device was measured using a VeecoDektak 150 surface profiler. GIWAXs experiments were performed at the National Synchrotron Radiation

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Research Center. Transmission electron microscopy (TEM) images of the

microscope operated at 120 keV.

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polymer:PC71BM films were recorded using a FEI T12 transmission electron

Fabrication and Characterization of Photovoltaic Devices

Indium tin oxide (ITO)–coated glass substrates were cleaned step wise in detergent,

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water, acetone, and isopropyl alcohol (ultra sonication; 20 min each) and then dried in an oven for 1 h; the substrates were then treated with UV ozone for 30 min prior to

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use. A thin layer (ca. 20 nm) of polyethylenedioxythiophene:polystyrenesulfonate

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(PEDOT:PSS, Baytron P VP AI 4083) was spin-coated (5000 rpm) onto the ITO substrates. After baking at 140 °C for 20 min under N2 atmosphere, the substrates were transferred to a N2-filled glove box. The polymer and PCBM were co-dissolved in DCB at various weight ratios, but at a fixed total concentration of 3 wt % (30 mg mL–1). The blend solutions were stirred continuously for 12 h at 80 °C and then filtered through a PTFE filter (0.2 µm); the photoactive layers were obtained by

ACCEPTED MANUSCRIPT spin-coating (600–2000 rpm, 60 s) the blend solutions onto the ITO/PEDOT:PSS surfaces. The thickness of each photoactive layer was approximately 90–120 nm. The

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devices were ready for measurement after thermal deposition (pressure: ca.1 × 10–6 mbar) of a 20-nm-thick film of Ca and then a 100-nm-thick Al film as the cathode.

The effective layer area of one cell was 0.04 cm2. The current density–voltage (J–V)

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characteristics were measured using a Keithley 2400 source meter. The photocurrent

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was measured under simulated AM 1.5 G illumination at 100 mW cm–2 using a Xe lamp–based Newport 150-W solar simulator. A calibrated Si photodiode with a KG-5 filter was employed to confirm the illumination intensity. External quantum efficiencies (EQEs) were measured using an SRF50 system (Optosolar, Germany). A

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calibrated mono-silicon diode exhibiting a response at 300–800 nm was used as a reference. For hole mobility measurements, hole-only devices were fabricated having

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the structure ITO/PEDOT:PSS/polymer/Au. The hole mobility was determined by

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fitting the dark J–V curve into the space-charge-limited current (SCLC) model,29,61 based on the equation

9 V2 J = ε0 εr µh 3 8 L

where ε0 is the permittivity of free space, εr is the dielectric constant of the polymer (assumed to be 3.0 for the conjugated polymers), µ h is the hole mobility, V is the voltage drop across the device, and L is the thickness of the active layer.

ACCEPTED MANUSCRIPT Results and Discussion Synthesis and Characterization of Polymers Scheme 1 outlines our synthesesof the designed polymers.We synthesized M1,46

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M2,46,69 M3,46,58 and M470 using methods reported in the literature. From Stille

couplings of M1–M3 with M4 in the presence of Pd2dba3 in CB at 130 °C for 48 h,

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we obtained the polymers PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO,

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respectively, in yields of 60–75%. Table 1 lists the number-average (Mn) and weight-average (Mw) molecular weights of these polymers, as determined through SEC, against polystyrene standards, in CHCl3 as the eluent.

C8H 17

C8H 17 S Sn S S C8H17 M1

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Sn

Br

S S Sn

Sn S

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C8H17O

OC8H 17 S

C8H 17O S

N

O

S N S C 8H17 Br

OC8H 17 S

S N

OC8H 17 M2

OC8H17

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PBDTT-C-BO

C8H 17O S

S

S

PBDTT-O-BO

C8H 17O S

S

Sn

OC8H 17 S

S N

SC8H17 M3

N

S

S S

O

C8H17S

S

S

N

S

S

C8H 17S

O

C8H17O

N

M4

OC8H 17 S

C8H 17O S

S

S

Sn

S

S SC8H17

O

N

PBDTT-S-BO

Scheme 1. Synthesis of the 2-D conjugated polymersPBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO.

ACCEPTED MANUSCRIPT Physical and Thermal Stability Table 1 summarizes the physical characteristics of the copolymers prepared in this study. They all exhibited high polymerization yields (60–75%) and molecular weights

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(Mn =35–43 kg mol–1) after a series of Soxhlet extractions, a result of the good

solubilities of the copolymers after adding the alkyl chains to both the D and A

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moieties. It has been reported previously that solubilizing alkyl chains can have a very

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significant influence on the polymerization yields and molecular weights of D–A copolymers.55,72 All of the copolymers had 5% weight-loss temperatures (Td) of over 300 °C (Figure 1), indicating good thermal stability. In addition, copolymers having the same polymer backbone exhibited similar values of Td. We ascribe this behavior

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to the thermal decomposition of the copolymers beginning at their soft parts (alkyl

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chains), which were identical for the same type of copolymer.

Table 1. Molecular weights, thermal properties, and solubilities of polymers Solubilityc

(°C)

(g L–1)

3.7

310

25.1

134.8k

3.1

301

12.4

112.3k

3.2

310

27.6

Mwa

(kDa)

(kDa)

PBDTT-C-BO

39.8k

147.2k

PBDTT-O-BO

43.5k

PBDTT-S-BO

35.1k

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Tdb

Mna

Polymer

PDIa

Values of Mn, Mw, and PDI of the polymers were determined through GPC (in CHCl3, using polystyrene standards). b The 5% weight-loss temperature (°C) under N2 atmosphere.c Concentration of the saturated solution in DCB at 25 °C, according to the Beer–Lambert law.

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PBDTT-C-BO PBDTT-O-BO PBDTT-S-BO

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90

80

70

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Weight Loss (%)

100

50 100

200

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60

300

400

500

600

700

o

Temperature ( C )

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Figure 1. TGA thermograms of the copolymers PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO, recorded at a heating rate of 20 °C min–1 under a N2 atmosphere.

Optical Properties. We recorded normalized optical UV–Vis absorption spectra

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of diluted polymer solutions (in DCB) at room temperature and of their films

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spin-coated onto quartz substrates. Figure 2a displays the absorption spectra of the solutions of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO in DCB at room temperature; each absorption spectrum, recorded from a dilute DCB solution, featured two absorption bands: one at 300–450 nm, which we assign to localized π–π* transitions, and another, broad band from 500 to 650 nm (i.e., in the long wavelength region) representing intramolecular charge transfer (ICT) between the acceptor (BO)

ACCEPTED MANUSCRIPT and donor (BDT) units. The absorption spectra of the three polymers in the solid state were similar to their corresponding solution spectra, with slight red-shifts (ca. 5–40

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nm) of their absorption maxima, indicating that some intermolecular interactions existed in the solid state. Table 2 summarizes the optical data, including the

absorption peak wavelengths (λmax,abs), absorption edge wavelengths (λedge,abs), and

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optical band gaps ( Egopt ) of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO.The

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absorption edge of PBDTT-O-BO was red-shifted by 10 nm relative to those of PBDTT-C-BO and PBDTT-S-BO, presumably because the electron-donating ability of alkoxy groupsis stronger than that of alkylthio groups.73,74 Figure 2b displays the absorption coefficients of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO films

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of similar thicknesses. Although the absorption profiles of the polymers were almost identical, the absorption coefficients of PBDTT-C-BO and PBDTT-S-BO reached

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6.5 × 104cm–1 and 6.1 × 104 cm–1 at their absorption maxima, while that of

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PBDTT-O-BO reached only 5.6× 104 cm–1. We suspected that the higher absorption coefficients of PBDTT-C-BO and PBDTT-S-BO would lead to enhanced values of Jsc for their corresponding devices.

Table 2. Optical properties of polymers

λmax,abs (nm) PBDTT-C-BO

Solution

Film

596

593, 640

λonset (nm)

FWHM (nm)

Film

Film

698

190

E gopt (eV)

1.77

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596

600, 640

710

172

1.74

PBDTT-S-BO

589

590, 635

698

183

1.77

(a)

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Solution

1.0

PBDTT-C-BO PBDTT-O-BO PBDTT-S-BO

0.8

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Abs

0.6 0.4

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0.2 0.0 300

400

500

600

700

800

700

800

Wavelength

4

7x10 6x10

PBDTT-C-BO PBDTT-O-BO PBDTT-S-BO

4

4x10

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4

5x10

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-1

Absorption coefficient (cm )

Film 4

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(b)

4

3x10

4

2x10

4

1x10

0 300

400

500

600 Wavelength

Figure

2.

UV–Vis

absorption

spectra

of

the

polymers

PBDTT-C-BO,

PBDTT-O-BO, and PBDTT-S-BO as (a) dilute solutions (1 × 10–5 M) in DCB and (b) solid films.

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Electrochemical Properties. To investigate the effects of the solubilizing side

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chains on the electronic energy levels of the polymers, we used CV to measure the electrochemical behavior of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO and, thereby, determine their HOMO energy levels. Figure S1 displays the electrochemical

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properties of films of the solid polymers; Table 3 summarizes the data. Partially

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irreversible p-doping/dedoping (oxidation/re-reduction) processes occurred for these ox polymers in the positive potential range. The onset oxidation potentials ( Eonset , vs

Ag/Ag+) for PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO were 0.62, 0.57, and 0.66 V, respectively. On the basis of these onset potentials, we estimated the HOMO

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energy levels, according to the energy level of the ferrocene reference (4.8 eV below vacuum level),56,75 of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO to

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be –5.42, –5.37, and –5.46 eV, respectively; these slight variations indicate minor

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modulations of the ICT strength induced by the electronic effectsof the solubilizing side chains of theBDT moieties. Because we did not directly observe the onset reduction potentials of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO during the reduction process, we calculated the energy levelsof the lowest unoccupied molecular orbitals (LUMOs) of these polymers from the HOMO energy levels and the optical energy gap ( E gopt ): LUMO =  E gopt + HOMO (eV). Figure 3 reveals that the LUMO

ACCEPTED MANUSCRIPT energy levels of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO were located within the range from –3.63 to –3.69 eV—that is, they were significantly higher than that (ca. –4.0 eV) of PC71BM; thus, we expected efficient charge transfer/dissociation

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to occur in their corresponding devices.76 Moreover, PBDTT-O-BO exhibited the highest HOMO energy level, due to strong electron donation from the alkoxy

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substituents. In addition, the HOMO energy level of PBDTT-S-BO (alkylthioside

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chains) was 0.09 eV lower than that of PBDTT-O-BO, indicating that the thiophene side chains in the 2-D copolymers were in conjugation with the polymer main chains and that the substituents on the conjugated thiophene side chains significantly

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influenced the electronic properties of these 2-D conjugated polymers.

Table 3. Electrochemical properties of polymers

HOMOa

E gopt

LUMOb

HOMOc

LUMOc

(V)

(eV)

(eV)

(eV)

(eV)

(eV)

PBDTT-C-BO

0.62

–5.42

1.77

–3.65

–5.02

–2.61

PBDTT-O-BO

0.57

–5.37

1.74

–3.63

–4.92

–2.61

PBDTT-S-BO

0.66

–5.46

1.77

–3.69

–5.15

–2.66

a

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ox E onset

HOMO energy levels determined from the onsets of the CV curves, using the equation HOMO = –(4.8 + Eoxonset) eV. b LUMO energy levels determined using the opt

equation ELUMO = EHOMO + Eg . c HOMO and LUMO energy levels calculated using DFT.

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Figure 3. Energy level diagram for the polymers.

Computational Study. We optimized the ground state geometries of the three polymers (PBDTT-C-BO, PBDTT-O-BO, PBDTT-S-BO) using density functional

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theory (DFT) at the B3LYP/6-31G(d,p) level. We performed vibrational analysis to check that the optimized geometries were local minima; only the lowest-energy

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conformations are reported herein. All calculations were performed using Gaussian

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09.77 We calculated the frontier molecular orbitals (HOMO, LUMO) at the same level of theory. We investigated the electronic nature of the absorption bands through time-dependent DFT (TDDFT)78 calculations at the B3LYP/6-31G(d,p) level in vacuo, up to 10 excited states. In addition, we also calculated the ground-to-excited state dipole change (∆µge). Table 4 lists selected dihedral angles in the main backbones and side chains of

ACCEPTED MANUSCRIPT the optimized ground state geometries of the three polymers (PBDTT-C-BO, PBDTT-O-BO, PBDTT-S-BO). For PBDTT-C-BO, the dihedral angle θ1 between

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the π-bridge thiophene and the acceptor unit of BO was 6.1°, suggesting high planarity of the π-conjugated systems. In contrast, the side chain thiophene group was twisted with atorsion angle θ2 of approximately 55.8°. In a previous study,70 we had

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observed a similar trend.

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To understand the nature of the charge transfer and the changes in electronic transitions that occurred upon varying the substituents on the donor moieties, we investigated the HOMO and LUMO energy levels along with frontier molecular

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orbital distributions; Tables 4 and Table S1 display the results. The HOMO and LUMO energy levels of PBDTT-C-BO were–5.02 and –2.61 eV, respectively, giving a HOMO–LUMO gap (HLG) of 2.41 eV. Substitution with a CH3S- group at the side

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chain of the thiophene unit in the BDT moiety of PBDTT-S-BO decreased the energy

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of the HOMO by 0.13 eV, while the LUMO energy level decreased by only 0.05 eV relative to that of PBDTT-C-BO. Table 4 displays the molecular orbital distributions of the HOMO and LUMO isosurfaces of all three polymers calculated at the B3LYP level. The calculated HOMO and LUMO energy levels are consistent with the experimental values in Table S1. The calculations reveal that the molecular orbitals of the HOMO have π character; with electron density delocalized mainly in the donor

ACCEPTED MANUSCRIPT side over the whole main chain with very minor contributions on the side chain of the thiophene units. For the LUMO, the electron density was distributed mainly in the BO

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acceptor unit, with no electron density contribution on the side chain.

at the B3LYP/6-31G(d,p)level.

Structure

HOMO orbital

LUMO orbital

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55.8

6.0

56.0

5.4

55.4

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PBDTT-S-BO

θ2 (°)

6.1

PBDTT-C-BO

PBDTT-O-BO

θ1 (°)

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Polymer

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Table 4. Dihedral angles in the polymers, determinedfrom theoretical simulations and frontiermolecular orbital distributions of the HOMO and LUMO isosurfacescalculated

Electronic Absorption Spectra. We performed computational studies to gain a

deeper understanding of the experimentally determined optical properties. Here, we used the TDDFT method at the B3LYP/6-31G(d) level to obtain the electronic absorption properties. From these calculations, we predicted the absorption

ACCEPTED MANUSCRIPT wavelengths (λabs), oscillator strengths (f), nature of the major transition descriptions of S1-S2, and transition state dipole moments from the ground to excited state (µtr)

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(Debye); Table S1 displays the results. There is good agreement with the experimentally observed values. The maximum absorptions of all three molecules

arose mainly from electronic transitions from the ground state (S0) to the first excited

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state (S1), resulting in major contributions from the HOMO→LUMO, contributing

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greater than 99%, with oscillator strengths of 0.36–0.56. For all of the polymers, the S2 state was contributed mainly by the H-1→ L configuration with low oscillator strengths, except for PBDTT-O-BO, where the absorption maximum appeared near

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584 nm with an oscillator strength of 0.561. Interestingly, substitution of a CH3S group at the BDT donor part, giving PBDTT-S-BO, caused the absorption maximum (λabs) to decrease by approximately 21 nm from that of PBDTT-C-BO. The oscillator

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strength in PBDTT-C-BO was smaller than that in PBDTT-S-BO. Significantly,

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substitution with a CH3O group at the BDT donor part, giving PBDTT-O-BO, caused the absorption maximum (λabs) to increase by 25 nm from that of PBDTT-C-BO. Furthermore, we calculated the ground state dipole moments for the three

molecules to be 3.5 D. Each dipole moment was in the direction of the y-axis, passing through the side chain. The calculated transition state dipole moment from the ground to excited state (µtr) for PBDTT-C-BO was 8.34 D, with charge transfer being

ACCEPTED MANUSCRIPT dominant mainly on the x-axis and perpendicular to the dipole moment, lying on the backbones of the molecule. Previous studies79,80 have indicated that the change in the

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dipole moment from the ground to the excited state (∆µge) of a conjugated polymer largely correlates with the efficiency of related polymer-based solar cells. We used

dipole (∆µge) along each coordinate axis:

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∆µge =[(µgx – µex)2 + (µgy – µey)2 + (µgz – µez)2]1/2

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the following equation to determine the overall change in the ground and excited state

We took the ground state dipole moments (µg) and excited state dipole moments (µe) from the ground states and optimized excited states (S1), respectively, calculated

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at the TDDFT level. The calculated excited state dipole of PBDTT-C-BO (19.2 D) was higher than that of PBDTT-S-BO (13.07 D). As revealed in Table S2, PBDTT-C-BO provided a wider range of values for ∆µge (3.56–18.73 D) when

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compared with those of PBDTT-S-BO (3.57–11.18 D). From these results, the large

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change in ∆µge for PBDTT-C-BO molecule is closely correlated with the overall PCE.71 Similarly, Table S2 reveals that the predicted change in ∆µge was 28.46 D for PBDTT-O-BO, but its experimental PCE was lower than those of the other two polymers. To further investigate this phenomenon, we examinedthe photovoltaic properties. Hole Mobility. Figure 4 displays the hole mobilities of devices incorporating the

ACCEPTED MANUSCRIPT polymer/PC71BM blends ata blend ratio of 1:2 (w/w). The hole mobilities, determined between the voltage drop of 0.4–1.5V, in the PBDTT-C-BO, PBDTT-O-BO, and

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PBDTT-S-BO blends with PC71BM were 9.1×10–3, 1.1×10–3, and 3.5×10–3 cm2 V–1s–1, respectively; thus, the hole mobility in the PBDTT-C-BO blend was almost one order

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of magnitude larger than that in the PBDTT-O-BO blends.

5

PBDTT-C-BO PBDTT-O-BO PBDTT-S-BO

2

Current density (mA/cm )

10

4

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10

3

10

2

1

10

TE D

10

V-Vbi (V)

1

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Figure 4. Dark J–V curves of hole-dominated carrier devices incorporating

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the polymers PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO blended with PC71BM [blend ratio, 1:2 (w/w)].

Photovoltaic Properties and Active Layer Morphology. Next, we investigated

the photovoltaic properties of the polymers in BHJ solar cells having the sandwich structure ITO/PEDOT:PSS/polymer:PC71BM (1:2, w/w)/Ca/Al, with the photoactive layers having been spin-coated from DCB solutions of the polymer and PC71BM. After testing several compositions, we found that the optimized weight ratio for the

ACCEPTED MANUSCRIPT polymer and PC71BM was 1:2. Notably, we did not use any processing additives nor employ any post treatment processes; therefore, we could directly investigate the

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effects of the various solubilizing side chains on the device performance. Figure 5 presents the J–V curves of these PSCs; Table 5 summarizes the PCEs. The devices prepared from the blends of PBDTT-C-BO, PBDTT-O-BO, and

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PBDTT-S-BO with PC71BM exhibited open-circuit voltages (Voc) of 0.74, 0.71, and

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0.86 V, respectively. The value of Voc of the PSC based on PBDTT-S-BO was 0.15 V higher than that of the PSC based on PBDTT-O-BO, benefitting from the lower HOMO energy level of PBDTT-S-BO. Because the electron-donating ability of a

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sulfur atom is weaker than that of an oxygen atom, the HOMO energy level of PBDTT-S-BO (containing alkythio side chains) was lower than that of PBDTT-O-BO. The value of Voc has previously been related to the energy difference

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between the LUMO of the fullerene acceptor and the HOMO of the donor polymer,82

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so long as there is good ohmic contact between the active layer and the device electrodes.83 The value of [ELUMO(fullerene) – EHOMO(polymer)]/e represents the maximum achievable voltage from a BHJ device, but it is rarely observed because of losses originating from band offsets, internal space charge, and molecular reorganizational energy characterized by the Stokes shift.84 We suspect that the Voc difference between the PBDTT-C-BO and PBDTT-S-BO device is owing to the

ACCEPTED MANUSCRIPT relative higher π–π stacking of PBDTT-C-BO than that of PBDTT-S-BO, which induces a slight loss in Voc.85

Polymer/PC71BM (1:2)

(V)

PBDTT-C-BO

0.74

15.7

7.5 (7.2)

64

PBDTT-O-BO

0.71

13.0

5.4 (5.3)

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PBDTT-S-BO

0.86

13.2

Jsc

–2

FF (%)

Mobility Thickness (cm2 V–1 s–1) (nm) 9.1× 10–3

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58

6.6 (6.4)

58

111

1.1× 10–3

113

3.5× 10–3

117

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0

PBDTT-C-BO PBDTT-O-BO PBDTT-S-BO

-5

-10

EP

2

Current density (mA/cm )

More than 10 devices were fabricated.

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a

Voc

PCEmax (mA cm ) (PCEaverage)a (%)

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Table 5. Photovoltaic properties of PSCs incorporating the three polymers

-15

0.0

0.2

0.4

0.6

0.8

Voltage (V)

Figure 5. J–V characteristics of PSCs incorporating PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BOblended with PC71BM [blend ratio, 1:2 (w/w)].

ACCEPTED MANUSCRIPT The short-circuit current densities of the devices incorporating blends of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO with PC71BM were 15.7, 13.1,

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and 13.2 mA cm–2, respectively. Figure 6 displays the EQE curves of the devices incorporating the polymer:PC71BM blends at weight ratios of 1:2. The empirical short-circuit current densities obtained from integrating the EQE curves of the

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PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO blends were 15.0, 12.8, and 12.9

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mA cm–2—values that agree reasonably (discrepancies: <5%) with the measured (AM 1.5 G) values.The highest value of Jsc (15.7 mA cm–2) was that for the device incorporating PBDTT-C-BO, presumably because of its large absorption coefficient,

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broadest absorption range of FWHMs, and high hole mobility (9.1×10–3 cm2 V–1 s–1). Although PBDTT-O-BO had, among the three polymers, the broadest absorption range from 500–700 nm, its device exhibited the lowest value of Jsc (13.0mA cm–2),

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presumably because of the low absorption coefficients and low hole mobility

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(1.1×10–3 cm2 V–1 s–1).

ACCEPTED MANUSCRIPT

100 PBDTT-C-BO PBDTT-O-BO PBDTT-S-BO

60

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EQE (%)

80

40

0 400

500

600

700

800

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300

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20

Wavelength (nm)

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Figure 6. EQE curves of PSCs incorporating polymer:PC71BM blends[blend ratio, 1:2 (w/w)].

The highest FF was that of the device incorporating PBDTT-C-BO in the active layer, most likely as a result of its high hole mobility. In the 2D GIWAX image of the

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polymer PBDTT-C-BO in Figure 7, we observe evidence for strong π–π stacking

AC C

[d(010) = 3.78Å] along the out-of-plane direction, indicating a mostly isotropic structure with little preference for the face-on orientation. This face-on orientation would bebeneficial for charge transport in the device. In contrast, the image for PBDTT-S-BO suggested an ordered structure with arc-shaped lamellar scattering [d(100) = 20.2Å] and π–π stacking [d(010) = 3.63Å] along the out-of-plane direction, indicating both face- and edge-on orientations. The image for PBDTT-O-BO

ACCEPTED MANUSCRIPT suggested a preference for the edge-on orientation along the out-of-plane direction [d(100) = 20.2Å]. Such a face-on orientation enhances vertical charge carrier transport in PSC devices.86–89

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SC

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(a)

(b)

PBDTT-C-BO PBDTT-O-BO PBDTT-S-BO

4

EP

TE D

4

1x10

AC C

Intensity

2x10

0

1

2

qz (Å-1)

Figure 7. GIWAXS images of pure films of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO: (a) 2-D images of each pure film; (b) X-ray diffraction patterns of pure films in the out-of-plane direction.

ACCEPTED MANUSCRIPT When exploring the decisive factors affecting the efficiencies of PSCs, we must consider not only the absorption behavior, energy levels, crystallinity, and orientation

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of the polymers but also the surface morphologies of their blends.90 Figure S2 display the surface morphologies of our blend systems, determined using AFM. We prepared samples of these polymer/PC71BM blends using procedures identical to those we

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employed to fabricate the active layers of the devices. In each case, we observed a

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smooth morphology for the blends of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO with PC71BM, with root-mean-square (rms) roughnesses of 1.1, 0.9, and 1.4 nm, respectively. Furthermore, Figure S3 displays TEM images of thin films of PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO blended with PC71BM (1:2,

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w/w); the polymer and PC71BM domains appear as bright and dark regions, respectively, owing to their different degrees of electron scattering. The bright

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polymer-rich and dark PC71BM-rich domains are well distributed and continuously

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interpenetrated for PBDTT-C-BO and PBDTT-S-BO (Figure S3a and S3c)—desirable factors for the improved exciton dissociation and charge transport. The image in Figure S3b reveals vivid PC71BM aggregates in the PBDTT-O-BO/PC71BM blend films.We suspect that PBDTT-C-BO and PBDTT-S-BO mixed well with PC71BM in the blend solution because of greater

ACCEPTED MANUSCRIPT solubility (Table 1); therefore, they underwent nanoscale phase separation even during

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such arapid evaporation process as spin-coating.

Conclusion

We have synthesized three medium-bandgap, 2-D conjugated D–π–A copolymers

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featuring different solubilizing substituents—namely linear alkyl, alkoxy, and

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alkythio moieties—on their thiophene side chains in an attempt to optimize the performance of these copolymers in PSCs. To explore the potential of these polymers as efficient active materials in solar cells, we systematically investigated the influence of their solubilizing substituents on the structural, optical, electrochemical, and

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photovoltaic properties of the polymers. Mobility measurements, morphological studies, quantumchemical calculations, and GIXRD patterns provided us with

EP

additional insight into the behavior of these polymers. Through modification of the

AC C

solubilizing substituents of the 2-D conjugated polymers, the resulting PCEs could vary from 5.4 to 7.5%, highlighting the fact that the solubilizing substituents on the side chains can play significant roles in modulating the values of Voc and Jsc of BHJ solar cells fabricated from polymers featuring an otherwise identical conjugated backbone. Among these polymers, a device incorporating PBDTT-C-BO exhibited the highest PCE of 7.5%. Notably, the excellent photovoltaic performance of this PSC

ACCEPTED MANUSCRIPT based on PBDTT-C-BO was obtained without any processing additives or post-treatment procedures, suggesting applicability in large-scale production and for

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the commercial application of PSCs. With its high value of Jsc (15.7 mA cm–2) and medium bandgap, the polymer PBDTT-C-BO appears to be a promising candidate

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material for use in tandem solar cells.

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Acknowledgment

We thank the National Science Council, Taiwan, for financial support (NSC:

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101-2923-E-009-003-MY3).

Appendix A. Supplementary data

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Supplementary data related to this article can be found at http://dx.doi.org

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ACCEPTED MANUSCRIPT Highlights • We synthesize a series of 2-D conjugated D–π–A polymers: PBDTT-C-BO, PBDTT-O-BO, and PBDTT-S-BO. • We investigated the effects of these polymers on the structural, optical, and electronic properties. PBDTT-C-BO exhibited the highest PCE of 7.5%.

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