Synthesis and characterization of novel indacenodithiophene-based narrow band-gap polymers with pendant isoindigo units for polymer solar cells

Accepted Manuscript Synthesis and characterization of novel indacenodithiophene-based narrow band-gap polymers with pendant isoindigo units for polymer solar cells Wenhong Peng, Hua Tan, Manjun Xiao, Jianhua Chen, Qiang Tao, Xiongwei Duan, Yafei Wang, Yu Liu, Renqiang Yang, Weiguo Zhu PII: DOI: Reference:

S0014-3057(16)30574-2 http://dx.doi.org/10.1016/j.eurpolymj.2016.06.013 EPJ 7392

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

12 April 2016 6 June 2016 13 June 2016

Please cite this article as: Peng, W., Tan, H., Xiao, M., Chen, J., Tao, Q., Duan, X., Wang, Y., Liu, Y., Yang, R., Zhu, W., Synthesis and characterization of novel indacenodithiophene-based narrow band-gap polymers with pendant isoindigo units for polymer solar cells, European Polymer Journal (2016), doi: http://dx.doi.org/10.1016/ j.eurpolymj.2016.06.013

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Synthesis and characterization of novel indacenodithiophene-based narrow band-gap polymers with pendant isoindigo units for polymer solar cells Wenhong Peng,1 Hua Tan,1,3* Manjun Xiao,1,2 Jianhua Chen,1 Qiang Tao,1 Xiongwei Duan,1 Yafei Wang,1 Yu Liu,1 Renqiang Yang,2* Weiguo Zhu1* 1

College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, China 2

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China 3

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510000, China

Correspondence to: W. Zhu (E-mail: [email protected]) or H. Tan (E-mail: [email protected]) or R. Yang (E-mail: [email protected])

Abstract: Two novel indacenodithiophene (IDT) based donor-acceptor (D-A) narrow band-gap polymers PIDTBDTID and PIDTBDT(ID)2 were designed and synthesized, where one/two isoindigo (ID) moieties were introduced into the backbone as the acceptor unit. The optical, thermal, electrochemical and photovoltaic properties have been investigated in detail. The PIDTBDTID containing one ID unit exhibited better light-harvesting properties and charge transport properties. On the other hand, the PIDTBDT(ID)2 with two ID units displayed lower the highest occupied molecular orbital energy levels (HOMO). Using these polymers as electron donors and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptor, polymer solar cells (PSCs) based on PIDTBDTID exhibited better photovoltaic performance with a power conversion efficiency (PCE) of 2.66 %, an open-circuit voltage (VOC) of 0.87 V and an enhanced short-circuit current (JSC) of 7.24 mA cm-2. While the PIDTBDT(ID)2-based PSCs shown a PCE of 2.50%, an improved VOC of 0.93 V and an JSC of 6.34 mA cm-2.

Keywords: Donor-acceptor; Conjugated copolymer; Isoindigo units; Side chain;

Polymer solar cells.

1. Introduction The advance in donor-acceptor (D-A) conjugated polymers with alternating electron-rich (D) and electron-deficient (A) units along the polymer main chain has been the major driving force for the improvement of the power conversion efficiency (PCE) of polymer solar cells (PSCs) up to 10% in single junction PSCs [1-3]. However, most of the present polymers are in main-chain D-A frame, which may suffer from lower hole mobility due to the influence of acceptor units in the main chain [4-10]. Side-chain D-A conjugated polymers is a new molecular design strategy, in which D moieties were employed to construct the main chain and A units were conjugatedly pended onto the D units in the side chain [11-16]. Compared to the corresponding main-chain D-A polymers, the side-chain ones mainly possesses three advantages. Firstly, the highest occupied molecular orbit (HOMO) energy levels of the side-chain polymers can preserve in a deeper position. Secondly, a two-dimensional-like structural feature renders this class of polymers with better solubility. Finally, there exists an internal charge transfer (ICT) behavior from the side-chain D-A structure, which can fine tune the band-gap (Eg) of the resultant polymers, as well as promote an isotropic charge transport along the polymeric main chain. As a result of these merits, PSCs based on these previously reported side-chain D-A polymers exhibited a maximal PCE of 8.04 % [17]. Nevertheless, most of these side-chain D-A conjugated polymers did not exhibit comparable device performance to their main-chain D-A counterpart polymers. Therefore, it remains a great challenge to the research community to prepare new high performance side-chain D-A conjugated polymers for organic photovoltaic (OPV) applications. In order to enhance the PCE of side-chain D-A PSCs, it is necessary to develop new catalog of side-chain D−A polymers for efficient PSCs. Benzodithiophene (BDT) as an electron-rich fused aromatic ring, which exhibits good planarity, is often introduced into the backbone of polymers to obtain high hole-mobility materials [18-20]. On the other hand, isoindigo (ID) unit is a strongly A

unit with a rigid and planar structure, and has been used to construct high mobility conjugated polymers for organic thin-film transistors (OTFTs) and low bandgap conjugated polymers for PSCs [21-24]. Herein, we designed and synthesized two new side-chain D−A polymers (PIDTBDTID and PIDTBDT(ID)2), where BDT and indacenodithiophene (IDT) groups make up the main chains and isoindigo units are connected to the main chains through a vinyl unit to form the side chains. Although side-chain D−A polymers attaching one acceptor unit onto the polymer main chain have been reported, there were few reports on side-chain D−A polymers attaching two acceptor units. We systematically investigate the optical, thermal, electrochemical, and photovoltaic properties of the polymers. Using PIDTBDTID or PIDTBDT(ID)2 as electron donor and PC71BM as an electron acceptor, respectively. PSCs based on PIDTBDTID exhibited better photovoltaic performance with a PCE of 2.66%, an open-circuit voltage (VOC) of 0.87 V and an enhanced short-circuit current (JSC) of 7.24 mA cm-2. While the PIDTBDT(ID)2-based PSCs shown a PCE of 2.50%, an improved VOC of 0.93 V and an JSC of 6.34 mA cm-2.

2. Experimental 2.1. Materials and methods All manipulations were performed under dry nitrogen flow. All reagents were obtained from Aldrich and directly used without further purification. Tetrahydrofuran (THF) and toluene were freshly distilled over sodium wire under nitrogen prior to use. The synthetic routes for monomers and polymers are shown in Scheme 1. Compound 1, 3 and 4, 5 were synthesized according to the literature methods [11,13 ]. 2.2. Synthesis of tetraethyl((5,5'-(2,6-dibromo-benzo[1,2-b:4,5-b']dithiophene-4,8diyl)bis(thiophene-5,2-diyl))bis(methylene))bis-(phosphonate) (2) A mixture of compound 6 (1.02 g, 1.46 mmol) and triethylphosphite (10 mL) was stirred at 160 °C for 8 h in a 50 mL flask. The excess triethylphosphite was distilled off at 100 °C under 20 mm Hg and the residue was purified by silica gel column chromatography with PE/acetic ether (1:1, v/v) as the eluent to get compound 7 as a

pale yellow solid (612 mg, yield: 31.0%). 1H NMR (400 MHz, CDCl3, δ): 7.59 (d, J = 5.6 Hz, 1H), 7.51 (d, J = 5.6 Hz, 1H), 7.31 (s, 1H), 4.20-4.15 (m, 4H), 3.52 (d, J = 20.9 Hz, 2H), 1.36 (t, J = 7.0 Hz, 6H). 2.3. Synthesis of 6-(thiophene-2’-carbaldehyde)-N,N’-(2-octyldodecanyl)-isoindigo (5) Compound 2 (1.00 g, 1.11 mmol) , Pd2(dba)3 (30 mg), P(o-tol)3 (20 mg), and K3PO4 (2.18 g, 11.1 mmol) in 30 mL of THF were heated to 50 °C under a nitrogen atmosphere for 15 min. A solution of 5-formylthiophen-2-yl boronic acid (0.17g, 1.11 mmol) in THF (5 mL) was transferred to the mixture through a septum. The mixture was then heated to 80 °C and stirred for 12 h. After cooling, the resulting solution was extracted with CH2Cl2 (3 × 10 mL). The organic portion was combined and washed with brine and water and dried with anhydrous MgSO4. The solvent was removed by evaporation and the residue was purified by column chromatography using silica gel (PE/DCM = 4/1-1/1, v/v) to give a wine-red solid (0.65 g, yield: 63.1%). 1H NMR (400 MHz, CDCl3, δ): 9.92 (s, 1H), 9.25 (d, J = 8.3 Hz, 1H), 9.17 (d, J = 7.8 Hz, 1H), 7.77 (d, J = 2.2 Hz, 1H), 7.48 (d, J = 2.4 Hz, 1H), 7.37 (s, 1H), 7.35 (d, J = 6.9 Hz, 1H), 7.06 (d, J = 8.0 Hz, 1H), 7.03 (s, 1H), 6.78 (d, J = 7.8 Hz, 1H), 3.69 (dd, J = 22.5, 7.0 Hz, 4H), 1.93 (s, 2H)，1.29 (d, J = 46.6 Hz, 64H), 0.85 (d, J = 4.8 Hz, 12H). MS (MALDI-TOF): Calcd for C61H92N2O3S [M] +, 932.28; found, 932.34. 2.4. Synthesis of M1 A solution of compound 3 (200 mg, 0.21 mmol) and compound 9 (145 mg, 0.21 mmol) in THF (10 mL) was heated to 90 °C for 15 min under nitrogen atmosphere, was dropwise added another solution of potassium tertbutoxide(28.8 mg, 0.26 mmol) in THF (2 mL). The reaction mixture was stirred at 90 °C for 12 h, respectively. After cooled to RT, the reaction mixture was extracted with DCM and washed with dilute aqueous HCl solution. The organic phase was dried over anhydrous MgSO 4, and the filtrated solution was distilled to remove off solvent by rotary evaporation. The residue was purified by silica gel column chromatography (DCM/PE = 1:1) to give nigger-brown solid (180 mg, yield: 58%).1H NMR (400 MHz, CDCl3, δ): 9.21 (d, J = 8.4 Hz, 1H), 9.16 (d, J = 7.9 Hz, 1H), 7.66 (d, J = 5.5 Hz, 1H), 7.61 (d, J = 5.6 Hz,

1H), 7.56 - 7.46 (m, 2H), 7.37 (s, 1H), 7.35 (d, J = 4.0 Hz, 2H), 7.32 (s, 1H), 7.29 (d, J = 7.9 Hz, 1H), 7.19 (s, 1H), 7.18 - 7.08 (m, 2H), 7.03 (dd, J = 16.6, 8.9 Hz, 2H), 6.77 (d, J = 7.6 Hz, 1H), 3.70 (dd, J = 22.5, 7.0 Hz, 4H), 2.53 (s, 3H), 2.02 - 1.81 (m, 2H), 1.49 - 1.03 (m, 64H), 0.86 (d, J = 6.4 Hz, 12H).MS (MALDI-TOF): Calcd for C81H102Br2N2O2S5[M]+, 1455.82; found, 1455.66. 2.5. Synthesis of M2 The same procedure was used as for monomer M1.Compounds used were potassium tertbutoxide (30.4 mg, 0.271 mmol), compound 3 (230 mg, 0.25 mmol), and compound 7 (100 mg, 0.12 mmol), THF (6 mL). After workup, a nigger-brown solid was obtained (90 mg, 31.0%). 1H NMR (400 MHz, CDCl3,δ): 9.20 (d, J = 8.4 Hz, 1H), 9.16 (d, J = 7.7 Hz, 1H), 7.69 (d, J = 5.6 Hz, 1H), 7.56 (d, J = 5.7 Hz, 1H), 7.39 (s, 1H), 7.35 (d, J = 2.9 Hz, 1H), 7.34 (s, 1H), 7.31 (d, J = 7.1 Hz, 2H), 7.19 (s, 1H), 7.18 - 7.08 (m, 2H), 7.03 (dd, J = 16.6, 9.1 Hz, 2H), 6.77 (d, J = 7.5 Hz, 1H), 3.70 (dd, J = 22.6, 5.7 Hz, 4H), 2.01 - 1.83 (m,2H), 1.49 - 1.00 (m, 64H), 0.86 (d, J = 5.8 Hz, 12H).MS (MALDI-TOF): Calcd for C142H192Br2N4O4S6 [M]+, 2371.27; found, 2371.46. 2.6. Polymerization for PIDTBDTID Under an argon atmosphere, a solution of

2,7-bis(trimethyltin)-4,9-dihydro-

4,4,9,9-tetrakis(4-hexylphenyl)-sindaceno[1,2-b:5,6-b′]dithiophene

(10)

(85

mg,

0.069 mmol), M1(100 mg, 0.069 mmol),Pd2(dba)3 (5 mg), and P(o-tol)3 (10 mg) were disolved in toluene(6 mL), stirred, and heated to 110 °C under argon for 24 h. After allowed the reaction to cool to RT, 0.1 mL of 2-bromothiophene was injected and the mixture was stirred at 110 °C under argon for another 2 h. The reaction was then cooled again to RT, 0.4 mL of 2-tributyl-stannylthiophene was injected. The mixture was stirred at 110 °C for a further 2 h. After cooling, the resulting polymers were precipitated into methanol (200 mL) and filtered off. The crude polymer was purified by Soxhlet extraction in methanol and acetone. Then the precipitated polymer was purified by silica chromatography with CHCl3 as eluent to give a brownish red solid (140 mg, yield: 92.7%).1H NMR (400 MHz, CDCl3, δ): 9.25 (br, 1H), 9.16 (br, 1H), 7.71 (br, 2H), 7.59 -7.39 (br, 8H), 7.34 (br, 4H), 7.17 (br, 8H), 7.15-6.85 (br, 18H),

6.78 (br, 1H), 3.68 (br, 4H), 2.72 (s, 3H), 2.54 (br, 12H), 1.92 (br, 2H), 1.57 (br, 4H), 1.24 (br, 88H), 0.85 (br, 24H). 2.7. Polymerization for PIDTBDT(ID)2 PIDTBDT(ID)2 was synthesized according to the same method as polymer PIDTBDTID, except using monomer M2 (84 mg, 0.035 mmol) to replace monomer M1. A brownish red solid was obtained (95 mg, yield: 86.4%).1H NMR (400 MHz, CDCl3, δ): 9.24 (br, 1H), 9.16 (br, 1H), 7.76 ( br, 1H), 7.54 (br, 2H), 7.47 (br, 2H), 7.30 (br, 6H), 7.09 (br, 16H), 6.77 (br, 1H), 3.67 (br, 4H), 2.53 (br, 6H), 1.92 (br, 2H), 1.56 (br, 2H), 1.28 (br, 76H), 0.85 (br, 18H). 2.8. Measurement and characterization Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker DRX 400 spectrometer using tetramethylsilane as a reference in deuterated chloroform solution at 298 K. MALDI-TOF mass spectrometric measurements were performed on Bruker Bifiex III MALDI-TOF. Thermogravimetric analysis (TGA) was conducted under a dry nitrogen gas flow at a heating rate of 20 °C min -1 on a Perkin-Elmer TGA 7. UV-Vis absorption spectra were recorded on a HP-8453 UV visible system. Cyclic voltammetry was carried out on a CHI660A electro-chemical work station in a three-electrode cell dipped in a 0.1M tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution under nitrogen protection at a scan rate of 100 mVs-1 and room temperature (RT). In this three-electrode cell, a platinum rod, platinum wire and saturated calomel electrode were used as a working electrode, counter electrode and reference electrode, respectively. 2.9.Device fabrication and characterization Polymer solar cells were fabricated using indium tin oxide (ITO) glass as an anode, Ca/Al as a cathode, and a blend film of polymers and PCBM as a photosensitive layer. After a 40 nm buffer layer of poly-(3,4-ethylenedioxythiophene) and polystyrene sulfonic acid (PEDOT: PSS) was spin-coated onto the precleaned ITO substrate, the photosensitive layer was subsequently prepared by spin-coating a solution of polymers and PCBM (1: X, w/w) in chlorobenzene on the PEDOT: PSS layer with a typical concentration of 20 mg mL−1, followed by annealing at 80 °C for 10 minutes

to remove chlorobenzene. Ca (10 nm) and Al (100 nm) were successively deposited on the photosensitive layer in vacuum and used as top electrodes. The current–voltage (J–V) characterization of the devices was carried out on a computer–controlled Keithley 236 source measurement system. A solar simulator was used as the light source and the light intensity was monitored by a standard Si solar cell. The active area was 7×10-2 cm2 for each cell. The thicknesses of the spun-cast films were recorded by a profilometer (Alpha-Step 200, Tencor Instruments). Hole mobilities of the polymers/PC71BM(w:w, 1:X) blend films were measured according to a similar method described in the literature

[11]

, using a configuration of

ITO/PEDOT:PSS(40nm)/active layer/MoO3(4nm)/Al(100 nm) by taking current-voltage current in the range of 0-8 V and fitting the results to a space charge limited form, where the space charge limited current (SCLC) is described by J = 9ε0εrµhV2/8L3, where J is the current density, L is the film thickness of active layer, µ h is the hole mobility, εr is the relative dielectric constant of the transport medium, ε0 is the permittivity of free space (8.85 × 10-12 F m-1), V is the internal voltage in the device and V = Vappl-Va-Vbi, where Vappl is the applied voltage to the device, Va is the voltage drop due to contact resistance and series resistance across the electrodes, and

Vbi is the built-in voltage due to the relative work function

difference of the two electrodes.

Scheme 1. Synthetic routes of PIDTBDTID and PIDTBDT(ID)2

3. Results and discussion 3.1. Polymer synthesis and thermal property The PIDTBDTID and PIDTBDT(ID)2 were synthesized by a palladium-catalyzed Stille coupling reaction, as shown in Scheme 1. The PIDTBDTID and PIDTBDT(ID)2 showed good solubility in common organic solvents such as chlorobenzene(CB), o-dichlorobenzene (ODCB), dichlomethane (DCM). The molecular weights of the resulting polymers were measured by gel permeation chromatography

(GPC).

The

PIDTBDTID

and

PIDTBDT(ID)2 have

a

number-average molecular weight (Mn) of 21.5 kDa and 11.7 kDa, with a polydispersity index (PDI) of 1.7 and 1.1, respectively ( Table 1). Table 1 Polymerization results and thermal properties of copolymers.

a

Copolymer

Mn (kDa)a

Mw (kDa)a

PDIa

Td (°C)b

PIDTBDTID

21.5

36.9

1.7

392

PIDTBDT(ID)2

11.7

13.3

1.1

340

Determined by GPC in THF based on polystyrene standards.

b

Temperature at 5% weight loss measured by TGA at a heating rate of 20°C min-1

under nitrogen atmosphere. Thermogravimetric analysis (TGA) was utilized to evaluate thermal properties of the PIDTBDTID and PIDTBDT(ID)2. As shown in Fig. S1 and Table 1, the onset decomposition temperatures with 5%

weight

loss (Td)

of PIDTBDTID,

PIDTBDT(ID)2 are 392 and 340 °C, respectively. The thermal stabilities of the polymers are adequate for PSCs and other optoelectronic device applications. Fig. S2 depicts the DSC curves of PIDTBDTID and PIDTBDT(ID)2. No melting and crystallization processes are observed during the heating and cooling processes for PIDTBDT(ID)2. However, a distinct endothermic peak at 93 oC are exhibited for PIDTBDTID during the heating process, which result from the melting process. And two exothermic peaks at 93 oC and 72 oC are simultaneously appeared during the cooling process, corresponding to the crystallization process. The different DSC results indicate that PIDTBDTID packed much orderly.

3.2. Optical properties The UV−Vis absorption spectra of the PIDTBDTID and PIDTBDT(ID)2 in chloroform (CF) solution and as thin films are shown in Fig. 1a. The optical properties of the polymers are summarized in Table 2. Two distinct UV–Vis absorption peaks from 350 nm to 750 nm are observed for the polymers in both solutions and films, which are a common feature of the D-A copolymers. The vibronic shoulder peak at longer wavelength about 580 nm is attributed to the strong intra-molecular charge transfer; the other peak at short wavelength about 428 nm is assigned to the localized π–π* transition of the polymeric backbone. Compared to the absorption spectra in solution, the PIDTBDT(ID)2 exhibited significantly red-shifted absorption peak relative to the PIDTBDTID. It implies that the π-electrons in PIDTBDT(ID)2 can be delocalized more effectively owing to the additional A units. However, as shown in Fig. 1b, the molar extinction coefficients (ɛ) of the low-lying peak for PIDTBDT(ID)2 was measured to be 1.6×105 M cm-1 which is lower than that of PIDTBDTID (2.7×105 M cm-1). Increased molar absorptivities were also observed from PIDTBDT(ID)2 to PIDTBDTID in their neat films (Fig. S4a). The low-lying peak for PIDTBDTID was measured to be 1.3×104 M cm-1 which is 1.46 times higher than that of the PIDTBDT(ID)2. As we all know, the strong absorption was highly favorable for effective solar photon harvesting and might lead to high photocurrent in PSC devices.

In solid films, the PIDTBDTID and PIDTBDT(ID)2 show red shifts at

the absorption maxima, indicating the enhanced interchain π−π stacking in the solid state. The optical band gaps (Egopt) of PIDTBDTID and PIDTBDT(ID)2 are 1.75 and 1.73 eV, respectively.

(a)

(b)

Fig. 1. (a) UV−vis absorption spectrum of PIDTBDTID and PIDTBDT(ID)2 in CF solution and thin film, (b) the absorption coeffcients of PIDTBDTID and PIDTBDT(ID)2 in CF solution. 3.3. Electrochemical properties Electrochemical properties of PIDTBDTID and PIDTBDT(ID)2 are explored by cyclic voltammetry (CV) measurement. Fig. S2 displays the electrochemical properties of the polymers; and the CV data are listed in Table 2. The onset oxidation potentials (Eoxon) for PIDTBDTID, PIDTBDT(ID)2 were 1.13, 1.19 eV, respectively; On the basis of Eoxon, we estimated the HOMO energy level, according to the energy level of the ferrocene reference (4.8 eV below vacuum level). Since we could not obtain reliable onset reduction potentials for the PIDTBDTID and PIDTBDT(ID)2, the optical band gaps (Egopt) of the polymer films were utilized to estimate the LUMO energy levels of the copolymers based on the relation of Egopt = ELUMO-EHOMO. The EHOMO and ELUMO of the PIDTBDT(ID)2 is about 0.1 eV lower than that of the corresponding PIDTBDTID copolymer, respectively. This result indicates that the introduction of additional A units onto polymer main chains can decrease the band gap and lower the energy levels for the resulting polymers. To further understand the photophysical properties of the copolymers investigated in this work, molecular simulations were performed on single donor-acceptor units using the density functional theory (DFT) at the B3LYP/6-31G* level. As shown in Fig. S5, it was found that the HOMO is mainly distributed on the BDT units, the HOMO energy levels were calculated to be -4.73 eV for the BDTID and -4.80 eV for BDT(ID)2. These results are consistent with the CV results (Fig. S6). Table 2 Optical and electrochemical properties of PIDTBDTID and PIDTBDT(ID)2. Polymer

λmax(nm)a

λonset (nm)

λmax(nm)b[εmax (M cm-1)]

Egopt Eoxon EHOMO ELUMO (eV)c (V) (eV)d (eV)d

solution

film

PIDTBDTID

416

417,708

0.27

1.75

1.13

-5.47

-3.72

PIDTBDT(ID)2

425

435,718

0.16

1.73

1.19

-5.53

-3.80

a

From solution and thin film absorption measurements

b

Measured in CHCl3 at a

concentration of 10 -6 mol/L c

Estimated according to the formula, Egopt = 1240/λonset, λonset is a onset wavelength

of the optical absorption in thin film d

EHOMO level was calculated based on the formula, EHOMO = (Eoxon + 4.34) eV, ELUMO

level was estimated using empirical equations ELUMO = EHOMO + Egopt 3.4.Photovoltaic properties The photovoltaic properties of PIDTBDTID and PIDTBDT(ID)2 were studied using PCBM as the acceptor in a conventional device configuration of ITO/PEDOT: PSS(40 nm)/active layer(~75 nm)/Ca(10 nm)/Al(100 nm). Further details for the device fabrication and characterization are given in the experimental section. Table S1–S3 summarized the photovoltaic responses of optimized devices. Usually, the polymers:PC71BM-based devices exhibited better photovoltaic properties as compared to the polymers:PC61BM-based devices, which can be attributed to the stronger absorption of PC71BM than PC61BM in the visible region from 440 to 530 nm (Fig. S4). The current-voltage (J-V) characteristics of the BHJ solar cells based on

PIDTBDTID and PIDTBDT(ID)2 were illustrated in Fig. 2, and the corresponding device parameters were summarized in Table 3. The PSCs based on PIDTBDTID gave the highest PCE of 2.66% (Jsc = 7.24 mA cm−2, Voc = 0.87 V, FF = 42.0%). The stronger absorption should be responsible for the good photovoltaic properties of PIDTBDTID. Notably, BHJ solar cells based on PIDTBDT(ID)2 exhibited a higher Voc of 0.93 V. which agreed well with the relatively deep-positioning HOMO level.

Fig. 2. J–V curves of optimized photovoltaic devices. Table 3 Photovoltaic properties of the PIDTBDTID and PIDTBDT(ID)2. Donor

Acceptor

Jsc(mA cm− 2)

Voc(V)

PIDTBDTID

PC71BM

7.24

0.87

42.0

2.66 (2.51)

PIDTBDT(ID)2

PC71BM

6.34

0.93

42.3

2.50 (2.33)

FF (%) PCE (%)(PCE max averag)

Fig. 3 shows the external quantum efficiency (EQE) curves of the device with the polymers: PC71BM blend. It was found that all of the resulting BHJ solar cells exhibited efficient photo-response with a broad range from 350 to 750 nm. Notably, PIDTBDTID-based BHJ solar cell demonstrated a more efficient photo-response with relative high EQE over 45% in a broad range from 400 to 550 nm, which is also agreed well with UV−vis absorption spectrum of the polymer/PC71BM blend films (Fig. S4). It is also explained the reason why the PIDTBDTID-based PSCs possessed a higher JSC value than that of the PIDTBDT(ID)2-based OSCs.

Fig. 3. EQE curves of optimized photovoltaic devices. Hole mobility To understand the photovoltaic behaviors of the resulting polymers in-depth, the hole mobilities of the polymers were measured by using SCLC method, which is widely used to evaluate the charge transporting ability of the resulting BHJ solar cells active layer. Fig. 4 shows the J-V characteristics of the PIDTBDTID and PIDTBDT(ID)2 devices with a hole-only device configuration of ITO/PEDOT: PSS (40 nm) /active layer (~75 nm)/ MoO3 (4 nm)/Al(100 nm).

The hole mobilities of

the PIDTBDTID: PC71BM (1: 4, w/w) and PIDTBDT(ID)2: PC71BM (1: 3, w/w) blended films were measured. The PIDTBDTID demonstrated a hole mobility of 3.9 × 10−5 cm2V−1s−1 which is higher than that of PIDTBDT(ID)2 (1.7 ×10−5 cm2V−1s−1). Including that the PIDTBDTID has the enhanced planarity geometry and better molecular packing to favor the charge transport in BHJ film.

Fig. 4. J-V characteristics of the hole only devices based on PIDTBDTID/PC71BM (1: 4) (a) and PIDTBDT(ID)2/PC71BM (1: 3) (b).

4. Conclusions In conclusion, two novel IDT based D-A narrow band-gap polymers PIDTBDTID and PIDTBDT(ID)2 were designed and synthesized, where one/two ID moieties were introduced into the backbone as the acceptor unit. The PIDTBDTID containing one ID unit exhibited better light-harvesting properties and charge transport properties. While the PIDTBDT(ID)2 with two ID units displayed lower the HOMO levels. PSCs based on PIDTBDTID exhibited better photovoltaic performance with a PCE of 2.66%, an VOC of 0.87 V and an enhanced JSC of 7.24 mA cm-2. While the PIDTBDT(ID)2-based PSCs shown a PCE of 2.50%, an improved VOC of 0.93 V and an JSC of 6.34 mA cm-2.

Acknowledgements This work was supported by the Program for Innovative Research Cultivation Team in University of Ministry of Education of China (1337304), the National Natural Science Foundation of China (51403178, 51573154, 21172187), the project of hunan natural science foundation (2015JJ3113), the Scientific Research Fund of Hunan Provincial Education Department (14C1099, YB2015B025, 13A102), the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, 2014-skllmd-10).

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Graphical Abstract

Two novel indacenodithiophene (IDT) based donor-acceptor (D-A) narrow band-gap polymers PIDTBDTID and PIDTBDT(ID)2 were designed and synthesized, where one/two isoindigo (ID) moieties were introduced into the backbone as the acceptor unit. The PIDTBDTID containing one ID unit exhibited better light-harvesting properties and charge transport properties. On the other hand, the PIDTBDT(ID)2 with two ID units displayed lower the highest occupied molecular orbital energy levels (HOMO). Using these polymers as electron donors and PC71BM as electron acceptor, polymer solar cells based on PIDTBDTID exhibited better photovoltaic performance with a power conversion efficiency (PCE) of 2.66 %, an open-circuit voltage (VOC) of 0.87 V and an enhanced short-circuit current (JSC) of 7.24 mA cm-2. While the PIDTBDT(ID)2-based PSCs shown a PCE of 2.50%, an improved VOC of 0.93 V and an JSC of 6.34 mA cm-2.

Highlights  Two side-chain D−A polymers pending one or two acceptor units of isoindigo were synthesized.  Polymers exhibited absorption ranges of 300–750 nm.  The highest PCE value of 2.66% was obtained in the solar cell device with PC71BM.