Efficient Fullerene-Free Polymer Solar Cells Based on Alkylthio

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Efficient Fullerene-Free Polymer Solar Cells based on Alkylthio Substituted Conjugated Polymers Qi Wang, Shaoqing Zhang, Bowei Xu, Sunsun Li, Bei Yang, Wenxia Yuan, and Jianhui Hou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11848 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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The Journal of Physical Chemistry

Efficient Fullerene-Free Polymer Solar Cells based on Alkylthio Substituted Conjugated Polymers Qi Wang1,2, Shaoqing Zhang1,2, Bowei Xu2,*, Sunsun Li2, Bei Yang2, Wenxia Yuan1,*, and Jianhui Hou1,2* 1. School of Chemistry and Biology Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China 2. State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing 100190, China KEYWORDS fullerene-free polymer solar cell, conjugated polymer, alkylthio substituent

ABSTRACT A series of medium and wide band gap polymers with different backbones and sidechains were designed and synthesized as electron donors for fullerene-free polymer solar cells (PSCs). The relationship between the molecular structures of the polymer donors and the photovoltaic properties of the fullerene-free PSC devices was fully investigated. The optical band gaps and the energy levels of the polymers were modulated by the copolymerization of the benzodithiophene (BDT) unit with the various comonomers. Moreover, by incorporating alkylthio substituents, the highest occupied molecular orbital (HOMO) levels of the polymer donors were effectively reduced, and thus the open-circuit voltages (Voc) of the fullerene-free PSC devices were

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strongly improved. Simultaneously, the extinction coefficients of the polymers were also enhanced by introducing the alkylthio groups, which ensured sufficient light-harvesting capability. The exciton dissociation probability P(E,T) in the blend films of the alkylthio substituted polymers was also higher than that of the alkyl substituted polymers. Among all the six polymers, poly[bis(5((2-ethylhexyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene]-alt-[1,3-bis(2-ethylhexyl)-5,7di(thiophen-2-yl)-4H,8H-benzo[1,2-c:4,5-c']dithiophene-4,8-dione] (PB1-S), exhibited the most suitable optoelectronic properties for fullerene-free PSCs because of its complementary absorption band with ITIC and the low HOMO level. The fullerene-free PSC based on the PB1-S:ITIC active layer exhibited excellent photovoltaic performance, with a power conversion efficiency (PCE) as high as 10.49%.

Introduction In recent years, great progress has been made in fullerene-free polymer solar cells (PSCs), in which the active layer consists of a conjugated polymer as electron donor and a non-fullerene (NF) organic compound as electron acceptor.

1-6

Compared with the traditional fullerene derivative

acceptors, NF-acceptors have significant advantages of broad absorption, tunable energy level and low-cost production. Based on advancements in developing superior non-fullerene acceptors, state-of-the-art fullerene-free PSCs have achieved a power conversion efficiency (PCE) approaching 10%, which is comparable with that of the fullerene-based devices. 7-9 In addition to developing suitable NF-acceptors, the appropriate selection of matched polymer donor is also very important to obtain high-performance fullerene-free PSCs. Currently, although a variety of new non-fullerene acceptors have been successfully developed, the selection of appropriate polymer donor still remains a great challenge because many traditional polymer donors do not match the photoelectric parameters of the newly developed non-fullerene acceptors. Therefore, developing


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polymer donors with suitable photovoltaic properties is of considerable significance for the field of fullerene-free PSCs. The requirements for polymer donors in fullerene-free PSCs are quite different from those in fullerene-based PSCs.

10-13

First, the polymer donor and NF-acceptor should possess

complementary absorption spectra to achieve optimal light-harvesting. Unlike fullerene derivative acceptors with the absorption bands located mostly within the wavelength range below 400 nm, the extensively used NF-acceptors, such as 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene

(ITIC)

14

,

2,2′-((2Z,2′Z)-((5,5′-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-

indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(4-(2-ethylhexyl)thiophene-5,2diyl))bis(methanylylidene))-bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEIC)

15

and 2,2′-((2Z,2′Z)-((5,5′-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-

b:5,6-b′]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)-oxy)thiophene-5,2diyl))bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO) 16, are all typical low band gap compounds with the absorption onsets at approximately 800 nm. Therefore, the medium and wide band gap polymer donors with absorption bands complementary to that of the NF-acceptors is desired in fullerene-free PSCs. In addition, a suitable energy level is also crucial to selecting polymer donors for fullerene-free PSCs. Since the opencircuit voltage (Voc) of the fullerene-free PSCs is directly proportional to the gap between the highest occupied molecular orbital (HOMO) level of the polymer donor and the lowest unoccupied molecular orbital (LUMO) level of the NF-acceptor, 17, 18 the HOMO level of the polymer donors should be adequately low. In addition, other parameters should also be considered simultaneously, including the high hole mobility of the polymer donor, favorable phase-separated morphology in


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the active layer, and appropriate molecular orientation between the polymer donor and the NFacceptor. 19-24 Chemical modification provides a favorable opportunity to adjust the photoelectric properties of the polymer donors,

25-34

improving their matching capacities with NF-acceptors. One

representative example is the polymer J51 (reported by Gao et al.) used as electron donor in the fullerene-free PSCs.

35

The introduction of a fluorine atom into the backbone of J51 effectively

lowered the HOMO level of the polymer donor, thereby enhancing the Voc of the fullerene-free PSCs. Furthermore, in view of the molecular energy level modulation, Zhang et al. reduced the HOMO level of polymer PBDD4T-2F to -5.39 eV by introducing two fluorine atoms in the 2,2’dithiophene unit in polymer backbone and attained a high Voc of 0.94 V in the fullerene-free PSCs. 36

Cui et al. found that the introduction of alkylthio substitutions could effectively decrease the

HOMO level of the benzodithiophene (BDT) and thiophene (TT)-based polymer and improve the Voc of the fullerene-based PSC devices, thus providing a new strategy for optimizing the chemical structure of the low band gap polymer donors. 37 Moreover, by replacing the alkyl side chain with alkylthio side chain, Wang et al. not only lowered the HOMO level of the polymer PBD-S but also improved the extinction coefficient of the polymer donor, which enhanced the short circuit current density (Jsc) of the PSCs.38 These encouraging results suggest that the introduction of alkylthio substitutions is an effective method to regulate molecular energy levels of the polymer donors and improve the photovoltaic performances of the fullerene-based PSCs. However, this strategy for molecular optimization of polymer donors has not been used in the field of fullerene-free PSCs. Therefore, the use of alkylthio substitutions for developing superior polymer donors for fullerenefree PSCs must be further explored.


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Herein, we selected a series of medium and wide band gap polymers with different backbones and side-chains for fabricating fullerene-free PSCs. Different units, such as 2,6-bis(trimethyltin)4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene

(BDT),

(4,8-bis(5-((2-

ethylhexyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl)bis(trimethylstannane) (BDT-S),

1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-

c']dithiophene-4,8-dione (DTBDD), 2-octyldodecyl-4,6-bis(5-bromothiophene-2-yl)-thieno[3,4b]thiophene-2-carboxylate (DTTT), and 4,7-bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-2,1,3benzothiadiazole (DTBT) were selected for designing the polymer donors. In particular, alkylthio side chains were introduced to optimize the properties of the polymers. As shown in Scheme 1, the polymers with the alkylthio side chains, namely PB1-S, PB2-S and PB3-S, were designed and synthesized for the use as electron donors in fullerene-free PSCs, and the polymers PB1, PB2, and PB3 with alkyl substitutions were also prepared as the reference materials. The alkylthio substituted polymers, PB1-S, PB2-S and PB3-S, possessed lower HOMO levels, higher extinction coefficients and higher hole mobilities than their alkyl substituted analogs. Fullerene-free PSCs were fabricated using these polymers as electron donors and ITIC as electron acceptors. The alkylthio substituted polymer donors provided much higher Voc in the fullerene-free PSCs in comparison with their alkyl substituted counterparts; this enhancement of Voc was not observed in the fullerene-based PSC in the previous report, suggesting the different effects of incorporating alkylthio substituents on improving the Voc between the fullerene-based and fullerene-free PSCs. Moreover, the Jsc of the fullerene-free PSCs based on the alkylthio substituted conjugated polymers were also improved compared to those of the alkyl substituted polymers. The comparisons among these polymers applied in fullerene-free PSCs provide fundamental


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information for understanding how the molecular structures of the polymer donors affect the photovoltaic properties of the fullerene-free PSCs.

Scheme 1. Chemical structures of PB1, PB2, PB3, PB1-S, PB2-S, PB3-S and ITIC.

Experimental Section Materials and methods, Crystallography. BDT, BDT-S and DTBDD monomers were prepared according to our previous reports. 38, 39 The monomers DTTT, DTBT and the compound ITIC were purchased from Solarmer Materials Inc. Pd(PPh3)4, Pd2(dba)3 and P(o-tol)3 were purchased from Frontier Scientific Inc. Chlorobenzene, chloroform, toluene and DMF were purchased from Tokyo Chemical Industry Inc. All of the commercial available chemicals and solvents were used without any further purification.


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1

H NMR spectra of the monomers BDT, BDT-S, DTBDD, DTTT and DTBT as well as the

polymers PB1, PB2, PB3, PB1-S, PB2-S, and PB3-S were provided in Supporting Information. Synthesis Preparation of PB1, PB1-S and PB2-S. The polymer PB1, PB1-S and PB2-S were prepared according to our methods.

38-40 1

H NMR spectra of polymers were available in Figure S2,

Supporting Information. Here were the Elemental analysis (the original elemental analysis reports of all the six polymers were provided in Table S1, Supporting Information). PB1, calculated for C69H86O2S8: C, 68.99; H, 6.64; found: C, 68.95; H, 6.72. PB1-S, calculated for C69H86O2S8: C, 65.44; H, 6.30; found: C, 65.23; H, 6.34. PB2-S, calculated for C69H86O2S8: C, 65.36; H, 6.84; found: C, 64.42; H, 6.37. Preparation of PB2. The bis-trimethyltin-monomer BDT (0.181 g, 0.20 mmol) and the dibromide monomer DTTT (0.146 g, 0.20 mmol) were dissolved in toluene in a flask with a reflux condenser. After flushed with argon for 5 minutes, 15 mg of Pd(PPh3)4 was added into the flask. The reactant was protected by argon for another 20 minutes, and then heated to 110 °C and stirred for 16 hours. Subsequently, the solution was cooled to room temperature and precipitated into methanol. After filtrated, the polymer was subject to Soxhlet extraction apparatus successively with acetone (4 hours), hexane (6 hours) and 200 mL of chloroform. The solution was concentrated by rotary evaporation under reduced pressure. The resultant polymer was precipitated from methanol and collected by filtration. The polymer PB2 was dried under vacuum for 12 hours with a yield of 61% (0.147 g). Elemental analysis: calculated for C69H86O2S8: C, 68.84; H, 7.20; found: C, 68.08; H, 6.66. Preparation of PB3. The BDT monomer (0.299 g, 0.33 mmol) and the acceptor monomer DTBT (0.226 g, 0.33 mmol) were used for polymerization (dissolved in 7 ml toluene and 1 ml


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DMF). After flushed with argon for 5 min, 18 mg of Pd(PPh3)4 was added to the flask. The reactant was purged by argon for another 20 min. Then, the reactant was heated to 110 °C and stirred at 110 °C for 17.5 hours under the protection of argon. The polymer was purified by the same procedure as mentioned above and dried under vacuum for 12 hours with a yield of 56% (0.203 g). Elemental analysis: calculated for C64H78N2S7: C, 69.90; H, 7.15; N, 2.55; found: C, 69.81; H, 7.05; N, 2.49. Preparation of PB3-S. The synthesized and purified processes of PB3-S were similar as PB2 and PB3. The amounts of the monomers and the catalyst as well as the time of polymerization were different. In details, BDT-S monomer (0.481 g, 0.50 mmol), DTBT monomer (0.343 g, 0.50 mmol), and Pd(PPh3)4 (25 mg) were dissolved in 10 ml toluene; the reaction time of the polymerization was 14 hours. Yield: 58% (0.338 g). Elemental analysis: calculated for C64H78N2S9: C, 66.05; H, 6.76; N, 2.41; found: C, 66.30; H, 6.65; N, 2.40. Instruments and measurements Molecular weight and polydispersity (PDI) of the polymers were acquired on high temperature gel permeation chromatography (HT-GPC) method at 145 °C using 1,2dichlorobenzene as the eluent. Thermogravimetric analysis (TGA) measurements were estimated by TGA-2050, TA Instruments Inc. UV-visible absorption spectroscopy measurements were conducted on a Hitachi U-3100 UV-vis spectrophotometer. The electrochemical cyclic voltammetry (CV) measurements were performed on a CHI650D electrochemical workstation in a 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution with a scan rate of 20 mV·s-1, using Pt disk, Pt plate, and Fc/Fc+ electrode as working electrode, counter electrode and reference electrode, respectively. The current density-voltage (J-V) characteristics were performed on an Keithley 2400 Source/Measure Unit and measured under the AM 1.5G


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spectrum at a calibrated light intensity of 100 mW·cm-2 from a XES-70S1 solar simulator (AAA grade, 70 mm × 70 mm photo-beam size, SAN-EI Electric Co., Ltd). The external quantum efficiency (EQE) curves were measured by Solar Cell Spectral Response Measurement System QE-R3011 (Enli Technology Co., Ltd). For photocurrent behavior analysis for exciton dissociation, the devices were biased sweeping from -6 to +1 V. Hole mobilities were estimated by SCLC method using hole-only device with a structure of ITO/PEDOT: PSS/polymer/Au (50nm). Atom force microscopy (AFM) images were performed on a Nanoscope III A (Vecco) in the tapping mode. Transmission electron microscopy (TEM) was measured by a JEM-2010 instrument at accelerating voltage of 200 kV. The Elemental analysis data were obtained by using FLASH EA1112 Elemental Analyzer. Fabrication of the fullerene-free PSC devices Fullerene-free PSC devices with a structure of ITO/PEDOT: PSS/polymer:ITIC/Mg/Al were fabricated as follows and measured using the solar simulator mentioned above and the same silicon reference cell as our previous reported. The clean indium tin oxide (ITO)-coated glass substrate with sheet resistance of 10 Ω/square was used as the substrate. After the substrate was treated with the UV-ozone for 15 minutes, a 35 nm thick PEDOT:PSS (Heraeus, PVP 4083) layer was spincasted onto the ITO substrate and then annealed at 150 °C for 15 minutes. The active layer consist of polymer and ITIC was casted directly from 1,2-dichlorobenzene (o-DCB) solution onto the PEDOT:PSS layer by spin coating without any additives and annealing treatment. At last, 20 nm Mg and 100 nm Al were successively deposited onto the active layers under high vacuum (ca. 1 × 10-4 Pa). The devices were fabricated after PEDOT:PSS deposition and characterized in Nitrogenfilled glovebox. The effective area of each fullerene-free PSC device was the overlap between the cathode and anode (ca. 0.2 cm × 0.2 cm), confirmed by an optical microscopy.


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Scheme 2. Synthesis routes of the polymer donors PB1, PB2, PB3, PB1-S, PB2-S and PB3-S together with all the relevant monomers.

Results and Discussion Table 1. Molecular weights from the HT-GPC and Td data of the polymers.

Polymer

Mn

Mw

PDI

Td (°C)

PB1

21.6 K

54.1 K

2.51

427

PB1-S

18.1 K

43.2 K

2.38

377

PB2

19.0 K

52.4 K

2.76

410

PB2-S

13.4 K

31.2 K

2.33

364

PB3

16.0 K

35.9 K

2.24

466

PB3-S

10.5 K

22.0 K

2.10

392

Synthesis and Thermal Stability


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Polymers PB2, PB3, and PB3-S were synthesized using the Stille coupling reactions according to our previous work, as shown in Scheme 2. Moreover, the Stille coupling reactions were employed for synthesizing all six of these polymers. The molecular weight and polydispersity index (PDI) of the polymers are listed in Table 1. All the polymers could be easily dissolved in common organic solvents, such as chloroform, chlorobenzene, and dichlorobenzene. The alkylthio substituted polymers showed slightly lower molecular weights compared to their alkyl counterparts; the difference in molecular weights is probably due to the different reaction activity of the monomers. Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability of the polymers, as shown in Figure S1, Supporting Information. The thermal decomposition temperatures (Td) at 5% weight loss of all these polymers are above 360 °C (see Table 1), which is adequate for the fabricating and storages processes of PSCs. In addition, the alkylthio substituted polymers showed slightly lower decomposition temperatures than their alkyl substituted analogs; this could be ascribed to the introduction of the carbon-sulfur bonds of the

1.2 1.0

PB1 PB2 PB3 PB1-S PB2-S PB3-S ITIC

a

0.8 0.6

(solution)

0.4 0.2 0.0 300

400

500

600

700

800

900

Normalized Absorption (a.u.)

alkylthio side chains. Normalized Absorption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2 1.0

PB1 PB2 PB3 PB1-S PB2-S PB3-S ITIC

b

0.8 0.6

(film)

0.4 0.2 0.0 300

400

500

600

700

800

900

Wavelength(nm)

Wavelength(nm)

Figure 1. (a) Normalized absorption spectra of the polymers and ITIC in the o-DCB solutions. (b) Normalized absorption spectra of the solid films of the polymers and ITIC. Optical Properties


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The normalized UV-vis absorption spectra of PB1, PB2, PB3, PB1-S, PB2-S, and PB3-S in o-DCB solutions and solid films are shown in Figure 1 and the corresponding optical parameters are listed in Table 2. The alkylthio substituted polymers PB1-S, PB2-S, and PB3-S exhibited similar absorption bands to their alkyl substituted analogs, suggesting that the introduction of alkylthio side chains did not significantly impact the absorption spectra of the polymers. Among all these polymers, PB2 and PB2-S showed the most red-shifted absorption spectra as a result of the strong electron “puss-pull” effect between the BDT and DTTT unit. For the solutions of polymer PB3 and PB3-S, three absorption peaks at approximately 320 nm, 425 nm, and 600 nm could be observed; these peaks were in coincidence with the peaks in the absorption spectra of similar polymers containing the BDT-DTBT conjugated backbones.

41

The absorption bands of

PB1 and PB1-S films were primarily located in the wavelength range from 300 to 675 nm. Apparently, the BDT-DTBDD-based polymer PB1 and PB1-S have complementary absorption spectra compared to that of the ITIC; as a result, a higher Jsc in fullerene-free PSCs might be expected for PB1 and PB1-S as polymer donors. The thin-film absorption spectrum of each polymer is almost identical in line shape to the solution spectrum, except that the absorption band of these polymers in solid films is slightly red-shifted compared with that in diluted solutions. The appearance of the absorption shoulders at approximately 680 nm in PB2 and PB2-S films indicated the existence of the strong inter-chain interactions in their solid states. In the case of the solutions, the polymer PB1 and PB1-S also exhibited good complementary absorption spectra with ITIC, indicating that the blend of PB1/PB1-S and ITIC is probably most suitable for use as the active layer in PSCs.

Table 2. Optical properties and electrochemical properties of the polymers.


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Egopt

HOMO

LUMO

λmax,sol

λmax,film

εfilm

(eV)a

(eV)

(eV)b

(nm)

(nm)

(10-2/nm)

PB1

1.79

-5.25

-3.46

565

625

0.48

PB1-S

1.78

-5.30

-3.52

621

630

0.56

PB2

1.63

-4.94

-3.31

624

617

0.61

PB2-S

1.64

-5.08

-3.44

625

618

0.63

PB3

1.70

-5.14

-3.44

568

600

0.50

PB3-S

1.71

-5.24

-3.53

565

595

0.59

Polymer

a

Calculated from absorption onsets of the polymer films.

b

Calculated from LUMO = HOMO + Egopt. The optical band gaps (Egopt) of the polymer films were estimated from their absorption

onsets, as listed in Table 2. The Egopt of PB1, PB2, PB3, PB1-S, PB2-S, and PB3-S are 1.79, 1.63, 1.70, 1.78, 1.64, and 1.71 eV, respectively. Clearly, the alkylthio substituted polymer PB1-S, PB2S, and PB3-S possessed similar optical band gaps as their alkyl substituted counterparts. Moreover, the extinction coefficients (εfilm) of the polymer films at their absorption peaks were calculated; the results are listed in Table 2. The extinction coefficients of the alkylthio substituted polymers are higher than their alkyl substituted counterparts. Apparently, the introduction of the alkylthio substituents could improve the extinction coefficient of these polymers; similar results were also observed in previous reports.

38, 42

The higher extinction coefficients of the alkylthio substituted

polymers could be ascribed to the auxochrome effect of alkylthio groups. As a result, the active layer based on the alkylthio substituted polymers can absorb more sunlight than their alkyl substituted counterparts at the same thicknesses; this is highly advantageous to the efficient lightharvesting in PSCs.


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2

Current (mA)

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1

PB1 PB2 PB3 PB1-S PB2-S PB3-S

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a

0 -1 -0.4

0.0

0.4

0.8

1.2

Potential ( V vs Fc/Fc+)

Figure 2. (a) CV plots of the polymers in thin-film form. (b) Molecular energy-level alignments of the PB-based polymer donors and the NF-acceptor. Electrochemical Properties The electrochemistry of the polymers PB1, PB2, PB3, PB1-S, PB2-S, and PB3-S was investigated by using cyclic voltammetry with a thin-film electrode. The cyclic voltammograms of these polymers are shown in Figure 2a, and the relevant data are summarized in Table 2. The onset oxidation potentials (φox) of PB1, PB2, PB3, PB1-S, PB2-S, and PB3-S are 0.45, 0.14, 0.34, 0.50, 0.28 and 0.44 V, respectively, corresponding to the HOMO levels of -5.25, -4.94, -5.14, 5.30, -5.08, and -5.24 eV, respectively. The LUMO levels of the polymers were calculated from the optical band gaps (LUMO = HOMO + Egopt) because the reduction waves were not clearly observed for these polymers. For a clear comparison, the electronic energy-level diagram of these polymers and ITIC is illustrated in Figure 2b. It is clear that all the alkylthio substituted polymers possess lower HOMO levels than their alkyl substituted analogs because of the electron-donating ability of the alkylthio substituent. Note that both PB1 and PB1-S have deep HOMO levels at approximately -5.25 eV; thus, these polymers are expected to obtain higher values of Voc when used in fullerene-free PSCs.


14

10 5

a

0 -5


80

b

60 EQE(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current Density (mA/cm2)

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-10


40 20

-15 -20 0.0

0.2

0.4

0.6

0.8

1.0

0 300

400

Voltage (V)

500 600 700 Wavelength(nm)

800

900

Figure 3. (a) J-V curves and (b) EQE curves of the fullerene-free PSC devices based on polymer:ITIC fabricated under the optimal conditions. Table 3. Photovoltaic properties of fullerene-free PSCs devices based on the PB-series polymers and ITIC.

Polymer/

D/A

Voc

Jsc

FF

PCE

Thickness

ITIC

ratio

(V)

(mA/cm2)

(%)

(%)

(nm)

PB1

1:1

0.907

16.10

65.5

9.56

96

PB1-S

1:1

0.941

17.84

62.5

10.49

94

PB2

1:1

0.628

12.41

47.8

3.72

89

PB2-S

1:1

0.730

14.61

50.1

5.35

91

PB3

1:1

0.822

13.47

59.8

6.61

90

PB3-S

1:1

0.873

16.93

56.0

8.28

88

Photovoltaic Device Performance To evaluate the photovoltaic performance of these polymers, bulk-heterojunction fullerenefree PSCs were fabricated with a conventional device structure of ITO/PEDOT:PSS/


15


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polymer:ITIC/Mg/Al. The active layers of the fullerene-free PSC devices were spin-coated from the o-dichlorobenzene blend solutions of the polymer donor and the ITIC acceptor, and the blend ratios of the polymer donor (D) to ITIC acceptor (A) were optimized by changing the D:A weight ratios from 1:0.8 to 1:1.5. In addition, the thicknesses of the active layers were regulated to be approximately 90~100 nm by controlling the spin speeds. Moreover, the polymer/ITIC blend films were prepared without the use of any solvent additive, and no thermal annealing treatment was required to modulate the morphology of the active layer. The details of the device optimization processes of the fullerene-free PSCs are presented in the Supporting Information. For a clear comparison, the current density-voltage (J-V) curves of the optimized devices are displayed in Figure 3a, and the corresponding photovoltaic performance data are listed in Table 3. The fullerene-free PSCs with PB2:ITIC and PB2-S:ITIC active layers only afford moderate PCEs of 3.72% and 5.35%, respectively. The large overlap between the absorption spectra of PB2/PB2-S and ITIC limits the light-harvesting of the active layers, resulting in the low Jsc. The fullerene-free PSC devices based on the PB3 and PB3-S polymer donors achieved PCEs of 6.61% and 8.28%, respectively. The combined absorption between PB3/PB3-S and ITIC covers a broader spectrum than that of the PB2:ITIC or PB2-S:ITIC blends; this is favorable to the Jsc of the PSC devices. When the BDT-BDD-based polymers, PB1 and PB1-S, were used as electron donors in the active layers, the performances of the fullerene-free PSCs were significantly improved. The PB1:ITICbased PSC device displayed a PCE of 9.56% with a Voc of 0.91 V, a Jsc of 16.10 mA/cm2, and a fill factor (FF) of 65.5%. The best performance of the fullerene-free PSC device was achieved when PB1-S was used as the electron donor, exhibiting a PCE of 10.49% with a Voc of 0.94 V and a Jsc of 17.84 mA/cm2, and FF of 62.5%. The higher PCE for the PB1-S-based photovoltaic device mainly resulted from the simultaneous enhancement of Voc and Jsc. The complementary absorption


16

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of the PB1-S donor and the ITIC acceptor ensured the sufficient light-harvesting, contributing to the enhanced Jsc of the PSC device. Moreover, from PB1 to PB1-S, the replacement of the alkyl group with alkylthio group in the polymer donors led to a significant increase in Voc of ～ 0.034 V in the corresponding fullerene-free PSC device. Note that this enhancement of the Voc was not observed in the fullerene-based PSC in the previous report, suggesting the different effects of incorporating alkylthio substituents on improving the Voc between the fullerene-based and fullerene-free PSCs. By comparison of the three groups of the polymer donors, i.e., PB1 and PB1S, PB2 and PB2-S, PB3 and PB3-S, we found that the introduction of the alkylthio substituent could effectively lower the HOMO levels and enhance the extinction coefficients of the polymer donors, and hence simultaneously improve the Voc and Jsc of the devices, i.e., by replacing the alkyl with alkylthio side chain, the Voc of 0.628 V and Jsc of 12.41 mA/cm2 for the PB2-based fullerenefree PSC device increased to Voc of 0.730 eV and Jsc of 14.61 mA/cm2 for the PB2-S-based device, respectively. Similarly, significant improvements in Voc and Jsc were also achieved in the PB3-Sbased fullerene-free PSCs. These results provide a new strategy in the development of superior polymer donors for fullerene-free PSCs. The external quantum efficiencies (EQE) curves of the optimal fullerene-free PSCs are shown in Figure 3b. The broad EQE spectra from 300 to 800 nm, of all the devices indicate that both the polymer donors and the ITIC acceptor contribute to charge generation. As shown in Figure 3b, although the fullerene-free PSCs based on the alkylthio substituted polymer donors exhibited EQE spectra similar to those of their alkyl substituted counterparts, the EQE values of the PSCs with alkylthio substituted polymer donors were much higher. Notably, the PB1-S-based device displayed high EQE above 60% in the wide response range of 430-750 nm, which in part reflects the high Jsc of the device.


17


b

-32

-1

10

0

10 Veff (V)

-34


3

10

PB1:ITIC PB2:ITIC PB3:ITIC PB1-S:ITIC PB2-S:ITIC PB3-S:ITIC

-1

2

2

2

100

-30

short-circuit condition

lnJL /V (Acm /v )

a

P(E,T)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-36 -38 0

100

200

300 0.5

400

500

600

700

0.5

(V/L) （ V/cm)

Figure 4. (a) Exciton dissociation probability [P(E,T)] versus Veff of the fullerene-free PSCs. (b) Hole mobility plots of the pure polymer films from the SCLC method. Although the extinction coefficient of PB1-S is lower than that of PB2-S and PB3-S, the EQE of PB1-S is higher than that of PB2-S and PB3-S in the broad wavelength range from 400 nm to 710 nm; this may be ascribed to the exciton dissociation probability rather than the extinction coefficient. Figure 4a shows the plots of exciton dissociation probability [P(E,T)] versus the effective voltage (Veff, Veff = V0 – Va, where Va is the applied voltage and V0 is the voltage when Jph equals zero) of the fullerene-free PSC devices. The P(E,T) under short-circuit condition of PB1S-based device is 86.8%, which is much higher than those of PB2-S (66.3%) and PB3-S devices (74.8%). The high P(E,T) might account for the superior EQE of the PB1-S PSC device. Hole mobilities of the polymers were also estimated from the dark J–V measurements of the hole-only devices by using the space-charge limited current (SCLC) method. As shown in Figure 4b, the hole mobilities of PB1-S (8.52 × 10-3 cm2 V-1 s-1) is higher than those of PB2-S (2.84 × 10-3 cm2 V-1 s-1) and PB3-S (5.17 × 10-3 cm2 V-1 s-1), which is in good agreement with the high Jsc of the PB1-S device. Moreover, compared to their alkyl substituted analogs, the alkylthio substituted polymers exhibited higher P(E,T) and hole mobilities in devices (Table S3, Supporting


18

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Information), which is correlated with the enhanced Jsc as well as the overall PCE in the corresponding fullerene-free PSCs.

Figure 5. AFM height images (a-f) TEM images (g-l) of polymer:ITIC blend films processed under optimal conditions. Film Morphology Nanoscale surface morphologies and bulk morphologies of the blend films prepared under the optimal conditions were investigated by atomic force microscopy (AFM) in the tapping mode and transmission electron microscopy (TEM). As shown in Figure 5(a)—(f), all the active layers exhibited smooth surface with the root mean square roughness (Rq) lower than 1.02 nm. The AFM images of all the polymer:ITIC blend films showed similarly smooth surfaces with low roughness. In the AFM phase images, obvious fibril-like structures could be observed in the PB1:ITIC and PB1-S:ITIC blends (see Figure S4, supporting information). Compared to their alkyl substituted analogues, alkylthio substituted polymers exhibited smoother surface in blend films with lower Rq values. As shown in Figure 5(g)—(l), the TEM images correlated well with the morphologies


19


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observed with AFM and no distinct phase separation could be observed from TEM images in all above polymer:ITIC blend films.

Conclusion In conclusion, we designed and synthesized a series of polymers utilizing different backbones with the respective alkyl and alkylthio side chains and applied them as electron donors in fullerenefree PSCs. The relationship between the molecular structures of the polymer donors and the photovoltaic properties of the fullerene-free PSC devices was fully investigated to select appropriate polymer donors. The optical band gaps and the energy levels of the polymers were modulated by the copolymerization of the BDT unit with the various comonomers. Compared to the other polymers, the DTBDD-based polymers PB1 and PB1-S exhibited more suitable optoelectronic parameters for matching with the ITIC acceptor in constructing high-performance fullerene-free PSCs. Moreover, the HOMO level of these polymers could be further reduced by introducing alkylthio substituents, and their extinction coefficients were also enhanced. Among all six polymers, PB1-S exhibited the most promising potential to serve as an efficient electron donor because of its low HOMO level, complementary absorption band with ITIC, and high hole mobility; hence, a high PCE of 10.49% was achieved in the PB1-S-based fullerene-free PSC device. This work demonstrated the significance of selecting appropriate polymer donors for improving the performance of the fullerene-free PSCs. Moreover, the results indicated that the introduction of alkylthio side chains is an effective method in the design of superior polymer donors for fullerene-free PSCs.


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ASSOCIATED CONTENT Supporting Information. 1H NMR spectra of all monomers and polymers, the TGA plots, the original element analysis reports of the polymers, the details of the device optimization data and J-V curves, and AFM phase images of the blend films. AUTHOR INFORMATION Corresponding Author * To whom correspondence should be addressed: 1. Prof. Jianhui Hou, E-mail: [email protected]; Bowei Xu, E-mail:[email protected]; Tel: +86-10-82615900; address: Zhongguancun North First Street 2, Beijing 100190, China 2. Prof. Wenxia Yuan, E-mail: [email protected]; Tel: +86-10-62332681; address: Xueyuan Road 30, Haidian District, Beijing 100083, China Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the financial support from National Basic Research Program 973 (2014CB643501), the National Natural Science Foundation of China (Nos. 21325419, 51173189, 21504095), and Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030200). REFERENCES (1) Sonar, P.; Lim, J. P. F.; Chan, K. L. Organic non-fullerene acceptors for organic photovoltaics. Energy Environ. Sci. 2011, 4, 1558-1574.


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Efficient Fullerene-Free Polymer Solar Cells Based on Alkylthio

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