Two-Dimensional BDT-Based Wide Band Gap Polymer Donor for

Subscriber access provided by PEPPERDINE UNIV

Article

2D-BDT-Based Wide Band Gap Polymer Donor for Efficient Non-Fullerene Organic Solar Cells Kai Zhang, Yunpeng Qin, Feng Li, Liangmin Yu, and Mingliang Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05815 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

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

The Journal of Physical Chemistry

2D-BDT-Based Wide Band Gap Polymer Donor for Efficient Non-Fullerene Organic Solar Cells Kai Zhang b, Yunpeng Qin c, Feng Li a*, Liangmin Yu d, Mingliang Sun b*

a. Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Engineering, Qingdao University of Science & Technology, 53 Zhengzhou Road, Qingdao 266042, China b. Institute of Material Science and Engineering, Ocean University of China, Qingdao 266100, China. c. School of Chemistry and Biology Engineering, University of Science and Technology Beijing, Beijing 100083, China d. School of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China.

ABSTRACT In this paper, in order to study the two-dimensional (2D) conjugated side chain effect on polymer donor for the non-fullerene polymer solar cells (NFPSCs), the thiophenothiophene (TT) side-chained benzodithiophene (BDT) and bithiophene copolymer (PBDTDT-TT) has been prepared by Stille coupling reaction, compared with the copolymer (PBDTDT-O) with alkoxy side group and the same polymer backbone. The 2D conjugated polymer PBDTDT-TT exhibits better thermal stability, 1

ACS Paragon Plus Environment


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

Page 2 of 26

wider and red-shifted UV-vis absorption spectra. These two copolymers were used as the donor materials, respectively, blended with a new non-fullerene receptor ITM, successfully

fabricating

the

ITO/PEDOT:PSS/polymer:ITM/PFN-Br/Al

positive

devices

with

structure.

Compared

with

PBDTDT-O:ITM blend film, the PBDTDT-TT:ITM blend film shows lower LUMO energy level, narrower optical band gap, higher electron and hole mobility in the devices. The best PCE is 8.43% for PBDTDT-TT polymer, while the PCE is less than 1% for PBDTDT-O polymer. Extending the conjugated plane through the 2D conjugated side chain, can help the PBDTDT-TT based PSCs devices show higher exciton dissociation efficiencies of 85.36% (Compared with PBDTDT-O based device of 13.12%), and then enhances the NFPSCs’ photovoltaic performance. INTRODUTION Due to the advantages of low cost, light weight, simple preparation process and flexible device, bulk-heterojunction (BHJ) structure polymer solar cells (PSCs) has attracted much attention from academia and industry.1-3 In recent years, the design and application of novel polymer donor materials have played an important role in the continuous improvement of power conversion efficiency (PCE).4-13 Researchers have put the work focus into the molecular structure design and optimization of the donor materials to modulate the material's energy level and band gap, increasing the PSCs devices’ PCE from the initial about 1% to now 11% or more.14-15 For example, in 2006, Hou5 et. firstly reported that taking advantage of the photovoltaic polymer design method with two-dimensional conjugated branch, they broadened the light absorption 2


Page 3 of 26

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


of polythiophene-based photovoltaic materials, enhanced the hole mobility of materials, and achieved high device performances. Donor-acceptor units (D-A) alternative structure is an effective molecular design method to reduce the energy level and optical band gap of the conjugated polymers in high performance PSCs donor materials. Among them, due to the good planarity and easy modification, polymers with benzodithiphene (BDT) as the building block have achieved excellent photovoltaic properties.13,

16

For instance, a PCE of 6.43% was obtained with PBDTTT-C, an

alternating polymer of BDT substituted with alkoxy and thienothiophene (TT). Whereas, the alternating copolymer PBDT-TS117-18 of BDT substituted with two-dimensional conjugated alkylthiothiophene and TT can obtain high PCE of 10%, which proves that extending π-conjugated side chain of the two-dimensional conjugated polymer can effectively improve the PCE of the PSCs. π-Conjugated side chains can broaden the conjugated plane of BDT, so that two-dimensional conjugated polymer has a lower HOMO energy level which contribute to a larger open circuit voltage (Voc) of PSCs devices. The conjugated side chains also can improve the interchain aggregation of the conjugated polymer, which enhance the corresponding PSC’s the photo-response and charge transport properties. As we all know, the optical band gap determines the light absorption limit of a particular polymer and it is a key parameter for the active layer material for PSCs. In the past decade, many researchers have paid attention to developing low band gap (LBG) polymers for PSCs, because they have broad absorption spectra which help to ensure efficient light-harvesting. This can increase the short circuit current density (Jsc) 3



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

of the devices,19-21 but LBG polymer-based PSCs show low Voc due to energy level limitation. 22-23 In the PSCs devices based on wide band gap (WBG) polymers donor and new acceptors, these shortcomings are well made up. Recently, Zhao et al. synthesized two new WBG polymer donor materials: PBT-TTz and PBT-S-TTz, the Egpot of them are all 1.95 eV, but the HOMO level of PBT-S-TTz was lowered to -5.45 eV. A PCE of 7.92% was then obtained in PSCs device based on PBT-S-TTz:PC71BM active layer. Furthermore, a high PCE of 8.22% were obtained from the PBT-S-TTz:ITIC-based PSCs device.24 The band gap of several typical WBG polymers and the photovoltaic parameter of the corresponding NFPSCs are listed in Table 1. Fullerene derivatives, such as PC71BM, is widely used as the acceptor material in high-performance PSCs devices, because of its good solubility, high electron mobility, good phase separation with the common donor polymer materials and etc.. However, fullerene derivatives are faced with huge challenge when the PCE of PSCs increased to 11%.15, 25 Due to the inactive properties of fullerene derivatives, it is difficult to narrow its band gap and tune the energy level by chemical structure optimization. In addition, fullerene derivatives have high charge separation energy as PSCs’ acceptor materials, usually above 0.6 eV,26 which limits the Voc of PSCs based on the narrow band gap donors. In recent years, non-fullerene acceptor materials have been developed rapidly. Especially in the last two years, the PCE of PSCs based on the non-fullerene acceptors has increased from 4% to 13%,27-41 almost close to or even high than the fullerene derivatives’ performances. Currently, there are some popular high-efficiency non-fullerene acceptors, such as naphthalimide-, perylene diimide- and IDT- organic 4


Page 4 of 26

Page 5 of 26

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


polymers or small molecules.33, 39 For example, the perylene diimide cored organic small molecule acceptor based organic solar cells can achieve PCE of 7% -8%;42 the PSCs using the N2200 polymer (naphthalimide and thiophene copolymer) as the acceptor, have achieved the PCE of more than 8%; the PCE of the ITIC and IDT materials has also been more than 10% reported by Zhan et al.43

Table 1. The band gaps of various WBG polymers and the photovoltaic parameters of the corresponding NFPSC devices. WBG Polymer

devices acceptor

band gap (eV)

Voc(V)

PCE(%)

Ref.

PBDTBDD-T

PNDI

---

0.87

5.8

45

J52

ITIC

1.96

0.74

5.18

36

J51

IDSe-T-IC

1.99

0.91

8.58

37

PBDTS-Se

SdiPBI-S

1.81

0.91

8.21

44

PBT-S-TTZ

ITIC

1.95

0.97

8.22

24

Based on these considerations, in this work, in order to study the application of two-dimensional conjugated side chain in non-fullerene-type solar cells polymer donor, we prepared the copolymer based on thiophenothiophene (TT) side chained BDT and the bithiophene monomer, compared with the alkoxy side chained copolymer.

5



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

Page 6 of 26

RESULTS AND DISCUSSION

HE

HE O

C12H25

S Sn

Sn S O

S

Br

S

Br

Pd(PPh3)4

C12H25

EH

O

C12H25 S

Toluene 110℃

S n

S

S O

C12H25

EH

PBDTDT-O EH

EH

S

S S

S

C12H25 S

Sn

Sn

Br

S

S S

S

C12H25 S

Toluene 110℃

Br

Pd(PPh3)4

C12H25

S

S

S S S

S

EH

EH

PBDTDT-TT

Scheme 1. Molecular structures and synthesis route of the copolymers PBDTDT-TT and PBDTDT-O.

Synthesis BDT-O, BDT-TT and DT monomers are commercially available. The 2-ethylhexyl alkyl chain is substituted on the side chain of the BDT, thus ensuring the solubility of the target polymer. And the 3-dodecyl DT monomer was selected to copolymerize with the BDT monomer in order to obtain wide band gap polymer. PBDTDT-O and PBDTDT-TT polymer were prepared by Stille coupling reaction between bis(trimethyltin) BDT monomers (BDT-O or BDT-TT) and bromide (DT), respectively, as shown in Scheme 1. The polymers were precipitated in methanol, filtered, dried, and then the crude product was purified by chromatography on a column of chloroform. The yield of the products was around 60-80%. 6


n

C12H25

Page 7 of 26

Thermal Stability and Absorption Spectra The thermal stability of the polymers under nitrogen atmosphere was evaluated by thermogravimetric analysis (TGA). The TGA curves of these two polymers are shown in Figure 1(a). The decomposition temperature (Td) of PBDTDT-O was about 342°C at 5% weight loss; and PBDTDT-TT had a Td of 410°C. It indicates that the stability of the polymer is greatly improved when the alkoxy side group in BDT unit is replaced by an alkylthienyl group.

1.0

80

PBDTDT-O PBDTDT-TT

60 40 20 0 100

200

300

400 o

Temperature ( C)

500

600

b

1.0 PBDTDT-TT PBDTDT-O

0.8

Normalized Abs (a.u.)

a Normalized Abs (a.u.)

100

Weight (%)

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


0.6 0.4 0.2

PBDTDT-TT PBDTDT-O

0.8 0.6 0.4 0.2 0.0

0.0 300

c

400

500

600

700

800

Wavelength(nm)

300

400

500

600

700

Wavelength(nm)

Figure 1. TGA plots of PBDTDT-O (■) and PBDTDT-TT (●) with a heating rate of 10℃min-1 under an inert atmosphere. (a) Normalized UV-Vis absorption spectra of PBDTDT-O and PBDTDT-TT in a solid film on quartz (b) and as a chloroform solution (c).

The UV-Vis absorption spectra of PBDTDT-O and PBDTDT-TT in solid films and solution are shown in Figure 1 (b) (c). The relevant absorption data, including the maximum absorption peak and absorption onset in the solid films and solution, are shown in Table 2. The absorption peaks of the PBDTDT-O and PBDTDT-TT in solution are 462 nm and 506 nm, respectively, and which are 511 nm and 531 nm in solid film. Obviously, the solid-state absorption spectra of the two polymers are 7


800


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

Page 8 of 26

red-shifted compared to that of the dilute chloroform solution, respectively, 49 nm and 25 nm. The absorption band of PBDTDT-TT is red-shifted and broadened, compared to alkoxy-substituted polymer PBDTDT-O, which can be attributed to the enhanced intermolecular π-π interaction induced by the alkylthiophene conjugated side chain. The absorption onset of the PBDTDT-O and PBDTDT-TT is 608 nm, and 664 nm respectively; thus, the PBDTDT-TT has a band gap of 1.8 eV which is lower than that of PBDTDT-O (2.0 eV) by 0.2 eV. Therefore, it can be concluded that extending two-dimensional π-conjugate by the alkyl thienyl side chain can help to improve the light-absorbing properties of this backbone polymer. Table 2. Absorption spectra properties and molecular energy level data of the polymers. λmax(nm)

λmax(nm)

λ (nm)

Egopt

HOMO

sol

film

film onset

(eV)

(eV)

Polymer

PBDTDT-O

462

511

608

2.0

-5.37

PBDTDT-TT

506

531

664

1.8

-5.27

Molecular Energy Levels The energy levels of the polymers are determined by electrochemical cyclic voltammetry (CV). The CV curves of the two polymers on the glassy carbon electrode are shown in Fig. 2, and the detailed HOMO energy levels are recorded in Table 2. According to the CV diagram, the onset of the p-doping process (first oxidation) of

8


Page 9 of 26

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


PBDTDT-O and PBDTDT-TT is 0.57 V and 0.47 V, respectively, corresponding to HOMO levels of -5.37 eV and -5.27 eV. The LUMO levels of the polymer were estimated by the HOMO energy level and the optical band gap of the solid state according to the following equation: ELUMO= Egopt +EHOMO. From the CV results, we can find that when the alkylthio group was substituted by the alkoxy group, the HOMO level of the BDT polymer was obviously changed by 0.1 eV. The theoretical calculation energy level results of PBDTDT-O and PBDTDT-TT are shown in the supporting information (Figure S1).

PBDTDT-O PBDTDT-TT

0.0

0.2

0.4

0.6

0.8

1.0

1.2

+

Potential (V vs Fc /Fc)

Figure 2. Cyclic voltammograms of PBDTDT-O and PBDTDT-TT films on the glassy carbon electrode in 0.1 mol/L Bu4NPF6 acetonitrile solution. Photovoltaic Properties In order to study the photovoltaic properties of these two polymers, we fabricated a conventional PSCs device based on ITO/PEDOT:PSS/polymer:ITM/PFN-Br/Al configuration and the devices performances are optimized by varying donor/acceptor (D/A) ratio. The energy level of the materials in PSCs devices are show in Fig 3. The detailed parameters of the polymer solar cell devices are listed in Table 3, and the 9



corresponding J-V curves of these devices are shown in Figure 4a. It could be seen from Table 3 that the optimum D/A ratio of the device based on PBDTDT-TT/ITM active layer material was 1:1, and the best devices parameters were higher than PBDTDT-O/ITM except open-circuit voltage. The high PCE was mainly attributed to the high short-circuit current density (Jsc), which was probably due to the higher mobility, the broadened absorption of the 2D-conjugated BDT polymer and the good phase separation of active layer. Devices based on PBDTDT-O/ITM have slightly higher open-circuit voltages (Voc), which might be due to the differences in the energy levels of the polymers donor.

LUMO Energy levels (eV)

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

Page 10 of 26

-3.37

-3.47 0.6

-3.98

0.5 HE S

-4.0

HE

O

PBDTDT-O C12H25 S

S S

S O

S

S O

n

CN NC

CN

S

PBDTDT-TT

S O Me

Hex

ITM

S

EH

Hex

-5.0

-5.27

-5.37 -5.58

HOMO

n C12H25

S

NC

S

S S

S

S

C12H25

EH

C12H25

Me

Hex Hex

Figure 3. The energy levels of PBDTDT-O, ITM and PBDTDT-TT. For high performance PSCs devices, the energy levels of the donor and acceptor materials should be matched, and the energy level difference between the LUMO of the donor and the acceptor must be larger than 0.3 eV to ensure sufficient driving force for exciton separation on the donor-acceptor interface. In the energy level diagram (Fig.3), 10


Page 11 of 26

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


it can be seen that the energy level difference between these two donor materials and the acceptor is 0.6 eV and 0.5 eV, respectively, so the driving force for exciton separation is enough. And the open circuit voltage (Voc) of the PSCs device is proportional to the energy level difference (ΔE) between the HOMO level of the donor and the LUMO level of the acceptor, so PDBTDT-O/ITM devices have higher Voc than PBDTDT-TT/ITM devices due to the low HOMO of PBDTDT-O polymer, as Table 2 and 3 demonstrated. Table 3. Photovoltaic properties of the PSCs based on the polymers. Polymers

D/A

Jsc (mA cm-2)

Voc(V)

FF(%)

PCE(%)

PBDTDT-TT

1.5:1

0.82

15.58

60.01

7.64

PBDTDT-TT

1:1

0.82

16.17

63.40

8.43

PBDTDT-TT

1:1.5

0.81

16.56

55.02

7.39

PBDTDT-O

1.5:1

0.87

1.56

31.07

0.42

PBDTDT-O

1:1

0.85

1.86

34.01

0.54

PBDTDT-O

1:1.5

0.82

1.70

28.63

0.40

Figure 4b shows the devices’ external quantum efficiency (EQE), which reflects the accuracy of the PV measurement. The EQE curve of the PSCs devices are measured at the optimum D/A ratio (1:1) under the same optimization conditions as used for the J-V measurement. From the EQE curve, it can be concluded that the integrated current density of PBDTDT-TT/ITM device is 16.10 mAcm-2, which is higher than that of PDBTDT-O/ITM (1.88mAcm-2), consistent with the result of J-V test result (about 1% 11



deviation). And the PBDTDT-TT/ITM and PDBTDT-O/ITM devices have a wide response range covering 300 to 800 nm with the EQE peaks of 72% and 9.8%, respectively. This experimental data shows that more incident photons in the PBDTDT-TT/ITM active layer are converted into electrons to the external circuit, which lead to a higher short-circuit current density of PBDTDT-TT/ITM device.

80

4

a

70

b

0

60 -4 -8 -12

PBDTDT-TT 1.5:1 PBDTDT-TT 1:1 PBDTDT-TT 1:1.5 PBDTDT-O 1.5:1 PBDTDT-O 1:1 PBDTDT-O 1:1.5

EQE (%)

-2 Current Density (mA cm )

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

Page 12 of 26

40 30 20

-16 -20 -0.4

PBDTDT-TT PBDTDT-O

50

10

0.0

0.4

0.8

0 300

1.2

400

500

600

700

800

900

Wavelength (nm)

Bias (V)

Figure 4. (a) J-V curves of the polymer solar cells based on PBDTDT-O and PBDTDT-TT under illumination of AM 1.5G, 100 mWcm-2. (b) EQE curves of the corresponding polymer solar cells. With respect to photo-generated current behavior, the high performance of the PBDTDT-TT/ITM PSCs device is explained by the relationship between the photo-generated current density (Jph) and the effective voltage (Veff). (Fig 5) According to the relationship Jph = JL-JD, JL and JD are the current densities under illumination and dark conditions, respectively. Veff = V0-Va, V0 is the voltage when Jph=0, Va is the applied voltage. The photo-generated current behavior of the blended films was analyzed from -5V to +3V in J-V diagram, in that the exciton dissociation probability

12


Page 13 of 26

(Pdiss) is defined as the key parameter in the research of exciton generation and dissociation in polymer solar cells, Pdiss=Jph/Jsat. The results show that the exciton dissociation efficiencies of PBDTDT-TT/ITM and PBDTDT-O/ITM blends are 85.36% and 13.12% respectively, indicating that PBDTDT-TT/ITM blend film has stronger exciton dissociation ability.

PBDTDT-TT PBDTDT-O

-2

(mA cm )

10

ph

1

J

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


0.1 0.1

1

V

eff

(V )

Figure 5. Photocurrent versus effective voltage plots under optimal conditions for different polymers. Mobility is an important parameter for evaluating the photoelectric properties of materials, and the electron mobility (μe) and hole mobility (μh) of the blend active layers of polymer and ITM are measured by the space charge-limited current (SCLC) method, as Figure 6 shows. After measurement, the electron mobility and hole mobility of the PBDTDT-TT/ITM blend film are 4.65×10-4 cm2V-1s-1 and 2.05×10-4 cm2V-1s-1, respectively, and that of the PBDTDT-O/ITM blend film are 8.31×10-5cm2V-1s-1 and 3.1×10-5cm2V-1s-1. The electron and hole mobility of PBDTDT-TT is one order of magnitude higher than that of PBDTDT-O. This indicates that the transport of holes and electrons in the PBDTDT-TT/ITM based device is easy and balanced, which can 13



effectively avoid the accumulation of charge, and this is the key to achieve higher fill factor (FF) polymer solar cells.

3

a

-37

PBDTDT-TT PBDTDT-O

2

3 -2 -2 ln(JL V )(AcmV )

4

2 ln J (mA/cm )

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

Page 14 of 26

1 0 -1 -2

b PBDTDT-TT PBDTDT-O

-38

-39

-40

-3 -4 0.0

0.5

1.0

1.5

2.0

2.5

-41 500

3.0

600

700

0.5

ln V (V)

(V/L)

800

0.5 (V/cm)

Figure 6. Electron (a) and hole mobility (b) of the polymer/ITM blend film. Morphology Analysis In PSCs, there is a direct relationship between the topography structure of the active layer and the PCE of the device. Especially in the bulk heterojunction structure, the interpenetrating network of bicontinuous phase is necessary to realize the effective exciton diffusion, separation and carrier transport. Therefore, we use atomic force microscope (AFM) and transmission electron microscopy (TEM) to analyze the morphology of the blend films formed by the polymers and ITM. As shown in Fig. 7, both of PBDTDT-O and PBDTDT-TT have good compatibility with ITM, and these two blends have similar morphology features. The PDBTDT-O/ITM and PDBTDT-TT/ITM blend films’ root-mean-square roughness (Rq) are 1.39 nm and 1.52 nm, respectively, in the AFM height images. This indicates that the PDBTDT-O/ITM blend film has smaller phase separation than PDBTDT-TT/ITM, which is

14


900

Page 15 of 26

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


disadvantageous for exciton dissociation and charge transport. This further explains why the EQE response of the PDBTDT-O/ITM blend film is lower. The intrinsic bulk morphology of the blended films was further studied by TEM, both images (Fig.7 a and d) showed similar homogeneous morphology, and no large-scale aggregation structure was observed. Observing carefully, the PBDTDT-O/ITM blend film is more evenly distributed and delicate than the PDBTDT-TT/ITM, indicating the phase separation of the PDBTDT-TT/ITM blend film is larger, which is consistent with the results of the AFM.

Figure 7. (a) TEM image, (b) AFM height and (c) phase images of PBDTDT-O/ ITM blend films. (d) TEM image, (e) AFM height and (f) phase images of PBDTDT-TT/ITM blend films.

15



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

Experimental Materials: the monomers, BDT-O, BDT-TT, and DT, were purchased from Solarmer Materials Inc. Pd(PPh3)4 was obtained from Frontiers Scientific Inc. All of these chemicals and reagents were used as received. Synthesis: The bis-trimethyltin-monomer (BDT-O or BDT-TT) (0.3mmol) and dibromide monomer (DT) (0.3mmol) were dissolved in 10 ml of toluene in a duplex flask. Pd(PPh3)4 was added quickly as the catalyst after being purged with argon for 5 min, and the mixture was purged with argon for another 25 min. The flask was put into the thermostatic oil bath and heated to 110℃ for 4h. Then cooling the reactant to room temperature, and precipitating the polymer into 100 mL of the methanol. The precipitated polymer was filtrated, dried, and dissolved in chloroform to further purification by the silica gel chromatography. We got the last polymer by precipitating again 100 mL of methanol. Both PBDTDT-O and PBDTDT-TT polymer is dark purple solid with a yield of 60-80%. And the 1H NMR of the polymers are depicted in Figure S2 (Supporting Information). PBDTDT-O. Elemental analysis calcd (%) for C59H90S4O2: C 73.90, H 9.39; found: C 73.43, H 9.41. Mn = 59k, PDI = 2.5. PBDTDT-TT. Elemental analysis calcd (%) for C70H92S8: C 70.70, H 7.74; found: C 70.66, H 7.64. Mn = 60k, PDI = 3.0. Instruments: 1H NMR spectra were measured on a Bruker arx-400 spectrometer. Absorption spectra were taken on a Hitachi U-3010 UV-Vis spectrophotometer. The electrochemical cyclic voltammetry experiments were conducted on a Zahner IM6e 16


Page 16 of 26

Page 17 of 26

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


Electrochemical Workstation with glassy carbon disk, Pt wire, and a Ag/Ag+ electrode as the working electrode, counter electrode, and reference electrode, respectively in a 0.1molL-1 tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution. The potential of Ag/Ag+ reference electrode was internally calibrated by using the ferrocene/ferrocenium redox couple (Fc/Fc+). TGA measurements were performed on a TA Instrument, Inc., TGA-2050. Fabrication of polymer solar cells: All structures of PSCs devices are ITO/PEDOT:PSS/Polymers:ITM/PFN-Br/Al.

Patterned

indium

tin

oxide

(ITO)-coated glass substrate was precleaned in an ultrasonic bath of acetone and isopropyl alcohol, and treated in an ultraviolet-ozone chamber (Jelight Company, Irvine, CA) for 20 min. A thin layer of PEDOT:PSS was deposited through spin-coating on ITO glass from at about 3000 rpm and dried subsequently at 150 ℃ for 15 min in air. Then the device was transferred to a nitrogen glove box, where the active blend layer containing a blend of PBDTDT-O (or PBDTDT-TT) and ITM was spin-coated from chlorobenzene solution onto the PEDOT:PSS layer. Afterwards, the methanol solution of 0.5 mg/mL PFN-Br was deposited atop the active layer at 3000 rpm for 30 s to afford a thickness of ca. 10 nm. Finally, 80 nm of Al were deposited onto the PFN-Br layer under high vacuum successively. The active area of the device was 4 mm2. Conclusions In this work, one 2D conjugated BDT and bithiophene copolymer (PBDTDT-TT) with thiophenothiophene (TT) side chain is compared with alkoxy group side-chained 17



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

polymer (PBDTDT-O) for donor materials of non-fullerene polymer solar cells (NFPSCs’). PBDTDT-TT polymer shows better thermal stability, wider and red-shifted UV-Vis absorption spectrum and lower band gap. NFPSCs’ devices is fabricated with ITO/PEDOT:PSS/polymer:ITM/PFN-Br/Al configuration, where ITM is one new non-fullerene acceptor. PBDTDT-TT polymer based NFPSCs’ devices show high PCE of 8.43%, while the PCE of PBDTDT-O polymer based device is less than 1%. The high performance of PBDTDT-TT polymer solar cells is attributed to the wide light absorption, high and balanced electron and hole mobility, good active layer phase separation and high exciton dissociation efficiencies. ASSOCIATED CONTENT Supporting Information. Theoretical calculation results and 1H NMR spectra of PBDTDT-O and PBDTDT-TT are provided in the Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Mingliang Sun , E-mail: [email protected]; Feng Li, E-mail: [email protected] Funding Sources Notes The authors declare no competing financial interest. Acknowledgements The authors gratefully acknowledge financial support from the NSFC (21274134). 18


Page 18 of 26

Page 19 of 26

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


REFERENCES 1. Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. L. Design Rules for Donors in Bulk-Heterojunction Solar Cells - Towards 10 % Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789-794. 2. Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photon. 2012, 6, 153-161. 3. Heeger, A. J. 25th Anniversary Article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26, 10-27. 4. Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mulhlbacher, D.; Scharber, M.; Brabec,

C.

Panchromatic

Conjugated

Polymers

Containing

Alternating

Donor/Acceptor Units for Photovoltaic Applications. Macromolecules 2013, 40, 1981-1986. 5. Hou, J. H.; Park, M. H.; Zhang, S. Q.; Yao, Y.; Chen, L. M.; Li, J. H.; Yang, Y. Bandgap and Molecular Energy Level Control of Conjugated Polymer Photovoltaic Materials Based on Benzo[1,2-b:4,5-b']dithiophene. Macromolecules 2008, 41, 6012-6018. 6. Chen, J.; Cao, Y. Development of Novel Conjugated Donor Polymers for High-Efficiency Bulk-Heterojunction Photovoltaic Devices. Accounts Chem. Res. 2009, 42, 1709-1718. 7. Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868-5923.

19



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

Page 20 of 26

8. Beaujuge, P. M.; Fréchet, J. M. J. Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications. J. Am. Chem. Soc. 2011, 133, 20009-20029. 9. Zhu, J.; Li, S.; Liu, X.; Yao, H.; Wang, F.; Zhang, S.; Sun, M.; Hou, J. Subtle Side-chain Tuning on Terminal Groups of Small Molecule Electron Acceptors for Efficient Fullerene-free Polymer Solar Cells. J. Mater. Chem. A 2017, 5, 15175-15182. 10. Henson, Z. B.; Mullen, K.; Bazan, G. C. Design Strategies for Organic Semiconductors Beyond the Molecular Formula. Nat. Chem. 2012, 4, 699-704. 11. Li, Y. F. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Accounts Chem. Res. 2012, 45, 723-733. 12. Ye, L.; Zhang, S.; Huo, L.; Zhang, M.; Hou, J. Molecular Design toward Highly Efficient

Photovoltaic

Polymers

Based

on

Two-Dimensional

Conjugated

Benzodithiophene. Accounts Chem. Res. 2014, 47, 1595-1603. 13. Yao, H.; Ye, L.; Zhang, H.; Li, S.; Zhang, S.; Hou, J. Molecular Design of Benzodithiophene-Based Organic Photovoltaic Materials. Chem. Rev. 2016, 116, 7397-7457. 14. Chen, J. D.; Cui, C.; Li, Y. Q.; Zhou, L.; Ou, Q. D.; Li, C.; Li, Y.; Tang, J. X., Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1035-1041.

20


Page 21 of 26

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


15. Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H., Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. 16. Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Accounts Chem. Res. 2012, 45, 723-733. 17. Ye, L.; Zhang, S.; Zhao, W.; Yao, H.; Hou, J. Highly Efficient 2D-Conjugated Benzodithiophene-Based Photovoltaic Polymer with Linear Alkylthio Side Chain. Chem. Mater. 2014, 26, 3603-3605. 18. Yao, H.; Zhao, W.; Zheng, Z.; Cui, Y.; Zhang, J.; Wei, Z.; Hou, J., PBDT-TSR: A Highly Efficient Conjugated Polymer for Polymer Solar Cells with A Regioregular Structure. J. Mater. Chem. A 2016, 4, 1708-1713. 19. Boudreault P. L. T.; Najari A.; Leclerc M. Processable Low-Bandgap Polymers for Photovoltaic Applications. Chem. Mater. 2011, 23, 456-469. 20. Kroon R.; Lenes M.; Hummelen J. C.; Blom P. W. M.; Boer B. D. Small Bandgap Polymers for Organic Solar Cells (Polymer Material Development in the Last 5 Years). Polymer Rev. 2008, 48, 531-582. 21. Duan C.; Huang F.; Cao Y. Recent Development of Push-Pull Conjugated Polymers for Bulk-Heterojunction Photovoltaics: Rational Design and Fine Tailoring of Molecular Structures. J. Mater. Chem. 2012, 22, 10416-10434.

21



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

22. Gevaerts V. S.; Furlan A.; Wienk M. M.; Turbiez M.; Janssen R. A. J. Solution Processed Polymer Tandem Solar Cell Using Efficient Small and Wide bandgap Polymer:Fullerene Blends. Adv. Mater. 2012, 24, 2130-2134. 23. Trupke T.; Green M. A.; Würfel P. Improving Solar Cell Efficiencies by Down-Conversion of High-Energy Photons. J. Appl. Phys. 2002, 92, 1668-1674. 24. Zhao, K.; Wang, Q.; Xu, B.; Zhao, W.; Liu, X.; Yang, B.; Sun M.; Hou J. Efficient Fullerene-Based and Fullerene-Free Polymer Solar Cells Using Two Wide Band Gap Thiophene-Thiazolothiazole-Based Photovoltaic Materials. J. Mate. Chem. A 2016, 4, 9511-9518. 25. He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y. Single-Junction Polymer Solar Cells with High Efficiency and Photovoltage. Nat. Photon. 2015, 9, 174-179. 26. Bin, H.; Gao, L.; Zhang, Z. G.; Yang, Y.; Zhang, Y.; Zhang, C.; Chen, S.; Xue, L.; Yang, C.; Xiao, M.; Li, Y. 11.4% Efficiency Non-Fullerene Polymer Solar Cells with Trialkylsilyl Substituted 2D-Conjugated Polymer as Donor. Nat. Commun.2016, 7, 13651. 27. Jiang, W.; Ye, L.; Li, X.; Xiao, C.; Tan, F.; Zhao, W.; Hou, J.; Wang, Z. Bay-linked Perylene Bisimides as Promising Non-Fullerene Acceptors for Organic Solar Cells. Chem. Commun. 2014, 50, 1024-1026. 28. Li S.; Ye L.; Zhao W.; Zhang S.; Ade H.; Hou J. Significant Influence of the Methoxyl Substitution Position on Optoelectronic Properties and Molecular Packing of

22


Page 22 of 26

Page 23 of 26

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


Small-Molecule Electron Acceptors for Photovoltaic Cells. Adv. Energy Mater. 2017, 1700183. 29. Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables Over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148-7151. 30. Zang, Y.; Li, C. Z.; Chueh, C. C.; Williams, S. T.; Jiang, W.; Wang, Z. H.; Yu, J. S.; Jen, A. K. Y. Integrated Molecular, Interfacial, and Device Engineering towards High-Performance Non-Fullerene Based Organic Solar Cells. Adv. Mater. 2014, 26, 5708-5714. 31. Li, H.; Earmme, T.; Subramaniyan, S.; Jenekhe, S. A. Bis(Naphthalene Imide)diphenylanthrazolines: A New Class of Electron Acceptors for Efficient Nonfullerene Organic Solar Cells and Applicable to Multiple Donor Polymers. Adv. Energy Mater. 2015, 5, 1402041. 32. Lin, H.; Chen, S.; Li, Z.; Lai, J. Y.; Yang, G.; McAfee, T.; Jiang, K.; Li, Y.; Liu, Y.; Hu, H.; Zhao, J.; Ma, W.; Ade, H.; Yan, H. High-Performance Non-Fullerene Polymer Solar Cells Based on a Pair of Donor-Acceptor Materials with Complementary Absorption Properties. Adv. Mater. 2015, 27, 7299-7304. 33. Nielsen, C. B.; Holliday, S.; Chen, H.-Y.; Cryer, S. J.; McCulloch, I. Non-Fullerene Electron Acceptors for Use in Organic Solar Cells. Accounts Chem. Res. 2015, 48, 2803-2812. 34. Zhao, J.; Li, Y.; Lin, H.; Liu, Y.; Jiang, K.; Mu, C.; Ma, T.; Lin Lai, J. Y.; Hu, H.; Yu, D.; et al. High-Efficiency Non-Fullerene Organic Solar Cells Enabled by A 23



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

Difluorobenzothiadiazole-Based Donor Polymer Combined with A Properly Matched Small Molecule Acceptor. Energy Environ. Sci. 2015, 8, 520-525. 35. Zhong, Y.; Trinh, M. T.; Chen, R.; Purdum, G. E.; Khlyabich, P. P.; Sezen, M.; Oh, S.; Zhu, H.; Fowler, B.; Zhang, B. et al. Molecular Helices as Electron Acceptors in High-Performance Bulk Heterojunction Solar Cells. Nat. Commun. 2015, 6, 8242. 36. Bin, H.; Zhang, Z. G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y. Non-Fullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2D-Conjugated Polymers Reach 9.5% Efficiency. J. Am. Chem. Soc. 2016, 138, 4657-4664. 37. Li, Y.; Zhong, L.; Wu, F.-P.; Yuan, Y.; Bin, H.; Jiang, Z. Q.; Zhang, Z.; Zhang, Z.-G.; Li, Y.; Liao, L. S. Non-Fullerene Polymer Solar Cells Based on Selenophene-Containing Fused-Ring Acceptor with Photovoltaic Performance of 8.6%. Energy Environ. Sci. 2016, 9, 3429-3435. 38. Liu, J.; Chen, S.; Qian, D.; Gautam, B.; Yang, G.; Zhao, J.; Bergqvist, J.; Zhang, F.; Ma, W.; Ade, H.; et al. Fast Charge Separation in A Non-Fullerene Organic Solar Cell with A Small Driving Force. Nat. Energy 2016, 1, 16089. 39. Liu, S.; Kan, Z.; Thomas, S.; Cruciani, F.; Brédas, J.-L.; Beaujuge, P. M. Thieno[3,4-c]pyrrole-4,6-dione-3,4-difluorothiophene Polymer Acceptors for Efficient All-Polymer Bulk Heterojunction Solar Cells. Angew. Chem. 2016, 128, 13190-13194. 40. Liu, T.; Guo, Y.; Yi, Y.; Huo, L.; Xue, X.; Sun, X.; Fu, H.; Xiong, W.; Meng, D.; Wang, Z.; et al. Ternary Organic Solar Cells Based on Two Compatible Nonfullerene

24


Page 24 of 26

Page 25 of 26

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


Acceptors with Power Conversion Efficiency >10%. Adv. Mater. 2016, 28, 10008-10015. 41. Yang, Y.; Zhang, Z. G.; Bin, H.; Chen, S.; Gao, L.; Xue, L.; Yang, C.; Li, Y. Side-Chain Isomerization on An n-Type Organic Semiconductor ITIC Acceptor Makes 11.77% High Efficiency Polymer Solar Cells. J. Am. Chem. Soc. 2016,138, 15011-15018. 42. Gao, L.; Zhang, Z. G.; Xue, L.; Min, J.; Zhang, J.; Wei, Z.; Li, Y. All-Polymer Solar Cells Based on Absorption-Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%. Adv. Mater. 2016, 28, 1884-1890. 43. Lin, Y.; Wang, J.; Zhang, Z.-G.; Bai, H.; Li, Y.; Zhu, D.; Zhan, X., An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170-1174. 44. Liu T.; Meng D.; Cai Y.; Sun X.; Li Y.; Huo L.; Liu F.; Wang Z.; Russell T.P.; Sun Y. High‐Performance Non‐Fullerene Organic Solar Cells Based on a Selenium‐Containing Polymer Donor and a Twisted Perylene Bisimide Acceptor. Adv. Sci. 2016, 3, 1600117. 45. Ye L.; Jiao X.; Zhou M.; Zhang S.; Yao H.; Zhao W.; Xia A.; Ade H.; Hou J. Manipulating Aggregation and Molecular Orientation in All‐Polymer Photovoltaic Cells. Adv. Mater. 2015, 27, 6046-6054.

25



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

TOC

26


Page 26 of 26

Two-Dimensional BDT-Based Wide Band Gap Polymer Donor for

Recommend Documents