Two-dimensional Copolymers Based on an Alkylthio-naphthyl

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Two-dimensional Copolymers Based on an Alkylthio-naphthyl Substituted Benzo[1,2-b:4,5-b']dithiophene for High-efficiency Polymer Solar Cells Gongyue Huang, Huanxiang Jiang, Jun Zhang, Fushuai Liu, Mengbing Zhu, Hua Tan, Ya-Fei Wang, Weichao Chen, Renqiang Yang, and Weiguo Zhu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00312 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 11, 2018

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ACS Applied Energy Materials

Two-Dimensional Copolymers Based on An Alkylthionaphthyl Substituted Benzo[1,2-b:4,5-b']dithiophene for High-Efficiency Polymer Solar Cells Gongyue Huang, †,‡,¶ Huanxiang Jiang, ¶ Jun Zhang, †,¶ Fushuai Liu, ¶ Mengbin Zhu, ‡ Hua Tan, †,‡ Yafei Wang,†,‡ Weichao Chen, *,§, ¶ Renqiang Yang,*, ¶ Weiguo Zhu*,†,‡ †

College of Chemistry, Xiangtan University, Xiangtan 411105, China

‡

School of Materials Science and Engineering, Jiangsu Collaboration Innovation Center of

Photo voltaic Science and Engineering, Jiangsu Engineering Laboratory of Light-Electricity -Heat Energy-Converting Materials and Applications, Changzhou University, Changzhou 213164, China §

College of Textiles & Clothing, Qingdao University, Qingdao 266071, China

¶

CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess

Technology, Chinese Academy of Sciences, Qingdao 266101, China Corresponding Author * Email: [email protected] * Email: [email protected] * Email: [email protected]

KEYWORDS: Polymer solar cells; Donor-acceptor polymer; Narrow-band polymer; Benzo[1,2-b:4,5-b’]dithiophene; Fullerene.

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ABSTRACT.

Two new D-A donor polymers of PBDTNS-DTBO and PBDTNS-DTBT are designed and synthesized with a two-dimensional alkylthio-naphthyl substituted benzo[1,2-b:4,5-b’]dithio phene (BDTNS) unit. The influence of the BDTNS unit and another modified acceptor unit of benzo-oxadiazole (BO) (or benzo-thiadiazole (BT)) on optical, electrochemical and photovoltaic properties is primarily studied. A stronger photo-response with a higher external quantum efficiency is observed in the PBDTNS-DTBO film. As a result, PBDTNS-DTBO with dialkoxy-substituted BO unit exhibits better photovoltaic properties than PBDTNS-DTBT with difluorine-substituted BT unit in their solution-processing bulk hetero-junction polymer solar cells (PSCs) using [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor. The maximum power conversion efficiency of 8.02% with a short-circuit current density of 13.05 mA/cm2 and a high filled factor of 71.5% is obtained in the PBDTNS-DTBO based devices. Our study indicates that PBDTNS-DTBO is a promising narrow-band photovoltaic polymer to construct high-performance PSCs.


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Polymer solar cells (PSCs) have been attracted global attention in recent years due to their unique advantages, like simple preparation, flexibility, low cost and large-scale production.1-4 Their commercial application is mainly limited by their unsatisfactory power conversion efficiency (PCE). There are three key parameters to influence the PCE value, which are open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF).5 Therefore, a promising donor material in solar cells should meet some basic requirements to improve its photovoltaic properties, such as broad absorption spectra to increase JSC, deep highest occupied molecular orbital (HOMO) energy level to maximize VOC, suitable lowest unoccupied molecular orbital (LUMO) energy level to efficiently promote charge separation with minimum energy loss, and high hole mobility to further increase JSC and FF.6, 7

Combining electron donor (D) and acceptor (A) units to construct D-A type polymers is an effective strategy to enhance photovoltaic performance currently, and lots of high-efficiency donor materials with D-A structure were developed.8-18 As one of excellent donor building blocks, benzo[1,2-b:4,5-b']dithiophene (BDT) with planar conjugated structure has become a focused spotlight to obtain high-performance D-A donor materials by inherent structural modification.19-23 For example, a classical D-A polymer of PTB7 based on alkoxy-substituted BDT unit exhibited a high PCE of 7% reported by Yu and coworkers in 2010.9 A breakthrough progress with a PCE of 9.29% was made by Yang and coworkers in 2016, in which an unsubstituted BDT was used to construct the novel D-A polymer of PBDT-DTFFBT.11 At the same time, two-dimension (2D) BDT polymers were also presented in recently years and showed the improved photovoltaic properties than 1D BDT polymers.24-27 Among these D-A polymers with BDT unit, it is found that different acceptor units, such as ester-substituted 3 ACS Paragon Plus Environment


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thieno[3,4-b]thiophene (TT), benzo[c][1,2,5]thiadiazole (BT), benzo [1,2-b:4,5-b']dithiophene -4,8-dione (BDD) played an important role in improvement of photovoltaic performance.28-33 Therefore, it is very important to alternate donor and acceptor units for constructing high-performance D-A photovoltaic polymers.

Inspired by these promising efforts, two D-A type polymers of PBDTNS-DTBO and PBDTNS-DTBT were designed and synthesized with a two-dimensional alkylthionaphthaleneyl substituted benzo[1,2-b:4,5-b’]dithiophene (BDTNS) unit. We selected BDTNS as a novel donor unit due to its stronger absorption than the previous derivative BDT unit.34 In order to study the influence of the alternative donor and acceptor units on optical, electrochemical and photovoltaic properties, we chosen 5,6-bis(octyloxy)benzo[c][1,2,5] oxadiazole (BO) and 5,6-fluorobenzo [c] [1,2,5]thiadiazole (BT) as acceptor units to construct novel donor polymers as these acceptor units have intense electron-withdrawing property. As expected, a BDTNS donor unit and a modified acceptor unit of BO (or BT) have a significant effect on optical, electrochemical and photovoltaic properties. PBDTNS-DTBO with alkoxy-substituted BO unit exhibited better photo -voltaic properties than PBDTNS-DTBT with fluorine-substituted BT unit in their solution-processing bulk hetero-junction polymer solar cells (PSCs) using [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor. The maximum PCE of 8.02% with a JSC of 13.05 mA/cm2 and a high FF of 71.5% is observed in the PBDTNS-DTBO based devices.

Synthesis and thermal property


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Two polymers of PBDTNS-DTBO and PBDTNS-DTBT were synthesized with Stille coupling reaction. Their general synthetic routes are depicted in Scheme 1 and their detailed synthetic routes are given in Experimental Section. It is found that both polymers are easily dissolved in common solvents for fabricating devices, such as chlorobenzene (CB) and o-dichlorobenzene (o-DCB). The number-average molecular weights (Mn) of PBDTNS-DTBO and PBDTNS-DTBT

Scheme 1. Synthetic routes of PBDTNS-DTBO and PBDTNS-DTBT.



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Figure 1. TGA plots of PBDTNS-DTBO and PBDTNS-DTBT at a heating rate of 20 °C min-1 under a nitrogen atmosphere.

were measured to be 25 and 21 KDa with a polydispersity of 2.3 and 2.5, respectively. Their thermal property was tested by thermogravimetric analysis (TGA) in a nitrogen atmosphere at a heating rate of 20 °C min−1. As shown in Figure 1, PBDTNS-DTBO and PBDTNS-DTBT showed an onset decomposition temperature (Td) of 307 °C and 365 °C at 5% weight loss, respectively. It is clear that PBDTNS-DTBT exhibits higher thermal stability than PBDTNS-DTBO, which means that introducing F atom into the BT unit is available to promote thermal stability for these D-A type polymers.

Optical and Electrochemical Properties.

UV-vis absorption spectra of PBDTNS-DTBO and PBDTNS-DTBT in o-DCB solutions and in their neat films are shown in Figure 2. Two maximum absorption peaks in the range of 500-700 nm are observed for both polymers, in which the peak at ca.580 nm comes from the intra- molecular charge transfer (ICT) between the molecular donor and the acceptor unit, another peak at ca.650 nm is attributed to the molecular aggregate.35,

36

In comparison to

PBDTNS-DTBO, PBDTNS-DTBT exhibits a slightly red-shifted absorption spectrum in o-DCB solution and in thin film owing to its stronger intermolecular D-A interaction. The optical band gaps of PBDTNS-DTBO and PBDTNS-DTBT are respectively calculated to be 1.79 eV and 1.76 eV according to the absorption edge of the films. However, PBDTNS-DTBO possesses a stronger absorption in the region of 400 ~ 600 nm than PBDTNS-DTBT, it is


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possible that the former has stronger molecular aggregate in its neat film.37 This feature is available to make PBDTNS-DTBO absorb more photonics.

Figure 2. Normalized absorption spectra of PBDTNS-DTBO and PBDTNS-DTBT in o-DCB solutions and in their solid films.

Figure 3. CV curves of the PBDTNS-DTBO and PBDTNS-DTBT films vs Fc/Fc+ in acetonitrile solution.

Figure 3 shows cyclic voltammetry (CV) curves of PBDTNS-DTBO and PBDTNS-DTBT. An onset oxidation potential of 0.97 eV for PBDTNS-DTBO and 1.04 eV for PBDTNS-DTBT 7 ACS Paragon Plus Environment


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is observed. However, their reductive potentials are not recorded. Based on the empirical equation, their corresponding HOMO energy levels are calculated to be −5.31 eV and −5.38 eV, respective -ly.38 The LUMO energy levels are derived from their HOMO levels and optical bandgaps, which are about −3.52 eV and −3.62 eV for PBDTNS-DTBO and PBDTNS-DTBT, respectively. The deeper HOMO energy level for PBDTNS-DTBT is attributed to the electron-withdrawing ability of F atom and can make the PBDTNS-DTBT based cells obtain high VOC.

Photovoltaic Performance The organic solar cells based on PBDTNS-DTBO and PBDTNS-DTBT were fabricated using PC71BM as acceptor. With a conventional structure of ITO/PEDOT:PSS/polymer: PC71BM /PFN/Al, the solar cells were optimized by changing weight ratio of the donor to acceptor (D/A, w/w) in the active layers. The J-V curves of the devices at different D/A ratios are shown in Figure S1. The J-V curves of the optimal devices are shown in Figure 4. And their detailed photovoltaic parameters are summarized in Table 1. A highest PCE of 8.02% with a JSC of 13.05 mA/cm2, a VOC of 0.86 V, and a FF of 71.5% is observed in the PBDTNS-DTBO based devices. The PBDTNS-DTBT based devices exhibit a higher VOC, but a slightly lower JSC and FF than the PBDTNS-DTBO based devices. Benefiting from the higher JSC and FF values, the PBDTNS-DTBO based devices show an increasing PCE value than the PBDTNS-DTBT based devices. Furthermore, in comparison with the similar reported 2D D-A polymers, both polymers based on the BDTNS unit obtained a larger JSC and FF, which lead to a preferable photovoltaic performance.26, 39


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Figure 4. J-V curves of the optimal polymer/PC71BM solar cells under simulated AM 1.5G irradiation (100mW/cm2).

Table 1. Photovoltaic parameters of the optimal polymer / PC71BM solar cells. Polymer

D/A ratio

PBDTNS-DTBO

1：2

PBDTNS-DTBT

PBDTNS-DTBT a a

VOC (V)

JSC (mA/cm2)

FF (%)

PCE b (%)

0.86

13.05

71.5

8.02

(0.86±0.004)

(12.99±0.05)

(70.8±0.95)

(7.95±0.10)

0.94

10.93

56.1

5.78

(0.93±0.005)

(10.81±0.03)

(54.5±1.3)

(5.69±0.20)

0.90

12.34

62.2

6.89

(0.90±0.004)

(12.10±0.04)

(61.1±1.1)

(6.77±0.13)

1：1

1：1

3% DIO; b The values in parentheses are average PCEs from 15 devices.

Figure 5 shows the external quantum efficiency (EQE) curves of both polymers/PC71BM blends. It is found that the PBDTNS-DTBO/PC71BM blend gives the maximum EQE of 74.43%, which presents stronger photo-response with a higher maximum EQE than the PBDTNS-DTBT/ PC71BM blend from 300 nm to 600 nm. This is one of the reasons that the PBDTNS-DTBO based devices exhibit higher JSC value and PCE. A calculated current density of 12.83 mA/cm2 and 12.17 mA/cm2 were respectively obtained by integrating the EQE curves 9 ACS Paragon Plus Environment


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for PBDTNS-DTBO and PBDTNS-DTBT,, which was consistent with the measured JSC with an error of

Two-dimensional Copolymers Based on an Alkylthio-naphthyl

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