Effect of cyano substituent on photovoltaic properties of quinoxaline-based polymers

doi:10.1016/j.jiec.2020.03.018

Journal of Industrial and Engineering Chemistry

Volume 86, 25 June 2020, Pages 244-250

https://doi.org/10.1016/j.jiec.2020.03.018 Get rights and content

Highlights

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Quinoxaline-based polymers were synthesized for polymer solar cells.
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The electron-donating indacenodithiophene and indacenodithieno[3,2-b]thiophene were linked to the electron-accepting 2,3-diphenylquinoxaline.
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The incorporation of CN substituent in DPQ unit can efficiently improve the photovoltaic performances.

Abstract

Three donor-acceptor type quinoxaline-based conjugated polymers, in which electron-donating indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) were connected to the electron-accepting 2,3-diphenylquinoxaline (DPQ) derivatives, were synthesized using the Stille coupling reaction. To investigate the effects of the strong electron-withdrawing cyano ( single bond CN) substituent on the photovoltaic properties of polymers, it was selectively introduced in the DPQ unit on the polymer backbone. The combination of IDT and the standard DPQ produced the reference polymer IDT-Qx, while the coupling of IDT and IDTT with mono-CN substituted DPQ provided the target polymers IDT-QxCN and IDTT-QxCN, respectively. Owing to the significant contributions of the CN substituent, IDT-QxCN and IDTT-QxCN exhibited better photovoltaic performances than IDT-Qx reference. Furthermore, the presence of more planar two thieno[3,2-b] thiophenes in IDTT than the two thiophenes in IDT allowed IDTT-QxCN to display the highest power conversion efficiency (5.47%) with an open circuit voltage of 0.95 V, short-circuit current density of 15.30 mA cm⁻², and fill factor of 56.95%.

Graphical abstract

Introduction

During last few decades, bulk-heterojunction (BHJ) polymer solar cells (PSCs) have attracted immense attention, owing to unique advantages such as simple preparation, low weight, good flexibility, and cost-effectiveness [1], [2], [3]. The favorable bicontinuous interpenetrating networks between p-type polymeric electron donors and n-type electron acceptors was simply achieved by forming a BHJ structure [4], [5], [6], [7], [8], [9]. Thereby, the separation and transport of photo-excited excitons were greatly facilitated in an active layer of the device. The recent breakthroughs in material synthesis and device technology have increased the power conversion efficiencies (PCEs) of PSCs to more than 16% [10]. In the case of p-type polymers in BHJ PSCs, various conjugated polymers with alternating electron-donating (D) and electron-accepting (A) components along their backbones have been considered and developed. In this D-A architecture, the bandgap of conjugated polymers can be significantly reduced by the formation of an intramolecular charge transfer (ICT) state between the D and A constituent [11], [12], [13]. Usually, benzothiophene, triphenylamine, and carbazole derivatives are utilized as D components [14], [15], [16], whereas benzothiadiazole, diketopyrrolopyrrole, and quinoxaline (Qx) units are adapted as A constituents to construct D-A type conjugated polymers [17], [18], [19].

One recent noteworthy finding intended to improve the photovoltaic performances of D-A type polymers is the incorporation of strong electron-withdrawing moieties, such as fluorine ( single bond F), and trifluoromethyl (–CF₃) groups into the polymer structures [20], [21], [22]. The existence of electron-withdrawing substituents, particularly onto the A-unit of the polymers, can efficiently reduce the energy levels of the polymers in both the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). This low-lying HOMO energy level of the polymers can readily increase the PCE of PSCs by enhancing the open-circuit voltage (V_oc) of the device [23], [24].

In this study, three D-A type quinoxaline-based conjugated polymers, in which electron-donating indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) were connected with electron-accepting 2,3-diphenylquinoxaline (DPQ) units, were prepared under the Stille coupling condition. In particular, the strong electron-withdrawing single bond CN substituent was selectively incorporated at the 6-position of a DPQ moiety to enhance the photovoltaic properties of the polymers. For example, the coupling of IDT and pristine DPQ produced the reference polymer IDT-Qx, while the combination of IDT and IDTT with CN-substituted DPQ yielded IDT-QxCN and IDTT-QxCN, respectively (Fig. 1). The photovoltaic performances of the three polymers were investigated using an inverted-type PSC with the configuration of ITO/ZnO/polymer:PC₇₁BM/MoO₃/Ag. Owing to the contributions of the electron-withdrawing CN substituents, the PSC based on IDT-QxCN (4.52%) exhibited higher PCE than did the device with the reference IDT-Qx (4.18%). Moreover, with further enhancement, a PCE of 5.47% was obtained with a device based on IDTT-QxCN. This enhancement was attributable to the presence of more planar two thieno[3,2-b] thiophenes in IDTT than the two thiophenes in IDT.

Section snippets

Materials and instruments

4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole (1), 5,8-di(5-bromothiophen-2-yl)-2,3-diphenylquinoxaline (3), 1,1′-[4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl]bis[1,1,1-trimethylstannane (IDT, 5), and 1,1′-[661,212-tetrakis(4-hexylphenyl)-612-dihydrodithieno[2,3-d:2′,3′-d']-s-indaceno[1,2-b:5,6-b']dithiophene-2,8-diyl]bis[1,1,1-trimethylstannane] (IDTT, 6) were synthesized according to previous reports [19], [25], [26], [27]. PC₇₁BM

Conclusion

Three quinoxaline-based D-A type conjugated polymers were synthesized by Stille coupling polymerization for photovoltaic applications. The strong electron-withdrawing single bond CN group, in particular, was systematically incorporated into the DPQ unit of the polymer backbone to investigate its influence on the diverse properties of the polymers. With respect to the reference polymer IDT-Qx, two CN-substituted polymers (IDT-QxCN and IDTT-QxCN) showed not only stronger ICT formation but also lower HOMO

Conflict of interest

None.

Acknowledgement

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20153010140030 and 20194010201840).

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¹: S. L. Handoko and H. C. Jin equally contributed to this research

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Effect of cyano substituent on photovoltaic properties of quinoxaline-based polymers

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and instruments

Conclusion

Conflict of interest

Acknowledgement

J. Ind. Eng. Chem.

Dye. Pigment.

Chin. Chem. Lett.

Org. Electron.

Sol. Energy Mater. Sol. Cells

Chem. Rev.

Nat. Photonics

Chem. Rev.

Adv. Mater.

Macromol. Res.

Dye. Pigment.

Nat. Commun.

Macromolecules

Chem. Commun.

Acc. Chem. Res.