Improved photovoltaic performance of a 2D-conjugated benzodithiophene-based polymer by the side chain engineering of quinoxaline

Author(s): Qunping Fan ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Manjun Xiao ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Yu Liu ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Wenyan Su ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Huishan Gao ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Hua Tan ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Yafei Wang ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Gangtie Lei ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ , Renqiang Yang ⁶ ^, ⁷ ^, ⁸ ^, ⁵ , Weiguo Zhu ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵

Publication date (Electronic): 2015

Journal: Polymer Chemistry

Publisher: Royal Society of Chemistry (RSC)

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Abstract

To investigate the influence of side chains of quinoxaline on the photovoltaic performances, a novel D–A-type polymer of PBDTDT(Qx-3)-T was synthesized and characterized, in which 5,8-dioctylthienyl substituted benzo[1,2- b:4,5- b′]dithiophene (BDT-T), thiophene (T) and 6,7-dioctyloxy-2,3-diphenylquinoxaline (Qx-3) were used as the donor (D) unit, π-bridge and acceptor (A) unit, respectively. The resulting polymer exhibited good thermal stability with a high decomposition temperature of 357 °C, a low optical bandgap of 1.78 eV with an absorption onset of 696 nm, a low-lying highest occupied molecular orbital (HOMO) energy level of −5.51 eV, and a high carrier mobility of 2.19 × 10 ⁻⁴ cm ² V ⁻¹ s ⁻¹. Compared to the reported analogues, polymer solar cells (PSCs) based on PBDTDT(Qx-3)-T/PC ₇₁BM demonstrated the highest open-circuit voltages ( V _oc) up to 0.96 V. The maximum power conversion efficiency (PCE) of 6.9% with a V _oc of 0.94 V, short-circuit current ( J _sc) of 11.28 mA cm ⁻² and a fill factor (FF) of 64.7% was obtained with a delicate balance among the above factors for the polymer in PSCs. On the basis of these results, it can be concluded that the appending two octyloxy side chains at 6,7-positions of quinoxaline in the BDT-T- alt-DTQx type polymers would be a feasible approach to improve photovoltaic properties.

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Most cited references 39

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Molecular design of photovoltaic materials for polymer solar cells: toward suitable electronic energy levels and broad absorption.

Yongfang Li (2012)

Bulk heterojunction (BHJ) polymer solar cells (PSCs) sandwich a blend layer of conjugated polymer donor and fullerene derivative acceptor between a transparent ITO positive electrode and a low work function metal negative electrode. In comparison with traditional inorganic semiconductor solar cells, PSCs offer a simpler device structure, easier fabrication, lower cost, and lighter weight, and these structures can be fabricated into flexible devices. But currently the power conversion efficiency (PCE) of the PSCs is not sufficient for future commercialization. The polymer donors and fullerene derivative acceptors are the key photovoltaic materials that will need to be optimized for high-performance PSCs. In this Account, I discuss the basic requirements and scientific issues in the molecular design of high efficiency photovoltaic molecules. I also summarize recent progress in electronic energy level engineering and absorption spectral broadening of the donor and acceptor photovoltaic materials by my research group and others. For high-efficiency conjugated polymer donors, key requirements are a narrower energy bandgap (E(g)) and broad absorption, relatively lower-lying HOMO (the highest occupied molecular orbital) level, and higher hole mobility. There are three strategies to meet these requirements: D-A copolymerization for narrower E(g) and lower-lying HOMO, substitution with electron-withdrawing groups for lower-lying HOMO, and two-dimensional conjugation for broad absorption and higher hole mobility. Moreover, better main chain planarity and less side chain steric hindrance could strengthen π-π stacking and increase hole mobility. Furthermore, the molecular weight of the polymers also influences their photovoltaic performance. To produce high efficiency photovoltaic polymers, researchers should attempt to increase molecular weight while maintaining solubility. High-efficiency D-A copolymers have been obtained by using benzodithiophene (BDT), dithienosilole (DTS), or indacenodithiophene (IDT) donor unit and benzothiadiazole (BT), thienopyrrole-dione (TPD), or thiazolothiazole (TTz) acceptor units. The BDT unit with two thienyl conjugated side chains is a highly promising unit in constructing high-efficiency copolymer donor materials. The electron-withdrawing groups of ester, ketone, fluorine, or sulfonyl can effectively tune the HOMO energy levels downward. To improve the performance of fullerene derivative acceptors, researchers will need to strengthen absorption in the visible spectrum, upshift the LUMO (the lowest unoccupied molecular orbital) energy level, and increase the electron mobility. [6,6]-Phenyl-C(71)-butyric acid methyl ester (PC(70)BM) is superior to [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) because C(70) absorbs visible light more efficiently. Indene-C(60) bisadduct (ICBA) and Indene-C(70) bisadduct (IC(70)BA) show 0.17 and 0.19 eV higher LUMO energy levels, respectively, than PCBM, due to the electron-rich character of indene and the effect of bisadduct. ICBA and IC(70)BA are excellent acceptors for the P3HT-based PSCs.

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Modification of a ZnO cathode by doping it with a hydroxyl-containing derivative - giving a ZnO-C60 cathode - provides a fullerene-derivative-rich surface and enhanced electron conduction. Inverted polymer solar cells with the ZnO-C60 cathode display markedly improved power conversion efficiency compared to those with a pristine ZnO cathode, especially when the active layer includes the low-bandgap polymer PTB7-Th. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Author and article information

Journal

Journal ID (publisher-id): PCOHC2

Title: Polymer Chemistry

Abbreviated Title: Polym. Chem.

Publisher: Royal Society of Chemistry (RSC)

ISSN (Print): 1759-9954

ISSN (Electronic): 1759-9962

Publication date Created: 2015

Publication date (Electronic): 2015

Volume: 6

Issue: 23

Pages: 4290-4298

Affiliations

[1 ]College of Chemistry

[2 ]Xiangtan University

[3 ]Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education

[4 ]Xiangtan 411105

[5 ]China

[6 ]Qingdao Institute of Bioenergy and Bioprocess Technology

[7 ]Chinese Academy of Sciences

[8 ]Qingdao 266101

Article

DOI: 10.1039/C5PY00305A

SO-VID: 3538aec4-37b5-4719-abb4-a7f86089b327

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