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Investigation of fluorination on donor moiety of donor-acceptor DTBTbased conjugated polymers towards enhanced photovoltaic efficiency Yonghai Li, Junyi Wang, Yan Liu, Meng Qiu, Shuguang Wen, Xichang Bao, Ning Wang, Mingliang Sun, and Renqiang Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08233 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016
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Investigation of fluorination on donor moiety of donor-acceptor DTBT-based conjugated polymers towards enhanced photovoltaic efficiency Yonghai Li,a Junyi Wang,a Yan Liu,b Meng Qiu,a Shuguang Wen,a Xichang Bao,a* Ning Wang,a Mingliang Sunb and Renqiang Yanga* a
CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess
Technology, Chinese Academy of Sciences, Qingdao 266101, China. E-mail:
[email protected];
[email protected] b
Institute of Material Science and Engineering, Ocean University of China, Qingdao 266100,
China KEYWORDS: fluorination, donor moiety, DTBT, conjugated polymers, photovoltaic
ABSTRACT: It’s known that fluorination on π-conjugated donor-acceptor (D-A) polymers can significantly affect the optoelectronic properties and fluorination on A moiety has been well established for design of efficient photovoltaic materials. For example, polymers based on 4,7dithienyl-5,6-difluorobenzothiadiazole (DTffBT) have been intensively investigated and exhibited excellent performance, but the corresponding DTBT-based polymers without fluorine often display an unfavourable efficiency. With the purpose of improving photovoltaic efficiency
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of DTBT-based D-A polymers, we design three polymers PDTBT-TxfBT (x=0, 1, 2) with fluorination on D moiety (TxfBT) and systematically investigate fluorination on the photophysical/electrochemical and photovoltaic properties. The results show that polymer solar cells (PSCs) based on PDTBT-TBT exhibit moderate power conversion efficiency (PCE) of 5.84%. But the bis-fluorination on TffBT moiety (PDTBT-TffBT) can greatly enhance the molecular planarity and intermolecular interaction, improve the charge transport and heterojunction morphology, and further suppress the charge recombination losses. PSCs based on PDTBT-TffBT demonstrate obviously improved photovoltaic efficiency with the best PCE up to 7.53% without any processing additives, which ranks among the top DTBT-based PSCs. However, it should be noted that unsymmetrical fluorination on TfBT moiety (PDTBT-TfBT) impairs the regularity of polymer backbone and intermolecular interaction, increases the recombination losses and seriously reduces the short-circuit current density and efficiency (5.44%). The results exhibit that fluorination on D moiety is a helpful strategy for design highperformance photovoltaic materials and the regularity of fluorination is crucial to improving efficiencies.
1. INTRODUCTION Polymer solar cells (PSCs) have attracted increasing attention in academy and industry due to their advantages of light weight, low-cost, flexibility and easy fabrication through large-scale roll-to-roll solution processing.1-3 Great progress has been achieved for PSCs driven by design and synthesis high-quality active materials, implementation of interfacial engineering, adoption of revolutionary device structures and optimizing processing conditions. Among them, research about PSCs based on donor-acceptor (D-A) conjugated polymers with bulk heterojunction (BHJ) devices has been proved efficient for both fullerene and non-fullerene systems. For example,
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power conversion efficiencies (PCEs) exceeding 10% have been reported for both single and tandem junction BHJ PSCs.4-11 While these efficiencies are impressive, there is still a significant gap between PSCs and state of the art inorganic technologies which needs to be close before a large scale commercialization of PSCs is feasible. The key factors that determine the PCEs of solar cells are open-circuit voltage (VOC), shortcircuit current density (JSC), and fill factor (FF). It’s reported that VOC is proportional to the offset between highest occupied molecular orbital (HOMO) of donor and lowest unoccupied molecular orbital (LUMO) of acceptor.12 So a lower HOMO level of donor material will be benefit for a high VOC. Furthermore, a broad absorption band, good hole mobility and desirable film-forming property would be crucial for the design of efficient PSCs donor materials. Fluorination on π-conjugated frameworks can significantly affect the optoelectronic properties of materials in multiple perspectives: (i) electron-drawing effect of F atom will reduce HOMO levels which can afford an improved VOC;13 (ii) strongly induced dipole in C-F bond will result in stronger intra- and intermolecular interactions and thus a higher hole mobility for an improved JSC and FF;14-16 (iii) however, the solubility and film-forming ability may be decreased which will negatively influence PCEs of solar cells. As for D-A conjugated polymers, fluorination on A moiety has been well established for design of efficient optoelectronic materials. For example, difluorobenzothiadiazole (ffBT) is reported as one of the most promising acceptor building blocks and polymer PffBT4T based on ffBT and quarterthiophene has been intensively studied for high efficient polymers solar cells. 5-6, 17-19
Theoretical study demonstrates that, compared to 4,7-dithienylbenzothiadiazole (DTBT),
4,7-dithienyl-5,6-difluorobenzothiadiazole (DTffBT) can significantly diminish structure torsion and increase the planarity of molecular backbone.20-21 Decreased bimolecular recombinations
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and enhanced crystallinity could be found for DTffBT based D-A polymers in blending films of PSCs.22 However, polymers based on DTBT (for example PBT4T) usually exhibit a relatively low efficiency and frequently used as comparison to illustrate fluorination on DTffBT-based polymers.21-27 Considering that fluorination on D moiety would produce a deeper HOMO levels and a better polymer backbone planarity than fluorination on A moiety, we will systematically investigate the fluorination on D moiety with the purpose of improving PCEs of DTBT-based DA polymers. In this work, we design and synthesize a series of DTBT based D-A polymers PDTBT-TxfBT (x=0, 1, 2) to investigate the fluorination on D moiety and meanwhile the effect of irregularity (for PDTBT-TfBT, induced by unsymmetrical fluorination of TxfBT moiety) on optoelectronic properties towards enhanced power conversion efficiencies. The results reveal that the number of F atoms in TxfBT unit has an obviously different impact on basic properties of materials and efficiencies of PSCs. It’s found that well-developed continuous network formed in blending film of PDTBT-TffBT/PC71BM which leads to the highest hole mobility and JSC. The PSCs based on PDTBT-TffBT show the best PCE up to 7.53% without any processing additives, much higher than those of PDTBT-TBT (5.84%) and PDTBT-TfBT (5.44%). It’s noted that the devices based on PDTBT-TfBT exhibit the lowest JSC and PCE, which should be attributed to the irregularity of polymer backbone and thus reduced optoelectronic properties. Molecular conformations, photophysical/electrochemical properties, molecular packing and crystallinity, film morphology, charge separation and recombination were systematically investigated to reveal the structureproperty relationships and provide valuable experiences for the design of new high-performance photovoltaic materials.
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Scheme 1. Chemical structures and synthetic routes to polymers. 2. RESULTS AND DISCUSSIONS 2.1 Synthesis and Characterization As depicted in Scheme 1, D-A conjugated polymers PDTBT-TxfBT (x = 0, 1 and 2) were prepared via Stille polymerization between 2BrTxfBT monomers and DTBT monomer in the presence of Pd2(dba)3 and P(o-tolyl)3 as catalyst and ligand, respectively. The synthetic pathways of 2BrTxfBT monomers are displayed in ESI (Scheme S1). PDTBT-TBT and PDTBT-TfBT with 2-octyldodecyl as side chains are highly soluble (>30 mg/mL) in common organic solvents including chloroform, tetrahydrofuran, toluene and 1,2-dichlorobenzene (o-DCB) at room temperature. It’s worth noting that polymer PDTBT-TffBT with 2-octyldodecyl as side chains is originally designed and synthesized, but it’s difficult to be dissolved even in hot o-DCB (
