Enhanced photovoltaic performance of benzodithiophene-alt-bis(thiophen-2-yl)quinoxaline polymers via π–bridge engineering for non-fullerene organic solar cells
Graphical abstract
The photovoltaic performance of quinoxaline-based polymers was greatly increased via the side and main chain engineering of the polymer backbone, and the PCE was maximum enhanced from 0.8% to 6.2%.
Introduction
Organic solar cells (OSCs) have been broadly recognized as efficient energy conversion devices based on their environmentally friendly energy production utilizing renewable energy resources, as well as low-temperature fabrication processes based on solution processability and the ability to fabricate lightweight large-area devices at low cost [[1], [2], [3], [4], [5]]. In OSC devices, a photoactive layer is sandwiched between hole and electron transportation materials, which are connected to an anode and cathode, respectively. Generally, the solar-to-electrical energy conversion efficiency of an OSC device is correlated with the properties of the organic materials utilized in the photoactive layer. This layer typically contains bi-continuous networks of electron donating conjugated polymers and electron acceptor materials [[6], [7], [8]]. In particular, fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]- phenyl-C71-butyric acid methyl ester (PC71BM) have been widely utilized as electron acceptor materials [[9], [10], [11], [12], [13], [14], [15], [16]]. Conventional OSCs with the fullerene derivatives as an acceptor provide maximum power conversion efficiencies (PCEs) of approximately 12% [[9], [10], [11], [12], [13], [14], [15], [16]]. Recently, wide-bandgap fullerene derivatives have been replaced with narrow- or ultra-narrow-bandgap ladder-type organic small molecule acceptors (non-fullerene acceptors, NFAs) in an effort to enhance the performance of OSCs further. The resulting NFA OSCs have exhibited significantly enhanced photovoltaic performance compared to conventional OSCs [[17], [18], [19], [20], [21], [22], [23], [24], [25]]. Additionally, efficient NFAs exhibit strong absorption in the low-energy portion of the solar spectra [[26], [27], [28]], meaning wide bandgaps are a good choice for maximizing PCE based on favorable complementary absorption from polymer:NFA blends. For example, the highest PCEs (14–17%) have been reported for NFA OSCs fabricated from blends containing wide-bandgap polymers and narrow-bandgap NFAs [17,18]. It is noteworthy that the use of wide-bandgap polymers in combination with narrow-bandgap polymers has yielded the highest recorded efficiency of over 17% for tandem-structured NFA OSCs [20]. Therefore, novel π-conjugated wide-bandgap polymeric donors have received significant attention in the area of NFA OSCs.
Heteroaromatic quinoxaline skeletons have been considered as promising building blocks based on their strong electron accepting nature and planar backbone, which are favorable for constructing efficient π-conjugated polymeric donors [[29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]], as well as π-conjugated organic small molecule acceptors [[41], [42], [43], [44]] for optoelectronic applications. Recently, our laboratory reported a series of 2,3-didodecyl-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (TFQ)-based polymers, namely P(BDTO-TFQ), P(BDTT-TFQ), and P(BDTSi-TFQ), containing benzo [1,2-b:4,5-bʹ]dithiophene (BDT) derivatives with three different substituents, namely 2-butyloctyloxy, 2-(2-butyloctyl)thiophene, and triisopropyl (prop-1-ynyl)silane, in their fourth and eighth positions [45]. All three polymeric donors exhibited favorable complementary absorption with electron accepting 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis (4-hexylphenyl)dithieno [2,3-d:2ʹ,3ʹ-dʹ]-s-indaceno [1,2-b:5,6-bʹ]dithiophene (ITIC). However, NFA OSCs fabricated from TFQ-based polymer:ITIC blends exhibited significantly different PCEs of 3.49% for P(BDTO-TFQ), 7.06% for P(BDTT-TFQ), and 0.75% for P(BDTSi-TFQ) [45]. P(BDTSi-TFQ) exhibited much worse photovoltaic performance compared to P(BDTT-TFQ) based on its low crystallinity, carrier mobility, and high bimolecular recombination rate of excitons [45].
It was noted that the steric hindrance generated between the alkyl groups attached to the BDT and TFQ backbone play a significant role in the property modulation of TFQ-based polymers. We expect that the insertion of additional π-spacers between BDT and TFQ units may improve planarity, which is expected to induce red-shifted absorption, higher crystallinity, and higher mobility for the resulting polymers. Therefore, in this study, we prepared and studied the properties of three new polymers, namely P(BDTO-TTFQ), P(BDTT-TTFQ), and P(BDTSi-TTFQ), incorporating BDT and 5,8-bis(5-(4-hexylthiophen-2-yl)thiophen-2-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (TTFQ) derivatives. We also compared the properties of the newly prepared TTFQ-based polymers to those of reported TFQ-based polymers with the goal of understanding the effects of incorporating additional π-spacers on TFQ-based polymer backbones. The chemical structures of TFQ- and TTFQ-based polymers are presented in Fig. 1.
Section snippets
Material synthesis and characterization
Monomer Dibromo TTFQ was prepared using the literature [46]. The Stille coupling reactions between dibromo TFQ and (4-hexylthiophen-2-yl)trimethylstannane yielded TTFQ. The di-bromination of TTFQ utilizing N-bromosuccinimide yielded dibromo TTFQ. Stille polymerization of distannyl BDTO, distannyl BDTT, distannyl BDTSi, and dibromo TTFQ yielded three new polymers: P(BDTO-TTFQ), P(BDTT-TTFQ), and P(BDTSi-TTFQ). The synthesis routes for dibromo TTFQ, P(BDTO-TTFQ), P(BDTT-TTFQ), and P(BDTSi-TTFQ)
Conclusions
We synthesized three new polymers, namely P(BDTO-TTFQ), P(BDTT-TTFQ), and P(BDTSi-TTFQ), containing different functionalized BDT derivatives and TTFQ units with the goal of improving the photovoltaic performances of structurally similar TFQ-based polymers, namely P(BDTO-TFQ), P(BDTT-TFQ), and P(BDTSi-TFQ). We determined that the incorporation of additional π-spacers (4-hexylthiophene) between the BDT and TFQ units of TFQ-based polymers, namely P(BDTO-TFQ), P(BDTT-TFQ), and P(BDTSi-TFQ), yields
CRediT authorship contribution statement
Vellaiappillai Tamilavan: Conceptualization, Validation, Methodology, Writing - original draft, Writing - review & editing. Soyeong Jang: Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Jihoon Lee: Formal analysis. Rajalingam Agneeswari: Data curation. Ji Hyeon Kwon: Visualization. Joo Hyun Kim: Resources. Youngeup Jin: Project administration, Funding acquisition, Supervision. Sung Heum Park: Project administration, Funding acquisition, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements \
This research work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy (MOTIE) of Republic of Korea (20193091010110). This work was also supported by KETEP and MOTIE (No. 20194010201840).
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