Exploring thieno[3,4-c]pyrrole-4,6-dione combined thiophene as π-bridge to construct non-fullerene acceptors with high VOC beyond 1.0 V
Graphical abstract
Introduction
Polymer solar cells (PSCs) as one of promising alternatives to convert solar energy into electricity have attracted enormous attention because of their unique advantages such as light-weight, low-cost, shape-adapted, flexible fabrication in large area and semitransparent devices [[1], [2], [3], [4]]. The power conversion efficiencies (PCEs) arrived over 18% benefiting from the rapid development of non-fullerene acceptors (NFAs) [[5], [6], [7], [8], [9]]. Compared with fullerene acceptors, the NFAs possess stronger light-harvesting capability, more easily-tuned energy levels and relatively simple purification method [[10], [11], [12]]. The typical chemical structure of NFAs can be simplified to A-π-D-π-A (or A-D-A). Upon modifying the D part, A group or π-bridge motif, the photoelectronic properties can be finely modulated. In particular, the introduction of π-bridge unit can further adjust the intra-molecular charge-transfer properties, energy levels and molecular packing styles [[13], [14], [15], [16]]. So far, the electron-donating units such as thiophene [17,18], thieno[3,4-b]thiophene [19,20], benzene [21] and the electron-accepting units such as benzothiadiazole (BT) [[22], [23], [24], [25]], benzotriazole (BTA) [[26], [27], [28], [29]], quinoxaline (Qx) [29] and thienopyrazine (TP) [30] and their derivatives have been used as the π-bridge units.
Compared with benzene-containing electron-deficient building blocks such as BT, BTA and Qx, thieno[3,4-c]pyrrole-4,6-dione (TPD) motif shows more efficient quinoidal character [31], and the intramolecular noncovalent S⋯O or H⋯O interactions between oxygen atoms on TPD and sulfur or hydrogen atoms of adjacent thiophene rings will improve the molecular planarity and promote the intra-molecular charge transfer (ICT) effect [31,32]. TPD has been chiefly developed in the p-type copolymers by copolymerizing with various electron-donating segments and high PCEs of 8–9% [33,34] with fullerene acceptors and that of 10–13% [35,36] with NFAs have been reported. In the n-type polymers or small molecules, TPD unit is rarely utilized. The representative materials previously reported are summarized in Scheme 1 [31,[37], [38], [39], [40], [41], [42]]. Taking n-type polymers for example, Michinobu group designed a TPD-based polymer acceptor by copolymerizing with 3,4-difluorothiophene and a PCE of 4.4% was achieved with polymer PCE10 as the donor [38]. Tsuyoshi et al. reported another TPD-based n-type polymer by combining naphthalenediimide (NDI) with TPD motif, which finally exhibited a high PCE of 6.62% with PBDT-TPD as the donor polymer [41]. Very recently, Guo group report a new n-type terpolymer acceptor, a remarkable PCE of 8.28% [42] could be achieved with PTB7-Th as the polymer donor. For the TPD-based SMA, a PCE of 3% was offered by a SMA comprising TPD core and N-annulated perylene diimide (PDI) side unit [40]. During the manuscript preparation, Chen et al. designed an A-D-A-D-A type SMA comprising of TPD as the central A unit, cyclopentadithiophene (CPDT) as the bridge D unit and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (IC) as the end A unit [31]. The number of n-type SMAs of TPD heavily dragged behind the p-type counterparts. In addition, TPD has never been used as π-bridge in A-π-D-π-A type SMAs. We believe TPD would play a significant role in fine-tuning the energy level, absorption spectrum, molecular crystallinity and electron transport properties for A-π-D-π-A type SMAs.
Based on the above considerations, we firstly designed two molecules with indacenodithiophene (IDT) as the central core, TPD as the bridge unit and rhodanine (R) or 2-(1,1-dicyanomethylene)rhodanine (RCN) as the terminal unit. However, the Vilsmeier–Haack and bromine-lithium exchange reactions cannot occur (summarized in Scheme S1). And then, two new A-π-D-π-A SMAs named as TPD-Th1 and TPD-Th3 were successfully obtained with TPD combing thiophene (Th) as the π-bridge unit (Scheme 2). The intra-molecular non-covalent H⋯OC interactions, between hydrogen atom in thiophene and the oxygen atom of carbonyl groups on TPD, improve the molecular planarity and promote the ICT effect. The preliminary photovoltaic results indicate TPD-Th3 is a promising electron acceptor and a PCE of 5.57% with a high VOC of 1.02 V and a relatively low Eloss of 0.58 eV can be achieved. This work not only firstly designs two TPD-based SMA with TPD as the π-bridge segment, but also demonstrates that TPD is an effective building block to construct A-π-D-π-A skeleton NFAs.
Section snippets
Results and discussion
The synthetic route of TPD-Th1 and TPD-Th3 are outlined in Scheme 2. Both TPD-Th1 and TPD-Th3 were synthesized through three-steps. The 1-bromo-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione was prepared as previously reported [43]. The final compound of TPD-Th1 and TPD-Th3 were obtained from 1-bromo-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione by successively using Still coupling, direct arylation [44] and Knoevenagel condensation reactions. TPD-Th1 and TPD-Th3 are fully characterized by 1H
Conclusion
Two new TPD-based symmetric A-π-D-π-A typed SMAs TPD-Th1 and TPD-Th3 were designed and obtained, where TPD was firstly incorporated as the π-bridge unit in NFAs. In the consideration of reaction, TPD combining Th was inserted between the IDT core and the end-capped group R or RCN. Although TPD-Th1 and TPD-Th3 have similar light absorption and energy level, they exhibit totally different photovoltaic performance with PTB7-Th as the polymer donor. PTB7-Th:TPD-Th1 device shows a much lower PCE of
CRediT authorship contribution statement
Jianfeng Li: Writing - original draft. Feng Li: Data curation. Yanfang Geng: Data curation, Writing - review & editing. Xiaochen Wang: Writing - review & editing. Xiaoyang Zhu: Data curation. Qingdao Zeng: Data curation. Xing Feng: Writing - review & editing, Data curation. Qiang Guo: Writing - review & editing. Erjun Zhou: Writing - review & editing, 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
The authors thank the support from the National Key Research and Development Program of China (2017YFA0206600), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH033), the National Natural Science Foundation of China(NSFC, Nos. 51673048, 51773046, 21602040, 51873044).
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