Elsevier

Organic Electronics

Volume 89, February 2021, 106026
Organic Electronics

Construction of simple and low-cost acceptors for efficient non-fullerene organic solar cells

https://doi.org/10.1016/j.orgel.2020.106026Get rights and content

Highlights

  • Four simple, low-cost non-fullerene acceptors were synthesized and systematic comparative investigated.

  • Both electron-donating donors and bridging groups have direct effects on the properties of non-fullerene acceptors.

  • PTB7-Th:DTC-T-F based OSC showed a high PCE of 8.8%, which is the highest value in the fluorene-based acceptors devices.

Abstract

The flexibility in structural design of organic semiconductors endows organic solar cells (OSCs) not only great function-tunabilities, but also high potential toward practical application. In this work, four simple and low-cost non-fullerene acceptors with fluorene or carbazole as central cores, 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c] thiophen-4-ylidene)malononitrile (TC) as terminal groups, and thiophene or furan as linkers, named DTC-T-F, DTC-F-F, DTC-T-C and DTC-F-C, are developed through twostep synthesis, and their photophysical properties, electrochemical behavior and photovoltaic performance are systematically and comparatively studied. The results revealed that fluorene-based acceptors exhibited superior photophysical properties and morphology characteristics than carbazole-based counterparts, and thiophene is more suitable as bridging groups. Combining the advantages of both, the BHJ-OSC based on PTB7-Th:DTC-T-F blend film showed a high PCE of 8.8%, with a Voc of 0.78 V, a Jsc of 17.46 mA cm−2, and an FF of 0.65, which is the highest value in the PTB7-Th and fluorene-based acceptors coupled devices, implying its potential application.

Graphical abstract

Four simple and low-cost non-fullerene acceptors were synthesized and systematic comparative investigated, in which the DTC-T-F based BHJ-OSC showed a high PCE of 8.8%.

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Introduction

Organic polymer solar cells (OSCs) have attracted considerable attentions due to the advantages of material structure diversity, simple manufacturing process, lightweight, flexible, ultra-thin and transparent, convenient transportation and flexible deployment, are expected to be a good candidate for the new generation of solar cells [1,2]. Over the past few decades, materials innovation, associate with device engineering and morphological optimization have led to continued progress in the field of organic solar cells [[3], [4], [5]], in which, the materials innovation is the fundamental and critical factor. The continuous emergence of new materials provides a basis for the improvement of OSCs performances [6]. Recently, the breakthrough of non-fullerene n-type organic semiconductors has brought a new engine to this field, driving OSCs to a new development period and achieving many landmark achievements [1,2]. Based on polymer donor materials/non-fullerene acceptor materials, the power conversion efficiencies (PCEs) of single junction devices are over 14% [7,8], even up to 16% [9], and the PCEs of tandem devices and ternary devices has exceeded 17% [[10], [11], [12], [13]], and successfully pass the threshold of commercial application.

On the eve of OSCs practical application, apart from high PCE, the easy accessibility and low cost of materials is another important issue to be addressed. Up to date, the most successful non-fullerene electron acceptors, such as ITIC [14], IDIC [15], IHIC [16], IEICO [17], COi8DFIC [18], etc., are almost adapted A–D–A molecular architecture with ladder-type fused-ring as central core, which enables bulk heterojunction (BHJ) layers to effectively utilize solar photons from visible to near-infrared range (NIR) and gain high carrier mobility. Nevertheless, the access of multi-ring-fused ladder structure inevitably involves synthetic complexities. The multi-step synthesis and purification consequently increase the material costs. Therefore, despite a large family of electron acceptors developed, there is still a strong requirement to explore the new structural design of simple and effective molecules, for accessing efficient and low-cost OSCs. Recently, some positive works have been done in this issue and encouraging results have been achieved [[19], [20], [21]], However, the samples are still rare, and more extensive research is urgently needed to enrich this field.

As classical raw materials for organic photoelectric materials, fluorene and carbazole are widely used as electron-donating building blocks due to their high hole mobility feature and inexpensive characteristics [22,23]. They were also be introduced to construct non-fullerene electron acceptors with wide bandgaps and deep-lying highest occupied molecular orbitals (HOMO) [[24], [25], [26]]. On the other hand, 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile (TC) reported by Zou and co-workers [27] was a potential excellent end group for acceptor, due to its strong light-harvesting from the electron delocalization and good charge transport originate from the strong intermolecular S⋯S interactions. ITTC utilized TC as end group was demonstrated to be a high-performance non-fullerene acceptor in OSCs.

Bearing these in mind, in this work, we designed and synthesized a serial of simple small molecule acceptors, named DTC-T-F, DTC-F-F, DTC-T-C and DTC-F-C (Fig. 1), comprising 9,9-dioctylfluorene and 9-octylcarbazole as the central cores, thiophene and furan as bridging units, and TC as terminal acceptor groups. The new molecules were synthesized from widely available and inexpensive commercial materials in only two steps with a high overall yield of 80%, indicating great potential for the large-scale production of this new acceptor material. All these acceptors showed relatively narrow absorption in 300–650 region with HOMO/LUMO levels approximately −5.8/-4.0 eV.

The fluorene-based acceptors exhibited superior photophysical properties and morphology characteristics than carbazole-based counterparts, and thiophene is more suitable as bridging groups. Considering the matching of energy levels and the partially complementarity of absorption spectral, a narrow-bandgap polymer poly[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene2,6-diyl)-alt-(2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate-4,6-diyl)] (PTB7-Th) was utilized as the donor material to fabricate inverted bulk-heterojunction solar cells (BHJ-OSCs) with the new synthesized acceptors. Among them, the BHJ-OSC based on PTB7-Th:DTC-T-F blend film showed a high PCE of 8.80%, with a Voc of 0.78 V, a Jsc of 17.46 mA cm−2, and an FF of 0.65, which is the highest value in the PTB7-Th and fluorene-based acceptors coupled devices, suggesting its potential application.

Section snippets

Materials synthesis and general characterization

The synthetic routes of DTC-T-F, DTC-F-F, DTC-T-C and DTC-F-C are shown in Scheme 1. All the raw materials were purchased from commercial suppliers and used without any further purification. All the two synthesis steps are simple and classical reactions of Suzuki coupling-condensation. The starting boric acid esters 1 and 7 were prepared from their corresponding brominates via an efficient Suzuki-Miyaura cross-coupling reaction. The key intermediates 3, 6, 8 and 9 were synthesized in over 90%

Conclusion

In summary, four simple, easy accessibility and low-cost fullerene-free acceptors were designed and synthesized through a twostep route with high yields. Systematical theoretical and experimental results show that these acceptors exhibited strong absorption in the visible light region and possess suitable energy levels. The structure-performance relationship revealed that the fluorene-based acceptors exhibited superior photophysical properties, carrier mobility and morphology characteristics.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgements

This work was financial supported by Natural Science Foundation of Guangdong Province (2018A030313304), Talent Research Start-up Fund of Guangdong University and Technology (GC300501-146), Guangdong Provincial Key Laboratory of Distributed Energy Systems, No. (2020B1212060075), Dongguan Social Science and Technology Development Project (20185071401607) and Guangdong Public Welfare Fund Project (2017A010103047).

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