Improving the performances of all-small-molecule organic solar cells by fine-tuning halogen substituents of donor molecule
Graphical abstract
Introduction
In the past decades, great progress has been made in organic solar cells (OSCs) with the constant emergence of novel organic photoactive materials and device preparation techniques [[1], [2], [3], [4], [5], [6]]. Recently, high power conversion efficiencies (PCEs) of over 18% have been achieved for both single junction and tandem OSCs [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. Such a remarkable breakthrough was mainly benefited from the rapid development of small molecular nonfullerene acceptors (SMNFAs) with low band gap. Compared with the traditional fullerene-based acceptors (i.e., PC61BM and PC71BM), SMNFAs possess several unique advantages including fine-tuned energy levels, near-infrared light absorption and facile synthesis [[17], [18], [19], [20], [21], [22], [23], [24], [25]]. The SMNFAs with near-infrared absorption could help to increase the short-circuit current density (Jsc) of the OSCs. However, the open-circuit voltage (Voc) could be decreased simultaneously due to the low-lying lowest unoccupied molecular orbital (LUMO) energy levels of the SMNFAs [26,27]. To address this issue, great efforts have been made in the molecular design and structural optimizations of the electron donor counterparts in OSCs, to achieve good matches between the donors and the SMNFAs. Among various strategies, introducing electron-withdrawing groups (e.g. halogen atoms) into the electron-donating units of the acceptor-donor-acceptor (A-D-A) type small molecular donors or D-A type polymer donors has been proved to be effective to downshift their HOMO levels and thus increase the gap between the HOMO level of the donor and the LUMO level of the acceptor, which is favor of improving the Voc of OSC devices [[28], [29], [30], [31], [32], [33], [34]]. Lots of promising donor materials designed on the basis of this principle, such as PM6, PM7, D18, BTR-Cl, show higher Voc and better performances than the non-halogenated counterparts [7,26,[35], [36], [37], [38]].
Fluorination is an effective approach in developing new organic photoactive materials for OSCs with high performances. Numerous studies have showed that introduction of fluorine atoms can effectively tune the molecular energy levels because of the strong electronegativity, and enhance the molecular planarity and crystallinity due to the non-covalent interactions between fluorine and hydrogen, or sulfur atoms [[39], [40], [41], [42]]. In most cases, the fluorinated conjugated molecules showed red-shifted absorption and high charge carrier mobilities. Nevertheless, fluorination does not necessarily lead to improved PCEs in practical terms, owing to the low charge transport and unfavorable morphology caused by the excessive intermolecular aggregation [34,43]. Moreover, fluorination suffers from the tedious synthesis and low overall yield, making it difficult to meet the criteria of industrial applications [44,45]. Therefore, finding an alternative approach has become an urgent and important task in recent years. In comparison to fluorine, it has been demonstrated that chlorine is also capable of lowering the molecular energy levels despite a weaker electronegativity than fluorine [28,46]. Most importantly, chlorinated molecules possess intrinsic merits, such as easy preparation, high overall yield and low cost. More and more chemists and material scientists have made attention in conducting systematic investigations of the chlorination strategy. To date, lots of chlorinated donor and acceptor materials have been reported, which show even higher PCEs than the fluorinated counterparts [31,47,48]. Such a great success of chlorination could be attributed to the appropriate phase separation and molecular packing caused by the steric hindrance of chlorine atom with a large atomic radius, which plays a big role in the morphology manipulation of the active layer [45,49].
The studies of chlorinated small molecular donors were less than the polymeric counterparts. And the corresponding all-small-molecule OSCs (ASM-OSCs) device performances lagged far behind chlorinated polymer based OSCs. Nevertheless, considering the advantages including well-defined chemical structures, easy purification and thus batch-to-batch reproducibility of small molecules, more efforts should be devoted to boosting the photovoltaic performances [50,51]. Although there is only a few chlorinated small molecular donors being reported, nearly all of which could work well with the mostly reported NFAs. BTR-Cl, a well-known small molecular donor originated from BTR by introducing chlorine atoms onto the thiophene side chains, set off a boom in ASM-OSCs due to its outstanding performance with a high efficiency of 13.61% when combined with Y6 [35]. Subsequently, a surprising PCE of 14.7% was obtained through further device optimizations concerning the solution concentration of active materials [52]. Therefore, it is no doubt that the development of chlorinated small molecular donors is of great significance to promote the industrialization of ASM-OSCs.
In our previous work, a fluorinated small molecular donor, namely DRTB-FT (as shown in Fig. 1a), was reported, which achieved a PCE of 7.66% with a high Voc of 1.07V when blended with F–2Cl acceptor (Fig. 1a) [34]. Obviously, the high Voc is originated from its low HOMO energy level (−5.64 eV) by introducing fluorine atoms on the thienyl substituent of central benzodithiophene (BDT) unit. However, both the fill factor (FF, 0.53) and the Jsc (13.46 mA cm−2) were low, which is mainly attributed to the poor morphology of active layer caused by fluorine-induced excessive aggregation and crystallization. Based on the above considerations, herein, we designed and synthesized a new small molecular donor named DRTB-CT (Fig. 1a), by replacing the fluorine atoms of DRTB-FT with chlorine atoms, in order to optimized the blend film morphology and maintain a low-lying energy level simultaneously. The systematic investigations reveal that such a small change plays a vital role in preventing the strong π-π stacking and intermolecular aggregation in donor phases, and thus achieving favorable morphology which is beneficial to enhancing the FF and the Jsc of the corresponding photovoltaic devices. In addition, DRTB-CT shows a slightly lower HOMO energy level of −5.65 eV in comparison to DRTB-FT (−5.64 eV), contributing to obtain a high Voc. The optimal ASM-OSC based on DRTB-CT:F–2Cl gave a PCE of 9.05% with a higher FF (0.568) and Jsc (15.07 mA cm−2) compared with the fluorinated counterpart.
Section snippets
Materials and synthesis
The donor DRTB-CT was prepared following the synthetic route in Scheme 1. The acceptor material F–2Cl was synthesized referring to the reported literature [53]. Other materials, including compound 1 and compound 2, were commercially available and used without further purification. The solvents that need to be purified and dried were carried out according to the standard procedures. All manipulations and reactions were carried under argon protection through the standard Schlenk technique.
Synthesis and characterization
The detailed synthetic route of DRTB-CT is presented in Scheme 1. Palladium-catalyzed Stille cross-coupling reaction between the two BDT derivatives gave dialdehyde 3 in 79% isolated yield. Subsequently, the target molecule DRTB-CT was prepared by the Knoevenagel condensation of the dialdehyde with 3-ethylrhodanine in high yield of 75%. The molecular structures of the intermediate and the final product were confirmed by 1H and 13C NMR spectroscopy and MALDI-TOF MS (Figs. S1–S3 in Supplementary
Conclusion
A new A-D-A type small molecular donor, named DRTB-CT, was designed and synthesized by replacing the fluorine atoms with chlorine atoms on the basis of DRTB-FT. Such a simple change makes an important impact on preventing the strong π-π stacking and excessive aggregation in donor phases owing to the steric effect of large radius, which contributes to forming favorable interpenetrating network and suitable domain size in the active layer. When blended with the same NFA, the optimal ASM-OSC
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.
Acknowledgments
The authors acknowledge the Research Fund of Jining University (2019BSZX01) and the Young Innovative Talents Introduction & Cultivation Program for Colleges and Universities of Shandong Province: Innovative Research Team on Optoelectronic Functional Materials.
References (53)
- et al.
18% Efficiency organic solar cells
Sci. Bull.
(2020) - et al.
Single-junction organic solar cell with over 15% efficiency using fused-ring acceptor with electron-deficient core
Joule
(2019) - et al.
All-small-molecule organic solar cells with an ordered liquid crystalline donor
Joule
(2019) - et al.
Synergistic effect of halogenation on molecular energy level and photovoltaic performance modulations of highly efficient small molecular materials
Nanomater. Energy
(2017) Semiconducting polymers: the third generation
Chem. Soc. Rev.
(2010)Organic solar cells: recent progress and challenges
ACS Energy Letters
(2019)- et al.
A history and perspective of non‐fullerene electron acceptors for organic solar cells
Adv Energy Mater
(2021) - et al.
Next-generation organic photovoltaics based on non-fullerene acceptors
Nat. Photonics
(2018) - et al.
Acceptor–donor–acceptor type molecules for high performance organic photovoltaics – chemistry and mechanism
Chem. Soc. Rev.
(2020) - et al.
Organic and solution-processed tandem solar cells with 17.3% efficiency
Science
(2018)