Background and Originality Content
In recent years, the development of organic solar cells (OSCs) has largely benefited from the revolution of non-fullerene acceptors, especially fused-ring electron acceptors (FREAs) with the acceptor-donor-acceptor (A-D-A) architecture, which were pioneered by Zhan et al. in 2015.[1-3] Later in early 2019, Zou et al. reported a new generation of A-DA′D-A structured FREAs (represented by Y6), containing electron-deficient units in the fused-ring cores.[4, 5] Due to the unique structural feature, FREAs possess superior absorption property and tight intermolecular π–π stacking in common.[6] With the continuous efforts on molecular tailoring and device engineering, the power conversion efficiencies (PCEs) of the state-of-the-art single junction OSCs have exceeded 19%.[7-14] However, due to the remarkable structural feature of highly fused conjugated backbones, FREAs generally require complicated synthetic approaches and suffer from low yields and high product costs, which may severely hinder their large-scale applications.
To address the disadvantage of FREAs, an alternative strategy for low-cost electron acceptors was proposed, namely nonfused-ring electron acceptors (NFREAs).[15] In general, this novel type of acceptors shares an unfused central core (a simply fused or even completely unfused π-conjugated core), thus making the synthesis route more concise.[16-21] More importantly, the noncovalently conformational locks (NoCLs) strategy is generally introduced in the molecular design of NFREAs, which can effectively restrict the rotation of single bonds and reduce conformational isomers raised from the unfused-ring cores, thus improving the molecular planarity and rigidity.[22-26] Therefore, NFREAs possess a simple-structured yet co-planar conjugated backbone, opening the opportunity for constructing cost-effective acceptor materials.[27-29] Regarded as tailoring from A-DA′D-A type FREAs, A-D-A′-D-A structured NFREAs have received much attention recently. Due to the key role of the A′ unit in regulating light-harvest ability, energy levels, intramolecular conformation, and intermolecular packing behaviors, a lot of work has been devoted to the selection of A′ units, including benzo[c ][1,2,5]thiadiazole,[30, 31]2H -benzo[d ][1,2,3]triazole,[32, 33] thieno[3,4-c ]pyrrole-4,6-dione,[34, 35] and benzo-[1,2-c :4,5-c′ ]dithiophene-4,8-dione.[36, 37] However, it is still challenging to rationally choose an electron-withdrawing motif as the A′ unit.
Thiazole is a widely-used electron-deficient heteroaromatic ring due to its electron-withdrawing nitrogen of imine (C=N),[38] which has been employed in constructing organic semiconductors for optoelectronics, such as OSCs,[39] organic field-effect transistors (OFETs),[40] and organic light-emitting diodes (OLEDs).[41] Compared with thiazole, 4-chlorothiazole should be a more electron-deficient unit, which is expected to build organic conjugated molecules with deep-lying highest occupied molecular orbital (HOMO) energy levels. Particularly, the N and Cl atoms in 4-chlorothiazole may be able to form S···N and S···Cl NoCLs with neighboring thiophene units to enhance the molecular planarity. Therefore, 4-chlorothiazole is an excellent candidate as the A′ unit to construct high-planarity and low-cost NFREAs.
Herein, thiazole and 4-chlorothiazole were utilized as the A′ units to construct two A-D-A′-D-A type NFREAs, namely Tz-H and Tz-Cl , respectively (Figure 1a). Compared with Tz-H , Tz-Clpossessed a facile and concise synthesis, due to the unexpectedly C-H direct arylation of 2,5-dibromo-4-chlorothiazole. Both density function theory (DFT) calculations and single-crystal X-ray diffraction (SC-XRD) data revealed that the simultaneous incorporation of S···N and S···Cl NoCLs can provide a more planar conjugated backbones, and thus a more compact and ordered packing than the sole introduction of S···N NoCLs. When blending with the polymer donor PM6, the Tz-Cl -based blend film afforded higher exciton dissociation efficiency, more balanced charge mobility, lower recombination loss, shorter charge extraction time, and more favorable morphology. Therefore, a PCE of 11.10% was achieved for Tz-Cl -based devices, much higher than that of theTz-H counterpart (6.41%). This work demonstrated the potential of NoCLs to construct low-cost NFREAs.
Results and Discussion
Synthesis and characterization
The synthesis routes of the two NFREAs were shown in Scheme S1, and the detailed synthesis procedures,1H-/13C-NMR spectrometry, and mass spectrometry were given in Supporting Information. Initially, we tried to obtain the dialdehyde intermediate 4 from 2,5-dibromothiazole (1 ) and compound 6 by Pd(0)-catalyzed C-H activation, but failed. Thus, we utilized compound1 with 2 equivalents of mono-organotin compound (2 ) to afford a nonfused-type conjugated core (3 ), followed by the Vilsmeier-Haack reaction to convert dialdehyde intermediate 4 . Different from the chloro-free thiazole derivation, 2,5-dibromo-4-chlorothiazole (5 ) can unexpectedly undergo a two-fold C-H activation with compound6 to directly afford dialdehyde intermediate 7 . Finally, Tz-H and Tz-Cl was obtained through Knoevenagel condensation between the dialdehyde intermediate (4or 7 ) and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H -inden-1-ylidene)malononitrile (IC-2Cl). Obviously, the synthetic route for Tz-Cl is more concise, which may be beneficial to reducing the production cost. BothTz-H and Tz-Cl showed good solubility in common organic solvents, such as chloroform, tetrahedron, and chlorobenzene.