LAUSR

Organic solar cells (OSCs) are promising to access flexible, light weight and semi-transparent photovoltaic devices by low-cost solution fabrication. Recently, the fused-ring nonfullerene acceptors play an important role in promoting the research progress of the OSCs. The power conversion efficiencies (PCEs) have been rapidly boosted to over 14% in single junction OSCs with the development of new nonfullerene acceptors and the related devices [1–3]. Although the PCEs of OSCs are still inferior to their inorganic counterparts, further improvement of the PCEs could be expected by the development of new photovoltaic materials. Very recently, Prof. Yingping Zou’s group [4] at Central South University reported a new nonfullerene acceptor Y6 on the basis of a fused-ring structure containing electrondeficient benzothiadiazole core, which enabled a breakthrough of a single junction OSC performance with PCE of 15.7% (Figure 1). In early 2017, Zou et al. [5] have firstly introduced a thieno[3,2-b]pyrrolo-fused pentacyclic benzotriazole central core into the non-fullerene acceptor BZIC. The OSC based on BZIC as acceptor and their own-made hexafluoroquinoxaline-based polymer (HFQx-T) as donor showed a PCE of 6.30%. In order to improve its photovoltaic performance, they further introduced stronger electronwithdrawing benzothiodiazole (BT) instead of benzotrizole as central core and used thienothiophene (TT) instead of thiophene as the end group of the central fused ring, to accomplish the synthesis of new acceptor Y6. Such design is beneficial for increasing electron mobility and broadening absorption of the new acceptor. Besides, the central core of Y6 is attached with alkyl side chains on the nitrogen atoms at the same side to prevent over aggregation of molecules, while maintaining an effective intramolecular contact for charge transport. In contrast to previous fused-ring ladder type structure with sterically-hindered side chains at sp carbon, the side chains on the central core of Y6 adopts an orthogonal conformation to the main plane. The grazing incidence wide/small angle X-ray analysis showed that there is a strong diffraction peak in the out-of-plane direction in the blend film of PM6:Y6. Therefore, significantly increased electron mobility was achieved with thicker films (electron mobility reached 2.73×10 cm V s for the blend film with thickness of 300 nm). As a result, the PM6:Y6-based OSC without extra treatments showed a high PCE of 15.3%. After optimizing the morphology, a record efficiency of 15.7% was achieved for both conventional and inverted devices. Inverted devices certified an efficiency of 14.9% at Enli Tech. Laboratory. Moreover, the Y6-based devices maintained a high efficiency of 13.6% with an active layer thickness of 300 nm. The results demonstrate a highly encouraging new nonfullerene acceptor for enabling high performance thick-film OSCs. The electron-deficient-core-based fused ring acceptor reported in this article opens a new access in the molecular design of high-performance acceptors for the future application of OSCs.