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Organic photovoltaics (OPVs) are one of the most promising technologies to power indoor electronic devices. However, the limited types and design strategies of medium band-gap acceptor materials hinder the development of indoor OPVs. Currently, the state-of-the-art non-fullerene acceptors (NFAs) are Y6 and its derivatives, which are based on an A–DA′D–A structure. In this paper, we report the modification of the Y6 structure by replacing Y6's D unit (thieno[3,2-b]thiophene) with benzothiophene, and synthesize two novel NFAs (LBT-DF and LBT-SCl) for indoor applications. The use of the benzothiophene unit reduces the intermolecular charge transfer effect, thus blue-shifting the absorption spectra and up-shifting the LUMO energy levels of the NFAs. As a result, LBT-SCl achieves a higher Voc and thus PCE (up to 25.1%) than Y6 (22.2%) under indoor conditions. Interestingly, these two NFAs exhibit a non-planar aromatic structure, which is rarely observed in the published NFAs, because a better coplanar molecular configuration is usually considered as the critical factor in realizing high PCE. Different from the above mentioned molecular design concept, we find that non-planar structure acceptors also have great potential in OPVs. Our work provides an effective OPV material design guideline for developing high-performance indoor OPVs and also opens a new direction for the design of NFAs.

A ternary strategy has been demonstrated as a promising method to further boost the performance of organic solar cells (OSCs). Herein, an efficient polymer donor S3 was synthesized and incorporated into a PM6:Y6 system to fabricate ternary OSCs. S3 possesses complementary absorption spectra and good compatibility with PM6, which is beneficial to fine-tune the photon harvesting and morphology of the ternary blend films, resulting in simultaneous enhancement of the short-circuit current density (JSC) and fill factor (FF). In addition, the highest occupied molecular orbital (HOMO) energy level of S3 is slightly lower than that of PM6, which enables lower nonradiative energy loss in ternary OSCs compared with that of PM6-based binary OSCs, leading to higher open-circuit voltage (VOC). The optimized ternary OSCs with 20 wt% S3 in the donors achieve a PCE of 17.53%, which should be among the highest values of ternary OSCs. This work provides an effective approach to fabricate high-performance ternary OSCs by synergizing two well-matched polymer donors.

Thiophene and its derivatives have been extensively used in organic electronics, particularly in the field of polymer solar cells (PSCs). Significant research efforts have been dedicated to modifying thiophene-based units by attaching electron-donating or withdrawing groups to tune the energy levels of conjugated materials. Herein, we report the design and synthesis of a novel thiophene derivative, FE-T, featuring a monothiophene functionalized with both an electron-withdrawing fluorine atom (F) and an ester group (E). The FE-T unit possesses distinctive advantages of both F and E groups, the synergistic effects of which enable significant downshifting of the energy levels and enhanced aggregation/crystallinity of the resulting organic materials. Shown in this work are a series of polymers obtained by incorporating the FE-T unit into a PM6 polymer to fine-tune the energetics and morphology of this high-performance PSC material. The optimal polymer in the series shows a downshifted HOMO and an improved morphology, leading to a high PCE of 16.4% with a small energy loss (0.53 eV) enabled by the reduced non-radiative energy loss (0.23 eV), which are among the best values reported for non-fullerene PSCs to date. This work shows that the FE-T unit is a promising building block to construct donor polymers for high-performance organic photovoltaic cells.