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In this work, we attempt to figure out alkoxy-substitution effects on two kinds of well-known non-fullerene acceptors (NFAs), IDT-series and Y-series acceptors, by placing alkoxy side chains on the β-positions of the outer thiophene units. The resulting molecules, named IDTN-O and Y6-O, exhibit different properties compared to the original acceptors named IDTN and Y6. The HOMO and LUMO levels of IDTN-O are slightly upshifted at the same time, which causes a slightly higher open-circuit voltage (Voc) without sacrificing the short-circuit current density (Jsc) of the devices. J71:IDTN-O blend films can also achieve a better blend morphology due to the conformational locking effect and higher dipole moment induced by the alkoxy groups, which helps achieve a higher fill factor (FF). In addition, Y6-O exhibits significantly upshifted LUMO levels compared to Y6, leading to a high Voc of 0.95 V. PM6:Y6-O blend films can also maintain an optimal blend morphology through side-chain engineering, which leads to an excellent FF of 78.0%. As a result of alkoxy substitution, both IDTN-O and Y6-O-based devices can achieve better performances of 12.1% and 16.6% than IDTN and Y6-based devices (10.9% and 15.7%), which indicates that this is an effective method to optimize these two types of NFAs.

Polymer acceptors with acceptor–donor–acceptor (A–D–A) building blocks have demonstrated great potential in achieving excellent power conversion efficiency (PCE) and stability in the field of organic solar cells (OSCs). Currently, most of the A–D–A building blocks are composed of benzene-fused end groups, while other aromatic-fused terminal groups are rarely reported, which makes material diversity fall behind. Herein, a newly designed polymer acceptor with thiophene-fused end groups, namely PIDTC-T, is synthesized, which works well with PM6 in all-polymer OSCs based on their combination (PCE = 8.35%), indicative of its good compatibility with PM6. Critically, efficient liquid additive-free OSCs have been realized by doping PIDTC-T as involatile solid additive into the PM6:Y6 blend, achieving a remarkable PCE of 16.76% compared to the original 15.87%, along with better shelf life, photostability and thermostability. A similar achievement is obtained in as-cast PM6:Y6 devices. The mechanistic study reveals tuned morphology, complementary absorption spectra and adjusted composition distribution in devices, leading to higher mobility and suppressed recombination, which concurrently contribute to the improved photovoltaic performance of PM6:Y6 system. Our work can be considered meaningful as it helps in enriching material for polymer acceptors and fabricating stable and efficient OSCs without liquid additives.

Over the past decade, fluorinated materials have become the dominant donors for achieving high power conversion efficiencies in organic solar cells (OSCs). Research on F substituents on N-heterocycle-based polymer solar cells was firstly carried out by introduction of the F atom into the pyrazine-based acceptor unit, which was successfully applied in constructing a polymer (BDT-FPY) for PSCs. MeIC-based polymer solar cells (PSCs) with BDT-FPY as a donor achieve a high power conversion efficiency (PCE) of up to 12.30%, significantly higher than that of a device based on BDT-PY:MeIC (the PCE of unfluorinated polymer donor BDT-PY is 10.2%). The higher PCE of BDT-FPY:MeIC is attributed to more balanced μh/μe, better molecular packing, and more appropriate phase separation features. This result shows great potential for creating high-performance N-heterocycle-based PSCs.

The inevitable energy loss (Eloss) in organic solar cells (OSCs) makes it a challenge to simultaneously enhance the open-circuit voltage (VOC) and short-circuit current (JSC). Herein, we designed and synthesized an asymmetrical non-fullerene small molecule acceptor (SMA), namely a-BTTIC, with a novel eight-fused ladder-type donor unit. Compared to its symmetrical analogue (BTTIC), a-BTTIC shows a red-shifted absorption (by over 20 nm) and an elevated LUMO level (by 0.07 eV), which was beneficial for boosting VOC and JSC concurrently. As a result, OSCs based on a-BTTIC achieved higher power conversion efficiencies (PCEs) of up to 13.60% with simultaneously increased VOC and JSC and a significantly lower Eloss of 0.526 eV than the BTTIC-based OSCs. We noted that the 13.60% PCE and the 0.526 eV Eloss were the best values among OSCs based on commercialized PBDB-T. Through various electrical and morphological characterizations, we observed that the simultaneous enhancement of VOC and JSC in a-BTTIC-based OSCs was attributed to the different molecular conformations and the small change in LUMO level from neutral to ion state of asymmetrical SMAs. Overall, our design route for asymmetrical SMAs serves the dual roles of minimizing the Eloss and promoting the PCE. These results shed light on how to further reduce Eloss for high-performance OSCs with Eloss below 0.6 eV, which thus provides a promising molecular design strategy for further PCE breakthroughs.