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Central π-core engineering of non-fullerene small molecule acceptors (NF-SMAs) is effective in boosting the performance of organic solar cells (OSCs). Especially, selenium (Se) functionalization of NF-SMAs is considered a promising strategy but the structure-performance relationship remains unclear. Here, we synthesize two isomeric alkylphenyl-substituted selenopheno[3,2-b]thiophene-based NF-SMAs named mPh4F-TS and mPh4F-ST with different substitution positions, and contrast them with the thieno[3,2-b]thiophene-based analogue, mPh4F-TT. When placing Se atoms at the outer positions of the π-core, mPh4F-TS shows the most red-shifted absorption and compact molecular stacking. The PM6 : mPh4F-TS devices exhibit excellent absorption, high charge carrier mobility, and reduced energy loss. Consequently, PM6 : mPh4F-TS achieves more balanced photovoltaic parameters and yields an efficiency of 18.05 %, which highlights that precisely manipulating selenium functionalization is a practicable way toward high-efficiency OSCs.

Summary The growth of the internet of things (IoT) is creating a demand for convenient energy sources, like organic photovoltaics, to power various small IoT devices. Here, we report a highly crystalline small molecular acceptor (named FCC-Cl) with an optical band gap of 1.71 eV suitable for indoor applications. The important design rationale of FCC-Cl is the combination of a weak electron-donating core and a moderate electron-withdrawing end group, which leads to needed band gap and high crystallinity. The OPVs based on D18:FCC-Cl achieved a high external quantum efficiency up to 85% and a high fill factor of 80% due to the high absorption coefficient and strong crystallinity of FCC-Cl. Consequently, an impressive power conversion efficiency of 28.8% was achieved under a 2,600 K LED lamp at 500 lux. It was also demonstrated that PM6:FCC-Cl-based devices can achieve high efficiencies over a wide range of active-layer thicknesses, which is a feature necessary for large-scale roll-to-roll printing processes.

Side-chain engineering has been shown to be an important strategy to optimize Y-series nonfullerene acceptors (NFAs). Most previous reports were focusing on changing the branching positions and size of the alkyl side chains on Y6. In this paper, we investigate the influence of the orientation of side chains on the properties of NFAs and the performance of the organic solar cells (OSCs). Three isomeric NFAs named o-BTP-PhC6, m-BTP-PhC6, and p-BTP-PhC6 are designed by changing the substitution positions and thus orientations of the side chains attached to the central core. Our studies show that the optimal side-chain orientation can be achieved by the meta-positioned hexylphenyl group (of the m-BTP-PhC6 molecule), which introduces significant beneficial effects on optical absorption, intermolecular packing and phase separation of the NFAs. By pairing a donor polymer PTQ10 with m-BTP-PhC6, device efficiencies of 17.7% can be achieved, which is among the best values for PTQ10-based nonfullerene OSC devices so far. These results reveal that regulating side-chain orientations of Y-series NFAs is a promising strategy to achieve favorable morphology, and high charge mobility and solar cell performances.