The evolution of organic photovoltaics (OPVs) has been significantly influenced by the development of non-fullerene acceptors (NFAs), which have emerged as superior alternatives to traditional fullerene derivatives. Unlike fullerene-based acceptors, NFAs offer tunable optoelectronic properties, broader absorption spectra, and enhanced morphological stability, leading to remarkable improvements in power conversion efficiencies (PCEs). This article examines the molecular design principles, optoelectronic characteristics, and performance advantages of NFAs, focusing on key material classes such as ITIC and Y6.

Fullerene derivatives, such as PCBM, were long the dominant acceptor materials in OPVs due to their high electron affinity and isotropic charge transport. However, their limitations—narrow absorption spectra, limited energy-level tunability, and morphological instability—hindered further efficiency gains. NFAs address these shortcomings through tailored molecular structures that enable precise control over optical and electronic properties. The shift toward NFAs has propelled OPV efficiencies from single digits to over 18% in recent years, marking a transformative advancement in the field.

A critical advantage of NFAs lies in their molecular design flexibility. Unlike fullerenes, which have fixed energy levels and absorption profiles, NFAs can be engineered to optimize light harvesting, charge generation, and transport. Key design strategies include the incorporation of electron-deficient cores, extended conjugation, and side-chain engineering. For instance, the ITIC family of NFAs features an indacenodithienothiophene (IDTT) core flanked by electron-withdrawing end groups. This design yields a narrow bandgap, enabling absorption into the near-infrared region, while the planar structure promotes efficient π-π stacking for enhanced charge mobility.

The optoelectronic properties of NFAs are central to their superior performance. Broad and intense absorption spectra are achieved by carefully selecting donor and acceptor units within the molecular framework. For example, the Y6 acceptor, a landmark NFA, combines a fused-ring electron-deficient core with alkyl side chains to balance solubility and crystallinity. Y6 exhibits strong absorption across 600–900 nm, complementing the visible absorption of common donor polymers like PM6. This complementary absorption maximizes photon harvesting, contributing to high short-circuit current densities (Jsc).

Energy-level alignment is another critical factor. NFAs enable fine-tuning of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels to minimize energy losses during charge transfer. The LUMO levels of ITIC and Y6 are typically higher than those of fullerenes, resulting in higher open-circuit voltages (Voc) without sacrificing charge separation efficiency. For instance, ITIC-based devices often achieve Voc values exceeding 0.9 V, compared to 0.6–0.7 V for fullerene-based counterparts.

Morphological stability is a key advantage of NFAs over fullerenes. Fullerene derivatives tend to undergo undesirable phase separation or crystallization over time, degrading device performance. In contrast, NFAs like Y6 form more stable blends with donor polymers due to favorable intermolecular interactions and tailored side-chain chemistry. This stability translates to longer operational lifetimes, a critical requirement for commercial applications.

The impact of NFAs on OPV efficiency is evident in the rapid progress of record PCEs. ITIC, one of the earliest high-performance NFAs, demonstrated PCEs exceeding 11% in 2015 when paired with donor polymers like PBDB-T. Subsequent developments, such as the Y6 acceptor, pushed efficiencies beyond 15% by 2019. The success of Y6 lies in its unique three-dimensional packing motif, which facilitates efficient charge transport while maintaining a favorable blend morphology. Recent iterations, including Y6 derivatives and new NFA families, have further elevated PCEs to 18–19%, approaching the thresholds required for practical deployment.

Material classes such as perylene diimides (PDIs) and fused-ring electron acceptors (FREAs) have also contributed to the NFA landscape. PDIs, with their rigid, planar structures, offer high electron mobility but face challenges in blend morphology. FREAs, including ITIC and Y6, address these challenges by incorporating flexible side chains and asymmetric structures to optimize packing and compatibility with donor polymers.

The rise of NFAs has redefined the prospects of OPVs, offering a pathway to efficiencies competitive with inorganic thin-film technologies. By leveraging molecular design to control optoelectronic properties and morphology, NFAs have overcome the intrinsic limitations of fullerenes. Key materials like ITIC and Y6 exemplify the potential of tailored acceptor systems, enabling unprecedented performance in organic solar cells. As research continues to explore new NFA architectures and combinations, the efficiency and stability of OPVs are expected to reach new heights, solidifying their role in the future of renewable energy.