Non-fullerene acceptors (NFAs) have revolutionized organic photovoltaics (OPVs) by offering tunable electronic properties, broad absorption spectra, and enhanced stability compared to traditional fullerene-based acceptors. Among these, ITIC, Y6, and their derivatives represent key milestones in molecular engineering, enabling significant improvements in power conversion efficiencies (PCEs). This article examines the structural design, optoelectronic properties, and performance metrics of these NFAs, focusing on their role in advancing OPV technology.

ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene) is a pioneering NFA that set the stage for high-performance OPVs. Its molecular structure consists of a fused-ring electron-deficient core flanked by electron-donating side chains, creating a push-pull configuration that enhances intramolecular charge transfer. The indacenodithieno[3,2-b]thiophene (IDTT) core provides rigidity and planarity, facilitating strong π-π stacking and efficient charge transport. ITIC exhibits a narrow bandgap (~1.6 eV) and broad absorption in the 600–800 nm range, complementary to many donor polymers. Early OPV devices incorporating ITIC achieved PCEs exceeding 11%, a marked improvement over fullerene-based systems. Key performance metrics include high fill factors (FF > 70%) and open-circuit voltages (Voc ~ 0.9 V), attributed to reduced energy losses and favorable morphology.

The success of ITIC spurred the development of derivatives such as ITIC-Th and ITIC-4F, where structural modifications further optimized performance. ITIC-Th introduces thienyl side chains, extending conjugation and red-shifting absorption. ITIC-4F incorporates fluorine atoms on the end groups, lowering the lowest unoccupied molecular orbital (LUMO) energy level and enhancing Voc. These modifications demonstrate the critical role of end-group engineering in tuning electronic properties and interfacial energetics.

Y6 (Y6: 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2,''3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) represents a leap forward in NFA design. Its fused-ring electron-accepting core paired with electron-deficient end groups enables exceptional light-harvesting and charge transport. Y6’s unique "A-DA'D-A" structure (A = acceptor, D = donor) narrows the bandgap to ~1.3 eV, extending absorption into the near-infrared (NIR) region. This design minimizes energy losses, with reported Voc values exceeding 0.8 V despite the low bandgap. Y6-based OPVs have achieved record PCEs over 18%, driven by high short-circuit current densities (Jsc > 25 mA/cm²) and FFs approaching 80%. The morphology of Y6 blends is notably favorable, forming nanoscale phase separation with donor materials, which enhances charge generation and reduces recombination.

Molecular engineering of Y6 derivatives has further pushed performance boundaries. Y6-BO, for example, employs bulky alkyl side chains to improve solubility and film morphology, while maintaining strong intermolecular interactions. Fluorination and chlorination of end groups, as seen in Y6-1O and Y6-Cl, fine-tune energy levels and crystallinity, enabling better compatibility with wide-bandgap donors. These modifications highlight the importance of balancing solubility, packing, and electronic structure in NFA design.

Beyond ITIC and Y6, other NFAs have emerged with distinct structural motifs. COi8DFIC features a ladder-type core with fluorine-substituted end groups, achieving PCEs over 14% via enhanced charge mobility and reduced voltage losses. IEICO-4F, with its ultra-narrow bandgap (~1.2 eV), excels in tandem OPVs by harvesting NIR light. The A-D-A framework common to these NFAs allows systematic optimization of absorption, energy levels, and blend morphology.

Performance metrics for NFAs are evaluated through several key parameters. The external quantum efficiency (EQE) spectrum reveals the wavelength-dependent photocurrent generation, with high-performing NFAs exhibiting EQE > 80% across broad ranges. Energy loss analysis, quantified as Eloss = Eg/q - Voc (where Eg is the bandgap and q is elementary charge), distinguishes NFAs with minimal non-radiative recombination (Eloss < 0.5 eV). Charge mobility, measured via space-charge-limited current (SCLC) or field-effect transistor (FET) techniques, indicates efficient transport (μe > 10⁻³ cm²/Vs for electrons). Morphological stability under thermal and illumination stress is another critical metric, with many NFAs demonstrating >80% PCE retention after 1000 hours of aging.

The table below summarizes key properties of select NFAs:

NFA Bandgap (eV) Voc (V) Jsc (mA/cm²) FF (%) PCE (%)
ITIC ~1.6 ~0.9 ~16 ~70 ~11
Y6 ~1.3 ~0.85 ~25 ~78 ~18
IEICO-4F ~1.2 ~0.7 ~20 ~65 ~12
COi8DFIC ~1.5 ~0.9 ~18 ~75 ~14

Molecular engineering strategies for NFAs focus on several levers. Core modification alters the central conjugated unit to adjust planarity and electron affinity. End-group engineering, particularly with electron-withdrawing units like dicyanoindanone or halogenated indanone, deepens LUMO levels for higher Voc. Side-chain optimization balances solubility and packing, with branched alkyl chains reducing aggregation while linear chains enhance crystallinity. Heteroatom incorporation (e.g., sulfur, nitrogen) can further tune optical and electronic properties.

The future of NFAs lies in continued refinement of these design principles. Multi-fused-ring cores, asymmetric structures, and 3D conformational control are emerging as promising directions. By systematically addressing energy loss, charge transport, and stability, NFAs like ITIC and Y6 have laid the foundation for OPVs to compete with inorganic photovoltaics in efficiency and application diversity.