Rylene diimides, particularly perylene diimides (PDIs) and naphthalene diimides (NDIs), represent a prominent class of n-type small-molecule semiconductors. Their rigid, planar aromatic cores functionalized with imide groups enable strong electron affinity, high electron mobility, and robust environmental stability. These characteristics make them suitable for applications in organic field-effect transistors (OFETs), all-polymer solar cells, and thermoelectric devices.

The synthesis of PDI and NDI derivatives typically involves the condensation of perylene or naphthalene tetracarboxylic dianhydrides with primary amines. Functionalization at the imide positions or bay regions allows tuning of solubility, packing, and electronic properties. For instance, core-unsubstituted PDIs exhibit strong π-π stacking due to their planar structure, whereas bay-substituted PDIs introduce steric hindrance, disrupting dense packing but improving solubility. NDIs, with a smaller conjugated core, often show higher crystallinity and tighter intermolecular interactions, leading to efficient charge transport.

Molecular packing significantly influences charge transport in rylene diimides. PDI derivatives typically adopt herringbone or slipped-stack arrangements, with π-π stacking distances ranging from 3.3 to 3.6 Å. These tight interactions facilitate electron delocalization and hopping transport. NDI-based materials, due to their smaller core, often exhibit closer π-π distances (3.2–3.4 Å), contributing to higher electron mobilities. Side-chain engineering plays a critical role in modulating packing motifs—bulky substituents can reduce crystallinity but enhance solution processability, while linear alkyl chains promote ordered thin-film morphologies.

Electron transport properties of rylene diimides are characterized by high electron affinities (EA ≈ 3.5–4.5 eV) and low-lying lowest unoccupied molecular orbitals (LUMO ≈ −3.7 to −4.3 eV), ensuring efficient electron injection and air stability. Single-crystal OFETs of unsubstituted PDIs have demonstrated electron mobilities exceeding 1 cm² V⁻¹ s⁻¹, while NDI derivatives achieve mobilities up to 12 cm² V⁻¹ s⁻¹ in optimized devices. Thin-film devices, however, often exhibit lower performance due to grain boundaries and defects, with mobilities typically in the range of 0.1–0.5 cm² V⁻¹ s⁻¹ for solution-processed films.

In OFETs, rylene diimides serve as the active channel material due to their high electron mobility and ambient stability. Device performance is influenced by film morphology, dielectric interface, and contact resistance. Top-gate architectures with polymer dielectrics often yield superior performance by minimizing interfacial traps. Recent advances include the use of blended films with insulating polymers to improve thin-film uniformity, achieving mobilities > 0.3 cm² V⁻¹ s⁻¹ in flexible OFETs.

All-polymer solar cells benefit from rylene diimides as non-fullerene acceptors, where their broad absorption and strong electron-accepting properties enhance photovoltaic efficiency. PDI-based acceptors, when paired with donor polymers like PBDB-T, achieve power conversion efficiencies (PCEs) of 6–8%, while NDI-based polymers reach up to 10%. Their aggregation tendency must be carefully controlled to prevent excessive phase separation, which can hinder charge separation and transport. Ternary blends incorporating PDI/NDI derivatives alongside fullerene acceptors have further improved device performance by optimizing morphology and energy-level alignment.

Thermoelectric applications leverage the high electrical conductivity and low thermal conductivity of rylene diimides. Doped NDI films exhibit Seebeck coefficients of 100–300 μV K⁻¹ and power factors up to 50 μW m⁻¹ K⁻², though their performance remains limited by low carrier concentrations compared to inorganic counterparts. Strategies such as molecular doping with hydrazine or metal complexes have enhanced conductivity without significantly compromising Seebeck coefficients.

Despite their advantages, challenges persist in achieving balanced solubility, crystallinity, and charge transport in rylene diimides. Future research may focus on novel core-expanded derivatives or advanced doping techniques to further improve device performance across electronics, photovoltaics, and energy harvesting applications.