High-mobility organic semiconductors have garnered significant attention due to their potential for flexible electronics, low-cost processing, and tunable electronic properties. The performance of these materials is largely dictated by their molecular structure, which influences charge transport, solubility, and film-forming properties. Molecular engineering strategies, such as fused-ring core design and side-chain optimization, play a pivotal role in enhancing charge carrier mobility while balancing processability requirements.

A critical factor in achieving high mobility is the molecular packing arrangement, which facilitates efficient π-π stacking and minimizes charge trapping. Fused-ring aromatic cores, such as those in rubrene and dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), exhibit extended conjugation, leading to strong intermolecular interactions and reduced reorganization energy. Rubrene, for instance, demonstrates hole mobilities exceeding 40 cm²/V·s in single-crystal form due to its herringbone packing motif, which promotes orbital overlap. DNTT derivatives, on the other hand, achieve mobilities of 10–15 cm²/V·s in thin-film transistors, attributed to their planar backbone and sulfur-rich heterocycles that enhance electronic coupling.

Side-chain engineering is equally crucial in optimizing solubility and film morphology without compromising charge transport. Linear alkyl chains are commonly employed to improve solubility in solution-processable semiconductors, but excessive chain length can disrupt π-π stacking. Branched or bulky side chains may enhance solubility but often reduce crystallinity and mobility. For example, the introduction of triisopropylsilylethynyl (TIPS) groups in pentacene derivatives improves solution processability while maintaining mobilities of 1–3 cm²/V·s. Similarly, alkylated DNTT variants balance solubility and mobility, with C8-DNTT exhibiting thin-film mobilities around 8 cm²/V·s.

The choice of heteroatoms within the molecular framework further modulates electronic properties. Sulfur-containing thienoacenes, such as benzothienobenzothiophene (BTBT), exhibit high mobilities due to strong intermolecular S···S interactions. Oxygen or nitrogen incorporation can alter energy levels and stability but may introduce charge traps if not carefully designed. Diketopyrrolopyrrole (DPP)-based polymers, for instance, achieve balanced ambipolar transport with mobilities of 1–5 cm²/V·s, leveraging intramolecular hydrogen bonding for structural rigidity.

Crystallinity and thin-film morphology are strongly influenced by deposition techniques. Vacuum-deposited small molecules often yield higher mobilities than solution-processed counterparts due to superior molecular alignment. However, advances in solution shearing, blade coating, and inkjet printing have narrowed this gap. For example, solution-processed C10-DNTT films achieve mobilities of 5–7 cm²/V·s, comparable to vacuum-deposited layers. Additives and solvent annealing techniques further enhance grain size and reduce defects, as demonstrated in TIPS-pentacene systems.

Trade-offs between mobility and processability remain a central challenge. High-mobility materials like rubrene suffer from poor ambient stability and limited solubility, necessitating protective encapsulation or hybrid designs. In contrast, polymers like poly(3-hexylthiophene) (P3HT) offer excellent solution processability but exhibit lower mobilities (0.1–0.5 cm²/V·s) due to conformational disorder. Recent efforts focus on asymmetric side-chain designs and conjugated spacers to decouple solubility from backbone rigidity, as seen in indacenodithiophene-co-benzothiadiazole (IDT-BT) copolymers.

The following table summarizes key benchmark materials and their properties:

Material | Mobility (cm²/V·s) | Processing Method | Notes
Rubrene | 40 (single crystal) | Vacuum deposition | High mobility but unstable
DNTT | 10–15 | Vacuum deposition | Sulfur-enhanced coupling
C8-DNTT | ~8 | Solution processing | Alkyl chain optimization
TIPS-pentacene | 1–3 | Solution processing | Bulky side chains
DPP-based polymer| 1–5 | Solution processing | Ambipolar transport

Future advancements hinge on predictive molecular design tools, such as machine learning-assisted screening, to identify optimal core-side chain combinations. Multi-fused heteroacenes and ladder-type structures show promise for further mobility enhancements, while environmentally friendly processing solvents may improve sustainability. The continued exploration of non-covalent interactions, such as halogen bonding or chalcogen bonding, could unlock new pathways for performance optimization without sacrificing manufacturability.

In summary, high-mobility organic semiconductors rely on a delicate interplay of molecular design, processing, and structural control. Fused-ring cores and tailored side chains remain central to achieving high performance, but the field must address inherent trade-offs to enable scalable production. Benchmark materials like rubrene and DNTT provide valuable insights, yet ongoing innovation in molecular engineering will be essential to surpass current limitations.