The central insight of the review is that fluorination acts on organic semiconductors through several distinct mechanisms at once. First, fluorine is highly electronegative, so attaching it directly to a conjugated backbone pulls electron density away from the π-system and lowers the lowest unoccupied molecular orbital, the LUMO. That single change can flip a material’s transport character from p-type, where holes carry the current, to ambipolar or even n-type, where electrons dominate. Because n-type and ambipolar materials are historically harder to make than their p-type counterparts, this energy-level tuning has opened the door to organic complementary circuits that were long impractical.

Second, fluorination dramatically improves ambient stability. Many high-mobility n-type semiconductors degrade in air because oxygen and water attack the electron-transporting state. Lowering the LUMO makes the material less vulnerable to this oxidative damage, so fluorinated derivatives of perylene diimides, naphthalene diimides, and related electron-deficient cores can keep working in unencapsulated devices. The review traces this theme from early air-stable n-channel materials through to modern polymers that maintain electron mobility even at very high relative humidity.

Third, fluorine engages in weak but consequential noncovalent interactions. Fluorine atoms can form halogen-bond-like contacts with sulfur, hydrogen, and other atoms in neighboring molecules, and these interactions act as conformational locks that planarize the backbone and guide how molecules stack in the solid state. Because charge-carrier mobility in organic semiconductors depends directly on how well molecules pack and how easily electrons hop or delocalize between them, these subtle structural effects translate into measurable gains in transistor performance.

The review organizes the field into two broad categories based on where the fluorine ends up. The first category covers fluorine atoms placed directly on the conjugated main chain. This includes fully perfluorinated systems, such as tetradecafluorosexithiophene and perfluoropentacene, which demonstrated that complete fluorination can produce genuine n-type transport and even p–n junctions and complementary circuits when paired with their hydrogenated analogues. It also includes molecules terminated with fluorobenzene groups and systems in which the number and position of fluorine substituents are varied systematically to fine-tune packing and transport.

Position matters as much as quantity. Studies of isoindigo-based polymers, diketopyrrolopyrrole copolymers, and fluorinated benzothiadiazole units show that moving a fluorine atom from one ring position to another can change backbone planarity, side-chain orientation, and thin-film microstructure, sometimes with opposite effects on hole and electron mobility. The review highlights cases where regioselective fluorination enabled ultrahigh intrinsic mobility by tightening crystal packing, alongside cautionary examples where fluorination introduced charge trapping and operational instability, a reminder that the technique must be applied with structural precision.

The second category shifts the fluorine away from the backbone and into the side chains, in the form of trifluoromethyl groups and semifluoroalkyl chains. Here the benefits are different in kind. Fluoroalkyl chains improve solubility and processability, enhance ambient stability by shielding the reactive backbone, and, remarkably, can drive self-organization within thin films. Because fluorinated and non-fluorinated segments tend to segregate, semifluoroalkyl side chains can promote edge-on orientation of polymer backbones relative to the substrate, the geometry that favors in-plane charge transport, and can encourage the growth of large crystalline domains during solution processing.

This side-chain engineering has produced some of the field’s most striking results. Semifluorinated side chains have been used to build polymers whose solubility switches reversibly with temperature, enabling environmentally friendly patterning and self-organized high-mobility films. Fluoroalkylated naphthalene diimide and naphthodithiophene diimide derivatives have delivered air-stable, high-electron-mobility transistors, while trifluoromethyl-substituted oligomers achieved very high electron mobility in early n-channel devices. The review notes that when stability and solution processability are the priority, fluoroalkyl substitution often outperforms direct ring fluorination.

The two strategies are increasingly combined. Recent work pairs backbone fluorination with side-chain fluorination to simultaneously lower energy levels, lock molecular conformation, and control film morphology, and fluorinated side chains terminated with polar groups have been applied to n-type organic thermoelectrics to improve both performance and air stability. The authors argue that this dual-modulation mindset, treating fluorination as a tunable design parameter rather than a binary choice, is what allows chemists to balance the competing demands of mobility, stability, and manufacturability.

For a field racing toward printable, flexible, and wearable electronics, the message of the review is that fluorine offers a rare combination of predictability and versatility. By showing precisely how fluorination position, density, and location, backbone versus side chain, map onto energy levels, packing motifs, crystallinity, and device metrics, the authors provide a practical design guide for the next generation of organic semiconductors. As OFETs move from laboratory curiosities toward commercial sensors and circuits, the smallest substituent in organic chemistry looks set to remain one of its most powerful.

Subject of Research: Fluorine modification of π-conjugated semiconductors for organic field-effect transistors

Article Title: Review: fluorine-modified π-conjugated molecules/polymers for organic field-effect transistors-a boost to devices performance

Article References: Li, Y., Wang, X., Ji, Y., Li, Q., Li, Z., Du, S., Feng, Y., Wu, T., Liu, S., & Yu, Z. (2026). Review: fluorine-modified π-conjugated molecules/polymers for organic field-effect transistors-a boost to devices performance. Journal of Materials Science, 61(42), 32391-32433. https://doi.org/10.1007/s10853-026-13597-9

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13597-9

Keywords: organic field-effect transistors, fluorination, organic semiconductors, π-conjugated polymers, charge transport, LUMO energy levels, ambient stability, fluoroalkyl side chains, molecular packing, n-type semiconductors, thin-film crystallinity, materials science

Rubrene has long been a benchmark material in the organic electronics community. In single-crystal form it can carry charges at mobilities of up to 20 square centimeters per volt per second, rivalling some amorphous inorganic semiconductors. The catch is that those spectacular numbers come from painstakingly grown individual crystals, not from the thin films that real devices need. When rubrene is evaporated directly onto silicon dioxide, the standard insulator of transistor technology, thermal annealing rarely produces uniform, large-area crystalline domains. The result is a patchwork of grains with wildly varying quality, and devices whose performance cannot be trusted from batch to batch. Reproducibility, not raw mobility, has become the bottleneck for commercialization.

The Korean team’s strategy was to insert an ultrathin polystyrene interlayer between the silicon dioxide and the rubrene, and then to engineer that interlayer’s chemistry with remarkable precision. They prepared three versions of the polymer surface. The first was plain, as-cast polystyrene. The second, which they call PS-x, was irradiated for twenty minutes with 365-nanometer ultraviolet light in ambient air. The third, dubbed PS-net, received the same UV dose and was then rinsed briefly in isopropyl alcohol. Twenty nanometers of rubrene were subsequently evaporated onto each surface under high vacuum, and the films were annealed in a nitrogen glovebox.

The ultraviolet treatment sounds crude, but it triggers a rich photochemistry. UV photons cleave carbon-hydrogen bonds in the polystyrene backbone, generating carbon radicals that react with oxygen in the air to form peroxy species. Some of these radicals link neighboring polymer chains together into a crosslinked network, while others drive the opposite process, beta-scission and ring-opening reactions that chop the polymer into small, polar, low-molecular-weight fragments. Fourier-transform infrared spectroscopy confirmed the chemical transformation: the characteristic benzene-ring and carbon-hydrogen absorption bands weakened after irradiation, while new carbonyl signals appeared between 1700 and 1850 per centimeter, marking the increased polarity of the modified surface.

The decisive evidence for crosslinking came from a clever solubility test. The researchers patterned UV light onto selected regions of a polystyrene film and then rinsed the whole sample in isopropyl alcohol, a solvent that normally cannot touch pristine polystyrene. Scanning electron microscopy showed that only the irradiated regions survived the rinse. Crosslinking had rendered the exposed polymer insoluble, while the untouched areas dissolved away. The subsequent alcohol wash on the PS-net samples serves a second, subtler purpose: it strips away the soluble chain-scission fragments, leaving behind a cleaner, purely crosslinked network at the surface.

When rubrene was deposited and annealed on these different surfaces, the contrast was dramatic. On bare silicon dioxide and on untreated polystyrene, no crystalline domains appeared at all, even after annealing. On PS-x, spherulitic crystals formed only partially and unevenly. But on PS-net, the film blossomed into highly uniform spherulitic domains that swept across the entire surface. Polarized optical microscopy, which exploits the birefringence of rubrene crystals, revealed a classic nucleation-and-growth mechanism: new nuclei appeared during annealing, expanded radially, and after roughly sixty seconds the whole film had crystallized. Prolonged annealing beyond that point caused no further ripening, indicating the crystals had settled into a deeply stable energy state, with the final grain size fixed by the early nucleation density.

The speed is the headline. Previous polymer-assisted approaches to rubrene crystallization were far slower. One study by Bronshtein, Rybtchinski and colleagues needed more than 300 seconds of rapid thermal processing under high vacuum with argon flow to grow single crystals from a rubrene-polystyrene blend. Another group required roughly ten minutes of solvent vapor annealing in chloroform. The new interlayer method achieves uniform, large-area crystal growth in just sixty to ninety seconds, a tenfold or better acceleration that matters enormously for manufacturing throughput and, crucially, avoids the solvent-blending route whose crystal growth is notoriously sensitive to solvent choice and ambient conditions.

Temperature proved to be a razor-thin dial. Below 130 degrees Celsius, nothing crystallized; the thermal energy was simply insufficient to mobilize the rubrene molecules. At 140 degrees, small domains emerged that previous work assigns to the triclinic crystal phase. Between 150 and 160 degrees, triclinic regions gradually gave way to orthorhombic domains, the crystal structure favored for charge transport. Only in the narrow window of 170 to 180 degrees did large orthorhombic spherulites blanket the entire film, with triclinic domains vanishing entirely. That window, consistent with earlier findings by Holmes and Fielitz, is punishingly tight, and it underscores how little slack process engineers have when optimizing organic transistor fabrication.

What makes the crosslinked interlayer so special? Surface measurements offered clues but no complete answer. Atomic force microscopy showed all three polymer films were remarkably smooth, with roughness below one nanometer, though PS-net was slightly rougher at 0.804 nanometers than its counterparts at roughly 0.49 nanometers, perhaps aiding nucleation. Contact-angle measurements traced a rising surface energy, from 27.5 millijoules per square meter for pristine polystyrene to 38.5 for PS-x and 44.1 for PS-net, the latter closely matching rubrene’s own surface energy. Yet the energy-matching story collapsed as a full explanation: silicon dioxide and PMMA, with even higher surface energies, supported no polycrystalline growth, while HMDS-treated silicon dioxide, whose surface energy resembles PS-net, also failed. Something beyond wetting physics was at work.

The missing ingredient, the authors argue, is the thermal dynamics of the interlayer itself. Differential scanning calorimetry showed a clear glass transition for pristine polystyrene but none for the UV-modified films, a signature of restricted segmental motion caused by crosslinking. In the 170-to-180-degree window, the crosslinked network apparently provides just enough surface mobility for rubrene molecules to diffuse and reorder into orthorhombic crystals. Too rigid a surface, like HMDS-treated silicon dioxide, blocks diffusion; too mobile a surface, like linear polystyrene, lets the polymer chains churn and disrupt ordering. The residual chain-scission fragments in unrinsed PS-x likely interfere with long-range molecular ordering, echoing earlier pentacene studies in which shortened polystyrene chains suppressed surface diffusion. The researchers caution that this mechanistic picture remains partly speculative, but the practical recipe is clear: remove the unstable fragments, tune the crosslinking, and the polymer underlayer becomes a template that guides small molecules into the crystal phase devices need. For organic field-effect transistors, where structural uniformity and batch-to-batch reproducibility are prerequisites for any commercial future, a sixty-second annealing step on a UV-crosslinked plastic film may prove to be exactly the kind of unglamorous enabling technology the field has been waiting for.

Subject of Research: Interfacial engineering of polymer interlayers to control rubrene thin-film crystallization for organic electronics

Article Title: Interfacial engineering of rubrene thin films with crosslinked polymer layers

Article References: Im, J., & Choi, H. H. (2025). Interfacial engineering of rubrene thin films with crosslinked polymer layers. Advances in Industrial and Engineering Chemistry, 1(1), Article 20. https://doi.org/10.1007/s44405-025-00023-y

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00023-y

Keywords: rubrene, organic semiconductors, polystyrene, crosslinking, spherulites, thin films, crystallization, surface energy, UV irradiation, organic field-effect transistors, polymer interlayers, thermal annealing