A new organic material holds its shape under light and solvents, allowing engineers to build flexible, reliable electronics and light-sensitive devices without complex processing steps.
(Nanowerk Spotlight) Organic semiconductors offer a promising way to manufacture electronic devices that are flexible, lightweight, and inexpensive to process. These carbon-based materials can be shaped from solution, printed onto soft surfaces, and tuned chemically in ways that silicon cannot match. But despite this versatility, they are notoriously difficult to pattern with the precision required for functional circuits.
Patterning is not optional. To build a working transistor, sensor, or photodetector, the active materials must be arranged in tightly defined geometries, with controlled spacing and alignment. Any disorder introduces electrical noise or leakage that degrades performance. In silicon electronics, these patterns are defined using photolithography, a process that uses light and chemicals to etch fine features into solid films. But most organic semiconductors cannot withstand that process. They dissolve in the solvents used to develop the image, degrade under the etchants, or deform under the heat.
To avoid this, researchers have turned to gentler fabrication techniques. Films can be printed, transferred, or assembled by manipulating surface properties. These methods can produce patterns, but the results are often fragile. A key problem is that organic semiconductor films, once deposited, are still vulnerable. If a second layer is applied using solution processing, it can wash away or disrupt the first. Patterns become blurred. Films dissolve at the edges. Devices fail before they are finished.
A workaround is to chemically fix the material in place to make it insoluble after it is patterned. This is done by introducing light-reactive chemical groups that form bonds under ultraviolet exposure, a process known as photo-crosslinking. It creates a network that resists solvents. But in most designs, the crosslinking groups are additives blended into the semiconductor. These additives can separate out during film formation, weakening structural order and reducing reproducibility.
A more precise approach is to build the crosslinking function directly into the molecule so that it acts as both a semiconductor and a photoresist, without added components. The challenge is making this work without disrupting the crystal packing that supports charge transport. In small-molecule semiconductors, that packing is highly sensitive to chemical modifications. Add the wrong group in the wrong place, and the material stops working.
In a study published in Advanced Science (“Intrinsic Photo‐Crosslinkable Semiconductive Small‐Molecule Crystals (i‐PSSCs) for Patterning Electronic Devices”), researchers report a material that resolves this tension. Their work describes a semiconducting molecule that forms ordered crystals and crosslinks under UV light, without additives or developers. Films made from this material can be patterned with micron-scale resolution and retain electrical function, offering a method for producing solvent-stable, structurally defined organic electronics.
How the new crystals are patterned with light. A thin film of the material is formed on a surface and exposed to ultraviolet (UV) light through a mask. The light triggers a chemical reaction that locks the molecules in place, while the unexposed regions are washed away, leaving precise patterns such as letters and shapes. Microscopy images confirm that the patterned edges are sharp and well-defined, showing that the material resists damage during the process and keeps its ordered structure. (Image: Adapted from DOI:10.1002/advs.202504711, CC BY) (click on image to enlarge)
The researchers designed their photo-patternable semiconductor by starting with a well-known molecular structure called BTBT, short for [1]benzothieno[3,2-b]benzothiophene. This compound has a rigid, conjugated core that supports efficient charge transport and strong intermolecular interactions. To give the molecule the ability to crosslink under light, they added diacetylene groups at its ends. Diacetylenes can undergo a reaction called topochemical polymerization, where they form new bonds in the solid state without disturbing the overall crystal structure.
Three related molecules were synthesized and labeled as compounds 6, 7, and 8. These variants differed only in the length and structure of their side chains, but those small differences turned out to be decisive. Only compound 6 packed in such a way that adjacent diacetylene groups were close enough to react when exposed to UV light. In compounds 7 and 8, the molecular distances were too large, or the packing too disordered, for crosslinking to occur effectively.
To test the material’s performance, the team deposited thin films of compound 6 onto silicon substrates coated with polystyrene to promote adhesion. The films were prepared using blade coating, a solution-based method that creates large, uniform films with well-aligned crystalline domains. After deposition, the films were exposed to shortwave UV light through a mask. The exposed areas underwent polymerization, while the unexposed regions were removed using a solvent rinse. This produced sharply defined patterns without requiring any external photoresist or chemical developer.
Characterization showed that the molecular order was preserved during crosslinking. X-ray diffraction patterns taken before and after UV exposure showed only a slight shift in the stacking distance between molecules, indicating successful polymerization without loss of alignment. Grazing-incidence wide-angle X-ray scattering confirmed long-range order, and atomic force microscopy showed clean, well-defined edges. The average line-edge roughness was about 65 nanometers, which is comparable to other state-of-the-art organic patterning methods.
Electrical measurements further supported the material’s stability. Organic thin-film transistors were fabricated using both unpatterned and patterned films of compound 6. The pristine films exhibited a maximum field-effect mobility of 0.46 square centimeters per volt per second, while the patterned films retained a mobility of 0.25. These values indicate that the photo-crosslinking process did not significantly compromise charge transport. The devices also showed consistent behavior across multiple samples, with narrow performance distributions and high on-off current ratios.
The same material was used to construct UV-sensitive phototransistors. These devices integrate light detection with signal amplification, and are useful in applications such as imaging and optical sensing. Under UV illumination at 365 nanometers, the current in the device increased in proportion to light intensity. When the light was switched on and off repeatedly, the device responded with stable changes in output current.
To test spatial selectivity, the team used a patterned mask in the shape of the letter H to illuminate a 7 by 7 device array. The output signals matched the input pattern, showing that the material could function as part of an artificial vision system capable of detecting structured light stimuli.
The researchers attribute the success of compound 6 to the short terminal methyl group on its side chain, which allows adjacent diacetylene units to overlap and react. This overlap does not occur in the other two compounds. The result is a semiconducting material that can be directly crosslinked by light, maintains crystal packing after patterning, and performs reliably in electronic and optoelectronic devices.
While the mobility values are lower than some high-performing BTBT derivatives, the tradeoff is clear. Compound 6 provides a patternable, solvent-resistant crystalline film without the need for additives or post-processing steps. This reduces complexity, improves reproducibility, and allows integration of organic semiconductors into device architectures where solvent exposure is unavoidable.
By embedding photo-crosslinking directly into the semiconducting molecule, this study presents a way to fix organic films in place without compromising their internal structure. The material performs both functions simultaneously, enabling applications that depend on both spatial control and electrical activity. As organic electronics move toward more complex architectures and multifunctional systems, such materials may help close the gap between molecular design and device fabrication.
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