Small-molecule semiconductors are gaining traction in flexible electronics due to their tunable electronic properties, processability, and compatibility with unconventional substrates. Among their applications, skin-compatible sensors stand out for healthcare monitoring, human-machine interfaces, and diagnostics. Unlike stretchable polymers or hybrid systems, small molecules offer precise control over molecular packing and charge transport, enabling mechanically robust and low-power devices without compromising performance.
Mechanical robustness is critical for skin-compatible sensors, as they must endure repeated bending, folding, and mechanical stress without degradation. Small molecules such as pentacene, rubrene, and C60 derivatives exhibit inherent flexibility when deposited in thin films on compliant substrates like polyimide or parylene. Studies show that films under 100 nm thick can withstand bending radii below 5 mm without cracking, owing to their molecular-scale adaptability. Additionally, grain boundary engineering through thermal annealing or solvent vapor treatment enhances mechanical endurance by reducing microcrack formation.
Low-power operation is another key requirement for wearable sensors, as energy efficiency prolongs battery life in continuous monitoring applications. Small-molecule semiconductors achieve this through high charge carrier mobility, which reduces the driving voltage needed for signal transduction. For instance, solution-processed small molecules like TIPS-pentacene exhibit mobilities exceeding 1 cm²/Vs, enabling operation at sub-1V biases. Furthermore, their low off-currents (below 10⁻¹² A) minimize static power consumption, making them suitable for intermittent sensing with energy harvesting integration.
Sensor performance hinges on the molecular design and thin-film morphology. Planar, π-conjugated molecules facilitate strong intermolecular interactions, improving sensitivity to mechanical strain or biochemical stimuli. Functional groups can be tailored for specific sensing applications:
- Redox-active molecules (e.g., quinones) enable electrochemical detection of metabolites like glucose or lactate.
- Ambipolar materials allow dual-mode operation, detecting both electrons and holes for multimodal sensing.
- Surface modifications with self-assembled monolayers enhance selectivity toward target analytes while rejecting interferents.
A critical challenge is maintaining performance under environmental stressors such as humidity and temperature fluctuations. Encapsulation strategies, including atomic layer deposition of Al₂O₃ or organic-inorganic multilayers, protect the active layers without compromising flexibility. Accelerated aging tests reveal that properly encapsulated small-molecule sensors retain functionality after 10,000 bending cycles or 7 days in 85% relative humidity.
Power efficiency is further optimized through device architecture. Bottom-gate thin-film transistors (TFTs) with hybrid dielectric layers (e.g., Cytop/Al₂O₃) achieve subthreshold swings below 100 mV/dec, reducing dynamic power consumption. For passive sensors, resistive or capacitive designs leverage the piezoresistive or dielectric properties of small molecules, operating at nanoampere-level currents.
Comparative studies highlight advantages over alternative materials:
- Unlike conductive polymers, small molecules avoid dopant migration, ensuring long-term stability.
- Compared to metal oxides, they offer superior mechanical flexibility and lower processing temperatures.
- Their synthetic versatility allows fine-tuning of energy levels, enabling direct interfacing with biological tissues without additional interfacial layers.
Future directions include integrating these sensors with wireless communication modules and energy-efficient readout circuits. Advances in vapor-phase deposition and roll-to-roll printing will facilitate scalable manufacturing. Research is also exploring biodegradable small molecules for transient electronics, addressing sustainability concerns.
In summary, flexible small-molecule semiconductors present a viable pathway toward high-performance, skin-compatible sensors. Their mechanical endurance, low-power operation, and molecular precision make them ideal for next-generation wearable technologies, provided encapsulation and integration challenges are addressed. Continued material innovation and device engineering will unlock their full potential in personalized healthcare and beyond.