Organic thin-film transistors (OTFTs) based on zinc phthalocyanine (ZnPc) have demonstrated remarkable sensitivity to tetrahydrocannabinol (THC) vapor, reaching part-per-billion detection levels through precise control of surface morphology and crystallinity. In the study, phthalocyanines (Pcs) with varying degrees of fluorination—CuPc, F16-CuPc, ZnPc, F4-ZnPc, and F16-ZnPc—were deposited as 400 Å films via physical vapor deposition (PVD) at 25 °C onto OTS-treated Si/SiO₂ substrates. GIWAXS and XRD confirmed α-phase orientations with co-facial herringbone stacking, while electrical characterization revealed n-type behavior for fully fluorinated variants.
Exposure to 4 ppm THC vapor over 90 seconds produced measurable changes in mobility, threshold voltage, defect density, hysteresis, on/off ratio, and XRD peak intensity. With the exception of F4-ZnPc, changes in crystallinity correlated strongly with mobility and defect density variations. No new polymorphs were detected, indicating that mobility reductions stemmed from altered α-crystallinity rather than phase transitions. THC interactions with the central metal proved critical: CuPc devices exhibited positive threshold voltage shifts, consistent with electron trapping, while ZnPc devices showed negative shifts, indicative of short-lived hole traps. Peripheral fluorination reduced both structural and electronic changes, likely due to steric and electronegative effects.
ZnPc devices exhibited the most pronounced responses, prompting further investigation into crystallinity effects. By varying deposition rate, substrate temperature, and using p-sexiphenyl (p-6P) as a patterning agent, films ranging from low to very high α-crystallinity were prepared. Low-crystallinity films, with small grains and high surface area, showed the greatest sensitivity to THC vapor, while highly crystalline films exhibited minimal response. Atomic force microscopy quantified surface area differences, confirming an inverse relationship with crystallinity.
Film thickness also influenced sensitivity. Low-crystallinity α-ZnPc films of 200 Å and 800 Å thickness were exposed to 40 ppb THC vapor. Electrical performance before exposure was similar across thicknesses, but thinner films displayed larger decreases in mobility and on-current, suggesting greater susceptibility to structural changes. The initially screened 400 Å α-ZnPc films showed negligible response at this low concentration, underscoring the 100-fold sensitivity gain achieved by optimizing morphology and thickness.
Real-time measurements revealed immediate, sharp drops in operating current upon THC vapor onset, followed by sustained decreases. Thinner films exhibited larger responses, consistent with higher analyte-to-semiconductor ratios and more extensive defect formation. Recovery after short exposures was partial, with repeated cycles diminishing both the magnitude of current changes and recovery. Tests with cigarette smoke produced only minor, noisy responses, highlighting ZnPc’s selectivity for THC.
To assess polymorphism effects, β-ZnPc films were prepared by toluene vapor treatment of α-ZnPc films, yielding large rectangular crystals and distinct XRD peaks at 2θ = 7.04° and 9.32°. Upon THC exposure, β-ZnPc OTFTs showed increased operating current, substantial threshold voltage shifts, and morphological changes visible by SEM, including sheet-like crystal formation. XRD indicated partial conversion toward α-like phases and the emergence of a new peak at 2θ = 7.38°, suggesting a tertiary morphology induced by THC incorporation.
β-ZnPc’s high crystallinity typically limits charge transport due to close intermolecular spacing and face-on molecular orientation. The observed “turn-on” response may result from THC-induced structural reorganization that improves transport pathways. Low defect densities post-exposure support this, while increased hysteresis likely arises from large interfacial sheet boundaries. Real-time characterization of β-ZnPc OTFTs under continuous THC exposure revealed rapid current decreases with decaying slopes, differing from α-ZnPc behavior, possibly due to heterogeneous crystal distribution between electrodes.
These findings demonstrate that every fabrication parameter—material choice, fluorination, deposition conditions, film thickness, and crystal phase—critically shapes OTFT sensing performance. For ZnPc-based devices, tuning nanostructure and morphology enables extraordinary sensitivity and selectivity to THC vapor, offering a model for engineering high-performance chemical sensors.