We report a concept fabrication method that helps to improve the performance and stability of copper phthalocyanine (CuPc) based organic field-effect transistors (OFETs) in ambient. The devices were fabricated using a trilayer dielectric system that contains a bilayer polymer dielectrics consisting of a hydrophobic thin layer of poly(methyl methacrylate) (PMMA) on poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) or polystyrene (PS) with Al2O3 as a third layer. We have explored the peculiarities in the device performance (i.e., superior performance under ambient humidity), which are caused due to the polarization of dipoles residing in the polar dielectric material. The anomalous behavior of the bias-stress measured under vacuum has been explained successfully by a stretched exponential function modified by adding a time dependent dipole polarization term. The OFET with a dielectric layer of PVA or PVP containing hydroxyl groups has shown enhanced characteristics and remains highly stable without any degradation even after 300 days in ambient with three times enhancement in carrier mobility (0.015 cm(2)·V(-1)·s(-1)) compared to vacuum. This has been attributed to the enhanced polarization of hydroxyl groups in the presence of absorbed water molecules at the CuPc/PMMA interface. In addition, a model has been proposed based on the polarization of hydroxyl groups to explain the enhanced stability in these devices. We believe that this general method using a trilayer dielectric system can be extended to fabricate other OFETs with materials that are known to show high performances under vacuum but degrade under ambient conditions.

A two terminal sensor device based on PDI-HIS was developed for room temperature vapor phase detection of ammonia at very low sub-ppm levels under ambient conditions.

The influence of structural ordering of methyl cyclohexane appended naphthalene diimide (NMeCy2) thin films and their correlation with enhanced device performances are presented here. The vacuum-deposited thin-film microstructure and morphology of NMeCy2 have been investigated using thin-film X-ray diffraction (XRD), atomic force microscopy (AFM), and field emission scanning electron microscopy (FESEM) and were comparable with the bulk-phase crystalline structure and packing of NMeCy2. The organic field-effect transistor (OFET) fabricated on a glass substrate consists of a bilayer polymer dielectric poly(methyl methacrylate) (PMMA) over poly(vinyl alcohol) (PVA) and an inorganic high-k dielectric Al 2 O 3 as the third layer. NMeCy2 thermally deposited at an optimized substrate temperature (T sub ) of 60°C displayed excellent molecular packing over a large area that resulted in the improved field-effect performance with electron mobility (μ e ) value of 0.6 cm 2 V −1 s −1 and current on/off ratio (I on/off ) of 10 6 via modifications in dielectric configuration. Furthermore, the device afforded an unprecedented threshold voltage (V Th ) of 5.23 V with this material. We have been successful in developing a facile, reliable, and cheap method to tune the dielectric features which can culminate in improved field-effect transport properties. ■ INTRODUCTIONElectronic devices based on small organic molecules and polymers have received considerable attention in recent years due to their low cost and milder operating conditions. Particularly, small molecule based organic semiconductors (OSCs) are versatile, inexpensive, reliable, easily functionalized, and used as active materials in organic light-emitting devices (OLEDs), 1 organic photovoltaics (OPVs), 2 and organic fieldeffect transistors (OFETs). 3 The key problems of these molecules are the low mobility and low environmental stability. High mobility in OFETs is a prerequisite for fast response in high speed device applications. On the other hand, OFETs impose serious limitations to their practical applications due to the high operating voltage and high threshold voltage. Serious efforts have been devoted by researchers to improve these factors. This can be achieved by designing molecules with superior properties, device engineering, controlling the roughness and morphology of the dielectric, and OSC layers. The growth of OSCs and the alignment of crystalline layers on the dielectric surfaces are the key requirements for achieving high mobility. 4 Generally, the hydrophobic surface is more favorable for the growth of OSCs than hydrophilic surfaces. Highly compact well-organized molecular packing among adjacent molecules with significant π-orbital overlap and the absence of grain boundaries facilitate efficient charge transport enabling high mobility. 5 The low operating voltage is one of the main criteria for the integrated digital circuits and biosensor applications. The reduction in the threshold voltage and operating voltage can be achieved by using high-k dielectri...

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We have studied the kinetic roughening in the growth of cobalt phthalocyanine (CoPc) thin films grown on SiO2/Si(001) surfaces as a function of the deposition time and the growth temperature using atomic force microscopy (AFM). We have observed that the growth exhibits the formation of irregular islands, which grow laterally as well as vertically with coverage of CoPc molecules, resulting rough film formation. Our analysis further disclosed that such formation is due to an instability in the growth induced by local diffusion of the molecules following an anomalous scaling behavior. The instability relates the (ln(t))(1/2), with t as deposition time, dependence of the local surface slope as described in nonequilibrium film growth. The roughening has been characterized by calculating different scaling exponents α, β, and 1/z determined from the height fluctuations obtained from AFM images. We obtained an average roughness exponent α = 0.78 ± 0.04. The interface width (W) increases following a power law as W ∼ t(β), with growth exponent β = 0.37 ± 0.05 and lateral correlation length (ξ) grows as ξ ∼ t(1/z) with dynamic exponent 1/z = 0.23 ± 0.06. The exponents revealed that the growth belongs to a different class of universality.