• MONICA KATIYAR

Articles written in Bulletin of Materials Science

• Processing and performance of organic insulators as a gate layer in organic thin film transistors fabricated on polyethylene terephthalate substrate

Fabrication of organic thin film transistor (OTFT) on flexible substrates is a challenge, because of its low softening temperature, high roughness and flexible nature. Although several organic dielectrics have been used as gate insulator, it is difficult to choose one in absence of a comparative study covering processing of dielectric layer on polyethylene terephthalate (PET), characterization of dielectric property, pentacene film morphology and OTFT characterization. Here, we present the processing and performance of three organic dielectrics, poly(4-vinylphenol) (PVPh), polyvinyl alcohol (PVA) and poly(methylmethacrylate) (PMMA), as a gate layer in pentacene-based organic thin film transistor on PET substrate. We have used thermogravimetric analysis of organic dielectric solution to determine annealing temperature for spin-coated films of these dielectrics. Comparison of the leakage currents for the three dielectrics shows PVA exhibiting lowest leakage (in the voltage range of −30 to +30 V). This is partly because solvent is completely eliminated in the case of PVA as observed by differential thermogravimetric analysis (DTGA). We propose that DTGA can be a useful tool to optimize processing of dielectric layers. From organic thin film transistor point of view, crystal structure, morphology and surface roughness of pentacene film on all the dielectric layers were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM).We observe pyramidal pentacene on PVPh whereas commonly observed dendritic pentacene on PMMA and PVA surface. Pentacene morphology development is discussed in terms of surface roughness, surface energy and molecular nature of the dielectric layer.

• Improvement in light-extraction efficiency of light emitting diode using microlenses fabricated by a novel and cost-effective method

A low cost, solution-based novel process was proposed, which avoids any photo-lithographically fabricated Si template and yet yields small dimension microlenses. The microlenses were fabricated using a well-known chemical technique called dewetting of polymer thin film. Polystyrene (PS) was used to make a polymer thin film with thickness ranging from 20 to 40 nm. Then, this PS film was kept in dewetting solution namely methylethylketone (MEK) to obtain the microlenses. Dimension of these microlenses was measured to be the order of 1–2 $\mu$m. The effect of various microlens parameters such as diameter and area fraction on light-extraction efficiency was systematically studied. Improvement of 4% in extraction efficiency was obtained by employing it on white light emitting diode. The area fraction of microlenses was increased up to 0.34 by reducing the spin speed. The light-extraction efficiency was further enhanced up to 7% uponincreasing the area fraction of microlenses.

• # Bulletin of Materials Science

Volume 43, 2020
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Posted on July 25, 2019

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