Soumya Dutta

The high cost of conventional surface acoustic wave (SAW) devices limits its widespread use as micro-sensor and actuator for lab-on-a-chip applications. Solution-processed, inexpensive SAW device based on poly[(vinylidenefluoride-co-trifluoroethylene] [P(VDF-TrFE)], a ferroelectric polymer, is an alternative route to address this issue. However, incompatibility of P(VDF-TrFE) films with photoresist strippers and temperature intensive steps, often used in the optical lithography process, impede the development of efficient SAW device. This paper describes suitability of methanol as an alternative photoresist stripper without compromising the ferroelectric property of P(VDF-TrFE). Furthermore, a novel Bilayer-IDT architecture based SAW device is presented, which can be easily poled after temperature intensive fabrication steps to restore its ferroelectric property prior to SAW characterization. The SAW device, presented here, shows resonant frequency in the ultra-high frequency (UHF) range (≈451 MHz), which is quite superior to the highest reported value for P(VDF-TrFE) based SAW. The result is in good agreement with FEM simulations.

The effect of substrate temperature and deposition rate on the film morphology, crystallinity, and electronic properties of Pentacene transistors treated with hexamethyldisilazane (HMDS) is studied. The gate bias dependence of mobility is used to estimate the width of the density of states and thereby quantify the disorder of the highest occupied molecular orbital. A low deposition rate and the substrate held at room temperature are shown to be the optimal conditions for good mobility (0.20 cm2 V-1 s-1) and low electronic disorder (50 ± 10 meV). X-ray diffraction measurements are performed to quantify the ratio of the two crystalline phases (thin-film phase and bulk phase) present in the film. The crystalline phases, rather than grain size, plays a significant role in determining the charge carrier mobility. Film deposition with the substrate at room temperature leads to low electronic disorder as the film is composed of one crystalline phase (thin-film phase), while high substrate temperature makes the film increasingly polymorphic, leading to increased electronic disorder (up to 230 meV). A high deposition rate leads to poor morphology of Pentacene near the source/drain electrode edge, thereby leading to increased contact resistance and electronic disorder. Hence, a low growth rate at room temperature is required for HMDS treated substrates to induce good crystalline properties of the film in the channel region, which results in enhanced electronic properties of the transistors.

In this work, we performed gated Kelvin Probe Force Microscopy on reduced graphene oxide thin-film transistors with time transient. This enabled us to probe the electronic density of states around the charge neutrality point in reduced graphene oxide thin film. The charge neutrality point is of significance to know the nature of the intrinsic doping of the thin film and the switching of the majority carriers in the transistor devices. We measured the transfer characteristics of the reduced graphene oxide transistor devices to estimate the intrinsic charge neutrality point. The results were in good agreement with the time-resolved gated surface potential measurements obtained using Kelvin probe force microscopy. We propose that gated time-resolved measurement of these semi-metals can be an effective tool to study the nature of electronic states.

In this report we address the open circuit voltage (Voc) variation with temperature (T) in details by using TCAD numerical simulation and by developing physics based model. The proposed model is in good agreement with the TCAD simulation and is able to explain the experimentally observed (Voc) variation with different temperature and intensity.

In this paper, we present the operation principle of an organic metal-insulator-semiconductor (MIS) capacitor where the organic semiconductor is undoped. In spite of a low charge concentration within the semiconductor, this device exhibits a capacitance variation with respect to the applied gate voltage yielding the capacitance-voltage characteristics similar to that of a traditional MIS capacitor based on the doped semiconductor. A physics-based model is developed to derive the charge concentration, surface potential, and the capacitance of the organic MIS capacitor. The model is validated with TCAD simulation results as well as with experimental data obtained from the fabricated organic MIS capacitor consisting of poly(4-vinylphenol) and poly(3-hexylthiophene-2, 5-diyl) as an insulator and a semiconductor, respectively.

A systematic study of the photoluminescence quenching efficiencies in P3HT: PCBM blends showed a non-linear dependence on the PCBM concentration. We find a faster decrease in PL emission initially which later flattens out around 1:1 composition which tallies well with sample compositions known to give the best power conversion efficiencies. This implies that the exciton dissociation rates dominate the photocurrent generation in these films. We obtained a maximum of 91% photoluminescence quenching for films with a 1:1 blend ratio. A mean field based phenomenological model is presented, which very well describes our experimental results. The generation of free carriers due to various proposed mechanisms like dissociation and delocalization are collectively considered in the model. The model helps us understand the underlying physics and dependence of the quenching efficiency on parameters like excitation intensities. The proposed model will be useful in predicting the behaviour of exciton dissociation in new organic blends.

The performance of organic semiconductor photodetectors depends on the choice of materials as the active layers. In this paper, we show that a 1:1 blend of PPDT2FBT-PC71BM blend gives high responsivity of about 0.33 A/W at 660 nm and is operable in the wavelength range of 350 to 720 nm. The causes for high efficiencies at low operating voltages are discussed.

Substantial improvement of organic field effect transistors (OTFTs) toward circuit applications certainly demands stable device performance upon varying frequency and temperature. Organic metal-insulator-semiconductor (MIS) capacitor has been considered to be an efficient model system to predict the device performance of OTFT under the influence of frequency and temperature. In this study we show that the capacitance dispersion with respect to frequency and temperature in organic MIS capacitor device is an inherent property, which arises due to the low conductivity of the semiconductor along with Schottky type contact at the semiconductor-metal junction. In addition we propose an equivalent circuit model to explain the dispersion of capacitance with respect to frequency and temperature.

The solution-processed inverted co-planar organic thin film transistor (OTFT) incorporating recessed source-drain-gate and polymer gate dielectric is demonstrated to realize a fully recessed device. The impact of recessed electrodes on the contact resistance ( R-{c} ) and the mobility ( \mu ) is thoroughly investigated by comparing the performance of fully recessed devices, partially recessed (source-drain) devices and standard non-recessed devices. A reduction in R-{c} by two orders of magnitude and 3-fold increase in \mu to attain the values of 60 k\Omega -cm (without any surface treatment) and 1.5\times 10^{-2}cm^{2}/Vs respectively are achieved in a fully recessed device as compared to the standard non-recessed device.

In this paper, a wet-dry hybrid technique to transfer patterned reduced graphene oxide (rGO) thin film to arbitrary substrates at predetermined locations without using any chemicals is reported. The transfer process involves water-assisted delamination of rGO, followed by dry transfer to an acceptor substrate using viscoelastic stamp. Patterned reduced graphene oxide films are transferred to silicon dioxide (SiO2/Si) substrate to begin with. Subsequently, the method is deployed to transfer rGO to different polymer substrates such as poly(methyl methacrylate) (PMMA), and crosslinked poly(4-vinylphenol) (c-PVP), which are commonly used as gate dielectric in flexible electronic applications. The credibility of the transfer process with precise spatial positioning on the target substrate leads to fabrication of freely suspended reduced graphene oxide membrane towards nanoelectromechanical systems (NEMS) based devices such as nanomechanical drum resonators.