Indrajit Mukhopadhyay

Structural, optical and electrical properties of spray-deposited CZTS thin films under a non-equilibrium growth condition

Spray pyrolysed thin films of quaternary Cu2ZnSnS4 (CZTS) were successfully deposited on soda lime glass substrates at 320 °C under a non-equilibrium condition (by varying Zn, Sn and S precursor concentrations) and without additional sulfurization. The effect of deficiency and enrichment of these three elements (normalized with respect to Cu) on the films microstrain, optical band-gap and Hall mobility was investigated. A large non-uniform microstrain of (5–6) × 10−3 and compressive nature were observed for both Zn- and Sn-enriched films from Williamson–Hall analysis of x-ray diffraction (XRD) data. However, a tensile strain of (2–3) × 10−3 was revealed in Sn-poor and S-rich samples. The optical band-gap (Eg) in stoichiometric CZTS was found to be 1.45 eV and Hall mobility (μH) in the range 87–92 cm2 V−1 s−1 was observed for S- and Zn-enriched films. The tensile nature of microstrain and inhomogeneities in Eg and μH were observed with greater magnitude due to the existence of other secondary phases, which were confirmed complementarily by FTIR spectroscopy and XRD.

Controlled Growth of Polyaniline Fractals on HOPG through Potentiodynamic Electropolymerization

Polyaniline (PANI) in fractal dimension has been electrodeposited reproducibly on highly oriented pyrolytic graphite (HOPG) from 0.2 M aniline in 1 M aqueous HCl solution by potentiodynamic sweeping in the range of -0.2 to 0.76 V vs Ag/AgCl at room temperature. Fractal growth of PANI dendrimers is affected by diffusion limited polymerization (DLP) at a sweep rate of 15 mV s(-1) for 43 min. This type of PANI dendrimer is prepared for the first time on such large area HOPG substrate by electrochemical technique using rather simple cell setup. The fractal dimension has been determined by chronoamperometry (CA) and box counting technique and is found to vary from 1.4 to 1.9 with the duration of electropolymerization. The sweep rate, terminal oxidation potential, and the diverse surface anisotropy of the HOPG surface are found to be crucial factors in controlling the growth of such PANI fractals.

Study of the junction and carrier lifetime properties of a spray-deposited CZTS thin-film solar cell

The Cu2ZnSnS4 (CZTS) thin-film solar cell fabricated entirely by a spray pyrolysis process was investigated under diffused white light in the present study. A CdS layer was developed as a heterojunction partner. The structural, morphological and photovoltaic characterization of an as-prepared stoichiometric CZTS film was carried out. The diode ideality factor n was found to be in the range of 1.2–5.6 in the forward bias region and it is explained by the Frenkel–Poole conduction model. The solar cell exhibited open-circuit voltage (Voc) of 157.25 mV, short-circuit current density (Jsc) of 3.024 mA cm−2 and fill factor (FF) of 24.77% at an incident irradiance of 200 W m−2 from the white LED source. The effective minority carrier lifetime of 263 μs was confirmed by the open-circuit voltage decay fitting under pulsed monochromatic LED excitation.

Electrochemical and electronic properties of flower-like MoS2 nanostructures in aqueous and ionic liquid media

In the present work, we report a facile strategy to synthesize uniform 3D flower-like MoS2 nanostructures prepared by a one-pot hydrothermal method and investigate their supercapacitive behavior. A field emission scanning electron microscopy, atomic force microscopy and X-ray diffraction study reveals the formation of randomly stacked layers of MoS2. The electrochemical properties of MoS2 nanostructures were investigated using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectra (EIS) techniques in aqueous and ionic liquid media. The CV measurement shows that the as-synthesized MoS2 electrode delivered a maximum capacitance of 218 F g−1 at a scan rate of 5 mV s−1 in aqueous medium. The GCD measurement shows a maximum specific capacitance of about 217.6 F g−1 at a discharge current density of 0.1 A g−1 in aqueous electrolyte. EIS with an appropriate electrical equivalent circuit was employed to understand the charge storage mechanism in the MoS2 electrode. Cyclic stability tests in aqueous medium reveal a capacitance retention of about 76% after 1000 cycles. This study reveals that a nearly pure capacitive behavior is observed for aqueous electrolyte and a diffusive behavior is observed for the MoS2 electrode in ionic liquid medium.

Elucidating different mass flow direction induced polyaniline-ionic liquid interface properties: insight gained from DC voltammetry and impedance spectroscopy.

This work describes the use of direct current (DC) cyclic voltammetry (CV) and alternating current (AC) electrochemical impedance spectroscopy (EIS) as a means to monitor an electrochemical interface of different mass flow direction induced polyaniline (PANI) film in IL (BmimPF6). Observed by SEM, vertical mass flow (VMF) and horizontal mass flow (HMF) induce porous nanorod and compact granular morphology of PANI, respectively. The present work explores in detail analysis of double layer capacitance, polarization resistance, diffusion mechanism, as well as other electrochemical features associated with the PANI-IL interface. A comparatively higher value of capacitance obtained for VMF PANI film from CV measurement confirms the higher electroactivity at the VMF electrode than the HMF film. Impedance spectroscopy, using a small amplitude perturbation, confirms the CV result. Impedance measurement gives a value of capacitance larger than that from CV where the amplitude of the perturbation is much larger. The implications of these results for its potential application in energy storage devices are discussed.

Influence of current collector electrode on the capacitive performance of electrodeposited PANI: insight gained from frequency and time domain analysis

This article focuses on the choice and effect of current collector electrode on the performance-indicating parameters of supercapacitors. The electrochemical properties of polyaniline (PANI) deposited on platinized fluorine doped glass (Pt) and fluorine doped glass (F : SnO2) current collectors are evaluated in depth by galvanostatic charge/discharge (GCD), cyclic voltammetry (CV) and impedance spectroscopy (IS). The GCD results show that the Pt-based electrode has better supercapacitive behavior than the F : SnO2 electrode. Scanning electron microscopy (SEM) results reveal that a porous granular array of PANI is induced on the Pt electrode, whereas a compact granular morphology is induced on the F : SnO2 electrode. Results from the CV are used to describe the dynamic variation of capacitance as a function of the applied bias. CV results reveal a higher value of capacitance for the Pt-based supercapacitor, which confirms the higher pore filling by electrolyte ions. Impedance measurements lead to determination of parameters such as equivalent series resistance, rate capability of electrodes and AC conductivity of supercapacitors. The results obtained here could aid understanding of how simple time and frequency domain experiments can be strategically used for quantitative characterization of supercapacitors.

Thermal decomposition of hydromagnesite

Non isothermal decomposition of synthetically prepared hydromagnesite phase with two different morphologies (2-D micro sheets and nests) was studied in dynamic nitrogen atmosphere by thermogravimetric analysis, differential thermogravimetric analysis, and differential scanning calorimetric techniques. Two different kinetic models, i.e. the Friedman isoconversion and the Flynn–Wall methods were employed for the analysis of thermal decomposition. The apparent activation energy (Ea) of the hydromagnesite phases having 2-D micro sheet and nest morphology were calculated and compared. The activation energy of nest morphology was found to be relatively higher than 2-D micro sheets. The higher activation energy for the relatively close packed ‘nest’ morphology is attributed to the difficulty of thermal transport in the core.

Influence of the magnitude and direction of electric field on the transport and growth property of deposited polyaniline films

Polyaniline (PANI) films were deposited by electrochemical polymerization of aniline monomer on a fluorine-doped glass substrate at room temperature under different electric field directions. The as-synthesized PANI films obtained at different growth cycles were characterized by AC impedance spectroscopy and scanning electron microscopy (SEM). The results revealed that the film morphology, transport kinetics, and electrical properties are strongly dependent on the electric field direction and magnitude of the applied field during electropolymerization. The SEM morphology and AC impedance (modulus spectroscopy) indicate that a more homogeneous, high-porous, and conducting PANI film is induced by horizontal electric field direction (HEFD) electrodeposition, whereas the modulus spectroscopy of the PANI film deposited by vertical electric field direction (VEFD) reveals that VEFD deposition favours two-dimensional growth of PANI. The obtained polymer is more of dielectric in nature due to preferable dendritic growth which is supported by SEM analysis.

On the electrical and interface properties of nanostructured CdTe Schottky diodes electrodeposited from an ionic liquid medium

A simple and cost effective method to fabricate nearly ideal Schottky diode out of p-CdTe semiconductor is discussed. The efficient re-use of ionic liquid for the deposition of nano-microstructures of CdTe is also disclosed. The I–V characteristic of the diode configured as Cu:FTO:p-CdTe:Cu showed the rectifying nature with a small forward voltage (0.8 V) and a rectification ratio of 6 × 103 at 4.8 V. Theoretical model suggests the diffusion controlled carrier transport process with an ideality factor of 1.1 up to a small forward voltage range whereas the thermionic transport with generation recombination dominates at higher voltages. The interface properties of p-CdTe and FTO were studied by impedance spectroscopy under varied bias conditions to electrically represent the diode.

Electrodeposition of Si from an Ionic Liquid Bath at Room Temperature in the Presence of Water

The electrochemical deposition of Si has been carried out in an ionic liquid medium in the presence of water in a limited dry nitrogen environment on highly oriented pyrolytic graphite (HOPG) at room temperature. It has been found that the presence of water in ionic liquids does not affect the available effective potential window to a large extent. Silicon has been successfully deposited electrochemically in the overpotential regime in two different ionic liquids, namely, BMImTf2N and BMImPF6, in the presence of water. Although a Si thin film has been obtained from BMImTf2N; only distinguished Si crystals protected in ionic liquid droplets have been observed from BMImPF6. The most important observation of the present investigation is that the Si precursor, SiCl4, instead of undergoing hydrolysis, even in the presence of water, coexisted with ionic liquids, and elemental Si has been successfully electrodeposited.