Electronic Devices Research Group

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Research

Prof. Vijay Singh heads the Electronic Devices Research Group at the Center for Nanoscale Science & Engineering, Department of Electrical & Computer Engineering, University of Kentucky. We have the expertise and facilities to fabricate and characterize nano porous films, nano crystalline semiconductor films, nano wires and nano fibers for opto-electronic device and sensor applications.

Prof. Vijay Singh has more than 35 years of experience working in opto-electronic devices. His main areas of interest have been Electroluminescent Flat Panel Displays and Photovoltaic Cells. Prof. Singh and his team pioneered the development of a Light weight and Flexible Solar cell on a Mo foil substrate (CdS/CdTe) with Voc of 824 mv and also hold a US patent for the same. The underlying theme of research in the group is to take advantage of the benefits offered by nano technology to make light weight, flexible and more efficient opto-electronic devices, characterize and model them. In particular, nanoporous metal oxide templates (Alumina, Titania) are used as enabling platforms to create a multitude of nanostructures for opto-electronic and sensing applications.

Nanostructured Solar Cells:

Performance of CdS/CdTe cells can be improved by incorporating nano-technology in to their device designs. Semiconductor nanocrystals exhibit a wide range of size-dependent properties. Variations in fundamental characteristics ranging from phase transitions to electrical conductivity can be induced by controlling the size of the crystals. For example, in the prototypical material, CdS, the band gap can be tuned between 2.5 and 4 eV. As a part of this project we are evaluating CdS nanowires, nano-crystalline CdS films as window layers in a CdS/CdTe heterojunction solar cells.

Recent work involved fabricating CdS/CdTe nanowire heterojunction solar cells utilizing porous alumina template grown on ITO/Glass substrate. We observed 6.5% efficiency, which is the highest efficiency of power conversion reported so far among all solar cells based on nanopillars, nanowires, nanodots and nanorods. The nanowire-CdS layer has higher transmittance than the traditional planar CdS window layer. It has been observed that the absorption peak of CdS nanowires is shifted towards the blue region, compared with bulk CdS. This enhances the number of sunlight photons incident on the CdTe absorption layer and increases the light-generated current and the overall efficiency of the solar cell. Furthermore, because aluminum oxide is an insulator with much higher optical transmittance and CdS nanowires only occupy a portion (depending on the porosity of the AAO template) of the window layer, the overall transparency is further increased and more photons can be absorbed in the CdTe layer.

From theoretical considerations, this leads to a window layer of higher optical transmission and a reduced junction area for the ‘lossy’, reverse saturation current. As a result, one can expect higher short circuit current and higher open circuit voltage values, resulting in an estimated 26.8% increase in the power conversion efficiency of the CdS– CdTe solar cell. In the initial experiments, a power conversion efficiency value of 6.5% was achieved. For this cell, open circuit voltage, short circuit current density and fill factor values were 705 mV, 25.3 mAcm−2 and 36.4%, respectively. Further process optimizations, are currently in progress.

This work was supported in part by grants from the National Science Foundation (NSF-NIRT-ECS-0609064) and NSF-EPSCoR (EPS-0447479) and the Kentucky Science and Engineering Foundation (KSEF–148-502-02-27 and KSEF-148-502-03-68). More details about this project can be found here.

Template assisted fabrication of CuInSe2 nanowire arrays:

We are the first group to report a simple template assisted approach for fabricating I–III–VI semiconductor nanowire arrays. Vertically aligned arrays of CuInSe2 (CIS) nanowires of controllable diameter and length were synthesized by pulse cathodic electrodeposition from a novel acidic electrolyte solution into anodized alumina (AAO) templates. Scanning electron microscopy revealed that the nanowires were dense and compact. Depending on the dimensions of the starting AAO template, the diameters ranged from 5 to 40 nm and the lengths ranged from 600 nm to 5 µm; the grain size was estimated to be less than 5 nm. The composition of the nanowires was analyzed by energy dispersive x-ray (EDX) spectroscopy, and was found to be close to stoichiometric CuInSe2 within the limit of the resolution of the EDX technique. High resolution transmission electron microscopy and x-ray diffraction revealed high purity CuInSe2 nanowires with a preferred [112] orientation. More details about this project can be found here.

Barrier Layer Free Alumina and Templated CdS nanowire Arrays:

Nanoporous anodic aluminum oxide (AAO) has been used widely as a template for device fabrication. In many nanostructured electro-optical device designs, AAO grown on an ITO substrate is the desired configuration. However, a residual thin aluminum oxide barrier layer between ITO and the AAO pores remains and process non-uniformities during the template fabrication can cause serious problems in the quality of nanowires deposited later in these pores. In this study, causes and remedies for this non-uniformity are investigated, including the effects of a thin Ti interlayer inserted between the ITO and AAO. Templates with different Ti layer thickness and annealing conditions were compared. Mechanisms for the formation of voids beneath the barrier layer were analyzed and studied experimentally. Reactive ion etch (RIE) was found to be the preferred method to mitigate process non-uniformities. Using the above methods, barrier-free AAO templates on ITO substrates were obtained; their thicknesses ranged from 200 to 1000 nm. The characteristics of CdS nanowires electrodeposited into the initial templates with non-uniform barrier layer thicknesses and into the processed, barrier-free templates were compared. More details about this project can be found here.

Multi-walled carbon nanotube arrays for gas sensing applications:

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays fabricated by xylene pyrolysis in anodized aluminum oxide (AAO) templates without the use of a catalyst were integrated into a resistive sensor design. A thin layer of amorphous carbon (5–50 nm), formed on both sides of the template during xylene pyrolysis, was part of the sensor design. The thickness of the conducting amorphous carbon layers was found to play a crucial role in determining the sensitivity of the resistive sensor. A study was undertaken to elucidate (i) the dependence of sensitivity on the thickness of amorphous carbon layers, (ii) the effect of UV light on gas desorption characteristics and (iii) the dependence of room temperature sensitivity on different NH3 flow rates. Variations in sensor resistance with exposure to oxidizing and reducing gases are explained on the basis of charge transfer between the analytes and the CNTs which were modeled as p-type semiconductors. More details about this project can be found here.

MWCNT–polymer composites as highly sensitive and selective room temperature gas sensors:

Multi-walled carbon nanotubes (MWCNTs)–polymer composite-based hybrid sensors were fabricated and integrated into a resistive sensor design for gas sensing applications. Thin films of MWCNTs were grown onto Si/SiO2 substrates via xylene pyrolysis using the chemical vapor deposition technique. Polymers like PEDOT:PSS and polyaniline (PANI) mixed with various solvents like DMSO, DMF, 2-propanol and ethylene glycol were used to synthesize the composite films. These sensors exhibited excellent response and selectivity at room temperature when exposed to low concentrations (100 ppm) of analyte gases like NH3 and NO2. The effect of various solvents on the sensor response imparting selectivity to CNT–polymer nanocomposites was investigated extensively. Sensitivities as high as 28% were observed for an MWCNT–PEDOT:PSS composite sensor when exposed to 100 ppm of NH3 and − 29.8% sensitivity for an MWCNT–PANI composite sensor to 100 ppm of NO2 when DMSO was used as a solvent. Additionally, the sensors exhibited good reversibility. More details about this project can be found here.

Template assisted fabrication and characterization of nano-scale heterostructures.

High quality nanoporous alumina films of controllable pore size and pore height were fabricated on ITO, aluminum, molybdenum (for the first time ever) and glass substrates; Free-standing, nanoporous alumina membranes of controllable pore size and pore height were fabricated. Methods for selectively depositing arrays of nanoscale materials and junctions in an insulating matrix were demonstrated. Potential applications of this technology include solar cells, magnetic storage, optical switching and sensors; Anodized alumina templates were used as hosts for catalyst-free, aligned carbon nanotube growth which has applications in sensors and field emission display devices. Furthermore AAO templates offer the flexibility to control the fabrication of nanostructures by varying both the pore size and the interpore distance; In addition, we have successfully fabricated of a wide variety of nanowires, nanotubes in AAO templates by electrodeposition/chemical vapor deposition: For ex: Semiconductors (CdS, CuInSe2, Cu2S, CuPc, C60,) metals (Ni, Co, Au, Pd) and carbon nanotubes. We are first group in the world to fabricate CuInSe2 nanowires. We have also observed the highest achievable Voc (1.19V) in CuPc nanowire based Schottky diode solar cells.

Any questions or comments about this web page should be addressed to Dr. Suresh Rajaputra