Suresh Rajaputra

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/SiO(2) 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 NH(3) and NO(2). 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 NH(3) and - 29.8% sensitivity for an MWCNT-PANI composite sensor to 100 ppm of NO(2) when DMSO was used as a solvent. Additionally, the sensors exhibited good reversibility.

Cadmium sulfide nanowires for the window semiconductor layer in thin film CdS–CdTe solar cells

Thin film CdS/CdTe heterojunction device is a leading technology for the solar cells of the next generation. We report on two novel device configurations for these cells where the traditional CdS window layer is replaced by nanowires (NW) of CdS, embedded in an aluminum oxide matrix or free-standing. An estimated 26.8% improvement in power conversion efficiency over the traditional device structure is expected, primarily because of the enhanced spectral transmission of sunlight through the NW-CdS layer and a reduction in the junction area/optical area ratio. In initial experiments, nanostructured devices of the two designs were fabricated and a power conversion efficiency value of 6.5% was achieved.

Barrier layer non-uniformity effects in anodized aluminum oxide nanopores on ITO substrates.

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. It was observed that in many templates, even the pores closest to each other could have their barrier layer thicknesses differ by as much as 10-20 nm. In this paper, 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.

Fabrication and characterization of CdS/CIS nanowire heterojunctions

Nanowire arrays of CIS were formed inside porous alumina templates by pulsed potential electrodeposition. Close to stoichiometric CIS phase in a chalcopyrite crystal structure with a = 5.782 Å and c = 11.619 Å. was obtained. Atomic weight fractions of Cu, In and Se were 20%:38%:42%. Arrays of cadmium sulfide nanowire were also fabricated inside the porous AAO templates. The XRD pattern of the CdS nanowires indicated the hexagonal lattice with the dominant crystal planes of <100>, <002> and <101>. The absorption spectrum of as-prepared CdS nanowires showed the blue shift of the absorption edge from the bulk crystal value of 515 nm to 490 nm. Al/CIS and Au/CdS Nanowire Schottky diodes were fabricated and their electrical properties were investigated. Diode analysis on the Au/CdS device reveled that the values of the effective diode ideality factor (A) and the effective reverse saturation current (J0,), were, 8.0 and 0.32 mA/cm2 in the dark, and 9.9 and 0.9 mA/cm2 under illumination. The fact that the measured values of effective diode ideality factor (A) were larger than 2.0 indicates that the dominant mechanism of electron transport across the junction is likely to be tunneling and/or interface state recombination, or a combination of these two.

CdS nanowire layers of enhanced transmittance for window layer applications in thin film solar cells

Arrays of n-CdS nanowires were investigated as replacement for the planar n-CdS film; the latter is presently used as a window layer in important photovoltaic devices like CdS-CdTe and CdS-CIGS. CdS nanowire arrays continued to exhibit substantial transmission even when the wavelength of incident radiation was well below 512 nm, which is the wavelength corresponding to the energy band gap of single crystal CdS. Theoretical calculation involving the number of excess photons transmitted through the CdS nanowire window layer indicated a potential enhancement in photocurrent by as much as 38% over the planar CdS film window layer device. Next, Au/CdS Schottky diodes were formed by depositing thin films of gold on the nanowire arrays. Analysis of the current-voltage characteristics of these Schottky diodes showed smaller diode ideality factors and higher carrier concentrations in devices with CdS nanowires than in devices with planar CdS films; these are desirable junction characteristics when the designer is trying to maximize the open circuit voltage and the power output.

Nanotube photovoltaic configuration for enhancement of carrier generation and collection

We report on the growth of geometric feature tuned semiconductor nanotubes on a transparent substrate through the application of an anodic aluminum oxide membrane-assisted method. Three-dimensional nanotube solar cells are developed in which semiconductor absorbers are not only used to fill the inner core of the nanotubes, but also to replace the membrane and to fill the intertube space between the nanotubes. The nanotube solar cells generate and separate carriers in three dimensions, namely, inside the cores of the nanotubes, in the intertube space between the nanotubes along the radial direction, and above the nanotubes along the axial direction. In preliminary experiments conducted to demonstrate the potential of this approach, nanotube CdS-CdTe solar cells were fabricated. CdS nanotubes with an inner diameter, wall thickness and intertube spacing of 35, 20, and 35 nm, respectively, were grown; the porosity and CdS nanotube density were 36.5% and 2.26 × 1010 nanotubes/cm2, respectively. These features of CdS nanotubes enable more efficient carrier collection because of the reduced recombination, especially in those cases in which the minority carrier lifetime is short, thus resulting in a diffusion length of less than 100 nm. Nanotube CdS-CdTe solar cells exhibit a wide and strong spectral response and quantum efficiency, indicating enhanced light absorption and carrier generation and collection. Without the benefit of an antireflection coating, the cells exhibited a wide and strong spectral response of quantum efficiency, and a short current density of 25.5 mA/cm2, an open circuit voltage of 750 mV, and a power conversion efficiency of 10.7% under 1-sun illumination. The materials and electro-optical characterizations indicated well-defined junction and interface behavior in these 3D nanotube solar cell configurations.

Nano-structured Cadmium Sulfide thin films for solar cell applications

A window layer made from Cadmium Sulfide thin films was constructed to enhance the electrical and optical properties of the solar cell. With the help of a Indium Tin Oxide layer and a Tin Oxide layer, a deposition of Cadmium Sulfide was performed by both Chemical Bath Deposition and Electro Deposition. Gold contacts were thermally evaporated on to the samples to test the successful creation of Schottky diodes that would make the layer more efficient for real world use. Measurements were made to test the samples thickness, absorption and transmittance percentages, and IV properties. Results show that the most efficient sample for use was the Chemical Bath Deposited sample. The sample revealed low absorption percentage of about 1%, while having a transmittance percentage of about 70%. IV measurements also revealed that a successful Schottky diode could be created with the use of gold contacts and Cadmium Sulfide. Future research is to be recorded to enhance the results and better create the window layer for the solar cell.

Nanoscale Engineering Approach for Enhancing the Performance of Photovoltaic Cell Technologies for Non-Fossil Energy Sources

Energy consumption in the world is increasing rapidly and the supply of fossil fuels will not be able to keep up with the demand for too long. Fortunately, two emerging technologies, photovoltaic (PV) cells and concentrated solar power (CSP), can deliver a large portion of United States’ energy needs in the next 40 years if they are properly developed. In this chapter, first, fundamental mechanisms of how electricity is generated by these two technologies are described. Next, recent developments in the application of nanotechnology for enhancing PV cell performance are presented. Among inexpensive solar cell technologies, copper indium diselenide (CuInSe2 or CIS) based thin film solar cells have achieved solar to electrical conversion efficiency of 19.5 %. However, further improvement of efficiency is needed for them to become competitive with traditional energy sources. Two major efficiency limiting factors are, less than optimal energy band gap and short carrier diffusion length. In our group, we have used nanoscale engineering to develop device designs that would counter these two limiting factors. Specifically, vertically aligned nanowire arrays of CuInSe2 of controllable diameter and length were produced by simultaneously electrodepositing Cu, In, and Se from an acid bath into the pores of anodized aluminum oxide (AAO) formed on top of an aluminum sheet. Ohmic contact to CIS was formed by depositing a 100 nm thick gold layer on top and thus a Schottky diode device of the Au/CIS nanowires/Al configuration was obtained. Analysis of the current–voltage characteristics of these devices yielded higher resistivity than those reported for CIS thin films, as expected from the size-dependent effects. Capacitance–voltage measurements were performed on the diodes to get the estimates of space charge density and the junction potential. Based on these experimental results, a nanowire-based solar cell configuration is proposed and illustrated.

Vertically aligned CuInSe 2 nanowire arrays on titanium coated glass substrates for photovoltaic applications

Nanowire arrays of copper indium diselenide (CuInSe2) were fabricated using an electrochemical deposition process. Custom anodized aluminum oxide (AAO) membranes on a glass substrate with a Ti interlayer served as templates for this electrodeposition. Typical diameter of electrodeposited nanowires was 60 nm although process parameters for anodization could be varied in a controlled way to obtain pore diameters as low as 20 nm. Elemental composition of these CuInSe2 nanowires on titanium substrate was studied using energy dispersive x-ray analysis (EDX). The atomic percentage ratio for as deposited nanowires was Cu: In: Se= 1:1.16:2.53. The electrolyte composition and other deposition parameters were optimized in order to yield slightly In-rich structures because it is well known that such films result in better photovoltaic devices. It is thought that, with material properties ideally suited for photovoltaic (PV) applications, the use of CIS nanowire arrays would enable a new generation of PV device architectures.

Fabrication and characterization of n-type cadmium telluride nanowires for thin-film solar cell applications

Thin films of n-type cadmium telluride nanowires were produced by electro-deposition into the pores of alumina membranes and were studied for their morphology, chemical composition, crystallite orientation and spectral transmission characteristics. Electrodes were deposited to form ITO/CdTe nanowire/Graphite Schottky diodes and ITO/CdTe nanowire/Indium conducting contact devices and their electrical characteristics were studied. Nanowires of 50 nm diameter and 200 nm lengths, embedded in a matrix of alumina, were obtained. CdTe crystallites in the nanowires were of cubic phase with a <111> preferential orientation. Film resistivity in the direction normal to the film was 2.4∗103 ω-cm. Spectral transmission measurements revealed that for wavelengths below 450 nm, a 200 nm thick CdTe nanowire film had higher transmission than a polycrystalline CdS film of same thickness.