V.P. Singh

Copper indium diselenide nanowire arrays by electrodeposition in porous alumina templates

A simple template assisted approach for fabricating I–III–VI semiconductor nanowire arrays is presented. 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, followed by annealing at 220u2009°C in vacuum. 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 40xa0nm and the lengths ranged from 600xa0nm to 5xa0µm; the grain size was estimated to be less than 5xa0nm. 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.

Copper-phthalocyanine-based organic solar cells with high open-circuit voltage

We report on a copper phthalocyanine (CuPc)/3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) organic solar cell with an aluminum cathode exhibiting an open-circuit voltage (VOC) of 1150 mV and a short-circuit current density (JSC) of 0.125mA∕cm2. For comparison, aluminum and silver were used as electrode materials and their effects on device characteristics were studied; silver yielded lower VOC but higher JSC and higher efficiency. Results could be understood through a model that hypothesized surface modification of CuPc by PTCBI and the formation of a thin insulating layer between aluminum and PTCBI when aluminum was used as electrode.

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.

Weed Management in Conservation Agriculture Systems

Conservation agriculture (CA) does have several advantages over conventional tillage (CT)-based agriculture in terms of soil health parameters. However, weeds are the major biotic constraint in CA, posing as a great challenge towards its adoption. The presence of weed seeds on the upper soil surface, due to no tillage operation, leads to higher weed infestation in CA, and so far, herbicides are the only answer to deal with this problem. Overreliance of herbicide use showed its consequence in terms of environmental pollution, weed shift and herbicide resistance development in weeds. Growing herbicide-tolerant crops using nonselective herbicides could be a broad-spectrum weed management technique to tackle weed shift, but the same is being resulted in the evolution of more problematic ‘super weed’. These observations indicate the need of integrated weed management technologies involving the time tested cultural practices, viz. competitive crop cultivars, mulches, cover crops, intercrops with allelopathic potential, crop diversification, planting geometry, efficient nutrient, water management, etc., along with limited and site-specific herbicide application. The modern seeding equipment, e.g. ‘Happy Seeder’ technology, that helps in managing weeds through retention of crop residues as mulches, besides providing efficient seeding and fertilizer placement, shows the promise of becoming an integral part of CA system.

Weed control in sesame with pre-emergence herbicides

Pendimethalin, imazethapyr, metribuzin, oxyfluorfen, imezemox were evaluated as pre-emergence herbicides in sesame during summer season of 2013 and 2014 at ICAR-Directorate of Weed Research, Jabalpur. All the herbicides reduced the weed population significantly as compared with weedy check but delayed the germination of sesame and caused sesame injury. Among all the herbicides, pendimethalin alone and in combination with lower dose of imazethapyr caused less injury to the sesame and produced higher sesame yield. None of the herbicides surpassed the yield what was achieved with 2 HW.

Schottky diode solar cells based on copper phthalocyanine nanowires

Copper phthalocyanine (CuPc) nanowires were electrodeposited in the pores of anodized aluminum oxide (AAO) templates. CuPc-Al Schottky diodes were formed on these nanowires and their materials and electro-optical characteristics were compared with Schottky diodes made from CuPc films electrodeposited on planar ITO-glass substrates. CuPc nanowires and the electrodeposited CuPc films (as prepared) had the preferred crystallite orientation of the α-phase. A thin layer of PEDOT: PSS, inserted between CuPc nanowires and the ITO electrode, improved the contact and reduced the series resistance by an order of magnitude. Under one sun illumination, Schottky diode on the planar CuPc film yielded a record high open circuit voltage of 1.19 V. On the other hand, no photovoltaic response was seen in case of nanowire Schottky diodes. Lack of photovoltaic response was attributed to the length (500 nm) of nanowires being much larger than the CuPc exciton diffusion length of only a few nm.

Copper Phthalocyanine Nanowire Based Solar Cells

Photovoltaic devices based on organic semiconductors are of interest because of their potential as flexible, lightweight and inexpensive devices. One of the promising devices, involves the heterojunction between copper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic bis-benz-imidazole (PTCBI). Earlier, we reported, the highest V oc (1.125V) in a single organic heterojunction solar cell in an ITO/PEDOT:PSS/CuPc/PTCBI/Al structure. Results were interpreted in terms of a modified CuPc-Al Schottky diode for this thin PTCBI layer case and a CuPc-PTCBI heterojunction for the thick PTCBI case. We also reported the device characteristics of Copper phthalocyanine (CuPc)/Aluminum (Al) Schottky diode solar cells. Here, open circuit voltages ( V oc ) increased from 220 mV at 15 nm to 907 mV at 140 nm. Analysis of the current-voltage characteristics indicated that tunneling and interface recombination mechanisms are important components of the current transport at the CuPc/Al junction. With the objective of higher short circuit current densities in mind, we have extended our earlier work on organic semiconductors to nanowire based designs. In this paper, we report the fabrication, materials and electrical characterization of CuPc nanowire based solar cells in AAO templates by electro-deposition. The CuPc nanowires were electrochemically deposited into the AAO templates using CHCl3 (Chloroform) containing 10-4 M CuPc with 0.5 ml of CF3COOH (Trifluoro acetic acid). The nanowires were characterized by XRD, UV-Vis absorption spectroscopy, electron microscopy and electrical measurements. CuPc nanowires were also electrodeposited onto ITO/glass substrates and compared with templated nanowires for their structural, optical and electrical properties. Effect of the PEDOT: PSS buffer layer on the nanowire based device characteristics was also investigated.

Analysis of CuPc-based organic solar cell with high photovoltage

Photovoltaic devices of the configuration ITO-PEDOT:PSS/CuPc/PTCBI/AI were investigated. A high open circuit voltage (V/sub OC/) of 1.125 V was obtained when the PTCBI layer was 7 nm thick. Value of V/sub OC/ was lower when silver, copper or gold electrode was used instead of aluminum. However, short-circuit current density (J/sub SC/) with these electrodes was much higher (4 mA/cm/sup 2/) than in the case of aluminum (0.12 mA/cm/sup 2/). These results could be understood in terms of a modified CuPc/AI Schottky diode model for the thin PTCBI case and a CuPc/PTCBI heterojunction model for the thick PTCBI case. Also, the formation of a thin, protective aluminum oxide layer under the aluminum electrode was postulated. For devices with silver, copper and gold electrodes, absence of this protective layer was thought to be the cause of the relatively lower V/sub CO/ and higher J/sub SC/.