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This paper uses impedance spectroscopy as a simulation tool to analyze grain boundaries properties in poly-crystalline materials. The interfacial structure at grain boundaries in poly-crystalline materials plays a decisive role in their electrical behaviors and degrades the performance in integrated circuits, gas sensors and solar photovoltaics. A numerical simulation is carried out to optimize grain size of poly-crystalline material to behave as mono-like material using impedance spectroscopy technique. The grain boundary potential (Vb) obtained from thermionic emission diffusion theory based continuum and localized energy models are utilized for obtaining the resistance (R), capacitance (C) and angular frequency (ω) of the bulk of the grain (Rb, Cb and ωb) and that of grain boundaries (Rgb, Cgb and ωgb) using the brick layer model. Two markedly different impedance spectra are obtained, one for the bulk of the grain and another for the grain boundaries. It is found that with an increase in the grain size (Lg), the signatures of grain boundaries spectra diminish and at a particular grain size, disappear completely which indicates that the poly-silicon material behaves comparable to mono-crystalline material. In this simulation work, silicon material is taken as the reference due to its abundant availability and huge market potential for photovoltaic and semiconductor applications but the simulation can be implemented on any poly-crystalline materials. The effect of varying silicon material resistivity (1.0–3.5 Ωcm) and temperature (300-420 K) on grain size was also analysed using the impedance spectra. The findings of the paper will be helpful for the silicon ingot growth industries to decide the optimum grain size during the polysilicon materials growth.

Field effect passivation of crystalline silicon surface is controlled by tailoring the built-in charge in a bilayer dielectric system consisting of hafnium oxide (HfO2) and aluminium oxide (Al2O3). The effective surface recombination velocity (Seff, max) ~ 10 cm/s is achieved at intermediate bulk injection levels with thermal ALD deposited HfO2 (top)/Al2O3 (bottom) bilayer system on n-type silicon with individual layer thickness between 3 and 7 nm. The best realized Seff,max value is lower by a factor of ~ 2.5 with respect to the single Al2O3 layer of similar thickness deposited under the same experimental conditions. The improved field effect passivation is quantified by enhanced effective charge density for the bilayer system in comparison with the corresponding value for single Al2O3 layer. The introduction of extra trap centers at the interface of the bilayer system is primarily responsible for the enriched field effect passivation, however, the sequence of the dielectric layers is important.

Present study reports fabrication of silicon nanowires over micro-textured Si substrates. Silver assisted electroless chemical etching route has been adopted for fabrication of the nanowires. Influence of HF concentration on the formation kinetics has been investigated by using scanning electron microscopy. The hierarchical binary structures have been able to reduce solar weighted reflectance (SWR) to direction. However, with increase in the concentration, etching in both and non- directions such as also occurs on the micro-pyramids of similar dimensions and results in the binary structures with slanted silicon nanowires on 3-dimansional micro-pyramids.

A striking correlation is observed between the characteristic defect-mediated deep level luminescence and deposition temperature of the zinc oxide (ZnO) thin films made by atomic layer deposition (ALD) technique. Near-ideal and virtually defect-free zinc oxide films are deposited at a temperature of 150 °C. However, distinct long-wavelength broadband luminescence features are identified for yellow (maxima at 570 nm) and green (maxima at 500 nm) at lower and higher temperature regimes of 150 °C, respectively. The decay time for yellow and green luminescence is different, having slower decay components for the yellow luminescence. The multi-step relaxation process governing the luminescence is discussed under the theory of efficient trapping of a photo-generated hole at a surface species in the first step, and radiative recombination at a deep-level acceptor type defect in the final step. Excess non-radiative surface recombination at the surface species is responsible for the shortening of the decay time of green emission line.

In this paper, an attempt has been made to find the correlation between various parameters of the two-diode equivalent circuit model of silicon solar cells. The statistical analysis has been done to find the engineering fit models between these parameters. The solar cell parameter data of 82 solar cell samples has been estimated using the Particle Swarm Optimization method from the measured illuminated I-V characteristics of the cells. This data on estimated parameters has been used to find the Pearson’s correlation coefficient between different parameters and the significant outcome of this work is that it revealed a high correlation between the first diode’s reverse saturation current and its ideality factor and a medium correlation between the second diode’s reverse saturation current and its ideality factor in the two-diode equivalent circuit model of a silicon solar cell. An engineering fit model has been suggested between the reverse saturation current and ideality factor of the first diode, based on the data of 82 large area (∼154.8 cm2) silicon solar cells with AM1.5 conversion efficiency between 15% and 18.4%. The suggested engineering fit between the two would be a method to reduce the number of parameters needed for silicon solar cell modeling and to make it easier for predicting the module output.

An effective way to modify the photocatalytic activity of both anatase and rutile TiO2 nanoparticles by coating the surface with either an inorganic (SiO2/silica) or organic (green-tea) layer using a chemical approach is demonstrated. Tetraethyl orthosilicate (TEOS) was used to cover the surface of TiO2 with silica which facilitates the inhibition of photocatalytic activity, ensuring its application in sunscreens by blocking the reactive oxygen species (ROS). Green-tea extract, rich in epigallocatechin gallate (EGCG), was used to coat/stabilize nano-sized TiO2. The morphology of these coatings was revealed by transmission electron microscopy (TEM) and by energy dispersive spectroscopy (EDS) mapping. These studies showed good coverage for both types of coating, but with somewhat better uniformity of the coating layer on rutile TiO2 compared to anatase due to its more uniform particle geometry. The effectiveness of each coating was evaluated by photodegradation of an organic dye (methyl orange). These studies showed rutile_polyphenol exhibits the highest photocatalytic activity among rutile forms which validates its feasibility to be used in photodegradation.

The present work reports a new concept on how to diminish dark twist intramolecular charge transfer (TICT) states with the zero-twist D–A systems in order to design frameworks with dual solution and solid-state emission property. The study began with theoretical calculations to understand the structural needs followed by the chemical synthesis of conceptually new two molecular designs, 1 and 2, with zero-twist angle between electron donor and acceptor units linked through a covalent bond and finally their applications in OLED devices. Oxazole was used as an acceptor in combination with the phenothiazene core as the donor, and the effect of enhanced electron donation was studied using methyl and anisole donor groups. DFT studies indicated a partial segregation of HOMO–LUMO levels in molecular designs, and the photophysics of these planar charge transfer molecules have been investigated. Natural transition orbital (NTO) calculations were carried out to understand excited-state transition character in these ...

We have synthesized a new aluminium complex, [5-choloro-8-hydroxyquinoline] bis[2,2'bipyridine] Aluminium Al(Bpy)(2)(5-Clq) and characterized it for structural, thermal and photoluminescence properties. The prepared material was characterized by Fourier transformed infra-red spectroscopy (FTIR), thermal gravimetric analysis (TGA) and photoluminescence. The prepared material showed thermal stability up to 240 degrees C. The photoluminescence spectrum of Al(Bpy)2(5-Clq) in toluene solution showed peak at 515 nm. This material was used as an emissive layer in organic light emitting diodes (OLEDs). The fundamental structure of device is ITO/F4-TCNQ(1 nm)/alpha-NPD(35 nm)/Al(Bpy)(2)(5-Clq) (35 nm)/BCP(6 nm)/Alq(3)(28 nm)/LiF(1 nm)/Al(150 nm). The device emits an yellowish green light (CIE coordinates, x = 032, y = 0.52) with maximum luminescence 314 Cd/m(2) at 18 V. The maximum current efficiency of OLED was 0.09 Cd/A and maximum power efficiency was 0.03 lm/W at 9 V respectively. (C) 2015 Elsevier B.V. All rights reserved.

Organic/inorganic heterojunctions provide a viable option to replace the conventional high-temperature dopant diffusion-based p–n junction owing to their low manufacturing cost. Thus, there has been increasing interests in low temperature heterojunction solar cell concepts particularly polymer/silicon-based heterojunction solar cells. Here, we report fabrication of heterojunction silicon solar cells employing a relatively rapid and solution-based low temperature (~100 °C) process wherein heterojunctions are made by directly spin coating the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a p-layer on the micro-textured (µT) n-Si substrates. The micro-texturing enhances the surface area as well as reduces reflectance to ~11 % in the spectral range useful for Si solar cell. The role of dimethyl sulfoxide (DMSO) addition in PEDOT:PSS on the performance parameters of the solar cells has been investigated. The PEDOT:PSS layer also acts as a surface passivation layer for n-Si as confirmed by the minority carrier lifetime measurements. Almost threefold enhancement in the photocurrent density (J sc) and a fivefold improvement in the conversion efficiency (η) for an optimized DMSO addition in the polymer have been observed compared to that having no DMSO addition. As a result, a maximum η of 6.45 % and J sc of 27.28 mA/cm2 have been achieved under 100 mW/cm2 irradiation at 25 °C. In these cells, open circuit voltage and fill factor are found low, which is the reason for low device efficiency. However, there is a scope for further improvement in device performance by process optimization particularly metal electrodes, PEDOT:PSS/DMSO layer thickness, PEDOT:PSS/DMSO/µT-Si interface properties, and incorporation of back surface field to exploit the full potential of such concepts.

Recently, a lot of attention is being paid to combine silicon nanowires (SiNWs) and the conventional micro-pyramid silicon (µT-Si) structures leading to SiNWs/µT-Si binary structures for efficient light harvesting. We report large-area fabrication of vertical SiNWs array over three-dimensional micro-pyramid structured silicon substrate through silver (Ag)-assisted electroless wet chemical etching in aqueous HF and AgNO3 solution. The influence of AgNO3 concentration has been investigated over the formation of the SiNWs. Vertical SiNWs formation takes place for lower AgNO3 concentrations. For higher concentrations, random formation of NWs and depletion of the micro-pyramids is observed. The present study reveals strong influence of local morphology of micro-pyramids over NWs formation. The formation mechanism of such binary structures has been discussed in co-relation with dimensional features of the micro-pyramids and surface free energy. Such hierarchically textured binary structured silicon surfaces exhibit excellent light trapping properties reducing reflectance to as low as

Minimization of reflection losses is extremely important for efficient silicon solar cells made either on mono crystalline (c-) or multi-crystalline (mc-) silicon (Si) substrates. A simple and fast etching process yet effective for nano-scale texturing of mc-Si surface using silver assisted wet chemical etching is reported. This results in very low reflecting (

A systematic study of hydrogen sintered aluminum rich zinc oxide (AZO) films is made from the view point of their application as surface passivation layer for silicon solar cell. AZO films of various Al and Zn molar concentration ratios (0 4% vis-a-vis the control cell (without AZO layer) is seen after AZO layer is applied on the rear surface of the cell.

In2O3 nanocones were synthesized on silicon substrates using thermal evaporation of In2O3 precursor powder followed by annealing in oxygen ambient. The morphology and aspect ratio of the nanocones strongly depend on the thermal budget during processing. The grown structures had tapered morphology as observed from microscopic analysis. The layers showed a minimum reflectance of ∼3% over a wide range of spectrum from 300 nm to 1100 nm with a high transmittance of ∼90%. The material has an energy band gap of 3.93 eV. X-ray diffraction analysis revealed that the annealed layers were crystallized in (2 2 2) plane. Moreover, these layers showed photoluminescence at 464 nm, 481 nm, 487 nm and 558 nm under the excitation wavelength of 325 nm. This frequency (photon) shifting towards more favourable wavelength regime is expected to improve the blue response of the solar cell device.

We report the study of titanium dioxide films (TiO 2 ) using titanium di-isopropoxyl di-2ethyl hexanoate Ti(OC 3 H 7 ) 2 (C 7 H 15 COO) 2 colloidal precursor. This compound is less hygroscopic in nature and easy to use with processes like spin or dip coating. Thin films of TiO 2 are made on silicon substrates and their structural and optical properties are studied. The effect of Ti content in the precursor, sintering temperature and its duration on film thickness and refractive index are investigated. Refractive index shows an increasing trend with the rise in the sintering temperature but remains unchanged with the time. The film thickness decreases with both sintering temperature and time and increases with Ti content in the precursor. Reflectivity measurements show marked reduction in the reflection losses compared to bare silicon surface wherein the film thickness is altered by spin speed. XRD results show anatase phase in the samples sintered at lower temperature (

Six novel anthracene-oxadiazole derivatives, 4a (2-(4-(anthracen-9-yl)phenyl)-5-p-tolyl-1,3,4-oxadiazole), 4b (2-(4-(anthracen-9-yl)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole), 4c (2-(4-(anthracen-9-yl)phenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole), 8a (2-(4-(anthracen-9-yl)phenyl)-5-m-tolyl-1,3,4-oxadiazole), 8b (2-(3-(anthracen-9-yl)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole) and 8c (2-(3-(anthracen-9-yl)phenyl)-5-(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazole) have been synthesized and characterized for use as emitters in organic light emitting devices (OLEDs). They show good thermal stability (T(d), 297-364 °C) and glass transition temperatures (T(g)) in the range of 82-98 °C, as seen from the thermo gravimetric analysis and differential scanning calorimetric studies. The solvatochromism phenomenon and electrochemical properties have been studied in detail using UV-Vis absorption, fluorescence spectroscopy and cyclic voltammetry. TD-DFT calculations have been carried out to understand the electrochemical and photophysical properties. The spatial structures of 4b and 8c are further confirmed by X-ray diffraction analysis. Un-optimized non-doped electroluminescent devices were fabricated using these anthracene derivatives as emitters with the following device configuration: ITO (120 nm)/α-NPD (30 nm)/4a-4c or 8a-8c(35 nm)/BCP (6 nm)/Alq3 (28 nm)/LiF (1 nm)/Al (150 nm). Among all the six compounds, 8a displays the maximum brightness of 1728 cd m(-2) and current efficiency 0.89 cd A(-1). Furthermore, as an electron transporter, 8a exhibited superior performance (current efficiency is 11.7 cd A(-1)) than the device using standard Alq3 (current efficiency is 8.69 cd A(-1)), demonstrating its high potential for employment in OLEDs. These results indicate that the new anthracene-oxadiazole derivatives could play an important role in the development of OLEDs.

Aluminum rich zinc oxide (AZO) film was made on silicon substrates by spin coating using a precursor containing Al and Zn in the 20% atomic ratio. The films were annealed in air atmosphere and in hydrogen ambient. The minimum reflectance (Rλ) in air annealed (AZO:Air) samples was ∼2.5% in the vicinity (±50 nm) of 750 nm, however, the value was marginally lower (∼2%) in hydrogen annealed (AZO:H2) samples. The average Rλ over 300–1200 nm range was 18.5% and 14.1% respectively. An additional XRD peak of H2Al10O16 at 2θ = 51.5° corresponding to (1 2 1) reflection plane was observed in the AZO:H2 film. The AZO films were applied on the front and back surfaces of bifacial crystalline silicon (c-Si) solar cells where an improvement both in short circuit current density (Jsc) and open circuit voltage (Voc) was observed. The values of Jsc and Voc after AZO:Air were increased by 23% and ∼10 mV respectively w.r.t without AZO layer cells which on subsequent annealing in H2 raised to 32% and ∼21 mV. Jsc (AZO:Air) was used to calculate the expected enhancement in Voc which matched with the measured increase in Voc. The improvement in Jsc is due to reduced reflectance and, hence, the observed increase in Voc (AZO:Air) is attributed to it. On the other hand, increase in Voc after AZO:H2 step was much more and cannot be accounted only by Rλ, and therefore, the additional enhancement in Voc is attributed to back surface passivation that has occurred after annealing of the solar cells in hydrogen ambient. This attribution is substantiated by the internal and external quantum efficiencies (IQE and EQE) after back and front illumination of the solar cells and minority carrier lifetime measurements. The IQE data was used to calculate the diffusion length (Ln) of the base region where the value of Ln did not change after AZO:Air layer (=130 μm for Cell #1) but its value substantially increased (=244 μm) after the AZO:H2 treatment. This observation further reiterates our claim that the AZO film has surface passivation property.

Solar photovoltaic (SPV) is capable of providing the most feasible carbon-free route to the worldwide traditional power consumption. During the last decade, there has been tremendous development in silicon wafer based photovoltaic (PV) cells technology and today commercial silicon PV cells over 20 % efficiencies have been achieved. However, large-scale implementation of silicon wafer PV is currently not economical because of their high cost as compared to traditional power sources. One of the primary cost components for silicon PV cells is the starting silicon wafer, which requires extensive purification to maintain reasonable performance. Therefore, development of efficient and low cost PV devices is extremely important. Silicon nanowires (SiNWs) are a very promising candidate for next generation PV. The SiNW arrays exhibit low reflection, strong broadband light absorption and may be used as antireflection surface in solar cells. In addition to enhanced optical properties, nanowire arrays also have the potential for efficient charge carrier collection across the nanowire diameter for radial junction (homo/hetro p-n junctions) solar cells and therefore may relax high quality material requirement, enabling lower-cost PV cells. In the chapter, a short review of aperiodic SiNW arrays fabrication by silver assisted wet chemical etching method, their light trapping properties and PV applications with emphasis on SiNW arrays based solar cells would be presented. Finally, challenges in effective use of SiNW arrays in PV devices and future perspective would also be briefly discussed.

We report the surface passivation studies made on p-type single-crystalline silicon wafers using ethanolic solution of iodine and bromine. Minority carrier lifetime (τeff) is measured by the microwave photoconductance decay (μ-PCD) method and using a Sinton's lifetime tester. Measurements are carried out at different molar concentrations of iodine–ethanol (I–E) and bromine–ethanol (B–E) solutions to optimize the process parameters. It is found that good passivation (±5% of measured maximum lifetime) could be achieved for certain ranges of concentration, which in the case of I–E and B–E are 0.07–0.12 M and 0.05–0.07 M, respectively. The effect of pre-conditioning steps on surface passivation (silicon surfaces with and without native oxide) is investigated. It is shown that the quality of surface passivation can be improved by an optimized wet-chemical pre-conditioning treatment. The effect of bias light and passivation time is also studied. I–E solution provides better passivation than B–E solution in terms of τeff whereas B–E solution passivation exhibits better stability in comparison with I–E solution. The τeff measured by the Sinton method (WCT-120) and using a Semilab system (μ-PCD, WT-2000) are comparable if injection levels are matched.

A study of the effect of hydrogen annealing on antireflection and surface passivation properties of sol–gel derived aluminum rich zinc oxide (AZO) films coated on silicon wafers has been done in 300–600 °C temperature range. The minority carrier lifetime in silicon wafers coated with AZO films was measured using microwave photoconductive decay (μ-PCD) technique. After annealing of AZO coated silicon wafer in hydrogen ambient for 30 min between 400 and 600 °C the effective minority carrier lifetime τeff improved above the initial value of ∼16 μs and attained a maximal value of ∼71 μs for annealing at 500 °C. It may be that above 400 °C the molecular hydrogen gets dissociated into atomic hydrogen in presence of Al induced defects at the AZO coated silicon surface and passivates them by formation of Si–H–Al complex. The annealing in air in the same temperature range did not affect the lifetime. AZO films annealed at 500 °C in hydrogen or air show high transmittance in 400–1200 nm wavelength range and are suitable as AR coatings for silicon solar cells. We have applied a hydrogen annealed AZO antireflection coating on n+ front surface and an equally thick hydrogen annealed AZO coating on the p-back surface of an n+-p multi crystalline silicon solar cell. The observed improvement in Jsc, Voc values of the cell established that while AZO film on front acted as a good AR coating the AZO film on the back surface passivated the back surface effectively.

We report a relatively simple process for fast nano-texturing of p-type (1 0 0) silicon surface using silver catalyzed wet chemical etching in aqueous hydrofluoric acid and hydrogen peroxide solution at room temperature. In this approach, a fine textured black surface was achieved in about 15–30 s duration with reflectivity less than 4% in 400–1000 nm range. The reflectivity versus texturization time has been investigated and co-related with morphological characteristics of the nano-textured-silicon (nT-Si) surfaces. The nano-texturization process is also applicable in multi-crystalline silicon. Further, n + –p–p + structured solar cells have been fabricated on such nT-Si substrates following the standard fabrication protocol. Significant improvement in short circuit current density (>20%) and efficiency (∼1.25% absolute) has been achieved compared with planar control cell without deterioration of other performance parameters (such as open circuit voltage and fill factor). Light beam induced current measurements have also been carried out to show the spatial distribution of the light trapping and current in the planar and the nT-Si solar cells.

Antireflection titanium dioxide (TiO2) coatings have been developed on monocrystalline silicon by a sol–gel spin-coating process using titanium di-isopropoxidebis(acetylacetonate) colloidal precursor solution. The effect of titanium content in the precursor, spin rate, sintering duration and temperature have been studied and their effect on coating thickness and optical properties (i.e., refractive index and reflectivity) were investigated. The influence of post-deposition sintering temperature on the optical characteristics, composition and the microstructure of the coatings have been evaluated by UV–vis spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, atomic force microscopy and X-ray diffraction techniques. Solar cells made on silicon wafers with TiO2 as antireflection layer showed enhancement of more than 20% in short circuit current density in comparison to a cell devoid of the TiO2 coating.

We report fabrication of solar cell (n+–p–p+ structure) on black silicon substrates consisting of silicon nanowire (SiNW) arrays prepared by Ag induced wet chemical etching process in aqueous HF–AgNO3 solution. SiNW arrays surface has low reflectivity (

Porous silicon (PS) layers were formed on p-type, 〈1 0 0〉 oriented, 1–5 Ω cm resistivity Cz silicon wafers by electrochemical etching in an HF:C 2 H 5 OH (1:2 by volume) electrolyte at room temperature at a constant current density 20 mA/cm 2 . The etching duration was varied to achieve PS layers of different morphologies and thicknesses. Both the photoluminescence (PL) and the total diffused reflectivity spectra of the PS layers were measured. It was found that for the PS layers grown for etching durations of less than 90 s the PL emission is insignificant and reflectivity is quite low. Such PS layers can be used as antireflection coatings (ARC) on solar cells. The PS layers formed for etching durations greater than 90 s show a significant PL emission in 500–800 nm range with peak lying in 630–660 nm wavelength range. When etching duration increases from 90 s to 8 min the PL intensity increases and the PL peak shows a blue shift. With further increase in etching duration the PL intensity decreases and PL peak shows a red shift. The reflectivity of the photoluminescent layers increases with etching duration showing a highest value for a sample grown for 8 min. Further increase in etching duration up to 20 min the reflectivity decreases and then increases. Striking observation is that both the PL emission intensity and reflectivity in the wavelength range of 550–800 nm are maximum for the PS layer grown for the etching duration of 8 min.

Capacitance–voltage (C–V) measurements were carried out on conventional n+-p-p+ structure based silicon solar cells (SSC) of different thicknesses (40–300 μm) and on induced junction n+-p-p+ structures (IJS) under dark at room temperature. The capacitance is determined from the best fit of the measured data. It is shown that the capacitance under reverse and forward bias condition can be divided into two distinct regions, which are correlated to the quality of the junction and effectiveness of back surface field (BSF). It is found that the IJS has shallow junction and better BSF than the conventional solar cells.

Growth of high quality III-nitride films on high-k oxide/Si substrates is crucial for the fabrication of nitride based metal-oxide-semiconductor devices. Here, we report the effect of nitridation of HfO2 surface on the physical properties of gallium nitride (GaN) films grown on HfO2 (5 nm)/p-Si(100) substrates using laser molecular beam epitaxy (LMBE) technique. The grazing incidence x-ray diffraction (GIXRD) measurements showed the prominent GaN growth along (0002) direction. It is observed that the HfO2 nitridation significantly improves the surface coverage of GaN film with larger grains of size in the range 200 ~ 250 nm. Room temperature photoluminescence (PL) spectroscopy exhibits a sharp, intense near band-edge emission line at 3.4 eV with negligible defect-related deep bands for the GaN film grown with HfO2 nitridation. Also, the current-voltage characteristics of Au/GaN Schottky contacts show a reduced leakage current for HfO2 nitridation. Thus, the surface nitridation of HfO2 is found vital to improve the quality of GaN films for the development of GaN-on-Si technology.

We report a simple, inexpensive, and rapid process for large area growth of vertically aligned crystalline silicon nanowires (SiNWs) of diameter 40–200 nm and variable length directly on p-type (100) silicon substrate. The process is based on Ag-induced selective etching of silicon wafers wherein the growth of SiNWs was carried out using the aqueous HF solution containing Ag+ ions at room temperature in a Teflon vessel. Effect of etching time has been investigated to understand the evolution of SiNW arrays. It has been found that the length of SiNWs has a linear dependence on the etching time for small to moderate periods (0–2 h). However, etching rate decreases slowly for long etching times (>2 h). Scanning electron microscopy was used to study the morphology of the SiNW arrays. Structural and compositional analysis was carried out using Raman spectroscopy and high-resolution transmission electron microscopy equipped with energy dispersive X-ray spectroscopy. Orders of magnitude intensity enhancement along with a small downshift and broadening in the first-order Raman peak of SiNW arrays was observed in comparison to the bulk crystalline silicon.

The electrical properties of an n+–p–p+ structure-based single-crystalline silicon solar cell were studied by impedance spectroscopy, I–V and spectral response. The impedance spectrum is measured in dark, under different intensities (14, 43, 57, 71, 86, 100 mW/cm2) of illumination and wavelengths (400–1050 nm) of light. Under dark and at low intensities of illumination (

We report the fabrication of buried contact solar cells using porous silicon as sacrificial layer to create well-defined channels (for buried contacts) in silicon. In this paper, the salient features of the technology have been presented. No detrimental effect was found in the performance of buried contact solar cell with partially filled contact area compared to the solar cells having conventional planar contacts. However, a marked difference in the short circuit current density was seen when channel was fully filled with metal by screen printing, without degradation in the open-circuit voltage. It is expected that improved processing in combination with optimized buried metallic contact parameters may yield higher efficiencies that may result in substantial decrease in solar cell cost.

The correlation between the temperature-dependent electrochromic (EC) activity and other properties of galvanostatically deposited Prussian Blue (PB) films is presented here. Films subjected to annealing treatment in air at temperatures up to 500 °C were characterized by a variety of techniques which include TGA, XRD, FTIR, UV–vis spectrophotometry, SEM, XPS, cyclic voltammetry etc. The as-deposited X-ray amorphous hydrated PB films were blue in color and had Fe in both FeII and FeIII valence states and were electrochromically active. Consequent to changes in the valence state, degree of hydration and coordination environment of the iron ions upon annealing, EC activity and morphology of the films exhibited dramatic changes. Annealing at moderate temperatures retained the blue color of the films and decreased the EC activity consistent with dehydration and decreased the FeII content. Lack of EC activity at higher temperatures was consistent with dehydration and quenching of FeII states accompanied with change of color from blue to rust (FeIII) typical of Fe2O3. Independent of the annealing temperature, the films retained their amorphicity, however, prolonged annealing at 500 °C yielded hexagonal Fe2O3.

Ternary sodium–cobalt–phosphate glasses of the composition (50 − x )Na 2 O–50P 2 O 5 – x CoCl 2 with x varying between 0 and 15 mol% prepared by melt quenching have been characterized by Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) techniques. Thermal ( T g , T c ) and electrical properties have been investigated. Infrared spectra reveal the formation of metaphosphate glasses ( Q 2 tetrahedral units) with symmetric bridging oxygen (P–O–P) and non-bridging oxygen (P–O − ). The spectra also indicate the formation of P–O–Co bonds in the metaphosphate glasses that replace P–O − –Na + bonds. The results of thermal studies correlate with these FT-IR findings and support the formation of P–O–Co bonds and an increased cross-link density with increasing CoCl 2 . This results in enhanced chemical durability and increased T g and T c of the glasses. The electrical conductivity parameters upon changing the composition have been correlated with structural changes in the glass matrix.

Minority carrier lifeline, τ, is one of the most important parameters which has a decisive effect on the performance of silicon devices based on excess carriers. The value of τ is greatly affected by the presence of impurities and defects in silicon and its value provides a fair indication of quality of the material. Photoconductivity decay (PCD) and photocurrent generation (PCG) methods are simple and low cost methods of measurement of minority carrier lifetime in silicon wafers. However, their application requires care. The PCD method can give quite misleading results in case of polycrystalline wafers if there exists potential barriers at the grain boundaries which may affect majority carrier mobility significantly. PCG needs creation of an inducedp +-p-n + structure of substantially good quality that should not degrade with time. For PCG method the T measurement under vacuum conditions provides correct and consistent results.

Several samples of ion-conducting silver phosphate glasses doped with zinc/cadmium halides are prepared and subjected to X-ray diffraction analysis, transference number measurement, and electrical conductivity studies. The percentage transference numbers of these glasses are found to lie between 98.3 and 99.3, which suggests that the glasses are essentially ionic conductors. The conductivity values range from 1.66×10 −2 to 8.66×10 −2 S cm −1 at 300 °C. Solid-state batteries using these glassy systems as electrolytes with the cell configurations Ag||Ag 2 OP 2 O 5 (20%)Zn/CdX 2 ||(C+I 2 ) and Ag||Ag 2 OP 2 O 5 (20%)Zn/CdX 2 ||(C+I 2 +electrolyte) (where X is Cl, Br or I) have been fabricated and their discharge characteristics studied. Cells with undoped silver phosphate glasses as electrolytes have also been prepared and studied. The addition of Zn/CdX 2 to silver phosphate glass results in a substantial increase in the open-circuit voltage in the order MI 2 >MBr 2 >MCl 2 (where M is the dopant cation). Cells with Ag||Ag 2 OP 2 O 5 (20%)Zn/CdI 2 ||(C+I 2 +electrolyte) deliver the best performance.

A number of samples of glassy sodium polyphosphate and copolyphosphates of sodium — cobalt, sodium — copper, sodium — nickel, sodium-manganese and sodium — calcium were prepared and their optical properties were investigated. The ultraviolet and visible spectra of these glasses were recorded at the room temperature. It was found that the fundamental absorption edge of these glasses usually occurs in the ultraviolet — visible region. The optical absorption edges (E0) were calculated by using the relation α h ν = B (hν − E0)2 where B is constant. The linear variation of (αhν)1/2 with hν where α is the absorption coefficient and hν is the incident photon energy, is taken as evidence of non-direct interband transitions. The E0 values obtained for copolyphosphate glasses appeared to depend on the size of the counter cation. The infrared spectra of all these glasses appeared to be almost the same, indicating the presence of characteristic P  O  P linkages of linear polyphosphate chains and two non bridging oxygen atoms bonded to phosphorus atoms O  P  O (PO2) units in the polyphosphate tetrahedra.

Silver phosphate glassed doped with different concentrations of zinc and cadmium halides have been prepared and characterized by using X-ray diffraction, elemental analyses, DSC studies, ion transport and infrared spectral studies. FTIR spectra of all the samples were done which indicate the presence of characteristic P  O  P linkages of linear polyphosphate chain and two non-bridging oxygen atoms bonded to phosphorus atoms (O  P  O units) in the phosphate tetrahedra. On the basis of IR studies, a plausible resonance chain structure for these glasses has been established and discussed.

Silicon surface passivation is studied using Al2O3 thin film deposited by thermal process using atomic layer deposition (ALD) method. Minority carrier lifetime measurements showed that the film passivate the silicon surface effectively. Capacitance–voltage measurement confirms the activation of negative fixed charges after sintering at 400 °C.

Electrovoltaic (EV) effect provides a way of generating voltage across an unbiased junction under dark. Electrovoltaic (EV) cell in its simplest form is a device based on n+–p–n+ (or p+–n–p+) like structure in which if one p–n junction is subjected to an external forward bias, then, a voltage is developed across the other p–n junction such that the n-side gets a negative polarity with respect to the p-side. Connecting to a load across one of the n+–p junctions a bipolar transistor can be operated as a three-terminal EV cell. A new device henceforth known as electrophotovoltaic (EPV) cell wherein EV and PV effects could be expected to work cooperatively was also realized. It is based on a structure which is a combination of n+–p–n+ EV and n+–p–p+ photovoltaic (PV) cell structures having a common n+–p junction and is able to operate in EV, PV and EPV modes. We have developed one-dimensional physical models of EV and EPV cells and have applied them to explain the observed I–V characteristics of an n–p–n silicon bipolar transistor 2N3055 in EV mode and the EPV cell in EV, PV and EPV modes. While the photovoltaic efficiency ηPV decreases slowly with d/L, where d is the thickness and L is the diffusion length of minority carriers in the base region, the electrovoltaic efficiency ηEV has a strong dependence on d/L and decreases sharply with increase in d/L. Transistor 2N3055 with d/L=0.7 demonstrated ηEV>60%, whereas, our EPV cell with d/L>2.7 had ηEV

Large area silicon solar cells with screen printed contacts have been realized for the first time on 10 cm diameter, p-type, Cz silicon wafers which were bonded to silicon substrates by alloying of a suitably thick screen printed layer of Al on them. In cells made on 300 μm thick wafers without texturization, antireflection coating and passivation of the front surface, the values of the open-circuit voltage (Voc), the short-circuit current density (Jsc), curve factor (CF) and the efficiency (η) were found to be in the range 572–579 mV, 16–19.2 mA cm−2, 0.53–0.61 and 5.5–5.89%, respectively, under simulated tungsten halogen light of 100 mW cm−2 intensity. Using thinner wafers and having optical confinement, surface passivation and effective back surface field, the cell performance would be substantially improved. In fact, an efficiency close to 18% (AM1.5) would be realizable with this approach. Another attractive feature of this approach is that a low-cost silicon substrate could be used at the bottom that would act as support for the thin top surface without disadvantage to the cell performance. In this paper only the principle has been demonstrated experimentally. Possible improvements have been shown by computer simulation.

A new process of preparation of shaped multicrystalline silicon ingots in graphite molds based on the directional solidification technique is reported. The mold has a coating of silicon oxynitride instead of more commonly used silicon nitride on the inner side of its walls. The coating acts as a mold releasing layer and makes the mold reusable. It also enables to use of demountable molds made by assembling two or more parts together. It is expected that the repeatable use of molds made of graphite sheets will be able to promote a considerable savings in the production cost of multicrystalline silicon ingots in industrial practice.

A lectin was purified from the leaves of Allium altaicum and corresponding gene was cloned. The lectin namely Allium altaicum agglutinin (AAA) was ~24 kDa homodimeric protein and similar to a typical garlic leaf lectin. It was synthesized as 177 amino acid residues pre-proprotein, which consisted of 28 and 43 amino acid long N and C-terminal signal peptides, respectively. The plant expressed this protein more in scapes and flowers in comparison to the bulbs and leaves. Hemagglutination activity (with rabbit erythrocytes) was 1,428 fold higher as compared to Allium sativum leaf agglutinin (ASAL) although, the insecticidal activity against cotton aphid (Aphis gossypii) was relatively low. Glycan array revealed that AAA had higher affinity towards GlcAb1-3Galb as compared to ASAL. Homology analysis showed 57–94% similarity with other Allium lectins. The mature protein was expressed in E. coli as a fusion with SUMO peptide in soluble and biologically active form. Recombinant protein retained high hemagglutination activity.

Impedance Spectroscopy Technique (IST) has been employed for the average space charge potential and resistivity measurement at grain boundaries (GBs) in multi-crystalline silicon wafer's. It is found that IST is the most simple and accurate way to obtain space charge potentials developed at grain boundaries in semiconductor devices. In this work the individual and average grain GB measurement is carried out in mc-silicon wafers. For a single grain the impedance spectrum gives a single semicircle but over a GB the spectrum contains two semicircles.These semicircles can be fitted into two RC networks. The GBs potential is calculated by using the parameters resistance (R), capacitance(C) and relaxation frequency (f) obtained from the impedance spectrum. The effect of biasing and temperature on space charge potential obtained is calculated and found good agreements with theory.

Simple molecular structures capable of emitting over the entire visible range are still a challenge. Planar molecular structures have the drawback of fluorescence quenching in the solid state thus limiting their application fields. Combining long range excimer/exciplex emissions with a compound emission have been used to get white light. In this work, a series of new coumarin derivatives having a planar structure have been synthesized and characterized. The effects of systematic variation in alkyl chain functionalization providing morphological variations that permit interesting solid state emitting properties have been discussed simultaneously with electrochemical behavior and OLED (organic light emitting diode) device applications. Carbon chains containing 0–16 carbon atoms have been studied in order to conclude the results that systematic changes in alkyl group substitution can be utilized as a tool to tune the emitting color of these planar coumarins. Alkyl chains were introduced by O-acylation and O-benzoylation reaction on the hydroxyl group of parent coumarin 5. Thus the present strategy is also helpful in establishing a template to control the unproductive interchromophore electronic couplings. Solid state fluorescence properties support the crystal studies. Theoretical studies are also in agreement with experimental data. Electroluminescence of Device 2 with a turn on voltage (Von) around 5–6 V having s-CBP doped with 1% of 8 having alkyl substitution of 2-carbons is found to exhibit white emission with CIE co-ordinates of (0.29, 0.34) which is close to white emission while the alkyl substitution of 14-carbons (compound 17) in Device 7 (Von = 7 V) exhibited green emission. Thus a strategy helpful to tune the electroluminescence has been discussed. Refereed/Peer-reviewed

Vertically aligned silicon nanowire (SiNW) arrays have been fabricated over a large area using a silver-assisted single-step electroless wet chemical etching (EWCE) method, which involves the etching of silicon wafers in aqueous hydrofluoric acid (HF) and silver nitrate (AgNO3) solution. A comprehensive systematic investigation on the influence of different parameters, such as the etching time (up to 15 h), solution temperature (10–80 °C), AgNO3 (5–200 mM) and HF (2–22 M) concentrations, and properties of the multi-crystalline silicon (mc-Si) wafers, is presented to establish a relationship of these parameters with the SiNW morphology. A linear dependence of the NW length on the etch time is obtained even at higher temperature (10–50 °C). The activation energy for the formation of SiNWs on Si(100) has been found to be equal to ~0.51 eV . It has been shown for the first time that the surface area of the Si wafer exposed to the etching solution is an important parameter in determining the etching kinetics in the single-step process. Our results establish that single-step EWCE offers a wide range of parameters by means of which high quality vertical SiNWs can be produced in a very simple and controlled manner. A mechanism for explaining the influence of various parameters on the evolution of the NW structure is discussed. Furthermore, the SiNW arrays have extremely low reflectance (as low as

Minority carrier lifetime in silicon wafers has been measured by applying an impedance spectroscopy technique (IST). Induced p+–p and p–n junctions were formed on both sides of the silicon wafer by thermally evaporating semitransparent metal layers of palladium and aluminium respectively. As such, no thermal treatment was given to the device, and therefore there is no diffusion of impurities inside the semiconductor and the two junctions are induced in the form of accumulation and depletion regions of charge carriers respectively. Both generation and recombination lifetimes applicable under the reverse and forward bias conditions respectively have been measured. The generation lifetime was estimated to be around 73 µs, whereas the recombination lifetime has been found to be about 11 µs. It is shown that the effective recombination lifetime is determined mainly by surface recombination velocity at the silicon–palladium interface. The effective minority carrier lifetime as measured by the microwave-detected photoconductive decay method on the same sample is 12 µs which is close to the measured recombination value by the IST. This shows that impedance spectroscopy can be used to measure effective lifetime of the wafer using an induced junction structure prior to the formation of an actual device like the solar cell. Moreover, the series resistance (Rs), diode ideality factor (n) and barrier height (Vbi) obtained from C–V (using the IST) data as well as the I–V measurement of the device show agreement with the expected device parameters. Thus, the IST can be effectively employed as a tool in extracting many relevant characteristic parameters of the material and the device.