Sonali Das

Synthesis of DLC films with different sp2/sp3 ratios and their hydrophobic behaviour

Diamond-like carbon (DLC) films were deposited on glass substrate by sputtering of a vitreous carbon target in Ar + H2 plasma. The sp2/sp3 content in the films depended on the relative amount of hydrogen in the Ar + H2 plasma. The films were characterized by Fourier transformed infrared studies, Raman studies, scanning electron microscopy, atomic force microscopy and optical measurements. Hydrophobicity in these films was studied by measuring the contact angles of the water droplets and it was found that the films were extremely hydrophobic. The results are interpreted in terms of hybridization of carbon in these DLC films.

Hydrogen sensor based on thin film nanocrystalline n-GaN/Pd Schottky diode

Nanocrystalline n-GaN in thin film form was deposited onto fused silica substrates by high pressure dc sputtering of Si(1 at%) doped GaN target. An Al/n-GaN/Pd Schottky diode was realized by depositing n-GaN nanocrystalline layer onto Al-coated fused silica substrate under identical deposition conditions. Top Pd contacts were obtained by evaporating Pd using an appropriate mask. Corresponding current–voltage characteristics of the Schottky diodes were recorded before and after hydrogen gas exposure. I–V variations were analysed in the light of the existing theories. The ideality factor for the Schottky diodes varied between 3.4 and 4.5. The series resistance of the diode before gas and after gas exposure were found to be ~500 kΩ and ~300 kΩ, respectively. The barrier height could be seen to decrease significantly with increased hydrogen concentration while it increased with temperature.

Silica nanoparticles on front glass for efficiency enhancement in superstrate-type amorphous silicon solar cells

Antireflective coating on front glass of superstrate-type single junction amorphous silicon solar cells (SCs) has been applied using highly monodispersed and stable silica nanoparticles (NPs). The silica NPs having 300 nm diameter were synthesized by Stober technique where the size of the NPs was controlled by varying the alcohol medium. The synthesized silica NPs were analysed by dynamic light scattering technique and Fourier transform infrared spectroscopy. The NPs were spin coated on glass side of fluorinated tin oxide (SnO2: F) coated glass superstrate and optimization of the concentration of the colloidal solution, spin speed and number of coated layers was done to achieve minimum reflection characteristics. An estimation of the distribution of the NPs for different optimization parameters has been done using field-emission scanning electron microscopy. Subsequently, the transparent conducting oxide coated glass with the layer having the minimum reflectance is used for fabrication of amorphous silicon SC. Electrical analysis of the fabricated cell indicates an improvement of 6.5% in short-circuit current density from a reference of 12.40 mA cm−2 while the open circuit voltage and the fill factor remains unaltered. A realistic optical model has also been proposed to gain an insight into the system.

Role of metal and dielectric nanoparticles in the performance enhancement of silicon solar cells

The suitability of using spherical metal and dielectric nanoparticles on the top of a silicon solar cell has been investigated. An enhancement index factor (EIF) for each wavelength of light and an averaged EIF for the AM 1.5 solar spectrum, weighted by the photon flux, has been introduced. These factors estimate the effect of the nanoparticles in improving the performance of the solar cells, considering the absorption loss due to joule heating, fraction of radiation scattered into the substrate and the front scattered radiation pattern. A systematic comparison between silver and dielectric nanoparticles (silica, silicon nitride, titanium dioxide) shows that titanium dioxide and silicon nitride nano particles of sizes ≥100 nm exhibit larger enhancements compared to that of silver nanoparticles of similar sizes. Further, as the dielectric constant of the dielectric nanoparticles increases, the optimal particle size corresponding to maximal enhancement shifts towards lower value. At optimal particle sizes, the enhancement is 1.5–2 times greater than that due to silver nanoparticles.

A tunable band-stop filter using a metamaterial structure and MEMS bridges on a silicon substrate

A band-stop filter using a metamaterial structure (complementary U-shaped split resonator; CUSR) on a silicon substrate with a 13% tuning range is presented for Ka band applications. The metamaterial structure is used as a frequency-selective geometry on a coplanar waveguide (CPW) and tunability is achieved with the help of MEMS bridges. The rejection in the stop band is around 25 dB for the entire tuning range. A low insertion loss of 0.5 dB is obtained in the pass band. A simple electrical model of the proposed device and the design guidelines are presented. The filter is realized by a novel fabrication methodology involving the micromachining of two bonded silicon wafers and initial fabricated results are reported.

Effect of embedding silica nanoparticles and voids in the performance of c-Si solar cells

The effect of embedding nanoentities (silica and voids) on the optical and electrical performance of Si solar cells has been investigated in an attempt to decouple the Anti-Reflection (AR) properties of the standard nitride coated Solar Cells (SCs) and the scattering properties of the nanoentities. The decoupling will ensure the use of the scattering properties of the nanoentities without disturbing the optimized reflection characteristics of a standard SC. Lumerical® Finite Difference Time Domain Solutions software has been used to simulate the optical performance of solar cells after embedding nanoentities in the emitter region. Simulation results indicate that total decoupling of the AR properties and the scattering properties of the nanoentities is not obtained. Electrical performance evaluation of the system reveals a substantial relative improvement (1.7%) in the efficiency of thick (200 μm) SCs which further increases for thin (2 μm) film cells (23%) when 100 nm radius nanovoids having 30% area coverage are embedded at a depth of 200 nm from the silicon surface. The relative improvement is compromised if the changes in the material parameters due to embedding nanoentities are taken in to account.The effect of embedding nanoentities (silica and voids) on the optical and electrical performance of Si solar cells has been investigated in an attempt to decouple the Anti-Reflection (AR) properties of the standard nitride coated Solar Cells (SCs) and the scattering properties of the nanoentities. The decoupling will ensure the use of the scattering properties of the nanoentities without disturbing the optimized reflection characteristics of a standard SC. Lumerical® Finite Difference Time Domain Solutions software has been used to simulate the optical performance of solar cells after embedding nanoentities in the emitter region. Simulation results indicate that total decoupling of the AR properties and the scattering properties of the nanoentities is not obtained. Electrical performance evaluation of the system reveals a substantial relative improvement (1.7%) in the efficiency of thick (200 μm) SCs which further increases for thin (2 μm) film cells (23%) when 100 nm radius nanovoids having 30% area cov...

Enhanced optical absorption and electrical performance of silicon solar cells due to embedding of dielectric nanoparticles and voids in the active absorber region

Dielectric nanoparticles and voids have been embedded in the active silicon layer of standard silicon nitride-coated planar solar cells with a view to decouple the scattering properties of the nano entities (dielectric nanoparticles and voids) from their antireflection properties. However, it was found that complete decoupling between the two is not obtained. Our study shows that the additional reflection losses due to embedding may be partially or fully offset by the enhanced scattering, leading to net increase in absorption inside the solar cell for many embedding configurations depending on the nano entity material, size, area coverage, and depth of embedment. Optical simulation results were then incorporated into the electrical device model to obtain the solar cell parameters. Relative improvement of 7.1% in the efficiency of 20 μm thick solar cell has been obtained for 200 nm radius voids embedded 300 nm deep with 30% coverage. The relative improvement in efficiency is lower (1.9%) for 200 μm cells and higher (27.5%) for 2 μm cells.

Novel compact CPW filter for MICs using metamaterial structures

A band pass filter for Q-band MIC application is presented using two sets of resonator of different dimensions on Silicon substrate. The two resonators are implemented with the help of metamaterial structures and compactness is achieved by placing the resonators on both signal and ground planes of a Co Planar Waveguide (CPW). The resonators are realized using Complementary Split Ring Resonators (CSRRs). An equivalent circuit of the proposed design is presented along with the fabricated structures. Extraction of the LCR parameters of the proposed equivalent circuit is demonstrated. Further, it is seen that changing the metamaterial structure geometry from CSRR to Complementary U-shaped Split Resonators (CUSRs) tailoring of the pass-band can be achieved. By using different combinations of these two resonators, it is possible to tailor the bandwidth of the filter.

Optical properties of Si-doped GaN nanocrystals in SiO2/GaN/SiO2 thin film structure

SiO2/n-GaN/SiO2 composite films in the nanocrystalline form were deposited using a multi-target sputtering system onto fused silica substrates at a system pressure ~25 Pa. Different sets of nanocomposite films with different ratios of the sizes of the nanocrystallites (d) and intercrystallite distances (s) were deposited by changing the substrate temperature during sequential sputtering of the targets. The films were characterized by measuring the microstructural and optical properties. Photoluminescence measurements were carried out at 300 K. The experimental absorption spectra could be faithfully described by considering the combined effect of scattering by the ultra-small crystallites and broadening due to inhomogeneity in the sizes of the films.

Wide angle light collection with ultralow reflection and super scattering by silicon micro-nanostructures for thin crystalline silicon solar cell applications

Conventional c-Si solar cells employ micron-sized pyramids for achieving reduced reflection (~10%) and enhanced light trapping by multiple bounces (maximum 3) of the incident light. Alternatively, bio-mimetic, moth-eye sub-wavelength nanostructures offer broadband antireflection properties (~3%) suitable for solar cell applications in the optical regime. However, such structures do not provide any advantage in the charge carrier extraction process as radial junctions cannot be formed in such sub-wavelength dimensions and they have high surface area causing increased charged carrier recombination. The choice of the geometry for achieving optimum photon–electron harvesting for solar applications is therefore very critical. Cross-fertilization of the conventional solar cell light-trapping techniques and the sub-wavelength nanostructures results in unique micro-nanostructures (structures having sub-wavelength dimensions as well as dimensions of the order of few microns) which provide advanced light management capabilities along with the ability of realizing radial junctions. It is seen that an ultralow reflection along with wide angle light collection is obtained which enables such structures to overcome the morning, evening and winter light losses in solar cells. Further, super-scattering in the structures offer enhanced light trapping not only in the structure itself but also in the substrate housing the structure. Ray and wave optics have been used to understand the optical benefits of the structures. It is seen that the aspect ratio of the structures plays the most significant role for achieving such light management capabilities, and efficiencies as high as 12% can be attained. Experiments have been carried out to fabricate a unique micro-nanomaze-like structure instead of a periodic array of micro-nanostructures with the help of nanosphere lithography and the MacEtch technique. It is seen that randomized micro-nanomaze geometry offers very good antireflection properties (~1%) which are close to the expected optical behaviour of the periodic array at a much lower surface area.