Siva Sivoththaman

Heterojunction Silicon Microwire Solar Cells

We report radial heterojunction solar cells of amorphous silicon on crystalline silicon microwires with high surface passivation. While the shortened collection path is exploited to increase the photocurrent, proper choice of the wire radius and the highly passivated surface prevent drastic decrease in the voltage due to high surface-to-volume ratio. The heterojunction is formed by depositing a ∼12-16 nm of amorphous silicon on crystalline silicon wires of radius approximately equal to minority carrier diffusion length (∼10 μm). In spite of very short carrier lifetime (<1 μs), the microwire array devices generate photocurrent of ∼30 mA/cm(2), and the same time, voltages close to 600 mV are achieved, leading to efficiency in excess of 12% in extremely short carrier lifetime silicon. We also find that formation of nanocrystallites of silicon in the deposited film results in loss of the expected passivation.

Overview of solar cell technologies and results on high efficiency multicrystalline silicon substrates

Fabrication technologies for multicrystalline silicon (mc-Si) solar cells have advanced in recent years with efficiencies of mc-Si cells exceeding 18%. Intense efforts have been made at laboratory level to improve process technology, growth methods, and material improvement techniques to deliver better devices at lower cost. Deeper understanding of the physics and optics of the device led to improved device design. This provided a fruitful feedback to the industrial sector. Both screenprinting and buried-contact technologies yield cells of high performance. An increasingly large amount of research activity is also focussed on the fabrication of thin solar cells on cheap substrates such as glass, ceramic, or low quality silicon. Success of these efforts is expected to lead to high efficiency devices at much lower costs. Efforts are also being put on low thermal budget processing of solar cells based on rapid thermal annealing.

A Tunable RF MEMS Inductor on Silicon Incorporating an Amorphous Silicon Bimorph in a Low-Temperature Process

A novel tunable radio frequency microelectromechanical system inductor based on the bimorph effect of an amorphous silicon (a-Si) and aluminum structural layer is presented. The outer turns of the inductor have a vertical height of 450 mum when no voltage is applied. A 32% tuning range with high inductance (5.6-8.2 nH) is achieved by the application of a voltage, with the structure completely flattening at 2 V. With no actuation, the peak quality factor is 15, and the self-resonance frequency is 7 GHz. The fact that the device is fabricated on Si in a low-temperature (150 degC) process enhances the potential for system integration

Formation of nanoscale columnar structures in silicon by a maskless reactive ion etching process

We describe a maskless reactive ion etching process that employs CF4 gas plasma to create nanoscale structures in silicon. Process conditions are controlled to produce pillars of up to 2μm tall and less than 50nm wide. The contributing mechanisms are discussed based on the trends observed for varying plasma conditions. Higher pressures or lower self-bias voltages result in pyramidal structures. Lower pressure and higher voltage result in needlelike structures that resemble silicon wires. By carefully controlling the automasking process mechanism, columnar silicon structures were reproducibly formed with good uniformity all over the wafer. The regularity of the fabricated structures, process controllability, and process compatibility of dry etching are promising for potential photonic and optoelectronic applications.

Development of a low temperature MEMS process with a PECVD amorphous silicon structural layer

Amorphous silicon (a-Si:H) deposited at 150 °C by plasma-enhanced chemical vapor deposition (PECVD) is investigated as a structural layer for low temperature microelectromechanical system (MEMS) fabrication. The process development of depositing thick a-Si:H films and the material characterization of the film stress and hydrogen content is presented. To demonstrate a MEMS application, bimorph thermal actuators incorporating a-Si:H and aluminum were designed and fabricated resulting in tip deflection of hundreds of microns. The PECVD film coverage of various sidewall structures was also studied by scanning electron microscopy. Our a-Si:H films formed a sidewall coverage angle of 80° with the substrate. Mechanical simulations relate this angle within the range of minimum stress and maximum deflection of the actuator. Our results indicate that amorphous silicon is as an attractive structural material offering a low thermal budget for post-processing and integration of MEMS devices with complementary metal-oxide–semiconductor technology.

Trends in industrial silicon solar cell processes

Abstract The performance and technology of industrial silicon solar cells have improved considerably in recent years. Conversion efficiencies exceeding 18% are reproducibly obtained by cost-effective technologies on large area Cz-silicon. The performance of multicrystalline silicon cells is closing-in at 17.2%. Improved material casting techniques, a refined technology, and efficient in-process material improvement techniques are found to be the major causes behind such advancement. The trend to towards thinner substrates leads to considerable material cost reduction while yielding better performance. The major processing technologies and steps are critically discussed in this article, keeping in mind the priorities of todays PV industry: cost, and environmental issues. The future trends of the technology are outlined.

Three-dimensional modeling and simulation of p-n junction spherical silicon solar cells

A three-dimensional numerical model is presented to simulate spherical p-n junction silicon solar cells, which is a promising new technology for photovoltaic (PV) energy conversion for terrestrial applications. Material properties imposed by the sphere formation method, geometry of the device, and the specific device structure stemming from the fabrication technology are taken into account in the optical and electrical models of the device. The spherical device is numerically simulated based on these models using finite-difference method in a spherical system of coordinates, generating the internal quantum efficiency and current-voltage (I-V) characteristics of the device. It has been shown that the efficiency of a spherical solar cell is slightly lower than a conventional device; however, the slightly inferior performance does not outweigh the cost advantage. It has been also found that subsurface diffusion length from effective impurity segregation and the depth of the denuded zone in spherical devices are parameters that mainly affect the device efficiency. Based on the simulation and analysis, design guidelines have been presented for spherical PV devices.

Characterization of low permittivity (low-k) polymeric dielectric films for low temperature device integration

Spin-coated low-k dielectrics are now widely used in integrated circuit processing due to their low permittivity and planarization properties. Another area of potential application is in large area digital imaging using amorphous silicon (a-Si:H) technology where low-k dielectrics enable new integration schemes for thin film transistors (TFT) and sensors. In this work, the properties of spin-coated, polymeric, low-k dielectric materials, BCB (benzocyclobutene) and HSQ (hydrogen silsesquioxane), are studied after treating them with low temperature anneals. Fourier transform infrared spectroscopy (FTIR), high frequency capacitance–voltage, topographic planarization, and wafer deflection stress measurements have been used to characterize the films so as to correlate with processing conditions. Lower k values are obtained for lower process temperatures and are correlated by capacitance and FTIR measurements. Annealing in the presence of O2 appears to increase the permittivity. The BCB films yield low stress a...

Very High Brightness Quantum Dot Light-Emitting Devices via Enhanced Energy Transfer from a Phosphorescent Sensitizer

We demonstrate very efficient and bright quantum dot light-emitting devices (QDLEDs) with the use of a phosphorescent sensitizer and a thermal annealing step. Utilizing CdSe/CdS core/shell quantum dots with 560 nm emission peak, bis(4,6-difluorophenylpyridinatoN,C2) picolinatoiridium as a sensitizer, and thermal annealing at 50 °C for 30 min, green-emitting QDLEDs with a maximum current efficiency of 23.9 cd/A, a power efficiency of 31 lm/W, and a brightness of 65,000 cd/m(2) are demonstrated. The high efficiency and brightness are attributed to annealing-induced enhancements in both the Forster resonance energy transfer (FRET) process from the phosphorescent energy donor to the QD acceptor and hole transport across the device. The FRET enhancement is attributed to annealing-induced diffusion of the phosphorescent material molecules from the sensitizer layer into the QD layer, which results in a shorter donor-acceptor distance. We also find, quite interestingly, that FRET to a QD acceptor is strongly influenced by the QD size, and is generally less efficient to QDs with larger sizes despite their narrower bandgaps.

MEMS Multisensor Intelligent Damage Detection for Wind Turbines

Maintenance and repair of wind turbine structures have become more challenging and at the same time essential as they evolve into larger dimensions or located in places with limited access. Even small structural damages may invoke catastrophic detriment to the integrity of the system. So, cost-effective, predictive, and reliable structural health monitoring (SHM) system has been always desirable for wind turbines. A real-time nondestructive SHM technique based on multisensor data fusion is proposed in this paper. The objective is to critically analyze and evaluate the feasibility of the proposed technique to identify and localize damages in wind turbine blades. The structural properties of the turbine blade before and after damage are investigated through different sets of finite-element method simulations. Based on the obtained results, it is shown that information from smart sensors, measuring strains, and vibrations data, distributed over the turbine blades can be used to assist in more accurate damage detection and overall understanding of the health condition of blades. Data fusion technique is proposed to combine these two diagnostic tools to improve the detection system that provides a more robust reading with reduced false alarms.