Gregory N. Nielson

Dynamic pull-in of parallel-plate and torsional electrostatic MEMS actuators

An analysis of the dynamic characteristics of pull-in for parallel-plate and torsional electrostatic actuators is presented. Traditionally, the analysis for pull-in has been done using quasi-static assumptions. However, it was recently shown experimentally that a step input can cause a decrease in the voltage required for pull-in to occur. We propose an energy-based solution for the step voltage required for pull-in that predicts the experimentally observed decrease in the pull-in voltage. We then use similar energy techniques to explore pull-in due to an actuation signal that is modulated depending on the sign of the velocity of the plate (i.e., modulated at the instantaneous mechanical resonant frequency). For this type of actuation signal, significant reductions in the pull-in voltage can theoretically be achieved without changing the stiffness of the structure. This analysis is significant to both parallel-plate and torsional electrostatic microelectromechanical systems (MEMS) switching structures where a reduced operating voltage without sacrificing stiffness is desired, as well as electrostatic MEMS oscillators where pull-in due to dynamic effects needs to be avoided

Adiabatic Resonant Microrings (ARMs) with directly integrated thermal microphotonics

A new class of microphotonic-resonators, Adiabatic Resonant Microrings (ARMs), is introduced. The ARM resonator geometry enables heater elements to be formed within the resonator, simultaneously enabling record low-power (4.4 W/GHz) and record high-speed (1µs) thermal tuning.

Adiabatic thermo-optic Mach–Zehnder switch

In this Letter, we propose and demonstrate a high-speed and power-efficient thermo-optic switch using an adiabatic bend with a directly integrated silicon heater to minimize the heat capacity and therein maximize the performance of the thermo-optic switch. A rapid, τ=2.4 μs thermal time constant and a low electrical power consumption of P(π)=12.7 mW/π-phase shift were demonstrated representing a P(π)τ product of only 30.5 mW·μs in a compact device with a phase shifter of only ~10 μm long.

Integrated wavelength-selective optical MEMS switching using ring resonator filters

An integrated optical microelectromechanical system (MEMS) switch that provides wavelength selectivity is described. The switching mechanism is based on moving a MEMS actuated optically absorbing membrane into the evanescent field of a high-index-contrast optical ring resonator. By controlling the loss, and thus, the cavity quality factor, the resonant wavelength is switched between the drop and through ports.

Substrate-modified scattering properties of silicon nanostructures for solar energy applications

Enhanced light trapping is an attractive technique for improving the efficiency of thin film silicon solar cells. In this paper, we use FDTD simulations to study the scattering properties of silicon nanostructures on a silicon substrate and their application as enhanced light trappers. We find that the scattered spectrum and angular scattering distribution strongly depend on the excitation direction, that is, from air to substrate or from substrate to air. At the dipole resonance wavelength the scattering angles tend to be very narrow compared to those of silicon nanostructures in the absence of a substrate. Based on these properties, we propose a new thin film silicon solar cell design incorporating silicon nanostructures on both the front and back surfaces for enhanced light trapping.

Optimal cell connections for improved shading, reliability, and spectral performance of microsystem enabled photovoltaic (MEPV) modules

Microsystems enabled photovoltaics (MEPV) is a recently developed concept that promises benefits in efficiency, functionality, and cost compared to traditional PV approaches. MEPV modules consist of heterogeneously integrated arrays of ultra-thin (∼2 to 20 µm), small (∼100 µm to a few millimeters laterally) cells with either one-sun or micro-optics concentration configurations, flexible electrical configurations of individual cells, and potential integration with electronic circuits. Cells may be heterogeneously stacked and separated by dielectric layers to realize multi-junction designs without the constraints of lattice matching or series connections between different cell types. With cell lateral dimensions of a few millimeters or less, a module has tens to hundreds of thousands of cells, in contrast to todays PV modules with less than 100. Hence, MEPV modules can operate at high voltages without module DC to DC converters, reducing resistive losses, improving shading performance, and improving robustness to individual cell failures.

Microscale c-Si (c)PV cells for low-cost power

We are exploring fabrication and assembly concepts developed for Microsystems/MEMS technology to reduce the cost of solar PV power. These methods have the potential to reduce many system level costs of current PV systems including, among others, silicon material costs, module assembly costs, and installation costs. We have demonstrated a direct c-Si material reduction of approximately 20X (including wire-saw kerf loss and polishing loss). The cells have achieved efficiencies of almost 9% and Jsc of 30 mA/cm2. We are currently using integrated-circuit (IC) fabrication tools that will lead to higher efficiencies and improved yield. These advantages and the material reduction are expected to reduce the current module manufacturing costs.

Leveraging scale effects to create next-generation photovoltaic systems through micro- and nanotechnologies

Current solar power systems using crystalline silicon wafers, thin film semiconductors (i.e., CdTe, amorphous Si, CIGS, etc.), or concentrated photovoltaics have yet to achieve the cost reductions needed to make solar power competitive with current grid power costs. To overcome this cost challenge, we are pursuing a new approach to solar power that utilizes micro-scale solar cells (5 to 20 μm thick and 100 to 500 μm across). These micro-scale PV cells allow beneficial scaling effects that are manifested at the cell, module, and system level. Examples of these benefits include improved cell performance, better thermal management, new module form-factors, improved robustness to partial shading, and many others. To create micro-scale PV cells we are using technologies from the MEMS, IC, LED, and other micro and nanosystem industries. To date, we have demonstrated fully back-contacted crystalline silicon (c-Si), GaAs, and InGaP microscale solar cells. We have demonstrated these cells individually (c-Si, GaAs), in dual junction arrangements (GaAs, InGaP), and in a triple junction cell (c-Si, GaAs, InGaP) using 3D integration techniques. We anticipate two key systems resulting from this work. The first system is a high-efficiency, flexible PV module that can achieve greater than 20% conversion efficiency and bend radii of a few millimeters (both parameters greatly exceeding what currently available flexible PV can achieve). The second system is a utility/commercial scale PV system that cost models indicate should be able to achieve energy costs of less than

Ultrathin Flexible Crystalline Silicon: Microsystems-Enabled Photovoltaics

0.10/kWh in most locations.

Defect formation dynamics during CdTe overlayer growth

We present an approach to create ultrathin (<;20 μm) and highly flexible crystalline silicon sheets on inexpensive substrates. We have demonstrated silicon sheets capable of bending at a radius of curvature as small as 2 mm without damaging the silicon structure. Using microsystem tools, we created a suspended submillimeter honeycomb-segmented silicon structure anchored to the wafer only by small tethers. This structure is created in a standard thickness wafer enabling compatibility with common processing tools. The procedure enables all the high-temperature steps necessary to create a solar cell to be completed while the cells are on the wafer. In the transfer process, the cells attach to an adhesive flexible substrate which, when pulled away from the wafer, breaks the tethers and releases the honeycomb structure. We have previously demonstrated that submillimeter and ultrathin silicon segments can be converted into highly efficient solar cells, achieving efficiencies up to 14.9% at a thickness of 14 μm. With this technology, achieving high efficiency (>;15%) and highly flexible photovoltaic (PV) modules should be possible.