Publications

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Microelectronics and microsystems have been part of the key technologies that enabled the incredible pace of development we have seen over the last five to six decades. This paper presents a basic view in terms of technology development supporting new approaches to generation and transfer of knowledge for critical activities, one of the key ones being the ability to capture, store and more efficiently use energy. © 2012 SPIE.

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We report on the application of MEMS and other microsystem technologies to photovoltaic (PV) cells, modules, and systems, taking advantage of several, significant benefits that are realized as the size of solar cells decrease to sub-mm length scales. To demonstrate these effects, we have developed both crystalline silicon and III-V PV cells. These cells are from 2 to 20 microns thick and from 250 microns to one millimeter across. We have demonstrated conversion efficiencies of up to 14.9% for a 14 micron thick crystalline silicon PV cell. This work contributes to two broad PV applications: 1) highly flexible PV modules with conversion efficiencies greater than 20%, and 2) commercial/utility scale PV systems using moderate concentration flat plate modules with simple single-axis or coarse dual-axis tracking. Cost models indicate that systems based on these technologies can achieve unsubsidized energy costs of less than $0.10/kWh. © The Electrochemical Society.

In this paper, we demonstrate gate-all-around (GAA) single crystalline nanowires (SiNWs) that are fabricated using top-down standard CMOS front-end processes. The GAA silicon nanowires are fabricated in well-defined locations with high-quality electrical contacts, and controlled geometry and alignment. These SiNW FETs fabricated in this process have demonstrated repeatable electrical performance with threshold voltages of ∼0.2V and subthreshold slopes of ∼80mV/dec. The p-i-n silicon nanowires are highly sensitive to the intensity and polarization of the incident light. The results in this work demonstrate that individual SiNWs are good candidates for high resolution optical sensing and allow for tuning of the optical properties of the nanoscale devices by precise control of the nanowire geometry and orientation of the incident light. These top-down fabricated SiNWs can be easily integrated in high density arrays for enhanced light absorption, resulting in imaging sensors with nanoscale spatial resolution. © 2011 IEEE.

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We present an approach to create ultrathin (<20μm) 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 2mm 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 14μm. With this technology, achieving high efficiency (>15%) and highly flexible photovoltaic (PV) modules should be possible. © 2011 IEEE.

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Microsystem-Enabled Photovoltaic (MEPV) cells allow solar PV systems to take advantage of scaling benefits that occur as solar cells are reduced in size. We have developed MEPV cells that are 5 to 20 microns thick and down to 250 microns across. We have developed and demonstrated crystalline silicon (c-Si) cells with solar conversion efficiencies of 14.9%, and gallium arsenide (GaAs) cells with a conversion efficiency of 11.36%. In pursuing this work, we have identified over twenty scaling benefits that reduce PV system cost, improve performance, or allow new functionality. To create these cells, we have combined microfabrication techniques from various microsystem technologies. We have focused our development efforts on creating a process flow that uses standard equipment and standard wafer thicknesses, allows all high-temperature processing to be performed prior to release, and allows the remaining post-release wafer to be reprocessed and reused. The c-Si cell junctions are created using a backside point-contact PV cell process. The GaAs cells have an epitaxially grown junction. Despite the horizontal junction, these cells also are backside contacted. We provide recent developments and details for all steps of the process including junction creation, surface passivation, metallization, and release.

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