Photovoltaic Energy Harvesting for Millimeter-Scale Systems

The Internet of Things (IoT) based on mm-scale sensors is a transformational technology that opens up new capabilities for biomedical devices, surveillance, micro-robots and industrial monitoring. Energy harvesting approaches to power IoT have traditionally included thermal, vibration and radio frequency. However, the achievement of efficient energy scavenging for IoT at the mm-scale or sub mm-scale has been elusive. In this work, I show that photovoltaic (PV) cells at the mm-scale can be an alternative means of wireless power transfer to mm-scale sensors for IoT, utilizing ambient indoor lighting or intentional irradiation of near-infrared (NIR) LED sources through biological tissue. Single silicon and GaAs photovoltaic cells at the mm-scale can achieve a power conversion efficiency of more than 17 % for silicon and 30 % for GaAs under low-flux NIR irradiation at 850 nm through the optimized device structure and sidewall/surface passivation studies, which guarantees perpetual operation of mm-scale sensors. Furthermore, monolithic single-junction GaAs photovoltaic modules offer a means for series-interconnected cells to provide sufficient voltage (> 5 V) for direct battery charging, and bypassing needs for voltage up-conversion circuitry. However, there is a continuing challenge to miniaturize such PV systems down to the sub mm-scale with minimal optical losses from device isolation and metal interconnects and efficient voltage up-conversion. Vertically stacked dual-junction PV cells and modules are demonstrated to increase the output voltage per cell and minimize area losses for direct powering of miniature devices for IoT and bio-implantable applications with low-irradiance narrowband spectral illumination. Dual-junction PV cells at small dimensions (150 µm x 150 µm) demonstrate power conversion efficiency greater than 22 % with more than 1.2 V output voltage under low-flux 850 nm NIR LED illumination, which is sufficient for batteryless operation of miniaturized CMOS IC chips. The output voltage of dual-junction PV modules with eight series-connected single cells is greater than 10 V while maintaining an efficiency of more than 18 %. Finally, I demonstrate monolithic PV/LED modules at the µm-scale for brain-machine interfaces, enabling two-way optical power and data transfer in a through-tissue configuration. The wafer-level assembly plan for the 3D vertical integration of three different systems including GaAs LED/PV modules, CMOS silicon chips, and neural probes is proposed.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163261/1/esmoon_1.pd

High‐efficiency photovoltaic modules on a chip for millimeter‐scale energy harvesting

Photovoltaic modules at the millimeter scale are demonstrated in this work to power wirelessly interconnected millimeter‐scale sensor systems operating under low‐flux conditions, enabling applications in the Internet of things and biological sensors. Module efficiency is found to be limited by perimeter recombination for individual cells and shunt leakage for the series‐connected module configuration. We utilize GaAs and AlGaAs junction barrier isolation between interconnected cells to dramatically reduce shunt leakage current. A photovoltaic module with eight series‐connected cells and total area of 1.27 mm2 demonstrates a power conversion efficiency of greater than 26% under low‐flux near‐infrared illumination (850 nm at 1 μW/mm2). The output voltage of the module is greater than 5 V, providing a voltage up‐conversion efficiency of more than 90%. We demonstrate direct photovoltaic charging of a 16‐μAh pair of thin‐film lithium‐ion batteries under dim light conditions, enabling the perpetual operation of practical millimeter‐scale wirelessly interconnected systems.We demonstrate monolithic GaAs photovoltaic modules at the millimeter scale to efficiently power wirelessly interconnected millimeter‐scale sensor systems operating under low‐flux conditions. Eight series‐connected cells are used to provide an operating voltage of 5 V for direct battery charging. Module power conversion efficiency greater than 26% is achieved under weak 850‐nm near‐infrared illumination and 90% voltage up‐conversion efficiency utilizing AIGaAs junction barrier isolation as a critical technique in reducing shunt leakage current.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149283/1/pip3132_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149283/2/pip3132.pd