Indoor photovoltaic energy harvesting for mm-scale systems

Low power electronic circuitry, including wirelessly interconnected sensor nodes, is a transformational technology that can be applied to a broad range of applications. These low power systems still require electrical power, ideally from ambient energy sources. Ambient sources of light can provide sufficient energy for these applications. Stray sunlight is more than adequate, though it is not available in all locations. Indoor lighting may also provide a sufficient energy source, though the characteristics of the spectrum are significantly different than the solar spectrum, where irradiance is confined to a narrower window in the visible spectrum. Energy-autonomous operation in mm-scale sensors have been achieved using photovoltaics based on silicon CMOS [1,2]. Improvements in energy harvesting are necessary to increase the duty cycle of the microsystem and to facilitate wireless transceivers. Photovoltaic cells consisting of materials with larger bandgap energy, such as GaAs, provide a better match to the indoor light spectrum, reducing thermalization losses and increasing power generation. The larger voltage provided by higher bandgap materials such as GaAs can also improve the efficiency of the overall system, where higher voltages are beneficial for the battery storage system and DC-DC converter. While the cost of GaAs photovoltaics is significantly higher than for silicon, and is currently prohibitive for large area solar energy production, the small power requirements and associated size requirements for photovoltaic cells makes GaAs an affordable option. Requirements for active and standby power are 10μW and 0.5nW, respectively[1,2], where perpetual operation may be achieved using a photovoltaic cell with area on the order of 1 mm².