Spectrum-Dependent Photovoltaic Energy Harvesting

Indoor photovoltaic energy harvesting is a promising candidate to power millimeter (mm)-scale systems. Low-power photovoltaic energy harvesting allows for the deployment of fully autonomous small-scale sensors in environments not previously possible. Indoor lighting is one of those environments where the illumination intensity is typically below 1,000 lx and the spectrum is narrowly centered in the visible region. These conditions differ vastly from traditional solar cell testing conditions that have illumination intensities greater than 10,000 lx and contain significant ultraviolet and infrared light. Photovoltaics based on III-V compounds provide an outstanding choice for indoor lighting conditions due to their superior absorption, carrier transport, and corresponding high quantum efficiency in the visible spectrum. The theoretical efficiency and electrical performance of photovoltaics under typical indoor lighting conditions are analyzed. Commercial crystalline Si, amorphous Si, and fabricated GaAs and Al0.2Ga0.8As photovoltaic cells were experimentally measured under simulated AM 1.5 solar irradiation and indoor illumination conditions using a white phosphor light-emitting diode to study the effects of input spectra and illuminance on performance. The Al0.2Ga0.8As cells demonstrated the highest performance with a power conversion efficiency of 21%, with open0circuit voltages >0.65 V under low lighting conditions. The GaAs and Al0.2Ga0.8As cells each provided a power density of ~100 nW/mm2 or more at 250 lx, sufficient for the perpetual operation of present-day low-power mm-scale wireless sensor nodes. The path for achieving large light harvesting efficiencies will be discussed as well as the implementation of