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 mm(2) demonstrates a power conversion efficiency of greater than 26% under low-flux near-infrared illumination (850 nm at 1 mu W/mm(2)). 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-mu Ah pair of thin-film lithium-ion batteries under dim light conditions, enabling the perpetual operation of practical millimeter-scale wirelessly interconnected systems.