A Stacked-Photovoltaic-Cell Energy Harvester with >81% Indoor Light Harvesting Efficiency for Millimeter-Scale Energy-Autonomous Sensor Nodes

We propose a light energy harvester that achieves harvesting efficiency of 82 - 98% at indoor light levels (70lx � 1klx) and powers a millimeter-scale system even at 40lx. It improves the efficiency by connecting stacked photovoltaic (PV) cells directly to a battery without diode voltage drop and implementing essential battery protection functions, such as battery discharging/overcharging protection and initial-battery-charging delay, in 334pA. The proposed circuit monitors open-circuit voltage of the stacked PV cells indirectly with a tiny auxiliary cell (only 0.16% area penalty) for battery discharging protection without interrupting energy harvesting. It also delays the initial battery charging by 3.9 hours, long enough to avoid accidental battery charging during a system assembly process.