## 1 Introduction  
There is an increasing demand for energy-efficient wireless sensor networks in different sensing and monitoring applications. These emerging sensors that mostly rely on energy harvesting, incorporate energy source, energy storage and electronic circuits for power management, sensing and communication into a miniaturized system. In order to have autonomous operation, different energy harvesting sources, including piezoelectric, electro-magnetic, thermoelectric, radio frequency and solar sources can be used. Solar energy harvesting can be a good energy source for autonomous microsystems in the order of a few cubic centimeters' size. However, as energy harvested from solar cells is intermittent, an energy storage device, such as a rechargeable microbattery or a supercapacitor, should be used for reliable operation of electronic circuits that are used for power management, sensor interfacing and wireless data transmission. The system level design of energy harvesting system starts by selecting an appropriate energy harvesting source and energy storage device, and the energy harvester circuit is designed according to the target energy source and energy storage. The proposed area- and energy-efficient solar energy harvester circuit harvests energy from a photovoltaic (PV) module consisting of nanowire solar cells in series [1] and stores it in a Vatta V6 HR NiMH microbattery [2]. A miniaturized PV module should have high efficiency to provide enough power, while other characteristics, such as open circuit voltage, short circuit current and maximum power point of the PV module, should be also considered to design a highly efficient solar energy harvester. The microbattery not only provides power for the proposed sensor interface and power management circuits, but also provides the required supply current during wireless data transmission in the target TZ1053 chipset [3]. The DC voltage that a PV module provides depends on the environmental conditions, including illumination level and temperature. Meanwhile, the battery voltage may change from its nominal voltage during battery charging and discharging. At room temperature, the voltage of the target NiMH battery reaches to an end-of-discharge voltage (V\({}_{\mathrm{EOD}}\)) of 0.9 V when the battery is discharged and reaches an end-of-charge voltage (V\({}_{\mathrm{EOC}}\)) of 1.5 V when the battery is fully charged. As a result, the DC voltage that the PV module provides may differ from the voltage of the target battery, depending on the illumination level and remaining charge of the battery. Different DC-DC converters have been proposed to harvest energy from a miniaturized PV module to charge a battery, including switched-capacitor (SC) [4] and inductive DC-DC converters [5]. These DC-DC converters either use external capacitors or inductors or operate at high frequencies to have high efficiency. However, by using higher frequencies, the power consumption of energy harvester circuits increases and a high efficiency is not achievable under a reduced illumination level when input power is only a few micro-watts. In addition, since under reduced light intensity, harvested energy from the PV module may be lower than the power consumption of the microsystem, the energy management circuit should dynamically reduce power consumption of the sensor to avoid complete discharge of the battery during system operation. As a result, the microsystem can continue its autonomous operation at a lower speed. The proposed energy harvester circuit reliably measures energy stored in the target NiMH microbattery and scales the power consumption of the microsystem up or down according to the energy stored in the battery. This circuit consumes less than 350 nW and achieves more than 90% efficiency during battery charging, using the direct charging method already proposed in [6]. As the use of hydrogen fuels becomes more common, an increasing demand for miniaturized hydrogen sensors is expected.