An on-chip DC-DC converter for energy harvesting is proposed for a new battery-less solar cell tag applied to the Internet of Things (IoTs) devices. The main purpose of this design is to maximize the transfer efficiency and to stably provide a high enough supply voltage to the back-end loads. The overall circuit consists of a crosscoupled charge pump, a maximum power point tracking (MPPT) circuit, a timing control circuit and regulator. The input voltage range is from 0.45V to 3V. The output voltage can be boosted to the required level, in the range of 1V to 3.3V, with more than 60% power conversion efficiency. The throughput power is at least 500pW. A gated clock frequency modulation circuit is designed and adopted in the MPPT unit to lock the input resistance of the charge pump. In addition, a low-dropout (LDO) regulator circuit is used to provide a stable voltage to the load. The experimental results of chip show that the maximum power conversion efficiency (PCE) is 80% at 33pW input power condition.