Using green energy to charge Internet of Things (IoT) devices is becoming a technically viable option. Green far-field Wireless Power Transmission can help powering IoT terminal devices. Keeping the maximum number of IoT devices (IoTDs) in their normal working modes is to furthest prolong the lifetime of the IoT network. In this work, for a given charging area, we first determine association between IoTDs and the green energy base station (GEBS), and then efficiently power IoTDs wirelessly. In green far-field wireless charging, we propose a dual-threshold model for IoTDs to facilitate wireless charging. In the dual-threshold model, we define two thresholds, i.e., Alarm Threshold and Working Threshold. Based on the dual-threshold model, we then propose the Dual-Threshold Orderly Charging (DTOC) algorithm to efficiently charge their surrounding IoTDs in a specific order to improve the charging efficiency. Finally, we validate the performance of the proposed algorithm through extensive simulations.