Solar thermoelectric devices play a significant role in addressing the problem of global warming, owing to their unique features of converting both waste heat and solar energy directly into electricity. Herein, a flexible 3D Janus helical ribbon architecture is designed, starting from well‐aligned tellurium (Te) nanowire film, using an in situ redox process reacting with Ag+ and Cu2+ resulting in n‐type, p‐type, and photothermal sides in one film. Remarkably, the device shows all‐day electricity generation and large temperature gradient by coupling the cold side with a passive radiative cooling technique and the hot side with a selective solar absorption technique, showing a temperature gradient of 29.5 K, which is much higher than previously reported devices under a low solar radiation of only 614 W m−2. Especially, the device can still generate electricity even at night. The present strategy offers a new way for heat management by efficiently utilizing solar energy and the cold of the universe. A solar thermoelectric generator is fabricated with outstanding temperature difference and the property of synergistic utilization of solar energy and waste heat by in situ redox reaction to construct n‐type, p‐type, and photothermal sides in one nanowire film that is the reverse side of passive radiative cooling film. The ordered nanowire film also imparts the solar thermoelectric generator with better thermoelectrical performance.