Wireless sensor nodes spend most of their time in standby mode and wake up periodically to send the measurement data. Conventionally, the wake-up function is realized using a radio-frequency oscillator with an amplifier to recognize an activating radio signal. Therefore, even during standby operation, the sensor node utilizes a certain amount of energy, which can be critical for an energy-harvesting source. In this study, we propose a novel approach of an optical wake-up for autonomous sensor nodes, which employs a solar cell as a wake-up signal detector. The bright light flash coming from another node or a smartphone exposes a solar cell, which activates the sensor node. Unlike photodiodes or RF-antennas, solar cells do not require any additional energy to detect the light signal. Therefore, the proposed electric circuit allows the sensor node to wake-up from a complete power-off state. The solar cell of the novel wake-up receiver has a sensitive area of 8 mm x 10 mm. The wake-up signal can be recognized from a maximal distance of 25 cm at ambient illumination from 0 to 1600 lx with a transmitter optical power of 20 mW. At power-off state the power consumptions are the lowest among all existing off-the-shelf wake-up receivers: 248 pW at 0 lx and 627 nW at 1600 lx.