An electronic device is provided with a display and a solar cell ambient light sensor that receives light through a portion of the display. The solar cell ambient light sensor may include one or more thin-film photovoltaic cells. A voltage that accumulates within the thin-film photovoltaic cell in response to ambient light is sampled and converted into ambient light data. The device includes control circuitry that modifies the intensity of display light generated by the display based on the ambient light data from the photovoltaic cell. The solar cell ambient light sensor is attached to a transparent cover layer, a color filter layer, or any other layer of the display. When the accumulated voltage is not being sampled for ambient light measurements, the voltage may be used to provide charge to a battery in the device.