Even if a phone sensor is 5% as sensitive as a larger format sensor (I'd guess more like 50%, large format sensors have slower and less frequent development times), it's still receiving solar energy. Phone cameras still have a (plastic, crappy) lens which focuses light down and microlenses plated on the sensor itself to further focus light directly onto the photosensitive regions of the chip.

Phones don't have physical shutters because they take up space and add unnecessary cost. I can't think of any phone use cases where the device is left outside in the elements for days at a time with a semi-direct angle towards the sun.