Hallo Kelly,

your potis must not be referenced to pin 3, but to pin 8. Pin 2 must not be connected to ground.

When no light is falling on your sensor, output voltage is zero relative to potential at pin 8. So, your ADC gets 3.3V!! When sensor gets light, input of ADC increases to a voltage of >3.3V.

So, Steve is fully right: You need a differential amplifier stage performing Upin5 - Upin8, or the use of a real negative supply voltage, e.g. fabricated by a 'charge pump'.

Kai